One ball saves the race

Mark Fletcher digs into the facts behind a self-healing bearing arrangement

One ball saves the race

A single ceramic ball that repairs steel bearing raceways – first revealed in Eureka’s March 1998 issue – could have far reaching implications for power transmission products.
The concept behind the ‘self healing’ ball bearing was discovered by SKF researchers who noticed that during wear tests on bearings using steel rings and ceramic balls, the use of just one ceramic ball gave almost the same low wear results as that of an all-ceramic system.
The ball can repair minor damage to the raceway and reduce wear in steel bearings by almost 90 per cent. It is this major reduction of wear that should catch the attention of any designer of wear critical systems.
SKF suggests many applications, including gearboxes, conveyors, woodworking machines and automotive applications – all areas where bearing failure and replacement are major factors in the operation of the host unit.
Under test the wear was measured by calculating the precise weight loss of the bearing ring. For a single-ball hybrid this figure was nine times lower than its all-steel equivalent. The wear was also gauged by measuring the axial displacement of the bearing. In this case the single ball example showed a wear rate which was only 17 per cent of its all-steel counterpart.
The wear resistance depends on the hardness of the ceramic – in this case, silicon nitride. The ball actually burnishes the inside of the race and compresses the metal particles back into the surface of the raceway. This means that if the race gets pitted, the ceramic ball will ‘heal’ the damage by smoothing the edges. This, in turn, presents less of a wear risk to the other balls. The company claims that this effect can also double or even treble the fatigue life of the bearing.
In general, silicon nitride has a density 40 per cent lower than steel. This results in lower centrifugal loads, reduced contact stress and reduced skidding. Its modulus of elasticity is 50 per cent higher, which means deformation under load is a lot less than steel, which increases the overall stiffness of the bearing.
The material is very hard, has low rolling resistance on steel and does not suffer plastic deformation. It will not weld to steel, so virtually eliminates scuffing damage – a major cause, along with poor lubricatin, of bearing failure. Because it is inherently non conductive, it will also eliminate electrical damage to bearing raceways.

Design pointers
Wear in steel bearings can be reduced by almost 90 per cent through using a single ceramic ball
The price is twice that of a normal bearing but 20 per cent lower than an equivalent all-ceramic unit, making it cost effective in wear-critical applications

The wear properties, using just one ceramic ball, are comparable to an all-ceramic construction


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