WO2019017799A1

WO2019017799A1 - Shaft-coupling insert for direct-drive rotor

Info

Publication number: WO2019017799A1
Application number: PCT/NZ2018/050099
Authority: WO
Inventors: Russell Joseph Jackson
Original assignee: Fisher & Paykel Appliances Limited
Priority date: 2017-07-21
Filing date: 2018-07-21
Publication date: 2019-01-24

Abstract

A shaft-coupling insert (20) for overmoulding by, or insert moulding in, a polymeric hub (51; 58) of a rotor frame (50; 57, 58). The insert has a body with an axially-extending opening (21) for engagement with a rotor shaft (9). A radial interlock feature (23, 25; 31) provided on the body, radially outward of the axial opening, has an axially-extending wall (25), a portion of which has a component facing radially inwardly and is adapted for enabling the hub to resist outward radial movement relative to the insert. A rotational interlock feature (27; 31) is also provided on the body, having an axially-extending wall, a portion of which faces non-radially and enables the hub to resist rotational movement relative to the insert. The insert couples the shaft to the hub such that resistance to deformation of the hub during torque transfer between the shaft and rotor is improved.

Description

SHAFT-COUPLING INSERT FOR DIRECT-DRIVE ROTOR

TECHNICAL FIELD

This invention relates to electric motors and in particular though not solely to shaft coupling inserts of direct-drive electric motor rotors formed from relatively soft or low strength materials such as plastics or polymeric materials, motors or rotors including such inserts, and laundry washing machines including such motors or rotors.

BACKGROUND ART

As is well known, the primary purpose of a rotor frame and hub (the central part of the rotor frame which engages with an output drive shaft) is to transfer drive torque developed at the periphery of the rotor to the central output shaft. Constructing the rotor frame by plastics or polymeric moulding provides numerous advantages compared to a metallic pressing or casting as the moulding process can result in a cheaper, lighter part which may incorporate complex shapes and features compared to a pressing or casting. However, in applications such as laundry washing machines, due to the typically small diameter of the shaft (for example, less than about 50mm), the short axial rotor length (for example, less than about 50mm) available for shaft engagement and the relatively high torque being transmitted (for example, up to about 50Nm), the shaft to hub interface stresses produced are generally too high to be tolerated by a cost-effective polymeric material suitable for injection- moulding the entire frame. In such cases, at least some relative rotation between the shaft and hub may detrimentally occur. This problem is exacerbated in modern, high torque rotors of larger diameter, such as outer-rotor-type permanent magnet or BLDC electric motors where the rotational force provided by the rotor magnet ring is at a greater radial distance from the hub.

To overcome or alleviate this problem it is common to overmould or insert mould a reinforcing hub insert or shaft coupler formed from a higher strength material (such as a metal, for example steel) into the radial centre of the polymeric rotor frame. This enables the rotor torque to be transmitted via the polymeric frame at a greater radius than that of the shaft, enabling the polymeric interface stresses to be much lower. The insert-to-rotor frame interface, when viewed axially, is shaped to be non- circular so as to increase the contact area over which the rotor torque is transmitted and in order to avoid relative rotational movement between the rotor frame and insert. For example, the insert may have a hexagonal or octagonal profile or may be provided with a multiplicity of axially-extending teeth formed circumferentially about an outer axial surface of the insert which, when overmoulded by the polymeric frame, perform the function of transmitting torque between the insert and polymeric rotor frame. A problem can arise with connections between components made from different materials due to their differing co-efficients of thermal expansion. Electric motors can experience significant and cyclical temperature increase with extended use which causes greater expansion of polymeric components, such as the rotor frame, compared to metallic components, such as a metal hub insert. In the particular example of a metallic shaft-coupling rotor hub insert having teeth on a radially outward-facing surface it is possible for engagement between the torque-transmitting faces of the polymeric rotor frame and metallic hub insert to be compromised or lost due to the increase in diameter of the insert mounting hole in the rotor frame being greater than the increase in diameter of the hub insert. SUMMARY OF INVENTION

It is therefore an object of the present invention to provide a shaft coupling insert for the rotor of an electric motor that will go at least some way towards overcoming the above disadvantages or which will at least provide industry with a useful choice.

In a first aspect, the invention consists in a shaft-coupling insert suitable for overmoulding by or insert moulding in a polymeric hub of a motor rotor frame, the insert comprising:

a body having an axially-extending opening adapted for rotational engagement with a rotor shaft,

a radial interlock feature on the body, radially outward of the axial opening, having an axially- extending wall, at least a portion of which has a component facing radially inwardly and is adapted for enabling the hub to resist outward radial movement relative to the insert, and

a rotational interlock feature on the body, having an axially-extending wall, at least a portion of which faces non-radially and is adapted for enabling the hub to resist rotational movement relative to the insert.

In a second aspect, the invention consists in a motor rotor comprising:

a rotor frame having a central polymeric hub,

a shaft-coupling insert according to the first aspect overmoulded by or insert moulded in the hub,

wherein the frame includes an axially-extending wall at least a portion of which has a component facing radially outwardly and is in contact with the axially-extending wall of the radial interlock feature.

In a third aspect, the invention consists in an electric motor comprising:

a stator,

a rotor according to the second aspect mounted to the stator for rotation thereby, and a rotor shaft received in and rotationally engaged with the opening of the shaft-coupling insert.

In a fourth aspect, the invention consists in a laundry washing machine comprising:

a cabinet,

a stationary liquid container within the cabinet,

a rotatable drum for holding a laundry load mounted within the liquid container, and an electric motor according to the third aspect with its shaft coupled to enable the rotor to rotationally drive the drum.

The invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

The invention consists in the foregoing and also envisages constructions of which the following gives examples only. In particular, the invention is mainly described with reference to its implementation in an external- or outer-rotor (internal stator or "inside-out") motor but is equally applicable to a conventional internal-rotor-type motor. A motor incorporating a shaft coupling insert in accordance with the present invention is particularly suitable for use in a "direct-drive" arrangement wherein the rotor drive shaft is directly connected to the rotatable component without intermediary belts, chains or gearboxes. However, the invention is also compatible with drive arrangements incorporating such intermediary components. The invention is also exemplified with reference to electric motors for use in laundry machines (washing machines, drying machines and combination washer-driers) although it is not limited to that application only, and in that application, it is equally applicable to both horizontal- axis ("front-loading") and vertical-axis ("top-loading") machine types. It should also be noted that whenever terms such as "axially-extending" or "radially-directed" are used in this document, they are respectively intended to encompass directions that have at least some component that extends or is directed axially (parallel to an axis, in either axial direction) or radially (in a radial direction from or toward the axis) and are not intended to be limited only to directions that are entirely axial or entirely radial. BRIEF DESCRIPTION OF DRAWINGS

Preferred forms of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a cut-away and partially exploded perspective view of a top-loading laundry washing machine incorporating an electric motor having a rotor with a shaft-coupling insert according to an embodiment of the present invention,

Figure 2 is a perspective view from in front and below of the shaft-coupling insert of Figure 1, Figure 3 is a perspective view from in front and above of the shaft-coupling insert of Figure 2,

Figure 4 is a perspective view from in front and below of a vertical cross-section through the shaft-coupling insert of Figure 2,

Figure 5 is a perspective view from in front and below of a vertical cross-section through the motor rotor of Figure 1,

Figure 6 is a perspective view from in front and above of a vertical cross-section through the motor rotor of Figure 1 with the shaft-coupling insert removed, and

Figure 7 is a cross-sectional perspective view from below of the shaft-coupling insert shown in Figures 2 to 6 incorporated within a rotor according to an alternative embodiment to those figures.

DESCRIPTION OF EMBODIMENTS With reference to the accompanying drawings and in particular Figure 1 initially, a laundry washing machine 1 of the top-loading or vertical-axis variety is shown. The washing machine includes a cabinet 2 with a lid 3 allowing access to a perforated rotatable drum or spin tub 4. A non-rotatable liquid container 5 is suspended within the .cabinet and has the stator 6 of a Brushless DC (BLDC) or Permanent Magnet (PM) electric motor 7 fixedly attached to its base. An outer- or external-rotor 8 (only partially shown) is mounted with the poles of its ring of permanent magnets radially-inwardly facing and adjacent to radially outwardly facing pole tips of the stator. Rotor 8 is rotationally fixed to the lower end of a rotor shaft 9 mounted within a passage through the base of the liquid container 5 via at least one pair of spaced apart bearings (vertically spaced in this embodiment), such as roller bearings. A well-known mechanism may be provided to selectively rotationally interconnect rotatable drum 4 to rotor shaft 9 so that the drum rotates therewith when desired, such as during spin (dehydration) cycles of the laundry washing machine. At other times, the spin tub may be rotationally disconnected from the rotor shaft so that it is not rotationally driven thereby. An agitator or impellor 10 may be rotationally connected to the rotor shaft to turn therewith during both washing and spin cycles. A machine controller and/or motor controller (not shown) provides appropriate commutation signals to stator windings provided around the stator poles so that the rotor's rotational speed and direction may be dictated during the various cycles of a user-set wash programme of the machine. In some embodiments, particularly in some horizontal-axis washing machines, the drum and drum shaft may be permanently rotationally coupled to the rotor, for example, where the drum shaft is also the rotor drive shaft. In some embodiments, particularly in some vertical-axis washing machines, a gearbox may be mechanically coupled:

- between an agitator (or impellor) drive shaft and the rotor, and/or

- between a drum shaft and the rotor (this arrangement is also relevant to horizontal-axis machines).

Further details of the electric motor, and in particular rotor 8 will now be described with reference in particular to Figures 2 to 6. Rotor 8 is preferably formed (such as by injection moulding) from a relatively light-weight yet rigid and cost-effective polymeric or plastics frame 50 (see Figures 5 and 6). The rotor frame spans between and includes a central rotor hub feature, extending radially outwardly to an outer rotor portion carrying a circumferential ring of permanent magnetic poles, such as the exemplary ring of magnet elements 52. A backing ring 53 of magnetically permeable material such as steel may be provided radially outside the permanent magnet ring to provide a low magnetic reluctance path between magnet elements for magnetic flux directed at least partially radially outwardly therefrom. The exemplary rotor frame 50 includes a plurality of circumferentially-spaced "spokes" 54 extending between the hub and a radially outer portion wherein the radially inner ends of alternate spokes are spaced axially. An exemplary rotor frame similar to that shown in Figures 5 and 6 is also described in our prior patent US9325210B.

Although not visible in Figure 1, a shaft-coupling insert or coupler 20 is provided within a central hub 51 of rotor 8. In this embodiment hub 51 is an integrally-formed component of the rotor frame such that the entire rotor frame 50 is formed, preferably by injection moulding, from the same polymeric material in a single, or monolithic piece. The shaft-coupling insert has a generally annulus-shaped body with an axial opening 21 therethrough which is shaped and dimensioned to engage with the rotor shaft 9. In order that torque generated by the rotor can be transmitted effectively from insert 20 to the shaft, the wall of opening 21 is preferably provided with engagement features 22 such as a tooth or teeth formed as splines, knurls, keys or combinations thereof. The outer surface of shaft 9 is provided with complementary features for engagement with features 22 so that shaft 9 and shaft- coupling insert 20 are rotationally locked together. Shaft-coupling insert 20 may be moulded, cast, machined or otherwise formed from any suitable engineering material although a metal such as steel is preferred due to its desirable combination of strength and cost.

As may be best seen in Figure 5, hub 51 of rotor frame 50 not only overlaps axially with the rotor- coupling insert 20 but overlaps radially also. That is, the interface between insert 20 and hub 51/frame 50 includes both surfaces extending generally axially and generally radially. In order to reduce or eliminate the tendency of the rotor frame to radially separate from the rotor-coupling insert during high speed rotation, particularly at elevated temperatures due to differing thermal expansion coefficients, a radial interlock feature is provided. The radial interlock feature is provided by a radially inner portion of the rotor frame extending radially inwardly beyond the radially outer peripheral surface of the insert and engaging with an at least partially axially-extending wall of the insert which has at least a component facing toward the axis of the insert. In other words, the radially inner portion of the rotor frame is "hooked" over and engages with an inner wall or face of a generally axially- extending feature of the insert. In this way, radially outwardly-directed movement of the hub or entire rotor frame with respect to the insert may be resisted by the wall of the insert. Preferably this is achieved by injection moulding the rotor frame about the rotor-coupling insert such as by overmoulding or insert moulding techniques.

The axially-extending wall may extend completely circularly or circumferentially about the axis of the insert or may comprise one or more discrete wall portions spaced apart circumferentially and/or radially. The wall or walls may extend axially away from a base surface of the insert or may extend axially below a base surface of the insert. Although ideally the wall would extend annularly about the axis and entirely axially, it will be appreciated that some resistance to relative radial movement between the hub/frame and insert will be achievable by a wall that has at least some axial component and which is not entirely radially aligned. So, for example, an axially extending wall that spirals about the axis, or a single segment or spaced apart segments of such a spiral, would also offer some radial interlocking ability, even if it/they are slanted away from an axial plane, for example, on a frusto- conical surface about the axis.

In order to ensure effective transmission of torque from the outer, magnet-carrying portion of the rotor to the rotor-coupling insert 20, the insert is also provided with a rotational interlock feature in a similar manner to shaft-engagement features 22, for transferring torque between the insert and the rotor shaft. The rotational interlock feature resists relative rotational movement between the hub or entire rotor frame and the rotor-coupling insert and could, for example, comprise one or more features extending in an at least partially radial direction from a surface of the insert and having at least some, at least partial, axial extent.

For example, the rotational interlock feature could simply comprise a non-circular profile (when viewed axially) of the outer circumferential surface of the insert. The rotational interlock feature may be one or more at least partially radially-outwardly-extending projections (such as an outer or outward-facing tooth or teeth) from an outer circumferential surface of the insert and which project within, and engage with, the hub/frame structure. Alternatively, the rotational interlock feature could be provided by one or more radially-inwardly-directed feature of the insert that has at least some axially-directed extent. In each case, the hub of rotor frame 50 includes a corresponding mating feature or features that mate with the rotational interlock feature of the insert to couple the two components together for rotation. It will be appreciated that the torque transmission effectiveness and reliability of the rotational interlock feature can be improved by the provision of the radial interlock feature, and vice versa.

Turning again now to Figures 2 to 6, in one preferred embodiment the radial interlock feature and the rotational interlock feature are provided as the same, combined rotational and radial interlock feature. That is, the axially lower side of the insert (see Figures 2 and 4) has an axially-extending annular lip or rib 23 formed around its circumference. Lip or rib 23 has a radially-outer surface flush with the outer circumferential surface 24 of the body of the insert although this is not essential. Lip or rib 23 has an inner wall 25, surrounding opening 21 and its axis, and which also surrounds a recessed central region 26 of the insert. It will be appreciated that lip/rib 23 and radially-inwardly-facing wall 25 provides a radial interlocking feature for resisting radial separation of the hub from the insert. When overmoulded or insert moulded during formation of the rotor frame, wall 25 receives a flow of molten polymeric material that forms a mating wall 55 (see Figures 5 and 6) of the rotor frame and which extends radially inwardly of the lip or rib 23 and also extends at least partially axially into the recessed central region of the insert. The mating wall 55, once cooled and solidified includes a radially- outwardly-facing surface adjacent to and in contact with the radially-inwardly-facing surface of inner wall 25. Inner wall 25 is generally circular when viewed in an axial direction but is provided with circumferentially spaced apart radially-extending sections so that a series of circumferentially- extending, radially-inwardly-facing, teeth 27 are formed. These inner teeth 27 extend at least partially axially and provide a rotational interlocking feature against complementary radially-outward-facing teeth formed in mating wall 55 of the hub, while also providing the aforementioned radial interlocking feature. Teeth 27 could, for example, be formed as splines, knurls, keys or combinations thereof.

As may be seen in Figure 3, additional or alternative interlocking features may be provided on or in insert 20. The further interlocking features shown in Figure 3 are associated with a surface 30 of the insert that faces an opposite axial direction to the surface or side from which lip or rib 23 extends. At least one at least partly axially-extending depression 31 is formed in surface 30. Preferably, as shown, a plurality of depressions 31 are spaced circumferentially about the axis of the insert to effectively form a series of inner or inwardly-directed teeth, relative to the outer circumferential surface 24. Each depression 31 may comprise a blind hole and has a side wall or walls which extend(s) at least partially axially and has a wall portion forming a radial interlock feature as it is at least partially radially facing also. When overmoulded or insert moulded by polymeric material, the wall portions of depression 31 which are at least partially radially facing will provide resistance to radially-outwardly directed force from complementarily moulded teeth of the hub/rotor frame that extend axially into depressions 31. Such resistance could also, for example, be provided by an axially extending annular slot or recess formed in surface 30 which could provide an alternative radial interlock feature. However, because the side wall of each depression 31 also has a further portion that faces non-radially (so is not on a circle or tangent about the axis of the insert) then that/those further wall portion(s) provide a rotational interlock feature which, after the depressions 31 are filled with molten polymeric material as part of the hub/rotor frame 50 and solidified, will provide resistance to relative rotational movement between the insert and the hub/rotor frame. As may be seen in Figure 6, an annular ring of axially-directed teeth 56 is formed in the hub 50 of the rotor frame due to the depressions 31 in insert 20. Teeth 56 engage with depressions 31 which therefore provide the insert with both radial and rotational interlocking features, as explained above.

In an alternative embodiment to that shown in the drawing figures respectively, radial and rotational interlock features of the shaft-coupling insert 20 could be provided, respectively, on axially opposed sides of the body. For example, one axially-spaced side of the insert's body could be provided with a radial interlock feature of an annular groove/recess or rib/ridge while the other axially-spaced side of the insert could be provided with a rotational interlock feature of one or more purely radial slots/recesses, or outer-circumferentially-spaced teeth. In these situations the radial interlock feature (for example, a circumferential ridge) does not have any rotational interlocking functionality and the rotational interlock feature (for example, a radial ridge) does not have any radial interlocking functionality. Of course, either one or both axially-spaced side(s) of the insert could be provided with both radial and rotational interlocking features. Also, on the side or sides of the insert that have both radial and rotational interlock features, those features may be combined into a single radial and rotational interlocking feature (such as has been described above with reference to either axially- spaced side of the insert 20 shown in the attached drawing figures). Alternatively, when radial and rotational interlock features are provided on or in the same side of the insert, they may be distinctly separate features carrying out their own function only.

In Figure 7 an alternative embodiment of rotor, incorporating the above described shaft-coupling insert 20, is shown. Instead of an entirely plastics or polymer rotor frame as shown in Figures 5 and 6, the rotor shown in Figure 7 has a plastics or polymeric central hub 58 component which, as in the previous embodiment, has been moulded over insert 20 or insert 20 has been insert-moulded therein. Hub 58 is attached or fixed to a pressed outer frame component 57. The outer frame component 57 is dish-shaped and has a central opening which accommodates a portion of the insert and enables access thereto by the rotor shaft. As may be seen in Figure 7, hub component 58 may include an outer flange that contacts and is fastened (via screws or bolts, for example) to the surface of outer frame component 57 surrounding its central opening. The frame component 57 has a substantially flat region extending radially outwardly from its central opening to an axially extending wall at the periphery of the rotor. The axially extending wall supports the ring of magnets 52. This embodiment of the rotor encompasses the same advantages as the previous embodiment but, in addition, may have beneficial weight or cost (manufacturing and/or raw materials) savings.

Claims

1. A shaft-coupling insert suitable for overmoulding by or insert moulding in a polymeric hub of a motor rotor frame, the insert comprising:

2. The shaft-coupling insert as claimed in claim 1, wherein the radial interlock feature is formed in or on a side of the body which is substantially axially opposite to a side of the body in or on which the rotational interlock feature is formed.

3. The shaft-coupling insert as claimed in claim 1, wherein the radial interlock feature is formed in or on the same side of the body as the rotational interlock feature.

4. The shaft-coupling insert as claimed in claim 3, wherein the radial interlock feature and rotational interlock feature are part of the same interlock feature.

5. The shaft-coupling insert as claimed in claim 4, wherein the radial interlock feature and the rotational interlock feature comprise a series of axially-extending, radially inward-facing teeth extending, either continuously or intermittently, at least partially circumferentially about the axis.

6. The shaft-coupling insert as claimed in any one of the preceding claims, the body further comprising an at least partially-annular, axially-extending lip or rib, wherein the radial interlock feature and/or the rotational interlock feature is/are formed on a radially inner side of the lip or rib.

7. The shaft-coupling insert as claimed in claim 6, wherein the axially-extending lip or rib is formed at or adjacent to a radially outer surface of the body.

8. The shaft-coupling insert as claimed in any one of the preceding claims, wherein the body includes a first radial interlock feature extending from a surface of the body in a first axial direction in common with the rotational interlock feature, and a second radial interlock feature extending from a surface of the body in an axial direction opposed to the first axial direction.

9. The shaft-coupling insert as claimed in any one of claims 1 to 7, wherein the body includes a first rotational interlock feature extending from a surface of the body in a first axial direction in common with the radial interlock feature, and a second rotational interlock feature extending from a surface of the body in an axial direction opposed to the first axial direction.

10. The shaft-coupling insert as claimed in any one of the preceding claims, wherein the axially- extending wall of the radial interlock feature or of the rotational interlock feature comprises an outer wall of at least one axially-extending projection from the body.

11. The shaft-coupling insert as claimed in any one of claims 1 to 9, wherein the axially-extending wall of the radial interlock feature or of the rotational interlock feature comprises an inner wall of at least one axially-extending cavity in the body.

12. A motor rotor comprising:

a rotor frame having a polymeric central hub,

a shaft-coupling insert as claimed in any one of the preceding claims located at the hub and overmoulded by or insert moulded in the hub,

13. The motor rotor as claimed in claim 12, wherein the frame includes an axially extending wall, at least a portion of which faces non-radially and is in contact with the axially-extending wall of the rotational interlock feature.

14. The motor rotor as claimed in claim 12 or claim 13, further comprising a series of radially inwardly directed alternating permanent magnetic poles spaced radially away from the hub by the frame.

15. The motor rotor as claimed in any one of claims 12 to 14, wherein the co-efficient of thermal expansion of the frame is significantly different to the co-efficient of thermal expansion of the shaft- coupling insert.

16. The motor rotor as claimed in claim 15, wherein the shaft-coupling insert is formed from metal.

17. The shaft-coupling insert as claimed in any one of claims 12 to 16, wherein the polymeric hub of the rotor frame is attached to a radially outer portion of the frame, the radially outer frame portion formed from pressed steel.

18. The shaft-coupling insert as claimed in any one of claims 12 to 16, wherein the rotor frame, including the central hub, is polymeric.

19. An electric motor comprising:

a stator,

a rotor as claimed in any one of claims 12 to 18 mounted to the stator for rotation thereby, and

a rotor shaft received in and rotationally engaged with the opening of the shaft-coupling insert.

20. A laundry washing machine comprising:

a cabinet,

a liquid container within the cabinet,

a rotatable drum for holding a laundry load mounted within the liquid container, and an electric motor as claimed in claim 19 with its shaft coupled to enable the rotor to rotationally drive the drum.