# Air spring stiffness calculation

1. Aug 11, 2011

### serbring

Hi guys,

I must simulate with simulink an air spring, connected to a device. In many book is present this non linear formula:

k(h)=-(n*P0*Ae*h0^n)/(h^(n+1))

Where:
P0 is the air pressure at the equilibrium condition
h0 is the spring heigth at the equilibrion condition
Ae=F/P0 is the effective area
n is the polytropic coefficient

So for checking the formula, I try to compare my result with the stiffness provided by the supplier and unfortunately I get a different value (40% less) .

In this link you can find an air spring catalogue:
http://213.164.133.30/catalog/ShowBalgPDF/SZ 50-11.pdf

For example, at 3bar=3*10^5 Pa I got an force of 1200N, h0=0.15m

K(h0)=1.4*1200/0.15=11200N/m, instead of 18500N/m. What is wrong in my calculation?

Thanks

2. Dec 14, 2015

### kammieorama

hi were you able to get an answer to this ...?

3. Dec 14, 2015

### serbring

Unfortunately not

4. Dec 17, 2015

### kammieorama

are you working on Air Spring simulation ? was it for some college project ?

5. Dec 17, 2015

### serbring

I am working on air spring simulation for a multibody simulation project

6. Dec 18, 2015

### kammieorama

Oh thats good. Even i am doing same. What all are you investigating ? Maybe we can converse over mails, if that is ok with you .

7. Dec 18, 2015

### JBA

This is an obvious question; but, are you sure you are using the same value of "n" as the manufacturer is using in the equation for the type of gas in the cylinder?

8. Dec 18, 2015

### JBA

After viewing the air spring design configuration (which I should have done before entering my above post) I don't think that my question in that post is of any value.

Alternatively, I suspect that a curve based upon actual load vs deflection test results are the only way to establish a truly accurate curve for the sleeve deflections on that type of air spring. If you need to simulate such a spring for a project I recommend that you select a specific manufacturer's spring that suits your application and based upon the actual travel range you require; then, try to establish a curve fit equation for F vs d for that section of the manufacturer's supplied F vs d curve to use to determine the effective k values in your calculations.

9. Dec 23, 2015