GB2214721A - Brushgear in an electric motor - Google Patents

Abstract

An electric motor comprises a housing in the form of a casing 1 and end cap 14 carrying stator magnets 8, 9 and brushgear. A rotor 10 comprising a wound armature 12 and commutator 13 on a shaft 11 is rotatably mounted in the housing. The brushgear comprises a brushholder, which may be of liquid crystal polymer material, having brushes 25 slidable in cages 18 under the action of coil spring 26. Holes 41 are formed in the brushes 25 to enable them to be engaged by a divider or the like to urge them against the force of the springs 26, to keep the brush dear of the commutator 13 during assembly. <IMAGE>

Description

BRUSHGEAR IN AN ELECTRIC MOTOR The present invention relates to brushgear in an electric motor, in particular a fractional horsepower permanent magnet direct current (PMDC) motor.

PMDC motors typically comprise a stator housing comprising a can-like casing carrying permanent magnets and brushgear, and a rotor comprising a wound armature and commutator rotatably mounted in the housing. During assembly the motor is inserted into the casing, and the brushgear, which may be mounted on an end cap is then mounted on the casing. It is important to ensure that brushes of the brushgear are not damaged during assembly, and to this end a mechanism is provided to ensure that the brushes remain well clear of the commutator and other, adjacent rotor parts until the brushes can be released onto the commutator. It is particularly difficult to provide a low cost, reliable and simple mechanism to operate system for caged brushes.In such brushgear the brushes are housed in a tube-like cage and urged axially of the tube onto the commutator by a coil spring or leaf spring acting on the outer end of the brush.

GB2113924 shows a system in which the cage walls grip the brush to hold it against the spring pressure, the walls being urged apart to release the brush onto the commutator as the brush cage assemblies are inserted in the motor end cap. In GB1562986 a lug temporarily holds the brush against the force of the biassing spring, and the brush is then pushed beyond the lug after assembly. In US3628075 the brushes are held against the force of the biassing spring by a hook which engages the brush shunt wire, which carries current to the brush from the motor terminals. It is common practice to hold the brush back by pulling on the shunt wire, but this can weaken the connection of the brush to the wire.

According to the present invention there is provided an electric motor comprising a housing carrying a stator and brushgear, a rotor rotatably mounted in the housing and comprising a commutator and a wound armature mounted on a shaft, wherein the brushgear comprises a brush having opposite ends slidably mounted in a cage and projecting through an open end thereof to bear at one end on the commutator and biassing means for biassing the brush against the commutator, wherein a reentrant formation is provided in an outer surface of the brush between the opposite ends thereof to enable the brush to be held against the force of the biassing means during assembly of the motor by engagement of the reetrant formation with a tool.

Preferably the reentrant formation is a hole in the surface of the brush.

Preferably the cage has a wall having a slot therein, the slot being positioned adjacent the path of the brush hole as the brush moves in the cage, so that the hole can be accessed when the brush is withdrawn into the cage.

Other preferred features and advantages of the invention will be apparent from the following description and the accompanying- claims.

The invention will be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is an exploded perspective view of a fractional horsepower permanent magnet direct current motor in accordance with the invention with the brushgear removed; Figure 2 is a partly cut-away plan view of brushgear of the motor of Figure 1; Figure 3 is an underneath plan view of the brushgear of Figure 2; Figure 4 is a cross-sectional view of a brush holder of the brushgear of Figure 2 on line IV-IV of Figure 3; Figure 5 is a cross-sectional view on line V-V of Figure 3 of the brushholder; and Figure 6 is a partly cut-away side view of a brush of the brushgear of Figure 2.

Referring to the drawings, Figure 1 shows a fractional horsepower PMDC motor having a deep drawn can-like steel casing 1 which has a pair of opposed flat sides 2, 3, opposed curved sides 4, 5 and an end wall 6. End wall 6 carries a bearing 7 for a rotor shaft (vide hereinafter). A pair of magnets 8, 9 are held against the insides of the curved side walls 4, 5 by springs and/or ears formed in the casing walls (not shown) in conventional manner.

A rotor 10 of the motor comprises a shaft 11 carrying a wound armature 12 and commutator 13. The casing 1 is closed by a plastics end cap 14 which carries a bearing 15 and has a skirt 16 which fits snugly inside the casing 1. Shaft 11 of the rotor 10 is journalled in the bearings 7, 15 of the assembled motor. The end cap 14 is held on the casing 1 by lugs pressed from the casing sidewalls to engage in recesses in the outer surface of the end cap (not shown).

The construction thus for described is well-known in the motor art.

Figure 2 to 6 show brushgear for the motor of Figure 1. Referring to Figure 2, the brushgear comprises a brushholder 17 having integrally formed brush cages 18. The holder 17 is moulded from liquid crystal polymer material, such as XYDAR sold by Dartco Manufacturing Co. of the U.S.A.

The holder 17 comprises a flat annular ring 19 having inner and outer cylindrical walls 20, 21 extending perpendicular to the plane of the ring 19. The outer wall 21 is dimensioned to fit inside the casino 1 and to abut the magnets 8, 9. A gap is formed in a flat side 22 of the outer wall 21 and is bridged by a bridging piece 23 which extends above the plane of the upper edge 24 of the wall 21 (Figure 4).

The brush cages 18 each houses a brush 25 which is urged towards the centre of the ring 19 by a coil spring 26. The brushes each have a shunt wire 27 embedded in an upper wall 28 thereof. The shunt wires 27 are each electrically connected to a power supply lead 29 by the crimping portion 30 of an eyelet terminal 31. The eylets are attached to the brushholder 17 by rivets which engage in apertures 32 in the annular disc 19. The leads 29 pass through the gap in the outer wall 21, beneath the bridge 23.

Referring to Figure 5, the brush cages 18 are of box-like configuration having a bottom wall 33, formed by the annular ring 19, sidewalls 34, 35, a rear wall 36 formed by the outer wall 21 and an upper wall 37. The bottom wall 33 has two slots 38 separated by a rib 39. A slot 40 is formed in the upper wall 37. Slots 38, 40 are arranged so that mould members can penetrate the inner volume of the cage 18 from above and below the plane of ring 19, through the slots 38, 40, to exclude the moulding plastics from the inner volume, thus avoiding the need for any radially moving mould parts. The slot 40 is open at its radially inner end to allow a shunt wire 27 to be slid therein. A coil spring 26 is housed between the rear wall 36 of a cage 18 and the respective brush and urges the brush radially inwardly of the brushholder 17.

In accordance with the invention, a hole 41 is formed in the upper surface 28 of each brush 25. Each hole 41 is formed to be in line with the slots 40 in the upper walls 37 of the cages 18 and is positioned between the shunt wire 27 and an arcuate commutator contacting surface 42 of the brush. The brushes are generally square in cross-section and are chamfered at their corners 43.

To assemble the motor, a spring 26 is inserted in each brush cage 18. The brushes are connected to the supply leads 29 and inserted in respective cages 18, the eyelets 31 being rivetted to the ring 19.

Pointed ends of a brush separating tool (not shown) or the like are then inserted in the holes 41 in the brushes to urge them radially outwards against the force of the springs 26, compressing the springs The rotor 10 is inserted in the casing 1, and then the assembled brushgear is slid into the open end of the casing 1, the brush separating tool serving to keep the brushes 25 retracted within the cages 18 and so clear of the commutator 13 to avoid damage to the brush faces 42. The bridge piece 23 of the holder 18 is received in a cut-out 45 in a side wall 2 of the casing 1.

After the brushgear is slid home in the casing 1 against the magnets 8, 9 the brush separating tool is removed to allow the brushes to settle on the commutator 13. The end cap 14 is then slid over the free end of the shaft 11, a cut-out 46 in the skirt 16 fitting around the bridge piece 23.

In place of the holes 41, a recess in the form of a reentrant step or a channel which may extend to a sidewall or the radially inner face of the brush may be provided, the aim being to provide a reentrant surface which the brush separating tool can bear on to urge the brush radially outwards.

Various modifications may be made to the described embodiment and it is desired to include all such modifications as fall within the scope of the accompanying claims.

Claims (14)

1. An electric motor comprising a housing carrying a stator and brushgear, a rotor rotatably mounted in the housing and comprising a commutator and a wound armature mounted on a shaft, wherein the brushgear comprises a brush having opposite ends and being slidably mounted in a cage and projecting through an open end thereof to bear at one end on the commutator, and biassing means for biassing the brush against the commutator, wherein a reetrant formation is provided in an outer surface of the brush between the opposite ends thereof to enable the brush to be held against the force of the biassing means during assembly of the motor by engagement of the reetrant formation with a tool.

2. An electric motor as claimed in claim 1, wherein the reentrant-formation is a hole in the surface of the brush.

3. An electric motor as claimed in claim 1 or 2, wherein the biassing means comprises a coil spring positioned between a radially outer end of the brush and an end wall of the cage.

4. An electric motor as claimed in claim 2 or 3, wherein a radially extending slot is provided in a wall of the cage in alignment with the hole.

5. An electric motor as claimed in claim 4, wherein the slot is provided in a wall of the cage which is transverse to an axial direction of the motor and distal of the wound armature.

6. An electric motor as claimed in claim 4 or 5, wherein the brush is electrically connected to a supply lead via a wire which is embedded at one end in the brush, the wire being embedded in the brush at a position in radial alignment with the hole.

7. An electric motor as claimed in claim 6, wherein the wire is connected to the supply lead by a crimped terminal which is attached to a structural part of the motor.

8. An electric motor as claimed in any one of claims 1 to 7, wherein the brushgear comprises a brushholder which is integrally moulded.

9. An electric motor as claimed in claim 8, wherein the brushholder is moulded of liquid crystal polymer material.

10. An electric motor as claimed in claim 8 or 9, wherein the brushholder comprises a flat annular ring having generally cylindrical walls formed at inner and outer peripheries thereof.

11. An electric motor as claimed in claim 8, 9 or 10, wherein the brushholder is configured to be moulded between mould pieces which are relatively movable in a single, common axial direction.

12. An electric motor as claimed in claim 11, wherein a bottom and upper walls of a brush cage of the brusholder are slotted, the slots being juxtaposed to enable mould pieces to be inserted into the enclosed space of the cage to fill the space.

13. An electric motor as claimed in any one of claims 8 to 11, wherein the brushholder is separate to an end cap of the motor.

14. An electric motor substantially as hereinbefore described with reference to the accompanying drawings.