In order to take into account the effect of direct shear and change in coil curvature, a stress factor is defined known as Wahl's factor.
A helical spring made from the wire of circular cross-section is shown in Fig. D and d are the mean coil diameter and wire diameter respectively. The number of active coils in this spring is N. The spring is subjected to an axial force W.
Wahl derived the equation for resultant stress, which includes torsional shear stress, direct shear stress and stress concentration due to curvature.
This equation is given by,
Where K is called the stress factor or Wahl stress factor. The Wahl stress factor is given by,
Where C is the spring index. The Wahl factor provides a simple method to find out resultant stresses in the spring.
Effect of spring index on Wahl factor, indicates as spring index decrease Wahl factor increases.