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Magnetism and Bearings


Iron is one of the most abundant elements on earth, and iron oxide makes an excellent grinding media. If you have a precision piece of machinery like a bearing that’s also magnetic, you create a situation in which it can grind itself away. Also, as the bearing wears away, the resultant product is itself a grinding media multiplying the problem. Having a bearing that attracts a grinding medium is not a good thing.

There is not much material published on this topic; however, suggested that a measurement of no more than two gauss – magnetic flux density measurement unit – should be measured in the bearing area of the shaft.

For a typical maintenance department, such an instrument to measure gauss is not common. However, there is a clear recognition that whatever manufacturing processes there are for a bearing that will be mounted on a shaft, there must be some way of both measuring and eliminating magnetism that may be created by that process.

Another cause of magnetism is an induction heater, a common tool in shops that install bearings. The device creates an electric field that heats the bearing (like a kitchen induction oven), but it also causes magnetism in the bearing. More sophisticated induction heaters should be preferred that have an automatic demagnetization cycle (demag cycle).

Bearings induction heater - SKF

For induction heaters with demag cycles, the stated “norm” is that the magnetism must be below two ampere/centimetre (A/cm) (of interest, one A/cm, is about 0.000126 Tesla and one Tesla is 10,000 Gauss). These higher-grade induction heaters ensure that they do not leave the bearing with magnetism, and thus unintentionally shorten its life.

Having a bearing that has become magnetic can be detrimental to its operation. The sources of magnetism in the bearing manufacturing process are well understood and closely controlled. Therefore, the likelihood of a new bearing being magnetic from the package is highly unlikely. However, further handling of the bearing, such as using an induction heater without a demag cycle, or having the shaft itself become magnetic from its manufacturing process, can induce detrimental magnetism in the bearing.

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