Observe the Deformation  
scene 3 of 13
Diagram shows two vertical shafts A B marked 1 and C D marked 2 centered on the x and x prime axes respectively. A gear of 150 millimeter diameter is at A on shaft 1 at a height of 400 millimeters. A gear of 100 millimeter diameter is at C on shaft 2 at a height of 400 millimeters. Above the gear A a torque T is applied on shaft 1 in the counterclockwise direction. There is an inward twist on the shafts 1 and 2. The section on gear A is theta subscript A pointing in the counterclockwise direction. The section on gear C is theta subscript C pointing in the clockwise direction.

Torque T causes gear A to rotate, which in turn causes gear C to rotate in the opposite direction. When gears A and C rotate, they twist shafts (1) and (2), creating internal torques in each.

Note: Admittedly, this mechanism seems kind of pointless. Keep in mind that we are trying to illustrate principles and analytical procedures relevant to statically indeterminate torsion members. Consider this little mechanism to be a subset of a more significant mechanism.