Abstract:
An inner sleeve, having threads on each end, is formed on a motor or generator rotor. A pair of stub shafts are threadably attached to the inner sleeve at each end and a magnet is placed in between. The stub shafts are turned on the inner sleeve to bring them together, thereby creating an axial compressive force on the magnet. By keeping the magnet in a compressive state at all times, cracking of the magnet is prevented, thus keeping the rotor stable during use of the motor or generator. An outer sleeve is shrunk-fit around the magnet and stub shafts to provide a further radial force on the rotor. The magnet assembly of the present invention also enlarges the required radial shrink-fit tolerance of the outer sleeve on the magnet, thereby reducing costs conventionally required to match the outer sleeve to the magnet within close tolerances.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a divisional application of U.S. application Ser. No. 10/637,373, filed on Aug. 6, 2003, now U.S. Pat. No. 7,075,204 B2, which is hereby incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   The present invention generally relates a threaded inner sleeve for a magnetized rotor for the motor or generator and, more specifically, to a threaded inner sleeve that may apply an axial compressive force onto the magnet. 
   Permanent magnet rotors are frequently used in dynamo electric machines such as motors and generators. Permanent magnets are secured to a rotor hub or shaft by any of a variety of means and care must be taken to assure that such securement prevents the magnets from moving either axially or radially. If axial movement is permitted, one or more magnets may not properly align with an armature with the consequence that machine efficiency diminishes. If radial movement occurs, the probability of interfering contact between the rotor and the stator, and the resulting frictional drag and/or damage to machine parts come into existence. 
   Conventional motor or generator designs may use any shape magnet, so long as a hole is formed in the center of the magnet(s) for insertion of a rotor. As an example, and referring to  FIG. 1 , arc-shaped magnet sections  10  may be arranged to form a donut shape. A rotor shaft  12  may be inserted in the hole of the donut shape magnet. An outer ring  14 , sized slightly smaller than the donut shape of magnet sections  10 , is heated to cause its thermal expansion, allowing outer ring  14  to fit around an outside edge  16  of magnet sections  10 . Upon cooling, radial shrinkage due to the thermal coefficient of expansion of metal outer ring  14  holds magnet sections  10  frictionally in place. 
   These conventional motor and generator configuration assembly methods are complex and result in high initial manufacturing costs as well as high operational costs. Manufacturing costs are high due to the need for a close tolerance between outside edge  16  of magnet sections  10  and outer ring  14 , requiring machining of both outer ring  14  and magnet sections  10  to exacting standards. Operational costs are high due to possible rotor slippage and/or magnet cracking under operating conditions. 
   U.S. Pat. No. 4,433,261 concerns a structure for attaching magnets of a rotor for a synchronous motor of a permanent magnet type. Side plates ( 4 ) are fixed on a rotor shaft ( 2 ). Grooves ( 4   a ) are formed in side plates ( 4 ). Grooves ( 4   a ) match the size of one of the arc-shaped sections of magnet ( 1 ). Therefore, one magnet section ( 1 ) will fit into groove ( 4   a ) and the adjacent magnet section ( 1 ) will be offset, fitting into a corresponding groove ( 4   a ) of the opposite side wall ( 4 ). This configuration forces magnet ( 1 ) to move with rotor shaft ( 2 ), since side walls ( 4 ) are fixed to rotor shaft ( 2 ) ( FIGS. 3 and 4 ). This conventional configuration requires the use of magnet segments that are specifically sized to fit into grooves ( 4   a ). Should the magnet size or shape change, side plates ( 4 ) must be milled to different grooves ( 4   a ) to match this size change. Moreover, grooves ( 4   a ) must be milled to fairly close size standards to match the size of magnet sections ( 1 ). 
   As can be seen, there is a need for a motor or generator configuration wherein the magnet is contained or fixed in position on the rotor shaft, especially during the machine&#39;s worst operating conditions. Such a configuration should be simple in design and relatively low cost in its manufacture. 
   SUMMARY OF THE INVENTION 
   In one aspect of the present invention, a permanent magnet machine comprises a rotor; an inner sleeve fitting around a portion of the rotor; a first stub shaft attachable to an inner sleeve first end; a second stub shaft attachable to an inner sleeve second end; a magnet fitting between the first stub shaft and the second stub shaft when the first stub shaft and the second stub shaft are attached to the inner sleeve; and an outer sleeve fitting around the magnet and the first stub sleeve and the second stub sleeve. 
   In another aspect of the present invention, a permanent magnet machine comprises a rotor; an inner sleeve integrally formed around a portion of the rotor; a first stub shaft attachable to an inner sleeve first end; a second stub shaft attachable to an inner sleeve second end; a magnet fitting between the first stub shaft and the second stub shaft when the first stub shaft and the second stub shaft are attached to the inner sleeve; and an outer sleeve fitting around the magnet and the first stub sleeve and the second stub sleeve. 
   In yet another aspect of the present invention, a permanent magnet machine comprises a rotor; an inner sleeve fitting around a portion of the rotor; the inner sleeve having an inner sleeve first end and an inner sleeve second end, with both of the inner sleeve first end and the inner sleeve second end having male threads; a first stub shaft threadably attachable to the inner sleeve first end; a second stub shaft threadably attachable to the inner sleeve second end; a magnet fitting between the first stub shaft and the second stub shaft when the first stub shaft and the second stub shaft are threadably attached to the inner sleeve; and an outer sleeve fitting around the magnet and the first stub sleeve and the second stub sleeve. 
   In a further aspect of the present invention, a magnet assembly for placement on a rotor of a motor or a generator, comprises an inner sleeve fitting around a portion of said rotor; the inner sleeve having an inner sleeve first end and an inner sleeve second end, with both of the inner sleeve first end and the inner sleeve second end having male threads; a first stub shaft threadably attachable to the inner sleeve first end; a second stub shaft threadably attachable to the inner sleeve second end; a magnet fitting between the first stub shaft and the second stub shaft when the first stub shaft and the second stub shaft are threadably attached to the inner sleeve; and an outer sleeve fitting around the magnet and the first stub sleeve and the second stub sleeve. 
   In still a further aspect of the present invention, a method for making a permanent magnet machine comprises attaching an inner sleeve around at least a portion of a rotor of the permanent magnet machine; attaching a first stub shaft to an inner sleeve first end; attaching a second stub shaft to an inner sleeve second end; fitting a magnet against the inner sleeve between the first stub shaft and the second stub shaft; axially compressing the magnet by moving the first stub shaft toward the second stub shaft, with the magnet there between; and fitting an outer sleeve around the magnet, the first stub shaft and the second stub shaft. 
   In yet a further aspect of the present invention, a method for making a permanent magnet machine comprises threading an inner sleeve with male threads on an inner sleeve first end and an inner sleeve second end; attaching the inner sleeve around at least a portion of a rotor of the permanent magnet machine; threading a first stub shaft and a second stub shaft with female threads matable with the male threads; threadably attaching a first stub shaft to an inner sleeve first end; threadably attaching a second stub shaft to an inner sleeve second end; fitting a magnet against the inner sleeve between the first stub shaft and the second stub shaft; turning at least one of the first stub shaft and the second stub shaft along the male thread of the inner sleeve to axially compress the magnet by moving the first stub shaft toward the second stub shaft, with the magnet there between; and fitting an outer sleeve around the magnet, the first stub shaft and the second stub shaft. 
   These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional drawing, viewed radially with respect to the rotor, showing a conventional machine having a magnet on a rotor encased by an outer ring; 
       FIG. 2  is a perspective drawing showing a permanent magnet machine which may incorporate a magnet configuration of the present invention; 
       FIG. 3  is a cross-sectional drawing, viewed axially with respect to the rotor, showing a magnet configuration of the present invention; 
       FIG. 4  is a cross-sectional drawing taken along line  4 - 4  of  FIG. 3 , showing a radial cross-section with respect to the rotor; 
       FIG. 5  is a perspective view of a threaded stub shaft according to an embodiment of the present invention; 
       FIG. 6  is a perspective view of a threaded inner sleeve according to an embodiment of the present invention; and 
       FIG. 7  is a partially assembled axial cross-sectional diagram of the magnet configuration of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention. 
   Broadly, the present invention provides for an axial constraint on the magnet of a motor or generator to provide additional insurance for the rotor to be fixed inside the magnet. As discussed above, a conventional design may require close tolerance between the outer sleeve and the magnet because thermal shrinking of the outer sleeve frictionally holds the magnet to the rotor. The present invention enlarges the required radial shrink-fit tolerance, thereby reducing time and cost of accurately matching the magnet size with the size of the outer sleeve. 
   Moreover, the present invention provides a compressive state on the magnet at all times, thereby preventing magnet cracking and the resulting unstable rotor. In contrast, conventional devices employ the radial shrinkage provided by a warmed, slightly undersized metal outer sleeve as it cools to hold the magnet in place. This shrink fit provided by the outer sleeve may not be enough, however, to prevent the magnet from axial and radial motion at the maximum operating conditions, such as ultra high speeds of up to 70,000 rpm and extreme temperature ranges, for example, from about −40C to about 200° C. 
   More specifically, the present invention uses a threaded inner sleeve having ends that axially compress the magnet. As is described in more detail below, this axial compression allows for a lessening of the more exact fit that is required between the magnet and outer sleeve of conventional configurations. Typically in the prior art, the magnet and the outer sleeve are ground to meet very close tolerance requirements. The present invention relaxes these size requirements, resulting in a substantial cost and time benefit in the manufacture of motors and generators. 
   Referring to  FIG. 2 , there is shown a perspective drawing of a permanent magnet machine  70  which may incorporate the magnet assembly of the present invention within a housing  74 . Generally, permanent magnet machines are those having a permanent magnet mounted on rotor  50 . Permanent magnet machines  70  may be used as either a generator, wherein electrical current is supplied out of electrical lines  72  via manually turning rotor  50 , or as a motor, wherein electrical current is supplied in through electrical lines  72  to apply torque to rotor  50 . The permanent magnet machine  70  of the present invention is especially useful in high-speed applications such as in a high speed generator. 
   Referring to  FIGS. 3 through 6 , there are shown cross-sectional drawings, viewed axially ( FIG. 3 ) and radially ( FIG. 4 ) with respect to the rotor, showing a magnet assembly  60  (not labeled in  FIG. 4 ) of the present invention.  FIGS. 4 and 5  show perspective views of a second stub shaft  28  and an inner sleeve  20 , respectively. Inner sleeve  20  may be disposed around at least a portion of rotor  50  to provide for the attachment of a first stub shaft  24  and second stub shaft  28 . First stub shaft  24  and second stub shaft  28  are used to axially compress a magnet  30  installed on rotor  50 . Preferably, each of first stub shaft and second stub shaft has a flat surface  42  for contact with magnet  30  and an inside surface  44  having female threads  32 . Inner sleeve  20  may have male threads  22  on at least one of its ends  26  for attaching at least one of first stub shaft  24  and second stub shaft  28 . Preferably, inner sleeve  20  has a male thread  22  on both of its ends  26  for attaching both first stub shaft  24  and second stub shaft  28  on each end  26  of inner sleeve  20 . An inside diameter D 1  of inner sleeve  20  may be sized to fit a rotor  50  therein. Alternatively, rotor  50  may be machined to form inner sleeve  20  integrally thereon. 
   Either end  26  of inner sleeve  20  may be attached to first stub shaft  24 . At least one of first stub shaft  24  and second stub shaft  28  can have female threads  32  that mate with male threads  22  of inner sleeve  20 . Preferably, first stub shaft  24  is attached to inner sleeve  20  with female threads  32  that mate with male threads  22  of inner sleeve  20 . The other end  26  of inner sleeve  20  may be attached to second stub shaft  28 . Also preferably, second stub shaft  28  is attached to inner sleeve  20  with female threads  32  that mate with male threads  22  of inner sleeve  20 . 
   Referring now to  FIG. 7 , there is shown a cross-sectional view of first stub shaft  24  and second stub shaft  28  assembled with inner sleeve  20 . When assembled, first stub shaft  24 , second stub shaft  28 , and inner sleeve  20  may form a circular channel  46  having a substantially U-shaped cross-section into which a magnet  30  (not shown in  FIG. 7 ) may be inserted. 
   Referring back to  FIGS. 3 through 6 , magnet  30  is preferably a one-piece donut-shaped magnet. Magnet  30  may be, however, formed of a plurality of arc-shaped magnet sections, which, when combined, form donut-shaped magnet  30 . An inside diameter D 2  of magnet  30  may be sized so that rotor  50  and inner sleeve  20  fit therein. First stub shaft  24  and second stub shaft  28  may have a radial thickness r that is equal to or less than the combined radial thicknesses r of magnet  30  and inner shaft  20 , thereby allowing an outer sleeve  40  to exert a radial force, in a direction as indicated by arrow  34 , on magnet  30  toward rotor  50 , as discussed in more detail below. Preferably, radial thickness r is equal to the combined radial thicknesses r of magnet  30  and inner shaft  20 . 
   Outer sleeve  40  advantageously has a length L that can be equal to or greater than the combined lengths of first stub shaft  24 , magnet  30  and second stub shaft  28 , as shown in  FIG. 2 . An inside diameter D 3  (D 1 +2r) of outer sleeve  40  may be sized equal to or slightly less than the outside diameter of magnet  30 . In one embodiment, the inside diameter D 3  of outer sleeve  40  is slightly less than the outside diameter of magnet  30 . The amount that the inside diameter D 3  of outer sleeve  40  is less than the outside diameter of magnet  30  is chosen to allow for a shrink-fit of a warmed, thermally expanded outer sleeve  40  onto magnet  30  as outer sleeve  40  cools. The exact size differential chosen depends on the thermal coefficient of expansions of the material of the outer sleeve  40 . In other words, the size differential must be such that, when outer sleeve  40  is warmed, the coefficient of thermal expansion of the material of outer sleeve  40  causes it to expand to an inside diameter D 3  greater than or equal to the outer diameter of magnet  30 . 
   First stub shaft  24 , second stub shaft  28 , outer sleeve  40  and inner sleeve  20  may be formed of any materials suitable for the conditions that may result from the intended use. As an example, first stub shaft  24  and second stub shaft  28  may be formed of a Ni—Cr alloy, such as INCONEL® steel 718, or a regular steel, such as steel 4130 or steel 4330. Outer sleeve  40  and inner sleeve  20  may each be formed of the same materials as first stub shaft  24  and second stub shaft  28 . Usefully, outer sleeve  40  can be formed of the Ni—Cr alloy, INCONEL® steel 718, and inner sleeve  20  can be formed of titanium. Titanium is desired over steel or Ni—Cr alloys as inner sleeve  20  due to its lower coefficient of thermal expansion. Thus, during use at a temperature greater than ambient temperatures, such as a temperature from about 25° C. to about 200° C., outer sleeve  40  and magnet  30  may thermally expand more than inner sleeve  20 . This differential thermal expansion of outer sleeve  40  and magnet  30  over inner sleeve  20  may thereby create additional compressive force, as indicated by arrow  34 , onto rotor  50 . 
   The present invention also provides a method to build magnet assembly  60 . One or more donut-shaped magnets  30  can be slid onto inner sleeve  20 . A rotor  50  may be fit in inner sleeve  20  or, alternatively, inner sleeve  20  may be preformed integrally with rotor  50 . 
   The inside diameter D 2  of magnets  30  can be approximately the same as the outside diameter of inner sleeve  20  so that magnets  30  slide on inner sleeve  20  while contacting inner sleeve  20  along its circumference. The resulting inner sleeve  20 /magnet  30  assembly can be threaded into the female threads  32  of first stub shaft  24  and second stub shaft  28 . Stub shafts  24  and  28  may be turned along male threads  22  of inner sleeve  20  to induce an axial preload, as indicated by arrows  38 , in magnet  30 , thereby providing the desired axial constraint for magnet  30 . 
   The resulting inner sleeve  20 /magnet  30 /stub shafts  24 ,  28  assembly can be cooled, preferably in dry ice, to thermally shrink the assembly. Outer sleeve  40  can be thermally expanded by warming and slid over the cooled assembly. When the cooled assembly and outer sleeve  40  are removed from their respective heat and cold sources and returned to ambient temperatures, the resulting outer sleeve  40  shrink-fits onto magnet  30  to form magnet assembly  60  of the present invention. 
   When inner sleeve  20  is formed integrally with rotor  50 , further assurances of axial constraint of magnet  30  on rotor  50  are provided. When inner sleeve  20  is fixed by being integrally formed with rotor  50 , first stub shaft  24  and second stub shaft  28  are each axially fixed, with respect to rotor  50 , when threaded onto inner sleeve  20 . When magnet  30  is axially constrained between first stub shaft  24  and second stub shaft  28 , magnet is constrained from movement axially along rotor  50 . While the present invention has been described using a single donut-shaped magnet  30 , for a two-pole motor or generator, a single or multiple magnets  30  may be used depending on the size of the machine, the desired magnetic flux, and other such characteristics. 
   It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention.