Lubrication and Viscosity

[Merkava_4]

I've been meaning to run this thread for awhile now, but wasn't sure how to word the questions. What would you guys say for the reason behind the increased viscosity of HM oils?

I'm thinking maybe as engine parts wear, their clearances between each other increase, making it necessary for a thicker oil to fill in the gap; but why do we need to fill in the gap? Kind of like how gear oils have such a high viscosity. Is there a relationship between gears needing a high viscosity and the increased clearances of an engine with a 150,000 miles on it?

How does an increase in viscosity relate to better lubrication?

Conversely, what would happen to a set of rear differential gears being lubricated with a thin oil like 5W-20?

 

[bruce381]

Quote:

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I've been meaning to run this thread for awhile now, but wasn't sure how to word the questions. What would you guys say for the reason behind the increased viscosity of HM oils?

""""less leaks through worn seals and better hydrodynamic film formation aswell as a thicker oil film to cushion and quite things.""""

Is there a relationship between gears needing a high viscosity and the increased clearances of an engine with a 150,000 miles on it?


""" No gears run in a high load/sliding EHD regime engines run in a low load boundry regime to different lube requirements"""

How does an increase in viscosity relate to better lubrication?

"""depends is cleranec for example is to wide a low vis oil will not form a hydrdynamic wedge if you can not fix the clearenec then a higher vis oil will work""


Conversely, what would happen to a set of rear differential gears being lubricated with a thin oil like 5W-20?

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""welding and scuffing aswell as pitting etc now if a full GL-5 EP oil of a low vis like a 20wt then much better perfromance due to the EP additives helping to make up for low oil vis BUT low vis will still have MORE wear than a higher vis of the same add system""

bruce

 

[Merkava_4]

I appreciate your valuable insight Bruce. I'm still trying to get to the bottom of this issue once and for all. Just a few more questions:

What is a hydrodynamic wedge?

What's an EHD regime?

Quote:

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BUT low vis will still have MORE wear than a higher vis of the same add system""

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Why?

 

[bruce381]

Cranks run in a full Hydro dynamic regime no crank to bearing contact when runing. A Hydro dynamic wedge is when the oil is "pulled into" the rotating shaft in the direction of rotation and the oil will "lift" the crank and oil finds wedges the crank up off the bearing.

That is NO metal contact as long as crank turns and oil is fed to it at the right vis, If vis is too low the wedge will not form or if clearence to large as in a worn engine then metal to metal contact occurs because the oil is not "lifting" the crank off the bearing and it will rub "boundry regime". That is why HM oils are blended to the high end aswell as to slow leaks and quite the engine down all which could be done by just going 1 grade thicker.

A higher vis oil will form the "wedge" easier/sooner/lower RPM than a light oil will.

That is why there is a limit to how thin can you go. IMHO that is 30wt for all around use a "tight" NEW engine can go 10-20wt MAYBE but IMHO the light oils will lead to more "pounding" types of falures but then again that in my thoughts.
bruce

 

[bruce381]

EHD is elasto hydro dynamic which means that in the case of a gear set as they roll they trap oil in the teeth this gets compressed and at a certain load the oil will become soild and will prevent metal to metal contact which will reduce and control gear teeth wear.

gears and "plain" bearing are different to lube in that gears like very thick oil plain bearings thin oil no to mention the additives a very complex subject that takes years to learn.
bruce

 

[Quest]

You've summed it up nicely, bruce.

My hat off to you sir.

Q.

 

[Merkava_4]

Yes, thank you Bruce!

I'm just trying to understand the basics for now, I'm not even going to attempt to get into additives yet. You've cleared up a lot of things for me; I appreciate your help immensely.

 

[Lonnie]

The wedge under a spinning crankshaft has to deal with a crank that has a far more complicated motion that just spinning in a circle. It's motion is not perfect circle and there are a couple of other motions that get added in as the cylinders fire and the crank spins, twists, hogs and flexes. The resulting motion is the combinations of several motions and the result can be something called oil swirl vibration that can exist in side to side vectors as well as linear vectors like a guitar string being plucked. Several of these vectors can end up working together and loading a bearing journal and cause damage or at least increased wear. Lubrication is a big part of the puzzle, but so is the design of other factors that are players in the lubrication of an engine. Then you do lots of testing to find out if it all works.

 

[LouDawg]

Excellent question, Merkava. Great responses, good information, good thread!

 

[Ramblin Fever]

Quote:

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That is why there is a limit to how thin can you go. IMHO that is 30wt for all around use a "tight" NEW engine can go 10-20wt MAYBE but IMHO the light oils will lead to more "pounding" types of falures but then again that in my thoughts.
bruce

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Yes! Great thread/question - told me exactly what I've been curious of myself.

Thank you both!

 

[Tempest]

Great stuff from Bruce as always. Take the terminology he has given you and start Googling. There is plenty of reading out there.