looking for polymer as a compression spring 2

[bhays]

Greetings,
I am looking to use a polymer/rubber as a replacement for a spring in a device redesign.
Can someone give me some guidance on how to find a suitable compressible material?

It needs to be able to produce around 55 N over .40cm and occupy no more than 1.5 cm^3.
thank you
-bhays

 

[plasgears]

We used conical plastic washers for end play reduction in motors. There may be some mfrs out there who will send samples. Experimentally, try one or more of these conical belleville washers. These washers should be molded to shape, not formed.

 

[patprimmer]

How many do you need.

How many cycles must it withstand and how much tension can it lose over time.

Regards

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[bhays]

It needs to withstand at least 100,000 cycles. it can loose some of its force (<10%), and it needs to retain its length to within 1mm. the frequency and speed of the compression is low. it is providing return force (squeeze) in a hand operated instrument.

 

[patprimmer]

It sounds to me like you really need spring steel.

Regards

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[bhays]

steel can work. I am looking for alternatives to the flat steel spring that i'm currently using.
The amount of space available it pretty restricted and as a result, There is a lot of force in a small area where the spring is held.
I was thinking that employing the right polymer would present a better way to distribute the load.

 

[patprimmer]

Polymers will take a permanent set with time and lose tension.

Regards

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[Pud]

Quote Pat:

"Polymers will take a permanent set with time and lose tension."

And pretty darn quick too. There is only one choice of polymer for springs and it's Acetal.

Read this manufacturer's overview with a large pinch of salt and a good dose of sceptism!

http://tinyurl.com/5lpwsp

Lot of force in small area/volume = steel.

Cheers

Harry

 

[MikeHalloran]

An elastomer, e.g. molded silicone rubber, can work as a spring at low cycle rates. Please note that it's not compressible- it has to deform.

A bit of advice: If you're going to redesign the device, then redesign the device in its entirety, not one piece at a time. The real opportunity comes from eliminating parts by combining functions; nothing is cheaper than a part that isn't there.



Mike Halloran
Pembroke Pines, FL, USA

 

[Demon3]

Brilliant advice Mike. Many people don't realise that the benefits of plastics mainly come from integration of parts to simplify design and cut assembly costs.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[bhays]

Mike,

To use an elastomer as you suggest, would it be best to constrain it so as to control the amount and direction in which the silicone part can deform? i'm imagining a silicone rubber plug in a cavity or guide and a plunger pressing against the plug. Is this on track with the usage you had in mind?

-Bryan

 

[MikeHalloran]

Uh, no. You can't reduce the volume of the plug.

If you push on the end with a plunger, you have to leave room for the plug to expand in other directions, e.g. back up around a very loose fitting plunger, or radially through holes in the cavity, or radially in a loose fitting cavity, or just restrain part of its axial length and let the rest expand freely.

You need to get hold of a Pink Pearl and push on it for a while until you understand Poisson's Ratio a little better.



Mike Halloran
Pembroke Pines, FL, USA

 

[bhays]

Perhaps "constraining something so as to control its deformation" was not understandably different than "fully constraining" deformation.

Of course one must allow an elastomer to expand in one direction if it is compressed in another. However, one cannot exert a meaningful force on the elastomer without constraining it in at least one direction.

Controlling the direction of that expansion by design is what I would call constraining it so as to control its deformation.

 

[patprimmer]

If you analyse the suspension of an early model Morris Mini Minor you might get some useful data. They used partly constrained vulcanised rubber cones.

Most elastomers will actually compress a little even if restrained as their molecular structure is flexible and they can be compressed to very slightly reduced volume.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mighoser]

I'd use carbon fiber reinforced PEEK. It is quite elastic up to ultimate. Very good strength for a polymer, chemical and temperature resistance. If the steel part is highly stressed then don't bother without some type of redesign.