The Fan
The fan will turn if the wind is side-on to the fan blades, once the fan is in line with the wind the blades will slow and stop. The shaft holding the blades is connected to a vertical shaft by a worm gearing. This shaft has further gears (photo right) which lowers the speed and finally turns a gear connected to a gear rack running round the top of the Mill. When the fan turns the cap is slowly rotating to bring the sails into the wind.
The worm drive has yet to fitted to the Fulbourn Mill so the gearing will not turn the cap, this is the reason we have put in open blades to spill the wind.
Problems arouse if the wind changed 180 degrees quickly because the fan would not turn the cap and the mill would be tail winded which resulted in damaging the Mill..