0


TECHNICAL PAPERS

J. Tribol. 2003;125(4):685-691. doi:10.1115/1.1572515.

A hardness analysis based on finite element simulation results and contact constitutive models is presented for both homogeneous and layered elastic-plastic media. The analysis provides criteria for obtaining the real material hardness from indentation experiments performed with spherical indenters. Emphasis is given on the estimation of the hardness of thin surface layers. The critical (maximum) interference distance that yields an insignificant effect of the substrate deformation on the estimation of the layer hardness is determined from the variation of the equivalent hardness of the layered medium with the interference distance (indentation depth). A relation between hardness, yield strength, and elastic modulus, derived from finite element simulations of a homogeneous half-space indented by a rigid sphere, is used in conjunction with a previously developed contact constitutive model for layered media to determine the minimum interference distance needed to produce sufficient plasticity in order to ensure accurate measurement of the material hardness. An analytical approach for estimating the layer hardness from indentations performed on layered media is presented and its applicability is demonstrated in light of finite element indentation results for an elastic-perfectly plastic layered medium with a hard surface layer.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):692-699. doi:10.1115/1.1572516.

The effect of residual stress in the surface layer on the deformation of elastic-plastic layered media due to indentation and sliding contact loading and unloading was analyzed with the finite element method. A three-dimensional finite element model of a rigid sphere interacting with a deformable layered medium was developed, and its accuracy was evaluated by contrasting finite element results with analytical solutions for the surface stresses of an elastic homogeneous half-space subjected to normal and friction surface traction. Deformation of the layered medium is interpreted in terms of the dependence of the von Mises equivalent stress, first principal stress, and equivalent plastic strain on the magnitudes of residual stress and coefficient of friction. The effect of residual stress on the propensity for yielding and cracking in the layered medium is discussed in the context of results for the maximum Mises and tensile stresses and the evolution of plasticity in the subsurface. It is shown that the optimum residual stress in the surface layer depends on the type of contact loading (indentation or sliding), coefficient of friction, and dominant deformation mode in the layer (i.e., plastic deformation or cracking).

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):700-708. doi:10.1115/1.1573232.

As loading forces decrease in applications such as MEMS and NEMS devices, the size of the asperity contacts which comprise the real contact area tend to decrease into the nano scale regime. This reduction in size of the contacts is only partially offset by the nominally increased smoothness of these contacting surfaces. Because the friction force depends on the real area of contact, it is important to understand how the material and topographical properties of surfaces contribute to friction forces at this nano scale. In this investigation, the single asperity nano contact model of Hurtado and Kim is incorporated into a multi-asperity model for contact and friction which includes the effect of asperity adhesion forces using the Maugis-Dugdale model. The model spans the range from nano-scale to micro-scale to macro-scale contacts. Three key dimensionless parameters have been identified which represent combinations of surface roughness measures, Burgers vector length, surface energy, and elastic properties. Results are given for the friction coefficient versus normal force, the normal and friction forces versus separation, and the pull-off force for various values of these key parameters.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):709-712. doi:10.1115/1.1573234.

An important problem in thermo-mechanical contacts is the determination of the stress and displacement fields caused by heat flow. Heat flow may come from a difference in temperature between the contacting solids, or from frictional heating at the sliding interface. Generally, the distribution of heat flow in a contact area is unknown. In many cases, however, it is approximately uniform or one may divide the contact area into small parts, and in each part the heat flow may be treated as approximately uniform. This work provides closed-form solutions of the stress and displacement fields in a semi-infinite solid caused by uniform steady-state heat flow over a rectangular area on the surface. The material is assumed to be homogeneous and isotropic.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):713-719. doi:10.1115/1.1574518.

Based on real-time observation of the workpiece surface in a series of Lo and Tsai’s (2002) compression-sliding experiment, it is found that the asperity contact area is much greater than that evaluated by the existing theorems such as the junction-growth theorem. With the aid of finite element analysis, it is verified that the tool sliding motion along with the minute elastic deformation (microwedge) of the tool surface around the asperity peaks increase the asperity contact area significantly even in a frictionless sliding. The microwedge induces a component of force along the sliding direction on the asperity. A combination of flattening and smearing effects can therefore aid in expanding the contact area. The greater the wedge angle, the stronger the propellent effect. An incremental model has also been developed to predict the evolution of contact area during sliding. The numerical simulation compares well with the experimental results. The new mechanism not only introduces an important tribological variable to forming processes, but also brings in a new concept of surface quality control for processes having a considerable sliding distance between workpiece and tool such as ironing, forging, and extrusion. New processes performing high relative sliding velocity can therefore be developed to ameliorate the brightness of products.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):720-730. doi:10.1115/1.1574520.

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 *), 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 (σ) 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.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):731-738. doi:10.1115/1.1540121.

With actual and virtual materials, the effects of the thermal conductivity of contacting surfaces on EHL are investigated through experimental analyses using the optical interferometry technique and the Newtonian thermal EHL analyses in consideration of the variation of oil properties in all directions within the film. A mineral bright stock is used as a lubricant. It is found that the distributions of pressure and film thickness, including the minimum film thickness, are influenced very much by the entrainment velocity and the slide-roll ratio. One of the causes is the temperature-viscosity wedge action produced by the temperature variation across the oil film, and the other is an increase in oil temperature at the entrance of the contact due to the heat produced by the compression work and the shearing of the oil. The degree of both influences depends on the thermal properties of contacting materials.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):739-746. doi:10.1115/1.1572513.

A previous study of the behavior of friction in EHL contacts for the case of Eyring lubricant behavior resulted in a friction mastercurve. In this paper the same approach is applied to the case of limiting shear stress behavior. By means of numerical simulations the friction coefficient has been computed for a wide range of operating conditions and contact geometries. It is shown that the same two parameters that were found in the Eyring study, a characteristic shear stress, and a reduced coefficient of friction, also govern the behavior of the friction for the case of limiting shear stress models. When the calculated traction data is plotted as a function of these two parameters all results for different cases lie close to a single curve. Experimentally measured traction data is used to validate the observed behavior. Finally, the equations of the mastercurves for both types of rheological model are compared resulting in a relation between the Eyring stress τ0 and the limiting shear stress τL.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):747-755. doi:10.1115/1.1574519.

A mixed lubrication model is developed to investigate the lubrication of coupled journal-thrust-bearing systems. The governing equations are mapped into a computational domain with a conformal mapping technique developed in a previous study by Wang et al. (2002). In order to study the influence of the boundary conditions on the performance of the bearing system, either the Reynolds boundary conditions or the JFO’s mass conserving conditions are applied to the governing Reynolds equations. The performance of a typical coupled journal-thrust bearing system is numerically investigated and the effects of viscosity, misalignment angle and feeding scheme are discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):756-769. doi:10.1115/1.1575773.

In this paper, a mixed EHL analysis is presented for the variable torque slipping clutch with skewed rollers. It is characterized by a finite spatial curve contact and two dimensional rolling and slipping. A combination of the average roughness Reynolds equation developed by Patir and Cheng and the pressure-compliance relationship of roughness contact developed by Greenwood and Tripp is adopted to combine the effect of surface roughness in the lubrication. The average roughness Reynolds equation is extended to involve the two dimensional rolling and slipping while the pressure dependence of both the density and viscosity are considered. Based on the numerical techniques of the FEM discretization and relaxation iteration, the average roughness Reynolds equation is solved simultaneously with the GT contact pressure-compliance relationship under the equilibrium condition of the elements of the slipping clutch. The hydrodynamic and asperity contact pressures are then calculated and the interactions and mutual influences between these two kinds of pressures are shown. In addition, the influences of the rotation speed on the resistant torque are investigated both theoretically and experimentally. Concurring agreements are found between the theoretical and experimental data.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):770-779. doi:10.1115/1.1576425.

A full numerical analysis is carried out to simulate the thermal elastohydrodynamic lubrication (TEHL) of an eccentric-tappet pair. Comparisons between thermal and isothermal results are given to reveal the role of the thermal effect. Under various eccentricities, the influences of two surfaces moving in opposite directions on pressure and film thickness profiles are analyzed and explained by the mechanism of the temperature-viscosity wedge. Pressure and film thickness profiles, the temperature and velocity distributions at zero entraining velocity are discussed fully. Particular analyses are given on the entrapped immobile surface layers that influence the velocity distribution at zero entraining velocity. Furthermore, in a working cycle, variations of central and minimum film thicknesses and frictional coefficients, under different eccentricities, are discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):780-787. doi:10.1115/1.1540122.

To verify the tribo-microplasma concept proposed by Nakayama, who suggested that a microplasma is generated in the gap of a sliding contact due to electrical discharge of the ambient gas in the electric field caused by tribo-charging, we challenged to observe directly the tribo-microplasma and to measure spectral characteristics of the emitted photons. In experiments to observe plasma image (with a diamond hemispherical pin sliding on a sapphire disk) it was found that the plasma was generated in the several micrometer gap of the sliding contact. The plasma had a shape of an ellipse with a tail, surrounding the contact and spread to the rear of the sliding contact. The plasma image observed through the UV transmittable filter (UV image) had a horseshoe pattern, while the IR image had a shape of a ring on the ellipse. The strongest UV emission was observed in the center of the horseshoe pattern outside the sliding contact, while the IR photon image showed that the most intense emission occurred at the sliding contact. The electrical discharge origin of the photon triboemission was proved by comparing spectra of tribophotons with spectra of photons emitted from plasma by electrical discharge in parallel electrodes in various gases. The results showed that the spectra of photons emitted from the sliding contact and those of gas-discharge completely coincided for all gases tested, i.e., dry air, N2,O2,H2, He, CH4,C2H4 and C3H8, except peaks originated from the excited atoms of the sliding surfaces. It was concluded that microplasma is produced by electrical breakdown of ambient gas at sliding contact. These results corroborate the tribomicroplasma concept.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):788-796. doi:10.1115/1.1573229.

Modeling dynamic or kinetic friction for realistic engineering surfaces continues to be a challenge, partly due to the coupling between system dynamics and interfacial forces. In this paper, a dynamic friction coefficient model for realistic rough surfaces under external normal vibrations is developed. From the system dynamic model, the instantaneous time varying normal separation at the interface is obtained under normal harmonic excitation. Subsequently, the instantaneous dynamic contact and tangential (friction) forces are calculated as a function of the instantaneous normal separation. The dynamic friction coefficient defined as the ratio of the time varying friction to the interfacial normal forces that explicitly includes interfacial damping, is also calculated. The results show that a mean increase in the instantaneous normal separation may or may not lead to a decrease of the mean friction force and the mean friction coefficient, which is supported by published data. For unlubricated elastic sliding contact conditions considered in this paper, the effect of damping on the dynamic friction coefficient is found to be negligible, whereas loss of contact causes significant apparent dynamic friction force and dynamic friction coefficient reductions. Several different interpretations of the time varying dynamic friction coefficient are presented and the implications of using a simple constant value to represent the time varying dynamic friction coefficient are discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):797-803. doi:10.1115/1.1539059.

The paper deals with a dynamic analysis of a non-contacting seal with two flexibly mounted rotating rings. The kinematics of motion is described and the equations of dynamics derived. Hydromechanical forces and moments containing coupling elements of the equation of motion are also determined. Selected results of digital calculations show the influence of the coupling elements on the axial and angular vibrations of a flexibly mounted ring.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):804-813. doi:10.1115/1.1575774.

High-speed rotor systems use either fluid film or rolling element bearing supports, depending upon their design and operating constraints. Regardless of bearing type used, these systems require specific bearing and support stiffness and damping characteristics to achieve the desired stable and low vibration operation. Building upon the technology of thin metallic corrugated bump foils presently used in a particular class of film riding hydrodynamic bearings, a novel corrugated bump foil damped mount is introduced which provides stiffness and damping for application with rolling element bearings. These damping elements are capable of operating at elevated temperatures where implementation of conventional squeeze film dampers is ruled out. The frictional damping results from micro-slip motions between the bump foils and the mating surfaces. A semi-empirical model, based on a one degree of freedom model was developed in which damping is replaced by an equivalent frictional force in order to gain insight into the dynamic friction coefficient of the individual damping element interfaces. Experimental results, obtained in the form of hysteresis loops were compared to the developed model with good agreement. The variation in damping and dynamic coefficient of friction was found to be dependent primarily upon three factors: vibration frequency, amplitude of motion and applied static load. These parameters were tested within the range of 50–1400 Hz, 2.54–12.7 micron and 45–135 N, respectively. The tests were conducted at room and 538°C ambient temperatures under both dry and vapor phase lubricated conditions. Using the resulting empirical data, several bearing dampers were designed, built and tested in a small, high-speed gas turbine engine simulator. The tested novel foil dampers were capable of operating reliably under extremely high levels of shaft imbalance (i.e., 320 times greater than the air bearing supported with specification of 0.0002 oz-in) even while operating at temperatures to 560°C. These results show the great potential for wide application of these dampers on gas turbine engines and high-speed rotating machinery.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):814-823. doi:10.1115/1.1576427.

The use of PTFE-faced pads in large vertical axis hydro-generators was pioneered in Russia in the 1970s, prompted by a series of failures of conventional babbitt-faced bearings. Some advantages claimed include higher specific loading, lower power loss and the omission of oil-lift facilities. There is strong interest in the Industry concerning this material, but limited data are available on actual performance. Some results from extensive testing of PTFE-faced pads are given, for two sizes of pad. These are compared directly size-for-size with results for babbitt bearings of nominally the same area. The power losses for the two types of bearing were found to be almost identical. Some of the effects observed during testing are described and discussed, including the effect of creep. The test results are compared with predictions using the GENMAT analysis software. A method of allowing for creep in numerical modeling is discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):824-832. doi:10.1115/1.1576428.

The modernization of hydro-generators can involve the analysis of many different manufacturer’s designs of thrust bearings. Recent designs of bearing in common use are very reliable, but when failures do occur, it is often with older machines and within the first few minutes of start-up. This paper is a result of general design studies of various thrust bearing configurations subjected to transient operating conditions. It is shown that transient effects can induce an ‘overshoot’ of thermal deformation which can become unstable, leading to ‘thermal ratchetting.’ Examples are given of pads of various manufacturer’s bearings that have been subjected to this mechanism. Results from operating turbines, basic studies and measurements of the thermal bending of plates indicate that a peak deflection occurs well before thermal equilibrium is attained. The peaking phenomenon may be obscured in some designs or in cases where the run-up is gradual. The beneficial effects of using an oil-lift system during start-up are described. During shut-down it is important that the contact of hot, crowned pads against the runner be prevented. Minimum times for operation of the lift system are suggested, based on the thickness of the pads.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):833-841. doi:10.1115/1.1538619.

This paper presents a finite element model that describes the thermomechanical interactions of a journal bearing undergoing thermally induced seizure (TIS). Two Categories of TIS are studied; the first part deals with occurrence of seizure during the start-up period followed by an investigation of TIS due to a transient flow-disturbance. The paper outlines the finite element modeling and analysis procedure involved in simulating TIS. An extensive set of parametric simulations covering load, speed, shaft radius, operating clearance, bearing length, friction coefficient and thermal expansion coefficient are performed to gain insight into the phenomenon of TIS. A statistical procedure is applied to the simulated results and an empirical relationship is derived. Good agreement between the empirical and published results attests to the capability of the model and its potential for predicting thermally induced seizure during system start up and during flow disturbance.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):842-849. doi:10.1115/1.1573231.

A novel method for measuring the velocity-dependent adhesive force exerted on a magnetic disk has been developed using the micro-probe of a scanning probe microscope (SPM). The deformation of a cantilever having a diamond tip was measured based on the Michelson laser interferometry, wherein an interference fringe pattern was formed over the cantilever. The fringe images were captured by an ultra-high speed CCD camera, and then the deformation of the cantilever was accurately captured through image processing for extracting ridgelines from the fringe pattern. The high-speed camera enabled observation of the fringe image variation while the tip was being separated from a target surface, and identification of the transient response of the cantilever arising after separation by using the regression analysis. Then, the separation instant and the separation force were identified as the values of the initial condition of the transient response. Finally, this novel method was applied to the measurement of the separation force exerted on a magnetic disk coated with a molecularly thin lubricant film. As a result, the separation force was found to be approximately proportional to the square root of the retractive velocity.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):850-853. doi:10.1115/1.1573230.

Organosilane monolayer of 1H,1H,2H,2H-perfluorodecyltrichlorosilane and PFPE lubricant films were deposited on magnetic hard disk surfaces protected with amorphous carbon overcoats, and their frictional properties were investigated by a ball-on-flat tribotester. The thickness of PFPE films having only an immobile layer or both immobile and mobile layers was varied from about 0.6 nm to 4 nm. The friction coefficient of the organosilane monolayer coated surface is comparable to that of the PFPE coated surface with a 1.0 nm thick immobile layer and lower than that of the PFPE coated surface with a 0.6 nm thick immobile layer. The thickness of the lubricant has to decrease to less than 1 nm to further increase recording density. In that case, the mobile portion of PFPE lubricant is negligible. This study suggests that organosilane monolayers (with shorter chain lengths) may be used as an alternate lubricant in hard disk drive systems.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):854-858. doi:10.1115/1.1575775.

This paper describes a nonlinear model and analysis of the axial transient response of the sector-shaped hydrodynamic thrust bearing-rotor system in a turbo-expander under a suddenly applied step load. The model is comprised of a time-dependent Reynolds Equation for oil film forces, and a vibration equation for the axial shaft system. The time-dependent form of the Reynolds Equation is solved by a finite difference method with a successive over-relaxation scheme, and the vibration equation is solved by the fourth-order Runge-Kutta method and the Adams method. In addition, a linear analysis is attempted in order to evaluate its suitability for the situation under consideration. The result of the analysis has shown that the linear model is unsuited, while the nonlinear analysis appears reasonable. Two system parameters, the initial oil film thickness and the angle of the inclination of the tapered land in a thrust bearing, are shown to have significant impacts on the transient response under consideration, and to be possibly optimized to achieve a minimum axial transient response.

Commentary by Dr. Valentin Fuster

TECHNICAL NOTES

J. Tribol. 2003;125(4):859-862. doi:10.1115/1.1573233.

So far in the literature, the distribution of stationary temperature over the surface of a half space subjected to a moving circular heat source has been reported in an integral or asymptotic form. In this paper, an exact explicit analytical solution is provided, which allows the determination of the temperatures over the contact area with a very short computational time, regardless of the value of the Peclet number. The solution is based on special functions (Bessel and hypergeometric functions) that are pre-programmed under a formal calculation software (e.g., Maple). The results of the proposed solution are in agreement with the asymptotic models available in the literature.

Commentary by Dr. Valentin Fuster
J. Tribol. 2003;125(4):862-868. doi:10.1115/1.1574517.

Computing the thermoelastic displacement of three-dimensional stationary or moving bodies subject to frictional heating is an essential numerical procedure for the complex modeling of the contact of tribological components. Surface Roughness inevitably causes the irregularity of the frictional heat distribution, and thus complicates the process of the numerical simulation of contact problems. The surface normal thermoelastic displacement has been studied in previous papers for either stationary bodies with irregularly distributed heat or moving bodies with regularly distributed heat. In this work, irregularly distributed frictional heat is applied on the surface of a moving body. Temperature and surface normal thermoelastic displacement are solved by using an efficient numerical procedure involving the discrete convolution and fast Fourier transform algorithm and frequency response functions. The thermoelastic displacement due to frictional heat that is proportional to the contact pressure is comparable to the elastic displacement caused by the contact pressure and is not sensitive to the roughness texture. The transient performance of multiple heat sources with different shapes, as well as the mutual influence, is also studied.

J. Tribol. 2003;125(4):868-870. doi:10.1115/1.1576424.

A Dynamic capacity for oscillating rolling bearings was published in 1968 and correlated with available laboratory fatigue life data. That development of the Dynamic Capacity extended the classic fatigue life theory of Lundberg and Palmgren (1947 and 1952) to oscillating rolling bearings. The calculation of the Dynamic Capacity is simplified as a modification of present ABMA and ISO load rating and life standards for continuously rotating rolling bearings. The simplified formulas agree with the Harris, 1991, text book formulation for oscillation amplitudes (greater than the critical amplitude) which cause an overlapping of stressed contact areas by adjacent rolling elements. Oscillation amplitudes less than the critical amplitude result in separate, discrete contact areas on each raceway. Use of the Harris equations will lead to overestimation of the fatigue for oscillation amplitudes which are less than the critical amplitude.

J. Tribol. 2003;125(4):871-873. doi:10.1115/1.1576814.

Tandon,  N., and Choudhury,  A., 1999, “ A Review of Vibration and Acoustic Measurement Methods for the Detection of Defects in Rolling Element Bearings,” Tribol. Int., TRBIBK32, pp. 469–480.trbTRBIBK0301-679XHoward, I., 1994, “A Review of Rolling Element Bearing Vibration—Detection, Diagnosis and Prognosis,” Technical Report DSTO-RR-0013, Defense Science and Technology Organization, Australia.Prabhakar,  S., Monhanty,  A. R., and Sekhar,  A. S., 2002 “ Application of Discrete Wavelet Transform for Detection of Ball Bearing Race Faults,” Tribol. Int., TRBIBK35, pp. 793–800.trbTRBIBK0301-679XBurrus, C. S., Gopinath, R. A., and Guo, H., 1998, Introduction to Wavelets and Wavelet transforms, Prentice Hall.

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In