Plain Bearing Failure - Lubricant breakdown with lead-bronze bearing

PLAIN BEARING FAILURE

Plain bearing failure index

1.0 Lubrication breakdown

Lubrication breakdown index

1.5 Lubricant breakdown with lead-bronze bearing

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	Crack in crankpin of reciprocating compressor with lead bronze big-end bearings caused by lubrication breakdown failure.
Main Characteristics	The photograph shows a section through one 10 inch diameter crankpin from a 6-throw reciprocating compressor after a lubrication failure. The lead-bronze bearing had wiped, but, in contrast to the case with a white metal bearing that limits the temperature rise to about 240°C, much higher temperatures have been reached and this has resulted in quench cracking of the steel crankpin and total loss of a large crankshaft.
Cause	This is a lubrication breakdown failure (see summary sheet at end of section for possible reasons).
Note	When wiping occurs with a lead containing copper alloy (lead bronze, copper lead), only a small proportion of the bearing metal melts (the lead phase); high enough temperatures are generated by rubbing against the solid material leading to potential cracking of the journal by repeated quenching as it leaves the load zone.
Possible Confusion with Other Types of Damage	Fatigue cracking due to repetitive stress cycles, rather than repetitive thermal cycles. However fatigue cracks are likely to be found at positions of stress concentration such as sharp corners or changes in section thickness.
Comment	White metal bearings should be the first choice where loading conditions permit their use. Lead-bronze can be used for little-end bearings where loads tend to be high, but the lower speed oscillating conditions do not tend to give rise to quench cracking. Note that hardened steel journals have to be used with these harder bearing alloys. As a rule of thumb journal hardness should be at least three times that of the bearing metal.