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# Spring Calculation

01/29/2012 12:39 AM

Hi all

I want to make a spring similar to the one shown, out of strip. The OD is about 58mm and the height about 75mm. I would use carbon steel spring strip. The spring needs to fit between 2 tubes 51mm OD and 60mm ID. It needs to compress 20mm with 20kg. I can't find any formulas, they all seem to relate to round wire.

Thanks

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#1

### Re: Spring Calculation

01/29/2012 2:05 AM

This might work...I don't think the shape matters, it's a compression spring...

As long as they are not stretched or compressed beyond their elastic limit, most springs obey Hooke's law, which states that the force with which the spring pushes back is linearly proportional to the distance from its equilibrium length:

where

x is the displacement vector - the distance and direction the spring is deformed from its equilibrium length.

F is the resulting force vector - the magnitude and direction of the restoring force the spring exerts

k is the rate, spring constant or force constant of the spring, a constant that depends on the spring's material and construction.

This seems to be some free spring design software...

http://www.mwspring.com/software.html

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#2

### Re: Spring Calculation

01/29/2012 4:46 AM

In the equation for the round wire introduce instead of π*d^4/32 which is the inertia moment for the round wire the inertia TORSIONAL moment for the rectangular profile.

The kind of spring you want to design is based on the torsion of the beam under the moment F*R where R is the coil radius and F the axial force.

Although this was not considered as necessary the wire section geometry plays an important role as well in the deflection as in the computing of the maximal load. For this you should introduce in the shear stress equation instead of the π*d^3/16 which is the resistance module for the circular section c*a^3 which depends on c= forma coefficient depending on the ratio b/a (a small side of the rectangle). The maximal shear stress should be of same order of magnitude as for round wire if same heat treatment was applied.

It can be considered that the shear stress has to be under yield stress in tension /3^0.5.

If you have still troubles then ask again.

I always wrote and repeat that we have a responsibility toward the one who ask the question and if do not know the correct answer it is better to say that.

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#3

### Re: Spring Calculation

01/29/2012 11:18 AM

I agree, and also don't assume you are always correct....

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#4

### Re: Spring Calculation

01/30/2012 12:41 AM

This webpage from Roymech (any relation? ) gives one formula for helical springs of rectangular wire down the page a ways... maybe of some use?

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

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#6

### Re: Spring Calculation

02/01/2012 12:56 AM

Thank you all for your help, especially kvsridhar and the link to Roymech (no relation). I put the furmula in a spreadsheet and fed in the sizes 16mm x 3mm material 57mm diameter, 4 turns, 32 pitch. I made one in Mild Steel as G is constant, to try out the result. The predicted rate is 17 N/mm mine tested at 11 N/mm, ballparkish. I phoned the springmaker to buy some material, it's not available!!!, unless I buy a sheet from overseas, make them anealed, then harden and plate. Toooooooo hard, back to the drawing board!!

Tony

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#5

### Re: Spring Calculation

01/30/2012 11:37 AM

I sent to make like those years ago for trolley self-regulated brake system, and not problem so those are more common than it seems but being so complex the simple round type, i should ask directly to the manufacturer in case you already have one.-

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