Patent Publication Number: US-6667565-B2

Title: Planar carbon segment commutator

Description:
FIELD OF THE INVENTION 
     This invention relates to a planar carbon segment commutator for use with brushes which bear axially against planar contact surfaces of the commutator, instead of bearing radially as in the case of a cylindrical commutator. 
     BACKGROUND OF THE INVENTION 
     It is known, for example from EP 0583892, to provide a planar commutator in which a plurality of commutator terminals are mounted on a commutator base and overmoulded with carbon segments. However, the terminals of these known planar commutators each have tangs to which the armature winding of an electric motor has to be connected. 
     A number of known methods for effecting such connections are in popular use. Where the winding is formed of low temperature wire, it is usual to employ a soft solder and flux method. Alternatively a cold crimp onto wire that has been stripped of insulation is used in order effect a connection. When dealing with high temperature wires it is necessary to apply heat, and also possibly to apply flux so as to remove the coating of insulation from the ends of the wire. 
     However, there are a number of inherent problems and undesirable side effects associated with all of the foregoing methods. 
     Heat causes embrittlement of the copper wire which is used for most armature windings and encourages rapid oxidation. The use of heat also demands a strong structure to support the commutator in order to minimize plastic distortion. This requirement usually demands the use of high temperature compression grade molding material. A further common problem is caused by the accidental stripping of insulation during winding of the armature which is often automated. As the wire passes over the metal of the commutator damage can be caused to the wire insulation and such damage will often be manifest as a short circuited winding. Additionally, there is always a danger of slack in the winding wire causing fretting under the acceleration due to centrifugal and inertial forces. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a planar carbon segment commutator comprising a commutator base of insulating material, the base having a rotational axis, front and rear surfaces, extending, at least in part, transversely to the rotational axis, and a plurality of first apertures extending through the base, a plurality of commutator terminals each of which comprises a terminal portion and a contact portion, the contact portion of each terminal extending through a respective first aperture in the base and being bent to lie against or in close proximity to the front surface of the base and the terminal portion of each terminal having two cutting edges for cutting insulation on a connector portion of a winding and a slot which in use straddles and grips said connector portion, and a plurality of carbon segments formed on the front surface of the base and over the contact portions, respectively, of the terminals. 
     Preferably, the commutator includes a housing having a plurality of housing recesses for receiving respective terminal portions. 
     Preferably, each housing recess has associated therewith means for positioning connector portions of the winding relative to each recess, the base, the terminals and the housing being such that with a single translational movement of the base relative to the housing, the terminal portions enter the housing recesses, the cutting edges strip insulation from connector portions of the winding and the slots establish and maintain electrical contact with connector portions of the winding by insulation displacement. 
     Preferably, the base has a cylindrical skirt extending rearwardly of its rear surface for receiving the housing. 
     Preferably, the front surface of the base has a plurality of recesses and each contact portion overlies a respective recess and has at least one aperture through which material forming a respective commutator segment extends into the recess to assist in anchoring the segment to the terminal. 
     Preferably, the base has a plurality of second apertures communicating with the recesses and through which material forming the commutator segments extends to assist in anchoring the segments to the base. 
     Preferably, the base has a plurality of third apertures through which material forming the commutator segments extends to assist in anchoring the commutator segments to the base. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view from the front and side of a commutator base of one embodiment of a planar commutator according to a first aspect of the invention; 
     FIG. 2 is a perspective view from the rear and one side of the commutator base shown in FIG. 1; 
     FIG. 3 is a plan view of the assembled commutator; 
     FIG. 4 is an underneath plan view of the assembled commutator; 
     FIG. 5 is a section taken along the line A—A of FIG. 3; 
     FIG. 6 is a section taken along the line B—B of FIG. 4; 
     FIG. 7 is a perspective view of a commutator terminal on an enlarged scale; 
     FIG. 8 is a developed view of the terminal shown in FIG. 7; 
     FIG. 9 is a perspective view of a housing for the terminals; and 
     FIG. 10 is a fragmentary sectional view of part of the housing of FIG.  9 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The commutator shown in the drawings is intended for use with small electric motors, particularly permanent magnet dc motors. 
     Referring firstly to FIGS. 1 and 2, the commutator base  10  shown therein is of molded material and comprises a circular front wall  11  and a cylindrical skirt  12  extending rearwardly from the front wall  11 . The base  10  also has a central boss  13  by which the base  10  can be fitted to an armature shaft (not shown). 
     A plurality of circumferentially spaced axially extending ribs  14  are provided on the inner surface of the skirt  12 , for a purpose that will be explained later. 
     The front wall  11  has a central aperture  45  aligned with the boss  13 , eight, equi-angularly spaced, elongate radially extending recesses  15  and an elongate, slit-like, aperture  16  radially aligned with each recess  15 . 
     Each recess  15  communicates at its radially inner end with an aperture  17 . 
     Each recess  15  is also associated with two apertures  18 , one on either side of a respective recess  15  and adjacent its radially outer end. 
     The front wall  11  also has an outer ring of angularly spaced apart slots  19 . 
     The commutator terminal  20  shown in FIGS. 7 and 8 comprises a terminal portion  21  and a contact portion  22 . The contact portion  22  is in the form of a finger having three apertures  23 ,  24  and  25  therein. The terminal portion  21  is rectangular (as shown in developed view) with its minor axis coincident with the longitudinal axis of the contact portion  22 . The terminal portion  21  has a central cut out portion  26  which is symmetrical with respect to both the major and minor axes of the terminal portion  21 . The cut out portion  26  reduces from its largest width at the center of the terminal portion  21  to two slots  27 . Two cutters  28  project a short distance into each slot  27 . These cutters  28  form sharp edges for cutting insulation on a connector portion of an armature winding. The terminal portion  21  also has two barbs  29  for a purpose which will become apparent later. 
     To assemble the terminals  20  to the base  10 , the fingers  22  are pressed through respective apertures  16  in the base  10  and the fingers  22  are then bent over respective recesses  15  to extend radially inwards. 
     Carbon commutator segments  30  are then formed on the front wall  11  of the commutator base  10  over the fingers  22 . This may be achieved by hot pressing a disc of green graphite material onto the front wall  11  and then cutting the disc into eight individual segments  30 . Green graphite material is a graphite mixture prior to sintering or heat treating during which the binder material is set. During the hot pressing, the binder is softened (possibly liquified) and this allows the mixture to flow under pressure through the apertures  23 ,  24  and  25  in the fingers  22  and into the recesses  15 , into the slots  19  and through the apertures  17  and  18 , as best shown in FIGS. 5 and 6, to anchor the disc to the base  10 . The binder, being of thermoset material such as phenolic resin, once melted and cooled becomes heat resistant, creating a stable contact surface for the commutator. As an alternative to the hot pressing process an overmoulding process can be used. In this latter process, the components, namely the commutator base  10  and the terminals  20  are placed into a mould and graphite material is injected into the mould after the latter has been closed. The hot pressing or molding process creates a good electrical connection with the fingers  22 . 
     Referring now to FIGS. 9 and 10, there is shown therein a housing  35  for the terminal portions  21  of the terminals  20 . This housing  35  is of crown-like shape and has a central boss  36  for receiving the armature shaft and eight radially outwardly extending housing portions  37  equally spaced around the circumference of the boss  36 . Each of the housing portions  37  defines a housing recess  38  and is used to effect connection between a respective portion of the armature winding and one of the terminal portions  21  of the terminals  20 . Each housing portion  37  has side walls  39 , an end wall  40 , and a cover  41 . The side walls  39  are parallel to the longitudinal axis of the boss  36 . 
     A stump  42  projects centrally from the internal surface of the end wall  40  and extends within the housing portion  37  for approximately half the length of the side walls  39 . The stump  42  extends parallel with the longitudinal axis of the boss  36  and is only connected to the housing  35  by the end wall  40 . Each side wall  39  has a slot  43  which extends parallel to the longitudinal axis of the boss  36 , from the commutator end of the housing  35  for a length which terminates at the level of the free end of the stump  42 . A portion of an armature winding can be passed through the slots  43  so that the winding portion rests on the end of the stump. 
     During assembly of the armature of an electric motor, the housing  35  is placed on the armature shaft. The lead wire of the armature winding is inserted into one of the housing portions  37  by laying the end of the wire in the slots  43  provided in the side walls  39 . The wire is drawn back into the housing portion  37  until it rests against the stump  42 . From this start, the first armature coil is wound. At the end of the first coil winding, the armature is indexed and the wire is laid in the same manner in the next housing portion  37  without breaking the continuity of the wire. This process is repeated until all coils have been wound and the tail end of the winding is then laid in the slots  43  of the first housing portion  37  and pushed back until it is adjacent to the lead end which was placed against the stump  42  at the beginning of the winding operation. The wire is then cut and the armature removed from the winding machine. 
     The housing  35  now has a winding portion comprising insulated wire laying in each of the housing portions  37 . Each of the winding portions is under tension and is pulled tight against the respective stump  42 . The commutator base  10 , together with the terminals  20  and commutator segments  30 , is then slid along the armature shaft so that the terminal portions  21  of the terminals enter respective housing portions  37  and the housing portions lie between the ribs  14 . As each terminal portion  21  approaches a winding portion held in a housing portion  37 , the slots  27  move over the wire. The cutters  28  severe the insulation on the wire which is deformed as the slots move over the wire. Intimate metal to metal contact is thereby provided between the wire and the terminal portions  20 . The barbs  29  grip the cover  41  of the housing  35  and therefore retain the terminal portions  21  within the housing  35 . 
     This manner of manufacture of a commutator lends itself to an automated process. No application of heat is required and the associated risk of distorting the housing  35  is therefore avoided. No embrittlement of the winding wire is caused and problems associated with oxidation are also avoided. The use of flux is negated and there is no chemical reaction or consequent erosion resulting from the connection. The armature winding can be a single continuous winding and the danger of introducing slack by breaking the winding to effect a connection to each coil can be avoided. 
     The above embodiment is given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention as defined in the appended claims.