Machinery's Handbook, 31st Edition
2604 VARIABLE SPEED BELTS Rim Speed: The material and design selected for sheaves must be capable of withstand ing the high rim speeds that may occur in variable-speed drives. The rim speed is calcu lated as follows: Rim speed (fpm) = ( π /12) ( D o ) (rpm). Arc of Contact: Arc of contact on the small sheave may be determined by the formulas: Exact Formula: 2cos Arc of Contact (deg) 1 = − C D d 2 − a k Approximate Formula: C D d 180 60 Arc of Contact (deg) = − − ^ h where D = Pitch diameter of large sheave or flat pulley, inch or mm d = Pitch diameter of small sheave, inch or mm C = Center distance, inch or mm Table 36. Arc of Contact Correction Factors
Arc of Contact, θ , on Small Sheave, (deg)
Arc of Contact, θ , on Small Sheave, (deg)
C D d −
C D d −
Correction Factor
Correction Factor
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
180 174 169 163 157 151 145 139
1.00 0.99 0.97 0.96 0.94 0.93 0.91 0.89
0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50
0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50
0.87 0.85 0.82 0.80 0.77 0.73 0.70
0.65 60-Degree V-Belts.— 60-degree V-belts are ideal for compact drives. Their 60-degree angle and ribbed top are specifically designed for long life on small diameter sheaves. These belts offer extremely smooth operation at high speeds (in excess of 10,000 rpm) and can be used on drives with high speed ratios. They are available in 3M, 5M, 7M, and 11M (3, 5, 7, 11 mm) cross sections (top widths) and are commonly found in the joined configuration, which provides extra stability and improved performance. They are specified by cross section and nominal length; for example, a 5M315 designation indicates a belt having a 5 mm cross section and an effective length of 315 mm. Industry standards have not yet been published for 60-degree V-belts. Belt manu- facturers should be contacted for specific applications, specifications, and additional information. SAE Standard V-Belts.— The data for V-belts and pulleys shown in Table 37 cover nine sizes, three of which — 0.250, 0.315, and 0.440 — were added in 1977 to conform to exist ing practice. This standard was reaffirmed in 1987. V-belts are produced in a variety of constructions in a basic trapezoidal shape and are to be dimensioned in such a way that they are functional in pulleys dimensioned as described in the standard. Standard belt lengths are in increments of 1 ⁄ 2 inch (12.7 mm) up to and including 80 inches (203.2 cm). Standard lengths above 80 inches up to and including 100 inches (254 cm) are in increments of 1 inch (2.54 cm), without fractions. Standard belt length tolerances are based on the center distance and are as follows: For belt lengths of 50 inches (127 cm) or less, ± 0.12 inch; over 50 to 60 inches (127–152.4 cm), inclusive, ± 0.16 inch; over 60 to 80 inches (152.4–203.2 cm), inclusive, ± 0.19 inch; and over 80 to 100 inches (203.2–254 cm), inclusive, ± 0.22 inch. Belt Storage and Handling.— To achieve maximum belt performance, proper belt storage procedures should always be practiced. If belts are not stored properly, their performance can be adversely affected. Four key rules are: 1) Do not store belts on floors unless they are protected by appropriate packaging. 2) Do not store belts near windows where the belts may be exposed to direct sunlight or moisture.
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