Abstract:
An impeller for a molten metal pump having a cylindrical body comprised of a refractory material. The cylindrical body includes generally coplanar top and bottom surfaces. A central bore is provided in the top surface to provide a point for mating with a shaft. A plurality of circumferentially spaced passages extend from the top or bottom surface to a side wall of the impeller, each of the passages being separate and preferably having an inlet opening which is equal to or less than the corresponding outlet opening in size. A recess being optionally formed in the top or bottom surface, forming the initial inlet to the passages.

Description:
This application is a continuation-in-part of U.S. Ser. No. 08/842,004 filed Apr. 23, 1997 now U.S. Pat. No. 5,785,494. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to molten metal pumps. More particularly, this invention relates to an impeller suited for use in a molten metal pump. The impeller of the present invention is particularly well suited to be used in molten aluminum and molten zinc pumps. In fact, throughout the specification, numerous references will be made to the use of the impeller in molten aluminum pumps, and certain prior art molten aluminum pumps will be discussed. However, it should be realized that the invention can be used in any pump utilized in refining or casting molten metals. 
     In the processing of molten metals, it is often necessary to move molten metal from one place to another. When it is desired to remove molten metal from a vessel, a so called transfer pump is used. When it is desired to circulate molten metal within a vessel, a so called circulation pump is used. When it is desired to purify molten metal disposed within a vessel, a so called gas injection pump is used. In each of these types of pumps, a rotatable impeller is disposed within a pumping chamber in a vessel containing the molten metal. Rotation of the impeller within the pumping chamber draws in molten metal and expels it in a direction governed by the design of the pumping chamber. 
     In each of the above referenced pumps, the pumping chamber is formed in a base member which is suspended within the molten metal by support posts or other means. The impeller is supported for rotation in the base member by means of a rotatable shaft connected to a drive motor located atop a platform which is also supported by the posts. 
     Molten metal pump designers are generally concerned with efficiency, effectiveness and longevity. For a given diameter impeller, efficiency is defined by the work output of the pump divided by the work input of the motor. An equally important quality of effectiveness is defined as molten metal flow per impeller revolutions per minute. 
     A particularly troublesome aspect of molten metal pump operation is the degradation of the impeller. Moreover, to operate in a high temperature, reactive molten metal environment, a refractory or graphite material is used from which to construct the impeller. However, these materials are also prone to degradation when exposed to particles entrained in the molten metal. More specifically, the molten metal may include pieces of the refractory lining of the molten metal furnace, undesirables from the metal feed stock and occlusions which develop via chemical reaction, all of which can cause damage to an impeller and pump housing if passed therethrough. 
     With regard to earlier impeller designs, U.S. Pat. No. 3,048,384, herein incorporated by reference, displays a molten metal pump with a cup-like impeller having lateral openings in the sidewall for moving molten metal. Although the impeller of this design adequately pumps molten metal, it is prone to clogging when particles are drawn into the pump. More specifically, because the inlet to the impeller makes up the entire central top surface area and extends downwardly the entire depth of the radial openings to the circular base, large particles can enter the impeller but cannot exit through the smaller radial openings. Accordingly, a risk for catastrophic failure of the pump results if a large particle is jammed against the volute or the pumping chamber. In addition, small particles can slowly clog the radial openings and degrade the performance of the impeller by reducing the volume of molten metal that can be transferred. 
     In U.S. Pat. No. 5,586,863, a significantly improved molten metal impeller design is provided. More specifically, an impeller comprised of a spherical base, a central hub and radially directed vanes is described. This design achieves a significant advantage by providing a smaller inlet area than outlet area, which more readily passes particles without jamming and/or clogging. However, this design is slightly disadvantaged in that molten metal flow between adjacent vanes is difficult to control. 
     Accordingly, an impeller having low clogging characteristics, yet also providing high effectiveness would be highly desirable in the art. The current invention achieves these objectives. Moreover, the current invention achieves a number of advantages in directional forced metal flow. For example, the impeller of the current pump is not prone to clogging as in many of the prior impellers. Accordingly, catastrophic failure is much less likely to occur and the effectiveness of operation does not degrade rapidly over time. The design also achieves high strength by increasing the percentage of the body comprised of the refractory material. Furthermore, the impeller design can be prepared with relatively simple manufacturing processes. Therefore, the cost of production is low and accommodates a wide selection of materials, such as graphite or ceramics. 
     SUMMARY OF THE INVENTION 
     It is the primary object of this invention to provide a new and improved molten metal pump. It is a further object of this invention to provide a new and improved impeller for use in a molten metal pump. 
     To achieve the foregoing objects and in accordance with the purpose of the invention as embodied and broadly described herein, the molten metal pump of this invention comprises a motor having an elongated drive shaft with first and second ends. The first end mates with the motor and the second end is attached to an impeller disposed in a pumping chamber. The impeller is comprised of a cylindrical body of a refractory material and includes generally coplanar top and bottom surfaces, with a first central bore in the top surface that mates with the shaft. A plurality of circumferentially spaced passages extend from the top surface to a sidewall of the impeller. Each of the passages provides a separate duct from an inlet opening at the top surface to an outlet opening at the sidewall. 
     In addition, preferably each inlet opening has a cross-sectional area which is the same as or less than it&#39;s corresponding outlet opening. In a further preferred embodiment, the impeller is comprised of graphite. In a particularly preferred form, the impeller includes at least two passages, and more preferably six passages. Preferably, the impeller is provided with a bearing ring surrounding the edge of the bottom surface. In a further preferred embodiment, the top surface of the impeller is formed of a ceramic material and the body of the impeller is graphite. 
     In an alternative form of the invention, the impeller has a cylindrical graphite or ceramic body with opposed top and bottom surfaces and a radial sidewall. An annular recess is formed in the top or bottom surface, creating an outer ring and inner column. In a top feed embodiment a bore is formed in the inner column to accommodate a shaft. Preferably, the annular recess will extend to a depth between one-half the width of the recess and less than two-thirds, more preferably one half the overall height of the impeller body. In a particularly preferred embodiment, the width and depth of the annular recess are approximately equal. A plurality of passages extend from the sidewall and intersect the annular recess. Preferably, the passages have a height and a width greater than the dimension of the recess radially between the inner column and the outer ring. In this regard, any object or inclusion in the molten metal bath which is sufficiently small to enter the annular recess, will be easily passed through and out the passages in the sidewall. 
     In a preferred embodiment, the impeller will include four and more preferably six passages with a major portion of the passages disposed at a level below the annular recess, wherein the annular recess intersects only the top region of the passages. For example, the annular recess will extend through the top half of the impeller height and the passages will be located predominantly in the lower half of the impeller height. 
     In a particularly preferred form of the invention, a ceramic cap member will be secured to the top outer ring of the impeller to protect the top surface and a bearing ring will be secured to the outer lower edge. This form of the impeller has been found to effectively repel large objects in the molten metal bath away from the entry to the impeller, i.e., the annular recess, without significant damage to the impeller or pump housing. 
     In an additional alternative embodiment, the impeller will include passages which are substantially straight bores passing from the top or bottom surface of the impeller to the sidewall. Preferably the bores will be generally circular or oval in cross-section and will be angled at least 5° and more preferably about 45° from vertical. Preferably, the bores will widen from the inlet to the outlet. Furthermore, this straight bore embodiment can be combined with an annular recess, wherein each bore opens into the recess rather than the top or bottom surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the inventive impeller; 
     FIG. 2 is a top view of the inventive impeller, showing the passages in cross section; 
     FIG. 2A is a cross sectional view taken along lines A—A in FIG. 2; 
     FIG. 3 is a top view of alternative embodiment of the inventive impeller; 
     FIG. 3A is a cross sectional view taken along lines A—A in FIG. 3; 
     FIG. 4 is a cross-sectional view similar to that of FIGS. 2A, and  3 A, of an alternative embodiment of the inventive impeller. 
     FIG. 5 is a side elevation view of the inventive impeller secured to a drive shaft, partially in cross section; 
     FIG. 6 is an exploded view of a molten metal pump including the inventive impeller; 
     FIG. 7 is a perspective view of an alternative embodiment of the inventive impeller; 
     FIG. 8 is a top view of the inventive impeller of FIG. 7 (shaft removed); 
     FIG. 9 is a cross-sectional view of the inventive impeller of FIG. 8; 
     FIG. 10 is a cross-section of the impeller of FIG. 8 taken along lines B—B; 
     FIG. 11 is a cross-sectional view of the inventive impeller of FIG. 7; 
     FIG. 12 is a top plan view of the ceramic cap member; 
     FIG. 13 is a top view of the straight bore embodiment of the inventive impeller; 
     FIG. 14 is a side elevation view of the impeller of FIG. 13; and 
     FIG. 15 is a side elevation view of a bottom feed version of the impeller. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. While the invention will be described in connection with the preferred embodiment, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents that may be included within the spirit and scope of the invention defined by the appended claims. 
     This invention is directed to a new and improved impeller for use in molten metal pumps. In particular, the impeller is utilized in molten metal pumps to create a forced directional flow of molten zinc or molten aluminum. U.S. Pat. Nos. 2,948,524; 5,078,572, 5,088,893; 5,330,328; 5,308,045 and 5,470,201, herein incorporated by reference, describe a variety of molten metal pumps and environments in which the present impeller could be used. 
     Referring now to FIGS. 1,  2  and  2 A, the inventive impeller  1  is a generally cylindrical shaped body of graphite or ceramic and includes an upper face  2  having a recess  4  to accommodate a shaft. The upper face  2  also includes inlets  5  to passages  6  which extend downwardly from the upper face and outwardly through a sidewall  8 , to an outlet  9 . A bearing ring  10  of a ceramic, such as silicon carbide, is provided surrounding the outer edge of a lower face  12 . FIG. 1 also shows an optional ceramic cap  13 , which can be cemented to the top surface  2  of the impeller  1  to improve the wear characteristics of the device. With specific reference to FIGS. 2 and 2A, the passages  6  increase in diameter from the inlet  5  to the outlet  9 . In this manner, any particle which can enter the impeller will also exit. 
     FIGS. 3,  3 A, and  4  depict an alternative embodiment of the impeller. Particularly, in FIGS. 2 and 2A, the passages have an increasing diameter throughout their length. In contrast, the impeller  14  of FIGS. 3 and 3A includes passages  15  having a first diameter portion in a downward direction  16  and a second wider diameter portion  18  in an outward direction. Nonetheless, an inlet  17  has a smaller diameter than an outlet  19 . 
     FIG. 4 shows an impeller &#39; 14  wherein an inlet &#39; 17  and an outlet &#39; 19  have equivalent cross-sectional areas. Furthermore, the cross-sectional area of passages &#39; 15  are substantially equivalent in both the vertical component &#39; 16  and the horizontal component &#39; 18 . Nonetheless, absent any constriction of the flow path, the passages provide a “tunnel” which will accommodate the flow-through of any particle which can fit into the inlet. 
     FIG. 5 is included to depict the inventive impeller  14  attached to a shaft  20 . The shaft  20  is substantially encased in a protective sheath  21 , and includes a first end  22  which mates with a drive motor (see FIG.  5 ). The second end includes a tapered portion  24  which mates with the tapered walls of a central bore  26  in the impeller  14 . The shaft is secured in the bore  26  by cement (not shown) and several dowels  28 . A bearing ring  30  is also positioned on the shaft—cemented in place—to provide a wear surface. 
     FIG. 6 depicts the arrangement of the impeller  14  in a molten metal pump  32 . Particularly, a motor  34 , is secured to a motor mount  36 . A riser  38  (indicating this pump to be a transfer-style)through which molten metal is pumped is provided. The riser  38  is attached to the motor mount  36  via a riser socket  40 . A pair of refractory posts  42  are secured by a corresponding pair of post sockets  44 , a rear support plate  46  and bolts  48  to the motor mount  36 . At a second end, each of the posts  42 , and the riser  38 , are cemented into a base  50 . The base  50  includes a pumping chamber  52 , in which the impeller  14  is disposed. The pumping chamber is constructed such that the impeller bearing ring  10  is adjacent the base bearing ring  54 . The impeller is rotated within the pumping chamber via a shaft  59  secured to the motor by a threaded connection  60  pinned to a universal joint  62 . Of course, the skilled artisan is aware of many various coupling designs such as, but not limited to, pinned connections and quadralobal drives which are all suitable for use in the present pump. 
     The novel impeller has a generally cylindrical shape and is formed of a refractory material such as graphite or a ceramic such as silicon carbide. The cylindrical piece includes a cavity in its upper face suitable to accommodate a shaft. The shaft, in turn, is joined to a motor to achieve rotation of the impeller. The periphery of the upper face is machined to include a plurality of passages which extend downwardly and outwardly from the upper face to the sides of the cylindrical impeller. In the preferred embodiment, six passages are formed and provide a large fluid volume area. 
     Importantly, the passages are formed such that they provide a “tunnel” at the upper face of the impeller which effectively provides entrainment of any particular particles entering the impeller and prevents lodging/jamming between the rotating impeller body and the pump casing. Moreover, any inclusions which are too large to enter the passage will be thrown clear of the pump by centrifugal force, preventing catastrophic failure of the pump. Furthermore, in the preferred embodiment of the impeller, any inclusions or scrap contained in the molten metal which is small enough to enter this dimension of the passage will of necessity be sized such that it can exit the impeller. 
     Referring now to FIGS. 7-12, an alternative embodiment of the inventive impeller is depicted. In this regard, the impeller  101  again includes a main body  103  having a generally cylindrical shape. The cylindrical main body  103  includes a top surface  105  in which an annular recess  107  is formed. A shaft  109  is secured within bore  111  formed within centrally located column  113 , itself formed by annular recess  107  preferably, annular recess  107  extends less than one half the overall height of the cylindrical body  103 . Four passages  115  enter from radial side wall  117  and intersect the annular recess  107 . In this manner a plurality of passages are formed from the top surface  105  to the radial sidewall  117 . 
     In a particularly preferred embodiment, the impeller  101  includes a bearing ring  119  and a cap member  121  (see FIG.  12 ), each comprised of a refractory, high strength material which protects the graphite or ceramic main body  103  from wear, e.g. silicon carbide. 
     As most clearly seen in FIG. 11, the shaft assembly  109  is preferably provided with a diameter equivalent to that of the column  113  or, and as illustrated, is outfitted with a sheath member  123  to protect the shaft material and provide a consistent dimension with column  113  for effective mating of these two compounds. 
     It has been found that the impeller design of FIGS. 7-11 is particularly effective in expelling large occlusions in the molten metal bath away from the impeller shaft arrangement and away from the pump housing. More particularly, it has been found that objects are flung away from the impeller and do not become trapped between the impeller and shaft of impeller and housing—which otherwise results in excessive wear of the apparatus. 
     Referring now to FIGS. 13-14, a further alternative embodiment of the present invention is depicted. Particularly, the inventive impeller  201  is shown comprised of planar top and bottom surfaces  203  and  205 , respectively, and a generally circular in cross-section outer sidewall  207 . The sidewall  207  does not extend fully to bottom surface  205 , but rather a notch  209  is provided to which a bearing ring (not shown) can be affixed in the finished product. A bore  210  is formed in the top surface  203  to accommodate a shaft (not shown). 
     A plurality of passages  211  are provided. The passages  211  are generally straight bores passing from an inlet  208  in the top surface  203  to an outlet  212  in the sidewall  207 . The passages  211  generally have an oval cross-sectional shape and are inclined forwardly from vertical. The inlet  208  and outlet  212  are circumferentially offset. Since such a feature is shown in FIGS. 13 and 14, no new matter has been added. Particularly, during operation of the pump, the impeller rotation is generally in a direction of arrow  213 , from which the reference to forwardly inclined passages is derived. Generally the forward incline will be at least 5°, and preferably about 45° as shown in the figures. Of course, the passages are necessarily angled outwardly from inlet to outlet. 
     Finally, with reference to FIG. 15, a bottom feed impeller  301  is displayed. Moreover, the inlet  313  to the passages  305  is provided in the bottom surface  307  of the impeller  301 . Therefore, a plurality of passages  305  are included in this embodiment with outlets  309  being positioned in the sidewall  311  and inlet  313  being provided in the bottom surface  307 . 
     It is also noted that each of the impeller embodiments of this invention, including (i) mated horizontal and vertical passages (FIGS.  1 - 5 ), (ii) the annular intake recess (FIGS.  7 - 12 ), and (iii) the straight bore passages (FIGS.  13 - 15 ), can be advantageously combined and can be used in both top and bottom inlet pumps. 
     Thus, it is apparent that there has been provided, in accordance with this invention, a molten metal impeller and pump that fully satisfies the objects, aims and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. In light of the foregoing description, accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the impended claims.