The power transmitted by a shaft is related to the torque in the shaft and the speed of rotation.

• The torque in the shaft can be computed from the specified power and rotation speed.

• This torque along with the allowable shear stress will be used in the elastic torsion formula $(\mathrm{\tau }=\mathrm{Tc}/\mathrm{J})$ to determine the required shaft diameter.

• For a hollow shaft, the polar moment of inertia $\mathrm{J}$ will be expressed in terms of the ratio of inside diameter to outside diameter.