owing to more wrap on the sheaves, while others claim that tight ropes
are better. If a drive with all the ropes slack gave trouble by the
ropes slipping, the first remedy tried would be tightening the ropes.
But if the conditions were like Fig. 87, it would not be particularly
harmful to have some of the ropes longer than others; in fact, it might
be well, as the longer ropes would not make a complete circuit as
quickly as the shorter ones; consequently the position of the splices
would be continually changing. However, it seems more natural to have
about the same pull on all the ropes, that is, not have them as shown
in Fig. 88. In conclusion for the system, it should be noted that
it has no means of tightening the ropes except by resplicing; it is
not as well adapted to various conditions as the other forms; it is
the cheapest form to install and in some cases should give excellent
satisfaction.
With the continuous-wrap system having the rope from one of the grooves
pass over a traveling take-up, the latter has a tendency to produce
an unequal strain in the rope. In taking up, or letting out, the
rope must either slide around the grooves, or the strands having the
greatest pull will wedge themselves deeper into the grooves, producing
a smaller pitch diameter than the ones having less pull, making a
differential action on the ropes. It is therefore probable that it
is the differential action that takes up or lets out the ropes, the
take-up merely acting in a sense as an automatic adjustable idler. In
tightening, when the rope stretches or dries out, or even in running
normal, the greatest pull will be near the take-up, but if the drive is
exposed to moisture, and the rope shortens, it will be farthest from
the take-up, depending proportionately on the number of grooves the
take-up controls; so in large drives it is best to have more than one
take-up.
If one should use an unyieldable substance, as, for experiment, a
plain wire on two drums wrapped a number of times around and also over
a take-up, and the drums were moved together or apart, he would find
that the wire would have to slide around the drum; but, of course,
with a rope in a groove it is different. The rope will yield some.
It will also go deeper into the groove. This system costs more than
the preceding form, owing to extra expense for the traveling take-up,
but may be applied readily to different conditions and will be quite
satisfactory in general, if properly designed and installed.
The continuous-wrap system with a take-up or tightener acting directly
on all the wraps has practically none of the objectionable features
mentioned in the other two forms, and is quick in action, making it
applicable where power is suddenly thrown on or off. If the tightener
is made automatic, it may be controlled in numerous ways, as with a
weight or weight and lever or tackle blocks and weight, etc. It also
may be fitted with a cylinder and piston, with a valve to prevent too
quick action if power is suddenly thrown off or on. There is ordinarily
practically no unequal strain on the rope. This system may be applied
to different conditions as readily as the preceding form. Its cost is
more than that of either of the others, as the tightener must have as
many grooves as there are wraps. It must also have a winder to return
the last wrap to the first groove, and to give its highest efficiency
it must be properly designed and installed.
In either of the continuous-wrap systems, if a portion of larger rope
is used, it will produce a greater strain directly behind the large
rope, owing to its traveling around the sheave quicker. In angle work
there is always extra wear on the rope in the side of the groove, as
only the center or one rope may be accurately lined; so it is not
advisable to crowd the centers in angular drives, as the shorter the
centers and wider the sheaves the greater the wearing angle. It must
be remembered that the foregoing applies to ordinary simple drives
as shown in the sketches; where the drive is complicated, it may be
necessary to make other allowances.
XIV
A NEW SCHEME IN ROPE TRANSMISSION[10]
THE use of manila rope for transmitting power is becoming so common as
to attract no comment, and it possesses so many advantages in its own
field over any other method of conveying power that some objections
really existing are overlooked. When a rope drive is installed
according to modern practice, it is generally so successful and
furnishes such an agreeable and smooth running drive that any possible
objection is silenced by the many good qualities it evidently has. But,
as a matter of fact, the American continuous method of installing a
rope drive has a few serious drawbacks.
[10] Contributed to Power by Geo. F. Willis.
Were it possible to install a drive of say thirty ropes in such a
manner that each one of the ropes had exactly the same strain on it
that each other rope had, and this under varying conditions of speed
and load, it is evident that the thirty ropes would work exactly as a
belt of proper width to carry the load would, that the ropes would be
running with exactly the same tension clear across the width of the
drive, like the belt. But according to the best authorities on rope
transmission, this ideal condition is impossible to obtain.
It is given as desirable, by writers on rope transmission problems, to
use a take-up sheave for every twelve ropes, while ten is considered
even better. The best results have been secured by using a take-up
sheave for not more than eight ropes. But in any case the evil of
differential driving still exists.
In truth, the only drive in which perfect conditions can exist,
according to present practice, is one using but a single rope.
It is evident that when the load comes on the ropes, the entire number
of ropes in use are only able to ultimately reach the same tension from
the elasticity of the ropes themselves, as slipping in the grooves
rarely occurs. But there is a continued and uneven strain on the ropes
until the load becomes divided between them, and where ropes are used
to drive a varying load, this strain must and does reduce the life of
the ropes materially.
Many rope transmissions have been unsatisfactory because of this, and
when these drives have been so badly designed as to use one take-up
sheave for more than ten ropes, they are apt to be more expensive and
troublesome than could have been anticipated.
One rope drive is known where thirty ropes are used, with only one
take-up sheave. It has been a source of continual trouble and expense,
and has been replaced by the English system of multiple ropes. The
inherent troubles of this system have made the changed drive even
worse than the original. It will now be replaced by the system here
illustrated.
[Illustration: FIG. 89.]
In Fig. 89 is shown a plan view of the tighteners for a thirty-one
rope drive. As the ropes shown are 1-1/2 inches in diameter the main
tightener sheave is shown 60 inches in diameter or forty times the
diameter of the rope used. Mounted above the thirty-two groove sheave,
and in the same frame, is a single groove sheave of the right diameter
to reach the two outside ropes as shown, in this case 86 inches in
diameter. Further details are shown in the end elevation, Fig. 90, and
in the side elevation, Fig. 91. Allowing a working strain of say 250
pounds to each strand of the thirty-one ropes, we have a total weight
of 15,500 pounds which these two idler sheaves should weigh, including
the frame holding them.
These sheaves and the frame are mounted directly upon the ropes, on the
slack side of course, and just as a tightener is mounted on a belt. The
first rope passes around the thirty-two-groove sheave, on up over the
single-groove sheave, and back under the multiple-groove sheave again,
and is thus crossed over.
[Illustration: FIG. 90.]
It is evident that a rope threaded on this drive would, by the time
it had run ten minutes or so, have every strand in exactly the same
tension every other strand was in, and that the ropes would remain in
this condition in spite of variation of load and speed, as long as they
lasted.
The initial expense, including the erection, would probably be no
more than that for the necessary six or eight single-groove idlers,
with their shafts and boxes, tracks, etc., which would be necessary
according to established practice. The room taken up would evidently be
much less.
[Illustration: FIG. 91.]
In Fig. 92 an assembled drive of this character is shown. In Fig. 93
is shown a reverse drive, common in sawmill practice, where the two
sheaves described would preferably be mounted on a car, with the proper
weight to give the desired tension.
[Illustration: FIG. 92.]
[Illustration: FIG. 93.]
In a recent design is shown a cylinder with about 6 feet of piston
travel, provided with a reducing valve, so that the steam pressure
would remain constant at about 40 pounds. The cylinder is bolted to the
mill frame, while the piston rod is connected to the car carrying the
tightener sheaves. The cylinder is of the proper area, when furnished
with steam at 40 pounds pressure, to put the correct strain on the
ropes. A small steam trap is part of the equipment. This should give a
very elastic tension, and so long as steam pressure was at 40 pounds
or over, the tension would remain constant. With 6 feet piston travel,
it is evident that 372 feet of stretch could be taken out of the rope,
an amount entirely out of the question. A dog, or buffer, can be so
located as to prevent excessive back travel of the piston and car when
steam pressure is taken off.
It is evident that this method can be applied to a drive using any
number of ropes.
XV
HOW TO ORDER TRANSMISSION ROPE[11]
IT is probable that more different and erroneous terms are used by
purchasing agents and engineers when writing orders for transmission
rope than are used to describe any other article needed about a mill.
A knowledge of how to order clearly just the kind of rope wanted
would prevent delays and expense to many plants. Manufacturers of
transmission rope constantly receive orders so peculiar in their
wording that they dare not venture an immediate shipment, but must
first resort to the mails, telegraph or telephone to find out what is
really desired, and, of course, these mistakes, following the law of
"the general cussedness of things," usually occur after a breakdown at
the very time when every minute's delay means a considerable sum of
money lost.
[11] Contributed to Power by F. S. Greene.
There are in this country two manufacturers of cordage who make a
specialty of transmission rope, and the names under which their
rope is sold are fairly well known to all users of rope drives. In
addition to these two concerns, there are, perhaps, three or four other
cordage mills which make this grade of rope to some extent. From this
comparatively small source many different brands have sprung which,
rechristened, find their way to the market under a variety of names,
both poetic and classic. These many names lead to frequent delays in
ordering. The man who does the splicing at the mill has, at one time
or another, heard of a rope glorying in the possession of some fancy
title. It is more than probable that some salesman has told him most
wonderful stories of what this particular rope can do; consequently
when the time comes for a new rope, the splicer goes to the office
and asks that so many feet of such and such a rope be ordered. The
purchasing agent makes out the order, using this name, and sends it to
the manufacturer, who in all probability has never heard of the rope
and knows for a fact that it is not the brand under which any of his
fellow manufacturers are selling rope. Before the order can be filled,
two or more letters or telegrams must be sent and received.
It frequently occurs that manufacturers receive orders specifying
brands which never had existence at all, so far as their knowledge
goes. One firm recently found in the same mail requests for "Fern,"
"Juno," and "Elephant" transmission rope, though no such brands have
ever been on the market.
Another familiar mistake is the ordering of a certain color yarn
in the rope, as if this decoration possessed some peculiar virtue.
These colored yarns are simply a question of dye, and the rope in all
probability would be better and stronger were they left out.
Then again, we find peculiar wording as to the lubrication of a rope.
Some people insist that the rope shall be "tallow inlaid"; others
call for an "absolutely dry" rope or for a "water-laid" rope. All
transmission rope, to be of any service whatsoever, must be lubricated
and such a thing as a "dry" transmission rope or a "water-laid" one,
whatever that term might mean, would be of but small service to the
user. Each manufacturer has his own method or formula for lubricating,
and if this be a plumbago or graphite-laid rope, and he is asked for
an old-fashioned tallow-laid rope, he cannot fill orders directly from
stock.
It is unnecessary to name the number of strands, unless you wish a
three- or six-strand rope, for a four-strand transmission rope is
always sent, unless otherwise specified. It is also unnecessary to say
anything about the core, as the rope is always supplied with one, and
generally it is lubricated. Frequently five-strand rope is ordered.
This is very confusing, as there is such a thing as a five-strand rope,
but it is very rarely made. Ordering a five-strand rope is usually
brought about through the error of considering the core as a fifth
strand.
It is better, though not necessary, to order by the diameter instead of
the circumference, as transmission rope is made and usually sold upon
diameter specification.
By far the most frequent specifications received call for "long-fiber,
four-strand rope with core," and having done this, the purchaser
considers he has named all necessary requirements. At the present
price of manila hemp, which varies from 7 cents per pound for the
poorer grades to 12-1/2 cents per pound for the best, he may be quoted
for such a rope, with entire honesty, anywhere from 11 to 17 cents
per pound. To procure long-fiber manila hemp, and twist it into four
strands about a core, does not make a proper transmission rope. As
the rope will probably be required to run at a speed of from 3000 to
5000 feet per minute and be subjected to rapid and constant bending
throughout its entire length, the fiber should not only be long, but
the rope should be soft and pliable. Further than this, as the fiber,
yarns and strands must slip one upon another during the bending, the
rope should be so lubricated as to reduce to a minimum the frictional
wear from such slipping and rubbing, which is a much larger factor than
is generally supposed. Again, the unusual strength of manila fiber is
shown only when subjected to a longitudinal strain. Transversely, owing
to the cellular formation, the fiber is relatively weak; therefore,
in manufacturing transmission rope, the greatest care is necessary to
secure such proportion of twist in both yarns and strand as to render
the rope least vulnerable to crosswise strain. Nor will the term "long
fiber" insure the purchaser obtaining the proper material in his rope,
for the longest manila fiber, contrary to general belief, is not always
the best from which to make a transmission rope. Some of the extremely
long variety is coarse and brittle. The best fiber for transmission
rope is a particular grade of manila hemp known as Zebu, Fig. 94,
which is light in color, silky to the touch and exceedingly strong and
flexible.
[Illustration: FIG. 94.]
[Illustration: FIG. 95.]
[Illustration: FIG. 96.]
The accompanying illustration, Fig. 95, shows a close view of two
grades of hemp, that on the left being known in the trade as
"Superior 2ds," while the fiber to the right of the cut is "Zebu."
Fig. 96 shows a more distant view of the same two "heads" of hemp,
and the reader will see that in both the fiber is exceeding long,
and if anything, that of the Superior 2ds is longer than in the Zebu.
A transmission rope made from the latter, however, will cost the
manufacturer from 3-1/2 to 4 cents more per pound than if he had used
Superior 2ds, and will outlast two ropes made from the longer though
coarser fiber.
The reader, if he has perused this chapter to the present point, is
doubtless now asking himself: "How shall I word my order when I want
a first-class driving rope?" The safest road to follow is to write
to some manufacturer or firm whom you know to be reliable, and ask
for so many feet of their transmission rope, giving the name, if you
are certain on that point, and, of course, being sure to mention the
diameter. In case you do not know the name of his rope, word your order
as simply and briefly as possible; for example: "One thousand feet
1-1/2 inches diameter first quality manila transmission rope," and if
the concern to which you write is a reputable one, you will receive a
four-strand rope, made from Zebu manila hemp, put together with proper
twist and lay for the service required.
XVI
A BELTING AND PULLEY CHART[12]
RULE 1. _Pulley Speed._--When the diameter of both pulleys and the
speed of one is given, to find the speed of the other: Place the points
of spacing dividers upon the two given diameters in inches upon the
scale (Fig. 97); then raise the dividers, keeping the space obtained,
and place one point on the given speed and the other _above_ it for
speed of _S_, or _below_ it for speed of _L_ (_S_ and _L_ meaning
smaller and larger pulley, respectively). This point will fall upon the
required speed.
[12] Contributed to Power by A. G. Holman, M. E.
Example: If the two pulley diameters are 10 and 25 inches and speed of
larger pulley is 120 revolutions per minute, what is speed of small
pulley?
Place the points of dividers on 10 and 25 on scale _A_, then lift the
dividers and place one point on 120 and the other above it upon the
scale; the other point now rests on 300 as the speed of _S_. If the
speed of _S_ had been given, one point would have been placed at 300
and the other _below_ it, falling upon 120, the required speed of _L_.
Note.--In applying this rule, if the speed comes beyond the range
of scale _A_, the result may be read by carrying the space to the
revolution scale on scale _B_, and proceeding in the same way.
[Illustration: FIG. 97.]
Example: Diameter of pulleys 12 and 36 inches and speed of _L_ 500,
what is speed of _S_? Place points of dividers on 12 and 36. Now, if
dividers are raised and one point placed on 500 and the other above it
on scale _A_, it will come beyond the top of the scale. Hence go to
scale _B_, placing lower point on revolution scale at 500 and the other
point above, which will fall upon 1500, the answer.
RULE 2. _Pulley Diameters._--When the speed of both pulleys and the
diameter of one is given, to find diameter of the other: Place points
of dividers on the two speeds on scale _A_ or revolution scale _B_.
Then place one point of dividers on given diameter and the other above
it to find diameter of _L_, or below it for diameter of _S_. The figure
thus indicated is the required diameter.
Example: Speeds 180 and 450 and diameter of smaller pulley 20. What
must be diameter of _L_?
Place points of dividers on 180 and 450 on scale _A_. Then place one
point on 20 (the given diameter). The other point falls at 50, the
required diameter of _L_.
If the point falls between two graduations in any problem, the result
can be closely judged by the relative position.
The other and more labor-saving use for this chart is its application
to belting problems. It is generally conceded that there is no subject
of more general interest in practical mechanics and none on which there
is a greater difference of opinion than the proper allowance to be
made in the selection of belt sizes for given requirements. The general
formula for the horse-power transmitted by belting is
_HP_ = _WS_/_C_ in which _HP_ = horse-power,
_W_ = width of belt in inches, _S_ = speed of belt in feet per minute,
and _C_ = constant.
The proper values of this constant, or the feet per minute that each
inch of width must run to transmit a horse-power, under certain
conditions, is the point in question.
On the right-hand side of line _A_ on the chart is a series of lines
representing different values for this constant. The lower one,
marked 4, represents 400 feet belt speed per minute, the next above
is for 500, and so on. Against some of these values are suggestions
as to belts often recommended in connection with these constants. For
instance, 2 to 6 _S_ suggests the constant 1100 to be used for 2- to
6-inch single leather belt, 1000 for 6-1/2- to 10-inch single, 600 for
2- to 6-inch double, etc.
These suggestions practically agree with the advice of the Geo. V.
Cresson Company's catalog and the deductions of Kent's Handbook.
More power may be transmitted than these suggestions will allow, by
increasing the tension, but this is accompanied by the disadvantage of
requiring extra attention and undue pressure upon bearings.
The use of the chart for horse-power and width of belting is explained
by the following rules:
RULE 3. _Horse-power of Belting._--To find the horse-power that can
be transmitted when diameter and speed of pulley and width of belt
are given: Place one point of dividers on scale _A_ at the width of
belt in inches and the other point at the bottom of the line (at 1).
Next add this space to the hight representing diameter of pulley by
placing lower point of dividers upon the given diameter and allowing
the other point to rest upon the scale above. Then holding the upper
point stationary, open or close dividers until the other point falls
upon the proper constant on the scale at right-hand side of line _A_.
Now transfer this space last obtained to the scale _B_ by raising the
dividers, carrying them square across to _B_ and placing the point that
was on the constant upon the given speed on the revolution scale. Note
the location of the other point of dividers upon the horse-power scale,
which indicates the horse-power that can be transmitted under the given
conditions.
Example: What horse-power can be transmitted by an 8-inch double belt
running on a 40-inch pulley at 500 feet per minute? Place one point of
dividers on line _A_ at 8 (width of belt) and the other point at bottom
of line. Next raise dividers and place lower point on 40 (diameter of
pulley) and let the other point fall above upon the scale. Then close
dividers until lower point comes to the constant for 6-1/2 to 10 double.
Carry this space to scale _B_ with lower point on 500 on revolution
scale. Under point now falls upon 84 on horse-power scale, which is the
required horse-power.
RULE 4. _Width of Belting._--To find the necessary width of belting
when size and speed of pulley and the horse-power are given: Place one
point of dividers on scale _B_ upon the horse-power and the other point
upon the revolutions. Next transfer this space to scale _A_ by raising
the dividers, carrying them square across and placing the point that
was on revolutions upon the constant. Then holding the other point
stationary, raise the point that was on the constant and open dividers
until this point falls upon the given diameter. Now lift the dividers
and carry the lower point down to bottom of line (the point 1). The
upper point will now indicate the required width of belt.
Note.--If, in finding width of belt, there is doubt about the proper
constant to take, a medium value, say 6, may be assumed and a hasty
"cut and try" will show in what classification the required belt will
come.
Example: What width of belt for 100 horse-power with 40-inch pulley at
500 revolutions?
Place point of dividers on scale _B_ upon 100 on horse-power scale and
the other upon 500 on the revolution scale. Then carry the space to
scale _A_ with lower point on constant 5. Then resting dividers upon
upper point open them until lower point is at 40 (diameter). Finally,
raise dividers and place lower point at bottom of line. Upper point is
now at 9-1/2, indicating the nearest even width 10 as the answer.
A little practice will make one familiar with these rules, and it
will be seen that in the belting rules the four motions perform two
multiplications and a division.
XVII
SPLICING ROPE