Chapter XVIII: Wire Rope Transmission 143 (3)

One very common cause of trouble with engine belts is the fact that such belts usually run under the floor, where there is considerable moisture. Then the oil table under the average large Corliss engine will leak around dash-pots and rocker-arm shafts, and some oil will fly from the eccentric oil cups, get into the wheel, run around the rim and get to the belt; if the belt is not filled a very few drops of oil will make a large spot on it. Then, if an engine does not run the whole twenty-four hours, while it is off, watch. A few drops of water from a leaky valve stem whose bonnet drain is stopped up, as it will sometimes be, has a way of getting through the floor and falling on to the belt and running down the inclined inside of it until it finally comes to the flywheel, which, with the assistance of its crowning face, very kindly makes a nice pocket for said water and proceeds to drink it up. Result: the glue is loosened and the belt may come apart in consequence. Should there chance to be a point just at the bottom of this pocket, it will get the glue soft enough to slip but may not open up, which is much worse than if it did open up; for it may slip away from the shoulder of the splice for half an inch, and when the engine is put to work it may close down by running under the wheel and stick. If it does, the result is that at no very distant day you will find a break at that particular place, right across the face of the belt. The reason is that the load was all taken off the inside half of the belt by point slipping, thereby making the inside of the belt too long and putting all the load on the outside. The outside will continue to do all the work until it stretches enough to bring the inside back into service again. During this week or month you have been pulling your load with a single belt, not a double one, and after a short time you will find the break referred to above in the shape of a clean, well-defined crack extending across the belt parallel with the points of the laps. Now of course you are going to send for the man who sold you the belt and ask him to fix it. If he is a wise man and understands his business, he won't do a thing but show you right under that crack a point that does not come up to where it should come. Then the thing for you to do is to say to him that the belt is examined every time it is put into service and that you have noticed that the points he refers to all come loose during a "run," that any one knows that a few drops of water would not take any belt to pieces while it was running, and if it was water, why did it not take it apart everywhere, etc? And finally crush him completely by telling him that your men have no time to put a pair of clamps on a belt in order to pull back into its proper position every point that comes loose; that if they did do it they would have no time for anything else, especially in the present case, and that if his people had made the belt right the glue would have held, anyway.

After he has given you a new belt or repaired your old one, just take my advice and box that flywheel up above the top of the eccentric oil cup, at least 12 inches, and get some good, heavy tin or zinc and put a tight roof over the belt, under the floor.

First put in a ridge pole out of 1-1/2-inch pipe, starting at the face of the wheel and running in the direction of the main driven pulley, holding it firmly in place at each end with a strong iron clamp. Then solder into each edge of the strip of tin, which should be long enough to reach beyond any possible leak through the floor or oil table, a piece of 1/2-inch pipe, and put the tin over the ridge pole with a piece of small pipe on either side. Ordinarily the belt goes out past the cylinder; if it runs through a bricked-up runway on its route to the main driven pulley, just fasten the two pieces of 1/2-inch pipe to either wall and have the ridge about 6 inches higher than the outside ones. Then every drop of oil or water that comes through the floor will fall on to the roof and run down to the walls and be carried down to the floor of the pit and have no chance to touch the belt.

One of the most difficult things in the operation of large stations where a large number of belts are used is to keep them thoroughly clean and free from moisture and machine oil, the latter especially. One very hard problem that confronts all designers of machinery is the prevention of oil leakage from boxes. In several plants with as many as six dynamos of the same kind and the same design, at least four of the six have leaked oil every time they were run. The others did not leak as a usual thing, and all were equipped with the most modern methods of holding oil.

Now we come to the building of the belt, and we will notice only such points as interest the engineer or buyer. The first thing is to see that the laps are of uniform thickness, so that the belt will run quietly; and it should be absolutely straight when unrolled on the floor. If it has a long, graceful curve in it, look out; for it will not run straight on the pulleys until it has stretched straight, and by that time one of its edges may be ruined by coming in contact with the floor or some other obstacle. Next notice how long the leather is from which it is made. It should not show more than 52 inches, and then there will be 4 inches hidden by the point that is out of sight. Then see that the joints are broken properly. For instance, find the center of any piece of leather on one side of the belt, and then look on the opposite side and see if the joint is right under your center mark. It should be by all means, for right here lies the most important thing about the construction of leather belts. A belt whose laps are all the same length, and which has all its joints broken correctly, will put the same load on the glue throughout, and that is what must be done in order to get the best results. See Fig. 80. Here we have a belt that is 36 inches in width and a double ply. Now suppose there is a draft of 9360 pounds on this belt, that from point _A_ to point _B_ is 26 inches, and that the points are 4 inches long. Now we have 26 inches plus 4 inches plus 4 inches times 36 inches for the number of square inches in the glued joint. This equals 1224 square inches; the total pull on the belt divided by 1224 will equal the load on each square inch of glued joint, and will equal in this case 7.65 pounds. Now instead of assuming distance _A--B_ in Fig. 80 to be 26 inches, let the lower joint get out of step with the upper ones, and conditions get vastly different. We will suppose that the dimensions are as given in Fig. 81, as was the case with a new belt that was measured less than one month before the observation was made and we have the following: Joint _A B_ is now only 10 inches, and we have 10 inches plus 4 inches plus 4 inches times 36 inches which equals 648 square inches, and the lead on the joint is now 14.44 pounds. You will readily perceive what an important part in the life of the belt, and the life of everything around the belt as far as that goes, the proper breaking of the upper and lower joints is. Of course the belt maker will tell you that his glue is just as strong as the leather itself, and he is about right as long as you keep the belt free from oil and water; but when the belt becomes filled with oil the glue rots and loses its strength much faster than does the leather.

No good belt needs any posts along the sides to make it run straight and stay on the pulleys. If the pulleys are in line and the belt straight, it will run straight. All belts should be made to run perfectly straight on pulleys, first on account of the local advertisement that it gives to the man who has charge of them; second, if they do not run true, they will be on the floor or wrapped around the shaft in a very few minutes, should they ever slip. Another very important thing in the care of belts that carry heavy loads is that if any of the points do come loose so far back that they will not return to place without putting on the clamps, put them on by all means; as the restoring of this point to place means that you will still retain in service all of your belt, as you will not do if you glue it down where it is and thereby cut one side completely out of service.

HOW TO CLEAN BELTING

We submit the following as the best and proper way of cleaning a leather belt. It may seem simple, but it is safe and effective, as has been proved by many people who have thus restored old and dirty belting which had become almost or quite unfit for use.

Coil the belt loosely and place it on edge in a tank in which it may be covered with naphtha; a half barrel makes a good receptacle, but something with a tight cover would save the loss by evaporation. Put in enough naphtha to cover the belt completely and allow it to remain for ten or twelve hours; then turn the belt over, standing it upon the other edge. The vertical position of the belt surfaces allows the dirt to settle to the bottom of the receptacle as it is washed out, and permits naphtha to get at all the parts.

After the belt has remained in the naphtha another ten or twelve hours, or until sufficiently clean, raise it and allow the naphtha to drip back into the tank. Then lay the belt flat, stretching or shaking it until almost dry. You will find that the naphtha will not affect the leather nor the cement in the center of the belt, but may open the joints at the edges; in which case the old cement should be scraped off and the edges recemented. Your belt man will know how to do this. The belt will now be somewhat hard, and should be treated with a reliable belt dressing before being replaced on the pulleys.

X

BELTING, ITS USE AND ABUSE[7]

THERE is no class of appliances so little understood by the ordinary steam engineer and steam user as belts, which may be seen by the quantity of belting sold annually. Where one can point to a belt that has been in continuous use for twenty years, you can find hundreds that do not last one-fourth as long. Why? Not always because the buyer has tried to get something for nothing, but as a rule, when they do, they get nothing for something.

[7] Contributed to Power by Wm. H. McBarnes.

The average belt is a poor one, and the average buyer will not find it out till he has used it for some time. If you weigh the belt dealer up as a man who is trying to rob you, beat him down in price, then get him to give from 5 to 40 per cent. off, he will enter a protest, and, after some explanation, will come to some terms with you. Have you gained anything by your cleverness? Well, hardly. Belt dealers and makers, like almost all other dealers in supplies, aim to get nothing but first-class goods; but second and third, and even fourth-class goods, are made, and you get the quality you pay for. In the second place, belts wear out quickly because they do not get proper care. To let a belt run one moment after it gets too slack is bad practice, for it is apt to slip and burn all the staying qualities out of it. Another good reason why it should not be run slack is that the engineer or belt man, to save work, would be tempted to put on a dressing or, worse yet, put on resin to make it pull, and, in the language of Rex, "the man who will put resin on his belts is either a fool or a knave," for it is sure to spoil his belt if continued for any length of time.

In an emergency, as when some unforeseen substance has found its way to the belt, it may be necessary, to keep from shutting down between hours, to use some of the so-called dressing. We know from experience that engineers will go to almost any extreme to get out of a tight place--circumstances sometimes make it necessary to keep a belt running when it should not--but this should not be allowed to any extent. To allow a belt to run too tight is just as bad, for it will make short life for the belt, hot boxes and scored shafting. There is not one in twenty who takes the time or can splice a belt properly; it is generally done in a hurry, any way to make it hold together, with the understanding that it cannot talk; but it does. How often we see boards nailed up or rims tacked on to keep belts from getting off the pulleys. All of this is good for the belt dealers.

This is not all the fault of the engineer or the belt manufacturer. Often belts are made uneven, and soon get out of shape, even with the best of care. We sometimes find a belt that ordinarily runs easy on the pulleys and does its work with ease suddenly inclined to run to either one side or the other of the driven pulley. This is caused by one of two things--either the belt has been too slack, or the load increased for want of lubrication, or other causes. In either case it will run off if you insist on applying the power. The remedy would be to take up the belt, thoroughly oil the journals, or take off the extra load--maybe a combination of all. Still a little extra work making the belt tighter will enable it to run well and do the extra work just as long as the extra tension can be maintained. Then it may appear perplexing and run to one side of the driven pulley when the driven shaft gets out of line with the driving shaft. In a case of this kind the belt does not run to what is called the high side of the pulley, but to the low side. Another peculiar indication: If two shafts are parallel and there is a high place on the pulley, then a belt will run to the high place; but if the shafts are out of line, or, in other words, are not parallel, and the face of the pulley straight, then the belt will run to the low side or that closest to the driving shaft. The remedy would be to line up your shafting.

The object of this chapter is not to say how belts are made, but to impress upon the minds of belt users that to get the best results, belts, like all good servants, must be well cared for, and all responsibility should rest with one man, just as with your engine or any high-priced machine.

XI

A COMPARATIVE TEST OF FOUR BELT DRESSINGS[8]

DURING January, 1905, a comparative test of the working efficiency of four belt dressings and preservatives was made by T. Farmer, Jr., and the writer. The test was made on the regular belt-testing machine of Sibley College, Cornell University, a full description of which appeared on pages 705-707 of Vol. 12, _Trans. A. S. M. E._ This machine tests the belt under actual running conditions, though our belts were in somewhat better than average condition. The four belts were new 4-inch Alexander No. 1 oak-tanned single-ply, and were 30 feet long. Particular care was taken to keep them free from oil and dirt. The belts were first tested as received from the manufacturer, after which each belt was treated with one of the dressings and again tested.

[8] Contributed to Power by William Evans.

The dressings were two semi-solids, designated No. 1 and No. 2; a bar, No. 3, and neatsfoot oil, No. 4. As the first three are proprietary articles, it was not thought best to give their names, though any one familiar with the actions of belt dressings will readily recognize No. 1 from its peculiar curve. In applying the dressings, we followed directions carefully, and in the case of Nos. 2 and 3 exceeded them. The belt was given a five-hour run, during which two or three applications of the dressing were given, and then it was set aside in a warm place to allow it to absorb the applied dressing. After thus "soaking" for at least forty-eight hours, the belt was again run, this time for three hours, with one more application of the dressing. As No. 3 was a bar of sticky dressing, it will readily be seen that this precaution was not really necessary. No. 4, the neatsfoot oil, was not applied during the last run, as we were afraid of getting too much oil in the belt. As this oil is so extensively used by engineers for dressing belts, special care was taken to get the best possible results with it.

In Fig. 82, the result of the test with the neatsfoot oil is shown graphically. This curve is platted to show the relation between initial tension per inch of width and horse-power per inch of width. One reason for the drop in horse-power in the treated belt is that the slip was materially increased; in the lowest tension at which any power at all was transmitted, about 15 pounds per inch of width, the slip ran up as high as 25 per cent.

In Fig. 83, which shows the comparative value of the four dressings, the highest horse-power delivered to the belt was taken as the standard. The horse-power delivered by the belt was divided by this standard, and the result, expressed in percentage, was used as the percentage of available horse-power transmitted. This comparison shows the great superiority of dressing No. 1 at all times, and especially at low tensions. In looking at this chart, it is well to remember that No. 3 is a sticky dressing.

As the time of the test was so short, we were unable to determine the ultimate effect of the dressings on the leather of the belts. We could only approximate this by a chemical test and a close examination of the belts at the end of each test. The chemical analysis showed no ammonia or rosin in any of the dressings; No. 2 had a trace of mineral acid, and all had oleic acid as follows: No. 1, 0.27 per cent; No. 2, 29.85 per cent; No. 3, 3.5 per cent; No. 4, 0.7 per cent.

The practical test showed no ill effects except from No. 3, the sticky dressing, which ripped and tore the surface of the belt. The high initial tensions caused overheating of the journals, even though we kept them flooded with oil. On the low initial tensions there was no tendency to heat, even when the maximum horse-power was being transmitted by dressing No. 1. In the latter case we oiled the bearings once in every two or three runs (a "run" comprised all the readings for one initial tension), while in the former we oiled the bearings after each reading and sometimes between them; even then we were afraid that the babbitt would get hot enough to run. The readings for each run varied in number from two to a dozen, but only the one giving the maximum horse-power was used in drawing the curves. The belt speeds during the tests varied between 2000 and 2500 feet per minute, most of the tests being made at about 2200 feet per minute.

XII

BELT CREEP

THE question of the minimum amount of slip of a belt in transmitting power from one pulley to another reduces itself to a question of creep, for it is possible to have belts large enough so that with proper tensions there will be no regular slip. With a difference in tension on the two sides and of elasticity in the belt, creep, however, is bound to take place. What does it amount to and what allowance should be made for it? asks Prof. Wm. W. Bird of the Worcester Polytechnic Institute in his paper under the above title.

In Fig. 84 let _A_ be the driver and _B_ the driven, _T__{1} the tension in the tight side of the belt and _T__{2} in the slack side, the pulleys and belt running in the direction indicated. One inch of slack belt goes on to the pulley _B_ at _o_; at or before the point _p_ it feels the effect of increased tension and stretches to 1 + _s_ inches. It now travels from _p_ to _m_ and goes on to pulley _A_ while stretched. At or before reaching the point _n_, as the tension decreases, it contracts to one inch and so completes the cycle.

With a light load the belt creeps ahead of the pulley _B_ at or near the point _p_. If the load is heavy, the creep works towards the point _o_ and the belt may slip; this also takes place when the belt tensions are too light even with small loads.

The point may be easily appreciated by imagining the belt to be of elastic rubber. Professor Bird gives formulas for calculating the creep, and tests made at the Polytechnic to determine the modulus of elasticity. He concludes that the answer to his opening question is that for the common leather belt running under ordinary conditions the creep should not exceed one per cent. While this is sometimes called legitimate slip, it is an actual loss of power and cannot be avoided by belt tighteners or patent pulley coverings.

The smooth or finished side should go next to the pulley because the actual area of contact is greater than when the rough side is in contact; consequently, the adhesion due to friction is greater. Moreover, the smooth side has less tensile strength than the rough side, so that any wear on that side will weaken the belt less than wear on the other side would.

XIII

ROPE DRIVES[9]

THERE seems to be considerable difference in opinion regarding the various ways of applying rope to the sheaves in rope driving, viz., multiple- or separate-rope system, continuous-wrap or single-rope system with the rope from one of the grooves running on a traveling take-up device, continuous-wrap or single-rope system with the take-up working directly on all the wraps.

[9] Contributed to Power by R. Hoyt.

The multiple- or separate-rope system on a horizontal drive where the distance between centers is great enough so that the weight of the rope will give the required tension, having the tight or pulling part on the lower side and the sheaves of the same diameter, as in Fig. 85, should be very satisfactory, as old or worn ropes may be replaced by new ones of larger diameter, or some of the ropes may be tighter than others and still not alter the efficiency of the drive. It will be noticed in this case that a larger rope does not alter the proportional pitch diameters of the rope on the driving and driven sheaves; but if one of the sheaves is larger than the other, as in Figs. 86 and 87, and a new or larger rope is substituted for a worn or smaller one, or if some of the ropes are a great deal tighter than others, a differential action will be produced on the ropes owing to the fact that the larger or slack rope will not go as deeply into its grooves as the smaller or tight one. Consequently the proportionate pitch diameter on the rope on the driver and driven sheave will be changed. The action will depend upon whether the large or the small sheave is the driver. If the driver is the larger, and of course assuming that the slack or large rope is weaker than the combined tight or smaller ones, then it will have less strain on the pulling side; but if the driver is smaller, then the new or large rope will have greater strain on the pulling side. Whether the driver is larger or smaller, a large or slack rope affects the action oppositely to a small or tight rope. Fig. 87 shows how the action is reversed from Fig. 86.

For clearness we will exaggerate the differences in diameter in the sketches and figure the speeds that the different size ropes would produce. We will take _A_ as normal, _B_ 1 inch farther out of the groove, producing a difference in diameter of 2 inches; _C_ 1 inch deeper in the groove, producing a difference in diameter of 2 inches. In Fig. 85 assume for the normal diameter of driver and driven 40 inches, 42 inches for _B_ and 38 inches for _C_, with a speed of 200 revolutions per minute for the driver. Either _A_, _B_ or _C_ will give 200 revolutions per minute for the driven sheave, omitting slippage, of course. In Fig. 86 say the normal diameter of the driver for rope _A_ is 60 inches and of the driven 30 inches, a speed of the driver of 200 revolutions per minute will give the driven sheave a speed of 400 revolutions per minute; _B_, with the driver 62 inches and the driven sheave 32 inches diameter, will give the latter a velocity of 387-1/2 revolutions per minute. With _C_ the driver is 58 inches, the driven 28 inches, and the speed given the latter 414-2/7 revolutions per minute. In Fig. 87, the normal diameter of the driving sheave being 30 inches and the driven 60 inches, a speed of the driver of 200 revolutions per minute will give a speed of the driven member of 100 revolutions per minute. With _B_, if the driver is 32 and the driven 62 inches, the driven sheave will have a speed of 103-7/31 revolutions per minute; _C_, with the driver 28 inches and the driven sheave 58 inches, will give the latter a speed of 96-16/29 revolutions per minute. So it will be readily seen what effect a large or a small rope would have.

There are some who claim that slack ropes will transmit more power 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.

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.

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.

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.

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.

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.

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