In particular, the present invention relates to a brush holder assembly comprising an annular mounting plate positioned on the same axis as a commutator of an electric motor ; a plurality of tubular brush holders mounted radially on the mounting plate are distributed around the commutator, each being approximately as long as the mounting plate is wide.

Background art Normally, each brush holder is made from a substantially flat metal sheet obtained by blanking, has a substantially square shaped cross section and has two side faces that are substantially parallel to each other and to the mounting plate, and another two side faces that are substantially parallel to each other and perpendicular to the mounting plate, the side faces being substantially rectangular in shape.

Each brush holder is slidably engaged by a brush which is held in contact with the commutator by a spring and which is equipped with an electric connecting cable that normally extends from one end of it and is folded around the brush holder and hooked to the mounting plate.

One drawback of prior art brush holders of this type is that as the brush is used up and is pushed along the holder by the spring in order to keep it in contact with the commutator, the electric connecting cable may be damaged by coming into contact with the edge of one of the aforementioned side faces of the holder.

Another drawback of prior art brush holders is that the spring exerts a variable force on the brush, at first strong and then weaker, thus causing irregular wear on the brush.

Thus, the full length of the brush when new means that the force applied by the spring is initially very strong because the spring is fully compressed.

Subsequently, as the brush wears and gets shorter, the force of the spring diminishes considerably.

This means that the force applied by the spring is initially too high and causes excessive wear, whereas it subsequently becomes too low as the brush is used up, causing erratic contact between the brush and the commutator, contact bounce, sparking, etc.

A brush holder with a constant force spring is described in document DE-30 10 036 Al.

Disclosure of the invention One aim of the present invention is to provide an improved brush holder assembly that enables the brushes to wipe the commutator without causing excessive friction and without the risk of damaging the electrical connecting cable.

Another aim of the invention is to provide a brush holder assembly in which the brushes wear at a regular rate.

According to one aspect of it, the present invention provides an electric motor brush holder assembly as defined in claim 1.

The dependent claims refer to preferred, advantageous embodiments of the invention.

Brief description of the drawings Preferred embodiments of the invention will now be described, without restricting the scope of the inventive concept, with reference to the accompanying drawings in which: Figure 1 is a schematic front view of a preferred embodiment of the brush holder assembly according to the invention; Figure 2 is a schematic perspective view of a detail from Figure 1 ; Figure 3 is a side view of the detail of Figure 2; and Figure 4 is a schematic perspective view of another embodiment of the detail from Figure 1.

Detailed description of the preferred embodiments of the invention With reference to Figure 1, the numeral 1 denotes in its entirety a brush holder assembly for an electric motor (not illustrated) comprising an annular mounting plate 2 which, in this instance, is substantially flat and extends coaxially with a longitudinal axis 3 of a rotating commutator 4 of the electric motor (not illustrated).

In alternative embodiments which are not illustrated, the mounting plate 2 may differ from the substantially flat shape illustrated in Figure 1.

The unit 1 also comprises a plurality of tubular brush holders 5 (in this instance, four holders 5), mounted on the plate 2 at regular intervals around the axis 3 and each extending in a radial direction 6.

As illustrated in Figures 2 and 3, each holder 5 is made from a substantially flat metal sheet (not illustrated) which is slightly shorter in length, measured in a direction parallel-to the direction 6, than the width of the mounting plate 2, also measured in a direction parallel to the direction 6.

Each holder 5 has a substantially square shaped cross section and has two side faces 7 that are substantially parallel to each other and to the mounting plate 2, and two side faces 8 that are substantially parallel to each other and perpendicular to the faces 7.

Each holder 5 is slidably engaged by a contact brush 9 a first end 10 of which, under the action of a flat spring 11, is normally in contact with the commutator 4.

The spring 11 has a central portion 12 which extends between the holder 5 and the brush 9 and which is designed to engage a second end 13 of the brush 9, opposite the first end 10, and two rolled up end portions 14, each of which engages a tab 15 protruding outwards from the holder 5.

The tab 15 accommodates the end portion 14 of the spring 11 and is shaped in such a way as to allow the portion 14 to turn with the minimum of friction. For this purpose, the tab 15 comprises a first portion 15a that is substantially perpendicular to the direction 6, and a second portion 15b that is inclined by

an angle such that one end of it allows the spring portion 14 to be stably accommodated and the other end allows it to turn freely with the minimum of friction.

For guiding the central portion 12 of the spring 11 and allowing it to slide easily, the holder 5 also comprises a groove 8a on each of the side faces 8.

During use, as the brush 9 is used up by sliding contact with the commutator 4, the portions 14 roll themselves up in such a way that the spring 11 applies a constant force on the brush 9, causing the brush 9 to move along the holder 5 in the direction 6.

The brush 9 also has an electric connecting cable 16 that protrudes from the end 13, is folded over the mounting plate 2, and, during the movement of the brush 9 along the holder 5 towards the commutator 4, slidably engages an undercut 17, which is made in one of the faces 8, extends in the direction 6 and which is substantially V-shaped with a rounded stop end 18 whose radius of curvature is substantially the same as the radius of the cable 16.

The undercut 17 enables the cable 16 to move freely in the direction 6 along the holder 5 as the brush 9 is used up, thus preventing the cable 16 from being damaged by the holder 5.

Figure 4 illustrates an alternative embodiment of the holder 5.

In this embodiment, the holder 5 has a slot 7a made in its top face 7 for the passage of the cable 16. The slot extends in substantially the same direction as the direction 6 of the holder 5.

To facilitate insertion of the cable 16 and the brush 9 into the holder 5, the brush 9 has a recessed portion 9a for accommodating the cable 16 when it passes through the part of the holder 5 that is not slotted.

The operation of the brush holder unit 1 may be easily inferred from the foregoing and does not require further explanation.