Effects of helical groove on coil spring on fatigue life 1

[Shockdesigner331]

Hello all. I've got a question and I'm hoping that someone can point me in the right direction.

I've got a coil spring (17-7PH ~.5"diameter wire/bar) that has small grooves (~.015" deep) along the coils. These grooves are apparently the result of winding the spring. I'm concerened about the effects these grooves have on the fatigue strength of the spring. Has anybody ever dealed with this before and found information regarding this subject? Also, the surface finish is also in question. Is there any literature regarding levels of surface finish and fatigue life? (16mu, 32, 125, etc.) I'd appreciate any input.

 

[MikeHalloran]

You need to find another spring supplier.

Mike Halloran
Pembroke Pines, FL, USA

 

[israelkk]

We had similar problem and the springs failed in service. No tool markings are allowed on the spring wire.

 

[EdStainless]

I know that you can find data on the fatigue life of spring wire as a function of finish. As I recall a rough finish no not nearly as bad as tooling marks. These marks act as stress concentrations and can have serious impact on life.

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[TVP]

Like the others said, tool marks on a spring are just not acceptable. Were these springs shot peened? If the defects are on the OD, you may be able to shot peen the spring and diminish their effect. The surface roughness will be a function of shot peening, and to a certain extent, higher surface roughness is acceptable because it is more than compensated by the residual compressive stresses near the surface. If the spring has not been shot peened, then any surface roughness beyond what a high quality spring wire would have is unacceptable. For the latter case anything higher than Rz ~ 16 would be cause for concern in my opinion.

 

[swall]

Allowable wire surface imperfections are generally defined as a percentage of the wire diameter. ASTM A313, which covers 17-7 spring wire, allows surface imperfections up to 1% of the wire diamter. In your case, that would be around .005". That is for the wire. The coiling process can certainly introduce additional surface imperfections. Single point coiling will produce a mark on the spring I.D. This mark can very from a hardly noticeable burnish to a very prominent gouge. Proper coiling point maintenance and operator dilligence are necessary to prevent unacceptable I.D. imperfections. You need to communicate to your supplier to what is acceptable.