Maximum Spring Elongation 2

Calculate the torsional yield of the coil spring (basically a modified Tr/J calculation).

Spring force for given displacement
F = k*x

Shear in the wire:
tau = Ks*8*F*D/pi/d^3

Shear-Stress Augmentation Factor:
Ks = (2*C+1)/2/C

Spring Index:
C = D/d

If you have Joe Shigley's book: Mechanical Engineering Design, you can find a more thorough description of the problem.

In it's absence, you can find information at this link.

If you go with the ratings in the catalog, you are overstressing the spring (rated load = 7.86# @ 1.25" of extension). You're extending it 2.50" and would be producing twice its rated load.

BobM3 said:

If you go with the ratings in the catalog, you are overstressing the spring (rated load = 7.86# @ 1.25" of extension). You're extending it 2.50" and would be producing twice its rated load.

Click to expand...

Or you could do that.

Thanks for the input. I was definitely overthing this.

McMaster has a similar spring.

Rated for 8.9# at a deflection of 2.75". Should be more suited for my application.

Thanks.

Hi MDGroup

I used this site to calculate the stress on the spring you posted in your first post:-


Working through using spring details given above and Mcmaster site I calculate the maximum you can stretch that
spring body by 2.85" giving an overall 5.35" however thats taking the spring to failure so you wouldn't want to do that.
Normally for a safe working shear stress for these type of springs is about 0.4 multipled by the tensile strength of the material.
I think that you might be over stressing the new spring your thinking of using because from my calculations the load given at the deflected length is already hitting the 0.4 * tensile strength and if your cycling the spring it won't last very long.
There are other stresses to consider in the end loops and thats where the spring is most likely to fail.

desertfox

Hi MDGroup

Just looking at the spring you now propose to use it as a free length of 3" that means when the door is closed you will have no preload on the spring.

desertfox

Desertfox,
I am still designing all of the mounting brackets, so I can easily adjust these to pre-load the longer spring.
Cycling is minimal. A high estimate would be 100 cycles per day.

Thanks

Hi MDGroup

Okay thanks for the update.


desertfox

Another good source for tension springs is Century Springs. Here is a partial list at this site.