Effect of Thickener Particle Geometry and Concentration on the Grease EHL Film Thickness at Medium Speeds

The aim of this paper was to understand the parameters influencing the grease film thickness in a rolling elastohydrodynamically lubricated contact under fully flooded conditions at medium speeds. Film thickness measurements were taken under pure rolling for six commercial greases and their bled oils. The grease film thickness was found to be higher than corresponding bled oil, suggesting the presence of thickener in the contact. No rheological properties (characterized by steady and dynamic shear) showed any direct relation to the film thickness of the studied greases. AFM measurements of the thickener microstructure, from which the dimensional properties of the thickener particles (fibers/platelets/spheres) were estimated, showed that the relative increase in the film thickness due to entrainment of the thickener was proportional to the ratio of thickener volume fraction to the size of the fibers/platelets/spheres. Hence, with the same concentration, smaller thickener particles lead to the generation of thicker films than larger thickener particles. Next, this relation was used to establish the percentage of the thickener particles passing through the contact. Depending on the grease type, between about 1 and 70 % of the thickener particles were found to travel through the contact

Waviness Deformation in starved EHL circular contacts

By means of numerical simulations the deformation of transverse and isotropic harmonic waviness in EHL circular contacts under pure rolling has been studied in relation to the lubricant supply to the contact. In earlier work the deformation of waviness under pure rolling in a fully flooded contact was shown to depend on a single non-dimensional wavelength parameter. In terms of this parameter short wavelengths deform very little. In this paper the effect of starvation on this behavior is shown. First, the steady state smooth surface problem is discussed as an introduction and as a reference problem. It is illustrated in detail how the entire film thickness level decreases with decreasing lubricant supply. Subsequently, results are presented for the time dependent problem with waviness moving through the contact under pure rolling. The relative deformed amplitude of the waviness inside the contact is shown to depend on the same non-dimensional wavelength parameter as before, but also on the degree of starvation. A smaller lubricant supply leads to a larger reduction of the waviness amplitude inside the contact. Finally, it is shown that to an acceptable accuracy the relative deformed amplitude of the starved problem can be predicted by the formula for the fully flooded problem if the generalized wavelength parameter is modified using the reduction factor of the central film thickness for the starved steady state smooth contact. For this reduction factor an accurate formula is available and as a result also for starved contacts by means of a component wise approach a crude estimate of the deformed surface micro-geometry (roughness) inside a contact can be obtained quite easily now

Extension of the Friction Mastercurve to Limiting Shear Stress Models

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 0 and the limiting shear stress L

Influence of longitudinal roughness on friction in EHL contacts

The effect of longitudinal roughness on the friction in EHL contacts is investigated by means of numerical simulations. In the theoretical model the Eyring equation is used to describe the rheological behavior of the lubricant. First the relative friction variation caused by a single harmonic roughness component is computed as a function of the amplitude and wavelength for a wide range of operating conditions. From the results a curve fit formula is derived for the relative friction variation as a function of the out-of-contact geometry of the waviness and a newly derived parameter characterizing the response of the lubricant to pressure variations. Subsequently, the case of a superposition of two harmonic components is considered. It is shown that for the effect on friction such a combined pattern can be represented by a single equivalent wave. The amplitude and the wavelength of the equivalent wave can be determined from a nonlinear relation in terms of the amplitudes and wavelengths of the individual harmonic components. Finally the approach is applied to the prediction of the effect of a real roughness profile (many components) on the friction. From a comparison of the results with full numerical simulations it appears that the simplified approach is quite accurat

A generalized traction curve for EHL contacts

In this paper the subject of friction prediction is revisited, with the aim of obtaining a general formula predicting the coefficient of friction over a wide range of operating conditions. By means of full numerical simulations of the smooth isothermal elliptic contact, and assuming an Eyring non-Newtonian behavior, the coefficient of friction is computed for a wide range of operating conditions. It is shown that with respect to sliding friction, all results can be presented on a single generalized friction curve relating a reduced coefficient of friction to a characteristic nondimensional shear stress. Finally, it is shown that some measured data presented in the literature when presented in terms of the derived parameters closely follow the derived behavior, which provides a validation of the theoretical results

Effects of Surface Micro-Geometry On the Lift-Off Speed of an EHL Contact

With decreasing speed the film thickness in an EHL contact decreases. Below a certain speed asperity contact will take place and gradually the contact enters the mixed lubrication regime. Vice-versa, with increasing speed beyond a certain speed the film thickness has reached a level where asperity contacts have become so rare that the contact will be in the full film regime.\ud \ud It seems only logical to expect that the speed at which the first (or last) significant asperity interactions start to take place is influenced by the micro-geometry of the surfaces. In experiments performed on a two-disk rig under conditions of pure rolling, using one very smooth and one rough disk it was indeed observed that the “lift-off” speed defined as the speed above which full film lubrication prevails, differed significantly for surfaces with a different micro-geometry. The test results can be seen as ranking for the surface micro-geometries in terms of their film generating capability. In this paper the question is addressed if a ranking as observed in the tests can be predicted in advance, using the load conditions and the measured surface micro-geometry as input, without having to resort to full-scale numerical simulations of any sort.\ud \ud Based on the amplitude reduction formula proposed by Venner et al. (2000), for roughness in contacts under pure rolling a model is constructed that, given the load condition and a measured surface micro-geometry, determines the deformed micro-geometry and subsequently a measure of “probability of contact.” For a given contact this measure can be plotted as a function of speed to obtain a theoretical “lift-off” curve. For the different types of surface micro-geometries used in the tests such a curve is compared with the experimental results, showing a promising agreement in ranking. \u