Abstract:
Disclosed herein is a rotor assembly for an electric machine which includes a shaft having a slip ring end. A first pole segment rotatable with the shaft. A second pole segment rotatable with the shaft. A slip ring end fan disposed adjacent to the second pole segment and rotatable with the shaft. And, a ring adjacent to the slip ring end fan, press fitted onto the slip ring end of the shaft and rotatable with the shaft, wherein the ring is press fitted to the shaft contacts the slip ring end fan and applies a clamping force to the slip ring end fan.

Description:
TECHNICAL FIELD  
       [0001]     The application relates generally to an electrical apparatus. More specifically, this application relates to securing a rotor fan to a rotor assembly for an electric machine.  
       BACKGROUND OF THE INVENTION  
       [0002]     Electric machines are found in virtually every motor vehicle manufactured today. These electric machines, also referred to as alternators, produce electricity necessary to power vehicle electrical accessories, as well as to charge a vehicle&#39;s battery. Electric machines must also provide the capability to produce electricity in sufficient quantities to power a vehicle&#39;s electrical system in a manner that is compatible with the vehicle electrical components. Proper airflow and cooling is critical to improve heat dissipation and thus efficiency of the electric machine. One method to improve heat dissipation is to provide cooling fans within the electric machine to create a cooling airflow along the rotor, stator, and/or rectifier.  
         [0003]     Cooling fans are typically attached to the rotor by welding attachment points to either the pole segment face or bobbin towers. Another such method of attaching a cooling fan to a rotor utilizes a shouldered shaft to trap the fan between the shaft shoulder and the pole segment face and thus secure it in place. These conventional methods of attaching cooling fans to rotor shafts leads to a number of disadvantages. Such disadvantages include expensive and labor intensive manufacturing methods. Welding processes typically involve expensive fixturing, frequent equipment maintenance, and strict quality control. Likewise, a shouldered shaft is a more complex configuration as compared to a straight shaft and thus requires significantly more machining time and therefore is more costly to fabricate.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     Disclosed herein is a rotor assembly for an electric machine which includes a shaft having a slip ring end. A first pole segment rotatable with the shaft. A second pole segment rotatable with the shaft. A slip ring end fan disposed adjacent to the second pole segment and rotatable with the shaft. And, a ring adjacent to the slip ring end fan, press fitted onto the slip ring end of the shaft and rotatable with the shaft, wherein the ring is press fitted to the shaft contacts the slip ring end fan and applies a clamping force to the slip ring end fan.  
         [0005]     Further disclosed herein is a method for attaching a slip ring end fan to a rotor shaft. A shaft urged through a pole segment. A slip ring end fan is mounted on the shaft adjacent to the pole segment. And, a ring is attached to the shaft, adjacent to the slip ring end fan, wherein the ring is secured in place by a press fit between the linearly knurled portion of the shaft and an inside diameter of the ring, wherein a clamping force is applied to the slip ring end fan in response to the attaching of the ring to the shaft.  
         [0006]     Yet further disclosed herein is a rotor assembly for a vehicle alternator including a shaft having a slip ring end and a thread end. A first pole segment rotatable with the shaft. A second pole segment rotatable with the shaft. A slip ring end fan disposed adjacent to the second pole segment and rotatable with the shaft. And, a ring adjacent to the slip ring end fan, press fitted onto the slip ring end of the shaft and rotatable with the shaft wherein the ring contacts the slip ring end fan. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures:  
         [0008]      FIG. 1  is a perspective section view of a conventional rotor assembly;  
         [0009]      FIG. 2  is a perspective section view of a rotor assembly;  
         [0010]      FIG. 3  is a side view of a conventional shaft; and,  
         [0011]      FIG. 4  is a side view of a shaft and ring. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]      FIG. 1  illustrates a conventional rotor assembly  10   a  including a shaft  12   a , a first pole segment  15 , a second pole segment  16 , a core (not shown) integral with the pole segments  15  and  16 , wherein each pole segment  15  and  16  includes half of the core, a field winding  14  surrounding the core, slip rings  17 , a thread end internal fan  18  with integral fan blades  28 , and a slip ring end internal fan  19  with integral fan blades  27 . The shaft  12   a  serves as a mounting surface for these components and defines a central axis about which the rotor assembly  10   a  rotates. The shaft, further illustrated in  FIG. 3 , includes a shoulder portion  20  integrally formed as part of the shaft  12   a . The field windings  14 , when energized, create a magnetic field that saturates the surrounding first pole segment  15  and second pole segment  16 . The first pole segment  15  and the second pole segment  16  are secured to the shaft  12   a  and oriented such that the first pole segment  15  is opposed to and interdigitated with the second pole segment  16  as illustrated in  FIG. 1 .  
         [0013]      FIG. 2  illustrates an exemplary embodiment of a rotor assembly  10   b , which contains a straight shaft  12   b  and a press fit ring  21 . The straight shaft  12   b  and the press fit ring  21  are further illustrated in  FIG. 4 .  
         [0014]     The straight shaft  12   b  has a common diameter substantially throughout the axial length of the shaft  12   b , wherein the diameter may vary at different axial points along the axial length of the shaft, does not contain an integral shoulder portion, and is intended to be rotatable within an electric machine. A section of the shaft  12   b  adjacent to the slip rings  17  contains linear knurls  22 , which provide the interface for securing the ring  21  to the shaft  12   b . The linear knurls  22  provide a series of raised edges along the circumference of the shaft  12   b , which effectively increases the diameter of the shaft  12   b  in the section of the shaft  12   b  that contains the linear knurls  22 . The ring  21  is installed onto the shaft  12   b  by guiding the shaft  12   b  into a through-hole  23  of the ring  21 . The diameter of the ring  21  through-hole  23  is configured so that the ring will slide freely along the unknurled portions of the shaft  12   b , but will cause an interference fit, or a press fit, when positioned over the section of the shaft  12   b  that contains the linear knurls  22 . The press fit between the inside diameter, or through-hole  23 , of the ring  21  and the linear knurls  22  on the shaft  12   b  secures the ring  21  in a fixed position with respect to the shaft  12   b . Alternatively, the linear knurls  22  may be replaced by a smooth larger diameter portion of the shaft  12   b  wherein, for example, the smooth larger diameter portion may be a diameter similar to the knurl tip diameter of the illustrated embodiment. The smooth larger diameter portion of the shaft  12   b  would enable the ring to be press fitted to the shaft  12   b.    
         [0015]     During the assembly process, the slip ring end fan  19  is advanced axially along the shaft  12   b  until a first face  24  of the slip ring end fan  19  comes into contact with the ring  21 . The second pole segment  16  is then mounted axially along the shaft  12   b  until a first face  25  of the second pole segment  16  comes into contact with a second face, or an outside diameter  26 , of the slip ring end fan  19 . The slip ring end fan  19  is fabricated to have a cupped shape, wherein the cupped shape open end terminates in the outside diameter  26  of the slip ring end fan  19  and faces the second pole segment  16 . The first face  25  of the second pole segment  16  and ring  21  allow for pressure forces, applied during assembly, to act on opposite faces of the slip ring end fan  19  to secure the slip ring end fan  19  in place by compression. In other words, the slip ring end fan  19  is in a sprung relationship with, or tightly clamped between, the first face  25  of the second pole segment  16 , interfacing along the outside diameter  26  of the slip ring end fan  19  cupped end, and the ring  21 , interfacing along a mating surface of the first face  24  of the slip ring end fan  19 . This results in the slip ring end fan  19  being rigidly secured onto the rotor assembly  10   b . In an alternative embodiment, the sequence of the assembly process may differ in that the pole segments  15  and  16  are first mounted axially along the shaft  12   b , by inserting or pressing the shaft  12   b  through the pole segments  15  and  16 , followed by mounting the slip ring end fan  19  on to the shaft  12   b  between the second pole segment  16  and the linear knurls  22 , and lastly installing the ring  21  over the linear knurls  22 , wherein the same sprung relationship between the slip ring end fan  19  and the second pole segment  16  is achieved, thus rigidly securing the slip ring end fan  19  to the rotor assembly  10   b.    
         [0016]     Further, the disclosed ring  21  may serve as a bearing stop for a slip ring end bearing (not shown). After fabrication of the rotor assembly  10   b  is complete, the rotor assembly  10   b  is disposable within an electric machine wherein the ring  21  abuts the slip ring end bearing allowing for fixed axial positioning of the rotor assembly  10   b  with respect to the electric machine.  
         [0017]     Exemplary embodiments of the invention include internal fans such as the thread end fan  18  and the slip ring end fan  19  which may be manufactured by processes including stamping, drawing, or injection molding operations.  
         [0018]     The disclosed straight shaft  12   b  results in several benefits including ease of manufacture, reduced machining time and cost, and greater freedom with respect to fan blade  27  configuration without the concern for welding positions or welding tooling.  
         [0019]     While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims.