BeCu extension spring

[JohnPP]

I am working a project : To use BeCu wire to make an extension spring ( gasket ) for EMI shielding . I have asked the spring coil vendor to make the coil for me ( wire diameter is 0.19mm and to form coil diameter 1.0 mm with spring about 140 mm length ) I use BRUSHWELLMAN Alloy 25 with 3/4H , after coil forming , the springs were heat up 600 degree F for 1 hour . The out come is unexpected : The pitch of the coil extended to total length about 200 mm and discoloured .One of BRUSHWELLMAN engineer told me that it is the effect of stree relief .Please advise what were wrong ? The material spec ? The treatment temp and timing ? What will make the finished part with the original colour , with spring effect and not extended ?
And what is the proper procedure to make BeCu extension spring ?
Thanks

 

[israelkk]

This is the same post as in Metal and Metallurgy engineering forum please do not double post

First of all is this a real extension spring that is under deflection load? Or it just a coiled wire?

You can ask the spring maker to coil the spring with an initial force between coils (negative coiling pitch) that makes them to touch each other so that a predetermined force will be needed to separate the coils from touching each other. This may eliminate the phenomena or at least decrease it when some of the internal stresses will be relived during heat treatment.

http://israelkk.googlepages.com/home

 

[JohnPP]

I should say more correct term is to make BeCu Garter gasket ( extension type ) . Its function is as gasket , the force data really not very important . Brushwellman people also has mentioned - negative coiling pitch , but our spring making vendor seems doesn't know this term ? As I am not very familar the spring making , does it quite a common set up in spring coiling machine to have choice of positive coiling pitch , zero coiling pitch and negative coiling pitch ? If so , I may need to visit and work with them for the issue of negative coiling pitch .
As I have some good BeCu garter samples on my hand and I find that their pitchs are close to each other and flexible , I am sure it has been heat treated after cold work . Then what will it looks like prior to heat treatment ? ( I can imagine that these pitches are close pack as well ! But before and after stress released , they are same having close pitches !)
Thanks your opinions offer .

 

[israelkk]

If your spring manufacturer doesn't know this term then you should find another one. This is basic to extension spring making. There are limits to the initial force between the coils due to the negative pitch, otherwise the coils will collapse sideways. Therefore, a spring maker who is not familiar with this should not do the job.

More than that garter spring are another specialty, unless you use standard end coils (full round end) the ends should be manufactured such that the spring can be closed to take the shape of a circular closed spring. Another issue is that if the spring needs to be stretchable to form some holding force then it should be correctly designed not to be over stressed when it is stretched.

As far as I remember you can buy the spring wire in the already cord drawn and heat treated condition with bright and cleaned surfaces. However, it may not fit due to possible cracks as a result of the coiling operation for your spring coiling index of 4.

http://israelkk.googlepages.com/home

 

[JohnPP]

By the way , the current vendor has made the stainless steel gater spring for us with good ends ( front and rear )and no heat treatment needed after cold work , This time we have problem of heat treatment ( expand the space between pitches ! )
If I vist them and use negative pitch concept probably will solve pitch problem , but how's the discolouring problem ??