|Click on photograph to enlarge|| |
Minor rub in bottom of bearing subject to a vertical load
|Main Characteristics||Minor wiping - melting and resolidification of white metal at right hand side of damage patch (the black axial mark is a photographic
effect and should be ignored) - in centre of bottom half of bearing subjected to a vertically downward load.|
|Cause||Damage is caused by rubbing of journal before hydrodynamic lubricating film has developed.
For more information on causes of lubrication breakdown and possible actions
to take, see Lubrication
breakdown - Summary|
a hydrodynamic film is present, the load-line and the minimum film thickness is displaced in the direction of rotation. Hence any damage
during running due to small dirt particles or transient lubrication film breakdown
is likely to be displaced in the direction of rotation from the centre of the loaded half of the bearing.
|Possible Confusion with Other Types of Damage||Fretting damage caused by vibration from an external source when machine is not operating with the journal resting in the bearing also occurs on the bottom centre line.
However, fretting damage is likely to be symmetrical rather than smearing in
the direction of rotation (as shown in the photograph above). |
|Comment||This type of damage is limited to heavily loaded bearings, particularly in machines subject to frequent starts. The main effect is optical with the damage no more than a witness mark; it has no effect on the performance of the bearing and is only found when the bearing is examined at overhaul.
Minor wiping or plastic deformation can be a desirable feature, assisting run-in, the bearing generating its own profile to suit the operating conditions.
Similar witness marks may also be found at the edges of a bearing where there is misalignment or manufacturing error.