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Becoming higher in the front (leading edge) of the dent when the dented surface is travelling slower than the smoother one, and vice versa (see fig. 1 for nomenclature). Nélias and Ville, Ville and Nelias have carried out numerous experimental observations and numerical simulations of indentations in EHL contacts with conditions of pure rolling and rolling-sliding. The numerical simulations in Nélias and Ville with Newtonian fluids showed that any increase of sliding [bearings online] increases the maximum shear stress underneath the contact (e.g., higher sliding and higher friction as follows from a Newtonian fluid). It was observed that the higher the sliding imposed on the EHL contact, the higher the maximum shear stress in the subsurface and therefore the shorter the life around the indentation. 

In Ville and Nélias, further experiments were conducted and some simulations also carried out for S = ±0.015. The experiments confirmed earlier conclusions that the preferred site for spalls to develop depends on the skf bearings  friction direction. A dented slower surface tends to develop a spall at the leading edge of the dent, whereas a dented faster surface tends to develop the spall at the trailing edge of the dent. For pure rolling conditions, it is argued that the spall could appear on either side. From dry contact experiments (Xu et al.) it was observed that the spall appears in the back side (trailing edge) of the dent on the driving surface; thus Xu et al. concluded that it is the friction force on the surface that is the main mechanism promoting spalling of dented surfaces.

Indentations produced by different particles

The basic indent geometry has been idealized in fig. 2. Assuming that the particle is entrapped in the contact (very large particles are not entrapped and very small particles go through the film without causing indentations), different indent shapes are produced – depending on the hardness of the particle , its geom­etry and the hardness of the indented material. Indentations of large size (ø) or very deep (hp) and with large shoulders (sp) are the most dangerous. Fig. 3 shows different particle characteristics associated with the corresponding indentations. Soft or malleable (ductile) particles (fibre or metal) produce shallow bearings  indentations with shoulders. Brittle, hard particles shatter into many very small particles and prod­uce a cluster of tiny indentations. Friable tough particles produce a large agglomerate of material that dents the steel, producing sharp shoulders.

Interaction between indentations and lubricant To better understand the interaction of indentations with surrounding lubricant entrapped in a heavily loaded contact, SKF has developed detailed models and conducted experiments to study the phenomena associated with this problem. Here, the process of an indentation or any other geometric­al surface disturbance entering the contact is described.

Surface geometrical disturbance entering the heavily loaded lubricated contact When a surface disturbance enters the EHL contact, it generates two different pressure and film thickness waves. The first one is the associated pressure wave produced timken bearings by the steady-state elastic deform­ation of the surface disturbance. This wave is accompanied by an opposite wave in the film thickness, and it travels inside the contact with the same speed as the surface that produces it (e.g.,u2); it is called “particular integral”.