Machine Design

Traction Drives

Traction drives have been around for a long time but have not made a major impact on the power-transmission industry. The main obstacle to their increased use has been a lack of confidence in the fact that two smooth rollers can transmit force through a fluid. However, the block to using traction drives appears to be dissipating, aided by design changes that have improved capabilities. These advances, combined with ease of maintenance are making traction drives formidable competitors to conventional adjustable-speed drives.

Presently, the ball variator accounts for the majority of traction drives sold; however, Beier disc and ring cone drives also are widely used. In addition, planetary, toroidal, and ring roller devices have carved out niches as commercially proven traction drives. The friction drive (which some experts would not classify as a traction drive) rounds out the field of traction drives.

In general, traction drives are finding applications where compactness, ruggedness, and speed-control accuracy are primary concerns. The micrometer speed-setting dial on most traction drives attests to their inherent accuracy in maintaining a specific output speed. In addition, most conventional adjustable-speed drives require additional components to start smoothly. Traction drives, in contrast, automatically provide the extra torque needed at reduced speeds.

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