Bearing WEAR and its Causes.

Wear

In normal cases there is no appreciable wear in rolling bearings. Wear may, however, occur as a result of the ingress of foreign particles into the bearing or when, the lubrication is unsatisfactory. Vibration in bearings which are not running also gives rise to wear.

Wear caused by abrasive particles.

Small, abrasive particles, such as grit or swarf that have entered the bearing by some means or other, cause wear of raceways, rolling elements and cage. The surfaces become dull to a degree that varies according to the coarseness and nature of the abrasive particles. Sometimes worn particles from brass cages become verdigrised and then give light-coloured grease a greenish hue.

The quantity of abrasive particles gradually increases as material is worn away from the running surfaces and cage. Therefore the wear becomes an accelerating process and in the end the surfaces become worn to such an extent as to render the bearing unserviceable. However, it is not necessary to scrap bearings that are only slightly worn. They can be used again after cleaning.

The abrasive particles may have entered the bearing because the sealing arrangement was not sufficiently effective for the operating conditions involved. They may also have entered with contaminated lubricant or during the mounting operation.

Figure 1 - The outer race of a spherical roller bearing with raceways that have been worn by abrasive particles. It is easy to feel where the dividing line goes between worn and unworn sections.

Wear caused by inadequate lubrication

If there is not sufticient lubricant, or if the lubricant has lost its lubricating properties, it is not possible for an oil film with sufficent carrying capacity to form. Metal to metal contact occurs between rolling elements and raceways. In its initial phase, the resultant wear has roughly the same effect as lapping. The peaks of the microscopic asperities, that remain after the production processes, are torn off and, at the same time, a certain rolling-out effect is obtained. This gives the surfaces concerned a varying degree of mirror-like finish.
At this stage surface distress can also arise. If the lubricant is completely used up, the temperature will rise rapidly. The hardened material then softens and the surfaces take on blue to brown hues. The temperature may even become so high as to cause the bearing to seize.

Figure 2 - The outer ring of a spherical roller bearing that has not been adequately lubricated. The raceways have a mirror finish

Wear caused by vibration.

When a bearing is not running, there is no lubricant film between the rolling elements and the raceways. The absence of lubricant film gives metal to metal contact and the vibrations produce small relative movements of 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 well as the raceways.
Bearings with vibration damage are usually found in machines that are not in operation and are situated close to machinery producing vibrations. Examples that can be cited are transformer fans, stand-by generators and ships' auxiliary machinery. Bearings in machines transported by rail, road or sea may be subject to vibration damage too.

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 25. 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.

Figure 3 - The inner and outer ring of a cylindrical roller bearing that has exposed to vibration. The inner ring has changed position.

Figure 4 - The outer ring of a self aligning ball bearing damaged by vibration. The bearing has not rotated at all.

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