Looking for a Highly Fatigue Resistance Spring Material

[axleshox]

We're currently using a stainless 17-7 Condition C material.
The material needs to go through a 4 slide die for cold forming and cutting. Thus, we need it in a 2" width by 0.015" thick roll.

It is subjected to a stainless of about 0.005, 1/2% strain and must be resistant to fatigue for at least a 1 million cycles.

We've tried 304 and 316 alloys as well as nickel titanium alloys, i.e. Nitinol. The 304 and 316 were far less resistant to fatigue and came out slightly warped because of the heat treating proces. The Nitinol proved to be too difficult to manufacture and thus too expensive.

I'd appreciate any suggestions or leads.

Elmer Lee

 

[axleshox]

Hello All,

Since so many have ask for specifics of the design, I thought it would be best to spend some time explaining it.

The sping is a leaf spring that can be modeled as a cantilever beam rigidly supported on both ends. The beam however is not straight, but instead made up of 3 curves of different radiis. One end of the beam is held stationary and the other is displaced about 0.75" in total, i.e. .375" upward, and the same amount downward. As the spring is deformed, the 3 curves essentially change radiis. There is a formula:

strain=thick/(1/R1-1/R2)

where R1 is the orginal curvature of the beam and R2 is the resulting curvature after deformation.

I've written a computer algorithm that uses the above equation, and knowing some geometric constraints, i.e. total spring length, max and min radius, and determines all of the spring geometries that when deflected the desired amount will not give a strain of more than 0.005 which I thought was the upper bound of any metal's fatigue strain. So the resulting spring would only have a max strain of 0.0025 because it would be deflected 1/2 the desired amount upwards and the equal amount downards.

If anyone wants to know more about this code, my MSN messenging name is axleshox@hotmail.com. I'm almost always online. I'd be happy to discuss this further.

In the end, the algorithm spit out about 30 different sets of spring shapes. They were all quite similar and I pick the one that seemed easiest to manufacture, i.e. did not bend over on itself too much.

What I am finding is that the spring seems to be holding up to simple upward and downward deflection. However, we realized later that the spring was also being subjected to a torsional load. As a result, the torsional plus the vertical deflection is starting to produce fatigue failure in the springs.

Nicke, the suggestion about parallel spring is an excellent one. We tried that once and found that if we were to lay up say 3 springs on top of each other, the bottom one would deflect as desired but the top two would start to bend in un-predictable ways. I attribute the problem to not having the springs tied to one another. I think essentially the top two springs are buckling.

We're probably going to redesign a few tools that will allow us to bond the springs together with low durometer rubber or urethane. That way, the different layers can still move in shear with respect to one another, but no buckling can result.

I started this thread in hopes to find a stronger, more flexible material. That would have been the easiest fix.

Redesigning the shape of the spring seems like it would be unfruitful. The algorith was fairly complete. The only way to further reduce stress and strain whle still maintaining the same deflection is to use a longer spring. Unfortunately, the spring is enclosed in a housing and we've already used up every piece of space we could find.

Sorry for such a long post. I hope someone is willing to read it all. Appreciate all the comments thus far. It's geat to know there is a community out there willing to lend a hand. Thanks all.

Elmer

 

[CoryPad]

If the easiest change is to use a stronger material, then re-read TVP's post above about 301 stainless from Somer.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.