Elasto-Hydrodynamic Lubrication (EHL)

Figure 1: Rolling Element Bearing (SRB)

Elasto-hydrodynamic lubrication is the predominant lubrication mechnism between rollers and raceways in rolling element bearings, see figure 1. Due to the relative motion of the surfaces and the no-slip boundary condition for the fluid on the surface lubricant is dragged into the gap.Further aided by the deformation of the surfaces due to the small size of the ``contact area´´  and the strong dependence of viscosity on the pressure a lubricant film is formed that in many cases can fully separate the surfaces and protect against wear.  EHL problems are commonly modelled using the lubrication assumption that a ratio film thickness to length scale (h/L) is small. The flow is then described by the Reynolds equation (derived from Navier-Stokes equations) with additional equations for the surface deformation and force balance. The film thickness is varies from O(10) nm to O(200) nm depending on the loading conditions and the supply conditions:

• fully flooded (ample lubricant supply)
  
• starved (
  
• time varying load and/or speed
• surface roughness moving through the contact
• layer modification on surface due to overrolling
• layer modification on surface due to centrifugal effects.

In our research, in collaboration with researchers from other institutes,  we have studied many of these aspects for single contacts theoretically using numerical simulations, identified relevant dimensionless parameters and unveiled fundamental mechanisms. For this purpose Multigrid/Multilevell methods were developed. Owing to the efficiency of these algorithms nowadays realistic problems can be studied using very dense grids and simulation in time when needed. The results can be found in many publications, see list. Some illustrative results are given below. Pdf files of PhD Theses in which relevant models, solution methods, and results a can be downloaded below. Current research involves e.g. grease lubrication and mixed lubrication modelling.