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BEARINGS FEATURE Bearing design rolls with the changes


Thanks to their diamond-like carbon coating, NKE Austria’s cages for rolling bearings ensure durability even in severe operating conditions


A


t the end of 2013, NKE Austria introduced steel cages for rolling


bearings that are coated with DLC: diamond-like carbon. These cages are, the company claims, more resilient and durable than comparable brass cage designs, enabling their use in unfavourable operating conditions. The bearing cages have


now been used in a number of applications which highlight their advantages and demonstrate the transition from prototype to series-produced niche product. The first application


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), which had provided reliable service for many years. However, to increase the plant’s productivity, the customer planned to increase the machine’s operating speed from 2000rpm to a top speed of 3000rpm. 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. The bearings were also grease- lubricated and the higher speeds had a negative impact on the distribution of the lubricant, which further promoted wear. The DLC-coated inner ring-guided solid steel window cage (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. Identical but


Right: Deep groove ball bearing with outer ring guided solid-steel cage with DLC coating


Inner ring guided, solid-steel window type cage with DLC coating


uncoated steel cages were tested at the same time as the new, coated, cages. This test, however, was terminated after 1000 hours due to the significant wear and smearing on the contact surfaces.


MEETING DEMANDS The second application in which the new DLC- coated steel cages were trialled was a high-speed drum used to store kinetic energy in an emergency power supply unit. Radial ball bearings with outer ring guided two-piece solid brass cages (suffix MA) were originally used. 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 2500rpm – resulted in excessive wear of the cage’s


functional surfaces and subsequent cage fracture. Having looked into the operating conditions and analysed the damage pattern, the use of a two piece, outer ring guided solid-steel cage with DLC coating (suffix FA and SQ202C) was


recommended. During practical


testing of the 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 and is now scheduled for retrofitting in existing machines.


NKE www.nke.at Enter 205


SUPPORTING BULK MATERIAL GRINDING APPLICATIONS


From its site in Enns, Austria, Cemtec designs and manufactures systems for bulk material grinding. Its steel drums for tube mills are used to grind a variety of different materials (wet or dry) into fine powder, and these have diameters up to 12m, lengths up to 18m and weigh around 250 tonnes. To ensure reliability over a service life of 20 to 30 years, the company selected Schaeffler’s SNS plummer


block housed bearings, which support the drive shaft between the drive unit and the steel drum. Due to its design, the SNS plummer block housing is said to increase the bearing operating life by up to 50%. According to the company, the SNS plummer block housing for spherical rolling bearings


provides many advantages, including more even load distribution on the bearings. The optimised design of the housing is not only more cost effective to manufacture, but also easier to mount and dismount. Manufactured from high quality, spheroidal graphite cast iron as standard, the housing ensures a high level of robustness and operational reliability. In addition, due to the introduction of industry-standard mounting arrangements and dimensions, the housing can be installed as a direct replacement for a wide variety of conventional plummer block housings. By introducing some subtle geometrical changes, including a small arch-shaped cutaway at the


bottom centre of the housing, the load on the bottom roller (at the apex, 180˚) is now distributed more evenly across two or three rollers. By spreading the load from the shaft across a greater number of rolling elements, the operating life of the bearing is increased by up to 50%. In addition, a lubrication bore with a longer groove has been added to the inside of the upper


section of the housing, meaning different bearing types installed in each bearing position can now be supplied with sufficient lubricant at all times. Furthermore, to make mounting even easier, centre marks


that indicate the bearing centre line and shaft are provided in the lower section of the housing, which enables fast alignment of the housing. The lower section of the housing also incorporates high precision, machined locating surfaces. Rolling bearings of different diameters and widths can be


installed in the SNS plummer block housing. Schaeffler


www.schaeffler.co.uk Enter 206


/ DESIGNSOLUTIONS


DESIGN SOLUTIONS | FEBRUARY 2015


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