a similar tube and wick. Usually, however, the oil is fed in at the top
only, except in the case of locomotives, because in them all the weight
falls on the top brass; hence, the bottom may be utilised as an oil
receptacle. In English locomotive practice this receptacle as a rule
merely catches the oil that has passed through the bearing box, but
sometimes a roller is inserted and forced against the journal by springs
so as to rotate, by friction, with the rotating journal.
[Illustration: Fig. 2489.]
The bottom of the roller runs in oil so that the roller feeds the
journal with oil, but ceases to feed when the journal ceases to rotate,
an advantage not possessed by self-feeding oil cups, or by the cotton
wick syphons shown in Fig. 2489.
The oil ways or oil grooves are usually provided in small journal
brasses as follows:--
[Illustration: Fig. 2490.]
[Illustration: Fig. 2491.]
[Illustration: Fig. 2492.]
It is obvious that if the joint faces of the brasses are left open and
oil be supplied to one brass only, a great part of the oil supplied will
pass out between the joint faces before reaching the other brass, and
one brass will therefore be better lubricated than the other, unless
each brass be lubricated independently. Even in this event, however, a
great part of the lubricating material will be lost from finding rapid
egress through the opening of the brasses. This may be to some extent
prevented in brasses whose joint faces lie horizontally by chamfering
the edges of the bore so as to form a trough extending nearly to the
ends of the brass, as shown in Fig. 2492. Now it is obvious that the oil
hole must always be above the journal or bearing bore; hence when the
joint faces stand horizontal, the oil hole should come through the crown
of the brass, and oil grooves are necessary to convey and distribute the
oil along the bore. A single groove, as in Fig. 2490, is sufficient for
light duty, but for heavy duty a double groove, such as shown in Fig.
2491, is necessary.
[Illustration: Fig. 2493.]
When, however, the joint faces stand vertically and come brass and
brass, the oil hole may be filed half in the joint face of each brass,
and the edges chamfered off as in Figs. 2492 and 2493, A B representing
the chamfers and C the oil hole, the two brasses put together appearing
as shown in section in Fig. 2493.
This plan has the advantage that the oil is confined within the journal,
except in so far as it may in time work through the ends of the journal
bore, while there are two oil grooves provided without reducing the
bearing or bedding area of the brass. When the oil grooves run
diagonally, as in Fig. 2491, there is the advantage that the length is
greater, and lying nearer to the plane of rotation the oil flows along
the grooves easier, being assisted by its frictional contact with the
journal, but on the other hand the bearing area of the brass on the
journal is so much the more reduced.
Oil holes that are not provided with oil cups should be provided with
small wooden plugs, which will serve to keep the dirt and dust out; they
should be made of as small diameter as the quantity and nature of the
lubricant to pass through them will admit of, and should be left plain
at the top and not countersunk, because the countersinking simply forms
a dish that will collect dust, &c., which the oil applied will carry
down into the bearing.
In some cases there is provided an oil dish around the oil hole, and
this dish is filled with tallow that will not melt under the normal
temperature at which the brass is supposed to operate. But if from
defective oil lubrication or other cause the bearing begins to heat, the
tallow will melt, and flowing through the oil hole afford the needed
lubrication.
It is to be observed that the lubrication of a bearing in which the
pressure is moved alternately from one half of the bearing to the other
is far easier to attain, and more perfect, than in one in which the
direction of the journal pressure is constant, because in the latter
case the journal pressure acts to squeeze out and exclude the oil
continuously, whereas when the pressure is relieved alternately on each
brass, the oil has an opportunity to pass back between the relieved
surfaces. Again the lubrication is more perfect when the direction of
the journal motion is periodically reversed, as the passage of the oil
through the bearing is retarded by the motion, and yet again the
abrasion is reduced because, as stated when referring to rotating radial
surfaces, the particles of metal abraded add themselves together and
form cutting pieces when the motion is continuous in one direction,
whereas in a reversing motion the particles are kept separated and flow
out more freely with the oil that passes through the journal.
If a shaft having a continuous direction of rotation be given end play
so that while rotating it may move endwise, the particles abraded are
again kept separated, and the conditions of lubrication are such as to
give a minimum of wear, because the formation of fine rings or serration
is avoided, the end motion serving to cause the wear to smooth the
surfaces.
[Illustration: Fig. 2494.]
When a shaft has a collar, that is subject to end pressure, the oil way
may be carried up the face of the collar as in Fig. 2494 at B. So also
where very free lubrication is required, an oil groove may also be cut
in the journal itself, as at C in the figure. This plan is adopted by
some American engineers upon the crank pins of steam engines, the
grooves being cut on diametrally opposite sides of the pin in a line
with the throw of the crank.
Referring now to the oil itself, it is generally conceded that a pure
sperm or lard oil is equal to any that can be used for general journal
lubrication, but the ordinary purchaser has no means of knowing if the
oil is pure. The requirements of an oil for lubricating purposes are
given in the following paper on testing the value of lubricants, which
was read by Mr. W. H. Bailey before the Manchester (England) Institution
of Employers, Foremen and Draughtsmen:--
"A fact in connection with oil and lubrication is probably about as
difficult a thing to describe as anything which agitates the minds of
engineers and mechanical men. We appear to have very little published
information on the subject, except that which describes the labors of
Morin, of France, about forty years ago, and that which has been given
to us by Professor Rankine more recently in this country. Those
investigators who preceded Morin do not appear to have published
information of very much value, or which can be used with profit for the
discussion of lubricants, for their researches have been more concerning
the proportions of bearings, and the value of different materials of
construction, rather than the value of different lubricants.
"At the present moment so little is known generally concerning the
performance of different oils, that the public are much at the mercy of
the vendors of these oils, who can make almost any assertion they like
without fear of contradiction.
"The valuable discoveries of our distinguished townsman, Dr. Joule, have
enabled us to look upon the cost of friction and the cash value of heat
as mere questions of arithmetic. Dr. Joule's investigations have been
put into such forcible and elegant English by Professor Tyndall, and
other students of the science of force, as to cause us to understand
that when friction is produced heat is lost, and that all energy thus
wasted passes away in this heat, which may be measured and valued with
nearly as much facility as any article of commerce. We may gather from
this knowledge, when we apply it to workshop economy, that if a pedestal
or bearing becomes so hot through friction as to cause 1 lb. of water to
be raised only one degree Fahrenheit in temperature in one minute, that
heat has been lost equal to that which would be created by a weight of
_one pound falling through a space of 772 feet_. We are told that if we
apply this conversely, that heat has been lost which would lift 1 lb.
weight 772 feet; and if we apply these illustrations still further, and
imagine forty-two pedestals or bearings losing heat by friction in a
similar manner, we may inform ourselves that we are losing nearly 1
horse-power, because they represent 32,424 foot-pounds of force; and if
we know from our books what our coal costs, it will take very little
trouble to give us the exact cash value of this friction and destructive
action.
"What is friction? It may be described as the effect produced by two
bodies sliding one upon the other, which have upon their opposing
surfaces minute asperities, which interlock with each other. The sliding
movement which forcibly removes these minute irregularities creates what
we call friction. Friction is reduced when these asperities are small,
and lubrication is resorted to to prevent that loss of power caused by
motion under these conditions. The chief lubricants used are oil and
tallow, which have a less coefficient of friction than the parts in
contact. It may be well now to state that the term 'coefficient of
friction' is an expression which indicates the proportion which
resistance to sliding bears to the force which presses the surfaces
together. There is little friction when this amounts to only
one-twentieth, it is moderate when it is one-tenth, and it is very high
when it is a quarter or twenty-five per cent. of the force which presses
the surfaces, together, as I before said.
"QUALITIES OF LUBRICANTS.--Good lubricants should have the following
qualities: (1) Sufficient body to keep the surfaces free from contact
under maximum pressure. (2) The greatest possible fluidity consistent
with the foregoing condition. (3) The lowest possible coefficient of
friction. (4) The greatest capacity for storing and carrying away heat.
(5) A high temperature of decomposition. (6) Power to resist oxidation;
or in other words, the influence of the atmosphere upon them. (7)
Freedom from corrosive action on the metals upon which they are used. It
will thus be seen that many conditions have to be carefully taken into
consideration; and further, it may be stated that an oil which may be
good for heavy bearings may not be desirable for use on light spindles,
and for delicate machinery like clocks and watches, where very little
power is required to be transmitted beyond that of overcoming their own
inertia; and also that oil which is good for small machinery running at
quick speeds is very often useless for heavy pressures and large
shafting. For very heavy bearings tallow and other solid lubricants are
used, such as mixtures of sulphur and tallow, asbestos, soapstone with
asbestos, graphite, caustic soda, beeswax, and other similar mixtures,
which find favor among locomotive engineers and those in charge of heavy
machinery. The pressure that can be borne by a good lubricant for a
useful length of time depends upon the nature of the bearings as well as
upon the lubricant itself. The velocity of the rubbing action also must
be taken into consideration. The maximum of pressure that solid
lubricants will bear without destruction is unknown. For steel surfaces,
lubricated with best sperm oil moving slowly, 1,200 lbs. pressure per
square inch of bearing surface has been found permissible. Under the
pivots of swinging bridges several thousand pounds per square inch have
been found to work, and for iron journals 800 lbs. per square inch
should not be exceeded.
"Lubricants in the market vary much in cost as well as in quality, and
very often it is found that the varying prices bear little or no
relation to the value of the article purchased. Probably the best test
of value is one with which I was familiar some years ago. It consisted
of a small engine very much overworked, which stopped and refused to
move or go at the proper speed if the shafting had not been lubricated
with good oil.
"TESTING BY DESTRUCTION.--The instrument here illustrated, in Figs. from
2495 to 2501, to which I call attention, consists of a bed-plate, having
upon it a piece of shafting upon which friction is created by means of
two brass steps, the speed at which it is driven being about 300
revolutions per minute. The friction is brought to bear by levers and
weights somewhat after the manner of a friction brake as shown in Figs.
2495 and 2500. In the top step is a thermometer for indicating any
increase of temperature caused by the friction. A small index indicates
the number of revolutions that the shaft makes for any given temperature
which the friction causes the thermometer to indicate. The machines used
for testing oil have the friction shaft where the oil is destroyed three
inches in diameter. Those for tallow are of larger dimensions. It will
be seen that on ascertaining the number of revolutions which may be
obtained without generating heat, or with the lowest possible increase
of heat, that the value of the oil can be obtained. That oil which
allows the greatest heat to accumulate with the fewest revolutions must
be a bad lubricant. This tabular method of keeping an account of
experiments has been found useful. The machine is stopped when the
thermometer indicates 200 degrees, as it is considered that an oil has
not much lubricating power left if it permits that heat.
------------+--------+--------------+--------------+------------------
Name of oil.| Price. |Revolutions to| Temperature |No. of revolutions
| |200 degrees F.|of atmosphere.| to each degree.
------------+--------+--------------+--------------+------------------
| | | |
------------+--------+--------------+--------------+------------------
"When testing with this machine a definite quantity of oil should be
placed on the friction roller, the top step being removed for that
purpose; the quantity should be about five drops. A glass tube or small
tin measure should be used, as drops vary in size according to the
temperature of the oil, and also differ with the specific gravity. The
inventor of this machine is Mr. Heinrich Stapfer. I believe he may be
considered the inventor of the first instrument for testing oils by
destroying them by friction under the actual conditions in which oils
are used as lubricants. In using this machine I found that, although it
was supposed to test lubricants in the way in which they are used in
manufactories, a slight difference existed, which prevented accurate
results.
[Illustration: Fig. 2495.]
[Illustration: Fig. 2496.]
[Illustration: Fig. 2497.]
"BEHAVIOR OF THIN OILS.--The first machines were made with the brass
steps lipped or recessed, to prevent the oil running away, (see Fig.
2496), which, when thus tested, gave results very much different to
those which are accepted by those who are familiar with the use of
lubricants. For instance, some thin mineral oils were found to be quite
as valuable as, and in some cases superior to, sperm; and this was
caused by the lips on the sides, which prevented the oil from running
off the bearing when an increased fluidity was caused by friction, and
by any slight elevation of temperature. This is a very important quality
in lubricating oils, probably next to the capacity to resist oxidation,
the most important to be criticised by those who wish to value a
lubricant. Although this experiment points out to us that it may be
advisable to make the journals of heavy bearings similar to these, if we
wish to obtain the best results from cheap thin oils, yet, as oil should
be criticised and prepared to be used on bearings with parallel necks,
such as are used in works, it was considered proper to alter the tester
to that shape to make the conditions similar. This illustration (see
Fig. 2497) permits the oil when tested to run away from the bearing if
its increased fluidity gives it a tendency to do so. It is this severe
test which has enabled sperm oil to rise superior to all rivals, because
it has these two apparently opposite attributes--body or thickness,
which keep it on its bearing, combined with sufficient fluidity for
lubricating purposes. Permit me further to illustrate what I mean in
another manner. Suppose we take an oil, good as a lubricant in all other
respects, and place it on a bearing, and that 40 per cent. works quickly
away because of its extra fluidity when subjected to an increase of
frictional temperature, and then compare it with another oil under
similar conditions which only wastes, say, 5 per cent. This latter will
be 35 per cent. superior as an oil having body, and even if slightly
inferior as a lubricant, it may be the most valuable, because strong in
this one great quality of remaining at its duty when placed in position.
Still another illustration will inform us that in the one case we
obtain, say, 60 gallons of lubricating material out of every 100
purchased, and in the other we obtain 95 gallons.
[Illustration: Fig. 2498.]
"THE BEST METHODS OF USING THIN OILS.--This will show us that oils which
are deficient in body, but which are good in other respects, may be used
with good results if doled out in small quantities, as required, by
automatic oil-cups like the Lieuvain needle lubricator, Fig. 2498, or
any other means. Journals which cannot be fed by means of automatic
oil-cups in positions difficult of access should be fed with oil which
has a good body. If time permitted, much might be said of the proper
shape for bearings of machinery--a subject which would lead to valuable
results if discussed by the members of this Society, many of whom must
have great experience of those designs which have produced the best
results, as well as of those mixtures of metals which are the most
durable for light high speed and heavy slow shafting. If any member will
take up this subject, or if several members will read short notes,
giving their actual experience of different sorts of footsteps,
pedestals, and spindles, as well as of the use of different sorts of
oil-cups and lubricators, it will be highly advantageous knowledge,
which must be of great value to all who use machinery.
[Illustration: _VOL. II._ =OIL-TESTING MACHINE.= _PLATE X._
Fig. 2501.
Fig. 2502.]
"FLUIDITY OF OILS.--Continuing my remarks on the thinness or fluidity of
oils, I wish to call attention to an ingenious arrangement for testing
the fluidity when subject to a slight increase of temperature, and also
for detecting any tendency which they may have for combining with the
oxygen of the atmosphere; this latter quality being advantageous in oils
which are used to mix with paint, but which is a great evil when used
for lubricating purposes. A piece of plate-glass placed at an angle is
made warm to 200° Fahr. A drop of oil when placed on the upper end of
this glass will flow down a few inches and thus indicate its fluidity
when subjected to increase of temperature. Fig. 2499 shows a ready
method I have designed for testing oil in this way. It consists of a tin
box in which is fixed the glass, through which can be seen a
thermometer. A graduated scale at the side of the box enables the track
of the oil to be measured. The box has a door at the back which enables
a copper vessel full of boiling water to be introduced; the box is lined
with felt to prevent rapid radiation, and when the door is closed it
will be seen that several experiments may be conducted before the
apparatus becomes too cool for use. I think this a cleaner way than
using a lamp for the purpose. The copper may also be used by itself for
indicating the behavior of oil on copper when slightly warm in making
it discolored or otherwise. As I have before stated, there are many oils
which are good lubricants, but which become too thin when exposed to
slight heat, and I do not hesitate to reiterate the statement, as I wish
to have some influence on the future designs of bearing in this
district. A correspondent writing to _Engineer_ from Queensland says
that for six months in the year oil runs off the machinery like water
and seems to have no lubricating power; he says that the thermometer
registers in the summer 140° in the sun, and 110° in the shade. Great
difficulty seems to have been experienced by him in keeping oil on the
bearings; his experiments on locomotives show that it costs for
lubricating a locomotive there about a halfpenny to three farthings a
mile, according to the mixture used.
[Illustration: Fig. 2499.]
"INFLUENCE OF THE ATMOSPHERE ON OILS.--There are some oils which are
excellent lubricants for the first few hours of use, but which have a
low capacity for resisting the influence of the oxygen of the atmosphere
upon them. The warm glass test may be used for indicating this weakness.
If after the test for fluidity the oil be permitted to remain on the
glass any exhibition of a resinous or varnish quality may be observed.
Another test for this resinous or gummy quality is one which has been
suggested to me by Mr. F. R. Wheeldon, of Bilston. He has made many
experiments. He found that by permitting oil to remain on a Stapfer
friction tester after one test which had been recorded, he tested again
on the following day, without adding any fresh oil. This is a severe
test, as the thermometer was made to indicate 200° Fahr. each time.
[Illustration: Fig. 2500.]
"LONGEVITY OF LUBRICANTS.--Supposing an oil to possess all the qualities
which we think a good lubricant should have--that it has fluidity in
season, and that it does not combine with the atmosphere and become
varnish, that it does not become like water in summer and like mutton
suet in winter, and is in most respects satisfactory. We then want to
know its powers of endurance, its capacity to resist wear and tear--in
other words, its longevity. A good test for longevity or durability of
oil when subject to either heavy or light frictional pressure is one
suggested by Mr. W. H. Hatcher, a very careful investigator, and chief
of the Laboratory of Price's Patent Candle Company, who are extensive
oil manufacturers. It consists in taking away the bottom step of the
Stapfer tester and placing a small dish containing oil underneath the
friction roller (as in Fig. 2500). This oil is carefully weighed before
and after several hours' frictional wear and tear. The drawing (Fig.
2501) shows the application of this mode, which I have designed, for
testing solid lubricants, such as lard and sulphur and other railway and
steamship mixtures. It will be seen that the material is kept to its
duty by the weighted lever, and its progress of diminution can be tested
in its place by the scale-beam arrangement. When it is used with the
pressure on the top step it is advisable to drive it at about 2,000
revolutions per minute; otherwise much time will be occupied in
destroying a weighable quantity of oil. The large Stapfer tester (Fig.
2502) was designed a few months ago for this purpose for the Government
railways of New South Wales, and it is also used by the Manchester,
Sheffield, and Lincolnshire, the Lancashire and Yorkshire, and other
railways. I have not been able to get any results of these tests in time
for our subject on this occasion, but hope to do so at some future time.
The frictional roller is 6 inches in diameter, the pressure amounts to 1
cwt. on each step. As it takes a considerable time to wear away half a
pound of solid lubricant, it may be advisable to measure by minutes
instead of using the speed index. The speed should be at least 1,500
revolutions per minute. The Stapfer tester should be used in a room of
equal temperature, and should not be subject to draughts of cold air, as
it will be obvious these will interfere with the indications of the
thermometer. A recent alteration in the Stapfer tester permits the
quantity of oil used for testing to be measured with greater accuracy
than before. A small oil-hole is made in the top step (see Fig. 2502 at
_a_ and at _c_) in which is placed a glass tube. This only holds a few
drops, and can be filled by simply dropping the oil in, holding the
finger at the bottom to prevent it running away, and then place it in
the hole. If a small needle lubricator be weighed and then filled with
oil of a definite weight, and placed in this hole (see Fig. 2502 at
_b_), oil may be tested for longevity and for its anti-frictional
properties for a longer period than with the small tube. If oil be
placed in this at the same time that oil is placed in the lubricators in
the works and the oil tester be driven from the same shafting,
permitting it to stop and start when the engine stops and starts, the
effect of a week's work upon the weight of the oil may be seen; notice
should be taken of the difference of the temperature between the
thermometer on the instrument and the temperature of the atmosphere of
the workshop.
"TESTING FOR SALTS AND ACIDS, ETC.--It will be obvious that however good
as a lubricant an oil is, and however valuable its properties may be
when examined, if it possesses any corrosive quality which will be
injurious to the metals upon which it is placed, it will soon become
detrimental to the machinery, and may also cease to be valuable as a
lubricant. Mr. William Thomson, analytical chemist, of Manchester, read
a paper on this subject at the British Association at Glasgow, and he
stated the results of elaborate experiments conducted by him to discover
the influence of various oils of commerce upon bright strips of copper.
He permitted the copper strips to remain entirely covered by oil. He
also conducted similar tests with half of the strip below the surface of
the oil, and the other half exposed to the atmosphere, in order to see
what influence the oil had, when the surface line touching the metal
would, of course, be acted on by the atmosphere. After noticing the
effect upon the brightness or dulness of the copper, he carefully tested
the oils in order to detect the quantity of metal which had been
dissolved. Mr. Thomson found the following oils dissolved the largest
proportions of copper, leaving the surfaces of the copper slips
bright--rape, linseed, sperm, raw cod-liver, Newfoundland cod, and
common seal oils; and that the following dissolved much smaller
proportions of copper, also leaving the slips bright--seal, whale, cod,
shark, and East Indian fish oils; and that mineral oils seem to have no
dissolving power on the copper, the only effect being a slight
discoloration on the copper slip of a greyish color.
"SWISS WATCHMAKERS' TEST FOR FLUIDITY AND CAPACITY TO RESIST COLD.--It
seems, according to the _Watchmaker and Jeweller_ (a monthly trade
journal), that the plan I have described, and what may be called the
warm glass test, seems to be looked upon with favor for testing oil in
Switzerland. The degree of heat used for testing the fluidity of oil is
200° Fahr., and if this causes the oil to become a varnish two or three
days after the test the oil is considered unfit for use. Another test is
one to which I have not alluded, and that is, capacity to resist low
temperatures. Oils are tried for their capability to withstand low
temperature in the following manner: Fifteen parts of Glauber salts are
put into a small glass vessel, a small bottle of oil to be tested is
immersed into this; this done, a mixture of five parts of muriatic acid
and five parts of cold water is placed over the salt. By means of a
thermometer the temperature is indicated, and when it shows a very low
temperature, the behavior of the oil, subject to this freezing mixture,
may be observed and noted. Mr. Thomson, however, considers that this
mixture is not so good or so cheap as ice alone, or a mixture of ice and
common salt.
"BLOTTING-PAPER TEST.--It seems it is considered that the blotting-paper
test for fluidity is more reliable, according to the writer of the
article, than the inclined plane experiment. In order to use this test
we must saturate the strip of blotting-paper with oil, and watch whether
the drops fall off in pearls or have an inclination to spread out. The
latter is a certain sign, the writer says, of a viscid oil. Although
this may be considered viscid oil, and may not be valuable for watches,
it may, however, be a good oil for heavier machinery."
The amount of friction between a journal and its bearing varies with the
kind of metal of which the journal and bearing are composed; on the area
of surface in contact in proportion to the load or pressure sustained by
the bearing surfaces; on the nature or degree of the lubrication
afforded; on the diameter of the journal in proportion to its length; on
the manner in which the journal fits or beds to its bearing, and on the
kind of motion, as whether the same be continuous, intermittent,
rotatory, or reciprocating.
Referring to the friction as influenced by the nature of the metals in
contact: the friction varies with the hardness of the metal; thus, with
hard cast iron, there will, under equal conditions, be less friction
than with soft cast iron. The friction is greater when the surfaces in
contact are both of the same metal than when they are of different
metals. Mr. Rankine summarizes General Morin's experiments on the
friction of various bodies not lubricated as follows:--
GENERAL MORIN'S EXPERIMENTS ON FRICTION.
--------------------------------------+----------------+------------
| Angle of |Friction in
Surfaces. | repose. |terms of the
| | weight.
--------------------------------------+----------------+------------
| degrees. |
Wood on wood, dry |14 to 26-1/2|.25 to .5
" " soaped |11-1/2 " 2 |.2 " .04
Metals on oak, dry |26-1/2 " 31 |.5 " .6
" " wet |13-1/2 " 14-1/2|.24 " .26
" " soapy |11-1/2 |.2
" elm, dry |11-1/2 " 14 |.2 " .25
Hemp on oak, dry |28 |.53
" " wet |18-1/2 |.33
Leather on oak |15 " 19-1/2|.27 " .38
" metals, dry |29-1/2 |.56
" " wet |20 |.36
" " greasy |13 |.23
" " oily | 8-1/2 |.15
Metals on metals, dry | 8-1/2 " 11-1/2|.15 " .2
" " wet |16-1/2 |.3
Smooth metal surfaces occasionally | 4 " 4-1/2|.07 " .08
greased | |
" " " continuously | 3 |.05
greased | |
" " " best results | 1-3/4 " 2 |.03 " .036
Bronze on lignum-vitæ, constantly wet | 3(?) |.05(?)
--------------------------------------+----------------+------------
"The 'angle of repose' given in the first column is the angle which a
flat surface will make with the horizon when a weight placed upon it
just ceases to move by gravity. The column of 'friction in terms of the
weight' means the proportion of the weight which must be employed to
draw the body by a string in order to overcome its friction, and the
proportionate weight is sometimes called the _coefficient of
friction_."[34]
[34] From Bourne's "Handbook of the Steam Engine."
In the following table are given some of the results obtained from
Morin's experiments with unguents interposed.
------------------------------+-----------------+---------------------
Nature of surfaces in contact.| Coefficient of |
| friction during | Kind of
| motion. | unguent.
------------------------------+-----------------+---------------------
Brass upon brass | .058 | Olive oil.
Cast iron upon brass | .078 | "
" " " cast iron | .314 | Water.
Steel upon cast iron | .079 | Olive oil.
" " brass | .056 | Tallow or olive oil.
Wrought iron upon brass | .103 | Tallow.
" " " cast iron | .066 | Olive oil.
" " " wrought iron| .136 | "
------------------------------+-----------------+---------------------
Morin's experiments demonstrated that friction is always proportional to
the pressure and independent of the area pressed in contact, providing
that the pressure is not so great as to cause the surfaces to abrade in
the manner or to the degree commonly known as cutting, which occurs when
the area of bearing surface in proportion to the pressure is so small as
to press out the lubricating material.
Now, between the degree of abrasion that is sufficient to cause a
bearing to heat and the minimum, possibly lies a wide range that is very
difficult of classification, and that influences the friction of the
bearing and journal. Under any given dimensions of journal area and any
given pressure of the same to its bearing, the abrasion, and, therefore,
the friction, will be less in proportion as the fit of the journal to
its bearing extends over its whole area and with an equal pressure of
contact. Under these conditions, and with a bearing area ample for the
given pressure, the surfaces of a journal and bearing have a smooth,
glossy appearance, with a surface as glossy as plate-glass.
This degree of perfection, however, is only occasionally reached in
practice, because of imperfections in the fitting and lubrication.
Now, between this condition of glossy smoothness and the degree of
abrasion known to practical men as _cutting_ lies, as already stated, a
wide range of degrees of abrasion, and each of these has its own
coefficient of friction. This may be readily proved by freely
lubricating the bearings of a number of journals working under the usual
conditions of practice and smearing the oil just as it passes through
the bearings upon a sheet of white note paper, when it will be found to
contain fine particles of metal, the number and size of particles in a
given quantity of the oil decreasing as the surfaces of the bearings are
glossy, and increasing as those surfaces appear dull.
The order of value to resist wear is generally considered in practice to
be as follows:--
1st in value, hardened steel running on hardened steel.
2nd (and by some considered equal to the first when the pressure per
square inch of area is light), cast iron either upon cast iron, hardened
wrought iron, or hardened steel.
3rd, under light duty cast iron upon wrought iron or steel not hardened.
4th, wrought iron upon hard composition or brass.
5th, wrought iron upon some anti-friction metal, as Babbitt metal.
Cast iron appears to be an exception to the general rule, that the
harder the metal the greater the resistance to wear, because cast iron
is softer in its texture and easier to cut with steel tools than steel
or wrought iron, but in some situations it is far more durable than
hardened steel; thus when surrounded by steam it will wear better than
will any other metal. Thus, for instance, experience has demonstrated
that piston-rings of cast iron will wear smoother, better, and equally
as long as those of steel, and longer than those of either wrought iron
or brass, whether the cylinder in which it works be composed of brass,
steel, wrought iron, or cast iron--the latter being the more noteworthy,
since two surfaces of the same metal do not, as a rule, wear or work
well together. So also slide-valves of brass are not found to wear so
long or so smoothly as those of cast iron, let the metal of which the
seating is composed be whatever it may; while, on the other hand, a
cast-iron slide-valve will wear longer of itself, and cause less wear to
its seat, if the latter is of cast iron, than if of steel, wrought iron,
or brass. The duty in each of these cases is light; the pressure on the
cast iron, in the first instance cited, probably never exceeding a
pressure of ten pounds per inch, while in the latter case two hundred
pounds per square inch of area is probably the extreme limit under which
slide-valves work; and what the result under much heavier pressures
would be is entirely problematical.
Cast iron in bearings or boxes is found to work exceedingly smoothly and
well under light duty, provided the lubrication is perfect and the
surfaces can be kept practically free from grit and dust. The reason of
this is that cast iron forms a hard surface skin when rubbed under a
light pressure, and so long as the pressure is not sufficient to abrade
this hard skin, it will wear bright and very smooth, becoming so hard
that a sharp file or a scraper made as hard as fire and water will make
it will scarcely cut the skin referred to. Thus in making cast-iron and
wrought-iron surface plates or planometers, we may rub two such plates
of cast iron together under moderate pressure for an indefinite length
of time, and the tops of the scraper marks will become bright and
smooth, but will not wear off; while if we rub one of cast iron and one
of wrought iron, or two of wrought iron, well together, the wrought-iron
surfaces will abrade so that the protruding scraper marks will entirely
disappear, while the slight amount of lubrication placed between such
surfaces to prevent them from cutting will become, in consequence of the
presence of the wrought iron, thick and of a dark blue color, and will
cling to the surfaces, so that after a time it becomes difficult to move
the one surface upon the other. If, however, the surfaces are pressed
together sufficiently to abrade the hard skin from the cast iron, a
rapid cutting immediately takes place, which is very difficult to
remove.
To obtain the best results from cast-iron bearings the bedding of the
journal to the bearing must be full and perfect, and the surfaces bright
and smooth, in which case it will wear better than hardened steel,
unless it be very heavily loaded.
Again, a cast-iron surface will hold the lubricating oil better than
either steel, wrought iron, or brass of any kind. Indeed, if a cast-iron
surface be made very true and smooth so that it is polished and no marks
are visible upon its surface, it will take _much patient_ rubbing and
cleaning with a _dry clean rag_ to remove the oil entirely, whereas
other metals will clean comparatively easy. In testing this matter upon
surface plates the author has found that the only safe method, and by
far the quickest, of removing the oil from cast iron is an application
of alcohol or spirits of turpentine, because the oil will enter and to
some extent soak into the pores of cast iron and gradually work out
again as it is continuously wiped, so that if apparently quite clean
(after having been oiled and wiped) a short period of rest will cause
oil to again be present to some extent upon the surface.
As a general rule motion in a continuous direction causes more wear
under equal conditions than does a reciprocating one, because when a
revolving surface commences to abrade, the particles of metal being cut
are forced into and add themselves, in a great measure, to the particles
performing the cutting, increasing its size and the strain of contact of
the surfaces, causing them to cut deeper and deeper until at least an
entire revolution has been made, when the severed particles of metal
release themselves, and are for the most part forced into the grooves
made by the cutting.
In reciprocating surfaces, when any part commences to cut, the edge of
the protruding cutting part is abraded by the return stroke; which fact
is clearly demonstrated in either fitting or grinding in the plugs of
cocks, in which operation it is found absolutely necessary to revolve
the plugs back and forth, to prevent the cutting which inevitably and
invariably takes place if the plug is revolved in a continuous
direction. Furthermore, when a surface revolves in a continuous
direction, any grit that may lodge in a speck, hollow spot, or soft
place in the metal, will cut a groove and not easily work its way out,
as is demonstrated in polishing work in a lathe; for be the polishing
material as fine as it may, it will not polish so smoothly unless kept
in rapid motion back and forth. Grain emery used upon a side face, such
as the radial face of a cylinder cover, will lodge in any small hollow
spots in the metal and cut grooves, unless the polishing stick be moved
rapidly back and forth between the centre and the outer diameter. If a
revolving surface abrades so much as to seize and come to a standstill,
it will be found very difficult to force it forward, while it will be
comparatively easy to move it backward, which will not only release the
particles of metal already severed from the main body, and permit them
to lodge in the grooves due to the cutting, but will also dislodge the
projecting particles which are performing the cutting, so that a few
reciprocating movements and ample lubrication will, in most cases, stop
the cutting and wash out the particles already cut from the surfaces of
the metal.
In determining the metals to be used for a journal and bearing it is
preferable to use the softer metal, or that which will wear the most, in
the position in which it can be the most easily and cheaply replaced,
which is usually in the bearing rather than in the journal; and since
two metals of a different kind run better together than two of the same
kind, the bearing is usually of a different kind of metal from that
composing the journal. It may be stated, however, that under _light
duty_ cast iron will wear upon cast iron better than wrought iron or
brass upon cast iron (for reasons which have already been stated),
especially if the bearing area be broad and the lubrication ample and
perfect.
To facilitate the removal of the bearings, brasses or boxes are
provided, but in the case of small journals, as, say, of about 3 inches
and less in diameter, the duty being light, the lubrication ample and
equally distributed, and the journals an easy working fit when new, it
is found that solid cast-iron boxes will last for a great length of time
without sensible wear.
In some cases cast-iron boxes are cast with a receptacle for some soft
metal, such as the various compound metals known under the general name
of Babbitt metal.
Babbitt metal is composed of tin, antimony, and copper, mixed in varying
proportions. A good mixture for general use where the duty is light is
composed of 50 parts tin, 5 parts antimony, and 1 part copper. A harder
composition, sometimes termed white metal, is composed of tin 96 parts,
copper 4 parts, and antimony 8 parts. This mixture is especially
suitable for journal boxes or bearings. It is mixed as follows: Twelve
parts of copper are first melted, and then 36 parts of tin are added; 24
parts of antimony are put in, and then 36 parts of tin, the temperature
being lowered as soon as the copper is melted in order not to oxidize
the tin and antimony; the surface of the bath being protected from
contact with the air. The alloy thus made is subsequently remelted in
the proportion of 50 parts of alloy to 100 tin.
For brass bearings or boxes a mixture of 64 parts copper, 8 parts tin,
and 1 part zinc is found to answer well; but for bearings not requiring
so hard a metal, the quantity of zinc is increased, and that of the tin
diminished.
[Illustration: Fig. 2503.]
Bearings or boxes that are to be babbitted are usually cast as in Fig.
2503, there being a rib at A, B, and C, forming a cavity at D, into
which the melted metal is poured. The ribs (in new boxes) are sometimes
bored out, or for rougher work may be chipped and filed out to fit the
shaft, and hold it in line; and to prevent the ribs A, B, &c., from
bearing and cutting the shaft, a piece of pasteboard is laid on ribs A
and B, thus confining the journal bearing to the babbitt. The best
method is to pour the bearing and then rivet the babbitt well into the
cavity D, which is made wider at the bottom, to prevent the babbitt from
coming loose, and then bore out the bearing in the usual manner.
The principal advantage of a babbitted bearing is the ease with which it
can be renewed, and the fact that the metal will soon bed itself to the
journal. This is of great advantage in the case of solid bearings in the
framing of fast-running machines, and in situations where it would be
awkward or difficult to take brasses or bushes out to fit them, or align
them to the shaft, which in many cases would also require to be taken
out to remove the brasses. On the other hand, any particles of grit that
may find ingress to babbitted boxes are apt to become bedded into the
babbitt metal and cut or grind away the journal.
Since the babbitt metal in a bearing is apt to close across the bore
when cooling after being poured, a mandrel of slightly larger diameter
than the diameter of the journal should be used in place of the working
journal to run the bearing on. Some effect the same purpose by wrapping
writing paper around the journal; but it is wrong to use the journal,
for the following reasons: To get a good, sound, well-fitting babbitt
metal box, the metal should be poured as cool as possible, for if made
red hot it contracts so much in cooling that it does not fit well in the
box or frame of the machine. On the other hand unless the metal be well
hot it is apt to cool and set too soon and be unsound. To remedy this
the journal, or whatever represents it, must be heated. The heating is
very apt to bend it. It is obvious then that instead of the journal a
temporary bar of iron of slightly larger diameter than the working
journal should be used, heating it to a good black hot heat, so that the
babbitt metal may be poured less hot than would otherwise be
permissible, and the contraction of the babbitt in the box reduced to a
minimum. A little powdered resin sprinkled in the box will help the
babbitt to flow easily and make a sound casting.
The temporary spindle, or journal, should also be oiled, and as soon as
the metal has well set, the temporary journal should be revolved to free
it. Babbitt bearings cast in two halves should be fitted to the journal
as already described for brasses, which will well repay the cost and
trouble.
To prevent the metal from running out of the bearing, its ends are
closed by means of either clay or putty closely packed against the
bearing ends and the shaft, and in pouring in the melted metal it is
best to pour it on the top of the shaft, and let it run down its sides
into the cavity of the bearing. This heats the shaft equally, and
prevents it bending from unequal expansion, as it would do if it met the
heated metal on its lower half only, it being obvious that if the shaft
bends the bore of the bearing will not be cast in line; hence, the shaft
will bear at the end only, and will require to wear the babbitt down to
a bearing.
Babbitting is sometimes employed to refit parts that have worn loose, or
to bush the bores of work. Suppose, for example, that in a case of
emergency a pulley of a certain diameter is required, and that the only
one at hand has too large a bore, then we may take a mandrel or arbor of
the diameter of the shaft the pulley is required for, and drive on it
two thin washers and turn them to fit the bore of the pulley, and cut a
recess in each to enable the metal to be poured through. We may then put
the arbor and washers in the pulley (the washers serving to hold the
arbor true), and fill in the bore with babbitt metal, leaving the pulley
set-screw in place and set to just touch the arbor, so as to cast the
thread in the babbitt bushing, and thus save drilling and tapping.
PROPORTIONS OF JOURNALS.--It follows from what has been already said
that under a given amount of duty the friction will be less and the
durability greater in proportion as the bearing area of a journal is
increased. But it is an important consideration whether such area shall
be obtained in the diameter or in the length of the journal, or, in
other words, what shall be the proportions between the diameter and
length of a journal. It is found in practice that a journal wears better
in proportion as its length exceeds its diameter, providing that the
stress is not sufficient to cause sensible flexure, because in that case
the pressure is reduced at that part of the journal where the most
flexure occurs, and increased where the journal is most rigid. As a
result, the abrasion increasing with the pressure becomes locally
excessive, the glossy smoothness is lost and increased friction ensues.
If, however, the length of a journal is limited by the exigencies of its
location or the design of the machine, the diameter of journal must be
increased if necessary in order to obtain sufficient bearing area to
withstand the stress without causing undue abrasion.
Referring to the bearing area in proportion to the load, Prof. R. H.
Thurston writes, in an article in the _Railroad Gazette_ of January
18th, 1878, as follows:--
"A pressure of 800 pounds to the square inch can rarely be attained on
wrought iron at even low speeds, while 1,200 pounds is not infrequently
adopted on the steel crank-pins of steamboat engines. I have known of
several thousand pounds pressure per inch being reached on the
slow-working and rarely moved pivots of swing bridges. In my own
practice, I never, if I can avoid it, use higher pressures than 600 and
1,000 on iron and on steel, and, for general practice, make the pressure
less as the speed is greater."
W. Sellers and Co. state that under a pressure of 50 lbs. per square
inch, and with oil well distributed over the surface of the box, the
metal of the journal will not touch that of the bearing box bore.
In practice bearings are made with a length varying from that equal to
the diameter of the journal to about four times that diameter, and but
few cases occur in which these limits are exceeded in either direction.
It is to be observed, however, that diminishing the length is apt to
increase the abrasion unless the duty is very light indeed, while
increasing it increases the durability while not affecting the friction,
unless the shaft bends.
There are special cases in which within certain limits the proportions
of journals are nearly uniform in practice; thus the length of engine
crank-pin bearings rarely exceeds once and a half times the diameter,
while the main shaft bearings are often similarly limited in width from
the exigencies of designing the engine so that the eccentric shall come
in line with the slide-valve spindle. In the case of crank-pins the pin
cannot be held sufficiently rigidly to prevent spring or flexure; hence
it is desirable to limit its length so that its pressure shall be as
short a leverage as possible to the crank. The solid bearings in the
framing of machines usually admit of room enough to make their lengths
three or four times the diameter. Again, in the case of line shafting,
boxes having a length equal to three or four times the diameter may be
employed, providing that the alignment be made correct, or that the
boxes are self-adjusting. But in all cases the longer the bearings the
greater the necessity for correct alignment, so that the axis of the
bearing bore may be in line with the axis of the shaft, the error
manifestly increasing with the length of the bearing.
PLACING TWO CRANKS ON A SHAFT SO THAT THEIR CENTRE LINES SHALL STAND AT
A RIGHT ANGLE.--It is obvious that the keyways in both the crank and the
shaft must be cut accurately in their proper positions, because it is a
tedious operation to file out the sides of the keyways when the cranks
are placed upon the shaft. To mark the keyways in the absence of any
tools or appliances specially designed for the purpose we proceed as
follows: Placing the shaft upon a marking-off table, we plug up the
centres upon which the shaft has been turned by driving a piece of lead
in them, leaving the surface level with those of the shaft; and then
from the perimeter of the shaft we carefully mark, upon the lead plugs,
the centres of the shaft. From this centre we describe a circle whose
diameter will be equal to the required widths of the keyway, and then
taking a square we place its stock upon the face of the marking-table,
and bringing the edge of the blade even with the edge of the circle, we
mark a perpendicular line upwards from the circle to the perimeter of
the shaft, and then draw a similar line on the other side of the circle,
as shown in Fig. 2504, in which A represents the shaft and B the circle,
C the perpendicular line struck on one side of the circle, and D the
square placed upon the marking-table E, in position to mark the line on
the other side of the circle, F and G being wedges to keep the shaft A
from moving its position upon the table. We next mark with a
scribing-block or surface gauge the depth of the keyway as denoted by
the line H, and the marking at that end of the shaft is completed.
Passing to the other end of the shaft we find the centre of the shaft,
and describe around it a circle equal in diameter to the required width
of keyway, and from the edges of the circle to the perimeter of the
shaft draw two lines with a scribing-block, as shown in Fig. 2505, A
representing the shaft, B the circle, C D the breadth of the keyway, E
the marking-off table, F and G the wedges, and H the depth of the
keyway, which must, in this case, be marked with a square resting on the
table.
[Illustration: Fig. 2504.]
[Illustration: Fig. 2505.]
If, however, the shaft is too heavy or large to be placed on a
marking-off table, we may proceed as follows: Strike as before the
circle B, Fig. 2504, equal in diameter to the required width of keyway,
and adjust a straight-edge held firmly against the end face of the
shaft, so that its upper edge is coincident with the perimeter of this
circle, while the straight is horizontally level-tested by a
spirit-level. Draw a line along the shaft face, using the straight-edge
as a guide. This will give us the line C in Fig. 2505. By a similar
process the line D, Fig. 2505, may be drawn. At the other end of the
shaft similar lines, but standing vertical, may be marked, which will
give the positions of the keyways.
[Illustration: Fig. 2506.]
We have now marked off on the end faces of the shaft a keyway at each
end, one standing at a right angle to the other; but it must be borne in
mind that we have paid no attention as to which crank shall lead; that
is to say, suppose in Fig. 2506 A and B represent cranks placed upon the
shaft C, and running in the direction indicated by arrow D, it is
evident that the crank B leads in the direction in which the engine is
to run, and hence the keyway E stands in advance of the keyway F; and
therefore, as shown in the figure, the right-hand crank leads. To have
made the left-hand crank lead, when the engine runs in the direction of
the arrow D, we should, supposing the keyway F to be already cut, have
to cut the keyway E on the directly opposite side of the shaft; or, what
is the same thing, supposing the keyway E to be already cut, the keyway
F would require to be cut on the diametrally opposite side of the shaft.
It is obvious that if the engine ran in the direction of the arrow G,
the left-hand crank would lead, supposing in each case the cylinders to
stand at H. Here it may be necessary to explain the manner of
determining which is the right-hand and which the left-hand crank.
Suppose then that the figure represents a locomotive crank, the
cylinders being at H, then as the engineer stands in the cab, facing his
engine, A will be the left-hand and B the right-hand crank. It is usual
in locomotives to make the left-hand crank lead when the engine is
running forward, the practical difference being, that if the workman
were by mistake to make the right-hand crank lead, the engine would run
forward when the reversing lever was placed to run backward, and _vice
versâ_. It makes no difference whether the shaft can be turned end for
end or not: if the right or left crank is required to lead when the
crank is required to revolve in a given direction the keyways in the
shaft must be marked off in the relative positions on the shaft
necessary to obtain that result.
The keyways may be carried along the circumference of the shaft by a
square applied to its end face, or if that face is not flat by the
ordinary keyway marking tool.
[Illustration: Fig. 2507.]
To mark off the keyways in the cranks, we place a centre-piece in the
bore of the crank, as shown in Fig. 2507, in which A represents a crank
having a centre-piece of sheet iron B placed in the bore. On the face of
this centre-piece we mark the centre of the hole into which it fits, and
from that centre we describe the circle C, which must be of exactly same
diameter as the crank-pin if it is in its place, or otherwise of the
crank-pin hole. We then draw the lines D and E, using as a guide a
straight-edge placed one end upon the crank-pin journal, or even with
the edge of the crank-pin hole, as the case may be, and the other end
(of the same edge of the straight-edge) exactly even with the
circumference of the circle C. From D and E we find the centre of the
circle F, which must be central between D and E, and whose diameter must
be exactly equal to the required width of keyway; and we then mark the
circle G, describing it from the centre of the hole, and therefore of
the circle C. By drawing the lines H and I, which must be even with the
circumference of the circles F and G, using a straight-edge as a guide,
we shall obtain the correct position for the keyway K, and the whole of
the keyways may be cut, care being taken to cut them quite true with the
lines, and of an exact equal width.
To put the cranks on the shaft, first provide a temporary key, a close
fit on the sides, but clear top and bottom, so that it will bind just
easily on the sides of the keyways in both the shaft and the crank. The
shaft must be placed and wedged with its keyway downwards, so that in
putting the crank on, the pin end may hang downwards, which will render
it more easy both to put on, handle, and adjust. As soon as the shaft
has entered the crank, say a quarter of an inch, we must insert the
temporary key (which may have its end edges well tapered off to assist
the operation of entering it) sufficiently far into the keyway of the
shaft that it will not fall out, and we may then proceed to put the
crank on the shaft to the necessary distance, keeping the temporary key
sufficiently far in the keyway to enable it to act as a guide--that is
to say, up to at least half the length of the keyway.
[Illustration: Fig. 2508.]
[Illustration: Fig. 2509.]
To put on the second crank, we first place the shaft so that the crank
already on stands exactly horizontal, setting it by placing a
spirit-level, as shown in Fig. 2508, in which A represents either the
crank-pin journal or the crank-pin hole in the crank, and B a circle
struck on the end face of the shaft and from its centre, the diameter of
the circle B being exactly the same as that of A. If then we so adjust
the position of the crank that a spirit-level applied to the exact
circumferences of the circles A and B stands level, the crank will stand
level, and we have only to put the second crank on with its centre-line
standing perpendicular, and the two cranks will be at a right angle one
to the other. We now proceed to put on the second crank, pursuing the
same method employed in putting on the first one, save that the
temporary key need not be inserted so far into the keyway, because, if
the keyways have been cut the least out of true, it will make a great
difference at the crank-pin, because of the increased distance of the
latter from the centre of the crank-shaft. As soon as the second crank
is placed to its position on the shaft we must ascertain if it stands
vertical, which we may do by applying the spirit-level as shown in Fig.
2509, bringing its edges exactly fair with the edges of the circles A
and B, and moving the crank until the bubble of the level stands true,
and taking out the temporary key if it is necessary to adjust the crank
at all.
If, however, the crank is to be forced on by hydraulic pressure, this
latter adjustment should be made when the crank is just sufficiently far
on the crank shaft to enable it to bind enough to well support its own
weight, to facilitate which the end of the shaft is sometimes slightly
tapered for a very short distance--a practice which is sometimes
rendered unnecessary by reason of there being attachments fitted to the
hydraulic presses which of themselves adjust the position of the cranks,
and insure their being at a right angle one to the other.
After the cranks are on their places the keys may be fitted, care being
taken that, if the crank last put on had to be moved to adjust it, the
sides of the keyways be filed even, otherwise driving the key will tend
to move the crank.
FITTING ENGINE CYLINDERS.[35]--When engine cylinders are made in
quantities, as in locomotive building shops, a great deal of the fitting
work is saved by the machine work; but when a single cylinder or a pair
of cylinders only are to be fitted up it will not pay to make jigs and
appliances; hence, they are usually fitted up entirely by hand. The
first thing to do is to mark off all the holes requiring to be drilled,
and have the drilling done.
[35] From the "Complete Practical Machinist."
In marking the holes in the cylinder covers it is to be noted whether
that part of the cylinder cover which fits into the cylinder has a
portion cut away to give room for the steam to enter (as is usually the
case), and if so, first mark a line across the inside flange of the
cover, parallel to the part cut away, and then scribe each end of the
line across the edge of the flange. Then mark a similar line across the
cylinder end, parallel to the steam port where it enters the cylinder,
and scribe each end of this line across the cylinder flange, so that,
when the cylinder cover is placed into the cylinder and the lines on the
flanges of the cylinder and the cover are placed parallel to each other,
the piece cut away on the cover will stand exactly opposite to the steam
port, as it is intended to do. The cover may then be clamped to the
cylinder, and holes of the requisite size for the tap (the tapping
holes, as they are commonly called) may be drilled through the cover and
the requisite depth into the cylinder at the same time.
[Illustration: Fig. 2510.]
The cylinder covers must, after being drilled, as above, be taken from
the cylinder, and the clearing drill put through the holes already
drilled so that they will admit the bolts or studs, the clearing holes
being made 1/16 inch larger than the diameter of the bolts or studs. The
steam chest may be either clamped to the cylinder, and tapping holes
drilled through it and the cylinder (the same as done in the case of the
covers), or it may have its clearing holes drilled in it while so
clamped, care being taken to let the point of the drill enter deep
enough to pass completely through the steam chest, and into the cylinder
deep enough to cut or drill a countersink nearly or quite equal to the
diameter of the drill. If, however, the steam chest is already drilled,
it may be set upon the cylinder, and the holes marked on the cylinder
face by a scriber or by the end of a piece of wood or of a bolt, which
end may be made either conical or flat for the purpose, marking being
placed upon it; so that, by putting it through the hole of the chest,
permitting it to rest upon the cylinder face (which may be chalked so as
to show the marks plainly), and then revolving it with the hand, it will
mark the cylinder face. This plan is generally resorted to when the
holes in the chest are too deep to permit of being scribed. To true the
back face, round a hole against which face the bolt head or the face of
the nut may bed, in cases where such facing cannot be done by a pin
countersink or a cutter used in a machine, the tool shown in Fig. 2510
may be employed, _a_ being a pin provided with a slot at one end to
admit the cutter B, which is held fast by the key C, and is also
provided with a square end _f_, by which it may be turned or revolved by
means of a wrench, and with a thread to receive the nut E, _d_ being a
washer; so that, by screwing up the nut E, the cutting-edges of the
cutter are forced against the cylinder _g_, and will, when revolved, cut
the face, against which they are forced, true with the hole in the
cylinder through which the pin _a_ is passed.
After the drilling the cylinder should be placed on end and all the
holes that can be got at should be tapped. Then the cover joint,
supposing it to be a ground joint, should be made according to the
directions given for making ground joints, when the cylinder may be
turned upside down and the other cover fitted. Then the holes for the
cylinder cocks and for the steam and exhaust pipe should be tapped, and
the faces for these pipe joints fitted as required.
The steam-chest holes should then be tapped and the ports marked out and
chipped and filed to the lines, such lines being marked as described in
the remarks on lining out work.
The face for the steam-chest seat and the steam-chest cover may then be
prepared by filing, scraping, or grinding, as may be required, and
simultaneously the valve seat and valve face may be fitted. If the
cylinders are to be bolted together as in a locomotive, the holes for
holding them together should be drilled about 1/64 inch smaller than the
bolts, so that they may be reamed out together after the cylinder bores
are aligned.
One cylinder face should be marked and drilled first, and the two
cylinder bores being set to align true the other cylinder should be
marked from the other, or if there is a saddle between the two cylinders
both cylinders may be marked and drilled, and also the holes on one side
of the saddle. Temporary bolts may then be put through the holes that
are drilled in the cylinder and saddle and clamps used to hold the
undrilled cylinder to the saddle, when the cylinder bores may be set
true one to the other, and the holes on the remaining side of the saddle
marked through those already drilled in the cylinder. These latter holes
being drilled, temporary bolts of smaller diameter than the holes (so as
to give room to move the cylinders to align their bores) may be used to
bolt the cylinders together while their bores are accurately aligned,
which alignment may be effected as follows:--
[Illustration: Fig. 2511.]
The bores should be set as near true as possible, tested by a
spirit-level rested on the bore and placed as near true as can be judged
with the length of the bore, and a plumb rule may be applied to the end
faces where the cover joint comes. Then a straight-edge should be
applied, as in Fig. 2511, in which S is the straight-edge, and C and D
the two cylinder ends.
The method of testing is shown in Fig. 2511, where the straight-edge S
is shown in three positions, marked respectively 1, 2, and 3 at one end,
and F, G, and H at the other.
[Illustration: Fig. 2512.]
[Illustration: Fig. 2513.]
The first test should be made by simply placing the straight-edge across
the two cylinder faces, as at G 3; and when the cylinders are set
apparently true and the spirit-level applied to the respective bores
shows them true, greater accuracy may be secured by placing the
straight-edge in position 1 H, being pressed firmly to its cylinder face
with end 1 above the other cylinder face. Then, while end H is held
firmly to its cylinder, let end 1 lower until it passes entirely over
the face of cylinder C, whose face it should just touch; if on meeting C
the straight-edge strikes it or does not meet it, further adjustment of
the cylinder positions is necessary. Next place the straight-edge in
position 2, pressing end F firmly against cylinder D, and passing the
other end entirely over the end of cylinder C, which it should just
touch, and no more. It will then be necessary to repeat this process,
pressing the straight-edge against cylinder C and testing the other end
with cylinder D, and the cylinders thus set will be (if the end faces
are true, as they should be, and usually are) more truly aligned than is
possible by the use of the spirit-level. This method also brings the end
faces of the cylinders in the same plane, so that each piston head will
travel central in the length of the cylinder bore, approaching the
cylinder covers equally, and therefore keeps the clearance equal.
Incidentally, also, this secures accuracy in the cross-head traverse on
the guide bars (supposing these bars to be bolted to the cylinder
cover). The holes for bolting the cylinders together may then be reamed
and the bolts driven in and screwed up.
To guide the tap when tapping the cylinder cover and steam-chest holes
the guide stand S, shown in Fig. 2512, should be employed. It is bolted
to the cylinder face by the bolt B, which passes through a slot in the
stand.
The tap T is inserted through the two arms of the stand and its end
inserted in the hole to be tapped when bolt B is tightened up.
The stem of the tap should be of slightly larger diameter than the tap
thread, so as to fit in the holes of the guide or stand.
When, however, one end of the guide bars is carried on the cylinder
cover, it is necessary when setting that cover to be marked for the
drilling, to so set it that the seats for the guide bar ends shall be
horizontally level when the cylinder is on the engine; and when setting
the bores of the cylinder in line to mark the holes for bolting the
cylinders together or to the saddle, this point should also be looked
to, as if these seats are not in line the faces of the guide bars will
not be in line, and will not, therefore, bed fair to the cross-head
guide unless the error is in some way corrected.
It is desirable that these seatings be quite true and in line one with
the other on both cylinders, so that if liners require to be made, or if
the ends of the bars require to be filed to let the bars together at any
time, the surfaces may be filed true to the face of the bar, and thus be
set true and to fit the cross-head guides without requiring to put the
bars on and off to fit them true by trial.
[Illustration: Fig. 2514.]
[Illustration: Fig. 2515.]
REBORING CYLINDERS IN THEIR PLACES ON THE ENGINE.--When a cylinder bore
becomes so worn out of cylindrical truth, or becomes grooved or cut, as
it is termed, as to require to be rebored, it may be done with the class
of boring bar shown in Fig. 2513. It consists of a bar having journal
bearing in castings which bolt on to the two ends of the cylinder in
place of the cylinder covers. On the bar is fitted a sliding head
carrying the cutting tool and fed by a screw passing within the bar. To
operate the bar and simultaneously the feed screw, the hand-wheel and
worm-wheel is employed, giving rotary motion to the worm-wheel which is
fast upon the bar. Fast also upon the bar is the inside one of the two
small gears shown, which operates the inner of the two small gears shown
above it. The outer of the upper gears engages with the outer of the
lower ones, the latter being fast upon the feed screw. In the inner pair
the lower is of largest diameter, but in the outer pair the upper is the
largest, and as a result the outer of the lower rotates the fastest, and
hence rotates the feed screw, causing the tool to feed to its cut.
The proportions of these wheels are, first or inside pair, lower wheel
36, upper 37; outside pair, upper 37, lower 36, so that the feed per bar
rotation is in amount that produced by moving the outer lower gear a
part of a rotation equal to twice the pitch of the teeth, the cutting
tool motion depending upon the pitch of the feed screw.
To enable the rapid traverse of the head from end to end of the bar, the
upper pair of gears are mounted on an eccentric stud, so that by
operating the small handle shown they may be disengaged from the lower
feed gears and the feed screw operated direct by means of the handle
shown.
[Illustration: Fig. 2516.]
[Illustration: Fig. 2517.]
In setting such a bar to a cylinder bore it is to be remembered that two
methods may be employed. First, the bar may be set to accommodate the
cylinder bore, truing it out with as light a cut as possible. In this
case the bore of the cylinder may be made out of line with the guide
bars and with the centre of the length of the crank-pin journal.
In the second the bar may be set with a view to bore it out in line with
the guide bars and crank pin, and then taking as much cut as will be
necessary to true the bore.
The latter plan is the preferable of the two, unless the repairs are so
extensive as to require the guide bars to be redressed and the main
bearing renewed, in which case those parts requiring to be re-aligned,
the cylinder may be rebored with a view to take out as little metal as
possible, and the other parts set to suit the new bore.
To set the bar true to the guide bars and crank pin, and thus retain the
axis of the new bore true with that of the original bore, the bar should
be set true with the recessed counterbore at each end of the cylinder,
which being unworn remains true.
If, however, only one cylinder cover can be conveniently taken off, the
piece of wood will require to fit in the counterbore at the open end,
and in the cylinder bore at the closed end of the cylinder; hence we
make it large enough for the counterbore, and after having removed the
ridge at that end we cut the length of the wood down to fit the cylinder
bore, whereas if we made our rest to fit the bore at first we should
require to use wedges to make it fit the counterbore. In some cases
holes might be bored near the ends of the rest or fulcrum to serve the
same purpose as the notches.
The method of using the scraper, Fig. 2516, is shown in Fig. 2514, which
latter represents an engine cylinder. At B is shown the wooden rest or
fulcrum; and at C the lever scraper operating on the ridge at the closed
end of the cylinder. The lever C is worked on the pulling stroke only,
and is so held that the edge presents a keen scraping tool which will
cut very freely. The fulcrum B should be adjusted as closely as
convenient to the work, so as to obtain good leverage for the scraper.
It should be moved in its position, so that during the roughing out only
the lower notches in the fulcrum are used.
A plan was lately resorted to on the White Star Line of steamships for
re-boring a cylinder. The cylinder heads and piston follower were taken
off; a groove was cut from the outer end of the cylinder along the bore
as far and as deep as the counterboring was required to be done. The
counterboring was then accomplished in the manner shown in Fig. 2515.
The junk ring was provided with a small tool holder, such as is used
upon boring bars. The tool was fastened in the holder while its cutting
edge was in the groove referred to, cut as deep and as far up the
cylinder as the counterboring was to be. To the junk ring was fastened,
by two long bolts, a wooden lever extending above and across the
cylinder. Two men walked around pushing the lever, and when the tool at
each revolution arrived at the groove, a fresh cut was taken by moving
the engine so as to raise the piston the necessary amount. It is obvious
that the piston head may be steadied and held true in the bore of the
cylinder by means of a few wooden wedges. Thus we see that in this
operation the junk ring was made to serve as a boring bar head, the men
furnishing the necessary rotative motion, the feed motion to the tool
being obtained by advancing the piston toward the end of the cylinder
where the work was being done.
The ridges which in time form at the two ends of a cylinder bore are
usually removed by the hand-boring bar shown in Fig. 2513, but they may,
in cylinders of from 12 to 24 or 30 inches in diameter, be readily cut
out by hand as follows:--
Take a bar of steel 9/16 inch square and 3 feet 6 inches long; forge it
at one end to the shape shown in Fig. 2516, in which from A to B is the
forged end. This end must then be heated along its entire length to a
cherry red, and dipped vertically into cold water to harden it; after
which it must be ground from A to B on all four faces square across, and
as nearly of an even curve as can be ascertained by the eye. Next take a
piece of hard wood--oak, for instance--about an inch thick and 3 inches
wide; cut it to such a length that when placed upright its ends will
wedge tightly into the counterbore of the cylinder. Into the edges of
this piece of wood saw out a series of notches, making its finished
appearance to be such as shown in Fig. 2517. The object of fitting its
length tightly into the counterbore of the cylinder is as follows: If
both cylinder covers are off or can be conveniently taken off, the ridge
can be operated upon at each end of the cylinder; hence our piece of
wood, which is merely an improvised rest to act as a fulcrum for the bar
scraper shown at the top of the figure, would require to fit into the
counterbore.
CHAPTER XXIX.--ERECTING ENGINES AND MACHINERY.
In engines having suspended guide bars, it is not uncommon to set those
bars by the working parts of the engine, instead of by lines. This is an
advantage when the parts of the engines are not taken down, and, if care
is taken, will make a true and smooth working job; but otherwise, it is
likely to introduce errors in the lining of the engine, which throw it
out of proper line, and cause a great deal of friction.
The proper method of setting the bars depends upon the condition of the
engine as to wear. Suppose, for example, that a new piston head has been
put in, then, if the gland is new also, or is a good fit to both the
piston rod and the bar of the stuffing box, the bars may be set as
follows:--
[Illustration: Fig. 2518.]
Place the piston at the back end of the cylinder, and put the cross head
and guide blocks in proper place on the rod. Put up the bottom guide
bars so that they just touch the cross-head guides. Now, in adjusting
these bottom bars there are two essential points: first, that the plane
of their upper surfaces shall stand parallel with the axial line of the
main shaft, and secondly, to place the upper surface parallel with the
axial line of the cylinder (it being of course assumed that the cylinder
and crank shaft are in proper line). The first of these essential points
will be attained when a spirit-level, placed truly along the bore of the
cylinder, shows the bubble to stand in the same position in the tube, as
it does when placed upon and along the bar. The second will be attained
when a spirit-level, placed across the bars, as in Fig. 2518, at A,
shows the bubble to stand the same as it does when the level is placed
on a parallel part of the shaft, as in the figure at B. When the bars
are thus temporarily set, the liners, or pieces of iron, may be fitted
to the proper thickness, so that the gland will pass in and out of the
stuffing box easily by hand, no matter in what position the piston may
be in the cylinder.
To get the thickness of these liners, take wedges made of iron, wood, or
lead, and insert the thin end between the faces of the bars and those of
the supports, forcing the wedges in sufficiently to leave a mark upon
them. By chalking the faces of the wedges they will exhibit the marks
more plainly. The wedges should be inserted at each end and on both
sides of the bar, for one bar at a time, the liners being got out a
trifle too thick so as to allow some for fitting.
If the liners require to be very thin and are difficult to hold in the
vice without springing, take a piece of soft wood faced true, and grip
it in the vice, and fasten the liner on it by means of brads driven in
around the edge of the liner.
When the four liners are ready place them in position between the bars
and their seatings. Bolt the bars firmly in position, wipe them clean
and test them lengthwise with the spirit-level to ascertain if they are
parallel with the cylinder bore, and place the level across the bars at
each end to test parallelism with the engine shaft, as in Fig. 2518,
and, having noted where further adjustment is necessary, put marking
upon the bars and move the cross head back and forth to ascertain how
much the respective liners require reducing. If the gland is a fit upon
the piston rod and in the stuffing box, moving the gland in and out of
the stuffing box will be an admirable test of the guide-bar adjustment.
[Illustration: Fig. 2519.]
The straight-edge should also be applied to test if the surfaces of the
bars lead true one to the other; thus, in Fig. 2519, A and B are the
bars and E the straight-edge, which by being pressed firmly to the
surface of A discloses that the surface of A is not in line with B,
because if it were so the straight-edge would meet the face of B as in
Fig. 2519, where the straight-edge F pressed to the surface of C leads
true to the surface of the bar D. All four of the bars require testing
in this manner. If the seatings for the bars or the liners are not made
flat and of equal thickness, or if from any other cause the bars do not
bed properly upon the liners, then bolting up the bars will spring them
as shown in Fig. 2520, in which, at A, is shown a bar sprung in the
bolting up, because the liners fit at the ends B C only; while at E is
shown a bar sprung or bent because the liners fit at the ends D D only.
In either case the cross head would be forced to travel in a curve,
bending the piston rod, and inducing much friction. The way to test the
bars in this respect is, after the above operations, and before
loosening the bolts, place a long straight-edge lengthwise along each
bar and move it laterally at one end. If it swings from the centre the
bar is rounding, while if it shuffles across first at one end and then
at the other the bar is hollow in its length and we must find at which
end of the bar this spring exists. To do this, slightly slacken the
bolt or bolts at one end and again apply the straight-edge, if the
spring is removed the error lies in the bedding of the liner at that
end. If not removed, retighten the bolts at that end and slacken those
or that at the other end, and again apply the straight-edge, and thus
may it be determined how much of the spring is due to each of the
liners, and this must be remembered and allowed for in filing the liner
to its final adjustment. Before putting the liners in a second time it
is better to give them a light coat of marking to show where they bear.
At each trial of the bars the spirit-level and straight-edge should be
applied and the cross head should be moved back and forth to show by the
bearing marks how the cross-head guides fit to the bars. These marks are
a great deal finer test than any spirit-level adjustment, hence the last
part of the fitting should be performed with strict reference to the
bearing marks upon both the bars and the cross-head guides as well as
upon the liner, the cross-head flanges being adjusted and fitted at the
same time as the face fitting.
[Illustration: Fig. 2520.]
To set the top bars place the cross head in the middle of its stroke,
and place them upon the cross-head guides. Then, with the wedges applied
as before, ascertain the required thickness of the respective liners one
at a time, leaving them, as previously, a trifle too thick, and testing
them while fitting by marking placed upon their faces. The top bars may
be entirely adjusted from the contact marks left by the cross-head
guides when moved along the bars, thus dispensing with the use of the
straight-edge and spirit-level.
As the bolts supporting the bottom bars often require to be loosened to
get the top bars off, pieces of wood may be placed beneath the bottom
bars to retain them in position when the bolts are loosened. These
pieces must be removed during the testing, for if left so as to wedge
the bars they may spring them, and thus mislead in the adjustment. After
the top bars are adjusted the whole bearing surfaces should be oiled,
and the cross head pulled back and forth by hand without the use of a
lever, providing the size of the piston does not exceed about eighteen
inches diameter. The bars when set true should be clamped to their
seatings and the holes reamed out to receive the proper bolts, and,
finally, mark each bolt, bar, and liner to its place.
When the bars, tested with the straight-edge and spirit-level as
described, show true, if the gland will pass freely in and out of the
stuffing box with the cross head at any part of its stroke, the guide
bars are set.
In this operation let it be noted that the close fit of the piston to
the cylinder bore and of the gland to the stuffing box is alone depended
upon as a guide whereby to so set the guide bars that the axial line of
the piston rod and its plane of motion shall be in line with the centre
of the crank shaft.
[Illustration: Fig. 2521.]
Suppose, however, that the piston head is a new one, and the gland is
worn a loose fit to the stuffing box, then setting the bar to the gland
would produce the result shown in Fig. 2521, in which the dotted line A
A is a line or cord stretched axially true with the cylinder bore, and
coincident with the centre of the pillow block at B. The gland being a
loose fit permits the piston rod to fall below its proper level, and the
surface of the bars, if set by the gland only, without using the
spirit-level, would be set true to the full line C C, and therefore out
of true line. If the bars are set by spirit-level true to the length of
the cylinder bore, the gland becomes useless as a guide to set the bars
by. It is a not uncommon practice, when the gland has play, to insert in
the stuffing-box bore, at the bottom, a piece of tin or sheet brass,
equal in thickness to one-half the amount to which the gland is too
small, or to insert a similar piece beneath the piston head if it is too
small. As a rule, however, there will be at least as much play between
the piston rod and the gland bore as between the gland and the
stuffing-box bore; hence, if there is any play, it is better to discard
the use of the gland altogether.
The proper method of setting guide bars by a stretched line is as
follows:--
[Illustration: Fig. 2522.]
The cord or line is set true to the cylinder bore, and coincident with
the centre of the pillow block, as at A A in Fig. 2521, and the two
bottom bars are put up in line horizontally with the axial line of the
crank shaft, and at a distance below the stretched line equal to
one-half the height of the guides for the cross head, as in Fig. 2522,
in which A represents the stretched line, B, B the bottom bars, C C a
straight-edge, and D a piece of wire whose length from point to point is
equal to one-half the height or thickness of the guide blocks. The width
apart of the bars is set to suit the width apart of the flanges of the
guide blocks on the cross head, by means of a square. The square is
applied in the following manner: On a straight-edge mark two lines A and
D, Fig. 2523, a distance apart equal to the distance between the flange
edges of the cross-head guides. Midway between A and D mark the line C;
place the straight-edge across the bars as shown, and when the edge of a
square, placed on the straight-edge, coincides with C and the stretched
line, and the marks A and D coincide with the edges of the bars, the
latter are set true, and will come right for distance apart, and
distance from the centre line, supposing the flange edges of the
cross-head guides to be equidistant from the centre of the length of the
cross-head journal. If, however, such is not the case, the width from A
to C and from C to D must be made to suit, C representing the centre of
the length of the cross head journal, D the flange on one guide and A
the flange on the other guide. Here it may also be remarked that, if the
thicknesses of the cross-head guides vary, or if they are not central to
the axial line of the cross-head journal, the bars must be set for
distance from A in Fig. 2523, to suit the error, because in that figure
the straight-edge is supposed to stand parallel to the axial line of the
shaft, as it is also in Fig. 2522, the aim in both cases being to so set
the bars that the cross-head journal shall stand parallel with the crank
shaft.
[Illustration: Fig. 2523.]
It is the liability of variation in the thickness of the guide blocks,
and of their not being central to the cross-head journal, that
constitutes the disadvantage of setting the bars by lines, it being
obvious that the bars must be either set to suit any errors in the
guides, or those errors must be eliminated before setting the bars. The
top bars must be set parallel to the bottom ones, at a distance from the
bottom ones equal to the thickness of the guide blocks, and parallel to
one another. It is preferable to set the top ones with the cross head
and guides in place, observing all the precautions as to springing them
given in the case of the bottom bars.
[Illustration: Fig. 2524.]
The bars thus set will be in line with the crank axle, but unless the
piston accurately fits the cylinder bore, they will not long remain in
line with the line of motion of the piston rod. For example, Fig. 2524
shows a piston head too small for the cylinder bore, the guides fitting
the bars properly, and the gland and stuffing box fitting the piston
rod; the piston will be suspended in the cylinder, its overhanging
weight being supported by the guides B, the gland, and packing ring.
This would cause friction and rapid wear of the gland bore and guide
surfaces in a direction parallel to the line C, which would gradually
let the piston fall to the bottom of the cylinder bore, touching at the
end of D first. In some engines provision is made to adjust the piston
to take up its wear, which is, it will be seen, a great advantage.
THE HEATING AND POUNDING, OR KNOCKING, OF ENGINES.--The heating of any
part of an engine occurs from one of two causes, viz., either the fit of
the parts is too close, inducing undue friction, or the parts are not in
line.
When the former is the cause the remedy is to ease the fit. If the parts
are not in line, the heating may also be remedied by loosening the fit
of the parts; but this will often induce a pound or knock, hence the
true remedy is to properly align the parts.
The location of a pound may be discovered by placing a piece of metal
wire between the teeth, and resting the other end of the wire upon each
end of the cylinder, guide bars, and bearings of the main shaft,
repeating the operation in each place, and the sense of feeling will
distinctly indicate the location of the knock, by imparting a more
severe shock to the teeth when the vicinity of the knock is approached.
The most prominent location of the causes of a pound are, first, in the
crank pin, from causes to be hereafter explained, and from its wearing
oval at the cross-head journal; and second, at the ends of the
cylinders, or the ends of the guide bars, because of a ridge forming
there as the wear proceeds.
A crank pin cannot wear oval if the brasses are kept adjusted to fit it,
because in that case the brass bore must wear it round; but if there is
any play it wears oval, because the pressure of contact between the
journal and the brass bores is least when the pin is at and near the
points of dead centre, and the most when it is at and near half stroke.
The cross-head pin wears oval because the pressure between the pin and
its bearing is in a line with the connecting rod, and there is but
little wear on the pin in a direction at right angles to the rod.
Ridges form at the ends of the cylinder bore and at the ends of the
guides for the following reasons:--
Referring to the cylinder, the location of the piston stroke in the
cylinder bore alters as the connecting-rod keys pass through the rod,
because that alters the length of the connecting rod, and therefore the
path of the cross-head guides on the guide bars, and also that of the
piston in the cylinder.
As the piston rod is shortened there is less wear at the extreme end of
the cylinder bore farthest from the crank, and the same remark applies
to the guide bars.
If the piston head travels past the end of the cylinder bore and into
the counterbore at each end, a distance equal to the amount of taper on
the connecting-rod keys, or equal to the amount the connecting-rod
length will alter while those keys are passed through the rod (to take
up the journal and brass wear), the piston head will (if the rod is kept
to its original length within that amount) always travel to the end of
the cylinder bore, and no ridge should form on account of the length of
the rod altering; but even then a slight ridge may form because the wear
is naturally less at the ends. Thus in the middle of the cylinder length
the whole thickness of the piston head, piston rings, and of the
follower passes over the bore, while at the ends only the flange of the
piston head at one end and the follower at the other passes over the
metal of the bore; hence the friction and wear are less.
The ordinary cause of pounding and heating is a want of truth in the
alignment of the crank pin, or in that of the cylinder, main shaft, or
guide bars.
[Illustration: Fig. 2525.]
The method to be employed to line an engine, or to discover if it is out
of line, depends upon the design of the engine and its condition; thus
an engine having a Corliss frame has the slides to receive the cross
head made at a true right angle to the end face which meets the cylinder
end; equidistant from the centre of the gland hole or axis of the piston
rod, and the end of the frame fitting either the bore of the piston or
the turned flange of the cylinder cover; hence the guide bars must be
true if the frame is got up true, the fit of the frame end to the
cylinder end insuring truth in the guide or cross-head slides, providing
that the centre line of the frame, during the turning and planing
operations, leads from the centre of the cylinder end of the frame to
the centre of the crank-shaft brass; or, in other words, the planing and
boring of the frame must be true with a line running from the centre of
the cylinder end of frame to the centre of location for the crank shaft.
This will not only cause the outside of the frame casting to stand at
its proper level when the cylinder bore stands horizontally level; but
it will insure that the crank-shaft bearing brasses both be of equal and
of a proper thickness through the crown.
The engine being properly lined at first will not be liable to get out
of line, excepting so far as affected by the wear of the crank-shaft
bearing, which will cause the crank shaft to drop, as shown in Fig.
2525, where A A represents the true centre line of the cylinder and
guide bars, which, when the crank is in the position shown in the cut,
should be coincident with the centre line of the connecting rod and the
crank, but the crank brass having worn below the centre line of the
connecting rod and crank, the crank will get out of line as denoted by
the line B B.
[Illustration: Fig. 2526.]
As a result, a portion of the piston movement and pressure which should
be exerted on the crank after leaving the dead centre, will be exerted
on it before it reaches the dead centre, thus causing a back pressure,
involving a loss of power. Furthermore, the relative position of the
eccentric to the valve gear will be altered, impairing the proper set of
the valves; hence it follows that the wear of the crank bearing in this
direction should be taken up (by raising the lower brass) before it
becomes excessive. To find how much the bottom brass requires raising,
or whether it requires raising or not, find the centre of the crank
shaft, and from this centre strike the circle B, in Fig. 2526, whose
diameter must equal that of the crank pin A, and place the edge of a
spirit-level coincident with the perimeters of the crank pin and circle,
as shown in the cut. When the bubble of the spirit-level stands in the
same position as it does when the level is placed upon the bore of the
cylinder or along the piston rod, the crank will be in line with the
cylinder bore.
As a rule, the cylinder bore of a horizontal engine stands horizontally
true, and the crank centre line should also stand so when the crank is
on its dead centre, but if such is not the case the crank centre line
must nevertheless stand true with the axial line of the cylinder, when
the crank is on the dead centre.
[Illustration: Fig. 2527.]
If, instead of having a Corliss frame and fixed guide bars, the engine
has a flat bed and adjustable guide bars, as shown in Fig. 2527, the
operation is as follows:--
In setting up a new engine it is obvious that if the flanges of the
cylinder are planed parallel with its bore and at the proper distance
from its axial line, and the pillow block is made of the proper height,
a line stretched axially true with the cylinder bore will pass through
the centre of bore of pillow-block brasses, or be equal in height from
the engine bed; but the length of the cylinder being only about
one-fifth of the distance from the cylinder to the centre of pillow
block, any error in the planing of the cylinder flange true to the
cylinder bore becomes magnified five times at the pillow block; hence it
is necessary to stretch a line through the cylinder bore and set the
cylinder so that the line, being axially true with its bore, will pass
the pillow block at the centre of the bore of its brasses. This is
sometimes done by inserting thin pieces of sheet tin, metal, or even
paper beneath the cylinder flanges and the bed, and in the requisite
positions. The method of stretching the line is shown in Fig. 2527. F is
a device for holding the line at that end. It consists of a frame in the
form of a cross, with adjusting screws at the end of each arm, and a
small hole at its centre to receive the line. The other end of the line
A must be secured, under as much tension as the line will safely bear,
to a piece of wood clamped to the engine frame at R. The adjustment of
the line is made by measuring its distance from the walls of the bore of
the cylinder at one end and of the bore of the gland hole at the other
end, using a pair of inside calipers or a wire gauge. The latter should
be bent in its length to admit of adjusting the same by straightening to
increase, or still further bending to diminish, its length to suit the
requirements.
The wire, when applied, should only just meet or touch the line and not
bear the least hard, or it will spring the line, causing an error of
adjustment that will be serious when multiplied by the length of the
line to the pillow block as compared with the length of the cylinder
bore.
If the pillow block is planed on its bottom face and has its brasses
fitted, the latter may be marked off for boring from the line A, Fig.
2527, when stretched to set the cylinder, thus avoiding a second
adjustment of the line A A.
Suppose now that it is required to line the brasses in the pillow blocks
true to be bored (the pillow blocks being bolted in position). The
distance of the face P, of the brass from the stretched line A, in Fig.
2527, must equal the distance from the centre of the length of the
crank-pin journal, to the face of the large crank hub, and this distance
may be shown by a line marked on the edge of the brass flange.
Place a straight-edge C, Fig. 2527, having a line D parallel with its
edge E, so that this line will be in the centre of the width of the
pillow-block jaws, and at a right angle to the line A. The line D will
then represent the axial line of the crank shaft, and may be used as the
centre from which to mark the lines on the brasses used to set them by
for boring. To test if A and D are at a right angle, or to set D to A, a
large square should be used. If the side face P P of the pillow block
stands parallel to A, as it should do when it is true, it will serve to
chuck the pillow block by, thus boring the brasses in their places in
the pillow block, with the centre line of the bore at a right angle to
P. If otherwise, two flat places should be filed on the brasses, as
shown in Fig. 2528, in which C is the straight-edge, and A the
stretched line as before, H and I representing the flat places whose
distance from A, as shown at J J, may be made to represent the thickness
of the crank from its large hub face to the centre of length of
crank-pin journal; hence the depth of the flat places will show how much
to take off the face of the brass to leave it of the proper thickness.
[Illustration: Fig. 2528.]
A straight-edge placed across these flat places, or true to the lines H
I, must stand at a right angle to the line D, so that by setting the
brasses by the flat places they will be bored to stand at a right angle
to A. To set the brasses the other way a circle is struck from D, as a
centre, upon the faces of the brasses as in the end view, Fig. 2528, in
which the straight-edge C is shown wedged in the bore of the brasses,
which is the most convenient way when it can be done.
The line D is carried down on the end face of the straight-edge, and the
latter is used as a support for the compass points while striking the
circle M, which is defined more clearly by indenting it with fine
centre-punch marks. The height of the centre for bore of brasses may be
carried from the centre line of the cylinder A A to the end of the
straight-edge C, by placing another straight-edge across the engine bed
and measuring from the end of C to A.
Suppose now that the brasses are bored, and the position of the pillow
block is to be set, and the process is the same, the line D being marked
true from the bore of the brasses, and the pillow blocks adjusted until
D is at a right angle to line A A.
Though in a new engine every part may be made as true as possible in the
details of manufacture, yet when the parts come to be put together
errors of alignment will generally be found to exist. These errors may
be too minute for discovery in the separate piece, and yet form
important defects in the finished engine.
In rough practice these defects are left to remove themselves by
abrasion and wear, the process being to allow the parts to be somewhat
loose (wherever possible) in their adjustment, and adjust them closer as
the abrasion proceeds.
This is termed letting the parts _wear down to a bearing_. But the very
process of wearing _down to a bearing_ attests that the parts have not
been properly fitted to a bearing, whereas to attain the best possible
results the parts should be fitted to a bearing, because in wearing down
to a bearing, undue abrasion, and to some extent or in some degree,
roughness of the wearing surfaces, must ensue, because the strain
intended to be distributed over the whole _intended_ bearing area is
limited to the _actual_ bearing area. It is necessary, therefore, that,
in putting an engine together, each part be properly fitted to its
place, and that it be subsequently adjusted in its fit and position with
relation to the other parts to which it is connected.
The fitting of the single piece is a test of its individual or
disconnected truth; the subsequent or second adjustment is a test of its
truth with relation to the others. Thus a pair of brasses may fit a
journal perfectly, but that is no assurance that the brasses are so
bored as to bring the rod holding them in proper line to enable
connection at the other end without springing or bending the rod.
Furthermore, it often happens that the frame work of an engine does not
form a base for the whole of the parts, thus in a large stationary
engine, the end of the main shaft or crank shaft farthest from the crank
(generally called the _outboard_ bearing) is generally supported by a
bearing having an independent foundation, and as this foundation does
not exist until the engine comes to be permanently fixed for operation,
its alignment must be performed when setting the engine. In an old
engine this foundation may settle, or the wear itself may throw the
engine out of line, so that the lining of an engine becomes periodically
a necessity.
As a general rule a want of alignment induced by wear or incurred from
repairs to the parts principally affects the main shaft, the cross head
remaining more nearly true; and, with the exception of the crank pin,
the same holds good with reference to a new engine.
Now while an error of alignment may exist in any direction, it is true,
nevertheless, that an error in any direction will be discoverable if the
parts be tested at four equidistant parts of the stroke or revolution,
as, for instance, on the two dead centres of the crank and at the
highest and lowest points of the path of rotation of the crank pin;
hence attention may be confined to those four points.
Suppose then an engine already put together requires to be tested for
being in line, and we have to test--
1st. The alignment of the main or crank shaft vertically.
2nd. The alignment of the main shaft horizontally.
3rd. The axial truth of the crank pin with the main or crank shaft.
4th. The adjustment of the crank shaft for vertical height, with
relation to the cross-head journal.
Referring to this last, it may be necessary to remark that the axial
line of the main shaft may be parallel when viewed either vertically or
horizontally with the cross-head journal, and yet if a line be passed
through the centre of the cylinder bore, and prolonged past the crank
centre, the latter may fall above or below that line, but it will
generally be below, because from the weight of the crank shaft its
bottom bearings wear the most; and, further, to whatever extent those
bearings wear after being in proper line, the crank shaft will fall too
low.
We may now subdivide the errors of alignment of a crank shaft thus:--
1st. Its axial line, when viewed vertically, may form an acute angle to
the axial line of the cross-head journal.
2nd. It may form an obtuse angle with the cross-head journal when so
viewed.
3rd. It may, when viewed from the crank-pin end of the engine on about a
horizontal position, be too high or too low at the crank-pin end only.
4th. It may be too high or too low at the outboard end only.
5th. It may be too high or too low at both ends, although parallel to
the cross-head journals.
It will be found on consideration that with the exception of the
last-named case, the connecting rod forms the best test whereby to
discover an error in any of these directions, because it magnifies the
error and makes it more plainly discernible. It will further be found
upon careful observation, that although a combination of these errors
may exist, the connecting rod will serve to discover each error
separately, as well as the collective error, because, although in some
respects two distinct errors may have the same general result, yet the
result will be different if taken in detail, and it follows, therefore,
that the testing must be taken or made in detail first.
[Illustration: Fig. 2529.]
To test the parallelism of the axial line of the crank shaft with that
of the cross-head journal, when viewed vertically: In Fig. 2529, let A A
represent a line true axially with the bore of the cylinder, and B B a
line at a right angle to A A, and passing through the centre of the
pillow block or bearing spaces. If the engine were in line, B B would be
coincident with the axial line of the crank. Suppose, however, that line
B C represents the actual centre line of the crank, not then being at
right angles to A, the end E of the connecting rod, if connected to the
crank pin as shown, and made a good working fit so that there is no play
of the pin in the brasses, will not come fair laterally with the bearing
in the cross head. The amount of the error is the amount it is out of
true in the length of the crank-pin journal, multiplied by the product
of the length of the connecting rod (from centre to centre of the bores
of the brasses) divided by the length of the crank-pin journal. It is
apparent, however, that if the crank shaft be set to have its axial line
at B B instead of at B C, the error at E D will be corrected, and thus
we may employ the connecting rod to set the crank shaft in line.
It is, however, not sufficient to try the crank on one dead centre only
(as will be seen presently), hence we place it on the other, and move
the cross head to the other end of its stroke, and again try the end E
of the connecting rod with the cross-head journal, and if it falls to
one side, and _on the same side as before, but to a less amount_, it
demonstrates that the axial line of the crank forms with the line A A an
acute angle. If, however, instead of falling too much laterally towards
the side F of the cross head, it fell too much towards D, but more so
when tried with the crank on the dead centre nearest to the cylinder
than when tried with the crank on the other dead centre, then it is
proof that the axial line of the crank shaft forms with A A an obtuse
angle.
[Illustration: Fig. 2530.]
The reason that the error will be more plainly shown with the crank on
one dead centre than when on the other is shown in Fig. 2530, in which A
A is a line coincident with the axial line of the cylinder bore, and B B
the axial line of the crank shaft, from C to D is the plane of
revolution of the crank pin, while G represents the crank centre. The
points at C and F denote points central to the length and diameter of
the crank-pin journal. Now, the centre line of the connecting rod for
one dead centre is represented by E D, and for the other by F C, and it
will be seen that the point at E is farther from A than is the point at
F. It will be observed that the point D falls _outside_, while the point
C falls _inside_ of A A, and yet the centre line of the connecting rod
stands, in both cases, at the same angle to the centre line A A of the
engine, and in both cases throwing the end of the connecting rod,
represented by the points at E F, _outside_ the line A A.
If the connecting rod does not, when connected to the engine, as in Fig.
2529, fall true into the cross-head bearing, the error is the same _in
amount_ and comes on the outside of the cross-head journal with the
crank placed on each respective dead centre, it is proof that either the
flange of the crank-shaft brass (which is between the crank face and the
frame) is too thick, or the inside flange of the connecting-rod on the
crank pin is too thick, or else the crank is too thick, measured from
the crank-pin journal to its inside hub face, the error being in the new
crank or new brass, if one has been put in.
[Illustration: Fig. 2531.]
It may here be remarked that if the bore of the crank-pin brasses of the
connecting rod is not at right angles to the centre line of the rod
itself, the end E, Fig. 2529, might fall either inside or outside,
laterally, of the cross-head bearing, but in this case the error will
show more at one end of the stroke than at the other, for reasons which
are explained with reference to Fig. 2530; hence it follows that the
connecting-rod brasses should be properly fitted to their journals, and
made to lead true before using the rod to line the engine by. In some
cases it is more convenient to connect the rod at the cross-head end,
and try the other end with the crank-pin journal, as shown in Fig. 2531.
In this case, however, the connecting rod will (whenever the axial line
of the crank shaft is out of square, forming an acute angle with the
centre line A A, as in Figs. 2529 and 2530), fall laterally inside the
crank-pin journal when on one dead centre, as in Fig. 2531, and outside
when on the other dead centre, as in Fig. 2532, the respective amounts
of error being in this case equal for the two positions. The reason for
this is that the plane of revolution of the crank pin falls outside of
the centre line in one case, and inside for the other, as shown in Fig.
2530 at D C.
[Illustration: Fig. 2532.]
[Illustration: Fig. 2533.]
[Illustration: Fig. 2534.]
[Illustration: Fig. 2535.]
If the axis of the crank axle formed an obtuse angle to the engine
centre line A A in Fig. 2529, the connecting-rod end tried with the
crank pin, as shown in Fig. 2531, would fall outside of the crank-pin
journal when the latter was on the dead centre nearest to the cylinder,
as shown in Fig. 2534, and inside of the crank-pin journal when on the
other dead centre, as in Fig. 2535.
Now, suppose either of the errors to exist, and the alignment be
neglected, then if the brasses at each end be keyed up to fit their
respective journals, then the body of the rod must be bent into a bow
shape, and the strain of forcing or springing it into this shape will
fall upon the journals, which will heat and pound in consequence.
It is now to be explained how to test if the axial line of the crank
shaft is at a right angle to that of the cross-head journal, when viewed
from the crank-shaft end and horizontally.
From a want of parallelism in this direction, heating of the crank pin
and cross-head journals is _sure_, and a pound or thump is, to some
extent, liable to occur, and the cause, if the error is slight, is
difficult to discover, save by using the connecting rod to test it with.
When a thump occurs at the end of the stroke (when the crank is on a
dead centre), it may arise from a ridge at the cylinder, or at the
guide-bar end, or from the connecting-rod brasses being insufficiently
keyed up; but when it occurs while the crank is at half stroke these
causes are eliminated, and the cause must be looked for in either a
crank pin not parallel to the crank shaft, or, as in the case now under
consideration, because of one or the other of the crank-shaft journals
being too low.
Assuming the crank pin and crank shaft to be axially true, one with the
other, we may proceed to show separately the cause of the heating and
that of the pounding, if the crank journal is too low at either end.
[Illustration: Fig. 2536.]
In Fig. 2536, let A represent the cross-head journal, and B B a line
parallel to it. Let B C represent the axial line of the crank shaft
(being out of parallel because the crank end is too high or the other
end too low). Let F F represent the centre line of the crank pin when at
the top, and G G when at the bottom of its path of rotation, and it will
be observed that the vertical distance between the crank pin and the
axial line of the cross-head journal is less on one side than on the
other; thus in the figure distance D is less than E. We have in this
case measured these distances on a plane at a right angle to the
cross-head journal, but it will make no difference if we measure them on
a plane with the path of rotation of the crank pin, as will be seen in
Fig. 2537, in which the distance from the centre of the crank pin at two
opposite points in its path is represented by dots shown at E F, and
from E to H measures less than from F to H, H representing the centre of
the cross-head journal.
[Illustration: Fig. 2537.]
In Fig. 2537, let A represent the axial line of the cross-head journal,
B a vertical line at a right angle to A; C representing the crank shaft
extended by a dotted line, so as to enable comparison with A; D the
crank, E and F the centre of the crank-pin journal, and G G a line at a
right angle to cross-head journal A.
Now G, being at a right angle to A, represents what should be the plane
of rotation of the crank pin, whereas C, being out of parallel with A,
causes the path of rotation to be in the path from E to F, or as D
compared to B; supposing then that the bores of the connecting-rod
brasses to be axially parallel one to the other, and keyed up properly,
and when at E one bore of those brasses will stand parallel to E while
the other is parallel to A, or when at the bottom of the crank rotation,
one bore will be parallel to F and the other parallel to A. Thus the rod
will be twisted, and the strain due to this twist will cause the
bearings to heat. That this twisting is continuous throughout the whole
revolution may be seen by the want of parallelism of the dotted line
(representing the crank pin when on the dead centre) with A
(representing the cross-head journal).
It is now to be observed that if the plane of the crank rotation were at
a right angle to the axis of the cross head, as it should be, the path
of the centre of the crank-pin journal would be in the plane of G G,
whereas it falls outside as at E, and inside as at F, while at H it is
coincident; hence it appears that starting from a dead centre H, the rod
bends, passing at that end outward to E (when the crank has made a
quarter revolution), where it attains its maximum bend, thence
diminishing until finally ceasing, when the crank reaches the other dead
centre. As soon, however, as it passes the last dead centre a bend in
the opposite direction takes place, attaining its maximum at F, and
ceasing at H. This bending also causes undue friction and the consequent
heating of the journals; furthermore, if there be any _end_ play between
the brasses and the journals, there will be a pound, as the brasses jump
from one end of the journal to the other at different parts of the
stroke. It is obvious that if the crank end of the crank shaft was too
high instead of too low, as in our example, then the effects would be
the same, but E would fall on the inside instead of the outside of G,
while F would fall outside instead of inside.
[Illustration: Fig. 2538.]
[Illustration: Fig. 2539.]
[Illustration: Fig. 2540.]
To discover if the crank shaft is out of parallel in the direction here
referred to, connect the connecting rod to the cross-head journal,
setting the brasses up to a close working fit. At the other end of the
connecting rod put the strap keys and brasses in their places, but not
on the crank-pin journal. Place the crank in its highest position, and
lower the end of the rod down to the crank-pin journal, as shown in Fig.
2538, and if the crank shaft is parallel (in the respect here referred
to) to the cross-head journal, the brass flanges will just meet the
faces of the crank-pin journal, as shown in Fig. 2539. If, however, the
crank end of the crank shaft is too low, as in our example, the flanges
of the brasses will fall to one side of the crank-pin journal, and that
side will be toward B, Fig. 2540, when the crank pin is at the top, and
toward C, Fig. 2541, when it is at the bottom of its path of rotation.
The effects will be precisely the same, and in the same direction with
relation to the various parts of the crank's revolution, if the
crank-pin end of the shaft was of correct height; but the other end was
too high, hence, in correcting the error, it is desirable to place the
engine on the dead centre, so as to determine which end of the shaft to
operate on--that is to say, whether to raise the crank-pin end or lower
the other end. But suppose the error to be that the crank-pin end of the
shaft was too high instead of too low, then, the testing being continued
as before, the effects will be of the same general character, but
altered with relation to the specific parts of the revolution. Thus,
when the crank is at the bottom, the rod would fall towards A, Fig.
2542, and when at the top, it would fall in the opposite direction--that
is, towards D, Fig. 2542.
[Illustration: Fig. 2541.]
[Illustration: Fig. 2542.]
We now come to one of the most common errors in the alignment of the
parts of an engine, and to the one that it is the most difficult to
locate or discover, namely, a want of parallelism between the axial line
of the crank pin and that of the crank shaft.
This generally arises from improper methods in the chucking of the crank
to bore it, or from errors induced in fastening the crank to its shaft.
The results are precisely alike in both cases, supposing, of course, the
errors to exist in the same direction in the two cases.
The error in chucking usually consists in planing one surface of the
crank, and bolting the planed surface against the chuck to bore both
crank holes. In this case the crank holes will be out of true to twice
the amount the lathe face plate may be out of true, and to whatever
amount the crank may alter its form from having its surface metal
removed.
To avoid these errors the large bore and its hub face should be turned
at one chucking, and this hub face should be bolted to the face plate
for the second chucking, the small end swinging free, except in so far
as the ends of the plates may touch against it to steady it.
[Illustration: Fig. 2543.]
The error in putting the crank on may occur from the key springing the
crank out of true, and if the crank is shrunk on from too great an
allowance for shrinkage or improper heating for the shrinkage or
contraction, as it is sometimes termed. Referring to the error in
keying, it is more liable to occur when the crank bore and its seat upon
the shaft are made taper, than when made parallel, because it is a
difficult matter to insure accuracy in the fit of the taper, and the key
pressure will spring the crank over on the side at which it is the
easiest fit. In Fig. 2543 let A represent the end of the crank shaft; B
the key, and C the crank shown partly in section: suppose the crank bore
(whether made taper or parallel) has a slightly easier fit on the side D
than on the side E, and the pressure of the key (supposing it to fit
properly top and bottom) would spring the crank over in the direction
shown in the figure, the axial line of the crank pin standing at the
angle denoted by the line F, instead of parallel to the axial line of
the shaft. Suppose the crank to be put on by hydraulic pressure, and the
key to fit on the sides and not on the top and bottom, then its fit to
its seat on the shaft would depend on the truth and smoothness of its
bore and seat on the shaft, the amount allowed for the forcing fit and
the amount of the error. If the latter amount was so small that the
crank would fit at both ends, but simply fit tighter at E E than at D,
the crank would remain true, but might possibly get loose in time. This
would be especially liable to occur if the tool marks on the bore and
seat were so deep that the contact was mainly at the tops of those marks
or ridges which would be apt to compress. But if the surfaces were
cylindrically true and smooth, and the amount allowed for forcing was
sufficient as stated to give the bore and seat contact at D, with a key
fitting sideways, the crank would probably remain tight and true.
Were the bore and its seat parallel the crank would remain true, no
matter whether the key fitted on the sides or at the top and bottom,
providing the key fitting top and bottom were bedded fairly from end to
end.
When the surfaces are not smooth, but contain tool marks or ridges, an
unequal pressure of the key at one end, as compared to the other, sets
the crank over, as shown in the figure, because the key pressure
compresses the ridges and lets the crank move over.
[Illustration: Fig. 2544.]
Supposing the strain of the key, or keys, to be depended upon to hold
the crank, they must fit top and bottom, and their accurate fit becomes
of the first importance; because not only is it necessary that they fit
equally at each end, but they must also fit equally across the width of
the key at each end. For example, in Fig. 2544 is a key binding most at
the opposite corners, as denoted by the dotted surfaces A B, and the
result will be that the key pressure would tend to twist the crank in
the direction of D E, having C as a centre of motion, providing that the
error was equal at A and B; but in proportion as the error was greatest
and the fit tightest at A, or at B, would the centre of motion be moved
nearer to either point.
Supposing now that the crank is to be shrunk, or contracted on, then the
points of consideration are (supposing the crank to fit properly to its
seat, whether the same be either parallel or taper) that the hub of the
crank opposite to the throw is the weakest and is likely to give most
in the process of contraction, so that if one part (as F) of the crank
be made hotter than another (as G) it will give way more, and this will
twist the crank. This is specially liable to occur if an excessive
amount of difference in the bore and seat diameters has been allowed for
contraction.
[Illustration: Fig. 2545.]
It may not happen that a crank pin is out of truth in a direction in
which the error will show plainest when the crank is on its dead
centres, or at half-stroke; but if a crank pin, tested in those four
positions, fails to show any error when tested by the connecting rod, it
will be true enough for all practical purposes, and true enough to avoid
heating and pounding, both of which evils accompany an untrue crank pin.
Suppose, now, that a crank pin stands out of true in the direction shown
in Fig. 2545, in which A A represents the axial line of the cylinder
bore prolonged, and B B the axial line of the crank shaft (the two being
parallel or in proper line). Let E E represent the centre line of the
connecting rod when the crank is on one dead centre, the axial line of
the crank pin being at C C. Then the brasses being keyed up to fit the
crank pin, the centre or axial line of the connecting rod would stand as
denoted by E E. But the brasses at the other end of the rod being keyed
up to fit the cross-head journal, and their lines being at a right angle
to the line A A, we have that the rod is at that end endeavored to be
held parallel to A A; hence, keying up the connecting-rod brasses on the
crank pin would tend to bind the rod, one end standing parallel to A A,
and the other parallel to E E.
This would place great strain on the outer radial face of the cross-head
journal, as well as on the cylindrical body of the journal.
When, however, the crank pin arrives at the opposite dead centre, as
denoted by the dotted lines in Fig. 2545 (G G representing its axial
line, and F F the centre line of the connecting rod at a right angle to
G G), the want of truth in the pin throws the cross-head end of the
connecting rod against the inside face of the cross-head journal. Hence,
twice in each revolution is the connecting rod bent, and twice does it
jam from side to side of the cross-head journal.
It may now be pointed out that if we take either dead centre singly, and
connecting the rod at the crank-pin end, try it at the cross-head end,
and it will be a difficult matter to determine whether any want of truth
at the latter end is caused by the crank pin being out of axial truth,
or whether it is the crank shaft itself that is out of line. But there
is this difference between the two cases. When the error is due to want
of alignment in the crank shaft, the connecting rod will show the error
_on the same side_ of the cross head, no matter on which dead centre the
crank pin stands; but when it is due to the crank pin, the rod will fall
inside the cross head on one dead centre, and outside when tried on the
other dead centre, as is shown by the respective lines E and F, in Fig.
2545; E being at a right angle to C, and F at a right angle to G.
[Illustration: Fig. 2546.]
Again, it has been shown that when the shaft was out of line, a point on
the crank-pin journal passed outside of the cylinder centre line at one
dead centre and inside at the other; but when the pin is axially out of
parallel, the path of a point on its journal will remain in the true
plane, as is shown in Fig. 2546, the point being taken at the
intersection of E and C C. A A represents the path of rotation of the
same, which is parallel to the true face B of the crank.
From the angle of the axial line of the pin being in opposite
directions, when on opposite dead centres to the axial line of the crank
shaft, the bore of the brasses cannot wear to suit the error, which,
therefore, only diminishes by the wear of the crank pin. Suppose the
error to be 1/64 inch in a crank-pin journal 3 inches long, and that the
connecting rod is 6 feet long, the error at the cross-head end of the
rod will amount to 3/8 inch.
[Illustration: Fig. 2547.]
In Fig. 2546 the error is shown to exist in an opposite direction,
throwing the rod to the other side of the cross-head journal. But, in
this case, the crank, when on the dead centre nearest to the engine
cylinder, throws the connecting-rod end against the inside face of the
cross-head journal, as denoted by the line E, which is on the opposite
side of A A to what it is in Fig. 2545. Again, when on the other dead
centre, the line F F, in Fig. 2546, falls _outside_, while F F, in Fig.
2545, falls _inside_ of A A, and it is by this difference that we are
enabled to know in which direction the crank pin is out of true. To find
the amount to which it is out of true in the length of its journal,
place the crank on one dead centre, and with the connecting-rod brasses
keyed up firmly home on the crank pin, and the other end of the
connecting rod entirely disconnected from the cross head, mark on the
latter a line coincident with the side face of the rod end, as at D,
Fig. 2547. Then, with the crank pin placed on the other dead centre,
mark another line on the cross head, coincident with the other side face
of the rod, at C, Fig. 2547. Now, suppose that the line D shows the rod
to fall 3/8 too much on that side, and line C shows it to fall (when on
the other dead centre) 3/8 too much on the other side of the journal,
and that the length of the rod is 6 feet, while that of the crank-pin
journal is 3 inches, then the latter, divided into the former, gives 24,
and this sum divided into the 3/8, the rod end falling out of true at C
and D, Fig. 2547, gives us 1/64-inch as the amount the crank pin stands
out of true in its length; hence, to correct the error, we may file on
the crank pin a flat place at each end, as shown in Fig. 2548 by the
lines C D, and then file on the top and the bottom of the crank pin a
flat place B, 1/128-inch deep, and of equal depth all along the journal;
by then filing the crank pin round and bringing the flat places just up
to a circle, we shall have reduced the diameter of the crank pin by 1/64
inch, and have made it axially true with the cross-head journal. It is
important, however, to bear in mind that, in this case, the crank pin is
supposed to be out of true in the direction shown in Fig. 2545, and to
stand axially true with the cross-head journal, when the crank is placed
at half stroke, top and bottom, the crank shaft being in proper line.
[Illustration: Fig. 2548.]
If the axial line of the cross-head journal stands truly horizontal, the
flat places on the crank pin may be filed horizontally level, with the
crank placed on the corresponding and respective dead centres. But as
the length of the cross-head journal is so short, it is difficult to
gauge, if it does stand axially exactly horizontal, hence it is better
to try the rod, or follow the above directions; especially as the
cross-head journal and crank shaft may be in line without being axially
horizontal.
Suppose now that the axial line of the crank pin stands true with that
of the cross-head journal when the crank is on either dead centre, but
out of true when at the top and bottom half stroke. The connecting rod,
connected as before, and tried with the cross head, will fall first to
one and then to the other side of the cross-head journal, and the
direction in which the crank is out of true may be known from the
position of the crank pin when the error shows itself.
[Illustration: Fig. 2549.]
[Illustration: Fig. 2550.]
If the error exists to an extent that is practically measurable, a pound
in the journals, as well as their heating, is the inevitable result. In
Fig. 2549, for example, the rod end is shown in section, and it will be
noted that the error being in the direction there shown, and the crank
pin in the respective positions there shown, the brass bore only
contacts with the journal at each end, and that the diameter of the bore
of the brasses is greater than the diameter of the crank pin journal to
_twice_ the amount the crank pin is out of line. Now let us place the
crank at the top of its revolution, as in Fig. 2550, and as its axial
line then stands parallel to that of the cross-head journal, the brass
bore is too large to fit the crank pin journal and there is lost motion.
From the time the crank pin passes the dead centre this lost motion
increases in amount until it becomes sufficiently great to slam the rod
over against the side of the cross-head journal, while at the same
instant the crank pin pounds in the connecting-rod brasses. At what
precise part of each quarter crank revolution this action will occur,
depends upon the amount the crank pin is out of line; but the more it is
out the nearer to the dead centre it will be, and, conversely, the
nearer true it is the nearer the crank will approach its highest and
lowest positions before the pound takes place. If it is attempted to key
up the brasses so as to spring the rod and let them close along the
journal, the brasses will heat in proportion to the amount of error;
hence when the crank pin pounds with the brass properly adjusted, and
heats while keyed up enough to stop the pound, the crank pin is out of
true.
To test the alignment of an engine with stretched lines take out the
piston and rod, and take off the connecting rod, then fasten a piece of
iron at the open end of the cylinder so that it will hold a stretched
line true with the axis of the cylinder bore. Provide at the crank end
of the engine bed a fixed piece of wood to hold the other end of the
line, and then with a piece of wire as a gauge set this line (tightly
stretched) true with the cylinder bore. Then place the crank pin at the
top of its path of rotation and drop a plumb line from the centre of its
journal length, and this line should, if the crank shaft is horizontally
level, just meet the stretched line. If it does not do so place a spirit
level on a parallel part of the crank shaft, and if the shaft is not
level it should be made so, and so adjusted that the line from the
centre of the length of the crank pin journal just meets the stretched
line from the cylinder bore.
To test if the axial line of the crank shaft is at a right angle to the
cylinder bore axis move the crank pin nearly to its dead centre, and
measure the distance from the middle of its length to the stretched
line. Then move the crank pin over to nearly the opposite dead centre,
and (by means of the plumb line) measure the distance of the plumb line
from the stretched line. To be correct the plumb line from the crank pin
will during this movement just touch the stretched line.
To test if the stretched line is fair with the centre of the crank shaft
place a square on the end of that shaft and even with its centre, and
the blade should then just meet the stretched line.
The edges of the guide bars may also be tested with the stretched line,
and the top and bottom of the guide-bar flanges may be tested to prove
if the bars are of the correct height.
To further test the bars place a spirit-level across them and lengthwise
on them.
If the piston rod and connecting rod are in place the alignment may be
tested as follows; Let the piston rod be as far out of the cylinder as
possible, and stretch a line to one side of it, just far enough off to
clear the guide bars, &c. Set this line as follows: Let it be in line
with the rod as sighted by the eye when standing some few feet away from
it but horizontally level with the centre of the rod, set it parallel to
the rod with a rule or its equivalent. Then the centre of the crank-pin
journal should measure from the stretched line, the distance of the line
from the piston rod added to half the diameter of that rod. This test,
however, is not very accurate on account of the difficulty in setting
the line, and because the piston rod may not have worn equally on each
side.
SETTING SLIDE-VALVES--An engine slide-valve may be so set as to
accomplish either one of three objects. First, to give equal lead for
each stroke; second, to cause the live steam to be cut off and expansion
to begin at an equal point in each stroke; and third, for the exhaust to
begin at an equal point in each stroke.
If we, set the eccentric so that the exhaust will begin at corresponding
points for the two strokes, the valve lead will not be equal, and the
exhaust opening will be greater when the piston is at one end of the
cylinder than it will be when the piston is at the other end.
If the eccentric be set to cut off the steam at corresponding points for
the two strokes, then the lead, the admission, and the exhaust of the
steam at one port will differ (with relation to the piston movement)
from that at the other. It is generally preferred to set the eccentric
so as to give equal lead for the two ports when the piston is at the
respective ends of its stroke, which gives an equal amount of exhaust
opening when the piston is at the respective ends of its stroke.
The only operations properly belonging to the setting of a slide-valve
are those of finding the true dead centres of the crank pin, and setting
the eccentric to give the valve the desired amount of lead. It is
generally found, however, that the length of the eccentric rod requires
a little correction, and as this must be done before the eccentric can
be set, the setting operations should be conducted with a view to making
the correction as early as possible.
In many of the instructions given by various writers it is directed to
first square the valve, which is to attach the parts and move the engine
crank, or fly-wheel, through one revolution, to ascertain if the valve
moves an equal distance on each side of the centre of the cylinder
ports, correcting the length of the eccentric rod until this is the
case. This is an error, because on account of the angle of the eccentric
rod the valve does not, when set to have equal lead at each end of the
stroke, move an equal distance on each side of the cylinder ports, but
travels farther over the port nearest than it does over that farthest
from the crank.
When the travel of the valve is equal to twice the width of the steam
port, added to twice the amount of steam lap, the valve does not fully
open the farthest port from the crank. When the valve-travel is more
than this amount both ports may open fully, but the error due to the
unequal valve-travel from the angularity of the eccentric rod is
increased. That the amount of error induced by squaring the valve is
appreciable, may be seen from the fact that with 1-1/4 inch steam ports,
3/4 inch steam lap, and 4-1/2 inches of valve-travel, it amounts to
about 1/8 inch with an eccentric rod 4 feet long. As the eccentric rod
has (if a solid one, as in the case of a locomotive) to be operated upon
by the blacksmith to alter its length, and requires some accurate
setting for alignment after having its length corrected, it is obviously
preferable to obtain its exact length at once. This may be done with
less work than by the squaring process, which is entirely superfluous.
[Illustration: Fig. 2551.]