compression spring fatigue/life 1

[tajger8]

OD: .296 in
fl: .630 in
sl: .380 in
k: 148 lbs/in
material: sst

The spring is being used in a "detent" mechanism.
It's usually preloaded at least .06" but can be adjusted to a total of .150" When the mechanism goes through the detent the spring also travels .085" of further deflection.
Total deflection can range from .145" to .235"
This is a low cyclic application, however I need a rather good estimate on how many cycles it can take. Can someone help me get the results for .145" to .235" deflection every .020"? Any help appreciated.

P.S. is there any programs that do these calculations? Many thanks.

 

[israelkk]

What is the wire diameter?

 

[tajger8]

ahh forgot that, .054" and its closed and ground
thanks

 

[israelkk]

Check your numbers, at 0.235 deflection the force will be 34.78 lb. This spring O.D. seems too small for this force.

 

[tajger8]

I am aware that the .235 deflection is out of the springs linear range, just looking for some numbers for now.

 

[israelkk]

This spring can not give even 1 lb at 0.38" compressed length not to mention any life cycle. You are asking for 34 lb. Something is wrong. How did you arrived to this spring for the deflection and rate you ask for? The fl-fs=0.25" how it is related to the 0.235" deflection?

 

[israelkk]

Sorry, this spring can give 20lb at ~0.38" compressed length however something is wrong with the fl and fs

 

[tajger8]

Ok I just want to make sure we understand each other.
I am not designing a spring or anything. It is a catalogue spring.
I attached a quick sketch, since im not at work anymore, not to scale, sizes are greatly exaggerated

free length is .630"
solid length is .380"

At the position you see in the sketch the spring is preloaded to a total length of .570" (.06 deflection) This length can be adjusted to a total length of .480" (.150 deflection). You can see object 1 moves horizontally, as it passes by the triangular extension of object 2, object 2 is pushed it up. This object pivots on the pin (3). Now the total length it can move vertically (at the spring location) is .085". So now the spring deflects another ~.085". So the least the spring could be deflected is .145 (.485 total spring length), and the most .235 (total spring length).
Now I would like to know how many times can object 1 pass horizontally through object 2 at the specified preload positions.
Hopefully this makes it clear, or am I missing something?.
Sorry for the bad explanations, English is my second language

 

[israelkk]

If this is a catalog spring then why not give the part number and the maker name?

The 0.235" is the total deflection of the spring not the total spring length. The spring length after 0.235 deflection is 0.63"-0.235" = 0.395".

According to my calculations this spring can not be used for cycling in any of the two modes, 0.06" to 0.145" (~8.8 lb to ~21.5 lb) and the 0.15" to 0.235" (~22 lb to ~34 lb) at those loads the stresses are too high for any guarantied number of cyclic operations.

 

[tajger8]

sorry I-16 on century spring
yes I know its the deflection, I forgot to include the .395" in the paranthesis,
would peening have any "good" affect?
Isrelkk, do you have a good source for fatigue calculations?
thanks

 

[israelkk]

http://www.fatiguecalculator.com/constamp/index.htm

 

[jackpot]

SMI's ASD 6.0 software performs fatigue calculations for valve quality wire, music wire and some SS. I'm assuming that the material is 302 SS. According to the ASD software the fatigue life for this spring is less than 100K cycles.

You can download a free three day trial of the software at

http://www.uts.us.com

Shot peening can commonly impart up to 50% of the UTS in surface compression stresses at a depth of about 0.003 to 0.005" - depending on application an increase of at least 50% in fatigue life is common. SS springs should be shotpeened with either SS shot or ceramic balls and passivation per ASTM A380 after peening is recommended.