OIL "Cushion" for bearings..

[Gary Allan]

Quote:
If this means that for any decrease in viscosity, equal journal bearing protection can be obtained by increasing oil flow rate through it, this is one of the things that is not true. The eccentricity ratio increases (off-centeredness) due to the less viscous oil and what increasing flow rate would do is mostly: increase flow rate.





Now you're saying that that original offset will never be restored by increased volume? ..or (what you really implied) that the thinner fluid will always have this altered offset regardless of the volume applied.

That is, 1gpm with cold oil producing (let's say) 40 lb of back pressure ...that is reduced to 12lb of back pressure at hot visc ..has the same offset? ..and that increasing flow to produce 40lb of back pressure results in NO restoration of the offset toward how it was at the 1gpm, higher visc, flow rate?



(I'll continue to attempt text integration as we move on here. This, however, is the best medium for me).

 

[Bryanccfshr]

I think I just popped a brain vessel.

 

[Gary Allan]

Originally Posted By: Bryanccfshr
I think I just popped a brain vessel.


Reason it with me here, Bryan. If our static concept of visc is all that determines the offset ..and, as Jag asserts, "volume at visc (aka pressure)" has nothing to do with offset (did I get that implication right?) ...then the offset for a higher visc fluid at idle although altered at idle from it's high visc state (same flow rate .but cold) .. can not be restored by pumping more of the (now lower visc) fluid through the journal, right?

Everything that I read on that paper (that I can integrate) all shows linear relationships in shear stress and what have you. If there's no equivalency in "visc at volume" in apparent "forces" ..then some non-linear property or principle must be employed to rationalize it.

See my

 

[XS650]

Originally Posted By: Gary Allan




Now you're saying that that original offset will never be restored by increased volume? ..or (what you really implied) that the thinner fluid will always have this altered offset regardless of the volume applied.

For such simple looking devices, there is lot going on in a journal bearing. They are even more fun when you have to design one to run with water as the working fluid.



For all practical purposes, yes pump pressure has no direct effect on bearing offset.

Pump pressures run on the order of 40 psi, bearing operating pressures on the loaded side can be a couple hundred psi.

If you think in terms of pump oil pressure getting enough oil to the bearing to keep the cavity full with some surplus for cooling and then the bearing developing it's own pressure, you won't be far off.

Something to think about. If you increase pump pressure you are increasing pressure on the unloaded side of the journal, thereby adding load to the loaded side

I suspect the effects of that are negligible, but real.

 

[Bryanccfshr]

OK I think I understand the dilemma you bring up. I have not waded through the entire document. ( I am in and out of the office this morning monitoring a couple of activities).
So far you have not come across a non linear relationship described in the paper such as JAG asserts.

I personally cannot rationalize how viscosity can effect this bearing deflection/ offset. I am still thinking of it as a flat bottom boat hull on a fluid.
The draft (depth) that the hull displaces that fluid is dependent on a few variables.
The mass of the hull(or journal in this case) The velocity( the faster the hull is moving over the fluid the less draft. the load( a heavier load will increase the draft) and the density of the fluid (saltwater for example is more dense than fresh water and therefore the same objects are more buoyant in salt water).

Now here is another analogy(which is the best way I can structure concepts for my mind to understand.

A shallow water boat on plane can travel in 4-6 inches of water over a shallow sand bar with no contact with the bar). However if you cut power the boat will sink to the bottom and ground as velocity is reduced. This is similar to what must happen in a hydrodynamic bearing no velocity no lift, therefore the need for barrier film lubrication.
A hydrostatic bearing has an oil pressure supply that should float the journal.

 

[JAG]

Gary, yes you understood what I meant to say. Take a look at this presentation of how to design hydrodynamic bearings in conjunction with a particular oil. It uses an iterative process. Note the absence of oil feed pressure as an input/independent variable.
http://www.utm.edu/departments/engin/lemaster/Machine Design/Lecture 26.pdf

There are hydrostatic bearings that use oil feed pressure to lift the journal bearing but that is not what we are talking about here. This is about hydrodynamic bearings where the force separating the journal from the bearing comes from pressure/force developed within the oil caused by the rotation of the journal (in oil of course). The oil feed pressure is just there to keep oil flowing through the bearing to provide cooling, primarily. The oil feed pressure is superimposed on the journal-centering pressures and as XS650 said, it adds fluid pressure ALL AROUND the journal which mostly cancels out in the radial direction (not really helping to center the journal).

Here, http://www.roymech.co.uk/Useful_Tables/Tribology/Liquid_Lubrication.htm, scroll down to the hydrodynamic lubrication section. It may help.

 

[glennc]

Originally Posted By: Steve S
Getting too complicated here the "cushion" is just the time it takes for the oil to get pushed aside or out . Like water skiing...


Or to put it another way, would you rather fall several feet onto a water bed or onto a hard floor? The non-compressibility of a fluid does not preclude it having a cushioning effect. It is a viscous cushion. Very important for engine parts. G-loads would be astronomical without this cushion and parts would get hammered into oblivion.

To actually figure the quantitative level of cushioning would be complicated because you would have to integrate the viscous behavior of the oil film, which does not behave in a simple newtonian manner. However, I'm sure it is safe to say that the measured viscosity of the oil would be the most important variable, and my guess is that the cushioning ability would be about proportional to the square of the viscosity.

 

[XS650]

Originally Posted By: Bryanccfshr


A shallow water boat on plane can travel in 4-6 inches of water over a shallow sand bar with no contact with the bar). However if you cut power the boat will sink to the bottom and ground as velocity is reduced. This is similar to what must happen in a hydrodynamic bearing no velocity no lift, therefore the need for barrier film lubrication.

A hydrostatic bearing has an oil pressure supply that should float the journal.



That's not a bad analogy as long as you realize that automotive journal bearings aren't hydrostatic bearings.

 

[Gary Allan]

Thanks for trying this pony style, XS650.

It's hard to reason. That is, lets throw out pressure and just look at a constant volume of flow at a constant bearing speed with variable viscosity. Sure, we'll see that 1gpm @ 100Cst fluid will provide X offset ..and that as the visc retreats to 15Cst that this offset will be more. We (what's this "we" stuff whiteman?) tend to integrate this as it a function of "emptied easier" due to the lower visc fluid that has less resistance to flow (or inhibition to leaking down). That is, not only do I see thinner shearing sheets ..I see lower total volume suspending the bearing.


Bryan: Let me attempt to show you my dilemma.

Flatten out the bearing and forget any "leakage". As I apply fluid to separate the two plates (remember, we have a "magic seal" that only allows fluid to exit out the end of the bearing) and I begin to move it, I should see it lift at a given pressure as the viscosity of the fluid at volume reaches the needed level of force to perform the task. The plate may not be parallel with the other ..but since the end of the plate is the only exit point ..a visc/volume/pressure composite should result in the same effects.

Check out Section 1.3 of Jag's suggested document. Is this not a flattened journal bearing??

 

[Bryanccfshr]

I got that.

I am going to transcribe a short piece from the book(the hard way). The reference is already in the thread. Although I haven't found relevant viscosity data I think this is important to have in text..


" According to hydrodynamic theory, a very thin fluid film is generated even at low journal speed. but in practice, due to surface roughness, vibrations, and disturbances, a certain high minimum speed is required to generate an adequate film thickness so that occasional contacts and wear between the sliding surfaces are prevented. The most severe wear occurs during starting because the journal is accelerated from Zero velocity, where there is relatively high static friction. The lubricant film thickness increases with speed and must be designed to seperate the journal and the sleeve completely at the rated speed of the machine. During starting, the speed increases, the fluid film builds up its thickness, and friction is reduced gradually."

Still reading and learning.

 

[Bryanccfshr]

Originally Posted By: XS650
Originally Posted By: Bryanccfshr


A shallow water boat on plane can travel in 4-6 inches of water over a shallow sand bar with no contact with the bar). However if you cut power the boat will sink to the bottom and ground as velocity is reduced. This is similar to what must happen in a hydrodynamic bearing no velocity no lift, therefore the need for barrier film lubrication.

A hydrostatic bearing has an oil pressure supply that should float the journal.



That's not a bad analogy as long as you realize that automotive journal bearings aren't hydrostatic bearings.


Yes I do( thank you for following up to make sure I understood the difference, it is easy to get the terms mixed up and these are important differences)) they are hydrodynamic with the pump simply supplying fresh oil. The practical pressure is built up by the hydrodynamic process.

 

[Gary Allan]

Quote:
Gary, yes you understood what I meant to say. Take a look at this presentation of how to design hydrodynamic bearings in conjunction with a particular oil. It uses an iterative process. Note the absence of oil feed pressure as an input/independent variable.
http://www.utm.edu/departments/engin/lemaster/Machine Design/Lecture 26.pdf


Help me get around step 11

There, one could reason (if they were challenged

) that this is a result of all the constants and the applied variables...one of which includes the average visc. If I merely swapped the variables so that volumetric flow rate was the variable ..why wouldn't (varied) visc be the resultant product?

 

[Bob The Builder]

Originally Posted By: Steve S
Originally Posted By: Lazy JW
It's my understanding that oil actually IS compressible albeit a very tiny amount and not really enough to change the practical dynamics in an automobile engine. But if we are going to pick nits lets get really picky.
Joe
Getting too complicated here the "cushion" is just the time it takes for the oil to get pushed aside or out . Like water skiing. When you let go of the rope you start to sink ,the water is displaced ."some what of a poor analogy". Or maybe better is like in a shock absorber "dampner" the resistance of the oil being forced through the orifaces. Or put a glop of grease on a hard thick metal surface then hit the grease with a hammer the grease cushions the hammer blow as the grease is pushed aside untill there is no grease then it is metal to metal.



The only problem I have with that analogy about grease is the fact that in an engine, you have continuous flow of oil, hence you "always" have a "cushion" of oil protecting and essentially preventing direct contact between bearings and rotating metal parts. Because there is continuous oil occurring between the parts, the oil never truly gets pushed out...or aside.

Would you agree that if there weren't a "cushion" of oil, that direct contact and hence...abnormal wear or even failure of the parts would be imminent?

 

[D-Roc]

Good thread....

 

[Shannow]

Originally Posted By: Gary Allan
I tend to view (in Gary's adaptive view® via proprietary watershed osmotic absorption®) a journal bearing as a turbo charged crescent pump that has no suction head (although, at a certain point - I think the view may be slightly altered).


If you have a look at the pressure profile around a journal, there's a significant region of sub-atmospheric pressure on the return side (after the wedge)...

When watching vibration probes on a run-up, you will see the rotating shaft try to "climb" the bearing, setting up the "wedge".

The wedge is formed because the oil in contact with the journal does not move relative to it, and that next the the bearing is stationary, there's a gradient of shear between the rotating and stationary parts.

The rotating journal actually drags the oil into the gap.

Thicker oil, bigger gap, thinner oil, smaller gap.

 

[Bob The Builder]

Originally Posted By: Shannow
The rotating journal actually drags the oil into the gap.

Thicker oil, bigger gap, thinner oil, smaller gap.



Hmmmm....

Shouldn't that be reversed, since the only thing we can really control after the mfg process is the oil?

Bigger gap, use thicker oil, smaller gap, use thinner oil...on second thought, maybe we meant the same thing.

 

[Shannow]

Bob,
I'm referring to the gap that's created by the journal dragging the oil in (the thickness of the "wedge" if you will)...feed it thicker oil, and the "wedge" will be thicker.

 

[Gary Allan]

Quote:
The wedge is formed because the oil in contact with the journal does not move relative to it, and that next the the bearing is stationary, there's a gradient of shear between the rotating and stationary parts.


I'm there. Easy enough to see.

Quote:
The rotating journal actually drags the oil into the gap.


..and it would not drag more of a thinner, more easily sheared fluid? (many more thin shears than lessor thick shears)

Quote:
Thicker oil, bigger gap, thinner oil, smaller gap.



Apparently not

 

[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.

 

[Bob The Builder]

Originally Posted By: Shannow
Bob,
I'm referring to the gap that's created by the journal dragging the oil in (the thickness of the "wedge" if you will)...feed it thicker oil, and the "wedge" will be thicker.


Are you talking about an oil wedge....sort or an oil wave as it comes away from the metal?