Patent Publication Number: US-2013233955-A1

Title: Shredder hammers

Description:
FIELD OF THE INVENTION 
     The present invention relates to industrial shredding systems. More particularly, this invention relates to shredding systems that include shredder hammers. 
     BACKGROUND OF THE INVENTION 
     Industrial shredding equipment typically is used to break large objects into smaller pieces that can be more readily processed, for example as in the recycling industry. Commercially available shredders range in size from those that shred materials like rubber (e.g., car tires), wood, and paper to larger shredding systems that are capable of shredding scrap metal, automobiles, automobile body parts, and the like. 
     The core of most industrial shredding systems is the shredding chamber, where multiple shredder hammers are spun on a rotary shredding head, and repeatedly impact the material to be shredded against an anvil or other hardened surface. Shredder hammers are therefore routinely exposed to extremely harsh conditions of use, and so typically are constructed from hardened steel materials, such as low alloy steel or high manganese alloy content steel (such as Hadfield Manganese Steel). Shredder hammers may each weigh several hundred pounds (e.g., 150 to 1200 lbs.), and during typical shredder operations these heavy hammers slam into the material to be shredded at relatively high rates of speed. Even when employing hardened materials, the typical lifespan of a shredder hammer may only be a few days to a few weeks. In particular, as the shredder hammer blade or impact area undergoes repeated collisions with the material to be processed, the material of the shredder hammer itself tends to wear away. 
     It should be appreciated that the greater throughput that the shredding equipment can process, the more efficiently and profitably the equipment can operate. Accordingly, there is room in the art for improvements in the structure and construction of shredder hammers and the machinery and systems utilizing such hammers. 
     Examples of shredder hammers and industrial shredding equipment are disclosed in U.S. Pat. Nos. 1,675,464, 1,940,116, 1,954,175, 1,760,097, 2,534,301, 2,678,794, 2,716,526, 2,768,794, 2,750,124, 3,236,463, 3,738,586, 3,844,494, 4,141,512, 4,142,687, 4,310,125, 4,558,826, 4,805,842, 5,002,233, 5,073,038, 5,381,975, and 7,416,144; U.S. Patent Publication Nos. US20090250539, and US20100213301; and Japanese patent publication JP2007283243A. The disclosures of these and all other publications referenced herein are incorporated by reference in their entirety for all purposes. 
     SUMMARY OF THE INVENTION 
     The invention includes shredder hammers having first and second major surfaces on opposing sides, and a circumferential edge. The hammer includes a proximal portion and a distal portion. The proximal portion defines a mounting aperture extending from the first major surface to the second major surface of the hammer to receive a hammer mounting pin. The distal portion of the hammer includes at least one recess in at least one of the first and second major surfaces to provide additional surfaces by which to further shred the material. 
     In the various preferred embodiments shown in the drawings, the recesses are formed in the working portion or region of the hammer and spaced from the primary impact face for efficient, reliable operation. Some of the illustrated recesses intersect with the circumferential edge of the hammer body. The invention further includes shredding systems incorporating shredder hammers having such recessed features. 
     In an additional aspect of the invention, the invention includes shredding systems, where the shredding system includes a rotary shredding head, a shredding chamber enclosing the rotary shredding head, and a plurality of shredder hammers pivotally coupled to the rotary shredding head. Each shredder hammer includes first and second major surfaces on opposing sides, a circumferential edge, and a proximal portion and a distal portion. The proximal portion of each hammer defines a mounting aperture to receive a hammer mounting pin, and the distal portion of at least one hammer includes at least one recess in at least one of the first and second major surfaces. 
     In a preferred embodiment, the invention includes a shredder hammer including a pair of major surfaces and a circumferential surface connecting the major surfaces. A hole extends through the hammer and opens in each of the major surfaces to receive a support pin for mounting the shredder hammer in the shredding machine. A working or distal portion of the hammer is remote from the hole and includes at least one recess that opens to a major surface. The recess includes opposing walls that extend from the wear edge of the working end. 
     The working portion of the hammer includes the wear edge and the section of the hammer proximate to the wear edge with the primary contact faces for impacting target materials to be separated. The working portion is subject to wear during operation and is a sacrificial part of the hammer. 
     Other aspects, advantages, and features of the invention will be described in more detail below and will be recognizable from the following detailed description of example structures in accordance with this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic depiction of a shredding system according to an exemplary embodiment of the present invention. 
         FIG. 2  is a front elevation view of a rotary shredding head according to an exemplary embodiment of the present invention. 
         FIG. 3  is a perspective view of the rotary shredding head of  FIG. 2 . 
         FIG. 4  is a front perspective view of a shredder hammer according to an exemplary embodiment of the present invention. 
         FIG. 4A  is a front elevation view of a shredder hammer according to an exemplary embodiment of the present invention. 
         FIG. 5  is a rear perspective view of the shredder hammer of  FIG. 4 . 
         FIG. 6  is a front elevation view of the shredder hammer of  FIG. 4 . 
         FIG. 7  is a view of a cross-section of the shredder hammer of  FIG. 6  along line  7 - 7 , as indicated in  FIG. 6 . 
         FIG. 8  is a view of a partial cross-section of the shredder hammer of  FIG. 6  along line  8 - 8 , as indicated in  FIG. 6 . 
         FIG. 9  is a view of a partial cross-section of the shredder hammer of  FIG. 6  along line  9 - 9 , as indicated in  FIG. 6 . 
         FIG. 10  is a front perspective view of a shredder hammer according to an alternative exemplary embodiment of the present invention. 
         FIG. 11  is a rear perspective view of the shredder hammer of  FIG. 10 . 
         FIG. 12  is a front elevation view of a shredder hammer according to another alternative exemplary embodiment of the present invention. 
         FIG. 13  is a view of a cross-section of the shredder hammer of  FIG. 12  along line  13 - 13 , as indicated in  FIG. 12 . 
         FIG. 14  is a view of a partial cross-section of the shredder hammer of  FIG. 12  along line  14 - 14 , as indicated in  FIG. 12 . 
         FIG. 15  is a view of a partial cross-section of the shredder hammer of  FIG. 12  along line  15 - 15 , as indicated in  FIG. 12 . 
         FIG. 16  is a rear elevation view of the shredder hammer of  FIG. 12 . 
         FIG. 17  is a front perspective view of a shredder hammer according to another alternative exemplary embodiment of the present invention. 
         FIG. 18  is a rear perspective view of the shredder hammer of  FIG. 17 . 
         FIG. 19  is a front elevation view of a shredder hammer with inserts according to another alternative exemplary embodiment of the present invention. 
         FIG. 20  is a view of a cross-section of the shredder hammer of  FIG. 19  along line  20 - 20 , as indicated in  FIG. 19 . 
         FIG. 21  is an end view of the shredder hammer of  FIG. 19  showing inserts. 
         FIG. 22  is a front elevation view of another embodiment of a shredder hammer in accordance with the invention. 
         FIG. 23  is an end view of the hammer shown in  FIG. 22 . 
         FIG. 24  is flowchart setting forth a method of manufacturing according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Forming one or more recesses on one or both of the major surfaces of a shredder hammer advantageously creates one or more auxiliary impact faces, thereby enhancing the operational efficiency of the shredder hammer. Improved shredding of the materials enhances post-processing providing efficient sorting of the ferrous and non-ferrous metals and other materials. During operation, the material is sheared, torn and crushed by the primary impact face of the shredder hammer and between the hammer and the anvil and by the recess surfaces. In the present invention, additional recesses provide edges on the working portion of the hammer that further grip and engage the target material, to improve throughput and separation. 
     Even though the forming of recesses in the major surfaces of the hammer remove material from the hammer, work hardening due to additional material impacts, especially at edges of recesses, extend the work hardening more deeply into the hammer body. As a result a larger percentage of the volume of the hammer body has improved operational material characteristics. 
       FIG. 1  schematically illustrates an exemplary industrial shredding system  10 . The typical components of such a shredding system include a material intake (such as chute  12 ) that introduces material  14  to be shredded to a shredding chamber  16 . The material  14  to be shredded may be of any desired size or shape. The material  14  is optionally pretreated, such as by heating, cooling, crushing, baling, etc. before being introduced into the shredding chamber  16 . The material intake  12  may optionally include feed rollers or other machinery to facilitate feeding material  14  to chamber  16 , and/or to control the rate at which material  14  enters chamber  16 , and/or to prevent the material  14  from moving backward up the chute  12 . 
     Within shredding chamber  16  is a rotary shredding head  18 . Although the disclosure depicts a rotor or rotary shredding head, it should be appreciated that there are a variety of rotor configurations, including disc rotors, spider rotors, barrel rotors, and the like, that may also be used in the present shredding systems. Rotary shredding head  18  is equipped with a plurality of shredder hammers  22  according to the present invention, and is configured to rotate about a shaft or axis  20 . Each shredder hammer  22  is independently pivotally mounted to the rotary shredding head, so that as shredding head  18  rotates, centrifugal forces acting on the shredder hammers  22  urges each hammer to extend outwardly, tending toward a position where the center of gravity of each hammer is as far as possible from rotation axis  20 . 
     In this way, as rotary shredding head  18  rotates, the shredder hammers impact the material  14  to be shredded, and crush material  14  between hammer  22  and an anvil  24  (or other suitably hardened surface), breaking the material apart. As the shredder hammer  22  is rotatably mounted on the mounting pin  32 , contact with the material  14  to be shredded may cause the shredder hammer  22  to slow down or even rotate in the opposite direction as it crushes the material  14  to be shredded against the anvil  24 . 
     The resulting shredded materials may be discharged from the shredding chamber  16  through any one of the outlets  26  leading from the shredding chamber. As shown in  FIG. 1 , suitable outlets  26  may be provided in the bottom, top, or one or more sides of the chamber  16  walls. The shredded material may then be transported for collection and/or further processing. 
     The wide variety of applications for these machines, from clay processing to automobile shredding, results in a wide range and variety of shredder configurations.  FIG. 2  shows one typical example of a shredding head  16 . Rotary shredding head  16  includes a plurality of rotor disks  28  that are separated from one another by spacers that are configured to be mounted around the drive shaft  20 . While any number of rotor disks  28  may be utilized in a rotary shredding head, the illustrated example of shredding head  16  includes ten disks  28 . Disks  28  may be fixedly mounted with respect to the shaft  20 , for example by welding, mechanical coupling, etc., to allow the disks  28  to be rotated when shaft  20  is rotated by an external motor or other power source (not shown). In addition to providing a spacing function, spacers can also help protect the shaft  20  from damage, due to contact with material  14  as it is being shredded, or fragments of broken shredder hammers  22 , and the like. 
     The rotary shredding head  16  further includes a plurality of hammer mounting pins  32  that extend between at least some of the rotor disks  28  and/or through the entire length of the shredding head  16 . The shredder hammers  22  are rotatably mounted on the hammer mounting pins  32  so that they are capable of freely and independently rotating around the mounting pins. In this illustrated example, the shredding head  18  includes four mounting pins  32  around the circumference of the rotor disks  28 , and shredder hammers  22  are shown mounted on selected pins  32  between each adjacent pair of rotor disks  28 . It is recognized that three, four or more hammers can be mounted between adjacent disks depending on the specific application. The particular distribution of hammers may be modified as required by the end user, depending on end-user needs, although the hammers are typically positioned so that the shredding head is balanced with respect to rotation. 
     The mounting pins  32 , shredder hammers  22 , and rotor disks  28  may be structured and arranged so that, in the event that a shredder hammer  22  is unable to completely pass through the material  14 , it can rotate to a location between adjacent disks  28  and thereby pass by the material  14  until it is able to extend outward again under the effect of the rotation of the shredder head  18 . Alternatively, or in addition, the shredder hammer  22  may shift sideways on its mounting pin  32  as it passes by or through the material  14  to be shredded. If desired, the various parts of the shredder head  18  may be shaped and oriented with respect to one another such that a shredder hammer  22  can rotate 360° around its mounting pin  32  without contacting another mounting pin  32 , the drive shaft  20 , another hammer  22 , etc. 
     Shredder hammers used in the art of industrial shredder construction and operation typically are constructed from especially durable materials, such as hardened steel alloys. Exemplary materials suitable for the fabrication of shredder hammers include low alloy steel or high manganese alloy content steel, among others. Particularly preferred are so-called work hardening steel alloys, a family of steel formulations that become harder the more it is subjected to impacts and/or compressive forces. 
     One such manganese alloy is Hadfield Manganese steel, which contains about 11% to about 14% manganese, by weight. Although Hadfield Manganese steel typically exhibits a Brinell hardness of approximately 220 Bhn, after continued impact and/or compression it may surface harden to a Brinell hardness of over 550 Bhn. Significantly, only the outer skin surface of the shredder hammer will typically harden, if the hammer is made from Hadfield Manganese steel, even under heavy use. The under layer typically remains ductile and tough. However, as the surface of the shredder hammer wears, the layer of material exhibiting increased hardness is renewed, gradually increasing in depth as the hammer surface is worn away. 
     An exemplary shredding hammer  22  is depicted in  FIGS. 4-9 . Shredder hammer  22  includes a plate-like hammer body  34  that has a first major surface  36  and a second major surface  38  that define opposite sides of the hammer body  34 . Major surface  36  and major surface  38  are separated by the thickness  40  of the hammer body  34 . The thickness  40  of the hammer body  34  may be substantially constant, or it may vary over the area of the hammer body. 
     The shape of the hammer is largely defined by a circumferential edge  42  which extends between the first and second major surfaces  36 ,  38 . The circumferential edge  42  is typically substantially perpendicular to at least one of the planes defined by the first major surface  36  or second major surface  38 , or is substantially perpendicular to both the first major surface  36  and second major surface  38 . The circumferential edge typically includes a plurality of edge segments, including one or more curved edge segments, so as to define the overall outline of the hammer. In one embodiment of the present invention, the outline of the hammer is mirror-symmetric with respect to a plane of symmetry  43 , where the plane of symmetry  43  includes a longitudinal axis  44  of the hammer, and is perpendicular to at least one of the first and second major surfaces. Hammer  22  defines a midplane  45  that pass through longitudinal axis  44  and between first and second major surfaces  36  and  38 . In another embodiment of the present invention, the circumferential edge  42  delineates an outline that is approximately bell-shaped. In another embodiment of the present invention, the distal portion of the circumferential edge  42  is made up of a series of linear faces that intersect one another at obtuse angles. 
     The hammer  22  includes and defines a mounting aperture or opening  50  that is configured to receive the hammer mounting pin  32  in order to rotatably mount the shredder hammer to the rotary shredding head  18 . The mounting aperture typically extends from the first major surface  36  to the second major surface  38  of the hammer, and forms a passageway through the hammer  22 . The interior surface  52  of mounting aperture  50  may be of any geometry that is compatible with the desired mounting pin and rotary shredding head with which the shredder hammer is intended to be used. Interior surface  52  may be shaped so that the mounting aperture  50  is approximately cylindrical. Alternatively, the interior surface  52  of mounting aperture  50  may define one or more curving surfaces, such as are described in U.S. Pat. No. 8,308,094 (hereby incorporated by reference). 
     The hammer  22  may be characterized in having a proximal or mounting portion  46  and a distal or working portion  48 . The proximal portion  46  of hammer  22  may include a lifting eye  54 . The lifting eye  54 , when present, is typically disposed on the circumferential edge  42 , for example where the longitudinal axis  44  intersects the circumferential edge. The lifting eye  54  may be used to facilitate the handling and movement of the shredder hammer  22 , which may be both extremely heavy and relatively unwieldy. 
     The distal portion  48  of hammer body  34  is bounded by the distal portion of circumferential edge  42  including wear edge  56 . In a preferred example, the wear edge  56  is defined as a convex arc along the distal edge of hammer  22 . The shape of wear edge  56  as a convex arc helps prevent any undesired contact between the shredder hammer  22  and the walls of shredding chamber  16 , particularly the anvil  24 , as the shredder hammer rotates around mounting pin  32 . The distal arc may be an arc of a circle that defines a radius. The center of curvature defining the arc is at or near the axis of rotation of the hammer and the center of pin  52 . Alternatively, the wear edge is defined by one or more straight segments. In another alternative example the wear edge is defined by one or more straight segments combined with one or more curved segments. 
     The distal or working portion of hammer  22  may be differentiated from the proximal end of the hammer by a transverse axis  47  that passes through a reference point RP on the longitudinal axis between the wear edge and the center of gravity CG. The transverse axis may be a straight line  47 A that passes through the reference point perpendicular to longitudinal axis  44 . Alternatively, the transverse axis may be an arc  47 B passing through the reference point. The arc is defined by an axis of curvature  47 C along the longitudinal axis at or near the circumferential edge at the proximal end with a radius R 1 . Reference point RP on the longitudinal axis is spaced a distance d 1  from the center of gravity and a distance d 2  from the wear edge. Reference point RP in a preferred example is half way between the center of gravity and wear edge  56 . Alternatively, the distance d 1  may be one third the distance between the center of gravity and the wear edge along the longitudinal axis. 
     The transverse axis can define the separation of the distal portion  48  and the proximal portion  46  of hammer  22 . The distal working portion of the hammer accomplishes most of the shredding and wears away during operation. 
     The wear edge  56  may be bounded on one or both sides by a segment of the circumferential edge, forming an impact face  58  for the shredder hammer  22 . The shredder hammer may include at least one impact face  58 , and the shredder hammer  22  may be mounted so that the impact face  58  faces the direction of rotation of the rotary shredding head  18 . In another embodiment of the invention, the shredder hammer  10  includes two impact faces  58 . The impact faces are a portion of the circumferential edge located at each end of the arc or wear edge  56 . In this embodiment, the hammer is symmetric with respect to mirror plane  43 , so that if a first impact face should become excessively worn, the shredder hammer may simply be rotated and remounted, presenting a second, unworn impact face to the direction of rotation; thereby extending the life of the shredder hammer  22  and rendering it more economical. In an alternative embodiment, the hammer is not symmetric with respect to plane  43 . 
     At least one of the first and second major surfaces  36 ,  38  of hammer  22  may include a plurality of channels or recesses  60 . The size and shape of each such recess  60  is defined by its walls  62 , one of which can be considered the recess floor  64 . That is, the particular conformations of the recess walls  62  determine the overall length  66  and depth  68  of a recess  60 . Recess walls  62  refer to the side walls of a given recess, as well as a terminal or inner recess wall. The terminal recess wall or the floor  64  may not be present if the sidewalls of the recess taper toward each other and meet. 
     The recess walls include an upstream wall  60 A and a downstream wall  60 B defined by its orientation to an impact face  58  or the flow of material during operation. In some embodiments the upstream and downstream walls of a recess may have different configurations to take advantage of an associated impact face acting as the leading edge described further below. The upstream and downstream walls are opposed in that they are inclined to face each other or are parallel in extending from the recess floor  64 . The upstream and downstream walls extending away from wear edge  56  may be parallel, may diverge or may converge. 
     In one embodiment of the invention, one or more recesses  60  are disposed on the distal portion  48  of the hammer  22 , and extend from the wear edge  56  of the hammer into the interior of hammer  22 . In another embodiment of the invention, the distal portion of the circumferential edge  42  includes a wear edge  56 , and one or more of the plurality of recesses is oriented along a line normal to the curve of the wear edge  56  which is equivalent to the radius of the arc. Alternatively, one or more recesses or portions of recesses, may be oriented parallel to the longitudinal axis  44  of the hammer  22 . Alternatively, one or more recesses may be formed in the major surfaces  36 ,  38  without extending to the wear edge  56 . Alternatively, a hammer may have one or more recesses in accordance with any of the recesses described above. 
     The particular size and shape of recesses  60  may vary on a single shredder hammer, as well as between different shredder hammers. Exemplary recesses  60  for shredder hammer  22  are shown in cross-section in  FIGS. 7-9 . As shown in  FIG. 8 , the recess depth  68  may vary across the extent of a particular recess  60 . Where the recess depth varies over the length of the recess, the recess depth may increase as the recess extends toward the interior of the hammer from circumferential edge  42 . Preferably recesses  60  have a maximum depth that does not exceed the midplane  45  to ensure sufficient hammer strength and reliability, but they could exceed the midpoint in certain circumstances. 
     The presence of one or more recesses  60  on the major surfaces of the hammer advantageously creates one or more auxiliary impact faces, which enhance the operational efficiency of the shredder hammer. During use, not only is material  14  impacted and crushed by a primary impact face  58  and between curved face  56  and anvil  24 , the additional recesses  60  provide additional edges to engage the material, improving the ripping, tearing, impacting, and folding of the material between adjacent hammers and/or between the hammers and the anvil  24  or other chamber surface. 
     Recesses that extend to wear edge  56  typically cause wear edge  56  to become uneven as it wears. The recesses can reduce the exposed working area of edge  42 . This area wears at a higher rate than surrounding portions that have a greater exposed working area. This creates a rippling effect to edge  56  as it wears. The rippling of the curved face as it wears creates another edge to contact the material for improved shredding and a more effective gripping surface that can dislodge material stuck in the mill grates  26  so it can be processed by subsequent hammer strikes. 
     In addition, and also advantageously, the recesses  60  formed in the major surfaces of the hammer extend the work hardening of the hammer. The recesses provide additional impact points on the side of the hammer and additional areas of hardening. This additional hardening results in a larger percentage of the volume of the hammer  22  becoming work hardened. 
     One or more of the major surfaces  36 ,  38  of the hammer  22  may include one or more concavities  70  in mounting portion  46  to reduce the volume of metal needed for the hammer in locations where the hammer does not wear. Concavity  70  is such a weight saving and cost saving recess proximate to aperture  50 . Concavity  70  is remote from the primary metal impact zone (i.e. the working portion) of the hammer. Concavities  70  are predominantly within the mounting portion (though they could extend into the working portion) and are primarily for weight and cost reduction. Recesses  60  are predominantly within the working portion (though they could extend into the mounting portion) and are primarily for shredding the target material. Removal of metal at this location (i.e. in the mounting portion) does not limit the service life of the hammer. Moreover it can enhance the shredding by moving the center of gravity for the hammer closer to the wear edge  56 . Such concavities may be of any suitable size and shape, provided that they are not of such size and/or depth that the recess compromises the structural integrity of the shredder hammer. As shown in  FIGS. 5 and 7 , concavity  70  of shredder hammer  22  has the outline of a rounded rectangle but it could have other shapes. 
     In one embodiment of the invention, recesses  60  are formed to have a sufficient recess length  66  and depth  68  so that the recesses will be maintained, and therefore confer their additional operational advantages, throughout the useable life of the hammer. For example, in one embodiment of the invention, each recess has a depth of from approximately one-tenth to approximately one-half of the thickness  40  of the hammer body. In alternative embodiments, one or more recesses in the hammer may extend beyond the center cross section of the hammer. 
     As exemplified by the shredder hammer of  FIGS. 4-9 , the shredder hammer  22  may include a plurality of recesses  60  on both the first and second major surfaces  36 ,  38  of hammer  22 . Where there are recesses on both faces of the hammer, the recesses may be disposed symmetrically or asymmetrically. Furthermore, the recesses may be disposed symmetrically with respect to the plane of symmetry  43 , or disposed asymmetrically with respect to mirror plane  43 . In one embodiment of the invention, the shredder hammer has recesses on both major surfaces, and the recesses do not superpose. For example, as shown in  FIG. 6 , the recesses  60  of shredder hammer  22  are disposed symmetrically with respect to mirror plane  43 , but are asymmetrical and nonsuperposable with respect to the midplane  45  of the hammer  22 . 
       FIGS. 10-11  depict an alternative embodiment of a shredder hammer according to the present invention. Similar to shredder hammer  22 , shredder hammer  72  includes a plate-like hammer body  34 , a first major surface  36  and second major surface  38 , and a circumferential edge  42 . Also similarly, the distal portion of circumferential edge  42  defines a wear edge  56  that is modified by a plurality of recesses  60 . However, shredder hammer  72  additionally includes alternative fluted recesses  74  on major surfaces  36  and  38 . That is, a plurality of shallow fluted recesses  74  is superimposed upon the recesses  60 . Similarly to the recited advantages of forming recesses on the presently disclosed shredder hammers, the addition of the recesses  74  to shredder hammer  72  creates additional features to enhance gripping and tearing of the material to be shredded, as well as permitting work hardening to extend into the hammer body. 
       FIGS. 12-16  depict another alternative embodiment of a shredder hammer according to the present invention. Shredder hammer  76  is similar to previously discussed shredder hammers, but differs in the particular arrangement of recesses formed in the hammer  22 . As shown in  FIGS. 12-16 , the first major surface  36  of shredder hammer  76  includes recesses  78  that have a distal portion  80  that extends along a line normal to wear edge  56 , and a proximal portion  82  that is oriented parallel to a longitudinal axis of the hammer  22 . Recess  78  is further configured to increase in depth as the recess extends from the circumferential edge into the hammer body. Shredder hammer  76  also includes a relatively short and broad recess  84  disposed at the center point of the wear edge  56 . First major surface  36  further includes two symmetrically disposed recesses  85 , which have trapezoidal outlines, but could be any shape including triangular or square. 
     Major surface  38  of hammer  76  in  FIG. 16  includes recesses  86  predominantly in the working portion, which are defined by a first recess wall  88  that extends along a line normal to wear edge  56 , and a second arcuate and convex recess wall  90  that, in combination with recess wall  88 , forms recess  86 . The second major surface of shredder hammer  76  includes two surface features: a concavity  92  predominately in the mounting portion having a rounded rectangular outline, and a recess  94  predominately in the working portion having an oval outline. 
     Concavity  92  like concavity  70  is predominately in the mounting portion  46  of the hammer, which reduces the overall weight of the hammer without substantial reduction in operational effectiveness. During operation as the hammer spins at high speed, mass at the distal end travels at a much higher velocity with greater momentum than mass in the mounting portion. The reduction in mass at mounting end  46  has limited effect on the impact provided by the hammer and reduces the mass that is scrapped at the end of the service life of the hammer. 
     At the corners formed by the distal edge, the hammer face and a recess wall, the hammer material is not as well supported by surrounding material and is exposed to the full impact against target materials. These corners can tend to break away particularly on initial operation of the hammer when the material has not work hardened. The configuration of recess  78  where the recess is shallower at the edge  42 , and increases in depth moving away from the edge, provides better support and limits cracking at the wall of the recess and proximate to the circumferential edge  42 . As the hammer wears away exposing the deeper part of recess  78 , the hammer material has hardened and is less vulnerable to cracking at the recess walls. The hammer also has less mass and therefore less impact energy during use. 
       FIGS. 17-18  depict another alternative embodiment of a shredder hammer according to the present invention. Shredder hammer  94  is similar to previously discussed shredder hammer  22 , but differs in the particular arrangement of recesses formed in the hammer. As shown in  FIGS. 17-18 , the first major surface  36  of shredder hammer  96  includes four recesses  98  that each extend along a line normal to wear edge  56 . The second major surface  38  of shredder hammer  94  includes three recesses  100  that also extend along lines normal to wear edge  56 . Although recesses  98  and recesses  100  are each symmetrically disposed, they are non-superposable with each other. 
       FIGS. 19-21  depict another alternative embodiment of a shredder hammer according to the present invention. Hammer  102  is similar to previously discussed hammer  22 . Here the hammer includes inserts  104 ,  106  and  108  of a different material than the hammer body. The insert may comprise a hardened material such as diamond, tungsten carbide or carbon nitride. The inserts shown are located proximate to recesses  110 ,  112  and  114  preferably adjacent the downstream edge. 
     The inserts may be inserted into cast or drilled holes in the edge of hammer  102  and secured in place by gluing or soldering or any other similar method that retains the inserts. In an alternate aspect, inserts may be cast in place when the hammer is manufactured. In another alternative aspect, the hammer could be cast with recesses open at least on a side of the hammer and configured to accept an insert. The inserts could be positioned in the side of the hammer and the insert then secured in place again by gluing, soldering or some other method. 
     The inserts provide an additional engagement point together with the recesses to engage the consolidated material and exert shearing forces to separate it into smaller portions. 
       FIGS. 22-23  depict yet another alternative embodiment of a shredder hammer according to the present invention. Shredder hammer  130  is similar to previously discussed shredder hammers, but differs in the particular arrangement of recesses formed in the hammer body. 
     The shredder hammer  130  includes one recess  134  opening to surface  36  and edge  42  and one recess  132  opening to opposite surface  38  and edge  42 . Recess  132  includes a recess transition configured as a bevel portion  136 . Recess  134  includes a recess transition configured as a bevel portion  138 . These bevels are typically formed on the upstream side of the recess and permit better material access in the recess for improved shredding. The bevel provides a less pronounced approach to the recess than a right angle transition from the surface into the recess. 
     The transition portion can be any configuration that provides a less abrupt and more extended transition from the hammer surface to the recess. Here the bevel portion is a planar surface that extends from the bottom of the recess to the hammer surface along line  136 A at the edge  42  at an obtuse angle to the surface. Extending away from edge  42  the edges of the bevel converge to a point  136 B on the hammer surface so the bevel plane forms a triangular shape. Again, the recess transition could be another configuration such as a rounded edge or a bevel that does not extend to the bottom of the recess. At least a portion of the recess transition will form an obtuse angle to the surface of the hammer that opens up the recess at the upstream edge, and with metals that flow is less subject to formation of a cornice. 
     Some hammer materials exhibit a tendency to flow under specific circumstances. A sharp edge of a recess where it transitions from a hammer face to a recess wall has been shown in embodiments without a transition surface  136  and  138  as a right angle. Under repeated impacts the material of the hammer face can deform and deflect to create an overhang extending partially or entirely over the recess that limits the size of or closes the recess opening. This can reduce the amount of material impacted by the downstream edge of the recess. Modifying the leading or upstream edge of the recess from a right angle to a more extended transition reduces the tendency to form these features. 
     In addition to the advantages of the presently disclosed shredder hammers with respect to increased functional efficiency, the shredder hammers of the present invention may also offer advantages with respect to their manufacture. Although the recesses of the shredder hammers of the present invention may be machined into a shredder hammer body after casting, these features are preferably incorporated into the casting mold used to fabricate the shredder hammer from molten metal. The presence of recesses increase hammer surface area, which in turn increases cooling effects during casting resulting in better metal grain structure and depth of hardness, particularly for large hammers (e.g., those of 4 inches of thickness or more). The recited features of the disclosed shredder hammers are designed to improve freeze-off, solidification, and quenching during the casting process and heat treatment to improve material properties and product reliability. The use of casting molds incorporating these features result in improved material properties during the casting process, in turn resulting in greater wear performance and reliability for the resulting shredder hammers. 
     The presently disclosed shredder hammers may be manufactured using standard steel casting processes, as reflected by flowchart  150  of  FIG. 24 . These processes may include preparing a mold for the hammer body, at  152  including features or inserts corresponding to recesses adjacent a circumferential edge of the mold and extending from a face without extending past a midplane of the mold; preparing molten steel of an appropriate composition to yield an intermediate steel product having the desired initial mechanical properties upon cooling, at  154 ; filling the prepared mold with the molten steel, at  156 ; cooling the filled mold at  158 ; removing the intermediate shredder hammer from the mold, at  160 ; and heat treating the intermediate shredder hammer to adjust the mechanical properties to yield the desired hardened shredder hammer, at  162 . 
     In some cases it may be advantageous to manufacture the hammer so that the shredding recesses (i.e., those predominately in the working portion) are proximate or adjacent to edge  42  but do not open to the edge. The hammer may be manufactured with a thin wall or partition between the edge  42  and the recesses so the recess is spaced from wear edge  56 . When installed and initially operated, this partition is either worn away or quickly separates from the hammer providing the advantages of a recess on initial operation and through the service life of the hammer. All of the advantages of the recesses are realized in operation though the recesses are not initially open at edge  42 . Alternatively, the shredding recesses can be completely open (i.e. through the entire thickness) for a span (such as along wear edge  56 ) so long as most of the recess extends only part way through the thickness of the hammer for sufficient strength and reliability. 
     It should be appreciated that although selected embodiments of the representative shredder hammers are disclosed herein, numerous variations of these embodiments are possible. This presently disclosed shredder hammer design lends itself to use for manganese, steel alloy and composite hammer types, and the resulting hammers are well suited to a variety of shredding applications beyond metal shredding and metal recycling. 
     It is believed that the disclosure set forth herein encompasses multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. Each example defines an embodiment disclosed in the foregoing disclosure, but any one example does not necessarily encompass all features or combinations. Where the description recites “a” or “a first” element or the equivalent thereof, such description includes one or more such elements, neither requiring nor excluding two or more such elements. Further, ordinal indicators, such as first, second or third, for identified elements are used to distinguish between the elements, and do not indicate a required or limited number of such elements, and do not indicate a particular position or order of such elements unless otherwise specifically stated.