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.