Viscosity and lubrication film thickness

[rickwsykes]

Is there a calculation/equation that will determine the thickness of the oil-lubricating film based upon the viscocity of the oil? I realize that temperature, load and surface area of the load-bearing area are essential factors, but given those factors, how do we determine the effect of viscosity to provide a thick enough lubricating film? Too thick would skid, too thin would allow the surfaces to touch. How do we determine what viscosity would please Goldilocks?
Can anyone provide the chemical reasons that one oil is thicker (more viscous) than another and therefore can resist squeezing out of the load zone?

 

[EnglishMuffin]

Is this elastohydrodynamic or hydrodynamic lubrication ? I assume the former. In things like roller bearings or gears, the viscosity of the oil film at the contact ellipse or contact zone increases locally by hundreds of times - the amount depends on the "pressure - viscosity coefficient" - and that's part of the reason it doesn't squeeze out. But I don't know the chemical/physics reason for why it increases - I bet someone else reading this does. That's why water is such a lousy lubricant for things like bearings and gears - it doesn't behave the same way as oil. Nobody understood this scientifically until the work of Grubin in Russis about fifty years ago. There are now mucho books and papers on this subject.

 

[EnglishMuffin]

Further thoughts :
Here is a reference that might interest you

http://www.herguth.com/technical/PHYSICAL.HTM#vpc

In researching your question about why oil behaves the way it does, the best I have been able to come up with after a brief search is that oil consists of large molecules that are relatively close together and have large forces between them. So the viscosity is highish to begin with, and increses rapidly as the molecules begin to interlock with one another. Actually, that may be as good an explanation as you'll get - although I could be wrong. You would think, wouldn't you, since we are continually told by physicists that they now understand all the four basic forces of nature (or is it down to three now?), and quantum electrodynamics is the most accurate theory ever devised by man, blah blah blah, that it would be a simple matter to predict exactly the properties of any material from theory alone, especially with the power of modern computers at their disposal. But it is not so - they still haven't got much of a clue when it comes to actual numbers.

Regarding the film thickness calculations, there are many texts that enable you to caculate it for elastohydrodynamic lubrication - I have one called "ball bearing lubrication" by Hamrock and Dowson. You can also get the EHD manual from Mobil, for example.
For hydrodynamic lubrication, see a text on journal bearings.

 

[electricpete]

Qualitatively, I think that film thickness increases roughly "in proportion" to viscosity times speed over load (psi).

Hi speed, hi viscosity, low load all push toward higher film thickness.

Low speed, low viscosity, hi load all push toward smaller film thickness.

 

[EnglishMuffin]

With hydrodynamic lubrication, film thickness increases with increasing speed, and decreases with increasing load, although not linearly. With elastohydrodynamic lubrication, film thickness increases with speed, although again not linearly, but is relatively insensitive to increases in load.

 

[fluidfilm]

Thick oils have longer/larger moleculer chains than thin oils and oils have vastly larger molecules than water. Think of a very, very long train at rest. As the engine gets underway, time will elapse before the rear carriages move in sinc with the front, yet all are connected and eventually the entire train will travel at the same speed.
At a speed where one surface impinges on another faster than the oil can displace, the oil behaves (in a temporary way) much the same as an elastic solid i.e. when stress is applied there is elastic strain and proportional resistance to (rapid) increased stress.