Programme aims for next-generation bearings

A tribology programme aims to reduce bearing friction and wear via a combination of improved bearing materials, optimised surface finish and enhanced lubrication. Continuing advances in the design of pumps, rotating machinery, medical equipment, automotive components and others, mean that the demands on bearings for higher speed operation, improved reliability and longer life are set to increase. Barden is meeting these demands with a tribology programme aimed at reducing bearing friction and wear via a combination of improved bearing materials, optimised surface finish and enhanced lubrication. The programme is designed to enable rolling bearings to compete with aerostatic, hydrostatic and magnetic bearing systems, providing an economical solution for the next generation of high performance machines. Tribology is defined as the science of friction, lubrication and wear. Essentially it is all about ensuring moving surfaces work together efficiently by keeping them apart. For example, rolling bearings will perform satisfactorily when an elastohydrodynamic film of lubricant is established between their moving surfaces. But if the film is too thin - as a result of slow speeds and/or low lubricant viscosity - then what is known as boundary lubrication will occur. In this condition the load is carried almost directly on the moving surfaces. The result can be excessive friction leading to wear and premature failure of the bearing unless the correct lubricant is selected. The tribological performance of bearings is significantly dependent on the extent of internal stressing in the rolling contact area. In many applications it is not possible to generate a full elastohydrodynamic lubricant film, so Barden has developed processes and a capability to create a system resilient to material stressing. At the core of Barden initiative is a stress on new materials such as Cronidur 30. Cronidur 30 offers significant advantages over 100Cr6 and 440C, the standard rolling steels for super precision bearings. It offers 40% higher dynamic capacity, improved wear and fatigue life, and is corrosion resistant. The large carbides and carbide networks (80-100um), which are a feature of competitive super precision bearing steels, such as 440C, do not exist in Cronidur 30. Instead there is a homogeneous structure of finely dispersed carbonitrides, usually less than 10um in size. As a result, Cronidur 30 does not have the problem of large carbides concentrating at grain boundaries and providing potential 'weak spots' which are susceptible to mechanical stressing and corrosion. In tests carried out under a defined condition of mixed friction, super precision bearings manufactured from Cronidur 30 have achieved a tenfold service life, compared with bearings made of the standard material 100Cr6. Added to this, the wear behaviour of the bearing is considerably improved with Cronidur 30. This fact becomes particularly apparent in the case of hybrid bearings, using Cronidur 30 rings and ceramic balls, where the wear rate is considerably below the former usual values. The tribological properties of ceramic balls also mean that the operating temperature for hybrid bearings is much lower than for bearings using steel balls. This is due to the low thermal conductivity of the ceramic material (Silicon Nitride Si3N4), which enables the balls to 'keep cool'. Additionally the rate of thermal expansion of the ceramic ball is much lower than that of a steel counterpart, which results in minimal operational preload change, and friction remains small (approximately half that for an all steel super precision bearing). Experience has shown that this cool running can extend the working life of the bearing grease and, hence, the life of the hybrid bearing by a factor of five. In its development of new materials for bearing technology Barden has also concentrated on surface technology. This is of particular importance because, ultimately, it is the properties of the surface layers, not the bulk material, which determines and controls system performance. In the past, only the surface roughness (Ra value) was used for evaluation. However, more recent fundamental studies have revealed that the direction of 'grooves' and the 'sharpness' of individual roughness peaks also play an important role. The consequence of this is that a number of new parameters are considered in the evaluation process. These include percentage contact area, which indicates how much material there is at a certain depth beneath the surface, and Rsk value: a yardstick of a profile's symmetry (do peaks project from the surface, or are there grooves in the material?). In addition, there is also a value known as delta-Q, which indicates sharpness and roughness of the peaks. All of these parameters can be determined by means of modern measuring instruments. The key value of the measurement process is in helping to develop asymmetrical surface profiles that provide a good basis for lubricant film development. The objective is to achieve percentage contact area of 99.6% at a surface depth of 0.2um. Today, all Barden super precision bearings are produced using technologies that enable this quality benchmark to be consistently achieved. Achieving the ideal surface profile is one of the key elements in achieving a separating lubricant film to reduce surface stressing. The second such element is the lubricant itself. Over the past few years Barden has been actively collaborating with a leading lubricant manufacturer to produce a superior grease for use with super precision bearings. The result of this collaboration is Arcanol L75, a new, polyalfaolefin/ester based high speed bearing grease that excels for its noncritical run-in behaviour, high temperature stability, non toxicity, resistance to slumping and favourable viscosity- temperature behaviour. Run-in is the ability of a lubricant to reach a steady operating state in as short a time as possible. L75 does this in less time than half the time of barium-based greases. Importantly, the temperature of L75 during the critical run-in period is typically half that of barium-based greases when tested under the same conditions. What these results reveal is that, in the critical phase of grease distribution with L75, the damaging effects of thermal stress conditions and lubricant degradation are clearly eased and a significantly higher degree of operational reliability is achieved as a result.