Abstract:
A screening deck for the screening of crushed stone material includes a plurality of screening elements arranged adjacent one another. At least one side of each screening element is non-parallel with respect to a longitudinal direction of the screening deck. The screening deck includes at least two different types of screening elements which are arranged at different heights in the screening deck for creating narrowing passages and/or winding paths and/or steps for the material traveling on the screening deck.

Description:
The present application claims priority under 35 U.S.C. § 119 to Patent Application Serial No. 0400337-2 filed in Sweden on Feb. 13, 2004. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a screening deck for the screening of material, such as crushed stone, gravel or the like, that will herein be referred to as crushed stone, which expression is not intended to imply that the stone or gravel is of a particular size. The screening deck comprises screening elements through which the material falls. 
   In the mining and stone industries, it is in many cases important to fractionate (separate) crushed stone and gravel into fractions of different sizes. Ideally, each fraction would comprise particles of a prescribed size, but in practice each fraction typically includes particles that are somewhat larger or smaller than the prescribed size. Normally, the deviation from the prescribed size that is permitted according to industry standards is defined, e.g., 10 percent for oversized particles and 15 percent for undersized particles. It is, however, important that each fraction comprises a blend of particles within the permitted deviation range, since mixtures that deviate from the standard blends are prized lower. 
   In most cases, fractionating is done by supplying an unfractionated stream of crushed stone or gravel to a vibrating screen provided with screening elements including screening holes for allowing stones smaller than the screening holes to pass through the holes. The vibration pattern and the inclination of the vibrating screen are arranged so that the crushed stones continuously flow in one direction on the screen, ultimately exiting one side of the screen or falling through the holes in the screening elements. 
   In this way it is possible to fractionate the crushed stone stream into stones smaller than the screening holes and stones larger than the screening holes. For most applications, such a fractionating is not sufficient, since the resulting crushed stone fractions range in size from stone powder up to the screening hole size and from the screening hole size up to the largest stones entering the screen, respectively. One way of further fractionating the crushed stone into finer fractions is to run one fraction leaving the screen to a further screen, but a more common way of solving the problem is to use a screen with multiple screening decks on top of each other. 
   On a screen with multiple screening decks, the screening decks are provided with ever smaller screening holes the lower the deck is located. Due to gravity, stones smaller than the screening holes in an upper deck will fall down to the neighboring lower deck. Stones smaller than the screening holes in that deck will fall through the screening holes, either to a further lower deck or to a surface below the lowermost screening deck. Hence, as the crushed stones leave the screen, the fraction between two decks will contain stones ranging in size from larger than the hole size of the lower screening deck to smaller than the hole size of the upper screening deck. 
   A problem with screening decks is the wear which they undergo. As is well known by people skilled in the art, crushed stones are very abrasive, especially when they are vibrated in order to flow slowly over a screen. In order to reduce the wear, virtually all surfaces contacting the crushed stone can be clad with, or made of, rubber or polyurethane. The areas most exposed to wear are the edges of the screening holes. Hence, most screening decks are provided with exchangeable screening elements. This not only allows exchange due to worn elements, but also for exchange between screening elements of various screening hole sizes. 
   A system for exchanging screening elements in a vibrating screen for the screening of crushed rocks or gravel is described in SE-A-0 460 340 (corresponding to U.S. Pat. No. 5,085,324). The screen according to that invention includes a multitude of screening elements. The elements are at one end provided with snap locks for interaction with elongated stanchions provided on transverse carriers reaching across the screen. The other ends of the screening elements that are not provided with snap locks are jammed in place by means of an extension of a neighboring screen element. 
   One major problem with all screening decks is that the crushed stone material to be screened, i.e. stones or gravel, travel along a longitudinal path in the screening deck. The travel path of the material is also called the traveling direction. At the edges of the screening elements, there are no screening holes. Hence, the longitudinal connection area between two adjacent screening elements is not provided with holes. This means that if the material starts to travel close to the edges of the screening element, where no holes are placed, the material may travel over the entire length of the screening deck without encountering a screening hole. This problem is worsened by the fact that the screening elements are rectangular or square having symmetrically located holes, thus creating longitudinal paths without holes. One way of decreasing this problem has been to provide wedge-shaped obstacles on the screening element or on the edges of the screening elements that cause the material to change direction or at least move it transversely to the traveling direction. 
   Further, it is important that the material to be screened does not move so quickly and undistorted over the screening element that the material that should fall down through the holes has the possibility to pass over the holes. 
   SUMMARY OF THE INVENTION 
   The above-mentioned shortcomings and/or other problems are solved in that at least one side of each screening element is non-parallel with respect to a longitudinal direction of the screening deck; that the screening deck includes at least two different types of said screening elements; and that different screening elements are arranged at different heights in the screening deck for creating narrowing passages or winding paths for the material on the screening deck. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, a preferred embodiment of the invention will be explained with reference to the accompanying drawings. 
       FIG. 1  is a schematic perspective assembly view of a screening deck according to a first embodiment of the present invention. 
       FIG. 2  is a perspective view of a screening element according to the present invention. 
       FIG. 3  is a perspective view showing the underside of a screening element according to the present invention. 
       FIGS. 4   a - 4   d  are simple plan views of respective alternative embodiments of screening elements according to the present invention. 
       FIG. 5  is a schematic perspective assembly view of a screening deck with the screening elements arranged perpendicularly to the longitudinal direction of the screening deck. 
       FIG. 6  is a section view of a first embodiment of a carrier in a screening deck according to the present invention. 
       FIG. 7  is a section view of a second embodiment of a carrier in a screening deck according to the present invention, and 
       FIG. 8  is a perspective assembly view specifically illustrating an adapter for enabling a screening element according to the present invention to be used in a conventional screening deck assembly. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  schematically shows a screening deck  100  for the screening of crushed stone. As used herein the expression “crushed stone” includes stone, gravel, and the like that has been crushed to any suitable size. The screening deck comprises three short screening elements  110   a , three tall screening elements  110   b  and three transverse carriers  120 . The screening elements  110   a  and  110   b  differ in height, but otherwise they have a substantially identical shape. In  FIG. 1  the top surface of screening element  110   a  is lower than that of screening element  110   b . The screening elements  110   a  and  110   b  are normally alternately placed so that the neighboring screen element always will be of the other type. Each carrier  120  includes two elongated stanchions  130 ,  130 ′, extending parallel to the other carriers  120 . 
   A longitudinal direction of the screening deck is indicated with an arrow A in  FIG. 1 . The longitudinal direction of the screening deck is also the traveling direction for the crushed stone material in the vibrating screen. As shown especially well in  FIG. 2 , each screening element  110   a  is provided with snap locks  140  at its underside. The snap locks interact with the elongated stanchions  130 ,  130 ′ for fastening the screening element to the transverse carriers  120 .  FIG. 2  shows a larger-scale perspective view of the screening element  110   a  including the snap locks  140 . Through holes H have been provided in a screening membrane  115  for fractionating the crushed stone into fractions of stones or gravel of different sizes. The screening element  110   b  is substantially similar to the screening element  110   a , except for the difference in height, as noted earlier. 
   In  FIG. 3 , an underside of a screening element  110   a ,  110   b  is shown. As can be seen, the screening element comprises a framework  111 , including both longitudinal frame portions  112  and transversal frame portions  113 . The screening membrane  115  is provided between the frame portions  111 ,  112  and  113 . 
   Four embodiments  160 ,  170 ,  180  and  190  of the screening element according to the present invention are shown in  FIGS. 4   a - 4   b , respectively. The screening element  160 , which is shown in  FIG. 4   a , is similar to the screening element shown in  FIGS. 1-3 . The screening element  160  has two sides  161 ,  162  that are not parallel with either the longitudinal direction A of the screening deck or the traveling direction of the material. The sides deviate at an angle α from the longitudinal direction A. The angle α should be in the range of 1 and 45 degrees, more preferably in the range of 1 and 15 degrees. The angles have an effect on the traveling of material that will be discussed later. 
   The screening element  170 , shown in  FIG. 4   b , has one side  172  parallel with the longitudinal direction A, and has one side  171  that is not parallel with the longitudinal direction A. 
   In  FIG. 4   c , a screening element  180  that has two sides  181 ,  182  that are not parallel with the longitudinal direction A is shown. The two sides  131 ,  182  are, however, parallel with each other. 
   The screening element  190 , shown in  FIG. 4   d , is rotated 180 degrees compared to the screening element  160 . It has two sides  191 ,  192  that are not parallel with the longitudinal direction A of the screening deck or with the traveling direction of the material. 
   In  FIG. 5  an alternative orientation of the screening elements  110   a  on the screening deck  100  is shown. Alternatively, the screening element  110   b  could have been shown to demonstrate this. According to this embodiment, the carriers  120  are parallel with the longitudinal direction A of the screening deck  100 . Only one type of screening element is used, i.e., all screening elements have the same height, creating a substantially flat screening deck  100 . The screening elements  110   a  are alternately orientated so that a continuous screening deck  100  is created, and the screening elements  100   a  can be fastened to the carriers  120 . 
   In  FIGS. 6 and 7 , cross-sections of two respective embodiments of the carriers  120  are shown. According to the first embodiment, in  FIG. 6 , the stanchion  130  is lower than the stanchion  130 ′, which together with the different heights of the screening elements  110   a ,  110   b  results in “steps” (i.e., adjacent portions of different elevations) being formed on the screening deck  100 , as indicated by the arrow B in  FIG. 1 . According to the second embodiment, shown in  FIG. 7 , the stanchions  130 ,  130 ′ have the same height, which results in a flat screening deck  100 , shown in  FIG. 5 , provided that the height of the screening elements  110   e ,  110   b , as measured from the snap lock  140  to the screening membrane  115 , does not differ. 
   In  FIG. 8 , an adapter  200  is shown for fitting a screening element  110   a  according to the present invention to a prior art assembly according to SE-A-C 460 340. The adapter  200  comprises a lower surface  210  for interaction with a shelf  220  of a prior art carrier  230 . The adapter further comprises a stanchion  240  for interaction with the snap locks  140  of the screening elements  110   a  according to the present invention. During operation, the adapter  200  is kept in its place by a force exerted by a screening element fastened on the stanchion  240 , since the screening element is fastened to a stanchion  250  in its other end. 
   In practice, the carriers  120  are fastened by bolting, welding or other suitable fastening means to support beams (not shown) arranged in a vibrating screen. The screening elements  110   a ,  110   b  are fastened to the elongated stanchions  130 ,  130 ′ with the snap locks  140 . The combination of screening elements  110   a ,  110   b  being fastened on the stanchions results in a screening deck  100 . Even though the shown embodiments include the feature of fastening both ends of the screening elements  110   a , it would be possible to fasten only one end of the screening element. Likewise, the invention has only been shown with the snap locking method for fastening the screening element as it provides flexible fastening means, but other means of fastening are also possible, e.g., bolting, screwing, jamming or clamping. 
   As implied in  FIGS. 1-3  and  5 , but clearly shown in  FIGS. 4   a - 4   c , the screening elements  110   a ,  110   b  according to the present invention in most cases have a non-rectangular shape seen from above, i.e., the screening elements have one narrow end  110 N and one wide end  110 W. The screening element  180  in  FIG. 4   c  differs from this by having two ends with the same width. As earlier stated, the screening elements  110   a ,  110   b  are alternately fastened on the carriers  120 , i.e., one wide end  110 W of one screening element  110   a ,  110   b  is neighbored by two narrow ends  110 N of the neighboring screening elements  110   a ,  110   b . By this arrangement, the sides of the adjacent screening elements do not form any straight paths from one end of the screening deck  100  to the other end of the screening deck  100  parallel to the travel path, which minimizes the risk that stones or gravel may travel all the way from one end of the screening deck  100  to the other end of the screening deck  100  without encountering a hole H. 
   As is well known to people skilled in the art of screening, a screening membrane is provided with holes H having varying respective sizes and shapes to fractionate crushed stone into different-size fractions of stones or gravel. According to the invention, the holes H are also arranged with a transversal displacement so that the stones or gravel cannot travel in the longitudinal direction of the screening deck without encountering a screening hole. As shown in, e.g.,  FIG. 2 , the holes H could be grouped in different groups H 1 -H 3  as the width of the screenings element varies. In  FIG. 2  the holes are mutually parallel, having a greater density of holes in the group of holes H 1  located close to the wide end  110 W of the screening element  110   a , than in the group of holes H 3  located close to the narrow end  110 N.  FIG. 2  further shows that the holes of each of the groups H 1 -H 3  are displaced (offset) in relation to the holes of the next group in the direction of travel and in relation to the longitudinal direction A of the screening deck. Every row of holes H could be transversely displaced in relation to the most of the other rows (not shown), and not merely transversely displaced in relation to other groups of rows of holes H as shown in  FIG. 2 . 
   As mentioned above, the angle α can vary in the range of 1 and 45 degrees. It is preferable to have a relatively large angle α, since with increasing angle α the traveling speed of the stones and the gravel over the screening deck is reduced, and the likelihood for a stone or piece of gravel to fall into the screening holes is thereby increased. A larger angle α, however, causes a larger wear on the screen element, necessitating that the screen elements be replaced more often. The preferred angle α is therefore between 1 and 15 degrees. 
   The size of the screening elements can vary, but is adapted to fit as many vibration screens as possible. To facilitate the assembly of the screening decks the different screening elements  110   a ,  110   b  with different heights can be colored differently, e.g., grey for the screening element  110   a  and blue for the screening element  110   b.    
   The preferred material of the screening elements is polyurethane (PU) or rubber. In a preferred embodiment, the framework  111 ,  112 ,  113  is manufactured from relatively unresilient PU, whereas the screening membrane  115  of the screening element  110   a ,  110   b  is manufactured of a more resilient PU. The preferred materials for the framework  111 ,  112 ,  113  have a hardness that preferably is in the range from about 90 Shore A to about 75 Shore D, and the preferred materials for the screening membrane have a hardness of about 30 Shore A to about 95 Shore A or, more preferred, from about 40 Shore A to about 80 Shore A. 
   Preferred materials are, e.g., PU, metal, rubber, PVC, polyethylene, polyamide, polyester or the like for the framework  111 ,  112 ,  213  and urethane rubber, suitable natural rubber compounds or other rubber materials for the screening membrane. The invention is, however, not limited to screening elements without a separate framework, but also applies to screening elements with a frame like prior art screening elements. 
   The height of the stanchions  130 ,  130 ′ can, as mentioned, be varied. By having a larger height difference between the stanchions  130 ,  130 ′, the step height between each row of screening elements increases. The difference in stanchion height corresponds to the step height B, shown in  FIG. 1 , on the screening deck  100 . 
   As an alternative to the embodiment in  FIG. 1 , every screening element can be shaped as if rotated 180 degrees in the vertical plane whereby the narrow end of the screening element  110   a  would be located upstream and the wide end located down-stream. The screening elements  110   b  would have the wide end located upstream and the narrow end located downstream. This provides a screening deck, where material from the screening element  110   b  will fall down to screening element  110   a  and create turbulence in the material. The screening element  110   a  will alter the direction of the material much less due to the widening shape. It is, however, possible for the material membrane to be slightly thinned out since the screening element is widening along the traveling direction. 
   In the foregoing it has been described that the non-flat structure of the screening deck, i.e., the steps and difference in level, is provided by screening elements of different height and by stanchions of different height, but it could of course be provided in other ways as well. 
   The invention should not be limited to the shown embodiment; modifications within the scope of the appended claims are possible. For example, there could be used more than two types of different-height screening elements.