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. 

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.