Sping Steel

HELP! how do you compute the torque of a SPIRAL SPING (steel), length of sping, need formulas or websites?

i have a spiral sping from the rewind spring of a power saw. if the rewind sping is made out of steel , copper, alluminum. what is its torque, torsion if it has a diameter of 7.5 cm

You need more information. It sounds like this is a torsion spring so the formula is farily simple. You need the diameter of the spring wire and the number of turns in the spring.

Stiffnedd of a torsion spring is calculated exactly like a cantilever beam with a pure bending moment applied.

The only material property you need is the moulus of elasticity (E), it is:

30,000,000 psi for steel
18,000,000 psi for copper
10,000,000 psi for aluminum

Once you know the wire diameter (d), figure the moment of inertia (I) by:

I = (pi d^4)/64

Next figure the length of wire (L) from the spring diameter (D) and the number of wraps (n):

L = pi*n*D

Now the stiffness is just k = EI/L

The units on stiffness will be inch-pounds meaning the amount of torque in inch-pounds needed to rotate the end of the spring by one radian. It takes 2 pi k torque to go one revolution or pi k/180 to go 1 degree.

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