Novice Spring Steel Question

Hi Group,
I first of all apologise for my lack of knowledge and details! I manufacture a tape measure type device which has an internal (clock style)
return spring. This spring is supplied to us wound and trimmed to length (2100mm x 8mm x 0.127mm) with a 40mm internal Diameter. We reverse wind this spring into our product and the spring is used to recoil the measure. The material is a bright cold rolled high carbon strip 95% carbon (ASA1095) and sorry to say that is all the information I have on the material. This spring is too strong for our application and I wish to reduce it's power. The question is:- is it possible to heat treat this spring to reduce power? I cannot request deferent material or maybe thinner material mainly due to the fact that I purchase fairly small quantities (10,000/year) and costs for a non standard from the supplier are too high.
Regards
Simon
Cube3 - Creative Product Design
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Simon Miller wrote:

One way to reduce the spring return power is to reduce the thickness, as you have pointed out.
Another way is to reduce the width.
The above are geometric modifications. These leave the mechanical properties of the material itself unchanged.
In general, modifications attempting to reduce the elastic modulus of the material by heat treatment won't work. You can reduce the yield or failure strength by heat treatment, but what you really need to do is to reduce the elastic restoring force of the spring.
The three obvious ways to do this are the two mechanical ones of reducing thickness or width of the spring, and finding a return spring material of a much lower elastic modulus would be the third.
There might be a way to change the stress-free state of the spring material from straight to partially wound configuration. In that case, you would find that the restoring force is decreased because the amount of tension depends upon the degree of departure from the stress free state induced by extending the tape.
Someone else may have some clever ideas on how to do this, or experience doing this in other applications.
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Simon Miller wrote:

do you have room to use two springs in series? that will reduce "power" accordingly. otherwise you need to re-spec the spring as per jim's post.
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wrote:

Simon
Punch (or cut) single (or several) elliptical holes down the middle of the tape to reduce the part (tapes) elastic "compliance" of the tape. Unless you good at FEA/ FEM (finite element analysis/modeling) or even FDA (finite Difference Analysis - using Excel) your best bet is to experiment - use a round paper puncher if the strip is thin enough.
Jim was right about the "modulus" that is set for steel as ~ 29,000 ksi 500 ksi by atomic bonding and cannot be changed by heat treat.
Ed
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