Extension Spring Design

Suggest you talk to a spring design/manufacture company.

Don't know where you're from; in the UK I can recommend http://www.leespring.com/contact.asp. You may find their website informative anyway.

https://jdparts.deere.com/servlet/com.deere.u90.jdparts.view.publicservlets.HomeUnsigned

Go to "order parts by specification" in the bottom left corner

this could help you alot if you can follow the format

The major spring manufacturers (Lee, Raymond, Century) offer very usefull spring design apps on their web sites

A check reveals that with the data you want, it may not be possible to get a spring which can be taken beyond yield point manually.

d = 4mm

Dm = 40-4 = 36mm

The spring rate (gradient) = 0.35 N/mm if it is made from Spring steel

You would need to stretch it to 2000 mm (extension of 1300mm) to take it beyond stress limit (assuming 800 MPa, with Wahl stress correction factor included). The spring force at this point would be 455 N or about 46kGf. Maybe doable by a strong man ? Not me !

You may need to change to d = 3 mm if you want to be able to stretch it to yield easily.

i may have goofed up in some calculations, please check it out.... caveat emptor

Here is what i get with 3mm wire...using my own calculators (force unit kgf)

Clearly this all depends on the number of turns per unit length. If the spring has no gaps when un-stretched then he can clearly get 175 turns. I don't know anything about this sort of thing but I'd expect to get each turn to at least twice its diameter before it reached its elastic limit: that would be just over 12½ m.

Just thinking on the fly here but if the wire has to be at least 4mm could the spring be made from thin wall tube instead of wire?

thank you all for your posts.

Just to clarify something:

I only need some of the turns to fail, not the whole length of the spring. So I can have 65cm unstretched and then just stretch the central part of the spring so perhaps have 3-4 tunrs disfigured.

i haven't come across any partial yield by design. The stress is supposed to be uniform across the entire length of the wire. However, reality is never that perfect, and yielding will take place at random, where either the wire is weaker, or some extra stresses have come during coiling.

Springs have well-established design formulae. You can easily calculate various parameters yourself. As others have said, many manufacturers give some calculators on their websites. Depending on where you are, you can call them with your requirements if you do not want to do the calculations yourself.

i wanted to understand spring technology long ago, so made my own calculator

Because you dictate a 4 mm minimum for the wire diameter, 40 mm outside diameter and 700 mm free length you basically dictated the maximum number of coils. Therefore, the spring rate is dictated. The only way to control the spring rate is to play with the spring material or look for hollow spring wire (I have never encountered such a wire except remarks in articles).

PlanetSpring, you're walking a very fine line here between being helpful and just pimping your wares.

I apologize if I offended anyone. I removed my signature from my future posts, but thought the calculator would be helpful in this situation.