Investigation of Oil Additives for Boundary Lubrication of Railroad Journal Bearings

The boundary friction properties of the lubricated system of steel sliding on lead-base babbitt were investigated as a phase of the Association of American Railroads' sponsored project on freight car hot boxes. A friction pendulum was developed for determining the coefficients of boundary friction as a function of temperature with the specified metals lubricated with various lube oil additive blends.

A group of general relationships dealing with additives for the steel-babbitt system was developed. It is concluded that the extreme pressure agents containing sulfur or chlorine are generally unsuited for railroad journal bearing use. Lubricity additives which function by simple physical adsorption were found to be ineffective at moderate or high temperatures existing in railroad operations. Those lubricity additives which chemisorb to metal surfaces and form a close-packed layer were found to be effective antifriction agents.     

The influence of transverse roughness in thin film, mixed elastohydrodynamic lubrication

The Spacer Layer Imaging (SLIM) method has been used to investigate the influence of transverse roughness on the thickness and shape of elastohydrodynamic (EHD) films. The effects of entrainment speed and lubricant viscosity on film distribution are shown for three distinct asperity heights over a wide range of lambda ratio (ratio of lubricant film thickness, separating two contacting surfaces, to their combined RMS roughnesses). Subsequently, the behaviour of film distribution for a range of mixed rolling–sliding conditions is also studied for both thin and thick film conditions. This study provides an estimate of how and when transverse asperities decompress and an indication of conditions under which these asperities cease to affect lubricant film formation.     

Increase of Traction Coefficient due to Surface Microtexture

Increasing the traction coefficient of a traction drive system is a key factor in obtaining a smaller, lighter unit and also greater torque capacity. This study focused on the microtexture of the rolling elements, and effect of microtexture was examined with the aim of improving the traction coefficient in the viscous region. Three textures—dimple, transverse, and longitudinal—were examined using a 4-roller tester that enabled tests to be conducted under high pressure and high rolling speed. As a result, it was found that the longitudinal surface texture is the best for improving the traction coefficient. The results obtained with EHL analysis showed that only the surface texture with longitudinal grooves increased the traction coefficient, just as in the tests conducted with the 4-roller tester. The longitudinal surface texture was optimized using the 4-roller tester. The test results made it clear that the groove depth, groove pitch, and also the radius of curvature of the convex portion of the rolling elements are important parameters of the longitudinal grooves for improving the traction coefficient while assuring high durability at the same time. An attempt was then made to increase the traction coefficient of an actual CVT variator by applying the optimized longitudinally grooved microtexture to the traction surfaces. The test results show that the traction coefficient can be increased without sacrificing durability by optimizing the surface microtexture.     

Experimental Study of Lubricant Film Thickness Behavior in the Vicinity of Real Asperities Passing through Lubricated Contact

This article presents an experimental study of the influence of real surface micro-geometry on the film thickness in a circular elastohydrodynamic (EHD) contact formed between a real, random, rough surface of steel ball and smooth glass disk. Phase shifting interferometry was used to measure in situ initial undeformed rough surface profiles, whereas thin film colorimetric interferometry provided accurate information about micro-EHD film thickness behavior over a wide range of rolling speeds. Two real roughness features were studied in detail—a 56-nm-high ridge and a 90-nm-deep groove, both transversely oriented to the direction of surface motion. It was shown that the ridge is heavily deformed in a loaded contact and its height increases with increasing rolling speed. The asperity tip film thickness behavior is quite similar to the contact average film thickness when the film thickness is higher than the undeformed ridge height. However, below this limit the film is thicker than what the EHD theory predicts. For the groove, a local reduction in film thickness at the leading edge was observed. When the groove is passing through the EHD conjunction, it maintains its undeformed shape. The behavior of both roughness features studied shows good agreement with previous experimental observations conducted using an artificially produced ridge and groove.     

Investigating the Effect of Surface Finish on Mixed EHL in Rolling and Rolling-Sliding Contacts

Surface finish may significantly affect the lubrication performance of a tribological interface through the influence of topography on micro/nanoscale fluid flows around localized contacts at surface asperities. This paper aims to study the mixed lubrication performance of a group of engineered surfaces, including turned, isotropically finished, ground, and dimpled surfaces, under different operation conditions by means of a deterministic mixed elastohydrodynamic lubrication (EHL) model. The honed surface was used to mate with other surfaces. The results indicate that a longitudinal contact ellipse favors longitudinally oriented mating surface roughness and that a transverse contact ellipse, as well as a line contact, prefers a transversely orientated mating surface roughness for lubrication enhancement.     

A Multiscale Framework for EHL and Micro-EHL

In this article, a heterogeneous multiscale method is introduced to analyze the microelastohydrodynamic lubrication (micro-EHL) of bearings with topological features. Two scales are adopted in the analysis: the large-scale simulations describe the entire bearing domain, and the small-scale simulations describe the fluid–structure interaction (FSI) at the small-scale features. Conservation of mass and momentum of the lubricant and the bearing's elastic deformation are solved for. The relationship between the pressure gradient and mass flow is obtained from homogenized small-scale FSI simulations and applied on a global scale via a scattered data interpolation method. When the micro structure is periodic the exact model at micro scale is replaced by an effective derived equation, i.e., homogenized model. The elastic deformation of the textured bearing surface is addressed at both the large and small scales, by decomposing the displacement influence matrix into the diagonal terms and nondiagonal terms (sorted at the small scale and large scale, respectively).

The multiscale method was demonstrated as being capable of modeling the global pressure and film thickness for a bearing with surface texture while maintaining the accuracy of the small-scale modeling features. The illustrative geometry was that of a linear converging pad bearing in two dimensions. The solutions were compared with those obtained using lubrication theory for the smooth surface case, and good agreement was obtained. The method was then demonstrated for geometries incorporating topographical features.     

On Mechanisms of Fatigue Life Enhancement by Surface Dents in Heavily Loaded Rolling Line Contacts

This paper studies mechanisms of surface dents in enhancing the fatigue life of rolling bearings previously reported in Akamatsu et al. (1). First, transient micro-EHL analyses of heavily loaded contacts between rough surfaces with multiple dents are conducted under near rolling conditions. Contacts with various dent dimensions, dent arrangements under different loading and kinematic conditions are investigated. Results show that surface dents generate no favorable micro-EHL effects to enhance the contact fatigue life. Subsequent analyses, in conjunction with other published studies, suggest that the fatigue life enhancement likely comes from the reduced local traction at asperity contacts through the “oil pots” effects of the dents. The effects of the surface dents on contact fatigue life may depend on the lubrication regime in which the contact is operating being favorable in poor lubrication conditions but adverse in well-lubricated contacts. Since rolling bearings are usually designed to operate in a healthy regime of lubrication, fatigue life enhancement by artificially introducing dents on bearing surfaces may not extend to field applications.     

Effects of Surface Roughness on Point Contact EHL

The effects of orientation of surface roughness, entrainment (rolling) velocity, and slide/roll ratio on micro-elastohydrodynamic lubrication (micro-EHL) are investigated under pointcontact conditions using the optical interferometry technique. Long bumps with constant height and wavelength produced artificially on the surface of a highly polished steel ball are used as a model roughness. It is shown that the asperities are elastically deformed and the magnitude depends on the film factor A, defined by the ratio of the central film thickness based on smooth surfaces to the composite surface roughness, as well as the surface kinematic conditions and the orientation of the asperities. It is also found that a thin or thick oil film formed at the inlet of the contact by a moving rough surface travels through the contact region at a speed very close to the average speed of the contacting surfaces. The possible mechanism is discussed.     

Wavelength Dependence of Micro-EHL Pressure Rippling with Random Surface-Roughness Profiles

This paper studies line-contact elastohydrodynamic lubrication (EHL) between two rough surfaces of random roughness profiles. A transient micro-EHL model is used to simulate the problems. Numerical results are obtained for a range of practical operating conditions and for relatively small specific film thicknesses (i.e., the A-values). The study reveals the wavelength dependence of micro-EHL pressure rippling. For the same A-value, the size of the pressure rippling is shown to be significantly smaller than those predicted earlier using more idealized system models. Furthermore, the pressure distribution is essentially unaffected by the short-wavelength components in the random surface-roughness profiles, except under near rolling conditions. It is shown that lubricant non-Newtonian shear-thinning is the mechanism that suppresses the short-wavelength pressure ripples. With a Newtonian lubricant, sharp pressure rippling is generated by the small-scale surface roughness textures. Since EHL lubricants exhibit significant shear-thinning and since the surfaces possess random roughness profiles, the results reported in this paper reflect a main feature that may prevail in realistic EHL conjunctions. Future work will establish a theoretical basis for the numerical analysis conducted in this research.