How windmills work

Should it be called a starmill?The windmill is driven by energy from the star which we call the sun. The sun is 71% hydrogen and 27% helium. The very high temperature and pressure inside the sun cause nuclear fusion: hydrogen and helium nuclei combine and produce vast quantities of heat. Fortunately, the sun is 150,000,000 kilometres away from us, and has enough hydrogen left to burn for billions of years.As the earth rotates and orbits the sun, radiation from the sun warms the atmosphere, the clouds, the surface of the ground, and the surface of the sea. As a result, different parts of the atmosphere are at different temperatures. This causes differences of pressure. The attempt to equalise the pressure in different parts of the atmosphere is known as wind.So a windmill could be called a starmill, a sunmill, or a nuclear fusion mill, but we call it by the energy source which is closest and most familiar to us, the wind.

How the energy of the wind is capturedThe diagram on the left shows a board – seen from above – being held up in the wind. If you hold the board still, the moving air will mostly flow round the left-hand side. If you let the board go, the wind will push it back and to the right. If you only allow the board to move to the left or the right, the board will be pushed to the right.

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Windmills usually have four boards held at an angle to the wind.The sweep on the left is taken from the photograph on the Home Page. Its right edge is closer to you than the left edge. The wind blows from where you are, and will tend to flow round the left edge. The sweep cannot be blown backwards, because it is fixed to a central axle (called the windshaft) which points into the wind. If the windshaft is free to rotate, the sweep is pushed to the right. In other words, the windmill’s sweeps or sails always rotate anticlockwise.In the Middle Ages, sails were made of cloth attached to lengths of wood. Later, they were made entirely of wood, and since the eighteenth century have often been called sweeps.

To work efficiently, the sweeps must always face directly into the wind, the direction of which often changes. The sweeps must also be able to deal with changes in the strength of the wind, which are unpredictable. In early mills, both problems required constant attention from the miller, pushing the mill around on a central post to face it into the wind, and changing the area of cloth which made up the working surface of the sails. Two inventions in the eighteenth century made the life of the miller easier.

The fantail was invented in 1745. This is like a small windmill fixed to the back of the cap, but the shaft on which it rotates faces across the wind instead of into it. The fan on Stone Cross Mill, seen on the left, has eight paddle-shaped blades. When the sweeps face directly into the wind, the wind tries to blow the blades above the axle backwards, but it also tries to blow the blades below the axle backwards. Because the areas of wood above and below the axle are equal, the forces cancel each other out, and the fan stays still.Should the direction of the wind change, however, balance will be lost. If the wind comes more from your left, the bottom blade will begin to move backwards and the fan will spin. If the wind comes more from you right, the upper blades will begin to move backwards and the fan will spin in the opposite direction. On the right side of the fan’s axle is a worm gear, which connects to another worm gear, the wheel of which forms the top of the tower. As the fan spins, the cap of the mill turns on rollers until the fan regains its balance and the sweeps are facing directly into the wind again.

In 1772 the spring sail was developed. This works like a venetian blind. The wooden shutters which make up most of the area of the sweeps can be closed, or they can be opened at different angles to reduce the force of the wind on the sweep. This can be done by pulling on a rope or chain, or if the mill is working, by hanging sufficient weight on the rope or chain to allow the shutters to be kept closed in a gentle wind. A strengthening wind begins to blow the shutters open, as its force more than counteracts the weight. As the shutters open, the force on the sweeps is reduced.

How the energy is put to workThe sweeps cause the windshaft to rotate. Around the shaft is bolted an iron brakewheel 2.62 metres in diameter. This is fitted with a large wooden brake which can hold the sweeps stationary. The brakewheel is also a gear wheel, fitted with wooden teeth.A small section of the brakewheel can be seen at the back of the photograph on the left. The brown area beneath the teeth is part of the brake.

The teeth on the brakewheel engage with teeth on a smaller wheel, the “wallower” (seen on the left), which is connected by a vertical shaft to the great spur wheel two floors below it. It also drives the sack hoist, seen to the right of the picture. The sack hoist lifts sacks of grain up through trap doors from ground level.

The grain is then tipped through apertures in the floor. The storey beneath is the totally enclosed bin floor, which holds the grain ready for processing. From here it is directed down chutes to apertures at the centre of the rotating upper millstones on the floor below. These are suspended a small but adjustable distance above the stationary lower stones, in the surface of which fine channels were incised to provide a cutting action.Pictured in the centre of the photograph on the left is the great spur wheel, which drives three “stone nuts”, the smaller gear wheels which turn the upper millstones. Two of the smaller gear wheels can be seen top left and bottom right.On the left and right are two of the large wooden beams which help the brick tower support the weight of the heavy machinery. The beams beneath the next floor down take the weight of six millstones.

The shaft descending vertically from the stone nut in the picture above (bottom right) ends in a fork which engages with a metal bracket fitted to the bottom of the upper stone. Beneath the bracket is a socket, into which a shaft fits from below. The upper stone is balanced on the shaft. On the top of the stone are visible the maker’s plate, three wooden rectangles, and a fourth rectangle which reveals what is beneath the three wooden covers: a depression in the stone which is filled with lead, to balance the stone.Metal hoops encircle the stone for strength. The octagonal shape on the floor shows where the stone’s enclosure fits. Within the octagon, just left of centre, is the small aperture through which the flour falls to the floor below.

The lower stone (top centre) is supported on beams, with wedges used for levelling. The upper stone is balanced on the vertical shaft upper centre, which rotates with the stone. The shaft’s lower bearing is in the metal beam which crosses from lower left to upper right. This is supported on the right by a fixed metal bracket hanging from a beam, and on the left by a bracket which pivots about the far end, so that raising or lowering the other end changes the distance between the pair of millstones.The governor (on the right) is driven via a leather belt by a pulley on the stone’s supporting axle. As speed increases the balls rise, the curved lever is pushed down. The lever pivots about a fulcrum close to its left end, and lifts the beam above the handwheel on the left, used for adjustments.

The aperture in the floor at the front of the picture of the millstones leads down to the chute seen top centre of this photograph. There is another bin at the back of the one seen here. The flour or animal feed produced by grinding could be collected in the bins themselves, or in sacks attached to the chutes.


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