By J. A. Drake. Originally published in The Practical Mechanic, c. 1879. pp. 42-47
While the use of belts for the transmission of power is not an American invention, the numerous improvements made in this country have caused it to be known in Europe as the American system. In Europe the greater part of the power is transmitted by cog-wheels, but in this country 99 per cent. is transmitted by belting. The latter is used everywhere, from the sewing-machine to the 500 horse-power engine.
BELTS can be run in any way, at any angle, of any length, and at any speed, and can be put up by any one of ordinary skill. They can be made of any flexible material - leather, rubber, gutta-percha, or cloth; yet while so handy and so popular, they have one fault, they are not positive. If the motor makes a certain number of revolutions, a portion of them are lost with every belt used. This is the only fault of the system. It is noiseless, yielding, and regular, but unlike cog-wheels, it is not positive. The number of revolutions that are lost may, and do, vary continually by changes of the load, or the atmosphere.
BELTS DERIVE THEIR POWER to transmit motion from the friction between the surface of the belt and the pulley, and from nothing else, and are governed by the same laws as in friction between flat surfaces. The friction increases regularly with the pressure. The great difference often observed in the friction of belts is due simply to their elasticity of surface; that is, the more elastic the surface, the greater the friction.
IN TAKING POWER from any source of motion there are two points which control us; all the others we can control and modify to a certain extent. Ordinary belts will sustain safely a working tension of 45 pounds per inch in width. The rule to determine the width of belt and size of pulley required to transmit a given horse-power is easily found; since a horse-power is 33,000 pounds, raised one foot high per minute, we must adjust the width and velocity of belts so as to effect the required result. Thus, if the belt moves with the velocity of 733 feet per minute, a belt five inches wide will transmit five horse-power, provided the effective tension is 45 pounds per inch. If the velocity be increased to 1,466 feet per minute, the same belt, with the same tension, will transmit ten horse-power. So that a five-inch belt applied to a five-foot pulley, making 120 revolutions per minute, would transmit ten horse-power, when the effective tension is 225 pounds.
BY TAKING THE ACTUAL EFFECTIVE TENSION of the belt, and multiplying it by the actual velocity, we get what may be called the indicated horse-power of the belt, which corresponds to the indicated horse-power of the engine. And, finally, by measuring the actual power transmitted, which may be done by means of a dynamometer, we can get the actual power transmitted. Rules based upon the amount of belt surface in contact with the pulley, and on similar data, cannot be made to give reliable results. For practical purposes, velocity and power to resist tension are the only available elements of the calculation. Actual tension, adhesion, friction, etc., can all be varied at will, and consequently form no certain dependence for the calculations of the machinist and engineer.
ON THE SCIENTIFIC PRINCIPLE that the adhesion, and consequently the capability, of leather belts to transmit power from motors to machines, is in proportion to the pressure of the actual weight of the leather on the surface of the pulley, it is manifest that, as longer belts have more weight than shorter ones, and that broader belts of the same length have more weight than narrower ones, it may be adopted as a rule that the adhesion and capability of belts to transmit power is in the ratio of their relative lengths and breadths. A belt of double the length or breadth of another, under the same circumstances, will transmit more than double the power. For this reason it is desirable to use long belts. By doubling the velocity of the same belt its effectual capability for transmitting power is also doubled.
GOOD STOCK is the first requirement of a belt, which if spongy, will not meet that demand. It must be firm, but pliable; the grain or hair side should be free from wrinkles; the stock should show no inequalities in dressing, but be of an even thickness throughout; the splices should be mathematically true, and if rivets are employed they should be inserted on the hair side, and the burrs sent home before riveting; the edges should be parallel and perfectly straight. In handling a belt examine it carefully, double it up the hair side out, and press it together. If it crack under this treatment it should be rejected, as rational use of a belt consists in utilizing the whole amount of power it will transmit.
BELTS are sometimes used having a transmitting power of double the capacity necessary where they are employed, while quite as often they are much too narrow for the work required of them. The first instance shows a useless waste of material, the latter poor economy, for, in order that it may perform the work required, it is necessary frequently to take it up, as a result of which the weak points succumb to the strain and it is torn asunder, or, if not, the shaft is likely to be drawn out of line, or the bearing overheated.
IN USING A NEW BELT a few days, if it present a mottled appearance on the side next to the pulleys, it may be set down that it is not furnishing the full capacity of its power. The spots referred to indicate that certain portions of the belt do not touch the pulley, and that its entire transmitting power is not utilized. If the face of the pulley is true, and the belt as nearly perfect as possible, the defect may be remedied by the judicious application of rendered tallow and fish oil, two parts of tallow to one of oil, melted, and allowed to cool. A new belt should be used a day or two before it is oiled, and frequent applications of small quantities are better than too liberal oiling at long intervals.
If a belt of the proper size for the work it has to do, slip on the pulley, it is caused by the centrifugal force, which tends to throw it outward; a corresponding degree of tension will check the defect.
BELTS SHOULD BE PUT ON by a person acquainted with their use, as the wear of the belt depends considerably on the manner in which it was put on. Therefore the following suggestions, if practised, will be of much service to persons employed in this capacity. The ends to be joined should be cut perfectly square, in order that one side may not be drawn tighter than the other. Good lace leather, if properly used, will give better satisfaction than any patent fastening.
WHERE BELTS RUN VERTICALLY they should always be drawn moderately tight, or the weight of the belt will not allow it to adhere closely to the lower pulley, but in all other cases they should be slack. In many instances the tearing out of the lace holes is unjustly attributed to poor belting, when in reality the fault lies in having a belt too short, and trying to force it together by lacing; and the more the leather is stretched while being manufactured, the more liable it is to be complained of.
TO OBTAIN THE GREATEST AMOUNT OF POWER from belts, the pulleys should be covered with leather; this will allow the belts to be run very slack, and give 25 per cent. more wear.
More power can be obtained from using the grain side of a belt to the pulley than from the flesh side, as the belt adheres more closely to the pulley; but it should be remembered that the belts will not last quite so long, for when the grain, which is very thin, is worn off, the substance of the belt is gone.
DOUBLE LEATHER BELTS are frequently used, but it is clearly a mistake, as a single leather one will transmit more of the power than a double one. Double leather belts run straighter than single ones, as the flank side of one part can be put against the back of the other. A double leather belt will stand a greater tension than a single one, but a single belt will stand all that should be put upon any belt.
In cases where a belt is incapable of transmitting the required amount of power, and circumstances preclude the possibility of substituting a wider one, the difficulty may be overcome by using two belts of the same width, one on the top of the other. Two belts run in this way will transmit nearly as much power as one belt the width of the two.
HOW TO TEST THE QUALITY OF LEATHER FOR BELTING
Cut a small strip of the leather about one-sixteenth of an inch in thickness, and place it in strong vinegar. If the leather has been thoroughly tanned and is of good quality, it will remain for months even, immersed, without alteration, simply becoming a little darker in color. But, on the contrary, if not thoroughly tanned, the fibres will quickly swell, and after a short period become transformed into a gelatinous mass.
HOW TO MAKE BELTS RUN ON THE CENTRE OF PULLEYS
It is a common occurrence for belts to run on one side of the pulleys. This arises from one or two causes. 1. One or both of the pulleys may be conical, and of course the belt will run on the higher side. The most effectual remedy for this would be to straighten the face of the pulleys. 2. The shafts may not be parallel, or exactly in line. In this case the belt would incline off to the side where the ends of the shafts come the nearest together. The remedy in this case would be to slacken up on the hanger bolts, and drive the hangers out or in, as the case may be, until both ends of the shafts become parallel. This can be determined by getting the centres of the shafts at both ends by means of a long lath or a light strip of board.
The tighteners should be placed as close to the large or driving pulley as circumstances will permit, as the loss of power incurred by the use of the tightener is equal to that required to bend the belt and carry the tightening pulley. Consequently there is a greater loss of power by placing it near the small pulley, as the belt is required to be bent more than when it is placed near the large one.
The reason why belts run to the highest side of a pulley is due in part to a centrifugal force, and also to the fact that the part of a belt nearest to the highest part of a rounded pulley is more rapidly drawn because the circumference of the pulley is greater at that point.
RUBBER AND LEATHER BELTS¶
Rubber belts will transmit nearly as much power as leather belts with the same tension; and they have this advantage, that they may be made of any length, width, or thickness, and yet always run straight, providing the pulleys are in line. Besides, their first cost is much less than those of leather, but they will not last over half as long. They cannot be run in situations where the belt rubs, nor as cross-belts, or through forks, as shifting belts, and when they give out it is almost impossible to repair them.
IF A RUBBER BELT RUNS OFF and becomes entangled in the machinery, ten chances to one that it will be completely ruined, whereas a leather belt, under like circumstances, will sustain very little injury. When saturated with oil they soon rot, and when situated in cold damp places they are liable to freeze, which has a tendency to separate the different thicknesses and ruin the belt; besides, they often freeze to the face of pulleys when standing still, and when started up the gum facing is torn off, which ruins the belt.
A LEATHER BELT, if made of good stock, not overstrained and properly treated, will last for twenty years. When partly worn out it may be cut and used over again for a narrower or shorter belt; and when entirely unfit for the transmission of power it may be used for different purposes around a factory, but when rubber belts are worn out they are of no value whatever.
TO PREVENT ACCIDENTS by shafts revolving within reach of operatives' garments in mills and factories. - Cover the shaft with a loose sleeve of sheet tin or zinc, and insert a rim of thick gum or leather at each end to prevent rattling. Should it become entangled with the garments of any of the operatives the resistance will cause the sleeve to stand still while the shaft is rotating within it, by which means the person may be extricated and accident averted.
RULE FOR FINDING THE LENGTH OF BELT WANTED
Add the diameters of the pulleys together, divide the sum by 2, and multiply the quotient by 3 1/4. Add the product to twice the distance between the centres of the shafts, and the sum will be the length required.
RULE FOR FINDING THE WIDTH OF BELT TO TRANSMIT A GIVEN HORSE-POWER
Multiply 36,000 by the number of horse-power. Multiply the speed of the belt in feet per minute by one-half the length in inches of belt in contact with smaller pulley. Divide the first product by the second; the quotient will be the required width in inches.
RULE FOR CALCULATING THE NUMBER OF HORSE-POWER
A belt will transmit its velocity, and the number of square inches in contact with the smaller pulley being given. - Divide the number of square inches in contact with the pulley by 2; multiply this quotient by the velocity of the belt in feet per minute, and divide by 36,000. The quotient is the number of horse-power the belt will transmit.
Another rule to calculate horse power: Divide the number of square inches of belt in contact with the pulley by 2; multiply this quotient by the velocity of the belt in feet per minute; divide this amount by 32,000, and the quotient will be the number of horse-power.
RULE FOR CALCULATING LENGTH
For finding the change required in the length of a belt when one of the pulleys on which it runs is changed for one of a different size. - Take three times the difference between the diameters of the pulleys and divide by 2. The result will be the length of belt to cut out or put in.
HOW TO MEASURE A COIL OF BELTING
Add the diameter of the whole in inches to the outside diameter of the roll; multiply by the number of coils in the roll; then multiply this by the decimal .1309, and the product will be the number of feet in the roll. To have the exact length, the average diameter must be used, if the roll is not perfectly round, and fractional parts of an inch must not be omitted in the calculation.
HOW TO PUT ON A BELT
Never place a belt on the pulley in motion; always place it first on the loose pulley, or the pulley at rest; then run it on the pulley in motion. If the belt is very heavy, and the pulleys run at a very high speed, it is advisable to slack on the speed of the engine; but when this is impracticable or inconvenient, care must be taken to mount the belt on the exact face. The person engaged in so doing must have a firm footing, and prevent his clothes from getting in contact either with the belt or pulley. Where the belt is heavy, and the location such that it is impossible to get a solid footing and exert strength in running on the belt, it is best to stop the engine and mount the belt on the pulley as far as possible. Then take a small rope, double it, slip one end through the arms and around the belt and rim of the pulley, and the other end through the loop formed by the double of the rope; then stand on the floor on the opposite side and draw on the rope, when the belt will be hugged to the periphery of the pulley. When motion is communicated it may be slipped on without any trouble, while, by letting go the end of the rope when the belt is on the pulley, the noose will be undone and the rope thrown off.