Mill & Silo Manual
About the year 1880, when Flour Mill Engineering and Flour Manufactured
entered a new era, Millers and Engineers were under a great disadvantage,
having no literature or publications in book form for reference purposes.
Since that time engineering generally has made immense and rapid advances,
and it soon became evident that improvements on old methods of grain transport,
storage, cleaning and reduction of flour were possible and necessary.
The late James Donaldson in the year of 1885, issued the small book called
"The Roller Mill Annual," containing 162 pages. This ultimately
developed into the "Roller Mill Manual," published in 1890. A
second edition revised and enlarged, was issued in 1901, and a third one,
in which many additional illustrations and references were added, was published
in 1912. The present edition, revised, brings the work up-to-date. We are
indebted to Mr. Gerald Fitzgibbon for the article on "Oat Milling."
In a manual of this size, it is impossible to treat fully the subjects referred
to, as will be clear on reference in the Syllabus given on page 163.
The methods of manufacturing flour cannot be taught or learned from books,
the only true, practical, and permanent place being the Mill, where, the
sometimes complex and elaborate methods may be seen in full operation.
ACTION. The action of a millstone is that of a tearing, cutting,
or grinding nature without appreciable compression of the feed, unless overfed.
At starting it is advisable to supply a large feed, and so allow the stones
to be thoroughly filled up before commencing to set their usual work.
ADJUSTMENT GEAR. This usually consists of a lever or bridge tree,
rods, hand wheel, etc., used for adjusting the runner stone, to regulate
the fineness of the reduction of the feed by raising or lowering the spindle
and the stone. With an under runner, the usual plan is to have the adjustment
made by moving the upper fixed stone, by three or four screws at the periphery.
The adjustment should be possible either on the stone floor, or below it,
AUTOMATIC ALARM. This is an appliance situated in the feed hopper
or its adjacent parts, to give some warning when the feed has cease to flow,
or when the stones are running empty, as avoiding any injury to the millstones,
or any chance of firing. One form adopted is that of a floating disc, upon
which the feed falls, so retaining it in a certain position which, if varied
by the cessation, cause an alarm bell to be sounded.
BACK. On an upper runner this is the top part, and consists of a
thickness of plaster mixed with alum water, in which are bedded spalls or
pieces of burr stones, the top surface being turned true and even, whereby
the balancing is facilitated. In this back the balance boxes are embedded
BACKLASH. It must be noticed that the moment of inertia of the Prime
Mover must be exceed that of the machine driven, to avoid backlash. With
gearing this work denotes the clearance between the teeth.
BALANCING. As the materials used in the construction of millstones
are not uniformly distributed as regards weight, and as the stone is subject
to gravitational and centrifugal forces acting at the same time when the
stone is in motion, irregular, undulatory or wobbling motions occur, and
the working face will not keep perfectly horizontal, then some adjustments
must be made to obtain uniformity of action at all points of the face.
After attaining the standard balance, should be the part DW shown in Figure
#1 rise when running, then the weight BW must be raised in the box, so that
its downward action may be increased, until the upward tendency of DW is
neutralized. To meet the influence of gravity, the standing, stationary,
or static balance must be first made, afterwards the centrifugal force is
provided for, whereby the running or dynamic balance is attained without
any interference with the already made static balance. These corrections
are obtained by a process of trial, error and correction, not always an
BALANCE BOXES. Four or more iron boxes to hold adjustable weights
are fixed near the outer periphery of the stone, in order to get the standing
and running balance, that is to adjust the center of gravity so that it
shall be directly over the point of support. In some cases there are two
boxes, the outer one fixed in the stone, and the inner one, with center
screw and weight fitting inside. Vertically adjustable weights affect the
running balance without altering the standing balance. theoretically, the
standing balance, is better obtained by adjustable weights placed in the
horizontal plane passing through the center of gravity, that be a neutral
BALANCE RYYND. The cross bar to carry the running stone, and fixed
to it. The working centers should be in the same horizontal plane as the
point of support, or suspension on the spindle, as the case may be, to avoid
an unnecessary torque or twist on the rhynd, as is possible with the arrangement
shown in Figure #3 where, in case, the bearing points of the rhynd on the
driver, as o and d, may not be always in contact at he same time, due to
bad fitting or wear when these will be a twisting leverage equal to the
distance a, b, and the stone consequently, will be thrown out of level.
BEHRN'S EXHAUST. This consisted of a dust strainer of long haired
flannel stretched above the millstones, and was much in use.
CASTING HOOP or CURB. This is the cover, enclosing the millstones
and supporting the feed mechanism; when an upper runner is used it is made
of hard wood tongued and grooved joints, iron bands, all varnished on the
outside and should never be painted on the inside. Sheet iron hoops are
made for special work.
CENTER OF GRAVITY. This is a point where the whole weight of the
material forming the millstone may be considered as concentrated, and is
used for calculating forces, stability, and etc. Its position may be ascertained
by calculation or by experiment, and is assumed to be at the vertical axis
of the stone at the eye. In a doomed back stone it will not be at the geometrical
center of the transverse section, but some distance below it. The position
of "Center of Gravity" determines the point of support or suspension,
which for a balanced runner stone, will be above "Center of Gravity."
In a millstone there are three positions deserving consideration. (1) -
If the point of support be below "Center of Gravity," the stone
will be unstable and take up an incline position, and except at very high
speeds will wobble. (2) - If the point of support coincides with the "Center
of Gravity" the stone will be in an indifferent or in a neutral state
of equilibrium, attaining a horizontal position when running, with the possibility
of undulating. (3) - If the point of suspension be above the "Center
of Gravity," as us usual, then there will be a reluctance to depart
from a horizontal position whether at rest or running. See the first drawing.
French Burr weights about 160 pounds. per cubic foot, but the more honeycombed
variety may weight from 90 to 120 pounds per cubic foot. In the backing
of a built up millstone consisting of plaster, in which are placed pieces
of burr stone - spawls - for binding the mass, the weight may be about 130
pounds per cubic foot, in which the plaster is assumed to weigh 100 pounds
per cubic foot. This lighter weight of the backing lowers the position of
the "Center of Gravity," as compared to what the position would
be were the whole body of the stone of uniform weight and of burr stone.
CENTRIFUGAL FORCE (C). This will be equal to W x V2 divided by 32.3
x r, where W = weight in pounds, V - velocity in feet per second, r = radius
of gyration. For a 4 foot millstone C = 7,900.
COCKHEAD. Upon this rests the rhynd. Figure #3 and 3a show two
forms. See first drawing.
CONSTRUCTION. They are constructed of a great variety of stones,
such as Derbyshire Peak, Newcastle Grit, Belgian dark marble, German Cullen
stones, Porous lava, and etc., but the usual and most suitable stones are
quarried at La Ferte sous Jonare in France.
Blocks of various sizes are used of those burrs to build up the stone, the
joints being symmetrical, but differing for the runner and fixed stone.
For stones grinding various vitrous materials the radiating joints are made
to coincide as near as possible with the bottom line of the furrows, so
preventing the breaking away of small angular pieces when in use. With Composition
stones this need not be considered. The jointing material is of cement,
lead, or plaster, and is good work the joints should be fine, even, and
extend with the stone from the face towards the back for about 9 or 12 inches,
for a runner stone.
Composition stones constructed of concrete, spalls, Portland cement, and
etc., have special facing of emery, corundum or other material of a hard
The better class of burrs are in small pieces. The individual stones for
intensive work, should be built end on, that is with the grain square to
Burr stones vary in hardness and porosity, the usual qualities are hard,
soft, close or open, and of white, brown, violet and other shades of color.
Small size millstones are made of one piece. For oats, the honeycomb variety
is suitable. The stones at the bosom or breast are made of a harder quality
than for the skirt for single purpose. If the millstones happen to be out
of balance and touch, you would have hard stone touching hard stone. Rather
than soft touching soft, or hard touching soft. This way hard stone touching
hard stone would do less damage to the stones and to the dress on the millstones.
CRACKING. Lines of indentations on the lands, running parallel with
the furrows. They are cut with a mill bill or diamond on a special tool
for that purpose, and are spaced so that from 6 to 30 lines may be made
per inch, according to the material to be treated.
CRANE. Where there is no overhead traveler, a crane is required for
fixing or removing the millstones. If there is no possibility of using the
guy stay, the jib must be of sufficient strength to carry the stone, and
the top socket deep and well fixed. Square thread on the screw so the weight
of the stone will not unscrew the thread downward. The whole made so as
to be easily fixed or removed.
DIRECTION RUNNING. This is stated as, revolving with the sun, like
the fingers (hands) of a watch; or against the sun.
DISCHARGE SPOUT or CHUTE. This is so constructed that an examination
of the reduced material may be made at hand holes, either level with the
millstones floor or below it.
DISENGAGING GEAR. For this purpose clutches, fast and loose pulleys
are required, so that any pair of stones may be put in or out of action
as required. Remember raise the millstone first to allow all water to empty
out of the buckets of the water wheel so potential energy is not stored
within the system so the raising of the disengaging gear will not release
that energy, damaging the machinery or injuring or killing individuals.
DRAFT or DRIFT or LEAD. This is the amount of eccentricity
of the furrows, and is the radius of a circle at the eye to which a tangent
line is drawn to fix the position of the back edge of the furrow. The old
rule was, one inch per foot diameter of stone for the master furrow. If
the speed is increased, the draft may be less. Some authorities consider
that draft is not of much importance, and that to the centrifugal action
must be attributed the chief cause for the flow of the feed between the
Increasing the draft will permit of shallower furrows. Increasing the draft
results in a greater shearing action on the feed. It should be noted that,
although the master furrow as sown in Figure #7, say has a draft of 4 inches,
the successive furrows have drafts of about 6 3/4, 9 5/8, and 12 1/2 inches
respectively, for a 4 foot stone with a ten quarter dress.
DRESS. Figure #6 of the first drawing. "A" shows a 9.4
quarter less for a four foot stone. "B" is a sickle dress, in
which the interesting angles are 38 degrees, a particle "P" is
shown in the furrows. "C" is a dress of 24 leading furrows, and
24 skirt furrows, half of the surface being land and half being furrows
of 1 1/2 inches width.
There is no uniformity in the dress for millstones. The following examples
WHEAT (SOFT). Open Burr 4 feet in diameter, 22/2; furrows 1 1/2 inches by
3/16 inches; bosom 11/16 inches deep to 6 inches from skirt, 4 inch draft
furrows and lands equal.
WHEAT (HARD). Close Burr 4 feet in diameter, 22/2 furrows 2 inches by 3/16
inches; 3/16 inches deep, 2/3 furrows 1/3 land, 16 cracks per inch.
WHEAT. Burr 4 feet in diameter, 12-3; furrows 1 1/4 inches to 1 3/8 inches
by 3/16 inches deep, draft 4 1/2 inches.
INDIAN CORN. Burr 4 feet in diameter, 12 to 14/4; furrows 1 1/2 inches by
5/8 inches, 3 1/2 to 4 1/2 inch draft.
BARLEY. Peak stones, 12 to 14/3; furrows 3/8 inches deep, draft 4 to 4 1/2
inches, 10 cracks per inch.
OATS. Peak stone, close grain, no dress.
HOG MEAL. Burr 4 feet 2 inches in diameter, furrows 2 1/4 inches by 1/2
inch. 2 inch lands, draft 3 1/2 inches, 8 cracks per inch for 6 inches from
MIDDLINGS. Burr 4 feet in diameter, 14/3; furrows 1 3/8 inches by 3/15 inches
deep at eye to 3/32 at skirt, draft 4 inches.
MIDDLINGS. Burr 4 feet in diameter, 14/3; furrows 1 1/2 inches by 1/4 inches,
2 inch lands, draft 3 1/2 inches, 10 cracks per inch on 8 inch face.
MIDDLINGS. Burr 4 feet in diameter, 23/3; 6/10 furrow, 4/10 lead.
MIDDLINGS. Burr 3 feet in diameter, 12/3; furrows 1 1/8 inches, 16 to 18
cracks per inch.
BUCKWHEAT. Open or closed Burr 4 feet in diameter, 22/2; furrows 1 1/2 inches
by 3/16 inches; bosom 11/16 inches deep to 6 inches from skirt, 4 inch draft
furrows and lands equal, 16 to 18 cracks per inch.
RYE and RYE MEAL. Open Burr 4 feet in diameter, 22/2; furrows 1 1/2 inches
by 3/16 inches; bosom 11/16 inches deep to 6 inches from skirt, 4 inch draft
furrows and lands equal..
DRIVER. A cast steel block fitted and keyed on the top end of the
spindle, into this the rhynd or bail is fitted. It also may be forged and
formed by a blacksmith but not welded together out of smaller pieces.
DRIVING. These are several methods adopted for transmitting motion
and power from the line shaft to the millstone spindle. Spur wheels, beveled
wheels, pulleys and belts, friction wheels, and direct action motors. Gearing
may be noisy, the teeth of mortise wheels rapidly wear if out of alignment
or when not lubricated, and they may require complete renewing because of
neighbor worn teeth. Belts and ropes run smoothly. The pulley on the spindle
should be as large as possible to prevent slip of the belt, it should be
well crowned to retain the belt in the center of the pulley. The spindle
neck and toe bearings should be extra long on amount of the transverse pull
of the belt. The belt running horizontally will be wider than for a vertical
position for the same power, and from 2 to 3 per cent more must be allowed
for slip. The larger horizontal pulley may require a lip to keep the leather
belt from slipping off. A pulley covered with leather or canvas will transmit
20 25 per cent more power than one not so covered. With a quarter twist
drive the distance apart of the driver and driven pulleys should be not
less than three times or more than four times of the diameter of the larger
pulley, in order to avoid the use of riggers or jockey pulleys. For stones
of the following dimensions the sizes here given for pulleys, etc., serve
as a guide.
Diameter of Stones.........2 ft 6 in...3 ft 0 in...3 ft 6 in...4 ft 0 in....4
ft 6 in.
Diameter of Pulley...........1 ft 8 in...2 ft 2 in...2 ft 4 in...2 ft 8
in....3 ft 0 in.
Width of Pulley..................0..7.....0..8 1/2..0..10 1/2....1..0.....1..2.
DUTY. Millstones are used for reducing materials. Since 1890 Chilled
Iron Rollers have been generally adopted for reducing wheat, etc., but many
substances continue to be treated by stones, disintegrators, etc.
EYE. The opening at the center where the feed enters. A large eye
needs less swallow. Sometimes the eye is made larger in diameter at the
face than at the back to assist the flow of the feed. With high velocities,
the feed may adhere to the eye surface, due to centrifugal action, which
may be avoided by fixing a sleeve or conical metal bonnet to direct the
FURROWS. These are the grooves or channels cut in the face of the
stones. The master furrows begin at the eye, or thereabouts, and extend
to the verge. The tributary or branch furrows are those placed at an angle
with the master furrows. The purpose of the furrows is that granulation,
ventilation, distribution and conveyance of the feed towards the verge.
The more numerous the furrows the greater will be the quantity passed between
the stones. Each furrow consists of a back edge, the cheek, the slope and
the feather or cutting edge. The usual forms of section are shown in the
second drawing, and the one found most efficient, is the form number 1.
The depths and widths vary considerable, according to the nature of the
material to be reduced. Deep furrows work cooler than those that are shallow.
Some times the branch furrows are cut into the master furrow to ensure cooler
and quicker work. When small transverse furrows are cut into the master
furrows, the centrifugal action is somewhat resisted. The surface of the
furrows should be even and smooth, for an uneven furrow does irregular work.
A bend or elbow in a furrow extending a few inches from the skirt will accelerate
the rate of discharge of the feed as it becomes finer. But furrows are not
always necessary, for surfaces merely cracked or nicked may suffice for
reducing some fine materials. Complicated forms of dress, such as the Sickle
and the Hyperbolic Spiral, are difficult to make and expensive to keep in
order. For cement grinding only master furrows are made, the remaining being
land roughened, see the second drawing. The face parts not furrowed are
GYRATION. Taking the section of a runner stone as being approximately
a rectangle, there is a horizontal circle where the whole of the material
forming the stone may be considered as concentrated. Thus for a 4.0 millstone
with a 10 inch eye, R=1.415 feet or 17 inches, and at 120 revolutions per
minute the angular velocity at this radius will be 1,080 feet per minute,
or 18 feet per second.
HOOPS or BANDS. Neglecting the adhesion of the cement at the
stone joints, the metal bands at the circumference for binding the stones
must be made to exceed the centrifugal force by a factor of safety of over
15. There are usually three hoops 1 1/8 by 3/8 thick, placed about 1 1/2
to 1 3/4 apart, also another small band near the face, about 5/78 by 1/4.
Sometimes a band with vanes or wipers is used for ventilation the space
between the stone and the casting, and for directing the materials ground
to the discharge spout. The hoop joints should be dovetailed and welded,
then heated and shrunk on the stone.
JACK STICK. A tool used for adjusting the bed stone for level and
the spindle for a vertical position.
NECK BEARING. In the eye of the bed stone is fitted an iron box, in
which are fitted three or more adjustable bronze, gunmetal, wood, or white
metal pieces, upon which the spindle revolves, so forming the neck bearing,
and is so placed that it may be as near as possible to the point, where
the resistance of the running stone can be best received. The bearing must
be made as dust proof as is possible. With an upper running and overhead
drive, the neck bearing will be replaced by a small toe step bearing sufficient
to keep the stone and spindle in position.
PATH OF TRAVEL. Once the feed enters between the stones its path
during the process of reduction is not radial, but more or less a spiral
one, for an upper runner stone. Much depends upon the type of dress, the
speed, and the material being ground.
POWER. Once actual horse power is required to reduce one bushel of
wheat per hour on a 4.0 stone, but the power will increase with additional
feed and pressure.
RUNNER STONE. This may be either the under or upper stone, or both.
With an under runner stone there is no accumulation of the feed at the eye
and the upper stone is adjusted easier.
When the upper stone is the runner there is a certain amount of buoyancy
when the feed is between the stones, so reducing the weight on the top step
to the extent of 60 to 70 per cent, and possibly with an excessive feed
the spindle with the attached gear may be the only weight upon the toe.
There is an old saying that the weight on the toe is lessened by one hundred
weight for each bushel of corn (wheat) ground per hour. For reducing vitreous
materials, with an upper runner the overhead drive is preferable, as shown
in figure number 5. "A" is a spindle for adjusting of the runner.
SAUCER. This is a metal disc, plain or dished, revolving with the
spindle upon which the feed falls, and is distributed in a spray towards
SHAKING SHOE. This consists of a small box placed at an angle, into
which the feed is supplied and is supported on a flexible rod, and etc.
By the use of a damsel attached to the spindle the shoe is made to shake,
and so keep the feed flowing inot the eye. This is now dispensed with, and
what is termed a silent feed tube used in its place.
SPINDLE. The central steel revolving axle by which motion and power
are received for driving the millstone. The parts are, the toe, body, neck,
driver seat, and cockhead. They are made from 6 to 11 feet long and the
diameter vary from 3 1/2 to 4 1/2 inches, and must be free from any transverse
or tortional deflections.
It is usual for the top stone to be the runner, but several arrangements
are made with the bottom stone as the runner. The position of the spindle
may be either above or below the running stone. The usual position is below.
For stones applied in the reduction of vitreous substances, the top position
has advantages, as in that case, all the bearings and drive are clear for
injury. With an under runner the spindle below, there is the advantage of
having free and clear opening in the eye of the upper fixed stone for feeding.
A balanced running stone with the usual free irons works better than a runner
on fixed irons, the adaptability of the latter being considerable destroyed,
except for very coarse work.
SPLINES. These are templates to mark off the furrows and lands.
STAFF. This is required for truing the face of stones. When used
it is coated with a semi-liquid paste, so that the prominences on the stone
may be indicated. An annular iron staff is superior to a straight hard wood
one. The straight hard wood staff is called a "paint staff," and
the iron staff is called the "proof staff." The proof staff is
used to keep the paint staff true and is only used as a gauge instrument.
SWALLOW. Various forms are shown in the second drawing.
TIME (Removing and Refixing). The time required to remove the stone,
brushing, rubbing, cleaning, staffing, dressing, cracking and refixing may
take from 4 to 8 hours, depending upon the quality of the stone and appliances
TOE BEARING. The toe must be of such diameter that the pressure upon
it does not exceed 260 pounds per square inch, with ordinary lubrication,
to avoid any expulsion of the lubricant and consequent hearing or seizing.
The lubricant should circulate, and a single diametrical groove under the
toe has been determine upon as most efficient. The weight of a 4.0 millstone
with spindle and all parts attached to it make be taken at 2,300 pounds,
and assuming the bearing to be 3 1/2 inches diameter the pressure will be
about 240 pounds per square inch., See the results of some experiments on
the bearings published in the Proceedings of the Institute Mechanical Engineers,
Ball bearings have been about perfected and applied to many machines. Figure
4 in the second drawing, shows a form for and thrust only, and is made by
the Hoffman Manufacturing Company, Limited, Chelmsford, England. The steel
balls being 5/8 inches diameter, to take safe working pressure of 2,500
pounds at 120 to 150 revolutions per minute. A pedestal or journal close
above this bearing is necessary to take the side thrust and to hold the
spindle concentric. A heavy lubricant or, if flooded in oil or an oil bath,
may be used. The friction is about one-tenth that of ordinary bearings.
TRUNNION SOCKETS. These are fixed about 6 inches from the face at
opposite points of a diameter passing through the center of gravity of the
stone and receive the trunnions or pins on the crane bows (bails), upon
which the stone is supported and turned for inspection, dressing, etc.
VENTILATION. To prevent or lessen undue trituration of the material,
wind under a pressure of about one inch of water is applied at the eye which,
flowing through the furrows catches up the finer stuff, and with the assistance
of the centrifugal action conveys it to the surrounding space between the
cover and the stone, absorbing excessive moisture, and cooling the material
and the stone surfaces, decreasing the duration of the treatment, increasing
the output, and with some materials, leaving it in a better condition for
dressing. To a small extent the power required for working the stones is
Another arrangement included the use of a suction fan connected to the casting.
A flexible sleeve or tube attached is required at the eye to isolate the
eye form the casing space.
ZONES OF FACE. These are, beginning at the eye, the swallow, the
bosom, breast, and skirt. At the swallow the feed enters and subjected to
its first treatment, is further softened (reduced by increasingly more furrows)
at the bosom, and the operation of reduction completed at the skirt as shown
first drawing. The outer periphery of the stone is the verge. This is called
"the flouring of the stone" where the stones come very close together
and where the actual flour is made.
The angular velocity of a 4.0 stone at 120 revolutions per minute or 2 revolutions
per second at a radius of 12 inches is 12.5 feet per second; at 18 inches
about 19 feet per second, and at or near the skirt, 25 feet per second;
and a particle of feed passing between the stone will be subject to cutting
or tearing forces, applied at these velocities.
NOTE: REMEMBER THIS IS ENGLISH! Some terms are different, and
others that may be the same, but are spelled differently than American milling
terms. The above text has been edited for technical errors.
The above book does not contain a "Table of Contents." However,
it is usable by its index. The book is a collection of articles loosely
bound together, and does not flow very well. It is of little practical use.
James Donaldson is the implied author.
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