Coil spring breakages

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Boustrophedon
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Coil spring breakages

Post by Boustrophedon » Wed Aug 03, 2022 10:11 pm

One of the things I treasured about the engineering department at one of the better universities I attended, was the off hand, throw away comments from the lecturers, these were always offered up without proof and often there was no proof, just informed supposition, with perhaps the hope that someone would provide the proof.

The other day the front suspension spring on my Skoda Yeti broke, I felt it let go and heard it as it bounced about under the car as we ran over it. I stopped to pick the broken end up. This brought to mind one of those throwaway comments: 'Coil springs alway break 3/4 of a turn from the end.' Sure enough 3/4 of a turn from the end is what I picked up. I have a few other pieces picked up off the road of similar sized pieces of spring. (Nice high carbon steel for forging, worth picking up.)

So does anyone have any idea why coil springs should break at 3/4 of a turn? My intuition says that half a turn is maximum stress.
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dyqik
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Re: Coil spring breakages

Post by dyqik » Thu Aug 04, 2022 12:59 am

Boustrophedon wrote:
Wed Aug 03, 2022 10:11 pm
One of the things I treasured about the engineering department at one of the better universities I attended, was the off hand, throw away comments from the lecturers, these were always offered up without proof and often there was no proof, just informed supposition, with perhaps the hope that someone would provide the proof.

The other day the front suspension spring on my Skoda Yeti broke, I felt it let go and heard it as it bounced about under the car as we ran over it. I stopped to pick the broken end up. This brought to mind one of those throwaway comments: 'Coil springs alway break 3/4 of a turn from the end.' Sure enough 3/4 of a turn from the end is what I picked up. I have a few other pieces picked up off the road of similar sized pieces of spring. (Nice high carbon steel for forging, worth picking up.)

So does anyone have any idea why coil springs should break at 3/4 of a turn? My intuition says that half a turn is maximum stress.
3/4 of a turn is 90 degrees around from your assumed maximum stress point, so maybe that's the point that gets torsionally worked back and forth the most? Or is your half a turn already that point?

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Boustrophedon
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Re: Coil spring breakages

Post by Boustrophedon » Thu Aug 04, 2022 3:50 pm

dyqik wrote:
Thu Aug 04, 2022 12:59 am



3/4 of a turn is 90 degrees around from your assumed maximum stress point, so maybe that's the point that gets torsionally worked back and forth the most? Or is your half a turn already that point?
I don't know really. A very rigid thick coil spring could be loaded at just the tips of the spring and such an asymmetric load would give a maximum stress at 1/2 a turn in, but a real flexible spring bends and settles and perhaps a portion of the end of the string lies flat on the spring seat, moving the maximum load round the coil a bit.
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Gfamily
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Re: Coil spring breakages

Post by Gfamily » Thu Aug 04, 2022 4:05 pm

A Google image search for Broken Coil Spring shows a variety of break points; maybe people say anything between 210 and 330 degrees is 'about 3/4', and if you do that you'll up the percentage for which that's true.

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