George K. Nikas

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.

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.

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.

Experimental Study of Leakage and Friction of Rectangular, Elastomeric Hydraulic Seals for Reciprocating Motion from −54 to +135°C and Pressures from 3.4 to 34.5 MPa

Hydraulic seals for reciprocating motion are used in mechanisms, machines, and devices most commonly in automotive, aerospace, marine, and general industrial sectors. Applications vary from those of a cheap medical injector and tire pump to mechanisms controlling ultra-expensive equipment in power stations, ships, and space vehicles. Unfortunately, elastomeric seals are flexible solids with nonlinear response to changes in their environment involving stress or strain, heat transfer, interaction with fluids, and aging. Unsurprisingly, research into their performance is ongoing for more than 80 years. The present experimental study is a step toward a better understanding of sealing performance in a broad range of temperatures and sealed pressures. Hundreds of experiments were conducted in conformance to international standards and in controlled conditions within tight tolerances of all parameters, including mechanical properties, solid dimensions, and operating conditions. Rectangular elastomeric seals for aerospace applications were studied under sealed pressures of 3.4 to 34.5 MPa (500 to 5,000 lb/in2) and in ambient temperatures of −54 to +135°C. The combined range of pressures and temperatures exceeds what is available in the literature, particularly on the low temperature side. Other parameters varied in the experiments include the seal dimensions and radial interference, the surface roughness of the cooperating shafts, and the support of seals by one or two back-up rings. The results of the parametric study, summarized in eight tables and two figures, have been sorted for ascending leakage and friction force at each of the studied ambient temperatures for quick selection of optimal values.

Surface coatings and finite-element analysis of layered fretting contacts

Abstract A fundamental, two-dimensional finite-element analysis (FEA) of a coated flat surface in a fretting contact with a flat-rounded punch is presented with a detailed listing of all modelling steps. A large number of basic results from a parametric study has been derived, focusing on the stress analysis and the effect of coatings on the life expectancy of the coated solid, without actually getting into the ambiguities of life evaluation. The parameters included in the study are the metallic coating thickness (5, 20, and 50 μm) and elastic modulus, the endurance limit of the coated substrate, and the normal and tangential loads applied. A summary of the stress analysis results is presented in a figure with nine diagrams, which assists the quick selection of the optimum parameter values for maximizing the coating performance and minimizing the risk of fatigue within the design limits used in the study. The model is realistic in that it does not use idealized load distributions in the fretting contact but actual geometrical solids of finite dimensions in contact under the normal operating conditions, letting the FEA software resolve the loads and stresses, which are subsequently validated.

Nonlinear elasticity of rectangular elastomeric seals and its effect on elastohydrodynamic numerical analysis

Elastomeric seals used in hydraulic systems obey complex stress-strain laws at high sealed pressure and large temperature differentials. For elastohydrodynamic numerical analysis, it is important that the rubber behavior under stress is analyzed with a suitable stress-strain model. A nonlinear model has been developed here and tested to produce numerical results for seal performance. The nonlinear model is then compared with the classic Hookean model of linear elasticity, to establish the differences between the models, and the limits of applicability and validity of each for a wide range of operating conditions.

A study of lubrication mechanisms using two-phase fluids with porous bearing materials

A potential, novel lubrication mechanism involving two-phase fluids and porous bearing materials is examined in this study both theoretically and experimentally. Lubricating oils with a secondary particulate phase have been considered operating in oil-saturated porous media with the elements of the particulate phase acting as one-way valves on the pores, blocking and unblocking them, depending on the direction of contact load. The goal is to enhance the lubrication, mainly in starved contacts and in some ‘difficult’ applications, as for example in big end bearings for marine engines. The theoretical part of this work deals with the mathematical analysis of an oil-saturated, porous slab, in contact with a solid sphere and lubricated with a two-phase oil containing thin and soft platelets. The problem is analysed using Biots theory of consolidation in porous media. A subsequent study deals with the effects of the maximum contact load, size, and number of pores of the porous medium, elastic modulus of the medium, and the viscosity of the oil used to carry the particulate phase, on the lubrication performance of the system. Furthermore, a rig was built to study the concept on a macroscale and the results are reported in the paper. Both the theoretical and the experimental work gave good indications that the concept works, at least in some cases. Potential problems with application of this lubrication mechanism have also been identified and are briefly discussed in the paper.

Finite-element analysis of layered rolling contacts

A fundamental, two-dimensional finite-element analysis (FEA) of coated surfaces in rolling contact is presented with detailed listing of all modelling steps. A large number of basic results from a parametric study has been derived, focusing on the stress analysis and the effect of coatings on the life expectancy of coated surfaces, without actually getting into the ambiguities of fatigue life evaluation. Parameters included in the study are the metallic coating thickness (from zero to 50 μm) and elastic modulus (from 180 to 240 GPa), the Coulomb friction coefficient (0, 0.05, and 0.10) and the endurance limit of the coated substrate. A generalization involves the application of traction to simulate driving or driven wheels. The effect of that traction on the stress results is thoroughly examined. The set of results, summarized in a figure with nine diagrams, assists the quick selection of the optimum parameter values for maximizing coating performance and minimizing the risk of contact fatigue within the design limits used in the study. The model is realistic in that it does not use idealized load distributions in the rolling contact but actual geometrical solids in contact under normal operating conditions, letting the FEA software resolve the loads and stresses, which are subsequently validated.