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Plain Bearing Failure
7.0 Thermal ratcheting of Tin-Rich White Metals and Thermal Fatigue
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Thermal ratcheting of Tin-Rich White Metals and Thermal Fatigue - index
7.3 Thermal Ratcheting and Thermal Fatigue - Summary
Characteristics
Thermal ratcheting of tin-rich white metal produces a chequer board appearance on the trailing edges of thrust pads subject to thermal cycling. The effect is similar to that produced by chemical etching used for metallographic examination. It is largely an optical effect resulting from variations in the light reflected from slight differences in the orientation of neighbouring crystals. The scale of the pattern shows the size of the tin-rich crystals. The effect is limited to tin-rich white metals.
Possible Causes
Thermal ratcheting arises from thermal cycling of the bearing metal and, when this occurs, the effect is most obvious on the trailing edges of thrust pads where temperature changes are greatest. It occurs because of differential rotations of the tin-rich crystals caused by differences in the thermal coefficient of expansion in different directions in the tin-rich crystals.
Where dovetails are used thermal cycling can lead to thermal fatigue because of the difference in thermal expansion of white metal and steel, particularly where the use of dovetails results in to poor bonding of the white metal to the backing (see Section 3).
Actions
No corrective action is normally required for thermal ratcheting, though the very slight damage can easily be removed by a light polish with abrasive paper. Excessive temperature rise can, of course, result in wiping failure as described in Section 1.
Dovetails can inhibit the production of a sound metallurgical bond between steel and white metal and should not be used. See Section 3: Premature Fatigue.