[Footnote: Lecture delivered at Wells Memorial Institute, Boston, in
the Lowell Free Course for Engineers. From report in the _Boston
Journal of Commerce_.]
By THOMAS HAWLEY.
RIDING CUT-OFF VALVES--PECULIARITIES AND MERITS OF THE DIFFERENT
STYLES.
In considering the slide valve in its simple form with or without lap,
we find there are certain limitations to its use as a valve that would
give the best results. The limitation of most importance is that its
construction will not allow of the proper cut off to obtain all the
benefits of expansion without hindering the perfect action of the
valve in other particulars. At this economical cut off the opening of
the steam port is very little and very narrow, and although this is
attempted to be overcome by exceedingly wide ports, sixteen inches in
width in many cases in locomotive work, this great width adds largely
to the unbalanced area of the valve. The exhausting functions of the
valve are materially changed at the short cut off, and when much lap
is added to overcome this defect, there usually takes place a choking
of the exhaust port. You might inquire, why not make the port wider,
but this would increase the minimum amount of load on the valve, and
this must not be overlooked. Then the cut off is a fixed one, and we
can govern only by throttling the pressure we have raised in the
boiler or by using a cut off governor and the consequent wastes of an
enormous clearance space. You will observe, therefore, that the plain
slide valve engine gives the most general satisfaction at about
two-thirds cut off and a very low economic result. The best of such
engines will require forty-five to fifty pounds of steam per horse
power per hour, and to generate this, assuming an evaporation of nine
pounds of water to a pound of coal, would require between five and six
pounds of coal per horse power per hour. And the only feature that the
valve has specially to commend it is its extreme simplicity and the
very little mechanism required to operate it.
Yet this is of considerable importance, and in consideration of some
special features at its latest cut off, the attempt has been many
times made to take advantage of these features. For instance, at 90°
advance, the valve opens very rapidly indeed and fully satisfies our
requirements of a perfect valve. This is one good point, and in this
position also the exhaust and compression can be regulated very
closely and as desired without much lap, and as the opening of the
exhaust port comes with the eccentric at its most rapid movement the
release is very quick and as we would have it. This is only possible
at the most uneconomic position of the valve as regards cut off.
The aim of many engineers has been to take advantage of these matters
by using the valve with 90° angular advance of eccentric ahead of
crank, for the admission, release, and compression of the steam, and
provide another means of cutting off, besides the one already referred
to, viz., cutting off the supply of steam to the chest, and overcome
the objection in this one of large clearance spaces. This is done by
means of riding cut off valves, often called expansion valves, of
which, perhaps, the most widely known types in this vicinity are the
Kendall & Roberts engine and the Buckeye. The former is used in the
simplest form of riding cut off, while the Buckeye has many peculiar
features that engineers, I find, are too prone to overlook in a casual
examination of the engine. In these uses of the slide valve, too,
means are suggested and carried out of practically balancing the
valve.
The origin of the riding cut off is most generally attributed to
Gonzenbach. His arrangement had two steam chests, the lower one
provided with the ordinary slide valve of late cut off, and steam was
cut off from this steam chest by the expansion valve covering the
ports connecting with the upper steam chest. This had the old
disadvantage that all the steam in the lower chest expanded with that
in the cylinder, at a consequent considerable loss. This was further
improved by causing the riding cut off to be upon the top of the main
valve, instead of its chest, and resulted in a considerable reduction
of the clearance space.
This is the simplest form, and is shown in Fig. 1. The steam is
supplied by a passage through the main valve which operates exactly as
an ordinary slide valve would. That is, the inside edges of the steam
passage are the same as the ordinary valve, the additional piece on
each end, if I may so term it, being merely to provide a passage for
the steam which can be closed, instead of allowing the steam to pass
the edge. The eccentric of the main valve is fastened to the shaft to
give the proper amount of lead, and the desired release and
compression, and the expansion valve is operated by a separate
eccentric fastened in line with or 180° ahead of the crank. When the
piston, therefore, commences to move from the crank end to open the
port, D, the expansion valve is forced by its eccentric in the
opposite direction, and is closing the steam port and would have
closed it before the piston reached quarter stroke, thus allowing the
steam then in the cylinder to do work by expansion. The eccentric
operating this expansion valve may be set to close this steam port at
any point in the stroke that is desired, the closing occurring when
the expansion valve has covered the steam port. Continuing the
movements of the valves, the two would move together until one or the
other reached its dead center, when the movements would be in opposite
directions.
[Illustration: FIG. 1.]
There are three ways of effecting the cut off in such engines, the
main valve meanwhile being undisturbed, its eccentric fastened
securely so as not to disturb the points of lead, release, and
compression. All that is required is to cause the edge of the
expansion valve to cover the steam port earlier in the stroke, and
this can be done, first, by increasing the angular advance of the cut
off eccentric; second, by adding lap to the cut off valve; and third
by changing the throw of the eccentric. In all these instances the
riding valve is caused to reach the edge of the steam port earlier in
the stroke. We will take first, as the simplest, those methods by
which the lap of the cut off valve is increased.
It will be noted that there is but one edge of this valve that is
required to do any work, and that is to close the valve. The
eccentrics are so placed that the passage in the main valve is opened
long before the main valve itself is ready to admit steam to the
cylinder, so that only the outer edges are the ones to be considered,
and it will be readily seen that the two valves traveling in opposite
directions, any lap added to the working edge of the cut off valve
will cause it to reach the edge and therefore close the port earlier
than it would if there was less lap. And we might carry it to the
extreme that we could add lap enough that the steam passage would not
be opened at all.
In Fig. 2 is shown the method by which this is accomplished, in what
is called Meyer's valve, and such as is used in the Kendall & Roberts
engine. We have only one point to look after, the cut off, so we can
add all the lap we wish without disturbing anything else. In this
engine the lap is changed by hand by means of a little hand wheel on a
stem that extends out of the rear of the steam chest. The valve is in
two sections, and when it is desired to cut off earlier, the hand
wheel is turned in such a direction that the right and left hand
screws controlling the cut off valve move one valve portion back and
the other forward, which would, if they were one valve and they should
be so considered, have the effect of lengthening them, or adding lap
to them. The result would be that the riding valve would reach the
edge of the steam port earlier in the stroke, bringing about an
earlier cut off. If the cut off is desired to be later, the hand wheel
is so turned that the right and left hand screws will bring the valve
sections nearer together, thus practically taking off lap. Now this
may be done by hand or it may be done by the action of a governor.
[Illustration: FIG. 2.]
In the latter case the governor at each change of load turns the right
and left hand screws to add or take away lap, as the load demands an
earlier or later cut off; in other cases the governor moves a rack in
mesh with a gear by which the valve sections are brought closer
together or are separated. The difficulty with the case where the hand
wheel is turned by hand is that the cut off is fixed where you leave
it, and governing can only be at the throttle. For this reason
anywhere near full boiler pressure would not be obtained in the
cylinder of the engine. If the load was a constant one, and the cut
off could be fixed at about one-third, causing the throttle to open
its widest, very good results would be obtained, but there is no
margin left for governing.
If the load should increase at such a time the governor could not
control it under these conditions, and it would lead to a decrease in
speed unless the lap was again changed to give a later cut off. On
this account the general practice soon becomes to leave the cut off at
the later point and give range to the throttle, and we come back once
more to the plain slide valve cutting off at half stroke, and the only
gain there is, is in a quick port opening and quick cut off. But these
matters are more than offset by the wire drawing between the steam
pipe and chest, through the throttle, and the fact that there is added
to the friction of the engine the friction of this additional slide
valve and a considerable liability to have a leaky valve.
In the case where the governor changes the position of the cut off
valve a greater decree of economy would result. In this engine, of
which the Lambertville engine is a type, the main valve is a long D
slide, with multiple ports at the ends through which the steam enters
the cylinders. It is operated from an eccentric on the crank shaft in
the usual manner. The cut off valve is also operated from the motion
on an eccentric fixed upon the crank shaft. The rod or stem of the cut
off valve passes through the main valve rod and slide. Upon the outer
end of the cut off valve rod are tappets fastened to engage with
tappets on the eccentric valve rod. Connection between the cut off
eccentric, therefore, and the cut off valve is only by means of the
engagement of these tappets. The eccentric rod is fastened to a rocker
arm having motion swinging about a pin or bearing in the governor
slide, which may be raised or lowered by a cam operated by the
governor. The cut off slide is of cylindrical shape and incloses a
spring and dash pot with disks attached by means of which the valve is
closed. The motion for operating the valves is relatively in the same
direction, the cut off eccentric having the greatest throw and greater
angular advance to cause it to open earlier and quickly before the
main valve is ready to admit steam. The cut off eccentric rod swinging
the rocker arm, the tappets thereon engage with those upon the cut off
valve rod and open the passages to the main valve, and in their
movement compress the spring in the main valve. According as the speed
of the engine, the rock arm will be raised or lowered so that the
tappets upon the eccentric rod may keep in engagement a shorter or
longer time before they disengage, thus allowing the spring that has
been compressed by the movement of the cut off valve to close that
valve quickly and the supply of steam to the engine, the cut off valve
traveling with the main valve for the balance of the stroke. This
device will give a remarkably quick opening and a quick cut off, but
in view of the fact that the governor has so much to do, its delicacy
is impaired and a quick response to the demands of the load changing
not so likely to occur. The cut off cannot be as quick as in some
other engines, because the valves are moving in opposite directions,
and while this fact would help, so far as shortening the distance to
be traveled before cut off, the resistance of the valves to travel in
opposite directions, or rather the tendency of the valve to travel
with the main valve, hinders its rapid action.
[Illustration: FIG. 3.]
This is one great objection to the rack and gear operated by the
governor, that two flat valves riding upon each other and sliding in
opposite directions at times require a considerable amount of force to
move them, and as only a slight change in load is required by the
load, the governor cannot handle the work as delicately as it should.
It is too much for the governor to do well. To overcome this
difficulty the Ryder cut-off, shown in Fig. 3, was made by the
Delamater people, of New York. The main slide valve is hollowed in the
back and the ports cut diagonally across the valve to form almost a
letter V. The expansion valve is V-shaped, and circular to fit its
circular-seat. The valve rod of the expansion valve has a sector upon
it and operated by a gear upon the governor stem, which rotates the
valve rod, and the edge of the valve rod is brought farther over the
steam port, thus practically adding lap to the valve. Little movement
is found necessary to make the ordinary change in cut-off, and it is
found to be much easier to move the riding valve across the valve than
in a direction directly opposite. It would require considerable force
to move the upper valve by the governor faster than the lower, or in a
direction opposite to that in which it is moving, but very little
force applied sideways at the same time it is moving forward will give
it a sideways motion. In this device the governor has only to exert
this side pressure and therefore has less to do than if it were called
upon to move the upper valve directly against the movement of the
lower.
Something similar is the valve of the Woodbury engine, of Rochester,
N.Y. The cut-off valve is cylindrical, covering diagonal ports
directly opposite, and is caused to be rotated by the action of the
governor that operates a rack in mesh with a segment. Very little
movement will effect a considerable change in the lappage of the
valve, the valve turning about one-quarter a revolution for the
extremes of cut off. The cut off valve rod works through a bracket and
its end terminates in a ball in a socket on the end of the eccentric
rod. In this case the governor has not as much to do as in other
instances.
[Illustration: FIG. 4.]
Still another method of effecting this change in cut off, but hardly
by increasing the lap of the valve, is shown in the next drawing, Fig.
4. The cut off valve is held upon the main valve by the pressure of
steam upon its back and rides with it until it comes in contact with
the cut off wedge-shaped blocks, when its motion is arrested, and the
main valve continuing its movement the steam port is closed by the
main valve passing beneath the cut off valve. Thus the main valve
travels and carries the cut off valve upon its back again until the
cut off valve strikes the wedge on the other end and the cut off is
effected. The relative positions of the blocks are determined by the
governor, that will raise or lower them so that the cut off valve will
engage with them earlier or later as desired. This device was designed
specially as an inexpensive method of changing the common slide valve
into an automatic cut off. The cut off would not be as quick as in
other cases we have cited, depending here upon the movement of the
lower valve alone, and that, too, is in its slowest movement; whereas
in the other cases, the edges approaching each other, by the differing
movement of the valves the cut off is very rapid, provided the
distance to travel is not long. In this device considerable noise must
result by the cut off valve striking the cut off blocks, and a
considerable amount of leakage is likely to occur past this valve.
But there is one great objection in the valve gears thus far cited,
that the travel of the expansion valve upon the main valve is
variable. I have in mind the case of a Kendall & Roberts engine, which
had been run for a long time at no better economy than would be
obtained from a plain slide valve engine, and when it was attempted to
get an earlier cut off by separating the two cut off valves, they had
worn so much in their old place on the valve that shoulders were found
sufficient to cause a disagreeable noise and a leaky valve. This is
very apt to occur, not only where the valve is run for a long time on
one seat, but in cases of variation of the travel of the expansion
valve. The result is that a change will bring about a leaky valve,
something that every engineer abhors.
The construction of the Buckeye engine, which is also of this type, is
such that the travel of the valve on the back of the main valve is
always the same, no matter what the cut off may be. Then this engine
makes use of our second proposition as a means of effecting the cut
off, viz., by advancing the eccentric. You will readily observe that
anything that will cause the cut off valve to reach a certain point
earlier in the stroke will bring about an earlier cut off as it
hastens everything all around. This is the plan pursued in the
Buckeye, in which the governor, of the shaft type, turns the eccentric
forward or back according as the load demands. Then, in addition, the
valve is balanced partially, the attempt not being made to produce an
absolutely balanced valve, on the ground that there should be friction
enough to keep the surfaces bright and to prevent leakage. The most
perfect valve will, of course, be entirely balanced under all
conditions of pressure so as to move with perfect ease. With the
riding cut off valve in connection with the plain slide valve, this is
not accomplished, and it does not matter whether it is partially
unbalanced to prevent leakage or not, the fact that it is not entirely
balanced prevents it reaching the ideal valve.
[Illustration: Fig. 5]
This valve, Fig. 5, differs from the others also in this particular,
that the exhaust takes place at the end of the valve instead of under
the arch. Two eccentrics are used, the one for the main valve being
fastened to the shaft and the other riding loosely upon it and
connected to the fly wheel governor, by which it may be turned forward
or back as the load requires. The three points of lead, or admission
and exhaust and compression, are fixed and independent of the changes
and cut off. The motion of the main eccentric is given to a rocker
arm, the pivot of which is at the bottom, and from the upper end the
valve rod transfers the motion to the valve without reversing the
motion, as is done sometimes in the slide valve to overcome the
effects of the angularity of the connecting rod. The action of the
rocker arm, therefore, so far as the main valve in the Buckeye is
concerned, is no different than that which would occur if no rocker
arm intervened. The motion of the cut off eccentric, through its
eccentric rod, is given to a rocker rocking in a bearing in the center
of the main rocker arm (see Fig. 6). The motion of this eccentric is
reversed, so far as the cut off valve is concerned, and when the cut
off eccentric is moving forward, the cut off valve is being pushed
back. The main valve rod is hollow, and the cut off valve rod passes
through it.
[Illustration: Fig. 6]
The cut off eccentric can be placed in any position to cause it to cut
off as desired, and by drawing the valve forward, by increasing the
angular advance of the eccentric, the cut off valve is caused to reach
and cover the steam passage in the main valve earlier in the stroke.
Instead of being ahead of the crank, the main eccentric in this
arrangement follows the crank, on account of the exhaust and steam
edges being exactly opposite from those in the ordinary slide. What is
the steam edge of the common slide is in this the exhaust edge, and
what is the exhaust edge in the common valve is the steam edge in this
one. The valve, therefore, must be moved in the opposite direction
from what is ordinarily the case, the main eccentric being not 90 deg.
behind the crank. It has a rapid and full opening just the same, for
it is at this point behind the crank, or ahead of it, that the
eccentric gives to the valve its quickest movement, or between the
eccentric dead centers. The cut off eccentric is considerably ahead of
the main eccentric, and about even with the crank. If it was not for
the reversal of motion of the cut off valve through the rocker arm
this eccentric would be about in line with the crank, but on the other
end. The movement of the cut off valve, therefore, at the time of port
opening is very little, being about on its dead center, passing which,
it immediately commences to close.
The object of the peculiar construction of the rocker arm, and the
pivot for the cut off rocker being placed thereon, is to provide equal
travel on the back of the main valve, no matter what the cut off. I
have already explained, in connection with the slide valve, that
advancing the eccentric does not change the movement of the valve on
its seat, but simply its relation to the movement of the piston. You
will see that this is unchanged as using the main valve as a seat or
any other seat. If the main valve was to remain stationary, and only
the cut off valve to be operated by its eccentric, the movement of
this cut off valve on a certain plane would be the same for all
positions of the eccentric.
Moving the main slide does not affect the matter in any way, for it
moves at the same time the pivot of the cut off, and while the cut off
seat has assumed a different position with reference to the engine, it
is still as though stationary so far as the cut off valve is
concerned. This is the object of this peculiar construction, and not,
as some engineers suppose, simply to make an odd way of doing things.
And the object of it all is to give at all cut offs the same amount of
travel, so that there might be no unequal wear to bring about a leak,
to prevent which a perfect balancing has been sacrificed.
Referring to the valve and this engine as to how it will satisfy our
requirements of a perfect valve gear, we find that the first
requirement of a rapid and full opening is met, in that the opening
occurs when the main eccentric is moving very rapidly, yet not its
fastest, and while this opening will be very satisfactory, it is not
so rapid an opening as is obtained in some other forms of valves and
valve gears, but this could be overcome very readily by increasing the
lead a trifle, and in my experience with these engines I find that the
practice is very general by engineers and by builders themselves to
give them a considerable amount of lead. As to the second requirement,
the maintenance of initial pressure until cut off, giving a straight
steam line, cards from this engine will not be found to show that the
engine satisfies this requirement, and for this reason, that the
cut-off valve commences to close the port immediately after the piston
commences to move. The cut off eccentric you will remember is set to
move with the crank or very nearly so, and the lighter the load, the
greater will this fact appear. For the lightest loads the governor
places the eccentric in advance of the crank, so that the cut off
valve will commence to close the port before steam is admitted by the
main valve to the engine. Now, the later the cut off, the less will
this wire drawing appear at first, and the shorter the cut off, the
amount of wire drawing increases sensibly. The operation of the valve,
therefore, in this particular, cannot be considered as meeting our
requirement that the port shall be held open full width until ready to
be closed. Many men claim for this engine that the closing occurs when
the cut off eccentric is moving its fastest. This is a fact, and if we
consider the point of cut off only to be the point of absolute cut
off, the cut off must be instantaneous, for there is an instantaneous
point where the cut off is final only to be considered. The reasoning
applied here would hold good also to a less extent on the slide valve,
but is not the point of absolute cut off. We want to note how long it
is from the time the valve commences to close at all until finally
closed, and, as I have shown you, this is considerable in this engine.
Referring to the point of cut off finally, it is determined upon by a
governor of the fly wheel type. The eccentric is loose about the
shaft, and arms projecting therefrom are connected by other arms to
the extremity of an arm upon which is mounted a weight, and which is
attached to the spokes of the fly wheel, or special governor wheel in
this case, and which is fastened to the crank shaft. As the speed
increases through throwing off a portion of the load the governor
weights fly out, and this movement is transferred through the lever
connections to the eccentric, causing it to be turned ahead, and the
manner hastening the movement of the cut off valve on its seat and
causing it to reach and cover the edge of the steam port earlier in
the stroke. This engine was the pioneer in governors of this
character, the advantage being, in addition to its necessity for the
work of turning the eccentric ahead or back, that the liability of the
engine to run away, as very often happens from the breaking of the
governor belt or a similar cause, was not possible.
The cut off valve has a travel considerably beyond the edge of the
steam passage after the valve is closed, and this has one advantage,
that the valve is less liable to leak, and to this must be added the
loss from the friction of this moving valve, and moving too in
opposition to the main valve. In our perfect valve, as we outlined it,
the valve does not move after the port is closed. The exhausting
functions of the valve are very good, giving a quick opening and a
full opening, because this opening occurs when the eccentric is moving
its fastest. The engine also possesses a distinct advantage in having
remarkably small clearance spaces. The length of the steam passage is
very small in comparison with any form of engine, and having but two
ports instead of four, as in the Corliss and four valve type.
In these there must be included in the clearance, that to the exhaust
port as well as the steam port, adding a considerable amount where the
piston comes close to the head. As the engines leave the maker's hand
the engines are provided with a considerable amount of lap to give
plenty of compression, but are, of course, capable of having more
added to increase compression, or some planed off to decrease it.
One of the peculiar things about this engine is the failure to realize
anywhere near boiler pressure, noticeable in every case that has come
under my notice. The considerable lead gives it for an instant, but it
soon falls away, indicating the steam chest pressure only by a peak at
the junction of the admission and steam lines. This is probably due to
the fact that the cut off valve commences closing the steam passage so
soon after steam is admitted, and in this particular does not satisfy
the requirements of a perfect valve. There is this about the engine,
that above all others of this type there has come under my notice
fewer engines of this type with a maladjustment of valves from
tampering by incompetent engineers.
* * * * *