Neale Consulting Engineers - Plain Bearing Failure - Thermal Ratcheting of White Metal

PLAIN BEARING FAILURE

Plain bearing failure index

7.0 Thermal ratcheting of Tin-Rich White Metals and Thermal Fatigue

Thermal ratcheting of Tin-Rich White Metals and Thermal Fatigue - index

7.2 Thermal Fatigue of White Metal

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	Thermal fatigue of white metal thrust pad. Note the absence of a tinned layer on the steel backing
	Thermal fatigue of white metal thrust pad. Note how the fatigue lines up with the dovetail
Main Characteristics	The Photographs show typical fatigue of white metal. Normally, however, there is insufficient axial vibration in industrial machines to cause fatigue failure of thrust bearings. This is much more likely to be caused by thermal cycling.
Cause	Thermal cycling can lead to cracking because of the difference in the thermal coefficient of expansion between steel and white metal. Differences in thermal conductivity may also be a factor, particularly where dovetails are used because the different thermal paths accentuate the temperature difference between the white metal and the steel. Note the way the fatigue damage lines up with the dovetail in Photograph 'b'. Poor bonding accentuates the situation. The absence of a tinned layer on the exposed steel in Photograph 'a' shows poor bonding. The use of dovetails tends to make it more difficult to achieve a sound metallurgical bond between the white metal and the steel backing.
Note	Notice particularly the coincidence in location of the fatigue with the dovetails in the steel backing
Possible Confusion with Other Types of Damage	Normal fatigue (see Section 3) can occur in thrust bearings, but is very uncommon.
Comment	Dovetails should not be used in white metals bearings. The differences in the thermal paths tend to disrupt the bond when cooling after pouring on the molten white metal because of the different coefficients of thermal expansion between the two metals. (See also Section 3: Premature Fatigue)