Formula for Nitrogen Volume in Shock

[ZeroDegreeC]

I am looking for a formula to figure out the required volume of Nitrogen (using Nitrogen for its near Inert properties)in relation to suspension design. Any information any of you have on design characteristics for suspension internals would be great.

 

[Slowzuki]

You may want to expand, do you mean you are using the nitrogen to pressurize the oil and account for the volume change from the shaft moving in and out?

Nitrogen behaves very close to an ideal gas so some ideal gas calcs should do it. Pick a maximum pressure, set a min pressure you want on the oil, find the change in volume due to rod travel solve for nitrogen volume to meet those numbers.

You could then check the maximum pressure with the temp rise from heating of the shock in use.
Ken

 

[GregLocock]

You also have to account for the nitrogen absorbed by the oil. Cheers

Greg Locock

 

[ZeroDegreeC]

Slowzuki your post is exactly what I wanted but I am frustrated on the formula to use. perfect gas law is the only formula that I can think of that would work but I am having trouble sorting out the solution. Industry standard for nitrogen charge is 200psi but I am unsure of the MOP used in the shock calculation. The nitrogen is not mixed with the oil in this application, I am using a floating "puck" for the seperation of chambers so no mixing is involved.

 

[Slowzuki]

MOP?- not familiar with this acronym

Anyone care to comment on the effectivness of the pucks in monotube shocks? Leakage by the seals over time?

I don't have any references with me but you will have to assume adiabatic or isothermal conditions to find your second state. If the pressure change is small, it won't really be a large difference between the two assumptions.

Fast changes in volume in the situation you describe would be closer to adiabatic, slower changes would be isothermal. I don't think you'd want to get into looking at the heat transfer there.
Ken

 

[mloew]

ZeroDegreeC,

pV=nRT

p = pressure
V = volume
T = temperature
n & R are constants

Holding T constant:
p₁v₁ = p₂v₂

v₂ = v₁ + rod volume (positive in extension; negative in compression)

Hope this helps. Best regards,

Matthew Ian Loew

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[harrisj]

Puck? We don't play ice hockey here so we call it a separator piston. As Greg says, solubility of nitrogen in oil is significant, and depends on temperature. Also foaming can occur without a SP and gives you compressible oil, which reduces damping effect.

Yes they do work and you should use one if you can accommodate the extra length. Providing you have a low friction seal and bearings, the SP floats with no significant differential pressure and merely separates oil from nitrogen. Talk to Busak & Shamban or Greene Tweed about seals. You could also consider a nitrile bag as used in accumulators. Either way, make sure you can bleed air from below the piston / bag.

Don't make your piston shorter than 60% of the bore diameter, or it may cock over. (We don't want a cocking puck, do we?)

Gas index: for nitrogen adiabatic is 1.4 and isothermal is 1. For off-road, we use 1.27 as a typical figure for heavy vehicles; maybe 1.3 for lighter ones. But as Ken says, it doesn't make a huge difference.

If you keep units consistent, the gas laws work fine, just as Mr Charles and Mr Boyle predicted.

John

 

[ZeroDegreeC]

Harrisj what do you mean when you talk about the >60% bore Diameter? as in the "separator piston" depth? I dont understand that part of your answer. The figure I am trying to calculate is the depth of the "separator piston" for a shock with a 2.75" stroke shock, what is the volume needed for the Nitrogen if the Min pressure is 200psig (I dont know the MOP (Max Opperation Pressure)) I would need to know the MOP of the shock to figure out the separator piston depth I am guessing, I will have to make some SWAG (SCIENTIFIC wild ass guess) calculations to figure this out.

 

[harrisj]

Zero: My comments on pucks (easier to type) were in response to Slowzuki's question. The puck doesn't do anything clever except separating oil from gas. My 60% is just a rule of thumb to stop the puck jamming in the bore. So if your bore is 2 inch, your puck length should be 1.2 inch or more.

To get down to basics, what are you designing? Is it a spring (eg hydropneumatic strut) or a 'shock absorber' (damper)?

If it's a spring, the nitrogen is the elastic medium and the pressure and volume give the spring charactersitic.

If however, it's a gas-filled damper, the pressure is only there to prevent cavitation and doesn't need to change. Any significant change in pressure will add to your main spring cahracteristic. To keep the change in force low, you need as much N2 volume as possible. Just determine fluid volume change from one extreme to the other, choose a minimum gas volume, and apply the gas law to the two extreme conditions. Then see what variations in MOP you get.

Alternatively, guesstimate your MOP and see what gas volumes you get.

The 'best' answer is usually a trade-off between what you would like and what you've got room for.

John

 

[harrisj]

My comments on pucks (easier to type) were in response to Slowzuki's question. The puck doesn't do anything clever except separating oil from gas. My 60% is just a rule of thumb to stop the puck jamming in the bore. So if your bore is 2 inch, your puck length should be 1.2 inch or more.

To get down to basics, what are you designing? Is it a spring (eg hydropneumatic strut) or a shock absorber (damper)?

If it's a spring, the nitrogen is the elastic medium and the pressure and volume give the spring charactersitic.

If however, it's a gas-filled damper, the pressure is only there to prevent cavitation and doesn't need to change. Any significant change in pressure will add to your main spring cahracteristic. To keep the change in force low, you need as much N2 volume as possible. Just look at fluid volume change from one extreme to the other, choose a minimum gas volume, and apply the gas law to the two extreme conditions.

See what variations in MOP you get. The 'best' answer is usually a trade-off between what you would like and what you've got room for.

John

 

[Slowzuki]

Thanks for the "pucking" answer! My interest in it is due to a design for coupled suspension in off-highway/forestry/contruction equipment that I have been tinkering with.

The "sliding piston" would prevent the gas spring medium from being removed from the accumulator to the reservoir when ride height adjustments are made.

I would prefer to use a sealed sphere type accumulator with bladder but they are very expensive and provide no easy method of sensing the volume.