Method of Making a Planar Carbon Segment Commutator

The invention relates to a planar or faceplate commutator in which the contact brushes bear axially against planar contact surfaces of the commutator, instead of bearing radially as in the case of a cylindrical commutator, and in which the contact surfaces are provided by a plurality of segments (normally arranged in an annular ring) having carbon outer layers for engagement with the contact brushes.

One known planar carbon segment commutator, disclosed in the specification of German Utility Model G 89 07 045.3, in the joint names of Deutsche Carbone AG and Robert Bosch GmbH, comprises a base member of insulating material having a rotational axis, a front surface extending, at least in part, transversely to the rotational axis, and a plurality of apertures extending rearwardly from the front surface; a plurality of circumferentially spaced contact members mounted on the front surface; and a plurality of circumferentially spaced overmoulded carbon segments respectively formed on the contact members and each having integral anchor means which extend rearwardly into said apertures.

Although the anchor means provide some radial support for the carbon segments mounted on the contact member, against centrifugal force, and some support against axial withdrawal from the contact member, this support is dependent upon the shear strength of the carbon in the anchor means and on the frictional engagement between the anchor means and the apertures in which they extend. Therefore, to ensure adequate radial and axial support for the contact members and the carbon segments, the contact members have to be provided with rearward extensions insert moulded into the front surface of the base member and undercut recesses for receiving integral rear portions of the overmoulded carbon segments. This involves relatively complex and time-consuming forming and pre-assembly operations. Moreover, the resultant products are not particularly robust.

The purpose of the present invention is to provide a planar carbon segment commutator which, at least to some extent, avoids the disadvantages of known planar carbon segment commutators and to provide a planar carbon segment commutator in which the carbon segments are more firmly secured to the base member and help to fasten the other components of the commutator.

This is achieved by providing a commutator in which each carbon segment is aligned with at least one aperture; at least one aperture aligned with each segment is provided with a first part and, rearwardly of the first part, with a second part having an abutment surface laterally offset from the first part; and the anchor means extending into said one aperture have locking means disposed within the second part of the aperture and engaging the abutment surface to thereby resist axial withdrawal of the anchor means from the aperture.

Thus, according to the invention, there is provided a planar carbon segment commutator for an electric motor, comprising a base member of insulating material having a rotational axis, a front surface extending, at least in part, transversely to the rotational axis, and a plurality of apertures extending rearwardly from the front surface and each provided with a first part and, rearwardly of the first part, with a second part having an abutment surface laterally offset from the first part; a plurality of circumferentially spaced contact members mounted on the front surface; and a plurality of circumferentially spaced overmoulded carbon segments, respectively formed on the contact members, each aligned with at least one aperture and each having integral anchor means which extend rearwardly into said one aperture and have locking means disposed within the second part of the aperture and engaging the abutment surface to thereby resist axial withdrawal of the anchor means from the aperture.

The invention also provides a method of manufacturing a planar carbon segment commutator for an electric motor, comprising the steps of providing a base member of insulating material having rotational axis and a front surface extending, at least in part, transversely to the rotational axis; forming the base member with a plurality of rearwardly extending apertures, including at least one aperture which is aligned with each segment and is provided with a first part and, rearwardly of the first part, with a second part having an abutment surface laterally offset from the first part; mounting a plurality of circumferentially spaced contact members on the front surface; and overmoulding a plurality of circumferentially spaced carbon segments respectively on the contact members so that each segment has integral anchor means which extend rearwardly into said apertures, and include anchor means, extending into the apertures provided with the first and second parts, which are provided with locking means disposed within the second parts and engaging the abutment surfaces to thereby resist axial withdrawal of the anchor means from the apertures.

A single layer of carbon is preferably overmoulded on the front surface of the base member and then divided into said segments by cutting radial grooves which pass through the single layer into the base member.

Thus, when the contact members have been mounted on the base member, a mould is fitted to the base member, the rear ends of the apertures are closed and a mouldable mixture of carbon powder and carrier material is injected into the space between the base member and the mould. This ensures that the mouldable mixture fills those parts of the apertures which are not occupied by the contact members. Any known carrier material, such as phenolic resin, may be used with the carbon powder to form the mouldable mixture, but the choice of carrier material and any subsequent heat treatment will depend on the operating requirements of different commutators, in accordance with known technology which forms no part of the present invention.

Where two anchor means extend rearwardly from a carbon segment, each may be of uniform cross-section provided the two anchor means extend along non-parallel axes. Even when the first of two anchor means extend along an axis parallel with the rotational axis of the base member, the second, non-parallel anchor means will extend into a uniform cross-section aperture having a first part, a second part rearwardly of the first part, and an abutment surface laterally offset from the first part and the second anchor means will have locking means disposed within the second part of the aperture which engage the abutment surface provided by the second part of this aperture to thereby resist axial separation of the carbon segment from the base member. Similarly, because the first of the anchor means extend parallel to the rotational axis of the base member, this first anchor means will resist non-axial separation of the carbon segment from the base member.

An analogous locking effect is also obtained when both anchor means extend rearwardly along non-parallel axes which are also both non-parallel to the rotational axis of the base member.

However, in preferred embodiments of the invention, at least one (and, for ease of manufacture, preferably each) aperture formed in the base member has a second part of greater cross-section than its first part, and the anchor means integral with at least one (and, for ease of manufacture, preferably each) overmoulded carbon segment comprise a single anchor piece which extends into one of these undercut apertures. Both the apertures and the anchor pieces may extend parallel to the rotational axis.

Each such undercut aperture may be frusto-conical in shape, in which case the rearward end of the inner surface of the aperture provides the abutment surface of the second part of the aperture. However, in a preferred form of commutator according to the invention, each undercut aperture formed with first and second parts has an internal bounding surface extending around the periphery of the aperture; at least one portion of said aperture bounding surface defines an inwardly directed, peripherally extending projection; and the locking means of the anchor piece extending into the aperture may comprise a locking portion having an edge disposed rearwardly of and in axial abutment with the peripherally extending projection. This provides a more positive axial attachment of each segment and contact member to the base member.

Advantageously, the base member has side and rear surfaces, each contact member has a radially outer part and at least one gripping part, and each gripping part has a first portion extending rearwardly from the radially outer part of the contact member, along the side surface of the base member, and a second portion extending radially inwards from the first portion, along the rear surface of the base member, to secure the contact member axially to the base member. Moreover, each contact member may have two such gripping parts on opposite sides of a terminal extending from the radially outer part of the contact member.

To provide additional or alternative means of positioning the contact members relative to the base member, prior to overmoulding, the base member is formed with a plurality of axial recesses, each contact member has at least one rearwardly extending locating portion, and the locating portions are respectively disposed within said axial recesses. Moreover, the locating portions may be formed with barbs which engage inner surfaces of the axial recesses to thereby resist axial withdrawal of the locating portions from the axial recesses. This not only helps secure the contact members to the base member in the final assembly, it also ensures that the contact members are firmly located on the base member during the overmoulding process when a mouldable mixture of carbon powder and carrier material is applied to the base member and the contact members.

Preferably, each contact member has at least one abutment surface facing away from the rotational axis and engaging the carbon segment formed on the contact member to resist radial outward movement of the contact member and, conveniently, each contact member is formed with at least one hole having a internal bounding surface and part of said bounding surface constitutes the abutment surface.

In this case, each aperture preferably has a third part disposed forwardly of the first part and having a cross-section which fully overlaps each hole formed in the contact member engaging the carbon segment having the anchor piece extending into the aperture. This ensures maximum radial interlocking between the overmoulded carbon and the contact members.

An embodiment of invention and its method of manufacture are hereinafter described, by way of example, with reference to the accompanying drawings.

FIGS. 1 and 2 are perspective views, respectively, of a base member and a contact member forming part of a planar carbon segment commutator according to the invention;

FIG. 3 is a perspective view of a sub-assembly comprising a base member, as shown in FIG. 1, fitted with a plurality of a contact members, as shown in FIG. 2;

FIG. 4 is a perspective view of the sub-assembly shown in FIG. 3 provided with overmoulded carbon forming an outer layer for the formation of carbon segments;

FIGS. 5 and 6 are perspective views of a planar carbon segment commutator, according to the invention, showing, respectively, the front and rear surfaces of the commutator; and

FIG. 7 is a sectional side elevational of the commutator shown in FIGS. 5 and 6.

A circular base member 1, as shown in FIGS. 1, 3 and 7, has a rotational axis and a front surface including a central annular ring 27, radial strips 2 and support surfaces 28, perpendicular to the axis. The strips 2 are disposed between circumferentially spaced apertures 3 which each comprise a central first part 7 and second and third parts 8 and 24 respectively disposed rearwards and forwards of the first part 7.

As shown in FIGS. 1 and 7, each aperture 3 has a bounding surfaces 11 defining an inwardly directed peripherally extending projection 12 which ensure that the second and third parts 8 and 24 of each aperture 3 have greater cross-sectional areas than the first part 7.

Support surfaces 28, forming part of the front surface of the base member, are provided around the apertures 3 for supporting contact members 4, as shown in FIG. 3, and axial recesses 19 are formed in the base member 1 to receive locating portions 20 provided at the radially inner extremity of each contact member 4, as shown in FIG. 2. Barbs 29 on the locating portions 20 engage with internal surfaces 30 of the axial recesses 19 to resist withdrawal of the locating portions 20 from the axial recesses 19.

A terminal 31 extends from the radially outer edge 16 of each contact member 4 for attachment of an armature lead. Two fingers extend from the radially outer edge 16 of each contact member 7, on opposite sides of the terminal 31, to form gripping parts for securing the contact member 4 axially to the base member 1. As shown in FIG. 6, each gripping part has a first portion 17 extending rearwardly from the radially outer edge 16 of the contact member 4, along a side surface 14 of the base member 1, and a second portion 18 extending radially inwards from the first portion 17, along a rear surface 15 of the base member 1.

When all the contact members 4 are mounted in position on the base member 1, as shown in FIG. 3, the sub-assembly is placed in a mould (not shown) defining an annular cavity surrounding the central annular ring 27 and the second parts 8 of the apertures 3 in the base member 1 are blanked off on the rearward side of the base member 1. A mouldable mixture of carbon powder and carrier material, such as phenolic resin, is then injected into the annular mould cavity. This injected material flows through holes 22 formed in the contact members 4 and fills the apertures 3 in the base member 1 to form integral anchor pieces 6.

As shown in FIG. 7, the rearmost end of each anchor piece 6 forms a locking portion 10 disposed within the second part 8 of the aperture 3 in which the anchor piece 6 is disposed and has an edge 13 disposed rearwardly inwardly of and in axial abutment with an abutment surface 9 provided by the peripherally extending projection 12 to thereby secure the anchor piece 6 against axial withdrawal from the aperture 3.

As shown in FIGS. 3 and 7, the contact members 4 are each formed with three holes 22 having internal bounding surfaces 23. The radially inner portion of each of these bounding surfaces 23 therefore faces away from the rotational axis and serves as an abutment surface 21 with engages the carbon forming segment 5 enclosing the contact member 4 thereby to resist prevent outward radial movement of the contact member 4 relative to the segment 5. Similarly, to the extent that each contact member 4 is otherwise attached to the base member 1, for example, by means of the locating portion 20, the radially outer portion of the bounding surface 23 of each aperture 22 abuts the carbon forming the segment 5 enclosing the contact member 1 to thereby prevent outward radial movement of the segment 5 relative to the contact member 4.

This interlocking of the carbon with the contact members 4 is enhanced by ensuring that the cross-section of the third part 24 of each aperture 3 is large enough to encompass fully all of the holes 22 in the adjacent contact member 4.

The base member 1 and contact members 4 are thus formed separately, assembled together, and then locked together in an overmoulding process-in which a carbon layer 25, for the carbon segments 5, is formed with integral anchor pieces 6 which hold the carbon layer 25 in place on the base member 1. The carbon layer 25 is then separated into segments 5 by cutting radial slots 26 which pass right through the carbon layer 25 into the underlying base member 1. As shown in FIG. 5, these slots 26 also pass through the central annular ring 27 and into the strips 2. The segments 5 are therefore circumferentially separated and insulated from each other.