You will often see two stiffness values specified for rubber springs: static stiffness and dynamic stiffness.
Static stiffness relates to the characteristics of the spring with either steady state loading, or a slowly applied load.
Dynamic stiffness relates to the effective stiffness of the spring during dynamic loading, and is always higher than the static stiffness. The stiffening effect is caused by the natural hysteresis (internal damping) present in rubber materials, and low hysteresis rubbers like natural rubber exhibit relatively low dynamic stiffening.
The degree of stiffening is dependent on the rubber properties, but for natural rubber the dynamic stiffness coefficient lies between 1.1 and 1.3 for rubbers between 35 and 65 shore.
Note that the dynamic stiffness values should always be used when calculating the dynamic response of a system.
The references I mentioned in your earlier thread( basics of rubber technology thread 335-100189) in ths forum will give you more information on dynamic stiffness and damping.
Thank you very much for your valuable information for the Static and Dynamic stiffness.
At present, I am doing a task where i already have some spring rate values for NR 71 thru testing and i need to maintain the same spring rate and stress values for the part with NR 65.
I had Increased the cross sectional area along the axis in which i need the spring rate and take FEA engineers help to check the values using Marc software.
As u knew, FEA results as compared to Test results are not so near. So I thought of Doing the FEA for the current tested part NR 71 store the results and compare it with the new cross section with NR 65 and try to maintain the same results before going for a proto type so that i can be near my target.
Do u feel this is the best way of reaching my target or u have some suggessions for me.
If you are going for a softer rubber, this usually ( but not always) means a larger rubber cross section, so you should have the oppotunity to reduce stresses, and improve the fatigue life of the part.
There are a nummber of ways to achieve your target stiffness with a softer rubber, but this depends on how much space you have, the mounting type, and how complicated you want your mould tool to become.
For example:
If you have a simple compression mounting then make the part bigger, or stiffen the rubber section by including a interleaf plate( or 'rate plate' in the US), and keep the size the same.
For bushings loaded in the radial direction, use a duplex or interleaved bush, but these can be expensive.
For conical sections, you can also use interleaves, or make the cone angle more obtuse.
As a general rule of thumb, with the static load applied, try to keep the compressive stress to about 10%, and the shear stress to less than 50%. Maximum compressive stress should be limited to 20%.
ALWAYS avoid tensile stresses.
I always like to design new parts with rubber in the 55-60 shore range, as this allows the design to be fine tuned during development. If you start out with 71NR, and the part isn't stiff enough, or 35NR on the part is too stiff, then you're really stuffed. (been there, got the T shirt).
If you can tell me a little more about the application and the type of mounting, I can make some more comments.
I will try to reply to any new posts next week.
Centre bearing supports are usually a plain rubber bush, or a bush with a convolution if a very low stiffness is required.
If it is a plain bush, then reducing the radial depth and/or increasing the axial length will increase the stiffness.
If the bush has a convolution ( quite common for centre bearing supports) then the rubber section works partly in a bending mode, so calculation is less straightforward. Different (inreased) thicknesses for the convolution would have to be tried or modelled.
