Since extension springs have an initial tension in their resting state, they also have a shear stress in their coils while at rest. The maximum shear stress (at rest) ti occurs on the inner face of the coils, and is given by the equation,
where D is the nominal diameter of the spring, d is the wire diameter, and W is the Wahl Correction Factor.
After the initial tension is overcome, the extension spring can be analyzed as a compression spring with a negative force. The maximum shear stress (tmax) in the spring increases with the load and is given by,
The spring extension D is given by,
where G is the shear modulus and nt is the total number of coils.