Torques Based on Gear Radii  
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Two gears A and B are connected. A is larger than B. A counter clockwise arrow indicates T subscript large on gear A. Radius of gear A is R subscript large. A downward force F is applied on A at the connecting teeth of A and B.

Consider a free-body diagram (FBD) of the large gear. A torque from shaft A is applied to the large gear in the direction shown.

The teeth of the small gear exert a force on the teeth of the large gear. This force opposes the direction of rotation of the large gear.

The large gear must be in equilibrium; therefore, the sum of moments about shaft A must equal zero.

∑MA=Tlarge-F×Rlarge=0