Machinery's Handbook, 31st Edition
2566 PULLEY SPEEDS AND BELT LENGTHS Power Transmitted by Belts.— With belt drives, the force that produces work acts on the rim of a pulley or sheave and causes it to rotate. Since a belt on a drive must be tight enough to prevent slip, there is a belt pull on both sides of a driven wheel. When a drive is station- ary or operating with no power transmitted, the pulls on both sides of the driven wheel are equal. When the drive is transmitting power, however, the pulls are not the same. There is a tight side tension T T and a slack side tension, T S . The difference between these two pulls ( T T − T S ) is called effective pull or net pull . This effective pull is applied at the rim of the pulley and is the force that produces work. Net pull equals horsepower (HP) × 33,000 ÷ belt speed (fpm). Belt speed in fpm can be set by changing the pulley, sprocket, or sheave diameter. The shaft speeds remain the same. Belt speed is directly related to pulley diameter. Double the diameter and the total belt pull is cut in half, reducing the load on the shafts and bearings. A belt experiences three types of tension as it rotates around a pulley: working tension (tight side − slack side), bending tension, and centrifugal tension. The tension ratio ( R ) equals tight side divided by slack side tension (measured in pounds). The larger R is, the closer a V-belt is to slipping—the belt is too loose. (Synchro nous belts do not slip, because they depend on the tooth grip principle.) In addition to working tension (tight side − slack side), two other tensions are developed in a belt when it is operating on a drive. Bending tension T B occurs when the belt bends around the pulley. One part of the belt is in tension and the other is in compression, so com pressive stresses also occur. The amount of tension depends on the belt’s construction and the pulley diameter. Centrifugal tension ( T C ) occurs as the belt rotates around the drive and is calculated by T C = MV 2 , where T C is centrifugal tension in pounds, M is a constant dependent on the belt’s weight, and V is the belt velocity in feet per minute. Neither the bending nor centrifugal tensions are imposed on the pulley, shaft, or bearing—only on the belt. Combining these three types of tension results in peak tension, which is important in determining the degree of performance or belt life: T peak = T T + T B + T C . Measuring the Effective Length.— The effective length of a V-belt is determined by placing the belt on a measuring device having two equal diameter sheaves with standard groove dimensions. The shaft of one of the sheaves is fixed. A specified measuring ten - sion is applied to the housing for the shaft of the other sheave, moving it along a graduated scale. The belt is rotated around the sheaves at least two revolutions of the belt to seat it properly in the sheave grooves and to divide the total tension equally between the two strands of the belt. The effective length of the belt is obtained by adding the effective (outside) circumfer ence of one of the measuring sheaves to twice the center distance. Synchronous belts are measured in a similar manner. The following sections cover common belts used in industrial applications for power transmission and specified in Rubber Manufacturers Association (RMA), Mechani - cal Power Transmission Association (MPTA), and The Rubber Association of Canada (RAC) standards. The information presented does not apply to automotive or agricul- tural drives, for which other standards exist. The belts covered in this section are Narrow, Classical, Double, and Light-Duty V-Belts, V-Ribbed Belts, Variable-Speed Belts, 60 deg V-Belts, and Synchronous (Timing) Belts. Flat Belting Flat belting was originally made from leather because it was the most durable material available and could easily be cut and joined to make a driving belt suitable for use with cylindrical or domed pulleys. This type of belting was popular because it could be used to transmit high torques over long distances and it was employed in factories to drive many
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