- Jul 16, 2002
- Alamogordo, NM
I've been reading in the book, "Lubrication Fundamentals" about a lubrication regime not discussed much on this board--elastohydrodynamic lubrication (EHL). These very thin EHL films (.25 to 1.25um compared to >25um for hydrodynamic films) are discussed separately from hydrodynamic and barrier lubrication. EHL films are typically formed under very high pressure in gear teeth and along roller and ball-bearing surfaces. Wierd things happen in EHL films. The viscosity of the oil increases under pressure. In the case of a ball bearing, hydrodynamic pressure is sufficient to separate the surfaces at the leading edge of the contact area, but as pressure rises the viscosity of the oil increases so much that the oil can't flow out of the contact area. The oil roughly doubles in viscosity for every 5,000psi applied. Finally, the pressure becomes so great and the viscosity of the oil so high that the oil actually becomes harder than the metal(!). Wierder still, when the films become this rigid, increases in pressure have very little effect on film thickness. I guess with viscosity that high one layer doesn't stick to one moving surface and move with that surface while another layer sticks and moves with the other surface as in hydrodynamic lubrication. I'm having trouble visualizing EHL. Is EHL a higher friction regime than boundary lubrication since viscosity is so high? Can anybody shed more light on EHL films?