Challenge
A prospective customer wanted to replace a traditional torque-limiting coupling, which was expensive, delicate, and inflexible, with an inexpensive, robust, flexible coupling.

The coupling needed to completely decouple driving and driven sides at approximately 150 inch pounds of torque, in order to protect the driven side of the system, in the event of a failure of one of the components on the driven side. This sort of failure would be infrequent, so some maintenance to the coupling after such an event would be acceptable.

Traditional torque-limiting couplings are expensive, intricate assemblies, and although they generally have good repeatability in their decoupling torque, they can typically only handle so many decoupling cycles before wearing out; after that, some designs might decouple only after excessively high torque, if at all, and this might have catastrophic results in this customer’s device.

Other requirements in this application included a fairly substantial accomodation for radial misalignment, from both stackup in assembly and from thermal growth; angular misalignment of approximately 0.5 degrees; and an elevated ambient temperature.

Solution
In a sense, all shaft couplings are torque-limiting, in that they will decouple the driving and driven sides after failing, after their maximum rated torque is exceeded. Some couplings are damaged in the process, but others, like our types OC and UC, have an inexpensive replaceable break element.

Because angular misalignment was small, we focused on type OC. The diameters of the two shafts determined the size of the coupling; the static break torque of this particular size was about three times the torque at which we needed it to break, so we added features to the midsection that would cause the mid to break at a specified torque, and to break in such a way that the driving and driven sides of the system would be immediately and completely decoupled. We also engraved the static break torque into the midsection.

Factors like misalignment, ambient temperature, vibration, and the difference between static break torque and dynamic break torque all required that the customer experimentally determine what static break torque rating would work in their application. After some experimentation, it was determined that 150 inch pounds of torque would be appropriate. Repeatability of rated static break torque was plus or minus 5%.

So we were able to provide the customer with a coupling that was a very small fraction of the cost of a traditional torque-limiting device. After an over-torque event, the inexpensive midsection is replaced; and since the hubs have through-bores, this can be done without moving any components in the system — after loosening the set screws, one hub is slid back over the shaft, the new mid is installed, the hub is slid back into position, and the set screws are retightened. After replacing the midsection, the coupling is as good as new.

Any customer considering a torque-limiting coupling in their design might want to consider this option: We can produce an inexpensive alternative with their specified static break torque, plus or minus 5%, with accommodation for misalignment, that’s easily “reset” after over-torque.