Now that the six bearing failure modes and eleven pre-operational and operational causes have been defined and identified respectively, we can proceed and help you identify the cause of your specific bearing problems. The pattern or load zone produced by the applied load and the rolling elements on the internal surfaces of the bearing can be an indication of the cause of failure. How- ever, to benefit from a study of load zones, one must be able to differentiate between normal and abnormal loading patterns. Figure 4 and Figure 5 illustrate how an applied radial load of constant direction is distributed among the rolling elements of a
rotating inner ring bearing. The large arrow in the 12 o’clock position represents the applied load and the series of small arrows from 4 o’clock to 8 o’clock represent how the load is shared/supported by the rolling ele- ments in the bearing. The rotating ring will have a rotating 360° load zone while the stationary outer ring will show a constant or stationary load zone of approximately 150°. Figure 6 and Figure 7 illustrate how an applied load of constant direction is distrib- uted among the rolling elements of a rotat- ing outer ring bearing. The large arrow in the 12 o’clock position represents the applied load and the series of small arrows from 10 o’clock to 2 o’clock represent how the load is shared/supported by the rolling elements in the bearing. The rotating outer ring will have a rotating 360° load zone
while the stationary inner ring will show a constant or stationary load zone of approxi- mately 150°. These load zone patterns are also expected when the inner ring rotates and the load also rotates in phase with the shaft (i.e. imbalanced or eccentric loads). Even though the inner ring is rotating, its load zone is stationary relative to the inner ring and vice versa for the outer ring. Figure 8 illustrates the effect of thrust load on a deep groove ball bearing load zone pattern. In addition, it also shows the effects of an excessive thrust load condition which forces the ball set to roll up towards the shoulder edge. Excessive thrust load is one condition where the load zones are a full 360° on both rings.
Figure 9 illustrates a combination of thrust and radial load on a deep groove ball
Figure 7 Figure 8
Figure 4 Figure 5 Figure 6
360° 150° d d d d d d d d d d 360° 150°
Load distribution within a bearing Normal load zone inner ring rotating relative to load Outer ring rotating load zone, e.g. boat trailer wheel
Normal load zone outer ring rotating relative to
bearing. This produces a load zone pattern that is somewhere in between the two as shown. When a combined load exists, the load zone of the inner ring is slightly off cen- ter and the length of the load zone of the outer is greater than that produced by just radial load, but not necessarily 360°. For double row bearings, a combined load con- dition will produce load zones of unequal length. The thrust-carrying row will have a longer stationary load zone. If the thrust load is of sufficient magnitude, one row of rolling elements can become completely unloaded. Figure 10 illustrates an internally pre- loaded bearing that is supporting primarily radial load. Both rings are loaded through 360°, but the pattern will usually be wider in the stationary ring where the applied load
is combined with the internal preload. This condition can be the result of excessive interference fits on the shaft and in the housing. If the fits are too tight, the bearing can become internally preloaded by com- pressing the rolling elements between the two rings. Another possible cause for this condition is an excessive temperature differ- ence between the shaft and housing. This too will significantly reduce the bearing internal clearance. Different shaft and hous- ing materials having different thermal expansion coefficients can also contribute to this clearance reduction condition. A discus- sion of fitting practices appears on page 51.
Figure 11 illustrates the load zone found in a bearing that is radially pinched. The housing bore that the bearing was mounted into was initially out-of-round or became out-of- round when the housing was bolted to a non- flat surface. In this case, the outer ring shows two load zones. However, two or more load zones are possible in some cases depending upon the chuck that holds the housing during machining. An example would be a 3-point out-of-round condition. Multiple load zones will dramatically increase the bearing operat- ing temperature as well as the internal loads. Figure 12 illustrates the load zone pro- duced when the outer ring is misaligned relative to the shaft axis. This condition can occur when the shaft deflects or if the bear- ings are in separate housings that do not have concentric housing bores.
Figure 9 Figure 10
Figure 11 Figure 12 Figure 13
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Load zones produced by out-of-round
housing pinching outer ring Load zone when outer ring is misaligned relative to shaft axis (e.g. shaft deflection) Load zones when inner ring is misaligned rela-tive to shaft axis (e.g. bent shaft)
Load zone when thrust loads are excessive Load zone from internally preloaded bearing supporting radial load
Figure 13 illustrates the load zone pro- duced when the inner ring is misaligned relative to the shaft axis. This condition can occur when the shaft is bent and gener- ates what is referred to as a dynamic mis- alignment condition.
Being familiar with the basic load zone patterns and descriptions, the following damage mode causes should be more meaningful. As mentioned earlier, most bearing failures can be classified into two damage modes: pre-operational and opera- tional. Pre-operational damage modes that occur prior to or during bearing installation, are discussed first.