Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: A SILO UNLOADER
DESCRIPTION
The invention relates to storage silos and to
bottom unloaders ~or such storage silos.
Many bottom unloading devices for silos are known.
For free flowing materials like dry grain, it is usually
sufficient to open an outlet at the bottom o~ the silo
and let grain flo~ out. The size of the outlet must
however be sufficient to prevent the grain from forming a
stable arch or dome over the outlet.
To enable a silo filled with granular material to
empty under gravity it is usual to provide the silo with
a hopper at its base with sides sloped at between ~5 and
45 to the horizontal. Recently steep sided hoppers with
a slope of 70 to 80 to the horizontal have been used
to ensure "mass flow" conditions for the discharge o~
powders and the like.
There are some materials which, because they are
fibrous and/or cohesive, show a marked reluctance to
flow. These include silage, sawdust, seaweed, and soya
meal. They can form a stable arch or dome over distances
of several feet and occasionally across the whole
diameter of a silo, although after a while the arch or
dome usually collapses.
Known methods of bottom unloading non free-flowing
materials are based on altering the properties of the
material, e.g. fluidisation of cement or flour with air,
or the use of an agitator or cutter to break down the
stable arch and restart flow7 e.g. vibrators, augers
and chain cutters, or the use of a steep sided mass flow
hopper.
The majority of bottom unloaders for silage and for
the most intractable industrial materials have a central
discharge outlet in the silo base fed by a centrally
mounted sweep arm chain cutter or auger which undermines
the material in the base of the silo and conveys the
material to the discharge outlet. There are some in
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which a centrally mounted rotating flail forms a hopper
in the material and undermines the arch, A disadvantage
of existing radial auger or cutter unloaders is that
they are relatively inaccessible for maintenance and
repair.
When a silo is unloaded the distribution of
pressures in the silo changes. The vertical friction
wall load increases and the distribution of the la$eral
pressure~ changes with the formation of high local
pressures where mass flo~s starts to converge and where
the thrust from the arching of the dome reacts onto the
wall.
The prediction of these pressures is not easy as
their magnitude is very sensitive to the pattern of
material flow in the silo and changes in that pattern due
to varying material properties. However for central
filling and unloading, e.g. so-called clock-hand type
unloaders, reasonable estimates can be made for design.
~Then +he unloading is non-concentric as is the case with
a side-mounted sweep arm unloader, the loads in the silo
are not evenly distributed so that the wall of the silo
tends to be forced out of round. The patterns of flow
in the silo then become more complex and the development
of stable arches harder to predict. As a result, the silo
needs to be strongly constructed if it is not to be
dama~ed in use.
We consider that an important feature in the design
of an efficient bottom unloader is to arrange for the
formation of a marginally stable arch or dome across
most of the base area of the silo, so that the prime role
of the unloader is to collect the loose material that
falls from the underside of the dome, rather than the
Gutting of the compacted column of material. The end
of the unloader should go only sufficiently close to
the wall for it to undermine the edge of the arch or
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dome just enough to allow more material to drop
therefrom. In such an arrangement only the tip of the
unloader is cuttlng the dense material and the power
requirement is minimised as most of the energy requlred
to break up the mass of material comes from its own
weight as it descends in the silo. The gradual descent
of the mass ensures that with each pass of the cutter
new material has settled into position to be cut.
The ideal situation outlined above is not easy to
achieve with currently av~ilable unloaders which have no
simple method of adjustment. If the cutter length is too
long the whole dome collapses and the cutter has to work
in dense material. If the unloader arm is too short
the output rate falls æince the material can form a
stable dome or arch.
It is an object of the invention to provide an
arrangement in which the problems outlined above are at
least mitigated.
From one aspect the present invention provides the
combination of a storage silo al~d a bottom unloader
therefor wherein the base of the silo is formed as a
hopper having a substantially central discharge outlet
the unloader being arranged to project into the silo
through the discharge outlet and comprising a sweep arm
piYoted $or rotation in the hopper about a vertical
axis disposed centrally of the discharge outlet and for
vertical movement away from the surface of the hopper.
From another aspect the invention provides the
combination of a storage silo and a bottom unloader
therefor wherein the base of the silo is formed as a
hopper having a substantially central discharge outlet,
the unloader comprising a sweep arm arranged to project
into the silo through the discharge outlet and means
for varying the effective length of the arm comprising
a mounting supporting the arm for pivotal movement about
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a vertical axis disposed centrally of the discharge
outlet, the mounting being arranged such that the length
of the arm projecting into the silo can be altered.
Pre~erably the invention provides the combination
of a storage silo and bottom unloader therefor wherein
the base of the silo is formed as a hopper having a
substantially central discharge outlet the unloader
being arranged to pro;ect into the silo through the
discharge outlet and comprising a sweep arm pivoted for
rotation in the hopper about a vertical axis disposed
centrally of the discharge outlet and for vertical
movement away from the surface of the hopper and means
for varying the effective length of the arm comprising
a mounting supporting the arm for pivotal movement about
the centrally disposed vertical axis the mounting
arrangement being such that the length of the arm
projecting into the silo can be altered. Pre~erably
the hopper extends over æubstantially the whole of the
base of the silo and advanta~eously is conical in shape.
The angle of the cone of the hopper will be chosen to
suit the particular material to be stored but because
of the assistance provided by the unloader can be
substantially less than would be expected of a
conventional hopper which must have a sufficiently steep
angle to self empty. An important feature of the hopper
is that there must be su~ficient slope for the sweep arm
to be withdrawn through the central discharge outlet.
The term 'hopper' as used in the specification and claims
is to be construed accordingly.
Normally the unloader is mounted in a chamber below
the silo. Means may be provided in the chamber for
rotating the arm and for moving it vertically.
Preferably the vertical movement is an angular movement
about a central pivot.
The sloping of the floor of the silo to form a
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hopper with a central discharge outlet permits the
unloader to be introduced and withdrawn into the silo
through the discharge outlet thus rendering the
unloader accessible for maintenance and r~pair. Such an
arrangement also permits adjustment of the effective
length o~ the unloader so as to permit unloading with
relatively low power consumption. The hoppsr bottom of
the silo is a basically strong structural form whether
it is of steel restrained by a ring beam or o~
reinforced concrete or composite double wedge construction.
The central unloading of the silo together with a
concentric undercutting of material within the silo
renders more predictable the loads on the structure of
the silo making the silo design cheaper and simpler.
With the wetter longer chopped silages it is
possible for the material to form a stable arch or
dome across the whole silo diameter. When the cutter
length is at its maximum the edge material supporting
the arch or dome becomes progressively denser giving
extremely high local lateral loads. To avoid such high
lateral loads unloading is commenced with the arm at its
minimum length and the unloader arm is progressively
lengthened to leave a thin 'wall' o~ silage adjacent to
the silo wall and is then angled up into the underside
of the arch or dome until the band becomes unstable
and slowly collapses. The provision of a ~illet in the
form of a steeply sloping section between the hopper
bottom and the vertical silo wall may assist the process.
While the unloader configuration may allow the
effective length of the arm to be adjusted continuously
it may be simpler and more robust to provide a limited
range of settings.
The sweep arm may be mounted on a floor stand in a
chamber below the silo, independent o~ the hopper, and
incorporating the motor drive etc. Alternatlvely the
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sweep arm may be mounted on a floor stand in the chamber
but with a hopper reinforcing ring being used to steady
and drive the cutter assembly. Alternatively again the
sweep arm may be suspended from the hopper.
The sweep arm may comprise one or more vibrating
or reciprocating bars; this i9 simple but is only really
suitable for materials with hesitant flow properties
which only require small energy inputs to keep them on
the move. However vibration enables shallow hoppers to
self empty. The vibrating or reciprocating bars may also
be arranged to aerate the material in the silo.
Alternatively the sweep arm may be an auger. This is
simpler than a chain cutter. The size and variation of
pitch can be adjusted to suit the material to be stored.
An auger can work with well chopped silage and other
fibrous materials but it is best with granular/cohesive
materials. Alternatively the sweep arm can be a chain
cutter since such wi]l handle any material, th~ugh with
silage it prefeIs it chopped. The sweep arm may either
bc a Il~ee c~ntile~er unsupported on the hopper flo~r or
may be partly skid-supported on the hopper sur~ace.
Steel hoppers are strong and light but require a
strong ring beam especially if they have a shallow
angle. Reinorced concrete hoppers can either be fairly
massive but li~htly reinforced or of more heavily
reinforced twin we~ge configuration. Composite hoppcrs
of fabricated steel which can be assembled on site and
then filled with concrete may be attracti~e since this
arrangement would avoid the need for complex concrete
formwork.
W'ith materials which can on occasions flow freely,
a large central outlet in the hopper could produce
uncontrolled flow. This can be obviated either by fitting
a cone on the unloader above the outlet or by controlling
the discharge below the unloader. With this type of
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material the unloader could be used when free flow
stopped. However continuous use of the unloader would
give more controlled discharge and would prevent
"rat-hollng". Powders can 'flood' lf they become
fluidised and for these control below the unloader would
be necessary.
The invention is diagrammatically illustrated by
way of example in the accompanying drawings in which:-
Figure 1 is a cross-sectional side elevation ~hrough
the base of a hopper bottomed storage silo equipped with
a centrally mounted sweep arm unloader;
Figure 2 is a scrap section on the line X - X of
~igure l;
~ igure 3 is a partly sectioned perspective view of
a storage silo;
Figures 4a to 4c are scrap sectional side views of
the base of the silo of Figure 1 showing the unloader in
dif-ferent positions of operation; and
Figures 5a to 5e show five alternative forms of the
~0 base of the storage silo.
In the drawlngs Yigures 1 to 3 show diagrammatically
a cylindrical storage silo 1 formed at its base with a
conical hopper 2 having a central discharge outlet 3. A
chamber 4 through which the contents of the silo can be
removed is disposed below the hopper 2. An unloader 5
is disposed centrally of the discharge outlet 3 on a
mounting 11 on the floor 19 of the chamber 4. The
unloader comprises a body 7 pivotally mounted on the
stationary support 11, the body carrying at its upper
end a sweep arm conveyor 6 which projects into the silo
through the discharge outlet. The sweep arm 6 is
mounted on the body 7 by means of a forked member 15
which is clamped to its lower surface near its rear end
and which straddles and is pivoted to a $ongue-like
upward extension of the body 7 by means of a pin 14.
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The sweep arm 6 thus projects into the silo through the
discharge outlet 3 and is arranged to lie generally
parallel to the floor of the hopper 2 so that it can be
rotated in the hopper to remove material from the silo
through the discharge outlet. The sweep arm conveyor
is driven in rotation by means of a motor lO (e.g. an
hydraulic motor) mounted on the stand 11 which drives a
gear 9 which in turn engages a ring gear 8 fixed to the
body 7. In the present embodiment the sweep arm 6 has
an endless chain 16 trained around its periphery which
chain carries cutters 17 which cut and remove material
from the silo, the chain 16 being driven in rotation by
means of a motor 12 (e.g. an hydraulic motor) mounted
on the rearward end of the arm 6. Bolts 18 by means of
which the forked member 15 is clamped to the arm 6
permit the length of the arm 6 projecting into the silo
through the discharge outlet to be altered as desired.
Also the pl~otal mounting o~ the s~eep arm on the body 7
by mealls o:E pin 14 permits the sweep arm to be moved in
a generally vertical plane away from the floor of the
hopper, the inclination of the sweep arm being controlled
by means of a screw jack 13.
As shown in Figure 4 of the drawings the length of
the sweep arm projecting into the silo may be adjusted
as desired to minimise the power requirements in removing
dense materials such as silage from the silo. In this
connection Figure 1 of the drawings illustrates a
situation in which the silo contains a mass of silage 22
which will weigh many tonnes and can thus tend to trap
the sweep arm unloader. It will be seen that the wall
of the silo at the junction betu~een the hopper and the
silo proper is inclined upwardly at a steep angle to
form a fillet 21 to facilitate the creation o~ a semi-
stable dome or arch of compacted silage over the sweep
arm which can be made to collapse in a controlled manner
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by operation o~ the sweep arm so that the power
requirement in operating the unloader is minimised.
Reference 24 indicates an area of loose silage below
the dome of compacted silage 23. Initially the sweep
arm is operated partly retracted and as more material is
removed from the silo the arm is gradually lengthened
until it approaches the arch creating fillet 21 where
the silage can be expected to be at maximum density. If
the dome or arch of silage fails to collapse into the
area 24 where it can readily be removed from the silo the
arm can be pivoted upwardly as indicated in Figure 4c to
cause the collapse. To prevent damage to the arm which
may be caused by the sudden collapse of the arch when
the arm is raised away from the floor of the hopper by
means of the screw jack 13 we prefer that the arm is
resiliently biased upwardly so that it is free to pivot
downwardly into contact with the floor of the hopper
under the weight of silage collapsing on the arm. As
shown in Figure 4a of the drawings the adjustment of the
working lengtll o the cutter arm 6 may be effected by way
of a screw 25 instead of by means of the clamp shown in
Figures 1 and 2, The material in the silo is conveyed by
the cutters 17 on the chain 16 to the central discharge
outlet 3 where it drops onto a conveyor 20 for removal
from the silo.
Referring to Fi~ure 5a of the drawings there is
shown a silo having a steeply inclined hopper defining a
central discharge outlet and from which hopper the
unloader 6 is suspended. The extremity of the hopper
carries an outwardly extending annular flange 28 which
forms a track for a carriage 26 which is supported for
rotation on wheels 27 on the track 28. The carriage 26
supports the unloader 6 ~or lengthwise adjustment and for
pivotal movement away from the surface of the hopper in
*he manner described în Figures 1 to 3. Rotation of the
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unloader may be achieved by a circular rack and pinion
device.
In Figure 5b of the drawings there is shown
an arrangement of the kind described with reference to
Figures 1 to 3 with the exception that the hopper is of
a more pronounced slope.
With reference to Figure 5c of the drawings there
is shown an arrangement o~ the kind shown with reference
to Figures 1 to 3 of the drawings except that the
unloader comprises a screw auger instead of a cutter
chain.
In Figure 5_ of the drawings there is shown an
arrangement of the kind generally described above except
that the transitional area or fillet 21 between the
vertical wall 1 and the hopper 2 is more pronounced than
that described above.
In Figure Se oi~ the drawings there is shown an
arrangement generally simi]ar to that of Figure 4a but
modified for use with a relatively shallow hopper. In
this èmbodiment the track 28 is formed by the lower
flange of an annular channel member 29 surrounding the
discharge outlet 3 o~ the hopper.
It will be appreciated that in each of the
embodiments described above the sweep arm of the unloader
can be removed from the silo through the dischnrge outlet
for maintenance and repair and can in simple manner be
extended or retracted to suit the particular operating
conditions prevailing within the silo and can in simple
manner be moved vertically should the operating conditions
demand it.
