Spring stress calculations - unique compression spring loading
Spring stress calculations - unique compression spring loading
(OP)
I have a compression spring that is failing in application. The spring meets industry standard design criteria for axial loading. However, in application this spring also has a moment load generated as the opposite ends rotate relative to each other (see attachment). The questions I have are:
1. How do I calculate the increase in stress throughout the range due to the "pushing" effects on the coil?
2. How do I calculate the load change if the spring helix were in the opposite direction. The load requirements on the spring are important for the design performance.
1. How do I calculate the increase in stress throughout the range due to the "pushing" effects on the coil?
2. How do I calculate the load change if the spring helix were in the opposite direction. The load requirements on the spring are important for the design performance.





RE: Spring stress calculations - unique compression spring loading
Spring formulations are based on analytical and long time empirical experience but in your case you can not use that. You need to treat your spring as any mechanical products but you can not rely on a any spring specific knowledge.
If you can provide detailed information of the spring dimensions, loads, deflections, etc. and mode of failure may be it will be possible to analyse the spring independently as compression and torsion spring and see which mode is more detrimental to the failure.
RE: Spring stress calculations - unique compression spring loading
We had a similar problem on some of our textile type machines where we were experiencing the same type of spring failure as the OP. This problem was eliminated by initially installing a thrust bearing and if our recall correctly it ended up as a bronze thrust washer.
Could this type arraignment possibly help the OP?
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RE: Spring stress calculations - unique compression spring loading
Spring Specifications
Ends: Closed ground
Grade: Commercial
Wire: Chrome Silicon 280 ksi min
Wire dia: .192 in
Rate: 50 lb/in
Spring Index: 13.0625
Mean coil diameter: 2.508 in
Coil OD: 2.7 in
Active coils: 2.477
Total coils: 4.477
Pitch: 1.5408
Pitch angle: 11.0649
Free Length: 4.2 in
Load point 1 at installed length: 105 lbs at 2.1 in (stress 105.1 ksi considering compression only)
Load point 2 at full travel length: 162.5 lbs at .95 in (stress 162.6 ksi considering compression only)
RE: Spring stress calculations - unique compression spring loading
Theory says that if the two spring edges are fixed and can not rotate the spring rate will be ~25% higher than a case where at least one end can freely rotate. In your case the spring can rotate but a friction torque doesn't allow it to freely rotate. Therefore the rate will be higher than the calculated rate but below the 25% extra.
My quick calculations show that this spring should be presetted/scragged and supposes to give ~12000 cycles if repeatedly loaded between the two points of loads. This is a conservative estimation but it indicates that this spring has "limited" life cycle. Any scratch, dent, tool mark or surface imperfection will cause a large shift in the life cycle of this spring. The calculations assumed only compression without any torsion load. Any additional load due to friction will seriously decrease the calculaetd life cycle.
You should note that with fatigue calculation you need to assume a factor of 4 to 10 to be sure that all batches of spring will survive the designed life. Therefore, you can only guaranty ~1200 to ~3000 cycles.
You may need to test every batch of production to see if it match the design goal.
RE: Spring stress calculations - unique compression spring loading
RE: Spring stress calculations - unique compression spring loading
The torsional moment on the spring due to friction is about 20 in-lb at max compression (assuming a Mu=.10), and that would equate to a bending stress of about 29ksi in your coils. And that friction torque alone should produce a "twist" in your spring of about 23deg. At least according to my rough calculations.......
Terry
RE: Spring stress calculations - unique compression spring loading
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RE: Spring stress calculations - unique compression spring loading
I ran a test this weekend with a needle bearing under the spring. So far, the spring has completed 150,000 cycles with the max load matched. I had to reduce the stroke 7mm due to the bearing/washer thickness. This is more than sufficient for this application. So this leads me back to my original questions.
- Will the LH helix reduce the stress/increase the cycle life? How much?
- What will be the load difference between the RH, LH given the same deflection with the 45 twist?
We are working on samples to test, but I would like to have a good idea of what to expect before the samples arrive.
RE: Spring stress calculations - unique compression spring loading
LH should not change anything.
RE: Spring stress calculations - unique compression spring loading
With reversing sliding frictions, like you would have at your helical spring wire's end, there is a difference in frictions between the "approach" and "recess" actions. The friction created by the approach action (ie. where the sliding is in the direction of the wire end) will be higher than the recess action (ie. where the sliding is away from the wire end).
Other than that, the direction of helix should not matter, Since the loads are reversing.
Terry
RE: Spring stress calculations - unique compression spring loading
RE: Spring stress calculations - unique compression spring loading
What is the end condition of the far end (lower)? Is this free to slide against the member?
Also, pass the numbers and geometry of the Spring. Maybe something a little more freaky (internal to your geometry).
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada