Machinery's Handbook, 31st Edition
Variable Speed Belts
2601
α b g / b e
S g
a
h g
Companion Sheave Multiple Groove
Companion Sheave
Fig. 11. Companion Sheaves The factors that determine the amount of pitch diameter change on variable-speed sheaves are belt top width, belt thickness, and sheave angle. This pitch diameter change, combined with the selected operating pitch diameters for a sheave, determines the pos- sible speed variation. The range of output speeds from a variable-speed sheave drive is established by the companion sheave and is a function of the ratio of the pitch diameter of the compan- ion sheave to the maximum and minimum pitch diameters of the variable sheave. Speed variation is usually obtained by varying the center distance between the two sheaves. This type of drive seldom exceeds a speed variation of 3:1. For a single variable-speed sheave drive, the speed variation ( ) PD PD Speed variation Min = Max of variable sheave For a dual variable-speed sheave drive, which is frequently referred to as a compound drive because both sheaves are variable, the speed variation is dr dn DR DN Speed variation = ^ ^ h h With this design, the center distance is generally fixed and speed variation is usually accomplished by mechanically altering the pitch diameter of one sheave. In this type of drive, the other sheave is spring loaded to make an opposite change in the pitch diameter and to provide the correct belt tension. Speed variations of up to 10:1 are common on this type of drive. Speed Ratio Adjustment: All speed ratio changes must be made while the drives are running. Attempting to make adjustments while the unit is stopped creates unneces- sary and possibly destructive forces on both the belt and sheaves. In stationary control drives, the belt tension should be released to allow the flanges to adjust without belt force interference. Cross Section Selection: Selection of a variable speed belt cross section is based on the drive design horsepower and speed variation. Table 33 shows the maximum pitch diam- eter variation (2 av ) that each cross section can attain. Horsepower Ratings: The general horsepower formulas for variable-speed belts are: . . . . d r d r d d r r K 1422V HP 04907 0 8378 0 000337 08378 1 1 . p p p p SR 009 2 = − − + − − ^ ^ c h h m ; E where DR = max driver PD DN = max driven PD dr = min driver PD dn = min driven PD
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