OIL "Cushion" for bearings..

[Gary Allan]

Originally Posted By: Shannow
Gary,
the load carrying capacity of a journal bearing is determined by the minimum film thickness.

The minimum film thickness for any given bearing is related to the Sommerfeld number, which is proportional to viscosity, proportional to rotational speed. (Inversely proportional to clearance ratio - i.e. clearance to diameter)

Higher viscosity...higher gap (note that gap is inverse to eccentricity), higher speed...higher gap, higher clearances... lower gap.





I leak oil at higher velocity. If not for my sponsor link, I'd use that as my signature.


Any comment on the "Q" with factoring visc in exchange for the oil velocity factor???

Some math only works one way.

 

[Shannow]

Gary,
I'm referring only to the viscosity in the active part of the bearing with my thicker lubricant/thicker film.

Is your question about what happens when a thicker oil with less flow spends more time in the bearing ?

 

[Gary Allan]

Go here: Scroll down to step 11

Now factor as a normal equation where the flow rate doubles and the bearing speed/size/etc are exactly the same. just what's left to be manipulated in the equation?

So ..some math must only work one way. Take it to step 12 ..and between the two of them ..if the (apparently) immutables and constants stay constant ..this has to be thrown out of whack.

 

[Bryanccfshr]

Here come some more references. The links are very valuable. The pictures are nice eye candy.
http://www.tech.plym.ac.uk/sme/desnotes/lube1.htm
http://www.tech.plym.ac.uk/sme/desnotes/LubeCalcEG1.htm
http://www.tech.plym.ac.uk/sme/desnotes/lube2.htm



Assumptions:

1 The lubricant obeys Newton's laws of viscous flow.
2 Intertia effects of the lubricant are neglected.
3 The lubricant is incompressible.
4 The viscosity of the lubricant is constant throughout the film.
5 The pressure does not vary in the axial direction.
6 The curvature of the bearing can be ignored.
7 There is no flow in the axial (z) direction.
8 The film pressure is constant in the 'y' direction, and depends upon 'x'.
9 The velocity of a lubricant particle depends on its x and y coordinates.

I can take from some of this reading on the sommerfield number that the viscosity does play a role in the calculation but the calculation is meant to come up with a standardized tolerance of film thickness.

"The minimum film thickness acceptable depends upon surface finish and should allow expected particles to pass through without causing damage. For some applications, eg in automotive engines, filtering is provided to remove particles whose size would be likely to exceed the minimum film thickness. The following ho values have been suggested:

0.0000025 m for finely bored small bronze bushes.
0.00002 m for commercial babbitted bearings.
0.0000025 < ho < 0.000005 m for automotive engines with fine surface finish bearings and lubricant filtering. "

It could be as easy as running a somerfield calculation and checking the eccentricity ration on a particlar engine to see if it would be safe to run a lower or higher viscosity.
..Easy if you are an bearing designer.

 

[Bryanccfshr]

Gary, that slide show is much better than the papers I was studying.

 

[Shannow]

Just to demonstrate how tiny things can make a big impact.

We've been battling a particular bearing for nearly 2 years. 20"diameter, 19" long, about 30 tonnes load, 3000RPM, oil supply temperature 45C, oil exit temperature 75C.

Bearing started vibrating heavily at 1/4 times running speed. Amplitude of vibration was up to 6 thousands of an inch, 12.5 times per second. Bearing whip, caused by bearing design (Length to Diameter too large). The bearing surface had already been partitioned with a circumferential groove to reduce the effect.

Whip is caused when a natural frequency of the shaft, and the dynamics of the bearing get in phase, and the wedge forms and collapses.

Changing the oil supply temperature 10C caused the problem to go away. Increasing the oil supply pressure 5% made it noticeably worse. Could also be removed by introducing the jacking oil, lifting it off the wedge, then letting it settle back down.

Realigned the machine (400 tonnes lifted 4 thousandths, and moved across 1), and the problem now only occurs on hot days, and vastly lower than previously.

 

[Shannow]

Originally Posted By: Gary Allan
Go here: Scroll down to step 11

Now factor as a normal equation where the flow rate doubles and the bearing speed/size/etc are exactly the same. just what's left to be manipulated in the equation?

So ..some math must only work one way. Take it to step 12 ..and between the two of them ..if the (apparently) immutables and constants stay constant ..this has to be thrown out of whack.


I don't think that there's an asymptote in there where all lubricants end up with the same operating viscosity due to decreased side leakage...otherwise BITOG would disappear in a flash of nothing.

 

[XS650]

Originally Posted By: Gary Allan
Go here: Scroll down to step 11

Now factor as a normal equation where the flow rate doubles and the bearing speed/size/etc are exactly the same. just what's left to be manipulated in the equation?

So ..some math must only work one way. Take it to step 12 ..and between the two of them ..if the (apparently) immutables and constants stay constant ..this has to be thrown out of whack.



You need to go through the calculation process several times while observing what goes into the variables to get a grip on it.

Note: the last un-numbered step (after 14), which usually means, Return to Start, Do Not Pass Go, Do Not Collect $200.

It's a messy process. Works pretty good though. I've even used that method to designe a couple of journal bearings that used water as their working fluid with good results.

 

[Shannow]

XS650, one of our fluids lecturers made a spinning top (essentially thrust bearing) that you spun up with your fingers, and it sat on a film of air...brilliant device, and superb machining.

 

[Bryanccfshr]

What I am starting to see as relevant is "minimum film thickness" in the hydrodynamic wedge. So long as the lubricant falls in a viscosity range at the expected operating temperature of the oil that provides this minimum range( that has enough clearance to allow the largest expected particle) we have have good bearing protection. Theoretically te minimum film thickness (and the high pressure film area) can be increased with more viscosity. Depending on the overall clearance volume of the area between the journal and the bearing this may or may not be a significant number with the rather narrow ranges of viscosity's we use.

 

[demarpaint]

The math gets a little too deep for me. But common sense tells me that too thick an oil will eventually work its way in but at a cost. The cost being less life of the parts concerned. I would want something that can get to work right away, keeping the parts from rubbing together, cooling, etc. Having something too thick that takes to long to get to work is NG, especially when the parts are cold during winter starts.

Frank D

 

[Gary Allan]

Quote:
You need to go through the calculation process several times while observing what goes into the variables to get a grip on it.


I imagine it would show me that certain things that I'm suggesting cannot occur.


I do thank you and Shannow for your patience. I'm arguing points that are conceptual road blocks that will take a longer process to piece together and get through. No one is taking the rhetorical bait

Thinner oil/more offset/more leakage
Double volume/same offset/much mo leakage

A very intuitive concept

 

[Steve S]

Shannow is the 10c change in temps a drop or increase?

 

[Steve S]

Originally Posted By: demarpaint
The math gets a little too deep for me. But common sense tells me that too thick an oil will eventually work its way in but at a cost. The cost being less life of the parts concerned. I would want something that can get to work right away, keeping the parts from rubbing together, cooling, etc. Having something too thick that takes to long to get to work is NG, especially when the parts are cold during winter starts.

Frank D
Just think of sucking a real thick milkshake through a straw as compared to a soda. The thick milkshake takes a real long time to reach your mouth. Too thick is bad. Too thin is bad!!!

 

[JAG]

I'm thankful that others have responded to Gary's question to me. I've been off-line. Gary, I agree with what you said:
"I imagine it would show me that certain things that I'm suggesting cannot occur."

I don't know for sure but I imagine that as well. I learned in college that questioning some things in science/math/engineering TOO MUCH can get me wrapped around the axle. Trying to develop an alternate method as your question would lead you to, would be a hair-pulling exercise.

This has been an unusually informative thread, thanks to many.

 

[Gary Allan]

At least the stratospheric types here are friendly toward those with operational ceilings that are much closer to sea level ...like mine

 

[Bryanccfshr]

I agreee with Gary. It takes patience to deal with us non engineers muddling through difficult concepts. Thanks for sticking with it.

 

[demarpaint]

Yes thanks for sticking with us. This could be like rocket science for some of us........like me..........

Frank D

 

[Shannow]

Originally Posted By: Gary Allan
I do thank you and Shannow for your patience. I'm arguing points that are conceptual road blocks that will take a longer process to piece together and get through. No one is taking the rhetorical bait

Thinner oil/more offset/more leakage
Double volume/same offset/much mo leakage

A very intuitive concept

Seeing as I'm having the day off, I worked through the example at the end of the spreadsheet that Gary and JAG posted, of the SAE 10 and 40 weight oils in the same.

Results were (note, too many significant figures for the printed graphs)
SAE 40 had 63% increase in film thickness over SAE10
SAE 40 had 61% increase in power requirement over SAE10
SAE 40 had 5F increase in Delta T (10F in outlet temp)
SAE 40 had 7.5% lower side flow (leakage and cooling)
SAE 40 had 113% more viscosity than SAE10 at working temp.

And having used the graphs in the spreadsheet, I'm Soooo glad that I studied in S.I....no wonder things run into Mars every now and then.

 

[Shannow]

Originally Posted By: Steve S
Shannow is the 10c change in temps a drop or increase?


Increase, viscosity drops, wedge gets thinner and stiffer.