Tower Mill - Design

Design

The advantage of the tower mill over the earlier post mill is that it is not necessary to turn the whole mill ("body", "buck") with all its machinery into the wind; this allows more space for the machinery as well as for storage. However, select tower mills found around Holland were constructed on a wooden frame so as to rotate the entire foundation of the mill along with the cap. These towers were often constructed out of wood rather than masonry as well. A movable head which could pivot to react to the changing wind patterns was the most important aspect of the tower mill. This ability gave the advantage of a larger and more stable frame that could deal with harsh weather. Also, only moving a cap was much easier than moving an entire structure.

In the earliest tower mills the cap was turned into the wind with a long tail-pole which stretched down to the ground at the back of the mill. Later an endless chain was used which drove the cap through gearing. In 1745 an English engineer, Edmund Lee, invented the windmill fantail – a little windmill mounted at right angles to the sails, at the rear of the mill, and which turned the cap automatically to bring it into the wind.

Like other windmills tower mills have normally four blades. To increase windmill efficiency millwrights experimented with different methods:

• automated patent-sails instead of cloth spread type sails didn't need the sail cross to be stopped to spread or remove the cloth sails because they altered the surface from inside the mill by means of a controlling gear.
• more than four blades to increase the sail surface.

Therefore engineer John Smeaton invented the cast-iron Lincolnshire cross to make sail-crosses with five, six, and even eight blades possible. The cross was named after Lincolnshire where it was most widely used.

There are several components to the tower mill as it was in the 19th Century in Europe in its most developed stage, some elements such as the gallery are not present in all tower mills:

• Stock – the arm that protrudes from the top of windmill holding the frame of the sail in place, this is the main support of the sail and is usually made of wood.
• Sail – the turning frame that catches the wind, attached and held by the stock. The traditional style found on most tower mills is a four-sail frame, however in the Mediterranean model there is usually an eight-sail frame. An example of this in St. Mary's Mill on the Isle of Sicilly constructed in 1820.
• Windshaft – A particularly important part of the sail frame, the windshaft is the cylindrical piece that translates the movement of the sail into the machinery within the windmill.
• Cap – The top of the tower that holds the sail and stock, this piece is able to rotate on top of the tower.
• Tower – Supports the cap, the main structure of the tower mill.
• Floor – Base level of the tower inside, usually where grain or other products are stored.
• Gallery – Deck surrounding the floor outside the tower to provide access around the tower mill if it is raised, not present in all tower mills. The gallery allowed access to the sails for making repairs because they could not be easily reached from the ground in larger mills.
• Frame – Sail design that forms the outline of the sail, usually a meshed wood design that then is covered in cloth. The Mediterrenean design is different in that there are several sails on the sail-frame and each supports a draped cloth and there is no wooden frame behind it.
• Fantail – Orientation device that is attached to the cap, allowing it to rotate to keep the sails in the direction of the wind.
• Hemlath – Thick wooden sailbar on the side of the frame that keeps the narrower sailbars inside the sail.
• Sailbar – Elongated piece of wood that forms a sail.
• Sail cloth – Cloth attached to a sail that collects wind energy; a large sail cloth is used for weak winds and a small sail cloth for strong winds.