Showing papers in "Journal of Tribology-transactions of The Asme in 2008"

An Investigation of Rolling Element Vibrations Caused by Local Defects

Abstract: In this paper, a shaft-bearing model is developed in order to investigate the rolling element vibrations for an angular contact ball bearing with and without defects. The shaft-bearing assembly is considered as a mass-spring system. The system shows a nonlinear characteristic under dynamic conditions. The equations of motion in radial and axial directions were obtained for shaft and rolling elements, and they were solved simultaneously with a computer simulation program. Additionally, the effect of localized defects on running surfaces (i.e., inner ring, outer ring, and ball) on the vibration of the balls is investigated. Vibration of rolling elements in the radial direction is analyzed in time and freguency domains. Characteristic defect frequencies and their components can be seen in the frequency spectra of rolling element vibrations. Comparison of the obtained results with similar studies available in literature showed reasonable qualitative agreement.

A Full-System Approach of the Elastohydrodynamic Line/Point Contact Problem

Abstract: The solution of the elastohydrodynamic lubrication (EHL) problem involves the simultaneous resolution of the hydrodynamic (Reynolds equation) and elastic problems (elastic deformation of the contacting surfaces) Up to now, most of the numerical works dealing with the modeling of the isothermal EHL problem were based on a weak coupling resolution of the Reynolds and elasticity equations (semi-system approach) The latter were solved separately using iterative schemes and a finite difference discretization Very few authors attempted to solve the problem in a fully coupled way, thus solving both equations simultaneously (full-system approach) These attempts suffered from a major drawback which is the almost full Jacobian matrix of the nonlinear system of equations This work presents a new approach for solving the fully coupled isothermal elastohydrodynamic problem using a finite element discretization of the corresponding equations The use of the finite element method allows the use of variable unstructured meshing and different types of elements within the same model which leads to a reduced size of the problem The nonlinear system of equations is solved using a Newton procedure which provides faster convergence rates Suitable stabilization techniques are used to extend the solution to the case of highly loaded contacts The complexity is the same as for classical algorithms, but an improved convergence rate, a reduced size of the problem and a sparse Jacobian matrix are obtained Thus, the computational effort, time and memory usage are considerably reduced

Bearing Signature Analysis as a Medium for Fault Detection: A Review

Abstract: Rolling element bearings find widespread domestic and industrial application. Defects in bearing unless detected in time may lead to malfunctioning of the machinery. Different methods are used for detection and diagnosis of the bearing defects. This paper is intended as a tutorial overview of bearing vibration signature analysis as a medium for fault detection. An explanation for the causes for the defects is discussed. Vibration measurement in both time domain and frequency domain is presented. Recent trends in research on the detection of the defects in bearings have been included.

A Statistical Damage Mechanics Model for Subsurface Initiated Spalling in Rolling Contacts

Abstract: Fatigue lives of rolling element bearings exhibit a wide scatter due to the statistical nature of the rolling contact fatigue failure process. Empirical life models that account for this dispersion do not provide insights into the physical mechanisms that lead to this scatter. One of the primary reasons for dispersion in lives is the stochastic nature of the bearing material. Here, a damage mechanics based fatigue model is introduced in conjunction with the idea of discrete material representation that takes the effect of material microstructure explicitly into account. Two sources of material randomness are considered: (1) the topological randomness due to geometric variability in the material micro-structure and (2) the material property randomness due to nonuniform distribution of properties throughout the material. The effect of these variations on the subsurface stress fields in rolling element line contacts is studied. The damage model, which incorporates cyclic damage accumulation and progressive degradation of material properties with rolling contact cycling, is used to study the mechanisms of subsurface initiated spalling in bearing contacts. Crack initiation as well as propagation stages are modeled using damaged material zones in a unified framework. The spalling phenomenon is found to occur through microcrack initiation below the surface where multiple microcracks coalesce and subsequent cracks propagate to the surface. The computed crack trajectories and spall profiles are found to be consistent with experimental observations. The microcrack initiation phase is found to be only a small fraction of the total spalling life and the scatter in total life is primarily governed by the scatter in the propagation phase of the cracks through the microstructure. Spalling lives are found to follow a three-parameter Weibull distribution more closely compared to the conventionally used two-parameter Weibull distribution. The Weibull slopes obtained are within experimentally observed values for bearing steels. Spalling lives are found to follow an inverse power law relationship with respect to the contact pressure with a stress-life exponent of 9.35.

Performance of Spur Gears Considering Surface Roughness and Shear Thinning Lubricant

Abstract: A model is developed for predicting the performance of spur gears with provision for surface roughness. For each point along the line of action, the contact of pinion and gear is replaced by that of two cylinders. The radii of cylinders, transmitted load, and contact stress are calculated, and lubricant film thickness is obtained using the load-sharing concept of Johnson et al. (1972, "A Simple Theory of Asperity Contact in Elastohydrodynamic Lubrication, " Wear, 19, pp. 91-108) To validate the analysis, the predicted film thickness and the friction coefficient are compared to published theoretical and experimental data. The model is capable of predicting the performance of gears with non-Newtonian lubricants-such as that of shear thinning lubricants-often used in gears. For this purpose, a correction factor for shear thinning film thickness introduced by Bair (2005, "Shear Thinning Correction for Rolling/Sliding Electrohydrodynamic Film Thickness, " Proc. Inst. Mech. Eng., Part J: J. Eng. Tribal., 219, pp. 1-6) has been employed. The results of a series of simulations presenting the effect of surface roughness on the friction coefficient are presented and discussed. The results help to establish the lubrication regime along the line of action of spur gears.

Pressure Buildup Mechanism in a Textured Inlet of a Hydrodynamic Contact

Abstract: A flow analysis is carried out for an inclined slider bearing with the aim of showing the governing mechanism at conditions where an optimum in load carrying capacity is achieved. The effects of surface texture on pressure buildup and load carrying capacity are explained for a textured slider bearing geometry. Numerical simulations are performed for laminar, steady, and isothermal flows. The energy transferred to the fluid from the moving wall is converted into pressure in the initial part of the converging contact and into losses in the second part. The convergence ratio can be increased, in order to get the greatest pressure gradient, until the limiting value where flow recirculation begins to occur. The texture appears to achieve its maximum efficiency when its depth is such that the velocity profile is stretched at its maximum extent without incurring incoming recirculating flow. The wall profile shape controlling the velocity profile can be optimized for many hydrodynamic contacts.

Thermomechanical Analysis of Elastoplastic Bodies in a Sliding Spherical Contact and the Effects of Sliding Speed, Heat Partition, and Thermal Softening

Abstract: A thermomechanical analysis of elasto-plastic bodies is a necessary step toward the understanding of tribological behaviors of machine components subjected to both mechanical loading and frictional heating. A three-dimensional thermoelastoplastic contact model for counterformal bodies has been developed, which takes into account steady state heat flux, temperature-dependent strain hardening behavior, and interaction of mechanical and thermal loads. The fast Fourier transform and conjugate gradient. method are the underlying numerical algorithms used in this model. Sliding of a half-space over a stationary sphere is simulated with this model. The friction-induced heat is partitioned into two bodies based on surface temperature distributions. In the simulation, the sphere is considered to be fully thermoelastoplastic, while the half-space is treated to be thermoelastic. Simulation results include surface pressure, temperature rise, and subsurface stress and plastic strain fields. The paper also studies the influences of sliding speed and thermal softening on contact behaviors for sliding speed ranging three orders of magnitude.

CFD Modeling of a Thermal and Shear-Thinning Elastohydrodynamic Line Contact

Abstract: In this paper a computational fluid dynamics (CFD) approach for solving elastohydrodynamic lubrication using the freely available package OPENFOAM is introduced. The full Navier―Stokes equations are solved, which enables the entire flow domain to be modeled and all gradients inside the lubricated contact to be resolved. The phenomenon of cavitation is taken into account by employing a homogenous equilibrium cavitation model, which maintains a specified cavitation pressure inside the cavitating region. The energy equation used considers the effects of heat conduction and convection, viscous heating, and the heat of evaporation. The developed method has been applied to a series of cases of lubricated metal-on-metal line contact with an entrainment velocity of u ent = 2.5 m/s, viscosities η 0 = [0.01, 1] Pa s, and slide-to-roll ratios SRR =[0, 1, 2] under both thermal and isothermal conditions. The isothermal results are compared to the Reynolds theory and most results agree very well. Only the high-viscosity pure rolling case shows small differences. The combined effects of temperature, pressure, and shear-thinning are studied for the thermal cases. A temperature-induced shear band occurs in the case of sliding combined with very large viscosity compared to the isothermal case, which results in significant pressure variations across the thickness of the film. The impact of temperature on the friction force is discussed, showing differences of up to ―88.5% compared to the isothermal case. The developed method is capable of giving new insights into the physics of elastohydrodynamic lubrication, especially in cases where the usual assumptions of the Reynolds theory break down.

A Model for Contact and Static Friction of Nominally Flat Rough Surfaces Under Full Stick Contact Condition

Abstract: A model for elastic-plastic nominally flat contacting rough surfaces under combined normal and tangential loading with full stick contact condition is presented. The model incorporates an accurate finite element analysis for contact and sliding inception of a single elastic-plastic asperity in a statistical representation of surface roughness. It includes the effect of junction growth and treats the sliding inception as a failure mechanism, which is characterized by loss of tangential stiffness. A comparison between the present model and a previously published friction model shows that the latter severely underestimates the maximum friction force by up to three orders of magnitude. Strong effects of the normal load, nominal contact area, mechanical properties, and surface roughness on the static friction coefficient are found, in breach of the classical laws of friction. Empirical equations for the maximum friction force, static friction coefficient, real contact area due to the normal load alone and at sliding inception as functions of the normal load, material properties, and surface roughness are presented and compared with some limited available experimental results.

A New Approach to Modeling Surface Defects in Bearing Dynamics Simulations

Abstract: A dynamic model for deep groove and angular contact ball bearings was developed to investigate the influence of race defects on the motions of bearing components (i.e., inner and outer races, cage, and balls). In order to determine the effects of dents on the bearing dynamics, a model was developed to determine the force-deflection relationship between an ellipsoid and a dented semi-infinite domain. The force-deflection relationship for dented surfaces was then incorporated in the bearing dynamic model by replacing the well-known Hertzian force-deflection relationship whenever a ball/dent interaction occurs. In this investigation, all bearing components have six degrees-of-freedom. Newton's laws are used to determine the motions of all bearing elements, and an explicit fourth-order Runge―Kutta algorithm with a variable or constant step size was used to integrate the equations of motion. A model was used to study the effect of dent size, dent location, and inner race speed on bearing components. The results indicate that surface defects and irregularities like dent have a severe effect on bearing motion and forces. Furthermore, these effects are even more severe for high-speed applications. The results also demonstrate that a single dent can affect the forces and motion throughout the entire bearing and on all bearing components. However, the location of the dent dictates the magnitude of its influence on each bearing component.

Fully Coupled Frictional Contact Using Elastic Halfspace Theory

Abstract: The effect of dry metallic friction can be attributed to two major mechanisms: adhesion and ploughing. While ploughing is related to severe wear and degradation, adhesion can be connected to pure elastic deformations of the contacting bodies and is thus the predominant mechanism in a stable friction pair. The transmitted friction force is then proportional to the real area of contact. Therefore, a lot of effort has been put into the determination of the fraction of real area of contact under a given load. A broad spectrum of analytical and numerical models has been employed. However, it is quite common to employ the so-called Mindlin assumptions, where the contact area is determined by the normal load only, disregarding the influence of friction. In the subsequent tangential loading, usually the contact pressure distribution is kept fixed such that the coupling between the tangential and normal solutions is neglected. Here, a numerical solution scheme based on elastic halfspace theory for frictional contact problems is presented where full coupling between the normal and tangential tractions and displacements is taken into account. Several examples show the influence of the coupling effects, but also the limitations for the analysis of rough contacts.

A Finite Element Based Study on the Elastic-Plastic Transition Behavior in a Hemisphere in Contact With a Rigid Flat

Abstract: An axisymmetrical hemispherical asperity in contact with a rigid flat is modeled for an elastic peifectly plastic material. The present analysis extends the work (sphere in contact with a flat plate) of Kogut―Etsion Model and Jackson―Green Model and addresses some aspects uncovered in the above models. This paper shows the critical values in the dimensionless interference ratios (ω/ω c ) for the evolution of the elastic core and the plastic region within the asperity for different Y/E ratios. The present analysis also covers higher interference ratios, and the results are applied to show the difference in the calculation of real contact area for the entire surface with other existing models. The statistical model developed to calculate the real contact area and the contact load for the entire surfaces based on the finite element method (FEM) single asperity model with the elastic perfectly plastic assumption depends on the Y/E ratio of the material.

Optimization of Groove Geometry for Thrust Air Bearing to Maximize Bearing Stiffness

Abstract: Hydrodynamic gas film bearings are widely used for very-high-speed, lightly loaded rotating machinery. In the design of hydrodynamic gas film bearings, it is of cardinal importance to enhance the stiffness of gas films to minimize vibration due to external excitations. Among various types of hydrodynamic gas film thrust bearings, grooved bearings have an advantage of high stiffness and load-carrying capacity, but the stiffness of the bearings strongly depends on groove geometry. Therefore, when the groove geometry is suitably designed, it is expected to considerably improve the stability characteristics of the bearings. However, conventional bearing geometries are based on a fixed logarithmic spiral curve, and there is no literature on how to effectively change the groove geometry to drastically improve the bearing characteristics. In this paper, the entirely new optimum design methodology, in which the groove geometry can be flexibly changed by using the spline function, is presented to maximize the stiffness of gas films for grooved thrust bearings. The effectiveness of the methodology is experimentally verified.

Experimental Study of Adhesive Static Friction in a Spherical Elastic-Plastic Contact

Abstract: The elastic-plastic contact between a deformable sphere and a rigid flat during presliding is studied experimentally. Measurements of friction force and contact area are done in real time along with an accurate identification of the instant of sliding inception. The static friction force and relative tangential displacement are investigated over a wide range of normal preloads for several sphere materials and diameters. Different behavior of the static friction is observed in the elastic and in the elastic-plastic regimes of sphere deformation. It is found that at low normal loads, the static friction coefficient depends on the normal load in breach of the classical laws of friction. The presliding displacement is found to be less than 5% of the contact diameter, and the interface mean shear stress at sliding inception is found to be slightly below the shear strength of the sphere material. Good correlation is found between the present experimental results and a recent theoretical model in the elastic-plastic regime of deformation.

Carbon-Fiber Reinforced Paper-Based Friction Material: Study on Friction Stability as a Function of Operating Variables

Abstract: Carbon-fiber-reinforced paper-based friction material (CFRPF), as a new type of wet friction material for automatic transmission, was prepared by a paper-making process. The frictional response of CFRPF is highly complex under a set of dynamically variable operating conditions. To better understand the effect of operating factors (braking pressure, rotating speed, oil temperature, and oil flow rate) on friction stability of the material, tests were carried out using a single ingredient experiment and the Taguchi method. Experimental results show that the braking stability and the dynamic friction coefficient (μ d ) decrease as braking pressure, rotating speed, oil temperature, and oil flow rate increase. The influence of braking pressure on μ d is largest among the four operating factors. μ d declines gradually during the first 3000 repeated braking cycles and changes very little subsequently due to the surface topography change in friction material.

A Numerical Model for Life Scatter in Rolling Element Bearings

Abstract: Fatigue lives of rolling element bearings exhibit a wide scatter due to the statistical nature of the mechanisms responsible for the bearing failure process. Life models that account for this dispersion are empirical in nature and do not provide insights into the physical mechanisms that lead to this scatter. One of the primary reasons for dispersion in lives is the inhomogeneous nature of the bearing material. Here, a new approach based on a discrete material representation is presented that simulates this inherent material randomness. In this investigation, two levels of randomness are considered: (1) the topological randomness due to geometric variability in the material microstructure and (2) the material property randomness due to nonuniform distribution of properties throughout the material. The effect of these variations on the subsurface stress field in Hertzian line contacts is studied. Fatigue life is formulated as a function of a critical stress quantity and its corresponding depth, following a similar approach to the Lundberg-Palmgren theory. However, instead of explicitly assuming a Weibull distribution of fatigue lives, the life distribution is obtained as an outcome of numerical simulations. A new critical stress quantity is introduced that considers shear stress acting along internal material planes of weakness. It is found that there is a scatter in the magnitude as well as depth of occurrence of this critical stress quantity, which leads to a scatter in computed fatigue lives. Further the range of depths within which the critical stress quantity occurs is found to be consistent with experimental observations of fatigue cracks. The life distributions obtained from the numerical simulations are found to follow a two-parameter Weibull distribution closely. The L 10 life and the Weibull slope decrease when a nonuniform distribution of elastic modulus is assumed throughout the material. The introduction of internal flaws in the material significantly reduces the L 10 life and the Weibull slope. However, it is found that the Weibull slope reaches a limiting value beyond a certain concentration of flaws. This limiting value is close to that predicted by the Lundberg-Palmgren theory. Weibull slopes obtained through the numerical simulations ranee from 1.29 to 3.36 and are within experimentally observed values for bearing steels.

Free Surface Thin Layer Flow on Bearing Raceways

Abstract: In this paper, the effects of rotation on the distribution of a layer of oil on a bearing raceway are analyzed in relation to the geometry of the raceway. The research is motivated by the need to understand the behavior of grease lubricated bearings. Some specific aspects of grease lubrication can be understood by approximating the contact as a starved lubricated elastohydrodynamic lubrication contact. In such a contact, the shape and thickness of the inlet layer of oil, supplied to the contact on the running track, are of crucial importance to the film formation and contact performance. Small changes in the distribution of lubricant on the rolling track, as a result of reflow or redistribution, may have a large (local) effect on the film thickness inside the contact. Starting from the Navier–Stokes equations, the free surface thin layer flow equation for axisymmetric rotating surfaces is derived. For the case of bearing applications, it is shown that a simple quasilinear equation can be derived for the layer thickness, as a function of location and time, which can be solved analytically. Experiments have been carried out, measuring the changes of a layer of oil on rotating raceways in time in relation to the rotational speed and the raceway geometry. It is shown that the simplified model accurately predicts the thin layer flow, except in a region near the outflow boundary, where the effect of the boundary condition on the layer shape is crucial. This is further illustrated by results of numerical simulations.

Combined Effects of Shear Thinning and Viscous Heating on EHL Characteristics of Rolling/Sliding Line Contacts

Abstract: The combined influence of shear thinning and viscous heating on the behavior of film thickness and friction in elastohydrodynamic lubrication (EHL) rolling/sliding line contacts is investigated numerically. The constitutive equation put forward by Carreau is incorporated into the model to describe shear thinning. An extensive set of numerical simulations is presented. Comparison of the film thickness predictions with published experiments reveals good agreement, and it is shown that thermal effect plays an important role in the precise estimation of EHL film thickness and friction coefficient. Parametric simulations show that thermal effect in shear-thinning fluids is strongly affected by the power-law index used in the Carreau equation. Comparisons of prediction of the Newtonian fluid model are presented to quantify the degree to which it overestimates the film thickness.

Numerical Modeling of Mixed Lubrication and Flash Temperature in EHL Elliptical Contacts

Abstract: Highly loaded ball and rolling element bearings are often required to operate in the mixed elastohydrodynamic lubrication regime in which surface asperity contact occurs simultaneously during the lubrication process. Predicting performance (i.e., pressure, temperature) of components operating in this regime is important as the high asperity contact pressures can significantly reduce the fatigue life of the interacting components. In this study, a deterministic mixed lubrication model was developed to determine the pressure and temperature of mixed lubricated circular and elliptic contacts for measured and simulated surfaces operating under pure rolling and rolling/sliding condition. In this model, we simultaneously solve for lubricant and asperity contact pressures. The model allows investigation of the condition and transition from boundary to full-film lubrication. The variation of contact area and load ratios is examined for various velocities and slide-to-roll ratios. The mixed lubricated model is also used to predict the transient flash temperatures occurring in contacts due to asperity contact interactions and friction. In order to significantly reduce the computational efforts associated with surface deformation and temperature calculation, the fast Fourier transform algorithm is implemented.

A Pin-on-Disk Experimental Study on a Green Particulate-Fluid Lubricant

Abstract: The present investigation analyzes a green, petroleum-free lubricant that is produced by mixing two environmentally benign components—canola oil and boric acid powder. To study the influence of boric acid particle size and solid volume fraction on the proposed lubricant performance, pin-on-disk experiments were conducted with spherical copper pins (radius 6.5 mm) and aluminum disks (Ra = 1.35 μm). Friction coefficient measurements were taken at more than 20 distinct operating conditions while varying the lubrication condition (unlubricated, boric acid, canola oil, boric acid/canola oil mixture), boric acid volume fraction, and boric acid particle size. Based on the experiments, it was determined that a solid volume fraction of 7% with 350―700 μm particles was the optimum green particulate lubricant candidate for minimizing the friction at the conditions tested. This work also uncovered an inverse relationship between the friction coefficient and boric acid particle size (in canola oil at 7% solidfraction). Micrographs of the pin and disk wear track were analyzed to study this frictional behavior of the interface materials. Additionally, rheological tests were conducted to measure the viscosity of the canola oil and boric acid powder mixture as a function of particle size, and it was found that the viscosity increased with particle size over the size range tested. Finally, the results indicated that the boric acid-canola oil lubricant mixture demonstrated excellent potential for use as lubricants in industrial applications such as sheet metal forming.

The Static Performance Analysis of Foil Journal Bearings Considering Three-Dimensional Shape of the Foil Structure

Abstract: To obtain the foil bearing characteristics, the fluid film pressure must be coupled with the elastic deformation of the foil structure. However, all of the structural models thus far have simplified the foil structure without consideration of its three-dimensional shape. In this study, a finite element foil structural model is proposed that takes into consideration the three-dimensional foil shape. Using the proposed model, the deflections of interconnected bumps are compared to those of separated bumps, and the minimum film thickness determined from the proposed structural models is compared to those of previous models. In addition, the effects of the top foil and bump foil thickness on the foil bearing static performance are evaluated. The results of the study show that the three-dimensional shape of the foil structure should be considered for accurate predictions of foil bearing performances and that too thin top foil or bump foil thickness may lead to a significant decrease in the load capacity. In addition, the foil stiffness variation does not increase the load capacity much under a simple foil structure.

Thermal Study of the Dry Sliding Contact With Third Body Presence

Abstract: The objective of this paper is to highlight the influence of the rheology of a third body in dry sliding contact conditions. It has been shown that the local cohesion of the third body can create an asymmetric dissipative field through its thickness. The present study puts forward the consequences from a thermal point of view, overcoming the inherent experimental difficulties at this microscopic scale.Copyright © 2008 by ASME

EHL Circular Contact Film Thickness Correction Factor for Shear-Thinning Fluids

Abstract: An extensive set of full elastohydrodynamic lubrication point contact simulations ha been used to develop correction factors to account for the effect of shear-thinning lubri cant behavior on the central and minimum film thickness in circular contacts under pun rolling condition. T h e film thickness for a shear-thinning lubricant can be easily obtained by dividing the corresponding Newtonian film thickness by the appropriate correction factor. Comparisons of the film thickness values obtained using the correction factor have been matched with the published experimental results pertaining to shear-thinning lubricants with a variety of realistic flow and piezoviscous properties under a wide range of operating speed. The good agreement between them establishes the validity and ver satility of the correction factors developed in this paper.

Tribochemical Interactions of Friction Modifier and Antiwear Additives With CrN Coating Under Boundary Lubrication Conditions

Abstract: In recent years, the optimized use of low friction nonferrous coatings under boundary lubrication conditions has become a challenge to meet the demands of improved fuel economy in automotive applications. This study presents the tribological performance of chromium nitride (CrN) coating using conventional friction modifier (moly dimer) and/or antiwear additive (zinc dialkyl dithiophosphate (ZDDP)) containing lubricants in a pin-on-plate tribometer. Using surface analysis techniques such as the atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS), both topographical and chemical analyses of tribofilms were performed. This paper shows that ZDDP and moly dimer both give a positive effect for both low friction and antiwear performance in CrN/cast iron system. Both AFM and XPS analyses give evidence of the formation of ZDDP and moly dimer derived tribofilms on the CrN coating and thus support friction and wear results.

Numerical Model of a Tandem Reciprocating Hydraulic Rod Seal

Abstract: A numerical model of a tandem reciprocating hydraulic rod seal, consisting of two elastomeric U cup seals, has been constructed. It is applicable to cases in which the stroke length is significantly larger than the seal width. The model consists of coupled steady state fluid mechanics, deformation mechanics, and contact mechanics analyses, with an iterative computational procedure. The behaviors of the two seals are coupled through the pressure/density in the interseal region and through flow continuity. Results for a typical tandem seal are compared to those of a single seal and a double lip seal.

Transient Elastohydrodynamic Lubrication of Hip Joint Implants

Abstract: A general transient elastohydrodynamic lubrication model was developed for artificial hip joint implants, particularly in which the three-dimensional time-dependent physiological load and motion components experienced during walking conditions were considered in the theoretical formulation, although only a predominantly vertical load combined with a flexion-extension motion was actually solved. A nominal ball-in-socket configuration was adopted to represent the articulation between the femoral head and the acetabular cup in both simplified and anatomical positions. An appropriate spherical coordinate system and the corresponding mesh grids were used in the general transient lubrication model. Additionally, an equivalent discrete spherical convolution model and the corresponding spherical fast Fourier transform technique were employed to facilitate the evaluation of elastic deformation of spherical bearing surfaces in hip joint implants. The general lubrication model was subsequently applied to investigate the transient lubrication performance of a typical metal-on-metal hip joint implant. The effects of both cup inclination angles in either anatomical or horizontally simplified positions and different lubricant viscosities on the transient elastohydrodynamic lubrication were analyzed under the predominant components of vertically dynamic loading and flexion-extension motion. It was found that the general lubrication model and the numerical methodology were efficient for the transient elastohydrodynamic lubrication analysis during walking condition in hip joint implants. Furthermore, the significant effect of squeeze-film action on maintaining and enhancing the total thin film thickness formation was discussed for the transient lubrication study of the typical hip joint implant.

A Scaling Parameter and Function for the Accurate Correlation of Viscosity With Temperature and Pressure Across Eight Orders of Magnitude of Viscosity

Abstract: Quantitative calculations of film thickness and friction in elastohydrodynamic lubrication will require that the low-shear viscosity, μ, be described with far greater accuracy than it is today The free volume model has the advantage, over those currently used, of reproducing all of the trends that were known 80 years ago, although not necessarily to experimental accuracy A scaling parameter, ϕ∝TV γ based on the repulsive intermolecular potential having exponent ―3γ allows the viscosity to be written as a function of temperature, T, and volume, V, only, as μ = F(ϕ) The appropriate function for lubricants appears to be a Vogel-like form, μ∝ exp(B F ϕ ∞ /(ϕ ― ϕ ∞ )) Parameters are presented here for seven liquids When the dynamic crossover is present, two such functions are required A low molecular weight dimethyl silicone having high compressibility is an exception

Analyses of the Long-Term Performance and Tribological Behavior of an Axial Piston Pump Using Diamondlike-Carbon-Coated Piston Shoes and Biodegradable Oil

Abstract: This work reports on the performance enhancement of a real-scale hydraulic system consisting of diamondlike-carbon (DLC)-coated components in combination with biodegradable oil in long-term experiments under conditions simulating those in an actual application. The performance of a hydraulic axial piston pump with DLC-coated piston shoes was evaluated in a newly designed, dedicated hydraulic test system using fully formulated biodegradable, synthetic ester oil. For comparison, an equal but separated hydraulic system with a conventional commercial pump and stainless-steel shoe surfaces was tested. The tests were run at 85% of the maximum pump load and an oil temperature of around 80°C for a period of 2000 h, which corresponds to more than 1 yr of continuous 8 h/day operation in an application. A major abrupt oxidation-induced degradation of the oil did not occur in either system; however, the oil from the system comprising the DLC-coated shoes showed noticeably and consistently better results. The wear of the DLC-coated shoes, especially during the running in, was much lower than that in the conventional steel system. Only minor polishing wear was observed on the DLC shoe's sliding surfaces during the test period, while on the steel shoe's surfaces, many scratches were found and some erosion of the edges was detected.

Investigation of Tribological Behavior of Al–Si Alloy Against Steel Lubricated With Ionic Liquids of 1-Diethylphosphonyl-n-propyl-3- Alkylimidazolium Tetrafluoroborate

Abstract: A series of room temperature ionic liquids bearing with phosphonyl groups on the imidazolium cations, namely, 1-(3'-O,O-diethylphosphonyl-n-propyl)-3-alkylimidazolium tetrafluoroborate, were prepared and their physical properties were determined. They were also evaluated as promising lubricants for the contacts of aluminum on steel by using a SRV test rig. The tribological test results show that the synthetic ionic liquids exhibit better friction-reducing and antiwear abilities than the unsubstituted ionic liquid of 1-ethyl-3-hexylimidazolium tetrafluoroborate (coded as L206) and phosphazene (X-]P). Both the anions and the side substitutes attached to the imidazolium cations affect the tribological performance of lubricants. The scanning electron microscopy, energy-dispersive x-ray analysis, and x-ray photoelectron spectroscopy analyses of the worn surfaces show that complicated tribochemical reactions are involved in the sliding process. The anion decomposition and chemical adsorption of cation took place on the worn surface of aluminum alloy during the sliding process. As a result of the generation of boundary lubrication films which are composed of metal fluorides, B 2 O 3 , BN, nitrogen oxide, and FePO 4 help to effectively reduce the friction and wear of the contacts.