Nick Weedon explains why being a cog in a wheel is not such an insignificant achievement
The first and most impressive sight for visitors on a guided tour of Brixton Windmill is that of the sails stretching out into the sky. But what connects these with the heavy grey millstones on the second floor below?
The transfer of motion from the sails sweeping around at a majestic rate to the spinning of the millstones at high speed is made possible by several sets of cogwheels, or gearing. The picture shows two of these wheels – the great spur wheel and the stone nut.
Historians believe that cogwheels were familiar to Egyptian and Mesopotamian engineers over 2,000 years ago. But the millstones in Mesopotamian windmills were driven directly by sails attached to the stones, without the use of cogs. For fine flour you need fast millstones, but these early stones could only rotate at the same speed as the sails – something that would change with the advent of cogwheels.
One of the earliest recorded uses of cogwheels goes back to chariot construction in ancient China. A model in the London Science Museum of a chariot from the 4th century BC shows that, rather than being part of the drive mechanism, the cogwheels were connected with steering that enabled a pointer to stay in the same direction whichever way the chariot turned. The use of automatic navigation devices is nothing new!
Gearing was more commonly used in astronomy and timekeeping, a notable early example being the Antikythera device from 150-100 BC, used to calculate astronomical positions. This was a working mechanical model of the heavens. Timepieces brought remarkable precision to gearing, as differently-sized cogwheels were linked to hands that went round at different speeds.
Meshing together cogwheels of different sizes enables a multiplication of power and speed from one end of a mechanism to the other. In a clock, the second hand goes around 60 times faster than the minute hand, from the same power source.
During the days of windpower, every turn of the sails at Brixton Windmill caused the vertical drive shaft to rotate about three times, thanks to gearing. This would still be quite slow for turning the stones, so more gearing at the bottom of the main shaft (which can still be seen) would drive the stones four times faster still – that is, twelve times faster than the sails.
The video above, taken on National Mills Weekend 2016, shows the sails turning one revolution about every six seconds. If all the machinery were connected through the cogwheels, this would result in the stones turning two revolutions per second. For those who remember using record players, that’s about 120 revolutions per minute – two and a half times the speed of a 45 single.
So the windmill doesn’t just offer a lesson in windpower and milling: the cogwheels are an example of the power of mathematics and engineering.
The cogwheels in Brixton Windmill each have a name: the brake wheel (with 66 cogs, driving all the others), wallower (with 22 cogs, transferring the motion from horizontal to vertical), great spur (with 86 cogs), and stone nut (the smallest one, attached to the millstones).
Each pair consists of a wheel with metal cogs which meshes with a wheel with timber teeth. This arrangement was quieter, and avoided the risk of sparks igniting flour dust in the air – just one of the many hazards for a busy miller.
The large size of the brake wheel that drives all the others is important: if it were much smaller, it would be harder for the sails to turn the machinery. The same is true of bicycles and cars.
When setting off, the biggest cogwheel is used to get going, and once there is good momentum the gear changes can allow the same speed to be kept up using less power. This facility was never developed in windmills.
When in operation, a windmill’s cogwheels are always engaged, since disengaging and engaging different gears while milling would quickly cause damaging wear and tear on the cogs, and be dangerous for the operator.
In addition to the drive machinery, the windmill contains another set of cogs that help the top of the building to revolve, turning the sails into the wind. This section is called the cap, recognisable by a weatherboard timber construction that looks like an upside-down boat.
The cap sits on top of the brick wall of the tower on a flat steel ring that is kept greased. Held down by its own dead weight, the cap is restrained from slipping off sideways by guide wheels set against the top of the interior wall.
A set of cogs in a ring around the top of the wall enables a hand crank in the cap to make it revolve. The crank handle is attached to a large cogwheel to make it easier to drive a small wheel against the fixed cogs. Despite its own gearing, it takes two people to turn the handle. Each turn of the handle rotates the cap two degrees, meaning that it takes 90 turns of the handle to get the cap to face the opposite direction.
As an occasional operator of the cranking handle, I can testify that we usually get tired after about 10 turns. But for the original millers, it would all have been part of the “daily grind”!