Cyndi Nyberg 
Former EASA Technical Support Specialist

There are a number of ways that the shaft of an electric motor can become magnetized in service. The most likely culprit is electric current through the motor and shaft, either from internal dissymmetry, welding or from a variable frequency drive. It can also be caused by electrical faults in the system, or even a lightning strike. 

We of course know that shaft voltages and the associated currents can cause bearings to fail. A typical ball bearing failure from shaft currents is shown in Figure 1. when a shaft is magnetized, it can further lead to bearing failures, unless something is done to elimi­nate the residual magnetism. The first reason for bearing failures is that the residual magnetism can cause shaft currents, which can quickly lead to bearing failures. But in addition, a magnetized shaft will attract bits of metal to the bearings. This reduces bearing life because it damages the bearing surfaces. 

The magnetism in the shaft may be strong enough that a screwdriver that sticks to the shaft. In fact, this is the most simple test to check for a magnetized shaft. 

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Categories: Bearings, Repair procedures & tips, Shafts

Tags: Shaft currents Shafts

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