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Monitoring bearing wear
In a bearing, normal bearing loads cause an elastic deformation ofthe elements in the contact
area, which give a smooth elliptical stress distribution. But bearingsurfaces are not perfectly
smooth.
For this reason, the actualstress distribution in the contact area will be affected by a random
surface roughness. In the presence of a lubricant film on a bearing surface, there is a dampening
effect on the stress distribution and the acoustic energy produced will be low.
Should lubrication be reduced to a point where thestress distribution is no longer present, the
normal roughspots will make contact with the race surfaces and increase the acoustic energy.
These normal microscopic disuniformities will begin to produce wear and the possibilities of small
fissures may develop which contributes to the „Pre-Failure“ condition. Therefore, a side from
normal wear, the fatigue orservice life of abearing is strongly influenced by the relative film
thickness provided by anappropriate lubricant.
Ultrasonic inspection and monitoring of bearings is by far the most reliable method for detecting
early stages of bearing failure and conditions such as lubrication starvation. The ultrasonic
warning appears priorto a rise in temperature or an increase in lowfrequency vibration levels.
Ultrasonic inspection of bearings is useful in recognizing the beginning stages of fatigue failure
relatedto flooding of orlack of lubricant.
In ball bearings, as the metal in the raceway, roller or ball bearing begins to fatigue, a subtle
deformation begins to occur. This deforming of the metal will produce irregular surfaces, which
will cause an increase in the emission ofultrasonic soundwaves.
As a ball passes over a pit or fault in the race surface, it produces an impact. A structural
resonance of one of the bearing components vibrates or „rings“ by this repetitive impact. The
sound produced is observed as an increase in amplitude in the monitored ultrasonic frequencies
of the bearing.
When listening to a bearing, it is recommendedthat a user become familiar with the sounds ofa
good bearing. A good bearing is heard as a rushing orhissing noise. Crackling or roughsounds
indicate a bearing in the failure stage. Louder rushingsounds similar to that of a good bearing
only slightly rougher,can indicate lack of lubrication.
A change in amplitude from the original base line reading can be an indication of:
a. lack oflubrication or
b. incipient bearing failure.
When a reading exceeds any previous reading by 8-10 dB with no change in the sound quality (a
“rushing” noise) this is an indication of lubrication starvation, an increase of greaterthan 12 dB
can beconsidered to be an indication ofthe beginning of the failure mode.
