Abstract:
An electromagnetic clutch includes: a rotary member; an output mechanism including an electromagnetic coil and an armature moved toward the electromagnetic coil by electromagnetic force; a cam mechanism operated by rotation of the rotary member in the electromagnetic coil energized state; and a coil housing having a coil accommodating portion open toward the armature and accommodating the electromagnetic coil. The coil housing and the armature have frictional engagement faces. The frictional engagement faces of the coil housing and the armature are frictionally-engageable. At least one of the coil housing and the armature has a contact pressure reducing portion for reducing contact pressures of the frictional engagement faces of the coil housing and the armature, generated based on cam thrust generated through an operation of the cam mechanism when the frictional engagement faces of the coil housing and the armature are frictionally-engaged.

Description:
INCORPORATION BY REFERENCE 
       [0001]    The disclosure of Japanese Patent Application No. 2011-226015 filed on Oct. 13, 2011 including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to an electromagnetic clutch that is used to control torque transmission between rotary members or braking of a rotary member. 
         [0004]    2. Description of Related Art 
         [0005]    There is a conventional electromagnetic clutch that includes an output mechanism and a cam mechanism (see, for example, Japanese Patent Application Publication No. 2004-17807 (JP 2004-17807 A)). The output mechanism generates electromagnetic force to output actuating force. The cam mechanism operates along an axis of the output mechanism through driving of an electric motor. 
         [0006]    The output mechanism includes an electromagnetic coil and an armature. The electromagnetic coil generates electromagnetic force. The armature is actuated upon energization of the electromagnetic coil. The output mechanism is arranged around an output shaft. 
         [0007]    The electromagnetic coil is accommodated in a coil housing, and is fixed to a vehicle body-side member. The coil housing is formed of a first housing element and a second housing element. The first housing element rotates together with the output shaft. The second housing element is open toward the first housing element. 
         [0008]    The armature is arranged at such a position as to face the electromagnetic coil via the coil housing. The armature is configured to be frictionally engaged with the coil housing when the output mechanism outputs actuating force. In addition, the armature is configured to be moved away from the coil housing by the spring force of a return spring when the output mechanism stops outputting actuating force. 
         [0009]    The cam mechanism includes the above-described armature, and includes a gear, serving as a cam member, and cam followers. The gear is rotated through driving of the electric motor. The cam followers are interposed between the gear and the armature. The cam mechanism is arranged along the axis of the output mechanism. 
         [0010]    The gear is rotatably arranged around the output shaft. In addition, the gear is coupled to an input shaft (a motor shaft of the electric motor) via a speed reducing gear row. 
         [0011]    The cam followers each are formed of a spherical member. The cam followers are rollably arranged between the gear (cam grooves) and the armature (cam grooves). 
         [0012]    With the above configuration, when the electromagnetic coil is energized while the electric motor is driven, the armature is moved toward the electromagnetic coil and frictionally engaged with the coil housing. Accordingly, the cam mechanism operates. Therefore, due to cam action carried out through the operation of the cam mechanism, the armature is frictionally engaged with the coil housing more firmly than before the cam mechanism is actuated. Thus, driving torque of the electric motor is transmitted to the output shaft (differential side) via, for example, the cam mechanism. 
         [0013]    On the other hand, when the electromagnetic coil is de-energized while the electric motor is stopped, frictional engagement between the armature and the coil housing is cancelled by the spring force of the return spring, so the cam mechanism does not operate. Therefore, transmission of driving torque from the electric motor to the differential side is interrupted. 
         [0014]    In addition, there is a conventional electromagnetic clutch (brake) that includes a fixed portion and a rotary portion (see, for example, Japanese patent Application Publication No. 2000-179583 (JP 2000-179583 A)). The fixed portion has a coil housing that is open toward an armature and that accommodates a coil. The rotary portion has a hub that is rotatable with respect to the fixed portion. 
         [0015]    With the electromagnetic clutch described in JP 2004-17807 A, during an operation of the cam mechanism, the armature receives cam thrust from the cam followers at its radially inner portion (the bottoms of the cam grooves) not at its radially outer portion, and is frictionally engaged with the first coil housing. Therefore, if the coil housing described in JP 2004-17807 A is open toward the armature as in the case of the coil housing described in JP 2000-179583 A, the armature elastically deforms in such a manner that the radially outer portion moves away from the coil housing with the radially inner portion in contact with the opening periphery (edge) of the coil housing. Due to stress concentration on the edge of the coil housing, a maximum contact pressure against the coil housing is increased. 
       SUMMARY OF THE INVENTION 
       [0016]    It is an object of the invention to provide an electromagnetic clutch that makes it possible to reduce a maximum contact pressure against a coil housing by relaxing stress that acts on the coil housing during an operation of a cam mechanism. 
         [0017]    An aspect of the invention relates to an electromagnetic clutch that includes: a rotary member; an output mechanism that is arranged along a rotation axis of the rotary member, and that includes an electromagnetic coil that generates electromagnetic force and an armature that is moved toward the electromagnetic coil by the electromagnetic force; a cam mechanism that is arranged next to the output mechanism along the rotation axis, and that is operated by rotation of the rotary member in an energized state of the electromagnetic coil; and a coil housing that is arranged along an axis of the cam mechanism, and that has an annular accommodating recess that is open toward the armature and that accommodates the electromagnetic coil. An open end face of the accommodating recess of the coil housing is a frictional engagement face of the coil housing. The armature has a frictional engagement face of the armature. The frictional engagement face of the coil housing is frictionally-engageable with the frictional engagement face of the armature. At least one of the coil housing and the armature has a contact pressure reducing portion that reduces a contact pressure of the frictional engagement face of the coil housing and a contact pressure of the frictional engagement face of the armature, which are generated based on cam thrust generated through an operation of the cam mechanism when the frictional engagement face of the coil housing and the frictional engagement face of the armature are frictionally-engaged with each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
           [0019]      FIG. 1  is a plan view that schematically shows a vehicle in which an electromagnetic clutch according to a first embodiment of the invention is mounted; 
           [0020]      FIG. 2A  is a sectional view that shows an actuated state of the electromagnetic clutch according to the first embodiment of the invention; 
           [0021]      FIG. 2B  is a sectional view that shows a non-actuated state of the electromagnetic clutch according to the first embodiment of the invention; 
           [0022]      FIG. 3  is a sectional view that shows a main portion of the electromagnetic clutch according to the first embodiment of the invention; 
           [0023]      FIG. 4  is a sectional view that shows a state where an armature is frictionally engaged with a coil housing in the electromagnetic clutch according to the first embodiment of the invention; 
           [0024]      FIG. 5A  is a sectional view that shows an actuated state of an electromagnetic clutch according to a second embodiment of the invention; 
           [0025]      FIG. 5B  is a sectional view that shows a non-actuated state of the electromagnetic clutch according to the second embodiment of the invention; 
           [0026]      FIG. 6  is a sectional view that shows a main portion of the electromagnetic clutch according to the second embodiment of the invention; and 
           [0027]      FIG. 7  is a sectional view that shows a state where an armature is frictionally engaged with a coil housing in the electromagnetic clutch according to the second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0028]    Hereinafter, an electromagnetic clutch according to a first embodiment of the invention will be described in detail with reference to the accompanying drawings. 
         [0029]      FIG. 1  shows a hybrid vehicle  100 . As shown in  FIG. 1 , the hybrid vehicle  100  includes an engine  101 , a first motor generator MG 1 , a power split mechanism  102 , an output gear  104 , and a second motor generator MG 2 . The engine  101  and the first motor generator MG 1  are coupled to the power split mechanism  102 . The output gear  104  outputs torque to drive wheels  103 . The second motor generator MG 2  is coupled to the output gear  104  via a speed reduction mechanism  105 . Torque of the output gear  104  is transmitted to the right and left drive wheels  103  via a differential mechanism  106 . 
         [0030]    The engine  101  is configured as a spark-ignition multi-cylinder internal combustion engine. Torque of the engine  101  is transmitted to the power split mechanism  102  via an input shaft  107 . A damper  108  is interposed between the input shaft  107  and the engine  101 , and fluctuations in the torque of the engine  101  are absorbed by the damper  108 . 
         [0031]    The first motor generator MG 1  includes a stator  109   a  and a rotor  109   b . The stator  109   a  is fixed to a casing  6  that serves as a fixed member. The rotor  109   b  is arranged radially inward of the stator  109   a  so as to be coaxial with the stator  109   a . Similarly, the second motor generator MG 2  includes a stator  110   a  and a rotor  110   b . The stator  110   a  is fixed to the casing  6 . The rotor  110   b  is arranged radially inward of the stator  110   a  so as to be coaxial with the stator  110   a . The casing  6  has a through-hole  6   a  of which the axis coincides with a rotation axis O (shown in  FIG. 2 ). 
         [0032]    The power split mechanism  102  is formed of a single-pinion planetary gear mechanism that includes three elements that are differentially rotatable with respect to each other. The power split mechanism  102  includes a sun gear S 1 , a ring gear R 1  and a carrier C 1 . The sun gear S 1  is an external gear. The ring gear R 1  is an internal gear arranged coaxially with the sun gear S 1 . The carrier C 1  holds pinion gears P 1  such that the pinion gears P 1  are able to rotate about their axes and turn around the sun gear S 1 . The pinion gears P 1  are in mesh with the sun gear S 1  and the ring gear R 1 . 
         [0033]    In the present embodiment, the input shaft  107  is coupled to the carrier C 1 , the first motor generator MG 1  is coupled to the sun gear S 1  via a rotary member  2 , and the output gear  104  is coupled to the ring gear R 1 . 
         [0034]    The rotary member  2  is coupled to the rotor  109   b  of the first motor generator MG 1 . The entirety of the rotary member  2  is formed of a hollow member, through which the input shaft  107  is passed. The details of the rotary member  2  will be described later. 
         [0035]    The speed reduction mechanism  105  has three elements that are differentially rotatable with respect to each other. The speed reduction mechanism  105  is formed of a single-pinion planetary gear mechanism that reduces the speed of rotation of the second motor generator MG 2  and that transmits the rotation with a reduced speed to the output gear  104 . The speed reduction mechanism  105  includes a sun gear S 2 , a ring gear R 2  and a carrier C 2 . The sun gear S 2  is an external gear. The ring gear R 2  is an internal gear. The ring gear R 2  is arranged coaxially with the sun gear S 2 . The carrier C 2  holds pinion gears P 2  such that the pinion gears P 2  are able to rotate about their axes and turn around the sun gear S 1 . The pinion gears P 2  are in mesh with the sun gear S 2  and the ring gear R 2 . 
         [0036]    In the present embodiment, the sun gear S 2  is coupled to the rotor  110   b  of the second motor generator MG 2 , and the ring gear R 2  is coupled to the output gear  104 . The carrier C 2  is fixed to the casing  6 . Thus, the speed of rotation of the second motor generator MG 2  is reduced, and the torque is amplified and then transmitted to the output gear  104 . 
         [0037]    An electromagnetic clutch  1  is mounted in the hybrid vehicle  100 . The electromagnetic clutch  1  functions as a brake device that applies a brake to the rotary member  2  with respect to the casing  6 . In this way, it is possible to selectively carry out a continuously variable shift mode and a stepped shift mode. In the continuously variable shift mode, a continuously variable shifting is executed electrically with the use of the first motor generator MG 1 . In the stepped shift mode, stepped shifting is executed without using the first motor generator MG 1 . 
         [0038]      FIG. 2A  and  FIG. 2B  respectively show an actuated state and non-actuated state of the electromagnetic clutch. As shown in  FIG. 2A  and  FIG. 2B , the electromagnetic clutch  1  is mainly formed of the rotary member  2 , an output mechanism  3 , a cam mechanism  4 , and a coil housing  5 . The rotary member  2  rotates together with the rotor  109   b  of the first motor generator MG 1  (both are shown in  FIG. 1 ). The output mechanism  3  is arranged along the rotation axis O of the rotary member  2 . The cam mechanism  4  is actuated by actuating force output from the output mechanism  3 , and converts rotational force from the rotary member  2  to cam thrust in a direction along the rotation axis  0 . The coil housing  5  is arranged along an axis (rotation axis O) of the cam mechanism  4 . 
         [0039]    The rotary member  2  is formed of a cylindrical hollow shaft. The rotary member  2  is coupled to the rotor  109   b  of the first motor generator MG 1  via a hollow shaft  109   c  (shown in  FIG. 1 ). In addition, the rotary member  2  is rotatably supported by the coil housing  5  via a bearing  7 . The rotary member  2  is configured to rotate together with the rotor  109   b  through driving of the first motor generator MG 1 . 
         [0040]    An annular support member  10  is arranged on the rotary member  2 . The support member  10  supports the bearing  7  and a return spring  8  at its respective end faces, at positions on the outer periphery of the rotary member  2 . In addition, a flange  11  is integrally formed with the rotary member  2 . The flange  11  protrudes radially outward and faces the coil housing  5  via the cam mechanism  4 . 
         [0041]    The bearing  7  is formed of a ball bearing, and is arranged between the outer periphery of the rotary member  2  and the inner periphery of the coil housing  5 . An inner ring of the bearing  7  is fixed to the rotary member  2  by a snap ring  12 , and an outer ring of the bearing  7  is fixed to the coil housing  5  by a snap ring  13 . 
         [0042]    The return spring  8  is, for example, formed of a coned disc spring. The return spring  8  is interposed between the support member  10  and an armature  31  (described later), and is arranged on the outer periphery of the rotary member  2 . The return spring  8  is configured to apply return force to the armature  31  in such a direction that the armature  31  moves away from an electromagnetic coil  30 . 
         [0043]    The flange  11  has cam grooves  11   a  that open toward the coil housing  5 , and is formed of an annular member as a whole. The flange  11  is configured to function as a fixed cam member in the cam mechanism  4 . Each cam groove  11   a  is formed of a recess of which the axial depth changes along the circumferential direction of the flange  11 . 
         [0044]    The output mechanism  3  includes the electromagnetic coil  30  and the armature  31 , and is arranged around the rotary member  2 . 
         [0045]    The electromagnetic coil  30  is arranged in the output mechanism  3  at a position close to the casing  6 . In addition, the electromagnetic coil  30  is accommodated in a coil accommodating portion  50  (described later) of the coil housing  5 . The electromagnetic coil  30  is configured to form a magnetic circuit M over the armature  31  and the coil housing  5  upon energization, and to generate electromagnetic force that is used as a pushing force P 1  with which the armature  31  is pushed against the coil housing  5 . The electromagnetic coil  30  is positioned with respect to the coil housing  5  by a snap ring  14 . 
         [0046]    The armature  31  has a straight spline fitting portion  31   a  on its inner peripheral portion. The armature  31  is arranged in the output mechanism  3  at a position close to the cam mechanism  4 . In addition, the armature  31  is coupled to a cam member  41  of the cam mechanism  4  through spline-fitting so as to be non-rotatable but movable relative to the cam member  41 . The entirety of the armature  31  is formed of an elastically deformable annular plate made of a magnetic material, such as iron. The armature  31  is configured to receive the electromagnetic force of the electromagnetic coil  30 , as an output from the output mechanism  3 , and move along the rotation axis  0  toward the coil housing  5 . In addition, the armature  31  is configured to be able to rotate around the rotation axis O upon receiving the rotational force of the rotary member  2 . 
         [0047]    A first frictional engagement face  31   b  is formed on a coil housing-side end face of the armature  31 . The first frictional engagement face  31   b  faces an open end face of the coil accommodating portion  50  of the coil housing  5 . A second frictional engagement face  31   c  is formed on a cam mechanism-side (cam member-side) end face of the armature  31 . The second frictional engagement face  31   c  faces a frictional engagement face  412   a  of the cam member  41 . 
         [0048]    The cam mechanism  4  includes the flange  11 , the movable cam member  41  and cam followers  42 . The flange  11  is non-rotatable with respect to the rotary member  2 . The cam member  41  faces the flange  11 . The cam followers  42  are interposed between the cam member  41  and the flange  11 . The cam mechanism  4  is arranged along the rotation axis O. The cam mechanism  4  is configured to operate through the rotation of the rotary member  2  in the energized state of the electromagnetic coil  30 . 
         [0049]    The cam member  41  includes a base portion  410 , a cam portion  411  and a pushing portion  412 . The cam member  41  is arranged around the rotary member  2  so as to be rotatable around and movable along the rotation axis O. The cam member  41  moves toward the coil housing  5  through cam action generated through the operation of the cam mechanism  4 . Thus, the frictional engagement face  412   a  of the pushing portion  412  is frictionally engaged with the second frictional engagement face  31  c of the armature  31  with a pushing force P 2 . 
         [0050]    The base portion  410  has a straight spline fitting portion  410   a  on its outer periphery. The base portion  410  is arranged at the radially inner side portion of the cam member  41 . The entirety of the base portion  410  is formed of a cylindrical member through which the rotary member  2  is passed. 
         [0051]    The cam portion  411  has cam grooves  411   a  that open toward the flange  11 . The cam portion  411  is located between the base portion  410  and the pushing portion  412 . The entirety of the cam portion  411  is formed of an annular member through which the rotary member  2  is passed. Each cam groove  411   a  is formed of a recess of which the axial depth changes along the circumferential direction of the cam member  41 . 
         [0052]    The pushing portion  412  has the frictional engagement face  412   a  that faces the second frictional engagement face  31   c  of the armature  31 . The pushing portion  412  is arranged at the radially outer side portion of the cam member  41 . The entirety of the pushing portion  412  is formed of an annular member. The annular member has an inner periphery that faces the outer periphery of the base portion  410 . 
         [0053]    The cam followers  42  each are formed of a spherical member. The cam followers  42  are rollably arranged between the cam grooves  11   a  (groove bottoms) of the flange  11  and the cam grooves  411   a  (groove bottoms) of the cam portion  411 . In addition, the cam followers  42  are retained by a retainer  15 . Ball retaining holes  15   a  are formed in the retainer  15 . The cam followers  42  are rollably retained in the ball retaining holes  15   a.    
         [0054]      FIG. 3  shows the armature  31  and the coil housing  5 . The coil housing  5  has the coil accommodating portion  50  and a contact pressure reducing portion  51 . The coil housing  5  is arranged along the rotation axis O. In addition, the coil housing  5  is fixed to the casing  6  with fastening bolts  16 . The entirety of the coil housing  5  is formed of a magnetic material. The coil housing  5  functions as a yoke, and is configured to form the magnetic circuit M together with the armature  31  upon energization of the electromagnetic coil  30 . 
         [0055]    The coil accommodating portion  50  has a frictional engagement face  50   a  at its open end face. The frictional engagement face  50   a  faces the first frictional engagement face  31   b  of the armature  31 . The entirety of the coil accommodating portion  50  is formed of an annular recess that serves as an accommodating recess that is open toward the armature  31 , as a whole. 
         [0056]    As shown in  FIG. 3 , the contact pressure reducing portion  51  is formed of an annular first recess  51   a  and an annular second recess  51   b . The first recess  51   a  is open at a radially inner-side inner periphery of the coil accommodating portion  50 , at a position close to an open end of the coil accommodating portion  50 . The second recess  51   b  faces the first recess  51   a , and is open at a radially outer-side inner periphery of the coil accommodating portion  50 , at a position close to the open end of the coil accommodating portion  50 . The contact pressure reducing portion  51  is formed in the coil housing  5 . The contact pressure reducing portion  51  is configured such that the frictional engagement face  50   a  receives pushing force (the pushing force P 1  based on electromagnetic force and the pushing force P 2  based on cam thrust) based on cam thrust generated through the operation of the cam mechanism  4  (shown in  FIG. 2 ) from the first frictional engagement face  31   b  of the armature  31  and thus the coil housing  5  is elastically deformed. In this way, during the operation of the cam mechanism  4 , the pushing force (P 1 +P 2 ) based on the cam thrust acts on the open end face (frictional engagement face  50   a ) of the coil accommodating portion  50  of the coil housing  5  via the armature  31 . Thus, the coil housing  5  elastically deforms, stress that acts on the coil housing  5  from the armature  31  is dispersed and relaxed. As a result, a maximum contact pressure of the first frictional engagement face  31   b  against the frictional engagement face  50   a  is reduced. 
         [0057]    The first recess  51   a  and the second recess  51   b  are arranged at positions that are at a predetermined distance t (for example, t=1 mm) from the open end face of the coil accommodating portion  50 . In addition, the dimensions of the first recess  51   a  and the second recess  51   b  are set such that the width w is, for example, 1 mm and the depth h is, for example, 1.3 mm. 
         [0058]    Next, the operation of the electromagnetic clutch according to the present embodiment will be described with reference to  FIG. 2A ,  FIG. 2B  and  FIG. 4 .  FIG. 4  shows a state where the armature is frictionally engaged with the coil housing. Note that, in  FIG. 4 , deformation amounts of portions are exaggerated for illustrative purposes. 
         [0059]    As shown in  FIG. 2B , when the first motor generator MG 1  (shown in  FIG. 1 ) is driven, the rotational driving force of the first motor generator MG 1  is transmitted to the rotary member  2 , and the rotary member  2  is rotated. 
         [0060]    Normally, at the time of starting the first motor generator MG 1 , the electromagnetic coil  30  of the output mechanism  3  is in the non-energized state. Therefore, the magnetic circuit M starting from the electromagnetic coil  30  is not formed. As a result, the armature  31  is not attracted to the coil housing  5 . 
         [0061]    Therefore, the pushing force P 1  that is used as clutch force is not generated in the output mechanism  3 , and the first frictional engagement face  31   b  of the armature  31  is not frictionally engaged with the frictional engagement face  50   a  of the coil housing  5 . As a result, braking force by the electromagnetic clutch  1  is not transmitted to the rotary member  2 . 
         [0062]    In this case, relative rotation between the flange  11  and the cam member  41  is restricted, and the cam mechanism  4  does not operate. 
         [0063]    On the other hand, as shown in  FIG. 2A , when the electromagnetic coil  30  is energized while the first motor generator MG 1  is driven (while the rotary member  2  is rotated), the magnetic circuit M starting from the electromagnetic coil  30  is formed, and the armature  31  moves from its initial position toward the coil housing  5 . 
         [0064]    Therefore, the first frictional engagement face  31   b  of the armature  31  is frictionally engaged with the frictional engagement face  50   a  of the coil housing  5  with the pushing force P 1 , and, accordingly, the cam mechanism  4  operates. 
         [0065]    When the cam mechanism  4  operates, the frictional engagement face  412   a  of the pushing portion  412  of the cam member  41  is frictionally engaged with the second frictional engagement face  31   c  of the armature  31  with the pushing force P 2  (P 1 &lt;P 2 ) that serves as cam thrust due to cam action generated through the operation of the cam mechanism  4 . In addition, the first frictional engagement face  31   b  of the armature  31  is frictionally engaged with the frictional engagement face  50   a  of the coil housing  5  with the pushing force (P 1 +P 2 ) more firmly than before the cam mechanism  4  is actuated. As a result, braking force by the electromagnetic clutch  1  is transmitted to the rotary member  2 . 
         [0066]    In this case, when the pushing force (P 1 +P 2 ) based on cam thrust generated through the operation of the cam mechanism  4  acts on the open end face (frictional engagement face  50   a ) of the coil accommodating portion  50  in the coil housing  5  via the armature  31 , the radially outer portion of the armature  31  is bent into the coil accommodating portion  50  at a curvature p 1  and is elastically deformed from the state indicated by a long dashed double-short dashed line in  FIG. 4  into the state indicated by a continuous line in  FIG. 4 . Then, the coil housing  5  elastically deforms such that an open peripheral edge of the coil accommodating portion  50  is crushed to close the first recess  51   a  and the second recess  51   b , and, while keeping these states, the first frictional engagement face  31   b  of the armature  31  is frictionally engaged with the frictional engagement face  50   a  of the coil housing  5 . In  FIG. 4 , the reference sign O 1  denotes the center of a circle of curvature having a radius of 1/ρ 1 . 
         [0067]    Therefore, in the present embodiment, during the operation of the cam mechanism  4 , it is possible to bring the armature  31  and the coil housing  5  into plane contact with each other with the coil housing  5  elastically deformed. In this way, it is possible to relax stress that acts from the armature  31  on the coil housing  5 . 
         [0068]    With the above-described first embodiment, the following advantageous effects are obtained. 
         [0069]    During the operation of the cam mechanism  4 , stress that acts on the coil housing  5  is relaxed. Thus, it possible to reduce the maximum contact pressure against the coil housing  5 . 
         [0070]    Next, an electromagnetic clutch  61  according to a second embodiment of the invention will be described with reference to  FIG. 5A ,  FIG. 5B ,  FIG. 6  and  FIG. 7 .  FIG. 5A  and  FIG. 5B  respectively show an actuated state and non-actuated state of the electromagnetic clutch  61 .  FIG. 6  shows the armature and the coil housing.  FIG. 7  shows a state where the armature is frictionally engaged with the coil housing. In  FIG. 5A ,  FIG. 5B ,  FIG. 6  and  FIG. 7 , the same reference numerals denote the same members as those in  FIG. 2A  to  FIG. 4 , and the detailed description thereof is omitted. Note that, in  FIG. 7 , deformation amounts of portions are exaggerated for illustrative purposes. 
         [0071]    As shown in  FIG. 5A  and  FIG. 5B , the electromagnetic clutch  61  according to the second embodiment of the invention has a distinctive feature that the armature  31  of the output mechanism  3  has a contact pressure reducing portion  62 . 
         [0072]    Therefore, the contact pressure reducing portion  62  is formed by forming an annular recess  31  d, which is open toward the coil housing  5 , in the armature  31 . 
         [0073]    As shown in  FIG. 6 , the open width W 1  of the recess  31   d  is set larger than the open width W 2  of the coil accommodating portion  50  (W 2 &lt;W 1 ). In this way, in the state where the armature  31  (first frictional engagement face  31   b ) is frictionally engaged with the coil housing  5  (frictional engagement face  50   a ), the armature  31  is arranged such that the bottom face of the recess  31   d  covers the entire open face of the coil accommodating portion  50 . In addition, the depth H of the recess  31   d  is set to, for example, 2 mm. 
         [0074]    The contact pressure reducing portion  62  is configured such that the first frictional engagement face  31   b  of the armature  31  receives reaction force of the pushing force (the pushing force P 1  based on electromagnetic force and the pushing force P 2  based on cam thrust) based on cam thrust generated through the operation of the cam mechanism  4  from the frictional engagement face  50   a  of the coil housing  5  and then the armature  31  is elastically deformed. Thus, during the operation of the cam mechanism  4 , when the reaction force of the pushing force (P 1 +P 2 ) based on the cam thrust is applied from the coil housing  5  onto the open end face (first frictional engagement face  31   b ) of the recess  31   d , the armature  31  elastically deforms. As a result, stress that acts on the coil housing  5  from the armature  31  is dispersed and relaxed. In this way, a maximum contact pressure of the first frictional engagement face  31   b  against the frictional engagement face  50   a  is reduced. 
         [0075]    As shown in  FIG. 5B , in the thus configured electromagnetic clutch  61 , when the first motor generator MG 1  (shown in  FIG. 1 ) is driven, the rotational driving force of the first motor generator MG 1  is transmitted to the rotary member  2 , and the rotary member  2  is rotated. 
         [0076]    Normally, at the time of starting the first motor generator MG 1 , the electromagnetic coil  30  of the output mechanism  3  is in the non-energized state. Thus, the magnetic circuit M starting from the electromagnetic coil  30  is not formed, and the armature  31  is not attracted to the coil housing  5 . 
         [0077]    Therefore, the pushing force P 1  that is used as clutch force is not generated in the output mechanism  3 , and the first frictional engagement face  31   b  of the armature  31  is not frictionally engaged with the frictional engagement face  50   a  of the coil housing  5 . Therefore, braking force by the electromagnetic clutch  1  is not transmitted to the rotary member  2 . 
         [0078]    In this case, relative rotation between the flange  11  and the cam member  41  is restricted, and the cam mechanism  4  does not operate. 
         [0079]    On the other hand, as shown in  FIG. 5A , when the electromagnetic coil  30  is energized while the first motor generator MG 1  is driven (while the rotary member  2  is rotated), the magnetic circuit M starting from the electromagnetic coil  30  is formed, and the armature  31  moves from its initial position toward the coil housing  5 . 
         [0080]    Therefore, the first frictional engagement face  31   b  of the armature  31  is frictionally engaged with the frictional engagement face  50   a  of the coil housing  5  with the pushing force P 1 , and, accordingly, the cam mechanism  4  operates. 
         [0081]    When the cam mechanism  4  operates, the frictional engagement face  412   a  of the pushing portion  412  of the cam member  41  is frictionally engaged with the second frictional engagement face  31   c  of the armature  31  with the pushing force P 2  (P 1  &lt;P 2 ) as cam thrust, due to cam action generated through the operation of the cam mechanism  4 . In addition, the first frictional engagement face  3   b  of the armature  31  is frictionally engaged with the frictional engagement face  50   a  of the coil housing  5  with the pushing force (P 1 +P 2 ) motor firmly than before the cam mechanism  4  is actuated. As a result, braking force by the electromagnetic clutch  1  is transmitted to the rotary member  2 . 
         [0082]    In this case, the reaction force of the pushing force (P 1 +P 2 ) based on the cam thrust generated through the operation of the cam mechanism  4  is applied from the coil housing  5  onto the open end face (first frictional engagement face  31   b ) of the recess  31   d  of the armature  31 . Then, the radially outer portion of the armature  31  is bent into the coil accommodating portion  50  at a curvature ρ 2  that is larger than the curvature ρ 1  (ρ 2 &gt;ρ 1 ) from the state indicated by long dashed double-short dashed line in  FIG. 7 . Then, the contact portion, which contacts the coil housing  5 , is displaced so as to be move away from the edge of the coil accommodating portion  50  and is elastically deformed into the state indicated by continuous line in  FIG. 7 , and, while keeping this state, the first frictional engagement face  31  b of the armature  31  is frictionally engaged with the frictional engagement face  50   a  of the coil housing  5 . In  FIG. 7 , the reference sign O 2  denotes the center of a circle of curvature having a radius of 1/ρ 2 . 
         [0083]    Therefore, in the present embodiment, during the operation of the cam mechanism  4 , it is possible to bring the armature  31  and the coil housing  5  into frictional engagement, with the armature  31  elastically deformed. Therefore, it is possible to relax stress that is applied from the armature  31  onto the coil housing  5 . 
         [0084]    According to the above-described second embodiment, similar advantageous effects to those of the first embodiment are obtained. 
         [0085]    The electromagnetic clutch according to the invention has been described on the basis of the above embodiment. However, the invention is not limited to the above embodiment. The invention may be implemented in various other embodiments without departing from the scope of the invention, and may be, for example, modified as follows. 
         [0086]    In the above-described embodiment, the description has been made on the case where the electromagnetic clutch functions as the brake device that applies a brake to the rotary member  2 . However, the invention is not limited to this configuration. The electromagnetic clutch may be configured to function as a driving force transmission device that transmits driving torque between a pair of rotary members. 
         [0087]    In the above-described first embodiment, the first recess  51   a  and the second recess  51   b  are formed in the coil accommodating portion  50  of the coil housing  5 . In the above-described second embodiment, the annular recess  31   d  that opens toward the coil housing  5  is formed in the armature  31 . However, the invention is not limited to the above-described embodiments. There may be employed a configuration in which the first recess  51   a  and the second recess  51   b  are formed in the coil accommodating portion  50  of the coil housing  5  and the annular recess  31   d  that opens toward the coil housing  5  is formed in the armature  31 . 
         [0088]    According to the invention, it is possible to reduce a maximum contact pressure against a coil housing by relaxing stress that acts on the coil housing during the operation of a cam mechanism.