US007.154200B2
`
`(12)
`
`United States Patent
`Neal
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,154,200 B2
`Dec. 26, 2006
`
`(54) MOTOR
`
`(75) Inventor: Griffith D. Neal, Alameda, CA (US)
`
`(73) Assignee: Encap Technologies, Inc., Alameda,
`CA (US)
`
`4,128,527 A 12/1978 Kinjo et al.
`4,352,897 A 10/1982 Ogata et al.
`4,387,311 A
`6/1983 Kobayashi et al.
`4,492.889 A
`1/1985 Fukushi et al.
`4,572,979 A
`2/1986 Haar et al.
`4,679,313 A
`7/1987 Schultz et al.
`
`(*) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(Continued)
`FOREIGN PATENT DOCUMENTS
`
`(21) Appl. No.: 11/439,733
`
`BE
`
`870878
`
`1/1979
`
`(22) Filed:
`(65)
`
`May 23, 2006
`Prior Publication Data
`|US 2006/0208580 A1
`Sep. 21, 2006
`
`Related U.S. Application Data
`(63) Continuation of application No. 10/874,142, filed on
`Jun. 21, ... * º ..". d S &l
`continuation of application No.
`,428, Illed On
`Dec. 22, 1999, now Pat. No. 6,753,628.
`(60) Provisional application No. 60/146,446, filed on Jul.
`29, 1999.
`
`(51) Int. Cl.
`(2006.01)
`H02K 35/06)
`(52) U.S. Cl. ........................................................ 310/43
`(58) Field of Classification Search ..................... None
`See application file for complete search history.
`References Cited
`
`(56)
`
`|U.S. PATENT DOCUMENTS
`
`2,961,555 A * 11/1960 Towne ......................... 310/43
`3,575,621 A
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`
`
`
`(Continued)
`OTHER PUBLICATIONS
`Product Information from Dupont Engineering Polymers entitled
`“Electrical/Electronic Thermoplastic Encapsulation,” (no date),
`Publ: Reorder No. H-58633 (R, 96.7), 20 pages.
`(Continued)
`Primary Examiner—Joseph Waks
`(74) Attorney, Agent, or Firm—Steven P. Shurtz, Brinks
`Hofer Gilson & Lione
`
`(57)
`
`ABSTRACT
`
`A motor has a stator substantially encapsulated within a
`body of thermoplastic material; and one or more solid parts
`used in the motor either within or near the body. The
`thermoplastic material has a coefficient of linear thermal
`expansion such that the thermoplastic material contracts and
`expands at approximately the same rate as the one or more
`solid parts. In another aspect, a motor for a hard disc drive
`comprises at least one conductor, at least one magnet, at least
`one bearing and a shaft; and a monolithic body of thermo
`plastic material substantially encapsulating the at least one
`conductor. The bearing is either encapsulated in the ther
`moplastic material, housed in a hollow cylindrical insert
`encapsulated in the thermoplastic material, or secured in a
`bore formed in the body of thermoplastic material. The
`motor has improved shock resistance.
`
`16 Claims, 14 Drawing Sheets
`
`ãº
`ZZZZZ.
`
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`US 7,154,200 B2
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`|U.S. PATENT DOCUMENTS
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`
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`º º † º et al. ............. 310/43
`52- ... < 3
`wang
`6.232.687 B1 *
`5/2001 Hollenbeck et al
`310/88
`52- .22-3
`-
`6,300,695 B1 10/2001 Neal
`6,362,554 B1
`3/2002 Neal
`6,433,448 B1 *
`8/2002 Hatton ..................... 3.10/67 R
`6,437,464 B1
`8/2002 Neal
`6,501,616 B1 12/2002 Neal
`6,518,683 B1* 2/2003 Hatton ........................ 310/42
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`9/2003 Neal
`6,753,628 B1
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`6,844,636 B1
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`6/2006 Lieu et al.
`7,067,952 B1
`6/2006 Neal
`3 x < * *
`
`- - - - - - - - - -
`
`FOREIGN PATENT DOCUMENTS
`
`BE
`DE
`DE
`EP
`EP
`FR
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`SU
`SU
`WO
`WO
`WO
`WO
`
`3/1982
`89 1258
`3/1977
`25 39 492 A1
`1/1993
`42 21 429 A1
`2/1998
`0 747 943
`() 883 171 A1 12/1998
`2 647 958
`12/1990
`35.2079.207 A1
`7/1977
`0 1 19 0.256
`7/1989
`() 129 1652
`11/1989
`() 312 8645
`5/1991
`05336.722 A * 12/1993
`40533.36722 A 12/1993
`() 818 6953
`7/1996
`409072362 A
`3/1997
`410271719
`3/1997
`09 172748 A * 6/1997
`409 172748 A
`6/1997
`10070870
`3/1998
`41024.3595 A
`9/1998
`10271719 A * 10/1998
`11082508
`3/1999
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`8/1987
`1494,148
`7/1989
`WO 92/06532
`4/1992
`WO 96/20501
`7/1996
`WO 96/33533
`10/1996
`WO 97/39870
`10/1997
`
`OTHER PUBLICATIONS
`The Epoxylite Corporation, article from the Internet entitled
`“Vacuum Pressure Impregnation (VPI) Systems”, Nov. 19, 1999,
`<http://www.epoxylite.com/EpoxyliteBduipment.htm>, 3 pages.
`
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`US 7,154,200 B2
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`Buchanan Motor Works, Inc., article from the Internet entitled
`“Epoxy Seal—Prevents Down Time and Keeps Equipment Running
`Longer,” Jul 14, 1999, «http://www.bmworks.com/VIPhtm>, 1
`page.
`Neeltran Inc., article from the Internet entitled “Vacuum Pressure
`Impregnation (VIP)”, Nov. 19, 1999, «http://www.neeltran.
`thomasregister.com/olc/neeltran?neel.9.htm> 2 pages.
`
`LNP Engineering Plastics, Press Release entitled “LNP Introduces
`First-Ever Line of Thermally Conductive Compounds,” Jan. 28,
`1999 (2 pages).
`LNP Engineering Plastics, Advertisement entitled “Konduit.TM.
`Thermally Conductive Composites,” undated (2 pages).
`* cited by examiner
`
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`U.S. Patent
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`Dec. 26, 2006
`Dec. 26, 2006
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`U.S. Patent
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`Dec. 26, 2006
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`Dec. 26, 2006
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`US 7,154,200 B2
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`1
`MOTOR
`
`REFERENCE TO EARLIER FILED
`APPLICATIONS
`
`The present application is a continuation of U.S. patent
`application Ser. No. 10/874,142, filed Jun. 21, 2004, issuing
`as U.S. Pat. No. 7,049,715, which is a continuation of U.S.
`patent application Ser. No. 09/470,428, filed Dec. 22, 1999,
`U.S. Pat. No. 6,753,628, which claims the benefit of the
`filing date under 35 U.S.C. § 119(e) of provisional U.S.
`patent application Ser. No. 60/146,446, filed Jul. 29, 1999,
`all of which are hereby incorporated by reference.
`
`FIELD OF THE INVENTION
`
`The present invention relates generally to a high speed
`motor. It relates particularly to a spindle motor such as used
`in a hard disc drive, and to the construction and arrangement
`of the body of the spindle motor to align and retain the
`respective component parts of the motor, as well as stator
`assemblies used in the motors and hard disc drives using the
`motors, and methods of developing and manufacturing high
`speed motors.
`
`BACKGROUND OF THE INVENTION
`
`10
`
`15
`
`20
`
`25
`
`2
`spindle motors is that a number of separate parts are required
`to fix motor components to one another. This can lead to
`stack up tolerances which reduce the overall dimensional
`consistency between the components. Stack up tolerances
`refers to the sum of the variation of all the tolerances of all
`the parts, as well as the overall tolerance that relates to the
`alignment of the parts relative to one another.
`In an effort to enable increased motor speed, some hard
`disc manufacturers have turned to the use of hydrodynamic
`bearings. These hydrodynamic bearings, however, have dif
`ferent aspect ratios from conventional bearings. An example
`of a different aspect ratio may be found in a cylindrical
`hydrodynamic bearing in which the length of the bearing is
`greater than it’s diameter. This results in more susceptibility
`to problems induced by differing coefficients of thermal
`expansion than other metals used in existing spindle motors,
`making it difficult to maintain dimensional consistency over
`the operating temperature that the drive sees between the
`hydrodynamic bearings and other metal parts of the motor.
`Hydrodynamic bearings have less stiffness than conven
`tional ball bearings so they are more susceptible to imprecise
`rotation when exposed to vibrations or shock.
`An important characteristic of a hard drive is the amount
`of information that can be stored on a disc. One method to
`store more information on a disc is to place data tracks more
`closely together. Presently this spacing between portions of
`information is limited due to vibrations occurring during the
`operation of the motor. These vibrations can be caused when
`the stator windings are energized, which results in vibrations
`of a particular frequency. These vibrations also occur from
`harmonic oscillations in the hub and discs during rotation,
`caused primarily by non-uniform size media discs.
`An important factor in motor design is the lowering of the
`operating temperature of the motor. Increased motor tem
`perature affects the electrical efficiency of the motor and
`bearing life. As temperature increases, resistive loses in wire
`increase, thereby reducing total motor power. Furthermore,
`the Arhennius equation predicts that the failure rate of an
`electrical device is exponentially related to its operating
`temperature. The frictional heat generated by bearings
`increases with speed. Also, as bearings get hot they expand,
`and the bearing cages get stressed and may deflect, causing
`non-uniform rotation and the resultant further heat increase,
`non-uniform rotation requiring greater spacing in data
`tracks, and reduced bearing life. One drawback with existing
`motor designs is their limited effective dissipation of the
`heat, and difficulty in incorporating heat sinks to aid in heat
`dissipation. In addition, in current motors the operating
`temperatures generally increase as the size of the motor is
`decreased.
`Manufacturers have established strict requirements on the
`outgassing of materials that are used inside a hard disc drive.
`These requirements are intended to reduce the emission of
`materials onto the magnetic media or heads during the
`operation of the drive. Of primary concern are glues used to
`attach components together, varnish used to insulate wire,
`and epoxy used to protect steel laminations from oxidation.
`In addition to such outgassed materials, airborne particu
`late in a drive may lead to head damage. Also, airborne
`particulates in the disc drive could interfere with signal
`transfer between the read/write head and the media. To
`reduce the effects of potential airborne particulate, hard
`drives are manufactured to exacting clean room standards
`and air filters are installed inside of the drive to reduce the
`contamination levels during operation.
`Heads used in disc drives are susceptible to damage from
`electrical shorts passing through a small air gap between the
`
`30
`
`40
`
`45
`
`Computers commonly use disc drives for memory storage
`purposes. Disc drives include a stack of one or more
`magnetic discs that rotate and are accessed using a head or
`read-write transducer. Typically, a high speed motor such as
`a spindle motor is used to rotate the discs.
`An example of a conventional spindle motor 1 is shown
`in FIG. 1. The motor 1 includes a base 2 which is usually
`made from die cast aluminum, a stator 4, a shaft 6, bearings
`35
`7 and a disc support member 8, also referred to as a hub. A
`magnet 3 and flux return ring 5 are attached to the disc
`support member 8. The stator 4 is separated from the base 2
`using an insulator (not shown) and attached to the base 2
`using a glue. Distinct structures are formed in the base 2 and
`the disc support member 8 to accommodate the bearings 7.
`One end of the shaft 6 is inserted into the bearing 7
`positioned in the base 2 and the other end of the shaft 6 is
`placed in the bearing 7 located in the hub 8. A separate
`electrical connector 9 may also be inserted into the base 2.
`Each of these parts must be fixed at predefined tolerances
`with respect to one another. Accuracy in these tolerances can
`significantly enhance motor performance.
`In operation, the disc stack is placed upon the hub. The
`stator windings are selectively energized and interact with
`the permanent magnet to cause a defined rotation of the hub.
`As hub 8 rotates, the head engages in reading or writing
`activities based upon instructions from the CPU in the
`computer.
`Manufacturers of disc drives are constantly seeking to
`improve the speed with which data can be accessed. To an
`extent, this speed depends upon the speed of the spindle
`motor, as existing magneto-resistive head technology is
`capable of accessing data at a rate greater than the speed
`offered by the highest speed spindle motor currently in
`production. The speed of the spindle motor is dependent
`upon the dimensional consistency or tolerances between the
`various components of the motor. Greater dimensional con
`sistency between components leads to a smaller gap between
`the stator 4 and the magnet 3, producing more force, which
`provides more torque and enables faster acceleration and
`higher rotational speeds. One drawback of conventional
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`media and the head surface. In order to prevent such shorts,
`some hard drives use a plastic or rubber ring to isolate the
`spindle motor from the hard drive case. A drawback to this
`design is the requirement of an extra component.
`Another example of a spindle motor is shown in U.S. Pat.
`No. 5,694,268 (Dunfield et al.) (incorporated herein by
`reference). Referring to FIGS. 7 and 8 of this patent, a stator
`200 of the spindle motor is encapsulated with an overmold
`209. The overmolded stator contains openings through
`which mounting pins 242 may be inserted for attaching the
`stator 200 to a base. U.S. Pat. No. 5,672,972 (Viskochil)
`(incorporated herein by reference) also discloses a spindle
`motor having an overmolded stator. One drawback with the
`overmold used in these patents is that it has a different
`coefficient of linear thermal expansion (“CLTE”) than the
`corresponding metal parts to which it is attached. Another
`drawback with the overmold is that it is not very effective at
`dissipating heat. Further, the overmolds shown in these
`patents are not effective in dampening some vibrations
`generated by energizing the stator windings.
`U.S. Pat. No. 5,806,169 (Trago) (incorporated herein by
`reference) discloses a method of fabricating an injection
`molded motor assembly. However, the motor disclosed in
`Trago is a step motor, not a high speed spindle motor, and
`would not be used in applications such as hard disc drives.
`Thus, a need exists for an improved high speed spindle
`motor, having properties that will be especially useful in a
`hard disc drive, overcoming the aforementioned problems.
`
`BRIEF SUMMARY OF THE INVENTION
`
`A motor has been invented which overcomes many of the
`foregoing problems. In a first aspect, the invention is a motor
`having a stator substantially encapsulated within a body of
`thermoplastic material; and one or more solid parts used in
`the motor either within or near the body. The thermoplastic
`material has a coefficient of linear thermal expansion such
`that the thermoplastic material contracts and expands at
`approximately the same rate as the one or more solid parts.
`In another aspect, a motor for a hard disc drive comprises
`at least one conductor, at least one magnet, at least one
`bearing and a shaft; and a monolithic body of thermoplastic
`material substantially encapsulating the at least one conduc
`tor. The bearing is either encapsulated in the thermoplastic
`material, housed in a hollow cylindrical insert encapsulated
`in the thermoplastic material, or secured in a bore formed in
`the body of thermoplastic material.
`In another aspect, the motor has improved shock resis
`tance and comprises an assembly comprising windings and
`laminations; and shock absorbing thermoplastic material
`substantially encapsulating the assembly. The shock absorb
`ing thermoplastic material has a vibration dampening such
`that the assembly has a reduction of harmonics in the range
`of 300–2000 Hz of at least 5 decibels compared to an
`assembly with the same windings and laminations not being
`encapsulated.
`The invention provides the foregoing and other features,
`and the advantages of the invention will become further
`apparent from the following detailed description of the
`presently preferred embodiments, read in conjunction with
`the accompanying drawings. The detailed description and
`drawings are merely illustrative of the invention and do not
`limit the scope of the invention, which is defined by the
`appended claims and equivalents thereof.
`
`4
`BRIEF DESCRIPTION OF SEVERAL VIEWS OF
`THE DRAWINGS
`
`FIG. 1 is an exploded, partial cross-sectional and perspec
`tive view of a prior art high speed motor.
`FIG. 2 is a perspective view of a stator used in a first
`embodiment of the present invention.
`FIG. 3 is an exploded, partial cross-sectional and perspec
`tive view of a high speed motor in accordance with the first
`embodiment of the present invention.
`FIG. 3A is an exploded, partial cross sectional and per
`spective view of an alternative embodiment of the motor
`shown in FIG. 3.
`FIG. 4 is a cross-sectional view of the high speed motor
`of FIG. 3.
`FIG. 5 is an exploded, partial cross-sectional and perspec
`tive view of a high speed motor in accordance with a second
`embodiment of the present invention.
`FIG. 6 is a cross-sectional view of the high speed motor
`shown in FIG. 5.
`FIG. 7 is an exploded, partial cross-sectional and perspec
`tive view of a high speed motor in accordance with a third
`embodiment of the present invention.
`FIG. 8 is an exploded, partial cross-sectional and perspec
`tive view of a high speed motor in accordance with a fourth
`embodiment of the present invention.
`FIG. 9 is a cross-sectional view of a high speed motor in
`accordance with a fifth embodiment of the present invention.
`FIG. 10 is a cross-sectional view of a high speed motor in
`accordance with a sixth embodiment of the present inven
`tion.
`FIG. 11 is a cross-sectional view of a high speed motor in
`accordance with a seventh embodiment of the present inven
`tion.
`FIG. 12 is a perspective view of the inserts used in the
`high speed motor of FIG. 11.
`FIG. 13 is a cross-sectional view of a high speed motor in
`accordance with an eighth embodiment of the present inven
`tion.
`FIG. 14 is an exploded, partial cross-sectional and per
`spective view of a high speed motor in accordance with the
`ninth embodiment of the present invention.
`FIG. 15 is a drawing of a mold used to make the
`encapsulated stator of FIG. 3.
`FIG. 16 is a drawing of the mold of FIG. 15 in a closed
`position.
`FIG. 17 is an exploded and partial cross sectional view of
`components used in a pancake motor, a tenth embodiment of
`the invention.
`FIG. 18 is a cross-sectional view of a high speed motor in
`accordance with an eleventh embodiment of the invention.
`FIG. 19 is a perspective view of a stator and shaft used in
`a twelfth embodiment of the present invention.
`FIG. 20 is an exploded and partial cross sectional view of
`a hard disc drive of the present invention.
`
`DETAILED DESCRIPTION OF THE DRAWINGS
`AND PREFERRED EMBODIMENTS OF THE
`INVENTION
`
`First Embodiment
`A first embodiment of a high speed motor of the present
`invention is shown in FIGS. 2–4. By “high speed” it is meant
`that the motor can operate at over 5,000 rpm. The spindle
`motor 10 is designed for rotating a disc or stack of discs in
`a computer hard disc drive. Motor 10 is formed using an
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`5
`encapsulation method which reduces the number of parts
`needed to manufacture the motor as compared with conven
`tional motors used for disc drives, thereby reducing stack up
`tolerances and manufacturing costs and producing other
`advantages discussed below.
`Referring to FIG. 2, a stator 20 is first constructed, using
`conventional steel laminations 11 forming a magnetically
`inducible core 17 having a plurality of poles 21 thereon, and
`wire windings 15 which serve as conductors. The conductors
`induce or otherwise create a plurality of magnetic fields in
`the core when electrical current is conducted through the
`conductors. In this embodiment, a magnetic field is induced
`in each of the poles 21.
`The stator 20 is then used to construct the rest of the
`spindle motor 10 (FIG. 3). The spindle motor 10 includes a
`hub 12, which serves as a disc support member, the stator 20
`and a body 14. Together the stator 20 and body 14 make up
`a stator assembly 13. The body 14 is preferably a monolithic
`body 14. Monolithic is defined as being formed as a single
`piece. The body 14 substantially encapsulates the stator 20.
`Substantial encapsulation means that the body 14 either
`entirely surrounds the stator 20, or surrounds almost all of it
`except for minor areas of the stator that may be exposed.
`However, substantial encapsulation means that the body 14
`and stator 20 are rigidly fixed together, and behave as a
`single component with respect to harmonic oscillation vibra
`tion.
`The body 14 is preferably formed of a phase change
`material, meaning a material that can be used in a liquid
`phase to envelope the stator, but which later changes to a
`solid phase. There are two types of phase change materials
`that will be most useful in practicing the invention: tem
`perature activated and chemically activated. A temperature
`activated phase change material will become molten at a
`higher temperature, and then solidify at a lower temperature.
`However, in order to be practical, the phase change material
`must be molten at a temperature that is low enough that it
`can be used to encapsulate a stator. Preferred temperature
`activated phase change materials will be changed from a
`liquid to a solid at a temperature in the range of about 200
`to 700°F. The most preferred temperature activated phase
`change materials are thermoplastics. The preferred thermo
`plastic will become molten at a temperature at which it is
`injection-moldable, and then will be solid at normal oper
`ating temperatures for the motor. An example of a phase
`change material that changes phases due to a chemical
`reaction, and which could be used to form the body 14, is an
`epoxy. Other suitable phase change materials may be clas
`sified as thermosetting materials.
`As shown in FIG. 4, a shaft 16 is connected to the hub or
`disc support member 12 and is surrounded by bearings 18,
`which are adjacent against the body 14. A rotor or magnet 28
`is fixed to the inside of the hub 12 on a flange so as to be in
`operable proximity to the stator. The magnet 28 is preferably
`a permanent magnet, as described below. The body 14
`includes a base 22. In addition, mounting features, such as
`apertures 25, and terminals comprising a connector 26 for
`connecting the conductors to an external power source are
`formed as a part of the stator assembly. The terminals 26 are
`partially encapsulated in the body 14.
`Referring to FIGS. 3–4, the base 22 of the body 14 is
`generally connected to the hard drive case (not shown).
`Connecting members (not shown), such as screws, may be
`used to fix the base 22 to the hard drive case, using holes 25
`as shown in FIG. 3. Alternatively, other types of mounting
`features such as connecting pins or legs may be formed as
`part of the base 22. The connector 26 is preferably a
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`through-hole pin type of connector 26 and is coupled
`through the hard drive case to the control circuit board
`residing on the outer surface of the base (not shown).
`Alternatively the connector may be a flexible circuit with
`copper pads allowing spring contact interconnection.
`The stator 20 is positioned in the body 14 generally in a
`direction perpendicular to an interior portion 30. Referring
`to FIG. 2, the stator 20 is preferably annular in shape and
`contains an open central portion 32. The poles 21 extend
`radially outward from this central portion 32. Faces of the
`poles 21 are positioned outward relative to the central
`portion 32 of the stator 20. The body 14 is molded around
`the stator 20 in a manner such that the faces of the poles are
`exposed and are surrounded by and aligned concentrically
`with respect to the disc support member 12. Alternatively,
`the poles may be totally encapsulated in body 14 and not be
`exposed. FIG. 3A shows such an alternate embodiment of
`the motor depicted in FIG. 3. The poles 111 are totally
`encapsulated by the body in the stator assembly 113. As a
`result, no external surfaces of the stator are exposed.
`Referring to FIG. 4, the body 14 has an upper portion 40
`that extends upwardly from the stator 20. The upper portion
`40 is also preferably annular shaped. The body 14 includes
`the interior portion 30. The interior portion 30 is generally
`sized and shaped to accommodate the bearings 18. The
`interior portion 30 includes an upper support portion 42 and
`a lower support portion 44. In the embodiment illustrated in
`FIG. 4 the interior portion 30 is preferably cylindrically
`shaped.
`The phase change material used to make the body 14 is
`preferably a thermally conductive but non-electrically con
`ductive plastic. In addition, the plastic preferably includes
`ceramic filler particles that enhance the thermal conductivity
`of the plastic. A preferred form of plastic is polyphenyl
`sulfide (PPS) sold under the tradename “Konduit” by LNP.
`Grade OTF-212 PPS is particularly preferred. Examples of
`other suitable thermoplastic resins include, but are not
`limited to, thermoplastic resins such as 6,6-polyamide,
`6-polyamide, 4 6-polyamide, 12,12-polyamide, 6,12-polya
`mide, and polyamides containing aromatic monomers, poly
`butylene terephthalate, polyethylene terephthalate, polyeth
`ylene napththalate, polybutylene napththalate, aromatic
`polyesters, liquid crystal polymers, polycyclohexane dim
`ethylol terephthalate, copolyetheresters, polyphenylene sul
`fide, polyacylics, polypropylene, polyethylene, polyacetals,
`polymethylpentene,
`polyetherimides,
`polycarbonate,
`polysulfone, polyethersulfone, polyphenylene oxide, poly
`styrene, styrene copolymer, mixtures and graft copolymers
`of styrene and rubber, and glass reinforced or impact modi
`fied versions of such resins. Blends of these resins such as
`polyphenylene oxide and polyamide blends, and polycar
`bonate and polybutylene terephthalate, may also be used in
`this invention.
`Referring to FIG. 4, the bearings 18 include an upper
`bearing 46 and a lower bearing 48. Also, each bearing 18 has
`an outer surface 50 and an inner surface 52. The outer
`surface 50 of the upper bearing contacts the upper support
`portion 42 and the outer surface 50 of the lower bearing 48
`contacts the lower support portion 44. The inner surfaces 52
`of the bearings 18 contact the shaft 16. The bearings are
`preferably annular shaped. The inner surfaces 52 of the
`bearings 18 may be press fit onto the shaft 16. A glue may
`also be used. The outer