Brush wear indicator

An indicator is provided in a dynamoelectric machine such as a DC electric motor wherein brushes are provided in brush holders to cooperate with a rotatable electrical conductor in the machine, for example, slip rings or a commutator. The indicator gives an indication when a brush has worn to a point where it should be replaced, and then an alarm sounds or a light is illuminated. The brush wear indicator includes an indicator contact which is electrically connected to the indicator means and it is insulated from but mounted to move in accordance with the brush wear movement. The contact is provided on the end of an electrical conductor which is embedded in the rear axial end of the brush so that the indicator contact will become exposed and the insulation covering it will be worn away upon sufficient brush wear. This completes an electrical circuit between the indicator contact and the commutator to thus provide an indication at the indicator means.

BACKGROUND OF THE INVENTION 
Brush wear indicators for dynamoelectric machines, primarily motors with 
commutators, have previously been suggested. U.S. Pat. No. 4,024,525 shows 
a construction wherein an axial groove is formed in the edge of the brush 
and a fixed insulated contact is disposed in this groove which normally 
touches nothing and an indicator lamp is off. The lamp is illuminated when 
the brush wears, so that the insulated contact touches the outer end of 
the groove. 
U.S. Pat. No. 2,813,208 discloses a cartridge-type brush holder wherein a 
brush has a rear central axial aperture and a spring-pressed insulated 
plunger is urged toward the commutator. When the brush wears, the 
insulated plunger breaks through the working face of the brush to engage 
the commutator and push the brush rearwardly away from the commutator to 
stop the motor. 
U.S. Pat. No. 3,523,288 discloses a structure wherein a continuous 
electrical circuit is normally made through a lamp to energize it while 
the brush length is satisfactory. This circuit is made through a spring 
contact arm on the side of the brush holder and through a pin to the side 
of the brush. When the brush wears sufficiently, the pin drops into an 
aperture in the side of the brush and the lamp is then de-energized. 
U.S. Pat. No. 4,172,988 discloses an extra contact arm electrically and 
physically connected to the rear axial end of the brush. Upon sufficient 
brush wear, this moving contact arm engages a fixed contact to energize an 
indicator. 
U.S. Pat. No. 2,691,114 illustrates an extra cantilever contact arm which 
is insulated from but moves with a brush spring arm which urges the brush 
toward the commutator. Upon sufficient brush wear, the extra cantilever 
contact arm engages the brush holder to illuminate an indicator lamp. 
U.S. Pat. No. 4,121,207 discloses a roller actuator on a fixed miniswitch 
and the roller will roll over the outer axial end of the brush upon 
sufficient wear thereof to close the switch and illuminate an indicator 
lamp. 
The first four of the above-mentioned patents have the deficiency that if 
the indicator lamp is ignored, the brush wear indicator will hold the 
brush away from the commutator and thus stop operation of the motor. This 
may be very disadvantageous where interruption of the motor rotation would 
be highly undesirable. Even the fifth-mentioned patent has this 
disadvantage because the force with which the extra cantilever arm engages 
the brush holder is effectively subtracted from the force with which the 
spring arm urges the brush toward the commutator, and upon sufficient 
further wear the brush will no longer be urged toward the commutator. The 
difficulty with the last-mentioned patent is the relatively high cost of 
the separate miniswitch plus the difficulties of mounting such a switch on 
parts of the motor which may be at a high voltage, for example, 700 volts. 
SUMMARY OF THE INVENTION 
The problem may be solved by a brush wear indicator for use in a 
dynamoelectric machine having a rotatable electrical conductor comprising 
in combination, a brush holder in the dynamoelectric machine and having an 
axis, a conductive brush movable axially in said brush holder and having a 
working face cooperable with and contactable with the rotatable electrical 
conductor, indicator means, and an indicator contact electrically 
connected to said indicator means and insulatedly mounted on said brush to 
move in accordance with brush wear movement and upon sufficient brush wear 
to engage the rotatable electrical conductor of the dynamoelectric machine 
to complete an electrical circuit between said indicator contact and the 
rotating electrical conductor and thus provide an indication at said 
indicator means. 
Accordingly, an object of the invention is to provide a brush wear 
indicator which is low in cost yet reliable in operation and which will 
not interrupt the operation of the dynamoelectric machine on which 
installed. 
Another object of the invention is to provide a brush wear indicator which 
is actuated upon an indicator contact coming electrically into engagement 
with the commutator. 
A further object of the invention is to provide a brush wear indicator 
which may be electrically connected to an indicator lamp or which may be 
electrically isolated from a brush wear indicator. 
Other objects and a fuller understanding of the invention may be had by 
referring to the following description and claims, taken in conjunction 
with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The drawing illustrates a brush wear indicator 10 which is for use in a 
dynamoelectric machine 11 which has one or more brushes 12 cooperable with 
a rotatable electrical conductor 13. The dynamoelectric machine 11 may be 
a generator or motor having a shaft 14 journaled in a frame 15, with the 
rotatable electrical conductor 13 mounted for rotation in accordance with 
the rotation of the shaft 14. This rotatable electrical conductor may be 
one or more slip rings or, as illustrated, may be a commutator. The brush 
12 is one of a plurality of brushes cooperable with and contacting this 
commutator 13, and the brush 12 is one having an axis and mounted for 
axial movement in a brush holder 16. The axis 17 of the brush and the 
brush holder are coincident. The brush 12 has a working face 19 and a rear 
axial face 20. Some means are provided to urge the working face 19 of the 
brush 12 against the commutator 13, and this may be by a spring-urged arm 
or, as shown, by a coiled clock spring 21 which acts against an insulator 
pad 22 secured to the rear face 20 of the brush 12. Such an insulator pad 
22 keeps the electrical current from flowing through the spring 21. 
FIG. 1 illustrates the dynamoelectric machine 11 as having eight brush 
holders 16, and this is typical with larger four-pole machines. A pair of 
brush holders 16 are mounted on a brush holder arm 23, although this may 
be only a single brush holder or more than two brush holders per arm. Each 
arm is then mounted on a brush holder ring 24, and this is typically 
provided in larger machines so that the ring 24 may be slightly rotated 
relative to the frame 15 to achieve the best commutation, e.g., placing 
the brushes on a neutral magnetic axis for minimum arcing at the brushes. 
Insulation is provided somewhere between the brush holders and the frame. 
Each brush is provided with a flexible power pigtail lead 27, which leads 
are connected to power terminals 28 on the brush holder arms 23, or some 
other convenient insulated position. The power pigtail lead is 
electrically connected to and fixed in the rear face 20 of each brush 12, 
which may be accomplished by tamping an electrically conductive cement 
into the aperture in the brush surrounding the pigtail lead 27 or, as 
shown, the pigtail lead 27 may be embedded in the carbonaceous material of 
the brush 12 at the time it is fabricated. Such lead 27 provides the usual 
supply of heavy current through the brush 12 to the commutator 13 to 
operate the dynamoelectric machine 11. 
The brush wear indicator 10 includes indicator means 31, such as the lamp 
31A of FIG. 4 or the indicator alarm 31B of FIG. 5. The brush wear 
indicator 10 also includes an indicator contact 32 which is insulatedly 
mounted on the brush 12 and connected to the indicator means 31 by an 
elongated electrical conductor 33. A forward portion of this elongated 
electrical conductor is fixed relative to the brush 12 to move with brush 
wear movement just as the indicator contact 32 moves with brush wear 
movement. To this end the indicator contact 32 is embedded within the 
brush 12 so as to be within the confines of the brush and also within the 
confines of the brush holder 16, or at least within the confines of the 
longitudinal extension of the cross sectional area of the brush holder 
perpendicular to the axis 17. 
The indicator contact 32 may be merely the end of the electrical conductor 
33 which is closest to the working face 19 of the brush 12, or it may be 
an end of a separate electrical conductor 34 which preferably is made from 
a soft electrically conductive material such as soft aluminum, so that 
upon brush wear and engagement of this indicator contact with the 
commutator 13 the commutator will not be damaged. Copper bar commutators 
quickly develop a copper oxide surface on which the brushes ride, and it 
is preferable for good motor operation that this copper oxide film be 
maintained unbroken. Where the separate electrical conductor 34 is 
provided, it may be electrically connected to the electrical conductor 33 
by a tamped electrically conducting cement 35, similar to the manner in 
which the power pigtail lead 27 may be connected to the brush 12. The 
electrical conductor 33 is provided with a terminal 36 and also provided 
with an insulation jacket 37. The electrical conductor 34 is electrically 
insulated from the brush 12 by means of a cup-shaped insulating sleeve 40 
which preferably has a closed inner end 41 covering the indicator contact 
32. In a preferred embodiment, this insulating sleeve is made from a heat 
shrinkable polyolefin. This assembly of insulated conductors 33 and 34 may 
then be inserted in a predrilled hole in the rear end of the brush 12 and 
held in place by cement 42 at the rear of the brush, or by cement 43 
surrounding the insulating sleeve 40. 
An alternative construction is shown at the right side of FIG. 2 wherein a 
second brush holder 16 as shown as an example of those instances wherein a 
dynamoelectric machine 11 has a large current capacity and requires more 
than one brush 12. This alternative construction still employs the 
insulated conductor 33, but in this case it is longer and extends farther 
into the brush 12 and the conductor 33 is embedded into a miniature 
cylindrical brush 44 of carbonaceous material and the inner end 45 of this 
brush constitutes the indicator contact. The miniature brush 44, indicator 
contact 45, and inner end of the conductor 33 are covered again with the 
insulating sleeve 40 and held in place in a predrilled hole in the brush 
12 by the cement 42 or 43. The material of the insulating sleeve 40 is 
sufficiently soft that the end covering the indicator contact 32 or 45 
will wear away upon sufficient brush wear movement so as not to damage the 
surface of the commutator 13. 
FIG. 3 illustrates a modified indicator contact 46 in a brush 12 and the 
longitudinal sectional view of FIG. 3 is taken at right angles to the 
sectional view of FIG. 2 to illustrate the curvature of the commutator or 
other rotatable electrical conductor 13. In this embodiment, the indicator 
contact 46 is on the end of an elongated rod 47 of carbonaceous material, 
which may be similar to the graphitic lead found in a mechanical pencil or 
drafting pencil. In order to make connection to the flexible conductor 33, 
a male banana plug 48 is mechanically and electrically crimped on the 
outer end of the rod 47. The connector portion of this male banana plug 
and the elongated rod 47, as well as the indicator contact 46, are again 
covered by an elongated cup-shaped, insulating sleeve 40. Again, this may 
be of a heat shrink polyolefin which rigidifies and strengthens the 
carbonaceous rod 47. This unit as thus far described extends out the rear 
of the brush so that a female banana plug 49 may engage the male banana 
plug 48. This female banana plug 49 is electrically and mechanically 
connected to the flexible electrical conductor 33. Preferably, a 
heat-shrinkable tube 50 covers the female banana plug 49 and even 
telescopes over the outer end of the sleeve 40 to completely insulate this 
indicator conductor 33. The heat-shrinkable tube or sleeve 40 may fit 
closely within a predrilled hole in the rear end of the brush 12, and be 
held in place by cement 42, as in FIG. 2. This unit has the advantage of 
requiring only a small diameter hole for the indicator contact 46. 
FIGS. 4 and 5 illustrate the indicator circuits which may be used with the 
indicator contacts 32, 45 or 46. FIG. 4 illustrates a circuit 51 wherein 
the indicator means 31A are warning lamps. The dynamoelectric machine 11 
is shown as a two-pole DC motor having brushes 12 connected across power 
buses 52 and 53, and these would be connected to the main or power pigtail 
leads 27 on the brushes 12. The indicator contacts 32, 45 or 46 from each 
of the two brushes would be connected through the indicator lamps 31A to 
the opposite power bus so that the full voltage across the commutator is 
applied to each warning light 31A. Such warning light would remain 
unenergized until the brush 12 wore down a predetermined amount so as to 
wear through the end cap of the insulating sleeve 40 and have the 
commutator 13 engage the indicator contact 32, 45 or 46. At such time, the 
voltage across the buses 52 and 53 would be applied to the respective 
warning light to indicate which one of the two brushes had worn to an 
extent at which replacement would be required. The motor remains 
operative, however, as distinct from several of the prior art systems, 
wherein the brush wear indicator could prevent the working face 19 of the 
brush 12 from engaging the commutator, which would stop operation of the 
motor. The motor 11 might be one wherein it is critical that motor 
operation be maintained, even though the brush has worn to a point at 
which replacement should be effected. Such warning lamp might be 
continuously illuminated at such time, or might be intermittently 
illuminated if the insulator contact 32, 45 or 46 is narrow relative to 
the insulation spacing between commutator bars. In such case, such 
intermittent flashing would help invite attention to recommended brush 
replacement. 
FIG. 5 illustrates a circuit 55 wherein the indicator 31B is an alarm which 
may be aural or visual, and is electrically isolated from the 
dynamoelectric machine 11. Such machine 11 has been illustrated as a 
four-pole machine similar to that shown in FIG. 1. This electrical 
isolation is achieved by relay coils 56 and 57 which control contacts 58 
and 59, respectively. Such contacts may be normally closed contacts, but 
are shown as normally open contacts connected in parallel, and this 
parallel arrangement connected in series through the alarm 31B between 
control voltage buses 61 and 62. The relay coil 56 is connected between 
the power bus 51 and the indicator contact 32, 45 or 46 on each of two 
diametrically opposite brushes 12 in the motor 11. As per usual custom in 
a four-pole machine, these two brushes are shown connected in parallel by 
a jumper 63. Likewise, a jumper 64 connects in parallel the other two 
brushes 12, which are diametrically opposite and which two brushes are 
midway between the first-mentioned set of brushes. It will readily be 
appreciated that four relays 56 or 57 may be provided, one for each brush, 
but in FIG. 5, it is shown that one relay is controlled by either of two 
brushes. As shown in FIG. 1, the several terminals 36 from the indicator 
contacts in all of the brushes in one set riding on one commutator bar are 
connected to a terminal block 66 which is mounted at some convenient 
location, for example, the brush holder arm, but insulated therefrom. 
In the circuits of FIGS. 4 and 5, it will be apparent that the indicator 
means 31A or 31B need not be at the motor; in fact, the indicator of the 
present invention permits remote indication of the condition of the brush 
wear. This is especially important where the motor may be in some 
relatively inaccessible location and management is concerned that proper 
maintenance of the motor may be overlooked because it is too difficult to 
properly observe brush conditions at such relatively inaccessible motor. 
In FIGS. 2 and 3, it has been illustrated that the miniature brush 44 or 
elongated rod 44 is substantially parallel to the brush axis 17. This is 
not mandatory; the essential feature is that the indicator contact 32, 45 
or 46 be connected in some manner to the indicator 31, and it is 
convenient to have the indicator conductor 33 exit from the rear end of 
the brush 12. This minimizes any interference with the spring 21 or brush 
holder 16. 
In FIGS. 4 and 5, it will be noted that a particular indicator contact 32 
or 46 is connected through the indicator 31A or 31B to the power bus of 
the opposite potential. This is a convenient means for obtaining a 
potential difference across the indicator lamp 31A or across the relay 
coils 56 or 57, and it is merely a requirement that such indicator be 
connected from the respective indicator contact through the indicator to a 
terminal of an electrical potential different from that of the indicator 
contact. 
The present disclosure includes that contained in the appended claims, as 
well as that of the foregoing description. Although this invention has 
been described in its preferred form with a certain degree of 
particularity, it is understood that the present disclosure of the 
preferred form has been made only by way of example and that numerous 
changes in the details of the parts and circuit and the combination and 
arrangement of parts and circuit elements may be resorted to without 
departing from the spirit and the scope of the invention as hereinafter 
claimed.