Patent Publication Number: US-11396023-B1

Title: Dual cutter assembly and submersible shredder pump having a dual cutter assembly

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a dual cutter assembly and high volume submersible shredder pump having a dual cutter assembly. These are for reducing the size of solids within a liquid which is to be pumped by chopping, shredding, cutting or grinding. 
     Description of the Related Art 
     There is great commercial interest in centrifugal pumps which are capable of pumping liquids and slurries containing solid matter such as small pieces of disposed items. These pumps have the capability of chopping, shredding, cutting or grinding solid matter in the liquid mixture permitting the output from the pump to be disposed of more readily. Shredding pumps are used in liquid transfer applications that require size reduction for solid or semisolid materials contained in a liquid, in order to cut or shred such materials. 
     The pump of the present invention includes an improved dual sided cutting assembly which is particularly useful when mounted to the suction side of submersible pumps that pump raw sewage, fish silage, byproducts of slaughter houses, waste water of paper mills and similar tough applications. These solid or semisolid materials are reduced in size such that a slurry is formed, which is more easily pumped than the solids themselves. Known grinder pumps have an inlet connected to a pumping chamber, or volute, and a driven shaft extending through the pumping chamber, or volute, and into the inlet. The shaft rotates a cutting cylinder or disk in proximity to a plate cutter, thereby effecting the cutting action of the pump. Many other variations and configurations of grinder pumps are known, which provide shearing action between parts operating cooperatively at close tolerances. Examples of such pumps are disclosed in prior U.S. Pat. Nos. 3,650,081; 3,961,758; 4,108,386; 4,378,093; 4,454,993; 4,640,666; 4,697,746; 4,842,479; 5,016,825; 5,044,566; 5,256,032; 6,010,086; and 6,190,121. U.S. Pat. No. 7,159,806 provides a cutting assembly for a grinder pump comprised of a rotary cutter rotatable against an opposing plate cutter. The cutting edges of the plate cutter include a plurality of V-slice cutting teeth, which create bridging spaces to pinch material which is being sucked in to ports and begin cutting along the V-slice and then for cut material to pass through and onward into the volute of the pump. 
     U.S. Pat. No. 8,784,038, which is incorporated herein by reference, describes a single cutter assembly and an associated high volume submersible shredder pump. It provides a cutter assembly employing cutting lobes having a grooved surface which mate with corresponding grooves of a circular plate cutter. A problem with the mechanism of U.S. Pat. No. 8,784,038 is that it has a flat stationary disc and a flat rotary cutter such that some of the waste contents which are ground up are too large and have a tendency to clog the associated shredder pump. 
     U.S. Pat. No. 10,487,835, which is incorporated herein by reference, shows an improved single cutter assembly and associated submersible shredder pump whose cutter assembly has a cup-shaped, concave, plate cutter. It improves upon the cutter assembly of U.S. Pat. No. 8,784,038 by providing a more aggressive cutter arrangement including a cup-shaped, concave, plate cutter which mates with a corresponding rotary cutter having a plurality of cutting lobes. The inside walls of the cup-shaped, concave, plate cutter has a plurality of spaced apart concentric grooves and a concentric dividing wall between adjacent grooves. The underside of each of the cutting lobes has corresponding concentric grooves and concentric dividing walls which juxtapose and mate with those of the cup-shaped, concave, plate cutter. The present invention provides a further improved cutting assembly and shredder pump having a dual sided plate cutter which engages first and second rotary cutters on alternate sides of a centrally positioned dual sided plate cutter. In operation the waste water contents have a much finer grind which improves efficient removal and reduces the likelihood of shredder pump clogging. 
     SUMMARY OF THE INVENTION 
     The invention provides a cutting assembly comprising: 
     a) a drive shaft alternately rotatable in a first direction of rotation and a second direction of rotation; 
     b) a centrally positioned plate cutter having a circular horizontal cross-section, the plate cutter adapted for mounting to an intake opening of a stationary volute; said plate cutter having a first cutting side and an opposing second cutting side; each of the first cutting side and second cutting side of said plate cutter having one or more side walls terminating at a floor, and a plate cutter bore through the floor; each of the side walls having a plurality of spaced apart concentric grooves and a concentric dividing wall between adjacent grooves; each of the grooves and each of the dividing walls having the shape of an arc of a circle which is concentric with a central axis of the bore of the plate cutter; the drive shaft being mounted for rotation within the bore of the plate cutter; the plate cutter having a plurality of holes through the one or more side walls;
 
c) a first rotary cutter comprising a circular hub having a bore through a central axis of the hub, the first rotary cutter fixed to said drive shaft with an implement for fixing the drive shaft through the circular hub of the first rotary cutter within the bore; a plurality of cutting lobes fixed around the circular hub, each of said cutting lobes having an upper surface, a lower surface opposite to the upper surface, a leading edge and a trailing edge opposite to the leading edge; each cutting lobe having an aperture therethrough extending from and through the upper surface to and through the lower surface; each of the cutting lobes extending outwardly from the hub such that a center line equidistant between the leading edge and the trailing edge of each cutting lobe is substantially perpendicular to the central axis of the hub; the cutting lobes being distributed around a periphery of the hub such that each of the distances from the leading edge of each cutting lobe to the trailing edge of a next adjacent cutting lobe are substantially equal; the lower surface of each cutting lobe having a plurality of spaced apart grooves and a dividing wall between adjacent grooves, the grooves and dividing walls of each cutting lobe extending either from its leading edge to its trailing edge or from its leading edge to its aperture and from its aperture to its trailing edge; each of said grooves and dividing walls having the shape of an arc of a circle which is concentric with the central axis of the hub; the grooves and dividing walls from the lower surface of each cutting lobe being juxtaposed with corresponding dividing walls and grooves from the first cutting side of the plate cutter;
 
d) a second rotary cutter comprising a second circular hub having a second bore through a central axis of the second circular hub, the second rotary cutter fixed to said drive shaft with an implement for fixing the drive shaft through the second circular hub of the second rotary cutter within the second bore; a plurality of cutting edges fixed around the circular hub, each of said cutting edges having an upper surface; each of the cutting edges extending outwardly from the hub substantially perpendicular to the central axis of the hub; the cutting edges being distributed around a periphery of the second circular hub; each cutting edge comprising a plurality of spaced apart grooves and a dividing wall between adjacent grooves; each of said grooves and dividing walls having the shape of an arc of a circle which is concentric with the central axis of the second circular hub; the grooves and dividing walls from the upper surface of each cutting edge being juxtaposed with corresponding dividing walls and grooves from the second cutting side of the plate cutter.
 
     The invention also provides a shredder pump comprising: i) a stationary volute having an intake opening, and a discharge opening; ii) a cutting assembly mounted in front of the intake opening, the cutting assembly comprising: 
     a) a drive shaft alternately rotatable in a first direction of rotation and a second direction of rotation; 
     b) a centrally positioned plate cutter having a circular horizontal cross-section, the plate cutter adapted for mounting to an intake opening of a stationary volute; said plate cutter having a first cutting side and an opposing second cutting side; each of the first cutting side and second cutting side of said plate cutter having one or more side walls terminating at a floor, and a plate cutter bore through the floor; each of the side walls having a plurality of spaced apart concentric grooves and a concentric dividing wall between adjacent grooves; each of the grooves and each of the dividing walls having the shape of an arc of a circle which is concentric with a central axis of the bore of the plate cutter; the drive shaft being mounted for rotation within the bore of the plate cutter; the plate cutter having a plurality of holes through the one or more side walls;
 
c) a first rotary cutter comprising a circular hub having a bore through a central axis of the hub, the first rotary cutter fixed to said drive shaft with an implement for fixing the drive shaft through the circular hub of the first rotary cutter within the bore; a plurality of cutting lobes fixed around the circular hub, each of said cutting lobes having an upper surface, a lower surface opposite to the upper surface, a leading edge and a trailing edge opposite to the leading edge; each cutting lobe having an aperture therethrough extending from and through the upper surface to and through the lower surface; each of the cutting lobes extending outwardly from the hub such that a center line equidistant between the leading edge and the trailing edge of each cutting lobe is substantially perpendicular to the central axis of the hub; the cutting lobes being distributed around a periphery of the hub such that each of the distances from the leading edge of each cutting lobe to the trailing edge of a next adjacent cutting lobe are substantially equal; the lower surface of each cutting lobe having a plurality of spaced apart grooves and a dividing wall between adjacent grooves, the grooves and dividing walls of each cutting lobe extending either from its leading edge to its trailing edge or from its leading edge to its aperture and from its aperture to its trailing edge; each of said grooves and dividing walls having the shape of an arc of a circle which is concentric with the central axis of the hub; the grooves and dividing walls from the lower surface of each cutting lobe being juxtaposed with corresponding dividing walls and grooves from the first cutting side of the plate cutter;
 
d) a second rotary cutter comprising a second circular hub having a second bore through a central axis of the second circular hub, the second rotary cutter fixed to said drive shaft with an implement for fixing the drive shaft through the second circular hub of the second rotary cutter within the second bore; a plurality of cutting edges fixed around the circular hub, each of said cutting edges having an upper surface; each of the cutting edges extending outwardly from the hub substantially perpendicular to the central axis of the hub; the cutting edges being distributed around a periphery of the second circular hub; each cutting edge comprising a plurality of spaced apart grooves and a dividing wall between adjacent grooves; each of said grooves and dividing walls having the shape of an arc of a circle which is concentric with the central axis of the second circular hub; the grooves and dividing walls from the upper surface of each cutting edge being juxtaposed with corresponding dividing walls and grooves from the second cutting side of the plate cutter;
 
wherein the drive shaft is mounted for rotation through a wall of the stationary volute by a bearing and sealed by a mechanical seal; iii) an impeller in the stationary volute fixed around the drive shaft; iv) an electric motor attached to an outer portion of the stationary volute, and fixed to the drive shaft for rotating the drive shaft within the stationary volute.
 
     The invention further provides a method of shredding a solid within a liquid comprising: I) providing a shredder pump comprising: 
     i) a stationary volute having an intake opening, and a discharge opening; ii) a cutting assembly mounted in front of the intake opening, the cutting assembly comprising: 
     a) a drive shaft alternately rotatable in a first direction of rotation and a second direction of rotation; 
     b) a centrally positioned plate cutter having a circular horizontal cross-section, the plate cutter adapted for mounting to an intake opening of a stationary volute; said plate cutter having a first cutting side and an opposing second cutting side; each of the first cutting side and second cutting side of said plate cutter having one or more side walls terminating at a floor, and a plate cutter bore through the floor; each of the side walls having a plurality of spaced apart concentric grooves and a concentric dividing wall between adjacent grooves; each of the grooves and each of the dividing walls having the shape of an arc of a circle which is concentric with a central axis of the bore of the plate cutter; the drive shaft being mounted for rotation within the bore of the plate cutter; the plate cutter having a plurality of holes through the one or more side walls;
 
c) a first rotary cutter comprising a circular hub having a bore through a central axis of the hub, the first rotary cutter fixed to said drive shaft with an implement for fixing the drive shaft through the circular hub of the first rotary cutter within the bore; a plurality of cutting lobes fixed around the circular hub, each of said cutting lobes having an upper surface, a lower surface opposite to the upper surface, a leading edge and a trailing edge opposite to the leading edge; each cutting lobe having an aperture therethrough extending from and through the upper surface to and through the lower surface; each of the cutting lobes extending outwardly from the hub such that a center line equidistant between the leading edge and the trailing edge of each cutting lobe is substantially perpendicular to the central axis of the hub; the cutting lobes being distributed around a periphery of the hub such that each of the distances from the leading edge of each cutting lobe to the trailing edge of a next adjacent cutting lobe are substantially equal; the lower surface of each cutting lobe having a plurality of spaced apart grooves and a dividing wall between adjacent grooves, the grooves and dividing walls of each cutting lobe extending either from its leading edge to its trailing edge or from its leading edge to its aperture and from its aperture to its trailing edge; each of said grooves and dividing walls having the shape of an arc of a circle which is concentric with the central axis of the hub; the grooves and dividing walls from the lower surface of each cutting lobe being juxtaposed with corresponding dividing walls and grooves from the first cutting side of the plate cutter;
 
d) a second rotary cutter comprising a second circular hub having a second bore through a central axis of the second circular hub, the second rotary cutter fixed to said drive shaft with an implement for fixing the drive shaft through the second circular hub of the second rotary cutter within the second bore; a plurality of cutting edges fixed around the circular hub, each of said cutting edges having an upper surface; each of the cutting edges extending outwardly from the hub substantially perpendicular to the central axis of the hub; the cutting edges being distributed around a periphery of the second circular hub; each cutting edge comprising a plurality of spaced apart grooves and a dividing wall between adjacent grooves; each of said grooves and dividing walls having the shape of an arc of a circle which is concentric with the central axis of the second circular hub; the grooves and dividing walls from the upper surface of each cutting edge being juxtaposed with corresponding dividing walls and grooves from the second cutting side of the plate cutter;
 
wherein the drive shaft is mounted for rotation through a wall of the stationary volute by a bearing and sealed by a mechanical seal; iii) an impeller in the stationary volute fixed around the drive shaft; iv) an electric motor attached to an outer portion of the stationary volute, and fixed to the drive shaft for rotating the drive shaft within the stationary volute;
 
II) causing the electric motor to rotate the drive shaft in at least one direction of rotation; III) passing the liquid, and the solid, through the cutting assembly and into the stationary volute, and then causing the impeller to propel the liquid and the solid through the discharge opening.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view illustrating a shredder pump according to the invention. 
         FIG. 2  shows a perspective view of a lower surface of a rotary cutter. 
         FIG. 3  shows an assembly of circular plate cutter and a rotary cutter juxtaposed with one another. 
         FIGS. 4 ( a ), ( b ) and ( c )  show cross sections of the mating grooves and dividing walls of variants of a rotary cutter and a circular plate cutter. 
         FIG. 5  shows a bidirectional impeller which is useful for the present invention. 
         FIG. 6 a    shows an embodiment of the invention wherein the cup-shaped, concave, plate cutter having a circular horizontal cross-section which is generally conical in configuration. 
         FIG. 6 b    shows an embodiment of the invention wherein the rotary cutter rotatably mates with the conical cup-shaped, concave, plate cutter. 
         FIG. 6 c    shows a perspective view of an embodiment of the invention wherein the Circular plate cutter  40  is generally planar and a first side engages with first rotary cutter  16  and a second side engages with second rotary cutter  19 . 
         FIG. 7 a    shows an embodiment of the invention wherein the cup-shaped, concave, plate cutter having a circular horizontal cross-section which is generally semi-spherical in configuration. 
         FIG. 7 b    shows an embodiment of the invention wherein the rotary cutter rotatably mates with the semi-spherical cup-shaped, concave, plate cutter. 
         FIG. 8 a    shows an embodiment of the invention wherein the cup-shaped, concave, plate cutter having a circular horizontal cross-section is generally cylindrical in configuration. 
         FIG. 8 b    shows an embodiment of the invention wherein the rotary cutter rotatably mates with the generally cylindrical cup-shaped, concave, plate cutter. 
         FIG. 9  shows an embodiment of the invention wherein the cup-shaped, concave, plate cutter having a circular horizontal cross-section which is generally cylindrical in configuration mated with a corresponding rotary cutter. 
         FIG. 10  shows an exploded view of an embodiment of the invention wherein a dual sided cup-shaped central plate cutter engages first and second rotary cutters. 
         FIG. 11  shows a top view of an assembled cutting assembly showing a dual sided planar central plate cutter having first cutting side which engages with the under side of a first rotary cutter and a second cutting side engages the cutting edge of a second rotary cutter. 
         FIG. 12  shows a bottom view of an assembled cutting assembly wherein a dual sided planar central plate cutter has a first cutting side which engages with the under side of a first rotary cutter, and a second cutting side engages the cutting edge of a second rotary cutter. 
         FIG. 13  shows an exploded view of a cutting assembly according to the invention showing a dual sided generally planar central plate cutter engaging with first and second rotary cutters via a drive shaft retained by a socket head screw and end cap. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a shredder pump  10  according to the invention. It is shown to comprise a housing  1 , encompassing a central motor chamber  13 , in which is mounted an electric motor  3  which is powered by power cord  4 . Motor  3  is preferably a heavy duty, oil filled, and thermally protected motor, as is well known in the art. Motor  3  is securely mounted to the housing  1  via upper and lower ball bearings  2 . The motor rotates drive shaft  14  which in turn drives a non-wadding bidirectional impeller  8  within a stationary volute  5 . Such impeller being of a conventional unidirectional type or a vortex type, capable of pumping equally in either direction of rotation. The bidirectional feature can double the life of the shredding mechanism. Preferably the electric motor  3  is a bidirectional electric motor capable of rotating shaft  14  alternately in a first direction of rotation and a second direction of rotation responsive to controller  17 . The volute is attached to housing  1 , such as by bolts  9 . Volute  5  has an intake opening  7 , and a discharge opening  11 . Shaft  14  extends from motor  3  through a lower floor of housing  1 , through an upper wall of volute  5 , and through intake opening  7 . Motor  3  is separated from the volute  5  by mechanical seals  6  positioned around the shaft  14 . The mechanical seals  6  prevent the entry of liquid into the motor chamber. Thus the drive shaft  14  is mounted for rotation through an upper wall of the volute by the lower of bearings  2  and sealed by mechanical seal  6 . Rotatably fixed to an end of drive shaft  14  is a cutting assembly  12 . The cutting assembly  12  comprises the end of drive shaft  14  which is attached to a first rotary cutter  16 , a second rotary cutter, and a centrally positioned stationary circular plate cutter  40  having a first cutting side and an opposing second cutting side variations of which will be described below. 
     The first rotary cutter  16  is fixed to the drive shaft, and has a plurality of cutting lobes  24  juxtaposed with corresponding parts of the first cutting side of the plate cutter  40 . A second rotary cutter  19  is fixed to the drive shaft, and is juxtaposed with corresponding parts of the second side of the plate cutter  40 . 
     In one embodiment of the cutting assembly, the plate cutter  40  is generally planar in shape. In another embodiment of the cutting assembly, the plate cutter  40  is cup shaped and generally semi-spherical in shape. In another embodiment of the cutting assembly the plate cutter  40  is cup shaped and generally conical in shape. In yet another embodiment of the cutting assembly, the plate cutter  40  is cup shaped and generally cylindrical in shape. 
       FIG. 2  shows a detached, perspective view featuring a lower surface of a first rotary cutter  16  which is fixed to and rotatable driven by drive shaft  14  as seen in  FIG. 3 . First rotary cutter  16  comprises a circular hub  18  having a bore  20  through a central axis of the bore. It has a suitable implement for fixing the drive shaft within the bore such as keyway  22 , key  122 , and end cap  50 , which is shown in  FIG. 3 . Extending outwardly from hub  18  are a plurality of cutting lobes  24 . Each of the cutting lobes  24  have an upper surface  26 , a lower surface  28  opposite to the upper surface, a leading edge  30  and a trailing edge  32  opposite to the leading edge. Each cutting lobe has an aperture  34  therethrough extending from and through the upper surface  26  to and through the lower surface  28 . Each of the cutting lobes  24  extend outwardly from the hub  18  such that a center line equidistant between the leading edge  30  and the trailing edge  32  of each cutting lobe  24  is substantially perpendicular to the central axis of the hub  18 . The cutting lobes  24  are distributed around a periphery of the hub  18  such that each of the distances from the leading edge  30  of each cutting lobe to the trailing edge  32  of a next adjacent cutting lobe are substantially equal. It is most preferred that the lobes  24  be substantially equally spaced around the hub  18  so that the rotary cutter  16  is balanced when rotating. 
     As best seen in  FIG. 3 . the upper surface  26  of each cutting lobe  24  preferably has a convex curvature extending from its leading edge  30  to its aperture  34  and from its aperture  34  to its trailing edge  32 . As seen in  FIG. 2 , the lower surface  28  of each cutting lobe  24  has a plurality of grooves  36  and a dividing wall  38  between adjacent grooves  36 . The grooves  36  and dividing walls  38  of each cutting lobe  24  extend either from its leading edge  30  to its trailing edge  32  or from its leading edge  30  to its aperture  34  and from its aperture  34  to its trailing edge  32 . Each of said grooves  36  has the shape of an arc of a circle which is concentric with the central axis of the hub  18 . As seen in  FIG. 3 , a further part of the cutting assembly is a circular plate cutter  40 , having cutting edges in both of its sides. Circular plate cutter  40  is mounted in front of the intake opening  7  of volute  5 , such as by screws  42 . 
     The circular plate cutter  40  has a central plate bore for receiving shaft  14 , and a surface having a plurality of concentric grooves  44  and a dividing wall  46  between adjacent grooves  44 . Each of the grooves  44  have the shape of an arc of a circle which is concentric with a central axis of the bore, as well as the hub a rotary cutter  16 . The drive shaft  14  is mounted for rotation within the circular plate cutter bore. In this embodiment, the plate cutter  40  has a plurality of holes  48  extending completely therethrough from its top surface to its bottom surface. Although shown as circular holes in  FIG. 3 , the holes can be any of a variety of perforations of any shape, such as any geometrical shape, for example, circular, triangular, rectangular, polygonal, star-shaped, and the like, or they may be curved slots, slanted slots, radially arranged slots of any convenient length and width, or combinations thereof. In one embodiment, the holes  48 , regardless of their shape, may be tapered in the thickness between the top surface and the bottom surface of plate cutter  40 , thus providing each hole with an enhanced knife-like edge. When the cutting lobes  24  are mounted on shaft  14 , the grooves  36  and dividing walls  38  from the lower surface of each cutting lobe  24  are juxtaposed with corresponding dividing walls  46  and grooves  44  from the circular plate cutter  40 . Preferably the cutting lobes  24  are separated from the circular plate cutter  40  by a few thousandths of an inch by a metal spacer. 
       FIG. 3  shows an assembly  12  of circular plate cutter  40 , lobes  24  of rotary cutter  16  are juxtaposed with one another. The shaft  14 , and rotary cutter  16  are preferably held together by a keyed joint, for example a key along shaft  14  which enters a keyway through the respective central bore of circular plate cutter  40 , and rotary cutter  16 , and finally attached by an end cap  50  and a socket head screw  15 . The socket head screw  15  holds end cap  50  against rotary cutter  16 , and preferably threads right into shaft  14 . Although  FIG. 2  and  FIG. 3  show rotary cutters having three lobes, any desired number of lobes may be employed, for example from 2 lobes to 6 lobes, preferably 2 to 4 lobes. 
     The grooves  36  and dividing walls  38  from the lower surface of each cutting lobe  24  and corresponding dividing walls  46  and grooves  44  from the circular plate cutter  40 , may have any convenient shape. In one embodiment, each of the grooves and dividing wall between adjacent grooves form a generally rectangular shaped cross-section as seen in  FIG. 4( a ) . In another embodiment, each of the grooves and dividing wall between adjacent grooves form a generally V-shaped or triangular cross-section as seen in  FIG. 4( b ) . In yet another embodiment, each of the grooves and dividing wall between adjacent grooves form a generally semi-circular shaped cross-section as seen in  FIG. 4( c ) . 
     The mating grooves and dividing walls of the cutting lobes  24  and the circular plate cutter  40  present an improvement over the prior art by providing a much greater shredding surface area for size reduction of solids within liquids. In addition, the lobes having leading and trailing edges and central apertures in combination with the plurality of holes through the surface of the plate cutter, provide a much greater number of cutting edges for the reduction of solids. 
     In order to calculate at the net “Flow Area”, which is the equivalent area of a non-obstructed volute inlet opening, one deducts the area that is constantly being obstructed by the rotating cutter. Therefore, Net “Flow Area”=Total Area of the holes in the stationary circular plate cutter−Total Area being obstructed by the rotating rotary cutter. In the embodiment described herein having an aperture in each of the lobes of the rotating cutter lobes two distinct advantages are obtained, namely a reduction in the area being obstructed by the rotating cutter, which in turn increases the Net “Flow Area”, and an increase in the number of contact edges. Thus one prior art shredder pump claims 108 cuts per revolution. A non-exclusive example of the use of the inventive shredder pump may well shred solids at a rate of from about 1000 to about 3000 cuts per revolution. 
       FIG. 5  shows a bidirectional impeller  8  which is useful for the present invention. Impeller  8  is mounted for rotation around shaft  14  within volute  5 , as seen in  FIG. 1 . It is shown to comprise a plurality of vanes  52 . The number of vanes is easily determinable by the skilled artisan for the desired application.  FIG. 5  shows 6 vanes. Each vane  52  has opposing curved faces having a convex surface. Such a bidirectional impeller is capable of propelling fluids and imbedded solids within the liquid toward the discharge opening  11  of volute  5  when the shaft is rotating in either of the first direction of rotation or a second direction of rotation. In one embodiment the shredder pump of the invention further comprises a controller  17  for alternating the direction of rotation of the shaft  14  in a first direction of rotation and a second direction of rotation. This is particularly useful when the controller senses an overload caused by excessive solids jammed in the openings of plate cutter  40 , and automatically reverses the rotation of rotary cutter  16 . This enables some of the solids to become dislodged, and the remaining solids to be cut by the new leading edges of the cutting lobes. The reversing feature is also very useful to undo any stringy material wadded around the rotary cutter  16 . Such reverse direction controllers are well known in the art. 
       FIG. 6 a    shows a top view of an embodiment of an embodiment of the invention wherein the cup-shaped, concave, plate cutter  40  has a circular horizontal cross-section which is generally conical in configuration.  FIG. 6 b    shows an embodiment of the invention having the first rotary cutter  16  which rotatably mates with the generally conical cup-shaped, concave, plate cutter. 
       FIG. 6 c    shows a perspective view of an embodiment of the invention wherein the Circular plate cutter  40  is generally planar and a first side engages with first rotary cutter  16  and a second side engages with second rotary cutter  19 . 
       FIG. 7 a    shows an embodiment of the invention wherein the cup-shaped, concave, plate cutter  40  having a circular horizontal cross-section is generally semi-spherical in configuration. The second cutting side is not shown in this Figure.  FIG. 7 b    shows an embodiment of the invention having the rotary cutter  16  which rotatably mates with the generally semi-spherical cup-shaped, concave, plate cutter. 
       FIG. 8 a    shows an embodiment of the invention wherein the cup-shaped, concave, plate cutter  40  having a circular horizontal cross-section is generally cylindrical in configuration. The second cutting side is not shown in this Figure.  FIG. 8 b    shows an embodiment of the invention having the rotary cutter  16  which rotatably mates with the generally cylindrical cup-shaped, concave, plate cutter. 
       FIG. 9  shows an embodiment of the invention wherein the cup-shaped, concave, plate cutter  40  has a circular horizontal cross-section which is generally cylindrical in configuration mated with a corresponding rotary cutter  16 . A similar mating arrangement is made with cup-shaped, concave, plate cutters having a semi-spherical or conical configuration. 
       FIG. 10  shows an exploded view of an embodiment of the invention showing a dual sided cup-shaped central plate cutter  40  having first cutting side  41  and second cutting side  43 . First cutting side  41  engages with the under side  45  of first rotary cutter  16 . Second cutting side  43  engages the cutting edge  47  of second rotary cutter  19 .
           FIG. 11  shows a top view of an assembled cutting assembly according to the invention showing a dual sided planar central plate cutter  40  having first cutting side  41  which engages with the under side  45  of first rotary cutter  16 . Second cutting side engages the cutting edge  47  second rotary cutter  19 .     FIG. 12  shows a bottom view of an assembled cutting assembly according to the invention showing a dual sided planar central plate cutter  40  having first cutting side which engages with the under side of first rotary cutter  16 . Second cutting side  43  engages the cutting edge  47  of second rotary cutter  19 .     FIG. 13  shows an exploded view of a cutting assembly according to the invention showing a dual sided generally planar central plate cutter  40  having a first cutting side  41  which engages with the under side of first rotary cutter  16 . Second cutting side  43  engages the cutting edge  47  of second rotary cutter  19 . Drive shaft  14  extends through a bore  20  through a central axis of the first rotary cutter  16 ; a plate cutter bore  23  through its floor, and a bore  25  through the floor of the second rotary cutter  19 . Socket head screw  15  holds end cap  50  against rotary cutter  16 , and preferably threads right into shaft  14 .       

     In use, the invention further provides a method of shredding a solid within a liquid comprising first providing the above described a shredder pump; then causing the electric motor to rotate the drive shaft in at least one direction of rotation; then passing a liquid, and an optional solid, through the cutting assembly and into the volute, and then causing the impeller to propel the liquid and the optional solid through the discharge opening. 
     The configuration of these embodiments represent an advantage over the prior art cutter assemblies in that it keeps the solids captive while being shredded. The other types shred a portion of the solid item as it is sucked in, but then slings it out and returns into the liquid until is sucked in again by the pump. The cutter of this invention demonstrates that once a solid, such as a garment, gets sucked in the cup-shaped cutter assembly, it stays there until it is completely shredded. 
     While the present invention has been particularly shown and described with reference to preferred embodiments, it will be readily appreciated by those of ordinary skill in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. It is intended that the claims be interpreted to cover the disclosed embodiment, those alternatives which have been discussed above and all equivalents thereto.