Page General Torsion Equation Shafts of circular cross-section G T J r L          π π d J r π J r ‐ r π d d For Solid Shaft For Hollow Shaft T torque or twisting moment in newton metres J polar second moment of area of cross ‐ section about shaft axis τ shear stress at outer fibres in pascals r radius of shaft in metres G modulus of rigidity in pascals θ angle of twist in radians L length of shaft in metres d diameter of shaft in metres Fundamental Bending Equation M E I y R    For Rectangle M external bending moment in newton metres I second moment of area in m σ bending stress at outer fibres in pascals y distance from centroid to outer fibres in metres E modulus of elasticity in pascals R radius of curvature in metres I BD For Solid Shaft  π d I