8.1.4 Factors affecting bearing life
Wear of journal bearings mainly occurs with an oil film of very small minimum thickness and mild local wear, especially as new bearings 'bed-in', is to be expected. However, severe wear is a symptom of trouble and can be caused by:
- Contacting asperities on substandard journal and bearing surfaces.
- Scoring by oil-borne solid particles.
- Excessive local pressure or temperature.
- Electrical pitting.
Fatigue cracking of the white metal surface can be caused by heavy vibration or cavitation within the oil film; the latter is usually caused by a restricted supply of oil.
Corrosion rarely occurs, due to the control by treatment plant of moisture in the turbine lubricating-oil system (see Chapter 2).
Electrical erosion of bearings and journals has occasionally given trouble. Small shaft voltages are set up by electromagnetic induction in the generator rotor. Higher voltages sometimes occur in the turbine, produced electrostatically. Bearing erosion is prevented by effective insulation of the generator bearings from earth and the application of an earthing brush to a suitable point on the turbine rotor system. Both insulation and earthing brushgear require good maintenance as well as good design to assure their correct functioning.