Abstract:
An electronically-controlled turbocharger (ECT) includes a rotor (of an electric motor) on the shaft of the turbomachine. There are a variety of embodiments for securing the rotor onto the shaft including, but not limited to: having a taper on both the shaft on the rotor and using a nut engaging with threads in the shaft to put the two tapers together; a press fit; splines and grooves also using a nut; and a shoulder on the shaft that the rotor abuts when a nut is engaged with threads on the shaft. To accommodate these retaining features, the diameter of the shaft is greater near the turbine end of the shaft. A first bearing installed on the shaft near the turbine end of the shaft has a larger inside diameter than a second bearing installed on the other side of the rotor.

Description:
FIELD 
       [0001]    The present disclosure relates to bearings of a shaft of an electric motor. 
       BACKGROUND 
       [0002]    An electronically-controlled turbomachine (ECT) includes an electric machine (or motor) mounted between turbine and compressor sections of a turbomachine. The turbomachine rotates at speeds up to 350,000 rpm. 
         [0003]    The rotor of the electric machine should be mounted to resist relative rotation between the rotor and the turbocharger shaft. The rotor may be press fit onto the shaft as described in commonly-assigned patent application PCT/US14/17455 filed 20 Feb. 2013, which is incorporated herein in its entirety. Other rotor retention embodiments are disclosed in commonly-assigned provisional patent application 61/895,632 filed 25 Oct. 2013. 
         [0004]    The rotordynamics, which include the bearings, of such a system are critical to provide the desired performance and durability. 
       SUMMARY 
       [0005]    To provide desirable rotordynamics, an ECT is disclosed that has: a shaft having a turbine end and a compressor end with a turbine wheel coupled onto the turbine end of the shaft, a rotor mounted onto the shaft, a first bearing having a first internal diameter, the first bearing being mounted on the shaft between the turbine wheel and the rotor, and a second bearing having a second internal diameter, the second bearing being mounted on the shaft between the rotor and the compressor end of the shaft wherein the first diameter is greater than the second diameter. 
         [0006]    In one embodiment the shaft has an exterior taper on the shaft with a diameter of the taper decreasing monotonically in a direction moving away from the turbine wheel. An end of the rotor core that engages with the taper has an interior taper section which mates with the exterior taper. The interior diameter of the interior taper decreases monotonically in a direction moving away from the turbine wheel. 
         [0007]    In some embodiments at least one of the interior and exterior tapers is roughened by one of: knurling, bead blasting, etching, sand blasting, laser vapor deposition, laser etching, and applying a coating. 
         [0008]    In another embodiment the shaft has a shoulder located between the rotor and the turbine wheel and an end of the rotor abuts the shoulder. 
         [0009]    In another embodiment, the shaft has a taper located proximate an end of the rotor proximate the turbine wheel. In an alternative embodiment the shaft has a should located proximate an end of the rotor proximate the turbine wheel. The shaft has threads along a portion of the length of the shaft proximate an end of the rotor away from the turbine wheel. The ECT also includes a nut that engages with the threads and abuts the rotor on the end of the rotor away from the turbine wheel. 
         [0010]    In yet another embodiment, the rotor is press fit onto the shaft between the first and second bearings at least for a portion of the rotor. In some embodiments, the shaft is cutback in the center of the portion that the rotor is over the shaft so that the inside of the rotor and the shaft do not contact each other in the area of the cutback. 
         [0011]    In another embodiments, an end of the rotor has grooves defined therein; the shaft has splines defined therein; the splines mate with the grooves upon assembly of the rotor onto the shaft to thereby prevent relative radial motion of the rotor with respect to the shaft; and the shaft is threaded upon a portion its length at a location near the compressor end of the shaft. The ECT may also have a nut that engages with the threads of the shaft and abuts the rotor near the compressor end of the shaft. 
         [0012]    In an alternative embodiment: the rotor has internal threads; the shaft has external threads; and the rotor is mounted on the shaft by engaging the threads of the rotor with the threads of the shaft. 
         [0013]    The first and second bearings are fully floating bearings and include an oil groove in some embodiments. 
         [0014]    The ECT is further includes a housing that is installed over the rotor. The first bearing is mounted on the shaft between the turbine wheel and the rotor taken in a direction parallel to an axis of the shaft and mounted between the shaft and the rotor housing taken in a radial direction perpendicular to the axis of the shaft. The second bearing is mounted on the shaft between the compressor wheel end of the shaft and the rotor taken in a direction parallel to the axis of the shaft and mounted between the shaft and the rotor housing taken in a radial direction perpendicular to the axis of the shaft. 
         [0015]    Also disclosed is a method to assemble an ECT including: sliding a first bearing of a first diameter over a shaft of the ECT, installing a rotor onto the shaft, engaging threads of a nut onto threads formed in the shaft (in embodiments with a nut) and sliding a second bearing of a second diameter over a shaft of the ECT. The first diameter is greater than the second diameter. In embodiments in which the rotor has threads, the installing is accomplished by spinning the rotor onto the shaft. In embodiments with a nut that engages with threads on the shaft, the nut is installed to secure the rotor on the shaft prior to installing the second bearing. In most embodiments in which a press fit is used involves heating the rotor, cooling the shaft, or both to allow the rotor to slide over the shaft, which could not be accomplished at room temperature due to an interference fit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a cross-sectional view of an ECT; 
           [0017]      FIGS. 2  is a cross-sectional view of an ECT shaft, turbine wheel; and bearings; 
           [0018]      FIG. 3  is a cross section of a portion of an ECT: shaft, rotor, and two bearings; 
           [0019]      FIG. 4  is a portion of an ECT shaft with bearings; 
           [0020]      FIG. 5  is an ECT shaft that engages with rotor via splines and grooves; 
           [0021]      FIG. 6  shows an ECT shaft that engages with the rotor via threads; and 
           [0022]      FIG. 7  is a flowchart of assembling an ECT. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce alternative embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desired for particular applications or implementations. Those of ordinary skill in the art may recognize similar applications or implementations whether or not explicitly described or illustrated. 
         [0024]    ECT herein is used to denote both electronically-controlled turbocharger and electronically-controlled turbomachine, with the electronically-controlled turbocharger being one type of electronically-controlled turbomachine. In  FIG. 1 , an ECT is shown in cross section. The ECT has a compressor section  10 , an electric machine section  12 , and a turbine section  14 . A shaft  16  passes through sections  10 ,  12 , and  14 . A turbine wheel  18  is affixed to shaft  16  by welding, by mechanical fasteners, or any other suitable manner of coupling rotating members. 
         [0025]    Electric machine section  12  includes an electric machine that includes a rotor  20  and a stator  22  enclosed within two housing portions: a turbine-side housing portion  24  and a compressor side housing portion  26 . The electric machine can be operated as either a motor, in which electrical energy is applied to the motor to cause the shaft to rotate faster than it would otherwise, or as a generator, in which an electrical load is applied to the motor to cause the shaft to rotate slower than it would otherwise. The terms electric machine, motor, and generator are used herein interchangeably with the understanding that depending on the embodiment, the electric machine may be operated as a motor, generator, or neither if no electric current is applied to windings associated with the rotor. In some embodiments, the electric machine may be adapted to operate only as a motor or only as a generator. Bearings  28  and  30  are disposed in housing portions  26  and  24 , respectively, to support shaft  16 . Considered axially, bearing  30  is located between rotor  20  and turbine section  14  and journal bearing  28  is located between rotor  20  and compressor section  10 . 
         [0026]    A compressor wheel  32  is provided on the end of shaft  16  distal from turbine wheel  18  with a thrust washer  36  located between compressor wheel and bearing  28 . Compressor wheel  32  is held onto shaft  16  via a nut  34  in the embodiment of  FIG. 1 . The compressor wheel  32  is typically manufactured from a light alloy dissimilar from the turbo shaft  16  preventing a weldment. Compressor wheel  32  is typically secured onto the shaft via a fastener or threaded feature. Any suitable coupler may be used. 
         [0027]    In  FIG. 2 , a shaft  50  is welded to a turbine wheel  52 . A rotor core  54  of a rotor  40  is placed over shaft  50 . Permanent magnets  58  surround rotor core  54  with an outer containment sleeve  56  containing permanent magnets  58 . In the present embodiment, the rotor core is a stiffener sleeve. In other embodiments, the permanent magnets sit directly on the shaft. Rotor core  54  is shown in  FIG. 2  as a single piece. However, the rotor core may be made up of a plurality of sections, such as a center section and two end caps. Shaft  50  has threads  70 . Nut  60  engages with threads  70 . An inner surface of an end of rotor core  54  proximate turbine  52  is tapered to mate with a taper  64  on shaft  50 . A bearing  74  is mounted on shaft  50  at a location where the diameter is Da and a bearing  76  is mounted on shaft  50  at a location where the diameter is Db. Due to Db being to the right of taper  64 , Db is greater than Da, which means that the inner diameter of bearing  76  is greater than the inner diameter of bearing  74 . 
         [0028]    The embodiment in  FIG. 2  shows a rotor of a permanent magnet electric motor. However, this is not intended to be limiting. A rotor of any suitable electric motor may be mounted on shaft  50 . 
         [0029]    In various embodiments, some mating surfaces may be roughened to increase friction to resist disassembly. The mating surfaces may be roughened by laser surface treatments, sand blasting, knurling, ball peening or any other suitable technique. In one embodiment, at least one of the end of the rotor core  54  proximate turbine  52  and taper  64  has a roughened surface. 
         [0030]    In  FIG. 3 , a portion of a shaft  80  and a rotor  82  for an ECT is shown. The shaft has a diameter D 0  at the left hand side, which in some embodiments can be the compressor end of shaft  80 . To the right of the D 0  section is a section of greater diameter, D 1 , that forms a press fit with the inner surface of rotor  82 . To the right of the D 1  section is a cutback portion of the shaft with diameter, D 2 . The cutback can make it easier to press the rotor onto the shaft by reducing the length of the press fit. Furthermore, in some cases, the shaft rotordynamics are improved with a cutback. To the right of the D 2  section is a portion of the shaft with diameter, D 3 . D 3  can be equal to D 2  or greater. To locate rotor  82  onto shaft  80  during assembly, a small shoulder is provided on shaft  80  between the D 3  section and a section of slightly greater diameter, D 4 . A nut  89  engages with threads in shaft  80  to secure rotor  82  to shaft  80 . End  84  of rotor  82  abuts the shoulder of shaft  80  when nut  89  is tightened. The shoulder exists due to D 4  being of a greater diameter than D 3 . All of the transitions between varying diameters may be chamfered or use any suitable stress relief feature. Bearing  86  that is placed over the D 0  section has a smaller inside diameter than bearing  88  that is placed over the D 4  section of shaft  80 . D 4  is greater than D 3  which is greater than or equal to D 1  that is greater than D 0 . D 2  is less than any of D 1 , D 3 , and D 4 . D 2  may be the same as, greater than, or less than D 0 . 
         [0031]    A portion of a shaft  100  is shown in  FIG. 4 . The shaft has a section  90  with outside diameter D 0 ′ that has a bearing  112  mounting onto it. Section  92  has a diameter D 1 ′ that press fits with an inside portion of a rotor (not shown). Section  94  is cutback and has a diameter D 2 ′ which is less than either D 1 ′ or D 3 ′, the outside diameter of section  96 . Section  98  of shaft  100  has an outside diameter D 4 ′. Shaft  100  has a shoulder  110  that abuts a rotor that may be press fit or secured in any suitable manner onto shaft  100 . A section  91  may be provided with threads for a nut to secure the rotor. A bearing  114  is mounted on section  98 . The inside diameter of bearing  114  is greater than the inside diameter of bearing  112 . Bearings  112  and  114  have a central oil groove. However in other embodiments, other suitable bearing types are employed. 
         [0032]    In an embodiment shown in  FIG. 5 , a shaft  250  is provided with a plurality of splines  252  and a rotor  254  is provided with a plurality of fingers  256  and grooves  258 . Splines  252  mate with grooves  258 . A first bearing (not shown) is placed over a section  262  of shaft  250  that has a diameter, Dj, and a second bearing (not shown) is placed over a section  260  of shaft  250  that has a diameter, Di. Diameter Dj is greater than diameter Di, which means that the inside diameter of the first bearing is greater than the diameter of the second bearing. 
         [0033]      FIG. 6  shows yet another embodiment in which a shaft  150  has outer threads provided in the region  154  of engagement between shaft  150  and a rotor  152 . Rotor  152  has inner threads that mate with the threads of shaft  150 . Because of the threads in region  154 , the diameter of shaft  154  at De is greater than the diameter of shaft  152  at Dc. Thus, the inside diameter of a bearing  156  on shaft  150  is smaller than the inside diameter of a bearing  158  on shaft  150  on that is on the opposite end of rotor  152 . 
         [0034]    In  FIG. 7 , a portion of the assembly of an ECT is shown. The turbine wheel is friction welded to a blank shaft in block  170 . Any suitable assembly method can be alternatively used. In block  172 , the shaft is machined to include the desired features, which could include one or more of: threads, tapers, shoulders, chamfers, stops, and cutbacks, as a non-limiting list. In block  174 , the first bearing is installed by sliding it over the shaft. In block  176 , the rotor is slid onto the shaft. This may be a press fit in which case the shaft is cooled, the rotor is heated, or both. In a different embodiment, the rotor and the shaft have threads and those threads are engaged to cause the rotor to be located on the shaft, possibly with a stop or shoulder for axial location. Or in yet another embodiment, splines on one of the shaft or rotor engage with grooves on the other element. Block  178  only refers to embodiments in which a nut is used to secure the rotor onto the shaft. Threads of the nut are engaged with the threads of the shaft in such embodiments. During the engagement, the rotor hits a stop or shoulder, in some embodiments, or engages with a taper as far as desired, in other embodiments. In block  180 , the second bearing is slid onto the shaft. In block  182 , the thrust washer is slid onto the shaft. The thrust washer has not been discussed previously, but is shown in  FIG. 1 . The compressor wheel is affixed to the shaft via any suitable technique in block  184 . 
         [0035]    Several alternatives have been described above for retaining the rotor onto the shaft including at least: tapers, a shoulder, splines, a nut, and a collar. Further, many examples of surfaces that may be roughened to prevent relative rotation of adjacent members. And, many types of electric motors may be used in place of the permanent magnet motor disclosed herein. Not every suitable combination has been illustrated in the drawings. The drawings are not intended to be limiting and additional combinations than those explicitly shown and described are within the scope of the disclosure. 
         [0036]    While the best mode has been described in detail with respect to particular embodiments, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. While various embodiments may have been described as providing advantages or being preferred over other embodiments with respect to one or more desired characteristics, as one skilled in the art is aware, one or more characteristics may be compromised to achieve desired system attributes, which depend on the specific application and implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. The embodiments described herein that are characterized as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.