BELTS, endless strips of flexible material, usually leather or India rubber, to trans mit motion or power from one pulley to another. Hopes and chains serve a similar pur pose. When chains are used, the pulleys are provided with projections which engage in the links of the chains and prevent slipping, and the mechanism has the positive rela tions of a rack and pinion. Ordinary flexible belts transmit power by the friction between them and their pulleys. The pulley which communicates motion is the driving pulley; that which receives, the driven pulley; that part. of the belt which runs from the driven pulley to the driver is the driving part of the belt, since it is pulled by the eriver. and iu turn pulls on the driven pulley; the part of the belt which runs from the t river to the driven pulley is the slack belt. The strain on the driving bell is the sum of the strain'of the belt on the pulleys when there is DO motion, plus the strain of the fric tion; that on the slack belt is the same strain on the pulleys less the friction. Thus. if a belt is stretched over its pulleys with a strain of 10 lbs. per in. of width, arahit requires 5 lbs. to make it slip, then the strain on the loving belt is 10+5=15 lbs., and the strain on the slack belt is 10-5=5 lbs., per in. of belt. As the two parts of the belt are unequally strained there will be a tendency to move, or creep, towards the driving belt over the driven pulley. Hence, the velocity ratio of the two pulleys will not. exactly fol low the inverse ratio of their radii, and the belt cannot be relied upon for giving unifor mity of motion. For driving most machinery, the fact that the belt is elastic, and will slip if unduly strained, makes it a favorite method of communicating power. Rubber belts transmit about 25 per cent more power than leather, because the surface of the rub ber conforms more perfectly to the minute inequalities of the pulley surface, and thus acquires a closer grasp. The texture of a rubber belt is more uniform than can be had in leathe2, and therefore a wide rubber belt will wear more evenly. In damp
and exposed places, rubber is more durable than leather. If, however, the belt is to be shifted back and forth, as in the stopping and starting of some machines, or in cross belting—wherever the edge of the belt is liable to wear—leather is preferable. If the pulley be higher in the center than at the side, or higher at one side than at the other, the belt will creep towards the . highest part; for this reason the surface of the pulley is usually made not cylindric, but of greater diameter at the center. If this be overdone, the belt does not pull, except along its central part. The pulleys usually lie in the same plane; and with their axes' parallel; but this is not necessary, provided that the course of each part of the belt—the driving and the slack part alike—be in the plane of the pulley toward which that part of the belt runs; the belt being always delivered by one pulley into the plane of the other.
Transmission of power by B. is more common in the United States than in Europe. As extreme cases may be noted: a leather belt of the New Jersey zinc works, 4 thick nesses, 48 in. wide and 102 ft. long; a rubber belt in Chicago, 6 ply, 48 in. wide and 320 ft. long; a leather belt for a paper mill in Wilmington, Del., 60 In. wide and ft. long. Hempen or wire ropes, running over large pulleys with V shaped edges, are used to transmit power to long distances. The U. S. arsenal at Rock Island. Ill., carries more than half a mile by one rope the power of 4 large turbine wheels, sufficient for all the present need of the machine shops. Such cables have been called teleodynamic (rubles. They can he on as fast as one mile per minute, and without covering will last three years. Intermediate sheaves are required at every 300 or 400 feet. For informa tion concerning the length of B. and the power transmitted, see RANKINE, 3IACIIINEHY AND MILL WORK, etc.