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Anonymous Poster #1

Axially-Twisted Springs

03/01/2017 3:38 AM

If you took a linear section of wire and twisted it and indeterminate amount of degrees about it's axis, then formed it into a conventional spring--would the tensile strength be greater, or would it tend to yield quicker under commensurate strain than a normal spring?

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#1

Re: Axially-twisted springs

03/01/2017 8:25 AM

If you form a wire into a coiled spring, you have to apply a torsion (twist), just as a coiled up garden hose has a twist and kinks when straightened out.

So a straight wire has to be twisted to form a helix, and the torsion will have to be maintained to stay in the helical form. When it is compressed, the torsion will become less and when stretched it increases, and at some, point failure may occur.

For a wire to maintain the helical form without an external torsion being applied, it has to be annealed (softened) to relieve the stress and then rehardened. If this is done, the helical form will be at the neutral point and the spring will be less likely to fail.

http://www.thecartech.com/subjects/machine_elements_design/helical_springs.htm

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Anonymous Poster #1
#2
In reply to #1

Re: Axially-twisted springs

03/01/2017 12:46 PM

Let me clarify: If you twist axially the section of wire from which the spring is made 25o--after formation, when the spring is subjected to stress/strain would that axial twist have any quantitative effect on the limits of it's elastic and plastic deformation?

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#3
In reply to #2

Re: Axially-twisted springs

03/01/2017 1:53 PM

If you take a piece of garden hose or a rope and twist it, two things will happen: it will try to untwist (torsional force) and it will deform into a helical shape.

If you compress or extend a helical spring, you are applying torsional force. If you created the helical spring by twisting a straight wire, extending this spring will cause the two torsional forces to add. If you compress the spring, they will act in opposition.

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#4
In reply to #2

Re: Axially-twisted springs

03/01/2017 2:10 PM

..." For most materials, the strain experienced when a small stress is applied depends on the tightness of the chemical bonds within the material. The stiffness of the material is directly related to the chemical structure of the material and the type of chemical bonds present. What happens when the stress is removed depends on how far the atoms have been moved. There are broadly two types of deformation:

  1. Elastic deformation. When the stress is removed the material returns to the dimension it had before the load was applied. The deformation is reversible, non-permanent.
  2. Plastic deformation. This occurs when a large stress is applied to a material. The stress is so large that when removed, the material does not spring back to its previous dimension. There is a permanent, irreversible deformation. The minimal value of the stress which produces plastic deformation is known as the elastic limit for the material.

Any spring should be designed and specified such that it only ever experiences elastic deformation when built into a machine under normal operation."...

https://www.khanacademy.org/science/physics/work-and-energy/hookes-law/a/what-is-hookes-law

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#5
In reply to #4

Re: Axially-twisted springs

03/01/2017 3:58 PM

Alright--cut to the chase: I referring as to whether a Ni-Ti-Cu alloy spring would acquire any advantage by applying an axial twist before the subject is formed--since the spring itself has to rotate axially when it is stretched.

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#6
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Re: Axially-twisted springs

03/01/2017 6:55 PM
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#7
In reply to #6

Re: Axially-twisted springs

03/02/2017 1:09 AM

Interesting. 1976. Doesn't your invention have to have proven validity before they grant you a patent? The patent holder doesn't provide data, but the assertions seem to corroborate with my hypothesis. Thanks and kudos for the info.

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#12
In reply to #7

Re: Axially-twisted springs

03/02/2017 12:19 PM
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#11
In reply to #5

Re: Axially-twisted springs

03/02/2017 11:00 AM

I think I understand your question, though I do not know the answer.

First off I would say the grain of the material might be affected by per-twisting if the grain of the material was axial to begin with. For instance, you are twisting an extruded bar, then wrapping it into a spring.

Second, If the material is layered, then there may be an effect as well. You may introduce internal stresses in the per-twisting that may or may not effect the final spring.

Third, I would be interested in the direction of pre-twist versus post winding of the spring. A twist in one direction may increase a factor for a spring wound one way, and decrease a factor if wound the other way.

If the bending force is changed after twisting, then you may have something to use as a guide for testing.

Tension and compression springs may need twists in opposite directions for winding in a given direction.

If I take a piece of extruded bar stock and twist it clockwise and I keep going, it will eventually want to twist up on itself again and wrap up like a spring, re-inforcing the same twist, like winding a rubber band powered plane to a double-row.

The direction of twist the rubber band makes is significant to your question, and apropos to the decision to twist the bar one way or the other.

All of this is Dependant on the answer to the first point, the grain of the material. If the grain is not axial, then the process may not affect it or it may make it worse.

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#8

Re: Axially-Twisted Springs

03/02/2017 7:51 AM

Warning! No theory reference in this post.
It seems to me that if you twist the wire, you add stress prior to the stresses caused by winding. Unless it is treated after winding, those stresses would remain.
I would think (hypothetically) that the additional stresses may increase the force required for compression, or expansion; furthermore, I believe it would change the preload and working length of the spring to keep from overloading these increased stresses.
On an economic standpoint, it would seem cost prohibitive to add more processes and test your exact spring length to failure for the required acceptance quantity.

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#9

Re: Axially-Twisted Springs

03/02/2017 10:13 AM

Greater , it would work harden. Having done some return yield tests at Boeing normal Cs will harden when repetitive below yield point loads are applied.

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#10

Re: Axially-Twisted Springs

03/02/2017 10:43 AM

I'm thinking that the answer is likely to be yes, but not because of any change in the tensile strength of the materials, but because of the residual stresses after the initial onset of yield. In torsion, the surface of the wire section yields first and takes an inelastic set with material below the surface reaching onset of yield.

From that point onward, when unloaded the wire surface is in compression. As the load increases, the compression decreases and changes back to tension along with the material below the surface, exerting a larger torsion force. The spring will act stronger.

However, the alternating stress if the spring actually cycles to unloaded condition will shoot the hell out of the fatigue life of the spring. So for a constantly loaded or low cycle life spring, pre-straining will be good, but for a high cycle spring, it will break sooner.

You see this in the difference in life time and spring characteristics of cold-wound vs hot-wound springs, annealed and heat treated vs cold finished spring wire and even shot blasted springs.

I worked on one project on earth moving equipment cutter teeth where the initial plastic deformation of the teeth in use was planned for in the performance and life of the cutting tooth.

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