Bearing Cage Damage and Its
Causes.
Cage damage
If, on examination of a
failed bearing, the cage is found to be damaged, it may in many cases prove difficult to
ascertain the cause. Usually other components of the bearing are damaged too and this
makes it even more difficult to discover the reason for the trouble. However, there are
certain main causes of cage failure, viz. vibration, excessive speed, wear and blockage.
Vibration
When a bearing is exposed to vibration, the forces of inertia may be so great as to cause
fatigue cracks to form in the cage material after a time Sooner or later these cracks lead
to cage fracture.
Excessive speed
If the bearing is run at speeds in excess of that for which the cage is designed, the cage
is subjected to heavy forces of inertia that may lead to fractures. Frequently, where very
high speeds are involved, it is possible to select bearings with cages of special design.
Wear
Cage wear may be caused by inadequate lubrication or by abrasive particles. The idea with
rolling bearings is of course to avoid sliding friction. However, where the cage is
concerned, sliding cannot be eliminated in the contacts with the other components of the
bearing. This explains why the cage is the first component to be affected when the
lubrication becomes inadequate. The cage is always made of softer material than the other
components of the bearing and consequently it wears comparatively quickly. As the cage
pockets increase in size, due to wear, the rolling element guidance deteriorates and this
also applies to the cage in cases where the cage is centred on the rolling elements. The
resultant forces may lead to cage failure within a short space of time.
Blockage
Fragments of flaked material or other hard particles may become wedged between the cage
and a rolling element, preventing the latter from rotating round its own axis. This leads
to cage failure.
Other causes of cage
damage
If the rings of a deep groove ball bearing are fitted out of alignment with each other,
the path of the balls has an oval configuration. If the cage is centred on the balls, it
has to change shape for every revolution it performs. Fatigue cracks then form in the
material and sooner or later they lead to fractures. There is a similar case when a thrust
ball bearing is fitted together with radial plain bearings. If clearance arises in the
plain bearings, the washers of the thrust bearing become displaced in relation to each
other. Then the balls do not follow their normal path and heavy stresses may arise in the
cage. Cages in bearings subject to severe acceleration and retardation, in conjunction
with fluctuations in speed, are affected by forces of inertia. These give rise to
considerable pressure between the contacting surfaces, with consequent heavy wear.
Figure 1
Left hand image :
Fractured surface of the cage shown in the right hand image. The fatigue cracks are
clearly visible.
Right hand image : Cage of a spherical roller bearing. Fatigue cracks have formed
in the fillets.
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MIET FIDiagE
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