Rolling element bearings

of hydrostatic and hydrodynamic lubrication may provide an economic and valuable role for rotating spindle applications that not only require high precision and low wear but may also require the ability to withstand high overloads for short periods. A heavy duty milling machine spindle employing hybrid bearings might cope with the roughest use and still maintain the stiffness required for long life and low chatter susceptibility. W.B. Rowe

Gas bearings Understanding of gas bearings has progressed significantly over the last ten years. In the late sixties, the vast majority of design methods were based on approximate mathematical models for flow in the bearing clearances and control devices. Solutions were mainly restricted to steady state operation. Conventional orifice flow equations were used for the control devices, based on an adiabatic process. The majority of design methods did not take account of the recovery of pressure in the pocket. Laminar flow in the clearance was described by a suitable form of the Reynold's equation and by assuming that flow into the bearing clearance was a line source, for convenience, rather than the true case of disc r e t e p o i n t s 79 . Much of the early design work has been shown since to be reasonably accurate for short bearings at low eccentricity ratios where flow is predominantly axial. Correction factors to take into account circumferential flow and empirical factors derived from a vast amount of experimental data were used to improve accuracy. Developments in computer power and computational techniques lead to finite difference and finite element methods being applied to gas bearing design and a considerable number of papers resulted from many workers throughout the world. A considerable proportion of the work in the early seventies was related to orifice controlled bearings and porous pad bearings, but slot entry bearings were also receiving interest 8 o. Serious discrepancies between the various design methods were high-lighted in a paper by Pink 8~ which indicated the need for'accurate and reliable experimental results, particularly in the region of high eccentricities. Much of the latest work has been directed at improving the accuracy of experimentation and theoretical methods, including attempts to model the pressure loss in the orifice, leading to useful papers 82 . In addition, several workers have analysed the effects of manufacturing variations on performance, in an attempt to establish the reliability of design methods. A detailed contribution included procedures intended to simplify the designer's work 83 A criticism which may be levied at many workers is the lack of consideration given to information presentation and this is the area greatest attention should now be directed. Design methods have improved greatly so the majority of new work will either lie in extending the range of bearing types covered by current design, investigating new applications, or most important, the transfer of the existing knowledge into easily useable data for designers. There is a requirement for simple graphical design procedures for gas bearings which significantly reduce tedious calculation. This W.B. Rowe, Liverpool Polytechnic, Byrom Street, Liverpool L 3 3AF,

UK

24

TRIBOLOGY international February 1978

type of design presentation would allow interactive selection and adjustment of design parameters a4. With the improved design techniques and design procedures will come increased application of these bearings. The new applications are likely to arise in high technology machine tools in addition to laboratory equipment. Examples can be seen in the high precision grinding machine and earth rate tables being developed at the Cranfield Unit for Precision Engineering. Further developments may be anticipated in areas associated with energy resources and aerospace research. K.J. Stout

Rolling element bearings A high degree of standardisation on product and application design, coupled with the fact that performance and life can be predicted with remarkable accuracy, have made the rolling element bearing a cost effective solution to many bearing problems. Behind the universal acceptance of a critical component, the failure of which in application inevitably causes distress if not disaster, lies decades of effort in hammering out the design, improving the materials, and evolving a highly specialised manufacturing process. It also owes much of its succes to fundamental research in the fields of lubrication and rolling contact fatigue. The past decade has seen a further increase in the speed of rotating machinery and bearing loads. The former is perhaps more noticeable in North America and the latter in Western Europe and Japan. This has been brought about by: the need to improve the efficiency of turbines and pumps with a consequent increase in the speed of the associated transmission systems; the replacement of the tradit:'onal fluid bearing in, for example, steel rolling mills, high speed centrifugal pumps, etc; a significant increase in the power to weight ratio of motor cars, particularly in the Western Europe; and the demand for versatile machine tools capable of a wide range of operating conditions, controlled by a micro-processor. Designers' consciousness of equipment noise, either through market demand or legislation, has had a far reaching effect on bearing vibration standards. Rolling bearings from modern production facilities have a vibration level a mere fraction of what it was ten years ago, and new test equipment has had to be developed to measure it. For all their apparent simplicity, a wide range of improvements have been incorporated in the current rolling bearings to meet the new challenge. A few of these will be mentioned. Some relate to the way that bearings are being applied, others to the application of a new technique in characterisation of surface topography for smooth running bearings, and yet others to the improvement on materials and manufacturing techniques. Recent work has shown that, if the roller bearing races have a suitable combination of ribs, a thrust load of appreciable size can be carried on these without visible wear. The thrust is distributed among the rollers which are also radi-

K.J. Stout, Leicester Polytechnic, PO Box 143, Leicester LE2 9BH, UK

ally loaded, causing them to tilt slightly to react to the couple applied by the ribs: this makes it possible for a lubricant film to be formed at the ends. Using roller bearings to take thrust load can be an attractive solution in many applications; they can take high radial load and are simple to assemble. The rollers are suitably profiled to overcome the disadvantage of the slight tilt of misalignment. The effect of stress peaks at the roller ends has long been recognised as a problem even in cylindrical roller bearings subject to pure radial loads, but only recently a true optimised roller profile over a range of conditions becomes possible by the development of a complex computerised stress analysis procedure. Bearing cages or separators have long been treated merely as accessories, but their key role in high speed bearings and in bearings with difficult lubrication conditions is now beginning to be appreciated. Maach effort has gone into the theoretical modelling of dynamic interactions between rolling elements and cages, the development of special cage material, and design. This has improved our understanding of aero-engine bearing designs for gyroscopes and spaceship bearings, and has made possible the design of high speed spindle bearings with a minimum temperature rise above ambient, high speed fail-safe bearings in the event of lubricant failure, etc. There is room for improved cage design in greased-for-life bearings to extend their range of applications. 'Superfinish' bearing tracks by honing have been used in the past for high grade bearings and their use is now spreading to all bearings. Advanced surface topographical measurement techniques, are being applied to the study of the grinding and honing processes in relation to surface integrity and bearing vibration. This has led to a fundamental re-appraisal of these processes and will benefit the rolling bearing industry in further improving the quality and reliability of the product, particularly when it is coupled with material improvement. Since the mid 1960's conventional through-hardening high carbon chrome steel (1% C, 1.5% Cr) has been produced by electric arc melting with subsequent vacuum degassing to remove the non-metallic inclusions which act as stress raisers and reduce fatigue life. "~ore recently there has been a wide interest in the production of bearing rings by sinter forging and metal forming routes to eliminate wasteful machining operations: a new process of ball manufacturing by hot rolling gives a high product rate, better final form, and hence less subsequent grinding and better fatigue performance. Manufacturers are also alert to the possible benefit in better fatigue life or alternative materials through surface treatment or ion implantation. The rolling element bearing has, in the past ten years, more than kept pace with the technological demand. The next ten years will see new developments in manufacturing processes come to fruition to further enhance quality and reliability, to improve material utilisation, and to reduce costs. It will also see the realisation of the speed capability of rolling element bearings, though well established in the aerospace industry, in many applications in rotational machinery which are currently using fluid film bearings. Improvements in cage material and design for greased-for-life bearing for high

speed or general purpose application with enhanced life and reliability will reduce both capital and service costs. S. Y. Poon

Thrust bearings During the last decade the improvements in hydrodynamic thrust bearings have been mostly of a consolidating nature. The pivoted pad is an efficient and stable configuration and attempts at improvement have been concerned with the overall assembly or with special operating conditions. Methods of analysis have received some attention s 5 ,s 6 in that thermal and elastic distortion of the pads can now be more comprehensively treated than in the earlier studies of the nineteen sixties. Iri particular, Robinson s s included simultaneous solutions of the biharmonic equation for pad bending and their deflection, the energy equation for heat generation, and the Reynolds equation. If is now realised, for example by Kawaike et al 87, that thermal distortion of the pads becomes of critical importance in large installations. This can be shown in general terms by the simple example of a uniformly loaded cantilever (constituting the leading or trailing portion of a pad). The end deflection of a cantilever of unit width, thickness, and loaded with intensity p is given by 1.5 p L 4 l e d 3 , so that ifL is increased, and p and the ratio L i d is maintained, the end deflection increases with L. However, the minimum f'tim thickness tends to increase in the same proportion, so distortion from purely elastic causes is unlikely to give inefficient fdm shapes. The same cannot be said of thermal distortion, since if the temperature gradient through the pad thickness is maintained at larger sizes the thermal deflection increases as the square of the distance from the pivot. In practice this can give a thermal ratchetting effect in large installations since a n inefficient film shape tends to give a high pad temperature with consequent increased thermal distortion. The effect can be overcome with forced cooling of the pads a s, the use of material with a high thermal conductivity s9 , or the support of the pad to give elastic distortion in the opposite sense 86. In studies of the experimental performance of bearing assemblies at high speeds 9° ,9 ~, mean sliding velocities of 150 m/s were achieved. It appears that the onset of super laminar flow (in the region of 70 m/s) can cause a decrease in pad maximum temperature although the power loss of the assembly then becomes excessive. A method of sharply reducing the power loss by 'directed lubrication' is described by Bielec and Leopard 92 in which jets of oil are directed at the leading and trailing edges of the pads, allowing the assembly to operate on the 'dry sump' principle without excessive churning losses. A strong derating effect on thrust bearings is the carry over of hot oil from the exit of one pad to the entrance of the next. A calculation method to allow for this effect would be an advantage if design procedures are to proceed beyond the present moderately satisfactory mixture of empiricism and experience. Methods of calculation include a 'groove mixing' approach ss or a hot oil carry over 'coefficient 's 6 based on thermal boundary layer theory. Both methods appear to work reasonably well at low to medium speeds. S. Y. Poon, RHP Bearing Research Centre, PO Box 7, Chelmsford, Essex, UK

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