Abstract:
A method of assembling a rotor assembly is provided, without the use of a rotor steel hub. The method comprises forming first, second and third lamination stacks by stacking individual rotor laminations together. The first, second and third lamination stacks are cast to lock the individual rotor laminations together. The rotor laminations in the first, second and third lamination stacks have a generally ring-like shape with a variable inner diameter and a substantially common outer diameter. The rotor laminations in the first, second and third lamination stacks have substantially the same outer diameter, whereas the rotor laminations in the second lamination stack have a substantially larger inner diameter than the first and third rotor laminations. The elimination of the hub results in reducing total cost and manufacturing cycle time, and in improved features, performance and efficiency of the rotor assembly.

Description:
TECHNICAL FIELD 
     This invention relates to a structure and method of assembling a rotor assembly, without the use of a rotor steel hub. 
     BACKGROUND OF THE INVENTION 
     Conventional electric motors generally comprise of a cylindrical stator and a rotor assembly located within the stator. Typically, the rotor assembly is constructed with the aid of a heavy steel hub. The hub is used to ensure that the rotor laminations are connected to external structures and support the structure of the rotor assembly. The use of a hub requires special machining for locking rotor components or tabs to the hub. In the use of a hub to assemble the rotor assembly, the rotor components or tabs must be perfectly aligned to complete the assembly process. 
     In order for the hub to slide over the rotor lamination stack, a gap must be created. The hub is ordinarily contracted with the use of liquid nitrogen at −300° F. while the rotor laminations are expanded with the use of an oven at 400° F. As the temperatures normalize, the gap dissipates and a “shrink-fit” between the hub and rotor laminations results. 
     Other typical rotor assemblies include a supporting rotor shaft extending through a central hole in the stack of rotor laminations. The rotor shaft may also be welded onto either end of the stack of rotor laminations. 
     SUMMARY OF THE INVENTION 
     The present invention eliminates the use of a hub in assembling the rotor assembly. There is no supporting rotor shaft extending through the center of the stack of rotor laminations or attached at the ends of the stack of rotor laminations. 
     A structure and method of assembling a rotor assembly is provided. The first step comprises forming a first lamination stack by stacking individual first rotor laminations. The first lamination stack has a first end and a second end. The second step comprises stacking individual second rotor laminations onto the second end of the first lamination stack, thereby creating a second lamination stack. The second lamination stack has a first end and a second end. The third step comprises stacking a set of third rotor laminations onto the second end of the second lamination stack thereby creating a third lamination stack. The third lamination stack has a first end and a second end. 
     In another aspect of the invention, the first, second and third lamination stacks are cast to lock the first, second and third rotor laminations together. The first, second and third rotor laminations have a generally ring-like shape with a variable inner diameter and a substantially common outer diameter. The first, second and third rotor laminations have substantially the same outer diameter, whereas the second rotor laminations have a substantially larger inner diameter than the first and third rotor laminations. 
     The elimination of the hub results in the elimination of the use of liquid nitrogen at −300° F. and oven at 400° F. in the “shrink-fit” process described above. Fewer parts are needed with the method of this invention than are typically required, resulting in less inventory problems. The cost of assembling the rotor is reduced as is the time required to manufacture and assemble the rotor. 
     The above method also leads to improved rotor heat transfer for improved rotor oil cooling. As a result of the elimination of the rotor hub, the oil contained within the rotor has better contact with the rotor laminations, leading to better heat transfer. A cooler motor draws less electric current to operate, thereby reducing electric power consumption. The improved method also leads to better concentricity between the outer and inner diameters of the rotor. The gap between the outer diameter of the rotor and stator inner diameter will be more uniform and consistent since the flange bearing journals on both ends are more concentric. 
     The above features and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary schematic perspective view of the rotor assembly, with a casting mold shown in phantom; 
         FIG. 2  is a fragmentary schematic cross sectional view of the rotor assembly; and 
         FIG. 3  is a schematic perspective view of the bolt used in the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the embodiment described below, the rotor assembly is used in an electric motor in a hybrid transmission for a vehicle. However the method and structure described below is suitable for use with rotors for induction machines, permanent magnet and switch reluctance machines as well as other suitable machines. 
     Referring to  FIG. 1 , a perspective view of part of a rotor assembly  10  is illustrated.  FIG. 2  is a schematic cross sectional view of part of the rotor assembly  10 . The rotor assembly comprises a first lamination stack  20 , second lamination stack  30  and a third lamination stack  40 . The first lamination stack  20  is made up of individual first rotor laminations  22 . The first lamination stack  20  has a first end  24  and a second end  26 . The rotor assembly  10  also comprises a second lamination stack  30 , attached to the second end  26  of the first lamination stack  20 . The second lamination stack  30  is made up of individual second rotor laminations  32 . The second lamination stack  30  has a first end  34  and a second end  36 . The rotor assembly  10  also comprises a third lamination stack  40 , attached to the second end  36  of the second lamination stack  30 . The third lamination stack  40  is made up of individual third rotor laminations  42 . The third lamination stack  40  has a first end  44  and a second end  46 . The first lamination stack  20  and the third lamination stack  40  are substantially identical in size, see discussion below. 
     The first, second and third rotor laminations  22 ,  32  and  42  respectively are generally circular disks which are made of flat sheets of silicone steel. The sheets, which may be made of other suitable materials, are fitted into a punching die (not shown) which punches holes into the sheet resulting in a generally ring-like shape. Other non-circular shapes that are suitable for use in various electric machine rotor assemblies may also be employed. 
     As shown in  FIG. 1 , the first, second and third rotor laminations  22 ,  32  and  42  have an opening with center  50 , an inner circumference  52  and an outer circumference  54 . The distance between two points on the outer circumference  54 , going through the center  50  of the opening represents the outer diameter OD of each rotor lamination, as shown in  FIG. 1 . The distance between two points on the inner circumference  52 , going through the center  50  of the opening represents the inner diameter ID of each rotor lamination, as shown in  FIG. 1 . Each of the first, second and third rotor laminations  22 ,  32 ,  42  have annulus regions A 1 , A 2  and A 3 , respectively, between their respective inner and outer circumferences  52  and  54 . 
     In the embodiment shown, the first, second and third rotor laminations  22 ,  32  and  42  are stacked in an axial direction. The first lamination stack  20  has an axial length L 1 . The second lamination stack  30  has an axial length L 2 . The third lamination stack  40  has an axial length L 3 . There are approximately 25 first rotor laminations  22  in the first lamination stack  20 . There are approximately 220 second rotor laminations  32  in the second lamination stack  30 . There are approximately 25 third rotor laminations  42  in the third lamination stack  40 . Thus, in the preferred embodiment, the second lamination stack  30  has an axial length L 2  greater than either of the first and third lamination stacks  20  and  40 , i.e. the second lamination stack  30  contains a greater number of individual rotor laminations than either of the first and third lamination stacks  20  and  40  or the laminations of the combined laminations of the first and third lamination stacks  20  and  40 . Existing automated lamination feeding and stacking machines can be utilized for this assembly process. 
     In terms of the sizes of the individual rotor laminations, the first, second and third rotor laminations  22 ,  32  and  42  have substantially the same outer diameter OD. However the first and third rotor laminations  20  and  40  have substantially smaller inner diameters ID than the second rotor laminations  30 , as shown in  FIGS. 1 and 2 , to allow for the placement of bolt holes  60 . Thus the annulus region A 1  and A 3  is larger for the first and third rotor laminations  20  and  40  than the annulus region A 2  of the second rotor lamination  30 . 
     As shown in  FIG. 1 , slots  62  extend along the periphery of the outer circumference  54  of the first, second and third rotor laminations  22 ,  32  and  42 . The first, second and third lamination stacks  20 ,  30  and  40  are cast together by first being placed in a die cast mold fixture or casting mold  59  (shown in phantom in  FIG. 1 ). The first, second and third lamination stacks  20 ,  30  and  40  are molded by applying pressure (compression may be shown by arrows  61 ) to lock the first, second and third lamination stacks  20 ,  30  and  40  together. 
     Molten aluminum  63  or other suitable material is injected, as shown at  65  in  FIG. 1 , into the slots  62 . The molten aluminum  63  flows through the slots  62  from the first end  24  of the first lamination stack  20  to the second end  46  of the third lamination stack  40 . A means of pressure such as hydraulic back pressure (shown by arrow  67 ) is applied against the molten aluminum  63  forcing the molten aluminum  63  into the slots  62  to lock the first, second and third rotor laminations  22 ,  32  and  42  together in a unitary configuration, thereby avoiding air gaps, porosity and bubbles. For example, the first, second and third lamination stacks  20 ,  30  and  40  may be compressed together in a die-casting machine or casting mold  59  (shown in phantom in  FIG. 1 ) so as to lock the first, second and third rotor laminations  22 ,  32  and  42  together. The molten aluminum  63  solidifies to create a first end ring  64  at the first end  24  of the first lamination stack  20  and a second end ring  66  at the second end  46  of the third lamination stack  40 , see  FIG. 2 . The aluminum or other suitable material first and second end rings  64  and  66  serve to enhance the conductivity of the rotor assembly  10 . 
     As shown in  FIGS. 1 and 2 , bolt holes  60  extend along the periphery of the inner circumference  52  of the first and third rotor laminations  22  and  42 . The bolt holes  60  are configured to receive a corresponding bolt  68 . 
       FIG. 3  is a schematic perspective view of a bolt  68 . The head of each bolt  68  has a flat side  67  that may be used to wedge and lock the bolt  68  in the first and third lamination stacks  20  and  40 . Each bolt  68  has serrations  69  that prevent the bolt  68  from rotating and interact with complementary slots on the inside of each bolt hole  60 . The flat side  67  of the head of the bolt  68  and the serrations  69  provide anti-rotation when the first and second flange  74  and  76  (see discussion below) is assembled and the nut  78  is tightened. There are twelve bolt holes  60  in the preferred embodiment, however any number of bolt holes may be used. A representative first bolt  70  and representative second bolt  72  is shown in  FIG. 2 , attached onto the first lamination stack  20  and the third lamination stack  40 , respectively. 
     A first flange  74  is attached onto the first end  24  of the first lamination stack  20 , using the first bolt  70  for orientation. The first bolt  70  goes through a first flange hole  75  in the first flange  74 . A second flange  76  is attached onto the second end  46  of the third lamination stack  40 , using the second bolt  72  for orientation. The second bolt  72  goes through a second flange hole  77  in the second flange  76 . As stated above, the preferred embodiment describes a rotor assembly  10  used in an electric motor (not shown) in a hybrid transmission. The first and second flanges  74  and  76  are mechanical devices that provide a means of attachment for the first, second and third lamination stacks  20 ,  30  and  40  to the gears and/or other parts of the electric motor in the transmission. The first and second flanges  74  and  76  help transmit power to the mechanical components of the electric motor. A set of nuts  78  are placed over the edges of the first and second bolts  70  and  72  for secure attachment of flange to rotor assembly. 
     Alternatively, a single flange with multiple holes to mate with respective first and second bolts  70  or  72  may be used. Any number of multiple flanges may also be used. The flange may be constructed of steel or other suitable materials. The physical structure or configuration of the flange may be varied depending on the layout and design of the components to be attached to the rotor laminations  22 ,  32  and  42  through the flange. 
     Method 
     A method for assembling the rotor assembly  10  described above is provided. The first step comprises forming a first lamination stack  20  by stacking individual first rotor laminations  22 . The first lamination stack  20  has a first end  24  and a second end  26 . The second step comprises stacking individual second rotor laminations  32  onto the second end  26  of the first lamination stack  20  thereby creating a second lamination stack  30 . The second lamination stack  30  has a first end  34  and a second end  36 . The third step comprises stacking a set of third rotor laminations  42  onto the second end  36  of the second lamination stack  30  thereby creating a third lamination stack  40 . The third lamination stack  40  has a first end  44  and a second end  46 . 
     The first, second and third lamination stacks  20 ,  30  and  40  may be cast together in a die cast mold fixture or casting mold  59  (shown in phantom in  FIG. 1 ) and molded to lock the first, second and third rotor laminations  22 ,  32  and  42  together. A dowel pin or a guide bar (not shown) may be used to ensure the proper alignment of the first, second and third lamination stacks  20 ,  30  and  40 . The guide bar may be used to align the stacking of the first, second and third lamination stacks in the casting mold  59  prior to molding. The dowel pin or guide bar may be inserted into a respective one or more slots  62  of the rotor lamination  42  and extend axially through the slot to the first end  24  of the first lamination stack  20  or from the first end  24  to the second end  46 . 
     A first bolt  70  may be attached onto the first lamination stack  20  as well as a second bolt  72  onto the third lamination stack  40 . A first flange  74  may be attached onto the first end  24  of the first lamination stack  20 , using the first bolt  70  for orientation; and a second flange  76  may be attached onto the second end  46  of the third lamination stack  40 , using the second bolt  72  for orientation. 
     Other Steps 
     A further step may comprise machining the outer circumference  54  of the first, second and third rotor laminations  22 ,  32  and  42 . Machining involves grinding the outer circumference  54  for smoothness and precision of dimensions. This process may be done at the final rotor assembly, which involves assembling both flanges. 
     A further step may involve fine-tuning a speed sensor wheel  80 . Certain rotor assemblies may contain a speed sensor wheel  80 , as shown in  FIG. 2 . The speed sensor wheel  80  may be an integral part of the flange structure or it may be mounted separately by welding, using bolts or other means. If mounted on the structure, the speed sensor wheel  80  must be attached securely so it does not vibrate. 
     Finally, a next step may be final balancing of the rotor assembly  10 , which involves removing and adding extremely small amounts of weight at either end of the rotor assembly  10 . This serves to balance the weight of the rotor assembly  10  from one end to the other, leading to reduced vibration and noise. 
     The thickness of the hub used typically in constructing a rotor restricts the width A 2  of rotor laminations to what can be fitted within the hub. Eliminating the hub allows for the cross-sectional area of the rotor laminations to be increased. This allows for a greater electromagnetic flux path and increased efficiency of the motor. Furthermore, the electric motor performance and efficiency is improved as a result of the elimination of the contact pressure applied by the hub on the rotor laminations. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.