Machine Design
In the calculation of induced shear stress in helical springs, the Wahl's stress factor is used to take care of

Combined effect of transverse shear stress and curvature of wire
Combined effect of bending stress and curvature of the wire
Combined effect of transverse shear stress and bending stress in the wire
Combined effect of torsional shear stress and transverse shear stress in the wire

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Machine Design
The contact ratio is the ratio of

Length of path of contact to the circular pitch
Length of arc of approach to the circular pitch
Length of arc of recess to the circular pitch
Length of arc of contact to the circular pitch

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