GB2276572A

GB2276572A - Screen frame assembly with frame-bonded screen cloth and removable ball tray

Info

Publication number: GB2276572A
Application number: GB9306681A
Authority: GB
Inventors: Roger Francis Sanders
Original assignee: FILTER SCREEN SUPPLY Ltd
Current assignee: FILTER SCREEN SUPPLY Ltd
Priority date: 1993-03-31
Filing date: 1993-03-31
Publication date: 1994-10-05
Anticipated expiration: 2013-03-31
Also published as: GB2276572B; GB9306681D0

Abstract

A screen frame for use in a screening machine which separates material of different sizes, comprises a graded screen cloth 13 resin-bonded to frame 12. A ball tray 14 is removably attached to the frame 12 by quick release fastenings in the form of quarter turn screws 15. The resin-bonding ensures that good tension is maintained in the cloth 13, whilst the releasable tray 14 provides access to the balls. Ridge elements (20 and 21, Fig. 3) may be integral with the tray 14 to define compartments for the balls. <IMAGE>

Description

Screen Frame Assemblies The present invention relates to screen frame assemblies for screening machines.

Screening machines separate material according to particle size by passing the material over a vibrating screen cloth, such as a wire mesh. The cloth is graded so that only particles below a set size may pass through, either into a collection hopper or on to a further cloth of finer grade for further separation. The larger particles which are unable to pass through the cloth migrate to the cloth edge where they also fall into a hopper. The machines may be used for solid/solid or solid/liquid separation, and have various names, including sieves, sifters, separators and shakers. The term screening machine should be taken to cover all such machines.

Material can become stuck in the cloth, which can prevent proper separation of the material and reduce machine efficiency. Accordingly, resilient wearresistant balls are often provided below the cloth on a perforated ball tray, so that when the cloth is vibrated, the balls bounce back and forth between the tray and cloth, and dislodge (deblind or depeg) material from the cloth. The perforations in the tray are sized to allow the material to pass freely through, but to retain the balls in place.

One type of screening machine uses horizontally mounted circular cloths. In these, the material is fed to the cloth centre and moves outwardly to the cloth periphery. Typically, the machine is built up from cylindrical housing sections mounted one above the other, each having a discharge port on its peripheral wall and a circumferential recess about its top edge. A flanged dish-shaped ball tray is provided at the top of each section, with the flange sitting in the recess. The circular screen cloth is stretched over a tensioning ring which also sits in the recess, over the ball tray flange.

The sections are held together by toggle clamps, and this action also clamps the cloths and trays in place.

Another type of screening machine uses rectangular screen cloths inclined at an angle to the horizontal. In these machines, the material is fed on to the cloth at the higher end, and moves along the cloth to the lower end. Due to the structure of these machines, and to the vibratory motion employed by them, the screen cloth and ball tray need to comprise a single integral unit (hereafter referred to as a screen frame). Such machines generally have a rectangular box housing within which the desired number of screen frames are stacked one above the other, with suitable spacer elements in between. Each screen frame is longer than the one below, so that a hopper can be provided at the lower end of each frame to collect the separated particles.

One type of known screen frame comprises a wooden rectangular tray in which are placed a plurality of balls, and over which is stretched a wire mesh screen cloth nailed along its periphery to the top edges of the tray side walls. A problem with such a frame is that the cloth loses its tension over time and sags, which causes a reduction in efficiency and quality of separation.

Moreover, when the balls or cloth wear out, either the whole frame has to be discarded, or all of the nails have to be removed in order to replace the screen or balls.

This is an arduous and time consuming task.

A further screen frame is known in which spring clips are provided about the side walls of the frame.

The screen cloth has eyelets in an edging around its periphery, and is attached to the frame by looping the eyelets over upwardly extending arms of the spring clips, which then act to keep the cloth taut and in place. Such a screen frame offers improved performance, but the screen cloth can still take quite some time to fit and replace, and can still suffer a loss in cloth tension over time.

The present invention aims to overcome the above problems, and provides a screen frame assembly comprising a frame, a screen cloth stretched across one face of the frame and permanently adhered, for example resin-bonded, to the frame, and a perforated deflection plate spaced from and opposed to the screen cloth and connected to the frame by releasable fixing means.

As in the prior art, deblinding elements such as resilient wear-resistant balls, pads or cylinders are provided in the space between the screen cloth and plate, and the plate perforations are sized to allow material being screened to flow freely through, but to retain the deblinding elements in place.

In a significant and advantageous departure from the prior art, the present invention uses a removable deflection plate instead of a removable screen cloth to allow access to the deblinding elements, and combines this with the permanent adhesion of the screen cloth to the frame, for example by resin bonding, so that the tension in the cloth is kept uniform and constant throughout its life.

It should be noted that resin-bonding has been used in machines in which the cloth is carried by a circular tension ring which constitutes a separate component from a ball tray, but the use of this technology is a new departure in screen frames, where the cloth and tray form an integral assembly, but access is nonetheless needed to the deblinding elements,.

In the prior art screen assemblies, the cloth had to be connected to the frame at numerous points on its periphery, in order to keep the tension across the cloth uniform. Therefore, to remove the cloth, a large number of fastenings had to be undone. In the present invention, however, the plate need only be connected to the frame at a relatively few number of points, and so only a few fastenings need be undone to remove the plate and have access to the deblinding elements.

The plate may be attached to the frame by screws or any other suitable fixing means. It is preferred, however, to use quick release connections, in order to improve still further the plate release time. These may take any suitable form, and could, for example, comprise spring clips. Preferably, they comprise lock means in which a key element passes into a hole in the frame, and in which a turn of the key element causes it to engage with a portion of the frame to hold it in place. This locking preferably entails a cam action in which the plate is urged into engagement with the frame by the key turning action, and the lock could comprise, for example, a quarter turn screw as used in the aero-industry for fastening inspection covers.

As said, integral screen frames are normally used with rectangular machines, where the cloths are inclined to the horizontal. With such systems, the balls or other deblinding elements would tend to fall to the lower end of the screen frame, if not stopped. Accordingly, it is known to provide ridge elements, welded to the tray, which run along and across the tray, and extend upwardly to the cloth, to divide the tray into compartments within each of which a few balls may be held.

Such ridge elements are also preferably provided in the present screen frame assembly, and are preferably removable from the frame with the plate, most preferably by being integral with the plate. The elements might also be provided as a separate structure which is placed within the frame before the plate is attached. These arrangements provide particularly advantageous embodiments in that a single plate and ridge structure may be used with one of a number of different frames each having a cloth of a different grade. Accordingly, a user need only purchase a minimum number of plate and ridge structures, and is able to use these with any number of different frames to which may be bonded cloths of required grades for particular applications. This provides an inexpensive separation system of great flexibility, coupled with ease of use and fast cloth change.Further, if a cloth becomes worn, only the frame and cloth need be sent away for repair, with the plate and ridge elements meanwhile being used with a different frame and cloth. Also, the frame and cloth sent away are light and easy to handle.

Where the ridge elements are provided on the plate, the longitudinal ridge elements could be stamped out in the plate, and the transverse ridge elements could be welded to the plate, and have cut-outs therein to fit over the longitudinal elements, or vice versa.

Some of the elements may have sloped sides so that the deblinding elements are urged back into the centre of the compartment and upwardly towards the screen cloth, and others may have T-shaped cross-sections with the flat portion of the T helping to support the screen cloth.

The screen frame assembly elements may be made of any suitable materials, such as aluminium or stainless steel. In the prior art, the screen frames are made from extruded elements with vertical side walls. In the present invention, however, it is preferred to make the frame from box-sections, and to have the inner wall of the frame sloped in a similar manner to the ridge elements. The box-section construction facilitates the forming of this slope.

A groove is preferably provided in the lower surface of the frame to receive a female fastening element of the fixing means. Preferably a sealing means is provided so that when the screen frame assemblies are stacked one above the other in the screening machine, a seal is provided between them. Further, it is often preferable to not only stack the assemblies one above the other, but also to abut them adjacent one another, instead of, for example, using a single very long or broad assembly.

Accordingly, sealing means may be provided along outer side walls of the frame. Preferably, a sealing strip is provided along the top edge of one outer side wall, and along the bottom edge of the opposing outer side wall, so that when two assemblies are butted together, the upper and lower sealing strips cooperate to provide a good seal between the two frames.

When the screen frames are mounted in a box housing, spacer frames or strips, also preferably of box-section construction, may be provided between them. These spacers may have sealing means along their top and/or bottom edges.

The use of the screen frame with a removable deflecting plate is, in itself, a particularly advantageous and inventive concept, and, viewed from a second aspect, the present invention provides a screen frame assembly comprising a frame, a screen cloth stretched across one face of the frame, and a perforated deflection plate spaced from and opposed to the screen cloth and connected to the frame by releasable fixing means.

Use of quick release fixing means, and ridge elements separate from the frame, either mounted on the deflecting plate or provided as a separate structure, are especially advantageous and preferred features of this aspect of the invention.

The invention also extends to screening machines incorporating the novel screen frame assemblies discussed above.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a side elevation of a known screening machine; Figure 2 is a top plan view of a screen frame in accordance with an embodiment of the invention, with the screen cloth cut away; Figure 3 is a perspective view of a corner of the screen frame of Figure 2 with the screen cloth removed for clarity and with the frame and deflection plate separate; Figure 4 is a schematic cross-section through a portion of the screen frame of Figure 2; and Figure 5 is a schematic diagram showing how the screen frames of Figure 2 may cooperate with one another and with spacer elements.

A screening machine is shown in Fig. 1. Such machines are well-known in the art, and therefore only a brief description of it will be made.

An inclined rectangular box housing 1 is mounted on a base 2. It is attached at its lower end to a slide bearing 4, and at its upper end to the eccentric shaft 5 of a gearbox 6. The shaft 6 causes the box housing 1 to oscillate. Sleeves 3 guide filtered material to an underlying conveyor (not shown).

Screen frames 7 are stacked one above the other in the box housing 1, and are separated by spacer strips 8.

The box housing 1 is closed by a cover 9. Each screen frame 7 is longer than the one below to allow a hopper 10 to be placed at each one's end.

Material to be separated is fed into the machine through a feed hopper 11, and travels along the screen frames 7 under the influence of their incline and the vibrations imparted to them by the motor 6.

Each screen frame 7 carries a graded screen cloth (see below) through which only particles below a set size may pass. Thus, as the material moves along the screen frames 7, the material is separated, so that the material fed to each hopper 10 by each screen frame 7 is within a specific size range.

The present invention is concerned with an improved screen frame assembly, with an example being shown in Figs. 2 to 5.

The screen frame 7 comprises a frame member 12 to which is resin-bonded a screen cloth 13. A deflection plate 14 is releasably attached to the frame 12 by quarter turn screws 15, similar to those used in the aero-industry for mounting inspection plates. As seen in Figure 4, the screw 15 has a keyed portion 16 which is insertable into an elongate hole 17 in a receiving element 18 in one orientation, but which latches against the top of the element 18 when turned through 90" to secure the plate 14 in place. The screw 15 may have a cam action to urge the plate against the frame by its turning action.

In use, deblinding elements, such as resilient wearresistant balls, are placed in the space between the screen cloth 13 and the deflection plate 14. These deblinding elements bounce back and forth between the screen cloth and deflection plate to dislodge material in the screen cloth 13.

The deflection plate 14 is perforated by holes 19 sized such that the material to be separated can flow freely through, but which are too small to allow the deblinding elements to pass through.

In order to ensure that the deblinding elements act over the entire screen cloth area, transverse and longitudinal ridge elements 20 and 21 respectively are provided on the plate 14 to define compartments for the deblinding elements. The transverse elements 20 are triangular in cross section, so that the balls may bounce off the angled walls back towards the centre of the compartment and up towards the screen cloth 13. The longitudinal ridge elements 21 are T-shaped, with the flat portion of the T supporting the screen cloth.

The inside wall 22 of frame 12 may also be sloped to deflect the deblinding elements, and the frames lower edge 23 may have a recess 24 running therearound to help locate the female fitting of the fixing means. A sealing element 25 is secured to the underside of the edge of the plate 14. The frame 12 is advantageously formed from box-sections, as these facilitate the making of the sloped walls and the recessed lower edges.

The spacer elements 8 between each screen frame 7 in the box housing 1 are also formed of a box-section and provided with a seal 26 on its lower edges.

Where the screen frames 7 are not only stacked one above the other, but also adjacent one another, sealing means 27 can be provided toward the top of the outer frame wall 28 on one side of the frame 12, and toward the bottom of the outer frame wall 29 opposing outer frame wall 28. This provides an effective seal between adjacent screen frames 7.

It should be noted that the above is only one particular embodiment of the present invention, and that many variations on this embodiment are possible. For example, the cloth need not be resin-bonded to the frame.

Also, other fasteners such as screws or spring-clips could be used to mount the plate to the frame, and the ridge elements could be formed as a separate structure from both the frame and the plate.

Claims

1. A screen frame assembly comprising a frame, a screen cloth stretched across one face of the frame and adhered to the frame, and a perforated deflection plate spaced from and opposed to the screen cloth and connected to the frame by releasable fixing means.

2. A screen frame assembly according to claim 1, wherein the screen cloth is resin-bonded to the frame.

3. A screen frame assembly according to claim 1 or 2, wherein the plate is attached to the frame by quick release connections.

4. A screen frame assembly according to claim 3, wherein the quick release connections comprise lock means in which a key element connected to the plate passes into a hole in the frame, and in which a turn of the key element causes it to engage with a portion of the frame to hold the plate in place.

5. A screen frame assembly according to claim 4, wherein the locking entails a cam action in which the plate is positively urged into engagement with the frame by the key turning action.

6. A screen frame assembly according to any of claims 3 to 5, wherein the quick release connections comprise quarter turn screws.

7. A screen frame assembly according to any preceding claim, wherein the assembly further comprises ridge elements separable from the frame which divide the assembly into compartments.

8. A screenframe assembly according to claim 7, wherein the ridge elements are integral with the plate.

9. A screen frame assembly according to claim 7, wherein the ridge elements are separate from the frame and plate.

10. A screen frame assembly according to claim 8, wherein some of the ridge elements are stamped out from the plate.

11. A screen frame assembly according to claim 10, wherein further ridge elements are provided across the stamped-out ridge elements and are profiled to fit over the stamped-out elements.

12. A screen frame assembly according to any of claims 7 to 11, wherein some of the ridge elements have sloped sides so that deblinding elements provided in the assembly are urged towards the centre of the compartment and upwardly towards the screen cloth.

13. A screen frame assembly according to any of claims 7 to 12, wherein some of the ridge elements have T-shaped cross-sections, with the flat portion of the T helping to support the screen cloth.

14. A screen frame assembly according to any preceding claim, wherein the frame is made from box-sections.

15. A screen frame assembly according to any preceding claim, wherein inner walls of the frame are sloped so that deblinding elements provided in the assembly are urged towards the centre of the assembly and upwardly towards the screen cloth.

16. A screen frame assembly according to any preceding claim, wherein a groove is provided in the lower surface of the frame to receive a female fastening element of the fixing means.

17. A screen frame assembly according to any preceding claim, wherein a sealing means is provided so that when two or more screen frame assemblies are stacked one above the other in a screening machine, a seal is provided between the assemblies.

18. A screen frame assembly according to any preceding claim, wherein a sealing means is provided along outer side walls of the frame.

19. A screen frame assembly according to any preceding claim, wherein a sealing strip is provided along the top edge of one outer side wall of the assembly, and along the bottom edge of the opposing outer side wall, so that when two assemblies are butted together, the upper strip of one frame and the lower strip of the other cooperate to provide a seal between the two assemblies.

20. A screen frame assembly comprising a frame, a screen cloth stretched across one face of the frame, and a perforated deflection plate spaced from and opposed to the screen cloth and connected to the frame by releasable fixing means.

21. A screening machine incorporating one or more screen frame assemblies as claimed in any preceding claim.

22. A screen frame assembly as hereinbefore described with reference to Figs. 1 to 5.

23. A screening machine as hereinbefore described with reference to Figs. 1 to 5.