Abstract:
A generator rotor shaft includes a shaft body extending through a first axial distance, and being hollow. Spline teeth are formed in an inner bore of the hollow shaft body. The ratio of a distance from one end of the hollow body to a remote axial end of the spline teeth and compared to the overall length of the shaft body is between 0.3 and 0.6. A rotor balance assembly, a generator, and a method of forming a generator rotor shaft are also disclosed and claimed.

Description:
BACKGROUND OF THE INVENTION 
     This application relates to a rotor shaft for use in a generator, wherein the rotor shaft is configured to more efficiently utilize space. 
     Generators are known and typically include main windings that rotate with a rotor shaft. The rotor shaft is selectively driven to in turn drive the main windings adjacent to a stator, and the rotation of the windings relative to the stator generates electricity. 
     Modern generators have a number of additional accessories. As an example, an exciter rotor and a permanent magnet rotor are also attached to the rotor shaft. In addition, a rectifier assembly may communicate with the exciter rotor. The rectifier assembly has been mounted within a hollow rotor shaft, however, the exciter rotor has typically been at the end of the shaft. A wire can communicate to the rectifier assembly from the exciter by merely extending around the end of the shaft. The requirement of mounting the exciter rotor at this location provides an undesirable design constraint. 
     In addition, there have been compromises with regard to the size of the shaft. Typically, a clutch member includes splines that selectively transmit rotation to the rotor shaft. The clutch member must be able to slide such that it can be engaged or disengaged from a drive input. The splines have typically been provided at one end of the rotor shaft. Having the spline connection at either a remote end or an adjacent end of the rotor shaft results in a clutch member that is either too long or too short for many design applications. 
     Furthermore, generators are typically designed such that they will always operate at a frequency that is below a first natural frequency of the overall assembly. To achieve this goal, it is desirable to increase the rotor outer diameter, and to decrease the distance between supporting bearings for the rotor. However, the requirement of including several operational components has made achieving these goals challenging. 
     SUMMARY OF THE INVENTION 
     A generator rotor shaft includes a shaft body extending through a first axial distance, and is hollow. Spline teeth are formed in an inner bore of the hollow shaft body. A ratio of a distance from one axial end of the hollow body to a remote axial end of the spline teeth, compared to the overall length of the shaft body, is between 0.3 and 0.6. 
     A rotor balance assembly for a generator includes a main winding mounted on an outer periphery of a rotor shaft. The rotor shaft includes a shaft body extending through a first axial distance, and is hollow. Spline teeth are formed on an inner bore of the body. A ratio of a distance from one axial end of the body to a remote axial end of the spline teeth, to the first axial distance, is between 0.4 and 0.5. 
     A generator includes a stator and a main winding mounted to a rotor shaft for rotation adjacent to the stator. The rotor shaft includes a shaft body extending through a first axial distance, and is hollow. Spline teeth are formed on an inner bore of the body. A ratio of a distance from one end of the body to a remote axial end of the spline teeth, to the first axial distance, is between 0.4 and 0.5. 
     A method of forming a generator rotor shaft comprises the steps of forming a generally hollow body having an inner bore, and forming spline teeth within the inner bore at an intermediate location such that a ratio of the entire length of the generally hollow body relative to a length from one end of the hollow body to a remote end of the spline teeth is between 0.3 and 0.6. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of a generator. 
         FIG. 2  is a cross-sectional view through the  FIG. 1  generator. 
         FIG. 3A  shows a rotor shaft. 
         FIG. 3B  shows a detail of the  FIG. 3A  view. 
         FIG. 4  is an outer view of the rotor shaft. 
         FIG. 5  is an end view of a generator. 
         FIG. 6  shows a clutch member incorporated into the generator. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a generator  20  including a stator  21 , shown schematically. A clutch member  22  has teeth that will selectively engage teeth on a drive input  300 . The clutch member  22  may also be driven to engage or disengage from the drive input. 
     Main winding section or core  24  rotates near the stator  20 . An exciter rotor  26  and a permanent magnet rotor  28  provide control and safety functions, and also are associated with their own stators (not shown). Bearings  30  and  32  are placed on opposed axial ends of the main winding section  24 . 
     A drive input  300  is shown in phantom in  FIG. 1  and a yoke  301  is shown schematically. The yoke  301  operates to cam surfaces on the clutch member  22  toward and away from the drive input  300  such that drive can be selectively transmitted. Aspects of the clutch member, the yoke and the drive input are found in co-pending patent application Ser. No. 12/436,159, filed on even date herewith, and entitled “Decoupler Shaft for High Speed Generator” and owned by the assignee of this application and “High Speed Clutch Design with Jaw Tooth Profile to Reduce Separating Load,” assigned Ser. No. 12/436,168, filed on even date herewith, and assigned to the assignee of the present invention. In addition, a rotor gear which is driven by the rotor shaft is disclosed in co-pending patent application Ser. No. 12/436,190, filed on even date herewith, and entitled “Rotor Gear for a Generator.” This rotor gear serves to assist in driving an oil pump for delivering oil to components within the generator. In addition, a permanent magnet rotor is attached to the rotor shaft in a manner disclosed below, but further disclosed in detail in a co-pending patent application Ser. No. 12/436,164, filed on even date herewith, and entitled “Axial Retention of Permanent Magnet Rotor in High Speed Generator.” 
       FIG. 2  shows bearings  30  and  32  mounted at each end of the main winding section  24 . Windings  37  and laminations  38  are received within this main winding section  24 . 
     An enlarged portion  54  of the clutch member  22  is a close fit within an inner diameter of a rotor shaft  42 . Spline  52 / 36  supports the opposite end of the inner diameter shaft. A rectifier assembly  48  is shown schematically, and is positioned within a bore in the rotor shaft  42 . An element  40  is an oil distribution transfer tube which provides oil to the disconnect shaft or clutch member  22 . The oil may then be distributed to the rotor shaft. As can be seen, the oil tube  40  is received within an inner diameter  201  of the clutch member  22 . 
     Spline teeth  52  are formed at an axially intermediate portion of the inner bore of the shaft  42 , and mate with spline teeth  36  on the clutch member  22 . A spring  50  biases the clutch member  22  outwardly, and against the input shaft  300 . 
     Generally, a bar is machined by a lathe to include the basic shape of the rotor shaft  42 . Teeth  52  are formed at an internal surface by some machining process, and in one embodiment, a broaching operation. The intermediate member is then heat treated and final machined. 
     A wire  44  communicates from the exciter rotor  26  to the rectifier assembly  48 , and extends through slots  46  in the shaft  42 . 
     A method of replacing a generator rotor shaft into a generator includes the steps of moving the rotor shaft  42  such that a central portion  105  supports a main winding section of the generator, and such that smaller diameter portions  104  and  106  are supported by bearings, and moving a wire through slots in the shaft, and utilizing the wire to connect a rectifier assembly to an exciter rotor. 
       FIG. 3A  is a cross-sectional view through the shaft  42 . As shown, surfaces  104  and  106  are formed to be at a smaller diameter relative to a central portion  105 , which supports the main winding section  24 . The bearings  30  and  32  are received on surfaces  104  and  106 . Slots  46  are shown to include a plurality of slots, which are circumferentially spaced. As can be seen, a ledge  102  is positioned slightly axially inwardly from the end  111 . This ledge will provide a stop for the clutch member  34  when the generator is selectively disengaged. 
     An end  111  of the shaft, which receives the clutch member  34 , is spaced from an opposed end  110  by a first distance d 0 . The end of each spline  52  is spaced from the end  111  by a second distance d 1 . The spline  52  extends over a third distance d 2 . 
     Lubrication holes  107  are spaced along the rotor shaft  42 . 
     In one embodiment, the distance d 0  was 12.441″ (316.00 mm), the distance d 1  was 5.835 (148.2 mm) the distance d 2  was 0.635″ (16.1 mm). 
     A ratio of d 1  to d 0  is preferably between 0.3 and 0.6, and more preferably, 0.4 and 0.5. The ratio of d 2  to d 1  is preferably between 0.1 and 0.125. 
     As can be best seen in  FIG. 3B , at end  110 , there are screw threads  500 , and a piloting diameter  502 . As disclosed in the above-referenced U.S. patent application entitled “Axial Retention of Permanent Magnet Rotor in High Speed Generator,” the permanent magnet motor has threads which mate with threads  500 , and a pilot surface that pilots on surface  502 . These portions of the above-referenced application are incorporated herein by reference. As can be appreciated from  FIG. 3B , a first diameter D 1  to the root of the screw thread teeth is defined, as is a second diameter D 2  to the tip of the screw thread teeth. A third diameter D 3  is defined to the outer diameter of the pilot surface  502 . Although not shown as diameters in  FIG. 3B , it should be appreciated that these are diameters. In one embodiment, the diameter D 1  is between 1.8730″ (47.574 mm) and 1.8798″ (47.746 mm). The diameter D 2  is between 1.9294″ (49.007 mm) and 1.9375″ (49.212 mm) and the diameter D 3  is between 2.0165″ (51.219 mm) and 2.0160″ (51.20 mm). In embodiments, a ratio of D 3  to D 1  between 1.07 and 1.08. 
     In one embodiment, the screw threads were  20  threads per inch. A self-locking thread available under the trade name Spiralok was used on the permanent magnet rotor. 
     As shown in  FIG. 4 , the slots  46  are circumferentially spaced. The slots preferably extend over more than 180° of the circumference of the shaft  42  at that location. In embodiments, the slots may extend over more than 270°. 
       FIG. 5  is an end view of the shaft  42  showing the slots  46  and the wires  44  passing into the shaft  42 . Note the threads  500  are eliminated from the view for illustration simplicity. 
       FIG. 6  shows the clutch member  22  having splines  36  at one end, the portion  54  that is to be tightly received within end  111 . Teeth at the opposed end selectively engage teeth on the input shaft  300 . 
     The rotor shaft as disclosed and claimed in this application provides several benefits, including providing freedom as to the location of the exciter rotor, and to further allow a disconnect clutch shaft to have a length that is at a more optimal dimension relative to the prior art. In addition, a worker of ordinary skill in the art would appreciate many other benefits from the disclosed application. 
     Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.