Can the spring rate of rubber components be determined from hardness? 2

[markarnold]

I have a simple "donut" shaped buna rubber bumper of known hardness (70 durometer). I would like to know what the spring rate is so that I can calculate its kinetic energy absorption capability. Can the spring rate be determined from the hardness?

 

[MintJulep]

It is necessary, but not sufficient.

 

[markarnold]

Is it then more of an experimental determination? Or are there other factors I can calculate?

 

[MintJulep]

From "Mechanical Springs" by Wahl:

"...the calculation of rubber springs by available methods is at best only approximate."

If it's important, test.

Note also that with rubber springs, if the energy goes in faster than it can go out, they melt.

 

[smckennz]

Maybe these links will help

http://www.npl.co.uk/force/guidance/hardness/rubber.html

http://www.simritna.com/catalog/o-ring/98physics_rubber.htm

Have you considered testing the rubber doughnut by applying known loads and measuring deflection? It may be easier to measure than to calculate.

Cheers

Steve

 

[desertfox]

Hi markarnold

From a little book I have called "designing with rubber" the formula for the stiffness of rubber doughnuts is as follows:-

K= (4/3)* Eo*3.142*Dm*(b/t)*(1+k (D-d/(4*t))^2


where K= stiffness

Eo= youngs modulus based on hardness

k= numerical factor based on hardness

Dm= mean diameter ie D+d/2

D= external diameter

d= internal diameter

b= radial width of section ie- D-d/2

t= thickness

Now for a Durometer reading of 70 which unfortunately is not listed, however for a shore hardness of 65 (shore A) the Youngs modulus is given as 830 lbf/in^2 and for 75 the Youngs modulus is given as 1340 lbf/in^2 these figures are based on natural rubber (above 48 IRHD) with SRF black filler. Now for little "k" the numerical factor above, is given as 0.54 for shore hardness 65 and 0.52 for 75.
Note also that average design limits are 15%compression and 50% shear.


regards desertfox

 

[RichLeimbach]

Adding a little extra to the good posts above...
Don't forget that there will be quite a bit of damping involved if you are hitting this with any kind of speed at all. Just doing force / deflection tests might not give you a good understanding of the actual forces at work in real operation (but it's still not a bad place to start).
Not sure what your application is, but you might also want to consider the effect of temperature on your numbers. The spring rate for buna will go way up when it is cold and down when it is hot (like if the stop gets worked at a high cyle rate without cooling).

- Rich

 

[markarnold]

The application is to dampen the noise from the impact of a pneumatic cylinder.