Compression Spring Design

For those who know about compression springs.

My friend Mike just bought a recoil spring for his semi-auto pistol. Before installation he measured it at 5". He then installed it, cycled the slide a few times, removed the spring, then measured it at 4-1/2". When he told me this I was more than surprised by this.

I am a mechanical engineer with no specific training in spring design and operation. But for a brand new spring to "instantaneously" suffer 10% plastic deformation after just a few compression/relaxation cycles seems, well, ridiculous.

And I just read a post elsewhere where the writer stated that is "normal" for a recoil spring to "take a set" and become shorter almost immediately after installation.

So I'm hoping someone can shed light on this for me. Namely, is such behavior normal for brand new springs in general ??

Thanks for your help,
Carl

The material permanently yielded on cycling.. It is possible this was by design as in torque-to-yield bolts however, my first guess is that it is just cheap spring material..

You should contact the manufacturer of the firearm and ask...

Lots of compression springs are subjected to a manufacturing operation known as 'presetting'. The spring is overloaded or compressed to closed height so that all of the material has been taken to a stress level a bit above that which it will see in normal operation. This actually imparts some residual stress that is beneficial in subsequent cycling as long as the loading is in the same direction.

Your posting suggests that the recoil spring had not been through such a manufacturing stage.

It is to be hoped that this was by design and that the spring was not just stretched to obtain a larger coil spacing.

With the firearm cleared, Mike might push the slide down on a bathroom scale to get an idea of the starting and fully cycled loads with each spring. Thereby giving him an idea of the relative change in his recoil spring loading.