D. Dowson

A Numerical Solution to the Elasto-Hydrodynamic Problem

This paper presents a solution to the problem of hydrodynamic lubrication of highly loaded clastic cylinders under isothermal conditions. A numerical method is developed which enables a pressure cu...

Tribology of Total Artificial Joints

The tribology of total artificial replacement joints is reviewed. The majority of prostheses currently implanted comprise a hard metallic component which articulates on ultra high molecular weight polyethylene surface. These relatively hard bearing surfaces operate with a mixed or boundary lubrication regime, which results in wear and wear debris from the ultra high molecular weight polyethylene surface. This debris can contribute to loosening and ultimate failure of the prostheses. The tribological performance of these joints has been considered and a number of factors which may contribute to increased wear rates have been identified. Cushion bearing surfaces consisting of low elastic modulus materials which can articulate with full fluid film lubrication are also described. These bearing surfaces have shown the potential for greatly reducing wear debris.

Elastohydrodynamic and micro-elastohydrodynamic lubrication

Abstract Elastohydrodynamic lubrication is the dominant mode of lubrication in many critical, highly stressed machine elements such as gears, rolling bearings, cams and followers. It also governs the effective operation of many highly deformable or ‘soft’ bearing systems such as elastomeric bearings, seals and synovial joints. The major developments in the understanding of this exciting mode of fluid film lubrication during the latter half of the twentieth century is reviewed and attention is drawn to continuing topics of investigation.

Thin Films in Tribology

In this Keynote Address the author reviews briefly the full spectrum of thin film tribology, ranging from surface modification and coatings to the behaviour of thin, protective lubricating films. Attention is drawn to the remarkable way in which the effective minimum film thickness in highly loaded, reliable and efficient lubricated machine elements has fallen from tens of microns to tens of nano-metres throughout the 20th century. The near coincidence of studies of ‘solidified’ lubricants and ‘boundary’ lubricants in tribology is noted.

Some new evidence on human joint lubrication.

Theoretical consideration has been given to the use of pendulum machines which are used to examine the frictional properties of human joints by incorporating them as fulcra. As a result, a new type of pendulum machine has been built which incorporates the facility to apply sudden loads to the joint on starting the swinging motion, and also the ability to measure directly the frictional torque experienced by the joint. The results obtained from natural hip joints indicate the presence of squeeze film lubrication under conditions of sudden loading of a joint. In addition, a self-generated fluid film process was observed at low loads while at higher loads boundary lubrication appeared to be important. These results have been used to describe the lubrication regimens occurring in a normal activity such as walking. A single experiment carried out on a hip from a patient suffering from severe rheumatoid arthritis has also been reported and the frictional resistance was seen to be increased fifteenfold compared to a normal hip.

Behaviour of synovial fluid on surfaces of articular cartilage. A scanning electron microscope study.

Much attention has been given to the mechanics of synovial joints from a descriptive aspect and to studies relating to joint lubrication. The synovial fluid has been analysed and details of its composition are known (Ropes and Bauer, 1953). The principal constituent is hyaluronic acid-protein complex, the acid molecule having been shown to be a non-branched chain of repeating units with hydroxyl end-groups on each unit (Meyer, 1931). The acid-protein complex is responsible for the viscous properties of the synovial fluid (Negami, 1964; Davies, 1966) and it is fairly certain that it plays an important function in protecting the cartilage surfaces. A reasonably good picture has been obtained of the structure of articular cartilage, which is understood to consist of bundles of collagen fibres in a ground substance, chondroitin sulphate, with a high water content. The collagen bundles on the surface are believed to lie parallel to it. Those below are randomly or perpendicularly orientated to the surface depending upon the age and condition of the cartilage. (Benninghoff, 1930; Martin, 1953; Little, Pimm, and Trueta, 1958.) The remarkable lubrication characteristics of joints have been found to be due to the combination of properties of synovial fluid and articular cartilage. From experiments using a reciprocating friction machine and by measuring the topography of the surfaces of cartilage with stylus equipment, it has been concluded that the cartilage surfaces are prevented from contacting primarily because of the formation of trapped pools of lubricant (Dowson, Longfield, Walker, and Wright, 1969). Furthermore, it has been shown that, under some conditions,

Past, present and future studies in elastohydrodynamics:

Abstract The development of understanding of the phenomenon of elastohydrodynamic lubrication (EHL) throughout the twentieth century is reviewed. The development of solutions for both line and point contacts is considered for both fully flooded and starved conditions. Particular attention is given to the introduction of non-Newtonian rheological models and numerical methods. Progress in the analysis of impact and general non-steady-state conditions is reviewed, together with the consideration of rough surface and micro-EHL. Attention is drawn to the need to consider realistic models of real surfaces and real fluids in future studies.

The Effect of Material Properties on the Lubrication of Elastic Rollers

The effect of the significant variables on solutions to the problem of isothermal lubrication of elastic rollers is examined. It is found that for realistic materials, speeds, and loads, the outlet pressure peak predicted by Grubin (4) is always present. The magnitude of this pressure peak may exceed the maximum Hertzian pressure. It varies only slightly with load, but markedly with speed and materials. The influence of these variables on the shape of the outlet pressure curve explains the difference between Petrusevichs results (5) and those presented earlier by the authors. The stress concentrations produced near the surface by the outlet pressure peak would have a significant effect on the fatigue life of rolling elements.It is found that the centre-line film thickness varies hardly at all with load, significantly with material properties and considerably with the product of speed and inlet viscosity.Pressure distributions and film shapes are presented for five cases to illustrate the influence of loa...

Mode of aggregation of hyaluronic acid protein complex on the surface of articular cartilage.

There is a great deal of experimental evidence that the frictional forces between sliding cartilage surfaces lubricated with synovial fluid are very low under normal conditions. Considering coefficient of friction as the meaningful measure of friction (i.e. tangential force resisting sliding divided by the normal load forcing the sliding surfaces together), typical values for cartilage on glass have been found to be 0-002 to 0-01 (McCutchen, 1962; Walker, 1969). A similar range was found for the ankle joints of dogs (Linn, 1968). However, under certain conditions, the coefficient of friction can be much higher than the range quoted. For instance, a test on a dog ankle joint was run for up to 6 hours under conditions of starved lubrication (Linn, 1968), and the coefficient of friction was found to be 0 1. For cartilage on glass, even higher figures have been obtained (Walker, Dowson, Longfield, and Wright, 1968).

Quantitative study of stiffness in the knee joint.

Data from a knee arthrograph, which imparts a passive sinusoidal motion to the joint and measures the resistive torque and displacement, have been shown to be reproducible. Tests carried out on joints (from 49 males, 21 females) showed that the energy loss per cycle of the joint increased with age for both sexes but the peak to peak torque (elastic stiffness) did not vary with age. Muscle bulk had a large influence on the stiffness (both dissipative and elastic) of the knee as did the size of the joint itself. Womens knee joints exhibited lower values of dissipative energy loss and peak to peak torque than mens for all ages and sizes of joints.