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Applications involving highly loaded elastohydrodynamic lubrication (EHL), particularly when the lubricant experiences shear thinning, operating with small film thicknesses may necessitate consideration of surface asperities. A modified Reynolds equation with provision for surface roughness and shear thinning is treated to predict the pressure and surface asperity effect in an EHL line-contact. The unknown in the Reynolds equation is the hydrodynamic pressure instead of the total pressure to ensure that the pressure boundary condition at the outlet is properly posed. The Carreau viscosity model is used for characterizing the shear thinning behavior, Patir and Cheng flow factors for taking into the influence of roughness on the lubricating film, and Greenwood--Trip for determination of pressure at the asperity level. The modified Reynolds equation is solved for the hydrodynamic pressure instead of the total pressure with appropriately defined boundary conditions. [DOI: 10.1115/1.4001787]

Misalignment affects nearly all the bearing performance parameters including the cavitation, thermal field, leakage flow-rate, and moments. The present paper provides a comprehensive analysis of misaligned journal bearings based on a three-dimensional mass-conservative thermohydrodynamic model that appropriately takes into account the film rupture and reformation. An extensive set of numerical solutions are presented to closely examine the effects of misalignment in journal bearings. [DOI: 10.1115/1.4000280] Keywords: misalignment, thermohydrodynamic analysis, mass-conservative, cavitation

This paper is devoted to a study of the enduring contact between granules of powder lubricants in an effort to better understand the flow characteristics of powder lubricants. Appropriate formulation of the governing equations is reported that can be used for prediction of the flow velocity, pseudo temperature, and volume fraction distribution of powders for a wide range of operating speeds. A set of parametric simulations and a limiting analytical solution is presented for predicting the behavior of a powder lubricant under low operating speeds when the enduring contact tends to dominate the kinetic regime. The limiting solution shows that below a certain sliding speed the volume fraction remains unchanged due to the effect of the enduring contact. It is also shown that below this limiting speed the enduring contact plays a major role and should not be neglected. [DOI: 10.1115/1.2114933]

A model is developed to investigate the mechanism of thermoelastic instability (TEI) in tribological components. The model consists of two thermally conducting bodies of finite thickness undergoing sliding contact. Appropriate governing equations are derived to predict the critical speed beyond which the TEI is likely to occur. This model takes into account the surface roughness characteristics of the contacting bodies as well as the thermal contact conductance at the interface. Analytical expressions are provided for the special cases neglecting the disk thickness and the thermal contact conductance. An extensive series of parametric simulations and discussion of the implication of the results are also presented. The simulations show that the difference in material properties and geometry of the two conducting bodies has a pronounced influence on the critical speed. A special case of the model shows that the threshold of TEI critical speed is pushed to a much higher level when the conducting bodies have identical material properties and are geometrically symmetric. It is also shown that the perturbed wave generally tends to move with the body with higher thermal conductivity.

Thermoelastic instability (TEI) is known to be directly related to the occurrence of hot spots in friction systems, such as brakes, seals, and clutches. TEI is a failure process where the local frictional heat, thermal expansion, contacting pressure and temperature grow rapidly over a certain critical value of the operating speed. In this article, we examine the rate of growth of instability and establish a link between the wave speed and the configuration of hot spots as well as its penetrating depth. The analysis includes provision for surface roughness and is capable of treating bodies of finite thickness with or without liquid lubrication. [DOI: 10.1115/1.1631013]

A comprehensive model is developed for analyzing the onset of thermoelastic instability in a wet clutch. For this purpose, appropriate governing equations are derived that take into account the porosity and deformability of the friction material. The effect of the thickness of the separator disk and that of the friction material are also included. The model is general and can be used to describe TEI in a variety of other systems such as in a mechanical seal, as a special case. A series of simulations are presented that predict the thermoelastic behavior of a wet clutch from an instability viewpoint.

A thermoelastic instability (TEI) analysis is presented for determining the threshold of instability in f friction pain consisting of a conducting surface rubbing against a rough surface with very low thermal conductivity. The theory predicts that TEI is initiated when the operating speed exceed a limiting value of U(super *) given by: U(super*) / U(sub s) = 1 + lambda [Mathematical Expression Not Reproducible in the ASCII] Where U(sub s) is identical to the critical equation developed by Burton (1980) for highly loaded surfaces in contact defined at: U(sub s) = 2 omega k(sub s)/f alpha E. The dimensionless parameters on the right-hand-side of the above equation emerge from the consideration of the surface roughness and are defined as lambda = sigma omega E/2 chi, and [Mathematical Expression Not Reproducible in the ASCII] = H(sub o)/F'. The results indicate that the magnitude of the applied load and the surface roughness characteristics tend to play an important role in the prediction of the onset of TEI instability.

A comprehensive formulation of the governing equations, boundary conditions, and numerical solution technique is presented for modeling the thermal aspects of the engagement process in a wet clutch. The thermal model includes full consideration of the viscous heat dissipation in the fluid as well as heat transfer into the separator, friction material and the core disk. Roughness, wariness, deformability and permeability of the friction material are taken into account. It is shown that very large temperatures develop in the fluid during the engagement process which takes place on the time scale of one second. It is also shown that thermal effects influence the engagement time and the torque behavior of a clutch and should be included in the analytical studies.

An experimental apparatus was designed and tested to study the thermally induced seizure in bearing. The setup consists of a simple, unloaded journal bearing configuration which lends itself to useful physical interpretation without the complexities that are present in the control of flow rates and eccentricity in a loaded journal bearing in which the clearance would vary with time. The motor was fitted with a current limiter which stopped the motor when the torque exceeded a certain limit. Experiments revealed that with this particular system, the journal speed undergoes a significant reduction with time until the operation is halted by the current limiter, which signifies the occurrence of a seizure. The time of seizure is appreciably influenced by this behavior. A parallel theoretical analysis, which takes into account the speed variation with time, was developed. The analysis includes the derivation of the appropriate governing equations which involve the transient analysis of flow velocity, heat transfer, and thermomechanical expansion of the surfaces, together with the numerical solution. The results of the simulations compare favorably to those obtained experimentally both in trend and magnitude. Finally, general behavior of the system in terms of its time-to-seizure characteristic is illustrated through a series of parametric studies.

A detailed analysis of the flow characteristics and lubrication effectiveness of grease in journal bearings is presented. Based on the ideal Bingham plastic model, the appropriate governing equations are developed which effectively handle the formation of a floating core as well as a core which may attach itself to either the shaft or the stationary bushing. The formulation and numerical solutions presented pertain to three-dimensional flows in finite journal bearings. Moreover, an extensive amount of computational results are presented, which take thermal effects into account according to the thermohydrodynamic theory.

Thermohydrodynamic Design Charts are developed for slider bearings that enable one to rapidly determine a conservative estimate for the bearing maximum temperature and a pertinent mean temperature from which all of the bearing performance parameters are computed. A number of examples are presented which illustrate the utility of the design charts.

The results of extensive amount of thermohydrodynamic simulations are presented in the form of design 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.

A fully lubricated Petroff's-type journal bearing has been modeled to analyze thermomechanical expansion and determine the conditions in which bearing seizures occur. The model was used to develop ways to avoid thermally induced seizures and guidelines for evaluating the occurence of seizure. The model, which was based on the lumped-system analysis, is ideal for hydrodynamic bearings with small Biot numbers.