Machine Design
A bolt of M 24 × 2 means that

The effective diameter of bolt is 24 mm and there are 2 threads per cm
The nominal diameter of bolt is 24 mm and pitch is 2 mm
The pitch of the thread is 24 mm and depth is 2 mm
Cross-sectional area of the threads is 24 mm²

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

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