Patent Publication Number: US-6986644-B2

Title: Hard material impeller and methods and apparatus for construction

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to impellers of centrifugal pumps used in industrial applications. More specifically, this invention relates to impellers made of very hard materials which are conventionally structured with a lead babbitt to receive a drive shaft. The present invention provides structures and methods for eliminating the babbitt to provide an impeller that is environmentally safe and less expensive to produce. 
   2. Description of Related Art 
   Certain industrial processes involve the pumping of extremely abrasive materials. Such industrial processes include, for example, raw sewage treatment and mining and dredging where the slurries being pumped contain highly abrasive solids. While all pumps that process slurries are eventually subject to wear and degradation, those pumps that are used to process highly abrasive slurries are susceptible to faster and more significant degradation. 
   Responsive to the wear imposed by processing such highly abrasive slurries, pump impellers have been made of more durable material to withstand the wear. Many such impellers, for example, are made from very hard metals selected to be harder than the most common and abrasive grit particle, which is silica sand. The materials are generally selected, therefore, to have a hardness greater than 570 Bhn on the Brinell Hardness scale, or the equivalent thereof. Materials having a hardness greater than 570 Bhn include Ni-Hard and Hi-Chrome. The use of hard materials in the formation of pump impellers significantly improves the life of the impeller, but also imposes difficulties in the manufacture of the impeller. 
   Pump impellers are typically rotated within a pump casing by connection to the drive shaft of a motor. Impellers are generally formed with a central cavity or opening into or through which the terminal end of the drive shaft extends. The exact design and construction of the connection of the impeller to the drive shaft varies widely between types and models of impellers. Impellers that are made of softer metals may typically be machined to form a central cavity that will accommodate the end of the drive shaft. However, impellers that are made of very hard materials (i.e., greater than 570 Bhn), are very difficult to machine and, therefore, present a problem with fitting the drive shaft to the impeller. 
   It has been the conventional practice with very hard material impellers to form a babbitt in the central cavity of the impeller to receive the terminal end of the drive shaft. The babbitting is typically lead and the softness of the lead babbitt allows it to conform to the drive shaft to provide comprehensive contact between the babbitt and the drive shaft. The babbitt may be formed with a given configuration to accommodate the drive shaft. 
   In known casting techniques, the impeller is made in a mold which is shaped to produce a central cavity in the impeller. The central cavity of the casting is of imprecise dimension and finish which is permissible since the babbitt formed in the central cavity compensates for any dimensional or finishing imprecisions. Once the molten material of the impeller has hardened and the casting is removed from the mold, the central cavity is ready for the formation of the babbitt. The center of the cavity is determined and a post-like implement or mandrel is positioned at the center of the cavity. Molten lead is then poured into the cavity and around the mandrel. When the lead has hardened, the mandrel is removed. The babbitt may be formed with a particular shape that is dictated, at least in part, by the machining of the end of the drive shaft. 
   The need to use a lead babbitt, necessitated by the extreme hardness of the impeller material, results in significant additional labor which increases the time and cost of manufacturing hard material impellers. More importantly, however, is the fact that lead babbitts cannot be used in many applications because the lead seeps into and contaminates the water being processed. Also, formation of the lead babbitt is a very toxic and dangerous process and is very costly as a result. 
   An alternative method of mounting very hard material impellers to drive shafts is to provide a soft metal insert into the center of the impeller mold prior to pouring the molten material to form the impeller. The soft metal insert can be configured to provide contact with the impeller and to accommodate the drive shaft, but is also machined to receive the drive shaft. Soft inserts are used when the type of fluid being pumped is incompatible with the lead of a babbitt. The use of soft inserts, however, also represents added cost and labor to the manufacture of the impeller because of the machining required to manufacture the insert and the additional machining required to form the insert to the drive shaft. 
   Thus, it would be advantageous in the art to provide means for producing impellers from hard material which eliminates the need for a lead babbitt or any other type of insert to accommodate the drive shaft and which eliminates the need to machine the impeller or drive shaft, significantly reduces manufacturing costs, simplifies manufacture and provides a more environmentally safe impeller of very hard material. 
   BRIEF SUMMARY OF THE INVENTION 
   In accordance with the present invention, a centrifugal pump impeller formed from very hard materials is made in a manner which eliminates the need for a lead babbitt or other insert, and further eliminates the need to machine the impeller to receive the drive shaft, thereby reducing the cost of manufacturing and producing an environmentally-safe impeller. The impeller of the present invention is particularly configured to receive the terminal end of a drive shaft to provide comprehensive contact between the impeller and the drive shaft to improve the operational life of the drive shaft and the impeller. While the configuration and method of forming a hard material impeller in accordance with the invention is adaptable to any type or style of hard material impeller, the invention is described herein with respect to vortex type impellers as merely one exemplar application of the invention. 
   In accordance with the present invention, impellers formed of very hard material having a Brinell Hardness number of equal to or greater than 570 Bhn, or the equivalent thereof, are made by methods that provide an impeller having a central cavity that is ready to receive the terminal end of a motor drive shaft without requiring machining or the formation of a babbitt. 
   Prior art methods of making impellers of very hard material produce an impeller having a central cavity that is of imprecise dimension and finish. Such imprecision is not critical in prior art casting methods because machining and the babbitt formed in the central cavity will compensate for such imprecisions. In the present invention, use of a babbitt, soft inserts and the machining of the impeller casting is eliminated by forming the central cavity of the impeller with a selected configuration and finish during the casting process so that the central cavity is ready to receive the drive shaft following casting of the impeller. In the methods of the present invention, an impeller mold is used which has a core of selected configuration and finish which renders the central opening of the impeller casting ready to receive the terminal end of the drive shaft. The need for machining the casting or employing a lead babbitt is eliminated. 
   The core of the present invention which is used to form the impeller casting comprises a generally cylindrical form that is structured for attachment to the box in which the impeller mold is formed for casting the impeller. The configuration of the core is selected to determine the ultimate configuration of the central opening in the cast impeller which will receive the drive shaft. The configuration of the core, while variable, is formed with a portion that provides at least one contact surface in the central opening of the impeller for contacting the drive shaft. The core may also be formed with a portion that shapes the impeller in a manner that facilitates the attachment of the drive shaft to the impeller in assembly of the pump. The core may also be selectively formed from materials that will provide a desired finish to the interior wall of the central opening of the impeller. 
   Impellers of the present invention formed by the described methods may be configured in a number of ways to receive a drive shaft of a given configuration. A number of configurations may be used that improve the operational life of the impeller and drive shaft as compared to prior art impeller and drive shaft arrangements. Various embodiments of the cores and the impellers made by such cores are described herein in accordance with the invention. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention; 
       FIG. 1  is a view in cross section of a vortex impeller pump of the prior art illustrating the use of a lead babbitt; 
       FIG. 2  is a view in elevation of the suction side of a vortex impeller of the type shown in  FIG. 1 ; 
       FIG. 3  is a perspective view of a core used in accordance with the invention to form an impeller of the invention; 
       FIG. 4  is a view in elevation of a first embodiment of the core of the invention; 
       FIG. 5  is a view in elevation of the core illustrated in  FIG. 4 , the core having been rotated counterclockwise on its axis ninety-degrees; 
       FIG. 6  is a view in cross section of the core illustrated in  FIG. 4  taken at line  6 — 6  thereof; 
       FIG. 7  is a view in radial cross section of an impeller of the present invention, the central opening of which is configured by the core configuration shown in  FIGS. 4–6 ; 
       FIG. 8  is a view in axial cross section of the impeller shown in  FIG. 7  taken at line  8 — 8  thereof; 
       FIG. 9  is a view in axial cross section of the impeller shown in  FIG. 7  taken at line  9 — 9  thereof; 
       FIG. 10  is a view in elevation of a second embodiment of the core of the present invention; 
       FIG. 11  is a view in axial cross section of the core embodiment shown in  FIG. 10  taken at line  11 — 11  thereof; 
       FIG. 12  is a view in radial cross section of an impeller, the central opening of which is formed by the core embodiment shown in  FIGS. 10 and 11 ; 
       FIG. 13  is a view in axial cross section of the impeller shown in  FIG. 12  taken at line  13 — 13  thereof; 
       FIG. 14  is a view in elevation of a third embodiment of the core of the present invention; 
       FIG. 15  is a view in axial cross section of the core shown in  FIG. 14  taken at line  15 — 15  thereof; 
       FIG. 16  is a view in radial cross section of an impeller, the central opening of which is configured by the core shown in  FIGS. 14 and 15 ; 
       FIG. 17  is a view in axial cross section of the impeller shown in  FIG. 16  taken at line  17 — 17  thereof; 
       FIG. 18  is a view in elevation of a fourth embodiment of the core of the present invention; 
       FIG. 19  is a view in axial cross section of the core shown in  FIG. 18  taken at line  19 — 19  thereof; 
       FIG. 20  is a view in radial cross section of an impeller, the central opening of which is formed by the core shown in  FIGS. 18 and 19 ; 
       FIG. 21  is a view in axial cross section of the impeller shown in  FIG. 20  taken and line  21 — 21  thereof; 
       FIG. 22  is a view in axial cross section of an alternative embodiment of an impeller formed by core shown in  FIGS. 18 and 19 ; 
       FIG. 23  is a view in elevation of a fifth embodiment of the core of the present invention; 
       FIG. 24  is a view in axial cross section of the core shown in  FIG. 23  taken at line  24 — 24  thereof; 
       FIG. 25  is a view in axial cross section of an impeller, the central opening of which is formed by the core shown in  FIGS. 23 and 24 ; 
       FIG. 26  is a view in elevation of a sixth embodiment of the core of the present invention; 
       FIG. 27  is a view in radial cross section of an impeller of the present invention, the central opening of which is formed by the core shown in  FIG. 26 ; 
       FIG. 28  is a view in axial cross section of a seventh embodiment of the impeller of the present invention; and 
       FIG. 29  is a view in elevation of a seventh embodiment of the core of the present invention used to form the impeller shown in  FIG. 28 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates, for reference purposes, a known centrifugal pump  10  having a vortex-type impeller  12 . The pump  10  generally comprises a pump casing  14  having a suction side wear plate  16  and a drive side casing  18  which enclose the impeller  12 . In this particular kind of pump  10 , the impeller  12  is cradled in a suction side wear plate  20  that is secured to the drive side casing  18 . A drive shaft  22  extends through the drive side casing  18  and is supported by a bearing system  24  positioned within a bearing housing  26 . A seal assembly  28  (only partially depicted) also surrounds the drive shaft  22  at the drive side casing  18 . 
   The impeller  12  illustrated in  FIGS. 1 and 2  is structured with a plurality of upstanding vanes  30  which each extend from a suction side hub  32  to a circumferential rim  34 . Cupped portions  36  are formed between adjacent vanes  30  and the circumferential rim  34 . The cupped portions  36  receive fluid entering into the pump  10  through the pump inlet  38 . 
   An opening  40  is formed through the center of the impeller  12  to accommodate the drive shaft  22 . More specifically, the opening  40  may be considered to have a drive side portion  42 , which is sized to receive the terminal end  44  of the drive shaft  22 , and a suction side portion  46  formed in the hub  32  of the impeller. The suction side portion  46  of the opening  40  is generally sized to receive a lock nut assembly  48  and bolt  50  which fits through the lock nut assembly  48  and threadingly engages the terminal end  44  of the drive shaft  22  as shown. 
   The illustrated prior art impeller  12  of  FIG. 1  is made in a conventional manner well-known in the art which involves the formation of a mold into which molten material is poured to form the impeller casting. The mold, comprising two halves, is made by securement of a wooden and/or plastic pattern of the impeller to a platform followed by the placement of a box about the pattern. The pattern determines the outer configuration of the impeller but also produces a centered cavity for later defining of the central opening  40  of the impeller. 
   Fluidized sand containing a binder is packed into the two halves of the mold box and around the impeller pattern. When the sand has hardened, the impeller pattern is removed from the mold halves leaving an impeller impression. A core made of sand is connected to one of the two halves of the box and is centrally located within the centered cavity in the impeller impression. The core is of generalized shape and imprecise dimension. The two halves of the impeller mold are then secured together with the centrally-positioned core attached to the box and molten material, such as Ni-Hard, is poured into the mold to form the cast impeller. After the molten material has cooled, the two halves of the mold are removed from about the impeller casting and the core is removed. 
   It should be noted at this point that in such prior art casting techniques, the core, by virtue of its mode of placement and attachment to the impeller mold, is not entirely secured within the mold and the core may shift as the molten material is being poured into the impeller mold. A core shift of plus or minus one sixteen of an inch from the center line of the impeller is allowable within the industry because it can be compensated for by the lead babbitt described hereinafter and will allow the impeller to rotate without adverse affect. Core shifts of greater than that amount produce an impeller casting that cannot be used because the impeller will wobble when it rotates. 
   The conventional casting process as described produces a central opening  40  in the prior art impeller casting  12  which is of imprecise configuration, dimension and interior finish, rendering it unsuitable to receive the terminal end  44  of the drive shaft  22 . The imprecision of the central opening  40  is compensated for, however, by use of a relatively soft lead babbitt  54 , as shown in  FIG. 1 , which allows the babbitt  54  to conform to the drive shaft  22 . The central opening  40  may be cast with a certain configuration which, in this example, includes a plurality of axial and radial grooves  58  which receive the lead babbitting. 
   A mandrel-type tool, the shape of which is identical to the terminal end  44  of the drive shaft  22 , is positioned in the opening  40 . Molten lead is then poured into the opening around the mandrel-type tool. The molten lead fills the axial and radial grooves  58  formed in the impeller  12  and hardens to form the babbitt  54 . The mandrel-type tool is removed and the resulting opening is ready to receive the terminal end  44  of the drive shaft  22 . In the prior art impeller  12 , the soft lead babbitt  54  conforms to the shape of the terminal end  44  of the drive shaft  22 . The lead babbitt  54  eventually deforms or deteriorates over time and is no longer operative. A new lead babbitt  54  must then be poured to continue use of the impeller. 
   In accordance with the present invention, an impeller is manufactured from very hard materials in a manner which eliminates the need to machine the impeller or to employ a babbitt, thereby rendering the impeller environmentally-safe and considerably less expensive to produce. Impellers of the present invention are produced by preparing a two part impeller mold from sand in the conventional manner previously described. However, in the present invention, a selectively configured and precisely-formed core is used in the impeller mold so that the resulting impeller casting has a central opening which is precisely configured to render it suitable for direct attachment to the drive shaft. The need to machine the central opening or employ a babbitt is thereby eliminated. In addition to being selectively configured, the core is made of a selected material that produces a relatively smooth surface, or interior wall of the central opening of the impeller, to further eliminate the need for any machining or finishing. 
     FIG. 3  illustrates one exemplar core  60  used in the process of the present invention. The core  60  is illustrated as being made of sand, but the core  60  may be made of any other material, including ceramic material, that is capable of being formed into a selected configuration and is able to withstand contact with the molten material used to form the impeller. Moreover, the material used to form the core  60  may be particularly selected to produce an interior surface in the central opening of the impeller casting which has a desired character of finish. Thus, for example, a sand of a particular grain size may be particularly selected to achieve a desired finish in the impeller casting. 
   As one exemplar means of forming the core  60 , a mold, or core box, is formed using two plates of cast iron that are suitably machined to form the structural members of the core box. The two plates are drilled and pinned, and the mating surfaces of the two halves are ground to assure that when the two halves of the core box are separated and then rejoined, the halves will mate precisely. 
   A cavity or impression is machined into each half of the core box corresponding to one lateral half of the finished core configuration. The two halves of the core box are joined and the shape of the cavity or impression is checked to assure that the impression is will accurately produce the desired shape or configuration of the finished core. Additional machining is then performed on the core box halves to enable sand to be blown into the core box. 
   The core box is mounted in a machine that heats the two halves of the core box. Fluidized sand containing a binder is then blown into the core box. The heat of the cast iron core box causes the binder to solidify the sand. The two halves of the core box are then separated and the finished sand core is removed. The finished core  60 , an example of which is shown in  FIG. 3 , is then ready for use in formation of the impeller casting. 
   The impeller of the present invention, as shown generally in  FIG. 7  and described more fully hereinafter, may be cast in a sand mold or other suitably formed mold. The mold, comprising two halves, is formed by securement of a wooden or plastic pattern of the impeller on a platform and the pattern is then surrounded by a box as is done in conventional mold-forming methods. However, in accordance with the present invention, the impeller pattern used in making the impeller mold is especially structured to produce two centralized cavities which will receive the ends of the core  60  to stabilize the core  60  within the center of the impeller impression while the molten material is being poured into the mold to form the impeller casting. 
   Referring to  FIG. 3 , it can be seen that the first end  62  of the core  60  has associated therewith an elongated cylindrical section  63  and the second end  64  of the core  60  has associated therewith a frustoconically-shaped section  65 . The cylindrical section  63  and frustoconically-shaped section  65  constitute, respectively, a first core print  66  and a second core print  68 . In a particularly suitable embodiment, the length of the first core print  66  may be between three and about five times the length of the second core print  68 . The pattern that is used to form the impeller impression in the mold box is specifically structured to form two centralized and axially aligned core print cavities in the mold halves to accommodate the first core print  66  and second core print  68 , as described hereinafter. 
   When casting the impeller, the bottom half of the mold box is placed on a surface and the first core print  66  is positioned within the core print cavity that is formed in that half of the mold box. The selected shape, length and taper of the first core print  66  assure that the core  60  will be securely positioned within the mold box and along the center line of the mold to prevent shifting of the core  60 . The second half of the mold box is then positioned above the bottom half of the mold box and the core point cavity that is formed in the second, or upper, half of the mold box is aligned to receive the second core print  68 . Again, the selected shape, length and taper of the second core print  68  facilitates the accurate placement and alignment of the top mold half on the bottom mold half and assures the securement of the core  60  along the center line of the impeller impression provided by the mold. Moreover, the length and taper of the first core print  66  provide alignment and stability to the core  60  during placement of the top half of the mold on the bottom half of the mold. 
   The selectively configured core  60 , therefore, provides an improvement in the art of hard material impeller casting in that it assures that a precisely centered and configured central opening will be formed in the impeller casting for immediate receipt of the drive shaft. No compensation need be made with respect to the central opening of the impeller, such as machining or use of a babbitt or soft insert, to assure that the impeller will adequately receive the drive shaft or to assure that the impeller will rotate correctly. 
     FIG. 7  illustrates an impeller  70  of the present invention that is cast by the described method of the present invention. The impeller  70  is similar in overall shape to the prior art impeller  10  shown in  FIG. 1 , except that the central opening  72  of the new impeller  70  is configured differently in accordance with the invention, as described more fully hereinafter. The vortex impeller  70  of the present invention has a central opening  72  which, similar to the prior art impeller  10 , may be designated as having a drive side portion  74  which is sized to receive the drive shaft  22  and a suction side portion  76  formed in the hub  78  of the impeller  70 . The drive side portion  74  of the central opening  72  has a configuration selected for accommodation of the drive shaft  22  and the configuration of the drive side portion  74  is provided by the particular configuration of the core  60 , as described more fully hereinafter. 
   Referring again to  FIG. 3 , the core  60  of the present invention comprises at least one configured region which is selectively shaped and dimensioned to determine the configuration and dimension of the central opening  72  of the cast impeller  70 . Accordingly, a first region  80  of the core  60  dictates the configuration and dimension of the drive side portion  74  of the central opening  72 . The configuration of the first region  80  may vary depending on the shape of the terminal end of the drive shaft and on the desired contact between the central opening  72  and drive shaft. 
   A second region  82  of the core  60  may also be provided to determine the configuration of the suction side portion  76  of the central opening  72 . Consistent with the vortex-type impeller illustrated in  FIG. 7 , the second region  82  of the core  60  may be formed with a first cylindrical portion  88  having a selected circumferential dimension and a second cylindrical portion  90  having a circumferential dimension that is less than the circumferential dimension of the first cylindrical portion  88 . The resulting impeller casting is provided with a suction side portion  76  which accommodates a bolt and lock nut assembly of the type shown in  FIG. 1 . It should be noted that some hard material impellers may not be formed with a suction side portion  76  as illustrated in the vortex-type impeller shown herein and a core used to form such impellers would lack a second region  82  as described. 
     FIGS. 4–6  illustrate a first embodiment of the core  60  of the present invention where the first region  80  of the core  60  has a first selected configuration. It should be noted that the configuration and dimension of the second region  82  of the core  60  which ultimately determines the configuration and dimension of the suction side portion  76  of the central opening  72  of the impeller  70  may be presumed henceforth to be the same throughout the various embodiments of the core  60  and impeller  70  hereinafter described. 
   In  FIG. 4 , the first region  80  of the core  60  has a configuration which is generally conical in shape to receive a similarly shaped drive shaft. More specifically, the configuration of the first region  80  is comprised of a first conical portion  92 , a second conical portion  94  and a third conical portion  96 . The surface  93  of the first conical portion  92  and the surface  95  of the third conical portion  94  may, in a preferred embodiment, both lie in a plane  97  that intersects a plane through the axis  98  of the core  60  at an angle α thereto. The angle α of the plane  97  may, most suitably, be from about five degrees to about ten degrees. The surface  93  of the first conical portion  92  and the surface  95  of the third conical portion  96  need not lie in the same plane, however. 
   The surface  99  of the second conical portion  94  is spaced from the surfaces  93 ,  95  of the first conical portion  92  and third conical portion  96 , respectively. The surface  99  of the second conical section  94  lies in a plane  100  which intersects a plane formed through the axis  98  of the core  60  at an angle β. Where the surface  99  of the second conical portion  94  is parallel to surfaces  93  and  95 , which may be preferred, angle β is equal to angle α and, therefore, may range from about five degrees to about ten degrees. 
   A flat, indented portion  101  is formed in the second conical portion  94  of the core  60 , as best seen in  FIGS. 5 and 6 . Thus, the second conical portion  94  is not entirely conical throughout its length, as illustrated in  FIG. 6 . The flat, indented portion  101  of the core  60  provides a flat surface in the cast impeller for contacting a corresponding flat surface formed in the terminal end of the drive shaft.  FIG. 7  illustrates the configuration of the drive side portion  74  of the central opening  72  that is produced by the core  60  configuration shown in  FIGS. 4–6 . The resulting central opening  72  has a first tapered surface  102  and a second tapered surface  104  which is spaced from the first tapered surface  102  in a direction away from the axis  106  of the impeller  70  and drive shaft  22 . 
   As best seen in  FIGS. 8 and 9 , the flat, indented portion  101  of the core  60  produces a corresponding flat surface  108  in the central opening  72  of the impeller  70  which is spaced generally radially inward from the first tapered surface  102  of the central opening  72 . The drive shaft  22  has a corresponding flattened surface  110 . It can be seen in  FIGS. 7–9 , that the drive shaft  22  contacts the first tapered surface  102  of the central opening  72  at contact points  112  and  114  located on either side of the second tapered surface  104 . Friction between the drive shaft  22  and the impeller  70  at the contact points  112 ,  114  of the central opening  72  provide a primary drive mechanism for rotation of the impeller  70 . The rough casting of the impeller at the contact points  112 ,  114  allows the impeller to embed into the metal of the drive shaft  22  providing greater gripping potential for rotation of the impeller  70 . The contact between the flat surface  108  of the central opening  72  and the flattened surface  110  of the drive shaft  22  provides a secondary drive mechanism for rotating the impeller  70 . Therefore, the configuration provides improved operational life of the impeller and drive shaft. 
     FIG. 10  illustrates a second alternative embodiment of the core  60  of the present invention where, again, the first region  80  is generally conical in shape and the configuration is comprised of a first conical portion  116 , a second conical portion  117  and third conical portion  118 . The surface  119  of the first conical portion  116  and the surface  120  of the third conical portion  118  may preferably lie in the same plane as described for the first core embodiment of  FIG. 4 . The surface  121  of the second conical portion  117  is spaced from the surfaces  119 ,  120  of the first conical portion  116  and third conical portion  118 , respectively. The second embodiment of the core  60  illustrated in  FIG. 10  is similar to that shown in  FIG. 4 , except that the first region  80  is configured without a flat surface.  FIG. 11  further illustrates that the second conical portion  117  has a circular circumference in axial cross section. 
     FIG. 12  illustrates an impeller  70  having a drive side portion  74  of the central opening  72  that is formed by the core  60  shown in  FIG. 10 . The resulting central opening  72  is configured with a first tapered surface  124  and a second tapered surface  126  which is spaced from the first tapered surface  124  in a direction away from the axis  106  of the impeller  70  and drive shaft  22 . The drive shaft  22  is configured with a conically-shaped terminal end  127  (i.e., having no flattened surface) having essentially the same angle of taper as the first tapered surface  124  of the central opening  72 . Consequently, the drive shaft  22  contacts the first tapered surface  124  of the central opening  72  at at least two points, namely a first contact point  128  and second contact point  130  located on either side of the second tapered surface  126 , which provides greater gripping potential for rotation of the impeller  70 . 
     FIG. 14  illustrates a third embodiment of the core  60  of the present invention where the first region  80  of the core is generally conically shaped with a single conical portion  132 . The single conical portion  132  has a surface  134  which lies in a plane  136  that intersects a plane formed through the axis  98  of the core  60  at an angle α. The angle α may be from about five degrees to about ten degrees. An indented flat surface  138  is formed in the surface  134  of the single conical portion  132 , as best seen in  FIG. 15 . 
     FIG. 16  illustrates an impeller  70  of the present invention, the central opening  72  of which is formed by the core  60  shown in  FIGS. 14 and 15 . The drive side portion  74  of the central opening  72  is configured with a tapered surface  140  and a flat surface  142  formed along a portion thereof. The terminal end  144  of the drive shaft  22  is similarly configured with a conical shape having a flattened surface portion  146  that is positioned to contact the flat surface  142  of the drive side portion  74  when the drive shaft  22  is connected to the impeller  70 . As seen in  FIGS. 16 and 17 , this embodiment of the impeller  70  produces contact between the drive side portion  74  of the central opening  72  and the terminal end  144  of the drive shaft  22  virtually everywhere, except at a point  148  on one side of the flat surface  142  of the central opening  72 . As such, the fit between the drive shaft  22  and the impeller  70  lies more critically in the precision configuration of the core  60  as compared with other embodiments described heretofore. 
     FIGS. 18 and 19  illustrate a fourth embodiment of the core  60  of the present invention where the first region  80  of the core  60  is configured with a substantially cylindrical portion  150 . In this embodiment, the first region  80  is further configured with a linear projection  152  which extends outwardly from the surface  154  of the cylindrical portion and extends a selected length  156  of the cylindrical portion  150 . In the embodiment shown, the linear projection  152  extends the full length  156  of the cylindrical portion  150 , but may be less than the full length  156 . The configuration of the core  60  shown in  FIGS. 18 and 19  produces a drive side portion  74  of the central opening  72  that is substantially cylindrical and has a keyway  160  formed in the surface  162  of the drive side portion  74  as shown in  FIG. 20 . 
   In the embodiment of the impeller  70  of the present invention shown in  FIGS. 20 and 21 , the terminal end  166  of the drive shaft  22  is substantially circular in axial cross section and is formed with a keyway  168  that is located for alignment with the keyway  160  formed in the impeller  70 . At assembly, a key  170 , here shown as constituting a bar of rectangular cross section, is inserted to be received in both the keyway  160  of the impeller  70  and the keyway  168  of the drive shaft  22 . In this embodiment, contact is made between the surface  162  of the central opening  72  and the outer surface  172  of the terminal end  166  of the drive shaft  22  to provide a primary drive mechanism in rotation of the impeller  70 . The key  170  extending between the impeller  70  and the drive shaft  22  provides a secondary drive mechanism. 
     FIG. 22  illustrates a further means of connecting the impeller  70  illustrated in  FIG. 20  with a drive shaft  22 . In this embodiment, the terminal end  176  of the drive shaft  22  is cylindrical in axial cross section and is formed with a spline  178  that extends outwardly from the outer surface  180  of the terminal end  176  of the drive shaft  22 . The spline  178  is sized and shaped to be received in the keyway  160  of the impeller  70 . Again, contact between the surface  162  of the impeller  70  and the outer surface  180  of the terminal end  176  of the drive shaft  22  provides a primary drive mechanism for rotating the impeller  70 . The interaction of the spline  178  and keyway  160  provide a secondary drive mechanism. 
     FIGS. 23 and 24  illustrate a fifth embodiment of the core  60  of the present invention where the first region  80  of the core  60  is configured with a substantially cylindrical portion  184 . A linear channel  186  is formed along a selected length  188  of the cylindrical portion  184  extending inwardly from the surface  190  thereof. The channel  186  is shown as extending the full length  188  of the cylindrical portion  184 , but the channel  186  may extend less than the full length  188 .  FIG. 24  illustrates in axial cross section the configuration of the first region  80  of the core  60  shown in  FIG. 23 . 
     FIG. 25  illustrates an axial cross section of an impeller  70  of the present invention where the drive side portion  74  of the central opening  72  is formed by the core  60  shown in  FIGS. 23 and 24 . The drive side portion  74  is formed with a substantially circular surface  192 , but is further configured with a spline  194  produced in the casting by the channel  186  of the core  60 . The terminal end  196  of the drift shaft  22  is consequently configured with a keyway  198  that is sized to receive the spline  194  formed in the impeller  70 . The contact between the surface  192  of the of the drive side portion  74  of the impeller  70  central opening  72  and the outer surface  200  of the terminal end  196  of the drive shaft  22  provides a primary drive mechanism in rotation of the impeller  70 . The interaction of the spline  194  in the keyway  198  of the drive shaft  22  provides a secondary drive mechanism for rotation. 
   Because the core formation method of the present invention allows the core to be more precisely shaped or configured than was possible in prior art methods, it should be noted that first region  80  of the core  60  may be other than cylindrically-shaped in axial cross section as shown in the fourth and fifth embodiments of the core described previously. That is, the first region  80  of the core may be any suitable size, dimension or shape, including square, rectangular, triangular, hexagonal, oval, bilobular, etc., when viewed in axial cross section. 
   Additionally, with respect to those embodiments of the invention which employ a spline or key and keyway as a secondary drive mechanism, it should be noted that the first region  80  of the core  60  need not be limited to a cylindrical portion as previously described. For example, as shown in  FIG. 26 , the first region  80  of the core  60  may be conical in shape and may be configured with either a linear projection  202 , thereby producing a core  60  having an axial cross section as shown in  FIG. 19 , or having a linear channel  204  (shown in phantom in  FIG. 26 ), thereby producing a core  60  having an axial cross section as shown in  FIG. 24 . 
   The impeller  70  cast from the core  60  shown in  FIG. 26  is shown in  FIG. 27  and would have the same general elements of configuration as described with respect to  FIG. 20 , except that the drive side portion  74  of the central opening  72  is conical in shape and is configured to receive a drive shaft  22  having a terminal end  206  that is similarly conical in shape, having a keyway to receive a spline formed in the impeller or, alternatively as shown, a spline  208  to interactingly fit with a keyway  210  formed in the impeller  70  as shown. 
   The embodiment shown in  FIG. 16  illustrates an impeller  70  having a central opening  72  configuration which includes a flat surface  142  that extends only partially along the length of the tapered portion  140  of the central opening  72 . In an alternative embodiment shown in  FIG. 28 , the flat surface  220  may extend a length essentially equal to the length of the tapered portion  222  of the central opening  72 . The core  60  used in casting the central opening  72  of the impeller  70  is shown in  FIG. 29  where the first region  80  of the core  60  is generally conically shaped with a single tapered portion  224  similar to the core embodiment shown in  FIG. 14 . However, a flat surface  226  is formed along the length  228  of the single tapered portion  224 . In axial cross section, the single tapered portion  224  appears as shown in  FIG. 15 . Referring again to  FIG. 28 , the terminal end  144  of the drive shaft  22  is similarly configured with a tapered end having a flattened surface portion  230  that is positioned to contact the flat surface  220  of the drive shaft portion  74  of the impeller  70 . The axial cross section of the impeller  70  is as shown in  FIG. 17 . 
   The method of core formation and impeller casting disclosed herein may be used to form any type or configuration of impeller that is made of very hard material (i.e., greater than 570 Bhn), and is not limited to use in the formation of cupped vortex impellers as has been described herein as merely exemplar. Thus, the configuration of the core and the resulting configuration of the central opening of the impeller may be modified and adapted to any type or style of impeller for a centrifugal pump. Therefore, reference herein to specific details of the core and impeller configurations are by way of example only and not by way of limitation.