They protect downstream machinery and tools. But with ball-detent limiters, a commonly used design for emergency protection, every overload causes wear. Most don't survive more than 2,000 overloads before needing to be replaced. This makes them ill-suited for applications in which overloads are an integral part of a process.

The ESL limiter from R+W America, Bensenville, Ill. (rw-america.com), uses a principle borrowed from air ratchets to survive significantly more overloads before wear becomes a problem.

The ESL uses spring-loaded ball bearings that snap out of alignment when torque exceeds the spring's force. But instead of slamming into a conical detent, the bearings roll over another layer of roller bearings nested beneath the first. This second layer of bearings reduces impact and distributes forces evenly over both sets of bearings, resulting in less wear and longer life. The device handles overloads from 0.1 to 1,000 Nm and shafts with bore diameters of 3 to 70 mm. And a frictional clamping hub can replace the keyed connector to provide zero backlash.