In a windmill, a Crown Wheel is an auxiliary gear on the Upright Shaft.
• A wheel with cogs or teeth set at right angles to its plane; -- called also a contrate wheel or face wheel.
The escape-wheel in a verge watch is sometimes so called because of the shape of its teeth.
The Crown Wheel is a driving wheel located at the top of the Upright Shaft
Crown' wheel` (hw?l`). [ Named from its resemblance to a crown .] (Machinery) A wheel with cogs or teeth set at right angles to its plane; -- called also a contrate wheel or face wheel . 
A crown wheel is a type of cog with the cogs arranged at a right angle to the plane.
Crown Wheel and Upright ShaftThe Crown Wheel is a driving wheel located at the top of the Upright Shaft
a wheel next to the winding knob, having two sets of teeth, one at right angles to its plane. · any of various wheels having the form of a hoop or short cylinder with a serrated edge, as the escape wheel in a verge escapement.