Electrodynamic machine with brush leaves

Abstract: Each of the two brush leaf means of an electric motor comprises a first resiliently flexible conductive section (1), attached to a terminal (11), and an additional resiliently flexible section (5) extending an acute angle to the first section (1). The first and second ends (2 and 4) of the first section (1) are disposed on opposite sides of a contact portion (8) of the additional section (5) which engages a commutator (9) forming part of the motor. It is therefore possible to increase the length of the first section (1) to thereby provide robust brush leaf means which are nevertheless capable of flexing sufficiently to accommodate discrepancies in the mounting of the brush leaf means and in concentricity of the commutator (9). Such brush leaf means are fabricable of a single stamped sheet.

Description: The invention relates to a rotary electrodynamic machine such as an electric motor and more particularly to the brush leaves thereof. In one known electric motor construction, the first ends of two relatively rigid brush arms are pivotally mounted on an end cap and the opposite ends of a tension spring are connected to the second ends of the brush arms to urge these second ends towards the commutator. As a result, the brushes are suitably pressed into engagement with the commutator. However, installation of the brush arms is complicated, particularly if automated procedures are adopted. It is also normally necessary to fit insulators between the brush arms and the opposite ends of the tension spring, thus increasing the cost and adding to the difficulty of installing the brush arms. To overcome these problems, it has been proposed, in GB 987 375, to replace the tension spring with two blade springs pressed, respectively, from the two brush arms and arranged to engage the brush arm support means to urge the brush arms resiliently towards the commutator. Another solution is to replace the relatively rigid brush arms with resiliently flexible brush leaf means. Brush leaf means of this form each comprise a first end, which is rigidly attached to a brush leaf support in the form of a terminal, and a contact portion. Hnwever, as a result of a manufacturing imperfections, it is impossible to ensure concentricity of an electric motor commutator or precise mounting of the first end of the brush leaf means. The brush leaf means must therefore be mounted so that the contact portion is resiliently urged towards the commutator and the brush leaf means must have sufficient resilient deformability to accommodate the maximum anticipated spacial discrepancies while maintaining adequate pressure between the commutator and the contact means incorporated in or supported by the contact portion of the brush leaf means. One way to accommodate constructional tolerances is to provide brush leaf means of very thin material which can easily flex. However, in this case, the brush leaf means are not normally sufficiently robust and are not always capable of providing sufficient resilient loading on the commutator. An alternative solution is to form the brush leaf means of thicker material, but of greater length from a first, terminal end of the brush leaf means to the contact portion. However, the extent to which this solution can be applied is limited by the size of the electric motor and, normally, by the size of the end cap in which the brush leaf means are supported. It has therefore been proposed to increase the effective length of the brush leaf means accommodated within a prescribed space by forming the brush leaf means as a plurality of sections which are connected to each other, but extend in different directions. Thus. as disclosed in GB-A-2 180 999, brush leaf means, for an electrodynamic machine, comprise a first resiliently flexible conductive section which has a first end, for fixed, non-pivotal attachment to a brush leaf support, and a second end; and at least one additional resiliently and independently flexible conductive section, which each have first and second ends, wherein the first end of each additional section is attached to the second end of another resiliently flexible conductive section; at least one said additional section extends in spaced relation to an adjacent resiliently flexible conductive section in a direction, from first end to second end, having at least a major directional component opposite the direction in which said adjacent section extends from the first end to the second end of said adjacent section; and one additional section has a free second end, to which no first end of any other additional section is connected, and a contact portion. However, the construction disclosed in GB-A-2 180 999 is limited to brush leaf means in which the first resiliently flexible conductive section extends, from first end to second end, away from the contact portion. As a result, the first and second ends of the first resiliently flexible section are disposed on the same side of the contact portion. This arrangement imposes severe limitations on the effective length of the brush leaf means in that there is normally very little space between the support to which the first end of the first resiliently flexible conductive section is attached, and the outer enclosure of the electrodynamic machine. US-A-2852653 discloses a miniature DC motor in which the first and second ends of the first resiliently flexible section of each brush leaf are disposed on opposite sides of the contact position. This arrangement allows the brush leaves to have a greater effective length. However, the two brush leaves extend from the same side of the motor. Thus, in operation when one leaf is in tension, the other is in compression. This shifts the point of contact between the brushes and the commutator, resulting in a shift in the 180 electrical degrees phase shift between the brush leaves. This could lead to greater electrical noise and greater wear by electrical corrosion in one or both brushes. Also, the first and second sections of each brush leaf are joined by an arcuate section and extend parallel to one another. This is difficult to mass produce. It is the purpose of the invention to at least reduce the disadvantages of known brush leaves. According to the invention, there is provided an electrodynamic machine comprising two brush leaves and a brush leaf support, each brush leaf comprising a first resiliently flexible conductive section which has a first end for fixed, non-pivotal attachment to the brush leaf support and a second end; and a second resiliently and independently flexible conductive section which is connected at one end to the second end of the first section and which is directed opposite to the first section at an acute angle, the second section being free at its other end and having a contact portion between its ends for sliding engagement with a rotary contact of the electrodynamic machine, the first ends of the first sections of the two brush leaves being attached to the brush leaf support on opposite sides of the rotary contact so as to extend away from the brush leaf support in opposite directions to one another, wherein the first end of each first section is disposed on the opposite side of said contact portion to its second end and on the same side of said contact portion as the second end of the other first section. This form of construction provides brush leaves in which the first resiliently flexible conductive sections can extend from one side of the outer enclosure of the electrodynamic machine to the other, thus providing a longer and more flexible conductive section to accommodate discrepancies in the positioning of the brush leaves and radial inaccuracy in the position of the rotary contact, such as a commutator, engaged by the brush leaves, whilst avoiding the drawbacks referred to in connection with the arrangement disclosed in US-A-2852653. The contact portion of each brush leaf may be adapted to support contact means, such as a carbon brush, for engagement with a rotary contact forming part of the electrodynamic machine. However, in a simpler form of construction, the contact portion may be adapted for direct engagement with the rotary contact and, in a preferred embodiment, the second resiliently flexible conductive section having the contact portion comprises at least two fingers. Preferably, each resiliently flexible conductive section of each brush leaf extends rectilinearly, but at least one section may be curved. It is also possible that adjacent sections extend laterally alongside each other. In a preferred embodiment, both sections are parts of a single stamped sheet. The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:- Figures 1 to 3 are plan, section side elevation and end elevation views of a brush assembly of an electric motor according to the invention; Figure 4 is an axial view of the interior of an end cap of an electric motor fitted with two brush assemblies as shown in Figures 1 to 3 and showing a commutator of the electric motor engaged by the brush assemblies; and Figures 5 and 6 are schematic sectional views of part of an electric motor, in side elevation and plan, showing the position of one of the brush assemblies relative to the other parts of the electric motor. As shown in Figures 1 to 3 each brush leaf of the electric motor comprises a first resiliently flexible conductive section 1 attached, at a first end 2, to a laterally extending tab 3 of a brass sheet terminal 11 by means of rivets 12. The brush leaves are stamped from a sheet of beryllium copper and include a second resiliently flexible conductive section 5, in the form of three laterally spaced fingers 10, having a first end 6 attached to a second end 4 of the first section 1, remote from the first end 2 riveted to the tab 3, and a second end 7 remote from its first end 6. As shown in Figures 3 and 4, the second section 5 formed by the laterally spaced fingers 10 extends at an acute angle to the first section 1 attached to the tab 3 and, as shown more clearly in Figures 4 and 6, each of the fingers 10 forming the second section 5 is formed with a contact portion 4 which engages a commutator 9 of the electric motor. As shown in Figure 4, the first ends (2) of the first section (1) are supported on opposite sides of the rotary contact (9) so as to extend away from respective tabs 3 in opposite directions to one another. As shown in Figures 4 to 6, the electric motor has a canister 13 fitted with an end cap 14. The motor shaft 15 carrying the commutator 9 is mounted in a selfaligning bearing 16 seated in the end cap 14 and the terminal 11 extends through a slot 17 formed in the end cap 14. Spurs 18 formed on opposite sides of the terminal 11 engage opposite sides of the slot 17 to secure the terminal 11 more firmly in the slot 17. Although the brass sheet terminals 11 are thicker than the beryllium copper sheet from which the brush leaf means are stamped, in order to provide a rigid external connection, the beryllium copper sheet is relatively thick, thus providing robust brush leaf means. However, because of the extended length of the first section 1 of each brush leaf, the brush leaves are quite flexible and capable of being resiliently loaded gainst the commutator 9 even though the mounting of the terminal 11 in the end cap 14 and the concentricity of the commutator 9 are subject to variation, in different motors, as a result of manufacturing tolerances.

Claims: An electrodynamic machine comprising two brush leaves and a brush leaf support, each brush leaf comprising: a first resiliently flexible conductive section (1) which has a first end (2), for fixed, non-pivotal attachment to the brush leaf support, and a second end (4); and a second resiliently and independently flexible conductive section (5), which is connected at one end to the second end (4) of the first section (1) and which is directed opposite to the first section (1) at an acute angle, the second section (5) being free at its other end and having a contact portion (8) between its ends for sliding engagement with a rotary contact (9) of the electrodynamic machine, the first ends (2) of the first sections (1) of the two brush leaves being attached to the brush leaf support on opposite sides of the rotary contact (9) so as to extend from the brush leaf support in opposite directions to one another; characterised in that: the first end (2) of each first section (1) is disposed on the opposite side of said contact portion (8) to its second end (4) and on the same side of said contact portion (8) as the second end (4) of the other first section (1). An electrodynamic machine as claimed in claim 1, characterised in that the second section (5) of each brush leaf comprises at least two fingers (10). An electrodynamic machine as claimed in claim 1 or claim 2, characterised in that the first and second sections (1 and 5) of each brush leaf form part of a single stamped sheet.

Inventors: STEINER JEAN PIERRE, STEINER, JEAN PIERRE
Applicants: JOHNSON ELECTRIC SA, JOHNSON ELECTRIC S.A.
Application year: 1993
Main classification: H01R39
IPC codes: H02K13, H01R39, H02K5