Showing papers in "Journal of Tribology-transactions of The Asme in 1996"

A Numerical Three-Dimensional Model for the Contact of Rough Surfaces by Variational Principle

Abstract: A new numerical method for the analysis of elastic and elastic-plastic contacts of two rough surfaces has been developed. The method is based on a variational principle in which the real area of contact and contact pressure distribution are those which minimize the total complementary potential energy. The present variational approach guarantees the uniqueness of the solution of the contact problem and significantly reduces the computation time as compared with the conventional matrix inversion method, and thus, makes it feasible to solve 3-D contact problem with large number of contact points. The model is extended to elastic-perfectly plastic contacts. The model is used to predict contact statistics for computer generated surfaces.

Spectral Analysis of Two-Dimensional Contact Problems

Abstract: Contact problems can be converted into the spatial frequency domain using Fast Fourier Transform (FFT) techniques. Spectral analysis is used to develop an algebraic relationship between the surface displacement and the contact pressure. This relationship can be used to find the contact pressure or displacement for the contact of smooth surfaces or the complete contact of rough surfaces. In addition to providing rapid, robust solutions to contact problems, the algebraic relationship contains details of the relationship between surface displacement and contact pressure on different length scales. In particular, it is shown that the frequency composition of pressure is similar to that for slope of the surface displacement. Thus, the high frequency content of the surface profile gives rise to high localized contact pressure, in some cases singular pressure for complete contact. However, measurement limitations always lead to the omission of certain high frequency components of the surface profile. Assuming that the high frequency content of the surface profile obeys a power law, spectral analysis is also used to estimate partial contact parameters. This result relates the exponent of the power law to the contact pressure and implied surface integrity. It is concluded that spectral analysis can be combined with the FFT to provide a useful technique for classifying rough surface contacts.

Tribological Behavior of Amorphous Carbon Nitride Overcoats for Magnetic Thin-Film Rigid Disks

Abstract: Amorphous carbon nitride coatings of thickness of 5 and 30 nm were deposited onto 65 and 95 mm magnetic thin-film rigid disks surfaces using single-cathode and dual-cathode magnetron sputtering systems containing nitrogen-argon plasmas. Under optimum deposition conditions, amorphous carbon nitride coatings can be synthesized on ultrasmooth thin-film disks with no significant pinholes at thickness down to 5 nm, with hardness 22-28 GPa (compared to 7-12 GPa for amorphous carbon), and r.m.s. roughness as low as 0.25 nm. These amorphous carbon nitride coatings were shown to have better contact-start-stop performance and three-to-four times better pin-on-disk contact durability compared with amorphous carbon overcoats under identical testing conditions. Amorphous carbon nitride appears to be a promising candidate overcoat material for replacing amorphous carbon in the next-generation magnetic thin-film rigid disk systems.

Finite Element Modeling of Engagement of Rough and Grooved Wet Clutches

Abstract: A finite element model has been developed to investigate the engagement of rough, grooved, paper-based permeable wet clutches. The finite element (Galerkin) approach was used to discretize the modified Reynolds and force balance equations, and the solution domain geometry was described using an isoparametric formulation. Surface roughness effects were modeled via the Patir and Cheng (1978) average flow model, while asperity load sharing was calculated using the Greenwood and Williamson (1966) approach. The finite element model developed was used to investigate the effects of applied load, friction material permeability, and groove size on the engagement characteristics of wet clutches (i.e., torque, pressure, engagement time, and film thickness). The results indicate that the applied load, friction material permeability, and groove width significantly influence the engagement characteristics. Higher facing pressures increase peak torque and decrease engagement time. Higher permeability of the friction material significantly decreases engagement time but dramatically increases peak torque. Wider grooves decrease the peak torque and increase the engagement time. Groove depth does not significantly affect engagement characteristics for this model.

Abnormal Phenomena Appearing in EHL Contacts

Abstract: In a previous paper the authors showed that when the surface velocity of a body having lower elastic modulus is faster than that of a body having higher elastic modulus, and when the radius of curvature of the former is larger than that of the latter, a deep conical depression (dimple) is produced in the contact surface. This dimple occurs in place of the flat plateau predicted by the point contact elastohydrodynamic lubrication (EHL) theory. In this paper, the effects of surface kinematic conditions on the formation of such abnormal phenomena, which cannot be predicted by the present EHL theory, are investigated under rolling/sliding conditions using the optical interferometry technique. Transient behavior of the dimple caused by a groove passing through the EHL conjunction is also discussed based on direct observations. It has been found that the dimple occurs when the lubricating oil in the contact area is composed of solidified and unsolidified parts, and the dimple itself corresponds to the solidified part. On the basis of this finding, it has been asserted that the establishment of a new EHL theory is needed.

Numerical Analysis of the Influence of Waviness on the Film Thickness of a Circular EHL Contact

Abstract: Surface roughness and/or surface imperfections are well known to significantly affect the performance of concentrated contacts. Any deviation from the smooth surface will act as a stress raiser for itself (bump) or of its neighborhood (dent), and will therefore reduce the fatigue life of the component it is part of. These imperfections can also act as initiation sites of other types of contact failure such as scuffing, when contact conditions such as load, speed and film thickness become more and more severe. With the help of increasing computer speeds and more efficient numerical techniques, a theoretical analysis of the failure of concentrated contacts becomes possible. The full answer will involve many aspects of the contact, including the generation of heat, thermal response of the lubricant and solids, non-Newtonian as well as surface chemistry effects. This paper concentrates on the way the lubricant film thickness is affected by waviness and tries to identify the locations and the conditions where the film thickness is minimal. The lubrication ofnonsmooth surfaces is a transient two-dimensional problem, which will be treated without any geometrical simplification. More precisely, this paper focuses on the influence of rolling speed and the slide-to-roll ratio on the film thickness separating a smooth surface and one with transverse waviness.

On the Generalization of Thermohydrodynamic Analyses for Journal Bearings

Abstract: The results of extensive amount of thermohydrodynamic simulations are presented in the form ofdesign charts for journal bearings. These charts enable one to evaluate the maximum temperature and an effective bearing temperature. Numerous examples are presented that show the applicability of these charts in predicting bearing performance parameters.

Thermal EHL Analysis of Circular Contacts With Measured Surface Roughness

Abstract: Time dependent thermal EHL circular contact results with measured surface roughness were obtained to analyze the effects of roughness on pressure, film thickness, temperature, and coefficient of friction. Both contact surfaces are considered to be rough. Multilevel multigrid techniques (with multigrid integration) were used to solve the system of two dimensional Reynolds, elasticity and three dimensional energy equations simultaneously. The effects of surface roughness under various loads, speeds, and slip conditions have been studied. Surface roughness causes pressure and temperature spikes and increases the coefficient of friction, and surface roughness flattens due to the high pressure in EHL contact. The higher the load, speed and slide to roll ratio, the more significant the effect of the surface roughness. A comparison between rough EHL and smooth EHL results indicates that surface roughness cannot be ignored in EHL analysis.

The effects of surface texture on EHL point contacts

Abstract: The effect of surface texture on EHL point contact is studied numerically by using the multigrid method. Numerical simulations have been performed for waviness and random roughness with three different orientations, transverse, oblique and longitudinal. Results reveal a strong domination of unidirectional Couette flow in the EHL conjunction. The geometrical variations at inlet of the contact are transported downstream throughout the EHL conjunction. As a consequence, the oblique surface roughness striations are largely distorted, forming nearly longitudinal wary passages. Results show that the oblique roughness induces local three dimensional EHL pressure fluctuations. The maximum pressure is higher than that of the transverse roughness. For sinusoidal waviness, oblique orientation gives the smallest minimum film thickness as compared with those of longitudinal and transverse waviness.

Non-Newtonian Lubrication With the Convected Maxwell Model

Abstract: In certain applications where the lubricant is subjected to rapidly changing conditions along its flowing path (such as an elastohydrodynamic contact), the time dependent nature of the lubricant may be significant. One of the simplest types of models to account for such fluid time dependence is the Maxwell model. The time derivative used in such a model must be written with respect to coordinates which translate and rotate with the fluid, or coordinates which deform with the fluid. Unfortunately, such derivatives greatly complicate problems and are rarely used, due to nonlinear coupling of stresses. An admissible formulation of the Maxwell viscoelastic fluid model using the convected derivative has been applied to lubrication flow. Using a regular perturbation in the Deborah number, with the conventional lubrication solution as the leading term, a solution can be obtained. Viscoelasticity may raise or lower pressure depending on combinations of surface slope and curvature.

On the Accuracy of Rolling Bearing Fatigue Life Prediction

Abstract: Ball and roller bearings are designed to meet endurance requirements basically determined according to the Standard fatigue life calculation method This method is based on the Lundberg-Palmgren fatigue life theory as modified by reliability, material, and lubrication factors As application load and speed requirements have increased, the Lundberg-Palmgren method has resulted in bearings of increased size, adding unnecessarily to the size and weight of mechanisms This is a critical design situation for weight and size-sensitive components such as aircraft gas turbine engines and helicopter power transmissions The bearing life prediction method developed by loannides and Harris recognizes the existence of a fatigue limit stress If the stresses an operating bearing experiences do not exceed the limit stress, the bearing can achieve infinite life In any case, the method tends to predict longer lives than the Lundberg-Palmgren method This paper evaluates the life prediction accuracies of the Lundberg-Palmgren and loannides-Harris methods by comparing lives calculated according to these methods and to those actually experienced in 62 different applications As a result of the investigation, the loannides-Harris method is shown to more accurately predict bearing fatigue endurance

Magnetic Bearing Design for Reduced Power Consumption

Abstract: Magnetic bearings have relatively low power consumption compared to fluid film and rolling element bearings. They are now candidates for supporting gas turbines and aeropropulsion engines. This paper describes the design and construction of permanent magnet biased, actively controlled magnetic bearings for a flexible rotor. The rotor was originally supported in fluid film bearings consuming as much as 3000 watts of power. For the magnetic bearing, both permanent magnets and electromagnets are used in a configuration which effectively provides the necessary fluxes in the appropriate air gaps to support the rotor. The theoretical development related to the bearing design is presented along with some experimental performance results. The results include measurements of power consumption, load capacity, bearing linearized coefficients, and the dynamic response of the rotor. The measured total power consumption, excluding shaft losses, was 210 watts in the permanent magnet biased bearing.

Low speed mixed lubrication of bulk metal forming processes

Abstract: A simple analysis for the lubrication of saw-tooth surfaces with longitudinal lay under conditions of high fractional contact area is developed. This is then coupled with Wilson and Sheu's asperity flattening model to treat the mixed lubrication of a bulk metal forming process (rolling). It is shown that, even under low speed conditions where the inlet zone does not generate significant hydrodynamic pressures, relatively high hydrodynamic pressures can be generated in the work zone. This explains the persistence of hydrodynamic effects noted in low speed experiments.

Turbulent Flow, Flexure-Pivot Hybrid Bearings for Cryogenic Applications

Abstract: The thermal analysis of flexure-pivot tilting-pad hybrid (combination hydrostatic-hydrodynamic) bearings for cryogenic turbopumps is presented. The advantages of this type of bearing for high speed operation are discussed. Turbulent bulk-flow, variable properties, momentum and energy transport equations of motion govern the flow in the bearing pads. Zeroth-order equations for the flow field at a journal equilibrium position render the bearing flow rate, load capacity, drag torque, and temperature rise. First-order equations for perturbed flow fields due to small amplitude journal motions provide rotordynamic force coefficients. A method to determine the tilting-pad moment coefficients from the force displacement coefficients is outlined. Numerical predictions correlate well with experimental measurements for tilting-pad hydrodynamic bearings. The design of a liquid oxygen, flexure-pad hybrid bearing shows a reduced whirl frequency ratio and without loss in load capacity or reduction in direct stiffness and damping coefficients.

Ball Bearing Dynamic Analysis Using Computer Methods—Part I: Analysis

Abstract: An analytical ball bearing dynamics model was developed that rigorously models all of the significant kinematic, structural, and dynamic effects. The model can analyze bearings of any material combination for the races, balls and ball cage. This model analyzes the stresses and deflections of the loaded elements due to (I) preload, (2) external axial, radial and moment loads, (3) centrifugal and gyroscopic ball loads. A rigorous six-degree-of-freedom model of ball cage motions was developed to analyze ball and cage dynamics. The ball cage equations of motion were written in a rotating coordinate system, which greatly simplifies the equations, resulting in a highly efficient, but rigorous, model of bearing dynamics. A computer program was developed, incorporating the algorithms, to solve the multiple simultaneous quasi-static ball-to-race load equations using modified Newton-Raphson methods. The Lawrence Livermore Ode package (LSODA) is employed for numerical integration of the dynamic equations of motion. This method assures convergence, while controlling the accuracy of the calculations as a junction of computer run time and automatically selects the appropriate integration method for stiff and non-stiff system of ODE. The program analyzes ball and cage motions in time domain, wear life, fatigue life, lubricant film effects, ball-to-cage forces, torque noise and many other bearing parameters.

An EHD Inlet Zone Analysis Incorporating the Second Newtonian

Abstract: An apparently new approach to the solution of EHD film thickness is presented for line contact. This approach can readily accept non-Newtonian response as it is not based upon the Reynolds equation. Thermal effects and sliding may also be incorporated, although for large departure from Newtonian the pressure criterion should be reexamined

Hardness of Thin-Film Media: Scratch Experiments and Finite Element Simulations

Abstract: Experiments and finite element simulations are presented pertaining to the effective hardness and the mechanics of indentation and sliding contact on elastic-plastic layered media. Hardness measurements obtained from scratch experiments are presented for thin-film rigid disks with 30 nm carbon overcoats. Reproducible results are obtained for residual scratch depths greater than approximately 8 nm. A simple force balance model is used to calculate the effective hardness of the layered medium. Hardness values for the surface layer are calculated by fitting a relationship between the hardness, scratch geometry, and layer thickness to the experimental data. The experimental results are compared with three-dimensional finite element simulations of rigid spherical indenter sliding over a half-space with a stiffer and harder surface layer. The finite element results are used to verify the hardness model applied to the experimental data and to provide insight into the observed experimental behavior in the context of the associated elastic-plastic deformation characteristics of the layered medium.

A Study of the Oil-Lubricated Herringbone-Grooved Journal Bearing—Part 1: Numerical Analysis

Abstract: A numerical study of the oil-lubricated herringbone-grooved journal bearing is presented for the case of eight circular-profile grooves on the sleeve surface. The governing differential equation derived from the mass balance is solved by using the finite difference method. Some of the groove geometries are constrained because of the groove forming processes. Optimal values for various bearing parameters are obtained to maximize the radial force and to improve the stability characteristics. Results are compared with the plain and rectangular-profile grooved journal bearings. Radial force, attitude angle, stiffness and damping coefficients, and stability map are given for optimal configurations.

Reductions in Wear Rate of Carbon Samples Sliding Against Wavy Copper Surfaces

Abstract: Wear rates (μgm/s) versus rotor speed for carbon samples sliding against smooth and wavy copper rotors (250 μm thick copper sheets were attached to smooth and wavy steel and polycarbonate backings) were identical at some speeds, but at other speeds wear rates for the wavy rotors were almost half those of the smooth rotors. Slider vibrations (periodic, with period set by rotation) perpendicular to the sliding surface were measured and Fourier analyzed. Comparison of vibration spectral amplitudes to spectral amplitudes derived from surface profiles identified vibration modes dynamically enhanced by surface waviness on the wavy rotor. At speeds where wear rates on the wavy rotor were most reduced, amplitudes of certain modes in the vibration spectrum were most enhanced. For all these cases, the product of mode number times speed was nearly constant, suggesting resonance. Contact forces and contact voltage drop (due to a mA current flowing from slider to rotor) were measured and plotted versus time during all experiments. Friction coefficients rapidly varied between 0.1 and 0.4, but averaged 0.2. Traces of friction coefficient versus time for both wavy and smooth rotors were similar, even when wear rates plunged on the wavy rotor. There were no large jumps in the contact voltage drop data, suggesting that the slider never disconnected from any of the rotors. Photoelastic visualizations (Bryant and Lin, 1993) of slider-rotor interfaces revealed concentrated contact on the smooth rotors, but none on the wavy rotors. The absence (induced by vibration) of concentrated contact may have caused differences in wear rates. Appreciable reductions (up to 50 percent) in wear rate are possible by adding small surface waves to a rotor that induce micro-vibrations of the slider-spring-rotor contact system. The effect appears most pronounced at resonance.

Self-Excited Oscillations in Sliding With a Constant Friction Coefficient—A Simple Model

Abstract: The sliding of two surfaces with respect to each other involves many interacting phenomena. In this paper a simple model is presented for the dynamic interaction of two sliding surfaces. This model consists of a beam on elastic foundation acted upon by a series of moving linear springs, where the springs represent the asperities on one of the surfaces. The coefficient of friction is constant. Although a nominally steady-state solution exists, an analysis of the dynamic problem indicates that the steady solution is dynamically unstable for any finite speed. Eigenvalues with positive real parts give rise to self-excited motion which continues to increase with time. These self-excited oscillations can lead either to partial loss-of-contact or to stick-slip. The mechanism responsible for the instability is a result of the interaction of certain complex modes of vibration (which result from the moving springs) with the friction force of the moving springs. It is expected that these vibrations play a role in the behavior of sliding members with dry friction.

Generalized Boundary Interactions for Powder Lubricated Couette Flows

Abstract: Application of grain flow theories to powder lubrication of hydrodynamic bearings requires careful consideration of slip of the flow velocity at the boundaries together with appropriate formulation of the fluctuation (thermal) velocity conditions at the boundaries. Generalized boundary conditions that can be applied to a wide variety of surface and grain material characteristics are developed for a Couette flow configuration. The equations account for both the energy loss at the boundary due to inelastic grain-wall collisions and the energy supplied by the boundary due to collisional momentum supply. The predictions of the model are compared to a number of published results that are based on direct computer simulation of grain-grain collisions. The model's flexibility is demonstrated by simulating the effects of unequal surface roughnesses.

On the Dynamic Thermal State in a Hydrodynamic Bearing With a Whirling Journal Using CFD Techniques

Abstract: A thermohydrodynamic analysis, based on computational fluid dynamics (CFD) techniques, that accounts for conduction in the rotating and orbiting shaft of hydrodynamic bearing is presented. No restrictions apply to the circumferential or axial variation of journal/shaft temperature. Dynamic cavitation effects are also introduced, such that pressures in the cavitation region are predicted rather than set. The model predictions are validated against analytical and published experimental results. For the case of a centrally located synchronous forward circular whirl orbit, it is demonstrated that the journal does not behave as a circumferentially isothermal element and that significant steady temperature differentials across the journal may occur.

An Extended Three-Control-Volume Theory for Circumferentially-Grooved Liquid Seals

Abstract: Circumferentially-grooved seals are used in centrifugal pumps to reduce leakage flow. They can also have a significant impact on pump rotordynamic characteristics. Florjancic (1990) developed an analysis for leakage and rotordynamic coefficients, using a partition of the seal into three control volumes. This paper presents a new theory, based on an extension of Florjancic's work (1990) for circumferentially-grooved liquid seals. The current theory differs from Florjancic's analysis in the retention of transfer momentum terms and the introduction of diverging flow in the through-flow section within a seal groove. Validation of the new analysis is achieved through a comparison with existing experimental data taken from Kilgore (1988), and Florjancic (1990). Theoretical results are reasonable and consistent ; i.e., a modification in the seal parameters induces a correct evolution of the rotordynamic coefficients. Direct and cross-coupled stiffness coefficients are slightly underpredicted, whereas the direct damping coefficient is underpredicted within 40 percent. Leakage flow predictions are very good.

Crack Path Prediction Under Fretting Fatigue—A Theoretical and Experimental Approach

Abstract: In this paper, the direction of crack growth under fretting fatigue loading is studied through an experimental and theoretical approach. The experimental work enabled the fretting conditions to be known and the site of initiation and crack trajectory to be viewed ; theoretical work permitted a prediction of those processes. Fretting wear and fretting fatigue loadings induce non-proportional mixed mode loading at the tip of the cracks initiated within the contact zone. The classical criteria predicting the direction of crack growth cannot account for the non-proportional loading. Tests were carried out to study the cracking phenomena under cumulative effects of contact and external loadings, i.e., fretting fatigue loading. The fretting contact between the two contacting bodies is modeled to evaluate the operating contact loading conditions. The response of the cracked body is determined in terms of stress intensity factors using the continuous distribution of dislocations theory coupled with a unilateral contact analysis with friction. The angle of crack extension is then predicted, at different stages of crack life, according to a new approach. The correlation of the predicted angle of crack extension with the experimental observation enables the conclusion that, under fretting fatigue loading, cracks propagate by a mode I process.

Ultrathin Perfluoropolyether Films—Influence of Anchoring and Mobility of Polymers on the Tribological Properties

Abstract: The tribological behavior of ultrathin films of hydroxyl terminated perfluoropolyethers attached to smooth silicon surfaces was studied in pin-on-disk experiments. The frictional properties of the films, which consisted of either physically adsorbed or chemically bonded polymer or a combination of both, was measured in sliding contact and lubricant loss was studied simultaneously by scanning microellipsometry. Prior to these experiments the kinetics of attachment of the polymer to silicon surfaces and the behavior of the polymer at high temperatures was investigated in order to obtain reproducible conditions for the deposition of the lubricant films and to rule out the occurrence of thermal degradation during film deposition. The tribological investigations showed that the lifetime of the surfaces with the attached boundary lubricant was strongly dependent on the mode of attachment of the polymer. The best tribological behavior was observed in films which contained both chemisorbed and physically adsorbed material.

A Model for Elastohydrodynamic Film Failure in Contacts Between Rough Surfaces Having Transverse Finish

Abstract: The paper describes an elastohydrodynamic lubrication (EHL) model for collapse of the film in a contact of finite width between surfaces which have roughness aligned transverse to that of lubricant entrainment. The failure mechanism proposed is that of sideways leakage of the lubricant in the gaps that are present between the surfaces due to the valley features of the surface roughness. Under typical high temperature conditions with surfaces finished by conventional grinding, it is shown that the gap between the surfaces when lubricated is almost identical to that between the same dry surfaces in contact with the addition of a small land clearance equivalent to the nominal EHL film thickness. Analysis of idealized valley geometries leads to criteria for complete cavitation or significant loss of pressure between asperity contacts, but application of these criteria to a real contact suggests that scuffing occurs under conditions which are less severe than predicted by either of these simple failure models. Detailed analysis of leakage from the valley features in the transverse direction at the edges of a real elliptical contact shows that this can explain the complete loss of the film in a real contact, and this suggests a physical mechanism of scuffing.

Thermohydrodynamic Design Charts for Tilting-Pad Journal Bearings

Abstract: Design charts are presented which allow one to predict the maximum temperature and a realistic effective temperature of five-shoe tilting-pad bearings. The charts utilize two dimensionless parameters which characterize the temperature rise in the film based on the ISOADI boundary conditions. A number of examples are presented to illustrate the utility of the design chart where the results are compared to both experimental measurements and full THD simulations.

Transient analysis of plain and tilt pad journal bearings including fluid film temperature effects

Abstract: The paper considers vibration response of spinning shafts supported by flexible fluid film bearings to sudden mass imbalance (blade loss). A time transient study of the plain journal bearing with thermal effects is performed. A comparison between three transient analyses is performed. The three transient analyses studied are, the full nonlinear analysis, linear analysis using dynamic coefficients, and pseudo transient analysis using static application of dynamic loads. The validity of the nonlinear transient analysis is checked by matching the lower unbalance results with the linear analysis and the static equilibrium position results with Newton-Raphson iterative scheme. A nonlinear transient analysis of the tilt pad journal bearing is also performed, and a comparison is drawn between the three approaches.

Scale Effects of Elastic-Plastic Behavior of Microscopic Asperity Contacts

Abstract: Scale effects of elastic-plastic asperity microcontact behavior are studied theoretically using a new simulation method developed by the authors. Contact plastic deformation is described in terms of discrete dislocations, which allows the model to be made scale-sensitive. The present work deals with two-dimensional asperity microcontacts. It is found that when asperity size decreases and becomes comparable to the characteristic microstructural length, contact plastic deformation becomes increasingly difficult, and finally impossible. The effect of a simultaneous decrease in the asperity size and the microstructural length is also studied.

Thermal Analysis of Noncircular Bearings

Abstract: The present work is on prediction of temperature distribution in noncircular journal bearings and the surrounding solids Three forms are studied, viz two-lobe, elliptical and orthogonally displaced bearings For comparison purposes, a circular bearing with two different groove locations is analyzed The investigation includes the effects of recirculation and oil mixing at the grooves The cavitation zone has also been investigated The work is based on a two-dimensional treatment following Mc-Callion's approach The results are presented for various geometries of journal bearing configuration, including the conventional circular bearing