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Plain Bearing Failure
Incompatible Journal or Thrust Collar Material
9.3 Unsuitable Journal or Thrust Collar Materials : Summary
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There are two quite separate types of bearing damage that arise from using unsuitable materials for the journal or thrust collar.
1. Damage with Nitrided Steel Shafts
Characteristics
- The bearing surface of a nitrided steel shaft has to be cleaned and polished before going into service otherwise there is a risk of scoring of the bearing, with a characteristic vee-shape pointing against the direction of rotation
Causes
- Failure to clean the surface after nitriding. This is in fact a manufacturing error (see Section 10), though for descriptive purposes it is most conveniently treated here.
Characteristics
- This is the only form of bearing failure that results in extensive damage to both bearing surfaces. It is readily identified by the presence of a mass of 'wire wool' in the bearing housing.
Causes
- Embedding of a hard particle of dirt or swarf in the bearing material that remains proud and rubs against the steel surface creating high temperature through friction. Wire-wool failure is limited to alloy steels containing more than 3% chromium operating with surface speeds greater than 25 m/sec.
Actions
- The best action is to avoid using steels containing more than 1.5% chromium in bearings with surface speeds greater than 25 m/sec. The optimum solution with existing machines is either to sleeve the journal and replace the thrust collar with a plain low carbon steel; though hard chrome plating of the bearing surfaces will also remove the risk. Careful control has to be exercised over the chromium plating to ensure that the chromium is properly bonded to the steel surface.