Induction motors are fitted with ball and or roller bearings. These bearings are robust and reliable and should give very little trouble provided they are properly fitted, kept absolutely clean and lubricated correctly.
Many engineers argue that if a bearing seems to be operating correctly it should not be tampered with.
Portable vibration detection results, sampled periodically and analysed can be a very useful way to recognise the onset of a bearing failure. Bearing temperature, e.g. using embedded detectors or with portable Infra Red (IR) spot checks, is another indicator the general health of a shaft bearing.
Otherwise, it is not easy to predict (with any degree of certainty) the unexpired life of bearings that have already run for some time. Also, inspection may not show damage to raceways and rolling elements in areas hidden from view.
The best policy is to renew the bearings as part of a planned maintenance programme. If this is not possible because of cost or a shortage of replacements, then bearings should be removed, cleaned and inspected for signs of damage before a decision to refit or renew is taken.
Before opening up a bearing, make sure that the complete area around the housing is clean and dry. Manufacturers recommend that bearings should be removed from the shaft as seldom as possible, but cleaning and inspection is best done with the bearing off the shaft. If the correct size of wedges or pullers is used, then removal should not cause any damage.
Bearings should be cleaned by immersion in a solvent such as clean white spirit or clean paraffin, then thoroughly dried in a jet of clean, dry compressed air. Bearings should not be spun by the air jet because skidding can damage the rolling elements and raceways.
Once dry, the bearing must be lightly oiled. Any traces of metal particles, such as brass, indicate cage wear and the bearing must be replaced. If there is no evidence of metal particles, carefully examine the raceways and rolling elements for signs of wear or damage. Hold the inner race in one hand and slowly turn the outer race. Any sticking or unevenness in the rotation requires a rewash of the bearing and rotation in the cleaning fluid.
If the sticking persists the bearing must be rejected. Similarly, bearings with visible signs of corrosion, overheating or damage, and those with a noticeable degree of roughness in rotation should also be replaced.
When fitting a bearing to a shaft, first clean the shaft and apply a thin film of light oil. Set the bearing square on the shaft and, with a tubular drfit (pipe), force the bearing against the shaft shoulder. The drift should bear on the inner race as close to the shaft as possible. Large bearings can be heated for 10-15 minutes in clean mineral oil up to 80’C to facilitate fitting.
Lubricate the bearings with the correct type and quantity of grease as recommended by the manufacturer. Fill the bearing about one third to one half full with grease. Overgreasing causes churning and friction which results in heating, oxidation of the grease and possible leakage through the seals.
On account of the high ambient temperature and excessive vibration which many marine motors endure, grease life can be short and fresh grease should be applied at regular intervals. Unless the bearing housing has a vent hole to allow excess grease to escape, it will be necessary to clean out the bearing housing before charging it with fresh grease. Because of the vibration on ships, bearings can be damaged when the motor is not running.
The shafts of stationary motors should be periodically rotated a quarter turn to minimise vibration damage to the bearings.