GB1578948A - Sifting screens - Google Patents

Description

(54) IMPROVEMENTS RELATING TO SIFTING SCREENS (54) IMPROVEMu (71) We, UNITED WIRE GROUP LIMITED, a British Company of Granton Park Avenue, Edinburgh EH5 1HT, Great Britain do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a sifting screen assembly for use in a vibratory sifting machine and to a method of making said assembly.

According to the present invention there is provided a sifting screen assembly for use in a vibratory sifting machine comprising a frame, and two screens superimposed one on the other and adhesively secured at marginal edge portions thereof to said frame, the upper screen having a relatively fine mesh, the lower screen having a relatively coarse mesh, both screens being tensioned, and the upper screen being at a lower tension than the lower screen, so that, when the frame is vibrated, the screens strike each other so as to effect or assist in effecting clearing of blockages or to prevent or reduce the possibility of blockage of the screens.

Further, according to the present invention there is provided a method of making a sifting screen assembly as aforesaid, comprising tensioning a relatively coarse mesh screen on a frame, adhesively securing said screen to the frame, superimposing on said screen a relatively fine screen, tensioning the latter to a lesser degree than said coarse mesh screen, and adhesively securing said fine mesh screen to the frame.

Preferably, the lower relatively coarse mesh screen is woven from stainless steel wire, and the upper relatively fine mesh screen is woven from nylon monofilament strands.

The mesh of the upper screen may be, for example, about 100 to 200, that is, about 100 to 200 weft strands per inch and about 100 to 200 warp strands per inch, and the mesh of the lower screen may be about 10 to 20.

In one example of the invention, a screen assembly for use in a vibratory sifting machine consists generally of a rectangular frame and two rectangular screens. One screen is superimposed on the other, and the two screens are adhesively secured at their marginal edge portions to the frame. The screens are under tension, and the tension in the upper relatively fin screen is less than the tension in the lower screen.

When the screen is fitted to a vibratory sifting machine, and the machine is in use, the upper and lower screens respond differently to the vibratory force, so that the screens intermittently strike each other with sufficient force to prevent or reduce the possibility of the upper screen becoming or remaining blocked or plugged. The vibrations of the screens may differ in frequency, amplitude or phase or a combination of any two or all of these.

The members forming the frame are preferably angle-section members, each member having one flange arranged vertically and the other flange projecting horizontally outwards from the top thereof.

The frame may include at least one T-sectio reinforcing bar dividing the frame into parts of equal area and arranged parallel to the longer sides of the frames with the upper surface of the horizontal limb in the same plane as the upper surfaces of the horizontal flanges of the side and end members of the frame. There are preferably two such reinforcing bars.

The screen assembly may be made as follows.

The frame is supported horizontally on a tensioning device, and the upper surfaces of both the horizontal flanges and the horizontal limbs of the T-bars are coated with a heat curable adhesive. The latter is preferably an epoxy resin adhesive and that sold under the Trade Mark "Araldite".

A relatively coarse mesh screen woven from stainless steel strands is fitted to the frame on top of the adhesive, the screen being larger in both linear dimensions than the frame. The overlapping marginal portions of the screen are clamped in the tensioning device, and the latter is operated to tension the screen to the required degree.

The screen assembly with the screen still tensioned by the tensioning device, is placed below a heater to cure the adhesive, and, when curing has been completed, the tensioning device is removed from below the heater. The overlappmg marginal portions of the screen may be cut-off at this stage. Adhesive is then applied to the screen in the zones where it adheres to the frame and T-bars, and a relatively fine mesh screen woven from nylon mono filamellt is placed on top of the stainless steel screen. The nylon screen is then tensioned to the required degree, which is less than that of the stainless steel screen, the adhesive is cured, and the oveilapping marginal portions are cut ffl as above described for the stainless steel screen.

Various modifications may be made without departing from tlie scope of the invention.

For example, if time factors permit, the heat curing of the adhesive may be omitted, and it may be left to cure at ambient temperatures, and suitable adhesives other than epoxy resins may be used.

In order to reduce or prevent wear on the screens due to frictional contact with inner peripheral edges of the frame, the upper faces of the horizontal flanges and the inner faces of the vertical flanges of the frame members may be covered by strips of resilient material such for example as rubber or neoprene.

The lower screen, in addition to assisting in preventing or removing blockages in the upper screen, also provides additional support for the upper screen.

The screen assembly above described is especially, but not exclusively, suitable for use in a vibratory sifting machine as disclosed in our co-pending Patent Application No. 49795/ 76 (Serial No. 1 575 312).

The frame and screens need not be rectangular, but may be of any other suitable shape, such for example as circular, dependent upon the design of the machine to which it is to be applied, or its intended use. Also, the frame members, irrespective of the shape of the frame need not be of angle-section, but may be of any other suitable shape in cross-section, such for example as box-section.

It is possible for both screens to be made of stainless steel or from nylon.

Synthetic resinous materials other than nylon, and metal wire other than stainless steel may alternatively be used.

The tensioning device above mentioned is preferably operated by hydraulic rams, and, in this case, suitable tensions for the screens may be obtained when the pressure in the rams is about 6 or 7 bars, for the higher tensioned screen, and about 3 bars for the lower tensioned screen.

WIlAT WE CLAIM IS:- 1. A sifting screen assembly for use in a vibratory sifting machine comprising a frame, and two screens superimposed'one on the other and adhesively secured at marginal edge portions thereof to said frame, the upper screen having a relatively fine mesh, the lower screen having a relatively coarse mesh, both screens being tensioned, and the upper screen being at a lower tension than the lower screen, so that, when the frame is vibrated, the screens strike each other so as to effect or assist in effecting clearing of blockages or to prevent or reduce the possibility of blockage of the screens.

2. A sifting screen assembly as claimed in Claim 1, in which the upper screen is woven from nylon monofilament strands, and the lower screen is woven from stainless steel wire.

3. A sifting screen assembly as claimed in Claim 1 or 2, in which the upper screen has a mesh size of 100 to 200, and the lower screen has a mesh size of 10 to 20.

4. A method of making a sifting screen assembly as claimed in Claim 1 or 2 or 3, comprising tensioning a relatively coarse mesh screen on a frame, adhesively securing said screen to the frame, superimposing on said screen a relatively fine mesh screen, tensioning the latter to a lesser degree than said coarse mesh screen, and adhesively securing said fine mesh screen to the frame.

5. The method claimed in Claim 4 comprising coating the frame with a heat curable adhesive, fitting to the frame on top of the adhesive the relatively coarse mesh screen, tensioning the screen, curing the adhesive by heat, applying further adhesive to the screen in the zones where it adheres to the frame, superimposing on said screen a relatively fine mesh screen and tensioning the latter to a degree less than that of said coarse mesh screen, and curing said further adhesive.

6. The method claimed in Claim 4 or 5, in which each screen is tensioned by a tensioning device operated by hydraulic rams, the pressure in the rams to produce the tension in the coarse mesh screen being 6 or 7 bars, and the pressure in the rams to produce the tension in the fine mesh screen being 3 bars.

7. The method of making a sifting screen assembly as claimed in Claim 4 substantially as hereinbefore described.

8. A sifting screen assembly for use in a vibratory sifting machine substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. required degree. The screen assembly with the screen still tensioned by the tensioning device, is placed below a heater to cure the adhesive, and, when curing has been completed, the tensioning device is removed from below the heater. The overlappmg marginal portions of the screen may be cut-off at this stage. Adhesive is then applied to the screen in the zones where it adheres to the frame and T-bars, and a relatively fine mesh screen woven from nylon mono filamellt is placed on top of the stainless steel screen. The nylon screen is then tensioned to the required degree, which is less than that of the stainless steel screen, the adhesive is cured, and the oveilapping marginal portions are cut ffl as above described for the stainless steel screen. Various modifications may be made without departing from tlie scope of the invention. For example, if time factors permit, the heat curing of the adhesive may be omitted, and it may be left to cure at ambient temperatures, and suitable adhesives other than epoxy resins may be used. In order to reduce or prevent wear on the screens due to frictional contact with inner peripheral edges of the frame, the upper faces of the horizontal flanges and the inner faces of the vertical flanges of the frame members may be covered by strips of resilient material such for example as rubber or neoprene. The lower screen, in addition to assisting in preventing or removing blockages in the upper screen, also provides additional support for the upper screen. The screen assembly above described is especially, but not exclusively, suitable for use in a vibratory sifting machine as disclosed in our co-pending Patent Application No. 49795/ 76 (Serial No. 1 575 312). The frame and screens need not be rectangular, but may be of any other suitable shape, such for example as circular, dependent upon the design of the machine to which it is to be applied, or its intended use. Also, the frame members, irrespective of the shape of the frame need not be of angle-section, but may be of any other suitable shape in cross-section, such for example as box-section. It is possible for both screens to be made of stainless steel or from nylon. Synthetic resinous materials other than nylon, and metal wire other than stainless steel may alternatively be used. The tensioning device above mentioned is preferably operated by hydraulic rams, and, in this case, suitable tensions for the screens may be obtained when the pressure in the rams is about 6 or 7 bars, for the higher tensioned screen, and about 3 bars for the lower tensioned screen. WIlAT WE CLAIM IS:-

1. A sifting screen assembly for use in a vibratory sifting machine comprising a frame, and two screens superimposed'one on the other and adhesively secured at marginal edge portions thereof to said frame, the upper screen having a relatively fine mesh, the lower screen having a relatively coarse mesh, both screens being tensioned, and the upper screen being at a lower tension than the lower screen, so that, when the frame is vibrated, the screens strike each other so as to effect or assist in effecting clearing of blockages or to prevent or reduce the possibility of blockage of the screens.

2. A sifting screen assembly as claimed in Claim 1, in which the upper screen is woven from nylon monofilament strands, and the lower screen is woven from stainless steel wire.

3. A sifting screen assembly as claimed in Claim 1 or 2, in which the upper screen has a mesh size of 100 to 200, and the lower screen has a mesh size of 10 to 20.

4. A method of making a sifting screen assembly as claimed in Claim 1 or 2 or 3, comprising tensioning a relatively coarse mesh screen on a frame, adhesively securing said screen to the frame, superimposing on said screen a relatively fine mesh screen, tensioning the latter to a lesser degree than said coarse mesh screen, and adhesively securing said fine mesh screen to the frame.

5. The method claimed in Claim 4 comprising coating the frame with a heat curable adhesive, fitting to the frame on top of the adhesive the relatively coarse mesh screen, tensioning the screen, curing the adhesive by heat, applying further adhesive to the screen in the zones where it adheres to the frame, superimposing on said screen a relatively fine mesh screen and tensioning the latter to a degree less than that of said coarse mesh screen, and curing said further adhesive.

6. The method claimed in Claim 4 or 5, in which each screen is tensioned by a tensioning device operated by hydraulic rams, the pressure in the rams to produce the tension in the coarse mesh screen being 6 or 7 bars, and the pressure in the rams to produce the tension in the fine mesh screen being 3 bars.

7. The method of making a sifting screen assembly as claimed in Claim 4 substantially as hereinbefore described.

8. A sifting screen assembly for use in a vibratory sifting machine substantially as hereinbefore described.