The design of a transmission shaft involves understanding the stresses due to bending moments and torques.
When power is transferred through gears, the forces exerted equate to force-couple systems applied at cross-section centers, which subject the shaft to both transverse and torsional loading.
Although shearing stresses from transverse loads are smaller than those produced by torques and often neglected, the considerable normal stresses from transverse loads contribute significantly to the maximum shearing stress.
The normal stress exerted on the section is computed by examining a shaft's cross-section at a specific point and accounting for both the torque and bending couples.
The maximum normal stress occurs at the end of the diameter perpendicular to the resultant of the bending moment couple.
The minimum allowable polar moment ratio for the cross-section is determined by considering both the maximum resultant bending moment and torque in the shaft, along with the allowable shearing stress.
This approach facilitates the design of both solid and hollow circular shafts.
