Vibration an unexpected predator

Did you know that bearings can be damaged when at rest, and there will be no visible change apparent in the rolling elements?

Bearings with vibration damage are usually found in machines that are not in operation and are situated close to machinery that is producing vibrations. Examples are transformer fans, standby generators and ships’ auxiliary machinery. Bearings in machines transported by rail, road or sea may be subject to vibration damage too.

When a bearing is not running, there is no lubricant film between the rolling elements and the raceways. The absence of lubricant film allows metal-to-metal contact and the vibrations produce small relative movements of the rolling elements and rings. As a result of these movements, small particles break away from the surfaces and this leads to the formation of depressions in the raceways. This damage is known as false brinelling, sometimes also referred to as washboarding. Balls produce sphered cavities while rollers produce fluting.

In many cases, it is possible to discern red rust at the bottom of the depressions. This is caused by oxidation of the detached particles, which have a large area in relation to their volume, as a result of their exposure to air. There is never any visible damage to the rolling elements.

The greater the energy of vibration, the more severe the damage. The period of time and the magnitude of the bearing internal clearance also influence developments, but the frequency of the vibrations does not appear to have any significant effect.

Roller bearings have proved to be more susceptible to this type of damage than ball bearings. This is considered to be because the balls can roll in every direction. Rollers, on the other hand, only roll in one direction; movement in the remaining directions takes the form of sliding. Cylindrical roller bearings are the most susceptible.

The fluting resulting from vibrations sometimes closely resembles the fluting produced by the passage of electric current. However, in the latter case, the bottom of the depression is dark in colour, not bright or corroded. The damage caused by electric current is also distinguishable by the fact that the rolling elements are marked, as are the raceways.

Where machines subject to constant vibration are concerned, it is essential that the risk of damage to the bearings be taken into consideration at the design stage. Consequently, where possible, ball bearings should be selected instead of roller bearings.

The ability of ball bearings to withstand vibrations without being damaged can also be considerably improved by applying axial preloading with the aid of springs (see Fig. 4). An oil bath, in which all rolling elements in the load zone are immersed in the oil, has also proved to provide satisfactory protection. A vibration-damping base helps to prevent damage too.

The bearings in machines that are to be transported can be protected by locking the shaft, thus preventing the small movements that have such a damaging effect on the bearings. MRO

This article is an edited excerpt from ‘Bearing Failures and Their Causes’, an SKF guide. For more information, visit SKF Canada at www.skf.com/portal/skf_ca/home or use the online request number below at www.mromagazine.com/rsc.

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