Does Oil Pressure Help Film Strength?

[Jetronic]

by the time you have enough pressure built up the rpm would've been high enough to form an oil wedge anyway.

only usefull if you've got a completely independant lubrication system...

 

[Shannow]

Originally Posted By: Pontual
Yep, but a rather stronger pressure will fill the groove in time to push the caps out of the journals creating a competent wedge.


You are talking volume again pontual...more volume from the pump will fill the galleries sooner...nothing to do with pressure, which as Jetronic rightly states, happens AFTER the galleries are already full.

Ever pulled an engine apart and found a dry bearing ?

They aren't, as capillary action keeps them charged with oil...that's what keeps them lubricated (and yes it can form a wedge) in the couple of seconds that oil supply takes to replenish it.

You've been reading to many "all the oil is drained back to the sump" tales from the diesel thread if you think otherwise.

 

[MolaKule]

Originally Posted By: Pontual
Yep, but a rather stronger pressure will fill the groove in time to push the caps out of the journals creating a competent wedge.


You need to study hydraulics.

There is no way the 60 psi pressure from an automotive oil pump can overcome the immense pressure in a journal-bearing interface.

Forced Oil flow in an engine is for two purposes:

1) lubricant replenishment due to leakage

2) cooling

Which is what Shannow and others have been trying to tell you.

 

[DWC28]

Another view is anti-friction bearings which operate in hydro-dynamic regime. It is possible to obtain pressure of 1 GPa. At those pressure oil film thickness is affected. PAOs with a lower pressure viscosity coefficient give better efficiency because of lower film thickness. Connect Barus (sp?) equation to film thickness with Stribeck.

 

[Shannow]

Are there any automotive bearings that can sustain 1GPa surface pressure.

Not disagreeing with the statement, just in this application.

 

[DWC28]

Viscosity increases with pressure because the molecules are squeezed together forcing greater interaction. In an EHL contact where the pressure can be 2.1 GPa (300,000 psi) The higher the temperature the lower the viscosity increase due to pressure.

Viscosity pressure coefficient is the slope of lines on graphs of the log of viscosity vs pressure. The unit for pressure viscosity coefficient is the reciprocal of pressure. The SI units are 1/Pa or m2 N-1. Reference gives the pressure viscosity coefficients of several mineral oil showing a variation from 1.6 to 2.68 X 10-8 Pa-1. The coefficient increases with viscosity, and can vary by a factor of 3.

Pressure viscosity coefficient can also be measured from oil film thickness and other parameters from a transparent disk-on-ball apparatus. Pressure viscosity coefficient is used in the calculation of oil film thickness in tribological contacts. For example, in EHL contacts, oil film thickness is directly proportional to the 0.74 power of the pressure viscosity coefficient.

For more information call 1-888-HERGUTH (437-4884)

 

[DWC28]

The ball-on-flat is known as a Hertzian contact, which is a point contact with extremely high pressure. As an example, a 19mm (3/4”) diameter steel ball on a flat steel surface has a maximum contact pressure of 950 MPa (138,000 psi) for a 30 N (6.7 lb.) load. We can see that the nonconformal contact can produce pressures that are large enough to temporarily deform the solid steel surface.

 

[HerrStig]

Originally Posted By: Pontual
I prefer to see pressure as voltage and flow as amperage. Thats the classic definition. Resistance could be the clearances... could see the bearings/journals as repelling magnets. But if you raise the voltage has no intensified reppel effect ?
Where are the repelling magnets in Ohm's Law? Hope I'm not patronizing.

 

[Joe90_guy]

I always equated oil pressure to blood pressure. High blood pressure isn't a good thing. It means your heart is having to work harder to overcome greater resistance caused by gunked up arteries. Of course extending this analogy would equate thinner oils with rat poison (warfarin)....yeah, I can go with that!

 

[Shannow]

Joe90_guy,
not too bad an analogy, the oil pressure is the artifact of the bearings taking only what they need...you don't jam oil through them, they draw off the supply what they need for the operating conditions (my turbines with 9:00 located oil feeds could draw 20kPa vacuum on the supply line).

Any oil that the pump supplies over this goes to build pressure, and the manufacturers put excess capacity in to drive things like tensioners and squirters, and provide capacity margin when things like temperature and viscosity cause an increase in leakage (your warfarin analogy), or an increase in leakage (wear and the like).

 

[zeng]

Originally Posted By: DWC28
Viscosity increases with pressure because the molecules are squeezed together forcing greater interaction. In an EHL contact where the pressure can be 2.1 GPa (300,000 psi) The higher the temperature the lower the viscosity increase due to pressure.
Viscosity pressure coefficient is the slope of lines on graphs of the log of viscosity vs pressure. The unit for pressure viscosity coefficient is the reciprocal of pressure. The SI units are 1/Pa or m2 N-1. Reference gives the pressure viscosity coefficients of several mineral oil showing a variation from 1.6 to 2.68 X 10-8 Pa-1. The coefficient increases with viscosity, and can vary by a factor of 3.
Pressure viscosity coefficient can also be measured from oil film thickness and other parameters from a transparent disk-on-ball apparatus. Pressure viscosity coefficient is used in the calculation of oil film thickness in tribological contacts. For example, in EHL contacts, oil film thickness is directly proportional to the 0.74 power of the pressure viscosity coefficient.
For more information call 1-888-HERGUTH (437-4884)


MOFT film thickness/strength correlates with pressure viscosity coefficients (PVC) of an oil that is operating in Elasto-Hydrodynamic EHL lubrication regimes.
It's applicable in roller/ball bearing lubrication in a typical automotive differential and manual/automatic/DCT transmission systems.
However, plain bearings in engine crankshafts and big-ends typically operates in full hydrodynamic FHD lubrication regimes , whereby the oil property of PVC is not quite applicable.
CMIIMW.

 

[Jetronic]

It might be applicable for the camshaft followers and tumblers, which have needle bearings.

 

[Shannow]

EHD is certainly applicable in the rollers and cam faces etc.

But the premise of the question was regarding actual gallery oil pressure versus oil film thickness...and the answer is little to nothing.

 

[zeng]

Originally Posted By: Jetronic
It might be applicable for the camshaft followers and tumblers, which have needle bearings.


You're right about cam rollers, and piston ring/liners too..

 

[CT8]

Originally Posted By: Joe90_guy
I always equated oil pressure to blood pressure. High blood pressure isn't a good thing. It means your heart is having to work harder to overcome greater resistance caused by gunked up arteries. Of course extending this analogy would equate thinner oils with rat poison (warfarin)....yeah, I can go with that!
Coumadin

People warfarin!