WO2000074209A1

WO2000074209A1 - Shrink-fit tubing for magnetic segments

Info

Publication number: WO2000074209A1
Application number: PCT/US2000/014789
Authority: WO
Inventors: Howard Schmidt
Original assignee: Mccord Winn Textron
Priority date: 1999-05-28
Filing date: 2000-05-26
Publication date: 2000-12-07
Also published as: WO2000074209A9; AU5302500A

Abstract

A rotor design for stepper and brushless motors is disclosed in which the magnetic segments of the motor are secured in place by a section of non-metallic heat-shrink tubing.

Description

SHRINK-FIT TUBING FOR MAGNETIC SEGMENTS This application claims the benefit of copending U.S. provisional patent application serial No. 60/136,506 filed May 28, 1999 and 60/145,772 filed July 27, 1999, the teachings of which are incorporated herein by reference. The present invention relates to a rotor design for stepper or brushless motors wherein heat-shrink type non-metallic tubing is applied over the magnet segments to thereby hold said magnets in place. A variety of permanent magnet rotor structures have been disclosed along with accompanying methods of manufacture. For example, U.S. Patent No. 5,325,009 discloses a rotor assembly which includes a one-piece permanent magnet which is formed as an I-beam structure. Retaining wedges are provided on opposing sides of the permanent magnet. A cylindrical sleeve made of Inconel is said to surround the main core and define an oil containment for the assembled rotor unit. The cylindrical sleeve is then heated to, for example, 1 100 F and then the slid over the core portion. In U.S. Patent Nos. 5,040,286 (1991) and 5,563,463 (1996) there is disclosed a permanent magnet rotor which has a core, a plurality of magnet segments based around the core, and a thin walled retaining shell which is stretched around the core and the magnet elements to hold the elements in position. This patent discloses the thin wall shell as being made from a non-magnetic material, such as stainless steel tubing. The shell is subjected to a cold pressing operation, so that the positions between the indicated magnet elements are retained. U.S. Patent No. 5,774,976 discloses a permanent magnet rotor for an electronically commutated motor (ECEM), which is said to contain a thin-walled retaining shell which has been stretched around the core and magnetizable elements to hold the elements in position. The core and magnetizable elements serve as a mandrel about which the shell is reformed in a cold working operation. Other patents uncovered in a search of the prior art include U.S. Patent No. 3,531 ,670, which recites an electric generator or motor comprising a multipole wound stator and a multi-pole rotor having an equal number of poles fabricated from a plurality of circumferentially-arranged, radially-magnetizable permanent magnets, secured to each other by an adhesive, and to both an inner flux conducting ring and supporting rotary structure, the rotor being further structurally stabilized against centrifugal forces by a sleeve tightly embracing the outer peripheral surface thereof. Attention is also directed to U.S. Patent Nos. 4,625,392; 4,594,525; 4,683,393; 4,757,603 and 4,954,736 as other related background art. In addition, with respect to the use of shrink fit tubing, which use forms a part of the invention described herein, attention is directed to U.S. Patent No. 4,078,910, which discloses the application of heat shrink tubing in glass sleeve fiber joining. In addition, U.S. Patent Nos. 4,418,453 discloses a heating apparatus for shrink tubing wherein heat shrink tubing is selectively placed over the length of a section positioned on the terminated end of a wire lead. U.S. Patent No. 4,858,075 discloses an RF shielded and electrically insulated circuit board structure which makes use of a first layer of shrink-wrap tubing wrapped on a circuit board substrate. Finally, U.S. Patent No. 5,661 ,842 discloses a method for providing submarine cable joint protection and insulation using heat shrink tubing without utilizing costly molding and x-ray equipment. Accordingly, the present invention uniquely appreciates the application of non-metallic shrink tubing to hold magnets in place and therein overcomes various of the problems associated with the above pπor art designs which rely in general on a high temperature shπnk-fit In addition, the invention herein relates to the use of a non-metallic or plastic shπnk-fit tubing either with or without adhesive on the inner wall surface to again retain a plurality of magnet elements The non-metallic shπnk- fit tubing also has the characteπstic of being magnetic-flux permeable, and therefore contemplates the use of vaπous types of non-metallic tubing mateπals as applied for shπnk-fit purposes For example, shπnk-fit tubmg can be manufactured of vaπous synthetic and non-metallic polymeπc resin mateπals More specifically, such mateπals specifically include polyolefin type resms (polyethylenes/polypropylenes) as well as polyester type mateπals (polyethylene terephthalate) A suitable tubmg is the TAT- 125 Seπes of tubing available from Raychem Corporation This mateπal has an operating range of -55°C to 1 10°C, a minimum shπnk temperature of 95°C, and a full recovery temperature of 121°C An alternative tubmg for higher temperature applications is available from Raychem Corporation under the tradename KYNAR This material has an operating range of -55°C to 175°C Shπnk fitting is also specifically achieved by application of heat to a temperature sufficient to cause the shπnking herein to retain the magnets in place duπng manufacture The temperature sufficient to cause the shπnking is typically less than 200°C However, as noted, m the broad context of the present invention, such tubing can be any non-metallic type resin, characteπzed that said resm has substantial permeability to magnetic flux In addition, it is also worth emphasizing that a specific utility of the present invention lies in the fact that the shπnk-fit tubing herein is employed at temperatures below that of the high-temperatures normally required for shπnk-fittmg the metal sleeves of the previously noted prior art design, which high temperatures can lead to seπous damage to the magnetic elements Accordingly, the invention herein provides a unique and novel low-temperature non-metallic shnnk-fit tubing and cage structure for the manufacture of a rotor In view of the above, it can been seen that one object of the present invention is to provide a novel magnet assembly having a plurality of substantially identically shaped magnetizable elements that are secured together by a non-metallic heat-shrunk mateπal Another object of the present invention is to provide a method of making a permanent magnet rotor by aπanging a plurality of substantially identically shaped magnetizable elements, surrounding the magnetizable elements with a heat shπnkable mateπal, and heating the heat shrinkable mateπal to a temperature sufficient to cause the heat shπnkable mateπal to shπnk. Another object of the present invention is to provide a novel rotor assembly of the type having a plurality of permanent magnets, wherein the improvement compπses a non-metallic material that upon the application of heat reduces in size to secure the permanent magnets together. Further objects and features of the invention will become more apparent by reference to the following descπption taken in conjunction with the following figure, in which: Figure 1 is an exploded perspective view of the rotor assembly in accordance with the present invention; and Figure 2 is an exploded perspective view of a second embodiment of the rotor assembly in accordance with the present invention FIG 1 herein illustrates a basic and preferred configuration for the present invention As shown therein, the rotor assembly 10 contains a cage section 12 that acts to loosely hold the magnetic segments 14 in place In a prefeπed embodiment, the magnetic segments 14 are a plurality of substantially identically shaped magnetizable elements More preferably, the elements have an arcuate transverse cross-sectional configuration A core 16, if required, can be inserted within the magnetic segments 14 The magnetic segments 14 are secured together by an appropπate section of heat shπnk tubing 18 To make a permanent magnet rotor, a plurality of magnetizable elements are arranged, preferably in a cyhndπcal fashion, the elements are then surrounded with a heat shπnkable mateπal, and the heat shπnkable mateπal is then heated to a temperature sufficient to cause the heat- shπnkable mateπal to shnnk This retains the magnetizable elements Preferably the temperature sufficient to cause the heat shπnkable mateπal to shπnk is less than 500°C, more preferably less than 200°C, and within the range of 100°C and 200°C In the context of such upper limit of preferred temperature, the invention herein relates to temperatures less than such indicated maximum values, in decreasing 1°C temperature increments In a most preferred embodiment the temperature for shπnking is about 150°C Shnnk tubmg 18 may or may not have an inner coating of an appropπate heat activated adhesive In that regard, heat activated adhesives can optionally be selected from the broad family of thermoset type adhesive formulations, including the general family of epoxy type adhesive resins, which epoxy type resin adhesive is itself preferably designed to be particularly suited for an electrical application Other heat activated systems can make use of polyimide type resins, as well as acrylic type mateπals, including mixtures thereof Additionally, as shown in Fig 2, a worm gear 20a or 20b can be attached to the cage 12 Worm gear 20a or 20b can include specific gear arrangements 22 for a given application and/or desired rotation rate Preferably, worm gear 20a or 20b and cage 12 are formed as an integrated assembly Alternatively, worm gear 20a and 20b and cage 12 can be adapted to be removably affixed to one other (via, e g , snap fit, fπction fit, or other attachment means) to provide mterchangeability Worm gear 20a and 20b can be provided to integrate with a tooling system or rotation mechanism (not shown) as is understood in the art In sum, as descπbed and illustrated herein, the current invention allows for lower tooling cost in the preparation of a selected rotor design, as well as a much wider range of tolerances and much lower capital equipment cost as related to rotor manufacture The use of the shπnk-fit tubing as descπbed above therefore finds utility m both stepper and brushless motor designs, as well m other related applications wherein a plurality of magnets must be retained for any given electπcal motor application The shπnk-fit tubmg mateπal selected herein is such that it does not require the use of a temperature which would result in damage to the magnetic elements , such as a loss in their magnetic flux output Although the present invention has been descπbed in relation to particular embodiments thereof, many other vaπations and modifications and other uses will become apparent to those skilled in the art It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims

Claims

We claim: 1. A magnet assembly, comprising: a plurality of magnetizable elements secured together by a non-metallic heat- shrunk material.

2. The magnet assembly of claim 1 , wherein the non-metallic heat shrunk material shrinks at a temperature less than 200 C.

3. The magnet assembly of claim 1 wherein the plurality of magnetizable elements are substantially identically shaped

4. The magnet assembly of claim 1 , wherein the non-metallic heat shrunk material comprises a polymeric resin material.

5. The magnet assembly of claim 4, wherein the polymeric resin material is a material selected from the group consisting of polyolefin type resins and polyester type materials.

6. The magnet assembly of claim 1 , wherein the non-metallic heat-shrunk material comprises an adhesive layer in contact with said magnetizable elements.

7. The magnet assembly of claim 1, further comprising a cage to hold the magnetizable elements.

8. The magnet assembly of claim 7, further comprising a worm gear permanently affixed to the cage.

9. The magnet assembly of claim 7, further comprising a worm gear removably affixed to the cage.

10. A method of making a permanent magnet rotor comprising the steps of: arranging a plurality of magnetizable elements, surrounding the magnetizable elements with a heat shrinkable material, heating the heat shrinkable material to a temperature sufficient to cause the heat shrinkable material to shrink.

1 1. The method of claim 10, wherein the step of surrounding the magnetizable elements comprises inserting the magnetizable elements within a heat- shrinkable tube.

12. The method of claim 10, further wherein the step of arranging the plurality of magnetizable elements comprises holding the magnetizable elements in a cage.

13. The magnet assembly of claim 10 wherein the plurality of magnetizable elements are substantially identically shaped

14. The method of claim 13, further wherein the plurality of substantially identically shaped magnetizable elements each comprise an arcuate transverse cross sectional configuration.

15. In a rotor assembly of the type having a plurality of permanent magnets, wherein the improvement comprises a non-metallic material that upon the application of heat reduces in size to secure the permanent magnets together.

16. The magnet assembly of claim 15, wherein the non-metallic material reduces in size at a temperature less than 200°C.

17. The rotor assembly of claim 15, wherein the non-metallic material comprises a polymeric resin material.

18. The rotor assembly of claim 17, wherein the polymeric resin material is a material selected from the group consisting of polyolefin type resins and polyester type materials.

19. The rotor assembly of claim 15, wherein the non-metallic material comprises an adhesive layer in contact with the permanent magnets.

20. The rotor assembly of claim 15, further comprising a cage to hold the permanent magnets.

21. The rotor assembly of claim 20, further compπsing a worm gear permanently affixed to the cage.

22. The magnet assembly of claim 21 , further comprising a worm gear removably affixed to the cage.