Rolling element bearings are critical to the performance of virtually every type of machinery butdespite centuries of continuous developmentbearings still account for countless premature failures.
Often this is due to the incorrect bearing being specified at the design stageflaws in the installation procedure or a lack of adequate maintenance. Neverthelessbearing 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 conditionssuch as vibrating machinery or steel millsas 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 programmethese 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 borewhich leads to improved wear behaviour when operating unlubricated or in conditions of mixed friction. Because the coating is resistant to weather influenceswatervapouralkaline solutionsphotographic developers and even some type of acidsit reduces frictional corrosion between the bearing bore and shaft andin most casesavoids 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 valuesthere 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 NSKknown as the HPS (high-performance spherical) roller bearings. Designed to meet a wide spectrum of needs for a variety of industriesthe 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 bearingsthe 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. Howeverunder rotation these geometrical forms interact to produce slippage at the contact surface points between the raceways and rollers. During the development of the HPSNSK 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. Moreoverthe company established that applying a special surface treatment to the outer ring is effective in achieving this.
Compared with conventional nitridingNSK’s treatment forms a finer and harder surface with more uniform hardness. Howeversince the treatment involves elevated temperaturesthe dimensional accuracy of the cage is inevitably affectedso NSK had to find a way to ensure high accuracy of the cage after treatment. As a resultthe problem of cage wear under conditions of continuous running at high speedswhich otherwise would have restricted any increase in the limiting speed of the bearinghas 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 industrySWR bearings also have a running life more than twice that of conventional (SUJ2) bearing types (Fig. 2).
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 steelSWRwhich provides superior wear resistance under severe lubrication conditions and also high resistance to surface-originated flaking.
SWR takes advantage of a special alloya carbonitriding technologya 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 hardfine-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 surfacesthereby 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 treatmentthe 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 programmeNSK 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¢ªCand automatic grease lubrication is recommended to ensure that optimal running conditions are maintained. Each bearing set is custom designed and manufactured to suit the applicationwith batches manufactured on specialised machines. As a result of a substantial investment in capital equipmentthe 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 additionthe bearings can now be finished on one machine instead of the three that were required previously.
Barden is undertaking a tribology research programme aimed at reducing bearing friction and wear by means of a combination of improved bearing materialsoptimised surface finish and enhanced lubrication (Fig. 1). The programme is intended to enable rolling element bearings to compete with aerostatichydrostatic and magnetic bearing systemsproviding 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 & 440Cthe standard rolling steels for super-precision bearings. It offers improved wear and fatigue liferesistance to corrosion and an increase in dynamic capacity of 40 per cent or more.
The large carbides and carbide networkswhich are said to be a feature of competitive super-precision bearing steels such as 440Cdo 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 frictionsuper-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 additionthe wear behaviour of the bearing is considerably improved with Cronidur 30. This fact becomes particularly apparent in the case of hybrid bearingsusing Cronidur 30 rings and ceramic ballswhere 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.
In its development of new super-precision bearingsBarden has also concentrated on surface technology. This is of particular importance becauseultimatelyit is the properties of the surface layersnot the bulk materialthat determine and control the overall system performance.
In the pastonly the surface roughness (Ra value) was used for evaluation. Howeverrecent 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 areawhich indicates how much material there is at a certain depth beneath the surfaceand 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 additionthe 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 behaviourhigh-temperature stabilitynon-toxicityresistance to slumping and a favourable viscosity-temperature characteristic.
Apart from exceptional circumstanceselectric motor bearings are not classified as super-precision. Butthey 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 Europeresearch 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; howeverit 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 outthey can break down as a result of thermal conditions or attacks from solventsand they can become contaminated with waterdustor 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 andin many lighter-duty applicationsenables bearings to last as long as the equipment in which they are installed. Howeverin more arduous applicationssealed-for-life bearings are inappropriateso the rate of bearing failures in these types of applications has not reduced significantly. In these applicationsthereforethe motor user must employ a suite of measures to ensure that motor bearing life is maximised."