Abstract:
The construction of an electromagnetic device, such as an electric motor, has a segmented stator with a plurality of stator teeth held in a circular pattern solely by a shell of the motor that has been hot dropped over the stator teeth.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to the construction of an electromagnetic device, such as an electric motor, having a segmented stator with stator teeth held in a circular pattern solely by a shell of the motor that has been hot dropped over the stator teeth. 
     BACKGROUND ON THE INVENTION 
     Current trends in the design of electromagnetic machines such as motors have led to compact designs of high efficiency motors. The motor designs have obtained high output power to volume ratios through their use of high magnetic flux density (or “high remanence”) magnets on their rotors and high density windings of their stators, increasing efficiency, and through optimized thermal design which increases the motor&#39;s ability to dissipate losses. 
     With the reduction in size of these high efficiency motors the precision with which their components&#39; parts are assembled becomes more important. Specifically, as the size of a motor becomes increasingly smaller, the size and accuracy with which the air gap (which separates the exterior surface of its rotor from the interior surface of its stator) must be similarly reduced in order to compare favorably to a larger model with similar performance characteristics. In addition, with decreasing motor size, the tolerances of the bearings and their associated mounting diameters, rotor shaft and stator bore center axes also decrease, and the slightest misalignment can result in negative effects on motor performance, in increased bearing wear which significantly decreases the operational life of the motor or in contact of the rotating rotor with the stator bore which prevents its proper functioning all together. 
     Compact high efficiency motors are constructed of basically the same component parts typical to most motors, those being the stator, which is the stationary electromagnetic component of the motor, the rotor, which is the rotating electromagnetic component of the motor, and the endbells, which locate the rotor in relationship to the stator. To achieve the necessary tolerances for the motor&#39;s compact size, each of the component parts of the motor must be machined and assembled with high accuracy relative to the other component parts of the motor. The stator must be assembled in the motor housing shell and the motor housing shell must be manufactured to align or register the center axes of the stator bore relative to the housing shell. The endbells are machined with reference to the stator center bore. By machining and assembling each of the component parts of the motor with reference to the other component parts of the motor, the center axis of the rotor is closely registered with the center axes of the bearings mounted in the endbells which, in turn, are registered with the center axis of the stator bore when the component parts are assembled in the motor. The precise machining and assembly of the motor component parts is necessary to properly position the rotor in the stator bore and the rotor bearings in the endbell bearing seats. The extremely precise machining and assembly of the motor component parts comprise a major portion of the expense involved in manufacturing compact, high efficiency motors. 
     SUMMARY OF THE INVENTION 
     The present invention is an electromagnetic device, such as a motor, having component parts and a method of assembly that provide a motor of compact size which provides higher output performance and higher efficiency than similarly sized motors. The novel features of the invention are in the constructions of its component parts and in their method of assembly and, although described as applied to a motor, they may also be applied to alternators and generators. The improvements accomplished by these specific design and manufacturing techniques give higher torque density and improved thermal conductivity (allowing the motor to dissipate any losses more effectively). The design concepts under consideration here result in a device which is optimized to minimize cogging and torque ripple and provide uniform back EMF, which are significant contributors to output motion quality. 
     The motor of the invention is basically comprised of a stator assembly consisting of a wound stator core contained in a housing shell with a pair of endbells attached to the opposite ends of the housing shell, impregnating resin or encapsulant, and a rotor assembly. The novel features of the motor are in its component parts and the method in which they enclose the electromagnetic device, i.e., the stator and rotor of the motor. Therefore, the stator construction and rotor construction are described in only general terms with it being understood that alternative stator and rotor constructions may be employed with the invention. 
     The stator is a segmented stator comprised of stacks of stator laminates with each stack surrounded by an individual winding. Wound stacks are arranged in a circle in preparation for their being assembled with the housing shell. 
     The housing shell is tubular having a hollow interior and openings to the interior in opposite first and second end surfaces of the shell. The interior of the shell is machined to a precise diameter, and then the opposite first and second end surfaces are machined flat and perpendicular to the center axis of the shell interior. A series of pin holes is machined into each of the end surfaces of a specific depth to be described below. The shell is heated, allowing it to expand slightly, and then is hot-dropped over the circular cluster of wound stator stacks in precise alignment to the orientation of the stacks. A printed circuit board is then connected to the terminals of the stator windings and is positioned so that it is adjacent the stator windings at the rear of the stator assembly. 
     Both the front and rear or first and second endbells are cast from aluminum (although other materials may also be used). Steel bearing support rings are centered in the endbells as they are cast with a larger of the two bearing support rings being cast into the forward or first endbell. The endbells then receive basic machining creating a series of fastener through holes and threaded holes, and creating mating surfaces on the endbells having pilot holes machined therein. Steel pins are inserted into the pilot holes. 
     The endbells are positioned so that they are adjacent the opposite first and second end surfaces of the housing shell with the front end bell positioned adjacent to the first end surface and the rear end bell positioned adjacent to the second end surface. The endbell center axises are aligned with the axis of the stator bore, with the steel shear pins closely related with the matching pilot holes drilled into the housing shell. The end bells are then pressed into position over the first and second end surfaces of the housing shell with the end bell pins broaching into the pin holes of the shell end surfaces providing a precise and tolerance independent fit of the end bells over the opposite first and second end surfaces of the housing shell. The depth of the receiving holes in the housing shell is such there will be sufficient space at the bottom of the drilled hole to receive the shavings produced by the broaching process. The pins resist relative shear and torsional forces between the endbells and the housing shell. Bolts are inserted through the through holes in the front endbell and are screwed into a fastener threaded holes in the rear endbell in order to further secure in tension the endbells on the opposite end surfaces of the housing shell. 
     A removable core fixture assembly is inserted through a shaft opening of one of the endbell bearing support rings, through the stator bore, and through the shaft opening in the opposite endbell bearing support ring in preparation for injection of the encapsulant. An impregnating resin or encapsulant is then injected through one or more of the series of injection openings in one of the endbells. The encapsulant flows axially through the stator assembly permeating the stator core and the endbells until it passes through the injection venting openings of the opposite endbell. The core assembly excludes this material from the bore and bearing regions of the stator assembly. The encapsulant is cured and the core and associated fixturing are removed. 
     The front or first end bell&#39;s bearing bore is then machined in the steel bearing support ring cast into the front endbell. The front bearing bore is machined with its center axis referenced from or coaxially aligned with the center axis of the stator bore and axially referenced from the front of the stator wound core assembly. Either simultaneously or in a subsequent operation, the rear bearing bore is machined in the steel bearing support ring cast into the rear endbell. The rear bearing bore diameter is referenced from the stator bore diameter. 
     Front and rear retainer features are then machined into the front and rear end bells, machined concentrically to and referenced from the stator bore center axis. The axial locations of these features are referenced from or are in register with the axial depth of the front bearing bore. 
     The rotor is comprised of a one piece magnetic steel rotor shaft and core combination having a series of magnetic rings bonded on its exterior. Ball bearings are pressed to precise locations on the opposite ends of the rotor shaft at opposite ends of the magnet rings with the bearing on the rear end of the shaft having a smaller diameter than either the bearing on the front end of the shaft or the magnet ring, and with the bearing on the front end of the shaft having a larger diameter than that of the rotor core which, for example, may be comprised of the outer diametral surface of the magnet ring. 
     The rotor is held in precise alignment with the stator assembly and inserted into the stator by first inserting the rearward end of the rotor with its smaller bearing through the larger bearing bore at the front endbell of the rotor. The rearward end of the shaft and its smaller bearing pass through the stator bore until the rear bearing is positioned adjacently to the bearing bore in the rear endbell and the front bearing is positioned adjacently to the bearing bore in the front endbell. The rotor is then pressed into place with a press fit of the outer race of the front bearing in its housing and with a transitional or close slip fit of the rear bearing in its housing. The front bearing is pressed into the bearing support ring in the front endbell until it engages against the annular shoulder formed in the bearing support ring. A front bearing retainer device is then installed at the front of the larger bearing to help prevent long-term creepage. A bearing preload spring is then placed over the rear end of the rotor shaft and against the outer race of the rear bearing. The rear bearing retainer is then placed over the rear end of the shaft and against the preload spring and is secured to the rear endbell. The rear retainer is positioned in an annular seat that has been precisely machined in axial relation to the front bearing seat, resulting in the virtual elimination of variation in bearing preloading due to tolerance stack up. The rear bearing retainer bore is machined to precise concentricity with the stator bore in order to allow the accurate location of feedback devices relative to the rotor and stator assemblies. 
     The construction of the motor and its method of assembly maintains precision positioning of the rotor at the center of the stator bore with a uniform air gap between the stator bore interior surface and the rotor exterior surface and with the rotor center axis precisely aligned with the center axis of the stator bore as well as the center axes of the rotor bearings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Further features of the invention will be discussed in the following detailed description of the preferred embodiment of the invention and in the drawing figures wherein: 
     FIG. 1 is an exploded view showing the housing shell and front and rear endbells of the motor enclosure of the invention; 
     FIG. 2 is a view similar to FIG. 1 but with the motor turned 180°; 
     FIGS. 3A-3C are various views of the front endbell; 
     FIG. 4 is a partially sectioned perspective view of the front endbell; 
     FIGS. 5A-5C are various views of the rear endbell; 
     FIG. 6 is a partially sectioned perspective view of the rear endbell; 
     FIGS. 7A and 7B are front and rear views, respectively, of the rear retainer cap; 
     FIG. 8 is a cross-section view of the housing shell containing the stator; 
     FIG. 9 is a side-sectioned view of the motor enclosure containing a stator prior to encapsulant injection and machining; 
     FIG. 10 is a view similar to FIG. 9 after encapsulant injection and after the front and rear endbell bearing bores and rear retainer cap seat have been machined; 
     FIG. 11 is an exploded view showing the component parts employed in assembling the rotor assembly into the motor, with a representative feedback device shown for illustrative purposes (although other devices will be used as well); 
     FIG. 12 is a section view taken in a plane along the line  12 — 12  of FIG. 11 which shows the method by which the endbells are secured to the stator assembly, with a representative feedback device included for clarity, similarly to FIG. 11; and 
     FIG. 13 is a side-section view of the completed motor assembly. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows the three primary component parts that make up the enclosure of the motor construction of the invention, those being a housing shell  10 , a first or front endbell  12 , and a second or rear endbell  14 . The front endbell  12  is the one through which the drive shaft of the motor projects. The labels “front” and “rear” are used for reference only and are not intended to have any limiting meaning. 
     The housing shell  10  is a rough-form aluminum extrusion having a cylindrical interior surface  16 . Four drilled holes  18  pass through the housing shell to allow threaded fasteners to be passed through the shell. The shell has opposite first  20  and second  22  end surfaces that are machined, flat surfaces perpendicular to the center axis  24  of the shell. Four pin holes  26  are machined into each of the first  20  and second  22  end surfaces of the shell. Each of the pin holes  26  extend a set distance into the aluminum of the shell from the first and second end surfaces  20 ,  22 , but do not extend entirely through the shell. 
     The shell exterior surface  28  has a generally cylindrical configuration with additional material present where necessary to conform to agency-standard form factors and to enclose and protect the through-bolts and shear pins as described above, resulting in the preferred embodiment in a generally rectangular arrangement with a mounting screw clearance region at each of the four corners. 
     The first or front endbell  12  is comprised of cast metal with a steel bearing support ring  32  cast into the center of the endbell. Several views of the front endbell  12  are shown in FIGS. 1,  3 A- 3 C and  4 . As shown in FIG. 3A, the bearing support ring  32  has a cylindrical interior surface  34  that defines a shaft opening with a center axis  36 . This interior surface  34  will later be machined to receive a ball bearing assembly. The cast endbell  12  has axially opposite exterior  38  and interior  40  surfaces, with the interior surface being machined flat and perpendicular to the center axis  36 . The bearing support ring protrudes forward of the leading surface of the endbell to form the basis of the mounting boss as well as a locating feature for an optional seal. Four bolt holes  46  are machined through the endbell between its exterior  38  and interior  40  surfaces to receive threaded bolts that extend through the endbell, through the drilled holes  18  of the shell housing, and into threaded holes in the rear or second endbell  14  (yet to be described) to hold the two endbells to the opposite end surfaces of the housing shell. Four additional fastener holes  48  are cast in place in the front endbell  12  to receive fasteners used in mounting the motor to another device. Four pin holes  50  are machined into the endbell interior surface  40  at spatially arranged positions corresponding to the positions of the pin holes  26  formed in the first end surface  20  of the housing shell. Like the pin holes in the housing shell, the pin holes  50  in the endbell do not extend entirely through the endbell but only to a set depth below the interior surface  40 . Steel pins  52  are inserted into these four pin holes  50 . The pins  52  have a predetermined length such that a specific length of the pin will protrude a set distance past the interior surface  40  when they are fully inserted into the pin holes  50 , and have diameters that are slightly larger than that of the pin holes. These pins will have a rounded or chamfered edge on one end which inserts into the endbell while having a sharp edge at the other end to enable the pin to broach into the shell material. The length of the pins  52  left projecting from the endbell interior surface  40  are determined so that they will not reach to the bottoms of the pin holes  26  in the housing shell first end surface  20 . The pin holes  26  in the shell end surface have a depth that is greater than the exposed length of the pins  52  so that there is room in the end surface pin holes  26  to receive the material broached from the interior sidewalls of the pin holes as the pins are inserted. As the pins  52  are inserted into the pin holes  26 , their leading edges broach the interior surface of the pin holes  26  cutting away material from the surface and pushing it into the remaining depth of the pin holes  26 . In this manner, the pins  52  are securely held in their respective pin holes  26 . 
     Referring to FIG. 4, it can be seen that the first or front endbell  12  is cast with a series of axially spaced spokes or spines  54 ,  56  connecting a center portion  58  of the endbell surrounding the bearing support ring  32  to an exterior portion  60  of the endbell. Large portions of the spokes  54  are positioned adjacently to the exterior surface  38  of the endbell where the other spokes  56  are axially spaced away from the endbell exterior surface  38  and are positioned more toward the endbell interior surface  40 . The circumferential and axial spacing of the spokes  54 ,  56  creates injection openings  62  in the endbell exterior surface  38  that communicate with injection passageways  64  that pass axially between the spokes  54 ,  56 . These injection openings  62  and passageways  64  are employed to allow an impregnating resin or encapsulant to permeate through the endbell surrounding the spokes  56  and securing the endbell  12  in the encapsulant as it cures, which will be described later. 
     The second or rear endbell  14  is constructed in a similar manner to that of the first or front endbell  12  and component parts of the rear endbell  14  corresponding to those of the front endbell  12  are labeled with the same reference numbers followed by a prime (′). Because the construction of the rear endbell  14  is similar to that of the front endbell  12 , it will not be described in detail. However, the construction of the rear endbell  14  differs from that of the front endbell  12  in that the rear steel bearing support ring  66  is smaller than the front bearing support ring  32  and its interior surface  68  defines a smaller shaft opening than that of the front bearing support ring. The rear bearing support ring  66  has a center axis  70 . Also, the rear endbell  14  is not machined with through bolt holes  46 , but with internally threaded bolt holes  72  that extend into the rear endbell  14  from its interior surface  40 ′ but do not extend entirely through the rear endbell. The internally threaded bolt holes  72  are spatially arranged on the rear endbell interior surface  40 ′ to correspond to the positions of the through bolt holes  46  of the front endbell  12  and to receive threaded bolts inserted through the front endbell bolt holes  46  when attaching the two endbells to the opposite end surfaces of the housing shell  10  as will be described. The rear endbell does not possess fastener holes or the associated shape, but rather has a swept surface which provides maximum bell strength along with cosmetic appeal. A rear bearing retainer annular seat surface  44 ′ and retainer cap fastener holes  42 ′ allow the placement of the rear bearing retainer (to be described later). Apart from these differences, the rear endbell  14  is constructed with opposite exterior and interior surfaces  38 ′,  40 ′, pin holes  50 ′, and steel pins  52 ′, circumferentially and radially spaced spokes  54 ′,  56 ′, connecting a center portion  58 ′ of the endbell with an exterior portion  60 ′ and defining injection openings  62 ′ and injection passages  64 ′ just as in the construction of the front endbell  12 . 
     A rear retainer cap  90  is shown in FIGS. 7A and 7B. The rear retainer cap has a circular exterior surface  92 , an opposite interior surface  94  and a cylindrical side surface.  96 . A center shaft opening  98  passes through the rear retainer cap and has a center axis  100 . The exterior surface  92  is registered perpendicular with the center axis  100  and the side surface  96  is registered with the center axis  100  a predetermined distance and is also parallel with the center axis, enabling the placement of the retainer to be held precisely enough to allow it to serve as the mounting for the motor&#39;s feedback device. Four mounting screw locating recesses  102  project radially inward from the side surface  96  and receive threaded fasteners used in attaching the rear retainer cap  90  to the rear endbell  14 . An extra pair of holes  106  pass through the rear retainer cap  90 . These extra holes  106  have enlarged recesses where they emerge from the cap interior surface  94 . The enlarged recesses enable a pair of screws  107  to be inserted through the holes  106  with the heads of the screws received in the enlarged recesses and with the lengths of the screws projecting from the holes outwardly from the exterior surface  92  of the cap. These projecting screws may be employed in attaching an external device to the retainer cap, for example in attaching an encoder assembly  109  to the retainer cap such as that shown in FIG.  11 . 
     The component parts of the motor enclosure described to this point together with an impregnating resin or encapsulant, and the method by which they are assembled together with a stator assembly and rotor assembly of the motor make up the subject matter of the invention. The stator assembly construction and the rotor assembly construction of the illustrative embodiment may be varied, as the motor enclosure of the invention and its method of construction may be employed with various different types of stator assemblies and rotor assemblies. Therefore, the stator assembly and rotor assembly of the illustrative embodiment will only be described generally. 
     The stator  108  is a segmented stator comprised of nine stacks (used as an example, although other numbers of stacks will also be used in other variants of this motor), of stator laminates  110 . Individual laminates  110  can be seen in FIG.  8 . Each laminate has a general T-shape with a head portion extending across its top and a pole extending downwardly from the head as is common in many segmented stator constructions. Each stack of laminates is insulated with insulator endcaps  112  positioned at opposite ends of the stack (see FIG. 9) and with film insulation slot liners  114  positioned along the opposite sides of the stack (see FIG.  8 ). Alternatively, the tooth stacks may be insulated by overmolding the part with plastic formed such that the same purpose served by the slot liners and endcaps, described above, is met. The rear endcap  112  has a pair of terminals (not shown) inserted into the molded plastic of the endcap. Each insulated lamination stack is precision wound with a high-density wire coil  116  with the opposite end of each wire coil being terminated on one of the two terminals projecting from the rear endcap. The wound laminate stacks are assembled into a circle, engaging mating tongue and groove connections on the opposite ends of the laminate heads of the stacks in preparation for their being assembled with the housing shell. The stacks are arranged such that the forward surfaces of all stacks are coplanar. The stator center bore  118  is finished when the laminate stacks are assembled in the circle, meaning that no machining of the stator bore  118  is needed. 
     The housing shell  10  is rapidly heated, causing the interior diameter of the shell to expand slightly, and is then dropped while still hot over the circular cluster of wound tooth stacks. The forward surface of the shell  10  will be aligned with the coplanar surfaces of the tooth stacks, resulting in a single planar surface, and the radial orientation of the shell in relation to the tooth stack cluster is precisely controlled. The shell  10  engaging around the cluster of wound stator stacks acts to hold the stacks in their circular arrangement with no welds required between the stacks or between the individual laminates. FIG. 8 shows the housing shell  10  surrounding the nine laminate stacks (for example, although other numbers will be used in variants of the design). A printed wiring board (not shown) is then connected to the pairs of terminals of each of the stator windings and is positioned adjacently the stator windings in the rear end of the housing shell. 
     The endbells  12 ,  14  are positioned adjacently to the opposite first and second end surfaces  20 ,  22 , respectively, of the housing shell with the bearing bores of either endbell precisely aligned with the bore of the stator, and the pins  52  projecting from the front endbell  12  being generally aligned with the pin holes  26  in the housing shell first end surface  20  and the pins  52 ′ projecting from the rear endbell  14  being generally aligned with the pin holes  26  in the housing shell second end surface  22 . The endbells  12 ,  14  are then pressed into position over the first  20  and second  22  end surfaces of the housing shell with the pins broaching into the pin holes of the shell end surfaces pushing material of the housing shell cut away from the interior surfaces of the pin holes  26  to the unused depth of the pin holes as described earlier. This step provides a precise and tolerance independent fit of the endbells over the opposite first and second end surfaces of the housing shell that would not be possible with fasteners inserted through the bolt holes  46  of the front endbell  12  which are dimensioned slightly larger than the fasteners they accommodate permitting some relative sheer and torsional movement of the front endbell  12  relative to the fasteners and the housing shell  10 . The pins resist relative sheer and torsional forces between the endbells and the housing shell and hold the endbells in precisely registered positions relative to the housing shell and the stator contained in the shell. This assembly method enables bearing bores and a retainer cap seat to be later machined into the endbells in precisely registered positions relative to the stator bore center axis. Threaded bolts  120  are then inserted through the bolt holes  46  in the front endbell  12 , through the housing shell channels  18  and are threaded and tightened into the internally threaded bolt holes  72  in the rear endbell  14 , securely holding the two endbells to the opposite end surfaces of the housing shell and providing additional resistance to sheer and torsional movement of the endbells relative to the housing shell. 
     A removable core (not shown) is then inserted through the shaft opening of the larger of the endbell bearing support rings, through the stator bore  118 , and through the shaft opening in the opposite endbell support ring in preparation for injection of the impregnating resin encapsulant. The impregnating resin is then injected through one or more of the injection openings  62 ′ in the rear endbell  14 . The resin surrounds the spokes  54 ′ as it passes through the injection passages  64 ′ of the rear endbell and flows axially through the stator assembly except for the stator bore which is occupied by the core. Injecting the resin axially through the endbells and stator facilitates its permeating the open voids of the stator. The injection of the resin is continued until it passes around the spokes  54  of the front endbell  12  and passes out of the enclosure through the injection openings  62  of the front endbell, thus filling all of the open voids in the stator and the endbells. The encapsulating resin is then cured. 
     Following curing, the core is removed and any cured encapsulant projecting from the injection openings  62 ,  62 ′ of the two endbells is removed giving the exterior surfaces of the two endbells a smooth appearance. At this point in the motor&#39;s construction, it appears as shown in FIG. 10 with the cured encapsulant  122  filling all open voids in the stator assembly with only the stator bore  118 , the rear shaft opening defined by the rear bearing support ring surface  68  and the front shaft opening defined by the front bearing support ring surface  34  being devoid of encapsulant. The stator bore  118  has a cylindrical interior surface defined by the end surfaces  124  of the stacked laminate poles and the cured encapsulant  122  surrounding these end surfaces. 
     As shown in FIG. 10, the front bearing bore  126  is then machined into the steel bearing support ring cast into the front endbell. The front bearing bore is machined with its center axis  128  referenced from or coaxially aligned with the center axis  130  of the stator bore. Simultaneously, or in a subsequent operation, the rear bearing bore  132  is machined into the steel bearing support ring cast into the rear endbell. The rear bearing bore center axis  134  is referenced from the front bearing bore axis  128 . 
     Front bearing retainer ring groove  135  and rear retainer seat  136  are then machined into the endbells. Both details are machined concentric to and referenced from the stator bore center axis  130 . The depths to which these details are machined into the respective front and rear endbells are referenced from or are in register with the stator bore center axis and the axial depth of the front bearing bore. 
     FIG. 11 illustrates the assembly of the rotor  138  into the stator and the motor enclosure. The rotor is comprised of a magnetic steel rotor shaft  140  having a series of magnetic rings  142  bonded on its exterior. Ball bearings  144 ,  146  are pressed onto fixed positions on the opposite ends of the rotor shaft at opposite ends of the magnet rings with the bearing  146  on the rear end  150  of the shaft being smaller than the bearing  144  on the front end  148  of the shaft. The larger bearing must be located on the end of the shaft which is expected to see the greatest radial load. 
     The rotor  138  is inserted into the stator by first inserting the rearward end  150  of the rotor with its smaller bearing  146  through the larger bearing bore  126  at the front endbell of the rotor. The rearward end  150  of the shaft and its smaller bearing are guided through the stator bore  118  until the rear bearing  146  is positioned adjacent to the bearing bore  132  in the rear endbell and the front bearing  144  is positioned adjacent to the bearing bore  126  in the front endbell. The rear bearing  146  is then located into the machined bearing support ring  66  in the rear endbell and the front bearing  144  is simultaneously pressed into the machined bearing support ring  32  in the front endbell until it engages against the annular shoulder  152  formed in the bearing support ring. 
     The front bearing retainer device  74  is then placed over the forward end  148  of the rotor shaft and is seated against the front bearing outer race and within the beveled groove  135 . An optional shaft seal  162  may also be inserted into the support ring  32 . A bearing preload spring  156  is then placed over the rear end  150  of the rotor shaft and against the outer race of the rear bearing  146 . The rear bearing retainer cap  90  is then placed over the rear end  150  of the shaft and against the preload spring  156  and is secured to the rear endbell by screws  158 . The rear retainer cap  90  is positioned in the machined annular seat  136  that is precisely located in relation to the front bearing bore  126 , resulting in the virtual elimination of variation in bearing preloading due to tolerance stack up. The completed motor is shown in FIG.  13 . 
     The method of machining described here enables the rear bearing retainer details to be kept parallel and perpendicular to the axis of the stator bore, allowing the rear bearing retainer to be used to accurately mount feedback devices. 
     The construction of the motor and its method of assembly maintains precision positioning of the rotor at the center of the stator bore with a uniform air gap  160  between the stator bore interior surface and the rotor exterior surface and with the rotor center axis precisely aligned with the center axis of the stator bore  130  as well as the center axes  128 ,  132  of the rotor bearings. 
     While the present invention has been described by reference to a specific embodiment, it should be understood that modifications and variations of the invention may be constructed without departing from the scope of the invention defined in the following claims.