CA1333083C

CA1333083C - Method and apparatus for making a permanent magnet rotor, and a rotor made thereby

Info

Publication number: CA1333083C
Application number: CA000614084A
Authority: CA
Inventors: William Howard Stark
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1988-06-08
Filing date: 1989-09-28
Publication date: 1994-11-15
Anticipated expiration: 2011-11-15
Also published as: FR2632788B1; FR2632788A1

Abstract

A permanent magnet rotor for an electronically commutated motor (ECM) has a core, a plurality of magnetizable elements spaced around the core, and a thin-walled retaining shell which has been stretched around the core and magnetizable elements to hold the elements in position. The rotor is made by an inventive method which involves axially aligning the core and surrounding magnetizable elements with the retaining shell, and cold-pressing the retaining shell over the core and elements to sandwich the elements between the shell and core. The core and magnetizable elements serve as a mandrel about which the shell is reformed in a cold working operation. Other aspects of the invention include the fixturing apparatus used to align the core and magnetizable elements with the retaining shell, and apparatus which is used to evenly space the magnetizable elements around the core and hold the elements in position during at least a portion of the cold-pressing operation. Additional features of certain embodiments of the rotor of the present invention include end plates axially disposed adjacent the ends of the rotor core and magnetizable elements, and the use of adhesives between the core and magnetizable elements and/or the end of the core and magnetizable elements and the end plates.

Description

METHOD AND APPARATUS FOR MAKING A PERMANENT
MAGNET ROTOR. AND A ROTOR MADE THEREBY

FIELD OF THE lNv~NllON
This invention relates generally to permanent magnet rotors and to methods and apparatus for making permanent magnet rotors for electric motors, and more specifically to methods and apparatus for making rotors for electronically commutated motors (ECMs), some of which may be operable at relatively high rotor speeds.
BACKGROUND AND SUMMARY OF THE lNv~NllON
Permanent magnet rotors for ECM motors typically comprise a solid or laminated iron core, surrounded by a plurality of magnetic elements. The materials typically used to form the magnetic elements include barium or strontium ferrite (traditional ferrite magnets) and samarium cobalt (rare-earth magnets). The former are often referred to as ceramic magnets since they are manufactured by a technique which includes pressing the individual magnet elements into the desired shape and "firing" or heating the elements in a kiln until cured. Permanent magnet rotors of this type are used in motors and applications, and in conjunction with control circuitry and other associated apparatus, of the types described in the following U.S. patents, all of which are commonly assigned to the assignee of the present application: Eldon R. Cllnn;ngham, U.S. Patent No. 4,456,845, issued June 26, 1984; Charles W. Otto, U.S. Patent No. 4,466,165, issued August 21, 1984; Doran D. Hershberger, U.S. Patent No. 4,476,736, issued October 16, 1984; David M. Erdman, U.S. Patent No.
4,654,566, issued March 31, 1987; and William R. Archer, U.S. Patent No. 4,686,436, issued August 11, 1987.
Rotors produced in accordance with the present invention can be used to particular advantage in hermetically sealed refrigeration compressor applications, where exposure to refrigeration and/or lubricating fluids is likely to occur. Such applications may also require relatively high efficiency ratings, which may be achieved by rotors constructed in accordance with the present invention.
Due to the relatively high mass of the materials used to form the magnetic elements disclosed above, and the relatively high (1,000-16,000 RPM) rotor speeds developed in these motors, retention of the magnetic elements in position adjacent the rotor core is a serious problem. A number of methods and tPchniques for retaining magnetic elements on the rotor core have previously been used or considered. One such method involves positioning the magnetic elements around the outer diameter of the core, and applying a wrap of KevlarTM or fiberglass to hold the magnets in place.
The KevlarTH or fiberglass used is typically a fine stranded material which is pulled through epoxy prior to wrapping. An adhesive filler may be applied to the wrapped rotor to fill voids and provide a more rigid construction. However, the ends of the stranded material are difficult to attach to the rotor core, and the nature of the materials involved may create problems in the working environment. Additionally, use of this method is relatively expensive and time consuming, and uniformity and tolerances of the finished product can be difficult to control.
Another method of retAi n; ng the magnets on the core involves wrapping a relatively fine wire, under tension, around the magnetic elements, followed by application of an adhesive or epoxy overcoat to the assembly to protect the metal wire. As with the method discussed above, proper attachment of the wire ends to the core is difficult and relatively expensive to achieve. This technique is also time consuming and labor intensive, and involves a number of distinct operations which may be difficult and costly to automate for the production setting.
Another te~hnique for retAining magnets around a rotor core involves the use of a cylindrically shAre~
shell or "can" which is disposed around the outer peripheral surface of the magnet/core sub-assembly to hold the magnets in position. The outer shell is typically formed of a non-magnetic steel such as INCO-718TM (InconelTM) or beryllium-copper. At least three tech~iques for assembling a assembling a permanent magnet rotor which uses such a retaining shell have been previously developed. One of these techniques is understood to have originated with Hitachi of Japan and is believed to involve placing a core and magnetic elements in a shell, die casting molten aluminum into the shell to lock the magnetic elements and core in place, and forming end rings at the ends of the rotor so as to completely cover the magnetic elements. The shell of these rotors appear to be formed from a 300 series stainless-steel, having a relatively thick-walled construction which had been machined after die-casting to a final wall thickness which varied between 0.015"
and 0.025", depending on point of measurement.
Another technique for assembling an outer 1 3330~3 retaining shell onto a core/magnet sub-assembly is described in U.S. Patent No. 4,549,341, issued October 29, 1985 to Kasabian and U.S. Patent No.

4,625,135, issued November 25, 1986 to Kasabian. These patents describe a rotor which is formed from a shaft (10) which is turned to provide a central area (20) having a relatively larger outer diameter, as measured relative to the axially adjacent end areas (22 and 24).
The larger diameter portion of the shaft then has a number of flat faces (30) machined uniformly around the shaft, the number of faces being equal to the number of magnets to be installed around the periphery of the shaft. A plurality of steel blocks (50) which are identical in size and shape to the permanent magnets to be installed around the shaft are then mounted onto the machined faces of the shaft, and a layer of non-magnetic material (60) (e.g., aluminum) is cast around the large diameter portion of the shaft and attached blocks. The outer diameter of the casting is slightly larger than the outer diameter of the shaft and blocks, as measured across the outer surfaces of opposing blocks. The casing is then machined to a diameter which is slightly larger than the desired finished diameter, and which is equal to the diameter of the shaft as measured across the blocks. The side areas (62) of the casting facing the respective ends of the shaft are also machined flat.
The blocks are then removed and replaced with permanent magnets (80), which are typically rare-earth permanent magnets formed of samarium cobalt or al-nickel. The magnets are rectangular in shape and have flat faces which fit against the flat faces machined onto the larger diameter portion of the shaft. The magnets fit exactly into the apertures (70) left in the casting by the steel blocks. The magnets may be retained in the aperture by magnetic attraction 1 3330~3 to-the-shaft or, alternat~vely, by~-an- adhesive material placed in the aperture before installation of the-magnets. After the magnets are installed, the shaft iæ turned to machine the flat--outer surfaces of the magnets to a round shape, and the casting is marhine~ to the desired finished diameter.
The final step in assemblinq the above-described rotor is installation of the outer shell around the casting and magnets. As noted, the outer shell ~90) is typically a-non-magnetic steel such as INCO-718. Shell thicknesses are described as being kept to a minimum to keep the distance be~ e.. the magnets and a motor stator as small as possible.
Typical shell thicknesses specified vary from 0.04~ to 0.28~, dep~n~ing upon the diameter of the rotor assembly, the operating speed of the rotor, and the related centrifugal force. The method described for installing the shell is to put the rotor assembly (100) in dry ice while the shell is heated, and then immediately install the heated shell over the chilled rotor (i.e., over the casting and magnets). The shell is then cooled by directing water onto the shell surface. The e~pansion and subsequent shrinkage of the shell provides for a high interference fit bet~r~en the shell and the underlying casting and permanent magnets. Other patents which disclose the use of a heat-shrink technigue for installing a retaining shell o~er a rotor core and surro~n~ing magnets include U.S.
Patents 3,531,670 and 3,909,677 which are assigned to the Ben~i~ Corporation, and U.S. Patents 4,242,610; 4,332,079;
4,339,874; 4,445,062; and a number of related patents assigned to the Garrett Corporation.
U.S. Patent 4,617,726 (also assigned to the Garrett Corporation) discloses an alternative technique for installing 1 333~83 an~ outer- shell- (110) over a--.rotor assembly (50). Th~s technigue involves- the- use of-~a housing ~120) which opens at-:
one end to. receive the--shell. which-- has a slightly larger diameter at- its ends than in :its central or--:intermediate portion. The shel~ is positioned within the housing adjacent-an annular recessed area (13D) which is vented. The interior o the housing.is_ then supplied with pressurized hydraulic fluid. to. cause the shell to ~pan~ outwardly, slightly increasing its inner diameter. The rotor assembly is placed on lQ a.hydraulic ram (140) which e~Lends through the housing and which has an integral plate ~142) which secures the rotor assembly to the ram and causes the rotor assembly to be forced into the espanded shell when the ram is shifted in the pressurized housing. The initial diameter of the shell, the amount by which that diameter is e~panded, and the overall outer diameter of the rotor assembly must be carefully controlled to assure a uniform fit beL./een the rotor assembly and surrounding shell.
Each of these techniques for installing a retaining shell over a core and surroun~i~q magnetic elements has limitations and disadvantages. ~he first involves separate casting and machining operations which are costly, time consuming, and potentially in~urious to the magnetic elements. It- is also believed that this method initially requires a shell having a relatively large wall thickness due, at-least in part, to the c~n~itions attendant the die-casting operation. A marhin;ng operation is believed to be necessary to reduce the wall thickness of the shell on the finished rotor to avoid relatively large losses in efficiency.

The second (heat-shrink) technique is also potent$ally in~urious to the magnetic elements since the heat to which the elements are e-~os~d by virtue of their contact with the heated shell can cause the magnetic materials to crack and chip.
Additionally, the r L of e~r~nsion which can be achieved b~
heating the shell is limited. For esample, a shell for- a three-inch diameter rotor will espand approsimately 0.017~ when heated. Accordingly, the machinin~ step required by Xasabian tor similarly effective measures) is necessary to ensure that a lQ high interference fit will be-achieved when a heated outer shell is cooled after installation on a rotor assembly.
Machininq operations of the type described by Kasabian are-generally not practical when ferrite or other ceramic magnetic elements are used, since these materials are very espensive to Irchi~e and cut. Moreover, the materials and techniques used in the manufacture of ceramic magnets lead to relatively wide variations in dimensional tolerances. For esample, ferrite magnetic elements of the size which might typically be used with a t~ree-inch diameter rotor may vary by 0.020~ in thickness. This means that the overall diameter of the core and surrounding magnetic elements may vary by up to 0.040~.
Thus, this type of element cannot be routinely used with the heat-shrink assembly method discussed above due to the inherent dimensional limitations of that method. Althouqh the hydraulic e~pansion technique avoids the possible damage to the magnets inherent in the hea~-shrink method, this technique does not address the problems associated with dimensional variations in the magnetic elements and core.
- An object of the present invention is to provide a method and apparatus for making a permanent magnet rotor which offers 1 333~8~
03~006178 optimum magnetic element Le~ehLion, low manuacturing cost, and ea~e of-man~ ture.
Another ob~ect- o~- the present invention is- to provide a method and apparatus for making a permanent magnet rotor which is_-highly tolerant of ~loose~ tole~ances, and which assures a high-interference tit b~t~ n a - plurality of~ magnetizable el~ ~ ~s and a retaining shell in-spite of relatively wide . variations in such tolerances.
Yet- another object of the present in~ention is to provide a rotor structure which i5 estremely durable, easy to manufacture and relatively ine~pensive.
A further ob~ect of the present in~ention is. to provide a precise fisturing apparatus which holds the rotor core and sur~ou~in~ magnetic elements square and true relative to the can or retaining shell, and which permits uniform pressure to be applied to a th$n edge of the shell to allow the shell to be plastically and elastically deformed as it-is cold-pressed over the core and magnetic elements.
These and other objects of the present invention may be 2a achieved in one method of making a permanent magnet rotor of the type depicted in the drawings which includes the steps of placing a plurality of magnetizable elements around a core and temporarily holding the elements in position, a~ially aligning the- core and surro~ ing magnetizable elements with the retaining shell, and.cold-pressing the retaining shell over the core and su~.ol~ding rl~5 etizable elements to permanentl retain the magnetizable elements in position around the core.
It- should be noted at- this point that the term ~magnetizable elements~ as used in this applicatio~ refers to elements which are, or~ which may be, -gnetized, and is specifically inten~ed to- in~luAe elements- such as magnetic-elements 2~: referrea to below, and functional equivalents thereof, whether or not such elements are in- a ~m~l.Ftized~
condition. It is specifically contemplated that, under certain conditions, it may be desirable to assemble rotors according--to the present invention using magnetizable elements which are in a ~magnetized~ condition prior to or at the time of assembly.
On-the other hand, it-may also be advantageous to assemble a rotor using ~J.a~izable elements wh~ch have not been magnetized, and subsequently magnetizing the elements prior-to-actual usage of the rotor.
In one embodiment of the invention, the permanent magnet rotor includes a layer of adhesive bet ~en the core and surroun~inq magnetizable elements, and one method includes the additional step of applying the layer of adhesive prior to placing the magnetizable elements around the core. In another embodiment of the invention, the rotor additionally includes at least one end plate which is positioned ad~acent at least one end of the core and su~ro ~di~g magnetizable elements. A layer of adhesive may be disposed between the end plate and the end of the core and surroun~ing magnetizable elements.
In yet another preferred embodiment of the invention, the step of placing the magnetizable elements around the core includes the additional step of positioning the magnetizable element so as to assure substantially equal radial spacing bet~En adjacent elements, and the step of temporarily holding the magnetizable elements in position around the core includes clamping each element-in position against the core. The step of cold-pressing the retaining shell over the core and surrounding magnetizable elements further includes applying a _ g _ substa-~tially uniformly ~jstribt~te~ -force to- a substantially conti~Jous edge surface--of=the~-retri~i~q-shell to plastically and elastically deform the shell to -fit over the magneti~able elements.
The method of the present invention is preferably performed-in-a press which has a working asis (i.e., an^asis along which the force generated by the press acts) in which the rotor core is-~moYnte~ such that a -central asis of the core is aligned substantially parallel to the working asis of- the press. The core is mounted on a bolt which has a first end, a second end, and a co~ec-ing shank. The first end of the bolt is disposed adjacent a first-- end of the core and is matingly received within a fisture of the press. The shank of the bolt is disposed within a central bore of the core, and the second end of the bolt is disposed adjacent a second end of the core. The apparatus further includes a guide means which is mounted on-the second end of the bolt and which is designed to receive the retaining shell and align the shell with the bolt, the core, and- a plurality of: sur-o ~Aing magnetizable elements. The-apparatus additionally comprises a fisture which is attached to a wor~ing element of-the press and which has an internal bore having a diameter substantially equal to the inner diameter of the retaining shell. The fisture has a sharp edge surface immediately adjacent the bore for contacting the continuous edge surface of the retaining shell and for applying a substantially uniorm force to the edqe surface of the shell.
An-upper edge of the guide means is provided with a chamfer to facilitate passage of-the sharp edge of the fisture over the body of the guide means.

1 3~3083 An--additional asE~ of~ one embodiment- of-- the p-~e ~
invention lies in-the apparatus for positioning the magnetic elements in spaced relation around the core. This apparatus comprises a plurality of gauges mounted to a fisture which is positioned in the press ad;acent and around the core and sul~s~ ing magnetic elements_ The gauges are radially movable inwardly and outwardly toward and away from the core and-elements. A mechanism is provided for moving the gauges into--position beL en adjacent magnetic elements to adjust and gauge the spacing be~._ell the elements. In an especially preferred embodiment, the gauges have wedge-shaped tips for displacing and adjusting adjacent elements, and are moved simult~neo~sly by an adjusting device. The adjusting device includes a ring which is provided with a plurality of generally circumferential slots. The opposing sides of- these slots act as camming surfaces which interact with a roller mounted to each gauge, such that when the ring is rotated, the roller and camming surface interact to move the gauge inwardly or outwardly in accordance with the direction of rotation of the ring and the contours of the camming surfaces.
A further aspect of the present invention relates to the means for temporarily securing the magnetic elements in spaced relation around the core. This apparatus preferably includes a plurality of clamps, each of which is disposed adjacent a respective one of the magnetic elements. Each clamp comprises an end portion for contacting the adjacent magnetic element, means for moving the end portion into contact with the magnetic element, and means for releasably locking the end portion in the engaged position. The end portion of each clamp preferably includes an elastic element (such as sponge rubber) for compressibly engaging the ad~acent magnetic elemsnt when the end portion is moved into the engaged position.
Yet another aspect- of= the apparatus of- the present invention comprises the use of: a plurality of dies for 5incrementally and plastically deforming the edge of the shell inwardly toward the central asis of the shell after the shell has been positioned over the core and- surrolj~Aing magnetic elements. In one embodiment of--the crimping apparatus, a first die deforms the edge of the shell through an angle of 10 approsimately 30-, a second die further deforms the edge through an angle of approsimately 60, and a third die deforms the edqe through an angle of appro~imately 90. The third die is preferably provided with a Teflon insert which surrounds an end of the metal shell during the crimping operation.
Yet another aspect of the present invention relates to the rotor which is pro~t~ce~ by the method and apparatus described above and below. Use of this method and apparatus allows for construction of a rotor which includes a retaining shell positioned around the magnetizable elements, which is highly tolerant of dimensional variations in the components, and yet which assures a uniform fit of the shell over the core and magnetizable elements. In the prior heat shrink and hydraulic espansion techniques discusse~ above, the dimensional variations in core diameter and magnetic element thickness must be controlled in a relatively e~act manner to assure a uniform, high interference fit b th_en the components of the rotor.
However, in the present invention, the retaining shell is deformed around the core and su~o~ g magnetizable elements which act as a mandrel to deform the shell to the estent necessary to assure a uniform fit. The initial diameter of the shell is slightly smaller than the smallest espected orerall diameter of the core and su.rou.ding magnetizable elements.
The amount by which the inner diameter of the shell can be-deformed is limited by- the elongation characteristic of the material. For the stai~l~ss-steel material which is preferably used, the elongation characteristic and associated ~upper~
limit of deformation far e~ee~ the variation in tolerances which would be- espected to- occur in the rotor core and-surrou~ding magnetizable eleménts. This allows for the use of relatively ine~pen~ive ceramic magnetizable elements which, in an element having a thickness ranging from 0.4~-0.5~ may have a tolerance of 0.020~ or more (i.e., 5% or greater). The resulting variations in overall diameter of the core and surrounding magnetizable elements cannot be tolerated by the heat-shrink and hydraulic e~pansion methods described above.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

2 0 R~T~ DF~TPTIO~ OF TFF D~WINGS
Figure 1 shows a perspective view of a permanent magnet rotor embodying the present invention in one form thereof, produced by and in accordance with apparatus and methods that further ~ aspects of the present invention in other forms 2 5 thereof.
Figure 2 shows an esploded view of the rotor of Figure 1.
Figure 3 shows a partial sectional view of the rotor of Figure 1.
Figure 4 shows an esploded view of part of the apparatus used to assemble the rotor of Figure 1.

F~gure S sb . D-` a part~al side view-of the apparatus used.to as~emble the.rotor-of~-Figure l ~
Ffgures...6(a), 6(b) and~ 6(c~ illustrate-- the---operation--of .
cold-pressing a shell over a subassembly of.rotor components i~
accordance with a method and apparatus embodying one form--of the present in~ention Figure 7 shows a detailed..view of a portion of Figure 6(b).
Figures 8(a)-8(f) illustrate a crimping operation of-..the-shell and rotor shown in Figures 6(a), 6(b), and 6(c).
Figures 9(a) and 9(b) show top and side views, respectively, of_ preferred embodiments of spacing and clamping devices used to.practice the in~ention in one form thereof.
F~gure 10 shows the apparatus of Figure 9~a) with the spacing device in a second operative position.
Figures ll(a) and ll(b) show top and side views-, respectively, of the apparatus of Figures 9(a) and 9(b) with both spacing and clamping devices in second operative positions.

nFTATr.~n DF~r~IpTI~N OF TPF D~WI~GS
Figure 1 shows a pe~s~ecLive view of a pe -n~nt magnet rotor 10 produced by and in accordance with the apparatus and methods r ~ ing one form of the present invention. Rotor 10 comprises (in addition to component parts which are not visible in Figure 1) a rotor core 12, end plate 14-and an-outer shell 16. These and. other components will be.-discussed in more detail below in cc,-~e~ion with the other drawing fi~ure. For purposes of-simplicity and clarity, like reference numerals are used for like components in the discussions which follow.
Figure 2 shows an e~ploded view o permanent magnet rotor 10. Core 12 of rotor 10 is shown as. a solid, cylindricallY

shaped core having a longitvAinally e~Le~ding bore 18 e~t~nding along a central a~is 20 thereof. Although core 12 is described and depicted in the present application as a solid metal core, a laminated structure may also be used and may be preferred in particular applications.
Immediately adjacent core 12 are magnetic elements 22 which, in the embodiment illustrated, are similarly shaped arcuate ceramic magnets. As noted above, magnetic elements 22 may also be characterized as magnetizable elements since, at the time o assembly into the rotor, these elements may or may not be in a magnetized condition. The radius of curvature of an inner surface 24 of each element is approsimately identical to the radius of curvature of outer surface 26 of the core, so that each of the magnetic elements 22 fits against core 12 when rotor 10 is fully assembled. It should be noted that the ceramic magnetic elements illustrated are non-precise parts in the sense that the tolerances associated with the dimensions of these elements are subject to considerable variation. For e~ample, a magnetic element which has been used to construct a rotor according to the present invention has a nominal wall thickness of 0.420~, a tolerance on the inside radius of +O.OOS~, and a tolerance on the outside radius of +0.005~
(i.e., an overall tolerance of 0.020~). This is due in large part to the manner of fabricating these elements, and the difficulty of machining, grindi~g~ or otherwise finishing the parts to maintain tighter tolerances. At least one end of magnetic elements 22 is provided with a chamfer 28 on an outer edge thereof to aid in the assembly process, as will be discussed in more detail below.

03LO061~8 Also shown in~Figure 2 are top and..bottom-end_plates 14 and 30~ In. the- embodiment- illustrated and: d~sc~ssed in th~s application, end plates 14 and 30 are aluminum rings or washers having an-outer radius 32 and an inner radius 34. For rea~on~
which will become apparent from the discussion which follows, outer- radius 32 is. preferably equal to or--slightly less than-the overall combined minimum radius of: core 12 and magnetic elements 22~:(as measured in the ass d led condition). Inner.
radius 34 is preferably larger than the radius of bore 18 in core 12,.: and is preferably smaller-than the radius of curvature of-.surface 24 of magnetic element 22, as measured from central asis 20 when rotor 10 is fully assembled. One function of-end plates 14 and 30 is to prevent pieces of the ceramic magnet material from contaminating the interior of a motor if ceramic magnetic elements 22 should crack during usage. End rings 14 and 30 also serve to maintain a cracked magnetic element in operational position and, in the embodiment illustrated, serve to prevent asial ~v~ - ts of magnetic elements 22 relative to core 12. ~ ver, it.-should be noted that, although these factors are important features of certain motor designs, end plates may be omitted in other motors constructed by and in accordance with the apparatus and method of the present invention.
The remaining component of rotor 10, as.illustrated in Figure 2, is shell 16. Shell 16-is preferably made from--a non-magnetic metal, such as 304 stairles~ steel tubing. The wall thic~ness of shell 16 is preferably less than 0.020~, with satisfactory results having been obtained with wall thic~nesses of 0.008~ and 0.012~. It is believed that wall thicknesses of 0.006~ and, quite possibly, 0.004~ can be.used. In general, a 1 333~

decrease in wall thic~ness rest~lts in a decrease in losses due to eddy currents flowinq in the shell and the ability to decrease the size of the air gap betuee- the rotor and stator when a thin-walled shell is used. u~ er, the particular material used for forming the shell and the retention strenqth desired are other factors which must be considered. A-successful design for a particular application is one in which trade-offs- beL~een tolerable magnetic losses, aesired magnet retention strength, the cost, availability, and workability of various materials and dimensional configurations, and other factors are balanced to achieve an optimal design for the particular application.
The initial ~i.e., prior to assembly) diameter 36 of shell 16 is slightly ~ess than the minimum combined diameter of core 12 and magnetic elements 22 when these latter components are in their respective assem~led positions. Shell 16 is provided with a continuous edge surface 38 which faces asially away from core 12 in the unassembled condition, but which may be-subsequently crimped inwardly toward central asis 20 of core 12 and shell 16 after final assembly, as discussed in connection with Figures 8(a)-8(f) discussed below. While crimping of the edge of shell 16 is generally desired in the embodiment of the invention illustrated in the drawinqs, it-should be noted that this feature may not be desirable in other applications and may be omitted therefrom. As~ will be-disc~ssed in more detail below, assembly of core 12, magnetic elements 22~and shell 16 i~volves plastically deforming shell 16 over core 12 and magnetic elements 22 to retain magnetic elements 22 in position ad~acent core 12.

F~yure 3 shows-a partial cross-æectional view of-rotor lQ
in-the-fully assembled condition. As-^can be-seen in-~Figure -3, the ~e~Qec~ive-lengths of- maqnetic-elements 22_are less than the-length of core 12, resulting in the formation of--a gap 40 at:one end of-the assembly. Gap 40 will vary by-the amount of--the combined tolerances in- length of core 12 and ~-.etic elements 22-. If- a solid core construction is used, the- -tolerances on- core 12_ are relatively easy to cont-ul.
Tolerances of magnetic elements 22, ~ er, are not easy to control and the length of these elements may vary by up to 1~8~-for elements that are 4.0~-6.0~ in length. In general, it is desired that end plate 14 rests on the relatively smooth end of-core 12, rather than the- relatively rough and non-precise end surfaces of magnetic elements 22. Accordingly, core 12: is typically dimensioned so that the overall length of the core is always greater than or equal to the overall length of magnetic elements 22 when the respec~ive tolerances reach their estremes, resulting in a gap 40 which will vary from 0.0~ 8~
or more. It should also be noted that, while core tolerances are relatively easy to maintain when a solid core construction is used, the same is not necessarily true with a laminated core structure in- which greater variations in tolerances will normally occur.
~ - Gap 40 is preferably filled with adhesive 42, such as EPON
828 eposy, prior to final assembly. Other adhesives may be used provided they are suitable for the environment in which the rotor- will be used. For esample, for rotors used in refrigeration applications, the adhesive selected should be compatible with the fluids to which the rotor will be esposed.
Adhesive may also be provided at--44 bet een core 12~ and magnetic. elements 22,- and at~--46-be~ e~~ end plate 30 and:
adjacent-ends of core 12 and magnetic elements 22.
Figure 4 shows an esploded.view of part-- of. the. apparatus.
used to assemble the. rotor of.-F~gures 1-3. The apparatus of Figure 4 includes a top fi~ture 48 which is adapted for attachment to a press or like-de~ice (not shown) for eserting a linear force F upon the components of rotor 10. Fisture 48 has an-inner bore 50 which has a diameter which is substantially equal to initial inner diameter 36 of shell 16. An edge portion 52 of surface 54 immediately ad~acent bore 50 i5 specifically adapted for contacting edge 38 of shell 16 so that force F may be uniformly applied to shell 16 at edge 38.
Also shown in Figure 4 is lower fisture 56 which is adapted to be mounted, in colinear relation with fi~ture 48, to the press or like device. Also shown is a bolt assembly 58 which is used for supporting core 12 and end plates 14 and 30 during-the assembly process. Bolt assembly 58 includes a first end or-head 60 which is provided with wrench flats 62 and which has a portion 64 which is adapted to be matingly received within a portion 66 of lower fisture 56. Lower fisture 56 is further provided with an alignment pin 68 which is matingly received within hole 70 in first end 60 of bolt assembly 58. First end 60 is also provided with an alignment pin 72 which mates with a hole 74 (Figure 3) in the end of rotor 12. First end 60 is further provided with horizontal surface 76 and vertical surface 78 which toqether provide for asial and lateral support of end plate 30. First end 60 is further provided with an elongated portion 80 which is adapted to be received within bore 18 of core 12.

~ 333~3 Bolt. assembly 58 further comprises shank 82 which may be~a th~ea~e~ or.-partially threaded rod.. Shank 82 e~ends through core-12 and end plates 14-and 30, and through a sncol~ end 84 of bolt assembly 58. Sscc end 84 has a portion 86 which is adapted to. be- received within bore 18 of:-. core 12, and horizontal surface 88 and. ve~tical surface 90 which matingly contact top and interior surfaces 92-:and. 94, res~ecLively, of end plate 14. Seco~ end 84 is secured to shank 82 by means of washer 96 and nut 98.
Also shown in Figure 4 is shell guide 100. Guide 100 is:a generally cylindr~cal device having an outer diameter 102 which is slightly less than initial inner diameter 36 of shell 16 50 that shell 16 will fit over guide 100 in a snug, but-non-interfering, manner. Guide 100 has an inner diameter 104 which is substantially equal to or- slightly greater than the diameter of portion 106 of second end 84 of bolt assembly 58.
When portion 106 of second end 84 is positioned within guide 100, as-illustrated in Figures 5 and 6(a)-6~c) below, and shell 16 is positioned on guide 100, precise alignment of.the central asis of shell 16 with the central a~is of core 12 is provided.
Guide 100 is further provided with a slight chamfer 108 on its top end to facilitate placement of shell 16 thereon, and to facilitate entry of guide 100 into bore 50 of fi~ture 48.
Figure 5 shows the apparatus of Figure 4 with bolt assembly 58 in- the fully assembled condition, with guide means 100 positioned on portion 106 of-second end 84 of bolt assemblY 58, and with retaining shell 16 disposed on guide 100. Also shown in Figure 5 is a portion of magnetic element spacing device 110, and a portion of magnetic element clamping device 112.
Spacing device 110 is used to simultaneously position and gauge the spacing be~rae.. magnetic elements 22:so-that the elements-are evenly distributed around core 12. CIamping de~ice ll~ is used to- temporarily clam~- magnetic elements 2Z~ in-- position after spacing by-spacing device 110, and prior to the pressing of shell 16 over elements 22. The structure and operation of spacing-device 110 and clamping device 112 are discussed fully below in- con~ec~ion with Figures 9(a), 9(b), 10, ll~a) and ll(b).
Figures 6(a), 6(b) and 6(c) illustrate the operation of cold-pressing shell 16 over the sub-assembly comprising core 12, surrG~ ing magnetic elements 22 and end plates 14 and 30.
For purposes of simplification and clarity, spacing device 110 and clamping device 112 are omitted from Figures 6(a), 6(b) and 6(c). In addition, many of the reference numerals previously used in the above discussion are also omitted in these figures.
Figure 6(a) shows core 12, end plates 14 and 30, and magnetic elements 22 mounted on bolt assembly 58, and bolt assembly 58 positioned in bottom fisture 56. Guide 100 is positioned on portion 106 of second end 84 of bolt assembly 58, and shell 16 is mounted on guide 100. Upper fisture 48 is shown immediately after application of force F, and prior to contact between surface portion 52 and edge 38 of shell 16. It--should be noted that edge portion 52 of fisture 48 is specifically designed to be sharp and well-defined to assure masimum contact with edge 38 of shell 16. Also, the contour of edge portion 52 is designed to match as closely as possible the contour of edge 38. In the preferred embodiment illustrated, the contours of both of these surfaces are essentially flat and lie within planes which are substantially perpendicular to central asis 114, which is coasially aligned with central asis 20 of core 12 (Figure 2).

Figure-6(b) show~ the~relative positions of-the sub~ect.
components- approsimately midway through the - cold-pressing operation in which shelL 16.is_plastically deformed over core 12~and suL~o~-~Ai~g magnetic elements 22. It-s~o~ be-noted at this point- that: chamfer 28 on the top edqes of magnetic elements 22_~Fiqure 2) is..int~n~e~ to facilitate the process.of plastically deforminq shelL 16 over elements 22. As: an.
alternative, the- lower portion of shell 16 (i.e., the portion.
which first contacts magnetic elements 22) may be flared slightly to facilitate initial fitting- of shell 16 around the-combined diameter of core 12 and magnetic elements 22.
F~gure 7 shows an enlarged view of area A in which the plastic deformation of shell 16 is most apparent. As can be seen in Figure 7, core 12 and magnetic elements 22 act as a mandrel for plastically deforming and reforming shell 16 to subsequently form the outer surface of the rotor. In addition to the plastic deformation that occurs in area A, some elastic deformation of shell 16 occurs as well, so that shell 16 is in a state of tension when the assembly is complete and eserts an inwardly directed force upon magnetic elements 22. The e~tent of_ deformation of shell 16, in certain embodiments of the invention, is such that edges, surface defects, and other features of magnetic elements 22 can be observed (e.g., visually, tactually, etc.) by observation of.the outer surface o.shell 16 following.final assembly of the rotor.
Figure 6(c) shows the sub~ect assembly immediately after completion of the cold-pressing operation and removal of force F.- As can be seen in Figure 6(c), ends 118 and 120 of shell 16 estend above and below, rL~pec~ively, end plates 14 and 30 for reasons that are disc~sed below in conn~ction with Figures 8(a)-8(f). In_ other applications~. it may- be- desirable to-.
eliminate the e~t~ing ends~118 and~120 of_ shell 16 by, for esample, decreasing the overall length of shell 16,- so that the operations disc~ssed in~ ction with Figures 8(a)-8~f) are-not ne~essary. In.such applications, the end plates (if used~
may be--held in place by-an interference fit beL ~en. the edge surfaces of.the end plates and the inner surface of shell. 16, or--by-adhesives.or other suitable means. Alternatively, the-en~ plates may be omitted entirely.
At this point in the.operation, top fisture 48 is raised, guide 100 is removed, and bolt assembly 58, with core 12, end plates 14 and 30, magnetic elements 22 and shell 16 in plsce, is removed from lower fisture 56. It should be noted that when epo~y is used bet~a~n core 12.and end plates 14 and 30 or magnetic elements 22, as described above, a. period of-appro~imately 24 hours may be reguired for curing the adhesive, depending upon the type of adhesive utilized, misture ratios employed, environmental conditions, etc. T'~ er, subsey~en-assembly, shipping, and other operations may continue during 20 this period, as shell 16 acts as-a ~clamp~ to firmly hold the assembled c.~ Dnts to~ether until the adhesive cures.
F~gures 8(a)-8(f~ illustrate the final operation in the method of the present invention. This final operation is_a crimping operation in which P-tendi~g end. 118 is deormed inwardly over end plate 14. This operation is preferablY
~o~ ted in incremental steps and, in an especially preferred method, is con~tlcted in a total of- three steps. These steps involve the use of crimping fistures 122, 124, and 126, as-de~cribed below. Other methods, such a roll forming, may be used to deform e~te~ding end 118 in the desired manner.

The first of these steps is illustrated in Fiqures 8~a) ana 8(b). Crimping fisture 12 Z has an- inner bore 127 which incln~es a first~ portion 128 having- an inner diameter at- 128 which is substantially equal to or slightly larger than the --i m outer diameter of--rotor 10. An--inner portion 130 of bore 121 has a tapered diameter which, in the preferred embodiment shown, is tapered at an angle of approsimately 30-.
With reference to Figure 8~b), application of force Fl to-crimping fisture 122 results in the crimping of end 118 inwardly toward the central asis of the rotor by an amount equal to the subject angle (i.e., approsimately 30-).
The second step of the crimping operation is illustrated by Figures 8(c) and 8(d). Crimping fisture 124 has a bore 132 which is similar to bore 127, escept that tapered portion 134 of fi~ture 124 is tapered at an angle of approsimately 60-.
When force F2 is applied to crimping fisture 124, end 118, which was previously deformed inwardly by fisture 122, is further deformed as illustrated in ~igure 8(d).
The final step in the crimping operation is illustrated by Figures 8(e) and 8(f). Crimping fisture 126 differs from fistures 122 and 124, in that fisture 126 does not include a tapered portion for further deforming end 118. Rather, fisture 126 has a substantially flat surface 136 which substantially flattens end 118 upon application o force F3, as illustrated in Figure 8(f). Furthermore, fisture 126 is provided with Teflon insert 138 which lies immediately ad;acent shell 116 when crimping tool 126 is in the position shown in Figure 8(f). The inside diameter of Teflon insert 1~8 is dimensioned to be slightly smaller than the masimum outer diameter of rotor 10 such that a press fit may be necessary to enqage fisture 126 on--the end of-the rotor. It=has been found that pro~ision of-~eflon insert 138 in crimping fisture 126 insures that shell- 16 is contained during the final step of the~crimping operation such that any bulging of the outer diameter of shell 16 which may result from the crimping operation does not e~ee~ the.
masimum overall outer diameter- of the rotor. The use of Teflon insert 138 f~rther acts to p~r ~en~ wedging of shell 116 within fisture 126 when farce F~ is applied to complete the crimping operation. It should also be noted that it is generally preferred to crimp both ends 118 and 120 of shell 16 simultaneously prior to removing rotor 10 from assembly bolt 58. The ends are crimped simultaneously to avoid possible asial displacement of magnetic elements 22 and shell 16 by application of forced Fl, F2, or-F3. The crimping operation is preferably performed prior to removal of rotor 10 from bolt assembly 58 because first end 60 and second end 84 of bolt assembly 58 provide support for end plates 14 and 30 during the crimping operation. This is particularly important in the case of.end plate 14 since, due to-the possible esistence of gap 40, displacement of end plate 14 during the crimping operation might otherwise occur if the adhesive is not in a cured state.
Referring now to Figures 9(a), 9(b), 10, ll~a) and ll(b), the structure and operation of spacing device 110 and clamping device(s) 112 will now be discussed. Figure sta) shows a top view of core 12, su~-G~Aing magnetic elements 22, spacing deYice 110, and three clamping devices 112. With reference to Figure 9(b), core 12~ suL~o~ ding -~netic elements 22, and end plates 14 and 30 are shown together with bolt assembly 58 mounted in fisture 56, which is attached to a press or like device (not shown). A side view of the apparatus of Figure 1 33~8~

9(a) is_;shown-- inc Figure- 9(b).- For-- purposes of- clar~ty-, assembly bolt- 58-and end plate 14, wh~ch-are illustrated in-Figure 9(b), are not-shown in the top-~view-of~Figure 9(a).
With reference to Figure 9(a), the structure and- operation o- sp~cing device 110 will now be-- disc~ssed. Device 110 includes three rad~ally movable wedges 140 which are spaced at 120--intervals around- core 12_snd sur.o~ding magnetic el~ ~s 22~- when these components are mounted in lower fisture 56 (see~
Figure 9(b)). Each gauge 140 includes a wedge-shaped tip 142 which is_positioned immediately adjacent, but radially spaced from, abutting (or-spaced) edges of adjacent maqnetic el- u ~s 22. As=can be seen in-Figure 9~a), magnetic elements 22 are une.cnly spaced around core 12, resulting in an abnormally wide gap or space be~w~e.~ two of the adjacent elements 22, and little or no gaps or spaces betwG0n the other abutting edges.
If assembly of the rotor were completed with this spacing arrangement, severe imbalances of the rotor would likely result and operation of the rotor would be impaired.
Spacing device 110 further includes a movable ring 144 which is provided with a plurality of generally circumferentially oriented slots 146 equally spaced around ring 144. F~ten~ing into each slot is a roller 148 which is attar~ via a shaft 150 to an-end of gauge 140. Roller 148 contacts camming surfaces 152 and 154 which form opposite vertical edges o slot 146. Gauge 140 is circumferentiallY
secured in position by-bracket 156 and is movable only radially inwardly and radially outwardly, relative to core 12-and su~.ou~ling magnetic elements 22.
Spacing device 110 is further provided with a manual operating handle 158 and an ad~ustable handle stop 160 which 1 333~8~

are used to- operate s~Jci~g device lI0, as-described below.
Although a manually po _~ed and operated device is shown and discv~s~ in th~s application, sF~cin7 device 110 may be~
~automatically~ operated and powered by- air, hydraulics, electric motor, or--any other suitable means.
operation of de~ice 110 is illustrated in Figure 10. In~-Figure 10, ring 144 has been rotated clockwise by applicat$on-of a clockwise directed force on handle 158. Rinq 144 andhandle 158 are rotated until handle 158 contacts adjustable handle stop 160. Upon clockwise rotation of-ring 144, rollers-148 interact with respective camming surfaces 152, mo~ing gauges 140 radially inwardly toward core 12 and su~.o rling magnetic elements 22. Wedge-shaped tips 142 interact with the gaps or abutting edges of magnetic elements 22 and reposition elements 22 to obtain an equal spacing bet~_e., the edges of-adjacent elements. The spacing is adjusted by the thickness of wedge-shaped tip 142 at the point of contact with the edges of magnetic elements 22, and by the amount of radially inward movement of gauges 140. The latter ~ t is controlled by ad~usting the position of- handle stop 160. Use of spacing device 110 as described results in equal sp7cing of- magnetic elements 22 to within tolerances of 0.015~-0.020~. To move gauges 140 radially outwardly away from core 12 and surrol~n~ing magnetic elements 22, handle 158 and ring 144 are rotated counterclockwise. This causes rollers 148 to interact with camming surface 154 and results in outward movement of gauges 140 to the positions shown in Figure 9(a).
Figures 9(a), 9(b), 10, ll(a) and ll(b) also illustrate the structure and operation of clamping devices 112. With reference to Figure 9(a), clamping devices 112 are equally 1 333~83 spaced at = app~imate 120- angles, and are approsimately equally spaced bet c gauges 140 of- sp~cin~ device 110.
Clamping de~ices llZ~are secured to a fisturing device which may be-mounted to a press or like device by bolts 162. Each clamping device 112 is provided with an end portion 164 which is-attached for radially i~ ~rd and outward movement to a shaft 166. An end 174 of- end portion 164, which lies immediately opposite magnetic element 22, is formed of a compressible and elastic material, such as sponge rubber. Shaft 166 is secured within a bracket 168 and is attached on its end to operating handle 170. Operating handle 170 is attached to linkage 172 which is also attached to tn end of shaft 166, and which causes shaft 166 to be moved radially inwardly and outwardly in response to operation of handle 170. Linkage 172 also allows shaft 166 to be releasably locked in the estreme radially inward position (i.e., the engaged position).
Figures 9(a), 9(b), and 10 show clamping devices 112 in the released or disengaged position. Following the positioninq of surrounding magnetic elements 22 by- spacing device 110, as esplained above with reference to Figures 9(a) and 10, handles 170 of clamping devices 112 are moved in the counterclockwise direction (i.e., toward core 12 and magnetic elements 22), causing shaft 166 to move radially inward, and causing elastic end 174 of end portion 164 to engage respective magnetic elements 22, clamping elements 22 against core 12. This position (i.e., the engaged Position) is illustrated in Figure ll(a) and ll(b). CIamping devices 112 remain in the engaged position throughout the subsequent steps of the process as described in co~nection with Figures 6(a) and 6(b~. When shell 16 has advanced to approsimately the position shown in Figure - 2~ -6(b), gauges 140 are mo~ed outwardly away- from the rotor and clampinq devices 112 are released and moved into the disengaged position to allow the process of cold-pressing shell 116 over magnetic elements 22 to proceed to completion. Thus, clamping-elements 112 provide a means for temporarily securing magnetic elements 22 in position against core 12, until shell 16 has advanced sufficiently to assume this function. Although clamps 112 are shown as manually powered devices in this embodiment, power driven clamps (with or without automatic controls) may also be used.
Following is a brief summary of how the abo~e-discussed apparatus may be used to construct a permanent magnet rotor according to the present invention. First, a layer of adhesive (eposy) is placed on the outer circumference of a cylindrically-shaped rotor core (12). A layer of adhesive is also applied to one surface of a bottom end plate (30), and the end plate is placed on one end of a bolt assembly (58). The end of the bolt assembly is then placed in a fisture (56) which is mounted in a press or like device. The core, which has a central bore, is then placed on a shank of the bolt assembly and aliqned with an aliqnment pin which is in the end of the bolt. The end of the bolt assembly is aliqned in the fisture by a similar arrangement. After the bolt end, end plate and core are mounted in the fisture, a plurality of ~qne~ic elements (22) are placed around the core. A spacing device (110) is then used to assure that the magnets are uniformly spaced around the core. The core is sliqhtly longer than the lengths of the magnetic elements, leaving a small space betuoen the ~top~ of each of the elements and the top surface of the core. This space is filled with eposy. A top end plate is ~ 333083 then. placed ad~acent the- ends- of---the-.core and. magnetic elements, and~ a se~on~-end of-~the-bolt is~ positioned over the bolt shank to- sec~e the top en~plate to the core. Acth~
stainless-steel shell (16) is then placed over a guide (100) which is positioned over the top end of the bolt assembly. The.
guide aligns the. shell with the core and surro~n~in~ magnetic elements. A top fisture is. mounted to the press in colinear relationship with the bottom fisture, the bolt assembly, and the core. The top fisture is provided with a bore having an inner diameter which is approsimately egual to: the inner:
diameter of the shell. The fisture is psovided with a sharp edge which conforms to and is used to apply a uniform pressure on a continl~ous edge (38) of the shell. The top fisture is then moved downwardly to press the shell over the core and.
~g-lZLic elements. The shell P~ten~s over the end plates on both edges, and is subsequently crimped inwardly to secure the end plates in position.
Prior to the development of this invention, it was not believed possible to cold-press a thin-walled shell over a rotor sub-assembly due to perceptions regarding the level of forces required, and due to perceived difficulties in avoiding crushing, wrinkling, or otherwise deforming the thin-walled shell. The inventor has found that the amount of force required to cold-press a shell on a three-inch diameter core and magnetic element assembly varies widely, daE 8 'i~g upon the relative dimensions of_ the components and the material properties (especially hardness and yield strength) of the shell, and is within ranges that may be readily pro~l~ce~ in a practical manufacturing operation. The ranqe of forces observed to date are believed to have varied from approsimatelY

200-2,000 pounds. In general, as t~e hardness and yield strength of the shell material increases, the force required to press the shell over the magnetic elements also increases.
Dimensional changes in the shell also influence the a - t of force reguired. The difficulties involved in maintaining overall shell configuration and integrity have been solved by a precise fisturing arrangement to align the components before pressing.
In the first embodiments of the invention, the shell was made from a piece of stainless-steel tubing having a three-inch inner diameter and a 3- V8~ outer diameter, which was achinDd to have a wall thickness of 0.008~-0.012~. This material initially had a 40,000 psi yield strength, and it is believed that a force of approsimately 500-1,000 pounds was reguired to press the shell over the magnetic elements. However, it was later deter~ine~ that the material had been work-hardened during the machining process, and that the actual yield strength of the finished material ranqed from appro~imately 100,000 psi to 120,000 psi. The nest embodiments were constructed using a shell made from a 304 stainless steel annealed tubing having a wall thickness of 0.010~ and a yield strength of 40,000 psi (as purchased~. It is believed that a force of approsimately 200-500 pounds was required to pres$
shells made of this material over the magnetic elements. The nest material tried was 304 stainless-steel material having a wall thickness of 0.010~, which was half-hard and which had a yield strength of approsimately 160,000 psi. It is believed that a force of appro~imately 1,000-2,000 pounds was required to press shells made of thig material over the magnetic elements. In addition, it was discovered that, when the ~ 333083 half-hard material was used~ annealing had. taken place in the area o-the. welded seam, so= as-to-..p.oduce.a longitudinally--o~ien~ed area of relatively soft- material in. the shell. When these shells were pressed.over the ~ ic elements, most of the stretrhing and deformation occurred. along the relatively soft:seam, resulting in-unsatisfactory overall results (such as~ fractures at: the weld seam) in some rotors. At present, the tubing is made from annealed 304 stainless-steel strip stock. After seam-welding, this tubing is cold-worked decrease the wall thickness to 0.008~ and to increase the yield strength of.=all portions of the tubing to approsimately 80,000 psi to 120,000 psi. It--must be emphasized that, while this material is the presently preferred material for practicing the present invention, satisfactory results have been obtained using the other materials described above, and satisfactory results can most surely be obtained by using materials having other characteristics, or combinations of characteristics, suitable for particular applications.
It- should be noted that, while the apparatus and method described involves the use of a movable fisture which is used to press a shell over the core and magnetic elements (which act as a mandrel to reform the shell), it is believed to be possible for the shell to be held in a stationary position, while the core and magnetic elements are pressed into the shell to accomplish the same result. Th~s would require some additional modification of -the above-described apparatus, but is not considered to be bc,ond the level of those skilled in this art.
From the preceding description of the preferred embodiments, it is evident that the objects of- the invention are attained. Although the invention has been described and 1 333~83 illustrated in detail, it-is-to be clearly unde~sLood that the same is intended by way o illustrat-ion and esample only and is not. to be-taken by-way of limitation. The spirit and~ scope of .
the invention are to. be~-limited only by the terms of. the appen~e~ claims.

Claims

1. A method of making a permanently magnetizable rotor which includes a core, a plurality of magnetizable elements, and a retaining shell, said method comprising the steps of:
a. placing the magnetizable elements around the core and temporarily holding the elements in position;
b. axially aligning the core and magnetizable elements with the retaining shell; and c. pressing the retaining shell longitudinally over the core and magnetizable elements in a manner to permanently retain the magnetizable elements in position around the core.

2. The method according to claim 1 wherein said method includes the further step of applying a layer of adhesive between the core and the magnetizable element.

3. The method according to claim 1 wherein said method comprises the further step of positioning an end plate adjacent an end of the core and the ends of corresponding magnetizable elements.

4. The method according to claim 3 wherein said method includes the additional step of applying a layer of adhesive between said end plate and said adjacent ends of the core and surrounding magnetizable elements.

5. The method as defined in claim 4 wherein said core has a generally annular outer circumferential surface, said method comprising the further step of applying a layer of adhesive between said circumferential surface of the core and the magnetizable elements.

6. The method according to claim 3 wherein said core has a generally annular outer peripheral surface, said end plate has an outer diameter and an inner diameter, and wherein the outer diameter of the end plate is smaller than an overall diameter of the core and magnetizable elements, said inner diameter being smaller than the outer diameter of the outer annular surface of the core.

7. The method according to claim 1 wherein the step of placing the magnetizable elements around the core includes positioning the magnetizable elements so as to assure substantially equal circumferential spacing between adjacent elements.

8. The method according to claim 1 wherein the step of temporarily holding the magnetizable elements in position around the core includes clamping each element against the core.

9. The method according to claim 1 wherein the retaining shell has an initial inner diameter slightly smaller than an overall outer diameter of the core and magnetizable elements, said step of pressing the retaining shell longitudinally over the core and magnetizable elements including plastically deforming the shell to fit over the magnetizable elements so as to firmly retain the magnetizable elements in position on said core.

10. The method according to claim 9 wherein the retaining shell has a substantially continuous edge surface, and wherein the step of pressing the retaining shell longitudinally over the core and surrounding magnetizable elements includes applying a substantially uniformly distributed force to the edge surface of the shell.

11. The method according to claim 1 further comprising the step of mounting the core in a press such that a central axis of the core is aligned substantially parallel to a working axis of the press.

12. The method according to claim 11 wherein the core has an axial bore, and wherein the step of mounting the core in a press includes mounting the core on a bolt, said bolt having a first end, a second end, and a connecting shank, said first end of the bolt being disposed adjacent a first end of the core and matingly received within a fixture of the press, said shank being disposed within the axial bore of the core, and said second end of the bolt being disposed adjacent a second end of the core.

13. The method according to claim 12 wherein the step of axially aligning the retaining shell and the core and magnetizable elements includes mounting guide means on the second end of the bolt and mounting the retaining shell on said guide means.

14. The method according to claim 11 wherein the step of mounting the core in a press includes positioning the core in predetermined angular relation to the first end of the bolt, and positioning the first end of the bolt in predetermined angular relation to a fixture of the press.

15. The method as defined in claim 1 comprising the further step of crimping an end edge of the retaining shell inwardly toward a central axis of the shell after the shell has been pressed into position over the core and magnetizable elements.

16. The method according to claim 15 wherein said crimping step comprises a plurality of sub-steps by which the end edge of the retaining shell is incrementally and plastically deformed inwardly toward the central axis of the shell.

17. The method according to claim 15 wherein said crimping step comprises three sub-steps, and wherein the end edge of the retaining shell is plastically deformed approximately 30 degrees inwardly toward the central axis of the shell in each sub-step.

18. The method of claim 1 including the further step of positioning an end plate directly against each opposite end of the core, said magnetizable elements being formed of longitudinal lengths less than the longitudinal length of said core so as to form a space between at least one of said end plates and the corresponding ends of said magnetizable elements.

19. The method of claim 18 including the step of forming opposite ends of said shell generally radially inwardly so as to retain said end plates against said core ends after said shell has been pressed over the core and magnetizable elements.

20. A method of making a permanently magnetizable rotor which comprises a core, a plurality of magnetizable elements, and a retaining shell, comprising the steps of:
a. placing the magnetizable elements around the core and temporarily holding the elements in position;
b. axially aligning the core and magnetizable elements with the retaining shell;
c. assembling the retaining shell over the core and magnetizable elements so as to retain the magnetizable elements in fixed longitudinal relation against an outer peripheral surface of the core;
d. positioning an annular end plate against each end of said core, each end plate having an outer diameter less than the inner diameter of the shell; and e. forming the ends of the shell generally radially inwardly so as to firmly retain the corresponding end plates against the ends of the core.

21. The method of claim 20 wherein the magnetizable elements have longitudinal lengths less than the longitudinal length of the core, said step of forming the ends of said shell generally radially inwardly being controlled so as to establish a space between at least one of said end plates and the corresponding ends of said magnetizable elements.

22. The method of claim 21 including the step of introducing a curable adhesive between the ends of said magnetizable elements and said end plates.

23. The method as defined in claim 1 wherein said step of pressing said retaining shell longitudinally over said core and magnetizable elements comprises cold-pressing said retaining shell over said core and elements.

24. A method of making a permanently magnetizable rotor which includes a core, a plurality of magnetizable elements, and a retaining shell, said method comprising the steps of placing the magnetizable elements around the core in predetermined relation; and pressing a retaining shell having a wall thickness less than about 0.020" axially over the core and magnetizable elements so as to sandwich the magnetizable elements between the core and shell while causing the shell to be stretched by the magnetizable elements so that the shell accommodates the actual radial dimensions of the magnetizable elements.

25. A method of making a permanently magnetizable rotor which includes a core, a plurality of magnetizable elements, a retaining shell, and a pair of end rings, said method comprising the steps of:
a. placing the magnetizable elements around the core and holding the elements in position;
b. placing the end rings axially against opposite ends of the core so that a gap is formed between at least one of the end rings and the adjacent ends of the magnetizable elements;

c. forming the retaining shell over the core and magnetizable elements so as to permanently retain the magnetizable elements in position around the core and cause opposite ends of the shell to extend longitudinally outwardly from the end rings and establish opposite end extensions; and d. deforming said opposite end extensions of the retaining shell inwardly toward a central axis of the shell to retain the end rings in place against the corresponding ends of the core.

26. The method of claim 25 including the further step of applying an adhesive between the opposite ends of the core and a corresponding end plate positioned against each end of said core.

27. A method of making a permanently magnetizable rotor which includes a core, a plurality of magnetizable elements, a pair of end plates, an adhesive material, and a retaining shell, in a press or similar device, said shell having a longitudinal length greater than the longitudinal length of the core plus twice the thickness of said end plates, said method comprising the steps of:
a. applying a layer of adhesive to an outer surface of the core and to a surface of one of the end plates;
b. mounting said one end plate and the core in the press such that the adhesive bearing surface of said one end plate is axially adjacent an end of the core;
c. placing the magnetizable elements around the core and temporarily holding the elements in position with first ends of said elements being disposed adjacent said adhesive bearing surface of said one end plate;
d. applying a layer of adhesive to opposite second end surfaces of the magnetizable elements and core, and positioning the other end plate adjacent said second end surfaces;
e. axially aligning the core and magnetizable elements with the retaining shell;
f. pressing the retaining shell over the core and magnetizable elements to permanently retain the magnetizable elements in position around the core with opposite ends of the shell extending longitudinally outwardly from the end plates; and g. deforming said extending ends of the shell inwardly over the respective end plates to retain the end plates against the respective ends of the core.

28. The method as defined in claim 27 wherein a gap is formed between said opposite ends of said magnetizable elements and said other end plate.

29. Apparatus for making a permanently magnetizable rotor which includes a core, a plurality of magnetizable elements, and a retaining shell, said apparatus comprising:
a. means for positioning and temporarily holding the magnetizable elements in spaced relation around the core;
b. means for axially aligning the core and magnetizable elements with the retaining shell; and c. means for pressing the retaining shell longitudinally over the core and magnetizable elements to permanently retain the magnetizable elements around the core.

30. Apparatus according to Claim 29 further comprising means for mounting the core in a press such that a central axis of the core is aligned substantially parallel to a working axis of the press.

31. Apparatus according to Claim 30 wherein the core has a central bore extending along its central axis, and wherein said means for mounting the core in a press includes a bolt, said bolt having a first end, a second end, and a connecting shank, said first end of the bolt being disposed adjacent a first end of the core and matingly received within a fixture of the press, said shank being disposed within the central bore of the core, and said second end of the bolt being disposed adjacent a second end of the core.

32. Apparatus according to Claim 31 wherein said means for axially aligning the core and magnetizable elements with the retaining shell comprises guide means mounted on the second end of the bolt for guiding the retaining shell as the shell is pressed over the core and magnetizable elements.

33. Apparatus according to Claim 29 wherein the retaining shell includes a generally cylindrical wall and a substantially continuous surface at an end of said generally cylindrical wall, and wherein said means for pressing the retaining shell over the core and magnetizable elements includes means for applying a substantially uniformly distributed force to said substantially continuous surface of the shell.

34. Apparatus according to Claim 29 wherein said means for mounting the core in a press includes means for positioning the core in predetermined angular relation to the first end of the bolt, and means for angularly positioning the first end of the bolt in predetermined angular relation to a fixture of the press.

35. Apparatus according to Claim 29 wherein the retaining shell includes a body with a generally cylindrical wall and a substantially continuous surface at an end of said generally cylindrical wall, and an initial inner diameter, and wherein said means for pressing the retaining shell over the core and magnetizable elements comprises a fixture which is attached to a working element of a press, said fixture having an internal bore having a diameter substantially equal to the initial inner diameter of the generally cylindrical wall of the retaining shell, and said fixture having an edge surface adjacent the bore for contacting the substantially continuous surface of the retaining shell for applying a substantially uniform force to the surface at the end of the generally cylindrical wall.

36. Apparatus according to Claim 35 wherein said means for axially aligning the core and magnetizable elements with the retaining shell comprises shell guide means having an outer diameter which is slightly smaller than the initial inner diameter of the generally cylindrical wall of the shell and the diameter of said internal bore in the fixture such that the retaining shell is received over said shell guide means, and said shell guide means is received within said internal bore with a tight but non-interfering fit.

37. Apparatus according to Claim 29 wherein said means for positioning the magnetizable elements around the core includes means for assuring substantially equal spacing of adjacent elements.

38. Apparatus according to Claim 29 wherein said means for temporarily holding the magnetizable elements includes means for individually clamping each element in position adjacent the core.

39. Apparatus according to Claim 29 wherein the core is substantially cylindrical in shape and has an outer radius, and wherein the magnetizable elements are arcuately shaped elements each of which have an inner radius substantially equal to the outer radius of the core, and wherein said means for positioning the magnetizable elements in spaced relation around the core comprises a plurality of gauges supported adjacent the core and magnetizable elements, said gauges being radially reciprocally movable relative to said core and magnetizable elements, and actuator means for moving said gauges into position between adjacent magnetizable elements so as to adjust and gauge the spacing between said elements.

40. Apparatus according to Claim 39 wherein said gauges include wedge-shaped tips cooperative with said magnetizable elements to achieve desired spaced relation between the elements.

41. Apparatus as defined in Claim 39 wherein said actuator means includes means for effecting simultaneous predetermined movement of said gauges toward the core and magnetizable elements to simultaneously adjust and gauge the spacing between adjacent elements.

42. Apparatus according to Claim 41 wherein each of said gauges has a camming surface, and wherein said actuator means includes an actuator ring having a camming surface cooperative with each of said gauge camming surfaces, said actuator ring being movable to cause interaction between cooperative camming surfaces so as to effect movement of each gauge in accordance with the contours of the mutually cooperative camming surfaces.

43. Apparatus according to Claim 41 wherein each gauge has a roller having an outer surface, and wherein said means for effecting simultaneous movement of said gauges comprises an actuator ring having a plurality of surfaces disposed adjacent corresponding ones of said gauge rollers, and means for moving the ring so as to cause interaction between the respective rollers and camming surfaces and cause simultaneous movement of the gauges in accordance with respective contours of the camming surfaces.

44. Apparatus according to Claim 43 wherein the camming surfaces of said actuator ring have substantially identical contours, resulting in substantially identical movement of the gauges.

45. Apparatus according to Claim 43 wherein said camming surfaces are defined by opposing side edges of slots formed in said actuator ring.

46. Apparatus according to Claim 29 wherein the core is substantially cylindrical in shape and has an outer radius, and wherein the magnetizable elements are arcuately shaped elements each of which has an inner radius substantially equal to the outer radius of the core, and wherein said means for temporarily holding the magnetizable elements in spaced relation around the core includes a plurality of clamps, each clamp being cooperative with a corresponding magnetizable element.

47. Apparatus according to Claim 46 wherein each clamp comprises an end portion for contacting the corresponding magnetizable element, and including means for moving said end portions into clamping positions with the corresponding magnetizable elements, and means for releasably locking said end portions in said clamping positions.

48. Apparatus according to Claim 47 wherein each end portion includes an elastic element for engaging the corresponding magnetizable element when the end portion is moved into clamping position.

49. Apparatus according to Claim 29 further comprising means for crimping an edge of the retaining shell inwardly toward a central axis of the shell after the shell has been pressed longitudinally over the core and magnetizable elements.

50. Apparatus as defined in claim 49 including an end plate disposed against each end of the core in axial alignment therewith, each end plate having an outer diameter slightly less than the inner diameter of said retaining shell, said magnetizable elements having longitudinal lengths less than the longitudinal length of said core, and said shell extending longitudinally outwardly from each end of said core and corresponding end plate, said crimping means being operative to form said ends of said shell radially inwardly against said end plates so as to retain said end plates against said ends of said core.

51. Apparatus according to Claim 49 wherein said means for crimping an edge of the retaining shell comprises a plurality of dies for incrementally and plastically deforming the edge of the shell inwardly toward the central axis of the shell.

52. Apparatus according to Claim 49 wherein said means for crimping an edge of the retaining shell comprises three sets of dies for incrementally and plastically deforming the edge of the shell inwardly toward the central axis of the shell, a first of said dies being configured to deform the edge of the shell through an angle of approximately 30° a second of the dies being configured to deform the edge of the shell through a further angle of approximately 30°, and a third of the dies being configured to deform the edge of the shell to an angle of approximately 90° relative to the axis of the shell.

53. Apparatus according to Claim 52 wherein said third die is provided with an elastically deformable insert for surrounding an end of the shell during the crimping operation.

54. Apparatus for use in making a permanently magnetizable rotor which includes a core and a plurality of magnetizable elements spaced around the core, said apparatus comprising, in combination, means for positioning adjacent magnetizable elements to achieve a desired predetermined spacing between said adjacent elements, and means for clamping the elements in position against the core after the desired spacing has been achieved.

55. Apparatus according to Claim 54 wherein said means for positioning said elements comprises a plurality of radially movable gauges disposed adjacent respective edges of adjacent magnetizable elements, and actuator means for moving the gauges into engagement with the edges of the magnetizable elements to adjust the position of said elements to achieve said desired predetermined spacing.

56. Apparatus according to Claim 55 wherein each gauge has a roller having a surface for contacting a camming surface, and wherein said actuator means comprises a movable member having at least one camming surface disposed adjacent each roller such that movement of the movable member causes corresponding movements of the gauges in accordance with contours of the respective camming surfaces.

57. Apparatus according to Claim 56 wherein said movable member has a plurality of generally circumferential slots defining said camming surfaces, and wherein each gauge roller is disposed in a corresponding one of said slots for interaction with the corresponding camming surfaces when said movable member is moved.

58. Apparatus according to Claim 54 wherein said means for clamping the elements in position comprise a plurality of clamps, each clamp disposed adjacent a respective one of said plurality of magnetizable elements.

59. Apparatus according to Claim 58 wherein each clamp comprises an end portion for contacting a corresponding one of said magnetizable elements, actuator means for moving said end portions into clamping relation with the Corresponding magnetizable elements, and means for releasably locking said end portions in said clamping relation.

60. Apparatus according to Claim 59 wherein each of said end portions includes an elastic element for engaging the corresponding magnetizable element when the end portion is moved into said clamping relation.

61. A permanently magnetizable rotor having a core, a plurality of magnetizable elements disposed about the core, and a retaining shell surrounding the core and magnetizable elements, said rotor being produced in accordance with the method comprising the steps of:
a. placing the magnetizable elements around the core and temporarily holding the elements in position;
b. axially aligning the core and magnetizable elements within the retaining shell; and c. pressing the retaining shell longitudinally over the core and magnetizable elements in a manner to permanently retain the magnetizable elements in position around the core.

62. A rotor for an electric motor comprising a ferromagnetic core having an axial bore, a plurality of magnetizable elements positioned about the core, and a retaining shell pressed longitudinally over the magnetizable elements so as to retain the magnetizable elements in longitudinally fixed relation on the core.

63. A rotor as defined in Claim 62 wherein said shell has a wall thickness of less than 0.020".

64. A rotor as defined in Claim 62, wherein the magnetizable elements have radial thicknesses within a predetermined range of dimensional tolerances, and wherein said shell is in a stretched condition to accommodate the actual radial dimensions of the magnetizable elements.

65. The rotor of Claim 64 wherein the nominal wall thickness of the shell in an unstretched condition is less than 0.012".

66. The rotor of Claim 65 wherein the thickness of the shell in an unstretched condition is in the range of 0.006"-0.010".

67. The rotor of Claim 62 wherein the shell is stretched over the external contours of the magnetizable elements so that at least some external surface contours of the magnetizable elements are observable at the outer surface of the shell.

68. A rotor as defined in Claim 62 wherein the magnetizable elements have longitudinal lengths less than the longitudinal length of the core, and including end plates axially engaging opposite ends of said core, said shell having end port ions formed generally radially inwardly so as to engage and retain said end plates against the opposite ends of said core with a gap being established between at least one of said end plates and the corresponding ends of said magnetizable elements.

69. A rotor as defined in Claim 68 including a curable adhesive interposed between the ends of said magnetizable elements and the corresponding end plates so as to establish a void between said at least one end plate and the corresponding ends of said magnetizable elements.

70. A permanently magnetizable rotor which comprises:
a core, a plurality of magnetizable elements circumferentially spaced around the core, a pair of end rings disposed axially against opposite ends of the core, and a retaining shell disposed so as to retain the magnetizable elements in position around the core and retain the end rings in position against the core ends, at least some of said magnetizable elements having shorter longitudinal length than said core so as to create a gap between at least one end of each of said shorter elements and the corresponding end ring.

71. A rotor according to Claim 70 wherein a portion of said retaining shell extends beyond the end rings, and wherein said extending portion of said shell is deformed inwardly toward a central axis of the shell to retain the end rings against the ends of said core.

72. An electronically commutated motor having a stator assembly with a centrally located bore and a rotor assembly disposed within said bore, said rotor assembly comprising a ferromagnetic core having an axial bore, a plurality of magnetizable elements positioned about the core, and a retaining shell disposed about the magnetizable elements and stretched to conform to the contours of the magnetizable elements, said shell being a thin-walled shell having a wall thickness of less than 0.020".

73. An electronically commutated motor as defined in Claim 72 wherein the shell is cylindrical and is configured such that the ratio of the nominal outer diameter of the shell to a nominal shell wall thickness is in the range of about 150:1 to about 525:1.