Generally we assume that shaft is subjected to torsion only but in actual practice due to weight of the pulley, couplings, pull in belts or ropes etc, the shaft is subjected to bending too. Thus actually in the shaft, both the shear stress due to torsion and direct stress due to bending are induced.
From bending equation,
From torsion equation,
(i) The maximum principal stress at the section
The bending moment corresponding to the maximum principal stress is termed as equivalent bending moment, Me . Thus;
Or
Where;
Me = Equivalent bending Moment
M = Maximum Bending Moment from the bending moment diagram
T = Torque transmitted
(ii) The maximum shear stress developed at the section
The twisting moment corresponding to the maximum shear stress on the surface of the shaft is called equivalent twisting moment, Te. Thus;
Or
Where Te = Equivalent Twisting moment
