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

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Machine Design
The ends of the leaves of a semi-elliptical leaf spring are made triangular in order to

Make M/I constant throughout the length of the leaf
Obtain variable moment of inertia (I) in each leaf
Have variable bending moment (M) in each leaf
Permit each leaf to act as an overhanging beam

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