Fatigue life of 200 Million cycles needed for compression spring 2

[markarnoldTN]

We have a customer requirement for a spring to last 200 million cycles. The current spring chosen by another engineer is Century Spring #70834S. The spring has the following specifications:
OD .300"
ID .240"
Free length .940"
Rate 7.600 lbf/in
Solid height .260"
Wire diamter .030"
Total coils 8.630
Material 302 Stainless Steel
Ends closed and ground
Installed height is .787"
Compressed height is .551"
The height is not super critical as the forces are not. However, the stroke must remain .246"

The spring is enclosed in an 8mm diameter housing made of hard anodized aluminum. Operating frequency is 800 cycles per minute.

Another supplier supplied this product with an uncoated steel spring. The spring fretted away some of the aluminum. This aluminum clung to the spring and caused dissimilar metal corrosion and ultimately failure. The other supplier change to zinc plated spring steel and is now undergoing retest.

Our engineer chose stainless as the material for the resistance to the dissimilar metal corrosion. My concern is that stainless will have no where near the fatigue life required.

I am currently in a sales position so it has been a while since I have calculated spring life. I want to be able to come up with an alternative if necessary and have data to back me up when I go to engineering.

With the small size of this spring, I do not think we can shot peen it to increase life. Aside from pre-setting the spring, are there any other ideas to help achieve the life cycle?

 

[markarnoldTN]

IggyD,
My understanding is that the fretting was due to rotation of the spring and the ground end was fretting the aluminum, not the outside diameter of the spring contacting the housing.