CHAPTER III.

 
HOW A WINDMILL WORKS
 
INTRODUCTION

Little change occurred in windmill design from their first use in Britain until the Industrial Revolution, when a number of significant advances were made in the design of sails and machinery.  But important though they were, these advances were slow to gain acceptance and too late to make a significant impact, for by then James Watt’s condensing steam engine was in the ascendant.  From about the mid-19th century, steam-powered milling began rapidly to supplant wind and water power.  This was followed by the introduction of industrial-scale flour mills in which steel rollers replaced grindstones to produce, in greater quantity, finer and more consistent quality products.  Mead’s (later Heygates) Flour Mill at Tring (Chapter VII) is an early example of an industrial-scale flour mill.

This chapter describes the general operation of a windmill and some of the advances in design that were made towards the end of the windmill era.  A brief description of the modern roller mill is given in the Appendix.

 
WHAT A WINDMILL DOES

Fig. 3.1: the fantail (where one is fitted) rotates the cap
automatically to keep the sails facing the wind.

The mill was the first engine invented by man.  For centuries, mills driven by water or wind were the only machines that could convert the power of nature into useful work.  In the case of the windmill, wind striking its sails exerts a force upon them that causes the shaft to which the sails are attached to rotate.  What then follows depends on what task the windmill is to perform.  Windmills have been used for many purposes, such as pumping water, sawing wood and as crushing machines in the preparation of oil, paper, spices, chalk and pottery.  Today, wind turbines are used increasingly to generate electricity.  But in Britain, the windmill’s most common use over the centuries was to grind grain.

In a grain mill, the wind’s energy, harnessed by the windmill’s sails, is transferred via a system of shafts, cogs and belts to drive one or more pairs of millstones.  Grain, fed between the rotating millstones is ground into meal.

The remainder of this chapter describes the main steps in the windmilling process; also the sails and the machinery that is usually found within a windmill.

 
THE FLOORS OF A WINDMILL

Fig. 3.2: schematic drawing of a tower mill.

Windmills do not follow a common design but they do share common features, not least of which is that windmilling is a gravity-driven process.  Milling begins at the top of the mill and each succeeding stage of the process is performed on the next floor down (in following this process, the need for a mechanically powered hoist to lift sacks of grain and meal up several floors of a windmill soon becomes clear!)

Windmills were built with different numbers of floors, [3] hence, the windmilling process is not always exactly as described below.  However, in general the following applies . . . .

i. The cap, uppermost part of a windmill (fig. 3.1), houses the windshaft, bearings, cogs and the top of the upright shaft, which transmits the windshaft’s rotary motion down through the mill to drive the machinery.

ii. The dust floor (fig.3.2), positioned under the cap, serves to keep dirt from above from falling into the storage bins and to keep dust rising up from below.

iii. The bin floor houses bins in which are stored grain for cleaning; cleaned grain for milling; and meal to be sifted.

iv. The stone floor houses grain-cleaning machinery, the millstones used to grind the grain, and machinery to sift the ground meal into various grades of fineness.

v. The meal floor houses chutes from the stone floor above, down which flows cleaned grain for milling, meal for sifting, and milled products, each of which is collected in sacks.

 
THE WIND MILLING PROCESS

The first step in the milling process is to hoist the grain to be milled up to the bin floor where it is loaded into a storage bin ready to be cleaned.  When required, the uncleaned grain is discharged down a chute to the stone floor, where it is mechanically cleaned, then discharged down a chute for collection on the meal floor.

Sacks of cleaned grain are hoisted up the mill to the bin floor, where they are stored in a bin ready for milling.  When required, the cleaned grain is discharged down a chute into a hopper on the stone floor, from where it is trickled into the millstones, ground and discharged down chutes for collection on the meal floor.

The sacks are then hoisted up the mill to the bin floor, from where the meal travels downwards, this time through a flour dresser, which sorts and distributes it according to its fineness, white flour being the finest and bran the coarsest.  All this machinery is powered by the thrust of the wind as harnessed by the windmill’s sails.

 
THE SAILS

A windmill’s sails are usually four in number, but five, six and eight-sailed windmills (fig 3.3) were also built.  More sails generate more power with a smoother torque, but at greater cost, weight and maintenance.

A windmill’s sails do not rotate in the vertical plane, but are slightly inclined to it, for it was discovered that a slight inclination of about 15° increases the wind’s effect (fig. 3.4).  This is due to wind currents near to the ground meeting more frictional resistance than higher up, to the extent that at a level of 43 ft above ground-level, wind velocity is some 10 per cent greater than at 20 ft.  To accommodate the tilt of the sails, the windshaft has also to be inclined at the same angle below the horizontal, with its rear end held in place by a firmly-embedded bearing to enable it to rotate while preventing it from sliding backwards (plate 17; plate 25).

A further discovery was that sails worked more efficiently if, rather than being set flat across the sail stock, a slight twist is applied, this being more accentuated nearest the windshaft (about 18°) lessening towards the tip (about 7°); this twist can be seen in the sails of Quainton mill (plate 26).

A major problem for the miller was to regulate the speed of rotation of the sails and thus of the millstones.  The optimum sail speed for a grain mill generally lies in the range 11 to 15 revolutions per minute; speeds much above that run the risk of over-driving the stones and burning the grain, while even higher speeds — sometimes referred to as the mill ‘running away’ — could damage the machinery and, indeed, the mill itself.
 

Fig. 3.4: flow of air currents near the ground.

Small differences in sail speed can be adjusted by changing the amount of grain fed to the millstones or changing the gap between them, both of which affect the load placed on the sails.  But a large change in wind speed has to be dealt with by altering the sail area exposed to the wind, either by increasing or reducing it, a process called reefing (fig. 3.5).

Fig. 3.5: different degrees of reefing a simple cloth covered sail.

For centuries before the development of more advanced and better controlled sail systems, sails comprised a lattice framework over which the sailcloth was spread (plate 10).  Such common sails required two men for reefing, one to climb on the sweeps to carry out the task and one to control the brake; should the brake fail during the operation, the man on the sweep was in for a spectacular ride.

Towards the end of the 18th century, developments in sail design eased the reefing process.  Roller reefing employed banks of cloth blinds mounted on rollers (comparable to a household roller blind) that could be adjusted with a manual chain from the ground without stopping the mill.  Other systems replaced the sailcloth with sets of wooden shutters (comparable to Venetian blinds) mounted along each sweep.  These systems employed some form of tensioning that caused the shutters to spill the wind automatically if its force exceeded a set limit.  A later invention, the air brake (fig. 3.6), comprised shutters placed longitudinally at the tip of each sweep that turned automatically if the wind exceeded a set strength, thereby disturbing the sail’s profile and slowing it.

Fig. 3.6: top, a sail fitted with shutters and air brake.
Bottom, a sail for simple cloth covering.

The development of hollow windshafts permitted control rods to be inserted through their centre (fig. 3.7).  This enabled sail settings to be adjusted from within the mill automatically under the action of counter-weights.

While these developments were not without their complexities when compared to common sails, they reduced manual effort while improving the windmill’s efficiency as a motive force.

 
THE MACHINERY

Different hardwoods were used in the construction of milling machinery.  Cogs were often of applewood, hornbeam or beech, wheels and shafts of oak, whilst the dowels used to join together wooden parts were of holly.  However, from the middle of the 18th century cast-iron parts were used increasingly, although iron-to-iron gearing was avoided, for it was found that iron-to-wood gearing ran more quietly and was easier and cheaper to maintain and, most important, it avoided the risks of sparks causing a dust explosion. [4] Problems associated with manufacturing and handling large iron castings were avoided by the use of small sections bolted together, an example being the dozen or so sections that make up the 18ft diameter iron rack in the cap of Wendover mill (plate 24).

Fig. 3.8: general arrangement of a windmill fitted with a cap and fantail, and two sets of stones.

In order to rotate the millstones, the (near) horizontal rotation of the windshaft must first be converted into a vertical rotation.  This is achieved using a form of bevel gearing.  The inner end of the windshaft is fitted with a large toothed wheel, the brake wheel, the teeth of which mesh with a cog, the wallower, which is set at an upright angle to it.  The brake wheel, when rotated by the windmill’s sails in the horizontal, causes the wallower to rotate in the vertical (fig. 3.8).

The wallower is mounted on the upright (or main) shaft, which transmits its rotary motion downwards through the mill.  Mounted on the base of the upright shaft is another large toothed wheel, the great spur wheel.  This in turn meshes with the cogs — called stone nuts — that drive the millstones.  In this way the wind’s energy is captured and then put to work to grind grain and drive other machinery for cleaning, sifting and hoisting grain.

As its name implies, the brake wheel’s other function is to stop the mill.  In fig. 3.8, the brake is the circular shoes that surround the brake wheel.  To stop the mill, a lever is pulled to tighten the brake-shoes causing them to grip the periphery of the brake wheel and thus slow the rotating windshaft or clamp it in place.

 
THE MILLSTONES

Windmills are generally equipped with several sets of millstones (plate 14).  Each set comprises a rotating runner stone and a stationary bed stone which, depending on their diameter, weigh in the region of two tons.  Different types of stone are used to grind different types of grain.  Stones of grey millstone grit from Derbyshire are used to grind coarse meal for stock feeding.  The millstones employed to grind wheat for flour are made of French Burr, a hard silicate found in the Seine valley.  Burr stones are constructed in segments, cemented together and bound with heavy iron bands.

Fig. 3.9: millstones and their associated equipment.

In operation, the millstones (fig. 3.9) are fed from a hopper, which trickles grain along a wooden trough — the shoe — into the eye (a hole through the centre of the runner stone) where it is ground between runner and bed stones.  The shoe is kept in a continual state of agitation by a rotating crank.  Called the damsel, due to its chattering sound when in operation, it serves to keep the grain flowing steadily down the shoe into the eye.

Each stone has a pattern of grooves cut into its surface (fig. 3.10).  The grooves act like scissors, cutting the grain as well as moving it outwards from the centre to the periphery of the millstones.  As the meal emerges from between the stones, it is swept inside the circular wooden container that encases them — the tun or stone case — into the top of a chute, or meal-spout, which funnels it down to the meal floor below where it is bagged.

In regular use, the grooves in a millstone wear down and need to be dressed periodically; that is, re-cut to keep their cutting surfaces sharp.  This is a tedious and exacting task, sometimes performed by the miller but often by an itinerant stone dresser (fig. 3.11).  The work was executed using various tools including a mill bill, a tempered steel blade clamped in a wooden handle, rather like a small pickaxe, and used as a chipping tool.  Today, power tools are often used.
 

Adjusting the gap between the stones — a process called tentering — is carried out using a system of screws or levers that move the runner stone up or down.  This gap, or nip as millers call it, helps to determine the fineness of the meal; the smaller the nip, the finer the meal.  Only slight adjustments are required, for the stones, when grinding, are only about the thickness of a postcard apart.

Fig. 3.12: the centrifugal governor.

Tentering can be performed manually, but the centrifugal governor (fig. 3.12 & plate 2) can perform the task more efficiently.  This ingenious device is driven from the rotating upright shaft.  As the speed of rotation of the sails — and thus of the shaft and the millstones — increases, a pair of heavy metal balls linked to the governor’s spindle begin to fly outwards under the increasing centrifugal force.  This causes their collar to rise up the spindle, and in so doing to move the levers that adjust the gap between the millstones.  An increased gap results in more grain being fed into the eye of the millstones, which increases the load on the sails, slowing their speed of rotation and that of the millstones.

The optimum speed of rotation of a runner stone depends on its diameter and on the quality of the output required.  A rule of thumb followed by millers was to divide the diameter of the stone (in inches) into 5,000 for flour and 6,000 for coarser meal.  Thus the optimum speed for a 48 inch runner stone, set to produce meal, would be 125 rpm — assuming, of course, there was sufficient wind to drive it at that speed.

 
THE FANTAIL

Even a small change in wind direction can result in a significant reduction in the torque the sails generate if they are not repositioned.  In older windmills, ‘winding the mill’ involved hard manual effort.  In the case of a post mill, the entire superstructure needed to be turned by pushing on a large beam (the tail post) that protruded from the rear of the mill.  In later smock and tower mills, only the top floor (fig. 3.1) needs to revolve, but this still required manual effort.  The fantail automates the process by using the wind itself to wind the sails.  Later windmills usually adopted this feature, which could also be built onto the tail posts of old post mills (plate 31).

A fantail comprises gearing driven by a small set of sails (vanes), which are aligned at right angles to the main sails and positioned at the rear of the mill.  Its purpose is to rotate the cap automatically when a change of wind direction occurs, which it achieves via a system of gears that mesh with a toothed rack around the inside of the cap (fig. 3.8).  When the sails are properly winded, the vanes of the fantail are at right angles to the wind, so they derive no thrust from it and do not rotate.  But should the wind change direction, its force then falls on one side or the other of the fantail, causing its vanes to rotate and drive the gearing that rotates the cap and winds the sails, a clever feat of automation.

However, while very useful in normal working conditions, the fantail provided no guarantee against a mill being tail-winded by a sudden change in wind direction, as sometimes accompanies a thunderstorm; thus fantails might be supplemented by a hand crank, an example being in the cap of Wendover mill (fig. 10.3).  And unless the fantail drive could be disconnected, a further problem for the miller was how to turn the windmill’s sails off the wind in a sudden squall in order to stop them.  Things were rarely straight-forward in windmilling.

 
ANCILLARY EQUIPMENT

Because windmills use gravity feed to clean and grind grain, filter the meal produced and bag the end product, a sack hoist is an essential piece of equipment if the drudgery of lifting sacks manually up the mill, on numerous occasions, is to be avoided.  The sack hoist takes the form a simple rotating chain, driven by an auxiliary shaft which is in turn geared to the upright shaft (plate 3).  As it rotates, the sack hoist is used to lift the sacks of grain or meal up through a succession of trapdoors to the bin floor of the mill.

The auxiliary shaft also powers other windmill machinery.  That used prior to the grain being milled might include a smutter, which removes the black spots of smut caused by a fungus disease that can grow on grain if its gets damp; a separator, used to separate grain from other foreign matter, such as stones, weeds, and sticks; a scourer, used to separate usable grain from debris such as dirt, dust, and chaff.

Following milling, a flour dresser (fig. 3.13) is used to sift the meal into its various grades of fineness.  The dresser consists of a cylindrical drum, covered in wire mesh of increasing grades of fineness, and set at an angle.  Inside the drum revolves a set of brushes.  Meal, fed into the upper end of the cylinder is rubbed against the mesh screens by the brushes as it falls through the cylinder under gravity.  The finest meal, white flour, can pass through the finest mesh screen; next comes semolina flour, which passes through the next grade of mesh, leaving the coarsest product, bran.  Each grade is ejected into canvas chutes which feed sacks on the meal floor below.

The mill might also drive an oat crusher, used to crush oats for animal feed.

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APPENDIX


THE ROLLER MILL

Although the subject of this book is windmills, some mention should be made of the technology that in the latter part of the 19th century was to sweep away both wind and water mills so rapidly.

The steam engine was the first major advance.  The first steam-powered mill, Albion Mill, was established in 1786 by Matthew Boulton and James Watt.  It employed a Watt steam engine of 150 hp to drive 20 pairs of millstones, and was capable of grinding 10 bushels of wheat per hour, all day and regardless of wind strength.  Albion Mill was the industrial wonder of the age, but in 1791 it burned down.  The cause was never discovered, but it was widely believed to be an act of arson by local millers and millworkers who believed their livelihood was threatened by the new technology.

Rotating millstones, sometimes steam-driven, continued to be used for grain milling until the late 19th century, when roller mills — a Swiss invention — appeared, the first being built in Hungary in 1874.  Edward Mead (Chapter VII.) is believed to have installed the U.K.’s first roller mill at Chelsea in 1881.  The combination of steam power and the roller milling process led directly to the flour mills of today.
 

Roller milling (fig. 3.14) crushes the grain, not between revolving millstones, but between a series of fluted steel rollers of about 12 inches in diameter.  The rollers are set with a specified gap between them and spin towards each other at high, but at different, speeds; the surface of each roller is also grooved with a different pattern.

In a modern flour mill, before milling commences, the grain is cleaned of any extraneous matter using a variety of techniques including sifters and magnets (to remove metallic particles).  It is then conditioned to ensure uniform moisture content and blended with other wheats to provide a mix capable of producing flour of the required character.  Then, using a reduction process, the grain is crushed by a succession of rollers, the fine flour particles being sifted out at each pass of the rollers while the residue (bran) is sent on to the next set of rollers in a repetitive manner.  Thus, roller milling is a series of crushing and sifting operations, ideally suited for making clean white flour.

Roller mills offer several other advantages over traditional milling methods.  They eliminate the cost of dressing millstones and enable the production of a larger amount of better-grade flour from a given amount of wheat, quicker and to a consistent standard.  Rollers are also superior for milling the harder wheats used for bread by reducing the wheat kernel slowly into flour fragments to separate out the bran.

Roller milling made possible the construction of larger, more efficient grain mills, hastening the abandonment of the small country wind and water mills, and of stone grinding.  Indeed, so successful were the roller mills that within 30 years of their introduction into Britain in 1881, more than three-quarters of the wind and water mills that had served for centuries so faithfully, if erratically, had been demolished or abandoned.  This is what Edward Bradfield, an old miller who was writing in 1920, had to say about this milling revolution . . . .

“Then came the changes. ‘High grinding,’ ‘gradual reduction’ and the ‘roller system’, one after the other, came to revolutionize the trade.  The flour was greatly improved by the new methods and the trade of the stone millers was decimated.  The new methods allowed the brittle stony wheats then coming into fame to be made into excellent flour which the stone millers could not equal.  Yet many of them, who clung to the traditional system which had brought them fame, wealth and honour, made heroic efforts to stay the onslaught, but failed.”

However, the roller mill revolution brought with it a drawback.  The friction of the rollers caused the meal to become hot, which led to some nutrients in the flour being damaged.  This was not realized at a time when essential dietary needs were little understood.

Today, the ‘Bread and Flour Regulations’ govern the use of additives as well as requiring the addition of certain nutrients to ensure that a wholesome product emerges from the flour mill.  This from the Federation of Bakers . . . . .

“The Bread and Flour Regulations require that flour should contain not less than 0.24mg. thiamin (vitamin B1), 1.60mg. nicotinic acid and 1.65mg. of iron per 100g. of flour.  These amounts are found naturally in wholemeal flour.  White and brown flours must be fortified to restore their nutritional value to the required level.  In addition calcium carbonate, at a level of not less than 235mg. and not more than 390mg. per 100g. of flour, is added to all flours except wholemeal and certain self-raising varieties.  This ensures the high nutritional value of all bread, whether it is white, brown or wholemeal.”