stretching a compression spring

[bullpuppy666]

I am a computer engineer not a mechanical engineer so my question is pretty basic.


What effect does strechiing a compression spring beyond it's limit has on it's rate. For example if I have a spring that is has a 2 inches free length with a rate of 8 lbs and I stretch it to so it that it's free length is 4 inches what would the rate of the spring be?

 

[sreid]

Desertfox,

Bullpuppy"s original post sounded theoretical. I never suggested he actually stretch a spring (although who of us havent done this to a hardware store spring).

Just for fun, I looked at how the diameter of a spring would change if you stretched it to double it's length. I used 5 turns, one inch diameter, 5 degree helix angle stretched to a helix angle of 10 degrees. I calculated a diameter reduction of about 1% creating a spring rate increase of about 3%.

 

[bullpuppy666]

I beleive I found the answer. I can order a long spring and cut to length or I can just get two short stock spring at double the rate and put them on top of each other. Several companies have ones that will work and fit into my constraints. Thanks everyone for your feed back.

 

[NickE]

YOu wnat a separating washer if you stack two shorter springs on top of each other, if you dont they will "fall" into each other.

Nick
I love materials science!

 

[desertfox]

Hi sreid

I agree with your calculation for a spring with a 1" dia and helix at 5 going to 10 degrees.
I did a similar calculation using 1" dia (mean dia) but used
10 turns with a wire dia of 0.1" then I stretched it from a 2" length to a 4" length in accordance with the original post (in terms of length anyway) the result was for the 2"
spring the rate was 14.375lb/in and reducing the mean dia for the 4" length gave a spring rate of 17.3lb/in thats an increase in rate of just over 20% in my book thats a significant increase. Now consider if you compress this spring by a given amount the shear stress in the wire increases by 20% again not an insignificant amount.
Anyway its acadamic now.

regards

desertfox

 

[bullpuppy666]

Thanks for the idea Nicke. That just gave me the idea in that I can adjust the size of the spacer to adjust my operating length.

 

[sbozy25]

Excuse my bluntness here, but come on....

You do not need a seperating washer to keep them from falling inside them selves... In fact that is a bad idea, I deal with customers ocassionaly who try that, and all they do is end up spacing it to much and causing failures. If they are the same size, they will not "fall" inside of eachother, they will but against eachother just fine.