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Motion System Design

Limiters without limits: Torque limiters

Q: Torque limiters and proximity sensors: What's the connection?

Ball-detent torque limiters are based on a simple torque-transfer mechanism, consisting of ball bearings held in conical detents on a hardened steel plate. When torque exceeds its preset limit, the balls overcome their spring-loaded holding force and roll out of the detents, allowing the output plate to spin freely. The process takes less than 3 msec and occurs within ±5% of the devices' preset torque value.

One drawback with this simple mechanism is that it cannot be disengaged repeatedly. In most cases, ball-detent torque limiters are good for about 1,000 to 2,000 overloads. The problem is that the detents in the output plate deform quickly because of repeated collisions with the balls. This limits their use primarily to emergency overload duty.

A similar mechanism, used in air ratchets for decades, addresses this shortcoming by replacing the dimpled plate with another set of ball bearings against which the drive bearings nest. During overloads, the ball bearings roll over each other, reducing impact and more evenly distributing the resulting stress throughout the active area. Tests indicate that the process is virtually wear-free, translating to unlimited disengagement cycle


On display: ESL torque limiting mechanism

Key feature: Mating sets of circumferentially arrayed ball bearings nest within each other to transfer torque from one shaft to another. Beyond rated torque, the bearings unseat and begin to roll over each other, uniformly distributing the relatively small stresses produced.

What it means to you: A wear-free, zero-maintenance overtorque mechanism with unlimited cycles that integrates easily into couplings and other rotary components.

What else: Fits shafts from 3 to 70 mm diameter; rated from 0.1 to 1,100 Nm; operates without backlash.

Innovator: R&W America, Bensenville, Ill.

For more info: or call (888) 479-8728

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