Equilibrium Equations  
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Diagram shows a vertical shaft which is twisted to the right with a gear marked A on the top end. The radius of the gear A is D subscript A over 2 and the diameter is 150 millimeters. A force F is acting vertically downwards on the circumference of the gear A on the top right. Above the gear A a torque T is applied on the shaft in the counterclockwise direction. An internal torque T subscript 1 is applied on the post vertically downwards along the shaft.

Draw a FBD around gear A, cutting through shaft (1). Assume a positive torque T1 in shaft (1).

The teeth of gear C exert a force F on the teeth of gear A which opposes the external torque T.

The moment equilibrium for gear A and shaft (1) is: