Do you think thick oil always protects rings better? Think cavitation!

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Originally Posted by Shannow
The question was with regard to "film strength".

So your thread title mentions ring wear, but neither of the papers offered that as a suggestion or conclusion?

Oil-film strength means the viscosity of the oil under pressure at a given temperature. So, it's the pressure - viscosity-coefficient-corrected (PVC-corrected) viscosity. You can also take the PVC to be the oil-film strength but remember to account for the ambient-pressure viscosity.

The main reason why the cavitation is studied is that it could cause wear or damage. That's why they are trying to minimize it. This is mentioned in the introduction of most papers, such as the beginning of the second paper:

"Elastohydrodynamic lubricated (EHL) machine elements working under reciprocating motions are very common in mechanical applications. The lubrication state of these machine elements is often influenced by cavitation phenomena, because after reversal of the rolling direction the outlet region becomes the new inlet region. This means that the cavity produced in the outlet region can be entrained into the conjunction, and break the fluid film, which may lead to surface failure."

More explanation of the phenomenon:

"The cavitation region shrinks considerably in the vicinity of the dead centres. However, it partly survives beyond the dead-centre reversal as a confined bubble at the leading edge of the contact. Although this bubble quickly shrinks being absorbed by the lubricant film, while located at the inlet, it depletes the available lubricant supply leading to starvation. Quickly after the dead centre, a new cavitation region forms at the trailing edge of the contact. Although these two cavitation regions only coexist for a brief period, together with a very low entrainment motion and high contact loads, lead to thinner films and higher friction forces."

Cavitation-induced starvation for piston-ring/liner tribological conjunction
 
I had to think about this for a while. I know about cavitation in pump impellers and boat propellers but I have a hard time relating it to the subject of piston rings.

Doesn't cavitation require a rotating mass?

Also, motor oil has anti-foaming agents like silicon added to prevent cavitation which I assumed would occur in the oil pump.

The only other place that cavitation would occur is at the crankshaft and only if the oil level is too low allowing air to be induced by the crank.

Am I way off?
 
Originally Posted by PimTac
... Also, motor oil has anti-foaming agents like silicon added to prevent cavitation which I assumed would occur in the oil pump.
The only other place that cavitation would occur is at the crankshaft and only if the oil level is too low allowing air to be induced by the crank.
Am I way off?
A little. I think you're confusing air entrainment (or bubbles) with true cavitation, which involves a region of absolute pressure so low the fluid separates.
 
Pimtac,
as I've explained earlier, cavitation is simply a column of fluid getting pulled apart by the inertial pressures at the time exceeding to local vapour pressure.

In rotating equipment, the cavitation bubbles form close to the surface, and when they collapse, localised pressures are so high metal is eroded...(rubber lining can correct that issue, LOL)….I've shattered a glass syringe, simply "pulling apart" the water column within it (that's cavitation), and releasing the plunger the pressure spike was catastrophic. It's not foaming, it's separation of the oil column.

In piston rings, the oil gets squeezed into the gap, then has to change velocity to keep the now expanding gap filled...it can't, and thus a void is formed...a cavitation void.

Some pics and explaination
https://www.matec-conferences.org/articles/matecconf/pdf/2018/47/matecconf_iceaf-v2018_04012.pdf

Bearings can do it, as the gap expands away from the loaded point (MOFT), the oil can be pulled apart. It can happen on poorly designed oil pumps.
 
Originally Posted by Gokhan
Originally Posted by Shannow
The question was with regard to "film strength".

So your thread title mentions ring wear, but neither of the papers offered that as a suggestion or conclusion?

Oil-film strength means the viscosity of the oil under pressure at a given temperature. So, it's the pressure - viscosity-coefficient-corrected (PVC-corrected) viscosity. You can also take the PVC to be the oil-film strength but remember to account for the ambient-pressure viscosity.



No you can't...viscosity controls MOFT.
Higher PVC leads to higher MOFT

(within the bounds of possibility, such as this cavitation thing)

PVC...aka viscosity...is NOT oil film strength...their units aren't even similar, strength being Pascals,
 
Originally Posted by PimTac
I had to think about this for a while. I know about cavitation in pump impellers and boat propellers but I have a hard time relating it to the subject of piston rings.

Doesn't cavitation require a rotating mass?

Also, motor oil has anti-foaming agents like silicon added to prevent cavitation which I assumed would occur in the oil pump.

The only other place that cavitation would occur is at the crankshaft and only if the oil level is too low allowing air to be induced by the crank.

Am I way off?


Oil can also cavitate in journal bearings ... Googling will find info on that.
 
From the paper I mentioned previously....
https://www.matec-conferences.org/articles/matecconf/pdf/2018/47/matecconf_iceaf-v2018_04012.pdf

Quote
Oil viscosity and oil film thickness are characteristics that affect friction losses. Higher viscosity oils provide adequate wear protection but movement creates more resistance and high frictional losses when measured for the total stroke length [3]. Lower viscosity oils reduce frictional losses and increase efficiency but high volatility leads to hydrocarbon emissions. The reduction of oil viscosity makes the lubrication around the dead center of the stroke much more severe [9]. It is, of course, a matter of great importance to further investigate lubricant properties and friction as there is always a trade off between performance and emissions control.
 
CR94, Shannow, Zee06,

Thanks for your explanations. I always heard about the subject but obviously I am not fully educated about the process. I have seen boat and ship propellers eroded by it but that was long ago.
 
Originally Posted by Shannow
Originally Posted by Gokhan
Originally Posted by Shannow
The question was with regard to "film strength".

So your thread title mentions ring wear, but neither of the papers offered that as a suggestion or conclusion?

Oil-film strength means the viscosity of the oil under pressure at a given temperature. So, it's the pressure - viscosity-coefficient-corrected (PVC-corrected) viscosity. You can also take the PVC to be the oil-film strength but remember to account for the ambient-pressure viscosity.
No you can't...viscosity controls MOFT.
Higher PVC leads to higher MOFT

(within the bounds of possibility, such as this cavitation thing)

PVC...aka viscosity...is NOT oil film strength...their units aren't even similar, strength being Pascals,

Yes, the PVC gives the MOFT in EHL. That's how they measure it.

What do you mean by the oil-film strength then? Are you referring to the AW/EP/FM additives? I'm referring to the rheological properties only.
 
BITOG favorite topic to argue over lol.
07.gif
 
Originally Posted by Shannow
Pimtac,
as I've explained earlier, cavitation is simply a column of fluid getting pulled apart by the inertial pressures at the time exceeding to local vapour pressure.

In rotating equipment, the cavitation bubbles form close to the surface, and when they collapse, localised pressures are so high metal is eroded...(rubber lining can correct that issue, LOL)….I've shattered a glass syringe, simply "pulling apart" the water column within it (that's cavitation), and releasing the plunger the pressure spike was catastrophic. It's not foaming, it's separation of the oil column.

In piston rings, the oil gets squeezed into the gap, then has to change velocity to keep the now expanding gap filled...it can't, and thus a void is formed...a cavitation void.

Some pics and explaination
https://www.matec-conferences.org/articles/matecconf/pdf/2018/47/matecconf_iceaf-v2018_04012.pdf

Bearings can do it, as the gap expands away from the loaded point (MOFT), the oil can be pulled apart. It can happen on poorly designed oil pumps.


Nice summary!

Perfect little cut & paste for future reference! No need to read 10+ pages when I don't need/want to.
Adding cavitation to the very bottom of my worries list ... lol
Meanwhile, long live high hths.
grin2.gif
 
Originally Posted by OilUzer

Nice summary!

Perfect little cut & paste for future reference! No need to read 10+ pages when I don't need/want to.
Adding cavitation to the very bottom of my worries list ... lol
Meanwhile, long live high hths.
grin2.gif



Thanks

...to help with the understanding of inertial effects and vapor phase, here's a neat video showing it's development and progression on a hydrofoil...
 
Originally Posted by Pinoak
BITOG favorite topic to argue over lol.
True, but this thread is more entertaining than most thickie-vs.-thinie arguments, isn't it?
 
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