Using Vibration Analysis to Detect Early Failure of Bearings

Vibration produced by rolling bearings can be complex and can result from geometrical imperfections during the manufacturing process, defects on the rolling surfaces or geometrical errors in associated components. Noise and vibration is becoming more critical in all types of equipment since it is often perceived to be synonymous with quality and often used for predictive maintenance.

Rolling contact bearings are used in almost every type of rotating machinery, whose reliable operation very much depends on the type of bearing selected and the precision of associated components such as shafts, housings, spacers and nuts.

Bearing engineers generally use fatigue as the normal failure mode, on the assumption that the bearings are properly installed, operated and maintained. Today, because of improvements in manufacturing technology and materials, bearing fatigue life, which is related to sub-surface stresses, is not the limiting factor and accounts for less than 3% of failures in service.

Unfortunately, many bearings fail prematurely in service because of contamination, poor lubrication, misalignment, temperature extremes, poor fitting, unbalance and misalignment. All these factors lead to an increase in bearing vibration and so condition monitoring has been used for many years to detect degrading bearings before they catastrophically fail, resulting in associated downtime costs or significant damage to other parts of the machine.

Rolling element bearings are often used in noise-sensitive applications, such as household appliances and electric motors, which often use small to medium size bearings. Bearing vibration is therefore becoming increasingly important from both an environmental consideration and because it is synonymous with quality.

It is now generally accepted that quiet running is synonymous with the form and finish of the rolling contact surfaces. As a result, bearing manufacturers have developed vibration tests as an effective method for measuring quality. A common approach is to mount the bearing on a quiet running spindle and measure the radial velocity at a point on the bearing’s outer ring in three frequency bands, 50-300, 300-1,800 and 1,800-10,000Hz. The bearing must meet RMS velocity limits in all three frequency bands.

In the process industries, vibration monitoring is now a well-accepted part of many planned maintenance regimes and relies on the well-known characteristic vibration signatures which rolling bearings exhibit as the rolling surfaces degrade. However, in most situations, bearing vibration cannot be measured directly and so the bearing vibration signature is modified by the machine structure. This situation is further complicated by vibration from other equipment on the machine such as electric motors, gears, belts, hydraulics, structural resonance, and so on. This often makes the interpretation of vibration data difficult other than by a trained specialist and can in some situations lead to a mis-diagnosis, resulting in unnecessary machine downtime and costs.

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