Durable and rugged under severe operating conditions – Two application examples for DLC-coated bearing cages

Steyr, Austria, November 2015. 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

NKE offers both standard and special bearings for all industrial applications. NKE’s core functions – engineering, product development, final processing of components, assembly, quality assurance, logistics, and sales and marketing – are centralised at its Steyr headquarters. In addition to product development and application engineering NKE provides a full range of technical services and training. NKE’s products are distributed through 15 international representative offices and more than 240 distribution outlets in over 60 countries.

Legend of figures:

Fig. 1: Inner ring guided, solid-steel window type cage with DLC coating

Fig. 2: Deep groove ball bearing with outer ring guided solid-steel cage with DLC coating

Company information:

NKE Austria GmbH is a bearing manufacturer with headquarters in Steyr, Austria. The company with about 140 employees was founded in 1996 by a group of senior staff members of former company Steyr Wälzlager. NKE offers both standard and special bearings for all industrial applications. Engineering, product development, production and final processing of components, assembly, quality assurance, logistics, and sales and marketing are centralised at its Steyr headquarters. The factory in Steyr is certified to ISO 9001:2008, ISO 14001:2004 and OHSAS 18001. Its wide range of standard bearings is available from stock or at short production lead-times. It also provides tailored products and solutions. In addition to product development and application engineering NKE provides a full range of technical services, consulting, documentation and training. NKE’s products are distributed through 15 international representative offices and more than 240 distribution outlets in over 60 countries.