spring design
August 8, 2007 11:55 PM   Subscribe

design and calcs of a helical torsion spring - how do i calculate the effect small design and production changes will have on the spring constant?

i am trying to do some simple calcs on a small helical spring.it is a simple 3 coil spring in a light application with in-frequent cycles. it's coil size is around 22mm and wire diameter is around 1.25mm. the input force is onyl around 22N at a moment arm of 15mm.
the spring is used to resist or assist movement between two small actuating plastic levers.
i want to calculate the difference in the spring rate for a smaller wire diameter (1.2 or 1.0) and if i went to 2 winds instead of 3.
can anyone help with where i would start with the calcs, or send me to some freewear that might help me?
sorry for such a boring question!
posted by edtut to Science & Nature (9 answers total) 2 users marked this as a favorite
I've never designed a spring or had a similar problem posed to me, but it seems to me that a few more given parameters would certainly help. Assuming that you don't have any experimental data points with which to do a calibration (say, for the 3 coil spring with a 1.25mm wire diameter), or anything else along those, you'll probably want to know other parameters like the pertinent modulus (shear? torsional?) of the material you're using and/or the pitch rate so you can at least theoretically calculate this. I'm not sure if these parameters are just scaling factors within some magic formula; it's possible that they are, in which case....I've been no help at all. I'm sure you're just looking for that magic formula, which unfortunately, I've never encountered.
posted by universal_qlc at 12:36 AM on August 9, 2007

Is this any use?
posted by flabdablet at 1:32 AM on August 9, 2007

I just downloaded that thing, and unzipped it, and double-clicked it, and it ran just fine on my Ubuntu box (wine is installed).
posted by flabdablet at 1:35 AM on August 9, 2007

hey great work! meta comes through again. ill do some calcs then post the results
posted by edtut at 1:44 AM on August 9, 2007

For your original spring (3 coils of 1.25mm music wire, outside diameter 22mm) it gives me a spring rate of 2.215Nmm/deg.

3 coils of 1.2mm wire: 1.877Nmm/deg

3 coils of 1.0mm wire: 0.896Nmm/deg

2 coils of 1.25mm wire: 3.323Nmm/deg

2 coils of 1.2mm wire: 2.815Nmm/deg

2 coils of 1.0mm wire: 1.345Nmm/deg

The 1.0mm wire results in a popup: WARNING !! The index of this spring is greater than 20. This slows production and can cause the spring to appear wavy due to the affect of initial tension on large index parts !
posted by flabdablet at 1:56 AM on August 9, 2007

Got the Google bit firmly between the teeth now. This looks cool too.
posted by flabdablet at 1:59 AM on August 9, 2007

my conclusions, compared to the standard 1.25 gauge spring are:

- reducing the spring to 2 coils will reduce the spring strength by 50%

- going to 1.2 gauge wire will reduce the spring strength by 4%

- going to 1.0 gauge wire will reduce the spring strength by 20%
posted by edtut at 2:40 AM on August 9, 2007

Um... I'd expect that reducing the spring to two coils would increase the strength, not decrease it, and the results I got from the spring design software bear that out. If you spread the same deflection over less wire, the wire has to bend more, and is going to push back harder.
posted by flabdablet at 7:25 AM on August 9, 2007

Your gauge adjustments don't square with the results I got, either. How do you make a reduction from 2.215Nmm/deg to 0.896Nmm/deg come out as 20%? That looks more like a (2.215 - 0.896) / 2.215 = 60% reduction to me.
posted by flabdablet at 7:29 AM on August 9, 2007

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