Screw Conveyer History
and
General Application
HISTORY
Apart from cavemen that may have used  branches under
rocks to replace sliding friction by rolling friction, and conceptually
inventing the roller conveyor, the first conveyor as such was designed by
Archimedes (287 to 212 BC)? A Greek mathematician, physicist and inventor
for removing water from the hold of a ship built for King Hiero of
Syracuse. Since this idea apparently worked and the same device was next
used to raise water from a river to irrigate farm land.
The Archimedean
conveyor was of the internal helical screw type, It was mounted at an
angle with its lower end in the water and the upper end arranged to
discharge the water to a flume or irrigation ditch. The device was powered
by a slave who turned a crank fixed to its upper end. Even today a similar
machine is said to be used in the Netherlands except for the substitution
of electrical power for muscle, power. In today’s industry, the
Archimedean screw exists in the form of a tubular conveyor, to the inner
surface of which is fastened a helical ribbon. The exterior of the tube is
supported on roils, and the tube is revolved by a pinion meshing with an
externally mounted ring gear.
It is said that Archimedes may have been
the originator of two other forms of screw conveyors. One, a tube formed
into a helix around a central shaft of core, is not now seen in use; the
other, a helix rotating within a stationary casing is the fore. runner of
the modern screw conveyor in its most common form. A little before 1790,
an American Mentor, John Fitch, designed a steam boat to be propelled by a
section of screw conveyor flighting that appears in the drawings of that
day to be almost identical to flighting used in present day screw
conveyors. It appears, though, that this method of ship propulsion was at
once a victim of technological obsolescence brought on by the success of
paddle wheels. The term, “screw” still lives on as the usual terminology
for a ship’s propeller. During the many centuries of individual or small
group self?sufficiency following the days of Archimedes, there was little
need for continuous mechanical handling devices because there was little
need for volume production, and even if there Mad been there was no
satisfactory source of power available. It was about 1900 years later that
screw conveyors again were proposed, when it became imperative that some
means be found to handle mechanically the grain harvests made necessary to
serve the needs of the rapidly growing American population. In 1783, the
man who might be called the patron saint of mechanized materials handling,
Oliver Evans laid out on paper his first mechanized flour mill which
incorporated not only screw conveyors but bucket elevators and belt
conveyors as well. AI! these devices were tied together by a system of
wooden toothed gears, wooden pulley and leather belts and all were driven
from a single water wheel.
The first mill built by Evans in 1785
actually was a reconstruction of a 1742 mill thought by some to have been
built by his grandfather. The screw conveyors first designed by Evans
consisted of a round wooden core on which were mounted in helical form a
series of wooden plows or flattened wooden pegs. The whole screw assembly
revolved in a wooden trough or “box” as it was called then. Appropriate
sliding gates in the trough bottom could be opened to deliver grain to the
mills as needed. Soon, though, Evans improved on his design by making the
screws of helically formed sheet metal sections mounted on a wooden core
that might be anywhere from five to twenty feet long. He still maintained
his trough of “close fitting” boards.
In Rock Creek Park, Washington,
D.C., visitors may inspect a restored mill of the Oliver Evans era. The
Pierce Mill was built around 1820 (the exact year is open to argument) by
one Isaac Pierce and his son, Abner. The mill is in running order and has
all of the types of conveyors that F?vans used including screw conveyors
with wooden flights on wooden cores an which wrought iron journals were
pressed.
During this period the country grain elevator evolved of
necessity to handle what then was thought to be vast volumes of grain
needed by the growing and hungry population. Conveyors of the types Evans
used in his “automatic” flour mills were ready made for grain elevator
service. The technology of mechanization was keeping pace with the demands
of the spreading population.
The metal screw conveyor flights were
originally of the sectional flight variety, formed from flat sheets cut in
circular form with a hole in the centre then split on one side and the two
edges pulled apart to form one flight section of a screw. Successive
flights were then joined by riveting, shingle fashion, to make a
continuous helix of whatever length was called for. At some unknown date,
the wooden core was replaced by an iron pipe when the proper sizes of such
pipes became available.
The next technological advancement of
importance in screw conveyor design was patented March 29, 1B98 by Frank
C. Caldwell under patent number 601429. This was a continuous, one piece
screw flight formed by rolling a continuous strip of steel into a helix.
This construction is now known as the “helicoid” flight, and simplified
manufacture and assembly by eliminating the joints in the sectional flight
screws. Both types of screws are still produced.
Early screw conveyors
used wooden bearings and there are still applications where such bearings
are specified. Cast iron support hangers for the bearings and cast iron
trough ends came along with the all?metal screws. The first use of metal
in a trough probably was a sheet metal box liner curved to follow the
periphery of the screw, and fastened in the wooden “box” or
trough.
Since the screw conveyor came into general use a little over a
century ago for moving grains, fine coal and other bulk material of the
times, it has come to occupy a unique place in a growing area in the
general field of materials handling and processing. Many refinements in
design materials and methods have come into general use. Welding has
supplanted rivets to provide smooth conveying surfaces along with greater
strength and rigidity in screws and troughs? Ball bearings for hangers
have become less bulky so they now occupy little more space than did the
older plain sleeve bearings. Such bearings in the box or trough ends
provide improved thrust capacity. Improved methods of sealing to keep out
foreign materials and to retain lubricants have greatly expanded the use
of anti?friction bearings in screw conveyors Enclosed drive speed
reduction units in place of open gearing greatly reduces hazards to
workmen and reduces maintenance work largely to a matter of periodic
inspection. The screw conveyor engineer has tremendous latitude in the
selection of materials to best meet the operating conditions of a
particular conveying job when it falls outside the broad capabilities of
standard screws made of ordinary steel.
Whole new families of bulk
products are being handled as a matter of course today that
were not
even thought of just a few years ago, and the advance of technology Is
such that additional new products are being discovered and developed
almost daily for industrial and agricultural use. Many such products are
toxic to human beings, or are toxic at certain stages of their processing.
Others are merely Irritating or unpleasant to work around. Screw conveyors
often are the answer to handling these products. Highly developed seals
and methods of using them help to confine the products conveyed along with
any dust, gas or fumes within the trough and out of contact with anyone in
the area. They also help to protect material ale from contamination by
foreign matter.
The versatile screw conveyor is no longer limited to
transporting materials on the horizontal. Standard designs with auxiliary
equipment are available for operating up or down a slope or for the
vertical elevating of some materials.
While screw conveyors are about
the. oldest form of conveyor known today, they are still among the most
useful of mechanical handling devices, a fact attested to in the past
twenty years by a seven?fold growth in the rate of annual manufacture. In
addition to the movement of bulk materials, screw conveyors are now
forming an integral part of many production processes where mixing,
blending, heating, cooling, dewatering, drying or similar operations must
be performed in transit. Thus, one of the oldest forms of conveyor on
record has become an important factor in Space Age industrial
technology.
APPLICATION OF SCREW CONVEYORS
Screw conveyors are bulk material transporting devices
capable of handling a great variety of materials which have relatively
good flowablility. Flowability is de fined in the CEMA Material
Classification Standard and denotes the degree of freedom of individual
material particles to move past each other. This characteristic is
important in screw conveyor operation as the screw helix, mounted on a
central pipe or shaft, rotates within a fixed trough or tube, pushes the
material along the bottom and sides, shearing the material in the radial
clearance between the helix and trough and causinq the material to tumble
upon itself as the moving face of the helical flight lends to lift the
material. The various applications of screw conveyors proceed naturally
from two factors: the characteristics of the material to be conveyed and
the operating advantages peculiar to this type of conveyor.
Among the
many advantages of screw conveyors is the feasibility of numerous feed and
discharge openings. each easily provided with a regulating gate? This
facility lends itself to the use of screw conveyors to receive and
distribute bulk materials for in plant material storage in such a manner
that different grades or different kinds of materials may be conveyed to
or from the proper storage bins. Screw conveyors likewise may be used for
unloading materials from cars, bins or piles often to initiate a material
process, Typical applications are grain storage plants, feed mills cereal
processing plants and chemical plants;
Screw conveyors are very
adaptable to the volume control of materials from the bottoms of bins,
hoppers, bag dumps, storage piles and the like. In this use they are
termed screw feeders and as such fill a most important place in industry.
Not only is the control of volume necessary for the proper orientation of
succeeding conveyors of any type, but also for the operation of processing
units such as dryers hammer mills oil expellers and countless other pieces
of processing machinery.
It is rather simple to arrange screw conveyors
for the heating or cooling of material in transit. The conveyor trough is
provided with a jacket through which the beating or cooling medium is
circulated to obtain the required degree of heat transfer. Another use is
the blending of several ingredients to make a finished product, or to make
a premix for some product. This may involve the blending of different
grades of the same material or the making of a blended mixture of
different materials. The conveyor screw can be so fashioned that materials
are well mixed or blended while in transit.
In the handling of some
toxic materials screw conveyors lend themselves very well because the
enclosing trough can be made tight enough to contain toxic dusts or
vapours, thus reducing personnel hazards. Conversely, materials that must
be kept free from contaminants may be satisfactorily handled. In some
processing operations, the fact that the material load in the conveyor
acts as a “nut” and the helical screw permits screw conveyors,
particularly the tubular types, to he used as air or vapour lock devices.
The conveyed solids enter and leave but the passage of gases or vapours
are restricted.
Screw conveyors may be operated horizontally, on an
incline or vertically. Frequently, inclined conveyors simplify a conveying
system because they can do in one conveyor assembly what otherwise would
require a more elaborate combination of horizontal and vertical units.
Several types of vertical Screw conveyors are available. These generally
have tubular troughs in which the screws operate at appreciably higher
speeds than in horizontal units. |
