CAUSES OF BEARING FAILURE       

1)    Fatigue - This results in cracking or crazing of the bearing alloy due to cyclic loading conditions. This is a common cause of failure, particularly where clearances are too large and cyclic loading occurs.

2)                   Wiping - Wiping or running, which is caused by overheating, involves the smearing or running of bearing metal due to high local temperatures which flows to a cooler area where it solidifies. Minor wiping occurs in many soft alloy bearings without serious results but if it involves a major area due to say, an interruption of oil supply, the entire metal lining may melt and run involving complete bearing failure.

3)                   Corrosion - Corrosion may attack steel bearing shell in addition to the alloy lining. Whitemetal and aluminium alloys do not appear to be susceptible to normal corrosive attack, where as alloys containing lead either as an overlay or in the alloy itself such as lead - base whitemetal and copper - lead alloys are most vulnerable.

4)                   Fretting Corrosion - This is less common in journal bearings than in rolling contact bearings. It may be regarded as the removal of material due mainly to vibration of high amplitude and small oscillatory motion, combined with chemical attack.

5)                    Wear - It involves the removal of part (or all) of a bearing surface by mechanical action. Normal wear is frequently classed as abrasive wear to distinguish it from other types of similar failures.

6)                    Electrical damage -This can take several form but electrolytic pitting is most common in electric generator and motor. It usually takes the form of pitting of varying degree of magnitude, depending upon time and the rate of electrical leakage across the bearing.
The backs of the bearing shells, especially if of steel and of thinwall type, may also become pitted where the electrical current leakage passes to earth via the bearing pedestal housing.

7)                    Cavitation erosion - This is confined mainly to shell - type bearings fitted to high or medium speed engines and to bearings subjected to fluctuating loads, such as crankpin bearings. Cavitation and cavitation erosion are identical and caused by an implosion of gas or air bubbles released from a lubricating oil film under particular conditons. The
trapped air bubbles collapse or implode due to fluctuating loads, creating a local pressure possibly exceeding 221 kg/cm² and may cause a cavitation pitting effect in that area.

8)                    Whitemetal hardening - Tin - rich matrix of whitemetal bearing reacts chemically with oxygen, forming extremely hard tin - oxides, mainly stannic oxide (SnO₂). New tin - based whitemetal has brinnel hardness of about 30 to 40, but samples of the hardened layer can reach up to brinnel hardness value of 230 or about twice that of mild steel. With this the alloy losses all its essential conformability and embeddability properties and become extremely brittle and susceptible to fatigue failure. Under such condition, if hard foreign matter enters the bearing, then the journal can be severely scored instead of the whitemetal.

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