Rating Basis

The mechanical adjustable-speed drive package is usually rated as a constant-torque, variable-power device. The power rating is based upon the capacity of the whole unit at maximum speed setting. When operating at any output speed below this maximum, the power capacity is reduced in direct proportion.

FIGURE 5 Typical motor pulley belt drive with adjustable center distance between driving and driven shafts (Reliance Electric)
FIGURE 6 Power versus output speed relationship for the motor, belt section, and reducer section of a mechanical adjustable-speed belt drive

A drive unit is made up of three major components, each of which has its own individual torque or power output characteristics. For example, the ac induction-drive motor or prime mover develops constant power at a constant rotational speed.

The adjustable-speed belt section has an output torque characteristic that is a mixture of both constant torque and variable torque over its speed range. This section has a constant-power, variable-torque characteristic when operating above a 1:1 belt position and a constant-torque variable-power characteristic when operating below a 1:1 belt position. Finally, a parallel-shaft or right-angle gear reducer section has a constant-torque, variable-power characteristic. Figure 6 graphically illustrates these relationships.

Because the gear reducer section has a constant-torque characteristic, this section defines the output characteristic for the entire mechanical adjustable-speed drive.

It should be noted that virtually all manufacturers rate parallel-shaft drives using output shaft power as a base, whereas right-angle output shaft drives are rated on the basis of input power to the reducer minus reducer efficiency.

Because of the relatively low efficiency of the right-angle worm gear reducers and right-angle combination worm and helical gear reducers, the power transmission industry follows the practice of rating these units in terms of power at the input shaft. From the earlier discussion on operating principle, you will note that the adjustable-speed output shaft of the belt section of the drive becomes the input shaft of the reducer section of the same drive.

One or more sections of a drive, usually the belt and reducer sections, may be service-factored by frame or case oversizing to permit the rating of these sections for constant power over all or a portion of their speed range. Because the drive motor is a constant-power device, as already mentioned, oversizing of its frame is unnecessary.

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