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Brisbine (Military) (OP)
3 Dec 05 12:47
Hello all,

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!

Thanks,
-Brian

israelkk (Aerospace)
3 Dec 05 18:00
From your description and after some calculations it seems that your problem is not simple at all.

It may be that in the space constraints you may not find a solution.

The maximum wire diameter that you can use is 0.5" (1.9-0.9)/2. In this wire size the spring index will be less than 3. Therefore, a more sensible maximum wire diameter is 0.4" or 0.45". However, even if is made of 17-7PH CH900 it will need a much longer spring to achieve the rate of 1000lbs/inch.

Even to achieve half of the force (and half of the rate) you will need a more than 0.3" wire diameter of 17-7PH CH900 and the spring at the maximum deflection will still be much longer than 1.8". So nesting is not going to help.

http://www.webspawner.com/users/israelkk/
desertfox (Mechanical)
4 Dec 05 5:03
Hi Brisbane

Have you considered making a volute spring out of flat rectangular bar.
These springs will store a lot of energy in a small amount of deflection and they have a constant spring rate until the first coil bottoms.
These springs were first used in the U.S.A. in WW11 tank suspensions.
There is a section on these springs in the book:-

   Spring Design  by W.R.Berry


regards desertfox
Brisbine (Military) (OP)
15 Dec 05 11:27
Israelkk & Desertfox,

Thank you both for your input. It helped steer me towards a solution. Just wanted to give you an update on the situation.

We went to a more aggressive lining (reduction in lining life was acceptable), which reduced the required force. Then we hammered on the designer to give us a 2.2 in. bore to work in. That combination gave me what I needed to make it work.

Thanks again for your help!

-Brian

israelkk (Aerospace)
15 Dec 05 16:49
Can you give new spring requirements, dimensions etc. compared to the spring in the first post?

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