Abstract:
A brush assembly having a brush wear indicator for use with electric actuating devices such as motors and generators that detects the worn condition of a brush and generates a signal indicating this worn condition.

Description:
RELATED APPLICATIONS 
     [Not Applicable] 
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     [Not Applicable] 
     FIELD OF THE INVENTION 
     The present invention relates to a brush assembly having a brush wear detector and indicator for use with an electric actuating device such as a D.C. motor or generator, and more particularly, relates to a brush wear detector which generates an electrical signal indicating a worn condition of a brush. 
     BACKGROUND OF THE INVENTION 
     Electric actuating devices, such as rotating or linear moving electric apparatus, dynamos, motors, generators, etc., typically include a moving commutator. The commutator is electrically coupled to an external electric circuit through one or more brushes which make physical contact with the commutator. As the commutator moves against the brush, the contact surface of the brush wears down to a point where replacement of the brush is required. 
     Brush wear detectors are known in the art and generally comprise various types of mechanical and electrical arrangements which act to signal the fact that the brush has worn away to a point at which replacement is required. Known detectors may comprise electrical leads inserted into the brush which signal that the brush is worn. An example of such an apparatus is shown and described in U.S. Pat. No. 5,870,026, entitled “Brush Wear Indicator,” which issued to Keith C. Challenger on Feb. 9, 1999. Detectors that utilize electrical leads inserted into the brush not only increase the cost and complexity of the detector system, but may also cause metal-on-metal structural damage if the brushes are not replaced and the leads contact the commutator for an extended period of time. 
     Another example of a brush wear detector is one in which a magnet moves towards the commutator as the brush wears down and activates a reed switch at a point when the brush needs to be replaced. An example of such an apparatus is shown and described in U.S. Pat. No. 4,739,208, entitled “Brush Assembly Including Brush Wear Detector,” which issued to Dan W. Kimberlin on Apr. 19, 1988. Reed switches, however, are mechanical devices and are susceptible to shock and vibration which may be encountered in electric actuating devices. 
     Other examples of brush wear detectors are those which depend on physical contact between metallic components of the brush assembly to complete an electrical circuit. Examples of such an apparatus are shown and described in: U.S. Pat. No. 6,255,955, entitled “Brush Warning Indicator and Methods For Indicating Brush Wear-Out,” which issued to Harald Edmund Blaettner on Jul. 3, 2001; U.S. Pat. No. 5,731,650, entitled “Dynamoelectric Machine With Brush Wear Sensor,” which issued to Walfried F. Scheucher on Mar. 24, 1998; and U.S. Pat. No. 4,950,933, entitled “Carbon Brush Holder utilizing a Worn Brush Detector,” which issued to James R. Pipkin et al. on Aug. 21, 1990. Such detectors are not only costly and complicated, but are susceptible to unreliability if the contact parts become corroded or are fouled by foreign particulates such as dust from worn brushes. 
     Therefore, it is an object of the present invention to provide an improved brush wear detector that does not depend on physical contact between metallic components of the brush assembly. 
     BRIEF SUMMARY OF THE INVENTION 
     This and other objects of the present invention are achieved in an improved apparatus for detecting the worn condition of brushes in electric actuating devices. The apparatus includes a magnet that is moved as the brush is worn. A Hall-effect device mounted adjacent to the path of travel of the magnet generates a signal at a particular point along its path indicating that the brush is worn to a percentage of its length. In one embodiment, the magnet is attached to the brush by means of a bracket. The magnet translates in the same direction that the brush moves as the brush wears. 
     The magnet and brush may be contained in a brush holder that encloses the magnet along its path of travel. A Hall-effect device is positioned adjacent to the brush holder such that when the magnetic field produced by the moving magnet is of sufficient strength to exceed the operative point threshold of the Hall-effect device (preferably when the magnet is aligned with the sensor of the Hall-effect device), the device generates a signal indicating that the brush has worn to a percentage of initial length. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic cross sectional top view of the right portion of the electrical input end of a D.C. motor having a brush wear detector apparatus according to the present invention. 
         FIG. 2  is a top planar view of a brush assembly of the apparatus of  FIG. 1 . 
         FIG. 3  is a top planar view of a printed circuit board of the apparatus of  FIG. 1 . 
         FIG. 4  is an isometric perspective view of a Hall effect device assembly of the apparatus of  FIG. 1 . 
         FIG. 5  is a perspective view of a brush assembly holder and a brush, of the apparatus of  FIG. 1 . 
         FIG. 6  is a perspective view of the electrical input end of the D.C. motor of  FIG. 1 . 
         FIG. 7  is an enlarged cross-sectional lateral view of the brush assembly and holder of the apparatus of  FIG. 1 . 
         FIG. 8  is a cross-sectional end view of the brush assembly and holder, and view of the Hall-effect device, of that apparatus of  FIG. 1 . 
         FIG. 9  is a perspective view of the brush assembly holder of the apparatus of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a D.C. motor  11  includes a rotatable commutator  32  against which a brush  14  is forced. Brush  14  is one component of a brush assembly  23  which is contained within a brush assembly holder  22 . Brush  14  slides within holder  22  and toward commutator  32  as the brush wears away due to its physical contact with the rotating commutator. 
     Referring to  FIGS. 1 and 5 , brush holder  22  includes a cylindrical sleeve  13  which carries an inner tube  15 . Sleeve  13  is securely mounted to the housing of motor  11  so as to align tube  15  with commutator  32 . Tube  15  is made of an electrically conductive non-magnetic material (for example, brass) and is encapsulated by an insulating material. 
     As shown in  FIG. 1 , brush assembly  23  is retained in brush holder  22 . Brush holder  22  is secured to a shield  42  of the motor by a fastener  44 , such as a set screw. 
     As shown in  FIG. 5 , the cross-sectional shape of tube  15  is cross-shaped and is of the type shown in U.S. Pat. No. 6,731,042, issued May 4, 2004, which is incorporated herein by reference. A pair of side relief channels  17 ,  19  of tube  15  are disposed along the two sides of brush holder  22 . Relief channels  17 ,  19  house a pair of magnets  10  ( FIG. 1 ) mounted opposite to one another. Magnets  10  are thus enclosed within tube  15  as the magnets move along a pathway defined by relief channels  17 ,  19 . As will suggest itself, brush holder  22  may have cross-sectional shapes different than a cross shape. 
     Referring now to  FIG. 2 , brush assembly  23  includes carbon brush  14  which is attached to one end of a flexible shunt wire  16 . A spring  18  is coiled around shunt wire  16 , and serves to force brush  14  outwardly from holder  22  ( FIG. 1 ) and against commutator  32  ( FIG. 1 ). A terminal  20  is soldered to the other end of shunt wire  16 . 
     As shown in  FIG. 2 , a bracket  12  is shaped to conform to the shape of brush  14 , and is made of a material such as aluminum or high temperature resistant plastic that will allow for a bond between bracket  12  and magnets  10 . Alternatively, magnets  10  may be adhered directly to brush  14  by a suitable adhesive, as for example, a Permabond brand cyanoacrylate. Instead of using an adhesive to attach magnet  10  to bracket  12  (or to the brush itself), a mechanical method of attachment may be used, such as screws or rivets. Alternatively, magnets  10  may be molded or pressed directly into brush  14  negating the need for an adhesive or mechanical fastener. The leading edge of the bracket  12 , proximate to commutator  32 , is bent away from brush  14 , as shown. In the event that one or both of magnets  10  come loose from the bracket, the bent edge prevents the magnets from exiting tube  15  and contacting and damaging commutator  32  (or other moving parts) by containing magnets  10  inside of tube  15 . 
     Bracket  12  pilots off the hub on brush  14  and is held in place by the force of spring  18 . The hub serves as a locating pilot for spring  18  and a retainer for shunt wire  16 . Spring  18  is a helical coil compression spring and is made of a stainless steel to increase its resistivity to current flow. Shunt wire  16  may be manufactured to allow for maximum flexibility which prevents brush  14  from binding within holder  22  when brush assembly  23  is compressed during installation. The fit of the terminal  20  in brush holder  22  facilitates assembly. 
     In one embodiment, two magnets  10  are used so as to eliminate the need to orient brush assembly  23  prior to installation into brush holder  22 . Two-pole rectangular, square, or circular permanent magnets  10  are adhered to either side of bracket  12  and are made of rare earth materials for increased magnetic field strength. 
     As shown in  FIGS. 3 and 4 , a Hall-effect device assembly  27  is soldered to a printed circuit board  28  that is attached to end shield  42  ( FIG. 1 ) by screws  30 . The Hall-effect device assembly  27  is secured to printed circuit board  28  by a fastener  45 , such as a plastic rivet. As will suggest itself, other circuitry to perform other tasks may be provided onto printed circuit board  28 , and make use of power provided to the board. 
     As shown in  FIG. 4 , Hall-effect device assembly  27  includes a Hall-effect device  24  and a housing  26 . Housing  26  is made of a high temperature non-conductive material such as plastic. Hall-effect device  24  may be protected from brush dust or other harmful foreign materials, if desired. For example, potting compounds, conformal coatings or housing structures may be used. Housing  26  establishes the proper height relationship between the Hall-effect device  24  and magnets  10 . 
     As brush  14  wears, spring  18  pushes bracket  12  and magnets  10  along tube  15  toward commutator  32 . Hall-effect device  24  is positioned adjacent to the path of travel of magnets  10 . Device  24  is preferably uni-polar so that it remains actuated only when the magnetic field is perpendicular to the face of device  24 . 
     In the absence of a magnetic field strength greater than the operative point threshold of Hall-effect device  24 , the output of device  24  remains in a high voltage state. The output of the Hall-effect device switches to a low voltage state when the magnetic field strength exceeds the operative point threshold of the Hall-effect device. This occurs when magnets  10  on the brush assembly  23  reach a position adjacent to Hall-effect device  24 . The low state output of Hall-effect device  24  indicates that brush  14  has worn to a particular percentage of its initial length. 
     Hall-effect device  24  is a 3-lead package (not shown): one lead is connected to a supply voltage (not shown); one lead is connected to the common (not shown); and one lead is connected to the output (not shown) of device  24 . When magnetic flux is detected such that it exceeds the operative threshold of device  24 , the output is turned ON and connects to common. A pull-up resistor is connected between the supply and the output. When the output is OFF (i.e., when a magnetic field is not detected), the potential is the same at the output and supply. When the output is ON (i.e., when a magnetic field is detected), the voltage at the output will equal the saturation voltage of the Hall-effect device. A low-voltage condition indicates that the brush  14  has worn to a percentage of initial length. 
     A perspective view of the electrical input end of the D.C. motor  11  is illustrated in  FIG. 6 . The commutator  32  is shown relative to two brush assembly holders  22 . Hall-effect device assembly  27  is shown positioned relative to one of the holders  22 . As will suggest itself, another Hall-effect device assembly  27  (not shown) may be positioned relative to the other holder  22 , if desired. 
       FIG. 7  illustrates an embodiment of the wear detector apparatus in which housing  26  is secured by a fastener  45  relative to brush  14 , so as to position the Hall-effect device  24  (not shown in  FIG. 7 ). 
       FIG. 8  is a cross-sectional end view illustrating an embodiment of the wear detector apparatus in which two magnets  10  are illustrated relative to Hall-effect device  24 . Magnets  10  are shown within the side channels of tube  15 . 
       FIG. 9  illustrates a perspective view of an embodiment of brush assembly holder  22 . 
     While particular steps, elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications can be made by persons skilled in the art, particularly in light of the foregoing teachings. It is therefore contemplated by the appended claims to cover such modifications as incorporate those steps or elements that come within the scope of the present invention.