The Tower

A key component of all windmills is the tower, which not only determines the height of the mill but also contains all of the machinery for cleaning, grinding, and bagging grist.  The towers of antebellum French and English windmills in Illinois likely were constructed of stone and would have been round in shape.  These early mills were only a few stories high and could grind small amounts of grain at a time.
 
Custom stage mills built after the Civil War, however, were much larger and were designed to grind tens of barrels of grain per day.  Their towers were typically octagonal (8-sided) and taper in diameter toward the top (as pictured to the right).  Dutch and German windmills also feature a “skirt” near the base of the tower.  The sloped tower and the skirt are designed to guide the flow of rainwater away from the sill plate that anchors the tower to its foundation.  Because the structure resembled smocks worn by farmers at the time, windmills of this kind are also called “smock mills.”
 
At each of their eight corners are cant posts.  These main beams often measure twelve inches square or more because they must carry the load of the equipment, including one to three sets of two-ton grinding stones.  These posts were also often marked with Roman numerals to identify their location (an important feature, especially on windmills in Holland that were often disassembled and relocated to areas with more wind).
 
Pegged into the cant posts are horizontal common beams that create the floors of the windmill.  The largest of these beams are usually located under the “dust floor,” the area of the mill where grinding took place.  These beams are sometimes made with bilinga wood, a tree indigenous to Africa known for its strength and lack of knots.
 
Note that the towers of grinding mills never contain metal parts.  Every beam is held in place by wooden dowels and specially cut joists because a spark generated from metal fasteners could ignite the flammable dust produced from grinding wheat (another reason for using bilinga wood, as it does not ignite as easily as trees indigenous to North America).

The Fabyan Windmill tower being reconstructed in 1915 at its present site in Geneva, IL.

Photo from Geneva Preservation Partners

A—Ribs provide structural framework for the curved roof of the cap to protect the windmill from the elements.
 
B—Tail beam and tail-bearing beam, support the cap structure, the tail pole, and the tail end of the wind shaft.
 
C—Dead curb ring, the circular track of the tower upon which the cap sits and rotates.  The ring on this and other Illinois mills is called a dead curb because the track and its contact points are both made of wood and are stationary.  Compare this to the caps of windmills with tail fans, which have live curbs.  The curb rings there have bearings to ease rotation as well as gearing so the tail fan can turn the cap automatically.
 
D—Cross beams, present on Dutch and German windmills, located between each floor and within each of the eight sections of the tower.  These beams do not exist in windmills from other European countries.  They are present to provide additional stability to the tower.
 
E—Cant post, one of the eight main timbers of the structure that give the tower its shape.  Traditionally, these are made from bilinga wood, indigenous to Africa, known to have few (if any) knots or imperfections.  This wood is also less susceptible to fire and damage from time. 
 
F—Knee braces, fastened with tree nails (large dowel rods) into the cant posts to support common beams.
 
G—Common beams, which connect cant posts on opposite sides of the tower and form the floors within the mill.  These are also heavy-duty timbers because they must support the weight of the gears, stones, and grinding machinery.
 
H—Structure of the skirt.  Traditionally, this is a detail added to protect the sill plate (the wooden frame that anchors the tower to its foundation) from rainwater.  However, in the case of this and other mills in Illinois, there is also a second skirt to protect the timbers supporting the stage.
 
I—Stage, also known as the reefing stage, or the platform that encircles the mill so that the miller can access the tail pole for luffing and the sails for reefing canvas.
 
J—Basement level, an area only present on the Fabyan Windmill.  Usually, this level would not exist, and all of the equipment found here would be packed into the first floor of the mill.
 
 
The phrase “all windmills are built differently, but are the same” perfectly summarizes the construction of these custom-built mills.  All tower smock windmills have sails, a rotating cap, a tower, and a stage; but the building materials used, the size of the mill built, and the style the miller chooses are all unique to each mill.

This cutaway shows the Fabyan Windmill as it appears today in Geneva. When it was originally constructed in Lombard, the basement level was not present

Illustration by Tom Haskell

The Cap

The first windmills developed along the coast of the Mediterranean were fixed in place because, generally, the wind only blew in one direction—from the sea.  Millers who were further inland, however, realized that the wind changed direction; unfortunately, early European windmills required the miller to turn the entire body of the windmill, which was balanced on a post (hence the term “post mill”), into the new direction of the wind. 
 
The Dutch revolutionized windmill design by constructing fixed towers with movable caps.  The cap, which contains the rotating sails, is the top floor of the windmill and can pivot 360 degrees.  Through a process called luffing (see Windmill Operation), the miller can easily keep the mill running no matter what direction the wind blows. 
 
Caps are not constructed upon a turntable, but rather rest on a ring encircling the top of the tower called the curb.  The oldest types of curbs, dead curbs, are nothing more than the cap’s sheer-tree beams resting on a lubricated circular track.  More “modern” live curbs, however, are often metal and employ ball or roller bearings to ease rotation (live curbs are more prevalent in automatically luffing caps with tail fans and are still used in today’s electric wind turbines).
 
Because the Dutch designed a mill that was easier to operate and luff into the wind, windmills themselves became larger—and heavier—over time.  The tail pole would easily snap under pressure if it alone attempted to pivot a 20 ton cap into the wind, which is why caps are constructed with tie beams and support bracing running to the tail pole, as pictured; the extra timbers allow the cap to be pulled and rotated with even weight displacement.
 
Caps tend to have unusual shapes that vary by region, in part because all must accommodate the large brake wheel located near the center of the wind shaft.  Like the skirt of the tower, the sloping shape of the cap is designed lead rain water away from the sheers, tie beams, and the curb ring.  The cap takes its shape from several ribs constructed around the machinery—similar to roof rafters in a house.  Oftentimes, a window or hatch is built into the cap so the miller can access the sails.

The Fabyan Windmill, left, requires the miller to turn the cap into the wind using the tail pole. The tail fan of the Brockman Windmill, right, accomplished this task automatically.

Left photo by Tom Haskell

Right photo from DigitalPast

The Sails and Wind Shaft

The sails of a windmill—and most of the terminology associated with them—evolved from methods of harnessing wind for ships at sea.  Early sails on stone tower mills could best be described as several ship masts mounted to a rotating shaft.  Catching the wind in those days meant using rigging and canvas to create a jib sail similar to that on a sailboat. 
 
An important element in sail construction is the angle of weather, demonstrated in the right photo.  When viewed from the side, the “twist” seen in the sails from the heel to the tip is present so the wind can push the sail in one direction.  It is, essentially, the similar but opposite principle to a ceiling fan blade, angled to push air away from its hub.
 
Illustrated below are five common windmill sail types.  These designs are also interchangeable: for example, there are double-sided patent sails (the Danish Windmill in Elk Horn, IA is an example of this), and some windmills employ multiple sail types (the Prairie Mills Windmill uses two patent and two Dutch sails).

This photo of De Immigrant's sail demonstrates the angle of weather, the "twist" in the sail

Photo by Tom Haskell

A large drive shaft, appropriately named the wind shaft, is turned by the sails.  This shaft runs through the brake wheel to the tail bearing beam of the cap.  On older windmills, this shaft is comprised of four wooden beams, each 12” or more in diameter, held together with sets of four iron rod ends fastened together with square nuts.  On newer mills, the wind shaft is a single piece of cast iron.
 
Contrary to many a miniature model windmill, the sails are not attached directly to the wind shaft, but rather to the sail stocks.  Two stocks, each roughly thirty feet long, run through the center of the wind shaft and are held in place with wooden wedges.  The sails themselves, then, are fastened to the sail stocks with square U-bolts.  This way, individual sails can be removed or repaired without dismantling the entire assembly.

Detail of the Fabyan Windmill's sail stocks passing through the neck of the wind shaft

Photo by Tom Haskell

The Stage

The platform that encircles a tower mill is known as the stage.  The sails are purposely designed to “sweep” near the stage floor so that the miller can climb them.  From the stage, the miller can also access the tail pole (to turn the sails into the wind) and the brake rope (to start or stop operation).  In most cases, the stage is located on the grinding floor; this way, the miller conveniently operates the mill from a single location.
 
The stage is often supported by diagonal braces attached to the base of the tower.  The shape of the stage tends to coincide with the shape of the tower: a round, brick tower mill will have a circular stage (see Dutch (North) Windmill in Golden Gate Park), whereas octagonal mills will have an octagonal stage.

A—One of the four sails, necessary to harness the wind. 
 
B—Sail bars, the horizontal rung of a sail, built like the steps of a ladder so that the miller can climb the sail.
 
C—The uplongs run vertically to support the sail bars.
 
D—Wind board, determines rotation direction.
 
E—The cap of the windmill.  It sits upon a circular track, called the curb ring, and can turn 360 degrees so that the sails can face into the wind, no matter which direction it is blowing from.
 
F—The external brake lever releases the brake within the cap. 
 
G—Tail pole, a structural timber used by the miller to turn the cap into the direction of the wind—also counter-balances the cap against the weight of the sails.
 
H—Tie beams, connected by diagonal braces to the tail pole, necessary for weight stress distribution when turning the cap.
 
I—A winch, located at the end of the tail pole, is hand-cranked by the miller to pull the tail pole along a taught chain to the direction he wants the cap to turn.
 
J—Stage, the platform from which the miller can access the sails, brake rope, and tail pole winch. 
 
K—Skirt, a structural detail to protect the sill plate.
 
Some windmills—like this one—also have buildings beneath the stage level, used for shipping, receiving, an office, a steam engine, or for extra storage. 

The Fischer Windmill in Mount Emblem Cemetery circa 1977, with its original sails intact and the mill open for visitation. Click here to read more about this windmill.

Photo by Joe and Jeanette Archie