Specifying a variable pitch spring on a drawing

[sbandis]

I have modeled a variable pitch spring in ProE. I am not proficient in designing these types of springs so I was hoping someone could help in specifying the proper technique in defining the spring on a drawing so that a spring vendor could duplicate the design. Also is there an equation for designing a variable pitch spring?

Thanks in advance.

 

[israelkk]

How did you selected the wire spec and diameter?
How did you selected the number of active and total coils?
How did you selected the spring OD diameter and free length?
How did you verify that the modeled spring will give the Force vs Deflection you requir?
How do you know that the modeled spring will do the job?

 

[sbandis]

Thank you israelkk for responding.
I started with a compression spring we designed and tested and that provided the original wire spec, diameter,
active and total coils, OD and free length.

The goal is to decrease the spring rate by 25% for 25% of the travel and increase the spring rate by 40% for the rest of the travel.

I increased the number of active coils to decrease the spring rate. I only increased the pitch for 25% of the length. The last 75% I decreased the active coils to increase the spring rate.

The obvious issue is how to calculate the exact values. The first samples I get will allow me to measure the new rates over the length.

I would like to get a better method of determining the exact force/deflection curve.

 

[israelkk]

Assuming the 25% of the designed travel is 1" and assuming you have 14 active coils, therefore, to increase the rate by 40% you need to reduce the number of active coils by factor of 1.4 therefore, the new active coils are 14/1.4=10 coils. Now divide 1"/(14)=0.071" which should be the pitch for the first 4 coils. Since you require only two different rates from the spring then the first four active coils should have 0.071" pitch and the rest 10 coils should have at least (3"/10)+0.071"=0.3071" pitch. This will give total of 4" travel where after the 1" the first four coils bottoms up and touch each other leaving only 10 active coils for the rest of the 3" travel. The trick is to remember that the first travel of the 1" is taken by all 14 active coils. The rest of the 3" travel is taken only by the remaining 10 active coils (3"/10=0/3" for each coils) but you need to add to that the previous 0.071" for each coils to reach the bottoming of the first 4 coils.