Windmills: Types and Operation
The Post or Bock Mill:
A Post or Bock Mill
The Post or Bockmill was the first form of windmill. The earliest ones date from the twelfth century. Bockmills were rare in Schlesvig and Holstein. Bock is the German word for the large upright post on which the mill body was balanced. They were designed so that the entire building could be revolved on the post. This allowed the miller to position the sails of the mill so that they faced into the wind. This design put them in danger from storms. The Bockmill that Cay Lorenz Bevensee operated at Bornstein (adl Güt Altenhof), in the late 1700's was blown over in a storm.
Angled quarterbars and horizontal crosstrees held the post upright. Under the base of the post there were 2 cross bars, at right angles to each other, and 4 quarterbars. Other arrangements, such as 3 crosstrees and 6 quarterbars, occured. The crosstrees sat on brick piers to prevent rot. In some mills, the crosstrees and quarterbars were openly exposed and the mill was known as an 'open postmill'. In other mills this area was enclosed by bricks or thatch, and was known as the roundhouse. The roundhouse protected the wooden structure from rot and provided dry storage space for grain and flour.
The postmill had to face directly into the wind at all times in order for the sails to turn. The miller put his weight against the tailpole, which extended from the rear of the body, and manually rotated the body. Often there was a circle of raised curb stones around the mill to give the miller leverage as he turned the mill into the wind. If the mill was 'back winded' it meant that the mill was facing away from the wind and it was in danger of being blown over. Later a fantail was attached to the back of the mill and acted like an 'auto pilot', turning the mill body automatically. A ladder was attached to the tailpole and provided access up and into the mill.
The machinery of the mill was housed inside the body. The machinery consisted of a large brake wheel which was surrounded by wooden blocks, held together by an iron band. With the aid of a rope, these blocks could be tightened or loosened. Tightening the blocks applied the brake and stopped the motion of the sails; loosening the blocks released the brake. The brake wheel was on one end of a long shaft, called the windshaft. The sails and sack hoist were also attached to the windshaft. From the windshaft, power was transferred to a small gear, called the wallower. The wallower shared a vertical shaft with the spur wheel, and from this wheel a 'stone nut' was used to drive the mill stones. As larger mills were built, additional pairs of stones could be driven by placing an extra stone nut on the spur wheel. The machinery was distributed between two floors, with one set of stones on each floor. A large wooden friction brake was often fitted around the outer edge of the brake wheel, to allow some extra control over the speed of the sails.
The body was raised a considerable distance above the ground. This allowed the sails to catch the stronger wind currents that exist 20-40 feet above the ground surface. Besides turning the sails, the force from the wind speed also created a rearward pressure on the sails and on the body of the mill. This pressure was absorbed by the post and the tail construction.
Inside a Bockmühle
At the rear of the mill was a ladder, descending from the mill body to the ground, and a heavy tail pole. The tail pole passed between two rungs of the ladder and then curved slightly downward, with its end fastened to the ladder by two posts. This sturdy system of ladder, tail pole and posts formed a counterweight to the great weight of the sails, and kept everything balanced. When the mill was working, the ladder and tail structure rested on the ground. This relieved the pressure on the sails and the body while providing access into the mill. At the top of the ladder was a small balcony with a handrail. A door giving access to the inside was located there. Over the door was an opening which was closable by a shutter. At the top of the tail was a 'pent roof'. The shape of this roof depended on the region, and it protected the sack hoist from the weather. The sack hoist could be operated from the bottom of the ladder by means of a rope and was used to hoist sacks of grain from the ground level to the balcony.
There were two vent holes in the sides of the mill body which could be closed by shutters. When the wind started to blow through a hole, it warned the miller that its direction had changed and that he should turn the mill. The front of the mill body, called the breast, was adorned by a 'prick post' and it's lower edge was gracefully curved.
Bockmills did not have much storage space inside them and the working areas were cramped. The loading and un-loading of grains and milled products always had to take place in the open.
The Smock Mill:
A Galerieholländer, one of the many types of Smockmills
Smockmills were a great technological improvement over the Postmills. Instead of having to rotate the whole body of the mill to face the wind, the Smockmill employed a design that consisted of two parts. One part was a fixed wooden body that held the milling machinery. The second part was a rotatable cap which held the sails, windshaft and brake wheel. Because only the cap had to be rotated, the body of the mill could be made much larger than in a postmill. Smockmills could house more milling machinery, provided increased storage space and allowed for other types of machinery to be employed. Because the body could be built taller, sails could be longer and catch more wind to create more power. They are said to have gotten their name from their likeness to the linen smocks that were once worn by British countrymen. The cap was covered with wooden boarding or thatch.
The Smockmill was a tapered tower, clad in weatherboard or thatch. They were usually octagonal in shape, but they could also have six, ten or twelve sides. Cant posts provided the main structure of these mills. These posts extended from the base of the mill to its cap. As they neared the cap, the posts steadily converged, leaning inward. The bottoms of these posts were secured to a wooden sill which was bedded in a brick foundation. This joint between the posts and sill always created problems. Because the posts leaned inwards, the weight of the mill was bearing down, and out, at the sill. If there was any weakness in the post/sill joint, a post could slip away and the mill would topple. Horizontal ledges were placed at intervals between the posts and spaced evenly up the structure. Extra vertical and diagonal struts were fixed between the posts and ledges to form a more rigid structure. Most Smockmills were built on a brick foundation which protected their wooden bases from rot.
The wooden gears of a mill which operated the machinery
Detailed Features of a Smockmill:
The wind shaft (some 75-95 feet in length) was slightly inclined from front to rear. The shafts front was supported by the neck bearing. This bearing rested on the breast beam, a structural part of the cap. At the rear it was supported in the tail bearing. This bearing rested on the inner tail beam. Beyond this bearing was the thrust flange which took the thrust on the shaft and prevented it from slipping backwards. The windshaft projected through the front of the cap and this outside section was called the 'poll end'. The poll end was painted with bright colors and decorated with a star.
Above the poll end was the 'baard' (beard). This was a highly decorated and carved wooden board, and it's purpose was to protect the poll end from the weather. The baard was painted in bright colors: green, red, white, gold. Millers used to vie with each other in having their mills fitted with a fine, skillfully carved beard which served as an outward sign of prosperity. The beard was the most striking part of a windmill, and distinguished it from other mills. Above the baard the name and construction date of the mill was written.
A catwalk, or stage, encircled a type of Smockmill known as a Galerieholländer. Horizontal tie beams projected radially from the tower. These beams were supported at their ends by oblique braces which projected downward and attached to the mill body. On top of the horizontal beams planks were layed to create a floor. Around the stage was a handrail.
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 area at the top of the mill where the cant posts were connected by beams). The upper roller track was fixed to the cap. Both tracks were enclosed by a circular wooden band.
Diagram showing the brake mechanism of a windmill
The brake handle projected from the back of the cap and on its end was a long rope that which hung down to the tail. By using this rope, the brakes could be applied or released. The construction/operation of the brake wheel in Smockmills was basically the same as in the Postmills.
The brake wheel was mounted to the wind shaft and had a number of cogs on it. Its rim was surrounded by a ring of wooden blocks which were held together by an iron band. To apply the brake, the miller contracted the band holding the brake blocks. If the brake was to be taken off, the miller pulled the brake rope slowly. The brake lever was slowly raised and the pin moved upward along the catch and past the slot. When the lever was slowly lowered, the pin would drop back into the slot of the catch. In this position the brake lever was in its highest position and the brake band remained clear of the rim of the brake wheel.
Showing the rear of the mills' cap. The brake handle protrudes at the top with its rope hanging down. Also visible is the transverse beam and braces which created the tail structure.
Passing through the rear of the cap was a transverse beam which projected beyond the cap on both sides. Two other beams extended downwards and created braces for the tail pole. The tail pole was secured in the middle of the rear of the cap and extended downward. Both the long and short braces were firmly joined at the bottom to the tail pole, and this whole system formed the 'tail' of the mill. The tail had the cap firmly in its grip, and when the tail was turned around the mill, the cap turned as well. The turning of the cap by means of the tail took place on the ground, or on the stage, with the aid of the capstan wheel. The capstan wheel was a large wheel with a number of spokes that served as handles. The capstan had a drum with a chain wound around it. One end of the chain was held by an anchor post. When the capstan wheel was turned, the tail moved around as the chain wound up. The wheel could be held in any desired position on one of the spokes. The chain could be shifted each time from one anchor post to another and attached to it. The anchor posts are sturdy and heavy posts which are sunk fairly deep into the ground, from which their round heads project a short distance. As a rule twelve of these posts, and sometimes more, are grouped at regular intervals around the mill. Their heads are finished in a simple and neat manner, and of course are properly painted, usually white, so that they are easily visible, even in the dark, and one does not run the risk of stumbling over them. Turning the capstan wheel was no easy job and as a rule was not done by hand, instead the milles stepped on the spokes and forced them down with his own weight, as on a treadmill.
The Capstan wheel
The sails are of course the principal components of a windmill, for they transmit the power to all those parts which together form a windmill. Without sails, a mill is a mill no more. Most mills had four sails, but some had five, six, and eight-sails. It is obvious that the shape and the construction of the sails are of primary importance, for they determine the proportion of the energy which can be transmitted from the wind to the mill.The sails of the mill always turn counterclockwise, when one faces the mill. The reason for this is because it is the most natural direction in connection with the handling of the sails by the miller. When the miller has to reef the sails, furl them up, take them in, or set them, he starts by climbing onto the sail. It stands to reason that he begins in the sail that is directed vertically downwards. He mounts the framework and - just as on board ship - has to use one hand to hold on and the other to handle the sail-cloth. It is obvious that he will use his right hand for the work; this implies that he must have the stock on his righthand side in order to detach the cloth or fasten it. And thus the sail, when directed vertically downwards, having the stock to the right and the framework to the left, the sails have to turn counterclockwise.
Sail types: A) common sail - the oldest type, which was covered by stretched canvas. B) normal old-fashioned Dutch type (one leading board taken away). C) shuttered type, with air brake. D) shuttered type, with sky scraper
The sail has a wing-shaped surface, integral with the stock and placed on its driving side. As mentioned, the stock is the long timber which is mortised through the poll end and tapers towards its ends. The stock may have a length of as much as 75 to 95 feet. Formerly the stocks were always made of pitch-pine, but in the second half of the 19th century iron stocks came to be used with increasing frequency. The plane of the sail has a slight twist. On the side of the shaft the first bar is mortised into the stock at an angle of about fifteen degrees, the last bar at an angle which is much smaller or almost zero (relative to the plane of rotation of the sails). The reason of this is as follows: any sailing-man knows that when a ship gathers speed with a given force of the wind and sailing direction, the sheets have to be hauled up and tightened. In the case of the sails of a windmill, when revolving, the part near the shaft has a relatively low speed, but the speed with which the stock cleaves the air becomes greater for each point that is further away from the shaft. The speed at the tip of the sail accordingly is many times greater than at points nearer to the shaft. Consequently the sail will have at the tip a much smaller angle than nearer to the shaft. Our ancestors knew this quite well from experience, although theoretically they may not have been able to fathom this problem. Thus the sails had developed in a form which could not be improved upon very much during the past few centuries and which therefore was not modified or improved in any way. Until - in the early years of the twentieth century - technology began to concern itself with aviation.
Because of the pressure on the sails, the miller had to make adjustments by furling or un-furling the sailcloth manually. The oldest type of sail was the common sail. The common sails comprise a trellis-like structure over which canvas sails are spread. This operation required a total of three men. Two men climbed up the sail framework to spread and secure the canvas, while the third man operated the brake in the top of the mill. This act took about half an hour and was, of course, undertaken in the wind. As wind speeds increased this could be a very dangerous job.
Later, sails were fitted with wooden boards instead of sail-cloths; these wooden boards were adapted to pivot about an axis parallel to the stock. These sails accordingly are self-reefing under the influence of centrifugal weights. It is possible to influence the position of the boards also at will at the tail by means of a transmission mechanism, even to such an extent that the boards will act as a brake, so that one can stop the sails in a 'noiseless' way before putting on the brake.
Apart from cloth and solid wood sails there were sails with hinged shutters, like those of a venetian blind. Used in England since 1772 they where called 'spring sails' or 'shutter sails.' The surface of such a sail consisted of inter-connected shutters, mounted at right angles to the stock and adapted to pivot about their own axes against spring action. These sails were self-reefing and presented the great advantage that the miller did not have to set, or reef, them, which saved time and work. In ordinary circumstances the shutters form one continuous surface, which, like the 'normal' sail, is somewhat twisted. When the wind gathers strength, the shutters are opened a little by the pressure, thus spilling the wind. This gaves rise to the automatic adjustment, so that - at least theoretically - the mill would perform a more or less constant number of revolutions in a light as well as a stronger wind.
The 'patent sail' was invented in England in 1807 by Sir William Cubitt. In this, all the shutters of all the sails were connected by a spider coupling in front of the poll end. This was fastened to a rod which passed right through a bore along the wind shaft, which was controlled at the tail by adding weights on a chain attached to a lever. The plane in which the sails rotate is not exactly vertical, as one would expect at first sight, since the wind brushes horizontally over the earth's surface. It was, however, found empirically at a very early date that it presented certain advantages to give the plane of the sails a slightly inclined position and that the effect of the wind was thus greater.
The process of milling grain:
Millstones:
Two mill stones were used together to grind grain. The runner stone was suspended over the stationary bed stone and turned by a spindle that came up through the eye of the bed stone. The miller adjusted the runner stone for to the type of grain being ground and the speed of the water wheel. A sack of grain was either poured into the millstone hopper, or delivered from bins on the floor above by a chute. Hoppers held 50-100 pounds of grain depending on the type. The millstones were housed under a covering that was shaped like an upside-down pyramid. The covering kept the stones and the grain clean. The hopper was attached to this covering. Some hoppers had a control gate to regulate the flow of the grain.
As the grain fell through the hopper it carried air in a downdraft. This draft helped to cool the stones and it moved the finished product down another chute. From the hopper the grain fell into the shoe, a device suspended below by leather straps. It was so named because it resembled a wooden shoe of the period. The shoe can be raised or lowered to control the flow of grain. The shoe was vibrated back and forth by a spinning device called the damsel. The damsel was used to maintain an even flow of grain. The damsel was mounted on the top of the balance rynd, and sat in a pocket located in the center hole of the runner stone. The top end of the damsel turns in a hole through the wooden frame that encloses the shoe and hopper mouth. Some damsels were adjustable up and down, and could be used on different millstones. Some damsels were metal shafts with wooden blocks attached to the shaft. Some were made of wood with large round wire staples placed around the shaft to create raised, or fluted, ridges. There were different damsels for different rates of feed. Damsel's had from 4-12 flutes.
To ensure that the shoe always made contact with the damsel the miller employed a miller's willow. A millers willow was a wooden spring usually made of Ash. A string was tied at one end of the spring, and at the other end it was looped through a small hole in the shoe. The grain falls into the eye (center hole) of the runner stone and the flow of grain is controlled by adjusting the shoe and damsel.
The gap between the two stones was regulated by the miller to adjust the fineness of the flour. As mentioned, the stones were hidden by the cover. After grinding, 20-25 pounds of ground material remained around the edges of the stones inside the millstone cover, and was known as the miller's mite. Usually, the miller would only take off the millstone cover to clean between the stones. This left-over material was usually fed to livestock. A dishonest miller might lift the millstone cover after each batch of grain he ground, and include this material in his charge. Sometimes the millstone cover was much larger than the stones, or had multiple corners, allowing a dishonest miller to trap more ground material.
It was rare to find stone chips in the flour and their presence suggested neglect by the miller or millstone dresser. After dressing (sharpening) a pair of millstones small particles of stone often remained. To clean the stones, the miller would usually grind a sack of old grain that he would have normally discarded and then discard the ground material. The miller also used this grain to test out the quality of the newly sharpened stones. The millstones were never allowed to touch. They would be ground away and this stone dust would contaminate the flour or meal, not to mention quickly wearing them out; by touching they would either stall the mill, or worse yet, create a shower of sparks. The dust from wheat, rye, barley and oats is more explosive than gunpowder and 35 times more explosive than coal dust. Corn does not produce explosive dust, while Buckwheat and Rye produce the most dust during the grinding process.
Furrows and Lands - the grinding surfaces:
A miller could tell by the quality of the ground grain if the stones needed to be sharpened, or dressed. A mill in constant use needed it's stones sharpened every 4-6 weeks. The stones were hoisted by a hand crane to a position where they could be worked on. Using a template and a tool called a mill bill, the dresser would deepen the furrows, or grooves, in the stone. Opinions varied about how the stones should best be sharpened. Then they were pock marked by another tool. These indentations helped to cool the grain as it was being ground. If the grain got too hot it could burn. From this we get the expression "keep your nose to the grindstone."
Millstones had two types of features on their grinding surfaces: furrows and lands. The furrow patterns on both stones were identical. When the runnerstone was placed back on the spindle and rotated, its furrow pattern became the reverse of the furrow pattern on the bedstone.The furrows were designed to cut the grain like a pair of scissors, and to move the grain from the centers of the stones to their outer edges. Additionally, the furrows increased the air space between the two stones and kept them cooler. The fineness of the flour depended on the sharpness of the furrows. A pair of dull stones would not properly separate the bran from the inner kernel in the usual large flakes, instead the bran was torn into small pieces which were difficult to sift out of the flour. Dull stones also took more power to operate.
The furrows were arranged in groups known as harps or quarters. Each harp covered 1/4th of the stone, hence the term quarters. Each harp contained a master furrow which ran from the eye of the stone to its outer edge. The master furrow is the primary furrow on the surface of the millstone. Also in each harp there was a variable number of secondary furrows, from 3-5. The first was called the journeyman furrow, and this was the second largest furrow. It is parallel and adjacent to the master furrow. The apprentice furrow is the third largest furrow and is parallel and adjacent to the journeyman furrow. The butterfly furrow is the smallest of the four furrows. Sometimes this furrow is also called the fly furrow. If the millstone has a fifth furrow it may or may not have a name. It depends on what the stone dresser gives it. The furrows at the eye of the millstone were from 1/4" - 3/4" in depth, and they gradually tapered up to the edge of the millstone. The furrows were between 1" - 1 1/4" wide. The lands were the portion of the millstone between the furrows and were the true grinding surfaces of the millstones. The large kernels of grain were trapped inside the furrows, and as the stone rotated they were carried up the sloped furrow until it met the lands. The sharp edges of the lands, passing over the corresponding lands of the stone below, acted like a pair of scissors, shearing or cutting the grains into fragments. Once the runnerstone was in movement, it's inertia supported it's weight. This kept the weight off of the bedstone and it's spindle bearing. If the stones were operated without grain between them, the weight of the runner stone would quickly destroy the spindle bearing.
The last 6" -10" of the millstones were very close to each other. The millstones are not flat but have slight dishing shape. At the very outer edge, the stones may only be a paper's thickness apart. This area was known as the flouring of the stone, where the grain finally became flour. Stitches or Cracks were also put into the lands at this area of the stone. They were small parallel grooves which made the flour even finer. A skilled stone dresser could put from 10-50 cracks in one inch, using the mill bill.
The grinding of the grain took place between two large millstones which were enclosed in a casing. The surfaces of the stones were carved with furrows. The lower bedstone was stationary and the upper runnerstone revolved above it. The runnerstone had a hole in the center called the eye. A bin, mounted on a floor above the runnerstone, fed a hopper. From this hopper grain fell into the 'shoe' and then into the eye of the runnerstone. It was ground between the stones, moved through the furrows to the outer edge, and passed as meal through the casing.
The furrows of a millstone
A wooden spout then moved the ground material to the meal floor below where it was sacked. These wooden spouts could be opened or closed at the lower end by a wooden gate.
The bedstone was kept in place by wooden clamps. The quant drove the runnerstone and had to be firmly anchored in it. This was achieved by a rynd, i.e. a cast-iron cross with equal arms, having in the center a square opening for the spindle to pass through. The four arms were cemented into the stone. In a strong wind the power of the mill increased and it could grind larger quantities. In order to make use of this greater capacity, the gap between the stones was adjustable, thus admitting larger amounts of grain between the stones. This adjustment (tentering) was carried out in a very simple way. The runner stone rested on the bridge tree via an iron 'stone spindle' passing through the bedstone. One end of the bridge tree was hinged to the wooden framework of the mill and the other end was held up by a suspension system so that the weight of the millstone was balanced by a counterweight. The force required to move the stone up or down was only slight. The miller raised or lowered the cord a little if he wanted to raise or lower the runnerstone. Later on, an automatic centrifugal governor came to be used for this purpose. The movement of the stones also vibrated the shoe, so that the flow of grain wasn't interrupted. The stones had a diameter of five feet and a thickness of twelve inches.
Diagram of mill stones and the tentering adjustment.
The sharpening of the furrows was called dressing and this was a highly skilled job since the quality of the meal depended on it. With use the stones became dull and had to be sharpened. For dressing, the runnerstone had to be raised, which was a difficult and dangerous job in the cramped spaces. The dressing itself took place by the light of an oil lamp hanging over the stone. The poor daylight entering through the small windows of the mill was too dim and cast shadows which made dressing difficult, so the shutters were closed. For the dressing, special hammers called "millbills" were used. They were drawn out to a chisel point on each end. Dressing was a difficult and lengthy job and formed an important event in daily mill routine. During dressing, no grain could be ground. By positioning the sails in a specific manner, the miller would inform everybody in the neighborhood that the stones had been raised, and that no grain could be accepted for grinding. During the sharpening process, flakes of metal from the millbill would become embedded in the skin of the user's hands and forearms. This metal would turn blue and gave rise to the expressions: "Show your metal" and "Are you worth your metal?"
In barley mills and rice-hulling mills, hullingstones were used instead of millstones. They were larger than the common millstones and the casings were also constructed differently. The barley or rice did not have to be ground, but instead was hulled, i.e. the outer covering of the grains was removed. The stones were usually gritstones and the runner stone only had a few deep, wide furrows, through which the grains were flung out as the stone revolved, without being ground between the stones. The wall of the casing was lined on the inside with tin sheeting. This sheeting was punched full of holes. The sharp points created by the holes were on the inside and formed a cylindrical grater. The grains were flung against it and rubbed by the circular edge of the stones, freed from their hulls or shells, and smoothed. Barley was once an important food, but in the early 1800's it was steadily replaced by rice. Rice too was treated in the hulling mills. For the grains to be flung out it was necessary for the hullingstones to revolve faster than millstones. This required alot of power and hulling mills required a strong wind for operation. Instead of a sack hoist these mills employed elevator buckets on a conveyor belt, which lifted the barley or rice and poured it into the hoppers. For this additional work a third spindle was used and took its drive from the upright shaft and a separate gear wheel.
There were also mills designed for other purposes besides milling and hulling grain. Some of the specialty mills included those designed for milling oil, snuff and chalk. Others were set up to produce lumber and plaster.
The miller's touch:
1. The miller would catch a handful of ground material as it came out of the millstones into the palm of his hand. Then he closed his palm, made a fist, and opened it. Ideally, the ground material held together. If it fell apart, like sand, it was too dry from not having enough moisture. If it held together like clay then it had too much moisture. It was important to temper and condition the wheat before it was ground. Tempering and conditioning toughened the bran and mellowed the wheat berry. The wheat was placed in a large bin and dampened with water, then turned over several times in a 24 hour period.
2. Then the miller held the ground material in one palm and gently rubbed it back and forth with his opposite index finger to judge the materials fineness and determine the size/shape of the bran. Ideally, the bran would be in large, broad flakes and not torn into small particles.
3. Then, by rotating the ground material between his thumb and index finder, the miller could judge the size of the particles to see if it was the right grind and if it would sift properly.
These methods were usually performed in dimly lit areas of the mill, so it was necessary to have a good sense of touch.
The miller knew that when the millstones generated a smell of burning stone they were too close together and did not have enough grain between them. He also knew when the millstones were out of balance because of the noise created when they began to bang together on one side of their rotation. This would also generate a bit of burning stone smell. It would damage the furrows as well as create sparks. Since the stones were hidden by the millstone cover, he needed to be within hearing range. The speed that the running stone turned at was determined by how fast the water wheel was turning. The more water they added, the more that could be ground, or the more machinery that could be operated.
The "feed" is the feed of the grind into the millstones, and the "cut" is the distance between the two millstones that determines how coarse or fine it grinds. The miller determines the feed and the speed of his grind for how much he wants to grind in a set time period, but this is compounded by the amount of moisture or lack of moisture in the grain. A pair of millstones 46 inches in diameter can grind 300 pounds of grain per hour. A pair of millstones 48 inches in diameter can grind 400 pounds per hour and a pair of millstones 56 inches in diameter can grind 500 pounds per hour. The millstones would rotate at 120 to 125 revolutions per minute and require 4 1/2 to 10 horsepower depending upon the sharpness of the stones. The larger the diameter, the slower the millstones would rotate; and the smaller the millstones the faster the millstone would rotate.
If you keep your nose in the direction of the stones that means your ears are also in that direction. You run and operate the mill by sight and sounds. One little sound out of place can mean a gear tooth coming out of place. The sudden picking up of the speed of the millstones means that your hoppers are getting low. If the hoppers run out of grain the millstones will not have the space between them and this loss of natural heat dissipation causes temperatures to rise. Over heating can crack the stones. A cracked millstone will burn the flour. When the millstones are grinding grain a natural distribution of heat occurs within the stone. The heat is drawn towards the cooler surfaces of the stone. Cracks in the stone prevent this and certain areas become very hot and burn the flour.
Another thing a miller hadto watch for was "tramp iron." Tramp iron is metal mixed in with the grain before it gets to the mill. Tramp iron can be coins, nails, staples, screws etc. If metal gets between the stones, it can cause the runner stone to jump off the spindle. A stone weighing up to two tons can do a lot of damage before it comes to rest. Some small pieces of metal will go between the stones and come out like a shooting star down the chute. They can damage the dress and surface of the millstone and ignite the flour dust. Because of the danger, millers began putting large magnets in the chutes to catch any tramp iron.
Originally the miller's practiced low grinding or flat grinding, where the millstones were close together with a lot of speed and pressure on the grain. The idea was to produce as much fine, white flour in a single grind. Re-grinding was avoided because of problems in re-feeding it through the hopper and because re-grinding generated heat. In the 1850's they developed "high grinding" or "half-high grinding" where the millstones were farther apart for a lighter treatment. The idea was to produce middlings and not all of the white flour in a single grind. The bulk of white flour comes from the re-grinding of the middlings.
Watermills, Types and Operation:
Interior of a watermill. From Andrew Gray's The Experienced Millwright, 1804.
All or part of the machinery and waterwheel would be housed in the Millhouse. This usually consisted of two chambers: the lower one in which the waterwheel and/or gear wheels rotated and the upper one in which the grain was milled. In other cases the water wheel was house under an addition built on the side of the mill and at the same level. These buildings were either of wood or stone.
For the majority of watermill sites the most prominent components are the earthworks which supplied the water to the mill. Although mills could be situated directly over a river, it was more common for the water to be directed away from the main river channel through an embanked, artificial leat (millrace).
There were three types of watermills: tub, undershot flutter and overshot.
Tub mills tended to be small constructions. They generally consisted of a vertical shaft with vanes positioned horizontally and placed underneath a natural waterfall. Tub mills were not very productive and were often superceded by a more substantial mill of either an undershot or an overshot wheel.
The undershot wheel was constructed as a paddle wheel with vanes radiating from a horizontal shaft. The end of the shaft holding the vanes would be positioned, like the tub mill, under a natural waterfall. The water would strike the flat boards set in the rim of the wheel at its base (the vanes) from the backside and flow under the shaft. The undershot wheel was often small and not much of an improvement, in terms of overall power, over the tub mill. The speed of the wheel was directly linked to the natural speed of the water rushing against it. A substantial and constantly flowing stream was required for the undershot wheel. In times of low stream flow it became almost useless.
The mill that produced the most power, required the least amount of stream force, and therefore was favored by millers who could afford the construction of them, was the overshot wheel type. The overshot wheel gristmill did not need to be built right alongside the waterway from which it was fed. A slow stream of water from a nearby dam, most often transported to the overshot wheel via a manmade trough, was all that was required to move the large overshot wheel. Therefore, the mill could be built some distance from the natural waterway. Water was fed onto the top of the overshot wheel, filling buckets which unbalanced it and caused it to turn clockwise.
The third type of wheel was the the breastshot wheel. Water was fed onto the wheel at intermediate levels causing it to turn counter-clockwise.
The rotary movement of the wheel would be transferred by a system of machinery to carry out the mill's function, corn grinding, fulling etc. On a horizontal mill the wheel was simply rotated on a spindle and in turn the millstone to which it was connected by a rynd, and which was situated above it, rotated. On a vertical mill the movement had to be transferred though 90 degrees by a system of interlocking gear-wheels which then rotated the spindle, rynd and millstone. Most of the gearing system and other parts was made of wood but in some cases they were made of metal, particularly the spindle and the rynd which were generally made of iron since they had to support the weight of the millstone. Bearings were also commonly of iron but could be of stone or brass.
The trough that carried the water from the dam to the mill was known as the race or "head" race. The point at which the water began to flow onto the wheel was called the raceway. At the raceway was a gate of solid wood which the miller raised in order to allow water in the race to pour over the wheel. The higher the gate was raised, the more water was allowed to flow onto the wheel. The gate, therefore, was called the "head flow control." The race was usually constructed of wood planking simply nailed or pegged together. At first the race would have leaked quite of bit of the water, but eventually the planks would swell up and water loss would become minimal. The continual movement of water over the wood planks kept them swelled up and tight.
Throughout the day, when the mill was being operated, the water was allowed to flow freely from the dam and down the race. The day's usage might draw the dam down pretty low, but at night, when the race was blocked off, the dam refilled.The overshot wheel type of mill utilized a large wheel, sometimes twenty feet in diameter, with small trough-shaped "buckets" encircling the outer edge. The actual structure of the wheel consisted of two "sides" formed in the shape of a circle. The trough shaped buckets were nailed in place between the two sides. If the head race was strong and constantly filled, the width of the wheel (i.e. the length of the buckets) might be the same as the diameter of the wheel. The entire structure was attached to a horizontally placed main shaft by means of spokes radiating from the shaft. The wheel was often entirely or partially exposed on the outside of the mill structure, but it was not uncommon to be enclosed. By enclosing the wheel, there was less chance of it freezing up in the winter.
The volume and speed of water pouring over the wheel did not need to be large and fast. The mechanism that caused the wheel to turn was the fact that as the buckets at the top became filled with water, they overbalanced the empty lower ones. The horizontal main shaft, onto which the wheel structure was built, extended into the mill structure. The main shaft was located at the bottom of the mill structure on a water powered mill. If it were located toward the top, as in the windmill, the race would have to be much higher. On the inside end of the main shaft were either carved or attached cogs. The cogs fit into the open spaces of a lantern gear assembly known as the "trundle head". From the trundle head rose the "spindle", a vertical shaft which extended the entire height of the mill structure, and onto which the mill stones were attached at the top. Various additional gears could be linked to the trundle head, and they in turn, linked to different pieces of machinery that needed to be operated. As the result of mechanics of attaching smallerand larger gears together, different speeds could be obtained for different pieces of machinery despite the fact that the speed of the turning main shaft remained constant.
In regard to speed, a large wheel, up to twenty feet in diameter, would make about two and a half revolutions per minute with only a small volume of water causing it to turn. The spindle tended to turn between five and eight times faster than the main shaft.Two stones, often three feet in diameter and nearly a foot thick, made up the grinding mechanism of the grist mill. The bottom stone, called the "bedder" had a large hole in its center through which the spindle passed without touching. This stone was called the bedder because it was bedded onto the floor and kept stationary. The top stone was called the "runner". An iron plate, called a rynd, was attached to the spindle. It was likewise attached to the top surface of the runner stone. That attachment enabled the top stone to be turned at the same speed as the spindle.
Once the water had passed the wheel it would be directed back to the river via the tail-race. As with the leats, these were frequently timber-lined. In some cases there was a by-pass channel and water would be directed through this if the mill was not in use.
Two ways of starting the mill
1. The two stones were placed together and the headgate was opened. The water flowed down a sluice box and began to fill the top buckets on the water wheel. As it filled the first bucket, it over-flowed into the second bucket, then to the third, etc. When the water was over-flowing the wheel, the miller would slowly begin to raise the runner [top] stone. This would allow the water wheel to begin turning, which activated the gears that turned the runner stone. The mechanisms in the mill would come alive and very quickly turn at a good speed. Then the grain was slowly fed into the stones. Next the miller lowered the runner stone closer to the stationary bed [bottom] stone. This caused the material coming out of the stones to get finer and finer. Gradually the miller adjusted the runner stone for the desired grind.
2. Before opening the headgate, the miller raised the runner stone up as far as it would go. The miller gradually opened the head gate, and the water slowly filled the buckets on the top of the water wheel. When enough buckets were filled, the top of the water wheel became heavier than the bottom, and it began to turn. As the wheel's speed increased, the miller slowly lowered the runner stone. Again, the miller adjusted the runner stone for the desired grind.
Two ways to stop a mill:
1. The head gate was closed and as the water stopped flowing, the water wheel stopped turning. As the machinery began to slow, the runner stone was slowly raised. This method works for both gear operated and belt driven stones. There is a time lag between the closing of the head gate and the halting of the machinery. This can be a very long time if part of the mill's machinery is broken or jammed, or if someone is caught in it.
2. An extra handful of grain was pushed in-between the stones. This added amount of grain causes the runner stone to stall. When this happened the miller gradually brought the two stones together with the added cushion of grain between them. The weight of the two stones together overpowered the weight of the water in the wheel and the mill suddenly stopped as if brakes were applied. This method was best when something broke, or if someone, or something, was caught in the machinery. In a mill where the runner stone was belt driven, the drive belt that powered the runner stone could suddenly jump off, or break when the stones stalled. This method of stopping works best with gear-driven stones. The mill gears were made of wood which over time got worn, and they could suddenly snap if stopped abruptly. But if someone was caught in the machinery, the miller could risk destroying the gear teeth.
Diagram showing how the waterwheel operates the mill stones
Printed Sources:
Molens, by Frederick Stokhuyzen, CAJ van Dishoek-Bussum-Holland, 1962, translated by Carry Dikshoorn.
Water, Wind and Wheels, by Ann Jolliffe, Hawthorn Books, New York, 1965.
The Experienced Millwright, by Andrew Gray, Archibald Constable, Edinburgh, 1804.
The Practical American Millwright and Miller, by David Craik, H. C. Baird, Philadelphia, 1870.
The Mill, by William Fox, McClelland and Stewart, Toronto, 1976.
The Mill's Life: From the Domesday Book to the Millennium, Charles Llewellyn, Robson Books, New York, 1998.
Flour Milling, by Joseph Lockwood, 1945.