Machinery's Handbook, 31st Edition
2568 Flat Belts and Pulleys Pulley face widths are nominally the same as the widths of the belts they are to carry. The pulley face should be approximately one inch more than the belt width for belts under 12 inches (30.5 cm) wide, 2 inches (5.1 cm) more for belts from 12 to 24 inches (30.5– 61 cm) wide, and 3 inches (7.6 cm) more for belts over 24 inches (61 cm) in width. Belts may be made to center themselves by the use of crowned pulleys. The usual amount of crowning is 1 ⁄ 8 inch per foot (10.4 mm per meter) of pulley width. Thus, the difference in maximum and minimum radii of a crowned 6-inch (152.4 mm) wide pulley would be 1 ⁄ 16 inch (1.59 mm). Crowned pulleys have a rim section either with a convex curve or a flat V form. Flanges on the sides of flat belt pulleys are in general undesirable as the belt tends to crawl against them. Too much crown is undesirable because of the tendency to “break the belt’s back.” This is particularly true for riding idlers close to driving pulleys where the curvature of the belt changes rapidly from one pulley to the other. Here, the idler should under no circumstances be crowned and the adjacent pulley should have very little crown. Pulleys carrying shifting belts are not crowned. Open belt drives connecting pulleys on short centers with one pulley considerably larger than the other may be unsatisfactory due to the small angle of wrap on the smaller pulley. This angle may be increased by the use of idler pulleys on one or both sides of the belt. V-Belts Narrow V-Belts ANSI/RMA IP-22.— Narrow V-belts serve the same applications as multiple, classical V-belts, but allow for a lighter, more compact drive. Three basic cross sections—3V and 3VX, 5V and 5VX, and 8V—are provided, as shown in Fig. 1. The 3VX and 5VX are molded, notched V-belts that have greater power capacity than conventional belts. Narrow V-belts are specified by cross section and effective length and have top widths ranging from 3 ⁄ 8 to 1 inch (9.525–25.4 mm). Narrow V-belts usually provide substantial weight and space savings over classical belts. Some narrow belts can transmit up to three times the horsepower of conventional belts in the same drive space, or the same horsepower in one-third to one-half the space. These belts are designed to operate in multiples and are also available in the joined configuration. Belt Cross Sections: Nominal dimensions of the three cross sections are given in Fig. 1. Belt Size Designation: Narrow V-belt sizes are identified by a standard belt number. The first figure of this number followed by the letter V denotes the belt cross section. An X fol lowing the V indicates a notched cross section. The remaining figures show the effective belt length in tenths of an inch. For example, the number 5VX1400 designates a notched V-belt with a 5V cross section and an effective length of 140.0 in. Standard effective lengths of narrow V-belts are shown in Table 1.
1 ”
5/8 ”
5/8 ”
7/8 ”
3/8 ”
5/16 ”
17/32 ”
17/32 ”
3VX
5VX
5V
8V
Fig. 1. Nominal Narrow V-Belt Dimensions Sheave Dimensions: Groove angles and dimensions for sheaves and face widths of sheaves for multiple belt drives are given in Table 2a and Table 2b, along with various tolerance values. Standard sheave outside diameters are given in Table 3.
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