Springs, how they work, their coiled structure

[fog37]

TL;DR Summary

Understand how a spring really works

Hello!

Springs are amazing devices: we take a straight piece of metal wire, we change its shape and get something that can compress or extended a finite length. We could not do that with the straight wire (too difficult to move atoms apart or close to each other in a significant way).

Why does the spring work the way it does? The coil/helical shape is key to the design. But I am not sure what the explanation is. Is the spring, in the coiled shaped, a prestressed material? If we pull it along its axis, does that longitudinal pull translate in torsional motion of the spring section? Is that what happens? Why would the torsion be helpful?

Thanks

 

[jrmichler]

The wire in coil springs loaded in tension or compression is stressed in torsion. The spring wire may be either prestressed or not, depending on the manufacturing process.

A good discussion of spring stresses is in Fundamentals of Spring Design, by Spring Manufacturers Institute: https://smihq.org/store/ViewProduct.aspx?id=8088348. This should answer your questions. From an earlier edition of Fundamentals of Spring Design:

If you want to know more, the definitive source on spring design and internal stress is Mechanical Springs, by A.M. Wahl: https://smihq.org/store/viewproduct.aspx?id=8088306.

Interestingly, both of these books are well known, but neither is available from Amazon.

 

[Mark44]

Springs are amazing devices: we take a straight piece of metal wire, we change its shape and get something that can compress or extended a finite length. We could not do that with the straight wire (too difficult to move atoms apart or close to each other in a significant way).

Wire can be stretched but can't easily be compressed. When the "strings" on a steel-string guitar are tightened, they produce a tone that is higher in pitch. If you tighten them too much, they break -- they can be lengthened only so much.

Springs don't have to be helical. There are many types of springs that aren't, such as leaf springs on trucks and some kinds of cars. All of them are based on the elasticity of the material used to make the spring.

 

[anorlunda]

I think it is easiest in two stages. First, understand the linear torsion bar.

https://commons.wikimedia.org/w/index.php?curid=66083695

Then, ask yourself how the properties change if the bar is bent into a more compact helical shape.

 

[fog37]

Ok, so let's take a mathematical helix, a curve which has both constant curvature and torsion.

1) The more curvature a curve has, the less straight it is. The more torsion the curve has, the less planar it is. When we elongate a helix, I guess its torsion increases and its curvature decreases.

2) When I think of torsion, I think of twisting something using shear stress to deform it permanently or temporarily.
Back to springs, elongating a spring produces more torsion. I see how torsion increases given the mathematical definition of torsion: the piece of the spring get more distant from each other...still the straight metal wire making the spring that we started with has the same material and same overall length as the spring. Having deformed it into the helical shape, we have essentially convert it into something with very different properties...

 

[Baluncore]

If you want to know more, the definitive source on spring design and internal stress is ...

Mechanical springs, by A.M. Wahl ...
Wahl, A. M. (Arthur M.), 1901-
Cleveland, O., Penton Pub. Co., 1944.
http://hdl.handle.net/2027/uc1.$b76475
"This work is in the Public Domain, meaning that it is not subject to copyright. Users are free to copy, use, and redistribute the work in part or in whole."

https://ipfs.io/ipfs/bafykbzacedqr3... Springs-Penton Publishing Company (1944).pdf