What is the best spring material?

[molds]

I have a situation where compression springs are failing at an accelerated rate. the situation is a 3.00 inch long, 1/2 diameter spring. it has a rate of 31.17 LB/IN. preloaded .200 of and inch, with a working stroke of .75 inches. the solid height of the spring is 1.560 inches. the spring material is 302 stainless steel. the spring is retained in an aluminum block at one end that has been hard coat anodized for a lenght of 1.875 inches. the oposite end is retained by the ejector bar which has a .200 deep bore for thr spring pocket. it is in a horizontal position. the spring cycles once every 7 seconds. the aluminum plate has cooling water runing through it that ranges from aprox 33f degrees to 45f degrees. room temprture is held at 68f. can anyone suggest what may help this situation? I can not increase the diameter due to space constraints, but could increase the length slightly.

 

[israelkk]

MikeHalloran has a good idea, this way with short springs you do not have the bucking problem as long as you find a stack of springs that will give the forces and cycles.