As spring design is not my forte, I was hoping to get some feedback from the experts on my little problem. I am designing a spring actuated park brake mechanism. The spring works over a shaft within an air chamber with the following constraints:
1) Max OD = 1.9 in., Max ID = 0.9 in.
2) 1350 lbf load at 1.80 in. height.
3) 500 lbf load at 2.65 in. height.
The design life is for <30k cycles, no corrosive environment, no extreme temperatures.
I am having trouble coming up with a spring combination that is not over-stressed. I have tried single/nested round wire springs, and single/nested rectangular springs. The closest I have gotten is a set of 2 nested round wire springs,~2/3-1/3 load split, with a torsional stress of ~130-135 ksi. Unfortunately, for the diameter wire I had to go with (to satisfy space constraints and spring rate) the allowable is only ~120 ksi for pretempered alloy steel wire, and that does not even consider fatigue yet.
Do the load and space claim requirements of this design really make this difficult, or am I missing something fundamental here? Are there other materials out there that can handle these stresses better than what I have chosen?
Am I trying to design an impossible spring? My ignorance in this field makes me hesitant to say that it can not be done. Any info would be greatly appreciated!