Custom Shaft Slip Couplings, Design and Testing

When one designs a plastic coupling to support shaft torques in the 10’s of Newton meters and confirms this with hand calculations and Finite Element analysis, some fellow engineers will still doubt a plastic coupling can handle such a high level of torque. So, only thing left to do is test them.

Step One:
Set up the plastic coupling between the two parts used in the final assembly, properly supported.

Step Two.
Attach a 1-meter-long cheater bar to the “input” shaft, add a luggage scale to the other end and start reefing until the coupling breaks. Stop when you feel plastic yielding. Yeilding/failure was detected around 160Nm (120lb*ft) of torque

Step Three:

Inspect the damage… but in this test, there hardly was any. This was very strange based on the amount of plastic deformation felt at the wrench. Wait, maybe…

Found it!!

Looks like the stainless-steel input shaft failed before the plastic coupling will. This means coupling passes the test, YES!!!