The Torq-Tender overload safety coupling from Zero-Max Inc., Plymouth, Minn. (http://www.zero-max.com/), relies on a spring-based mechanism to disconnect shafts in case of torque overloads. The torque at which the coupler disconnects can only be reset by changing springs in the unit. This should prevent tampering. It also eliminates costly and potentially risky calibration procedures. The devices are bidirectional and will disconnect at the specified torque regardless of direction of shaft rotation.

The force of a pair of springs determines the torque value at which the device disengages. The springs act on a pair of sliders that hold the drive key in an engagement slot in the outer housing. When torque exceeds the rating, the springs move a slider that forces the drive key to pivot out of the slot. This disengages the inner part of the coupler from the outer part. After disengagement, there is little resistance on the shaft, and, after one shaft rotation, the torque limiter tries to reengage if the overload has ceased. Once the overload is removed and speed reduced, the key snaps into the slot and the coupler is reset, ready for the next overload.

The couplers handle angular shaft misalignments up to 1.5° and parallel misalignments of 0.005 to 0.015 in. The couplers are constructed of heat-treated steel for durability. The devices come in a variety of sizes and styles, with torque limits ranging from 3 to 3,000 lb‑in.

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