Abstract:
A spring-set, electromagnetically released brake that requires a relatively small amount of current to release is provided. The brake includes a magnet shell disposed about a first axis. The shell includes a radially outwardly extending pole. The brake also includes an armature disposed radially outwardly of the pole and pivotable at a first axial end and a first engagement element, such as a brake shoe, coupled to the armature proximate a second axial end of the armature. The brake further includes a spring disposed about the pole. The spring biases the armature radially outwardly to urge the first engagement element towards a second engagement element such as a friction surface in a brake drum. Finally, the brake includes a coil disposed about the pole and means for selectively energizing the coil to urge the armature radially inwardly and the first engagement element away from the second engagement element. A bobbin for use in the brake is also provided. The bobbin includes a substantially cylindrical body portion disposed about a second axis and first and second discs extending radially outwardly from first and second ends, respectively, of the body portion. The bobbin further includes a flange extending radially inwardly relative to the second axis and a projection extending axially relative to the second axis. The flange and projection are configured to engage the magnet shell and prevent radial and axial movement of the bobbin and coil.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to brakes and, in particular, to a spring-set, electromagnetically released brake requiring a relatively small amount of current for use in releasing the brake. This invention also relates to an improved coil bobbin for use in the inventive brake or other electromagnetic assembly and, in particular, to a coil bobbin that may be secured against movement within an electromagnetic assembly without bonding. 
     2. Disclosure of Related Art 
     A conventional spring-set, electromagnetically released brake includes an armature on which a brake shoe or other engagement element is disposed. A spring biases the armature and the engagement element towards a second engagement element, such as a friction surface. The brake further includes a coil disposed about a magnet pole or other ferromagnetic structure and means for energizing the coil. By energizing the coil, a magnetic circuit is established between the pole and armature in order to draw the armature towards the pole and the brake shoe away from the friction surface, thereby releasing the brake. 
     Conventional spring-set, electromagnetically released brakes often require relatively large amounts of current in order to overcome the biasing force of the spring and release the brake. As a result, these conventional brakes are relatively inefficient. Further, conventional spring-set, electromagnetically released brakes suffer from an undesirable amount of backlash as the brake is set. 
     Conventional coil bobbins used for retaining the coil in spring-set, electromagnetically released brakes and other electromagnetic assemblies also have disadvantages. These bobbins are typically secured within the assembly by bonding to prevent movement of the bobbin and the coil. Bonding, however, is a relatively expensive and inefficient process. 
     There is thus a need for a spring-set, electromagnetically released brake and a coil bobbin that will minimize or eliminate one or more of the above-mentioned deficiencies. 
     SUMMARY OF THE INVENTION 
     The present invention provides a spring-set, electromagnetically released brake as well as a coil bobbin for use in such a brake or other electromagnetic assembly. 
     An object of the present invention is to provide a spring-set, electromagnetically released brake that requires less current to release the brake as compared to conventional brakes. 
     Another object of the present invention is to provide a coil bobbin for use in a brake or other electromagnetic assembly that will restrict movement of the bobbin—and the coil retained by the bobbin—without bonding to a magnet pole or other structure within the assembly. 
     In accordance with the present invention, one embodiment of a spring-set, electromagnetically released brake includes a magnet shell disposed about a first axis. The magnet shell may be configured to receive a motor shaft extending along the first axis and includes a radially outwardly extending pole. The brake also includes an armature disposed radially outwardly of the pole and pivotable at a first axial end. The armature may be mounted to the magnet shell at its first axial end by a pivot pin extending in a direction perpendicular to the first axis. The brake further includes a first engagement element, such as a brake shoe, coupled to the armature proximate a second axial end of the armature. The brake further includes a spring disposed about the magnet shell pole. The spring biases the armature radially outwardly to urge the first engagement element towards a second engagement element, such as the inner surface of a brake drum coupled to the motor shaft. Finally, the brake includes a coil disposed about the pole and means for selectively energizing the coil to urge the armature radially inwardly and the first engagement element away from the second engagement element. 
     The pivoting motion of the armature and the relative positions of the armature and magnet shell pole allow a brake in accordance with the present invention to operate more efficiently as compared to conventional spring-set, electromagnetically released brakes. In particular, because the armature pivots at its first axial end, the second axial end of the armature containing the engagement element will travel a greater radial distance as the armature moves between a set position and a release position than those portions of the armature intermediate the first and second axial ends. The magnet shell pole is preferably located such that the axial center of the pole is intermediate the first and second axial ends of the armature. As a result, the average radial distance between the magnet shell pole and the armature when the armature is in the set position is less than the radial distance traveled by the second axial end of the armature as the armature moves between the set and release positions. This positioning allows a relatively small level of current to be used to move the armature and release the brake because the magnet shell pole and armature are relatively close even when the armature is in the set position. 
     One advantage of a brake in accordance with the present invention is a reduction in the amount of backlash when the brake is set as compared to conventional brakes. This reduction may be accomplished in several ways. First, the first axial end of the armature may be secured on a pivot pin held between two mounting posts on the magnet shell. The pivot pin is perpendicular to the first axis and, therefore, the axis of rotation of the brake drum and motor shaft. This arrangement reduces backlash in either rotational direction. Second, the inventive brake may include: (i) a coupling shaft having a female end with an axially extending slit configured to receive a key on the motor shaft and (ii) a collar disposed radially outwardly of the female end of the shaft and configured to clamp onto the key of the motor shaft. Finally, where the engagement elements form teeth disposed on the armature and brake drum, backlash may be reduced by angular placement of multiple armatures and/or their teeth so that the teeth on one armature are configured to fully engage the teeth on the brake drum while the teeth on another armature are configured to partially engage the teeth on the brake drum. 
     Other advantages of the present invention may be obtained by using multiple magnet shell poles, armatures, springs and coils. The use of multiple springs allows a braking torque to be applied in the event of a failure in any one spring. The use of multiple poles and coils enables the brake to be operated using various input voltages. For example, if the coils are connected in parallel a first voltage can be used. If the coils are connected in series, a second voltage (twice that of the first voltage) may be used. 
     A brake in accordance with the present invention may further include a bobbin to retain the coil about the magnet shell pole. A coil bobbin in accordance with the present invention includes a substantially cylindrical body portion extending along a longitudinal axis and having first and second ends. The body portion may be disposed about the magnet shell pole. The bobbin further includes first and second discs extending radially outwardly from the first and second ends, respectively. The first and second discs receive the coil therebetween. In a first embodiment of a bobbin in accordance with the present invention, the cylindrical body portion includes a radially inwardly extending flange and the first disc includes an axially extending projection. The flange and projection are configured to be received within openings in, or secured against surfaces of, the magnet shell of the inventive brake. In a second embodiment of a bobbin in accordance with the present invention, the first disc includes an elastically deformable retention member extending axially from the first disc. The retention member terminates in a triangular flange that is configured to engage the magnet shell of the inventive brake. The inventive coil bobbin is not restricted, however, to use within the inventive brake. Rather, the bobbin may be used in a variety of electromagnetic assemblies. 
     A bobbin for an electromagnetic assembly in accordance with the present invention represents and improvement over conventional bobbins because the bobbin may be secured against axial and radial movement within the assembly without bonding. As a result, the cost and time required to install the bobbin is reduced. 
     These and other features and objects of this invention will become apparent to one skilled in the art from the following detailed description and the accompanying drawings illustrating features of this invention by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a first embodiment of a brake in accordance with the present invention. 
     FIG. 2 is a cross-sectional view of the brake of FIG. 1 taken substantially along lines  2 — 2 . 
     FIG. 3 is an exploded perspective view of the brake of FIG.  1 . 
     FIG. 4 is a perspective view of the magnet shell of the brake of FIG.  1 . 
     FIG. 5 is a plan view illustrating one embodiment of the engagement elements of the brake of FIG.  1 . 
     FIG. 6 is a perspective view of a first embodiment of a bobbin in accordance with the present invention. 
     FIG. 7 is a cross-sectional view of the bobbin of FIG. 6 taken substantially along lines  7 — 7 . 
     FIG. 8 is a perspective view of an electromagnetic assembly including a second embodiment of a bobbin in accordance with the present invention. 
     FIG. 9 is a cross-sectional view of electromagnetic assembly of FIG.  8 . 
     FIG. 10 is a perspective view of a bobbin assembly including a third embodiment of a bobbin in accordance with the present invention. 
     FIG. 11 is a cross-sectional view of the bobbin assembly of FIG. 10 taken substantially along lines  11 — 11 . 
     FIG. 12 is a partial cross-sectional view illustrating the operation of a brake in accordance with the present invention. 
     FIG. 13 is a cross-sectional view of a second embodiment of a brake in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views, FIGS. 1-3 illustrate a brake  20  in accordance with a first embodiment of the present invention. Brake  20  includes a housing  22 , and an endbell  24 . In accordance with the present invention, brake  20  also includes the following elements: a magnet shell  26 ; armatures  28 ,  30 ; engagement elements, such as brake shoes  32 ,  34 ; springs  36 ,  38 ; coils  40 ,  42 ; means, such as a conventional power supply (not shown), for selectively energizing coils  40 ,  42 ; bobbins  44 ,  46 ; a brake drum  48 ; a brake shaft  50 ; and a collar  52 . Referring to FIG. 3, brake  20  may be used in connection with a servo motor or stepper motor  54 . It should be understood, however, that the application of brake  20  is not limited to such motors. Rather, brake  20  may be used with a wide variety of motors and in a wide variety of applications. 
     Referring to FIG. 1, housing  22  and endbell  24  are provided to protect the internal components of brake  20  from foreign objects and elements and to protect motor  54  or other objects and individuals in the event of a failure of brake  20 . Housing  22  and endbell  24  also provide structural support to the components of brake  20 . Housing  22  is conventional in the art and may be made from an aluminum alloy. Referring to FIG. 2, housing  22  may be centered about an axis  56 . Referring to FIG. 3, the radially inner periphery of housing  22  defines a bore  58  sized to receive brake drum  48 . Housing  22  includes an aperture  60  in which a conventional fluid tight connector  62  may be inserted to allow the introduction of lubricants to brake  20 . Endbell  24  is also conventional in the art and may be made from an aluminum alloy. Referring to FIG. 2, endbell  24  may also be centered about axis  56 . Referring again to FIG. 3, endbell  24  includes an aperture  64  through which brake shaft  50  may extend. Endbell  24  may be connected to housing  22  by inserting a plurality of screws  66  or other fastening means, together with washers  68 , into corresponding apertures  70 ,  72  in endbell  24  and housing  22 . 
     Magnet shell  26  provides structural support for the components of brake  20  and forms part of a magnetic circuit used to selectively release brake  20 . Shell  26  may be formed from a steel alloy or other ferromagnetic material and is disposed about axis  56 . Referring to FIG. 4, shell  26  includes a body portion  74 , a mounting flange  76 , mounting posts  78 , and poles  80 ,  82 . Referring to FIG. 3, body portion  74  is received within bore  58  of housing  22  and includes a bore  84  through which brake shaft  50  and/or a shaft  86  of motor  54  may extend. Mounting flange  76  is integral with body portion  74  and is configured to secure brake  20  to housing  88  of motor  54  and housing  22  using screws or other conventional fasteners. Referring to FIG. 2, mounting flange  76  also provides a seat  90  for collar  52 . Referring again to FIG. 4, mounting posts  78  are integral with body portion  74  and mounting flange  76  and are provided to secure armatures  28 ,  30  as described in greater detail hereinbelow. Poles  80 ,  82  are integral with, and extend radially outwardly from, body portion  74 . Poles  80 ,  82  form part of a magnetic circuit used to selectively release brake  20  as described in greater detail hereinbelow. Although only two poles  80 ,  82  are shown in the illustrated embodiment, it should be understood that additional poles may be disposed about body portion  74  of shell  26 . The use of shell  26  is advantageous with respect to conventional brakes, because shell  26  integrates several components into a single part. In particular, the integration of mounting flange  76  with body portion  74  eliminates the need to secure a separate endbell to housing  22 . 
     Referring to FIG. 2, armatures  28 ,  30  are provided to move engagement elements (e.g., brake shoes  32 ,  34 ) into and out of engagement with a second set of engagement elements formed in or extending from the radially inner surface of brake drum  48  to thereby set and release brake  20 . Armatures  28 ,  30  may be made from a steel alloy or other ferromagnetic material. Armatures  28 ,  30  are disposed radially outwardly of poles  80 ,  82  and extend longitudinally along axis  56 . Armatures  28 ,  30  are each configured to pivot at a first axial end,  92 ,  94 , respectively. Referring to FIG. 3, armatures  28 ,  30  may be supported on pivot pins,  96 ,  98 , respectively, extending through apertures  100 ,  102  disposed proximate the first axial ends  92 ,  94  of armatures  28 ,  30 . Pins  96 ,  98  may be supported within mounting posts  78  of magnet shell  26 . Because pins  96 ,  98  extend in a direction perpendicular to axis  56 —and therefore the axis of rotation of drum  48 —and are held securely within mounting posts  78 , backlash at the second axial ends  104 ,  106 , of armatures  28 ,  30 , respectively, in either rotational direction is reduced and/or eliminated. Ends  104 ,  106  of armatures  28 ,  30 , respectively, may be curved and may be sized to fit radially inwardly of brake drum  48 . 
     The engagement elements disposed on armatures  28 ,  30  are provided to engage corresponding engagement elements formed in or extending from brake drum  48  in order to set brake  20 . The engagement elements on armatures  28 ,  30  may comprise conventional brake shoes  32 ,  34 . Alternatively, as shown in FIG. 5, the engagement elements may each comprise a plurality of teeth  108 ,  110  extending radially outwardly from armatures  28 ,  30 . It should be understood, however, that the illustration of brakes shoes  32 ,  34  and teeth  108 ,  110  is exemplary only and that the engagement elements on armatures  28 ,  30  may take on a variety of forms known in the art. Brakes shoes  32 ,  34  and teeth  108 ,  110  may be disposed proximate the second axial ends  104 ,  106  of armatures  28 ,  30 , respectively. 
     Springs  36 ,  38  are provided to bias armatures  28 ,  30 , respectively, radially outwardly to thereby urge engagement elements on armatures  28 ,  30  towards the engagement elements on brake drum  48 . Springs  36 ,  38  are conventional in the art and may be made from zinc plated music wire. Springs  36 ,  38  may be disposed about poles  80 ,  82  of shell  26 . The use of multiple springs in brake  20  is advantageous because brake  20  will remain set in the event of a spring failure. 
     Coils  40 ,  42  are provided to selectively generate a magnetic circuit between poles  80 ,  82  of shell  26  and armatures  28 ,  30  in order to attract armatures  28 ,  30  radially inwardly and their engagement elements away from the engagement elements on brake drum  48 . Coils  40 ,  42  are conventional in the art any may be disposed about poles  80 ,  82 . Coils  40 ,  42  may be energized through lead wires (not shown) extending from a conventional power supply (not shown) through bobbins  44 ,  46 . The use of multiple coils in brake  20  is advantageous because it allows the use of different input voltages. If coils  40 ,  42  are connected in parallel, a power supply having a first input voltage may be used to supply current to coils  40 ,  42 . If coils  40 ,  42  are connected in series, a power supply having a second input voltage—twice that of the first input voltage—may be used to supply current to coils  40 ,  42 . 
     Bobbins  44 ,  46  are provided to retain coils  40 ,  42 , respectively. In accordance with the present invention, bobbins  44 ,  46  are configured to be secured within apertures in, and/or against surfaces of, magnet shell  26  so that bobbins  44 ,  46 —and coils  40 ,  42 —may be secured against movement without bonding bobbins  44 ,  46  to magnet shell  26 . Referring to FIGS. 6 and 7, one embodiment of a bobbin  44  in accordance with the present invention and for use in brake  20  is shown. Although FIGS. 6 and 7 illustrate only one bobbin  44  of bobbins  44 ,  46 , it will be understood that bobbin  46  is substantially similar to the illustrated bobbin  44 . Bobbin  44  may be made from glass-filled nylon and includes a cylindrical body portion  112  extending along a longitudinal axis  114 . Bobbin  44  also includes a pair of discs  116 ,  118  extending radially outwardly from first and second ends of body portion  112 . Discs  116 ,  118  may be integral with body portion  112 . Disc  116  includes at least one projection  120  extending axially relative to axis  114 . Projection  120  may be used to locate the lead wires (not shown) that supply current to the coil  40  retained by bobbin  44 . Body portion  112  includes a radially inwardly extending flange  122 . Flange  122  is annular and disposed about the entire radially inner periphery of body portion  112  in the illustrated embodiment. It should be understood, however, that flange  122  may extend about only an angular portion of the radially inner periphery of body portion  112 . Flange  122  is preferably disposed at one end of body portion  112  and, in particular, at the end from which the disc  116  having projection  120  is disposed. 
     The inventive bobbin  44  represents an improvement as compared to conventional bobbins because bobbin  44 —and the coil  40  it retains—may be secured against both axial and radial movement in brake  20  without bonding bobbin  44  to magnet shell  26 . Referring to FIG. 2, projection  120  may be disposed within a recess  124  in magnet shell  26 . Flange  122  may be urged against a surface  126  of body portion  74  of magnet shell  26  by spring  36 . Because bobbin  44  may be secured to shell  26  without bonding, devices that include bobbin  44 , such as brake  20 , may be made more efficiently and less expensively. 
     Referring now to FIGS. 8 and 9, an electromagnetic assembly  127  incorporating bobbins  44 ′,  46 ′ in accordance with a second embodiment of the present invention is shown. Assembly  127  may form a portion of brake  20 . Like bobbin  44 , bobbins  44 ′ and  46 ′ include a body portion  112  disposed about an axis  128  and discs  116 ,  118 . Bobbins  44 ′,  46 ′ further include retention members  130  extending axially relative to axis  128  from the radially outer perimeter of disc  116 . Retention members  130  terminate in triangular flanges  132  and may be elastically deformable. Members  130  may be deflected radially outwardly relative to axis  128  as bobbins  44 ′,  46 ′ are placed over poles  80 ,  82  of magnet shell  26 . Members  130  will then return to form to engage flanges extending from magnet shell  26 . Like bobbin  44 , bobbins  44 ′,  46 ′ represent an improvement with respect to conventional bobbins because bobbins  44 ′,  46 ′—and the coils  40 ,  42  they retain—may be secured against axial and radial movement within brake  20  without bonding bobbins  44 ′,  46 ′ to shell  26 . 
     Referring to FIGS. 10 and 11, a bobbin assembly  134  including bobbins  44 ″ and  46 ″ in accordance with a third embodiment of the present invention is shown. Assembly  134  may be used within brake  20  or other electromagnetic assemblies. Like bobbins  44  and  44 ′ described hereinabove, each of bobbins  44 ″,  46 ″ includes a cylindrical body portion  112  disposed about an axis  136  and a pair of discs  116 ,  118 . Bobbin  44 ″ further includes a coupling pin  138  and a female coupling  140  extending in a first axial direction relative to axis  136  from disc  116 . Pin  138  and coupling  140  are angularly spaced about disc  116  of bobbin  44 ″ and are diametrically opposite in the illustrated embodiment. Bobbin  46 ″ further includes a coupling pin  142  and a female coupling  144  extending in a second axial direction relative to axis  136  from disc  116 . Pin  142  and coupling  144  are angularly spaced about disc  116  of bobbin  46 ″ and are also diametrically opposite in the illustrated embodiment. Each of couplings  140 ,  144  is configured to receive a corresponding pin  142  and  138 , respectively, therebetween. Both pins  138 ,  142  and couplings  140 ,  144  may extend through apertures (not shown) in magnet shell  26  that extend in a direction perpendicular to the axis  56  about which shell  26  is disposed. Like bobbins  44 ,  44 ′, bobbins  44 ″,  46 ″ of assembly  134  represent an improvement with respect to conventional bobbins because each of bobbins  44 ″,  46 ″—and the coils  40 ,  42  they retain—may be secured against axial and radial movement within brake  20  without bonding bobbins  44 ″,  46 ″ to shell  26 . 
     Referring to FIGS. 2 and 3, brake drum  48  is provided as part of a means for setting brake  20 . Drum  48  is conventional in the art and may be made of steel. Drum  48  may include an annular body portion  146  centered about axis  56  and an axially-extending skirt  148 . Referring to FIG. 3, body portion  146  may include one or more curved recesses  150  at its radially inner perimeter to allow drum  48  to be mounted on brake shaft  50 . Alternatively, drum  48  may be made integral with brake shaft  50 . Portion  146  may also include one or more angularly spaced apertures  152  in order to reduce the weight of drum  48 . Portion  146  may further include one or more fans  154  mounted thereto to aid in cooling brake  20 . Fans  154 , may alternatively be mounted to the radially outer surface of skirt  148 . Drum  48  further includes an engagement element or elements that are provided to engage the engagement elements of armatures  28 ,  30  in order to set brake  20 . Referring to FIG. 2, the engagement element may comprise the radially inner surface  156  of skirt  148 . Surface  156  may act in a conventional manner to frictionally engage brakes shoes  32 ,  34 . Referring to FIG. 5, the engagement element may alternatively comprise a plurality of teeth  158  extending radially inwardly from skirt  148  and configured to engage corresponding teeth  108 ,  110  disposed on armatures  28 ,  30 . In the latter embodiment, either armatures  28 ,  30  or the teeth  108 ,  110  on armatures  28 ,  30  may be angularly offset so that, when brake  20  is set, one plurality of teeth  108 ,  110  is fully engaged with teeth  158  of drum  48 , while another plurality of teeth  108 ,  110  is only partially engaged with teeth  158  of drum  48 . This construction will further reduce backlash within brake  20  and also provides significant braking torque. 
     Referring now to FIG. 13, an alternative embodiment of a brake  20 ′ in accordance with the present invention is illustrated. Brake  20 ′ is similar to brake  20  described hereinabove, but includes a modified brake drum  48 ′. Like drum  48  described hereinabove, drum  48 ′ includes a body portion  146  centered about an axis  160  and an axially extending skirt  148 . Drum  48 ′ further includes a substantially cylindrical mounting portion  162  extending axially from body portion  146  through bore  84  of magnet shell  26 . Portion  162  may include a keyway  164  configured to receive a key  166  on motor shaft  86  in order to couple drum  48 ′ to shaft  86  for rotation therewith. Drum  48 ′ may be rotatably supported within bore  84  of shell  26  by bearings  168 . Brake  20 ′ offers several advantages relative to conventional brakes and even brake  20 . First, brake  20 ′ does not require the use of brake shaft  50  to transmit braking torque to motor shaft  86 . Second, brake drum  48 ′ provides bearing support for motor shaft  86 . Third, brake  20 ′ may be easily assembled with motor  54  by sliding drum  48 ′ onto motor shaft  86  and bolting mounting flange  76  of magnet shell  26  to housing  88  of motor  54 . 
     Referring again to FIG. 3 and a description of brake  20 , brake shaft  50  is provided to transfer a braking torque to motor shaft  86 . Shaft  50  may be made from steel or other conventional materials. Shaft  50  is preferably made of a ferromagnetic or other material having a low magnetic reluctance, however, in order to increase the strength of the magnetic circuit formed between poles  80 ,  82  of magnet shell  26  and armatures  28 ,  30  upon release of brake  20 . For the same reason, the diameter of brake shaft  50  may be about equal to the diameter of bore  84  of magnet shell  26 . Referring to FIGS. 2 and 3, shaft  50  may be centered about axis  56 . Shaft  50  may include a plurality of lugs (not shown) configured to be received within recesses  150  of body portion  146  of brake drum  48  in order to couple drum  48  to shaft  50  for rotation therewith. Shaft  50  may be supported at one axial end within endbell  24  by bearings  170  secured by retainer clips  172 ,  174 . A second axial end of shaft  50  may include a female coupling  176  having one or more axially extending slits  178 . Slits  178  may be configured to receive key  166  of motor shaft  86  to rotatably couple brake shaft  50  and motor shaft  86 . Through slits  178 , coupling  176  allows axial movement of motor shaft  86  without effecting the performance of brake  20 . 
     Referring to FIG. 3, collar  52  is provided to further reduce backlash in brake  20  and motor shaft  86  when brake  20  is set. Collar  52  is conventional in the art and includes a substantially circular piece of steel defining an aperture with means, such as a screw  180 , for adjusting the size of the aperture. Referring to FIG. 2, collar  52  may be seated within mounting flange  76  of magnet shell  26  and may be disposed about coupling  176  of brake shaft  50  and key  166  of motor shaft  86 . Collar  52  may be adjusted to clamp down upon key  166  of motor shaft  86  when brake  20  is installed. 
     In addition to the advantages discussed hereinabove, a brake in accordance with the present invention significantly improves upon conventional brakes in the efficient use of electrical power to release brake  20 . As discussed hereinabove, and with reference now to FIG. 12, each of armatures  28 ,  30  is disposed radially outwardly of poles  80 ,  82  of shell  26  and is configured to pivot at a first axial end  92 ,  94 . As a result, the second axial ends  104 ,  106  of armatures  28 ,  30  (which include the engagement elements) travel a greater radial distance D 1  than those portions of armatures  28 ,  30  intermediate ends  92 ,  94  and  104 ,  106 , respectively. As long as the average radial distance D 2  between the radially outer surfaces of poles  80 ,  82  and the radially inner surfaces of armatures  28 ,  30  is less than the distance D 1  traveled by ends  104 ,  106  of armatures  28 ,  30 , the current required to release brake  20  will be less than the current required if all portions of armatures  28 ,  30  traveled a uniform radial distance. This condition will be satisfied as long as the axial centers of poles  80 ,  82 , relative to axis  56  are intermediate the first and second axial ends  92 ,  94  and  104 ,  106  of armatures  28 ,  30 , respectively. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it is well understood by those skilled in the art that various changes and modifications can be made in the invention without departing from the spirit and scope of the invention.