of leather cemented and riveted together throughout the whole length of the belt. Still heavier belts, consisting of three or four thicknesses of leather, and known as triple or quadruple belts, are sometimes made for heavy drives. Cotton belts are made up of a number of layers, or plies, sewed together and treated with a water-proofing substance. They are termed two-ply, three-ply, etc., according to the number of plies they contain. Four-ply cotton belting is usually considered equal to single leather belting. Rubber belts are particularly adapted for use in damp or wet places. They withstand changes of temperature without injury, are durable, and are said to be less liable to slip than are leather belts.

Sag of Belts. The distance between pulley centers depends on the size of the pulleys and of the belt; it should be great enough so that the belt will run with a slight sag and a gently undulating motion, but not great enough to cause excessive sag and an unsteady flapping motion of the belt. In general, the centers of small pulleys carrying light narrow belts should be about 15 ft. apart and the belt sag 1 to 2 in.; for large pulleys and heavy belts the distance should be 20 to 30 ft. and the sag 2 to 5 in. Loose-running belts will last much longer than tight ones, and will be less likely to cause heating and wear of bearings.

Speed of Belts.-The higher the speed of a belt, the less may be its width to transmit a given horsepower; consequently, a belt should be run at as high a speed as conditions will permit. The greatest allowable speed for a belt joined by lacing is about 3,500 ft., per min. for ordinary single and double leather belts. For belts joined by cementing, when the joint has about the same strength as the solid belt, the velocity may be as high as 5,000 ft. per min. Higher speeds than these have been used, but there is little to be gained by exceeding about 4,800 ft. per min. In choosing a proper belt speed, due regard must be paid to commercial conditions. Although a high speed of the belt means a narrow and cheaper belt, the increased cost of the larger pulleys that may be required may offset the gain due to the high speed of the belt, at least so far as the first cost is concerned. The speed of a belt, in feet per minute, may be found by multiplying the number of revolutions per minute of the pulley by 3.1416 times the diameter of the pulley, in inches, and dividing the product by 12.

Horsepower of Belts.-The pull on a belt is greatest on the tight, or driving, side, and least on the slack side. The difference between the tensions, or pulls, in these two sides is called the effective pull. The effective pull that may be allowed per inch of width for single leather belts with different arcs of contact is given in the accompanying table. The arc of contact is the portion of the circumference of the smaller pulley that is covered by the belt. The horsepower that can be transmitted by a single leather belt may be found by the formula CWV 33,000

in which

H=

H= horsepower of belt;

(1)

C-effective pull, taken from table;
W width of belt, in inches;

V = speed of belt, in feet per minute.

If it is desired to find the width of single belt required to transmit a given horsepower, the formula becomes

EXAMPLE 1.-What horsepower can be transmitted by a single leather belt 4 in. wide running at a speed of 2,500 ft. per min., if the belt covers one-third of the circumference of the small pulley?

SOLUTION.-The fraction of the circumference covered by the belt is = .333. From the table, the allowable effective pull corresponding to this value is 28.8. Substituting in formula 1,

H=

28.8X4X2,500
33,000

=8.7 H. P.

EXAMPLE 2.-A single leather belt is to run at a speed of 3,000 ft., per min. and is to transmit 18 H. P. Find the width of the belt, if the arc of contact is 150°.

SOLUTION. The effective pull corresponding to an arc of contact of 150°, from the table, is 33.8. Substituting in formula 2,

33,000 X 18

W=
33.8 X3,000

A 6-in. belt would be used.

= 5.9 in.

The horsepower of a double leather belt may be taken as 14 times that of a single leather belt of the same width running under the same conditions. Accordingly, the width of a double leather belt required for any service is only fo that of a single belt for the same service.

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Lacing of Belts.-A very satisfactory way of lacing belts less than 3 in. wide is shown in Fig. 1, in which A is the outside of the belt and B is the side that runs against the face of the pulley. The ends of the belt are first cut square, and then holes are punched in the ends, in corresponding positions opposite one another. The number of holes in each row should always be odd, in the style of lacing shown, using three holes in belts up to 2 in. wide and five holes in belts between 2 and 3 in. wide. The lacing is first drawn through one of the middle holes from the under side, or pulley side, as at 1. Then it is drawn across the upper side and is passed down through 2, across under the belt to 3, up through 3, across and down again through 2, back under the belt and up through 3 again, then across and down through 4 and finally up through 5, where a barb is cut in the edge of the lacing to prevent it from pulling out. This completes the lacing of one half. The other end of the lacing is then carried through the holes in the other half, in the same order.

FIG. 1

For belts wider than 3 in., the lacing shown in Fig. 2 may be used. In this case, there are two rows of holes in each end of the belt to be joined. The row nearer the end of the belt should have one more hole than the row farther away. For belts up to 4 in. wide use three holes in the first row and two holes in the second row. For belts up to 6 in. wide, use four and three holes, respectively. For wider belts, make the total number of holes in both rows either one or two more than the number of inches of width of the belt, the object being to get an odd total number of holes. For example, a 10-in. belt should have eleven holes, and a 13-in. belt should have fifteen holes. The outside holes of the first row should not be nearer the side edges of the belt than in. and not nearer the joint edge than 7 in. The second row should be at least 1 in. from the joint edge. In Fig. 2, A is the outside face and B the face next the pulley. The lacing is first drawn up through 1 from the pulley side, and then is carried through 2, 3, 4, 5, 6, 7, 6, 7, 4, 5, 2, 8, 8, and out at 9 to be fastened. The other end of the lacing is used on the other half of the belt in the same way.

FIG. 2

B

Care and Use of Belts.-It is a much disputed question as to which side of a leather belt should be run next to the pulley. The more common practice, it is believed, is to run the belt with the hair, or grain, side nearest the pulley. This side is harder and more liable to crack than the flesh side; by running it on the inside, the tendency is to cramp or compress it as it passes over the pulley, while, if it ran on the outside, the tendency would be for it to stretch

and crack. The flesh side is the tougher side, but for the reason just given the life of the belt will be longer if the wear comes on the grain side. The lower side of the belt should be the driving side, the slack side running from the top of the driving pulley. The sag of the belt will then cause it to encompass a greater length of the circumference. Long belts, running in any other direction than vertical, work better than short ones, as their weight holds them more firmly to their work.

It is bad practice to use rosin to prevent slipping. Rosin gums the belt, causes it to crack, and prevents slipping for only a short time. If a belt in good condition persists in slipping, a wider belt should be used. Sometimes, larger pulleys on the driving and driven shafts are of advantage, as they increase the belt speed and reduce the stress on the belt. Belts may be kept soft and pliable by being oiled once a month with castor oil or neat's-foot oil.

When rubber belts are used, animal oils or animal grease should never be used on them. If the belt should slip, it may be lightly moistened on the side nearest the pulley with boiled linseed oil.

Flapping of Belts. One of the most annoying troubles experienced with belting of all kinds is the violent flapping of the slack side. Flapping may be due to one or both of the pulleys running out of true, causing the belt to be alternately stretched and released. This will usually cause a belt to flap when running at a high speed. If the belt is rather slack, tightening it may lessen or stop the flapping. Another frequent cause of the flapping of a belt is the want of alinement of the pulleys. To remedy this, the pulleys should be brought in line; should this fail, the belt should be tightened if it is rather loose. If no improvement is noticed and it is not possible to turn the pulleys, the belt speed should be reduced a little, either by the substitution of smaller pulleys or by changing the speed of the driving shaft, according to circumstances.

With belts running at speeds above 4,000 ft. per min., flapping may occur when the pulleys are perfectly true and in line with each other, even when the belt has the proper tension. This is believed to be due to air becoming entrapped between the face of the pulley and the belt; in this case the trouble may be prevented by perforating the belt with a series of small holes. Perforated belts may now be bought in the market.

Another cause of flapping is that the distance between the pulleys may be too great. In general, the distance between the pulleys should not exceed 15 ft. for belts up to 4 in. wide; 20 ft. for belts from 4 to 12 in.; 25 ft., from 12 to 18 in.; and 30 ft., for wider belts. A belt that is not joined square will flap, especially when running at a rather high speed.

SPECIFIC GRAVITY, WEIGHT, AND OTHER PROPERTIES OF MATERIALS

DEFINITIONS

The specific gravity of a body is the ratio of its weight to the weight of an equal bulk of pure water, at a standard temperature (62° F. = 16.670° C.). Some experimenters have used 60° F. as the standard temperature, others 32° and still others, 39.1°. To reduce a specific gravity, referred to water at 39.1° F., to the standard of water at 62° F., multiply by 1.00112.

Rule I. Given the specific gravity referred to water at 62° F., multiply by 62.355 to find the weight of 1 cu. ft. of the substance.

Rule II. Given weight per cubic foot, to find specific gravity, multiply by .016037.

Rule III.-Given specific gravity, to find the weight per cubic inch, multiply by .036085.

To Find the Specific Gravity of a Solid Heavier Than Water.-Weigh the body both in air and in water, and divide the weight in air by the difference of the weights in air and water.

EXAMPLE. If a piece of coal weighs 480 gr. in air and weighs 82 gr. in water, what is its specific gravity?

SOLUTION.-As 480-82-398, or loss of weight in water. Then 480÷ 3981.206, the specific gravity of coal.

To Find the Specific Gravity of a Solid Lighter Than Water.-Attach to it another body heavy enough to sink it; weigh severally the compound mass and the heavier body in water, divide the weight of the body in air by the weight of the body in air plus the weight of the sinker in water minus the combined weight of the sinker and body in water.

To Find the Specific Gravity of a Fluid.-Weigh both in and out of the fluid a solid (insoluble) of known specific gravity, and divide the product of the weight lost in the fluid and the specific gravity of the solid by the weight of the solid.

The weight of 1 cu. ft. of water at a temperature of 62° is about 1,000 oz. avoir., and the specific gravity of a body, multiplied by 1,000, shows the weight of 1 cu. ft. of that body in ounces avoirdupois. Then, if the magnitude of the body is known, its weight can be computed; or, if its weight is known, its magnitude can be calculated, provided its specific gravity is known; or, of the magnitude, weight, and specific gravity, any two being known, the third may be found.

To Find the Weight of a Body, in Ounces, From Its Magnitude.-Multiply the magnitude of the body in cu. ft., by the specific gravity of the substance multiplied by 1,000.

To Find the Magnitude of a Body, in Cubic Feet, From Its Weight.-Divide the weight of the body in ounces by 1,000 times the specific gravity of the body. NOTE. The specific gravity of any substance is equal to its weight in grams per cubic centimeter.

SPECIFIC GRAVITY OF COMMON SUBSTANCES
SPECIFIC GRAVITY OF MINERALS AND EARTHS