Patent Publication Number: US-2023140075-A1

Title: Retention cap for permanent magnet rotor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     1. Priority Application 
     The present application claims the benefit of and priority from U.S. Provisional Patent Application No. 63/257,249, filed Nov. 3, 2021, the entire disclosure of which is hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The embodiments described herein relate generally to a rotor for an electric motor. More particularly, embodiments of the present invention concern rotor assemblies having rotor cores with permanent magnets disposed therein. 
     2. Discussion of the Prior Art 
     Permanent magnets are often used in rotors of electronically controlled motors. These magnets are often placed in openings or slots defined in laminated rotor cores at predetermined positions for best motor performance. The magnets must be retained in the rotor core to prevent movement thereof during motor operation and shipping and handling. This is conventionally achieved through any one or more of a variety of techniques. For instance, high strength adhesive might be applied between the rotor core and magnets. However, in addition to the cost associated with adhesives, adhesives require additional processing such as cleaning the parts for proper adhesion, adhesive curing equipment, and time to develop the proper bond strength. Overmolding and other processes requiring heat can de-magnetize or reduce magnet strength, reducing motor performance and/or necessitating post-assembly magnetization, and may require collaboration with third-party vendors. Overmolding is also particularly challenging when rotor cores are fluffy or spongy. Shrink wrap leads to cost and performance issues to due a large air gap requirement. Press fitting of magnets into the core may require servo presses and also may result in tolerance stack up issues. 
     Rotor core designs featuring inbuilt or integral magnet retention structures may also be used. For instance, rotor cores may include laminations having integral projections that generate mechanical interference with magnets to retain the magnets via friction. However, such laminated rotors are susceptible to manufacturing variations in the size of the laminations and magnets. 
     SUMMARY 
     According to one aspect of the present invention, a rotor for an electric motor includes a core rotatable about an axis and defining a plurality of magnet-receiving slots; a plurality of magnets, each of the magnets being received in a respective one of the magnet-receiving slots; and a magnet retention cap fixed relative to the core and including a resiliently deformable magnet retention element configured to restrict axial movement of the magnets relative to the core. 
     This summary is provided to introduce a selection of concepts in a simplified form. These concepts are further described below in the detailed description of the preferred embodiments. 
     Various other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention are described in detail below with reference to the attached drawing figures, wherein: 
         FIG.  1    is a perspective view of a motor in accordance with a preferred embodiment of the present invention, wherein an endcap of the motor is partially sectioned to expose a magnet retention cap; 
         FIG.  2    is an alternate perspective view of the rotor of the motor of  FIG.  1   ; 
         FIG.  3    is an exploded perspective view of the rotor of  FIG.  2   ; 
         FIG.  4    is a front view of the rotor of  FIG.  2   , with the shaft and bearings removed; 
         FIG.  5    is a side view, taken along line  5 - 5  of  FIG.  4   , of the portion of the rotor shown in  FIG.  4   ; 
         FIG.  6    is a fragmentary cross-sectional side view, taken along line  6 - 6  of  FIG.  4   , of a portion of the rotor of  FIG.  4   , particularly illustrating the interaction of fasteners and bosses with the rotor core; 
         FIG.  7    is a fragmentary cross-sectional side view, taken along line  7 - 7  of  FIG.  4   , of a portion of the rotor of  FIG.  4   , particularly illustrating the interaction of pins and bosses with the rotor core; 
         FIG.  8    is a fragmentary cross-sectional side view, taken along line  8 - 8  of  FIG.  4   , of a portion of the rotor of  FIG.  4   , particularly illustrating interaction of bosses and magnet retention fingers with the rotor core and a magnet, respectively; 
         FIG.  9    is an enlarged front perspective view of the magnet retention cap of the rotor of  FIGS.  1 - 8   ; and 
         FIG.  10    is a rear perspective view of the magnet retention cap of  FIG.  9   . 
     
    
    
     The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. While the drawings do not necessarily provide exact dimensions or tolerances for the illustrated structures or components, the drawings are to scale with respect to the relationships between the components of the structures illustrated in the drawings. 
     DETAILED DESCRIPTION 
     The present invention is susceptible of embodiment in many different forms. While the drawings illustrate, and the specification describes, certain preferred embodiments of the invention, it is to be understood that such disclosure is by way of example only. There is no intent to limit the principles of the present invention to the particular disclosed embodiments. 
     Furthermore, unless specified or made clear, the directional references made herein with regard to the present invention and/or associated components (such as top, bottom, upper, lower, inner, outer, and so on.) are used solely for the sake of convenience and should be understood only in relation to each other. For instance, a component might in practice be oriented such that faces referred to as “top” and “bottom” are sideways, angled, inverted, and so on relative to the chosen frame of reference. 
     Motor Overview 
       FIG.  1    illustrates an electric motor  10  in accordance with a preferred embodiment of the present invention. The motor  10  broadly includes a stator  12  and a rotor  14 . The rotor  14  rotates relative to the stator  12  about an axis of rotation. 
     The motor  10  further includes a motor housing  16  and a controller assembly  18 . The motor housing  16  includes an outer shell  20  and a pair of axially opposed endshields  22  and  24 . The housing  16  defines a motor chamber  26  in which the stator  12  and the rotor  14  are at least substantially received. 
     The controller assembly  18  preferably includes electronic components  28  for controlling operation of the motor  10 . The controller assembly  18  further preferably includes a controller housing  30  secured to the motor housing  16  and at least in part enclosing the electronic components  28 . 
     The stator  12  is generally toroidal in form and defines a stator axis that is coaxial with the axis of rotation of the rotor  14 . However, according to some aspects of the present invention, it is permissible for the axes to be non-coaxial. The stator  12  preferably includes a stator core (not shown) and a plurality of coils  32  wound about the stator core. In the illustrated embodiment, the stator  12  further includes a plurality of electrically insulative coverings  34  positioned between the stator core and the coils  32 . 
     In the exemplary embodiment, the electric motor  10  is an inner rotor motor, with the stator  12  circumscribing the rotor  14 . However, according to some aspects of the present invention, the electric motor may alternately be an outer rotor motor or dual rotor motor. 
     In a preferred embodiment of the present invention, the rotor  14  includes a rotor core  36  and a rotor shaft  38  to which the rotor core  36  is mounted. 
     The rotor core  36  includes a plurality of arcuately arranged pole segments  40  and defines a plurality of axially extending, arcuately spaced apart magnet-receiving slots  42 . The slots  42  preferably alternate arcuately with the pole segments  40 . 
     The rotor  14  further preferably includes a plurality of magnets  44 , each of which is received in a respective one of the magnet-receiving slots  42 . 
     The magnets  44  are preferably permanent magnets and may comprise ferrite, neodymium, and/or other materials suited for accomplishing the desired motor performance. 
     It is noted that, according to certain aspects of the present invention, the rotor core may be constructed for use in an electric generator or other electric machine that includes a stator, in contrast to use in a motor, as illustrated. 
     The rotor core  36  is preferably generally cylindrical in form and, in a preferred embodiment, is fabricated from steel. Other materials are permissible according to some aspects of the present invention, however. 
     The rotor core  36  may be of either solid or laminated construction according to some aspects of the present invention, although laminated construction is preferred and illustrated. Furthermore, the rotor core may also be segmented in form, although non-segmented construction is illustrated and preferred. 
     Detailed descriptions of a preferred rotor construction are provided in U.S. patent application Ser. No. 17/167,873, filed Feb. 4, 2021, and entitled “LAMINATED SPOKED ROTOR WITH MECHANICAL MAGNET RETENTION,” and U.S. Provisional Patent Application Ser. No. 62/970,031, filed Feb. 4, 2020, also entitled “LAMINATED SPOKED ROTOR WITH MECHANICAL MAGNET RETENTION” and from which the &#39;873 application claims priority. The &#39;873 application and the &#39;031 application are incorporated by reference herein in their entireties. 
     The electric motor  10  further preferably includes first and second bearing assemblies  46  that cooperatively rotatably support the rotor shaft  38  of the rotor  14 . The endshields  22  and  24  support respective ones of the bearing assemblies  46 . Alternative or additional bearing assembly supports may be provided without departing from the scope of the present invention, however. 
     The rotor  14  further includes a pair of magnet retention caps  48 , each of which includes a resiliently deformable magnet retention element  50  configured to restrict axial movement of the magnets  44  relative to the core  36 . 
     The retention caps  48  are positioned on respective axial ends of the rotor core  36  and fixed relative thereto. Furthermore, the retention caps  48  are preferably identical to one another. However, it is permissible according to some aspects of the present invention for only a single retention cap to be provided or for the retention caps associated with a given motor to vary from each other substantially or insubstantially. 
     It is further noted that the illustrated retention element  50  of each cap  48  is configured to restrict movement of all the magnets  44 ; however, according to certain aspects of the present invention, one or both of the retention elements may alternatively be configured to restrict movement of less than all of the magnets. 
     Magnet Retention Cap Structure 
     As noted above, the retention caps  48  are preferably identical to each other. Therefore, for purposes of clarity and conciseness, a single one of the retention caps  48  will be described below. The descriptions below show be understood to apply to both of the end caps  48 , however. 
     The magnet retention cap  48  is preferably generally toroidal in form and extends circumferentially continuously along a circular path. Although discontinuities and/or non-circular extension fall within the scope of some aspects of the present invention, it is generally desirable that the rotor core  36  and the cap  48  have generally complementary shapes and that discontinuities or openings in the cap  48  are generally minimized except as noted below. 
     Furthermore, the cap  48  preferably presents an outer diameter slightly less than or equal to the outer diameter of the rotor core  36 . Still further, the cap  48  preferably presents an inner diameter that is greater than that of the rotor core  36 . In this manner, the cap  48  does not increase the outer radial dimensions of the rotor  14  as a whole or decrease the inner radial dimensions of the rotor  14  as a whole. Thus, the cap  48  does not interfere with any components radially adjacent the rotor  14 . 
     The cap  48  preferably includes a radially and circumferentially extending body  52  and radially spaced apart inner and outer walls  54  and  56 , respectively, extending from the body  52 . 
     The walls  54  and  56  preferably extend circumferentially, and concentrically in relation to each other. Furthermore, the walls  54  and  56  preferably project orthogonally or at least substantially orthogonally from the body  52  (or axially or at least substantially axially relative to the rotational axis of the rotor core  36 ), although shape and projection angle variations fall within the scope of some aspects of the present invention. Alternatively described, the walls  54  and  56  preferably extend axially away from the core  36 . 
     It is permissible according to some aspects of the present invention for either or both of the walls to be omitted entirely. 
     In a preferred embodiment, the body  52  presents an inner surface  52   a  and an axially opposed outer surface  52   b.  The walls  54  and  56  project from the outer surface  52   b.    
     The outer wall  56  and the inner wall  54  each preferably vary arcuately in height. More particularly, the inner wall  54  has an axial inner wall height and presents a contoured form such that the inner wall height varies circumferentially between maximum and minimum inner wall heights. Similarly, the outer wall  56  has an axial outer wall height and presents a contoured form such that the outer wall height varies circumferentially between maximum and minimum outer wall heights. 
     The arcuate positions of the minimum inner and outer wall heights preferably correspond to one another, as do the arcuate positions of the maximum inner and outer wall heights. That is, the inner and outer walls  54  and  56  are each at their maximum heights in radially aligned locations and are likewise each at their minimum heights in radially aligned locations. 
     Each wall  54  and  56  preferably includes two (2) minimum height locations and two (2) maximum height locations. Most preferably, the minimum height locations are diametrically opposed, as are the maximum height locations. Still further, the minimum and maximum height locations are preferably evenly arcuately spaced apart from each other. 
     As best shown in  FIG.  5   , the retention cap  48  presents an overall contoured shape as a result a result of the contouring of the walls  54  and  56 . The functional significance of this contouring will be discussed in greater detail below. 
     The retention cap  48  also preferably includes at least one alignment element  58  for orienting the cap  48  relative to the core  36  and for orienting the caps  48  relative to each other. In the illustrated embodiment, a plurality of alignment elements  58  are provided and include both visual alignment elements and physical alignment elements. 
     The visual alignment elements preferably include two (2) pairs of alignment bars  60 , a pair of notches  62 , and a pair of pins  64  and corresponding openings  66 . 
     More particularly, in a preferred embodiment, two pairs of diametrically opposed, radially extending alignment bars  60  project from the outer surface  52   b  and provide visual positioning guidance to facilitate proper clocking or rotation of the caps  48  relative to one another. In the illustrated embodiment, the bars  60  include two (2) diametrically opposed inner bars  60   a  extending from corresponding locations on the inner wall  54  and two (2) diametrically opposed outer bars  60   b  extending from corresponding locations on the outer wall  56 . Each inner bar  60   a  is preferably disposed directly radially opposite a corresponding one of the outer bars  60   b.  It is permissible according to some aspects of the present invention, however, for the bars to be alternatively configured, present in varying numbers, or omitted entirely. 
     A notch  62 , most preferably extending along an arc of a circle, is preferably formed in the body  52  and outer wall  56  adjacent each outer bar  60   b,  such that two (2) diametrically opposed notches  62  are provided. Such notches  62  are provided for additional alignment assistance but may be omitted without departing from the scope of some aspects of the present invention. 
     Although the bars  60  and notches  62  may be used to provide visual alignment assistance during manual assembly of the caps  48  onto the rotor core  36 , such alignment elements  58  may also provide optical guidance during automated assembly or in a post-assembly verification process. That is, machine vision may be used for analysis of the visual alignment elements  58 . 
     The physical alignment elements preferably include a pair of diametrically opposed alignment pin bosses  68  projecting axially from the inner surface  52   a  of the body  52  and a pair of alignment pins  64  projecting from respective ones of the pin bosses  68 . 
     In a preferred embodiment, an axially extending opening  66  extends through each pole segment  44 . The openings  66  both reduce core material and, in a preferred embodiment, facilitate holding of the rotor core  36  during tooling. Upon placement of the cap  48  onto the rotor core  36 , each pin  64  is received in one of the axially extending openings  66 . 
     It is permissible according to some aspects of the present invention for one or more of the openings to instead be defined by the body or another portion of the cap and for one or more of the pins to instead be defined by the core. However, the illustrated configuration, in which the pins extend from the body and into the core, is most preferred. 
     A pair of diametrically opposed fastener bosses  70  also project axially from the inner surface  52   a  of the body  52 . The fastener bosses  70  each define a fastener-receiving opening or aperture  72 . A corresponding fastener  74  extends through each aperture  72  (that is, through the body  52 ) and into the core  36  to secure the cap  48  to the core  36 . The fasteners  74  are preferably screws  74 . More particularly, the fasteners  74  are preferably self-tapping (that is, thread-forming) screws  74 , although it is permissible according to some aspects of the present invention for other screw or fastener types to be used. 
     The fastener bosses  70  are preferably positioned arcuately intermediately between the alignment bars  60  and notches  62 . Alternatively stated, the fastener bosses  70  alternate arcuately with and are evenly arcuately spaced from each set of alignment bars  60   a,    60   b  and notches  62 . However, uneven spacing falls within the scope of some aspects of the present invention. 
     As will be apparent from the above, the fastener bosses  70  also preferably alternate arcuately with the pin bosses  68 . 
     In a preferred embodiment, as will be discussed in greater detail below, the maximum heights of the inner and outer walls  54  and  56 , respectively, occur adjacent corresponding ones of the fasteners or screws  74  or, alternatively stated, adjacent the corresponding fastener bosses  70 . In contrast, the minimum heights of the walls  54  and  56  occur at positions orthogonal to the fasteners or screws  74  and fastener bosses  70 . That is, the minimum heights of the walls  54  and  56  occur radially adjacent corresponding ones of the alignment bars  60   a,    60   b  and notches  62 . 
     Inner and outer arcuate reliefs  76   a  and  76   b  are preferably provided in the inner and outer walls  54  and  56  adjacent the fastener-receiving apertures  72 . Such reliefs  76   a  and  76   b  facilitate insertion of the screws  74  and, if necessary, any associated flange or head portions thereof. However, the reliefs might in some instances be omitted or alternatively configured. 
     As noted previously, each cap  48  preferably includes a resiliently deformable or deflectable magnet retention element  50  configured to restrict axial movement of the magnets  44  relative to the core  36 . 
     Preferably, the magnet retention element  50  includes a plurality of resiliently deformable fingers  78 . Alternatively described, the fingers  78  are resiliently deflectable or spring-like. As will be discussed in greater detail below, the resiliently deformable nature of the fingers  78  facilitates transmission thereby of a spring force to the associated magnets  44  and consequent reduction or prevention of magnet chatter. 
     Most preferably, each finger  78  corresponds to one of the magnets  44 . Each magnet  44  likewise corresponds to one of the fingers  78 . However, it is permissible according to some aspects of the present invention for the preferred one-to-one correspondence to vary (as noted previously with respect to the retention element generally). 
     In a preferred embodiment, the fingers  78  are distributed in oppositely oriented clockwise and counter-clockwise extending pairs. Other configurations fall within the scope of the present invention, however. 
     Each finger  78  preferably extends from the body  52  in a cantilevered manner. More particularly, the body  52  preferably defines a plurality of slots  80  extending generally circumferentially in keeping with the arcuate extension of the cap  48  and body  52  in a broad sense. The fingers  78  preferably extend from the body  52  and into corresponding ones of the slots  80 . 
     In greater detail still, each finger  78  preferably includes main form  82  and a contact projection or nub  84  extending from the main form  82 . The main form  82  includes a base  86  adjacent and extending from the body  52  and a tip  88  spaced from the base  86  and disposed in the corresponding slot  80 . The nub  84  preferably extends axially from the main body  52  at or adjacent the tip  88 , although alternative positioning falls within the scope of some aspects of the present invention. 
     In a preferred embodiment, each nub  84  is positioned arcuately centrally over a corresponding one of the magnets  44  of the rotor  14  to facilitate application of a retentive spring force thereto if the finger  78  is deflected. Such functionality will be discussed in greater detail below. 
     It is noted that each nub  84  is preferably rounded or frustoconical to facilitate secure contact with the associated one of the magnets  44 . Alternative shapes fall within the scope of some aspects of the present invention, however. 
     The tip  88  is preferably free of direct interconnection with any structures of the cap  48  except for those of the finger  78  itself. That is, the tip  88  is connected to the base  86  by the main form  82  and has the nub  84  extending therefrom, but is otherwise devoid of contact with or connection to the cap  48 . Those of ordinary skill in the art will recognize that the fingers  78  may therefore be understood to be cantilevered structures. 
     It is permissible according to some aspects of the present invention for the fingers to be non-cantilevered in form. For instance, the fingers might extend across the slots in their entirety or be connected to the body by one or more bridges. However, in such an embodiment, it would nevertheless be most preferred for the resiliently deformable or deflectable nature and functionality of the fingers be retained. 
     The slots  80  are preferably at least substantially rectangular but with slightly curved inner and outer sides in keeping with the aforementioned circumferential extension thereof, although other shapes and extension directions (for instance, fully rectangular shape and tangential extent) fall within the scope of some aspects of the present invention. Likewise, the fingers  78  preferably extend slightly arcuately, although straight fingers fall within the scope of the present invention. 
     Each finger  78  is preferably disposed radially centrally within the corresponding slot  80 , although offset configurations fall within the ambit of some aspects of the present invention. 
     It is also permissible according to some aspects of the present invention for one or more of the fingers to extend radially inwardly or outwardly from the body into the corresponding slot. The slots might also vary from the preferred circumferential extension. 
     Each finger  78  preferably has an arcuate length between the base  86  and the tip  88  thereof that is slightly greater than half the arcuate length of the corresponding slot  80 . Variations in length proportion are permissible according to some aspects of the present invention, however. 
     The base  86  is preferably broader (that is, has a greater radial dimension) than the tip  88 , such that the finger  78  tapers from the base  86  to the tip  88 . Alternatively described, each finger  78  is preferably generally triangular in form, with a rounded vertex at the tip  88  thereof. In a preferred embodiment, the base  86  presents a base width that is at least one and one half (1 ½) times the tip width. However, shape and proportion variations of the fingers fall within the scope of some aspects of the present invention. 
     Each finger  78  and, in particular, the base  86  thereof, is preferably shaped so as to reduce or eliminate sharp corners or other irregularities or features that might be associated with stress concentrations. In the illustrated embodiment, for instance, curved stress-relief transition regions  90  are provided at the interface of each base  86  with the body  52 . The broad, smooth-edged base  86  is thereby configured to efficiently absorb and distribute bending loads, rather than such loads resulting in distortion of the body  52  or causing irregular, high, and/or potentially damaging stresses in the finger  78 . 
     It is permissible according to some aspects of the present invention, however, for the fingers to be generally straight-sided or rectangular and/or for the fingers to be devoid of stress-relief transition regions. 
     The magnet retention cap  48  preferably comprises synthetic resin, although any one or more of a variety or materials or combinations of materials may be suitable, provided detrimental effects on motor performance or cost fall within acceptable limits or do not occur. It is particularly noted that the use of synthetic resin, as preferred, enables the cap  48  to be low in weight and therefore not associated with any substantive resulting motor balancing issues. Use of a synthetic resin also enables the cap  48  to be both magnetically and electrically insulative. 
     Most preferably, the magnet retention cap  48  comprises Nylon 66 glass filled (PA66-GA33). Stamped metal might also be used, although insulation (in the form of a Mylar® sheet, for instance) might preferably be added to eliminate or reduce untoward effects on flux. 
     In a preferred embodiment, the cap  48  is readily suited for use with a variety of rotor core inner diameters, such as one half (½) inch, seventeen (17) millimeter, five eighths (⅝) inch, and twenty-five (25) millimeter, without any modifications. Furthermore, the cap  48  can be easily redesigned (that is, resized) to accommodate other inner diameters as necessary. Adaptation for other outer diameters is similarly straightforward. 
     Certain relative or proportional dimensions of components of the cap  48  and of the cap  48  relative to the associated rotor core  36 , however, are preferably generally consistent among retention caps regardless of the overall size of the given cap. For instance, relative inner and outer diameters of the cap  48  relative to the associated rotor core  36  have been discussed above. Furthermore, relative dimensions of components of the magnet retention element  50  are preferably chosen for stress reduction purposes as pertain to the cap  48  itself and for adequate force application as pertains to the fingers  78  relative to the magnets  44 . For instance, it will generally be preferred that the radial widths of the bases  86  of the fingers  78  are at least one and twenty-five hundredths (1.25) times the radial widths of the tips  88  thereof, more preferably at least one and five tenths (1.5) times the widths of the tips  88  thereof, and most preferably about two (2) times the widths of the tips  88  thereof. 
     Assembly of Rotor and Function of Magnet Retention Caps 
     In a preferred method of assembly, the rotor core  36  is initially hot dropped onto the shaft  38 . The magnets  44 , which have been pre-magnetized, are then pressed into the core  36  via an automated process. A first magnet retention cap  48  is positioned on a first axial end face  36   a  of the core  36  and secured thereto, and a second magnet retention cap  48  is positioned on a second axial end face  36   b  of the core  36  and secured thereto. The bearing assemblies  46  may then be added axially outwardly from respective ones of the magnet retention caps  48 . 
     In greater detail, after the magnets  44  have been inserted into the slots  42  of the rotor core  36 , the first cap  48  is positioned such that the alignment pins  64  extend into a corresponding first pair of the pole segment openings  66 , such that and the fastener bosses  70  and pin bosses  68  are in overlying engagement with (or nearly in overlying engagement with) the first axial end face  36   a  of the rotor core  36 . 
     The screws  74  are then inserted through respective ones of the fastener-receiving openings or apertures  72  defined by the fastener bosses  70  (that is, through the body  52  of the cap  48 ) and into a second pair of pole segment openings  66  that are offset from the first pair of pole-segment openings  66 . As noted previously, the screws  74  are preferably self-tapping, such that insertion of the screws  74  into the second pair of pole-segment openings  66  generates complementary threads in the core  36  and facilitates a robust connection between the cap  48  and the core  36 . 
     Furthermore, tightening of the screws  74  results in bearing down of the pin bosses  68  and fastener bosses  70  against the rotor core  36 , such that engagement of the bosses  68  and  70  with the rotor core  36  is ensured. 
     As shown in  FIGS.  5 - 8   , the bosses  68  and  70  function as standoffs, spacing the body  52  of the cap  48  from the rotor core  36  and the magnet  44 . The bosses  68  and  70  thus facilitate a reduced lamination count and ensure the cap  48  is tightened into engagement with the end face  36   a  of the core  36 . 
     It is noted that, if no bosses  68  and  70  were provided, the screws  74  might detrimentally cause delamination (or separation of the rotor core laminations) upon tightening. This might be manifested by peeling up of the endmost ones of the laminations. 
     As a result of the positioning of the bosses  68  and  70  securely against the first end face  36   a,  the main forms  82  of the fingers  78  are spaced from the corresponding ones of the magnets  44 , but the projections or nubs  84  of the fingers  78  either nearly engage or do engage the corresponding magnets  44 . More particularly, each magnet  44  presents axially opposed end faces  44   a  and  44   b.  The nubs  84  associated with the first retention cap  48  either nearly engage or engage the first magnet end faces  44   a.    
     If a given nub  84  engages the corresponding magnet  44  in such a manner as to result in deflection of the main form  82  of the associated finger  78 , the finger  78  will provide a spring force (alternatively, a resistive, restrictive, oppositional, or corrective force) against the magnet  44  that acts against an axially outward shifting of the magnet  44 . That is, the spring force resists shifting of the magnet  44  toward the associated finger  78 . 
     Whether or not a given nub  84  actually engages or only nearly engages the corresponding magnet depends on factors such as the actual axial heights of the rotor core  36  and magnets  44  (as opposed to the nominal or specified heights, thereof, for instance) and the positioning of the magnets  44  within the corresponding slots  42  (for instance, axially centered or shifted axially in one direction or the other). 
     Regardless of whether or not initial contact is made after assembly, the fingers  78  and the cap  48  more broadly are configured such that sufficient subsequent axial shifting or chatter of a given magnet  44  will ultimately result in engagement thereof by a corresponding nub  84  (if engagement has not initially occurred), with further shifting in the direction of the nub  84  resulting in commencement of or further deflection of the finger  78  and consequent application of or increases to a resistive force to the magnet  44 . In  FIG.  8   , for instance, a pair of fingers  78  are illustrated in a deflected configuration, with the nubs  84  thereof in contact with respective end faces  44   a  and  44   b  of a given magnet  44 . 
     In a preferred embodiment, retentive forces ranging from three (3) lb to five and one half (5.5) lb may be provided by each finger  78 . However, nominal values of the retentive forces may vary without departing from the scope of some aspects of the present invention. 
     As noted previously, each retention cap  48  presents a contoured shape as a result of the circumferentially varying height of each of the inner and outer walls  54  and  56 . As also noted previously, the greatest heights of the walls  54  and  56  are adjacent the fasteners  74 . Such configuration helps minimize deflection of the body  52  and walls  54  and  56  of the cap  48  in order to maximize the force applied by the fingers  78  of the magnet retention element  50 . 
     More particularly, contouring of each cap  48  as described above helps transfer loads toward the fasteners or screws  74 , minimizing deflection of the remainder of the cap  48  and in turn maximizing the ability of the magnet retention element  50  and, more particularly the fingers  78  thereof, to apply retentive forces to the associated magnets  44 . That is, deformation of a given finger  78  through contact with a magnet  44  results in a force being applied by the magnet  44  to the finger  78  and vice versa. The force applied to the finger  78  is transferred to the retention cap  48  in a broad sense, urging it deform to some extent. By merit of the contoured arch shape of the walls  54  and  56  described above, however, any such force is transferred from thinner (shorter) areas of the cap  48  and, particularly, of the walls  54  and  56 , toward thicker (taller) areas adjacent the screws  74 . That is, loads received by the retention cap  48  are, by merit of the contoured shape thereof, transferred to portions of the cap  48  best able to absorb such loads with minimal deflection. As will be readily understood by those of ordinary skill in the art, if the cap were not contoured, forces would not be directed optimally and deflection of the body of the cap would potentially increase. The resistive forces of the spring fingers as applied to the magnets would therefore detrimentally decrease. 
     It is noted that the design of the cap  48 , including the contouring thereof, also enables the number of fasteners  74  to be reduced to exactly two (2). Absent contouring and the associated transfer of forces toward the existing fasteners, additional fasters might be needed to restrict undesirable deflection. 
     The second cap  48  is preferably installed such that it engages the second end face  36   b  of the core  36  and potentially the second end faces  44   b  of the magnets  44  in a similar manner to that described above for the first cap  48 . 
     Preferably, the second cap  48  is installed so as to align arcuately with the first cap  48 , as visually aided by the bars  60  and notches  62  and as physically aided by receipt of the alignment pins  64  in openings  66 . Correct alignment is advantageous for motor balancing, although misaligned rotors fall within the scope of some aspects of the present invention. 
     In a preferred method of assembly, the caps  48  are installed fully automatically, with machine vision using the various visual alignment elements  58  to confirm correct rotational alignment or clocking of the first and second caps  48  relative to one another. Fully manual, fully automated, and/or other combined manual/automated assembly methods all fall within the scope of the present invention, however. 
     Discussion of Selected Advantages 
     The present magnet retention cap design and rotor assembly method present numerous advantages, including but not limited to those already elucidated above. 
     For instance, in contrast to an overmolded design in which magnetization occurs after assembly, the present invention facilitates pre-magnetization of the magnets  44 . 
     Furthermore, contact with the magnets  44  via the projections or nubs  84  of the fingers  78 , rather than by the main forms  82  thereof, enables continuous contact to be achieved with less risk of associated forces causing bending of the entire magnet retention cap  48 . For instance, provision of axially thicker fingers or main bodies that are themselves in direct contact with the magnets might result in bending of the magnet retention cap in a general sense. 
     In addition, the contoured shape of the cap  48  helps reduce part deflection once installed and facilitates relatively stable retention forces to be applied by various ones of the fingers  78 . 
     The contoured shape also enables fixation by means of only two (2) diametrically opposed fasteners  74 . 
     Still further, the tapered shaping of the fingers  78  is such that maximum stresses at maximum deflection are concentrated at the respective bases  86  of the fingers  78 . 
     The retention caps  48  used on both ends of the rotor core  36  are also identical, reducing part count and eliminating potential errors that might otherwise be associated with asymmetrical assembly requirements. 
     Still further, the inventive retention caps  48  may be used with rotors of varying stack heights and magnet lengths. 
     Provision of a flexible magnet retention element  50  also enables accommodation of variations in rotor length, even within a given rotor, resulting from tolerance stack-up issues and so on. That is, the present end cap  48  is designed to readily facilitate minimum and maximum manufactured dimensions and is generally not affected by so-called “fluffy” or “spongy” cores in which laminations are spaced apart somewhat but interlocked. 
     As noted previously, the illustrated caps  48  work with a variety of rotor core inner and outer diameters and can be easily redesigned to accommodate other inner and outer diameters as necessary. 
     The caps  48  may also be used with both fully and semi-processed rotor lam steel and are unaffected by different lam thicknesses. Solid cores are also broadly compatible with the caps  48 . 
     The magnet retention caps  48  also provide secondary retention of rotor core laminations, should delamination occur. 
     The caps  48  also provide aesthetically pleasing and effective physical barrier protection to protruded magnets  44  against metal particles or debris, are cost effective, and are lightweight so as to minimize additional loads on the bearing assemblies  46 . 
     The light weight of the caps  48  also ensures that any contribution to unbalance issues that might occur, particularly in high-speed applications, is minimal. 
     The caps  48  may be easily and cost-effectively manufactured via a molding process, eliminating costly machining. 
     Finally, provision of readily removable, re-installable, and replaceable end caps  48  facilitates rework of existing rotors already fitted with the caps  48  but in need of modification, as well as retrofitting of existing rotors not yet featuring the caps  48 . 
     Conclusion 
     The preferred forms of the invention described above are to be used as illustration only and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
     Although the above description presents features of preferred embodiments of the present invention, other preferred embodiments may also be created in keeping with the principles of the invention. Furthermore, as noted previously, these other preferred embodiments may in some instances be realized through a combination of features compatible for use together despite having been presented independently as part of separate embodiments in the above description. 
     The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and access the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention set forth in the following claims.