3
$\begingroup$

While writing a report about a classical spring experiment I noticed that, if small forces were applied to our spring, this would stretch much less than expected.

Searching on the internet I've kinda understood that this is due to the "preload" of the spring, but I haven't found any sound article on the phenomenon.

Do you know something more about it?

$\endgroup$
  • $\begingroup$ How large an effect was this? One possible effect I can imagine causing this is the following: you measure the natural length of your spring whilst it is horizontal. You then hang it vertically and assume it has the same natural length but in reality it has stretched under its own weight. This basically amounts to a measurement error of the natural length of the spring though, so you should be able to tell if that's consistent or not with the data. $\endgroup$ – jacob1729 May 15 '18 at 19:49
  • $\begingroup$ yes we did take into account its own weight, thanks for pointing that out $\endgroup$ – Lucio Tanzini May 16 '18 at 1:07
3
$\begingroup$

I assume you have a helical spring (like a spiral staircase). If the turns of the spring are touching each other when the spring is unloaded, the spring might well need a small load to separate the turns. A greater load will then produce a gap between the turns, so the spring will start to gain an extension.

It's easy to understand what's going on once you realise that the main mechanism involved in stretching a spring is twisting (torsion) of the wire of the spring about the central axis of the wire (I don't mean the central axis of the spring). Suppose that anchoring the top end of the spring and pulling the bottom end of the spring produces an anticlockwise twist in the wire (seen from its bottom end). But if the spring has a built-in a clockwise twist in the wire, this has to be overcome before the turns separate and the spring starts to stretch.

$\endgroup$
  • $\begingroup$ Great answer. I would add that the built-in twist, which causes the initial tension in extension springs, is done on purpose, during spring winding, to keep the coils of the spring together. Without it, the length of the extension spring would not be well defined. $\endgroup$ – V.F. May 16 '18 at 0:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.