Three formulas are derived for predicting the central and the minimum film thickness as well as the asperity load ratio in line-contact EHL with provision for surface roughness. These expressions are based on the simultaneous solution to the modified Reynolds equation and surface deformation with consideration of elastic, plastic and elasto-plastic deformation of the surface asperities. The formulas cover a wide range of input and they are of the form f(W, U, G, r, V), where the parameters represented are dimensionless load, speed, material, surface roughness and hardness, respectively. [DOI: 10.1115/1.4005514]

Film Thickness and Asperity Load Formulas for Line-Contact Elastohydrodynamic Lubrication With Provision for Surface Roughness

A model is developed for predicting the performance of spur gears with provision for surface roughness. For each point along the line of action, the contact of pinion and gear is replaced by that of two cylinders. The radii of cylinders, transmitted load, and contact stress are calculated, and lubricant film thickness is obtained using the load-sharing concept of Johnson et al. (1972, "A Simple Theory of Asperity Contact in Elastohydrodynamic Lubrication, " Wear, 19, pp. 91-108) To validate the analysis, the predicted film thickness and the friction coefficient are compared to published theoretical and experimental data. The model is capable of predicting the performance of gears with non-Newtonian lubricants-such as that of shear thinning lubricants-often used in gears. For this purpose, a correction factor for shear thinning film thickness introduced by Bair (2005, "Shear Thinning Correction for Rolling/Sliding Electrohydrodynamic Film Thickness, " Proc. Inst. Mech. Eng., Part J: J. Eng. Tribal., 219, pp. 1-6) has been employed. The results of a series of simulations presenting the effect of surface roughness on the friction coefficient are presented and discussed. The results help to establish the lubrication regime along the line of action of spur gears.

Performance of Spur Gears Considering Surface Roughness and Shear Thinning Lubricant

The development of understanding of the phenomenon of elastohydrodynamic lubrication (EHL) throughout the twentieth century is reviewed. The development of solutions for both line and point contacts is considered for both fully flooded and starved conditions. Particular attention is given to the introduction of non-Newtonian rheological models and numerical methods. Progress in the analysis of impact and general non-steady-state conditions is reviewed, together with the consideration of rough surface and micro-EHL. Attention is drawn to the need to consider realistic models of real surfaces and real fluids in future studies.

Past, present and future studies in elastohydrodynamics

Mixed lubrication is a mode of fluid lubrication in which both hydrodynamic lubricant film and rough surface asperity contact coexist. Mixed lubrication problems are usually associated with signifi...

On the λ ratio range of mixed lubrication

Thermoelastohydrodynamic lubrication (TEHL) analysis for spur gears with consideration of surface roughness is presented. The model is based on Johnson’s load sharing concept where a portion of load is carried by fluid film and the rest by asperities. The solution algorithm consists of two parts. In the first part, the scaling factors and film thickness with consideration of thermal effect are determined. Then, simplified energy equation is solved to predict the surfaces and film temperature. Once the film temperature is known, the viscosity of the lubricant and therefore friction coefficient are calculated. The predicted results for the friction coefficient based on this algorithm are in agreement with published experimental data as well as those of EHL simulations for rough line contact. First point of contact is the point where the asperities carry a large portion of load and the lubricant has the highest temperature and the lowest thickness. Also, according to experimental investigations, the largest amount of wear in spur gears happens in the first point of contact. Effect of speed on film temperature and friction coefficient has been studied. As speed increases, more heat is generated and therefore film temperature will rise. Film temperature rise will result in reduction of lubricant viscosity and consequently decrease in friction coefficient. Surface roughness effect on friction coefficient is also studied. An increase in surface roughness will increase the asperities interaction and therefore friction coefficient will rise.

H. Czichos

Papers

Influence of Surface Roughness and Its Orientation on Partial Elastohydrodynamic Lubrication of Rollers