Brush wear detector

A brush wear detector comprised of an L-shaped leaf spring having the outer end of its longer leg mounted on a first insulating member in fixed relationship to a spring support for a self-winding, brush-biasing spring. A second dielectric member is secured to the leaf spring between the ends thereof and projects outward from the leaf spring into engagement with an uncoiled section of a brush-biasing spring. The free end of the leaf spring is held away from engagement with the brush-biasing spring support, thereby being electrically isolated from it, when a new brush is mounted in operating position under the brush-biasing spring. When the brush becomes worn a pre-determined amount the brush-biasing spring moves in a direction away from the second dielectric member and so enables the leaf spring to force its free end into electrically conducting engagement with the electrically energized spring support. Establishment of that conductive relationship produces a signal that actuates a brush wear indicating circuit that is operatively connected to the leaf spring.

BACKGROUND OF THE INVENTION 
The invention relates to a brush wear detector for use on a dynamoelectric 
machine and in combination with a wear indicating circuit. More 
particularly, the invention relates to a detector that when mounted in 
operating relationship on such a machine, is responsive to a predetermined 
degree of movement of a brush-biasing spring to close an electrical switch 
contactor and so cause the energization of a brush wear indicating signal 
circuit. 
The use of brush wear detectors and associated brush wear indicating 
circuits to alert operators of electric machines to the occurrence of a 
given degree of wear of brushes on the machines is generally well known. 
In applications of such machines where unscheduled outages are 
particularly undesirable, the extra costs associated with providing a 
brush wear indicating function for the machines is commercially 
acceptable. For example, in certain motors and generators used on vehicles 
that are often operated for extended periods in locations remote from 
garages or other convenient maintenance or repair facilities, the 
importance of warning operating or maintenance personnel to replace 
extensively worn brushes, in an appropriately timely manner, often 
mandates the use of brush wear indicators on the machines. The operation 
of brush wear detectors in such important applications makes it desirable 
that they be made to reliably respond to the occurrence of a predetermined 
degree of brush wear. Moreover, the detectors should be capable of 
energizing an indicating circuit at an accurately set point corresponding 
to a precisely predetermined degree of wear of an associated brush. 
In addition to such operating objectives, brush wear detectors must be able 
to perform effectively in the relatively harsh environments frequently 
encountered in their application. Considerable mechanical stress is 
usually transmitted to brush wear detectors due to both the shocks and 
vibrations to which an entire motor or generator on which a brush wear 
detector is mounted, and due to extensive variations in the electric 
current passing through the windings, communtator and brushes of the motor 
or generator on which the detector is applied. Finally, the common 
presence of contaminants, such as carbon dust and other particulate matter 
from the ambient, all combine to impose stringent requirements on 
designers of brush wear detectors. 
A relatively early type of brush wear detector and indicator arrangement is 
shown in U.S. Pat. No. 2,691,114, which issued Oct. 5, 1954. The mechanism 
disclosed in that patent uses an electrical contactor mounted on a 
brush-follower spring to close a signal circuit responsive to a brush 
being worn sufficiently to enable the contactor-carrying spring arm to 
move close enough to a fixed contactor in the path of movement of the 
spring arm to effect such a circuit completion. One disadvantage of such 
an arrangement is that the engagement of the movable contactor with the 
fixed contactor can be easily interrupted due to vibration of the 
associated brush, thereby causing the brush-follower arm to bounce and 
intermittently disengage the movable contactor from the fixed contactor. 
Such tentative contact closure is avoided by other types of brush wear 
indicators, one example of which is shown in U.S. Pat. No. 3,523,288, that 
issued Aug. 4, 1970. With the device disclosed in that patent positive 
switch opening or closing action is obtained for a brush wear indicator by 
providing an insulated pin that slides on one side of an associated brush 
until a recess that is machined into the brush at a predetermined point 
near its upper end is moved under the pin. When such a range of movement 
occurs, the pin drops into the recess, thus causing a switch in the 
indicator circuit to snap either open or closed, thereby providing a 
signal to an associated wear indicating circuit. One disadvantage of such 
an arrangement is that the use of such a pin as a switch actuator produces 
a constant drag on the associated carbon brush. Moreover, in that type of 
arrangement, the pin actually acts to arrest movement of the carbon brush 
once the pin has dropped into the recess in the brush and into contact 
with one wall thereof. 
The problem of undesirable drag on movement of a brush is avoided by 
another type of previously known brush wear indicator arrangement, as 
shown in U.S. Pat. No. 4,024,525, which issued May 17, 1977. In that 
patent, a brush wear detector probe is mounted on an insulating member 
adjacent to an elongated groove formed in the side wall of an associated 
brush. As the brush wears down during its normal operation, the probe is 
positioned within the groove and spaced from the walls thereof so that it 
does not conduct current from the brush through the probe to an associated 
brush wear indicating circuit. When due to wear of the brush, it has moved 
down sufficiently to bring the end of the probe into contact with the 
uppermost wall of the groove in the brush, current is conducted from the 
brush through the probe to the indicator circuit. Although the probe does 
not restrict brush movement during most of the wear cycle of the brush, 
the probe does arrest further downward movement of the brush when the 
probe finally engages the upper wall of the groove. Reduced or 
intermittant current through the brush, resulting from such a stoppage of 
brush movement, can limit or impair operation of the associated 
dynamoelectric machine, until a new brush is installed. 
A later U.S. Pat. No. 4,172,988, issued Oct. 30, 1979, discloses a brush 
wear indicating means that may partially alleviate the problem of downward 
movement of a brush being arrested by engagement thereof by a brush wear 
detector mechanism. In that patent, a movable contact is mounted on a 
relatively long lever arm and is driven into engagement with a fixed 
contact mounted on an insulating member positioned adjacent to the path of 
movement of the brush, as a brush-follower spring forces the brush and the 
movable contact downward. Due to the long lever arm mounting the movable 
contactor, it is possible for the brush to be pivoted slightly around the 
fulcrum point of that arm to enable further wear of the brush after the 
wear-indicating contactors have been engaged. However, the disclosed 
mechanism still has the inherent disadvantage of severely restricting 
movement of the brush after the contactors are engaged. Also, this 
disclosed mechanism is somewhat like the brush wear indicator disclosed in 
the first patent mentioned above, in that movement of a wear indicating 
contactor is achieved by using the direct driving thrust of a 
brush-follower spring to move the contactor. Accordingly, intermittent and 
weak wear-indicating signals may result due to vibrations being 
transmitted through the brush to the juxtaposed surfaces of the movable 
and fixed signal circuit contactors when a predetermined degree of brush 
wear has been achieved. 
It has been recognized that a brush-follower spring of the pre-stressed 
convoluted type now commonly found in commercial brushholder applications 
can be used to actuate a brush wear indicating circuit in a way that 
minimizes the undesirable transmission of vibration from an associated 
brush to the movable switch contactor of a brush wear indicating circuit. 
In that regard, U.S. Pat. No. 4,272,695, which issued June 9, 1981 
discloses a brush wear indicator mechanism in which a movable 
contactor-bearing leaf spring is mounted on a support structure to 
position a movable end of the leaf spring against an uncoiled portion of a 
brush-biasing spring that has its coiled portion positioned on the upper 
end of a brush to drive it against an associated commutator or slip ring. 
As the brush wears, the brush-biasing spring winds up and causes the 
uncoiled portion to move in a direction away from the free end of the leaf 
spring. Such movement enables the leaf spring to drive the contactor 
mounted on it into engagement with a fixed contactor that is mounted on a 
suitably insulated terminal in the path of movement of the movable 
contactor. Closure of the two contactors is effective to initiate 
transmission of a brush wear indicating signal to an associated indicating 
circuit. A disadvantage of the type of apparatus shown in this patent is 
that the leaf spring bears against the brush-biasing spring and is 
energized by it. That arrangement can cause the leaf spring to 
significantly interfere with the driving force of the brush-biasing 
spring. In addition, such an arrangement results in the movable contactor 
being energized at all times, so it is necessary to provide a separately 
insulated fixed contactor on the brushholder assembly to which the brush 
wear indicating circuit is connected. It would be desirable to eliminate 
the added expense inherent in providing such an additional insulated 
contactor. 
Finally, in an earlier filed U.S. patent application, Ser. No. 183,904, 
which was filed Sept. 4, 1980 and issued May 11, 1982 as U.S. Pat. No. 
4,329,683 which is assigned to the assignee of the present invention, 
there are disclosed several embodiments of brush wear indicators that, 
respectively, utilize various electro-mechanical transducers operably 
connected between the coiled portion of a brush-biasing spring and a brush 
wear signal means to actuate a signal in response to the coiled portion of 
the spring being moved a predetermined distance. In each of those 
mechanisms, the transducer is directly connected in line with the driving 
thrust of the coiled portion of the spring, as it responds directly to 
brush movement in its control of a movable contactor of a wear-indicating 
circuit. Means are provided in each of those devices to reduce friction 
interference between the transducers and the brush-biasing spring. 
However, due to the direct in-line engagement of the transducers with the 
respective coiled portions of associated brush-biasing springs, the 
devices disclosed in that patent do have the disadvantage of transmitting 
an undesirable amount of vibration from the brush to the wear-indicating 
circuit contactor. Moreover, the contactor mounting means and spring 
actuating means disclosed in that patent are relatively expensive to 
manufacture, complex in structure and space consuming to assemble. 
Frequently in the context of conventional brushholder assemblies, there is 
a very limited amount of space available for installing desired brush wear 
indicators and associated brush wear detector mechanisms, therefore, it is 
desirable to provide a brush wear detector that requires relatively little 
space for its installation and operation on a brushholder assembly. 
OBJECTS OF THE INVENTION 
A major object of the invention is to provide a brush wear detector for use 
with brushholder assemblies of conventional dynamoelectric machines, and 
with associated wear indicating circuits, to reliably and accurately 
detect the occurrence of a predetermined degree of brush wear, in an 
economical and efficient manner, without impairing or undesirably 
restricting other operating components of the brushholder assembly. 
Another object of the invention is to provide a brush wear detector having 
a minimum number of components, each of which are economical to construct 
and assemble. 
A further object of the invention is to provide a brush wear detector that 
is capable of being readily retro-fitted to a wide variety of existing 
commercial brushholder assemblies to achieve the foregoing objectives. 
Still another object of the invention is to provide a brush wear detector 
that is operable to positively move a contactor into engagement with an 
energized conductor responsive to the occurrence of a predetermined degree 
of brush wear, in a manner such that undesired disengagement of the 
contactor due to vibration of an associated brush is avoided, or 
significantly diminished, relative to the degree of such vibration-induced 
disengagement encountered in related prior art brush wear detectors. 
Additional objects and advantages of the invention will be apparent to 
those skilled in the art from the description of it presented herein, 
taken in conjunction with the accompanying drawings. 
SUMMARY OF THE INVENTION 
In one embodiment of the brush wear detector of the invention a leaf spring 
is mounted on a first dielectric member, which is operatively positioned 
on a brush-biasing spring support to arrange the free end of the leaf 
spring for reciprical movement between a position of engagement with said 
spring support and a position of disengagement from the spring support. A 
second dielectric member is secured to the leaf spring intermediate the 
ends thereof, and projecting outward therefrom, into engagement with an 
uncoiled portion of a self-winding, brush-biasing spring that is 
operatively mounted on said spring support. In operation, the uncoiled 
portion of the brush-biasing spring is moved toward the spring support 
thereby causing the second dialectric member to be forced against the 
biasing action of the leaf spring to move the free end thereof into its 
position of disengagement relative to the spring support, responsive to 
the brush-biasing spring being extended sufficiently to enable a 
relatively new brush to be positioned in operative relationship against 
the coiled portion of the brush-biasing spring. As the brush wears down to 
a predetermined length, with the brush-biasing spring holding the brush in 
conductive relationship with a moving conductor of an associated 
dynamoelectric machine, the uncoiled portion of the brush-biasing spring 
moves away from the spring support, thereby enabling the second dielectric 
member to release the free end of the leaf spring to be moved into 
engagement with the spring support, under the biasing force of the leaf 
spring. Such engagement of the leaf spring with the brush-biasing spring 
support causes the leaf spring to be energized, so that a suitable 
terminal means and associated brush wear indicator circuit connected 
thereto will also be energized to provide a signal corresponding to the 
occurrence of said predetermined degree of brush wear.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the invention disclosed herein will be 
described in connection with its application on a relatively conventional 
type of brushholder that is mounted in operating relationship to support a 
carbon-composition brush in electrically conducting, sliding relationship 
on a rotatably mounted commutator. It will be recognized that in such a 
conventional dynamoelectric machine arrangement the brush could equally 
well be mounted to ride on a slip ring or other movable conductor of the 
associated machine. Likewise, it will be apparent that a number of 
different kinds of conventional brushholders can be used with the brush 
wear detector of the invention, and many different commercially available 
brushes are adapted to be readily usable in combination with the 
invention. 
In FIG. 1 of the drawing there is illustrated a fragment of a rotatably 
mounted movable conductor 1, such as a slip ring or commutator of a 
conventional dynamoelectric machine. A suitable carbon brush 2, is 
supported in a brushholder 3 and is biased against the slip ring 1 by a 
brush-biasing spring 4 that is mounted with one of its ends fixed to and 
supported by a rigid electrically conductive brush-biasing spring support 
5. Any suitable means may be used for mounting the spring support 5 on the 
brushholder, such as those used in various commonly known applications of 
such brushholder mounting arrangements. In the structure shown in FIG. 1, 
the mounting means includes an integral arm on the spring support 5, which 
arm is disposed in an aperture 3A through the brushholder, to enable the 
spring support 5 to pivot, around the fulcrum defined by the end of the 
arm 5A, as the coiled portion 4A of the brush-biasing spring moves 
downward, responsive to the brush 2 being worn away during its normal 
usage. It will be understood that the spring 4 is held in an electrically 
conductive relationship with the brush 2, and with the spring support 5, 
when the spring is extended to position the coiled portion 4A thereof in 
driving relationship with the outer end of the brush 2. If the reader 
desires a more detailed explanation of a somewhat related type of suitable 
conventional brushholder arrangement that utilizes an alternatively 
mounted and operated self-winding spring for biasing a brush against a 
movable conductor, reference may be made to U.S. Pat. No. 3,526,797, which 
describes one such alternative mounting means and structural arrangement. 
The brush wear detector 6 of the embodiment of the invention described here 
comprises an electrically conductive leaf spring 7 that has one of its 
ends 7A mounted in fixed relationship to a first dielectric member 8, 
while its other end 7B is free to move within a desired range of movement. 
The leaf spring is formed of suitable spring material, such as spring 
brass, and is generally L-shaped in configuration. The outer end of the 
shorter leg 7B' of the L-shaped configuration is disposed to be moved from 
a disengaged relationship with the brush-biasing spring support 5 to an 
engaged, electrically conducting relationship with the spring support, 
responsive to the uncoiled portion 4B of the brush-biasing spring moving 
away from the spring support 5. Such movement enables the leaf spring 7 to 
bias its free end 7B' into such engaged, electrically conductive 
relationship with the spring support 5. 
A rivet 9 is ueed to secure spring 7 to the first dielectric member 8. Of 
course, other suitable means may be used for securing those components 
together in alternative forms of the invention. Similarly, although the 
first dielectric member 8 is shown as a generally L-shaped bracket, it may 
be made of other suitable configurations in constructing alternative forms 
of the invention, and any suitable insulating material can be made to form 
the member 8. In this embodiment the member 8 is made of a rigid, 
thermally stable, phenolic plastic material of any suitable commercially 
available type. 
A second dielectric member 10 is secured to a portion of the leaf spring 7 
between its ends. The second dielectric member is arranged to project 
outward from the leaf spring, through an aperture 5B in spring support 5 
(see FIG. 3), to engage an uncoiled portion 4B of the brush-biasing spring 
4 when the spring is extended a predetermined length, thereby positioning 
the second dielectric member to hold the free end 7B of the leaf spring 
disengaged from the brush-biasing spring support 5. 
In the form of the invention described herein, the second dielectric member 
10 has a configuration comprising two parallel leg portions 10A and 10B 
that are joined together by a third leg 10C that is integral with the 
first two leg portions. The leg 10C is effective to hold the parallel leg 
portions 10A and 10B in rigid spaced relationship to one another. As shown 
in the drawing, one of the parallel leg portions 10A extends in a first 
direction from one end of the connecting leg 10C, while the other leg 
portion 10B extends from the opposite end of the connecting leg 10C in a 
direction opposite to the first direction of extention of the leg 10A. One 
of the leg portions 10B is mounted in fixed relation on the leaf spring 7 
between its two ends, by being adherred thereto with a suitable commercial 
epoxy resin. Of course, other suitable bonding or adherring means may be 
used to secure the second dielectric member 10 to the intermediate portion 
of spring 7 in alternative embodiments of the invention. The other leg 
portion 10A is disposed to engage the uncoiled portion 4B of the 
brush-biasing spring, responsive to the spring being extended in operating 
position to bias a relatively new or unworn brush, that is mounted in the 
brushholder, against the electrically conductive slip ring of the 
associated dynamoelectric machine. 
The leaf spring 7 may be made of any suitable spring material, but in this 
embodiment of the invention it is formed of spring brass and is made 
approximately two mils thick. The leaf spring 7 is effective, when mounted 
on the first dielectric member 8, to move the free end 7B of the spring 
into electrically conductive engagement with the brush-biasing spring 
support 5, responsive to the spring 4 winding up as a predetermined degree 
of wear of the brush 2 occurs. That winding-up movement of the spring 4 
causes the uncoiled portion 4B thereof to move in a direction away from 
the spring support 5 and away from the leg 10A of the second dielectric 
member 10 of the brush wear detector 6. 
The brush-biasing spring support 5 is provided with an aperture 5B (see 
FIG. 3) that enables the connecting leg 10C and the first parallel leg 
portion 10A of the second dielectric member 10 to move freely through it. 
As shown in FIG. 3, the area of the aperture is made sufficiently large to 
readily accommodate reciprocal movement of the second dielectric member 10 
therethrough, to enable the brush wear detector 6 of the invention to 
operate in the desired manner, which will be more fully described below. 
Thus, when the leaf spring 7 biases the second dielectric member toward 
the uncoiled portion 4B of the brush-biasing spring, the parallel leg 
portion 10A is enabled to follow the path of movement of the uncoiled 
portion 4B of the spring away from the spring support 5. The width of the 
free end 7B of leaf spring 7 is made greater than the width of aperture 
5B, as best seen in FIG. 3. Accordingly, at least one of the outer edges 
7B" at the edge 7B' of the free end 7B of the leaf spring is positioned in 
electrical engagement with the spring support 5 responsive to the second 
dielectric member 10 being forced to follow the path of movement of the 
uncoiled portion 4B of spring 4 by the biasing effect of the leaf spring 7 
on the member 10. In this form of the invention, it can be seen that the 
width of the end 7B of the leaf spring is great enough to cause both ends 
of its edge 7B' to engage spring support 5. 
The brush wear detector 6, in this embodiment, includes terminal means 11 
for electrically connecting a variety of suitable, commercially available 
brush wear indicating circuits, such as the circuit 12, shown 
schematically in the drawing, to the leaf spring. Examples of some types 
of effective brush wear indicating circuits are described in U.S. Pat. No. 
3,523,228. It will be recognized that such circuits, as well as others, 
are suitable for use with the detector 6, of the present invention. The 
terminal 11 in this form of the invention is a simple spade-type terminal 
that is electrically and mechanically secured in conducting relationship 
with the spring 7 by the rivet 9. A suitable conventional conductor 13 of 
the brush wear indicating circuit is electrically connected to the 
terminal 11 by being crimped therein in a conventional manner. 
In operation of the invention, when the brush wear indicating circuit 12 is 
connected via conductor 13 to the terminal 11, it can be electrically 
disconnected from the spring support 5 by operation of the brush wear 
detector 6, responsive to the brush-biasing spring being extended to at 
least a predetermined length sufficient to position the brush in the 
brushholder. The indicating circuit 12 is energized through the leaf 
spring 7 and the conductor 13, responsive to the occurrence of a 
predetermined degree of wear of the brush and the resultant engagement of 
the end 7B of leaf spring 7 with the brush-biasing spring support 5. Such 
controlled energization, followed by accurately determined de-energization 
of the brush wear indicating circuit is effective to precisely regulate 
circuit 12 to provide an operator with an accurate indication of the 
occurrence of a predetermined degree of wear of of the brush 2 
corresponding to the movement of the outer end of brush 2 to a point where 
leaf spring 7 is moved to contact spring support 5. 
It will appreciated that various modifications and alternative forms of the 
brush wear detector of the invention may be made, in view of the 
disclosure presented herein. It is my intention to encompass such 
alternative embodiments within the scope of the following claims.