Bearing Failure

Product category:

It is extremely unlikely that bearing failure is due to poor bearing quality. Bearings from Japanese, European and Korean sources are manufactured in highly automated ways to the same international standards with strict quality control. We have supplied many different brands from these sources over more than twenty years with good results.

When bearings do fail it is almost always for reasons totally unrelated to the quality of the bearing as follows: -


Bearings are designed to operate with the inner ring running exactly (within close tolerances) parallel to the outer ring. Any movement away from parallel will increase internal temperature, take up internal clearances and generally reduce bearing life. Common causes of misalignment are: -

  1. Bent shaft.
  2. Motor end plate not square to shaft due to poor manufacturing or faulty reinstallation.
  3. Bearing housing not bored square to end plate.
  4. Bearing "cocked" in housing due to foreign material left in housing or damage to housing in area where bearing seats.

Excessive Loading

If bearings are loaded either axially (along the shaft) or radially (at right angles to the shaft) with forces greater than the bearings are designed for, then bearing life is reduced. Radial loading from drive belts or gearboxes should be kept within original specifications. Repeated high failure rate or short bearing life may suggest excessive belt tension or an original design problem.

Bearings which have a tight fit on the shaft and/or the housing, have reduced internal clearances. For these applications C3 bearings, with greater than standard internal clearance, may be necessary to prevent excessive radial loading.

Excessive axial loading can occur when motors are used vertically or at 45° when they were designed to operate horizontally.

If motors do not make provision for an increase in shaft length as the motor heats up then the bearings can be subject to high axial loads. Do not use Loctite between the bearing and the housing if the bearing is meant to slide, unless you want early bearing failure!

Excessive Temperature

When electric motors operate at high temperatures heat is transferred either from the rotor or the windings to the bearing. When bearings run at higher than normal temperatures lubricant is lost from the bearing and failure follows soon after.

The way the bearing is discoloured will give a clue as to the primary source of the heat. Bluing of the external ring suggests a winding problem or insufficient motor cooling. Bluing of the inner ring suggests a problem with the rotor. Either the rotor bars are cracked or the rotor is poorly designed/manufactured.

Using run capacitors, which are more than 15% larger than the original specification, can result in larger circulating currents in the auxiliary winding and the rotor than the original design allowed. As a consequence motor temperature is increased. Similarly, using run capacitors, which are smaller than the original specification, can de-rate the motor and increase the operating temperature under load.

Mechanical Damage

Bearings should always be mounted using a press. Only that part of the bearing in contact with the interference fit should be pressed. Pushing elsewhere results in load being transferred through the balls of the bearing, risking damage to them and/or the bearing race.

Bearings should never be hammered into place. Damaged balls/raceways are noisy and have a much shorter life.


All bearings we supply come pre-greased. One common reason for bearing failure is over greasing. The bearing cavity should be filled no more than 30% with grease, otherwise the race-way does not have enough room to move freely. Some newer high temperature greases can cause the bearing to be noisy initially. After the grease has had time to mix thoroughly (10 minutes maximum) the bearing ususally becomes very quiet.