Bearing Failures & Their Causes

Bearings are among the most important components in the vast majority of machines and exacting demands are made upon their carrying capacity and reliability. Therefore it is quite natural that rolling bearings should have come to play such a prominent part and that over the years they have been the subject of extensive research. Indeed rolling bearing technology has developed into a particular branch of science.
Among the benefits resulting from this research has been the ability to calculate the life of a bearing with considerable accuracy, thus making it possible to match the bearing life with the service life of the machine involved. Unfortunately it sometimes happens that a bearing does not attain its calculated rating life. There may be many reasons for this - heavier loading than has been anticipated, inadequate or unsuitable lubrication, careless handling, ineffective sealing, or fits that are too tight, with resultant insufficient internal bearing clearance.
Each of these factors produces its own particular type of damage and leaves its own special imprint on the bearing. Consequently, by examining a damaged bearing, it is possible, in the majority of cases, to form an opinion on the cause of the damage and to take the requisite action to prevent a recurrence.

How is bearing life defined?

Generally, a rolling bearing cannot rotate for ever. Unless operating conditions are ideal and the fatigue load limit is not reached, sooner or later material fatigue will occur. The period until the first sign of fatigue appears is a function of the number of revolutions performed by the bearing and the magnitude of the load.
Fatigue is the result of shear stresses cyclically appearing immediately below the load carrying surface. After a time these stresses cause cracks which gradually extend up to the surface. As the rolling elements pass over the cracks fragments of material break away and this is known as flaking or spalling. The flaking progressively increases in extent (figs 1 to 4) and eventually makes the bearing unserviceable.
The life of a rolling bearing is defined as the number of revolutions the bearing can perform before incipient flaking occurs. This does not mean to say that the bearing cannot be used after then. Flaking is a relatively long, drawn-out process and makes its presence known by increasing noise and vibration levels in the bearing. Therefore, as a rule, there is plenty of time to prepare for a change of bearing.

Figures 1 to 4 - Progressive stages of flaking.

Path patterns and their interpretation.

When a rolling bearing rotates under load the contacting surfaces of the rolling elements and the raceways normally become somewhat dull in appearance. This is no indication of wear in the usual sense of the word and is of no significance to the bearing life. The dull surface in an inner or outer ring raceway forms a pattern called, for the purposes of this paper, the path pattern. This pattern varies in appearance according to the rotational and loading conditions. By examining the path patterns in a dismantled bearing that has been in service, it is possible to gain a good idea of the conditions under which the bearing has operated.

By learning to distinguish between normal and abnormal path patterns there is every prospect of being able to assess correctly whether the bearing has run under the proper conditions. The following series of figures illustrates normal path patterns under different rotational and loading conditions (figs 5 to 11) as well as typical patterns resulting from abnormal working conditions (figs 12 to 18).

In the majority of cases the damage to the bearing originates within the confines of the path patterns and, once their significance has been learned, the appearance and location of the patterns prove to be useful aids in diagnosing the cause of the damage. Deep groove ball bearings and thrust ball bearings have been used for illustrative purposes as they display such characteristic path patterns. However, the figures are applicable, with some modifications, to other types of bearing as well.

Figure 5 - Uni-directional radial load. Rotating inner ring - fixed outer ring. Inner ring: path pattern uniform in width, positioned in the centre and extended around the entire circumference of the raceway.
Outer ring: path pattern widest in the load direction and tapered off towards the ends. With normal fits and normal internal clearance, the pattern extends around slightly less than half the circumference of the raceway.

Figure 6 - Uni-directional radial load. Fixed inner ring - rotating outer ring. Inner ring: path pattern widest in the load direction and tapered off towards the ends. With normal fits and normal internal clearance, the pattern extends around slightly less than half the circumference of the raceway. Outer ring: path pattern uniform in width,positioned in the centre and extended around the entire circumference of the raceway.

Figure 7 - Radial load rotating in phase with the inner ring. Rotating inner ring - fixed outer ring. Inner ring: path pattern widest in the load direction and tapered off towards the ends. With normal fits and normal internal clearance, the pattern extends around slightly less than half the circumference of the raceway Outer ring: path pattern uniform in width, positioned in the centre and extended around the entire circumference of the raceway.

Figure 8 - Radial load rotating in phase with the outer ring. Fixed inner ring - rotating outer ring. Inner ring: path pattern uniform in width, positioned in the centre and extended around the entire circumference of the raceway Outer ring: path pattern widest in the load direction and tapered off towards the ends. With normal fits and normal internal clearance, the pattern extends around slightly less than half the circumference of the raceway.

Figure 9 - Uni-directional axial load. Rotating inner or outer ring. Inner and outer rings: path pattern uniform in width, extended around the entire circumference of the raceways of both rings and laterally displaced.

Figure 10 - Combination of uni-directional radial and axial loads. Rotating inner ring - fixed outer ring. Inner ring: path pattern uniform in width, extended around the entire circumference of the raceway and laterally displaced. Outer ring: path pattern extended around the entire circumference of the raceway and laterally displaced. The pattern is widest in the direction of the radial loading.

Figure 11 - Uni-directiorial axial load. Rotating shaft washer - fixed housing washer. Shaft and housing washers: path pattern uniform in width, extended around the entire circumference of the raceways of both washers.

Figure 12 - Uni-directional radial load + imbalance. Rotating inner ring - creeping outer ring. Inner and outer rings: path pattern uniform in width, extended around the entire circumference of the raceways of both rings.

Figure 13 - Fits too tight - preloading. Uni-directional radial load. Rotating inner ring - fixed outer ring. Inner ring: path pattern uniform in width, positioned in the centre and extended around the entire circumference of the raceway. Outer ring: path pattern positioned in the centre and extended around the entire circumference of the raceway. The pattern is widest in the direction of the radial loading.

Figure 14 - Oval compression of outer ring. Rotating inner ring -fixed outer ring. Inner ring: path pattern uniform in width, positioned in the centre and extended around the entire circumference of the raceway. Outer ring: path pattern positioned in two diametrically opposed sections of the raceway. The pattern is widest where the pinching has occurred.

Figure 15 - Outer ring misaligned. Rotating inner ring - fixed outer ring. Inner ring: path pattern uniform in width, positioned in the centre and extended around the entire circumference of the raceway. Outer ring: path pattern in two diametrically opposed sections displaced diagonally in relation to each other.

Figure 16 - Inner ring misaligned. Rotating inner ring - fixed outer ring. Inner ring: path pattern in two diametrically opposed sections, displaced diagonally in relation to each other Outer ring: path pattern widest in the load direction and tapered off toward the ends. The internal clearance is reduced on account of the misalignment of the inner ring; the length of the path pattern depends upon the magnitude of the internal clearance Reduction.

Figure 17 - Housing washer positioned eccentrically relative to shaft washer. Rotating shaft washer-fixed housing washer. Shaft washer: path pattern uniform in width, extended around the entire circumference of the raceway.
Housing washer: path pattern extended around the entire circumference of the raceway and off-centre relative to raceway.

Figure 18 - Housing washer misaligned. Rotating shaft washer - fixed housing washer. Shaft washer: path pattern uniform in width, extended round the entire circumference of the raceway. Housing washer: path pattern in the centre of the raceway but wider around part of its circumference.

Different types of bearing damage

Each of the different causes of bearing failure produces its own characteristic damage. Such damage, known as primary damage, gives rise to secondary, failure-inducing damage - flaking and cracks. Even the primary damage may necessitate scrapping the bearings on account of excessive internal clearance, vibration, noise, and so on. A failed bearing frequently displays a combination of primary and secondary damage.

The types of damage may be classified as follows:

Wear
Indentations
Smearing
Surface distress
Corrosion
Electric current damage

Secondary damage

Flaking (spalling)
Cracks
Cage damage

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