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non linear behavior of cylindrical helical spring

non linear behavior of cylindrical helical spring

non linear behavior of cylindrical helical spring

(OP)
Hello all,
I have a steel helical springs with constant pitch. The spring is a tension spring with closed ends. These springs are to be used inside a tank, in a wall scraper assembly to ensure the Teflon scrapers are in contact with the wall due to a constant force acting on it.
The spring after showing peculiar behavior was horizontally stretched between two fixed supports. The surprising part was that the deflection per one coil was not same throughout the spring. That is, the spring with 20 coils, the difference was that one coil had opened 2.3mm but another coil had opened only 1mm at the same force.
Initial thought was that inconsistent heat treatment may be the cause but that only effects the strength, not stiffness of the wire and since it was tested well within the elastic limit of the wire, this idea did not fit.
Upon extensive research that lead nowhere, i am forced to look to you all for guidance

RE: non linear behavior of cylindrical helical spring

Is this a permanent deformation? Is this just one coil that is different and the other 19 are essentially identical?

What is the wire diameter and coil diameter?

RE: non linear behavior of cylindrical helical spring

(OP)
Apologies for the ambiguity.

"Is this a permanent deformation? Is this just one coil that is different and the other 19 are essentially identical?"
As it is a tension spring, the coils are initially all bound closed. after applying load, coils on one end of the spring deflect more, and as we move towards the other end, the deflection per coil decreases incrementally. A coil on the end that deflected more, showed a deflection of 2.3mm/coil. 19 coils away, the other end of the spring showed deflection of 1mm/coil. The change in stiffness across the length of the spring was not drastic from one coil to another, but gradual.

"What is the wire diameter and coil diameter?"
The spring is made of stainless steel 2mm wire, with spring diameter of 20mm.

RE: non linear behavior of cylindrical helical spring

I have seen many wall scrapers and never a coil spring on one. Scrapers are exposed to many things in the mix that can cause high lateral forces and mechanical damage. They are usually designed to be energized by the viscous forces acting on them as they are moving. Torsion springs are used to bring them into wall contact.

Your problem sounds like like the spring was damaged by mechanical damage from solids or by over-extension due to a lack of hard stops on the scraper movement.

RE: non linear behavior of cylindrical helical spring

It seems most likely that the spring winder did not supply a constant preload as the spring was wound. Unfortunately I think that the simplest way to find out is destructive testing - cut a single coil at each end and see how far the spring moves in the opposite of the coiled direction.

The only other thing the should make a difference but not as much as you are seeing is if the wire diameter changed. I think that is unlikely as wire is normally drawn and the amount of change would be pretty large to get a 2.3:1 ratio.

It also brings up another thought - while the elasticity, E, isn't affected by strength, the pre-load during winding is applied by plastic deformation where strength of the material does matter and it may be that the initial coiling failed because of a tempering/heat treating problem.

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