Machinery's Handbook, 31st Edition
Double V-Belts 2585 effective diameters and in the position when a new belt is applied and first brought to driv ing tension. Next, measure the tangents and calculate the effective arc length ( AL e ) of each sheave (see Table 15 for a glossary of terms): AL d 115 e e θ = The effective length of the belt will then be the sum of the tangents and the connecting arc lengths. Manufacturers may be consulted for mathematical calculation of effective belt length for specific drive applications. Table 15. Glossary of Terms for Double V-belt Calculations AL e = Length, arc, effective, in. R = Ratio, tight side to slack side tension 2 a p = Diameter, differential, pitch to outside, in. R /( R − 1) = Factor, tension ratio d = Diameter, pitch, in. (same as effective diameter) r = Angular velocity, faster shaft, rpm/1000 d e = Diameter, effective, in. S = Speed, belt, fpm/1000 2 h d = Diameter differential, nomenclature to outside, in. T e = Tension, effective pull, lbf K f = Factor, length-flex correction T r = Tension, allowable tight side, lbf L e = Length, effective, in. T S = Tension, slack side, lbf n = Sheaves, number on drive T T = Tension, tight side, lbf P d = Power, design, horsepower (transmitted horsepower × service factor) θ = Angle, arc of belt contact, deg
a p
h d
Nomenclature diameter
Effective diameter Outside diameter
Fig. 7. Effective, Outside, and Nomenclature Sheave Diameters Number of Belts Determination: The number of belts required may be determined on the basis of allowable tight side tension rating ( T r ) at the most severe sheave. The allowable tight side tensions per belt are given in Table 16 through Table 19, and must be multiplied by the length-flex correction factors ( K f ) listed in Table 20. To select the allowable tight side tension from the tables for a given sheave, the belt speed and effective diameter of the sheave in question are required. Double V-Belt Drive Design Method: The fourteen drive design steps are as follows: 1) Number the sheaves starting from the driver in the opposite direction to belt rotation; include the idlers. 2) Select the proper service factor for each loaded driven unit. 3) Multiply the horsepower requirement for each loaded driven sheave by the corre sponding service factor. This is the design horsepower at each sheave. 4) Calculate driver design horsepower. This hp is equal to the sum of all the driven design horsepower. 5) Calculate belt speed ( S ) in thousands of feet per minute: S = rd /3.820. 6) Calculate effective tension ( T e ) for each loaded sheave: T e = 33 P d / S . 7) Determine minimum R /( R − 1) for each loaded sheave from Table 21 using the arc of contact determined from the drive layout.
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