The shafting and machines are usually driven by belting. Leather is the material generally used, and the belting may be from single to six-ply in any suitable width. Single belting has a flesh and a grain or hair side, and should be run with the grain side in contact with the pulley. The ends are cut square, and fastened by hooks, coiled wire, or rawhide lacing.
Leather belting is injured by water, steam, oil, and temperature above 110° F. Where such conditions exist, cotton belts faced with thin leather, or rubber belts, may be used. These belts are cheaper than leather, are about as strong, and will transmit power as effectively; but they will not stand mutilation of the edges. This is a point of prime importance, and prohibits their use in many cases.
The power transmitted by a belt is directly proportional to its speed and width. A safe rule is to allow one horsepower for a speed of 1,000 feet per minute, with a single thick belt one inch wide. This is a more liberal allowance in favor of the belt than is usually given, but will increase the life of the belt in far greater proportion than the increase in first cost. Double belts will transmit about one and one-half times as much power as single belts. The above rule applies to belts running over pulleys of equal diameter, or, in other words, to cases where the arc of contact is 180 degrees. For smaller arcs of contact, use the coefficients found in the following tabulation:
To increase the power transmitted, either increase the speed of the belt by using larger pulleys, or use a wider belt.
A 3-inch single belt is running over a 24-inch driving pulley which makes 200 r.p.m. (revolutions per minute). How many h-p. will it transmit?
The circumference of the pulley in feet is 2 X 3.1416 = 6.2832 feet. As the speed of the pulley is 200 r.p m., the speed of the belt will be 200X6.2832 = 1,256.64 feet per minute. For every inch of width, it will transmit 1,256.64÷1,000 = 1.25664 h-p. Then a 3-inch belt will transmit 3X1.25664 = 3.76992 h-p.
Ans. 3.75 h-p. (approximately)
It is desired to increase the h-p. in the above example to 5 h-p. How may it be done?
(a) By using a wider belt in the proportion of 3.75 to 5. 3.75: 5:: 3: 4. Ans. By using a 4-inch belt
(b) By using a larger pulley in the same proportion. 3.75: 5:: 24: 32. Ans. By using a 32-inch pulley
(c) By using a double belt. 1:1.5:: 3.75: 5.63. This would give a little better result than required.
Courtesy of Fellows Gear Shaper Company, Springfiua, Vermont