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. Excessive impact-induced accelerations (hammering situation).
VibrationWhen 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.
Vibrations may be divided into two types:
- Eccentric motion of the bearing generating vibrations which give heavy accelerations. This type of vibration causes fatigue of the cage bars. A typical example is shaking screens.
- Vibrations from other parts of the machinery or the surroundings that generate peak accelerations. This type of vibration causes hammering of the cage pockets (fig ). By heavy hammering the shakedown limit for the cage material is exceeded and the cage bars are plastically deformed. This process leads to the cage pocket clearance increasing and consequently the hammering effect accelerates until the cage bars fracture (fig 2). Typical examples are impacts on bearings from rail joints and deck vibrations in ships.
Excessive speedIf 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.
WearCage 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.
BlockageFragments 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 damageIf 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.
Damage examplesCage of a spherical roller bearing, fatigue cracks have formed in the fillets (fig 3).
Fractured surface of the cage shown in previous figure, the fatigue cracks are clearly visible (fig 4).
The wear on the cage is caused by inadequate lubrication (fig 5).
Cage has been damaged owing to blockage (fig 6).
A spherical roller bearing cage with bars deformed by roller hammering (fig 7). The rollers have been exposed to very high impact accelerations. The cage pocket clearance has gradually increased and consequently also the hammering effect.
A spherical roller bearing cage pocket with corner crack caused by roller hammering (fig 8). The rollers have been exposed to excessive impact accelerations.
Wear of the outer part of both raceways of a spherical roller bearing inner ring (fig 9). This damage has been caused by the cages being exposed to, for instance:
- Inadequate lubrication.
- Excessive cage accelerations on starting-up.
- Excessive centrifugal accelerations (vibrations).
- Excessive impact-induced accelerations (hammering situation).