Springs

I would say that if you, "stretch it past it's return shape" you have ruined its properties.

Once you exceed the yield strength of the wire in either direction, it's no longer a spring. You'd have to heat treat/temper the wire again after that.

Of course, Milo's (one of) the experts here.

It probably reduces the spring constant, but might be usable at lighter loads.

1.The spring constant/gradient will remain the same if you don't cut off any turns. Since you intend to do so, it will increase. Formulae are available on the web..

2. As has been said, since you have taken it past its yield point, it is no longer homogeneous, and will have residual stresses (google "scragging of springs") May require stress relieving and heat treatment.

3. This is a murderous way to get a compression spring, i would implore you to shop for a new one from the millions available ex-stock from your nearest spring company. If you tell them what you need and what you intend to do, they will send some with alacrity, perhaps free. _{Just to prevent the coldblooded murder of a spring}

If it does work (a la other posts) how do you intend to stop it buckling in compression? Is it in a tube, or a rod up the middle?

Codey

I'm thinking to stretch it until it approximates the coil shape I'm looking for. They are for a model locomotive; weight per spring no more than 5-10 pounds. Where I live, tension springs are more plentiful than compression springs. This is not a very critical application; just wanted to know from the experts.

stretching a spring will alter it's compression rate and stress the spring. cutting the spring will also alter the spring compressison rate. it's not worth trying to alter it's oringinal characteristics.

Er.... small correction.

Stretching will not change the spring rate (aka gradient) since the formula is : k = G/8 * (d^4)/(Dm³)/n_eff

However, cutting the spring will change it of course, since n_eff will change, and since it is in the denominator, the gradient will increase.

When i am at it, i can as well give the stress formula : ζ = 8/Pi * D_m/d³ * F

Exceeding the elastic limit results in residual strain, which is not accounted for in those formulas. The Bauschinger Effect applies to this type of situation -- reversing the load after plastic deformation.

Thank you very much, i did not know the name of this effect, which occurs during scragging. GA to you.

However, while this increases the yield stress, i do believe that it does not alter the spring gradient (lbs/inch or N/mm)....am i missing something ?