Showing papers in "Tribology Transactions in 1996"

A Model for Mechanical Seals with Regular Microsurface Structure

Abstract: A mathematical model is developed to allow performance prediction of all-liquid noncontacting mechanical seals with regular microsurface structure in the form of hemispherical pores. Seal performance such as the equilibrium face separation, friction torque and leakage across the seal are calculated and presented for a range of sealed pressure, pore size and pore ratio of ring surface area. An optimum pore size is found that depends on the other variables and corresponds to maximum axial stiffness and minimum friction torque. Also, a critical pore size is found above which seal failure is possible. Presented as a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology Conference in Kisslmmee, Florida, October 8–11, 1995

The Development of a Spacer Layer Imaging Method (SLIM) for Mapping Elastohydrodynamic Contacts

Abstract: Optical interferometry has proved to be a valuable experimental tool in the study of elastohydrodynamic lubrication (EHD). It is a technique that gives detailed information on the lubricant film distribution within the contact; however, the sensitivity is limited and it is only recently, with the development of the spacer layer optical technique, that the study of the thin film lubrication regime has been possible. The limitation of the spacer layer technique is that generally only one measurement is taken from the center of the contact. The next logical step in the development of this technique is, therefore, a system that combines the mapping capabilities of the original optical method with the thin film capabilities of the spacer layer approach. This paper describes the development of a contact mapping technique that uses the spacer layer approach to visualize, and measure, thin lubricant films in concentrated contacts. The development of the technique is described and its application to both static an...

Boundary Film Formation by Viscosity Index Improvers

Abstract: Ultrathin film interferometry has been used to measure the film-forming properties of a range of viscosity index improver (VII) solutions in rolling, concentrated contacts. It has been shown that some VIIs form boundary lubricating films of thickness 10 to 30 nm in contacts. These films result from the presence of highly concentrated and thus very viscous layers of polymer solution formed on the two rubbing solid surfaces by polymer adsorption. These boundary films are formed only by some types of VII and can persist up to temperatures in excess of 120°C. The possible implications of this type of boundary lubrication are discussed. Presented as a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology Conference in Kissimmee, Florida, October 8–11, 1995

Impact of Plateaued Surfaces on Tribological Performance

Abstract: Plateaued surfaces are surfaces that have been machined to simulate those that result from normal running in and are said to have advantages over conventional or non-plateaued surfaces. However, the evidence is lacking. This study evaluates the tribological performance of plateaued and non-plateaued surfaces on a pin-on-disk tribometer. The honing pattern of an engine cylinder bore was simulated on the disks. These disks have similar average surface heights with either plateaued or non-plateaued surface finish. Friction, wear and scuffing resistance of plateaued and non-plateaued disks were evaluated. Results from the pin-on-disk tribometer show that in the hydrodynamic lubrication regime plateaued and non-plateaued disks have the same friction, while in the mixed lubrication regime the plateaued surface has less friction. The author's findings also reveal that plateaued surfaces tend to have higher wear resistance but lower scuffing resistance. It also confirms the conventional wisdom that plateaued surf...

Contact Analysis of Non-Gaussian Surfaces for Minimum Static and Kinetic Friction and Wear

Abstract: Most statistical contact analyses assume that surface heights and peak (summit) height distributions follow a Gaussian distribution. However, engineering surfaces are frequently non-Gaussian with a degree of non-Gaussian character dependent upon materials and surface finishing processes used. For example, magnetic rigid disk surfaces used in magnetic storage industry are highly non Gaussian. The use of a Gaussian analysis in such cases can lead to erroneous results. This study for the first time presents a method to carry out a statistical analysis of non-Gaussian surfaces. Real area of contact, number of contacts, contact pressure and meniscus force (in wet interfaces) are calculated for probability density functions having different skewness and kurtosis. From these curves, the optimum value of skewness and kurtosis can be predicted for minimum static/kinetic friction. It is found that a range of positive skewness (between 0.3–0.7) and a high kurtosis (greater than five) significantly lower the real are...

Behavior of Elastic-Plastic Rough Surface Contacts as Affected by Surface Topography, Load, and Material Hardness

Abstract: Surface roughness plays an important role in affecting friction, wear, and lubrication of contacting bodies. A small change in the distribution of the asperity heights and widths can have a significant effect on the performance variables which include the real area of contact, the average gap, and the average asperity contact pressure. A series of contact simulations were conducted in order to investigate the effects of the surface topography, material hardness and load on the deformation behavior of the rough surfaces. These surfaces were numerically generated by computer and varied widely in statistical roughness properties, ranging from isotropic to strongly anisotropic. The current simulations took into account the elastic-plastic deformation behavior of the asperities. Using the contact simulation results, the performance variables were curve-fitted to convenient analytical formulas as functions of the surface roughness parameters, material hardness, and load. Presented at the 49th Annual Meeting in ...

The Formation of Viscous Surface Films by Polymer Solutions: Boundary or Elastohydrodynamic Lubrication?

Abstract: The film-forming properties of a range of polymer solutions have been studied down to very thin film thickness using ultrathin film interferometry. It has been found that, at very slow rolling speeds, some polymers generate much thicker films than predicted from theory. It appears that these polymers form adsorbed layers between three and 15 nanometers thick on the two solid surfaces. These layers have a viscosity many times higher than that of the bulk solution. Therefore, under slow speed, low film thickness conditions, the contact effectively operates within a viscous boundary layer, generating an elastohydrodynamic-type film much thicker than predicted from the viscosity of the bulk lubricant. As the speed is raised the contact emerges from this boundary layer and reverts to elastohydrodynamic behavior based upon the viscosity of the bulk polymer solution. Presented at the 50th Annual Meeting in Chicago, Illinois May 14–19, 1995

Boundary Film Formation by Lubricant Base Fluids

Abstract: The measurement of lubricant film thicknesses in a rolling steel ball on glass flat contact, down to one nanometer is now possible by using ultrathin film interferometry. This technique has been used to study the film-forming properties of a range of synthetic base fluids in the mixed elastohydrodynamic and boundary lubrication regimes. For a very highly purified sample of hexadecane, it was found that classical EHD theory was obeyed down to less than one nanometer, indicating that any boundary film formed by this fluid was less than I nm thick. Most other synthetic fluids tested gave thicker films than predicted from EHD theory under very thin film conditions. This deviation from theory occurred at between 1 and 10 nm for different fluids. The effect can be interpreted as resulting from the presence of boundary layers, one or two molecular layers thick of the fluid on each solid surface, which are more viscous than the bulk lubricant. Presented at the 50th Annual Meeting in Chicago, Illinois May 14–19, 1995

Effect of Roughness on the Behavior of Squeeze Film in a Spherical Bearing

Abstract: The behavior of a hydrodynamic squeeze film between a non-rotating spherical surface and a hemispherical bearing under a steady load is analyzed. It is assumed that the bearing surface as well as the surface of the approaching sphere have a longitudinal random roughness which is distributed throughout the surfaces. The stochastic film thickness characterizing the roughness is assumed to be symmetric and its mean value is equal to zero. The modified Reynolds equation is solved and the expressions for pressure, load-carrying capacity and the response lime are obtained. The results are presented graphically. It is observed that the performance of the bearing is adversely affected by the composite roughness of the surfaces.

Starvation and Reflow in a Grease-Lubricated Elastohydrodynamic Contact

Abstract: The lubrication mechanisms of a grease in a rolling-element bearing has been studied through the measurement of film thickness in a rolling point contact. To simulate bearing conditions the contact runs under fully starved conditions; there is no attempt to maintain bulk flow of the grease into the inlet using an external supply. In consequence the film thickness drops off rapidly as the contact progressively starves. After a few minutes rolling (at constant speed) an equilibrium film thickness is attained which has two components: a residual film (hR) comprised of degraded grease thickener and a hydrodynamic component (hEHD) due to the liquid phase from the grease. The hydrodynamic contribution represents a balance between lubricant lost from the contact and replenishment from the grease close to the track. The ability of the grease to replenish the rolling track has been inferred from measurements of lubricant reflow around the static contact. These results are discussed in light of current starvation a...

Effects of Cu Powder, BaSO4 and Cashew Dust on the Wear and Friction Characteristics of Automotive Brake Pads

Abstract: Brake pads used in automotive disk brakes are generally made of many components such as phenolic resin, aramid fiber, Cu powder, BaSO4, and cashew dust. Using a slider-on-disk-type wear tester, the friction and wear characteristics of the pad against the disk made of cast iron at high disk temperature (above 80°C) were studied with varying the content of the components. In particular, the relation between the sudden decrease in friction (fade) and the wear rate were closely studied. The experimental, result of the wear test was also analyzed, systematically by the multiple regression method and the effects of the components were clarified quantitatively. Presented at the 50th Annual Meeting in Chicago, Illinois May 14–19, 1995

Comparison of Life Theories for Rolling-Element Bearings

Abstract: Nearly five decades have passed since G. Lundberg and A. Palmgren published their life theory in 1947 and 1952 and it was adopted as an ANSI/ABMA and ISO standard in 1950 and 1953. Subsequently, many variations and deviations from their life theory have been proposed, the most recent being that of E. Ioannides and T.A. Harris in 1985. This paper presents a critical analysis comparing the results of different life theories and discussing their implications in the design and analysis of rolling-element bearings. Variations in the stress-life relation and in the critical stress related to bearing life are discussed using stress fields obtained from three-dimensional, finite-element analysis of a ball in a nonconforming race under varying load. The results showed that for a ninth power stress-life exponent the Lundberg-Palmgren theory best predicts life as exhibited by most air-melted bearing steels. For a 12th power relation reflected by modern bearing steels, a Zaretsky-modified Weibull equation is superior. The assumption of a fatigue-limiting stress distorts the stress-life exponent and overpredicts life.

Tribological Performance of Diamond and Diamondlike Carbon Films at Elevated Temperatures

Abstract: In this study, the authors investigated the tribological performance of diamond and diamondlike carbon (DLC) films as a function of temperature. Both films were deposited on silicon carbide (SiC) by microwave plasma chemical vapor deposition and ion-beam deposition processes. Tribological tests were performed on a reciprocating wear machine in open air (20 to 30% relative humidity) and under a 10 N load using SiC pins. For the test conditions explored, the steady-state friction coefficients of test pairs without a diamond or DLC film were 0.7 to 0.9 and the average wear rates of pins were 10−5 to 10−7 mm3/N·m, depending on ambient temperature. DLC films reduced the steady-slate friction coefficients of the test pairs by factors of three to five and the wear rates of pins by two to three orders of magnitude. Low friction coefficients were also obtained with the diamond films, but wear rates of the counterface pins were high due to the very abrasive nature of these films. The wear of SiC disks coated with e...

Infrared and visual study of the mechanisms of scuffing

Abstract: The development of a reliable model for predicting scuffing requires an understanding of the mechanism of scuffing initiation. This study examines the process of scuffing directly within a contact and thus tests some of the existing, proposed mechanisms. A lubricated steel ball is loaded and rotated against a sapphire flat and the load increased in stages until scuffing occurs. Two methods of observation are employed. In one, the temperature of the steel ball across the contact is mapped continuously using an infrared microscope. A novel, nodding mirror set-up enables temperature profiles to be taken many times a second. This work shows that scuffing does not occur at either a critical maximum or at a critical inlet temperature. In the second approach, a solid-slate TV camera and video recorder are used to monitor the contact visually up to and during scuffing. This work suggests that, for a range of different lubricants, the onset of scuffing is always immediately preceded by the buildup of fine wear deb...

Description of abrasive particle shape and its relation to two-body abrasive wear

Abstract: Recent developments in the characterization of particle abrasivity are described in this paper. A new numerical particle abrasivity descriptor based on particle angularity has been developed and compared with a descriptor developed previously. The relationships of these descriptors and the reciprocated shape factor to two-body abrasive wear together with the effects of particle size, load and speed is investigated. Seven types of mineral abrasives were used in the tests. Two-body abrasive wear tests were conducted on specially manufactured disks from all types of mineral abrasives used in the tests. All the abrasives tested were characterized by a newly and previously developed abrasivity parameter and by the commonly used shape descriptor, i.e., reciprocated shape factor. In order to isolate the effects of particle shape on its abrasivity, the experiments were conducted with all operating variables except particle shape held at the same level. It was found that abrasive wear rates were linearly proportio...

Lubrication of Strip Rolling in the Low-Speed Mixed Regime

Abstract: An analytical model for strip rolling in the low-speed mixed lubrication regime is developed. An average Reynolds equation for longitudinal saw-tooth surfaces under conditions of high fractional contact area, is combined with an analysis for asperity flattening under conditions of bulk plastic flow, to treat lubrication in the mixed regime. Analyses for the inlet zone and work zone and the influence of pressure on viscosity are included in the model. The model indicates that hydrodynamic lubrication effects are important at much lower speeds than previously considered possible. The film thickness predicted by the model is somewhat smaller than that measured using the oil drop method in rolling aluminum alloy with a mineral oil. Presented at the 50th Annual Meeting in Chicago, Illinois May 14–19, 1995

Effect of Slide-to-Roll Ratio on Interior Stresses Around a Dent in EHL Contacts

Abstract: The present, study extends the transient EHL point contact model and subsurface stress field calculation model to examine the influence of a surface dent on interior stresses in an EHL point contact under various slide-to-roll conditions. Results revealed that under the pure rolling condition the effect of a surface dent on the stresses is quite negligible. The presence of a shallow surface dent is unlikely to reduce the contact fatigue, life so long as pure rolling motion and good lubrication conditions are maintained. Unfortunately, the same cannot be said of the contact if it is operating in the boundary lubrication regime. When sliding was introduced, the surface indentation generated significantly high pressure spikes with a strong directional preference. These high-pressure spikes cause severe stress concentrations either below the trailing edge of the dent, if it moves faster than the opposing surface, or below the leading edge of the dent, if it moves slower than the opposing surface. The maximum ...

An Efficient Finite Element-Based Air Bearing Simulator for Pivoted Slider Bearings using Bi-Conjugate Gradient Algorithms

Abstract: A computationally efficient air bearing simulator-for pivoted slider bearings has been developed that is based on finite element theory and uses bi-conjugate gradient algorithms in conjunction with a sparse matrix storage scheme. The air bearing simulator involves the simultaneous solution of the Reynolds equation and the slider equilibrium equations. The highly nonlinear problem requires the repeated solution of a nonsymmetric system of equations, typically with a large number of unknowns depending on the complexity of the slider geometry. Iterative solvers, such, as the bi-conjugate gradient algorithms used for this study, require significantly less core memory as compared to direct solvers and reduce the solution time if combined with a suitable preconditions. Of the bi-conjugate gradient algorithm/preconditioner combinations implemented, the Bi-CGSTAB algorithm combined with an ILU preconditioner provided the best performance in terms of smooth convergence and computational efficiency. Presented as a ...

Wear Characteristics of Aluminum-Silicon Alloy under Lubricated Sliding Conditions

Abstract: The wear characteristics of aluminum-silicon alloys were studied using a pin-on-disk-type wear tester with different types of lubricants under fully flooded condition. Aluminum-silicon alloy pin specimens were run against the same aluminum-silicon disc surfaces. Wear tests showed that wear resistance of aluminum-silicon alloy was largely influenced by the type of lubricant. Test results showed that hydrocarbon-based fluids such as mineral oils or poly-a-olefins exhibit better wear-protecting property and higher load-carrying capacity than polar base fluids such as polyol esters or polyalkylene glycols. This is contrary to the results of iron-iron systems. The effect of water on the wear and scuffing resistance of aluminum-silicon alloy was also investigated. Water was found to promote the formation of aluminum oxide and silicon oxide layers on the surface which reduced the wear rate and increased scuffing load for aluminum-silicon alloy. Presented at the 50th Annual Meeting in Chicago, Illinois May 14–19,...

The Effect of Perturbation Amplitudes on Eight Force Coefficients of Journal Bearings

Abstract: This paper investigates the relationship between eight linear oil-film force coefficients of circular journal bearings and the perturbation amplitudes. The force coefficients are calculated by the finite perturbation method and compared with those calculated by the infinitesimal perturbation method. Numerical experiments show that the calculated results from both finite perturbation and infinitesimal perturbation methods are very close (with a variation less than 0.1 %) when the perturbation amplitudes are less than 0.02c (displacement) or 0.02ωc (velocity) for normal bearing eccentricities. For eccentricities between 0.2 and 0.8, the calculated coefficients by the finite perturbation method will differ less than 2.5 % from the results by the infinitesimal perturbation method if the perturbation amplitude is less than 0.05c or 0.04ωc. The bearing coefficients calculated by the finite perturbation method under different perturbation amplitudes are presented graphically.

Performance of Plain Bearings with Circumferential Microgrooves

Abstract: Higher performance of recent automotive engines requires their bearings to operate under more severe conditions like higher speeds, higher loads and higher temperatures. To meet this requirement, plain bearings with circumferential microgrooves have been developed. Experimental evaluation as well as theoretical analysis has revealed that the ease of initial wear and deformation at the ridges leads to superior conformabilily, increased oil flow along the microgrooves achieves lower bearing temperature, and the oil retained in the microgrooves prevents seizure tinder starved lubricating conditions. Presented at the 50th Annual Meeting in Chicago, Illinois May 14–19, 1995

Tribological Properties of Hard Carbon Films on Zirconia Ceramics

Abstract: In this study, the authors investigated the tribological properties of hard diamondlike carbon (DLC) films on magnesia-partially stabilized zirconia (MgO-PSZ) substrates over a wide range of bads, speeds, temperatures, and counterface materials. The films were 2 μm thick and produced by ion-beam deposition at room temperature. Tribological tests were conducted on a ball-on-disk machine with MgO-PSZ balls, in open air of 30 to 50% relative humidity under contact loads of 1 to 50 N, at sliding velocities of 0.1 to 6 m/s, and at temperatures of 400°C. Al2O3 and Si3N4 balls were also rubbed against the DLC-coaled MgO-PSZ disks, primarily to assess their friction and wear performance and to compare it with that of MgO-PSZ balls. A series of long-duration lifetime tests was run at speeds of 1, 2, and 6 m/s under a 5 N load to assess the durability of these DLC films. Results showed that the friction coefficients of MgO-PSZ balls sliding against MgO-PSZ disks were 0.5-0.8, and the average specific wear rates of ...

A study of dynamically loaded engine bearings using a transient thermohydrodynamic analysis

Abstract: A comprehensive, transient thermohydrodynamic (THD) analysis for dynamically loaded journal hearings presented earlier (1) is used to study the performance of dynamically loaded engine crankshaft hearings, i.e., main hearings and connecting rod. bearings. Oil film temperatures are found, to vary considerably over time and space. This variation can he explained, from the journal orbit diagram. A simplified thermal analysis which uses a single effective oil film temperature calculated from overall, energy conservation is also developed. Comparisons are made between the predictions of the full THD analysis, an adiabatic THD analysis and the simplified thermal analysis. The adiabatic THD analysis compares well with the full THD analysis. The simplified thermal analysis compares reasonably well with the full THD analysis and is a significant improvement over an isothermal analysis. Presented as a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology Conference in Kissimmee, Florida...

Granular collisional lubrication : Effect of surface roughness, particle size and solids fraction

Abstract: The flow of granules has been proposed as a mechanism of lubrication suitable for high-temperature applications This study is primarily experimental with a simplified theory presented for interpreting the data Experiments are conducted in an annular shear cell at very high concentration where the major force contribution is due to particle collisions Normal and shear stress as a function of solids fraction with different drive surface roughness and particle size are studied The experimental data show that the stresses increase and friction coefficients decrease with increasing solids fraction A rough drive surface tends to increase the magnitude of the stresses compared with its smooth counterpart Large particles carry more stress than small ones, and the existence of the lubrication wedge effect caused by three sloping regions is confirmed Theoretical predictions are compared with experimental results for the case of flat surfaces and reasonable agreement is found Presented as a Society of Tribol

A Numerical Model for the Dry Sliding Contact of Layered Elastic Bodies with Rough Surfaces

Abstract: A numerical model for the two-dimensional dry sliding contact of two elastic bodies with real rough surfaces has been developed, where an elastic body contacts with a multi-layer surface under both normal and tangential forces The model uses surface profile data directly recorded with a stylus measuring instrument and it is suitable for use on a microcomputer Green's function for a unit normal load and a unit tangential load for the generalized plane strain problem are derived Verification of the accuracy of the model by reproduction of test case results is presented Contact pressure distribution for layers of varying coefficient of friction, thickness and elastic modulus is analyzed Presented as a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology Conference in Klssimmee, Florida, October 8–11, 1995

Comparison of the Relative Wear Performance of Neutral and Basic ZnDTP Salts

Abstract: Zinc dithiophosphates (ZnDTPs) have been used as the main antiwear additive in automotive engine oils for many years. They consist of mixtures of neutral and basic ZnDTP salts. Until now, the relative wear performance of the neutral and basic ZnDTP salts has not been reported, despite the fact that these materials are ubiquitous in commercial oils. In this paper, the relative wear performances of neutral and basic ZnDTP salts are compared using a pin-on-disk tribometer, with oils containing carbon black, a soot proxy. Examples of primary, secondary, and aryl ZnDTPs are studied; thus, the entire commercial range of ZnDTPs are examined. Pure neutral and basic ZnDTP salt molecules were synthesized for this work. The conclusion reached was that the friction behavior and the wear behavior of each neutral and basic ZnDTP pair were indistinguishable. A possible reason for this observation is given. Presented at the 50th Annual Meeting in Chicago, Illinois May 14–19, 1995

Thermoelastohydrodynamic Analysis of Fixed Geometry Thrust Bearings Including Runner Deformation

Abstract: A thermoelastohydrodynamic theory for the analysis of sector thrust bearings is presented. The analysis includes the generalized Reynolds equation and a fully elliptic three-dimensional energy equation in the film. In addition, full three-dimensional heat transfer is allowed in the pad while axisymmetric conduction is allowed in the runner. Three-dimensional elastic deformation due to mechanical and thermal loading is allowed in the pad while axisymmetric mechanical elasticity is allowed in the runner. The performance of a parallel tapered-land thrust bearing including runner deformation effects is analyzed as a function of different models. It is seen that mechanical deformation of the pad affects the operating characteristics only slightly while thermal deformation of the pad can cause large increases in operating temperature. The runner deformation effects can include a reduction of the maximum film temperature at slight deformation or a large increase in film temperature at the inner radius of the pad...

Bulk-Flow Model for the Transition to Turbulence Regime in Annular Pressure Seals

Abstract: An analysis for the prediction of the dynamic force response fur annular seals in the transition regime to flow turbulence is presented. A heuristic curve fit connects the laminar friction factor to Moody's turbulent friction factor curve in the transition regime from laminar to fully developed turbulent flows. A universal friction factor model is provided and included in the bulk-flow equation (zeroth- and first-order) governing the flow for small amplitude rotor motions about an equilibrium position. General equations are presented for the shear coefficients and their derivatives required in the bulk-flow model. Flow rates, drag torque and ratordynamic force coefficients are compared with those from traditional analyses for an annular seal experiencing wear and increased clearances leading to off-design operating conditions. Predicted seal force coefficients show physically sound, smooth vacations while passing through the transition zone and demonstrate a marked improvement over simpler models which predict sharp discontinuities.

Hydrodynamic Lubrication Analysis of Metallic Hip Joint

Abstract: Transient numerical analysis for the lubrication process of a metallic hip joint has been performed for both pure squeeze and combined motion of squeezing and oscillatory rotation. Lubrication performance for two types of cup, namely the single-radius cup and the multi-radius cup, has been examined. Results show that the multi-radius cup has a better lubrication performance than the single-radius cup. Increasing cup radius and decreasing the clearance between the cup and head can effectively increase the lubricant film thickness. A metallic hip joint is capable of achieving full-film lubrication if a relatively large cup radius and/or small clearance between the cup and head are used xuith proper cup geometrical configurations. Presented as a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology Conference in Kissimmee, Florida, October 8–11, 1995

Effects of lubricant borne solid debris in rolling surface contacts

Abstract: The role of particle morphology, hardness and toughness in the rolling surface fatigue and wear have been investigated using a thrust bearing in an accelerated laboratory bench lest. Ceramic, steel and metal powders of known physical properties with sizes of 10-20 micrometers were used as contaminants in a 3-cSt ester fluid. Initial results suggest that ceramic particles with high hardness but low toughness induce the most damage in metallic rolling contacts. Emphasis was placed on analysis of the functional relationships among the various parameters. Presented at the 50th Annual Meeting in Chicago, Illinois May 14–19, 1995