Show Gear Rotations  
scene 11 of 12
The angular displacement on gear with 100 millimeter diameter is phi subscript B. The angular displacement on gear with 200 millimeter diameter is phi subscript C. The angular displacement of shaft C D is phi subscript 2. A torque T in counter clockwise direction is applied on the shaft at D.

Let's review the rotations in the system, looking from another viewpoint:

The rotation angle at D is the sum of the rotation angle at C plus the twist angle in shaft (2).