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Plain Bearing Failure
5.0 Fretting
- Fretting index
5.3 Fretting damage - Summary
Characteristics
Fretting on white metal results in a blackening of the surface. With a journal bearing, it occurs when the shaft is stationary and hence the damage is in the direction of the gravity load. In this way it resembles start-up damage (see Section 1.1: Start-up wiping damage), but can be distinguished as a deposit rather than a witness mark or wipe.
Fretting between steel surfaces (for example between the backs of bearing shells fitted with inadequate interference fit in the housing, or at the points of contact between the pivots of tilting-pads and the carrier in tilting-pad bearings) leaves a reddish-brown stain on the steel surfaces.
Possible Causes
Fretting is caused by small scale oscillatory movements. With journal bearings this can occur through external vibration transmitted to a stationary machine. Fretting damage can also occur on the backs of journal bearing shells that are not given an adequate interference fit in the housing. The latter is a fitting error and is dealt with in Section 10: Manufacturing and assembly errors.
Fretting at the contact between the carrier and the pivot of a tilting-pad occurs if the pad is subjected to radial vibration (journal bearing) or axial vibration (thrust bearing) causing a 'fluttering' instability through the slight changes in load induced by the vibration. It can also occur with a lightly-loaded thrust pad, particularly the pads of the reverse thrust in a double thrust bearing if the axial float is such that the reverse pads are loaded by hydrodynamic action.
Actions
Fretting with journal bearings is uncommon. If it is a problem the cure is to isolate the machine from the source of the vibration or to rotate the shaft at weekly intervals so that the point of contact is changed.
When fretting occurs with tilting pads subject to vibration, the only remedy is to reduce the level of vibration. Double thrust bearings should be given an adequate axial float to prevent 'fluttering'. As a guide the float should not be less than 0.1% of the mean diameter of the thrust collar with a minimum value of 0.1 mm. With point (ball) contact pivots, a hardened carrier has to be used.
Analysis
Fretting damage is normally easy to identify by visual examination. Fretting between steel surfaces results in the formation of the reddish-brown ferric oxide, alpha-Fe2O3. If confirmation of fretting is required, alpha-Fe2O3 can be identified by x-ray diffraction.