Abstract:
A grate assembly having at least two generally arcuate bodies and methods of forming and assembling the bodies of the grate assembly. Each body has an outer perimeter and a plurality of openings passing therethrough. At least a portion of the outer perimeter of each body has a perforate edge surface. The perforate edge surface is arranged for mating engagement with the perforate edge surface of an adjacent body to define at least some of the openings through which wood particulate passes.

Description:
[0001]     This application claims the benefit of U.S. Provisional Application No. 60/493,701, filed Aug. 8, 2003. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to comminution machines, such as tub grinders, horizontal feed grinders, gravity fed grinders, or the like, and more particularly to grates for receiving material from hammer mills in a wood comminution machine wherein the grates determine the size of the particulate that is produced by the comminution machine.  
       BACKGROUND OF THE INVENTION  
       [0003]     It is known in the art to prepare ground wood product, commonly referred to as particulate or chips, having a predetermined particulate size utilizing tub grinders, horizontal feed grinders, and the like wherein a hammer mill rotates with a shaft to grind the wood between hammers on the hammer mill and a surface presenting an anvil. Typically, screens or grates having an arcuate shape are located beneath the hammer mill and have a plurality of openings that determine the ultimate size of the particulate that is produced by the machine. The openings are known to take on a variety of shapes and sizes, such as square, circular, triangular, hexagonal, octagonal, and so on. Typically, the overall grates span approximately 180 degrees, commonly requiring two or three removable abutting arcuate grates to span the 180 degrees. As shown in  FIG. 1  of the prior art, the grates are commonly initially formed having a continuous imperforate flat frame  1  around an outer perimeter. Once bent to a curve, one of the perimetrical frames  1  of one grate  2  abuts another perimetrical frame  1  of a mating grate  3  to establish a seam  4  between the mating grates  2 ,  3 . As a result, the seam  4  presents a continuous imperforate wall  5  that prevents throughput of ground particulate from the machine. Ultimately, by preventing or restricting throughput in the areas where abutting frame surfaces are created between mating grates, the potential throughput of ground particulate produced by the machine is reduced. To compensate for this, it is known to construct grates with webbing having a minimal material thickness in an attempt to provide either more openings or more open area within each opening, and thus, to gain more particulate throughput. Ultimately, a reduced web thickness reduces the useful life of the grates.  
       SUMMARY OF THE INVENTION  
       [0004]     A grate assembly constructed according to the present invention improves the throughput of wood particulate in a comminution apparatus. The grate assembly has at least two generally arcuate bodies, with each body having openings passing therethrough and having an outer perimeter. At least a portion of the outer perimeter of each body has a perforate edge surface. The perforate edge surface is arranged for mating engagement with the perforate edge surface of an adjacent body to define at least some of the openings through which wood particulate passes.  
         [0005]     Another aspect of the invention includes forming a grate for a grate assembly including at least two of the grates and being used in a comminution apparatus that produces wood particulate. The steps include providing a sheet of steel having a predefined outer perimeter configuration and forming a plurality of openings having continuous peripheral surfaces through the sheet. Further, forming a discontinuous edge along at least a portion of the outer perimeter for mirrored mating engagement with another discontinuous edge on an abutting grate. Then, bending the sheet into a predefined arcuate shape.  
         [0006]     Another aspect of the invention includes a method of assembling a grate assembly within a housing having curved support surfaces, the housing being part of a comminution machine that produces wood particulate. The steps include providing at least two generally arcuate rigid bodies each having a plurality of openings defined by continuous peripheral surfaces and an outer perimeter with at least one discontinuous edge surface. Further, placing at least two of the bodies in the housing in abutment with the curved surfaces and arranging the discontinuous edge surface of one of the bodies in mirrored mating engagement with the discontinuous edge surface of an adjacent one of the bodies to define a plurality enclosed openings between the adjacent bodies.  
         [0007]     Some of the objects, features and advantages of the invention include a grate assembly that increases the amount of throughput of wood particulate in a comminution apparatus, provides an increased number of openings extending therethrough while having an increased lateral cross-sectional area through a webbing of each grate, improves the size consistency of the wood particulate, reduces the amount of material in constructing the grate assembly, reduces the overall weight of the grate assembly, is of relatively simple and economical design and manufacture and has an extended life in use. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     These and other objects, features and advantages of this invention will become readily apparent from the following detailed description of the preferred embodiments and best mode, appended claims and accompanying drawings in which:  
         [0009]      FIG. 1  is a plan view of a grate assembly before bending constructed according to the prior art;  
         [0010]      FIG. 2  is a side elevation view of a tub grinder apparatus having a grate assembly constructed according to one presently preferred embodiment of the invention;  
         [0011]      FIG. 3  is a view looking generally in the direction of arrow  3  in  FIG. 2  showing a hammer mill;  
         [0012]      FIG. 4  is a plan view of the grate assembly of  FIG. 2  before bending;  
         [0013]      FIG. 5  is an assembled perspective view of the grates with the hammer mill of  FIG. 3  removed;  
         [0014]      FIG. 6  is an enlarged fragmentary view showing mating surfaces of the grates taken generally along line  6 - 6  of  FIG. 5 ; and  
         [0015]      FIG. 7  is an enlarged fragmentary perspective view of the encircled area  7  of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     As shown in  FIG. 2 , a comminution apparatus, represented here as a tub grinder  10 , has a pair of screens or grates  12 ,  13  ( FIGS. 5 and 6 ) constructed according to one presently preferred embodiment. The grates  12 ,  13  facilitate the throughput of ground wood chips or particulate, and ultimately enhance the efficiency of producing ground particulate. It should be recognized that though the grates  12 ,  13  are shown in a tub grinder  10 , that other comminution machinery may incorporate the grates  12 ,  13 , for example and without limitation, horizontal feed grinders, gravity feed grinders, and the like, which is operative to reduce coarse or unground material to a smaller size finished material.  
         [0017]     As shown in  FIG. 3 , a tub  14  of the tub grinder  10  has a generally cylindrical wall  16  that rotates relative to a stationary base or floor  18 . As the wall  16  rotates, the unground wood material, commonly in the form of used pallets (not shown), in the tub  14  is caused to move in the direction of wall rotation. As the unground material rotates, it is urged into contact with a rotating hammer mill  20 .  
         [0018]     The hammer mill  20  is supported by an elongate shaft (not shown) within a housing  22  ( FIG. 5 ) for rotation with the shaft. As the hammer mill  20  rotates, a plurality of hammers  24  forcefully engage the unground wood material to cut or pulverize the material against an anvil  26  ( FIG. 3 ) typically supported by the floor  18  of the tub  14 . Upon the material being cut, the material falls against the grates  12 ,  13 , and the material that is of the desired finish size falls through openings  28  in the grates  12 ,  13  and onto a conveyor or auger  30  ( FIG. 2 ). The material that is too large, and thus unable to fit through the openings  28  is re-circulated by the rotating hammer mill  20  so that it can be further cut until it is of the desired size, thereby allowing it to pass through the openings  28 .  
         [0019]     The grates  12 ,  13  are generally formed from a metallic material, and preferably are formed from steel. As best shown in  FIG. 4 , initially, the grates  12 ,  13  have a frame or body  32 ,  33  cut or stamped from a sheet of steel to define an outer perimeter  34 ,  35  of the bodies  12 ,  13 , respectively. The bodies  32 ,  33  then have the openings  28  formed through the thickness of the bodies  32 ,  33 . The openings  28  are preferably stamped or cut, such as by a laser, water jet, EDM, and the like, thereby defining webs or webbing  44  having a thickness (t) ( FIG. 7 ) outlining a continuous peripheral surface of each opening  28 . As shown in  FIG. 4 , the outer perimeters  34 ,  35  of the bodies  32 ,  33  have a pair of opposite ends  41 ,  43  terminating at a pair of opposite sides  36 ,  37 , respectively. At least one of the sides  36 ,  37  of each body  32 ,  33  has a discontinuous or partly open perforate edge surface  39  for mating engagement with one another in use. Upon forming the openings  28  within the bodies  32 ,  33 , the bodies  32 ,  33  are bent, preferably with the use of a press, to take on a predefined generally arcuate or curved mating shape ( FIGS. 5 and 6 ). The independent arcuate bodies  32 ,  33  are supported at their ends  41 ,  43  in the machine on complimentarily curved support surfaces  45  on which they rest and from which they may be separately removed. Additionally, the bodies  32 ,  33  are shown supported along their length (L) by complimentarily curved support surfaces  47  between the ends  41 ,  43 . Desirably, the bodies  32 ,  33  have solid imperforate ribs  48  for mating engagement and support against the support surfaces  47 .  
         [0020]     As best shown in  FIG. 5 , the grates  12 ,  13  in the tub grinder  10  typically span one-half of the entire grate surface. An enlarged view of the mating perforate edge surfaces  39  is shown in  FIG. 6 , better showing a discontinuous seam  38  being defined between the mating and preferably mirrored discontinuous edge surfaces  39  of the abutting bodies  32 ,  33 . The mating perforate edge surfaces  39  generally mate across the thickness (t) and define openings  40  ( FIG. 5 ) preferably similar in shape and size to the openings  28  throughout each grate  12 ,  13 . As such, ground material or particulate of the desired size is free to pass through the composite openings  40  formed partly in each grate  12 ,  13  along the seam  38 , thereby improving the efficiency in producing ground particulate. In the prior art, ground material is obstructed by the imperforate wall  5  formed by the mating imperforate edges ( FIG. 1 ), thereby reducing the amount of ground material throughput of a comminution apparatus incorporating the prior art grates  2 ,  3 . Upon the material passing through the grates  12 ,  13 , it falls onto the pair of augers  30  preferably rotating in opposite directions so that the material is fed to a conveyor  42  ( FIG. 2 ). The conveyor  42  then feeds the ground material into a pile or elsewhere for further use.  
         [0021]     The bodies  32 ,  33  of the grates  12 ,  13  are preferably manufactured from a metallic material such as steel, wherein the openings  28 ,  40  are defined by a web  44  of steel ( FIG. 7 ). Each opening  28 , except for those at the mating perforate edge surfaces  39 , is preferably formed as a fully enclosed opening by the web  44 . The majority of the openings  28  are shown here having a generally hexagonal shape, though it should be understood that other geometries are equally suitable, such as triangular, rectangular, square, circular, or the like, as desired in the application.  
         [0022]     In establishing the openings  40  along the seam  38 , not only is the throughput of the ground material improved, but further design benefits can be achieved. For example, in having additional openings for ground material to pass through, the webs or webbing  44  surrounding each opening can be constructed having an increased cross-sectional thickness (t) without having to reduce the area presented by the openings  28  from that of the prior art, and thus the amount of throughput of ground material can be maintained. As such, the grates  12 ,  13  can be constructed having a more durable and long-lasting life in use. In addition, by eliminating the continuous edge  1  ( FIG. 1 ), the grates  12 ,  13  can be constructed having a reduced material content, thereby reducing weight, and lowering the material cost of the grates  12 ,  13 .  
         [0023]     It is to be understood that other embodiments of invention which accomplish the same function are incorporated herein and are contemplated to be within the scope of any allowed claims. For example and without limitation, it should be recognized that shapes of the openings  28 ,  40  and sizes of the openings  28 ,  40  may be varied as desired to achieve the desired size of ground product. In addition, any number of grates may be constructed to abut one another, such as three mating grates commonly used in horizontal feed grinders. The invention is defined by the claims that follow.