Cavitation/Foaming Vs Viscosity

[kritter]

Can anyone enlighten me with a text or website or experience that will give me a correlation between viscosity of an oil and cavitation in a hydraulic system?

This is for a monotube shock absorber application and I am trying to determine the best viscosity oil to use of the 5 most widely used viscosities in the industry.

My thinking is that the lower the viscosity, the less caviation/foaming is going to occur since the fluid will flow easier through the piston/shimstack and not have such a large pressure differention which is what causes the caviation in the first place.

I may be way off, so please enlighten me with some sources or experience.

Thanks,

Kris

 

[patprimmer]

I see 2 conflicting arguments.

The one you present, but also, the higher the viscosity, the higher the boiling point, so the less inclined to cavitate at the same pressure drop.

If there is no gas in the system, bubbles can only form by volatiles coming from the oil.

I believe the purpose of gas pressurised shocks is to increase the boiling temperature of the volatiles in the oil.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
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[kritter]

It is a monotube shock that has the oil and nitrogen seperated by a dividing piston...not an emulsion shock which has the oil and nitrogen mixed.

I can foam/cavitate shocks that have been air bled, all day long on the dyno after only a few thousand cycles...

You say that viscosity and boiling temp are directly related but are they related along the same lines as viscosity and pressure?

If I decrease the pressure differential by X amount from using a less viscous fluid, is that smaller pressure differential going to be enough to keep the fluid from cavitating or would the pressure differential be directly proportional to the boiling temp and I would be in the same boat as a large pressure differential and a higher boiling point?

I understand what I am saying but does anyone else?

 

[hutch325]

Consider this: The pressure differential is what makes the force that opposes the spring force, so for a given damper with a given characteristic you will have the same pressure differential for a point on the F(V) curve.

Simply choose the working fluid with the highest boiling point and go with it. I suspect the fluid manufacturer would have some useful information.

Perhaps it's not correct to neglect pressure gradients within the damper and there are localized low pressure regions, in which case I suppose my argument may go out the window.

Food for thought anyway.

 

[patprimmer]

Hutch is right. It takes a certain force. If the viscosity is lower, it might need a tighter valve or smaller orifice to get the same damping effect.

Viscosity and boiling point are not directly related, but tend to follow a trend. If they are both straight pure hydrocarbons, the higher viscosity will be higher boiling point, but if there are additives, they may be more volatile and boil off first, so it will be formulation as well as viscosity dependant


Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[PTwizz]

As I understand it, boiling of the fluid is not the cause of cavitation. Cavitation occurs when absolute pressure falls to zero, at which point voids of vacuum open up. This probably does lead to some boiling, but in dampers, the cavitation occurs at the flow restriction and is very localised. This means that there is very little time for boiling to occur.
It is also woth noting that the oil will contain a certain amount of dissolved gas, which is difficult to remove. (We have tried using vacuum equipment at various temperatures over various time periods, with limited success) Dissolved gas will come out of solution where cavitation occurs and will quickly re-dissolve.
One solution is to increase the gas pressure in the damper reservoir, so that it is always in excess of the delta pressure across the piston.
Externally valved dampers have the option to place the reservoir between the bump and rebound flow restrictions, causing the damper to work with positive pressure only, allowing very low gas pressures with no risk of cavitation.

Pete.

 

[MoreWing]

Well, I don't know if you could say the absolute pressure falls to zero. That's over-stating it to some extent. The pressure only has to go below the vapor pressure for whatever temperature you're operating at.

A lighter fluid will produce less of a pressure differential across the piston for a given shim stack. Of course, in doing so will also produce less damping force. For a given damping force, you'll end up running a stiffer shim stack for a lighter fluid. Kinda a vicious circle.

Here's the good news. Unless you're working with really big cars or desert trucks, cavitation to a small extent doesn't seem to drastically hurt the overall damping characteristics. It starts very small, and, as the damper get hotter or worked faster, it increases.

For racing applications, I tend to run very light fluid and my canister pressures so that the shocks cavitate a little. Reducing shock pressures reduces seal drag, stiction, a bad thing. The compromise of a small amount of cavitation vs. reduced seal drag is usually a winner. The other part of this is that the reduced damping that cavitation gives you can be good if it is happening over high frequency (as opposed to simply high shaft velocity) bumps. This is on paved surfaces. If you're running a desert truck or MotoX bike, you're on your own. I can't help you.

 

[PTwizz]

Quote: "...cavitation to a small extent doesn't seem to drastically hurt the overall damping characteristics."

Cavitation introduces a gas spring effect (referred to in damping jargon as hysteresis) which is defenitely undesirable. We go to considerable lengths to avoid anything that gives a series spring effect.

A secondary effect of cavitation is to cause rapid wear of the valve components (in your case, the piston sealing faces and the edges of the shims).

If you can't achieve the damping forces you need without high gas pressure or cavitation, you could use a larger piston diameter (lower delta pressure for the same force).

There are now dampers on the market that are 'cavitation proof', inluding Ohlins TTX and Multimatic DSSV (I have to confess an interest here, having spent the last four years working on DSSV!)

Pete.

 

[patprimmer]

I would also suspect that higher viscosity oil would be more cavitation resistant at similar damping rates as it has a higher boiling point.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MoreWing]

There's many different paths to the same point. From what I've found, limited cavitation hasn't been an overall detriment. It depends on when and where the cavitation is occuring, but over high frequency bumps, I've found it to help things rather than hurt them. Yes, it does act as a series spring and not as a damper, I completely agree with that analysis. Is there a reason why you wouldn't want to decrease damping over a high frequency bump?

The wear on the shims is a real concern, especially for very thin shims. They live a tough life. I'm not advocating running in constant cavitation, just very limited. If you have a couple places around the track that you know moves the damper at speeds where cavitation will occur, but for the most part everything is acting as normal, I haven't seen any abnormal wear issues.

If you've been working with the DSSV for that long, then I assume you work for NHR? Would that make you PG?

I've never worked with the DSSV and only have very limited experience with the TTX. That's all getting ready to change in the next few days.

 

[PTwizz]

MoreWing,

I don't work for NHR, but for their damper supplier. If you're geeting ready to work with DSSV soon, you'll be one of the first to use the new generation dampers. I trust they will serve you well.

Pete.

PS: PG left NHR last year.

 

[ddriscoll]

I have obsreved that the normal boiling point of a hydrocarbon is related to 1/Ln(viscosity).