Diagram shows a shaft A B with aluminum segment marked 1 of length 400 millimeters inside which is a brass shaft marked 2. Both the shafts are centered on the central axis x. A torque T is applied at B. An internal torque T subscript 1 and T subscript 2 is applied on the post towards the inside.

Draw a FBD around end B.

Assume positive internal torques T1 and T2 in shafts (1) and (2).

The equilibrium equation obtained from this FBD is:

Mx=-T1-T2+T=0           T1+T2=T