GB2064426A

GB2064426A - Screen Plate

Info

Publication number: GB2064426A
Application number: GB7936675A
Authority: GB
Original assignee: HUTECH SERVICES Ltd
Current assignee: HUTECH SERVICES Ltd
Priority date: 1979-10-23
Filing date: 1979-10-23
Publication date: 1981-06-17

Abstract

A vibratory screen plate comprising a three-part laminate of top and bottom metal layers having apertures therethrough and having an intermediate visco-elastic layer comprises a stainless steel top layer and a mild steel bottom layer. Stiffening is optionally provided in one or both of two ways; one form of stiffening comprises spot welds interconnecting the metal layers through the intermediate layer, at least over the central region of the plate; the other form of stiffening comprises metal reinforcing strips or other stiffening members, attached to the underside of the plate, preferably by being welded to the mild steel layer in an arrangement which avoid the screening apertures in the plate.

Description

SPECIFICATION Screen Plate This invention relates to a screen plate for use in screening particulate material, and finds use, for example, in a vibrating screen of coal preparation plant in a coal-mine.

Although the screen plate of the invention finds use in screening other particulate material, reference hereinafter will, for convenience, be made to the screening of coal only.

Coal that has been hewn from a seam in a coal-mine comprises pieces of a wide variety of sizes. Different sized pieces have different applications and can be sold at different prices.

Accordingly, the coal has to be screened for deposit in separate storage bins containing pieces of predetermined sizes. This operation is carried out in a coal preparation plant that contains a series of vibrating screen plates. The plates are arranged over the storage bins and are apertured such that successive screens have apertures of increasing size. The mixed-sized coal is directed on to the first screen and the smallest pieces pass through the apertures therein whilst the larger pieces are vibrated onwards and downwards to the next plate. This process is continued until all the coal delivered to the plant has been separated into the various storage bins containing pieces of coal of different sizes.

Conventionally, screen plates are made from a single sheet, or two laminated sheets of stainless steel. When coal is tipped on to these known plates, and is subsequently urged therealong by their vibrating movement, a great deal of noise is generated. The noise from the coal preparation plant, which is usually located at the coal-mine itself, has been found to be of a ievel well above the acceptable limits of the Government Code of Practice.

Screen plates are also susceptible to "plugging", whereby pieces of coal that are slightly too big to pass straight through the apertures become trapped therein and quickly block the passage of following coal. The thinner the plate, and thus the smaller the depth of the apertures, the smaller is the risk of plugging, but the rate of wear of the apertures owing to coal passing therethrough is then increased. In this respect, it is to be appreciated that as the apertures in a plate increase in size, coal pieces will pass therethrough that ought to be delivered to the next storage bin. Since in general larger pieces command a higher price than smaller ones, this represents a financial loss to the mine owner.

Accordingly, one factor in determining the lifetime of the plate is the time taken for the aperture to increase by a given amount.

In addition to the problem of plugging, the continual flexing of the plate can lead to fatigue fracture by cracks extending from one aperture to another. This again leads to inefficient screening and is also a further factor in determining the lifetime of the plate.

Laminated steel plates do not have the necessary stiffness to resist delamination by bending under the continual impact and heavy loading of the coal, thus the two plates of the laminate quickly separate, terminating its life.

It is an object of the present invention to provide a screen plate that overcomes, or at least alleviates, some or all of the above-mentioned disadvantages.

In one aspect of the present invention, the apertured screen plate comprises a laminate having a top layer of work-hardened stainless steel, a bottom layer of mild steel, and an intermediate layer of visco-elastic material.

The laminate of the invention provides an acoustically-dampened screen plate that significantly reduces the amount of noise generated by coal impact, and whose workhardened top surface minimises wear.

Acoustic dampening and mechanical strengthening can also be provided by an apertured screen plate in accordance with a further aspect of the invention, wherein stiffening members are welded, or otherwise securely attached, to a bottom surface thereof.

It will be appreciated that the stiffening members should be positioned so as to avoid or at least minimise interference with coal passing through the plate, and to this end they may be disposed in straight lines extending substantially tangentially with respect to the arrayed apertures.

In a conventionally-arrayed screen plate, where successive rows along the length of the plate are offset, the stiffening members preferably extend diagonally along the plate extending on each side away from a central line in a V-shaped configuration, the apex of the V being upstream of the direction of flow of the coal, thereby to minimise plugging.

In a laminated plate of the invention, with or without the additional stiffening provided by the said further aspect, the two steel layers may be spot welded together through the visco-elastic core, at least in a central zone of the plate, further to increase the plate stiffness without any significant reduction in the damping characteristic of the whole screen plate.

The increased stiffness inherent in plates according to the invention provides the advantage that use of expensive steel, particularly stainless steel, is minimised at the outset, and furthermore the lifetime of the plate is lengthened owing to the increased resistance to wear as by fatigue failure.

A screen plate in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawing which shows the plate in plan.

The screen plate is rectangular of overall width 2101 mm and overall length, in the direction of flow A of the coal, 1064 mm. The plate is apertured over the major portion of its surface by a symmetrical array of apertures 2, successive rows of which are offset from each other, so that any pieces of coal passing over the apertured portion of the plate is directed to at least one of the apertures 2. In the particular example given, the apertures 2 have a diameter of 50 mm, and are set at a horizontal pitch of 63 mm and a vertical pitch of 54.56 mm.

The screen plate is secured by bolts passing through twelve peripheral holes 4 to the frame of a vibrating screen (not shown) of a coal preparation plant, and the leading edge 6 of the plate is higher than its trailing edge 8.

Accordingly, as the coal is directed downwards on to the leading edge 6 of the plate it is urged in the direction of the arrow A, and pieces having a maximum dimension less than 50 mm will pass through the plate whilst the larger pieces are urged onwards to the next plate of the coal preparation plant, which has large apertures therein.

The screen plate is a three-stage laminate whose upper layer consists of 3 mm thickness work-hardened stainless steel, and whose bottom layer consists of 2 mm thickness of mild steel, which is not subject to wear by impact from the coal. These two layers are interconnected by a 0.5 mm thickness visco-elastic core. Thus, although the plate is of total thickness 5.5 mm, as opposed to the 5 mm thickness of the single stainless steel plate conventionally used, it will be seen that only 3 mm of (exDensive) stainless steel is required.

The visco-elastic core contributes not only to acoustic dampening of the screen plate, but the plate is also inherently more resistant to fatigue failure. Vibrations induced by the passage of the mine product over the plate are damped successfully.

The stiffness of the plate is further enhanced by the provision of a plurality of stiffening members 10 which are spot welded to the underneath of the plate along diagonal lines that do not encroach on the apertures 2. For the aperture size given above, and an array having nine rows of thirty one apertures and eight rows of apertures offset therebetween, it is found that the stiffening members 10 can be located at an angle of 60O to the leading and trailing edges 6 and 8 without there being any interference with the apertures 2.

The stiffeners 10 are disposed on each side of the centre of the plate so as to be directed away therefrom, with the apices of the V-shapes so formed being upstream of the direction A of the flow of coal. By this arrangement, coal which falls through the apertures 2 and subsequently abuts a stiffening member 10 will be directed onwards by the stiffening member in the direction of its original motion. It will be appreciated that if the stiffening members 10 were to be arranged so as to converge towards the trailing edge 8, that is to say if the plate were to be reversed such that the stiffening members would extend outwards opposing the direction A, then coal passing through apertures 2 and engaging the stiffening member 10 would be impeded, with the risk that this could lead to plugging of the plate, thereby drastically reducing the screening efficiency.

It will be appreciated that the constribution to the stiffness of the plate made by the stiffening members 10 will depend on the size of the members 10, in particular on their depth.

Accordingly, it is envisaged that the depth of the stiffening members can be increased, and a reduction could be made in the overall thickness of the plate, thus further minimising the risk of plugging.

The stiffness of the screen plate is further enhanced by the provision of spot welds 12 located between the apertures 2 over a central zone 14 of the screen plate. The spot welds extend through the viscoelastic core to weld the top stainless steel layer to the bottom mild steel layer. Only a very small percentage of the nonapertured area of the plate is taken up by the spot welds. Although the spot welds can conveniently be made after the apertures have been provided in the laminated screen plate, it is envisaged that welding could be carried out before aperturing.

The spot welding thus tends to offset the small tendency to delaminate th6t is an unavoidable consequence of providing apertures in a laminated screen plate.

Although the spot welding is shown confined to a central area 14 of the plate, this being the area that receives the greatest loading, it is envisaged that the spot welded area could be extended, for example to the whole plate.

The screen plate thus provided is appreciably acoustically damped and stiffened with respect to known screen plates, and thus not only substantially reduces the noise generated in a coal preparation plant, but also extends the lifetime thereof by increasing its resistance to wear at the apertures and to fatigue failure.

Consequently, the actual cost of screening-plate per ton of coal screened is reduced. An economic advantage of using a screen plate in accordance with the present invention rather than an unstiffened plate of single sheet or laminated construction can be appreciated from the fact that, in use as a coal screening plate for example, many thousands of tons of coal will be screened much more efficiently during the lifetime of each plate, so that the monetary return per ton of coal produced by the coal mine will be increased. This increased return, in respect of the mine output over a whole year is an appreciable amount.

Claims

1. A vibratory screen plate for use in the vibratory screening of particulate material, which comprises a laminate having apertures therein, the laminate comprising a top layer of workhardened stainless steel, a bottom layer of mild steel and an intermediate layer of visco-elastic material.

2. A vibratory screen plate according to claim 1, having one or more stiffening members attached to its bottom surface.

3. A vibratory screen plate according to claim 2, wherein the stiffening members are straight and the apertures are arranged in the plate so that the stiffening members do not obstruct them.

4. A vibratory screen plate according to claims 2 or 3, wherein the stiffening members extend diagonally of the plate and symmetrically about its centre line in a V-shaped configuration, the apex being upstream of the direction of flow of particulate material on the plate.

5. A vibratory screen plate according to any preceding claim, wherein the top and bottom layers are welded to one another through the intermediate layer at least in a central zone of the plate.

6. A vibratory screen plate according to any preceding claim, wherein the top layer has a thickness of 3 mm, the bottom layer has a thickness of 2 mm and the intermediate layer has a thickness of 0.5 mm.

7. A vibratory screen plate according to claim 1, substantially as hereinafter described with reference to the accompanying drawing.