Abstract:
Embodiments of the invention are directed to a rotor for a molten metal pump and a molten metal pump including the rotor. The rotor has a main body and a top comprised of a material that is at least twice as hard as the main body. The top, among other things, may form a first portion of each rotor blade wherein the first portion directs molten metal into a pump chamber or other structure in which the rotor is mounted.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a rotor for pumping molten metal, the rotor having a hardened top wherein at least some of the ceramic top preferably forms part of one or more rotor blades. The purpose of the hardened top is to decrease wear on the portions of the rotor that are struck by dross or other hard objects found in molten metal. 
       BACKGROUND OF THE INVENTION 
       [0002]    As used herein, the term “molten metal” means any metal or combination of metals in liquid form, such as aluminum, copper, iron, zinc and alloys thereof. The term “gas” means any gas or combination of gases, including argon, nitrogen, chlorine, fluorine, freon, and helium, that are released into molten metal. 
         [0003]    Known molten-metal pumps include a pump base (also called a housing or casing), one or more inlets (an inlet being an opening in the housing to allow molten metal to enter a pump chamber), a pump chamber, which is an open area formed within the housing, and a discharge, which is a channel or conduit of any structure or type communicating with the pump chamber (in an axial pump the chamber and discharge may be the same structure or different areas of the same structure) leading from the pump chamber to an outlet, which is an opening formed in the exterior of the housing through which molten metal exits the casing. An impeller, also called a rotor, is mounted in the pump chamber and is connected to a drive system. The drive system is typically an impeller shaft connected to one end of a drive shaft, the other end of the drive shaft being connected to a motor. Often, the impeller shaft is comprised of graphite, the motor shaft is comprised of steel, and the two are connected by a coupling. As the motor turns the drive shaft, the drive shaft turns the impeller and the impeller pushes molten metal out of the pump chamber, through the discharge, out of the outlet and into the molten metal bath. Most molten metal pumps are gravity fed, wherein gravity forces molten metal through the inlet and into the pump chamber as the impeller pushes molten metal out of the pump chamber. 
         [0004]    A number of submersible pumps used to pump molten metal (referred to herein as molten metal pumps) are known in the art. For example, U.S. Pat. No. 2,948,524 to Sweeney et al., U.S. Pat. No. 4,169,584 to Mangalick, U.S. Pat. No. 5,203,681 to Cooper, U.S. Pat. No. 6,093,000 to Cooper and U.S. Pat. No. 6,123,523 to Cooper, and U.S. Pat. No. 6,303,074 to Cooper, all disclose molten metal pumps. The disclosures of the patents to Cooper noted above are incorporated herein by reference, as are U.S. Pat. Nos. 7,402,276 and 7,507,367. The term submersible means that when the pump is in use, its base is at least partially submerged in a bath of molten metal. 
         [0005]    Three basic types of pumps for pumping molten metal, such as molten aluminum, are utilized: circulation pumps, transfer pumps and gas-release pumps. Circulation pumps are used to circulate the molten metal within a bath, thereby generally equalizing the temperature of the molten metal. Most often, circulation pumps are used in a reverbatory furnace having an external well. The well is usually an extension of the charging well where scrap metal is charged (i.e., added). 
         [0006]    Transfer pumps are generally used to transfer molten metal from the external well of a reverbatory furnace to a different location such as a ladle or another furnace. 
         [0007]    Gas-release pumps, such as gas-injection pumps, circulate molten metal while releasing a gas into the molten metal. In the purification of molten metals, particularly aluminum, it is frequently desired to remove dissolved gases such as hydrogen, or dissolved metals, such as magnesium, from the molten metal. As is known by those skilled in the art, the removing of dissolved gas is known as “degassing” while the removal of magnesium is known as “demagging.” Gas-release pumps may be used for either of these purposes or for any other application for which it is desirable to introduce gas into molten metal. Gas-release pumps generally include a gas-transfer conduit having a first end that is connected to a gas source and a second submerged in the molten metal bath. Gas is introduced into the first end and is released from the second end into the molten metal. The gas may be released downstream of the pump chamber into either the pump discharge or a metal-transfer conduit extending from the discharge, or into a stream of molten metal exiting either the discharge or the metal-transfer conduit. Alternatively, gas may be released into the pump chamber or upstream of the pump chamber at a position where it enters the pump chamber. A system for releasing gas into a pump chamber is disclosed in U.S. Pat. No. 6,123,523 to Cooper. Furthermore, gas may be released into a stream of molten metal passing through a discharge or metal-transfer conduit wherein the position of a gas-release opening in the metal-transfer conduit enables pressure from the molten metal stream to assist in drawing gas into the molten metal stream. Such a structure and method is disclosed in a copending application entitled “System for Releasing Gas Into Molten Metal,” invented by Paul V. Cooper, and filed on Feb. 4, 2004, the disclosure of which is incorporated herein by reference. 
         [0008]    When a conventional molten metal pump is operated, the rotor rotates within the pump housing and the pump housing, inlet and pump chamber remain stationary relative to the rotor, i.e., they do not rotate. A problem with such molten metal pumps is that the molten metal in which it operates includes solid particles, such as dross and brick. As the rotor rotates molten metal including the solid particles enters the pump chamber through the inlet. A solid particle may lodge between the moving rotor and the stationary inlet, potentially jamming the rotor and potentially damaging one or more of the pump components, such as the rotor or rotor shaft of the pump. 
         [0009]    Many attempts have been made to solve this problem, including the use of filters or disks to prevent solid particles from entering the inlet and the use of a non-volute pump chamber to increase the space between the inlet and rotor to allow solid pieces to pass into the pump chamber without jamming, where they can be pushed through the discharge by the action of the rotor. 
         [0010]    Gas-release pumps generally include a gas-transfer conduit having a first end that is connected to a gas source and a second end submerged in the molten metal bath. Gas is introduced into the first end and is released from the second end into the molten metal. The gas may be released downstream of the pump chamber into either the pump discharge or a metal-transfer conduit extending from the discharge, or into a stream of molten metal exiting either the discharge or the metal-transfer conduit. Alternatively, gas may be released into the pump chamber or upstream of the pump chamber at a position where molten metal enters the pump chamber. 
         [0011]    The materials forming the components that contact the molten metal bath should remain relatively stable in the bath. Structural refractory materials, such as graphite or ceramics, that are resistant to disintegration by corrosive attack from the molten metal may be used. As used herein “ceramics” or “ceramic” refers to any oxidized metal (including silicon) or carbon-based material, excluding graphite, capable of being used in the environment of a molten metal bath. “Graphite” means any type of graphite, whether or not chemically treated. Graphite is particularly suitable for being formed into pump components because it is (a) soft and relatively easy to machine, (b) not as brittle as ceramics and less prone to breakage, and (c) less expensive than ceramics. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention relates to rotors used for pumping molten metal wherein the rotor has a hardened top to alleviate damage to the rotor caused by dross or other hard particles striking the rotor as molten metal enters the chamber of a molten metal pump in which the rotor is retained. The top is at least twice as hard as the body portion of the rotor, and the top preferably covers the entire top surface of the rotor including the tops of the rotor blades. 
         [0013]    In one embodiment, the hardened top extends to include all or part of the surface of the rotor blades that move molten metal into the pump chamber or that push molten metal outward towards the wall of the pump chamber. Aspects of the invention can be utilized on any molten metal rotor design. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a front, perspective view of a rotor according to the invention. 
           [0015]      FIG. 2  shows a top view of the rotor of  FIG. 1 . 
           [0016]      FIG. 3  shows a perspective, side view of the rotor of  FIG. 1  with the top not assembled to the body. 
           [0017]      FIG. 4  shows a perspective, side view of the rotor of  FIG. 1  with the top assembled to the body. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0018]    As used herein the relative hardness of materials is determined by the MOHS hardness scale. On the MOHS hardness scale, treated graphite may have a hardness between 1.5 and 2.5, whereas silicon carbide generally has a hardness of 9-10. 
         [0019]    Turning now to the drawings, where the purpose is to describe a preferred embodiment of the invention and not to limit same, systems and devices according to the invention will be described. 
         [0020]      FIGS. 1-4  show one preferred rotor according to aspects of the invention. Rotor  100  as shown preferably has three identical rotor blades (also called “vanes” herein)  102 . As used herein, a rotor blade (or “vane”) is a structure separate from and spaced from other rotor blades. In rotor  100  each blade is dual flow, meaning that it has a first portion  102 A that directs molten metal either downward or upward (if the rotor is used on a bottom feed pump) towards a second portion  102 B that directs molten metal outward. 
         [0021]    A rotor according to aspects of the invention has a body (or body portion)  101  with a hardened top surface  106 . Rotor  100  may have a flow blocking and bearing plate  110 . As shown, flow blocking and bearing plate  110  is cemented to the bottom  120  of rotor  100 . If rotor  100  is used on a bottom feed pump, the flow blocking and bearing plate  110  may be at the top of the rotor (in essence, the rotor would be turned upside down, with the blades at the bottom, but the rotor shaft attachment mechanism would still be at the top). The flow blocking and bearing plate  110  is preferably comprised of a hard, wear-resistant material, such as silicon carbide. Alternatively, a rotor according to the invention may not have a flow blocking and bearing plate. 
         [0022]    Rotor  100  further includes a connective portion  112 , which is preferably a threaded bore, but can be any structure capable of drivingly engaging a rotor shaft (not shown). It is most preferred that the outer surface of the end of the rotor shaft that is received in portion  112  has tapered threads and connective portion  112  be threaded to receive the tapered threads. 
         [0023]    The preferred dimensions of rotor  100  will depend upon the size of the pump chamber or other structure in which it is received. 
         [0024]    Preferably each vane  102  has the same configuration so only one vane  102  shall be described. Each vane  102  preferably includes a horizontally-oriented first portion  102 A and a vertically-oriented second portion  102 B. The respective vertical and horizontal orientation of the portions described herein is in reference to a rotor positioned in a standard pump having an input port in its top surface. The invention, however, covers any rotor for use in a molten-metal pumping application, whether the flow of molten metal is first contacting the rotor at the top or bottom or both. It will be therefore understood that the terms “horizontal” and “vertical” refer to the rotor as shown in the orientation in  FIGS. 1-4 . 
         [0025]    Top surface  106  is preferably flush with a pump chamber inlet, if used with a pump chamber. 
         [0026]    Section  102 A preferably has a leading edge  116  and an angled surface (or first surface)  118 . Surface  118  is angled (as used herein the term angled refers to both a substantially planar surface, or a curved surface, or a multifaceted surface) such that, as rotor  100  turns (as shown it turns in a clockwise direction) surface  118  directs molten metal towards second portion  102 B. Any surface that functions to direct molten metal towards second portion  102 B can be used, but it is preferred that surface  118  is substantially planar and formed at a 30°-60°, and most preferably, a 45° angle. 
         [0027]    Portion  102 B, which is preferably vertical (but can be angled or curved), extends from the bottom of section  102 A to the top of base (or bottom)  120 . Portion  102 B has a leading face (or second surface)  122 . Leading face  122  is preferably planar and vertical, although it can be of any configuration that directs molten metal outward, such as towards the wall of a pump chamber or other structure in which the rotor  100  is housed. 
         [0028]    A recess  130  is formed in top portion  104  and preferably extends from top surface  106  to at least as far as the trailing face  132  of second portion  102 B. As shown, recess  130  begins at a position on surface  106  slightly forward of face  132  and terminates at a position even with trailing face  132 . The purpose of recess  130  is to reduce the area of top surface  106 , thereby creating a larger opening for more molten metal to enter into the rotor  100  thus enabling rotor  100  to move more molten metal per rotor revolution. 
         [0029]    The hardened top  104  is shown in  FIGS. 1-4 . The hardened top (or entrance to the rotor, because what is shown as the top in the Figures may be at the bottom on a bottom-feed pump or on both the top and bottom if no flow blocking and bearing plate is used) preferably is at least twice as hard as the body portion  101 , or 2-3 times harder than the body portion  101 , or 2-4 times harder than the body portion  101 , or 2-5 times harder than the body portion  101 . In one preferred embodiment, the body portion  101  is graphite and the top  104  is silicon carbide. At least top surface  106  includes the harder material of the hardened top  104 , and as shown the hardened top includes the first portion  102 A of each rotor blade  102 , which includes surface  118 . Additionally, it is preferred that the hardened top  104  include a part of second portion  102 B (and surface  122 ) immediately beneath surface  118 , and recess  130 , and a part of trailing face  132  immediately beneath trailing face  132 . 
         [0030]      FIG. 3  shows hardened top  104  prior to being assembled to the body portion. In order to secure the top  104  and body portion, it is preferred that portions of the corners of each blade section on body  101  be cut out to create recesses or gaps  150  and that the top portion  106  has sections  152  designed to fill gaps  150  when cemented in place. The mating of sections  152  and gaps  150  helps secure the top  104  and body portion to alleviate the possibility that they will come apart during use. 
         [0031]    Additionally, gaps  150  may have openings  151  that mate with pins (not shown) in sections  152 , or gaps  150  and sections  152  may have openings that receive dowel pins (not shown) to help secure top  104  to the body portion. The center opening  112  in the body portion may also include a locating ring  112 A formed therein, which mates with an extending portion (not shown) in the top  104  to properly center the two. 
         [0032]    A similar hardened top may be utilized in a rotor device such as the one described in U.S. Pat. No. 7,402,276. 
         [0033]    Having thus described some embodiments of the invention, other variations and embodiments that do not depart from the spirit of the invention will become apparent to those skilled in the art. The scope of the present invention is thus not limited to any particular embodiment, but is instead set forth in the appended claims and the legal equivalents thereof. Unless expressly stated in the written description or claims, the steps of any method recited in the claims may be performed in any order capable of yielding the desired result.