Patent Publication Number: US-7909591-B2

Title: Eccentric screw pump equipped with erosion-resistant rotor

Description:
BACKGROUND OF THE INVENTION 
     A rotor for an eccentric screw pump or eccentric screw motor produced by cold deformation is disclosed in DE 198 52 380 A1. The pump or motor of this reference has a stator with a continuous helical opening over which the rotor rolls during displacement operation. The stator comprises a cylindrical tube provided with an elastomeric cladding. The elastomeric cladding defines the wall of the passage opening and acts as a seal relative to the stator. 
     The stator includes a core element and a shell formed around the core element. Beginning with a cylindrical tube, the shell is deformed into a helical configuration. The originally cylindrically shaped tube acquires not only the helical configuration, which is required for the rotor, but this deformation also firmly connects the tube to the core element. In the final state, the thread valleys of the shell of the stator form a tight firm friction fit with the core element. To improve the driving effect between the core element and the shell of the stator, the support element also can be provided with longitudinal ribs. 
     This prior art rotor can be produced in a cost effective manner in very large numbers. Lengths of up to 6 meters can be reached easily without requiring final machining of the surface of the stator. The surface of the rotor is very smooth and sufficiently stable in its dimensions. The core element present in the shell prevents the rotor from uncoiling when exposed to pressure. Uncoiling of the rotor could lead to a pitch error between the stator and rotor that would result in leaks. 
     This prior art rotor is made of a steel material that does not have sufficient wear strength for many applications, and also does not have sufficient corrosion-resistance for some applications. In other words, the rotor does not have sufficient erosion resistance. Erosion is understood to mean not only wear by corrosion, but also ablation by sliding abrasion of the transported material on the surface. 
     It is also known from the prior art to provide the stator with a shell that has a helical configuration similar to the helical configuration of the passage opening. With such stators, the elastomeric cladding, which again serves as sealing material, has an almost constant wall thickness. Larger pressures, or larger torques in the case of an eccentric screw motor, can be produced with such stators. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of the foregoing, a general object of the invention is to provide an eccentric screw pump or an eccentric screw motor in which the rotor is characterized by better erosion resistance. Another general object of the invention is to provide a method for producing a rotor having greater erosion resistance. 
     The rotor in the eccentric screw pump or screw motor according to the invention is designed like a sandwich. The rotor consists of a radially inner layer and a radially outer layer with the radially outer layer being especially adapted to provide higher erosion resistance. In particular, the radially outer layer can be more abrasion-resistant or more corrosion-resistant or both than the radially inner layer. 
     Since more corrosion-resistant materials having larger wall thicknesses are in some circumstances more difficult to deform and are much more expensive than the radially inner layer, the radially inner layer can primarily be chosen from a standpoint of strength and cost, so that it is possible to use a very thin radially outer layer. 
     The rotor can have a very homogeneous structure if the inner layer consists of a seamless tube. Such an arrangement helps avoid heterogeneities, which otherwise occur during welding. Such heterogeneities could continue outward as shape defects. However, it is also possible to use a wound tube as the inner tube in the present invention. Such a tube is preferably laser-welded at the helical butt joint. The coil should run opposite the coil of the outer layer. 
     The inner layer or the inner tube consists of an easily deformable steel material that is can transfer the recurrent forces and be cold-worked in the usual manner. The outer layer can consist of an attached tube. Such a configuration, however, is only suitable for rotors with a relatively short design length. In rotors with a relatively longer design length, the outer layer can be formed from a wrapped metal band. The metal band is wrapped with butt joints so that the individual windings abut each other without a gap. A particularly good arrangement is produced if the helically running joint where the windings abut is welded before cold deformation. The welding is preferably done with a laser. 
     Stainless steels V2A, V4A steel or other abrasion-resistant steels can be used as the outer material. Since these materials have a very much higher specific weight than normal steel, the two-layer design also results in a weight saving as compared to a rotor made only of stainless steel. This can play a role in rotors with a length of up to 6 meters. 
     The strength of the rotor can be improved if it includes a core element. The rotor can be molded around the core element so that a good connection with the core element is produced. The core element prevents uncoiling of the rotor under load at great lengths. In addition, additional torque can be introduced over the length of the rotor by means of the core element. The substantially rotationally symmetric and non-helical core is better suited for this purpose. The core element can be tubular or solid. In addition, the intermediate space between the tube or shell of the rotor and the core element can be either left open or filled with a mass. 
     According to the method of the invention, a cylindrical tube is prepared first. The tube is enclosed with a metal layer so that a double-walled structure is produced. The double-walled structure, which is still cylindrical, is then helically deformed. Covering of the cylindrical tube with the outer layer is very simple of the simple geometric shape of the already prepared tube. Because the stability of the rotor can be achieved under some circumstances primarily with the inner tube, the outer layer only has to be applied with a limited thickness and thus materials that can not be cold deformed at greater wall thicknesses can also be used for the outer layer. 
     A seamless tube can be used in the method according to the invention. The seamless tube advantageously has a bright metallic surface so that connection of the outer layer with the tube by cold deformation is not hampered by oxide residues. 
     The outer metal layer in the simplest case consists of a metal band wrapped around the tube. To increase the tension the metal band can be heated immediately ahead of the contact site before winding. Subsequent cooling ensures shrinkage that holds the metal band particularly tightly on the surface of the tube. The butt joint between the adjacent windings can be welded in order to prevent penetration of particles. 
     The resultant double-walled structure is cold deformed. During the deformation process, the outer layer is bonded to the inner tube in at least a point-like manner, as is also the case during lamination. As a result, the connection is particular durable, and also is not broken by fluctuating temperatures. According to the method of the invention, a core element can be inserted before deformation of the coated tube. 
     An embodiment of the invention is shown in the drawings. In the drawings: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         FIG. 1  is partially cutaway perspective view of an illustrative eccentric screw pump according to the present invention. 
         FIG. 2  is a longitudinal section view taken through the stator of the eccentric screw pump of  FIG. 1 . 
         FIG. 3  is a longitudinal section view taken through the rotor of the eccentric screw pump of  FIG. 1 . 
         FIG. 4  is a cross section view taken through the rotor of  FIG. 3 . 
         FIGS. 5-7  are schematic drawings showing some of the steps associated with the an exemplary method according to the invention for producing the rotor of the eccentric screw pump of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A schematized, oblique view an eccentric screw pump  1  according to the invention is shown in  FIG. 1 . The eccentric screw pump  1  includes a pump head  2 , a stator  3  in which a rotor  4  rotates, as well as a connection head  5 . The pump head  2  has a substantially cylindrical housing  6 , which includes a closure cover  7  on one end with a closure cover  7 . A drive shaft  8  is guided outward in sealed fashion through the closure cover  7 . A connector  9  discharges radially into housing  6 . The housing  6  ends in a fastening flange  11 . As is common in eccentric screw pumps, the coupling piece for torque-proof coupling the drive shaft  8  (which is connected to a drive motor) to the rotor  4  is situated inside the housing  6 . 
     The end of the housing  6  that is remote from the cover  7  is provided with a tightening flange  12  that has a diameter greater than the diameter of the substantially cylindrical housing  6 . The tightening flange  12  has a stepped hole  13  that is aligned with the internal space of housing  6 . A contact shoulder is formed in the stepped hole, against which one end of the stator  3  is pressed. 
     The connection head  5  has a tightening flange  14  that cooperates with the tightening flange  12 . The tightening flange  14  also contains a stepped hole in which the other end of the stator  3  is inserted. A discharge line  15  is aligned with the stepped hole. 
     The stator  3  is firmly tightened in sealed fashion between the tightening flanges  12  and  14  by in this case four tie bolts  16 . In order to accommodate the four tie bolts  16 , the two tightening flanges  12  and  14  are each provided with four aligned holes  17  that lie on a circular area larger than the outside diameter of housing  6  or tube  15 . The rod-like tie bolts  16  are passed through these holes  17 . Nuts  18  are threaded onto each tie bolt  16  on the side facing away from the opposite tightening flange  12  and  14 , by means of which the two tightening flanges  12  and  14  are tightened to each other. 
     As shown in  FIG. 2 , the stator  3  consists of a tubular shell  19  with constant wall thickness that surrounds an inner space  20 . The shell  19  consists of steel, steel alloy, light metal or a light metal alloy. The shell  19  is shaped so that its inside wall  21  acquires the outer configuration of a multiple start screw. The outside surface  22  of the shell  19  has a similar matching shape with a diameter greater than the diameter of the inner space of the shell  19  according to the wall thickness. The shell  19  terminates at end surfaces  23  and  24  which are oriented at right angles relative to the longitudinal axis  25  of the shell. The longitudinal axis  25  is the axis of the inner space  20 . 
     In the simplest case, the internal space  20  has the shape of a two-start screw. The cross-section enclosed by the outer surface  22  when viewed at a right angle to the longitudinal axis  25  also has the shape of an oval, similar to a racetrack. In order to adapt the geometry to the stepped hole  13 , a closure or reducing ring  26  is seated on each end of the shell  19 . Alternatively, the ends can also be formed as cylindrical tubes. The closure ring  26  has a passage opening  27  that coincides with the course of the outer surface  22  over the longitudinal extent of the closure ring  26 . In other words, the closure ring  26  acts in the broadest sense as a nut, which is screwed onto the thread defined by the shell  19 . The length of the thread corresponds to the thickness of the closure ring  26 . 
     The closure ring  26  is bounded in the radially outward direction by a cylindrical surface  28 , which transitions axially into a flat surface  29  that faces away from the shell  19 . On the inner side  21 , the shell  19  is provided over its entire length with a continuous cladding  32 . The cladding  32  consists of an elastically flexible, preferably elastomeric material (e.g., natural rubber or a synthetic material) and has roughly the same wall thickness at each location. 
     As shown in  FIG. 3 , the rotor  4  includes a core element  33 , a rotor jacket  34 , and a coupling head  35 . The core element  33  in the illustrated embodiment is a thick-walled steel tube with an at least originally cylindrical outer peripheral surface  36  and a continuous cylindrical internal space  37 . 
     The core element  33  has a straight configuration as well as a tubular configuration as the internal space makes no noticeable contribution to the strength, but merely increases the weight. However, the core element can also be solid. As shown in  FIG. 3 , the core element has one threaded end  38  (the right end with reference to  FIG. 3 ). The opposing end of the core element  33  includes a threaded hole  39 . 
     The jacket  34  of the rotor  4  also has a tubular configuration including an inner wall  40  and an outside surface  41 . The outside surface  41  forms a thread that continues over the entire axial length of the jacket  34 . The thread begins at  42  and ends at  43 . The number of turns of the thread formed by the outer surface  41  is one fewer than the number of turns in the passage opening  20  in stator  3 . As shown in the cross section of  FIG. 4 , the rotor  4  in the illustrated embodiment has a four-start thread, i.e., a total of four strips run helically along the jacket  34 . The passage opening  20  accordingly has five starts. The five-start threads in the passage opening  20  are formed with a total of five helically extending strips made of elastomeric material. 
     As shown in  FIG. 4 , the rotor jacket  34  is two-layered, including an inner layer  44  and an outer layer  45  situated on the inner layer. The inner layer  44  consists of an originally cylindrical steel tube having good deformability and strength for the given applications. The outer layer  45 , in turn, consists of an erosion-resistant material, which is a material that is resistant to being worn or ground off and/or chemically attacked by the medium being pumped. An appropriate material for the outer layer is, for example, stainless steel like V2A or V4A. The wall thickness of the inner layer  44  can be between 1 mm and 5 mm, while the wall thickness of the outer layer  45  also can be between 1 mm and 5 mm. Production of this rotor  4  is explained further below by means of  FIG. 5 . As previously discussed, the jacket  34  has a tubular configuration, and as a result, the inner surface  40  follows the outer surface  41  at constant spacing. 
     Because of the screw-like configuration of the jacket  34 , the outer surface  41  when viewed in the longitudinal direction, forms an alternating sequence of thread crests  46  and thread valleys  47 . As a result of the multiple starts, the thread valleys  47  and the thread crests  46  appear not only in the longitudinal direction, but also in each sectional plane in the circumferential direction as shown in  FIG. 4 . 
     The dimensions of the cylindrical straight tube from which the jacket  34  is cold-deformed are chosen so that after final deformation to the helical configuration, the jacket  34  at least touches the outside peripheral surface  36  of the core element  33  with its inside peripheral surface  40  in the area of the thread valleys  47  (with reference to the outer contour). During correspondingly stronger deformations it is also possible to slightly deform the outer peripheral surface  36  of the core element  33  so that its outer peripheral surface  36  acquires shallow grooves  48  that follow the contour of the thread valleys  47 . If deformation is continued in this way, then not only a frictional but also a form-fit connection results between the jacket  34  and the core element  33  in the region of the thread valleys  47  that curve toward the interior of jacket  34  with the core element  33 . Moreover, because of the deformation, cold welding between the jacket  34  and core element  33  can even occur at the contact sites. 
     Since the semifinished product from which the jacket  34  is produced is a cylindrical tube whose diameter is greater than the outside diameter of the core element  33 , intermediate spaces  49  are formed that extend helically between the core element  33  and the jacket  34 . The number of helical screw intermediate spaces  49  is equal to the number of thread crests  46 , which are apparent in the cross section of the rotor  4  in the circumferential direction. Depending on the application, these intermediate spaces  49  can either be left empty or filled with a mass. This mass, for example, can be a synthetic resin or synthetic resin filled with light metal powder. 
     One embodiment of the method of production of the rotor  4  including layers  44  and  45  is shown in schematic fashion in  FIGS. 5 to 7 . Initially, a bright drawn, seamless steel tube  51  with a suitable wall thickness and an appropriate length of several meters is prepared. The steel tube  51  is wrapped on the outside with a metal band  52 , which later forms the outer layer  45 . The metal band  52  is a band made of an appropriate stainless steel or another steel material. The band  42 , as shown in  FIG. 6 , is wrapped like a single-thread screw onto the outside of steel tube  51 . Windings  53  lying next to each other are then formed. The windings  52  are separated from each other by a helical butt joint  54 . The wrapping of the metal band  52  is performed so that the butt joint  54  is as closed as possible. 
     During winding or in a separate step, the butt joint  54  is welded by means of a laser beam  55  and filler material in order to achieve a smooth, homogeneous cylindrical surface. Other welding methods can also be used. The welding can be carried out such that the band  52  is joined to the support tube  51  with a substance-to-substance bond in the area of the butt joint  54 . 
     The metal band  52  is heated, for example, by a gas flame  56  or inductively, immediately before it is placed on tube  51 . This enables the metal band  52  to produce a significant pressure in the circumferential direction after it is wrapped onto the tube  51  and cooled. After the band  52  has been wrapped over the entire length of the tube  51  and the butt joint  54  has been welded over the entire length, the core element  33  is inserted according to  FIG. 7 . The structure is then brought to the desired helical shape by cold deformation, for example, by rolling with a plurality of rolls (one such roll is referenced in the drawings as  57 ). During rolling, the metal band  52  is bonded very intimately with the outside surface of the underlying steel tube  51 . 
     After the process step shown in  FIG. 6  is completed, the metal band  52  forms a second outer tube on the metal steel tube  51 . The metal band  52  is seated firmly and with circumferential tension in a friction fit with the outside peripheral surface of tube  51 . The two tubes, namely the tube formed by wrapping and the seamless inner steel tube, are already so firmly joined to each other after wrapping that they can no longer be separated from each other. 
     The subsequent rolling process shown in  FIG. 7  ensures more intimate bonding, which at least to a certain degree is similar to plating with a metal layer. The rolling, which generally leads to stretching of a metal piece, surprisingly does not separate the outer tube from the tube  51  situated beneath it. Instead, the two tubes are deformed together into the desired helical shape, intimate bonding with the core element  33  being produced at the same time. 
     Instead of just one metal band, several metal bands can also be wound like a multi-thread screw. In addition, the winding process can be repeated in order to produce several layers, one on the other. 
     The invention has been described relative to an eccentric screw pump. However, those skilled in the art will readily appreciate that the invention is in no way restricted to eccentric screw pumps. Instead, rotors for eccentric screw motors or mud motors can also be produced following the method of the invention shown, for example, in  FIGS. 5 to 7 . As a result of using this method, a displacement mechanism is obtained which contains a very resistant rotor. 
     According to the foregoing, in one embodiment the invention provides an eccentric screw pump or an eccentric screw motor that includes a rotor formed from an least two-layer tubular jacket. The outer layer of the jacket consists of material that is abrasion-resistant and/or corrosion-resistant.