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Detecting bearing failure
There are two basic procedures of testing for bearing problems:
COMPARATIVE AND HISTORICAL. The comparative method involves testing two or more similar bear-
ings and „comparing“ potential differences. Historical testing requires monitoring a specific bearing over
a period of time to establish its history. By analyzing bearing history, wear patterns at particular ultrason-
ic frequencies become obvious, which allows for early detection and correction of bearing problems.
For comparative test:
1. Use contact (stethoscope) module.
2. Select desired frequency. (If only one frequency is to be monitored, consider using 30 kHz.
3. Select a „test spot“ on the bearing housing and mark it for future. Touch that spot with the con-
tact module. In ultrasonic sensing, the more mediums or materials ultrasound has to travel
through, the less accurate the reading will be. Therefore, be sure the contact probe is actually
touching the bearing housing. If this is difficult, touch a grease fitting or touch as close to the
bearing as possible.
4. Approach the bearings at the same angle, touching the same area on the bearing housing.
5. Reduce sensitivity to hear the sound quality more clearly.
6. Listen to bearing sound through headphones to hear the „quality“ of the signal for proper inter-
pretation.
7. Select same type bearings under similar load conditions and same rotational speed.
8. Compare differences of meter reading and sound quality.
Procedure for bearing history (historical): Before starting with the HISTORICAL method for
monitoring bearings, the COMPARATIVE method must be used to determine a baseline.
1. Use basic procedure as outlined above in steps 1-8.
2. Save the reading for future reference.
3. Compare this reading with previous (or future readings). On all future readings, adjust
frequency to the original level. If the decibel level has moved up 12 dB over the base line, it in-
dicates the bearing has entered the incipient failure mode. Lack of lubrication is usually
indicated by an 8 dB increase over baseline. It is usually heard as a loud rushing sound. If lack
of lubrication is suspected, after lubricating, retest. If readings do not go back to original levels
and remain high, consider bearing is on the way to the failure mode and recheck
frequently.
Lack of Lubrication:
To avoid lack of lubrication, note the following:
1. As the lubricant film reduces, the sound level will increase. A rise of about 8 dB over baseline
accompanied by a uniform rushing sound will indicate lack of lubrication.
2. When lubricating, add just enough to return the reading to base line.
3. Use caution. Some lubricants will need time to uniformly cover the bearing surfaces. Lubricate
a small amount at a time. DO NOT OVER LUBRICATE
Over Lubrication:
One of the most common causes of bearing failure is over-lubrication. The excess pressure of the lubri-
cant often breaks, or “pops” bearing seals or causes a build up of heat, which can create stress and de-
formity.
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