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SERVICES
Failure investigation
PLAIN BEARING FAILURES
Index of plain bearing failure and damage mechanisms:
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01.0 - lubrication breakdown
02.0 - wear
03.0 - fatigue
04.0 - cavitation erosion
05.0 - fretting
06.0 - chemical effects
07.0 - thermal ratcheting
08.0 - electrical damage
09.0 - incompatible materials
10.0 - manufacturing faults
PLAIN BEARING DAMAGE
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The identification of the cause of damage or failure in plain bearings is crucial before remedial action can be taken to prevent further recurrence.
The first step is a visual examination of the damaged component. The following photographs can then be used to find a match. "A picture is worth a thousand words." However, do not stop at this stage. More than one mechanism of failure can lead to superficially similar bearing damage. Moreover, once failure has been initiated, the original mechanism may lead to other failure mechanisms and it is essential to identify the original cause.
We are primarily concerned with bearings from industrial machines. The vast majority of these are designed to operate with a hydrodynamic lubricating film and are based on white metals (Babbitts) - alloys of tin and lead. Most of the photographs are of damaged white metal bearings, but some examples of other materials are included.
| White metals are chosen because they are of low hardness and melting point, so that, in the event of failure, damage should be limited to the bearing, a comparatively cheap and readily replaced component, leaving the expensive rotor unharmed. These desirable characteristics, however, place limits on their operation (see Figure alongside). The effect of temperature on the hardness of white metals, and hence their ability to carry load, is critical: | Suggested Temperature Limits for Whitemetal Bearings |
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OPERATING LIMITS
| The Figure alongside gives a working envelope for steadily-loaded journal bearings in terms of the main operating parameters: load, speed, lubricant viscosity and bearing geometry. | Load Capacity Diagram for Steadily-Loaded Plain Journal Bearings |
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| EDSU 66023 (Reproduced with permission) |
| This Figure gives a working envelope for steadily-loaded thrust bearings in terms of load capacity and operating speed. | Limits of Safe Operation |
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Note. Both these figures are for steadily-loaded operation. Under alternating load conditions, such as occur in the bearings in reciprocating machines or in the bearings of rotating machines subject to vibration (rotor out-of-balance, rotor instability), a separate limitation is imposed by the fatigue strength of the bearing material. In the case of reciprocating machines, higher strength bearing materials can be used, but with the risk of damage to the bearing journal.