Flow Equals Lubrication

[Shannow]

I keep reading on BITOG how flow equals lubrication, and that it's obvious to anyone who has even a modest understanding of lubrication.

Could proponents of this science please post/link science based evidence that "flow = lubrication", particularly with respect to bearings ?

 

[yonyon]

So, um, are you feeling alright?

 

[jaj]

Originally Posted By: Shannow
I keep reading on BITOG how flow equals lubrication, and that it's obvious to anyone who has even a modest understanding of lubrication.

Could proponents of this science please post/link science based evidence that "flow = lubrication", particularly with respect to bearings ?


http://www.epi-eng.com/piston_engine_technology/engine_bearings.htm

If it's true, then it's in there somewhere.

 

[martinq]

I think what 'they' are getting at is that good flow = good lubrication.

Restriction of flow can be detrimental but the bottom line is that the engine and bearings are engineered. There is a range of operation that is safe, and a range that is optimal. Straying from this range leads to excess stress, wear and damage.

So yes, (flow = lubrication) because the engine is designed & engineered that way.

No-flow = No-lubrication

 

[Shannow]

I understand that without the flow to make up for that lost during lubrication, the lubrication is compromised...of course it is. Pressure is what gets the flow there.

In general the statement is as follows...pressure does not equal lubrication,flow equals lubrication. More flow equals more lubrications.

Specifically

Quote:
There is a one to one relationship between
flow and separation. If you double the flow you will double the
separation pressure in a bearing.
(1)

Quote:
Thinner oil also flows more at normal operating temperatures. And oil FLOW is lubrication
(2)

I'm just after evidence that flow equals lubrication, and how an increase in flow increases lubrication. Especially as I've been told that it's plainly obvious...

(1) Engine Oil 101
(2) 540 Rat's dissertation.

 

[yonyon]

...wait...but...flow has to be sufficient to maintain adequate lubricant supply and cooling...and...my brain implodes!

 

[JHZR2]

I'd recommend that people pick up the Applied Tribology book by Khonsari for some good basis.

There is a hydrodynamic wedge formed by the self-acting pumping action in the bearing.

Pressure and backpressure are important in oscillating loads because they create a time dynamic for lubricant to flow out, allowing for a higher load carrying capacity.

The localized viscosity of lubricants increases with pressures exhibited in bearings. That's neither a flow or lube pressure issue, but rather an external pressure issue (Boosear, 1995).

The finite time it takes for a lubricant to flow away from two surfaces closing in provides a substantial amount of inherent load carrying capacity, even in absence of a hydrodynamic wedge/sliding motion.

So the reality again is that it is a matter of having just enough viscosity to remain in the regime of hydrodynamic lubrication, and that amount becomes less and less as the surface roughness on well-finshed parts improves. If everything had a mirror polish, the viscosity requirements would be far lower than stuff that has a micron of surface roughness.

Under heavier loads, assuming that the fluid cannot be excessively easily displaced, there will be a natural cushioning. Without shock loads, the hydrodynamic wedge need not be excessively large, so the viscous requirement can drop.

However, oil is also utilized as coolant. So given that the viscosity will continue to drop with localized temperature, I believe the hypothesis on flow is more to control localized heating and viscosity drop, because with thinner lubes there is less freedom and room for error. It's an approach towards "optimum" versus having an n times overdesign. Of course, closer to optimal, without overdesign, the closer you are to going out of optimal conditions. Given that less viscous fluids flow better than high viscosity fluids, it is a natural synergy that is self-protecting in these conditions where typical engineering overdesign is being ignored for the sake of hitting a theoretical optimum of some sort (or enabled by improvements in one of the many knobs to turn).

Just my 2c.

 

[JHZR2]

Originally Posted By: Shannow
I understand that without the flow to make up for that lost during lubrication, the lubrication is compromised...of course it is. Pressure is what gets the flow there.

In general the statement is as follows...pressure does not equal lubrication,flow equals lubrication. More flow equals more lubrications.

Specifically

Quote:
There is a one to one relationship between
flow and separation. If you double the flow you will double the
separation pressure in a bearing.
(1)

Quote:
Thinner oil also flows more at normal operating temperatures. And oil FLOW is lubrication
(2)

I'm just after evidence that flow equals lubrication, and how an increase in flow increases lubrication. Especially as I've been told that it's plainly obvious...

(1) Engine Oil 101
(2) 540 Rat's dissertation.


I think this has to do with the theory of the finite time of a shock event versus the ability to re-create a wedge. I dont necessarily buy it, but I suppose the hypothesis is that if you can easily and readily keep the wedge renewed then any shock event that would displace the hydrodynamic regime would be renewed faster and thus re-lift and maintain better separation than if a slower flowing fluid that took longer to rebuild was present.

Of course the slower fluid would have more load carrying capacity and notionally keep better separation to begin with, so it would be a moot point.

I think that the case quoted with flow = separation is accuate in a hydrostatic bearing, but not in a hydrodynamic bearing. Hydrostatic bearings derive their load capacity not from shear flow driven effects (hydrodynamic wedge and surface sliding) but rather from the combination of pressure versus flow resistance effects through a feed restrictor and in the film lands.

 

[Shannow]

Hydrostatics also have much higher pressures, with the feed directly into the loaded point rather than an oil feed into the "suction" area of a hydrodynamic bearing.

 

[martinq]

Might as well put this here:

Some Myths about Engine Oils
http://www.valvolinecummins.com/tips-engine-oil-pressure.asp

Found this interesting:

"At normal driving speeds, there should be little difference in oil pressure between an SAE 10W 30 and SAE 20W 50 engine oil if the engine is in good mechanical condition. If engine oil pressure responds to a change in SAE grades, this is usually an indication that the engine is in need of mechanical attention."

 

[JHZR2]

Originally Posted By: Shannow
Hydrostatics also have much higher pressures, with the feed directly into the loaded point rather than an oil feed into the "suction" area of a hydrodynamic bearing.


I agree. Main thing was it is a bearing example that relies upon pumping (even if nearly dead-headed).

 

[Shannow]

True,
turbines are typically "jacked" for slow speed operation (small volume, high pressure) to lift them off the journal for barring (low speed keeps shafts straight), and run-up, up to about 4-500RPM. Normal oil feeds and flows are used, "jacking oil" provides initial separation.

Can run down from speed to a stationary shaft position without jacking without damage...run-up will run them

 

[Propflux01]

This is your brain. This post is your brain on drugs!

 

[spasm3]

Originally Posted By: JHZR2


However, oil is also utilized as coolant. So given that the viscosity will continue to drop with localized temperature, I believe the hypothesis on flow is more to control localized heating and viscosity drop, because with thinner lubes there is less freedom and room for error.
Just my 2c.


I have wondered how zinc level and additives such as MOS2 affect heat transfer in an engine oil. Does it have any effect on bearing temperature?

 

[JHZR2]

Originally Posted By: spasm3
Originally Posted By: JHZR2


However, oil is also utilized as coolant. So given that the viscosity will continue to drop with localized temperature, I believe the hypothesis on flow is more to control localized heating and viscosity drop, because with thinner lubes there is less freedom and room for error.
Just my 2c.


I have wondered how zinc level and additives such as MOS2 affect heat transfer in an engine oil. Does it have any effect on bearing temperature?


Well think about what they do. Zddp adsorbs to the metal surface in a chemical reaction of sorts. moS2 may not adsorb, but it is present so that as the oil film decreases, it is in there sliding between the two surfaces preventing boundary lubrication (eg what the zddp does). So I'd think of that solid moly as an additional line of protection.

In theory, the chemisorbed zddp or the mos2 sitting on top of it would add another composite layer, lowering heat transfer coefficient from the surface the oil is contacting. In practice, both are so infinitesimally small that they probably make no difference. If frictional losses are reduced, the lube should be cooler, both locally and overall. But the heat load from that is probably tiny as compared to the combustion heat load and any load from boundary lubrication events.

 

[Shannow]

Given that the bearing is hydrodynamically lubricated, the heat rise across the bearing is largely due to the work that the engine is doing against the oil.

I don't think surface active elements would do much at all to bearing temperature.

 

[Shannow]

Originally Posted By: Propflux01
This is your brain. This post is your brain on drugs!

Thanks for that...do you even understand the question ?

 

[Propflux01]

Originally Posted By: Shannow
Originally Posted By: Propflux01
This is your brain. This post is your brain on drugs!

Thanks for that...do you even understand the question ?


Oh, I understand the question just fine. Its the ANSWERS that's making that "processing - one moment please" flash across my forehead!!

 

[TechnoLoGs]

Just remember that Dr Haas's 20-weight 0W-20 oils are more like light to mid-grade 30-weights. Understand this, and his article will make sense.

 

[boxcartommie22]

one of my many reasons i use the stainless steel oil filter because of its excellent decrease in psid restriction. i learned that flow(inc. lubrication) is more important then filtration. besides this filter goes down to 17u(tested single pass) and flows 57gpm/square in.if engines were able too.absolutely no startup noises in frigid weather.