Dr Haas' Motor Oil University Article

[Solarent]

It was that or replace the engine. There was so much sludge build up on the oil pickup screen there was zero oil pressure.

He is always playing around with different additive and oil combinations in his truck. On this particular blend it is running fine - because it's a flush cycle, he's draining every 1500 km or so - but I've seen the inside of his engine and it seems to be working. But I wouldn't recommend it for anyone else who doesn't have the resources and the money to burn on an engine experiment.

 

[SteveSRT8]

Sounds like you could do him a favor and get him a quart of Kreen!

I'd also be very interested in the results...

 

[Shannow]

Originally Posted By: Blue_Angel
Originally Posted By: Shannow
* more flow = more lubrication (actual understanding of hydrodynamics is 100 years old, and it's not flow)


Does someone have a link to a good article on this subject as it applies to crank/rod bearings? Preferrably keeping the language and/or math involved at a basic level.


It's pretty hard to escape the math.

http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2663284

http://www.substech.com/dokuwiki/doku.php?id=hydrodynamic_journal_bearing

In short, the oil that's in contact with the crank is stuck to and moves with the crank, dragging the oil with it.

The oil that's in contact with the bearing is stuck to the bearing, causing a shear gradient across the oil film.

As the gap between the journal and the bearing reduces, the oil is dragged into the gap, increasing it's pressure and pushing the shaft and bearing apart...it also pushes oil out the sides (ends) of the bearing, which the oil pump has to make up.

For any given bearing design, this separation force is larger with increasing viscosity, and increasing shaft speed.

The oil will spends some number of revolutions in the bearing getting hotter, before leaving the sides of the bearing, meaning that actual make-up to the bearing is some fraction of the total oil circulating in the bearing.

 

[Clevy]

Originally Posted By: Shannow
Originally Posted By: Blue_Angel
Originally Posted By: Shannow
* more flow = more lubrication (actual understanding of hydrodynamics is 100 years old, and it's not flow)


Does someone have a link to a good article on this subject as it applies to crank/rod bearings? Preferrably keeping the language and/or math involved at a basic level.


It's pretty hard to escape the math.

http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2663284

http://www.substech.com/dokuwiki/doku.php?id=hydrodynamic_journal_bearing

In short, the oil that's in contact with the crank is stuck to and moves with the crank, dragging the oil with it.

The oil that's in contact with the bearing is stuck to the bearing, causing a shear gradient across the oil film.

As the gap between the journal and the bearing reduces, the oil is dragged into the gap, increasing it's pressure and pushing the shaft and bearing apart...it also pushes oil out the sides (ends) of the bearing, which the oil pump has to make up.

For any given bearing design, this separation force is larger with increasing viscosity, and increasing shaft speed.

The oil will spends some number of revolutions in the bearing getting hotter, before leaving the sides of the bearing, meaning that actual make-up to the bearing is some fraction of the total oil circulating in the bearing.


Now that's interesting. I always learn something from your posts.

 

[Trav]

Originally Posted By: Clevy
Originally Posted By: Shannow
Originally Posted By: Blue_Angel
Originally Posted By: Shannow
* more flow = more lubrication (actual understanding of hydrodynamics is 100 years old, and it's not flow)


Does someone have a link to a good article on this subject as it applies to crank/rod bearings? Preferrably keeping the language and/or math involved at a basic level.


It's pretty hard to escape the math.

http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2663284

http://www.substech.com/dokuwiki/doku.php?id=hydrodynamic_journal_bearing

In short, the oil that's in contact with the crank is stuck to and moves with the crank, dragging the oil with it.

The oil that's in contact with the bearing is stuck to the bearing, causing a shear gradient across the oil film.

As the gap between the journal and the bearing reduces, the oil is dragged into the gap, increasing it's pressure and pushing the shaft and bearing apart...it also pushes oil out the sides (ends) of the bearing, which the oil pump has to make up.

For any given bearing design, this separation force is larger with increasing viscosity, and increasing shaft speed.

The oil will spends some number of revolutions in the bearing getting hotter, before leaving the sides of the bearing, meaning that actual make-up to the bearing is some fraction of the total oil circulating in the bearing.


Now that's interesting. I always learn something from your posts.


+1

 

[demarpaint]

Originally Posted By: Trav
Originally Posted By: Clevy
Originally Posted By: Shannow
Originally Posted By: Blue_Angel

* more flow = more lubrication (actual understanding of hydrodynamics is 100 years old, and it's not flow)


Does someone have a link to a good article on this subject as it applies to crank/rod bearings? Preferrably keeping the language and/or math involved at a basic level.


It's pretty hard to escape the math.

http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2663284

http://www.substech.com/dokuwiki/doku.php?id=hydrodynamic_journal_bearing

In short, the oil that's in contact with the crank is stuck to and moves with the crank, dragging the oil with it.

The oil that's in contact with the bearing is stuck to the bearing, causing a shear gradient across the oil film.

As the gap between the journal and the bearing reduces, the oil is dragged into the gap, increasing it's pressure and pushing the shaft and bearing apart...it also pushes oil out the sides (ends) of the bearing, which the oil pump has to make up.

For any given bearing design, this separation force is larger with increasing viscosity, and increasing shaft speed.

The oil will spends some number of revolutions in the bearing getting hotter, before leaving the sides of the bearing, meaning that actual make-up to the bearing is some fraction of the total oil circulating in the bearing.


Now that's interesting. I always learn something from your posts.


+1

Yes

 

[Clevy]

Originally Posted By: SteveSRT8
Sounds like you could do him a favor and get him a quart of Kreen!

I'd also be very interested in the results...



HA
Awesome.
Yep

 

[jrustles]

Originally Posted By: demarpaint

+1

Yes


+5

Note to newer members: when Shannow speaks of hydrodynamic lubrication and/or bearing affairs, you listen! The man knows.

 

[rockydee]

Originally Posted By: jrustles
Originally Posted By: demarpaint

+1

Yes


+5

Note to newer members: when Shannow speaks of hydrodynamic lubrication and/or bearing affairs, you listen! The man knows.


+6 Good to know.

 

[JAG]

Originally Posted By: jrustles
Originally Posted By: demarpaint

+1

Yes


+5

Note to newer members: when Shannow speaks of hydrodynamic lubrication and/or bearing affairs, you listen! The man knows.

+ 7.

 

[turtlevette]

Originally Posted By: jrustles
With all due respect to Mr Haas, he was a medical doctor and high-end auto hobbyist, not a tribologist or chemist.

Even so, I'd swap brains with him in a heartbeat. Not with shannow though.

 

[OVERKILL]

Originally Posted By: JAG

Originally Posted By: jrustles
Originally Posted By: demarpaint

+1

Yes


+5

Note to newer members: when Shannow speaks of hydrodynamic lubrication and/or bearing affairs, you listen! The man knows.

+ 7.


Late to the party!

+8

 

[Blue_Angel]

Originally Posted By: Shannow
For any given bearing design, this separation force is larger with increasing viscosity, and increasing shaft speed.

The oil will spends some number of revolutions in the bearing getting hotter, before leaving the sides of the bearing, meaning that actual make-up to the bearing is some fraction of the total oil circulating in the bearing.


Thank you for the information sir! Much appreciated!

I still have a few questions I hope you can help me with:

1. Increasing shaft speed increases the separation force. Is this actually the case if the load stays constant, or does the separation force stay the same and the oil film thickness (wedge) just increase (less shaft deflection)?

2. Does increased shaft speed equal increased side leakage for a given load?

3. Does increased load equal increased side leakage for a given shaft speed?

4. How does the bearing's oil supply pressure relate to oil wedge thickness and side leakage for a given load?

Sorry for all the questions... I appreciate your time and based on the responses of others I'm not the only one learning something here!

 

[Shannow]

1) Yep, absolutely correct, the load is the load, and is supported by the bearing oil “film” pressure, under the usual force=pressure x area.

The pressures vary around the circumference of the bearing, from very high pressures (hundreds of psi, regardless of supply pressure) to partial vacuum as the minimum clearance point is passed, and the incompressible oil is trying to expand to fill the increased space. On the previous links, it’s the area marked cavitation zone.

This is the point that many manufacturers of turbines try to install the oil feed point, and it can be a couple of psi vacuum. If the oil was sitting in a tank at shaft level, it would suck its own oil in.

Side to side, the bearing has max pressure in the middle, and zero (gauge) at the sides

As speed goes up the thickness of the oil film increases.

On large machinery, the lift is often supplied by hydraulic pumps (hydrostatic lift), often only to provide enough clearance to run the first 400-800rpm, until the rotational speed is sufficient to provide enough separation to do the job.

2) Yes, as the speed “pumps” more oil into the minimum clearance point, more gets pushed out the side. The increased clearance that issues from the increased speed gives a bigger gap for side leakage also.

Oil leakage increases with speed, and positive displacement pumps shift more with speed too…matching to a degree the demand of the bearings.

3) Not sure, will have to look at that one for a bit…

4) Oil pressure ensures that the bearing has enough oil to make up for its side leakage, and should really be considered a column of oil, or a drum on a hose, providing a supply of oil for the bearing to draw off. Bearing will take enough oil to do the job.

Bearing temperature (metal) is considered to be equivalent to the temperature of the “working oil” inside the bearing. Given mass balance, the working oil temperature is half way between the oil supply temperature and the side leakage temperature. The heat difference is the amount of work that the shaft does shearing through the oil. The heat rise in between the oil supply temperature can commonly be 10-20C, so an oil that may be 10cst at an 85C sump temperature could be 8cst (average) in the bearing, with the oil leaving the bearing at 100-105C (I personally think it’s the high temperature oil flung from the bearing into crankcase fumes that chews the oxidation life from the oil, and is the general location for varnish production but can’t prove that).

If you force oil into the bearing beyond that which it will draw from the supply, you’ll force oil out of the wide part of the gap before it gets to the working part, and reduce the amount of oil circulating. This will drop the working oil temperature, reduce the maximum oil temperature, and increase the viscosity in the bearing. It will also increase the amount of shaft power wasted in the bearing somewhat….I’ve seen a big bearing become unstable when too much flow meant too high an operational viscosity, and it fell outside stability regions (vibration could unexpectedly rise to 0.001” movement every rev on a 60 tonne load at 3,000RPM)…change oil supply by 5C, or reducing supply head made it behave.

 

[simple_simon]

Originally Posted By: OVERKILL
Originally Posted By: JAG

Originally Posted By: jrustles
Originally Posted By: demarpaint

+1

Yes


+5

Note to newer members: when Shannow speaks of hydrodynamic lubrication and/or bearing affairs, you listen! The man knows.

+ 7.


Late to the party!

+8

+9 and be sure to ignore turtlevette

 

[Clevy]

Originally Posted By: Shannow
1) Yep, absolutely correct, the load is the load, and is supported by the bearing oil “film” pressure, under the usual force=pressure x area.

The pressures vary around the circumference of the bearing, from very high pressures (hundreds of psi, regardless of supply pressure) to partial vacuum as the minimum clearance point is passed, and the incompressible oil is trying to expand to fill the increased space. On the previous links, it’s the area marked cavitation zone.

This is the point that many manufacturers of turbines try to install the oil feed point, and it can be a couple of psi vacuum. If the oil was sitting in a tank at shaft level, it would suck its own oil in.

Side to side, the bearing has max pressure in the middle, and zero (gauge) at the sides

As speed goes up the thickness of the oil film increases.

On large machinery, the lift is often supplied by hydraulic pumps (hydrostatic lift), often only to provide enough clearance to run the first 400-800rpm, until the rotational speed is sufficient to provide enough separation to do the job.

2) Yes, as the speed “pumps” more oil into the minimum clearance point, more gets pushed out the side. The increased clearance that issues from the increased speed gives a bigger gap for side leakage also.

Oil leakage increases with speed, and positive displacement pumps shift more with speed too…matching to a degree the demand of the bearings.

3) Not sure, will have to look at that one for a bit…

4) Oil pressure ensures that the bearing has enough oil to make up for its side leakage, and should really be considered a column of oil, or a drum on a hose, providing a supply of oil for the bearing to draw off. Bearing will take enough oil to do the job.

Bearing temperature (metal) is considered to be equivalent to the temperature of the “working oil” inside the bearing. Given mass balance, the working oil temperature is half way between the oil supply temperature and the side leakage temperature. The heat difference is the amount of work that the shaft does shearing through the oil. The heat rise in between the oil supply temperature can commonly be 10-20C, so an oil that may be 10cst at an 85C sump temperature could be 8cst (average) in the bearing, with the oil leaving the bearing at 100-105C (I personally think it’s the high temperature oil flung from the bearing into crankcase fumes that chews the oxidation life from the oil, and is the general location for varnish production but can’t prove that).

If you force oil into the bearing beyond that which it will draw from the supply, you’ll force oil out of the wide part of the gap before it gets to the working part, and reduce the amount of oil circulating. This will drop the working oil temperature, reduce the maximum oil temperature, and increase the viscosity in the bearing. It will also increase the amount of shaft power wasted in the bearing somewhat….I’ve seen a big bearing become unstable when too much flow meant too high an operational viscosity, and it fell outside stability regions (vibration could unexpectedly rise to 0.001” movement every rev on a 60 tonne load at 3,000RPM)…change oil supply by 5C, or reducing supply head made it behave.



Wow.

Ok. So let's take a common v8 engine with ambient air temps from 10c-30c and the engine reaches oil temps of 100c at steady cruise but will increase if "playing"
The engine call for a 20 grade. As far as the bearing is concerned,knowing the oil temps it operates up to and at is a thicker oil better for the bearing or is the 20 grade better.
Highway rpm is 1950 or so. Considering the oil is basically sucked into the bearing as it's needed and not pressure dependent what would you use if it was yours.
The engine will see 5500rpm daily too.

 

[Shannow]

Can't tell...bearing particulars, sizes, clearances come into play.

http://www.aclperformance.com.au/us/Chrysler345HemiBearingsus.htm
http://www.aclperformance.com.au/us/Ford302WindsorBearingsus.htm

Compare both bearings (rod and main), and you can see that the area (diameterxlength) of the hemi is 9.1% and 9.3% bigger respectively, meaning that all things being equal, the hemi can handle higher cylinder pressures/loads, or thinner oil.

Compare the mains, and they are 80% stiffer on the hemi, the big ends about 10% less stiff. Bottom ends, a skirted (crossbolted) block, versus set on main caps on the 302...things will stay where they are supposed to be when loaded.

Assuming similar bearing clearances, Hemi can handle more pressure/load, or lower viscosity...than say a 302

OEM is for 20, 20 should be fine...just pick one with a decent HTHS..IMO

 

[Blue_Angel]

Another great read!

Originally Posted By: Shannow
The pressures vary around the circumference of the bearing, from very high pressures (hundreds of psi, regardless of supply pressure) to partial vacuum as the minimum clearance point is passed, and the incompressible oil is trying to expand to fill the increased space. On the previous links, it’s the area marked cavitation zone.

This is the point that many manufacturers of turbines try to install the oil feed point, and it can be a couple of psi vacuum. If the oil was sitting in a tank at shaft level, it would suck its own oil in.


Is there a reason why every bearing wouldn't have its supply located in this vacuum zone? Regarding your comments about excessive oil supply potentially reducing bearing efficiency, do you think car engines generally locate the oil feed to maximize efficiency? Or do you think manufacturing process/cost is more of a concern? Many crank bearings seem to feed from the top, as does the example in the article you linked to.

Originally Posted By: Shannow
On large machinery, the lift is often supplied by hydraulic pumps (hydrostatic lift), often only to provide enough clearance to run the first 400-800rpm, until the rotational speed is sufficient to provide enough separation to do the job.


Very glad you brought this up! A question in the back of my mind regarding pre-lube setups: With the oil feed at the top of the bearing and the shaft RPM at zero, is it possible for Hydrostatic lift to take place or does the oil feed require careful placement for a pre-lube application?

Crank bearings generally have a groove in the center (for supply to the rod bearings I assume), does this groove affect the way a pre-lube setup would function? Does this groove effectively act as a radial bearing supply, allowing oil wherever the bearing requires it most (vacuum)?

If oil feed location is not a concern, for a given bearing design and required lubricant (example, car engine crank bearing and oil to spec), how much pressure do you think it would take to get the crank floating prior to starter engagement and during cranking? How does the oil viscosity varying with temperature affect this situation?

Originally Posted By: Shannow
3) Not sure, will have to look at that one for a bit&#133


Looking forward to this one.

Originally Posted By: Shannow
The heat difference is the amount of work that the shaft does shearing through the oil. The heat rise in between the oil supply temperature can commonly be 10-20C, so an oil that may be 10cst at an 85C sump temperature could be 8cst (average) in the bearing, with the oil leaving the bearing at 100-105C


WRT oil viscosity, a heavier oil will have more shear drag than a lighter oil and will heat up more. Can we assume that the work done shearing the oil is in direct proportion to the oil's viscosity (i.e. a 15cst oil generates 50% more heat than a 10cst oil for a given application)? Or, does the resulting thicker fluid wedge with the thicker oil reduce the shearing somewhat, resulting in a non-linear work/viscosity relationship? Or, is it a linear relationship that varies at a different rate than the viscosity change?

On that note and using engine oils as the example, do different oil weights/blends generally have interchangeable specific heat values, or do lighter oils heat more/less for a given amount of work than heavier oils? Is this even a concern?

 

[turtlevette]

I'm a bit overwhelmed by all the data here and am trying to understand how many of these formulas and theorems apply to a combustion engine rather than a large turbine shaft sitting stationary.

A turbine bearing sees a steady torque load from the steam jets applying steady force on the blades. An engine sees impulse loading from the explosions applying a greatly varying force to the piston.

The rod bearings "see" centrifugal force that tries to force the oil around in all different directions.

There are probably other things going on in a highly revving engine that I'm forgetting.

I've been into old Pontiac V8s lately and the word is the rod journals are too large or larger than the chevy and others and that causes oiling problems. The larger the journal the larger the differential speed between the 2 sliding surfaces. A statement above says that differential speed builds more pressure to provide more separation. Does this formula break down at some point? What is the limit to shear if any?

 

[Clevy]

Originally Posted By: Shannow
Can't tell...bearing particulars, sizes, clearances come into play.

http://www.aclperformance.com.au/us/Chrysler345HemiBearingsus.htm
http://www.aclperformance.com.au/us/Ford302WindsorBearingsus.htm

Compare both bearings (rod and main), and you can see that the area (diameterxlength) of the hemi is 9.1% and 9.3% bigger respectively, meaning that all things being equal, the hemi can handle higher cylinder pressures/loads, or thinner oil.

Compare the mains, and they are 80% stiffer on the hemi, the big ends about 10% less stiff. Bottom ends, a skirted (crossbolted) block, versus set on main caps on the 302...things will stay where they are supposed to be when loaded.

Assuming similar bearing clearances, Hemi can handle more pressure/load, or lower viscosity...than say a 302

OEM is for 20, 20 should be fine...just pick one with a decent HTHS..IMO



Thank you very much Shannow. I appreciate your input.

Thanks again.