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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 bearings
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 ultrasonic frequencies
become obvious which allows for early detection and correction of bearing problems.
For Comparative Test
1. Use contact (stethoscope) module.
2. Select a "test spot" on the bearing housing and mark it for future reference by marking it with a center
punch or with dye or by epoxy bonding a washer to the spot. Touch that spot with the contact 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.
3. Approach the bearings at the same angle, touching the same area on the bearing housing.
4. Reduce sensitivity) until the meter reads 20 (if unsure of this procedure, refer to SENSITIVITY
SELECTION DIAL (See page 6
5. Listen to bearing sound through headphones to hear the "quality" of the signal for proper interpretation.
(Refer to page 17 for discussion of audio interpretation.)
6. Select same type bearings under similar load
conditions and same rotational speed.
7. Compare differences of meter reading and sound quality.
Procedure for Bearing History (Historical)
There are two methods to historically "trend" a bearing. The first is a very common, field proven method
called the "SIMPLE" method. The other provides greater flexibility in terms of decibel selection and
trending
analysis. It is referred to as the "ATTENUATOR TRANSFER CURVE method. Before starting with either of
the two HISTORICAL methods for monitoring bearings, the COMPARATIVE method must be used to
determine a baseline.
Simple Method
1. Use basic procedure as outlined above in steps 1-7.
2. Note frequency, meter reading, and sensitivity selection on your Reference Chart 1 (page 25).
3. Compare this reading with previous (or future readings). On all future readings, adjust level to the
original level recorded in the Reference Chart.
a. If the meter reading has moved from the original 20 mark up to or past 100, there has been a 12
dB increase (increments of 20 on the meter in the linear mode is about 3 decibels. e.g.: 20-40=3
dB, 40-60=3db, etc.). NOTE: Increase of 12 dB or greater indicates the bearing has entered the
incipient failure mode.
b. 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, re-test. 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.
