GB2189653A - Anti:chatter brush assembly - Google Patents

Abstract

To reduce or prevent brush chatter on rotation of a commutator 14 in either direction the brush 12 is urged in the axial direction of the commutator and shaft, by a leaf spring 16 inserted between the brush and the upper cage wall 17. Alternatively the spring may be inserted between the brush and lower wall. The brush is urged by spring 13 against the commutator 14 which rotates with an axial shaft (15, Fig. 2). <IMAGE>

Description

SPECIFICATION Brush assembly The present invention relates to a brush assembly for an electric motor, particularly small electric motors.

In brush assemblies using brush cages a clearance is provided between brush and cage to allow for manufacturing tolerances and to ensure the free movement of the brush in the cage as the brush wears.

When a motor is in operation in one direction the brush is forced to one side of the cage by the force of friction between the brush and the commutator. Some elastic deformation of the brush takes place and stores some of the energy of deformation. The friction between the brush and the commutator is not constant but varies with many factors of geometry and electrical conditions. This results in a stick-slip effect between the brush and the commutator. The slick-slip in turn allows the stored energy in the elastically deformed brush to be returned with the result that the brush moves in the cage, causing chatter. This chatter has a number of undesirable effects, in particular the audible noise which it makes.

In motors which rotate in one direction only a small spring leaf is inserted between the brush and cage at one side thereof to dampen some of the motion of the brush.

The present invention provides a brush assembly for an electric motor, the assembly comprising a brush slidably mounted in a cage, means for urging the brush into contact with a commutator rotating about an axis, and means for biassing the brush in the axial direction of the commutator.

Unexpectedly it has been found that a substantial decrease in noise attributable to brush chatter is achieved. The invention is applicable particularly to bidirectional motors but may also be used in unidirectional motors.

The invention will be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is a plan view illustrating a prior art brush assembly; Figure 2 is a perspective view illustrating an assembly according to the invention; and Figure 3 is a cross-section aiong the line Ill-Ill of Fig. 2.

The sketch of Fig. 1 illustrates the prior art assembly where a carbon brush 1 is slidably received in a housing 2 mounted on a motor end cap 3.

The brush 1 is biassed against the commutator 4 mounted on the motor shaft 5 by a torsion spring 6. If the commutator rotates anticlockwise as seen in Fig. 1, a leaf spring 7 of Beryllium/copper is inserted behind the brush with respect to the direction of rotation, between the housing side wall 8 and the brush 1, to reduce chatter. If the commutator rotates in the reverse direction, audible chatter will result.

Fig. 2 is a perspective view illustrating a brush assembly of the invention. A motor casing 9 carries an electrically insulating end cap 10. A brush housing 11 is secured to the end cap and a brush 12 is slidably mounted in the housing 11. Current is supplied to the brush via a lead (not shown) in the usual way.

A torsion spring 13 passes through a slot 18 in the housing 11 and bears on the outer end of the brush 12, urging it in contact with a commutator 14. Commutator 14 is mounted fast on a shaft 15 and supplies current from the brush 12 to the coils of an armature (not shown) as is well known in the art. The commutator and armature rotate with the shaft 15, which is journalled at one end in the casing 9 and at the other end in the end cap 10.

A leaf spring 16 of Beryllium Copper alloy is inserted between the upper wall 17 of the housing 11 and the brush 12, urging the brush towards the end cap 10. By using the spring in this position it has been found that a substantial reduction in noise can be achieved when the motor shaft and commutator rotate in either direction, clockwise or anticlockwise.

The spring 16 may be placed below the brush 12, between the brush and the housing wi 1 adjacent the end cap 10, however it is easier to assemble the brush assembly with the spring at the top.

Various modifications will be apparent to those skilled in the art, and it is desired to include all such modifications as fall within the scope of the invention as defined herein.

1. A brush assembly for an electric motor, comprising a brush slidably mounted in a cage, means for urging the brush into contact with a commutator rotating about an axis, and means for biassing the brush in the axial direction of the commutator.

2. A brush assembly according to claim 1, wherein the biassing means comprises a leaf spring positioned between the brush and a wall of the cage.

3. A brush assembly according to claim 2, wherein the cage comprises a housing mounted on an end cap and the leaf spring is positioned between the brush and the housing wall distai of the end cap.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Brush assembly The present invention relates to a brush assembly for an electric motor, particularly small electric motors. In brush assemblies using brush cages a clearance is provided between brush and cage to allow for manufacturing tolerances and to ensure the free movement of the brush in the cage as the brush wears. When a motor is in operation in one direction the brush is forced to one side of the cage by the force of friction between the brush and the commutator. Some elastic deformation of the brush takes place and stores some of the energy of deformation. The friction between the brush and the commutator is not constant but varies with many factors of geometry and electrical conditions. This results in a stick-slip effect between the brush and the commutator. The slick-slip in turn allows the stored energy in the elastically deformed brush to be returned with the result that the brush moves in the cage, causing chatter. This chatter has a number of undesirable effects, in particular the audible noise which it makes. In motors which rotate in one direction only a small spring leaf is inserted between the brush and cage at one side thereof to dampen some of the motion of the brush. The present invention provides a brush assembly for an electric motor, the assembly comprising a brush slidably mounted in a cage, means for urging the brush into contact with a commutator rotating about an axis, and means for biassing the brush in the axial direction of the commutator. Unexpectedly it has been found that a substantial decrease in noise attributable to brush chatter is achieved. The invention is applicable particularly to bidirectional motors but may also be used in unidirectional motors. The invention will be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is a plan view illustrating a prior art brush assembly; Figure 2 is a perspective view illustrating an assembly according to the invention; and Figure 3 is a cross-section aiong the line Ill-Ill of Fig. 2. The sketch of Fig. 1 illustrates the prior art assembly where a carbon brush 1 is slidably received in a housing 2 mounted on a motor end cap 3. The brush 1 is biassed against the commutator 4 mounted on the motor shaft 5 by a torsion spring 6. If the commutator rotates anticlockwise as seen in Fig. 1, a leaf spring 7 of Beryllium/copper is inserted behind the brush with respect to the direction of rotation, between the housing side wall 8 and the brush 1, to reduce chatter. If the commutator rotates in the reverse direction, audible chatter will result. Fig. 2 is a perspective view illustrating a brush assembly of the invention. A motor casing 9 carries an electrically insulating end cap 10. A brush housing 11 is secured to the end cap and a brush 12 is slidably mounted in the housing 11. Current is supplied to the brush via a lead (not shown) in the usual way. A torsion spring 13 passes through a slot 18 in the housing 11 and bears on the outer end of the brush 12, urging it in contact with a commutator 14. Commutator 14 is mounted fast on a shaft 15 and supplies current from the brush 12 to the coils of an armature (not shown) as is well known in the art. The commutator and armature rotate with the shaft 15, which is journalled at one end in the casing 9 and at the other end in the end cap 10. A leaf spring 16 of Beryllium Copper alloy is inserted between the upper wall 17 of the housing 11 and the brush 12, urging the brush towards the end cap 10. By using the spring in this position it has been found that a substantial reduction in noise can be achieved when the motor shaft and commutator rotate in either direction, clockwise or anticlockwise. The spring 16 may be placed below the brush 12, between the brush and the housing wi 1 adjacent the end cap 10, however it is easier to assemble the brush assembly with the spring at the top. Various modifications will be apparent to those skilled in the art, and it is desired to include all such modifications as fall within the scope of the invention as defined herein. CLAIMS

1. A brush assembly for an electric motor, comprising a brush slidably mounted in a cage, means for urging the brush into contact with a commutator rotating about an axis, and means for biassing the brush in the axial direction of the commutator.

2. A brush assembly according to claim 1, wherein the biassing means comprises a leaf spring positioned between the brush and a wall of the cage.

3. A brush assembly according to claim 2, wherein the cage comprises a housing mounted on an end cap and the leaf spring is positioned between the brush and the housing wall distai of the end cap.