Shafts connecting gears and pulleys are a common application involving torsion members. In this movie, expressions relating torque, shear strain, shear stress, and angle of twist will be derived for a prismatic shaft with a circular cross section.
In many situations, members that will be subjected to rotating and twisting actions must be designed. A twisting action is called torsion, and thus members that twist about their longitudinal axis are termed torsion members or shafts. Moments that twist an object about its longitudinal axis are termed torques.