Patent Publication Number: US-8978854-B2

Title: Rotational coupling device with integrated rotor and brake disc

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/527,474 filed Aug. 25, 2011, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     a. Field of the Invention 
     This invention relates to a rotational coupling device. In particular, the instant invention relates to a device having a variety of improvements intended to permit use of the device in applications requiring either a wet (i.e. including use of a lubricating fluid) or dry coupling device and in applications requiring either vertical or horizontal mounting arrangements. 
     b. Background Art 
     Rotational coupling devices such as clutches and brakes are used to control transfer of torque between rotational bodies. In an electromagnetic coupling device, an armature coupled to an input or output member is brought into engagement with, and/or disengagement from, a rotor coupled to the other of the input or output members in order to rotatably couple or decouple the input and output members. 
     Conventional electromagnetic coupling devices are often ill-suited to use in certain applications including those where a wet coupling device is desired and in applications where the device may need to be oriented vertically. For example, in devices that are oriented vertically, gravitational forces act on the armature and/or clutch plates between the armature and rotor and may cause undesirable movement and frictional engagement of these components at a time when the torque transfer is undesirable. Further, if the device is oriented vertically and a wet device is required, difficulties exist in insuring adequate lubrication of the device components in view of the gravitational forces acting on fluid in the device. It is also difficult to design a wet device in which the electromagnetic circuit provides desired operating characteristics. Use of existing devices in some environments—particularly those in which water or other liquids may be present—can also be problematic because of an inability to adequately remove such liquids and maintain the operability of the device. 
     The inventors herein have recognized a need for a rotational coupling device that will minimize and/or eliminate one or more of the above-identified deficiencies. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates to a rotational coupling device. In particular, the present invention relates to a rotational coupling device having a variety of improvements intended to permit use of the device in applications requiring either a wet (i.e. including use of a lubricating fluid) or dry coupling device and in applications requiring either vertical or horizontal mounting arrangements. 
     A rotational coupling device in accordance with another embodiment of the invention includes a hub configured for coupling to a shaft for rotation with the shaft about an axis of rotation. The device further includes a rotor supported on the hub and configured for rotation relative to the hub about the axis of rotation. The rotor includes a first member defining a radially extending wall having a first rotor coupling surface and an axially extending, radially inner rotor pole and a second member coupled to the first member for rotation therewith, but axially movable relative to the first member. The second member defines an axially extending, radially outer rotor pole, a second rotor coupling surface and a first braking surface. The device further includes an armature supported on the hub for rotation therewith and disposed on a first side of the radially extending wall of the rotor. The device further includes a field shell disposed about the hub on a second side of the radially extending wall of the rotor opposite from the armature. The field shell houses a conductor therein and defines radially spaced inner and outer field shell poles aligned with the inner and outer rotor poles, respectively. Energization of the conductor establishes an electromagnetic circuit among the field shell, the rotor and the armature to urge the armature in a first axial direction towards the radially extending wall of the rotor and couple the rotor to the hub for rotation therewith. The device further includes a brake plate defining a second braking surface aligned with the first braking surface of the second member of the rotor. The second member of the rotor is configured such that, upon energization of the conductor, the second member of the rotor is urged towards the first member of the rotor and the second rotor coupling surface is urged into engagement with the first rotor coupling surface and, upon deenergization of the conductor, the second member of the rotor is urged towards the brake plate, the first braking surface is urged into engagement with the second braking surface and the second rotor coupling surface is urged to disengage from the first rotor coupling surface to create a fluid flow opening between the first and second members of the rotor. A rotational coupling device in accordance with this embodiment of the invention is advantageous as compared to conventional devices because it provides a simpler design than conventional devices and also provides an efficient way for removing liquids from the device when the device is used in a wet environment. 
     The foregoing and other aspects, features, details, utilities, and advantages of the invention will be apparent from reading the following detailed description and claims, and from reviewing the accompanying drawings illustrating features of this invention by way of example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-2  are perspective views of a rotational coupling device in accordance with the present invention. 
         FIG. 3  is a cross-sectional view of the rotational coupling device in  FIGS. 1-2 . 
         FIG. 4  is a plan view of the hub of the device in  FIGS. 1-2 . 
         FIG. 5  is a cross-sectional view of the hub of the device in  FIGS. 1-2 . 
         FIG. 6  is an enlarged cross-sectional view of a portion of the device in  FIGS. 1-2  illustrating the interface between the hub and clutch plates of  FIGS. 1-2 . 
         FIG. 7  is a plan view of the rotor of the device in  FIGS. 1-2 . 
         FIG. 8  is a perspective view of a portion of the rotor of the device in  FIGS. 1-2 . 
         FIG. 9  is an enlarged cross-sectional view of a portion of the device in  FIGS. 1-2  illustrating a portion of the rotor of the device in  FIGS. 1-2 . 
         FIG. 10  is a plan view of the armature of the device in  FIGS. 1-2 . 
         FIGS. 11-13  are plan views of various clutch plates of the device in  FIGS. 1-2 . 
         FIG. 14  is a plan view of the field shell of the device in  FIGS. 1-2 . 
         FIG. 15  is an enlarged cross-sectional view of a portion of the device in  FIGS. 1-2  illustrating the venting of vapor in the device in  FIGS. 1-2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views,  FIGS. 1-3  illustrate a rotational coupling device  20  in accordance with one embodiment of the invention. Device  20  functions as a clutch to selectively transfer torque from a rotating shaft  21  driven by an engine, electric motor or other conventional power source. Device  20  also functions as a brake when torque is not being transferred. Device  20  may be provided for use in a lawnmower or similar device. It will be understood by those of ordinary skill in the art, however, that device  20  may be used in a wide variety of applications requiring a clutch and/or brake. Device  20  may include a hub  22 , a spacer  24 , a rotor  26 , a sealing plate  28 , an armature  30 , a spring  32 , clutch plates  34 ,  36 ,  38 ,  40 ,  42 , means, such as tubes  44 ,  46 , for transporting fluid from a sump  47  to clutch plates  34 ,  36 ,  38 ,  40 ,  42 , a field shell  48 , a conductor  50 , brake plates  52 ,  54  (see  FIG. 14 ), means, such as magnets  56  for urging a portion of the rotor  26  towards brake plates  52 ,  54 , and a fluid slinger  58 . 
     Hub  22  transfers torque from a rotating shaft  21  to armature  30  and clutch plates  36 ,  40 . Hub  22  may be made from conventional metals and metal alloys. Hub  22  is annular and is disposed about the rotating shaft  21  and an axis  60  of rotation for the shaft  21 . Referring to  FIG. 4 , a radially inner surface of hub  22  may define a radially inwardly extending key  62  configured to be received within a corresponding keyway in the rotating shaft  21  to couple hub  22  to the shaft  21  for rotation therewith. It should be understood that hub  22  may alternatively define a keyway configured to receive a corresponding key in the rotating shaft  21  or may be coupled to the shaft  21  using a variety of other conventional coupling methods. The radially outer surface of hub  22  may define a plurality of radially outwardly extending splines  64  configured to be received within corresponding notches in armature  30  and clutch plates  36 ,  40 . The circumferential spacing between splines  64  may vary as shown in  FIG. 4  for a purpose described hereinbelow. Referring now to  FIG. 5 , a vent bore  66  may extend between the radially inner and outer surfaces of hub  22  for a purpose described hereinbelow. Bore  66  may extend in a direction perpendicular to axis  60 . As shown in  FIG. 5 , the outer diameter of hub  22  may vary along its axial length. Hub  22  defines an elongate surface  68  at one axial end configured, as illustrated in  FIG. 3 , to receive field shell and rotor bearings  70 ,  72 , and a spacer  74  disposed between bearings  70 ,  72 . The same surface  68  is configured to receive fluid slinger  58 . Referring again to  FIG. 5 , a groove  76  formed near one axial end of surface  68  is configured to receive a retainer ring  78  (see  FIG. 3 ) used to facilitate assembly of device  20  and maintain the position of bearings  70 ,  72 , and other components of device  20 . The opposite axial end of hub  22  defines a pair of shoulders  80 ,  82 , configured, as illustrated in  FIG. 3 , to receive another rotor bearing  84  and a fluid seal  86 . Referring again to  FIG. 5 , hub  22  further defines a groove  88  configured to receive a retainer ring which anchors the end of spring  32 . 
     In accordance with one embodiment of the invention, hub  22  further includes means, such as shoulders  90 ,  92 ,  94 , for limiting movement of clutch plates  36 ,  40  and armature  30  in one axial direction. The outer diameter of hub  22  varies to define steps and shoulders  90 ,  92 ,  94 , with the diameter increasing moving axially from clutch plate  36  to armature  30  such that the radial distances from axis  60  to shoulder  90 ,  92 ,  94  are different. When device  20  is oriented vertically, gravitational force urges armature  30  and clutch plates  36 ,  40 , in one axial direction (to the right in  FIG. 3 ). When the clutch of device  20  is disengaged, therefore, gravitational force may result in undesirable contact among armature  30  and clutch plates  34 ,  36 ,  38 ,  40 ,  42 . Referring to  FIG. 6 , shoulders  90 ,  92 ,  94  limit movement of plates  36 ,  40  and armature  30  to prevent such contact. 
     Referring again to  FIG. 3 , spacer  24  is provided to retain bearing  84  in assembled relation to the other components of device  20 . Spacer  24  may be made from conventional metals and metal alloys. Spacer  24  is disposed about axis  60  and is generally cylindrical in shape. Spacer  24  is configured to receive a fastener (not shown) that extends through spacer  24  and into the rotating shaft  21 . Spacer  24  may define a head  96  at one axial end. Referring to  FIG. 1 , head  96  may have a plurality of flats  98  that allow the rotating shaft  21  to be secured while applying torque to the fastener. Head  96  also opposes shoulder  80  in hub  22  to prevent axial movement of bearing  84 . Spacer  24  may further define a keyway in a radially outer surface configured to receive key  62  of hub  22 . 
     Rotor  26  provides a means for transferring torque to an output member. Rotor  26  provides a reaction surface against which armature  30  compresses clutch plates  34 ,  36 ,  38 ,  40 ,  42  to couple rotor  26  to hub  22  for rotation with hub  22 . In accordance with another aspect of the invention as discussed below, rotor  26  may also function as a part of a brake for the output member when the clutch is disengaged. Rotor  26  is disposed about hub  22  and axis  60  and may include several members  100 ,  102 ,  104  which may be made from conventional metals and metal alloys. 
     Member  100  is annular in shape and disposed about axis  60 . Member  100  defines a radially extending wall  106 . Wall  106  defines radially outer and inner circular recesses  108 ,  110  in one axial side of wall  106 . Outer recess  110  is configured to receive a portion of member  102  of rotor  26 . Inner recess  108  is configured to receive sealing plate  28 . Wall  106  further defines a plurality of apertures  112  extending axially therethrough and in communication with outer recess  110 . Apertures  112  are configured to receive fasteners  114 ,  116  used to couple member  100  to member  102  of rotor  26 . In the illustrated embodiment, fasteners  114 ,  116 , comprise screws secured in place by nuts  118 ,  120 , respectively. It should be understood, however, that other types of fasteners such as bolts, pins, welds or adhesives could alternatively be used. Fasteners  114 ,  116  extend through a gasket  122  that functions as a seal between members  100 ,  102 . Fasteners  116  are longer than fasteners  114  and extend through spacers  124  used to support leaf springs  126 . Each of fasteners  116  is used to secure one end of a corresponding leaf spring  126  extending between members  100 ,  104  of rotor  26  for a purposed described hereinbelow. Referring to  FIG. 7 , wall  106  may further include one or more radially spaced rows of circumferentially spaced, banana shaped slots  128 . Upon energization of conductor  50 , slots  128  help to direct the flow of magnetic flux between wall  106  of member  100  of rotor  26  and armature  30 . In the illustrated embodiment, wall  106  includes a single row of three slots  128 . It should be understood, however, that the number of rows of slots  128  the number of slots  128  in any one row, and the size and shape of slots  128  may vary. Referring again to  FIG. 3 , slots  128  are in communication with inner recess  108 . Member  100  further defines an axially extending radially inner rotor pole  130  extending from a radially inward end of wall  106 . Pole  130  is supported on rotor bearing  72  and includes a radially inwardly extending flange  132  at one axial end opposite wall  106  to limit movement relative to bearing  72 . 
     In accordance with another aspect of the invention, member  100 , together with member  102 , forms a substantially enclosed chamber  134  housing armature  30 , clutch plates  34 ,  36 ,  38 ,  40 ,  42  and a lubricating fluid such as oil. Accordingly, device  20  is not dependent upon fluid integration with other devices to provide lubricating fluid and does not require the design of custom housings for different applications. Member  102  is annular in construction and has a diameter that generally increases from one axial end  136  of member  102  farthest from member  100  to an opposite axial end  138  adjacent member  100 . Member  102  defines an axially ending wall  140  proximate end  136  sized to receive bearing  84  and seal  86 . Wall  140  defines a radially inwardly extending flange at one end configured to retain bearing  84 . A wall  142  extends radially from the opposite end of axially extending wall  140 . Wall  142  forms the bottom of a fluid sump  47  when device  20  is oriented vertically. Fluid may be inserted or removed through a fill port  144  in which a plug  146  is disposed. Another axially extending wall  148  extends from the opposite end of radially extending wall  142 . Wall  148  may be configured to support an output member  150 . In the illustrated embodiment member  150  comprises a pulley. It should be understood, however, that member  150  could take on a variety of forms including gears. Another wall  152  extends from axially extending wall  148  in a generally radial direction, but at an angle to axis  60 . Wall  152  terminates in another axially extending wall  154 . Referring now to  FIG. 8 , in accordance with one embodiment of the invention, wall  154  may include means, such as shoulders  156 ,  158 ,  160  for limiting movement of clutch plates  34 ,  38 ,  42  in one axial direction. Wall  154  may define a plurality of axially extending grooves  161  in a radially inner surface configured to receive corresponding teeth  181  on clutch plates  34 ,  38 ,  42  (see  FIG. 11 ). At one or more circumferentially spaced sections, however, a set (equal in number to the number of clutch plates  34 ,  38 ,  42 ) of adjacent grooves are axially shortened to define shoulders  156 ,  158 ,  160 . In the illustrated embodiment, grooves  161  are axially shortened in three equally circumferentially spaced sections on wall  154 . It should be understood, however, that the number of sections may vary. The axial length of the grooves  161  in which shoulders  156 ,  158 ,  160  are formed varies to maintain a predetermined spacing between clutch plates  34 ,  38 ,  42  when the clutch is disengaged. When device  20  is oriented vertically, gravitational force urges clutch plates  34 ,  38 ,  42  in one axial direction (to the right in  FIG. 3 ). When the clutch of device  20  is disengaged, therefore, gravitational force may result in undesirable contact among clutch plates  34 ,  36 ,  38 ,  40 ,  42  and armature  30 . The shoulders  156 ,  158 ,  160  on wall  154  limit movement of plates  34 ,  38 ,  42  to prevent such contact. Referring again to  FIG. 3 , a radially outwardly extending flange  162  extends from wall  154  and is configured to be received within recess  110  of member  100 . Flange  162  defines a plurality of apertures configured to received fasteners  114 ,  116 . 
     Member  104  defines an axially extending, radially outer rotor pole  164 . Member  104  may comprise a unitary body (i.e. a single piece structure without any type of fasteners such as screws, adhesives or welds to couple multiple pieces together). Referring to  FIG. 9 , pole  164  defines a coupling surface  166  at one axial end configured to engage a corresponding coupling surface  168  formed in wall  106  of member  100 . Member  104  further defines a radially extending flange  170  extending from pole  164 . In accordance with one aspect of the invention, flange  170  defines a braking surface  172  configured to engage a corresponding braking surfaces on brake plates  52 ,  54 . Referring to  FIG. 2 , flange  170  further defines a plurality of apertures  174  through which fasteners  176  such as rivets may extend with each fastener  176  coupled to one end of a corresponding leaf spring  126 . When the clutch is disengaged, member  104  is urged in one axial direction towards brake plates  52 ,  54  (to the left in  FIGS. 3 and 9 ) as discussed hereinbelow such that coupling surface  166  disengages from coupling surface  168  and braking surface  172  engages the braking surfaces on brake plates  52 ,  54 . As a result, a fluid flow opening is created between the coupling surfaces  166 ,  168  on members  104 ,  100 , which permits drainage of any fluid (e.g. water) from operating in wet environments. When the clutch is engaged, member  104  is drawn in an opposite axial direction towards member  100  (to the right in  FIGS. 3 and 9 ) by electromagnetic attraction such that braking surface  172  disengages from the braking surfaces on brake plates  52 ,  54 , and coupling surface  166  engages coupling surface  168  on member  100 . The integration of the rotor outer pole  164  and braking surface  172  provides a simpler design than conventional devices and also provides an efficient way for removing liquids from device  20  when device  20  is used in a wet environment. Further, the design minimizes the time that both the brake is applied and the clutch is engaged during transitions between engagement and disengagement of the clutch thereby avoiding undesirable wear and heat. The design also increases the speed of disengagement of the brake and engagement of the clutch relative to conventional designs thereby providing an improved feel to the user. 
     Referring again to  FIG. 3 , in accordance with another aspect of the invention, means, such as sealing plate  28 , are provided to prevent fluid from exiting chamber through slots  128  in member  100  of rotor  26 . Plate  28  is ring shaped and configured to be received within recess  108  in member  100  of rotor  26 . Plate  28  is made from a non-magnetic material or a material having a greater magnetic reluctance than member  100  of rotor  26 . The construction of member  100  of rotor  26  and plate  28  allows the use of slots  128  in member  100 , and the increased strength in the electromagnetic circuit resulting from their use, in applications requiring a wet device. To prevent fluids from gathering in slots  128  on the opposite side of plate  28  from chamber  134  when device  20  is used in a wet environment, an epoxy may be used to fill slots  128 . 
     Armature  30  is provided to engage the clutch by urging plates  34 ,  36 ,  38 ,  40 ,  42  into frictional engagement. Armature  30  may be made from metals or metal alloys or other materials having relatively low magnetic reluctance. In one accordance with one aspect of the invention, however, the radially innermost portion of armature  30  may be made from a material having a higher magnetic reluctance than the rest of armature  30  to limit or prevent flux transfer to hub  22  and undesirable electromagnetic attraction between armature  30  and hub  22 . Armature  30  is disposed about hub  22  and axis  60  on one axial side of wall  106  of rotor  26 . Referring to  FIG. 10 , armature  30  is annular in shape and defines a plurality of notches  178  in a radially inner surface configured to engage splines  64  on hub  22 . Armature  30  further defines a plurality of circumferentially spaced fluid apertures  180  between the radially inner and outer surfaces of armature  30 . Apertures  180  may be sized to receive tubes  44 ,  46  as discussed in greater detail hereinbelow. Apertures  180  may be nearer a radially inner diameter or surface of armature  30  than a radially outer diameter or surface of armature  30 . 
     Referring again to  FIG. 3 , in accordance with another aspect of the invention, spring  32  may be used to bias armature  30  in one axial direction (to the left in  FIG. 3 ) to counteract an opposing gravitational force biasing armature  30  in the opposite axial direction (to the right in  FIG. 3 ) when device  20  is oriented vertically. Spring  32  may be made from conventional metals and metal alloys. One end of spring  32  may be positioned against a retaining ring disposed within groove  88  in hub  22  while the opposite end of spring  32  engages armature  30 . The use of spring  32  permits faster engagement of the clutch because armature  30  does not have to overcome the gravitational force biasing armature  30  away from clutch plates  34 ,  36 ,  38 ,  40 ,  42 . 
     Clutch plates  34 ,  36 ,  38 ,  40 ,  42  are provided to transfer torque from hub  22  to output member  150  by coupling rotor  26  to hub  22  for rotation. Clutch plates  34 ,  36 ,  38 ,  40 ,  42  may be made from conventional metals and metal alloys and be treated for wear and corrosion resistance using the process offered under the registered trademark “NITROTEC” by TTI Group Ltd. of the United Kingdom or equivalents. Clutch plates  34 ,  36 ,  38 ,  40 ,  42  are disposed between armature  30  and radially extending wall  106  of member  100  of rotor  26 . In the illustrated embodiment, three clutch plates  34 ,  38 ,  42  are coupled to member  102  of rotor  26  for rotation therewith, but are axially movable relative to member  102  of rotor  26  while two clutch plates  36 ,  40  are coupled to hub  22  for rotation therewith, but are axially movable relative to hub  22 . Each of plates  36 ,  40 , is disposed between a corresponding pair  34 ,  38 , and  38 ,  42 , respectively of plates  34 ,  38 ,  42 . It should be understood that the number and ordering of clutch plates may vary. Springs (not shown) may be disposed between plates  34 ,  36 ,  38 ,  40 ,  42  to prevent rattle. 
     Referring to  FIG. 11 , clutch plates  34 ,  38 ,  42  are annular in shape and defines a plurality of teeth  181  in a radially outer surface configured to engage corresponding teeth on a radially inner surface of wall  154  of member  102  of rotor  26 . Plates  34 ,  38 ,  42  further define a pair of radially spaced rows of circumferentially spaced, banana shaped slots  182 . Upon energization of conductor  50 , slots  182  serve to direct magnetic flux to travel between wall  106  of rotor  26  and armature  30  along the radially inner and outer peripheries of plates  34 ,  38 ,  42 . It should be understood that the number of slots  182  in any one row, and the size and shape of slots  182  may vary. 
     Clutch plates  34 ,  36 ,  38 ,  40 ,  42  are provided to transfer torque from hub  22  to output member  150  by coupling rotor  26  to hub  22  for rotation. Clutch plates  34 ,  36 ,  38 ,  40 ,  42  may be made from conventional metals and metal alloys and be treated for wear and corrosion resistance using the process offered under the registered trademark “NITROTEC” by TTI Group Ltd. of the United Kingdom or equivalents. Clutch plates  34 ,  36 ,  38 ,  40 ,  42  are disposed between armature  30  and radially extending wall  106  of member  100  of rotor  26 . In the illustrated embodiment, three clutch plates  34 ,  38 ,  42  are coupled to member  102  of rotor  26  for rotation therewith, but are axially movable relative to member  102  of rotor  26  while two clutch plates  36 ,  40  are coupled to hub  22  for rotation therewith, but are axially movable relative to hub  22 . Each of plates  36 ,  40 , is disposed between a corresponding pair  34 ,  38 , and  38 ,  42 , respectively of plates  34 ,  38 ,  42 . It should be understood that the number and ordering of clutch plates may vary. Springs (not shown) may be disposed between plates  34 ,  36 ,  38 ,  40 ,  42  to prevent rattle. 
     In accordance with another aspect of the invention, the circumferential spacing between notches  184 ,  186  on clutch plates  36 ,  40  varies such that a circumferential gap d 1a , d 1b  between a pair of adjacent notches  184 ,  186  on each plate  36 ,  40  is greater than a circumferential gap d 2a , d 2b  between other pairs of notches  184 ,  186  on each plate  36 ,  40 . Similarly, the circumferential spacing between adjacent fluid apertures  192 ,  194  on each of clutch plates  36 ,  40 , varies such that a circumferential gap d 3a , d 3b  between a pair of apertures  192 ,  194 , on each plate  36 ,  40  is greater than a circumferential gap d 4a , d 4b  between other pairs of apertures  192 ,  194 , on each plate  36 ,  40 . Further, the center of the circumferential gap d 3a  between apertures  192  in plate  36  is located at a point radially outwardly of a center of the circumferential gap d 1a  between notches  184  in plate  36  while the circumferential gap d 3b  between apertures  194  in plate  40  is located diametrically opposite a point that is radially outwardly of a center of the circumferential gap d 1b  between notches  186  in plate  40 . The different arrangement of the fluid apertures  192 ,  194  relative to notches  184 ,  186  in plates  36 ,  40  facilitates proper assembly of device  20  by providing a visual cue to the proper ordering of plates  36 ,  40 . 
     In accordance with another aspect of the invention, means, such as fluid transport tubes  44 ,  46 , for transporting fluid from the sump  47  to clutch plates  34 ,  36 ,  38 ,  40 ,  42  are provided. Referring to  FIGS. 3 and 10 , tubes  44 ,  46  extend through apertures  180  in armature  30 . In the illustrated embodiment two tubes are used that extend through diametrically opposite apertures  180  and are therefore diametrically opposite one another. It should be understood, however, that the number of tubes and their orientation may vary. Referring to  FIG. 3 , each tube  44 ,  46  has one end  196  defining a fluid inlet and submerged in the fluid in the sump  47 . Each tube  44 ,  46  further extends through a corresponding aperture  180  in armature  30  and has an opposite end  198  defining a fluid outlet disposed within apertures  180  in armature  30  or on an opposite side of armature  30  from the fluid inlet. The fluid inlet and outlet may be oriented such that a direction of fluid flow through the inlet is different than a direction of fluid flow through the outlet. In particular, the direction of fluid flow through the inlet may be perpendicular to the direction of fluid flow through the outlet. As shown in  FIG. 3 , fluid may exit the outlet in a direction parallel to the rotational axis  60 . Referring to  FIG. 10 , fluid may enter the inlet in a circumferential direction relative to the axis of rotation. Alternatively, tubes  44 ,  46  may be oriented such that fluid enters the inlet in a radial direction relative to the axis of rotation, particularly in applications where device  20  has a horizontal orientation. Rotation of armature  30  enables ends  196  of tubes  44 ,  46  to scoop fluid from the sump  47  and draws fluid through tubes  44 ,  46 , where it may be deposited onto clutch plates  34 ,  36 ,  38 ,  40 ,  42 . 
     Referring again to  FIG. 3 , field shell  48  is provided to house conductor  50 . Shell  48  also forms part of an electromagnetic circuit that causes the selective engagement of wall  106  of member  100  of rotor  26 , armature  30 , and clutch plates  34 ,  36 ,  38 ,  40 ,  42  to rotatably couple hub  22  and rotor  26 . Field shell  48  may be made from conventional metals and metal alloys, including steel. Shell  48  is cylindrical and is disposed about hub  22  and axis  60  and is supported on an outer race of field shell bearing  70 . Referring to  FIGS. 2 and 3 , shell  48  is fixed against rotation and includes members  200 ,  202 ,  204 ,  206 . In accordance with one aspect of the invention, shell  48  is disposed on an opposite side of wall  106  of rotor  26  relative to armature  30  and clutch plates  34 ,  36 ,  38 ,  40 ,  42  and is isolated from the fluid in chamber  134 . As a result, the design of device  20  is less complex than conventional designs because no fluid seals or other methods to prevent exposure to fluids are required around the conductor  50  or any electrical connections. 
     Member  200  is generally U-shaped in cross-section and defines axially extending, radially inner and outer poles  208 ,  210  with a radially extending end wall  212  extending therebetween. Member  200  is sized to receive member  202  and conductor  50  between poles  208 ,  210 . Inner pole  208  is axially aligned with inner pole  130  of rotor  26  while outer pole  210  is disposed radially inwardly of outer pole  164  of rotor  26 . In accordance with another aspect of the invention, the stampings for member  200  of field shell  48  and member  100  of rotor  26  share a common pocket design for field shell bearing  70  and rotor bearing  72 , respectively, and may be made from the same blank size with a common tool and then press and formed to the desired shape and form. 
     Member  202  is annular in construction and is disposed within member  200  radially between inner and outer filed shell poles  208 ,  210 . Member  202  is configured to receive conductor  50 . Member  202  is supported on inner field shell pole  208 . Member  202  forms a part of the electromagnetic circuit referred to hereinabove and, like inner pole  208  of member  200 , directs magnetic flux from inner rotor pole  130  to end wall  212  of member  200  as well as to the outer diameter of inner field shell pole  208 . The inner diameter of member  202  may vary to define a shoulder  214  intermediate the axial ends of member  202 . In accordance with one aspect of the invention, a portion  216  of the inner pole  208  of member  200  may be deformed (e.g. by staking) to limit axial movement of member  202  in one axial direction (to the right in  FIG. 3 ). The outer diameter of member  202  may also vary such that member  202  defines a radially outwardly extending flange  218  at one axial end configured to engage conductor  50  and prevent movement of conductor  50  in one axial direction (to the right in  FIG. 3 ). 
     In accordance with another aspect of the invention, members  200 ,  202  define a fluid passage  220  configured to remove moisture from one axial side of bearing  70 . When device  20  is used in wet environments and mounted vertically, the possibility exists for fluid to accumulate on one side of bearing  70  between hub  22  and field shell  48 . A fluid passage  220  is therefore formed including a bore extending from a radially inner surface of inner pole  208  to a radially outer surface of pole  208  and a space between members  200 ,  202  to divert fluid from bearing  70  and, ultimately, expel such fluid through the fluid flow opening between coupling surfaces  166 ,  168 , on members  104 ,  100  of rotor  26 . 
     Referring to  FIG. 14 , members  204 ,  206  provide a means for mounting brake plates  52 ,  54  and provide a means for fixing field shell  48  against rotation. Members  204 ,  206  extend radially outwardly from member  200  and are affixed to member  200  by welds or other fasteners. Members  204 ,  206 , may be identical in construction. Each of members  204   206  defines a slot  222  proximate one circumferential end configured to receive a bolt or other fastener to counteract field shell rotation. Each member  204 ,  206  further defines a pair of circumferentially spaced apertures configured to receive fasteners  224  extending axially therethrough and into brake plates  52 ,  54 . 
     Referring again to  FIG. 3 , conductor  50  is provided to create an electromagnetic circuit among rotor  26 , armature  30  and field shell  48  in order to cause armature  30  to move in one axial direction (to the left in  FIG. 3 ) and press clutch plates  34 ,  36 ,  38 ,  40 ,  42  into frictional engagement in order to couple wall  106  of member  100  of rotor  26  to hub  22  for rotation therewith. Conductor  50  may comprise a conventional aluminum coil although other conventional conductors may alternatively be used. Conductor  50  is disposed within a housing  226  that may be used to mount conductor  50  within field shell  48 . Housing  226  may be molded from conventional plastics and, referring to  FIG. 14 , includes anti-rotation pins  228  extending through apertures in end wall  212  of member  200  of field shell  48 . Housing  226  may further include an integral terminal connector  230  through which conductor  50  may be electrically connected to a power source. Referring again to  FIG. 3 , in accordance with one aspect of the invention, the terminal connector  230  may be configured such that it slopes away from axis  60  in order to allow gravity to direct any moisture radially outward when device  20  is operated in wet environments. Conductor  50  and housing  226  are generally annular and are disposed about hub  22  and axis  60  within field shell  48 . In particular, conductor  50  and housing  226  are disposed between the inner and outer poles  208 ,  210  of shell  48  and end wall  212  and are supported on member  202  of shell  48 . Upon energization of conductor  50 , an electromagnetic circuit is formed between wall  106  of member  100  of rotor  26 , armature  30  and field shell  48 . Magnetic flux flows from outer pole  210  of shell  48  across an air gap to outer pole  164  of rotor  26 . Flux then travels from outer rotor pole  164  to wall  106  of member  100  of rotor  26  across another air gap. Flux then travels across a radially outer portion of clutch plates  34 ,  36 ,  38 ,  40 ,  42  to armature  30  and back to wall  106  and inner rotor pole  130  across a radially inner portion of clutch plates  34 ,  36 ,  38 ,  40 ,  42 . A portion of this flux also travels into member  202  of field shell  48  across air gaps between wall  106  and member  202  and between pole  130  and member  202 . Flux then travels from inner rotor pole  130  and member  202  of field shell  48  across air gaps to inner field shell pole  208  and end wall  212  and returns through end wall  212  of field shell  48 . Although the illustrated embodiment employs an electromagnetic circuit having two poles, it should be understood that the circuit could alternatively employ more poles (e.g. four or six poles). 
     Brake plates  52 ,  54  provide a means for applying a braking torque to rotor  26  when conductor  50  is deenergized. Brake plates  52 ,  54  may be made from conventional materials having a relatively low magnetic reluctance including conventional metals and metal alloys such as steel. Referring to  FIG. 14 , brake plates  52 ,  54  extends about at least a portion of the circumference of device  20 , and preferably only a portion of the circumference of device  20 , and are coupled to members  204 ,  206 , of field shell  48 . In particular, brake plates  52 ,  54  are coupled to members  204 ,  206  and suspended therefrom using fasteners  224 . Referring to  FIG. 9 , brake plates  52 ,  54  define a braking surface  232  configured to engage braking surface  172  of member  104  of rotor  26  upon deenergization of conductor  50 . Brake plates  52 ,  54 , also define a plurality of bores  234  configured to receive magnets  56 . Bores  234  extend through an axial portion of brake plates  52 ,  54  and comprise closed bores as shown in the illustrated embodiment. Referring to  FIG. 14 , brake plates  52 ,  54  may further define a plurality of slots  236  between tabs  238  in which the magnets  56  are disposed in order to magnetically isolate each magnet from other magnets. 
     Magnets  56  are provided to create a magnetic circuit between brake plates  52 ,  54  and member  104  of rotor  26  to draw member  104  into engagement with brake plates  52 ,  54  and provide a braking torque to rotor  26  and output member  150 . Magnets  56  may comprise neodymium iron boron (Nd—Fe—B) magnets or other known permanent magnets. Magnets  56  may be embedded within bores  234  of brake plates  52 ,  54  and may be secured therein using an adhesive. Magnets  56  may be arranged such that one face of the magnet  56  is flush with one side (and the braking surface  232 ) of brake plates  52 ,  54 . By placing the magnets  56  such that one face is flush with the engagement surface of brake plates  52 ,  54  magnets  56  add to the wear surface of brake plates  52 ,  54  increasing their wear resistance and the braking surface. Magnets  56  may be circumferentially spaced from one another about the circumferential extent of brake plates  52 ,  54  and each magnet  56  may be disposed in a tab  238  of brake plates  52 ,  54 , separated by slots  236  in brake plates  52 ,  54 . Alternatively, more than one magnet  56  may be disposed in a single tab  238  (and/or slots  236  eliminated) provided that the magnets  56  are appropriately spaced from one another. Magnets  56  may also be disposed in every other tab  236  to increase wear surface. It will further be appreciated that the number and location of magnets  56  within brake plates  52 ,  54  may vary depending upon the characteristics of device  20  and related design requirements. Magnets  56  may be arranged such that the facing poles of adjacent magnets  56  are of like polarity thereby forming parallel magnetic circuits. Alternatively, magnets  56  may be arranged such that the facing poles of adjacent magnets  56  are of opposite polarity thereby forming a less efficient series magnetic circuit. Magnets  56  are axially aligned with flange  170  of member  104  of rotor  26  such that magnetic flux travels axially through magnets  56 . In particular, magnetic flux travels through one pole of each magnet  56  (located at the radial center of magnet  56 ) across an air gap into member  104  of rotor  26 . Flux continues to travel radially inwardly and outwardly along member  104  and then axially across an air gap and radially through brake plates  52 ,  54  to return to an opposite pole of each magnet  56  located on the opposite axial end of magnet  56 . 
     In accordance with another aspect of the invention, brake plates  52 ,  54 , and their associated magnets  56  are asymmetrically arranged. In particular, brake plates  52 ,  54  are not diametrically opposed from one another, and the circumferential center of brake plate  52  is less than one hundred and eighty degrees from the circumferential center of brake plate  54 . Upon energization of conductor  50 , a portion of member  104  of rotor  26  circumferentially opposite brake plates  52 ,  54 , is able to quickly engage wall  106  of member  100  of rotor  26 , tipping member  104  relative to axis  60  thereby increasing the air gap between the braking surfaces  172 ,  232  of member  104  and brake plates  52 ,  54  and weakening the magnetic circuit between magnets  56  and member  104  and reducing clutch engagement time. 
     Referring to  FIG. 15 , fluid slinger  58  is provided to direct the return of fluid to the sump  47 . Slinger  58  may be made from conventional metal and metal alloys having a relatively high magnetic reluctance including stainless steel or aluminum. Slinger  58  is disposed about hub  22  on an opposite side of clutch plates  34   36   38 ,  40 ,  42  from armature  30 . Slinger  58  is disposed radially between hub  22  and wall  106  of member  100  of rotor  26 . In accordance with one aspect of the invention, vent bore  66  is disposed on an opposite side of slinger  58  from clutch plates  34 ,  36 ,  38 ,  40 ,  42  to permit venting of vapor to equalize system pressure. Vent bore  66  is disposed axially between fluid slinger  58  and rotor bearing  72 . Vapor from chamber  134  travels through a small opening  240  between the radially outer periphery of slinger  58  and member  100  of rotor  26 , through a passage  242  formed between slinger  58  and bearing  72  and through vent bore  66 . In applications where device  20  is oriented horizontally, slinger  58  may be replaced by a conventional seal and fluid is distributed by the slinger action of clutch plates  34 ,  36 ,  38 ,  40 ,  42 . 
     While the invention has been shown and described with reference to one or more particular embodiments thereof, it will be understood by those of skill in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.