Shaft Keys & Keyways - "Be aware of the little details”

Guest Post: Jim Mabrey, Lovejoy Field Sales Representative 

An Original Equipment Manufacturer (OEM) reported having issues with the Lovejoy SX6 Disc Coupling, specifically citing that a coupling vibration existed and it appeared to be leading to premature bearing failure on a 250Hp AC Motor driving a Blower. 

The immediate assumption was that the disc coupling hubs were not sufficiently balanced and that they needed to be replaced. Unfortunately, upon replacement, the freshly balanced hubs did not eliminate or address the vibration issue at hand.

Lovejoy SX6 Disc Coupling

Diving deeper, the question was then asked about the shape of the keyway. It was at this point that it was discovered that the OEM was using a square key provided by their Motor OEM on the motor shaft. 

The prevailing North American industry bore standard (ANSI/AGMA 9002-B04), used by Lovejoy, is to supply hubs with a chamfer key slot in order to reduce the stress on the hubs. Unfortunately, the square key used by was distorting the hub enough to make the coupling unbalanced. Once an ANSI/AGMA 9002-B04 compliant shaped key was used, the problem went away. 

The Lovejoy OEM disc coupling user now requires all keys provided by motor manufacturers to be made with a chamfer per ANSI/AGMA 9002-B04. 

About the Author: Jim Mabrey has many decades experience in the mechanical power transmission industry and has been a longtime supporter and champion of Lovejoy coupling solutions. To find a Lovejoy representative in your neck of the woods, please visit Lovejoy's Sales Representative Search Tool. 

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.