Patent Publication Number: US-7213779-B2

Title: Rotary grinder apparatus and method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is a divisional of U.S. patent application Ser. No. 10/138,807 filed May 3, 2002, now U.S. Pat. No. 6,840,471 which is a continuation-in-part of U.S. patent application Ser. No. 09/513,011 filed Feb. 25, 2000, now U.S. Patent No. 6,422,495, issued on Jul. 23, 2002, which applications are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to rotary grinders used for grinding things such as waste materials. More particularly, the present invention relates to rotary grinders having rotating arrangements of hammers. 
   BACKGROUND OF THE INVENTION 
   Grinders for grinding waste material such as trees, brush, stumps, pallets, railroad ties, peat moss, paper, wet organic materials and the like are well known. An example of one such prior art grinder, known as a tub grinder, is shown in commonly assigned U.S. Pat. No. 5,507,441 dated Apr. 16, 1996. Another example is shown in U.S. Pat. No. 5,419,502 dated May 30, 1995. Another type of grinder is known as a horizontal grinder, examples can be found disclosed in U.S. Pat. No. 5,975,443, U.S. Pat. No. 5,947,395, U.S. Pat. No. 6,299,082. 
   There are 4 different types of grinders that can be identified as defined in U.S. Pat. No. 6,299,082 including chippers, hammer mills, hogs and shredders: Each including a type of a rotary grinding device. 
   Tub grinders typically include a rotary grinding devices such as a hammermill or hog that is mounted on a frame for rotation about a horizontal axis. The hammermill or hog function in cooperation with a shear bar or anvil and typically a screen; the assembly including the hammermill or hog, anvil and screen forming a grinding device. A rotating tub surrounds the grinding device. The tub rotates about a generally vertical axis. Debris is deposited in the rotating tub and the grinding device grinds the debris. 
     FIG. 1  illustrates one type of prior art hammermill  20  commonly used with conventional tub grinders. The hammermill  20  includes a plurality of hammers  22  secured to a plurality of rotor plates  24 . The rotor plates  24  are rotatably driven about a generally horizontal axis of rotation  26 . Cutters  25  (e.g., cutter blocks, cutter teeth, etc.) are mounted on the hammers  22  (e.g., with nuts  30  and bolts  28 ). The hammers  22  are secured between the rotor plates  24  by shafts or rods  31  aligned generally parallel to the horizontal axis of rotation  26 . For example, each hammer defines two holes  32  and  34  each positioned to receive a different shaft  31  (only one shown). Shims  36  are mounted between the hammers  22  and the rotor plates  24 . When the rotor plates  24  are rotated about the axis of rotation  26 , the hammers  22  are carried by the rotor plates  24  in a generally circular path. Material desired to be ground is fed into the circular path such that the material is impacted and reduced in size by the cutters  25  of the hammers  22 . The grinding device of a conventional tub grinder also typically includes a sizing screen that curves along a lower half of the hammermill.  FIG. 15  illustrates a grinding device typical of the prior art including a rotary grinder  20 , anvil  100  and screen  102 . In this particular embodiment the screen  102  is comprised of 2 portions to aid removal and replacement. They are made to be replaceable, as different screens are installed to achieve differing ground material sizes. 
   The screens  102  are supported in alignment with the rotary grinder by plates  104  that are located on the sides of opening  45  in the floor  44  corresponding to the ends of the rotary grinder  20 , and in the vicinity of the rotary grinder support bearings. They are supported by frame  48 . Anvil  100  is supported by the frame  48  and by the screen  102 . The screens  102  are available in the prior art in a variety of configurations. One variety include round holes, another includes square or rectangular holes. The size of the holes varies, and effects the maximum size material that is allowed to pass through. Other variations of the screens include varying circumferential coverage wherein the length of screen is reduced, thereby increasing the gap  106  between the screens. It is known to significantly increase the gap  106  to allow material to exit the grinding device to reduce drag and power requirements. This is typically done in applications wherein the size of the ground material is not critical. 
   A grinding chamber is formed between the screen and the hammermill. The screen performs a sizing function and defines a plurality of openings having a predetermined size. In use, material desired to be ground is repeatedly impacted by the hammers  22  against the screen, or crushed between the hammers  22  and the screen, causing the material to be reduced in size. When the material is reduced to a size smaller than the predetermined size of the openings defined by the screen, the material moves radially through the screen. Upon passing through the screen, the reduced material commonly falls by gravity to a discharge system located beneath the hammermill  20 . 
   The grinding device of a horizontal grinder typically includes an anvil and a screen. Many different configurations for horizontal grinders have been developed, but the basic grinding actions are similar to those found in tub grinders. 
   The typical prior art hammermills or hogs generally utilize block-shaped cutters mounted such that the effective cutting edge is parallel to the axis of rotation. This results in a surface of rotation for each cutter describing a cylinder, having a single effective cutting diameter that cooperates with the straight edge of the anvil. 
   Many other techniques have been developed to improve the cutting efficiency including U.S. Pat. No. 4,066,216 disclosing relatively narrow cutters with plates that project into the space between cutters and U.S. Pat. No. 3,580,517 disclosing sharp-pointed cutters with an anvil that matched the profile of the surface of rotation defined by the cutters. In both of these examples the cutters are not as robust as a standard block-type cutter, resulting in concerns related to durability. Hammer wear is a significant concern relating to hammermills. For example, hammer wear results in loss of hammer integrity, out-of-balance conditions, reductions in grinding efficiency, and increases in maintenance and service costs. With a conventional hammermill, it is difficult to replace the hammers because the hammermill must be disassembled. Disassembling a hammermill can be particularly labor intensive and time consuming because the rods used to connect the hammers to the hammermill are quite heavy. There are typically several rods per hammermill and frequently two rods must be removed to replace a single hammer. Furthermore, rods can be corroded in place or deformed thereby making it even more time consuming and costly to disassemble a hammermill. 
   Power requirements and resulting fuel consumption is also affected by the interaction of the screens and the hammers. The crushing characteristic is known to result in a significant amount of frictional drag. This drag results from to the tendency to trap the material between the stationary screen surface and the moving cutters or hammers while under significant load. This condition results in either the material moving with the cutters and sliding against the screen or the material being retained by the screen and the cutters sliding past the material or some combination. Any of these result in significant drag, thus grinders typically require significant power. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention relates to a rotary grinder having a cylindrical drum rotatable about its axis. The cylindrical drum has a cylindrical wall, a first end and a second end. The cylindrical wall defines a first receiving hole and a second receiving hole for receiving opposite ends of a through-member. The first end of the through-member extends to the outside of the cylindrical wall by passing through the first receiving hole such that the first end of the through-member comprises a first grinding portion (e.g., a hammer, cutter, blade, tooth, etc.) when the cylindrical drum is rotated. Likewise, the second end of the through-member extends to the outside of the cylindrical wall by passing through the second receiving hole such that the second end of the through-member comprises a second grinding portion (e.g., a hammer, cutter, blade, tooth, etc.) when the cylindrical drum is rotated. Thus, the through-member forms a duplex grinding member (e.g., a duplex hammer). 
   Another aspect of the present invention relates to a rotary grinder having a plurality of grinding members secured to a drum by a single retaining member that extends longitudinally through the drum. 
   Another aspect of the present invention relates to a replaceable through-member adapted for use with a rotary grinder in accordance with the principles of the present invention. A further aspect of the invention relates to a method of securing a grinding member to a hollow drum by using a longitudinal retaining member. 
   In accordance with another aspect of the invention, a method for replacing a drum in a rotary grinder is presented. The rotary grinder includes a rotatable drum having a first end and a second end and a cylindrical surface. The rotary grinder also includes a plurality of hammers attached to the cylindrical surface and a first end cap attached to the first end of the drum and a second end cap attached to the second end of the drum. The method comprises the steps of removing the first end cap from the rotatable drum; removing the second end cap from the rotatable drum; replacing the rotatable drum with a second rotatable drum; attaching the first end cap to the first end of the second rotatable drum; and attaching the second end cap to the second end of the second rotatable drum. 
   Another aspect of the present invention relates to a grinding device which includes a novel screen that works in conjunction with the rotary grinder to improve the efficiency of the grinding process to require less power and fuel. 
   Another aspect of this invention is a grinding device that includes the novel screen and rotary grinder to improve the grinding efficiency and thus to achieve improved ground material size consistency. 
   Another aspect of this invention is a novel screen adaptable to several types of cylindrical drums to improve the grinding efficiency 
   A variety of advantages of the invention will be set forth in part in the description that follows, and in part will be apparent from the description, or may be learned by practicing the invention. It is to be understood that both the foregoing general description and the following detailed description are explanatory only and are not restrictive of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate several aspects of the invention and together with the description, serve to explain the principles of the invention. A brief description of the drawings is as follows: 
       FIG. 1  is a perspective view of a prior art hammermill assembly; 
       FIG. 2  is a schematic illustration of a tub grinder incorporating aspects of the invention; 
       FIG. 3  is a top view of the tub grinder of  FIG. 2 ; 
       FIG. 4   a  is a perspective view of a cylindrical drum of one embodiment of the invention; 
       FIG. 4   b  is a cross-sectional view of the drum of  FIG. 4   a  taken along section lines  4   b — 4   b;    
       FIG. 4   c  is a perspective view of the drum of  FIG. 4   a  with mounting sleeves mounted therein; 
       FIG. 5   a  is a perspective view of one embodiment of a hammermill of the invention; 
       FIG. 5   b  is a partially exploded, perspective view of the hammermill of  FIG. 5   a;    
       FIG. 5   c  is a side view of a connection configuration for securing a cutter to one of the hammers of the hammermill of  FIGS. 5   a – 5   b;    
       FIG. 6  is a perspective view of one of the duplex hammers of the hammermill of  FIG. 5   a;    
       FIG. 7   a  is a side view of an alternative embodiment of a duplex hammer of the invention 
       FIG. 7   b  is a side view of the alternative embodiment of the duplex hammer of  FIG. 7   a  taken along a line perpendicular to the view of  FIG. 7   a;    
       FIG. 8  shows another duplex hammer adapted for use with the hammermill of  FIG. 5   a;    
       FIG. 9  is a schematic, elevational view of the hammermill of  FIG. 5   a;    
       FIG. 10  is a side view of a connection configuration for securing a cutter to one of the hammers of the hammermill of  FIGS. 5   a – 5   b;    
       FIG. 11  shows a modified end plate design for the hammermill of  FIG. 5A ; 
       FIG. 12  is an end view showing maximum and minimum cutting diameters for a grinding member that is an embodiment of the present invention; 
       FIG. 13  is a perspective view showing the maximum and minimum cutting diameters of  FIG. 12 ; 
       FIG. 14  is a perspective view showing the maximum and minimum cutting diameters for an entire hammermill; 
       FIG. 15  is an end view of a prior art grinder; 
       FIG. 16  is an end view of a grinder including a grinding device that is an embodiment of the present invention; 
       FIG. 17  shows the grinder of  FIG. 16  with the end plate removed; 
       FIG. 18  shows a grinding device in accordance with the principles of the present invention that includes an enhanced sizing screen; 
       FIG. 19  is a side view of the sizing screen included with the grinding device of  FIG. 18 ; 
       FIG. 20  is a perspective view of the sizing screen of  FIG. 19 ; and 
       FIG. 21  shows the spacial relationship between the grinding members and the sizing screen of the grinding device of  FIG. 18 . 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to exemplary aspects of the present invention which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   Referring to  FIGS. 2 and 3 , a tub grinder  40  is shown. The tub grinder  40  is being shown exclusively to provide an illustrative field or environment to which the various aspects of the present invention are applicable. It will be appreciated that the tub grinder  40  is but one example of a type of grinding machine to which the various aspects of the present invention can be applied, and is not intended to in any way limit the scope of the present invention. 
   The tub grinder of  FIGS. 2 and 3  includes a rotary tub  42  mounted above a horizontal floor  44  for rotation about a vertical axis z—z. The floor  44  and the tub  42  are secured to a frame  48  of a trailer  46 . The frame  48  includes a hitch  50  for attachment to a semi-tractor for towing the tub grinder  40 . Wheels  52  are mounted on the frame  48 . A rotary grinder member or hammermill  56  is secured to the frame  48  beneath the tub  42 . 
   As best illustrated in  FIG. 3 , the floor  44  includes a floor opening  45  for allowing an upper portion of the hammermill  56  to extend into the tub  42 . In the remainder of this disclosure the term hammermill is meant to be synonymous with hog or rotary grinder. The hammermill  56  is mounted for rotation about a horizontal axis x—x and includes a plurality of hammers  53  (shown schematically in  FIGS. 2 and 3 ) that engage and crush waste material deposited in the tub  42 . The hammers  53  are secured to a drum  61  of the hammermill  56  as described below. 
   The hammermill  56  is coupled via a shaft  54  to an engine  58  for rotating the hammermill  56 . In operation, the tub  42  is rotated about the vertical axis z—z by a motor  55  (shown in  FIG. 2 ). Simultaneously, the hammermill  56  is rotated about the horizontal axis x—x. 
     FIG. 4   a  shows the cylindrical drum  61  of the hammermill  56 . The cylindrical drum  61  is hollow and includes a cylindrical wall having a cylindrical exterior surface  65  and a cylindrical interior surface  67 . The cylindrical drum  61  defines a plurality of holes  70  arranged in a pattern that spirals around the cylindrical surface of the drum  61 . Each hole  70  has a corresponding hole  72  positioned on the opposite side of the drum  61  from the hole  70 . The holes  70 ,  72  extend through the drum  61  in a radial direction between the interior and exterior surfaces  65  and  67 . Preferably, the holes  70 ,  72  are positioned such that straight lines  69  drawn from the holes  70  to their corresponding holes  72  pass through the horizontal axis x—x of the drum  61 . In the depicted embodiments, the holes  70  are axially staggered or offset relative to their corresponding holes  72  such that the straight lines  69  extending between the holes  70 ,  72  intersect the horizontal axis x—x at an oblique angle θ (shown in  FIG. 4   b ). In certain non-limiting embodiments, oblique angle θ is in the range of 80–90 degrees, or about 83 degrees. Preferably, the angle is selected such that cutters/grinders mounted adjacent the holes define separate cutting paths. Thus, the angle selected is typically at least partially dependent of the diameter of the drum  61 . Of course, the angle θ need not be limited to oblique configurations, and could also be perpendicular. 
     FIG. 4   c  shows the drum  61  with sleeves  63  that extend radially between the holes  70 ,  72 . The sleeves  63  extend radially through the interior of the drum  61  and are preferably welded in place. Each sleeve  63  defines a channel  75  that extends from one of the holes  70  to a corresponding hole  72 . 
   The shape of the holes  70 ,  72  in the embodiment shown in  FIG. 4   a  is rectangular. However, the scope of this invention is not limited to holes  70  and  72  having a rectangular shape. For example, the holes  70  and  72  could be circles, ovals, triangles or any other shape. 
     FIG. 5   a  shows the hammermill  56  in isolation from the tub grinder  40 . The drum  61  of the hammermill  56  includes oppositely positioned first and second ends  108  and  110  that are respectively closed or covered by first and second end caps  104  and  106 . As best shown in  FIG. 5   b , the first and second ends  108 , 110  have threaded holes  112  that align with corresponding holes  114  in the first and second end caps  104 , 106 . The end caps  104 ,  106  are preferably removably connected to the drum  61 . For example, bolts  116  can be used to removably secure the end caps  104 ,  106  to the drum  61  by inserting the bolts through the holes  114  and then threading the bolts  116  into the openings  112 . The removability of the end caps  104 ,  106  is advantageous because the drum  61 , which has a greater tendency to wear than the end caps, can be replaced without requiring the end caps  104 ,  106  to be replaced at the same time. This also allows the drum  61  to be reversed (rotated end-to-end relative to the end caps  104 ,  106 ) to increase the useful life of the drum  61 . 
   As described above, the end caps  104 ,  106  are connected to the drum  61  by fasteners  116 . It will be appreciated that this is but one fastening technique that could be used. Other techniques include, among other things, providing mating threads on the end caps and the drum such that the end caps can be threaded onto or into the drum. Alternatively, a snap-ring configuration, as well as other configurations, could also be used to secure the end caps  104 ,  106  to the drum  61 . 
   A driven shaft  118  is provided on the second end cap  106 , and a non-driven shaft  130  is provided on the first end cap  104 . The shafts  118 ,  130  are preferably connected to their respective end caps  106 ,  104  by conventional techniques (e.g., the shafts  118 ,  130  can be welded to or forged as a single piece with their respective end caps  106 ,  104 ). The shafts  118 ,  130  are aligned along the axis of rotation x—x of the hammermill  56  and project axially outward from their respective end caps  106 ,  104 . The driven shaft  118  defines a keyway  120  or other type of structure (e.g., splines) for use in coupling the driven shaft  118  to the drive shaft  54  of the engine  58 . In this manner, engine torque for rotating the hammermill  56  can be transferred to the hammermill  56  through the driven shaft  118 . When mounted within the tub grinder  40 , the shafts  118 ,  130  are preferably supported in conventional bearings adapted for allowing the hammermill  56  freely rotate about the axis of rotation x—x. 
   Referring to  FIGS. 5   a  and  5   b , the hammermill  56  also includes a plurality of through-members  76  (e.g., bars) that extend radially through the drum  61  and include ends that project radially beyond the exterior surface  65  of the drum  61 . Each of the through-members  76  forms two hammers  53  positioned on opposite sides of the drum  61 . Hence, the through-members  76  can be referred to as “duplex hammers.” The particular embodiment shown in  FIGS. 5   a  and  5   b  includes eight through-members  76  that provide a total of sixteen hammers. However, any number of through-members  76  could be used. 
   As best shown in  FIG. 5   b , the through-members  76  each have a first end  78 , a second end  80  and a central portion  82 . The central portions  82  are situated in the interior of the cylindrical drum  61 . Each through-member  76  extends through one of the holes  70  of the drum  61 , and also through the corresponding opposite hole  72  of the drum  61 . Within the drum  61 , the through-members  76  extend through the channels  75  defined by the sleeves  63 . The holes  70 ,  72  allow the first and second ends  78 ,  80  to be situated outside the exterior of the cylindrical drum  61  so as to form exterior hammers. Each through-member  76  has a leading face  84  and a trailing face  86  on the first end  78 , and a leading face  88  and trailing face  90  on the second end  80 . The leading faces  84  and  88  and the trailing faces  86  and  90  extend radially outward beyond the exterior surface  65  of the drum  61 . The leading faces  84  and  88  are the surfaces that lead the through-member  76  as it rotates in a direction designated as R in  FIG. 5   b.    
   The leading faces will be subjected to the grinding loads and friction which will result in the through-member being subjected to an overhanging load situation and wear. The loading situation will have the tendency to deflect the through-member and has been seen to permanently deform the through-member. In certain cases the through-member is first deflected and later can fail, be broken. In that case the through-member can be difficult to remove. It has been found that manufacturing the through members from steel conforming to specifications SAE 4140 through-hardened to a minimum exterior surface harness of Rockwell C-Scale Hardness  32  provides a much improved performance. The resulting through-member has a higher yield point, than prior to being through-hardened, and experiences less permanent deflection prior to failure. Thus, if failure occurs, it has not been preceded by deformation, and subsequent removal is improved. Other specific manufacturing processes could be utilized. The design intent is for the through member to withstand normal loading without any permanent deflection, without exceeding its yield point and for the through-member to intentionally fail when its yield point is exceeded. This can be affected by the proper material and heat treatment as herein disclosed, and is also affected by the geometry of the through-member. For instance a stress concentration groove or undercut could be intentionally located to achieve this result. 
   In addition to the bending affect, the through-members are subjected to significant wear. The preferred embodiment of through hardening the through-members to an exterior hardness of Rockwell C-Scale Hardness  32  minimum also significantly improves the wear characteristics. Here again other material specifications could be utilized to achieve this result, such as utilization of a low carbon steel with a type of surface hardening such as carburization. However, this type of material would provide significantly different bending failure characteristics. Thus, the material and heat treatment is selected to provide improved bending characteristics combined with improved wear characteristics. 
   A cutter  92  is preferably attached to each of the leading faces  84  and  88  of the through-members  76 .  FIG. 5   c  shows one of the cutters  92  adapted to be attached to one of the leading faces  84 . A bolt  94  is adapted to pass through co-axially aligned holes  93 ,  96  respectively defined by the cutter  92 , and the through-member  76 . By inserting the bolt  94  through the openings  93 ,  96  and threading a nut  99  on the bolt  94 , the cutter  92  is securely clamped against the through-member  76 . It will be appreciated that the cutter  92  can be any type of cutter known in the art with the preferred form of cutter being dictated by the type of grinding to be performed as is well known in the art. In the preferred embodiment illustrated the cutter  92  is symmetrical, including 2 cutting edges. The effective cutting edge is located on the outside, at the extreme radial dimension of the assembly, defining the cutting diameter. In that position there is a second cutting edge on the opposite end of the cutter, that is located below the outside surface  65  of the drum  61 . In this manner the second cutting surface is protected by the outside surface  65 . 
   When the cutter  92  is clamped to the through-member  76  as shown in  FIG. 5   c , the cutter  92  opposes or engages a retaining shoulder  67  formed at the end of the sleeve  63 . In this manner, the cutter  92  fastener is protected from shear loads by transferring forces through the sleeve  63  to the drum  61 . Similar cutters  92  and retaining shoulders  67  are located at each end of each through-member  78 . Engagement between the cutters  92  and the shoulders  67  functions to center or align the through-members  78  such that central openings  125  of the through-members  78  align with the axis of rotation x—x of the hammermill  56 . The sleeves  63  also function to guide the through-members  76  through the openings  70 ,  72 . 
   An alternate mounting arrangement for cutter  92  onto through-member  78  is illustrated in  FIG. 10  wherein an additional backing plate  77  is added in the assembly. This additional backing plate is positioned to transfer a portion of the radial load on cutter  92  to the sleeve  63  through bolt  94 . The backing plate  77  is removable and is fastened to through-member  78  by bolt  94 . 
   This transfer of load, from a cutter to the sleeve  63  has been found to be sufficient to deform the end of sleeve  63 . This deformation is detrimental to the subsequent removal of through-member  76 . It has been found to be beneficial to manufacture the sleeves  63  from a material, which can be heat-treated to achieve material properties sufficient to resist such deformation. In a preferred embodiment the sleeves  63  are constructed from steel conforming to specifications of SAE 8620 carburized, quenched and tempered to a surface hardness of Rockwell C-Scale Hardness  40  with a case depth of 0.030 inches. The configuration of the sleeves  63 , and the method of retaining them in the drum  61  is such that they are first processed to the correct shape, then they are heat treated such that selective portions of the surface, those that are adversely affected by the change in material characteristics, are not affected. This is accomplished by applying a masking compound, that prevents carbon migration during the carburization process, to those areas. In the preferred embodiment, those areas correspond to areas that will later be welded. 
   Alternate embodiments could include sleeves  63  that are not welded. In that case, the selective heat treating may not be necessary, and in fact a medium to high carbon steel, for instance, may be utilized. However, in all cases the material properties of the sleeves  63  will be selected to prevent deformation resulting from the radial loading. 
   The hammermill  56  also can include a rod  126  (best shown in  FIG. 5   b ) that extends along the axis of rotation x—x as shown in  FIG. 5   b . The rod  126  extends through a longitudinal opening  122  defined by the non-driven shaft  130  and the first end cap  104 . The rod  126  also extends through the plurality of co-axially aligned, central openings  125  defined by the through-members  76 . The rod  126  also can include a threaded end that threads within an internally threaded opening  132  defined by the driven shaft  118 . In this manner, the rod  126  could be used to clamp the end caps  104 ,  106  together. The rod  126  functions as a hammer retention system for the through-members  76  within the drum  61 . A significant aspect of the invention is that a single retaining member (i.e., the rod  126 ) can be used to secure all of the through-members  76  to the drum  61 . 
   The through-members  76  can experience significant radial acceleration when a cutters is inadvertently lost. This loading is absorbed by the rod  126 , performing its function of securing the through-member to the drum  61 . It has been found that the rod  126  can be thus damaged, to the extent that the subsequent removal of the rod  126  by passing it through the opening  122  is made difficult.  FIG. 11  illustrates the addition of 2 bushings  127  in the assembly. Bushings  127  are sized to fit into the opening  122  and have an ID large enough to allow rod  126 , in its normal condition, to pass through. The bushings have an outer diameter slightly larger than the mating inner diameter which defines the opening  122 . Thus, they are pressed into place and are retained in their original location. If a damaged rod  126  is removed, the damaged section of the rod is typically not able to pass through the inner diameter of the bushing  127 . However, the press-fit bushing  127  is able to slide in opening  122  thus allowing the rod  126  to be removed. 
   In an alternative embodiment, the rod  126  can be used to retain shorter through-members (e.g., half the length of the through-members  76 ) that each extend through only one of the openings  70 ,  72 . Also, the rod  126  need not be threaded into the driven shaft  118 . For example, the rod  126  can be configured to thread within the longitudinal opening  122  of the non-driven shaft  130  (e.g., the rod  126  can have threads near its head). In such a configuration, the far end of the rod preferably fits within an unthreaded sleeve or opening defined by the driven shaft  118 . 
     FIGS. 6  shows one of the through-members  76  in isolation from the drum  61 . As shown in  FIG. 6 , the through-member  76  comprises a generally rectangular bar having the opening  125  defined at a central region of the bar, and the cutter mounting holes  96  defined at the ends of the bar. Of course, other shapes (e.g., octagonal, hexagonal, round with flats, etc.) could also be used. 
     FIGS. 7   a  and  7   b  show side views of an alternative embodiment of through-member  76 ′ adapted to be mounted in the drum  61 . The through-member  76 ′ has first and second ends  78 ′,  80 ′ that are adapted for mounting narrow faced cutters used for more aggressive grinding of certain types of material. 
     FIG. 8  shows another through-member  76 ″ adapted for use with the hammermill  56 . The through-member  76 ″ has hooked ends  78 ″,  80 ″ that form aggressive cutting teeth. Shims can be used at the sides of the through-member  76 ″ to stabilize the through-member  76 ″ within the openings  70 ,  72  of the drum  61 . Hardfacing can be used at the hooked ends  78 ″,  80 ″ to improve durability. Additionally, the through-members  76 ″ preferably include central openings  125 ″ for allowing the through-members  76 ″ to be connected to the drum  61  by a single retaining member (e.g., the rod  126 ) in the same manner described above with respect to the through-members  76 . 
     FIGS. 5   a  and  5   b  show that the through-members  76  of the hammermill  56  are skewed relative to the axis of rotation x—x of the hammermill  56  (i.e., the through-members  76  intersect the axis x—x at an oblique angle). The angled nature of the through-members  76  relative to the axis x—x causes the first end  78  of each through-member  76  to travel along a different grinding path than the its corresponding second end  80 . For example, as shown in  FIG. 9 , a first one of the through-members  76   a  has a first end  78   a  that travels along path  1 , and a second end ( 80   a ) that travels along path  2 . Similarly, a second one of the through-members  76   b  has a first end  78   b  that travels along path  3 , and a second end (not shown) that travels along path  4 . The remainder of the through-members are preferably arranged in a similar configuration. Hence, the 8 through-members provide 16 separate cutting paths spaced along the axis x—x of the drum  61 In certain embodiments, the hammers are adapted to provide full face coverage of the drum  61 . Full face coverage means that there are no substantial gaps between adjacent cutting paths. Thus, as shown in  FIG. 9 , path  1  terminates where path  2  begins; path  2  terminates where path  3  begins; path  3  terminates where path  4  begins; etc. The skewed configuration of the through-members  76  allows full-face coverage to be provided with a relatively small number of through-members  76 . The skewed configuration also allows hammers to be mounted directly at the far edges of the drum  61 . While paths  1 – 16  are non-overlapping, it will be appreciated that alternative embodiments can have overlapping paths. Additionally, for certain applications, gaps can be provided between adjacent cutting paths. 
   Still referring to  FIG. 9 , each of the cutting paths  1 – 16  is typically defined by a maximum width of a cutter corresponding to each path. For example, paths  1  and  2  have widths w (measured in an axial direction) that correspond to the maximum widths of the cutters that are swung through the paths. For certain embodiments, the sum of the widths of all the paths is equal to or greater than a length d of the drum  61 . As shown in  FIG. 9 , the sum of the widths equal the length d. However, if the paths overlap, the sum of the widths will be larger than the length d. By contrast, if gaps are provided between adjacent paths, the sum of the widths is less than the length d. 
     FIGS. 12 ,  13  and  14  illustrate a representative surface of rotation defined by the cutting surface or edge of the generally block-shaped cutters  92 , the edge located at the furthest radial dimension. This surface of rotation can be described as a series of aligned cones, with a varying effective cutting diameter for each cutter including a maximum diameter D-maximum and a minimum diameter d-minimum.  FIG. 12  illustrates the position of cutters  92  on a through-member  76 . Through-member  76  passes through  2  holes,  70  and  72 , in drum  61  such that the through-member is angled relative to the horizontal axis x—x at an oblique angle θ (as shown in  FIG. 4   b ). This angle results in the cutting edge of each cutter  92  being angled, thus defining the conical surface of rotation.  FIG. 13  illustrates the resulting surfaces of rotation defined by a pair of generally block-shaped cutters  92  mounted onto a through-member  76 . Locating several through-members on a common axis of rotation, will result in the overall surface of rotation of the entire hammer mill as illustrated in  FIG. 14 . 
   The rotary grinder  56  herein described can be used in a grinding device, as illustrated in  FIG. 16 , and will cooperate with the anvil and screens in much the same manner as the prior art rotary grinder  20 . However, the grinding characteristics of the grinding device with rotary grinder  56  will be different than with rotary grinder  20 . The differences are related to the fact that the surface of rotation of rotary grinder  56  is a series of aligned conical sections as opposed to the generally straight cylindrical surface of rotation. This fact will affect the grinding characteristics. 
   An additional difference between the rotary grinders is the presence of the cylindrical exterior surface  65 . This surface holds the material to be ground forcing all the material to pass closely to the grinding chamber  108 , previously defined as the space between the screen and the rotary grinder. In the prior art rotary grinder  20  material could travel between the rotor plates  24 , and avoid being reduced in size. However, with rotary grinder  56  the cylindrical exterior surface  65  prevents this and thus is effective in improving the grinding characteristics of the grinding device. 
   While it is preferred to use a skewed through-hammer configuration to angle the cutters  92 , the invention is not limited to this type of configuration. Instead, in other embodiments, more conventional type hammers can be modified so as to mount the cutters at an angle relative to the axis of rotation of the grinder. 
     FIG. 17  illustrates a modified grinding device of the present invention comprising the rotary grinder  56 , and only an anvil. In this embodiment the grinding action will take place exclusively between the anvil  100  and the rotary grinder  56 , including its cylindrical exterior surface  65  and cutters  92 . This embodiment will result in reduced load and power requirements. 
   Another embodiment of the present invention is illustrated in  FIG. 18 . In this embodiment the screen comprises improved screen  120 .  FIGS. 19 and 20  further illustrate the screen  120 . In this embodiment screen  120  consists of a frame  121 , anvil  100 , and  3  scalloped screen plates  122 . The scalloped screen plates  122  include an upper surface  123  that will serve as a shearing surface. This surface includes a series of tips  126  and valleys  128 . The portion of the upper surface  123  between each tip  126  and valley  128  will be aligned with a surface of rotation of a cutter of the rotary grinder  56 , as illustrated in  FIG. 16 . The surface of rotation of each cutter defines a D-maximum and d-minimum. In one embodiment, D-max of each cutter aligns generally with a valley  128  and D-min aligns generally with a tip  126 . While the embodiment has been depicted including 3 screens, it will be appreciated that more or fewer screens could be used. Certain embodiments may include only one screen. 
   While the screen  120  is preferred to be used in combination with the depicted grinding drum, it will be appreciated that the screen is applicable to any type of grinding apparatus. For example, the screen is applicable to skewed and unskewed hammers. Also, the screen  120  could be used with grinding elements of the type disclosed in the background of the invention. 
     FIG. 21  illustrates how the screen  120  is aligned with rotary grinder  56 . It is positioned such that there is a gap  130  between the minimum diameter d-minimum of each cutter and a tip  126  of the scalloped screen plate  122  and a gap  132  between the maximum diameter D-maximum of each cutter and a valley  128  of the scalloped screen plate  122 . The gap between the portion of the upper surface  123  of the scalloped screen plate  122  between each tip  126  and valley  128  and the cutters is approximately consistent. In this manner the upper surface  123  of each scalloped screen plate  122  serves as a shearing surface. 
   The interaction between this shearing surface and the cutters provides a scissors effect wherein the shearing action happens over a significant range of travel of each cutter.  FIG. 21  illustrates this range of travel as B. The resulting shearing action provides more consistent load requirement, while simultaneously providing increased shearing forces on the material being ground. 
   The surface  123  of the scalloped screen plates will be subjected to abrasive conditions. This surface can be manufactured with any known type of surface treatment to reduce wear and increase service life. Likewise some treatments such as carbide impregnated weld, will increase the aggressiveness of the surface resulting in more effective grinding. 
     FIG. 20  illustrates an additional feature of the scalloped screen plate  122 , its bottom surface  125 . This bottom surface  125  can be straight or contoured. If it is straight it will cooperate with the top surface of associated scalloped screen plates  122  to form approximately triangular shaped openings  124 . If it is contoured the openings will be more restricted as illustrated by openings  124   a . These openings  124  or  124   a  will function to allow ground material, of a certain size, to pass through and exit the grinding device. 
   Referring to  FIG. 21 , the plates  122  include plates  122   a ,  122   b  and  122   c  that overlap one another. The plates  122   a ,  122   b  and  122   c  are progressively angled toward vertical. For example, plate  122   a  defines a greater angle relative to vertical than plate  122   b , and plate  122   b  defines a greater angle relative to vertical than plate  122   c . Plate  122   c  is aligned substantially upright. 
   In a preferred embodiment, the plates  122  are oriented such that leading portions of the plates  122  are “generally perpendicular” (perpendicular plus or minus 30 degrees) relative to a radius of the rotary grinder that intersects the leading portions. For example, referring to  FIG. 21 , radius R is generally perpendicular to the leading portion of plate  122   c . In this embodiment, the tips  126  (i.e., teeth) of the plates  122  extend outwardly from the plates in a direction opposite to the direction of rotation DR of the grinder. In other words, the valleys  128  face toward the direction of rotation DR of the grinder. The phrase “leading portion” will be understood to mean the portion of each plate which is first passed by the cutters as the grinder rotates (e.g., the upper portions in the depicted embodiment). 
   Referring still to  FIG. 21 , the plates are shown generally tangent to D-maximum of the rotary grinder. In other embodiments, D-maximum can intersect (i.e., overlap) the plates  122  such that portions of the cutters  92  pass through the valleys  128  between the peaks  126 . Of course, the spacing between the hammers and the screen can be varied depending upon the material being processed and the size of the end product desired. In certain embodiments, a gap can exist between the screen and the cutters such that the paths of the cutters do not intersect the valleys. 
   The method of replacing parts for the rotary grinder of this invention will now be explained. These various methods include replacement of cutters, replacement of through-members, and replacement of drums. These methods are all made easier in this invention. 
   The cutters can be easily reversed or replaced by removing the bolt  94 . The old cutter  92  is removed and a new cutter  92  or a different type cutter is fastened to the through-member  76  with bolt  94 . 
   One of the through-members  76  can be individually replaced by removing at least one of the cutters  92  from the through-member  76  desired to be replaced. The rod  126  is then removed from the hole in the driven shaft  118  and removed from the holes  125  of the through-members  76  by sliding the rod  126  at least partially out of the drum  61 . The bushings  127  may need to be removed if the rod  126  has been damaged sufficiently to prevent it from sliding through the inner diameter of the bushing  127 . The through-member  76  to be replaced can then easily be slid out of the drum  61 . A new through-member  76  is then slid into the position previously occupied by the old through-member  76 . Next, the rod  126  is slid back through the holes  125  and is inserted into the hole  132  in the driven shaft  118 . Lastly, cutters  92  are secured to the ends of the new through-member  76 . An important advantage of the through-members  76  is that when each through-member  76  is removed, equal weights are concurrently removed from opposite sides of the drum  61 . Thus, during removal of the through-members  76 , there are no unbalanced forces that cause the drum  61  to inadvertently rotate. Instead, the drum  61  remains balanced at all times. 
   During use of the hammermill  56 , the leading faces  84 ,  88  of the through-members  76  can become worn or deformed such that flat surfaces are no longer provided for mounting the cutters  92 . If this happens to a particular through-member  76 , the through-member  76  can be removed by detaching the cutter  92  from the damaged end of the through-member  76 , and by sliding the through-member  76  from the drum  61 . Thereafter, the through-member  76  can be reversely mounted in the drum  61  such that the previous trailing faces  86 ,  90  of the through-member  76  become the leading faces  84 ,  88 . Once the through-member  76  has been re-inserted through the drum, the cutter  92  can be fastened to the new leading face  84 ,  88  (i.e., the face that was the trailing face before the through-member  76  was reversed). 
   The following steps outline the method for replacing the drum  61 . The drum  61  can be replaced along with the through-members  76  and cutters  92 . Alternatively, the drum  61  can be replaced alone, while keeping the old through-members  76  and cutters  92 . To replace the drum  61  along with the through-members  76  and cutters  92 , first remove the rod  126  as described above. Next, remove the first and second end caps  104 ,  106  by removing bolts  116 . The old drum  61  along with its associated through-members  76  and cutters  92  can then be discarded, and the end caps  104 ,  106  can be mounted on a new drum  61  with new through-members  76  and cutters  92 . Lastly, the rod  126  is mounted axially through the new drum. 
   The following method can be used when replacing the drum alone while keeping the old through-members  76  and cutters  92 . First, the rod  126  and the through-members  76  are removed. In removing the through-members  76 , at least one of the cutters  92  will be removed from each of the through-members  76  to allow the through-members  76  to be pulled from the drum  61 . Next, the end caps  104 ,  106  are removed as described above. Subsequently, the old drum  61  is removed and replaced with a new drum  61 . Finally, the hammermill is reassembled in reverse order to the disassembly described above. 
   If through-members  76 ″ are used with the drum  61 , it will be appreciated that some or all of the through-members  76 ″ may fall from the drum  61  when the rod  126  is removed. This occurs because the through-members  76 ″ do not have cutters for maintaining alignment with the rod  126 . Thus, during disassembly of the grinder, such through-members  76 ″ will typically be removed from the drum  61  in concert with the removal of the rod  126 . 
   With use, contact between the through-members  76  and the trailing shoulders of the sleeves  63  can cause the shoulders to deform or “mushroom.” When this occurs, the end caps  104 ,  106  can be removed as described above, and the drum  61  can be reversed end-to-end. Thereafter, the through-members  76  can be reversed such that the cutters  92  face in the appropriate direction. By reversing the drum  61 , the useful life of the drum can be increased. 
   With regard to the forgoing description, it is to be understood that changes may be made in detail, especially in matters of the construction materials employed and the size, shape and arrangement of the parts without departing from the scope of the present invention. For example, while the various aspects of the present invention are particularly applicable to hammermills, such aspects are also applicable to other types of rotary grinders that use hammers such as mining equipment, brush chippers, excavation equipment, concrete cutters, etc. As used herein, the term “grind” is intended to include terms such as chop, cut, crush, pulverize, etc. It is intended that these specific and depicted aspects be considered exemplary only, with a true scope and spirit of the invention be indicated by the broad meaning of the following claims.