Oil pressure vs bearings life 2

[sierra4000]

HI,

What is the relationship between the pressure oil deliveries vs crankshaft bearing life?

What is the minimum oil pressure required for reliable (safe) lubrication crankshaft bearings at different rpm (1000 -7000)?

Thaks for explained

Radek

 

[MikeHalloran]

At pressure = 0, life = 0.
At pressure > 0, life > 0.



Mike Halloran
Pembroke Pines, FL, USA

 

[sierra4000]

is direct correlation?
I think oil pump pressure vs bearing oil pressure?

 

[MikeHalloran]

The pressure within the bearings comes from hydrodynamic pressurization; look it up.

Pressure at the pump/ filter/ oil gallery only assures that there is oil available for the bearings to inhale and pressurize, i.e., the pump is supplying oil faster than it can flow out of the bearings. Any positive number should be acceptable. Customary minimum numbers are basically safety factors. Increasing the oil pressure, e.g. by shimming the relief valve spring, just makes the pump work harder and put more heat in the oil, for no defensible reason, as far as the crank and cam bearings are concerned.

There are other reasons why you might want more flow, i.e. to force more oil out of the various orifices and squirters that cool the bottom of the piston crown, or lubricate the piston skirts, or spray oil on the cam lobes, etc.

Stock systems are normally adequate, within the normal stock rpm range. If you are running your engine faster than stock, you might consider modifications to the oil system, but which mods are needed for competition depends on the specifics of the particular engine.

The simplest and cheapest way to find out what your engine needs is to find the people who are doing well with that engine and not blowing up, ask them what they did to the oil system, and do just that.


Mike Halloran
Pembroke Pines, FL, USA

 

[sierra4000]

Thank You Mike,
is sufficient answer![Thumbsup2]

 

[romke]

there is no direct relation between oil pressure and bearing life. the oil pressure you can watch on a oil pressure gauge or indicated by a low pressure warning light is merely a indication that their actually is some pressure in the feeding system (and it is then assumed that there is also sufficient flow to the parts to be lubricated - which need not necessarily be the case....)

a much more reliable indicator would be a instrument that actually would indicate the flow rate. what you want is sufficient supply of oil to the bearings, at least as much as the actual leakage out from the bearings to compensate those so the bearing stays fully "filled". the oil pressure in the bearing itself has nothing to do with the pressure in the feed line and can reach values up to several hundreds of bars.

you may have noticed that when using different oils the oil pressure shown varies. there may be a clear distinction between oil that is cold or at operating temperature, and also there may be differences visible between say a 10W-30 oil or a 20W-50 oil. none of the differences are important, as long as a sufficient stream of oil is fed to the bearings to build up the lubricant film to separate the crankshaft and bearings completely. a thickness of about 40 to 100 microns is sufficient for that - about the thickness of a human hair.

a thinner oil (and thus a lower feed pressure) may be advantageous - bearing leakage will increase, so more oil flows through the bearings. as long as the thinner oil still is capable of building up the necessary oil film, the cooling of the bearings may be improved, since more oil flows through them. bearings can get quite hot - with modern truck engines the temperature of the oil entering the bearings usually is somewhere around 80 - 95 degrees C, the temperature of the oil flowing out may be up to 150 degrees C when the engine is under full load.

 

[sierra4000]

Thanks Romke,

after longer hard driving with hot oil (I do not have oil cooler now and oil thermometer has failed)
I noticing partial pressure drop,but still 3.5 bar at 6000RPM and about 0,8 bar at 1200RPM idle


I going to mounting exchanger water-oil, hopefully not damage my engine.

here my oil:

http://www.millersoils.co.uk/automotive/tds-automotive.asp?prodsegmentID=926&sector=Motorsport

Radek

 

[Tmoose]

I think a carefully designed and fabricated oiling system will be necessary to provide a continuous supply of oil during hard cornering, etc.
Just selecting a proper oil pressure will not do that when the oil pump takes a big gulp of air.

 

[sierra4000]

Tmoose,
oil supply is constant even when hard and long cornering and high overload,there is no problem here.

The pressure decrease occurred after oil overheating and followed viscosity lowering.

 

[romke]

the pressure drop is indeed due to lowering of the viscosity - the oil feed pump has to generate less pressure to supply sufficient flow because the oil flows easier through the piping. that need not worry you, unless it drops that much that due to leakages in the supply system the oil flow is decreased to a level where no sufficient feed to the bearings can be maintained or when the viscosity drops to a level where no sufficient film thickness can be build up in the bearings.

oil is very temperature sensitive - assuming a viscosity of 100% at 20 deg C it will drop to about 15% at 50 deg C, to about 6% at 100 deg C and about 2% at 150 deg C for a typical SAE 10W-40 oil...

the remaining viscosity still is sufficient to lubricate the engine, viscosity at startup is way higher then actually required. in your case you might have some extra reserve because the oil you use will be somewhat less sensitive to temperature changes. that can mean say 3% left at 150 deg C - 50% more, but still only a fraction of what you start with.

 

[sierra4000]

Thank You!