Diagram shows an aluminum shaft A B and a brass shaft B C with a flange in between at B. Both the shafts are centered on the central axis x. The shaft is supported at A and C. A torque T is applied at B. The radial and longitudinal lines are shown on the shaft. The deformation of the shaft is shown with a counterclockwise segment at B of angle phi subscript B, angle of twist. A caption to the left reads: phi subscript 1 = phi subscript b minus phi subscript A.

Consider shaft (1). The torque T causes shaft (1) to twist as shown.

The angle of twist in a shaft can be determined by considering the angles of rotation at its two ends.

Moving in the +x axis direction from A to B, the angle of twist is simply the rotation angle at the forward end B minus the rotation angle at the aft end A:

ϕ1=ϕB-ϕA