Ultrasonic bearing inspection
May 1st 2008

Mark A. Goodman, VP Engineering, UE Systems Inc discusses ultrasonic bearing inspection methods

Ultrasonic inspection and monitoring of bearings is by far the most reliable method for detecting incipient bearing failure. The ultrasonic warning appears prior to a rise in temperature or an increase in low frequency vibration levels. Ultrasonic inspection of bearings is useful in recognising:

The beginning of fatigue failure. Brinelling of bearing surfaces. Flooding of or lack of lubricant. Beginning of Fatigue Failure.

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 of ultrasonic sound waves.

A change in amplitude from the original reading is an indication of incipient bearing failure. When an ultrasonic reading exceeds any previous reading by 12dB, it can be assumed that the bearing has entered the beginning of the failure mode.

This information was originally discovered through experimentation performed by NASA on ball bearings. In tests performed while monitoring bearings at frequencies ranging from 24 through 50 kHz, they found that the changes in amplitude indicate incipient bearing failure before any other indicators including heat and vibration changes. An ultrasonic system based on detection and analysis of modulations of bearing resonance frequencies can provide subtle detection capability, whereas conventional methods are incapable of detecting very slight faults. 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 the repetitive impact. The sound produced is observed as an increase in amplitude in the monitored ultrasonic frequencies of the bearing.

Brinelling of Bearing Surfaces Brinelling of bearing surfaces will produce a similar increase in amplitude due to the flattening process as the balls get out of round. These flat spots also produce a repetitive ringing that is detected as an increase in amplitude of monitored frequencies.

The ultrasonic frequencies detected by the Ultraprobe are reproduced as audible sounds. This 'heterodyned' signal can greatly assist a user in determining bearing problems. When listening, it is recommended that a user become familiar with the sounds of a good bearing. A good bearing is heard as a rushing or hissing noise. Crackling or rough sounds indicate a bearing in the failure stage. In certain cases a damaged ball can be heard as a clicking sound whereas a high intensity, uniform rough sound may indicate a damaged race or uniform ball damage.

Loud rushing sounds similar to the rushing sound of a good bearing only slightly rougher, can indicate lack of lubrication.

Short duration increases in the sound level with 'rough' or 'scratchy' components indicate a rolling element hitting a 'flat' spot and sliding on the bearing surfaces rather than rotating. If this condition is detected, more frequent examinations should be scheduled. In some instances a loud sound similar to an electric 'hum', referred to as a change in 'tonal quality' will indicate a bearing failure that can be confirmed with the use of a vibration analyser to show the fault frequency.

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.

Some general guidelines: 1. Minimise variables. Try to be as consistent from test to test as possible.

2. Select one test point and identify it for future tests.

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

4. Test at the same angle.

5. If the inspection instrument has frequency tuning, note and use the same frequency.

6. Compare differences of meter reading/dB and sound quality.

7. Establish a baseline by comparing similar bearings, using the lowest dB level for the baseline.

8. Save the baseline reading for future reference.

9. 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 8- 10dB over the baseline accompanied by a uniform 'rushing' noise, this is an indication of lack of lubrication. A 12 to 16dB rise over the base- line accompanied by crackling or popping noises will indicate the bearing has entered the incipient failure mode.

Slow speed bearings Monitoring slow speed bearings is possible with ultrasound technology. Most of the ultrasound instruments will have a wide sensitivity range and some will have frequency tuning. With these features it is quite possible to listen to the acoustic quality of bearings. In extremely slow bearings (less 25 RPM), it is often necessary to disregard the meter display 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 indication of deformity occurring.

On most other slow speed bearings, it is possible to set a baseline and monitor as described above.

Lubrication It is important to consider two elements of potential failure.

One is lack of lubrication while the other is overlubrication.

Normal bearing loads cause an elastic deformation of the elements in the contact area providing a smooth elliptical 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 face surfaces and increase the acoustic energy. These normal microscopic deformities 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.

The right amount of lubrication is very important. If a bearing is over-lubricated, the bearing can be pushed excessively by the lubricant causing additional wear of the bearing. On the other hand, if there is not enough lubricant, the bearing will rub on the solid surface…again causing friction and wear on the bearings.

Either case is detrimental to the life of the bearing. In using airborne / structureborne ultrasound, you can take the guess out of lubrication.

To avoid lack of lubrication note the following:

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.

When lubricating, add just enough to return the reading to baseline or until the reading goes down.

Use Caution! Some lubricants will need time to run to uniformly cover the bearings surface. Lubricate a little at a time.

An alternative method is to add lubricant until the sound level drops off and then add a small amount of grease to assure the bearing has enough grease to fill the cavity. It would be prudent to recheck the bearing within 24 hours to verify that enough grease has been added.

Over-Lubrication When too much lubricant is put into the bearing housing the pressure builds up and can lead to an increase of heat which can create stress and deformity of the bearing or it can break or 'pop' the bearing seal, allowing lubricant to spill out into unwanted areas such as a motor winding, or allow contaminants to enter the raceway, all of which can lead to bearing failure.

To prevent this from occurring: 1. Set a baseline dB level.

2. On subsequent inspections do not lubricate if the dB levels are equal to or less than 8dB over the established baseline level and the sound quality has not changed.

3. If a reading is 8-10dB over the established baseline level, add lubrication until the sound level drops and stop lubricating immediately at this point.

Conclusion Ultrasound instruments are quite versatile and are ideally suited to predictive/preventive maintenance programs. Their enhanced sensitivity makes them ideally suited to note early stages of bearing failure and especially lubrication conditions.

By electronically translating ultrasound emissions down into the audible range, these instruments enable users to hear and recognise when and when not to add lubrication thus preventing over lubrication.

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