Bearing the brunt

. Tom Shelley reports Inserting three slightly oversized – but marginally compliant – tubular rollers between conventional, solid cylindrical rollers can prevent low-load slippage in cylindrical roller bearings. Schaeffler has developed what it designates the ‘FAG tube roller bearing’ for arduous applications such as hot steel rolling mills, where bearings alternate between having to ensure very heavy loadings – when steel is going through the mill – and no loading at all, when steel is not present. Under such conditions, it is possible for bearing slippage to occur, resulting in damage. “The initial idea was that it should be on very high load bearings,” says Rainer Eidloth of Schaeffler. “There is only microns of difference between the sizes of the tube rollers and the solid rollers, for pre-tensioning the ring.” Under full radial load, the tube rollers are slightly elastically compressed – because they are hollow – until they are of the same diameter as the solid rollers. The load forces are then distributed among all the rolling elements in the load zone. The company says that, compared with axially preloaded solutions, they permit simplified designs and easier installation. Since their abutment dimensions are unchanged, they are freely interchangeable with standard cylindrical roller bearings. Eidloth says they are currently being beta tested with customers and that the relative dimensions and designs have to be tweaked for each individual application. A sample shown by Schaeffler at the recent Hanover Fair was quite substantial, but small compared to some bearings the company has made – the FAG bearings on the Millennium Wheel in London, for example, are more than 4m across. As well as hot rolling mills, the new bearings are intended for use in other severe duty applications, such as wind turbines and paper mills. At the same event, the company unveiled another bearing with impressive credentials under severe conditions. This is made of ‘Cronitect’ corrosion resistant, martensitic hardening steel. The ‘Cr’ in the name refers to Chromium, the ‘Ni’ to nitrogen and the ‘tect’ to the material’s corrosion protection. Martensitic bearing steels containing chromium and nitrogen have been known for some years. However, Schaeffler says its formulation offers very high hardness and maximum corrosion resistance through its chemical composition, in combination with a new thermochemical surface layer treatment process. Cronitect withstands salt spray testing, in accordance with DIN 50021 SS without any problems, even after 600 hours, states Schaeffler, while rolling bearings made of the material can be used in contact with water, acids or cleaning agents, or in dry running, and are all available in sealed or non-sealed versions. Pointers * Three slightly oversized tube rollers, interspersed between conventional rollers, pre-tension the ring and prevent slippage under low load * Under full load, the tube rollers compress, spreading the load across all rollers * For arduous service conditions, the company has also developed a new corrosion-resistant martensitic hardening steel called ‘Cronitect’ Tandem hubs with low friction A demonstration at Hanover showed how four row angular contact ball bearings caused far less frictional drag than equivalent tapered roller bearings that are traditionally used in vehicle wheel hub assemblies. Started off at the same speeds, the new bearings – called ‘twin tandem bearings’ by Schaeffler – continued to rotate for 51 revolutions, compared with the seven revolutions of the equivalent tapered roller products. The reason, said Schaeffler’s Norbet Metten, is the inevitable frictional contacts between the ends of the rollers and the sides of the races in the traditional tapered roller bearings, which does not occur with the double row ball bearings. The new bearings have been sized for SUVs, light trucks and delivery vans, but, said Metten: “They could be in cars as well and in mass production by BMW with a BMW patent – and in different parts, it doesn’t matter where.” Reduced friction also lowers operating temperatures and extends lifetime. The company believes that replacing tapered roller bearings with the new design can reduce fuel consumption by up to 1.5% for every day of driving. No changes are required to vehicle designs. The new bearings use the same installation space and all types of sensors, including ABS, can be accommodated.