Elastohydrodynamic Lubricating (EHL) Films

[Jay]

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?

 

[RB Shannon]

As far as I know the film has a very high traction coefficient with no sliperage between molecular layers,I'm not positive but I believe EHL only occurs in rolling type contacts. But what I find the most interesting is the contact region can elastically flatten and trap the lubricant(hertzian zone) then go back to it's original form, this is the reasoning for a thinner base oil grease in a higher RPM roller bearing.

 

[MolaKule]

Non-Newtonian oils exhibit a phenomenon called, "piezo-visco-elasticity" when subjected to high pressures, and turn into a glass phase. EHD theory incorporates the mechanical response of the bearing and supporting structures as well as that of the oil. [ July 24, 2003, 04:18 PM: Message edited by: MolaKule ]

 

[JGW]

Interesting.

 

[Shannow]

Originally Posted By: Jay

Is EHL a higher friction regime than boundary lubrication since viscosity is so high? Can anybody shed more light on EHL films?

Have seen an example of a bearing purposely oversized to enable installation ease (beat cutting an 8" shaft in half at bearing time). Low contact pressures have resulted in the bearing skidding rather than grabbing traction, and makes for a lot of heat. I may be wrong here (fuzzy textbook picture from 20+ years ago in back of head), but EHD seemed to be like a "plank" of oil riding a wave of distorted metal, getting energy at one end, and pushing it in the other, while the element rolled to keep up with it.

 

[mechtech2]

I have heard of ball bearing 'skidding' . Some people warn of MoS2 use with them, because they will skid, not roll -then grab. It seems the closer you look, the more complicated things can become. Kinda like quantum physics .