Most belts used in machine shops are made of oak-tanned leather (Fig. 141), but canvas is sometimes substituted for leather belting. Single belts are made from one thickness of leather or canvas, and are 3/16 of an inch thick. Double belts are made from two thicknesses cemented together, triple belts from three thicknesses, and quadruple belts from four thicknesses. As a rule it is not advisable to use anything but single belts on pulleys smaller than 12 in. in diameter. Double belts, however, transmit about 70% more power than single ones of the same width.
There are various formulas given for finding the horsepower that can be transmitted safely, but a common rule used by mechanics is: A single belt one inch in width, running at the rate of 1000 ft. per minute, will transmit one horse-power without making the belt so tight that undue strain from journal friction will result.