Bearings...how they work.

[Shannow]

In another thread, Trav posted

Originally Posted By: Trav
Shannow, is there any correlation between a cast crank and lightly reinforced block that are subject to flexing and this..
Quote:
Hydrodynamic journal bearings become unstable with small shaft eccentricity. It's the reason that high speed or relatively low loaded hydrodynamic bearings are to be designed less stiff.


Would the slight flexing be considered eccentricity?


Thought it would be timely to explain what the term eccentricity is, and how bearings work in general.

For those unfamiliar with hydrodynamics, the relative movement of surfaces, with a lubricant between them creates hydrodynamic forces that keep the parts separated...think a kid on a skim board down on the shoreline, he can't stand on the board and have the water support him, it doesn't work in deep water, but when the film of water is thin enough and he's going fast enough, there is enough force built up to keep the board and sand apart.

Same with a journal bearing

The oil that is in contact with the shaft is dragged along with the shaft, and the oil in contact with the bearing is stationary. The oil dragged along with the shaft is being dragged into the diminishing gap. As a result, it creates hydrodynamic forces which push the shaft and bearing apart.

The "eccentricity" that Trav has mentioned is the "off centredness" of the shaft, which creates the diminishing gap, that creates the forces that keep the shafts apart...it's the little "e" in the picture.

The vector diagramme to the right is the sum of forces that are generated. The load is shown as vertical down. There is a force perpendicular to the minimum film thickness point which is the direction of the internal pressure of the oil film, and the third is the equivalent drag that's produced by the turning shaft on the oil at MOFT (Minimum Oil Film Thickness).

 

[Shannow]

Now, the more load that is applied to a bearing, the greater the eccentricity that results, and the thinner the MOFT, and the stiffer the bearing behaviour is.

A lesser eccentricity means that there's not such a sharp delineation in the lines of force, and as a result, things can become unstable.

I've had it happen professionally, with a turbine bearing that was quite massive (22"dia, 19"long), and was insufficiently loaded, ad would become violently unstable. We moved the shell up 0.004" (to increase W), towards the left 0.002" (to increase "e"), and then had to reduce the oil supply marginally to make the average oil viscosity a little thinner....this is on a concrete pedestal some hundreds of tonnes, so we know full well where the alignment ended up.
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If you consider a spindly crank in a flexible block, the nature of Trav's question, the "foundations" are mobile, so as the whole system does the hootchy coo (Smokey Yunick terminology), the eccentricity, and MOFT go walkabout, and can be in any direction.

Thus the push these days for skirted block, crossbolting and the like.

 

[Trav]

Thanks for the explanation Shannow. How does a higher viscosity oil at the temperatures seen at the journal effect the hydrodynamic lubrication?
If i understand this correctly oil temp is substantially higher at the journal than the overall oil temp like what would be seen with an oil temp gauge?
Does the higher HTHS oil have a greater "cushioning" effect?

 

[Shannow]

Oil is fed into the bearing (typically at TDC), in the widest gap, as that'e where the pressure is lowest...in reality, you can supply nearly zero oil pressure, and the movement of the shaft will "suck" oil into it (Big turbine bearing that I described earlier pulls a couple of PSI on the oil feed)

As it traverses, down into the wedge (description of the reducing gap that leads to the MOFT point), it's pressure increases, and some wants to escape along the shaft, and out the ends of the bearing...some, the majority makes it through the MOFT, and is returned to the feed side, where it mixes with new, fresh, cold oil and repeats it's journey.

The oil that leaks out in the vicinity of MOFT is considerably warmer than the bulk oil supply to the bearing feed point, by some 10s of degrees C (again, the big turbine bearing has a feed temp of around 50C, and an exit temperature of nearly 100C)...as an aside, the journal temperature is the average of the feed and exit temperature...all of the heat in the crankshaft is there because of oil friction.

 

[Shannow]

Trav, from another thread some time back, here's the relationships between bearing dimensions and viscosity.

http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=3023239&page=1

Originally Posted By: Shannow
Bearings use another number, Sommerfeld's Number...
So= (r/c)^2 X uN/P
r = shaft radius
c = diametrical clearance (r/c becomes dimensionless)
u = absolute viscosity (Units PaS, or (Force Seconds/Length^2)
N = rotational speed (Units Radians per second, radians being dimensionless, the units are 1/s)
P = projected bearing pressure (Force/length^2)

So the uN/P becomes also dimensionless...the number is dimensionless, and is used to model bearings.

What it tells you is that to change anything on a correctly designed bearing, you have to look at what has to change also to keep So the same.

Double the bearing load, and you can do any of the following to keep So the same :
* double bearing area (brings P back to the same);
* double the rotational speed;
* increase the journal size (note, this has to iteratively play back into the bearing width to balance P and r);
* reduce bearing radial clearance by 29.3%;
* double the viscosity; or
* similar to the journal size, manipulate all of the above to keep within safe parameters.

Study the number, and get a feel for what the variables do in bearing behaviour.


If you look at the Stribeck curve (take my warmup wear thread), you'll see that the lower, X axis is the Sommerfeld number without the bearing dimension reference...

The viscosity that's applicable at the bearing MOFT point is the High shear rate viscosity (HTHS), not the bolstered Kinematic Viscosity.

 

[userfriendly]

The part I liked in the old thread was that if you double
the load, you can return to the original load carrying capacity
by either doubling the viscosity, bearing area or
bearing speed.
Jag's post was spot on. Point taken;
The application of VI ruins the calculation because it
is not linear. Especially so with VII containing
oils, as Jag pointed out, they affect the viscosity
change more at lower temperatures than they do at
high lubricant temperatures.
If you know the in and out temperature of the oil
during it's life-cycle in the bearing, and the
viscosity change of the oil between those two
temperatures, then the VI calculated between two
other temperatures cannot be applied accurately.

 

[Shannow]

used_Oil,
there's another couple of threads around (probably 2004 to 2006), where I went through the iterative design process for a steady state bearing with Newtonian lubricants...have tried to find it, unsuccessfully today.

You pick a point, "design" the bearing, and see what the heat rise is...use that to pick the new viscosity, then repeat the design, until the heat rise/outlet temperatures converge.

Three is about how many iterations is the minimum (using paper).

That's at design point.

The good thing about oils is that at temperatures below the design point, they are thicker and therefore provide greater protection and MOFT, much like I've described in the warm-up thread.

Problem is that with Non Newtonian Oils, and VII, the viscosity in the area of MOFT is in High Shear range, and the High Shear viscosity is very much less than the Kinematic Viscosity. For example in Supertech 5W30 is 35% lower than the kinematic viscosity at 100C...that's a very much added complicator.

Supertech data here KV100 is 11.02, HTHS100 is 7.26...the bearing sees 7.26cst in the area of MOFT.

The KV40, and KV100 form the basis for the VI, and are based on polymers "uncurling"....however in high shear conditions, they are stretched and flattened, leading to the differing HTHS100/KV100...some have posited that the High Shear Viscosity Index is parallel to the KV Viscosity Index, which is clearly not the case if you understand VIIs...

 

[AdamC]

Originally Posted By: Shannow
and then had to reduce the oil supply marginally to make the average oil viscosity a little thinner....


Firstly - thanks for the great information.

So is it true that installing a higher flowrate oil pump (say a part of a modified engine rebuild) increases the average viscosity of oil to provide more MOFT?

What is the effect of oil pump delivery pressure on MOFT? Or if no effect on MOFT, on what basis is it set?

 

[Shannow]

Hey Adam, one of my best efficiency engineers has been playing with an R32 for not long after the wet Bathurst.

Originally Posted By: AdamC
So is it true that installing a higher flowrate oil pump (say a part of a modified engine rebuild) increases the average viscosity of oil to provide more MOFT?


All things being equal,a higher flow rate pump doesn't do anything for MOFT.

The bearings will only draw from the oil galleries that which they need...trying to force more oil through them through a higher volume pump doesn't work, it just means that more oil flows through the oil pump relief, as it can't be forced through the bearings.

Originally Posted By: AdamC
What is the effect of oil pump delivery pressure on MOFT? Or if no effect on MOFT, on what basis is it set?


Like I said in an earlier post on the thread, a bearing can draw it's own oil, they can suck a couple of psi in their pumping action. More pressure in the delivery area will increase side leakage a tad, but not enough to see a significant change in bearing viscosity...delivery pressure of 80psi is tiny compared to the hundreds that are produced by the hydrodynamic process.

Set pressure in hydraulic systems is a design point. Positive displacement pumps will break things if there's nowhere for the oil to go.

More modern variable displacement pumps pump more oil at low RPM, then reduce delivery at higher RPM to prevent providing more oil then the engine needs and relieving it...it's more efficient, by a couple hundred watts than a constant volume pump on relief.

 

[sicko]

I love threads where you learn something new and interesting. Thanks for posting this.

 

[OVERKILL]

What a great thread

Very informative Shannow, thank you!

 

[dustyroads]

Originally Posted By: OVERKILL
What a great thread

Very informative Shannow, thank you!


Agreed! Thanks for this thread and the warm up thread as well!

 

[Oil Changer]

I'll pile on and say I love Shannow's posts too. I do know of one member that doesn't and will think Shannow is just further obfuscating. LOL!

 

[wemay]

on board with Overkill and dustyroads...

 

[wemay]

Originally Posted By: Oil Changer
I'll pile on and say I love Shannow's posts too. I do know of one member that doesn't and will think Shannow is just further obfuscating. LOL!


You typed what i thought, lol.

 

[Trav]

Originally Posted By: OVERKILL
What a great thread

Very informative Shannow, thank you!


+3 Great thread, thank you! Really interesting stuff.

 

[-SyN-]

Good Grief! Time for some advil!

Thanks Shannow!!

 

[The_Oil_Mole]

This is pretty interesting post. In around 2006 i was driving, and I smelled something but didn't think anything of it, b/c it was hot outside and there was a lot of roadwork (so it smelled like hot asphault already). I pulled over to look and my tire was actually smoking. And then FWOOSH! the whole wheelwell are caught on fire. By the time I got the fire truck to come put out my burning tire, the tire burst and I guess the force of air put the fire out. i'm positive it was the bearings rubbing up against each other. And that was the last time i borrowed grandpa's loaner boat of a chevy caprice ha!

 

[Shannow]

Originally Posted By: Oil Changer
I'll pile on and say I love Shannow's posts too. I do know of one member that doesn't and will think Shannow is just further obfuscating. LOL!


It's a rainy Easter Saturday morning, so will venture into the further realms of obfuscation.

One thought 'though....it's important to realise that the above is most applicable to the main bearings.

Consider the big end bearings for a moment. They have limited direct access to the oil feed supply/pressure and are using oil that's already been partially heated in the mains.

Just a thought bubble.

 

[Oil Changer]

Which brings up yet another point. Not all engine blocks have priority main oiling.

Originally Posted By: Shannow
One thought 'though....it's important to realise that the above is most applicable to the main bearings.

Consider the big end bearings for a moment. They have limited direct access to the oil feed supply/pressure and are using oil that's already been partially heated in the mains.

Just a thought bubble.