Mills are mechanisms taking advantage of the energy of air streams. They were used for milling, grinding or stripping. There are two types: the vertical and the horizontal axis mill. For grinding, windmills use the wind’s kinetic energy. According to our present knowledge, the first windmills working like this appeared in the 6th century B.C. in Persia.
What were these early windmills like?
THE VERTICAL AXIS WINDMILL
Its sails were fixed on a vertical axis and were moving parallel to the ground. The millstones and the rotating, vertical axis were connected and so the millstones fixed to the bottom of the axis ground the wheat. The problem with these mills was that their average rotation speed was not enough for efficient grinding.
The horizontal axis windmills
The post or bock mill
The other category of windmills is the horizontal axis mills. In this case the sails were fixed on a horizontal axis and were spinning perpendicularly to the ground.
The other end of the axis was connected to a wooden gear wheel which turned another axis, connected to the millstone in its turn. Even in average wind, the gear assured an adequate grinding speed. Typical to this kind of mill was that it was built on a single post (that’s where its name comes from) and a crab was fixed to the construction. Because the wind wasn’t always blowing from a favourable direction, the miller could grab the crab and turn the whole mill into the most suitable direction. The performance of these structures was roughly 2-8 horsepower, which means 1.5-6kWs. The problem with the post-mill was that it could easily fall apart in a gale. Later men put a vane on the mill due to which it automatically turned into the direction of the wind.
Some sources say that the vertical axis mills arrived in Europe during the Crusades and were later improved into horizontal axis mills. Others claim that they are original European inventions and have nothing to do with Persian ancestors. In Europe they were first mentioned in a document written in 1185 (Yorkshire, England), but by 1195 they had been so widely spread that the pope put a tithe on them.
The smock mill
These mills were somewhat more developed than the post-mills. They were composed of two parts, a wooden body and a rotatable cap. Because the whole building could not be turned and was built on the ground, it could be designed bigger. The house was able to store more machines and grain. The cap was rotated by a system of rollers. These rollers were placed in two concentric rings. They ran on tracks located above and below them. The lower roller track rested on the upper sill. The upper roller track was fixed to the cap. Both tracks were enclosed by a circular wooden band. The cap was turned from the ground with the help of a support. They are said to have received their name from their resemblance to the linen smocks once worn by British countrymen. They were usually built octagonal but they could have six or twelve sides too. Most Smock mills were built on a brick foundation which protected their wooden bases from rot. At the beginning they used sails to capture the wind. The installation of these sails required the help of three men and had to be made in complete lull, otherwise it would have been too dangerous. Because the sails-cloths were exposed to great pressure, they were later replaced by wooden boards.
The tower mill
Tower or Holland type mills appeared in the 17th century. They were made of loam or stone and, as in the case of the smock mill, only the cap could be turned. Tower mills usually had a balcony and were built on the city wall. The door of first floor was sized so big that even carriages could enter.
First - with the help of a winch - they pulled the fodder up into the granary. Between the floors there were self-closing trapdoors, which didn’t allow sacks to fall back, since they could move only upwards. From the sacks the grain was thorn into a hopper, and from there it got to the millstones to be grinded into flour which was later sifted.
On the vertical axis, known as the king-axis, two grand gear wheels were installed on separate floors. These gave forward the torque. The big gear wheels propelled the little ones which were fixed to the axis of the four pairs of millstones. The bent wings of the puddle-wheel used the energy of the wind efficiently. Depending on the changes in the direction of the wind, one, two, three or even four pairs of millstones could be functioning. Because the scaffolds of the millstones were moving on wheels, gear wheel connections had to be made or broken off.
During the centuries there were improvements regarding the windmills, they could automatically turn into the direction of the wind and some bigger ones were capable of even 30kW performance. Their handicap, however, still remained their dependence on the wind.