Diagram shows a shaft A B with an aluminum segment of length 1,000 millimeters and a brass shaft in the inside. Both the shafts are centered on the central axis x. 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 of angle phi subscript 1 = phi subscript 2 = phi subscript B, angle of twist.

The relationship between internal torque T in a shaft and its angle of twist ϕ is given by:

ϕ=TLJG

Therefore, the torque-twist relationships for shafts (1) and (2) can be expressed as:

ϕ1=T1L1J1G1    and    ϕ2=T2L2J2G2