Filter screen assembly

An assembly comprises a rectangular principal frame (12) of moulded plastics material and subdivided by crosspieces (14, 16) to form sub-areas (18). Each sub-area receives as a releasable snap-fit a plastics moulded insert (20) in the form of a sub-frame carrying one or more tensioned screen meshes (FIG. 1). A peripheral sealing gasket may seal the joint between each sub-area (18) and the insert (20).

FIELD OF THE INVENTION 
This invention relates to a filter screen assembly, and more especially a 
filter screen assembly of the kind which in use is located in a vibrating 
basket of a filtering machine for the treatment of slurries or the like. 
BACKGROUND TO THE INVENTION 
A filter screen assembly conventionally comprises a steel frame with one or 
more filtering meshes secured and tensioned over its area. In use, the 
mesh can become damaged, creating holes through which can pass material 
which the mesh is intended to hold back. Replacement of the mesh in such 
an event is expensive and time consuming, requiring removal of the frame 
from the machine, removal of the damaged mesh, replacement by a new mesh 
and tensioning of the latter, and replacement of the frame in the machine. 
It is known to effect patching of a damaged mesh with metal sheet patches 
or, in the case of a multiple mesh assembly, to bond the two meshes 
together at the site of damage. The former procedure results in diminished 
throughput, whilst the latter results in reduced efficiency of the 
so-called blinding action which is intended to take place in use, whereby 
the differential vibrations of the two meshes serve to dislodge material 
tending to clog the upper mesh. Both procedures, whilst more economical 
than replacement of a complete mesh, are just as time consuming as a 
complete mesh replacement. 
It has been proposed in U.S. Pat. No. 4219412 to Hassell to provide a 
screen assembly comprising a latticed support frame into the apertures of 
which are force fitted sieve elements. Such elements are made of a 
synthetic plastics material, such as polyurethane, and no tensioning of 
the sieve occurs. In fact, the disclosed fitting of the element in each 
aperture is such that if there were any tensioning in the sieve, it would 
tend to be reduced after fitting. Furthermore, there is no teaching in 
Hassell of any of the sieve elements being replaceable, eg for repair. 
Indeed the Hassell system seems to be aimed at the manufacture of the 
original product--not the repair or replacement of brother parts. 
In U.S. Pat. No. 7875065 to Rosenblum there is disclosed a tensioned 
circular filter screen adapted to be force fitted over an annular lip of a 
frame to increase the tension in the screen. The screen is intended to be 
permanently secured, and so cannot be readily removed from the frame. 
There is no suggestion in Rosenblum that a plurality of such screens could 
be used as sieve elements in a latticed support frame as in Hassell, even 
if the force fitting were compatible. 
It is therefore an object of the present invention to provide an improved 
filter screen assembly in which the filter mesh elements are readily 
replaceable in the principal frame, and are adapted in fitting to at least 
maintain the tension in the mesh. 
THE INVENTION 
According to one aspect of the present invention, there is provided a 
filter screen assembly comprising a principal frame subdivided into a 
plurality of equally sized sub-areas, each sub-area having an inwardly 
directed lip, and a corresponding plurality of screen inserts snap-fitted 
into engagement within said sub-areas, each insert having a sub-frame with 
a tensioned filter mesh securably attached thereacross, whereby 
interengagement with said lips serves to at least maintain the tension in 
the mesh. 
According to another aspect of the invention, there is provided a method of 
constructing a filter screen assembly comprising a principal frame 
subdivided into a plurality of equally sized sub-areas, and including a 
corresponding plurality of screen inserts engageable within said 
sub-areas, each insert having a sub-frame adapted to support a tensioned 
filter mesh thereon, comprising the steps of assembling the sub-frames in 
the principal frame, tensioning a filter mesh over the principal frame so 
as to cover each sub frame, bonding the mesh around each sub-frame without 
bonding the mesh to the principal frame, cutting the mesh around the 
periphery of each sub-frame, and removing the surplus portions of mesh. 
Preferably, the principal frame and the sub-frames of the inserts are of 
moulded plastics material. A preferred plastics material is a glass-fibre 
reinforced plastics material, such as a mouldable polyester compound. 
A preferred principal frame is of rectangular shape, divided by 
cross-pieces extending in two mutually perpendicular directions parallel 
to the sides of the frame. This principal frame thus takes the general 
form of a window frame divided by mullions and transoms. 
The invention is applicable to filter screen assemblies of the kind having 
only a single mesh layer or of the kind having two or more superimposed 
mesh layers. In the latter case, each sub-area of the principal frame may 
receive two or more inserts in overling relationship, or a single insert 
may carry two or more overlying meshes. 
If desired, each sub-area of the principal frame, or possibly each insert, 
may be equipped with a rubber or like seal for acting to seal the joint 
between the principal frame and the insert when the latter is secured in 
position.

Referring to the drawings, a rectangular principal frame 10 is moulded of 
glass-fibre reinforced polyester resin compound. The frame 10 consists of 
a border 12 sub-divided divided by integrally formed cross-pieces or 
interstices 14 into equally shaped and sized rectangular sub-areas 18. 
An insert for each sub-area comprises a sub-frame 20 moulded of like 
material and to which is bonded a screen mesh 22 tensioned over its area. 
Referring particularly to FIG. 2, the periphery of each sub-area 18 of the 
principal frame 10 and of the sub-frame 20 are mutually shaped to enable 
the insert to be secured as a press fit or snap fit into the sub-area. 
Thus the cross-pieces 14 and the border 12 which define each sub-area 18 
are formed with a peripheral rabbet 24 for receiving a correspondingly 
shaped upper portion 26 of the sub-frame 20. It will be seen that the top 
surfaces of the sub-frames are flush with the top surfaces of the 
principal frame. 
The bottom of each rabbet 24 is constituted as a lip 28 which engages with 
a corresponding groove 30 formed in the sub-frame 20. Below each groove 30 
is a dependent ledge or fir-tree 32 whose outer surface 34 tapers inwards 
towards the bottom. When the sub-frame 20 is inserted in the sub-area 18 
the outer surface 34 is cammed over the respective lip 28 until the groove 
30 snaps into place around the lip. Release of the sub-frame is effected 
by pressing in the four ledges 32 and pushing them upwards. This may be 
done by hand, but could more easily be performed by a suitable tool. 
It will be appreciated that any twisting of the sub-frame, by inward 
pressure on the surfaces 34, will tend to force apart the top surfaces of 
the portions 26 to which the mesh 22 is secured. Therefore, depending on 
the degree of press fit between the lips 28 and the grooves 30, the 
tension in the mesh 22 will be increased, or at any rate will be 
maintained constant. 
In order to ensure that adjacent ledges 32 do not interfere at the corners 
when they are pressed inwards, a gap 36 is left at each corner between 
adjacent ledges. 
In use, when any screen mesh 22 in the assembly becomes damaged, it is only 
necessary to replace the insert containing the site of damage by a fresh 
insert. In some instances this may be possible without even necessitating 
the requirement to remove the filter screen assembly from the vibratory 
basket of the filtering machine. 
It may not be necessary for the two opposite shorter ledges 32 to be formed 
with grooves 30, in which case the thickness of those ledges will remain 
constant. The sub-frames can then be more easily removed merely by 
applying finger pressure inwards and then upwards against the opposite 
outer surfaces 34 of the two longer ledges 32. 
Although not shown, it will be apparent that each insert could be provided 
with a plurality of overlying filter meshes. Alternatively, or 
additionally, each sub-area of the principal frame could be arranged to 
accept a plurality of inserts lying one over the other. 
It will be seen from FIG. 2 that the section of the cross-pieces 14 is 
symmetrical about a vertical axis. In the case of the border 12 of the 
frame 10, a deep groove 40 is formed along the shorter side, and this 
enables the entire screen assembly to be slid into the basket of the 
vibratory machine. 
Referring now to FIG. 3, there is illustrated one method of forming the 
tensioned filter meshes over the sub-frames 20 of the inserts. 
With all the empty sub-frames in position in the principal frame 10, a 
single piece of filter mesh 22 is laid over the frame 10, and is then 
tensioned as required. Next, the mesh 22 is bonded or glued on to the 
upper surface of the position 26 of each sub-frame 20, but not to the 
principal frame. The mesh is then cut around the periphery of each 
sub-frame, as shown by the arrows X, and the surplus pieces of mesh are 
discarded, leaving the complete assembly ready for use. 
Various modifications of the afore-described and illustrated arrangement 
are possible within the scope of the invention hereinbefore defined.