GB2203595A - A PMDC motor incorporating a liftable brush switch - Google Patents

Abstract

A PMDC motor is provided with a switch which operates by lifting a brush off the commutator, eliminating the need for an additional switch in the circuit. A cover 11 is mounted on the outside of the end cap 2 and connects with a finger 16 inside the end cap via a shaft 14. The shaft 14 and finger 16 rotate with the plate 11 on the axis of the shaft 14, and a nib 16d on the finger 16 engages a brushleaf 7a to lift the brush 8a clear of the commutator. During assembly, the other brush may be held clear of the commutator by a removable pin passing through holes 40,39. <IMAGE>

Description

PMDC motor incorporating a switch The present invention relates to a PMDC motor incorporating a switch.

In fractional horsepower PMDC (permanent magnet direct current) motors, often termed micromotors, running at low voltages, typically 1.2 to 4.8 Volts, the overall resistance of the motor and related circuitry must be kept low to generate a high power output. Typically resistance of the motor is 50 milliohoms and the resistance of the remaining circuitry including the internal resistance of the battery or cell is of the same order. The provision of a switch in the circuitry provides a point of resistance, at the switch contacts.

The present invention provides a PMDC motor having a housing, a rotor including an armature and a commutator, a pair of brushes biassed against the commmutator for feeding current to the armature, and switch means for lifting a said brush away from the commutator, wherein said switch means comprises a switch member mounted on the housing for movement between first and second positions, said switch member lifting the said brush clear of the commutator when in the first position and allowing the brush to contact the commutator when in the second position.

In this way the need for a switch in the circuitry can be eliminated. The switch member may be operated manually or remotely, for example by a linkage to a manual switch or by a solenoid etc.

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 a perspective view of a PMDC motor, embodying the invention; Figure 2 is a cross-sectional view on the line II-II of Figure 1 with a brush and brushholder removed therefrom for clarity; Figure 3 is an exploded view of part of an end cap assembly of the motor of Figure 1 showing a switch member; Figure 4 is a cross-sectional view along the line IV-IV of Figure 2; Referring to the drawings, Figure 1 shows a PMDC motor incorporating a switch according to the invention. The motor comprises a housing 1 comprising a metal casing 2 and a plastics end cap 3. A motor shaft 4 is journalled in bearings 5 in opposite ends of the housing 1 (the bearing in the end cap 3 only is shown, Figure 4).

The casing 2 supports permanent magnets (not shown) and a rotor comprising armature coils (not shown) and a commutator 6 mounted on the shaft 4 is contained in the housing. The end cap 3 supports brush gear comprising a pair of brushes 7 each supported by resilient brush leafs 8 which are held in the end cap 3, only one brush and brush leaf is shown in the drawings, Figure 2. The brush leafs 8 are rivetted by rivets 10 to the end cap in contact with tags 9 which extend out of the housing 1 for connection to a power suppply (not shown).

The construction thus far described is well known in the art and need not be described in more detail here.

Referring to Figure 2, the invention provides a switch in the motor which lifts one of the brushes, 7a, off the comnutator 6, thus switching off the motor by cutting the power supply to the armature.

Referring to Figure 3, the switch mechanism comprises a moulded plastics cover 11 which is mounted on the outside of the end cap 2. Cover 11 comprises a plate 43 having an elongate, kidney shaped aperture 12 which. surrounds shaft 4 and on its inside a kidney shaped collar 24 which surrounds a boss 13 on the end cap 2. Boss 13 forms a housing for shaft bearing 5.

Collar 24 and boss 13 cooperate to limit the pivoting movement of the plate 11. Aperture 12 is sized to be just clear of the shaft 4 to allow it to rotate freely.

Cover 11 has a shaft 14 which is fast with the cover 11. A lower portion 14a of the shaft is received in an aperture 15 in the end cap 2. A finger 16 is mounted fast in a bore 14c through the shaft 14.

Finger 16 has a first arm 16a having a bifurcated end 16b. Arm 16a is inserted in the bore 14c of shaft 14 and the bifurcated end 16b spread apart to lock the arm in the bore 14c. Extending perpendicularly from the arm 16a is a dog leg 16c carrying a nib 16d at its end extending perpendicularly down from the dog leg 16c.

A coil spring 26 is mounted about an upper portion 14b of the shaft 14. A first leg 26a of the spring extends through an aperture 27 in the cover 11. Leg 26a engages under a lip 28 of a stop 29 integrally formed on the end cap 2. A second leg 26b of the spring is received in a recess 29 formed in an upper surface 30 of the cover 11. Leg 26b engages under lips 31, 32 at the top of the recess. The spring is configured to urge the legs 26a, 26b apart, thus urging the cover 11 into first position, the direction of arrow A (Figure 1).

A recess 33 is provided in a side of the cover 11 to facilitate access to the spring arm 26b, and a cutout 34 in the cover side wall 35 allows access to the spring arm 26b and stop 29.

A flange 36 is formed on the cover 11 distal of the shaft 14 and slidably engages under a lip 37 on the end cap 2 to hold the cover on the end cap. Lip 37 is a snap fit under the lip 37.

Also shown in Figure 3 are ventilation holes 38 in the end cap 2 (these are omitted from Figure 2 for clarity), and a pin hole 39 for use in assembling the end cap on the casing 2, as will be described hereinafter. A hole 40 is provided in the cover 2 to align with hole 39 when the cover is in the first position.

To assemble the motor the rotor comprising shaft 4, the armature and the commutator 6 is mounted in the casing 2 with permanent magnets housed in the casing as is well known in the art.

The finger 16 is positioned inside the end cap with arm 16a projecting through aperture 15.

The brush gear comprising brushes 7, brush leaves 8 and terminals 9 is fixed in the end cap 2 by rivets 10, brush leaf 8a laying across dog leg 16c.

The spring 26 is placed over the post 14b, arm 26a being inserted through aperture 27 to lie between the collar 24 and side wall 35, and arm 26b clipped into the channel 29 below lips 31, 32.

The cover 11 is placed on the end cap, shaft 14 being slid over the finger arm 16a and through aperture 15. Flange 36 is snapped under the lip 37, collar 24 fitting around the boss 13.

The bifurcated end 16b of finger 16 projects up into shaft portion 14b and is spread apart to lock the finger in the shaft.

Spring arm 26a is clipped under the lip 28, access to the arm being gained through cut out 34 if necessary.

Spring 26 urges the cover 11 to the first position, thus urging brush leaf 8a and brush 7a away from the commutator region, as seen in Figure 2.

When assembling the end cap on the motor casing it is necessary to hold both brushes clear of the commutator to avoid damage to them. The second brush is held clear by means of a pin inserted through apertures 39, 40, the pin engaging the brush leaf in known fashion and being removed after the end cap 3 is installed on the casing 2. The end cap 3 is held on the casing by tabs 41 being turned over into half cylindrical recesses 42 in the end cap side walls, as is well known in the art.

Figure 2 shows the finger 16 in full outline with the switch in its first or off position, that is with the nib 16d of finger 16 engaging the brushleaf to move it to the right in Figure 2, lifting the brush off the commutator 6. Shown in dotted outline in Figure 2 is the finger 16 with the switch in the on position. The switch pivots with shaft 14 to the extent allowed by the collar 24 and boss 13, to allow the brushleaf 8a to bring the brush 7a onto the commutator 6. The switch is configured to move the finger 16 far enough to allow for wear on the brush 7a, i.e. to allow the brushleaf 8a itself to come into contact with the commutator 6 (if the brush 7a is worn away completely) without engaging the nib 16d when the switch is in the on position.

As the switch moves from the second to the first position the nib 18 engages the brushleaf 7a, carrying it with the switch as it rotates.

Preferably the pivot point of the finger 16, i.e.

shaft 14, is close to the end 19 of the brushleaf 7a, that is the end about which the brushleaf 7a tends to pivot as it flexes.

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 (11)

1. A PMDC motor having a housing, a rotor including an armature and a commutator, a pair of brushes biassed against the commutator for feeding current to the armature, and switch means for lifting a said brush away from the commutator, wherein said switch means comprises a switch member mounted on the housing for movement between first and second positions, said switch member lifting the said brush clear of the commutator when in the first position and allowing the brush to contact the commutator when in the second position.

2. A PMDC motor as claimed in claim 1, wherein the switch member is pivotally mounted on the housing for pivoting movement between the first and second positions.

3. A PMDC motor as claimed in claim 2, wherein the switch member is mounted on an end cap of the motor, the end cap carrying the brushes.

4. A PMDC motor as claimed in claim 3, wherein the switch member comprises an actuating member mounted on the outside of the end cap and a finger which extends from the actuating member to engage the brush, or a brushleaf supporting the brush, to move the brush away from the commutator.

5. A PMDC motor as claimed in claim 3, wherein the switch member comprises an actuating member on the outside of the end cap, a shaft extending from the actuating member into the housing through the end cap and rotatable therein, and a finger extending from an end of the shaft within the housing to engage the brush, or a brushleaf supporting the brush, to move the brush away from the commutator.

6. A PMDC motor as claimed in claim 4 or 5, wherein the brushes are mounted at ends of brushleafs mounted in the end cap and arranged to bias the respective brushes against the commutator, and the finger of the switch member engages the brush leaf when moving from the second to the first position, to move the brush leaf, lifting the brush away from the commutator.

7. A PMDC motor as claimed in anyone of claims 3 to 6, wherein the end cap has a boss projecting externally of the motor, and the switch member comprises a plate mounted on the outside of the end cap and having a collar which receives the boss, the collar and boss cooperating to guide the switch member.

8. A PMDC motor as claimed in anyone of claims 1 to 7, wherein the switch member is biassed into the second position.

9. A PMDC motor as claimed in claim 7, wherein the plate is pivotally mounted on the cover by means of a shaft extending from the plate through an aperture in the end cap, the finger having an arm received in a bore in the shaft to lock the finger in position relative to the shaft.

10. A PMDC motor substantially as hereinbefore described with reference to the accompanying drawings.

11. A motor driven device incorporating a PMDC motor as defined in any one claims 1 to 10, wherein the switch member is operated via manually operable push button means and arranged to be in the second, on, position while mannual pressure is applied to the push button means, the switch member moving to the first position when the push button means is released.