AEMT - Association of Electrical and Mechanical Trades

26 July 2022
Q&A
In this issue of Renew, Matt Fletcher, MD of Fletcher Moorland Ltd, answers questions around the correct approach to greasing electric motor bearings.

QUESTION: Should I grease the bearings on all motors at my site on a scheduled basis – such as every day, week or month?

MATT FLETCHER: Generally, bearings in electric motors do need greasing; however, just doing something because you believe it's right may have unintended consequences. Take the motor pictured here, which came into our workshop recently. The person tasked with greasing this electric motor's bearing thought they were doing the right thing. However, in this example, over-greasing has caused the bearing to fail. When my team stripped down the motor, they discovered the excess grease covering the motor internals and the windings. It's a common sight in many electric motor repair workshops, and this is not the worst we've seen.

In typical grease-lubricated electric motors, a chamber holds the bearing; the grease for the bearing is enclosed in this chamber. New grease is fed in through an external port when needed. Many motors have an exit for the old grease to escape, however, it seems that many motors don't. Grease doesn't just disappear; it has to go somewhere. If the grease isn't coming out of the front of the motor, then it's passing through the grease chamber and into the motor.

Too much grease is as bad, if not worse, than too little grease in a bearing. What happens is the grease starts to churn, and excessive heat is generated. The operator notices this and thinks the bearing needs even more grease – a vicious and destructive cycle starts, which typically ends with a failed bearing and a stopped motor.

A secondary issue can also be caused by the grease coating the motor's windings. The grease can degrade the conductors' enamel coating, which can lead to a winding insulation failure.

It may well be that the operator who did this was under production pressures to get as much life out of the motor as possible, but it was only delaying the inevitable. 

There is no one-size-fits-all approach to greasing motors, but greasing every day, week or month, whether the motor needs it or not, could be a recipe for disaster. What I would recommend is that you check that your greasing program is fit for your motors and their application. In all likelihood, this will mean different timescales for different motors. Most bearing manufacturers will offer charts that calculate recommended greasing intervals based on the bearing type, size, and speed of operation.

If you are feeding grease into a motor on a regular basis. Stop to think about where the grease is going. You could be doing more harm than good without realising it.

 

QUESTION: Do you think there could be a way to measure how much grease is in a motor? With sensors, perhaps?

MATT FLETCHER: From my experience, ultrasound offers a good indication of whether a bearing has enough, too much or too little grease in it. It measures the ultrasound response to friction. From there, with a condition-based approach, a set dosing of grease can be filled to the bearing. I don't think it's a matter of how much grease there is but 'the right amount', and that can be measured by sensors.

A lot of motors have 'sealed for life' bearings. That description is often misleading as it's the life of the bearing and lubricant that's referred to here (load and speed, of course, determine bearing life), not the life of the motor. So, there is a finite life for those bearings, typically of 2-5 years. I would much prefer to see an open bearing that can be properly lubricated, so the grease goes through the bearing completely and has an external grease relief. That way, a proper lubrication program can be followed, and the bearing can attain its design life – assuming proper fitting and alignment.

 

In this story: FLETCHER MOORLAND
Elenora Street, STOKE ON TRENT, ST4 1QG
+44 (0)17824 11021
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