R. S. Sayles

Edge loading in metal-on-metal hips: low clearance is a new risk factor.

The revision rate of large head metal-on-metal and resurfacing hips are significantly higher than conventional total hip replacements. The revision of these components has been linked to high wear caused by edge loading; which occurs when the head–cup contact patch extends over the cup rim. There are two current explanations for this; first, there is loss of entrainment of synovial fluid resulting in breakdown of the lubricating film and second, edge loading results in a large local increase in contact pressure and consequent film thickness reduction at the cup rim, which causes an increase in wear. This paper develops a method to calculate the distance between the joint reaction force vector and the cup rim – the contact patch centre to rim (CPCR) distance. However, the critical distance for the risk of edge loading is the distance from the contact patch edge to rim (CPER) distance. An analysis of explanted hip components, divided into edge worn and non-edge-worn components showed that there was no statistical difference in CPCR values, but the CPER value was significantly lower for edge worn hips. Low clearance hips, which have a more conformal contact, have a larger diameter contact patch and thus are more at risk of edge loading for similarly positioned hips.

Basic principles of rough surface contact analysis using numerical methods

A detailed account of the principles involved in using numerical elastic contact techniques on digitized measurements from rough surfaces is presented in relation to two- and three-dimensional topography data. The main results of such analyses are shown to include the detailed interface geometry and the subsequent contact pressure distribution involved. Methods of defining the resulting sub-surface stresses created by this contact pressure distribution are also presented for static normal loading, and for the case of a normal load in the presence of a frictional surface shear. The problems posed in dealing with plastic asperity contacts are also discussed, together with an outline of how the numerical methods described have been modified further to allow analysis of rough layered bodies of dissimilar materials, thus offering a very useful design tool for surface coatings.

Effects of debris particles in sliding/rolling elastohydrodynamic contacts

Abstract A theoretical simulation of the behaviour of debris particles in elastohydrodynamic (EHD) contacts is an effective means for obtaining information regarding the life and performance of lubricated machine elements compared with costly experimentation. The present work indicates that debris particles are often responsible for two failure modes: (a) scuffing caused by particle agglomeration in the inlet zone of an EHD contact and (b) local melting due to high heat produced by the friction of debris in sliding contacts. The present predictions are in agreement with experimental evidence in two ways: firstly, in that EHD contacts may fail because of scuffing if the lubricant becomes contaminated, where the failure due to inlet blockage by debris and eventually fluid starvation, and, secondly, in that sliding asperity contacts encounter high flash temperatures which may cause local melting and thus plastic deformations.

Thermal Modeling and Effects From Debris Particles in Sliding/Rolling EHD Line Contacts—A Possible Local Scuffing Mode

The damage caused by debris particles in concentrated contacts has been studied extensively in the past, both theoretically and experimentally. Most of the theoretical studies, in which the damage on the surfaces was calculated in the form of dents, were performed isothermally. It is known that sliding asperity contacts, which resemble third body contacts, reach high local temperatures that can affect local material properties which, in turn, will affect the way damage is generated on the surfaces of machine elements. In the present work the heat transfer of lubricated, rolling/sliding line contacts in the presence of a ductile spherical particle is modeled. The particle is assumed to be significantly softer than the counterfaces that squash it. The local flash temperatures due to the combined sliding and squashing of a debris particle are calculated. It is found that high temperatures caused from small and soft particles are rather the rule than the exception.

The Effects of Particulate Contamination in Rolling Bearings—a State of the Art Review

The role played by particulate contamination in rolling contacts has formed the subject of much research in recent years. Bearing-steel quality has improved to a level where the long established sub-surface-inclusion based fatigue failures are being replaced by surface or near-surface initiated failures caused by surface defects resulting mainly from rolled-in debris particles. The paper reviews the reasons and mechanisms involved, and shows how particle material, size, concentration and hardness can all have effects on bearing reliability.

A numerical study of the contact mechanics and sub-surface stress effects experienced over a range of machined surface coatings in rough surface contacts

The paper employs a rough-surface numerical elastic contact method designed to analyze Hertzian elastic contact effects of surface coatings. In particular the paper explores the differences in the surface contact mechanics and the resulting sub-surface stresses experienced over a range of differing coating material-properties, thickness, and machined roughness levels in a quantitative manner. The effect of a range of surface roughness properties and in particular root mean square roughness (σ) and correlation length (β*), on the magnitude and depth of maximum shear stresses in the layer under individual asperities is investigated. This is done for a hard and stiff, and also for a soft and compliant coating, and for two coating thicknesses in each case. The results suggest that the magnitude of the local shear stress increases with increasing ratio σ/β* approximately linearly. The depth of the maximum local shear stress is found to correlate best with β*, however a further clear trend is observed between this depth and the number of profile peaks. The depth also shows a relation to the ratio σ/β* but the correlation in this case is weaker with significant deviations. Neither the magnitude nor the depth of shear stresses shows any significant trend in relation to the roughness (a) alone. The tensile stresses at the interface, and the subsequent potential for delamination, are also investigated and found to be significant. Approximate correlation between the magnitude of interface tensile stress and root mean square roughness is achieved, but no clear trend in relation to correlation length is evident.

Modelling and Optimization of Composite Rectangular Reciprocating Seals

Abstract A computational model of rectangular reciprocating seals, previously developed for elastomeric seals, has been extended to cover the mechanics and elastohydrodynamics (EHD) of composite seals comprising two polytetrafluoroethylene (PTFE) and one central elastomeric part, chemically bonded together. The model is demonstrated on a rotary vane actuator application and shown to produce realistic results regarding the contact pressure, film thickness, leakage, hydrodynamic friction force, and seal extrusion. Total computational time is very short, typically up to five hundredths of a second (0.05 s) on a 1.5 GHz personal computer. The main purpose of the model is the analysis and optimisation of said composite seal such that it outperforms the elastomeric seal (of exactly the same dimensions) in terms of leakage, hydro-dynamic friction, and extrusion in a wide range of operating conditions. For this goal, the PTFE-to-seal volume ratio of the composite seal is initially varied between zero and 90 per cent, and the results on leakage, hydrodynamic friction force, average film thickness in the sealing contact, and extrusion length of the composite seal are plotted and compared with those of the elastomeric seal for nominal operating conditions at three temperatures (−54, +22, and +99 °C) and for both flooded and starved lubricating conditions in order to find a first approximant to the optimum PTFE-to-seal volume ratio that gives the composite seal better overall sealing performance. Then, using the selected approximant of the optimum ratio, a parametric study is performed to compute the effects of seal interference (proportional to contact pressure), contact velocity, seal corner radius, and degree of lubricant starvation of the sealing contact on leakage, friction, and seal extrusion at the previously mentioned three temperatures for both flooded and starved contacts, thus establishing the regions of operating conditions and design parameters that benefit one seal or the other. It was found that, overall, the composite seal, despite a small (insignificant) disadvantage on leakage, significantly outperformed the elastomeric seal in terms of hydrodynamic friction and extrusion. Further optimization is possible, depending on performance priorities (for example, if leakage is valued more than friction, or wear more than leakage, etc.).

Thermoelastic Distortion of EHD Line Contacts During the Passage of Soft Debris Particles

During the passage ofa debris particle through an EHD contact, mechanical stresses due to particle compression and thermal stresses due to particle frictional heating produce a thermoelastic/plastic stress field, which governs the way a possible damage is generated. In the present paper, the complete three-dimensional solution of the thermoelastic distortion of surfaces due to the compression of a soft, ductile debris particle in an EHD line contact is presented both theoretically and through a realistic example. It is found that thermal stresses increase the likelihood of yielding and produce a characteristic omega shaped thermoelastic displacement. The important outcome of this work is the construction of a map which shows the critical particle size to cause damage (plastic deformations) in combination with operational parameters as the lubricant film thickness and relative sliding velocity of the contact.

Surface damage on rolling elements and its subsequent effects on performance and life

The influence of lubricant contamination and subsequent surface damage on rolling bearing fatigue has formed the basis of several studies over recent years. Webster et al (1) calculated the elastic subsurface stress fields from real dent profiles and used these as input to a fatigue life model to determine the reduction in lives. In this paper a slip line field analysis has been used to calculate the subsurface residual stresses for different idealised dent/roller combinations. These stress fields were superimposed upon those calculated from a dry contact analysis of the overrolling of the dent which used novel multi-level techniques to accelerate convergence, and the resultant stress fields provided input to the fatigue life model. The influence of the dent itself on life appears to be small and only becomes significant on inclusion of the residual stresses. These have a particularly marked effect as the roller radius and load are reduced, suggesting expected lives may not increase as rapidly with decreasing load as would be expected from conventional models.

Modelling and optimization of rotary vane seals

Abstract The present study deals with the formulation of the mechanics and elastohydrodynamics, and performance of composite vane seals of rectangular cross-section and goalpost shape, such as those used in rotary vane actuators in aviation, automotive, and industrial applications. After constructing the mathematical and computational model of such seals, results are obtained for an application involving a rotary vane actuator for aircraft wing control surfaces. Examples are presented for both steady-state and transient conditions (steady and unsteady motion). These are followed by an extensive parametric study to establish the effects of various parameters on sealing performance, such as the effects of the seal interferences, sealed pressure, speed, design clearances, operating temperature, and the degree of lubricant starvation. Sealing performance is assessed on the grounds of the mass leakage rate, hydrodynamic friction force and extrusion size during operation. The parametric study resulted in the construction of 144 diagrams with 432 performance curves in total. Based on this, the design and operational sealing parameters were optimized to maximize sealing performance, that is, to minimize leakage, friction, and extrusion. There was an excellent agreement between the theoretically derived optimum values and those recommended semi-empirically by the seal and actuator manufacturers as initial estimates. The parametric study showed that vane seals are objects of complex mechanical behaviour, requiring attention to details in their design and application, not only to maximize performance, but, primarily, to establish sufficient sealing and avoid premature failure that could prove very costly indeed.