Carbon commutator

A carbon-segment face commutator assembly for an electric motor includes an annular array of copper conductor sections stamped from a single copper blank. The annular array is overmolded with an electrical-conducting resin-bonded carbon composition which mechanically interlocks the conductor sections and defines a circular commutating surface. During overmolding, radial grooves are formed in a bottom surface of the carbon overmold opposite the commutating surface. An annular hub is then formed by overmolding an insulator material around and under the carbon overmold and the conductor section array. The hub insulator material flows into the radial grooves of the carbon overmold and leaves only the circular commutating surface exposed. The carbon overmold is formed into an annular array of eight electrically-isolated carbon segments by machining radial slots inward from the commutating surface of the carbon overmold to the underlying radial grooves. The slots are cut slightly into the insulator material occupying the radial grooves to ensure that the carbon overmold is completely cut through and the carbon segments are electrically isolated from each other.

TECHNICAL FIELD 
This invention relates generally to a carbon-segment commutator for an 
electric motor and a method for its manufacture. 
BACKGROUND OF THE INVENTION 
Permanent magnet direct current motors are sometimes used for submerged 
fuel pump applications. These motors typically employ either face-type 
commutators or cylinder or "barrel"-type commutators. Face-type 
commutators have planar, circular commutating surfaces disposed in a plane 
perpendicular to the axis of armature rotation. Barrel-type commutators 
have arcuate, cylindrical commutating surfaces disposed on the outer 
surface of a cylinder that is positioned coaxially around the axis of 
armature rotation. Regardless of their commutating surface configurations, 
electric motors used in submerged fuel pump applications must be small and 
compact, have a long life, be able to operate in a corrosive environment, 
be economical to manufacture and operate and be essentially 
maintenance-free. 
Submerged fuel pump motors must sometimes operate in a fluid fuel medium 
containing an oxygen compound, such as methyl alcohol and ethyl alcohol. 
The alcohol increases the conductivity of the fuel and, therefore, the 
efficiency of an electrochemical reaction that deplates any copper motor 
components that are exposed to the fuel. For this reason, carbon and 
carbon compositions are sometimes used to form carbon segments with 
segmented commutating surfaces for the motors. This is because carbon 
commutators do not corrode or "deplate", as copper commutators do. 
Commutators with carbon segments also typically include metallic contact 
sections that are in electrical contact with the carbon segments and 
provide a terminal for physically connecting each electrical contact to an 
armature coil wire. 
It is known to form a carbon commutator by first molding and heat treating 
a moldable carbon compound or machining heat-treated carbon or 
carbon/graphite stock. Such an arrangement is shown in German Disclosure 
3150505.8. A commutator-insulating hub may then be formed to support the 
metallic substrate. The hub may be molded directly to the metallic 
substrate either before or after the carbon is bonded to the metallic 
substrate. Slots are then machined through the carbon article and the 
metallic substrate to separate the carbon article and substrate into a 
number of electrically isolated segments. An inner diameter, outer 
diameter and the commutating surface of the commutator may also need to be 
machined. 
After the completed commutator is assembled to an armature, a clamshell 
mold may be positioned over the newly assembled commutator-armature in a 
final overmolding process. An open end of the clam shell mold is made to 
seal around the commutator in a manner that leaves the commutating surface 
exposed. Insulator material is then injected into the clam shell mold. 
Once the insulator material has cured, the clam shell mold is removed. 
This final overmolding step protects copper armature windings and other 
corrosion-prone elements from chemically reacting with ambient fluids such 
as oxygenated fuels. The overmolding also secures wires to reduce 
potential for stress failures and to maintain a corrected dynamic balance 
level. Overmolding will also reduce windage losses in the pump. 
Where, in manufacturing such a commutator, cuts are machined into or 
through a metallic substrate, metal chips may be produced. These metal 
chips can lodge in the slots between segments causing electrical failures. 
Machining into a metallic substrate can also expose the cut portions of 
the substrate to the corrosive effects of oxygenated fuels. 
Where the carbon and metal substrate portions of a commutator are 
machined-through to form electrically isolated segments, some type of 
support structure must be provided to strengthen the commutator and 
mechanically bind the carbon segments and conductor sections together. 
Such support structures sometimes require substantial additional axial 
space for the commutator, which can increase the overall axial length of 
the armature-commutator assembly and or reduce the size and the quantity 
of wire wound in the armature. 
For some types of electrical-conducting resin-bonded carbon compositions, 
an insulating surface skin characteristically forms on exterior surfaces 
of the composition as it cures. This skin forms an impediment to 
electrical contact between the carbon composition and the metallic 
conductor sections. Therefore, a carbon commutator using such a 
composition must provide an electrical path through the insulating surface 
skin. 
One approach to solving these problems is disclosed in U.S. Pat. No. 
5,386,167 issued Jan. 31, 1995 to Strobi (the Strobi patent). The Strobi 
patent shows a carbon disk made up of an electrical-conducting 
resin-bonded carbon composition. To avoid problems associated with 
machining into metal substrates, the carbon disk is overmolded onto eight 
pie-piece-shaped copper segments then radially cut between the segments to 
form eight electrically isolated carbon segments. A plastic substrate 
holds the copper segments in position for carbon overmolding and provides 
mechanical interlock between the carbon segments. However, the plastic 
substrate increases the axial thickness of the commutator. In addition, 
the Strobi patent does not provide structures that would provide an 
electrical path through carbon composition skinning or structures that 
might otherwise reduce electrical resistance. 
What is needed is a carbon-segment commutator that is stronger and provides 
lower electrical resistance through increased carbon to copper contact 
within the carbon segments and through any insulating surface skin that 
might form. What is also needed is a method for manufacturing such a 
commutator that requires less machining time and provides longer tool 
life. 
SUMMARY OF THE INVENTION 
In accordance with this invention a carbon-segment commutator assembly is 
provided in which a carbon disk is molded over a pre-stamped metallic 
substrate having an upturned projection, and an insulator hub is molded 
over the carbon-overmolded substrate prior to cutting radial slots. The 
commutator assembly comprises an annular array of at least two 
circumferentially-spaced conductor sections arranged around a rotational 
axis and an annular array of at least two circumferentially-spaced carbon 
segments formed of a conductive carbon composition. Each carbon segment is 
molded onto at least one surface of a corresponding one of the conductor 
sections with the annular array defining a segmented commutating surface 
of the commutator. An overmolded insulator hub is disposed around and 
between the carbon segments. The insulator hub mechanically interlocks the 
carbon segments. Each conductor section has at least one conductor 
projection that is at least partially embedded in a corresponding one of 
the overmolded carbon segments. 
According to one aspect of the present invention a method is provided for 
making the carbon-segment commutator assembly described above. The method 
includes forming the annular array of conductor sections then forming a 
carbon overmold by molding an electrical-conducting resin-bonded carbon 
composition onto the annular conductor section array. During carbon 
molding, inner grooves are formed in an inside surface of the carbon 
overmold opposite the commutating surface. Next, the insulator hub is 
formed by overmolding the carbon overmold and conductor section array with 
insulator material that at least partially occupies the inner grooves and 
mechanically interlocks the carbon segments. Finally, machining slots 
inward from the commutating surface of the carbon overmold to the inner 
grooves forms the annular array of electrically isolated carbon segments. 
Unlike prior art commutators, the filled inner grooves of the present 
invention leave only a thin section of the carbon segment to be machined 
through to electrically isolate the carbon segments. This provides at 
least three benefits: shallow slots result in a stronger and/or an axially 
shorter commutator, less machining time is required to cut the slots, and 
tool wear is reduced resulting in extended tool life. 
In addition, the conductor projections of the present invention reduce 
electrical resistance by increasing surface area contact between the 
conductor sections and their corresponding carbon segments. The 
projections also provide lower electrical resistance through increased 
carbon to copper contact within the carbon segments and provide an 
electrical path through any insulating surface skin that might form over 
carbon segments made of certain carbon compositions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A planar face-type carbon-segment commutator assembly for an electric motor 
is generally shown at 12 in FIGS. 1-3 and 9. The commutator assembly 12 
comprises an annular array of eight circumferentially spaced conductor 
sections, generally indicated at 14 in FIGS. 1-11. Each conductor section 
14 is a thin, flat, roughly triangular piece of copper. The conductor 
sections 14 are arranged around a commutator rotational axis 16 as shown 
in FIGS. 1-9. Each conductor section 14 has the same general sectorial 
configuration as all the other conductor sections 14. In other words, and 
as best shown in FIG. 4, each conductor section 14 has the shape of a pie 
piece cut from a circular, radially-cut pie. 
As generally indicated in FIGS. 1, 2, 8 and 9, the commutator assembly 12 
also comprises an annular array of eight circumferentially spaced carbon 
segments 18. Each carbon segment 18 has the same general sectorial 
configuration as all the other carbon segments. The segments 18 are 
initially formed as a single annular carbon disk as shown at 20 in FIG. 6. 
The carbon disk 20 is made from an electrical-conducting resin-bonded 
moldable conductive carbon composition before being cut into eight equal 
segments 18. The carbon disk 20 or "overmold" is overmolded onto the 
conductor section 14 array so that when the disk 20 is cut, each carbon 
segment 18 is left formed onto an upper surface of a corresponding one of 
the conductor sections 14. The annular array of carbon segments 18 has a 
segmented, circular upper surface 22 that serves as the segmented 
commutating surface of the commutator. 
An overmolded insulator hub, generally indicated at 24 in FIGS. 1-3, is 
circumferentially disposed around, under and between the carbon segments 
18 and conductor sections 14. When cured, the insulator hub 24 
mechanically interlocks the carbon segments 18. The insulator hub 24 has a 
generally cylindrical shape with a cylindrical armature shaft aperture 26 
disposed coaxially along the commutator rotational axis 16. As shown in 
FIG. 9, the cylindrical armature shaft aperture 26 is shaped to receive an 
armature shaft 28. 
Each conductor section 14 has two integral upturned conductor projections, 
shown at 30 in FIGS. 4 and 5. The conductor projections 30 extend from 
opposing diagonal edges of an upper surface 32 of the conductor section 
14. When the carbon composition is overmolded onto the conductor section 
14 array, the upturned projections 30 are embedded in the overmolded mass 
20. After the carbon disk 20 is cut into segments 18, each of the upturned 
projections 30 of each conductor section 14 remains embedded in a 
corresponding one of the overmolded carbon segments 18. The embedded 
projections 30, because of their shape and location within the carbon 
segments 18, reduce electrical resistance by increasing surface area 
contact between each conductor section 14 and its corresponding carbon 
segment 13 as will be discussed hereinafter in greater detail. 
Each conductor section 14 in the conductor section 14 array includes a 
circular conductor section aperture, shown at 34 in FIGS. 2 and 4. A 
conductor section aperture 34 is disposed approximately midway between an 
inner apex 36 and an outer semi-circumferential margin 38 of each 
conductor section 14. As shown in FIGS. 4 and 6-8, at the inner apex 36 of 
each conductor section 14 is a rectangular apex tab 40. As is best shown 
in FIGS. 1-3, a tang 42 extends integrally and radially outward from the 
outer semi-circumferential margin 38 of each conductor section 14. 
As shown in FIGS. 4 and 5, the conductor projections 30 are bent-up 
portions that extend integrally upward from the conductor sections 14. 
Each conductor section 14 includes two such bent-up projections 30. Each 
bent-up projection 30 is elongated and rectangular in shape and is bent-up 
(i.e., bent axially outward) from its respective conductor section 14 
along a lower elongated margin. 
Each conductor section 14 is embedded between the insulator hub 24 and one 
of the overmolded carbon segments 18. The tang 42 of each conductor 
section 14 protrudes radially outward from the insulator hub 24. 
As is best shown in FIGS. 1 and 8, each carbon segment 18 has the general 
shape of a piece of a radially-cut circular pie, i.e., the same general 
shape as each conductor section 14. However, each carbon segment 18 is 
longer, wider and thicker than each conductor section 14. Each carbon 
segment 18 has an inner apex wall 44 and an outer semi-circumferential 
peripheral wall 46. Both the inner apex wall 44 and the outer 
circumferential wall 46 of each carbon segment 18 have stair-stepped 
profiles which define an inner shelf-detent 48 and an outer shelf-detent 
50, respectively. 
The carbon segments 18 are made of an injection-molded and hardened 
composition of graphite powder and carrier material with the graphite 
powder making up 50-80% of the total composition weight. The carrier 
material is preferably a polyphenylene sulphide (PPS) resin. While this 
composition is suitable for practicing the invention, other carbon 
compositions known in the prior art are suitable for use in the present 
invention depending upon the application in which the armature is used. 
In other embodiments, metal particles may be embedded in the composition of 
carbon powder and carrier material to reduce electrical resistance between 
each conductor section and its corresponding carbon segment by improving 
carbon segment surface conductivity. The total metal content of the 
composition in such embodiments would be less than 25%. The metal 
particles could have one or more of a number of different configurations 
to include powder flakes. The metal particles would preferably be made of 
silver or copper. 
Radial interstices, generally indicated at 52 in FIGS. 1, 2, 3, 7 and 8. 
separate the carbon segments 18. Each of the interstices 52 has an inner 
groove portion 54 and an outer slot portion 56. The inner groove portions 
54 are formed during carbon overmolding. The outer slot portions 56 are 
formed by machining the commutating surface 22. 
The insulator hub 24 has flat upper and lower surfaces disposed adjacent 
the upper and lower edges of the circumferential sidewall. The 
circumferential hub sidewall is disposed perpendicular to the upper and 
lower surfaces of the hub 24. As best shown in FIG. 2, the armature shaft 
aperture 26 includes upper 58 and lower 60 frusto-conical sections that 
taper inward from larger upper and lower outer diameters to a smaller 
inner diameter. An inner portion 62 of the armature shaft aperture 26 has 
a constant diameter, i.e., the smaller inner diameter, along its axial 
length. 
An alternative carbon segment commutator assembly construction is generally 
indicated at 12a in FIG. 2A. Reference numerals with the suffix "a" in 
FIG. 2A indicate alternative configurations of elements that also appear 
in the embodiment of FIG. 2. Where a portion of this description uses a 
reference numeral to refer to FIG. 2, I intend that portion of the 
description to apply equally to elements designated by numerals having the 
suffix "a" in FIG. 2A. As shown in FIG. 2A, each carbon segment 18a 
encases one of the conductor sections 14a. This arrangement maximizes both 
strength and electrical contact area between each carbon segment 18a and 
its corresponding conductor section 14a. 
The inner groove portions 54 of the interstices 52 are filled with the 
insulator material of the hub 24. Hub insulator material is also disposed 
around the circumference of the carbon segment 18 array and encases the 
outer shelf-detent 50 of each carbon segment 18. Hub insulator material 
that forms the armature shaft aperture 26 also encases the inner 
shelf-detent 48 of each carbon segment 18. 
As is best shown in FIG. 3, the insulator hub 24 includes a circumferential 
land 64 that extends completely around a circumferential sidewall of the 
insulator hub 24. The land 64 has an axial width that extends from the 
protruding conductor section tangs 42 to the unfilled outer slots 56 of 
the interstices 52. As shown in FIG. 9, the circumferential land 64 
provides a circumferential sealing surface to mate with a corresponding 
surface 65 of a clamshell-type mold 67. The clamshell-type mold 67 is used 
in a final insulation overmolding process that is explained in greater 
detail below. 
The hub insulator material comprises a glass-filled phenolic available from 
Rogers Corporation of Manchester Connecticut under the trade designation 
"Rogers 660." Other materials that would be suitable for use in place of 
Rogers 660 include high-quality engineering thermoplastics, i.e., 
thermoplastics that exhibit a high degree of stability when subjected to 
temperature changes. 
In other embodiments, the annular arrays of conductor sections 14 and 
carbon segments 18 may include either more or less than eight sections, 
respectively. Also, the carrier material of the carbon composition may 
comprise a phenolic resin with up to 80% carbon graphite loading, a 
thermoset resin or a thermoplastic resin other than PPS, such as a 
liquid-crystal polymer (LCP). Both PPS and phenol type resins withstand 
long term exposure to fuels and alchohols. Other embodiments may also 
employ a commutator assembly 12 of the cylindrical or "barrel" type rather 
than the face-type commutator shown in the figures. 
In other embodiments the conductor section projections 30 may have any one 
or more of a large number of possible configurations designed to increase 
carbon to copper surface contact. For example, rather than comprising 
single bent-up portions of the conductor sections as shown at 14 in FIGS. 
4 and 5, the projections may instead comprise separate elements, crimped 
into place under a bent-over finger 66 extending from the conductor 
sections 14' as shown in FIG. 10. As is also shown in FIG. 10, the 
separate elements 30' may take the form of a plurality of narrow elongated 
metallic strands. In FIG. 10, a wire brush-like bundle of metallic strands 
is shown crimped to a conductor section 14' by bending a metal finger 66 
away from the conductor section 14' and crimping the finger 66 over the 
wires. 
As shown in FIG. 11, other embodiments could include tangs 42" formed with 
terminations 68 that each include a pair of slots for receiving insulated 
electrical wires, i.e., "insulation displacement"-type terminations. When 
an insulated wire is forced laterally into one of these slots, metal edges 
defining the sides of the slot cut through and force apart the wire 
insulation to expose and make electrical contact with the wire. 
In embodiments using insulation-displacement type tang terminations 68, 
wires extending from the armature windings 69 could be forced into the 
respective terminals 42" either during or after armature winding process. 
This would eliminate the need to weld or heat-stake the wires to the tang 
terminations 68. 
In practice, the carbon commutator described above is constructed by first 
forming the annular array of conductor sections 14. This is done by 
stamping the annular array from a single copper blank 70 as shown in FIGS. 
4 and 5. The stamping process leaves each conductor section 14 connected 
by a thin, radially extending metal strip 72 to an unstamped outer 
periphery 74 of the copper blank 70. The thin copper strips 72 allow the 
outer periphery 74 to act as a support ring that holds the conductor 
sections 14 in position, following stamping, for the subsequent steps in 
the commutator construction process. 
The carbon overmold 20 is then formed, s shown in FIGS. 6 and 8, by molding 
the carbon composition onto an upper surface 32 of the annular conductor 
section 14 array. The carbon composition is overmolded in such a fashion 
as to completely cover and mechanically interlock the conductor sections 
14. 
In the carbon overmolding process the carbon composition flows into each 
conductor section aperture 34 and over each peripheral edge of each 
conductor section. However, as is best shown in FIGS. 4, 6 and 8, the apex 
tab 40 of each conductor section 14 is left exposed by the carbon overmold 
20. The apex tabs 40 extend radially inward into the armature aperture 26. 
The carbon composition also envelops the integral upturned conductor 
projections 30. This allows the projections 30 to extend through the 
thickness of an insulating surface skin that characteristically forms on 
exterior surfaces of a carbon overmold 20 as the carbon composition cures. 
By extending through the insulating skin, the projections 30 serve to 
reduce the electrical resistance of the contact by increasing the amount 
of surface area contact between carbon and copper. Also in the carbon 
overmolding process, the radial groove portions 54 of the interstices 52 
are molded into an inside or bottom surface 76 of the carbon overmold 20 
opposite the commutating surface 22 and between the conductor sections 14. 
The grooves 54 may, alternatively, be formed by other well-known means 
such as machining. 
As shown in FIGS. 1-3, the hub 24 is then formed by a second overmolding 
operation that covers the carbon overmold 20 and conductor section 14 
array with the hub insulator material. During this hub overmolding 
process, the hub insulator material surrounds the carbon overmold 20 and 
the conductor sections 14. The hub insulator material also completely 
fills the radial grooves 54 that were formed in the bottom surface 76 of 
the carbon overmold 20 in the carbon overmolding process, i.e., the inner 
groove portions 54 of the interstices 52. Only the commutating surface 22 
portion of the carbon overmold 20 is left exposed after the hub 
overmolding operation is complete. 
As the insulator hub 24 is being overmolded, insulator material that is 
formed around the circumference of the carbon segment 18 array also flows 
over the outer shelf-detent 50 of each carbon segment 18 as is best shown 
in FIG. 2. Insulator material that is formed around the armature shaft 
aperture 26 flows over the inner shelf-detent 48 of each carbon segment 
18. After the hub insulator material has hardened over the inner 48 and 
outer 50 shelf-detents of each carbon segment 18 and after the insulator 
has hardened under the carbon segments 18 and conductor sections 14, the 
hardened hub insulator material serves to mechanically retain the carbon 
segments 18 in relation to each other. In addition, the hardened hub 
insulator material secondarily retains the carbon segments 18 to their 
respective conductor sections 14. 
After the hub 24 has been overmolded onto the carbon overmold 20 and 
conductor section array, a portion of the outer periphery 74 of the 
unstamped copper blank 70 is trimmed away from around the overmolded 
insulator hub 24. Once the periphery 74 has been cut away, each strip 72 
is bent to form a short tang 42 of each connecting strip 72 that is left 
protruding radially outward from an outer circumferential surface of the 
hub 24. The tangs 42 are thus positioned and configured for use in 
connecting each conductor section 14 to an armature wire extending from an 
armature winding. 
As is best shown in FIGS. 1-3, the annular array of electrically-isolated 
carbon segments 18 is then formed by machining the shallow radial slots 56 
inward from the exposed commutating surface 22 of the carbon overmold 20 
to the underlying radial grooves 54. The slots 56 can be formed by contact 
or non-contact machining techniques including, but not limited to, those 
using serrated tooth saws. 
Because the radial slots 56 are in direct overlying alignment with the 
radial grooves 54, the radial slots 56 can be cut completely through the 
carbon overmold 20 and slightly into the insulator material that occupies 
the radial grooves 54. This ensures that the carbon overmold 20 is cut 
completely through and the carbon segments 18 completely separated and 
electrically isolated from each other. The insulator-filled radial grooves 
54 and the radial slots 56 therefore meet within the commutator and form 
the interstices 52 between the carbon segments 18 as described above. 
The insulator-filled radial groove portion 54 of each interstice 52 
constitutes approximately half of the depth of each interstice 52. 
Consequently, to cut the remaining half of the depth of each interstice 52 
requires only a relatively shallow slot 56. 
Finally, the completed commutator assembly 12 is assembled to an armature 
assembly 80 as shown in FIG. 9. The clamshell mold 67 is then positioned 
over the newly assembled commutator-armature assembly, generally indicated 
at 81 in FIG. 9. While positioning the clamshell mold 67 over the 
commutator-armature assembly 81, the sealing surface 65 of the clamshell 
mold 67 is made to seal around the circumferential land 64. Insulator 
material is then injected into the clamshell mold 67. Once the insulator 
material has cured, the clamshell mold 67 is removed. This final 
overmolding step is intended to protect copper armature windings 69 and 
other corrosion-prone elements from chemically reacting with ambient 
fluids such as gasoline. 
A commutator manufacturing process accomplished according to the present 
invention involves no copper machining and, therefore, produces no copper 
shavings and chips that can lodge between carbon segments 18. In addition, 
no copper is left exposed to react with ambient fluids such as gasoline. 
Because a commutator assembly 12 constructed according to the present 
invention requires only shallow slots 56 in its commutating surface 22 to 
electrically isolate its carbon segments 18, the completed commutator 
assembly 12 is stronger and better able to resist breakage. As an 
alternative to a stronger commutator assembly, the hub 24 of the 
commutator assembly 12 may be designed to be axially shorter, allowing the 
commutator-armature assembly to either be designed axially shorter or to 
carry more armature windings 69. In other words, designers can capitalize 
on the shorter hub length by either shortening the overall 
commutator-armature assembly or including more armature windings 69. 
One other advantage of the shallow slots 56 is that they allow for the 
circumferential land 64 between the tangs 42 and the slots 56. By 
providing a convenient sealing surface for a clam shell mold, the 
circumferential land 64 eliminates the need for a more complicated 
operation that involves masking the slots 56 to prevent the outflow of 
overmolding material into and through the slots 56. 
This is an illustrative description of the invention using words of 
description rather than of limitation. Obviously, many modifications and 
variations of this invention are possible in light of the above teachings. 
Within the scope of the claims, one may practice the invention other than 
as described.