Detecting Bearing Failure, For Comparative Test

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DETECTING BEARING FAILURE

COMPARATIVE TESTING. The comparative method involves testing two or more similar bearings and "com- paring" potential differences.

FOR COMPARATIVE TEST

1.Use contact (stethoscope) module.

2.Select a "test spot" on the bearing housing. 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 (if unsure of this procedure, refer to SENSITIVITY SELECTION DIAL)

5.Listen to bearing sound through headphones to hear the "quality" of the signal for proper interpretation.

6.Select same type bearings under similar load conditions and same rotational speed.

7.Compare differences of meter reading and sound quality.

It is important to consider two elements of potential failure. One is lack of lubrication while the other is over lubrication.

Normal bearing loads causes an elastic deformation of the elements in the contact area which give a smooth elliptical stress distribution. But bearing surfaces are not perfectly smooth. For this reason, the actual stress 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 the stress distribution is no longer present, the normal rough spots 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, aside from normal wear, the fatigue or service life of a bearing is strongly influenced by the relative film thickness provided by an appropriate lubricant.

SLOW SPEED BEARINGS

Monitoring slow speed bearings is possible with the Ultraprobe 100. Due to the sensitivity range, it is quite possible to listen to the acoustic quality of bearings. In extremely slow bearings (less than 25 RPM), it is often necessary to disregard the meter and listen to the sound of the bearing. In these extreme situations, the bearings are usually large (1"-2" and up) and greased with high viscosity lubricant. Most often no sound will be heard as the grease will absorb most of the acoustic energy. If a sound is heard, usually a crackling sound, there is some indica- tion of deformity occurring.

4. GENERAL MECHANICAL TROUBLE SHOOTING

As operating equipment begins to fail due to component wear, breakage or misalignment , sonic and more importantly, ultrasonic shifts occur. The accompanying sound pattern changes can save time and guess work in diagnosing problems if they are adequately monitored. Therefore, an ultrasonic history of key components can prevent unplanned down-time. And just as important, if equipment should begin to fail in the field, the ULTRAPROBE can be extremely useful in trouble shooting problems.

TROUBLE SHOOTING:

1.Use the contact (stethoscope) module.

2.Touch test area(s): listen through headphones and observe the meter.

3.Adjust sensitivity until mechanical operation of the equipment is heard clearly.

4.Probe equipment by touching various suspect areas.

5.To focus in on problem sounds, while probing, reduce sensitivity gradually to assist in locating the prob-

lem sound at its' loudest point. (This procedure is similar to the method outlined in LEAK LOCATION, i.e., follow the sound to its loudest point.)