Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to a sieve screen deck and
more particularly to such a sieve screen deck having a self
supporting sieve screen with side walls which sieve screen
may be vibrated over substantially its whole surface.
Screen decks are used to separate liquids from a wide
range of solid particulate matter and to separate fines from
flowable streams of particulate matter. A slurry or a
mixture of particles is flowed onto the upper end of the
screen deck. As the mixture flows down a sieve screen in the
deck the liquid or fines fall through the apertures in the
sieve screen. In conventional screen decks the sieve screen
is rigidly connected to a frame around its full periphery.
In a planar screen this is usually done by clamping the sieve
screen around its edges to the screen while in the case of a
curved sieve screen, i.e. a sieve bend, the curved edges are
supported and stiffly held in place by radius bows which are
generally wedged in place or fimrly located by springs.
It has been found in certain situations that the sieving
action of such a screen deck may be enhanced by vibrating the
screen or by rapping the screen i.e. by applying repeated
impacts to one face of the screen or to the screen frame.
Rapping is normally done by connecting a rapping bar to the
underside of the sieve screen and positioning on the frame,
or on the ground beneath the frame a mechanical impacting
means which imparts blows in rapid succession to the rapping
bar or by vibrating the complete screen and holding frame.
The application of the rapping or vibrating force to a curved
sieve screen which is rigidly held around its sides to a
frame results in only a small part o~ the sieve screen
~0 actually moving.
It has been known to vibrate sieve screens formea of a
fabric which is not self supporting. The fabric is held in a
state of tension by being rigidly connected to a surrounding
frame at its ends but not its sides. In this arrangement the
application of a vibrational force to sieve screens of this
type has not prevented blinding of the screen with fine
particulate matter. It is believed this deficiency arises
due to the nature of the apertures in the woven fabric sieve
and due to the poor transmission of the vibrational forces
throughout the screen due to its inherently non-rigid, i.e.
non self supporting, nature.
The present invention consists in a sieve screen deck,
comprising a self supporting sieve screen having two opposed
end edges, rapping or vibrating means to cause the sieve
screen to vibrate, and a frame to which the sieve screen is
connected, the sieve screen including sieving means defining
a plurality of substantially parallel narrow slots and an
array of backing bars connected to the sieving means which
render the sieve screen self supporting, the sieve screen
being connected to the frame only at or adjacent its two end
edges and at least one of the connections being through a
resilient bush or like means allowing limited vibrational
movement between the sieve screen and the frames, a side wall
being provided substantially along the full length on each
side of the sieve screen between its end edges so as to
prevent material which is passing across the sieve screen
between the two end edges from flowinq over an edge of the
sieve screen intermediate the said end edges, each side wall
being formed of a resilient synthetic plastics material or
rubber such that they do not impede substantially the ability
of the sieve screen to vibrate and flex under the influence
of the rapping or vibrating means.
Side walls to prevent material falling over the side
edges of the sieve screen ma~ be formed of a suitable
synthetic plastics material or the like and connected to the
side edges of the sieve screen. The plastics material should
have sufficient resilience that the vibration o the sieve
screen is not substantially impeded. In a preferred
embodiment of the invention the side walls are cast in-situ
on the side edges of the sieve screen. In other embodiments
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they may be clipped on, screwed on or attached in any other
suitable manner~ It has been found suitable to form the
side walls of a polyurethane material which may be cast in-
situ or formed separately and attached.
It is believed that the provision of flexible cast
in-situ side walls on the self-supporting but flexible
sieve screen prevents material f~owing over the edges of
the screen without impariny the ability of the screen to
vibrate and flex. It is believed that the freedom of the
sieve screen to not only vibrate relative to its supporting
frame but to also flex makes an important contribution to
the efficiency of the sieve screen.
m e sieve screen is preferably formed of a parallel
array of wed~e wires lying at right angles to ~he slope of
the screen. The wedge wires are preferably connected to an
array of backing parts running longitudinally of the screen
parallel to its fall line. A rapping or vibrating means is
preferably connected to the backing bars parallel to the
wedge wires. Ihe screen may be planar or may curve along
its length to form a sieve bend.
In an alternative embodiment the sieve screen comprises
a sheet of metal into which an array of slots has been
etched or otherwise formed~ In this embodiment the sheet
may be inherently self supporting or it may be supported
by backing bars as described above~
The sieve screen is preferably formed with slots having
a width of from 10 microns to 4mm, most preferably the slots
have a width of from 100 microns to lmm.
The frame of the sieve screen deck is of conventional
construction subject to the fact that the screen is only
connected along its end edges to the frame. The frame may
be welded up from steel or formed in any other suitable
manner. In one embodiment of the invention the frame may
completely surround the sieve screen, however, in other
embodiments a plurality of sieve screens may be disposed in
A
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side by side array with a single frame supporting all of them
in which case each sieve screen is not individual]y
surrounded by the frames.
The sieve screen is connected at or adjacent its end
edges to the frame. One or both of the ends of the sieve
screen are connected to the frame through resilient bushes or
like means which allow a limited amount of vibrational
movement between the sieve screen and the frame. In a
typical situation the bushes might allow a maximum movement
of the sieve screen relative to the frames of 2 or 3mm, as
compared with a maximum ~mplitude of vibration of the sieve
screen of 5mm. If desired one end of the sieve screen may be
rigidly connected to the frame. If the sieve screen is
curved it may be desirable to provide adjustable co~nection
means on the frame to attach the sieve screen to the frame.
This is due to the fact that the exact curvature varies
slightly between screens of the same nominal dimensions.
In one embodiment of the invention the impact means may
be mounted directly onto a rapping bar and be pivotally
connected thereto. This arrangement allows greater control
to be effected over the vibrations of the sieve screen which
can substantially alter the sieving action of the sieve
screen deck. The impact means preferably comprises a double
acting pneumatic ram having a lug at one end which may be
bolted to a corresponding double clevis on the rapping bar.
The angle of impact of the piston within the pneumatic ram
may be varied by loosening the bolt connecting the ram to the
rapping bar and rotating the pneumatic ram about the axis of
the bolt. If desired the magnitude of the rapping Eorce may
3n be varied by adding weights to the free end of the piston rod
projecting from the pneumatic ram. Wh~le a ram as described
is the preferred method of applying a rapping force to the
sieve screens according to this invention other mechanisms
such as electric motors carrying eccentric ~eights or
solenoid rappers could be used~
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In another embodiment of the invention vibration of the
sieve .screen is brought about by the oscillation of a
vibration bar connected to the underside of the sieve screen
across its free width intermediate its ends. The vibration
bar i5 pivotally connected to one end of each one of an array
of fingers which are rigidly connected at their other end to,
and extend radially from, a rod which is caused to
reciprocate about its longitudinal axis by the action of a
motor rotating an eccentric which actuates a further arm
extending radially from the rod.
~ ereinafter given by way of example only is a preferred
embodiment of the present invention described with reference
to the accompanying drawing in which:~
Fig. 1 is a side elevational view of a sieve screen deck
according to this invention; and
Fig. 2 is a rear elevational view of the sieve screen
deck of Fig. 1.
As seen in Yigs. 1 and 2 the sieve screen deck 100
comprises a curved sieve screen 101 mounted in a frame 102
~0 and a vibrating means 103.
The frame 102 includes a distribution box 104 on its
rear side, which serves to distribute an incoming slurry
entering the sieve screen deck 100 through inlet 105 evenly
across the width of the sieve screen 101, a pair of side
frame members 106 and transverse frame members 107. The
sieve screen 101 is formed of a large number of horizontal
~edge bars connected to an array of backing bars 110. The
screen 101 is bolted at its upper end to an upper transverse
frame member 107 while at its lower end the screen is
connected to a lower transverse frame member 107 through
rubber bushes 108.
The sieve screen 101 is formed on either side with a
flexible side wall 109 which is cast in-situ on the sieve
screen 101 from a flexible polyurethane resin. These side
walls serve to prevent material being passed across the sieve
~8~
screen 101 from flowing off the sides thereof intermediate
its ends.
A vibration bar 111 is connected to the backing bars
110 across the full width of the sieve screen 101. A
plurality of U-shaped saddles 112 are connected to the rear
surface of the vibration bar 111. Each saddle 112 is
pivotally connected to the face end of one of a plurality
of fingers 113 which are welded to extend radially from a rod
114. The rod 114 extends transversely across the sieve
screen deck 100 and is supported at either end by a bearing
115 such that it may rotationally oscillate about its
longitudinal axis. A further arm 116 extends radially from
rod 115 and at its free end has a bush which surrounds an
eccentric cam (not shown) driven by motor 117 which is
mounted on frame 102.
In use a slurry or like feedstock is fed into the
distribution bar 104 through inlet 105 and flows evenly
onto sieve screen 101~ As the slurry flows down the sieve
screen 101 the screen 101 is caused to vibrate due to the
motor driven eccentric causing the rod 114 to rotationally
oscillate which in turn causes the vibration bar 111 to
vibrate. It has been found that the vibrations applied to
the sieve screen 101 are transmitted throughout the screen
due to the manner in which it is mounted and due to its
semi-rigid, self-supporting nature. The sieve screen deck
100 has been found to be substantially less susceptible
to blinding than any of the conventional sieve screen decks
described in the preamble to this specification.
A
