Bearing Skidding

Over the last three years IRS has been monitoring motor bearings displaying various stages of fatigue. Most of these motors were taken out of service before a failure could occur but had their life significantly reduced as a consequence of the phenomena of bearing skidding.

Roller bearings appear to be particularly susceptible to skidding, which is the result of a combination of a rolling and a sliding motion that compromises the rolling integrity of the bearing because the forward force at the inner ring is smaller than the resistance to rotation of the roller/cage set.

Skidding may cause smearing and streaks in the circumferential direction of the rolling elements and raceways, which will lead to high running noise and may cause premature bearing failure due to excessive pitting formation.

Larger electric machines are equipped at the driven end with cylindrical roller bearings and in order to obtain a load carrying capacity as high as possible at the shaft end, the design engineer frequently selects a bearing for a heavier series load. A high load carrying capacity is however, actually required only with belt or gear take-off but in practice 90 per cent of today’s electric motors feature a coupling take-off. Thus the bearings are not subjected to high externally applied loads and will skid as a result.

Skidding problems occur mainly during test runs with unloaded bearings. In this case skidding is not only due to the low loads but will also be influenced by the unfavourable marginal conditions experienced during the test run. In a new bearing somewhat more grease is packed than the bearing will require for normal lubrication. This will affect the resistance to which the bearing cage is subjected and enhance its tendency to skid.

Bearing skidding can be determined relatively easily.

• Experience has shown that bearing noise created due to skidding will disappear for a short time when the bearing is relubricated.
• If the bearing cover can be removed skidding can be diagnosed with a stroboscope that will highlight the sliding, as opposed to rolling motion of the roller bearing element.

With critical skidding, it will be difficult to bring the cage to an apparent stop with a stroboscope. The rotational speed of the bearing cage will not be constant, but will be influenced by the varying friction contact and/or lubrication conditions in the contact area.

Skidding can be avoided in general by:

• Applying sufficient external load to force the bearing elements to rotate.
• Replacing the cylindrical roller bearings with deep groove ball bearings of identical dimensions. Deep groove ball bearings are less sensitive to skidding as the rolling elements have an additional degree of freedom and the bearing has a lower radial load carrying capacity than the cylindrical roller bearing of the same size.
• Reducing the grease charge applied to the bearing.

For more information related to bearing skidding contact Mark Drayton at Integrated Reliability Solutions on 0438 000 624.