GB2154073A

GB2154073A - Improvements in or relating to electric motors and components therefor

Info

Publication number: GB2154073A
Application number: GB08502853A
Authority: GB
Inventors: Alan Hayes; David Harris
Original assignee: Black and Decker Inc
Current assignee: Black and Decker Inc
Priority date: 1982-01-08
Filing date: 1985-02-05
Publication date: 1985-08-29
Also published as: GB8300456D0; GB8502853D0; GB2154073B; GB2113924B; GB2113924A

Abstract

An electric motor comprises a rotor 9 having a shaft 10 with a commutator 13 thereon, a stator assembly surrounding the rotor and including a laminated stator stack 1, first and second end cap 11, 12 of a plastics material and having first and second sets of location surfaces 16a, 16b, 28a, 28b for locating the end cap on the laminated stator stack with the location surfaces of the first set (16a, 28a) seated upon the end faces of the stack and the location surfaces of the second set (16b, 28b) contacting the side of the stack. The first and second end caps 11, 12 incorporating bearings 19, 134 rotatably mounting the shaft 10. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Electric Motors and Components Therefor The invention relates to electric motors, components thereof, and to methods of assembling electric motors and components thereof, and is particularly directed towards facilitating automatic assembly by machinery with the minimum of manual handling.

Over the years many different constructions of electric motors have been used or proposed. As the need to reduce the cost of motors and to facilitate their assembly became apparent, much attention has been given over the years to developing electric motor components and their assembly. However, the need to facilitate automatic assembly and minimise manual handling still exists. One ofthe problems that occurs is that as more components are made by mass production, accumulation of tolerances occurs during final assembly that can produce unsatisfactory products. To avoid this it is necessary to manufacture the components to closer tolerances at an increase in cost, and/or to include more manual handling of the components during the final assembly also at an increase in cost.

One of the objects of the present invention is to produce electric motor components of economic designs and to facilitate the automatic assembly of the electric motor by machinery.

A feature by which this is achieved is the provision of a brush box assembly that retains the brush in the brush box and allows the brush to be released out of the brush box only during assembly in the electric motor. This has the advantages of greatly facilitating handling of the brush box assembly and simplifying its insertion into the electric motor.

Another separate feature by which this is achieved is the provision of a choke in the form of a cartridge fuse which can be inserted between a field winding terminal and a brush terminal by simply inserting it in slotted contact tongues.

Another object of the invention is the provision of an electric motor which is of economic construction and which can be assembled automatically with the minimum of manual handling.

A feature by which this is achieved is the provision of location surfaces on the end caps and directly assembling the end caps over the end faces of the stator, and constructing the end caps with an open structure. This has the advantage of more accurately positioning the end caps and rotor bearings supported therein, and of allowing access for assembly of other components after assembly of the end caps.

Another feature by which automatic assembly is advanced is the measuring of the distance between an end face of the stator and a specific location on the rotor shaft during assembly of the second end cap on the stator, and subsequently moving the bearing in the other end cap by pressing it inwards an amount related to the measured distance to determine the end float of the rotor shaft. This has the advantage of controlling this end float to within tight tolerances even though there may have been an unacceptable build up of tolerances during earlier assembly steps.

Therefore, according to one aspect of the invention, there is provided a brush sub-assembly for an electric motor, comprising a brush holder, a brush located within the brush holder, resilient means stressed between the brush holder and the brush for urging the brush outwardly in a longitudinal direction from within the brush box, and said holder being stressed to grip and hold said brush against movement. When the stressing is relieved upon insertion of the brush box subassembly in the electric motor, the brush can move freely in said brush holder under the influence of the resilient means.

Preferably, the brush holder is a box with a longitudinal slot defined therein and disposed between side walls to enable those walls to be separated sufficiently when a member is forced into the slot. Conveniently the member can be part of a box support into which the brush box is inserted.

There may be more than one slot, for example two slots each in opposing peripheral portions of the brush box.

With this sub-assembly, the brush can be, initially, located completely within the holder and held in that position entirely by the gripping action of the holder, no other retaining or brush gripping means being required. The box may have at least two opposed side walls that are stressed to grip and retain the brush and which are separable to allow the brush to move freely. The side walls may be separated by a longitudinal gap.

Also, according to another aspect of the invention, there is provided a method of assembling an electric motor, comprising the steps of press fitting a first bearing into a first end cap, supporting the first end cap, locating an end face of a stator against the end cap, inserting a rotor having a shaft into the stator with one end of the shaft passing into the bearing, measuring the distance between the opposite end face of the stator and a location on the shaft, and positioning a second end cap containing a second bearing over the other end of the shaft. Then advancing the second end cap relative to the other end of the shaft to engage the shaft in the second bearing and until the second end cap is located against the opposite end face of the stator.

Subsequently pressing the second bearing into the second end cap in the direction of the first end cap an amount related to the measured distance to determine the end float of said shaft.

The location on the shaft which is measured relative to the stator end face may be a side of a thrust washer directed towards that end face and previously assembled on the shaft.

The invention also provides electric motors incorporating the above features, and electric motors made by the above methods.

For instance, the electric motor can have first and second end caps mounted directly on the ends of a laminated stator stack, with location surfaces on the end caps seated upon the end faces of the stator stack and an annular portion of each end cap engaging over the ends of the stator stack. The first end cap can incorporate a pair of brush box mountings of channel form with a wall of each channel having an upstanding member protruding therefrom and engaging in a slot in a brush box inserted in the channel. The upstanding member forces opposite walls of the brush box apart to release the brush in the brush box and allow the brush to move under the action of a spring into engagement with the commutator of the motor.

By way of example only, an electric motor embodying the invention and including a brush box sub-assembly, and a method of assembly such a motor will now be described in greater detail with reference to the accompanying drawings of which: Figure 1 is a longitudinal section through a motor according to the invention on the line I-I of Figure 3, Figures 2 and 3 are end views of the motor of Figure 1 looking in the directions respectively of arrows 1I--II and Ill-Ill in Figure 1, Figure 4 is a section of the end cap on the right in Figure 1, Figure 5 is a section of the end cap on the left in Figure 1, Figures 6,7 and 8 are, respectively, an end view, plan view and underneath view of a brush holder of the motor, Figure 9 is a section on the line IX-IX of Figure 6 with the brush omitted, Figures 10 and 11 are, respectively, a plan and underneath view of a further component, Figure 12 is a section on the line Xll-XIl of Figure 10, Figure 13 is a section on the line XlIl-XllI of Figure 10, Figure 14 is a section on the line XlV-XIV of Figure 15, Figure 15 is an underneath view of the component shown in Figures 10 to 13 with contact members in place and an electrical component present, Figure 16 is a section on the line XVI-XVl of Figure 15, Figures 17, 18 and 19 are, respectively, plan, underneath and side views of one of the contact members, Figure 20 is a section on the line XX-XX of Figure 17,and Figure 21 is an end view of the contact member of Figures 17 to 20 taken from the left in Figure 19.

Referring first to Figures 1 to 5, there is shown a commutator motor, for example a universal motor, comprising a stator having a laminated stator stack 1. The laminated stator stack is formed internally with pole pieces (not shown) around which are located the conventional field windings (not shown).

The individual laminations of the stator stack 1 are accurately formed of iron pressings assembled in carefully aligned manner and welded together along guide channels 3 formed in flat side edges 4 of the laminations (see Figure 3). In this way, the stator stack provides outer end faces 5, 6 which form reference surfaces for end caps to be described in detail below. The stator stack 1 has several, preferably at least two, longitudinal passageways 7 extending through the length thereof as are shown in Figure 1. Ring-like terminal blocks 2 of plastics material are attached to opposite ends of the stator stack 1 and form part of the stator assembly. The ends of the field windings are brought and attached to two pairs of terminal posts that are located in the terminal block 2 on the left in Figure 1.The terminal blocks 2 have mounting lugs that are located in additional longitudinal passages in the stator stack 1. A suitable form of terminal block is described in U.K. Patent Specification No. 1,482,470, the contents of which are hereby incorporated herein by reference.

The rotor or armature 9 of the motor is mounted upon a shaft 10 itself rotatably mounted in end caps 11, 12 of a suitable plastics material, for example glass filled nylon. Also secured to the shaft 10 adjacent one end thereof is a commutator 13, while a fan 14 is secured to the shaft 10 adjacent the other end thereof. The fan 14 is the conventional cooling air flow fan and may be, for example, a radial flow fan.

End cap 11 (Figures 2 and 4) comprises an annulus 15 from which axially extend four, equispaced fingers 16 that support a disc-like end member 17 in axial spaced relationship with the annular 15. Integral with the end member 17 is a hollow boss 18 containing a plain bearing 19.

Moulded into the surface of the end member 17 is a die-cast, metal heat sink 20 also of generally disclike form with a tubular extension 21 in thermal contact with the plain bearing 19. The heat sink had a plurality of apertures 20a which assist in retaining it in position in the end member 17. The bearing 19 is a press fit in the tubular extension 21 of the heat sink.

The end member 17 is cut away at diametrically opposed sites 22 to give access to bolts 23 that secure the end cap 11 to the end cap 12 via the stator stack 1. To locate the end cap 11 on the end face 5 of the stator stack 1, the end cap 11 has internal individual ledges 16aformed at the inner ends ofthe fingers 16 and these ledges contact the end face 5 when the cap 11 is correctly seated upon the end face with the annulus 15 closely encircling the end face 5 and the outer surface of the stator stack adjacent the end face 5, four pads 1 6b integral with the annulus 15 and on the inner face thereof locate the end cap 11 radially on the stator stack 1. The adjacent terminal block 2, the periphery of which is disposed between the stator stack 1 and the end cap 11, has cutawaysthat allow the ledges 16a to seat on the end face 5. Additionally, internal apertured bosses 24 provide the means for securing the end cap 11 to the end cap 12 via the stator stack 1 by bolts 23 passed through the holes in the bosses 24.

Further bosses 25 (see Figure 2) that project one on each side of the central boss 18 provide means for locating the end cap 11 within the casing of a powertool to be driven by the motor, location being assisted by centrally gapped flanges 26 that project outwardly from annulus 15 and extend over about 60 of arc.

End cap 12 (Figures 3 and 5) also comprises an annulus 27 from which axially extend four fingers 28 that converge inwardly to support an end boss 29 axially spaced from the annulus 27. The fingers 28 are of a channel-shaped cross-section in a plane normal to their length for strength. Each finger incorporates small studs 30 (see Figure 3) whose purpose will be described below.

Moulded into the end boss 29 is a die-cast metal heat sink and bearing support having an outer annulus 31 and an inner annulus 32 connected and supported in spaced relationship by a web 33 apertured as indicated at 34 in Figure 3. The inner annulus 32 supports a plain bearing 134 through which shaft 10 extends. The apertures 34 permit flow of cooling air through the web 33 of the heat sink.

Extending longitudinally from annulus 27 are supports 35 to whose inner ends are secured supports 37 that stem from the end boss 29. The supports 35 have tapered blind bores 38 into which are screwed the bolts 23, the latter having the self-tapping screw threads.

Formed internally of the annulus 27 at the ends of the fingers 28 are individually locating surfaces 28a which, when the end cap 12 is correctly seated with the annulus 27 closely encircling the end face 6, contact the end face 6, the cap 12 being held in position by the bolts 23. The adjacent terminal block 2 has cut-aways that allow the surfaces 28a to seat on the metal end face 6. Four pads 28b integral with the annulus 27 and on the inner face thereof locate the end cap 12 radially on the stator stack 1.

Two brush assembly mountings 36 are formed integrally with the end cap 12. The mountings 36 are of channel form open at both ends and are sized to receive brush boxes one only of which is shown in section in Figure 1 but which will be described below. The channels of the mountings 36 are diametrically aligned and the open side of each channel faces outwards away from annulus 27 in the axial direction of the end 29. The floor of each channel mounting 36 has an upstanding rail 39 wedge-shaped at its outer end as can be seen in the lower part of Figure 3. The side-walls of the channels each have a dent 91 in the form of a shallow recess or notch intermediate the length thereof as shown in Figure 3.

Figures 6 to 9 show a brush box 40. The box is a one-piece metal pressing with a longitudinal slotlike opening 41 along one side, the opening 41 being bounded by two outwardly turned flanges 42.

Pressed out of two opposite side walls of the box 40 are tongues 43, while a terminal tag 44 is pressed out of the fourth side wall of the box leaving a hole 45. The terminal tag 44 is formed into a cylindrical configuration, open at one side, as can be appreciated from Figure 7. End flanges 46 extend outwardly at the one closed end 47 of the box as can be seen in Figures 6 and 8. The two end flanges 46 extend in opposite directions from the two side walls having the tongues 43. As can be seen, the terminal 44 is intermediate the length of the box 40, and the tongues 43 are towards the opposite end of the box to the flanges 46.

Each box 40 contains a carbon brush 48 and a helical spring 49 (see Figure 1) which, in use, urges the brush into electrical contact with the commutator 13. The side walls of each brush box 40 are stressed inwardly slightly so that they grip the carbon brush 48 tightly and hold the latter wholly within the box against the action of the spring 49 located between the brush 48 and the closed end 47 of the box 40. At a point in the assembly cycle to be described below, when the box 40 is entered into the mounting 36, the rail 39 passes between the flanges 42, the wedge-shaped end of the rail 39 forcing the flanges and thus the side walls apart so allowing the brush to move more freely and assume an operative position under the influence of the spring 49 as shown in Figure 1, one end of the brush projecting from the box into electrical contact with the commutator.The height of the rail 39 from the floor of the channel support 36 is approximately equal to the distance by which the flanges 42 extend outwardly from the brush box 40 so that the rail or rib 39 does not contact the carbon brush 48.

Turning nowto Figures lotto 13, to enable the appropriate electrical connections to be made to both the field windings and the brushes, a carrier 50 is provided and this accommodates electrical connectors that make the desired connections.

Carrier 50 comprises a block of a suitable insulating material, for example polycarbonate.

Along the top of the block is a raised passageway 51 open at both ends and formed with a central keyway 52. As can be seen from Figure 13, the passageway 51 does not extend for the full length of the block. At one end, the block has a passageway 53 open not only at both ends but also along the length thereof as indicated at 54, the sides of the block bounding the opening converging as indicated in Figures 10 and 11. The passageway 53 is at right angles to the passageway 51.

Extending inwardly of the block 50 from the base thereof are blind passageways 55 and 56 whose lower ends are belled slightly as can be seen from Figures 11,12 and 13. Extending from the lower ends along the face of the block are shallow recesses 55a, 56a.

Bore-like passageways 55 and 56 communicate with slots 57, 58 respectively. Blind recesses 59, 60 of rectangular cross-section are formed in the bottom face of the carrier 50 adjacent passageways 55 and 56. As can be seen in Figures 11 and 13, the passageway 55 is approximately midway along the length of the carrier 50. The two passageways 55,56 and the passageway 53 are all parallel and extend parallel to the axis of the rotor 9 when the carrier 50 is mounted in position as shown in Figure 1.

Associated with the carrier 50 are three connectors of an electrically-conductive material, and an example of one pair of which is shown in Figures 14to 16, the third being shown in Figures 14 to21.

Figures 14to 16 show two connectors 63 pressed from sheet metal to form a first tubular portion 64 with a longitudinal slit 65 and a laterally-extending tongue 66. The tongue 66 is slotted as at 67, the walls 68 of the slot converging inwardly as can be seen from Figure 16. Tongue 66 is of considerably greater height than the tubular portion 64, the latter being belled as indicated at 69.

The third connector 72 is shown in Figures 14 to 21. The connector 72 is also a metal pressing having a first tubular portion 73 divided longitudinally by a slot 74 and a second tubular portion 65 also with a longitudinal slot 76 and orientated with its axis at right angles to that of portion 73. The portion 73 has contact tongues 77 cantilevered inwardly as shown in Figures 14,20 and 21, and external tongues 78.

The components described above are adapted to be assembled automatically by suitable machinery.

During assembly of the motor in accordance with the invention, the assembly machinery first presses the plain bearing 134 into the tubular extension 21 of the heat sink mounted into end cap 12. The end cap so fitted with its bearing is then shifted to a position suitable for subsequent assembly operations.

A previously assembled stator complete with laminated stator stack, field windings and terminal blocks is then offered up to end cap 12 and located in position with the internal ledges 1 16a referred to above seated squarely on the end face 6 of the stator stack 1 and the pads 16b in contact with the side of the stack. The end cap 12 and stator stack are located with the annulus 27 uppermost over a spigot which enters the plain bearing 134. A fibre washer is then placed over the spigot.

A previously assembled rotor 9 is racked into the assembled stator with the fan end uppermost, the rotor 9 entering via the open end of the stator unit.

During that operation, the rotor shaft 10 enters the plain bearing 134 displacing the spigot and in the process transferring the fibre washer from the spigot to the shaft, the washer being indicated at 136 in Figure 1.

Gauge fingers are now advanced to measure the distance between the fibre washer 136 and upper face 5 of the stator stack after which the fingers are withdrawn.

End cap 11 is then positioned over the now upper end of the rotor shaft 10 and advanced downwardly until the locating surfaces 16a and pads 16b are seated on the end face 5 of the stator stack and in contacting the side of the stack respectively.

The assembly machinery then inserts the selftapping screws 23 through the bosses 24 on end cap 11, through the appropriate passageways in the stator stack 1 and into the blind bores in the support bosses 35. The blades of the fan 14 have previously been aligned to give the necessary clearance to permit insertion of the screws 23 as just described.

The measurement obtained by the gauge fingers is now used to set the end float limits of the rotor 9.

This is accomplished by holding the motor assembly as so far described in a supporting cradle and exerting inward pressure on the outer end of the bearing 19 to move it towards the fan 14 by an amount determined from the gauge fingers measurement to allow a specified amount of axial movement of the rotor relatively to the stator. As will be appreciated, that movement is determined by the axial distance between the inner ends of the bearings 134 and 19.

In this way, the location of the end caps 11, 12 both axially and radially is determined from the end faces 5 and 6 and the side of the laminated stator stack 1, and the alignment of the rotor bearings 19, 134 in the end caps with each other is ensured and consequently the correct alignment and location of the rotor 9.

Brush box sub-assemblies, comprising brushes 48 and springs 49 located in brush boxes 40 previously sub-assembled by the machinery, are then inserted laterally into the outer open ends of the mountings 36, the motor assembly having been rotated through 180 prior to the insertion of the brush box sub-assemblies. At this stage it should be noted that the brushes 48 are a tight fit in the brush boxes 40 and are so retained totally within the brush boxes so greatly facilitating handling.

The openings 41 of the brush boxes 40 engage the tapered rails 39 as the boxes enter the mounting 36.

That engagement frees the brushes which move inwardly towards and into engagement with the commutator 13 under the bias of the springs 49. The insertion of the brush boxes 40 is continued, until the outwardly extending flanges 46 engage against the outer ends of the channel mountings 36, such engagement preventing further inward movement of the brush boxes. When in position, the boxes 40 are held in position against outward movement by the free ends of the tongues 43 engaging in the detent recesses 91 of the sides of the mountings 36.

Connector pins are now inserted via holes 15 (Figure 3) into the terminal posts on the terminal block 2 adjacent the commutator 13 to extend the posts to a level at which electrical contact can be made therewith, sub-assemblies comprising carriers 50 fitted with connectors are located across the upper edges of the mountings 36, one such carrier 50 being shown in Figure 1 and positioned in the manner just described.

In a typical example, the carrier 50 is fitted with a connector 72 and this is done automatically by feeding the portion 73 into the passageway 51 until the portion 75 locates fully in the passageway 53.

Entry of the portion 75 is by way of the lateral opening 54 in the passageway 53. The parts of the tongues 77 that lie outside the tubular portion 73 slide along the keyway 52 of the passageway 51.

Withdrawal of the connector 72 from the passageway 51 is prevented by the tongues 78 which dig into the wall of the passageway if removal is attempted.

At the same time as or subsequently to the insertion of the connector 72, connectors 63 are inserted from below the carrier 50 into the passageways 55 and 56. The tubular portions 64 are entered into the passageways, while the tongues 66 move into the respective slots 57. The belled lower end 69 of the portions 64 fit the correspondingly belled end of passageways 55 and 56.

Thus, as the carrier 50 with its connectors is moved into place, the pair of connector pins extending from the terminal block 2 that are aligned with the carrier enter the portions 64 and 75 of connector 63 in passageway 56 and connector 72 respectively. At the same time, the terminal tag 44 on the brush box 40 enters the portion 64 of the connector located in passageway 55. As can be understood from Figure 1,each brush box 40 is inserted in the appropriate mounting support 36 with the side having the slot 41 facing rearwardly towards the bolt support 35. This orientation of the brush box locates the terminal tag 44 extending forwardly parallel to the axis of the rotor 9 and towards the left end of the motor as seen in Figure 1.

Although not shown, the ring-like terminal block 2 nearest the commutator 13 has a forwardly extending, i.e. to the left in Figure 1, pair of terminals adjacent its top, and a similar pair adjacent its lower section. The upper pair of terminals, together with their previously mentioned connector pin extensions, are parallel to and in a common plane with the brush box terminal tag 44; the connector pins of this pair of upper field terminals being spaced apart opposite female connectors 64 and 76 in the carrier 50, and the terminal tag 44 being located opposite the middle female connector 64. Thus, as the carrier 50 is moved in a direction parallel to the rotor axis and towards the brush box assembly, the pair of terminal block connector pins and the brush box terminal tag 44 simultaneously enter their respective female connectors in the carrier 50.

Although only one carrier 50 is shown in assembled position in Figures 1 and 3, it will be understood that a similar second carrier is assembled diametrically opposite against the other brush box assembly, this second carrier engaging the terminal tag of the other brush box and the lower pair of connector pins of the same terminal block 2.

Electrical connection from one end of one field winding to the appropriate brush 48 is completed by a choke 79 of cartridge fuse form with end connectors located in slots 67 of connectors 63, as shown in Figures 15 and 16, and making electrical connection therewith via the tongues 66. The choke 79 is positioned on the carrier 50 after the connectors have been placed in position thereon.

The choke 79 comprises an induction coil of varnished or enamelled wire wound on a central core with a metal cap 179 at each end functioning as a connector to the wire. This choke of cartridge fuse form is simply pushed into the slots 67 in the metal tongues 66 which simultaneously physically support the choke and effect electrical connection thereto via the metal caps 179.

In some cases, a connector incorporating a conducting link can be used instead of the choke just described. The alternative form of link includes end connectors similar to those referred to in the preceding paragraph and is connected in a similar manner.

After the two carriers have been placed in position, either simultaneously or sequentially, the internal connections between the field windings and brushes of the motor is completed.

Connection to terminals by means of which connection is made to an external supply is effected by conductors inserted into the tubular portions 73 of the connectors 72, electrical contact being completed via one or other of the tongues 77 depending upon the direction of insertion of the conductors.

The above described assembly technique surprisingly enables the end float of the rotor shaft 10 to be controlled to within 0.4 mm, even though the various components such as the end caps, the bearings, the fibre thrust washer and the rotor shaft may have various production tolerances that could otherwise create an unwanted, and in some instances an unacceptable, build-up of tolerances.

This technique has the further advantage of allowing more freedom in the tolerances of the involved components so reducing their cost of manufacture.

It will be appreciated that all electrical tests and mechanical tests can be carried out on the assembled unit shown in Figure 1 priortothe removal of the unit from the assembly machinery before the unit is mounted in a power tool, and appropriate testing means are incorporated in the machinery referred to above.

Thus, the present invention provides a number of ingenious sub-assemblies and new methods for their assembly in the motor that further advance the automation of electric motor assembly.

The above described embodiments, of course, are not to be construed as limiting the breadth of the present invention. Modifications, and other alternative constructions, will be apparent which are within the spirit and scope of the invention as defined in the appended claims.

For example, reference has been made above to the provision of small extensions 30 (see Figure 3) on the fingers 28. Those extensions provide supports and locks for alternatively holding and retaining a printed circuit board with the electrical connections referred to above formed on the board.

The latter will also have sockets or connectors for making electrical contact with the terminal posts to which the ends of the field windings are connected.

The printed circuit board may then be used in place of the carriers 50 described above in some circumstances.

Claims

CLAIMS 1. An electric motor comprising a rotor having a shaft with a commutatorthereon, a stator assembly surrounding the rotor and including a laminated stator stack, first and second end caps of a plastics material and having first and second sets of location surfaces for locating the end caps on the laminated stator stack with the location surfaces of the first set seated upon the end faces of the stack and the location surfaces of the second set contacting the side of the stack, and in which the first and second end caps incorporate bearings rotatably supporting the shaft. 2. An electric motor as claimed in claim 1 and incorporating brush sub-assemblies each comprising a brush holder stressed to grip and hold against movement a brush located in the holder, resilient means located within the holderfor urging the brush outwardly therefrom and into contact with the commutator, and mountings in which the subassemblies are supported in a manner such that the stressing is relieved to an extent sufficient to allow the brushes to move, undertheinfluenceofthe resilient means, into contact with the commutator. 3. An electric motor as claimed in claim 2 in which the mountings are incorporated in the first end cap. 4. An electric motor, as claimed in claim 3, in which: each mounting is of channel form, a wall of the channel having an upstanding rail with a wedge shaped leading portion, and in which each holder has at least two opposed walls stressed to grip and hold against movement a brush located within the box, the latter having defined in the periphery thereof a slot disposed between said opposed walls, and said brush holders being received within said channel form mountings with said rails forced into said slots and sufficiently separating said opposed walls to enable the brushes to move in said brush holders. 5. An electric motor of claim 4, wherein said brush holder has a third wall between said opposed walls said third wall having outwardly extending flanges that define said slot. 6. An electric motor as claimed in any one of claims 1 to 5, wherein each cap comprises an annular portion that fits over the respective end of said stator stack, said sets of location surfaces being formed on the inner face of the annular portion. 7. An electric motor as claimed in any one of claims 1 to 6, wherein: said first end cap includes fingers formed integrally with its annular portion, said fingers extending axially and converging inwardly and terminating in a boss having an overall diameter smaller than the annular portion, a first heat sink mounted in said boss, said first bearing being mounted in said heatsinkfor assisting dissipation of heat generated in said first bearing, said heat sink comprising inner and outer annular members joined by a web apertured to permit the passage of air through the web, said second end cap including fingers formed integrally with its annular portion and extending axially therefrom and supporting at their ends a disc-like member having a boss, a second heat sink having a cylindrical portion mounted in said second end cap boss and carrying said second bearing, and having a disc portion connected to and forming part of said disc-like member, and a fan mounted on said rotor shaft inside said second end cap for drawing cooling airthrough said apertured web of said first heat sink and discharging said cooling air radially outwards over said disc portion of said second heat sink. 8. An electric motor as claimed in any one of claims 1 to 7 further comprising: a carrier having a pair of laterally extending contact tongues having slots therein, and a choke in the form of a cartridge fuse and comprising an induction coil wound on a core with the ends of the coil being connected to metal end caps covering each end of the choke, the choke being mounted on said carrier with said end caps engaging in the slots of said contact tongues. 9. An electric motor substantially as hereinbefore described with reference to the accompanying drawings. Amendments to the Claims have been Filed, and have the Following Effect: (a) Claims 1 to 9 above have been Deleted or Textually Amended. (b) New or Textually Amended Claims have been Filed as Follows:-

1. An electric motor comprising a rotor having a shaft with a commutator thereon, a stator assembly surrounding the rotor and including a laminated stator stack, first and second end caps of a plastics material mounted on opposite ends of the laminated stator stack, each end cap having first and second sets of location surfaces for locating the end cap on the laminated stator stack with the location surfaces of the first set seated upon an end face of the stack and the location surfaces of the second set contacting the side of the stack, and in which each end cap incorporates a bearing rotatably supporting the shaft.

2. An electric motor as claimed in claim 1 in which each end cap comprises an annular portion that fits over the respective end of the laminated stator stack, the first and second sets of location surfaces being provided inside the annular portion.

3. An electric motor as claimed in claim 2 in which the second set of location surfaces of each end cap is provided by pads projecting inwardly from the inner face of the annular portion of the end cap.

4. An electric motor as claimed in claim 2 or 3 in which the first set of location surfaces of each end cap is provided by end faces of fingers.

5. An electric motor as claimed in claim 4 in which the fingers extend from the annular portions of the end caps.

6. An electric motor as claimed in any preceding claim in which a terminal block mounted on an end face of the laminated stator stack has cut aways which allow the first set of location surfaces of the first end cap to seat upon the end face of the laminated stator stack.

7. An electric motor as claimed in any of claims 2 to 6 in which: the first end cap includes fingers formed integrally with its annular portion, the fingers extending axially, converging inwardly and terminating in a boss having an overall diameter smaller than the annular portion, a first heat sink is mounted in the boss, the bearing of the first end cap being mounted in the heat sink for assisting dissipation of heat generated in the bearing, the heat sink comprising inner and outer annular members joined by a web apertured to permit the passage of air through the web, the second end cap includes fingers formed integrally with its annular portion and extending axially therefrom and supporting attheirendsa disc-like member having a boss, a second heat sink has a cylindrical portion mounted in the second end cap boss and carries the bearing of the second end cap, the second heat sink having a disc portion connected to and forming part of the disc-like member, and a fan mounted on the rotor shaft inside the second end cap for drawing cooling air through the apertured web of the first heat sink and discharging the cooling air radially outwards over the disc portion of the second heat sink.

8. An electric motor as claimed in any preceding claim and incorporating brush sub-assemblies each comprising a brush holder stressed to grip and hold against movement a brush located in the holder, resilient means located within the holder for urging the brush outwardly therefrom and into contact with the commutator, and mountings in which the subassemblies are supported in a manner such that the grips of the holders on the brushes are relieved to an extent sufficient to allow the brushes to move, under the influence of the resilient means, into contact with the commutator.

9. An electric motor as claimed in claim 8 in which the mountings are incorporated in the first end cap.

10. An electric motor as claimed in claim 9 in which: each mounting is of channel form, a wall of the channel having an upstanding rail with a wedgeshaped leading portion, and in which each holder has at least two opposed walls stressed to grip and hold against movement a brush located within a box, the latter having defined in the periphery thereof a slot disposed between the opposed walls, and the brush holders being received within the channel form mountings with the rails forced into the slots and sufficiently separating the opposed walls to enable the brushes to move in the brush holders.

11. An electric motor as claimed in claim 10, wherein each brush holder has a third wall between the opposed walls the third wall having outwardly extending flanges that define the slot.

12. An electric motor as claimed in any preceding claim further comprising: a carrier having a pair of laterally extending contact tongues having slots therein, and a choke in the form of a cartridge fuse and comprising an induction coil wound on a core with the ends of the coil being connected to metal end caps covering each end of the choke, the choke being mounted on the carrier with the end caps engaging in the slots of the contact tongues.