Conclusiones

Points to Note Main

Characteristics

The bearing damage is in the form of sharp vee-shaped grooves pointing against the direction of rotation.

Cause The presence of hard particles loosely adhering to the nitrided surface.

Note Possible Confusion with Other Types of Damage

Both dirt erosion (see Section 2: Wear) and cavitation erosion (see Section 4:

Cavitation Erosion) produce arrow-shaped damage on the bearing surface; in these cases, however, the whole of the arrowhead has a matt appearance.

Comment Nitrided steels should be cleaned and polished before being used as bearing surfaces.

Wire-Wool Failure of Journal and Thrust Bearing

Wire-wool failure of journal bearing

Wire-wool failure: damage to journal and thrust collar corresponding to the bearing in Photograph ‘a’ above.

Points to Note Main

Characteristics

Identified by extensive damage to both bearing and the journal and/or thrust collar with wear products from the shaft collected in the bearing housing where they look like wire wool. A black scab is also often present on the damaged surface, hence the alternative name ‘black scab failure’.

Cause This failure occurs when a small particle of hard dirt or swarf gets embedded in the bearing material, but continues to rub against the steel counterface. At high speed the temperature generated by the frictional rub carburises the chromium in the steel in the presence of a hydrocarbon oil, producing hard chromium carbides that embed in the soft bearing material and act as cutting tools on the journal or thrust collar. The process continues by accretion of the embedded carbides and may result in a journal being turned completely through.

Note Although wire-wool failures have been most frequently encountered with whitte metals bearings, they can also occur with copper bearing alloys.

Possible Confusion with other Types of Damage

Comment The best solution is to limit the chromium content of the steel to 1.5% where surface speeds are greater than 25 m/sec.

Unsuitable Journal or Thrust Collar Materials Summary

There are two quite separate types of bearing damage that arise from using unsuitable materials for the journal or thrust collar.

Damage with Nitrided Steel Shafts Characteristics

The bearing surface of a nitrided steel shaft has to be cleaned and polished before going into service otherwise there is a risk of scoring of the bearing, with a characteristic vee-shape pointing against the direction of rotation

Cause

Failure to clean the surface after nitriding. This is in fact a manufacturing error (see Section 10), though for descriptive purposes it is most conveniently treated here.

Wire-Wool Failure Characteristics

This is the only form of bearing failure that results in extensive damage to both bearing surfaces. It is readily identified by the presence of a mass of ‘wire wool’ in the bearing housing.

Cause

Embedding of a hard particle of dirt or swarf in the bearing material that remains proud and rubs against the steel surface creating high temperature through friction. Wire-wool failure is limited to alloy steels containing more than 3% chromium operating with surface speeds greater than 25 m/sec.

Actions

The best action is to avoid using steels containing more than 1.5% chromium in bearings with surface speeds greater than 25 m/sec. The optimum solution with existing machines is either to

sleeve the journal and replace the thrust collar with a plain low carbon steel; though hard chrome plating of the bearing surfaces will also remove the risk. Careful control has to be exercised over the chromium plating to ensure that the chromium is properly bonded to the steel surface.

10. Manufacturing Faults and Assembly Errors

Bearing failures can occur as a result of manufacturing or assembly errors. A few examples are given, but obviously there is no general pattern as the failure mechanism depends on the type of error.

In the event of unusual effects that do not match with any of those illustrated in the previous Sections, a careful examination should be made of the whole bearing assembly for any

manufacturing defects or assembly error that could account for the damage. This should include the back of the bearing or bearing shell for peculiar markings and a check of the journal for taper, barrelling or corrosion damage..

There are two obvious categories:

manufacturing faults assembly errors

Manufacturing Faults

1. Barrelled Shaft

Points to Note

Main Characteristics

Cause

Note Possible Confusion with Other Types of Damage

Comment

Manufacturing Faults

2. Undersize Bearing Shells or Oversize Housing Bore

Points to Note

Main Characteristics

Cause

Note Possible Confusion with Other Types of Damage

Comment

Manufacturing Errors

3. Incorrect drilling of oil feed hole

Fatigue damage to journal bearing (a) in line with incorrectly positioned feed hole partially drilled in back of shell (b)

Points to Note Main

Characteristics

A small patch of fatigue damage at centre line of bottom half bearing, Photograph

‘a’. Fatigue would be expected to occur at the position of maximum, not at the bottom of the bearing

Cause Examination of the back of the bearing shell shows a partially drilled hole in line with the fatigue damage, Photograph ‘b’. This was obviously a machining error that was discovered before the oil feed hole was completely drilled through. This has resulted in inadequate support of the white metal that has failed in fatigue under the low loading at the centre of the bottom half of the bearing.

Note This was a main bearing from a reciprocating compressor, with sufficient alternating loading to cause the fatigue.

Possible Confusion with Other Types of Damage

While this is clearly a fatigue failure, the location of the fatigue patch suggests some abnormality.

Comment The back of the bearing shell should always be examined, particularly in cases of abnormal damage.

Manufacturing Errors

4. Incorrectly Inserted Thermocouple Causing Distortion of Bearing Surface

Points to Note Main

Characteristics

This bearing was removed because of a very high temperature when it was put into service.

Cause The surface of the pad was distorted by incorrect assembly of the thermocouple.

This led to reduction in the oil film thickness in the distorted area with abnormally high temperature resulting from the thin oil film.

Note It is normal to fit more than one pad with a thermocouple or RTD. This should help to indicate if one of the pads is behaving abnormally.

Possible Confusion with Other Types of Damage

Comment

Assembly Errors 1. Misalignment

Pair of bearing shells showing wiping at opposite sides of bottom and top shell

Points to Note Main

Characteristics

The wiping at opposite sides of the top and bottom halves of the bearing shows misalignment causing breakdown of the oil film by edge loading.

Cause Misalignment

Note A similar result could occur by running with a tapered shaft, but in this case the damage would be on the same side of each bearing half.

Possible Confusion with Other Types of Damage

Comment The low melting point and strength of white metal (compatibility) are valuable properties, allowing it to wipe or deform to compensate for a limited amount of misalignment.

Assembly Errors

2. Dirt Between Thin Shell Bearings and Housing

A thin-walled bearing shell showing local overheating in its bore caused by dirt trapped between the shell and the housing

Points to Note

Main Characteristics

Cause

Note Possible Confusion with Other Types of Damage

Comment