Rolled Key Example in a Coupling Hub

One issue that we see from time to time is the when a key has rolled within the coupling hub/shaft keyway & key joint.  While the joint might not experience any external issues (such as a crack) the joint does need immediate attention.  

It is just a matter of time before the joint will experience an issue as the joint may no longer able to transmit the full torque capacity.  The upper right hand corner of the image above is a good example of a rolled key, with a visible gap seen between the shaft and coupling near the keyway. 

The coupling pictured above was taken out of service and removed to allow for a more complete inspection & further determination as to what steps were required to bring the coupling hub/shaft keyway joint up to acceptable condition.

Recommended Follow-On Reading: To learn more about the importance of inspecting shafts, shaft keyways, and key prior to coupling installation, we recommend the following two articles: Shaft Inspection Prior to Coupling Installation and Shaft Keyway Damage.

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.