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Maximum Spring Elongation
2

Maximum Spring Elongation

Maximum Spring Elongation

(OP)
I apologize in advance for this question (as I should know this) but honestly can't figure out how to approach this.

I am using a standard extension spring to pull closed a door.

The free length of the spring is 2.5".
With the door closed, the spring is pre-stretched to 3.0".
With the door fully open, the spring is stretched to approximately 5.0".

Is it acceptable to continuously cycle this spring from 3" to 5"? Or will it stretch out over time and lose force.

Spring Spec's (McMaster PN 94135K18)

Type: Extension Springs
Material: 302 Stainless Steel
Overall Length: 2-1/2"
Outside Diameter: 3/8"
Wire Diameter: .048"
Load: 7.86 lbs.
Deflection at Load: 1.25"
Rate: 5.04 lbs./inch
Initial Tension: 2.54 lbs.

How do you determine the Maximum Allowed Elongation of a Spring before it will deform?

Thanks

RE: Maximum Spring Elongation

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.

RE: Maximum Spring Elongation

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.

RE: Maximum Spring Elongation

Quote (BobM3):

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.

Or you could do that.

RE: Maximum Spring Elongation

(OP)
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.

RE: Maximum Spring Elongation

Hi MDGroup

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

http://www.roymech.co.uk/Useful_Tables/Springs/Springs_helical.html#Extension

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

RE: Maximum Spring Elongation

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

RE: Maximum Spring Elongation

(OP)
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

RE: Maximum Spring Elongation

Hi MDGroup

Okay thanks for the update.


desertfox

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