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Durable and rugged under severe operating conditions

. Denise Thalmann

In December 2013 Austrian bearing manufacturer NKE Austria GmbH introduced cages for rolling bearings that are optimized for unfavourable operating conditions.

Coated with DLC (diamond-like carbon), the new steel cages are both more resilient and durable than comparable brass cage designs. Two application examples have been chosen to highlight the advantages of the new cages and demonstrate the transition from prototype to series-produced niche product.

The first application case provided the impulse for the new cage’s development: a special- cylindrical roller bearing for the large connecting-rod eye in a high-speed crank mechanism. In the original design the bearing was fitted with an inner ring-guided solid brass cage (suffix MPB) – a tried-and-tested solution that had provided reliable service for many years. To increase the plant’s productivity, the customer planned to increase the machine’s operating speed  from 2000 r.p.m. to a top speed of 3000 r.p.m. Tests showed that, under these operating conditions, the standard MPB cage had a life expectancy of only 300 hours before the cage guidance surfaces showed considerable wear  and the cage cross pieces started to disintegrate . This was attributed to the sharp rise in centrifugal forces and resulting loads on the cage. In addition the bearings were grease-lubricated and  the higher speeds had a negative impact on the distribution of the lubricant – especially in the cage guide gap –, which further promoted wear.

The DLC-coated inner ring-guided solid steel window cage (Figure 1) with suffixes FPB and SQ202C developed to solve this issue has, to date, exhibited a highly satisfactory service life of more than 3000 hours. In two years’ of long-term testing, the cages had no mechanical failures and – with the exception of a slight transfer of the DLC coating between the guide surfaces – showed no signs of  undue wear. For comparison, identical but uncoated steel cages were tested at the same time as the new, coated cages. This test was, terminated after 1000 hours due to the significant wear and smearing on  the contact surfaces.

The second application in which the new DLC-coated steel cages were trialled, and in which the experience gained in the first application were applied, was a high-speed drum used to store kinetic energy in an emergency power supply unit. The original bearings used were radial ball bearings with outer ring guided two-piece solid brass cages (suffix MA). Here, too, unfavourable operating conditions – a combination of low radial load with a high axial load and a high outer race speed of more than 2500 r.p.m. – resulted in excessive wear of the cages’ functional surfaces and subsequent cage fracture.

After consideration of the operating conditions and analysis of the damage pattern, the use of  a  two piece, outer ring guided solid-steel cage with DLC coating (suffix FA and SQ202C) was recommended. In the subsequent practical testing of two bearings fitted with these cages, which operated for  more than 900 hours, no wear or signs of mechanical failure of the cages were observed. The design received the go-ahead for use in series production (Figure 2) and is additionally scheduled for retrofitting in existing machines.