Abstract:
A submersible pump unit having a wet-running electric motor and only a single impeller can be driven by the electric motor with a rotation speed greater than 20,000 rpm. The rotor of the electric motor has a diameter of less than 25 mm.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Section 371 of International Application No. PCT/EP2006/007671, filed Aug. 3, 2006, which was published in the German language on Mar. 29, 2007, under International publication No. WO 2007/033726 A1 and the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a submersible pump assembly having a wet-running electric motor. 
     Submersible pump assemblies with wet-running electric motors are known, wherein several stages, i.e., several impellers arranged one after the other, are provided for achieving a greater delivery output. The disadvantage of these assemblies is the fact that the constructional size is increased on account of the number of stages. Furthermore, the friction in the whole assembly increases, so that the power loss is also increased. 
     BRIEF SUMMARY OF THE INVENTION 
     With regard to this, it is an object of the invention to provide an improved submersible pump assembly having a wet-running electric motor, which assembly has a smaller constructional size, and provides an increased efficiency. 
     The submersible pump assembly according to the invention is merely provided with a single impeller, i.e., it only has one stage. This single impeller is driven by the wet-running electric motor at a rotational speed of greater than 20,000 rpm, preferably greater than 25,000 or 30,000 rpm. A large delivery output may be achieved even with only one stage on account of this high rotational speed. In this manner, further stages of the submersible pump assembly may be spared, whereby a compact constructional size may be achieved, and the power loss of the submersible pump assembly may be simultaneously minimized, on account of the reduced friction. Furthermore, the electric motor according to the invention is designed such that the rotor has a diameter of smaller than 25 mm, preferably smaller than 20 mm. The friction between the rotor and can is reduced by this rotor with a small diameter, so that the efficiency of the motor and thus of the total pump assembly may be further increased. Simultaneously, one may achieve a compact constructional shape. The smaller the rotor diameter, the lower is the occurring friction. 
     Given a small rotor diameter, the electric motor, which is reduced in size with regard to its diameter, may be designed longer in the axial direction, in order to be able to provide an adequate power of the electric motor. Preferably, a very stiff rotor shaft is provided, in order to render this possible. Such a very stiff rotor shaft may be achieved by designing the rotor shaft, including the axial end on which the impeller is attached, as one piece, ideally as one piece with the complete rotor. 
     Preferably, the impeller too has a small diameter, whereby the constructional size of the pump assembly is reduced, and simultaneously the efficiency of the pump may be increased, in particular in combination with the high rotational speed, on account of the reduced friction. 
     An impeller is further preferably axially sealed in the region of the suction port. The axial sealing of the suction port has the advantage that the axial surface of the impeller may simultaneously serve as a sealing surface, so that the number of required sealing elements is reduced, and a simple seal may be formed in the region of the suction port. This further permits the minimization of the friction in the pump assembly and thus of the power loss. 
     Moreover, at least one axial end side of the impeller, particularly preferably, forms a thrust bearing surface. In this manner, the number of the required components for mounting the rotor is reduced, since the impeller itself may be part of the thrust bearing. This on the one hand permits a simplified and compact construction of the entire pump assembly, and on the other hand permits the power loss to be further minimized, and thus the efficiency to be increased. Particularly preferably, the bearing surface simultaneously serves as an axial sealing surface. This has the further advantage, that no additional pressing elements are required, in order to hold the seal in bearing. An adequately small gap, which ensures a reliable sealing and simultaneously ensures an adequate lubrication film on the bearing surface, automatically sets in the thrust bearing, which forms a sliding bearing. The gap preferably lies in the region of a few micrometers. This ensures a particularly good sealing at the suction port, which further contributes to increasing the efficiency of the pump assembly. 
     Further preferably, the impeller is designed in an open manner at its axial end side on which the impeller blades are arranged, and the axial end sides of the impeller blades form a thrust bearing surface of the impeller. This means that the axial, free end sides of the impeller blades serve as the axial mounting of the impeller and thus of the rotor shaft, and simultaneously the sealing of the impeller at its open end side. A particularly good sealing is achieved very simply in this manner, since the impeller blades are pressed by the occurring axial force, which is to be accommodated by the thrust bearing, against an opposite thrust bearing surface, for example an upthrust washer. A very small gap between the axial end sides of the blades and the upthrust washer is created by this, which simultaneously ensures a good sealing and an adequate lubrication film in the axial sliding bearing. 
     Usefully, the impeller is fixed on the rotor shaft in the axial direction, so that the impeller may assume the axial mounting of the whole rotor. This means that the axial mounting of the whole rotor is effected at the impeller, preferably in a sliding bearing, whose thrust bearing surface is formed by the axial end side of the impeller, preferably by the axial end sides of the impeller blades. 
     According to a further preferred embodiment, the axial side of the impeller, which faces the electric motor, is designed as a sealing surface for sealing the rotor space of the electric motor. This means that here, an axial sealing surface is also preferably provided, on which a stationary sealing element, for example a sealing ring, bears. This sealing ring may be pressed against the sealing surface by spring bias or elastic internal stress. The sealing of the rotor space is preferred, in order to prevent contamination from the fluid to be delivered by the pump assembly, which is preferably water, from penetrating into the rotor space, and there, leading to an undesired friction, or possibly even to damage to the rotor. The rotor space may be pre-filled with fluid at the factory. Alternatively, it is possible for the fluid to penetrate into the rotor space with the first starting operation of the pump assembly. This may be ensured by the seal not being designed in a completely fluid-tight manner between the impeller and rotor space, but merely being designed such that no contamination or only small quantities of fluid may enter into the rotor space. Thus, the fluid exchange between the pump space, in which the impeller rotates, and the rotor space inside the can, is minimized or prevented. A very simple sealing with a minimal number of components may be ensured due to the fact that the sealing surface is provided directly on the impeller. Moreover, on account of the adequate sealing, one may ensure that no friction losses may occur due to contamination, whereby a greater efficiency of the pump assembly may be ensured in a permanent manner. 
     The impeller, particularly preferably, comprises at least one surface of carbide or ceramic, and is preferably completely manufactured of carbide or ceramic. This design permits the wear of the impeller blades on account of contamination in the fluid, such as sand particulates for example, to be minimized or prevented. Moreover, the particularly hard and wear-resistant design of the impeller surfaces permits the use as sliding bearing surfaces or thrust bearing surfaces, so that one may do away with additional bearing shells or bearing elements. The wear-resistant design of the impeller furthermore permits the rotational speed of the impeller to be further increased, without excessive wear occurring. This permits the increase of the efficiency of the pump assembly without further stages having to be provided. Simultaneously, the impeller may be designed in a very small manner. A small impeller diameter leads to a reduction of friction losses, whereby the efficiency of the pump assembly may be increased further. Alternatively to the design of carbide or ceramic, or to the surface coating with carbide or ceramic, one may also apply other methods or coatings, for the surface hardening of the impeller, assuming that an adequate wear-resistance of the surfaces is achieved. Preferably, a hardness of the impeller surface is, for example, greater than 1000 HV. The design of the impeller completely of carbide or ceramic may be effected with a sintering method, for example, wherein the impeller blades are preferably subsequently ground, in order to design the end sides of the impeller blades as a defined thrust bearing surface and sealing surface. If the opposite side of the impeller is likewise to be designed as a sealing surface, this too is preferably ground, in order to provide a defined contact surface. 
     According to a further preferred embodiment, the rotor of the electric motor is designed as a permanent magnet rotor. This permits a simple design of the electric motor. In order, despite this, to be able to achieve a high efficiency with a small rotor diameter, particularly preferably strong permanent magnets, for example neodymium magnets, are arranged in the rotor. 
     An upthrust washer facing the impeller is further preferably provided, which bears on an axial side of the impeller, preferably the axial end which faces away from the electric motor, in a manner such that it forms a thrust bearing surface. Thus, a sliding bearing is formed between the axial end of the impeller or the impeller blades and the upthrust washer, the sliding bearing being able to serve as a thrust bearing of the impeller and of the whole rotor. 
     The upthrust washer preferably likewise comprises at least one surface of hard metal or ceramic material, in order to be able to ensure the wear characteristics, which are required for a sliding bearing surface and sealing surface, even at high rotational speeds. It is also possible to design the upthrust washer completely of carbide or ceramic material. Particularly preferably, only the part of the upthrust washer which faces the impeller is formed of such a material. The part which faces away from the impeller may be designed of another material or of metal, and may be bonded, for example, to the part facing the impeller. Alternative methods or designs which ensure an adequate hardness or wear resistance of the surface of the upthrust washer may also be applied here. 
     The axial side of the upthrust washer which faces away from the impeller is preferably spherical, i.e., is preferably designed in a hemispherical manner. This permits the mounting of the upthrust washer in a corresponding spherical or hemispherical receiver, so that a self-centering or self-alignment of the upthrust washer parallel to the impeller or the axial end side of the impeller is achieved. On the one hand, this simplifies the assembly, and on the other hand ensures a wear-free and secure operation of the pump assembly even at high rotational speeds. 
     Preferably, the impeller is surrounded by spiral housing or diffuser, whereby the delivered fluid radially exiting the impeller is deflected, such that it is led most preferably in an axial direction, and may be led out of the pump assembly into a connection conduit. 
     For this, the impeller is, particularly preferably, surrounded by a spiral housing, which extends helically in a manner such that the exit opening of the spiral housing is aligned in the axial direction to the impeller, i.e., parallel to its rotation axis. This has the effect that the fluid, which exits from the impeller in a tangential, radial direction, is deflected by the spiral housing, with as little loss as possible, to an axially directed exit opening of the pump assembly. 
     Further preferably, the wet-running electric motor of the submersible pump assembly comprises a can which is manufactured of a non-metallic material, wherein the non-metallic material is provided with at least one additional, hermetically sealing layer. 
     The can according to the invention thus consists of a non-metallic material, i.e., of a material which influences the magnetic field between the rotor and the stator as little as possible, or not at all. A worsening of the efficiency, on account of the arrangement of the can between the stator and the rotor, is avoided due to the fact that the magnetic field remains uninfluenced by the material of the can. The hermetically sealing layer, which is preferably deposited on the outer or the inner peripheral surface or on both peripheral surfaces, permits the application of a material for the can, which per se does not have the adequate diffusion sealing ability. This means that one may select a material which primarily ensures an adequate stability of the can. 
     The diffusion sealing ability, to the extent that fluid located in the inside of the can, i.e., in the rotor space, may not penetrate through the can into the stator space, is achieved by the additional layer, which is preferably deposited on the surface of the non-metallic material. Also, several layers of different materials may be used in combination, in order to achieve the desired hermetic sealing between the inner space of the can and the outer peripheral region of the can. Thus, the can wall may be constructed in a multi-layered manner from the non-metallic material and one or more layers of further materials, which ensure the diffusion sealing ability. For example, the diffusion-tight layer, which ensures the hermetic sealing, may be formed of a special plastic or paint. The diffusion-tight layer may further be formed, for example, as a tube, film or film pot, in particular of metal. After manufacture and shaping of the non-metallic material, these may be deposited onto this material. Furthermore, it is possible to incorporate a film or a tube into the non-metallic material at the time of shaping this material, so that the hermetically sealing layer covers the tube or film on one or both sides or peripheral sides. Thus, the tube or the film may be arranged in the inside of the non-metallic material. This may be effected, for example, during an injection molding of the non-metallic material. 
     Further preferably, the at least one layer is designed as a coating on the inner and/or outer peripheral surface of the non-metallic material. Such a coating may be deposited on its surface after the manufacture or shaping of the part of the non-metallic material, for example by spraying on or vapor deposition. 
     Preferably, the coating is designed as a metallization of the non-metallic material. This means that a metal layer is deposited onto the inner and/or outer peripheral surface of the can, for example by vapor deposition. This metal layer then ensures a hermetic sealing. The coating of the non-metallic material, for example by metallization with a suitable material, is effected usefully such that the complete peripheral surface, which forms the separation between the rotor space in the inside of the can and the surrounding stator space, is accordingly coated in this region, so that no fluid, for example water from the inside of the can, may penetrate through the can wall into the surrounding stator space. In this manner, it is possible to apply stators without a casting mass. 
     The can is, particularly preferably, manufactured of plastic and preferably of a fiber-reinforced plastic. Plastic permits an inexpensive manufacture of the can, for example in an injection molding method. Furthermore, plastic has no magnetic properties whatsoever, and therefore does not influence the magnetic field between the stator and the rotor. Furthermore, plastic is suitable for coating or for being provided with further, surrounding and inner-lying plastic layers, in the manner of co-extrusion. A metallization of plastic is also possible without any problem. The fiber-reinforced construction may improve the stability or the pressure strength of the can. 
     Preferably, the can is manufactured of a tubular component and a base element, which closes the tubular component at a first axial end. This permits a simplified manufacture of the can, which for example also permits the manufacture of thin-walled plastic cans with an injection molding method. With the injection molding of the can, it may be useful for a core forming the cavity inside the can to be held at both axial ends of the can, in order to achieve a very thin-walled design of the can. Thus, first, the tubular component is manufactured, and then later the base element is inserted into this tubular component, in order to close an axial opening of the tubular component and to form a canned pot. The opposite axial end of the can is designed in an open manner, so that the rotor shaft may extend to the pump space through this axial end. The base element may be inserted into the tubular component with a non-positive fit, a positive fit, and/or a material fit, so that a firm, stable and preferably sealed connection between the tubular component and the base element is created. 
     The base element is particularly preferably molded with the tubular component. 
     Thereby, after the manufacture of the tubular component, the base element, in a second manufacturing step in the injection molding method, may be injected or cast onto the tubular component, or molded into the tubular component, so that a permanent, sealed connection between the two elements is created. 
     The tubular component and the base element are further preferably both manufactured of a non-metallic component, preferably plastic, and after the assembly are commonly provided with the additional layer or coating. In this manner, the region of the base element and, in particular, the transition region between the tubular component and the base element, are also additionally hermetically sealed by the additional layer or coating. For example, the tubular component and the base element may be metallized together. Alternatively, the additional layer on the base element may also be deposited separately or integrated into this base element. 
     According to a further preferred embodiment, a radially outwardly extending, preferably metallic collar is formed on the outer periphery at one axial end of the can, preferably at the end which faces the pump space and the impeller of the pump. This metallic collar serves, for example, for the end closure of the stator housing, in which the stator winding is arranged. The stator housing, in particular when applied in a submersible pump, is preferably hermetically encapsulated, so that no fluid may penetrate into the inside of the stator housing. Thus, the coils inside the stator housing are protected, in particular from moisture. The metallic collar, which is attached on the outer periphery of the can, serves for the connection to the outer parts of the stator housing, and permits the can to be welded to the remaining stator housing. 
     The collar is preferably connected to the non-metallic material with a positive fit and/or material fit, and together with this, is provided with the additional layer or coating. Alternatively, a non-positive fit connection is also conceivable, inasmuch as an adequate strength and sealing ability is ensured. The common coating of the non-metallic material of the can and of the collar has the advantage that, in particular, the transition region between the non-metallic material and the collar is also hermetically sealed on account of the coating. In order to ensure a permanent sealing in this region, a particularly firm connection between the metallic collar and the non-metallic material of the can is preferred, so that movements between the two elements, which could lead to a tearing of the coating, are avoided. 
     The metallic collar is preferably connected directly to the non-metallic material on manufacture of the can, in order to achieve a particularly firm connection between the metallic collar and the non-metallic material. In the case of injection molding the can of plastic, the metallic collar, for example, may be inserted into the tool before the injection molding, and the plastic injected onto the collar, or a part of the collar is peripherally injected with plastic, so that a positive fit connection and material fit connection between the two elements is achieved directly on injection molding. 
     One surface of the collar is preferably structured or roughened before the connection to the non-metallic material of the can, in order to further improve the connection between the collar and the non-metallic material. This may be effected, for example, by laser radiation, wherein small recesses and/or crater-like raised parts are incorporated into the surface of the collar by a laser beam, into which the non-metallic material, plastic for example, flows when molding, and thus creates a firm connection to the collar, on the one hand via a large surface area, and on the other hand via a positive fit. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
         FIG. 1  is a longitudinal, sectional view of a pump assembly according to an embodiment of the invention; 
         FIG. 2  is an enlarged, longitudinal, sectional view of the can of the electric motor of the pump assembly of  FIG. 1 ; 
         FIG. 3  is an enlarged, sectional view of the circled cut-out of  FIG. 2 ; 
         FIG. 4  is an enlarged, longitudinal, sectional view of the stator of the electric motor in the pump assembly of  FIG. 1 ; 
         FIG. 5  is an enlarged, perspective, end view of the impeller of the pump assembly of  FIG. 1  showing the impeller blades; and 
         FIG. 6  is an enlarged, perspective, axial view of the side of the impeller of  FIG. 5 , which faces away from the impeller blades. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a sectional view of the upper end of a submersible pump. The lower end, in which the electronics for the control and regulation of the pump are accommodated, is not shown in the Figure. The pump assembly at its upper end comprises a connection stub  2  with a return valve  4  arranged therein. A spiral housing  6 , which surrounds the impeller  8 , connects upstream to the connection stub  2 , inside the pump assembly. The impeller  8  is arranged at the axial end of the single-piece rotor shaft  10  of the electric motor  11 , or its permanent magnet rotor  12 . The impeller  8  is firmly fixed on the rotor shaft  10 , in particular is also firmly connected in the axial direction X. The permanent magnet rotor  12  runs inside a can  14 , which is annularly surrounded by the stator  16  on its outer periphery. The stator  16  is designed in a known manner as a lamination bundle with coils windings. The stator  16 , as a whole, is hermetically encapsulated in a stator housing  18 . The rotor shaft  10  is mounted in the radial direction in two radial bearings  20 . These radial bearings  20  are preferably designed in a self-centering manner, so that a simple assembly and a secure operation are ensured, even at high rotational speeds. 
     The can  14 , as shown in detail in  FIGS. 2 and 3 , is designed of plastic in the shown example. The can is formed of a tubular component  22 , which is manufactured of fiber-reinforced plastic with an injection molding method. In order to be able to manufacture the tubular component  22  in a particularly thin-walled manner with the required precision, the tubular component  22  is first formed with open axial ends  24  and  26 . This permits a core, which forms the inner space  28  of the can  14 , which later forms the rotor space, to be able to be fixed at both axial ends in the tool. After injection molding the tubular component  22 , this is then closed at the axial end  24  by a base element  30 , so that a canned pot is formed. The base element  30  preferably may be formed likewise of plastic, and be molded into the previously injected tubular component  2 . Alternatively, the base element  30  may be manufactured separately and be inserted later into the tubular component  22 . As shown, a positive-fit connection is created between the base element  30  and the tubular component  22 , by the inwardly bent, axial peripheral edge of the tubular component  22  engaging into a peripheral groove  32  of the base element  30 . 
     A collar  34  is applied on the outer periphery of the tubular component  22 , at the opposite axial end  26  which faces the impeller  8 . The collar  34  is made of metal, preferably of stainless steel, and is annular, wherein its inner diameter is matched to the outer diameter of the tubular component  22  at the axial end  26 . The ring of the collar  34  has a U-shaped cross-section, wherein the transverse limb faces the axial end  26 . The inner wall  36  of the collar  34  bears on the peripheral wall of the tubular component  22  in a parallel manner, and is connected to this. 
     The connection between the inner wall  36  of the collar  34  and the tubular component  22  is already effected during the manufacturing process, i.e., the molding process of the tubular component  22 , by previously inserting the collar  34  into the tool, so that the tubular component  22  is molded directly onto the inner wall  36  of the collar  34 . Thus, a firm, positive-fit connection and/or material fit connection between the plastic of the tubular component  22  and the inner wall  36  of the collar  34  is created. In order to improve this connection, the inner wall  36  is previously roughened or structured on its inner periphery. This may be effected preferably by laser machining, whereby small recesses may be incorporated into the metal or sheet-metal of the collar  34  on the surface, into which recesses the plastic of the tubular component  22  then flows during injection molding. These recesses, particularly preferably, may comprise undercuts, whereby an even firmer connection between both elements is created. 
     After the injection molding of the tubular component  22 , wherein the collar  34  is firmly directly connected with the tubular component  22 , and after the subsequent insertion of the base element  30 , the can  14  created in this manner is metallized. Thereby, a thin metal layer  38  is deposited on the outer surface of the can  14 , as shown in  FIG. 3 . The metal layer  38  coats the complete outer surface of the tubular component  22  and the base element  30 , as well as the collar  34 . In this way, it is particularly also the transition regions between the collar  34  and the tubular component  22 , as well as between the base element  30  and the tubular component  22 , which is covered by the metal layer  38 . The metal layer  38  ensures that a hermetic sealing of the can  14  and, in particular, of the peripheral wall of the tubular component  22  is created. This hermetic sealing by the metal layer  38  has the effect that fluid, which is located in the rotor space  28 , may not penetrate through the can  14  into the inside of the stator housing  18 , in which the stator  16  is arranged. The metallization or coating  38  thereby permits the use of a plastic for the tubular component  22  and the base element  30 , which per se is not diffusion-tight. Thus, here, the plastic may be selected purely accordingly to the requirements with regard to the stability for the can  14 , as well as according to manufacturing aspects. 
     A can  14 , which is provided with a metal layer  38  on its outer side, was previously described. Alternatively, it is also possible to provide the can  14  on its outer side, as well as the inner surfaces of the inner space  28 , with a metal layer by metallization. Moreover, it is alternatively also possible to metallize the can only on the inner walls of the inner space  28 . 
     The metallic collar  34  serves for connecting the can  14  to the remaining part of the stator housing  18 . This, in particular, may be effected by a weld seam  39  on the outer periphery of the metallic collar  34 . The collar  34  thus creates the connection to the other metallic components, from which the stator housing  18  is formed, as shown in  FIG. 4 . 
     The use of the can  14  of plastic, i.e., of a non-metallic material without magnetic properties, has the advantage that the can  14  influences the magnetic field between the stator  16  and the permanent magnet rotor  12  only little, or not at all, whereby the efficiency of the electric motor  11  is increased. 
     With regard to the pump assembly according to the invention, the diameter of the permanent magnet rotor  12  and the impeller  8  is kept small, in order to reduce the friction in the system, and thus the power loss, as much as possible. In order, despite this, to ensure a high efficiency of the electric motor  11 , the permanent magnet rotor  12  is equipped with particularly strong permanent magnets, for example neodymium magnets. In the shown example the rotor diameter is 19 mm. The shown electric motor is designed for very high rotational speeds &gt;20,000 rpm, in particular between 25,000 and 30,000 rpm. One may thus obtain a high delivery output with only one impeller  8  with a comparatively small diameter. 
     The impeller  8 , which is shown in  FIGS. 5 and 6  as an individual part, is manufactured of carbide, in order to ensure a high wear-resistance. The impeller blades  42  are formed on an axial end  40 , which in the installed condition faces away from the electric motor  11 . The impeller  8  is designed in an open manner, i.e., the impeller blades project from the axial end  40  of the impeller  8  and are not closed by a cover disk at their end sides  44 . 
     The end sides or end edges  44  of the impeller blades  42  are ground, and thus form a thrust bearing surface and sealing surface of the impeller  8 . The end sides  44 , in the installed condition, bear on an upthrust washer  46 , which annularly surrounds the suction port  48  of the pump. The complete rotor  12  is supported via the impeller  8  in the axial direction on the upthrust washer  46  by the firm connection of the impeller  8  to the rotor shaft  10 . That is, the end face of the upthrust washer  46  which faces the impeller  8  and the end sides  44  of the impeller blades  42  form an axial sliding bearing. The end sides  44  of the impeller blades  42  are pressed against the upthrust washer  46  by the axial pressing force of the impeller  8 , such that a particularly good sealing between the impeller blades  42  and the upthrust washer  46  occurs. The losses in the pump are minimized by this, and the delivery output of the pump assembly is further increased, indeed at the high rotational speed of the motor described above. One may achieve a very high delivery output with the described, very small impeller in this manner, even with a single-stage design of the pump assembly. The impeller  8  thereby assumes the axial-side sealing with respect to the upthrust washer  46 , on the suction port  48 , and simultaneously assumes the thrust bearing function, so that here too, the number of components and the occurring friction may be minimized. 
     The rear side  50  of the impeller  8 , which faces away from the impeller blades  42 , comprises a further annular sealing surface  52 , which annularly surrounds the opening  54  for receiving the rotor shaft. The sealing surface  52  bears on a seal  56 , which surrounds the rotor shaft  10  in a stationary manner, and seals the rotor space  28  inside the can  14  to the pump space, in which the impeller  8  is arranged. This seal  56  is held bearing on the sealing surface  52  by spring action. The sealing  56  ensures that contamination in the fluid, which is delivered by the impeller  8 , may not penetrate into the rotor space  28  inside the can  14  and may not lead to undesired friction or damage there. 
     The upthrust washer  46  is preferably likewise designed of carbide or of ceramic. The side  58 , which faces away from the impeller  8 , is designed in a spherical shape (not shown in  FIG. 1 ), and is mounted in a spherical receiver in the pump housing  60 , so that the upthrust washer  46  may align itself parallel to the impeller  8 . This part of the upthrust washer, which forms the rear side  58 , may be designed of a material other than carbide or ceramic, and may be connected to the part of the upthrust washer  46  which faces the impeller  8 , for example by bonding. 
     The impeller  8  is peripherally surrounded by the spiral housing  6 . The spiral housing  6 , proceeding from the peripheral region of the impeller  8 , extends helically to the connection stub  2 , so that a flow deflection in the axial direction is effected. That is, the flow, which exits in the radial/tangential direction at the outer periphery of the impeller  8 , is first deflected by the spiral housing  6  in a purely tangential direction or peripheral direction of the impeller  8 , and is then deflected in the axial direction with as little loss as possible, on account of the helical winding of the spiral housing  6 , so that the flow at the connection stub  2  may exit from the pump assembly in the axial direction. The spiral housing  6  is preferably likewise manufactured of plastic as an injection molded part. The spiral housing  6 , moreover, at its lower end which faces the impeller  8 , further comprises the likewise spherical receiver for the upthrust washer  46 , and centrally forms the suction port  48  of the pump, through which the fluid is sucked by rotation of the impeller  8 . The outer housing of the pump assembly, in the region in which the spiral housing  6  is arranged in its interior, comprises entry openings  62  in its outer peripheral wall, through which the fluid enters from the outside, flows around the spiral housing  6  from the outside, and then enters the suction port  48 . 
     One may create a very powerful, compact submersible pump assembly, which with only one stage, achieves a large delivery output with a high operational speed, having all the elements which have been previously described, i.e., with a can  14  of plastic with a metallization, with a small pressure sensor of the rotor  12 , with an impeller  8  made of carbide and having a small diameter, which simultaneously assumes the sealing and axial mounting. 
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.