Rolling element bearings are critical to the performance of virtually every type of machinery but, despite centuries of continuous development, bearings still account for countless premature failures.

Often this is due to the incorrect bearing being specified at the design stage, flaws in the installation procedure or a lack of adequate maintenance. Nevertheless, bearing manufacturers still spend a considerable proportion of their research and development budgets investigating ways to improve the life expectancy of their products.

This is particularly so for bearings that operate in extreme conditions, such as vibrating machinery or steel mills, as well as super-precision bearings and those in commonplace applications such as electric motors.

One example of a specialist bearing type that has received close attention is FAG’s spherical roller bearings for with vibrating screens – which are typically used for sorting and classifying goods or materials. Thanks to a recent development programme, these now offer a service life that is twice as long as that of conventional bearings of the same size. This is due the bearings’ new thin-layer chromium-plated bore, which leads to improved wear behaviour when operating unlubricated or in conditions of mixed friction. Because the coating is resistant to weather influences, water, vapour, alkaline solutions, photographic developers and even some type of acids, it reduces frictional corrosion between the bearing bore and shaft and, in most cases, avoids it completely.

The special coating is bonded to the base material at a molecular level and resists high loads. As the bearings’ principal dimensions and tolerances are unchanged from standard values, there is no need to alter the design of adjacent components or the assembly procedure.

Another range of spherical roller bearings with double the life of standard versions was recently announced by NSK, known as the HPS (high-performance spherical) roller bearings. Designed to meet a wide spectrum of needs for a variety of industries, the HPS series also benefits from a maximum limiting running speed that has been improved by up to 20 per cent.

Compared with other types of bearings, the spherical roller bearing (SRB) has a high load capacity and a self-aligning feature that means some misalignment of the axis of rotation can be tolerated.

SRBs are self-aligning by virtue of the rollers’ spherical profile that acts in the bearing inner and outer ring raceways. However, under rotation these geometrical forms interact to produce slippage at the contact surface points between the raceways and rollers. During the development of the HPS, NSK claims to have clarified – for the first time in the world – the mechanism that causes slippage and creates high friction on the surface – leading eventually to damage due to surface fatigue. NSK also used theoretical analysis and bench testing to verify that the friction can be reduced by controlling the motion of rotating rollers. Moreover, the company established that applying a special surface treatment to the outer ring is effective in achieving this. 

Compared with conventional nitriding, NSK’s treatment forms a finer and harder surface with more uniform hardness. However, since the treatment involves elevated temperatures, the dimensional accuracy of the cage is inevitably affected, so NSK had to find a way to ensure high accuracy of the cage after treatment. As a result, the problem of cage wear under conditions of continuous running at high speeds, which otherwise would have restricted any increase in the limiting speed of the bearing, has been solved. This is evidenced by the 20 per cent increase in limiting speed for the HPS series.

Initially 42 sizes with outside diameters ranging from 80mm to 260 mm will be marketed.

NSK is claiming another ‘world-first’ by combining carbides technology with a special alloy to create a new Super Wear-Resistant (SWR) bearing steel for rolling bearing applications. Designed for use in the guide rolls of continuous casting machines used in the steel industry, SWR bearings also have a running life more than twice that of conventional (SUJ2) bearing types (Fig. 2).

New materials

NSK has developed a bearing whose life can be extended by means of the latest materials and heat treatment technologies. Key to this development is the new type of steel, SWR, which provides superior wear resistance under severe lubrication conditions and also high resistance to surface-originated flaking.

SWR takes advantage of a special alloy, a carbonitriding technology, a heat treatment and a surface hardening process. The special alloy is combined with NSK’s carbonitriding technology to produce a material with large amounts of hard, fine-grained particles in the material surface. The major benefit of this is that it provides superior resistance to wear – about seven times that of conventional AISI 52100 steel.

The second key element in the development of the SWR material is NSK’s patented TF Technology. This includes a special heat treatment process developed by NSK to optimise the level of retained austenite in the bearing steel. Austenite is important because it minimises stress concentrations around defects on the bearing surfaces, thereby improving flaking life performance under conditions of contaminated lubricant. When compared on the basis of L10 basic rating life (at which the cumulative damage probability becomes 10 per cent), the life of the SWR steel is about five times that of conventional AISI 52100 steel.

In addition to heat treatment, the SWR steel is also case hardened so that its core toughness is about five times as high as that of an SUJ2 through-hardened steel.

Following the excellent results obtained from its

in-house testing programme, NSK has undertaken field evaluations of SWR bearings on continuous casting lines in co-operation with several major steelworks. The results show that SWR bearings provide two or three times more life when compared to conventional bearings.

An alternative approach to extending the life of bearings in continuous casting machines is adopted by the Cooper Roller Bearing Company. Water cooling is used to maintain the bearing temperature at 60 to 80¢ªC, and automatic grease lubrication is recommended to ensure that optimal running conditions are maintained. Each bearing set is custom designed and manufactured to suit the application, with batches manufactured on specialised machines. As a result of a substantial investment in capital equipment, the company has been able to modify the internal geometry of the bearings to give a 50 per cent increase in bearing life and a 20 per cent increase in static load capacity. In addition, the bearings can now be finished on one machine instead of the three that were required previously.

Super-precision bearings

Barden is undertaking a tribology research programme aimed at reducing bearing friction and wear by means of a combination of improved bearing materials, optimised surface finish and enhanced lubrication (Fig. 1). The programme is intended to enable rolling element bearings to compete with aerostatic, hydrostatic and magnetic bearing systems, providing an economical alternative for the next generation of high-performance machines.

At the core of Barden’s initiative are new materials such as Cronidur 30. This offers significant advantages over 100Cr6 & 440C, the standard rolling steels for super-precision bearings. It offers improved wear and fatigue life, resistance to corrosion and an increase in dynamic capacity of 40 per cent or more.

The large carbides and carbide networks, which are said to be 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. Cronidur 30 therefore does not have the problem of large carbides concentrating at grain boundaries and providing potential 'weak spots' that are susceptible to mechanical stressing and corrosion.

In tests carried out under defined conditions of mixed friction, super-precision bearings manufactured from Cronidur 30 have achieved a service life 10 times greater that that of bearings made of the standard 100Cr6 material. In addition, 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 silicon nitride material that enables the balls to remain at a relatively low temperature.

Surface finish

In its development of new super-precision bearings, 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, that determine and control the overall system performance.

In the past, only the surface roughness (Ra value) was used for evaluation. However, 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 the percentage contact area, which indicates how much material there is at a certain depth beneath the surface, and the Rsk (skewness) value – a measure of a profile’s symmetry (whether peaks project from the surface or there are grooves in the material). In addition, the Delta-Q value indicates sharpness and roughness of the peaks.

The key value of the measurement process is in helping to develop asymmetrical surface profiles that provide a good basis for lubricant film development. Achieving close to the ideal surface profile is one of the main 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. Arcanol L75 is a new polyalfaolefin/ester-based high-speed grease that features excellent non-critical run-in behaviour, high-temperature stability, non-toxicity, resistance to slumping and a favourable viscosity-temperature characteristic.

Motor bearings

Apart from exceptional circumstances, electric motor bearings are not classified as super-precision. But, they represent an important segment of the overall market for bearings and premature failure could be costing Europe millions of Euros. According to Steve Wilkinson of NSK Bearings Europe, research in the USA has found that bearing systems failures in electric motors are costing the electric utility industry $150 million annually. An equivalent figure for Europe is open to conjecture; however, it would be reasonable to assume that downtime costs resulting from motor bearing failures are substantial.

The quality of modern bearings means that failures in industrial environments are now less frequently caused by fatigue. Instead they are more likely to be due to problems with lubrication; lubricants can leak out, they can break down as a result of thermal conditions or attacks from solvents, and they can become contaminated with water, dust, or rust from the bearings themselves or adjacent parts.

One way in which motor manufacturers are addressing these problems is to equip their products with grease-sealed-for-life bearings. This overcomes the cost of re-lubrication and the problem of over-lubrication and, in many lighter-duty applications, enables bearings to last as long as the equipment in which they are installed. However, in more arduous applications, sealed-for-life bearings are inappropriate, so the rate of bearing failures in these types of applications has not reduced significantly. In these applications, therefore, the motor user must employ a suite of measures to ensure that motor bearing life is maximised.