Synthetic rubber mechanical properties

[dmapguru]

I am a mechanical engineer, so rubber materials are exotic for me and outside my traditional choice of materials.

I have an existing mechanical system I want to modify by adding a spring. I could just add a traditional steel coil tension spring, but space is limited. The configuration lends itself well to the form of a flat elastic band, yes the type you might use to keep a bunch of letters together. However, the system is inside the workings of an internal combustion engine, so it will be subjected to exposure to engine oil, and trace amounts of gasoline as well as a temperature environment of -20C startup, 90C operating, 130C maximum.

I considered 2 synthetic rubbers, Nitrile (NBR/BUNA-N) or Viton.

My question is about the mechanical properties of these rubbers. I am aware of the relationship between Shore hardness and elastic modulus, but I have no feeling at all for the elastic limit and fatigue resistance. How far can you stretch these rubbers and how well do they perform, in tension, under high numbers of cyclic loads. Is there a recommended strain range for best operation?

 

[Compositepro]

Polymers will perform very poorly as springs in that environment. Properties change dramatically with temperature. They creep significantly and rapidly compared to metals. Where rubber is used as a spring, it is normally in a low stress state and will quickly bounce back to that state.

 

[dmapguru]

Thank you very much for your quick and clear response.

 

[btrueblood]

Rubber fatigue life in tension is very poor compared to just about any metal, and its properties will vary from batch to batch, and part to part, much more than typical metals. In compression (shock mounts) it survives fairly well, with caveats as stated by Cpro.

 

[TugboatEng]

NBR is absolutely the worst choice of material for that application. It will last just long enough for you to take it in to production and then start failing in use creating a warranty nightmare. While most data sheets rate it for 100°C operation you will find its life to be very short in any application above about 60°C. Viton is an expensive alternative but HNBR rubber is being used successfully for in crankcase timing belts on some current production engines. You may want to look there.