Shaft Keyway Damage

During inspection of a shaft prior to installing a replacement coupling, a technician found the shaft keyway had some damage. I was asked if this damage would have any impact on the coupling. (The image above shows the shaft keyway in question).

First, I commended the technician for inspecting the equipment and asking about its condition. Second, we dove into what the impact the damaged shaft keyway could have on the system once a new coupling was installed.

For this application (the coupling hubs having interference fit bores), torque is transmitted between the shaft and coupling hub through two methods: (1) the friction caused by the shaft interference fit and (2) via the surface of the key.  When the surface of the keyway is damaged, the key no longer has proper contact and the shaft & hub connection/joint may not be able to fully transmit to the torque level as designed.

For this application, as the new coupling might not be able to transmit a full torque capacity as rated, a recommendation was made that the shaft should be reworked/replaced to ensure a proper keyway fit.