Note: Descriptions are shown in the official language in which they were submitted.
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WEAR RESISTANT SCREEN, SCREEN PANEL OR THE LIKE
FIELD OF THE INVENTION
This invention relates to a wear resistant screen, screen panel, or the like,
which,
for use, are usually attached to a supporting steel framework or other
structure,
such as a vibrating frame, and which are replaceable as and when the need
arises
in consequence of wear having taken place with resultant deleterious effect on
apertures through the screen.
In this specification, for convenience, the term "screen panel" will be used
hereinafter, but it is to be understood that this term extends to a screen
panel which
constitutes the entire screen area of a screen; one which forms a part of the
screen
area, usually together with others in a tiled configuration on a support
frame; and
also includes a screen element which may only define one aperture through the
screen, with such screen elements also being arranged in a tiled configuration
in
plan view.
BACKGROUND TO THE INVENTION
Screen panels are widely used for the purpose of grading and sorting mined
ores,
minerals or other natural products which have been sub-divided in order to
separate such materials into two fractions.
Screens used for this purpose often comprise one or more screen panels
supported, interchangeably, on a rigid support structure such as a frame. The
entire panel, which may optionally be reinforced, is commonly cast or moulded
from
polyurethane material which has desirable wear resistant and somewhat
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elastomeric properties which are particularly well suited to a screening
application.
Examples of such screening panels are shown in South African Patent 73/5815 of
Screenex/Freissle and Canadian Patent 1 323 602 October 26 1989 B07B-001/46
who claim a screen panel manufactured of polyurethane and reinforced with
steel.
However, these panels are difficult to remove and invariably deform the panel
on the
removal thereof. The support structure generally made of steel is damaged by
the
fine grit which makes its way between the panels and comes in contact with the
steel
structure. Drawbacks of U.S. Patent 5,829,599 Woodgate 11/1998 is dependent on
a clip bar which holds the panel in place when the bar is forced down into the
clip rail.
Each clip rail has side formations opposite and equal side formations along
the length
of the screen panel.
This is cumbersome and tends to creep forward in the direction of the
amplitude of
the screening machine. The clip bar appreciably reduces the screening open
area
and the clip channel is held by a steel bolt embedded into the channel and
fastened
through an aperture of the screening support structure by means of a nut,
which can
become loose through the vibration of the machine.
It is also extremely difficult to access the bolt if such a loosening should
occur.
The further drawback of this system is that there is no apparent locking
mechanism
which prevents the panel from being forced in the direction of the flow of the
material
being screened (Known in the industry as bunching).
U.S. 5,377,846 Askew 01/1995 - This design has a drawback in that the side
holding
down bar is dependent on wedges which can become dislodged due to the
movement of the screening machine. Secondly, the clip rail retainer is
fastened down
by means of a bolt which is fastened to the main steel frame. While making use
of
the entire section of the steel frame, it also reduces the open area of the
screen.
This structure also makes use of steel reinforcing bars embedded in the
screening
element, making it heavy and therefore inefficient.
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U.S. 5,755,334 Wojcik et al 05/1998 - This design is similar to Woodgate
5,829,599
mentioned above. It is dependent on a clip bar holding down the screen panels.
This system is inefficient due to the clip rail arrangement taking up a large
portion of
the screening surface. It is also clear that this screening system is re-
inforced with
steel making it heavy and expensive to manufacture and also not
environmentally
friendly because of the steel that is lost when it is replaced. It is also
obvious that the
screening surface constitutes a small percentage of the entire screen panel.
The screen panels are costly and are regarded as disposable items. Such costs
are
a significant part in running a screening installation. Also it is to be noted
that
polyurethane materials cannot practically be recycled and only a relatively
small
amount of the material forming a panel is usefully sacrificed. An example of
this
screening element is shown in U.S. patent 3,980,555 Freissle of Screenex of
Sept.
14 1976. The drawback of this patent is that on extracting for the purpose of
replacing or rotating, these panels are extremely difficult to remove and the
securing
pins fitted to the panel often break off, rendering it useless for re-
inserting. The
further drawback of this type of panel is that it is reinforced with steel and
this steel
is practically non re-claimable and invariably discarded and goes to
landfills.
Unfortunately the entire panel was paid for by the end user. The extremely
wide
periphery of this screening element reduces the desired open area and
efficiency of
the screen panel.
Further cost of extensive damage of the rigid steel support frames are often
overlooked and in a relatively short time these frames are destroyed beyond
repair
and have to be replaced. These costs are of great concern to the end user. An
example of such a screen panel is patented by John Alexander Deschamps South
African Patent 75/2483 is a polyurethane modular panel which is dependent on a
steel clip rail running in the entire length of the steel support frame. The
drawback
of this particular system is that the grit passes between the panels and gets
lodged
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in the clip rail and eventually forces to panel to become dislodged.
Another example of a screening panel patented in South Africa 77/0386 and
Germany P2622 709.8 on 21 May 1976 by Kurt Wolff of Steinhaus and in Canada,
Patents 1 157 426 of Nov 1983 B07B-1/46 of Wolff and Canadian Patent 1 148 503
dated June 1983 B07B -1/46 of Wolff. This screening panel has a drawback of
securing pins which are forced between the screen panels into the steel
support
structures. These pins are extremely difficult to remove after the screen has
been
in operation since the grit around the pin must be removed by chipping before
the pin
can be removed.
Another type of screen panel which is commonly used in various industries is a
unit
known as a side tensionable screen panel. These panels are usually fitted with
hooks running parallel with the flow of the machine. The hooks are parallel
with
interlocking hooks from the side of the screening machine and are tensioned
over an
arc across the width of the machine. This type of polyurethane panel is
extremely
expensive and the scope of interchanging panels is limited and time consuming
and
the amount of surface material being discarded by far outweighs the material
being
worn away by the ore being screened.
Such a tension screen is mentioned in U.S. patent No 3,684,091 (Albert Wehner)
of
15 August 1972. The drawbacks are that the entire screen panel structure
extends
across the entire screening machine making it extremely expensive, since by
material
passing over a screening panel, the wear surface never wears out evenly and it
is
possible that a large screen panel needs replacing when only a small portion
is worn
out.
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OBJECT OF THE INVENTION
It is the object of this invention to provide a screen panel which can be
produced
appreciably less expensively than existing polyurethane screen panels and
which will
5 provide a decrease in cost of screen panels when related to the quantity of
ore
processed thereby.
SUMMARY OF THE INVENTION
In accordance with this invention there is provided a screen panel comprising
a
moulded or fabricated base made of structural plastics material and defining
at least
a basic frame for a screen panel having apertures there through and wherein
the
base supports, permanently or removably secured thereto, a body of wear
resistant
elastomeric material defining, at least in the unworn condition, the entire
operative
surface of the screen panel.
Further features of the invention provide for the wear resistant elastomeric
material
to be secured to the base either by bonding, or by means of mechanical
formations,
or both; for the base to be an injection moulded unit moulded of a suitably
strong
polymeric material such as polypropylene; for the body of wear resistant
material to
be moulded onto the prefabricated base, such body preferably being a
thermoplastic
elastomer; for the screen panel to be of the type in which a rectangular, or
more
commonly, a square screen panel has a series of rectangular, either elongate
or
square, apertures passing there through; and for the base to assume either a
lattice
form of construction defining, at least in outline, the screen apertures and
wherein the
lattice members are of Tee-shape in cross-section and, at the periphery, of
angle or
rectangular shape in cross-section to thereby provide an enlarged upper
surface to
which said body of wear resistant elastomeric material is secured or for the
base to
define only a surrounding frame and optionally one or more transverse
subsidiary
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frame members.
It will be understood that, by employing the invention defined above, a
relatively
inexpensive, but strong structural plastics material can be used for the base
of the
screen panel and this can carry the body of wear resistant elastomeric
material which
is substantially more expensive than the structural plastics material. The
latter
provides the required characteristics of the apertures at the position where
the
material being screened enters the same. A substantial saving can thus be
achieved
whilst not forfeiting any advantages, whatsoever, as regards performance of
the
screen panel.
The plastics materials chosen must either be suitable for mutual bonding
together
whether with the use of a bonding agent or simply in consequence of their
coming
into contact during manufacture, or the base will have to have formations,
preferably
a multitude thereof, for mechanically locking the wear resistant elastomer to
the base.
In the former case, for example, certain structural grades of polypropylene
have been
found to become adequately bonded to a thermoplastic elastomer material
moulded
thereon. One such thermoplastic elastomer material is sold under the trade
name
"Monoprene" by QST Inc. of the State of Vermont in the United States of
America.
Another, which applicant believes is presently under development, is a
thermoplastic
polyurethane.
In the latter case, the formations would be a multitude of laterally extending
fingers
or spike moulded integral with the base.
The invention also provides a screen panel assembly in which the outer
surfaces of
two opposed side edges of a screen panel have longitudinally extending
recesses
therein which co-operate, in clipping relationship, with a lateral head region
of co-
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operant retaining members in turn releasably attached to a screen supporting
frame.
Further features of this aspect of the invention provide for the clip rails to
extend
along substantially the entire length of the recessed region of the side edges
of each
screen panel and for each of the clip rails to have integral nodules passing
downwardly through co-operating apertures in the screen supporting frame.
In order that the invention may be more fully understood, various embodiments
thereof will now be described with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:-
Figure 1 is a schematic plan view of one form of a screen panel to which the
invention may be applied;
Figure 2 is a schematic cross-section thereof taken along line II to II in
Figure 1;
and
Figure 3 is an exploded isometric view of part of a screen assembly
illustrating
the mounting of a screen panel to a screen supporting frame;
Figure 4 is a cross-section through one clip rail illustrating its attachment
to the
screen supporting frame;
Figure 5 is a similar view to Figure 4, but illustrating an alternative form
of
attachment of a clip rail to the screen supporting frame;
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Figures
6A and 6B are sectional elevations taken through one form of screen panel at
right-angles to each other along lines (A) to (A) and (B) to (B) in Figure
3;
Figure 7 is an exploded view illustrating component frame members of the base
of the screen panel illustrated in Figures 6A and 6B; and
Figures
8A and 8B are similar to Figures 6A and 6B but of a different embodiment of
the
invention.
DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS
In the implementation of the invention illustrated in Figures 1 and 2 of the
drawings,
the invention is applied to a square screen panel (1) dimensioned to be
compatible
with screen support frames of substantially conventional and currently used
type.
The screen panel has a series of square apertures (2) there-through when
viewed
from the top face (3).
As provided by this invention the panel has a base (4), in this case made of
injection
moulded polypropylene material. The outer fame members (5) of the base are
each
of inverted angle shape in cross-section to provide a flat face (6) directed
upwardly,
and each of the intermediate frame members (7) defining the square apertures
is of
Tee-shape in cross-section so as to provide a structurally strong base with an
upper
surface (7) defining the square apertures.
Moulded onto the top surface (6) and (7) of the base is a body of wear
resistant
thermoplastic elastomeric material (8) and, in this case, the material is that
sold under
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the trade name "Monoprene" as identified above. It has been found that
"Monoprene" automatically bonds itself onto polypropylene when it is
thermoplastically moulded onto the latter.
The moulding of the body (8) of the wear resistant elastomeric material is
carried out
such that the apertures (2) each have a divergent configuration as indicated
by
numeral (9) at the operatively upper region of the aperture so as to conform
with the
normal tapered shape of apertures through currently available polyurethane
panels.
However, at the depth in the aperture determined by the depth of the body (8)
of
elastomeric material, there is, unlike the prior art, a step increase in size
of aperture
as defined by the stem of the Tee and shown most clearly in Figure 2. This, it
is
envisaged, could be particularly advantageous in that the walls of the
aperture below
the operative size determining region thereof are protected from damage, at
least to
some extent.
The embodiment of the invention described above may well only be appropriate
where the aperture size of the screen is fairly large. In instances where the
aperture
size is smaller or, at least narrower in one direction, it may be that the
base moulded
of polypropylene, only defines basically the surrounding frame and,
optionally, one
or two transverse subsidiary frame members. In this case the actual apertures
are
defined exclusively by the elastomeric material bonded to the moulded
polypropylene
face. Such an arrangement is illustrated in Figures Sa and 8b in which the
elastomeric material (10) is simply bonded onto the top of an injection
moulded
surrounding frame (11 ) to form the entire upper surface of the screen panel
with the
apertures (12) being formed only in the elastomeric material itself. Figures
8a and
8b also illustrate one subsidiary Tee-sectioned frame member (13) extending
across
the frame in one direction and forming part of the base.
It is also within the scope of this invention that the elastomeric material
defining the
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apertures themselves need not be bonded to the supporting base but could be
mechanically held relative thereto. Such is the case in respect of the panel
illustrated
in Figures 6a and 6b as well as Figure 7 of the drawings.
5 In this case the base is composed of a pair of opposite and parallel
extruded frame
members (14) each having longitudinally extending dove-tail shaped recesses
(15)
into which the elastomeric material flows when it is moulded onto the frame.
This is
shown clearly in Figure 6a. In addition each frame member (14) has a Tee-
shaped
extension (16) on top whereat the cross-bar of the Tee also becomes removeably
10 embedded in the moulded material (17).
The ends of these two frame members (14) are interconnected by a pair of
injection
moulded frame members (18) which can be made of substantially the same
material
as the extruded frame members (14). The injection moulded frame members (18)
have dove-tail formations (19) for engaging in the ends of the dove-tail
formations
(15) in the extruded frame members so that a square or rectangular frame can
be
fabricated extremely easily as will be apparent from the illustration of
Figure 7. The
injection moulded frame members (18) also have an extension (20) of Tee-shape
in
cross-section extending along their top edges.
Figure 7. It stands to reason that in order to complete a full basic frame,
the
requirements would be two frame members (18), two frame members (14) which
will
form the periphery of the said frame and at least one of frame member (21 )
which
would act as a reinforcing frame member strategically positioned between two
frame
members of (18) and connected to two frame members of (14)
In Fig 7 it should be understood that no (36) is a groove which would act as a
mechanical fitting should it be desired for the screening surface to be
removable.
Also illustrated in Fig. 6A and Fig 6B.
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Subsidiary frame members (21 ) which can also be injection moulded, can be
employed to interconnect the central regions of the extruded frame members
(14)
and each of these subsidiary frame members (21 ) has dove-tail formations (34)
at
their ends such that they can be slid along the dove-tail shaped grooves (15)
in the
extruded frame members (14) to the required position. One such subsidiary
frame
member (21 ) is illustrated in Figure 6b.
In addition, the injection moulded frame members each have a recess (22)
extending
along the length on the outside thereof (see Figure 6b and Figure 3). These
recesses extend along the entire length of the frame member but terminate
short of
the ends thereof so that an end stop is defined at each end of the recess (22)
as
indicated by numeral (23).
The elastomeric material (17) can, in this case, be any suitable thermoplastic
elastomer and in particular, a thermoplastic polyurethane exhibiting the
desired
properties and is simply moulded onto and over the frame members of the base.
The
apertures (24) are, once more, defined exclusively in the elastomeric material
and are
not directly connected to any supporting part of the base as in the case of
the
embodiment illustrated in Figures 1 and 2.
As provided by the second aspect of this invention, the panels illustrated in
Figures
6a, 6b and 8a and 8b are adapted to be supported on a screen support frame
(25)
(see Figure 3) through the medium of short lengths of retaining member in the
form
of a clip rail (26).
As shown clearly in Figures 4 and 5, the clip rails have a cross-section
providing a
foot (26) for engaging the upper surface of the frame and a head (27)
extending
laterally outwardly from a waisted central region (28).
Each of the clip rails (26) has at least two retaining nodules (29) extending
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out of their under surface with the retaining nodules either being simply
resiliently
deformable to pass through a smaller diameter hole (30) in the screen support
frame
(25) (as illustrated in Figure 5) or, alternatively, a bore (31 ) may pass
through the rail
and associated retaining nodules (29) such that the latter can be introduced
through
the hole (30) by allowing the central region of the retaining nodules to
collapse and
thereafter installing a locking pin (32) in the hole (31) as shown in Figure
4,
The head (27) is, in each case, adapted to allow a panel, generally indicated
by
numeral (33), to be snapped into position between two adjacent clip rails (26)
with the
laterally projecting head extending into the recess (22) described above in
the
injection moulded frame members. Conveniently the clip rails are made of
polyurethane to allow sufficient resilient deformation to take place for this
to happen.
It will be understood that the foot portion (26) of each rail will protect the
metal frame
(25) against abrasion in use and the recesses (22) are dimensioned such that
the
edges of adjacent panels (33) touch each other to prevent material from
passing
adjacent edges of adjacent panels.
(35) is a headed formation to accept (23) end stop.
It will be understood that numerous variations may be made to the embodiment
of the
invention described above without departing from the scope hereof which is
limited
only to the screen panel having a base made of relatively inexpensive
structural
plastics and carrying at its operatively upper region a body of wear resistant
elastomeric material.
