Coil Spring Force Application Area on Rubber Insulator/Isolator

[jmarkus]

Okay, this is probably just a geometry problem, but I can't wrap my head around it.

If I have a coil spring sitting on an isolator pad the surface of the spring which makes contact with the isolator pad will increase as the force on the spring is increased and the rubber compresses and deforms around the spring end. I would like to predict how much the isolator will bulge out radially, based on the force applied, but I am hung up on the first statement. How can I determine the deformation/deflection of the rubber isolator given a certain spring (and spring force) and applied load to the spring?

Thanks,
Jeff

 

[israelkk]

Is this a real problem? This seems odd. Why not use a steel washer between the spring and the rubber to avoid direct contact between the spring coil and the rubber? This should be the correct way to go. To your question, the only way to solve the actual problem is using a large deflection (of the rubber) non linear FEA analysis.

 

[israelkk]

If you go your way, the force on the spring may not align along the spring axis which will force you to use the FEA and bucking analysis on the spring too. The direction of the force on the spring will vary from system to system depends on the rotational/circumferential direction of the spring assembly with respect to his axis.

 

[GregLocock]

This is a big real world issue since the road springs on a car use (shaped) rubber pads between the spring and the cap. Among other things the take up between the coil and the pad affects spring durability (if you get it completely wrong) and the effective point of application and direction of the spring load, which is of great interest in some suspensions. The shaped isolators also help to prevent the spring from rotating, which is important.

To be honest I've never worked on the isolator side of this arrangement, we typically modify the spring profile, a non linear FEA seems like a modern approach. We did it on a rig.




Cheers

Greg Locock


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[jmarkus]

Ok, so I guess the consensus is to use FEA to determine the amount the isolator will bulge. I guess that puts me in the usual iterative design loop...start with an assumed thickness, diameter and shape and apply the spring geometry and force and see how much it bulges, then revise as necessary and re-analyze.

What did we do before FEA? Oh yeah...build, test and break, build, test and break...sigh...

Jeff