Patent Publication Number: US-9835209-B2

Title: Control method and control device for dog clutch

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2016-004252 filed on Jan. 13, 2016 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control method for a dog clutch that transmits a torque by meshing and a control device for a dog clutch. 
     2. Description of Related Art 
     A dog clutch that can transmit a torque by meshing is used to lock (differentially lock) a differential action between a pair of side gears of a differential device of a vehicle, for example (e.g., see Japanese Patent Application Publication No. 2006-29579 (JP 2006-29579 A)). 
     The differential device described in JP 2006-29579 A includes an electromagnetic coil, a plunger that moves due to a magnetic force of the electromagnetic coil, and a clutch ring having meshing teeth configured to mesh with one side gear out of the pair of side gears, as a structure to lock the differential action between the pair of side gears. When a current is applied to the electromagnetic coil, the meshing teeth of the clutch ring mesh with the one side gear along with a movement of the plunger, so that a relative rotation between a differential case and the one side gear is restricted. Accordingly, a pair of pinion gears pivotally supported by a pinion shaft rotating integrally with the differential case cannot rotate, so that the differential action between the one side gear and the other side gear is restricted. 
     When a differential-lock operation switch operated by a driver enters an ON state, a control device for controlling the differential device supplies an exciting current to the electromagnetic coil. The control device detects by a signal of a position switch that the meshing teeth of the clutch ring mesh with the one side gear. The control device is configured such that, when the meshing is detected, the control device starts a timer, and when a timer value reaches a predetermined threshold, the control device decreases a current to be supplied to the electromagnetic coil from a current necessary to mesh the clutch ring to a relatively small current to such an extent that a meshing state is maintained. 
     SUMMARY OF THE INVENTION 
     In a control method described in JP 2006-29579 A, while a differential-lock state is maintained, power consumption is restrained and heat generation of the electromagnetic coil is also restrained in comparison with a case where a current necessary to continuously mesh the clutch ring is kept flowing. However, the heat generation of the electromagnetic coil increases according to a product of a current amount and a time, so some restrictions might occur in mountability to a vehicle in consideration of a heat dissipation property. Further, when a relatively large current flows for a long time, the electromagnetic coil may become unusable within an intermittent rating. In this case, it is necessary to use a larger-capacity electromagnetic coil. Thus, the method described in JP 2006-29579 A left room for improvement in those points. 
     In view of this, the present invention provides a control method and a control device for a dog clutch that is able to further reduce power consumption and to restrain heat generation of an electromagnetic coil. 
     A first aspect of the present invention relates to a control method for a dog clutch, the dog clutch including: a first rotational member; a second rotational member placed rotatable relative to the first rotational member around a common rotating axis; a clutch member restricted from rotating relative to the first rotational member, the clutch member having meshing teeth meshing with the second rotational member, the clutch member being movable between a connecting position where the meshing teeth mesh with the second rotational member and a non-connecting position where the meshing teeth do not mesh with the second rotational member; an electromagnetic coil configured to generate a magnetic force to move the clutch member from the non-connecting position to the connecting position; and a position detecting portion configured to detect a position of the clutch member. The control device includes: supplying an exciting current to the electromagnetic coil when the clutch member is moved from the non-connecting position toward the connecting position; and promptly reducing the exciting current when the position detecting portion detects that the clutch member has moved to the connecting position. 
     Further, a second aspect of the present invention relates to a control device for a dog clutch, the dog clutch including: a first rotational member; a second rotational member placed rotatable relative to the first rotational member around a common rotating axis; a clutch member restricted from rotating relative to the first rotational member, the clutch member having meshing teeth meshing with the second rotational member, the clutch member being movable between a connecting position where the meshing teeth mesh with the second rotational member and a non-connecting position where the meshing teeth do not mesh with the second rotational member; an electromagnetic coil configured to generate a magnetic force to move the clutch member from the non-connecting position toward the connecting position; and a position detecting portion configured to detect a position of the clutch member. The control device includes an ECU. When the clutch member is moved from the non-connecting position toward the connecting position, the ECU is configured to supply an exciting current to the electromagnetic coil, and when the position detecting portion detects that the clutch member has moved to the connecting position, the ECU is configured to promptly reduce the exciting current. 
     According to the control method and the control device for the dog clutch according to the above aspects, it is possible to reduce power consumption and to restrain heat generation of the electromagnetic coil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic view illustrating an exemplary configuration of a vehicle equipped with a differential gear in which a dog clutch is incorporated, according to an embodiment of the present invention; 
         FIG. 2  is a sectional view illustrating an exemplary configuration of the differential gear according to the embodiment of the present invention; 
         FIG. 3  is an exploded perspective view of the differential gear; 
         FIG. 4A  is a perspective view of a clutch member constituting a pressing mechanism of the dog clutch; 
         FIG. 4B  is a perspective view of the clutch member constituting the pressing mechanism of the dog clutch; 
         FIG. 5A  is a sectional view illustrating a part of the differential gear in an enlarged manner; 
         FIG. 5B  is a sectional view illustrating a part of the differential gear in an enlarged manner; 
         FIG. 6A  is an explanatory view schematically illustrating an operation of a cam mechanism; 
         FIG. 6B  is an explanatory view schematically illustrating the operation of the cam mechanism; 
         FIG. 6C  is an explanatory view schematically illustrating the operation of the cam mechanism; and 
         FIG. 7  is a time chart illustrating an example of change of a supply current to an electromagnetic coil, a position of a clutch member, and a detection signal of a position sensor when a lock mode selecting switch is switched on/off. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described with reference to  FIGS. 1 to 7 . Note that the embodiment described below shows a preferred concrete example on performing the present invention. There are some parts that specifically exemplify various technical matters that are technically preferable, but the technical scope of the present invention is not limited to such concrete examples. 
       FIG. 1  is a schematic view illustrating an exemplary configuration of a vehicle equipped with a differential gear in which a dog clutch is incorporated, according to an embodiment of the present invention.  FIG. 2  is a sectional view illustrating the exemplary configuration of the differential gear.  FIG. 3  is an exploded perspective view of the differential gear.  FIGS. 4A and 4B  are each a perspective view of a clutch member constituting a pressing mechanism of the dog clutch.  FIGS. 5A and 5B  are each a sectional view illustrating a part of the differential gear in an enlarged manner. 
     A vehicle  100  includes: an engine  101  as a driving force source; a transmission  102 ; a differential gear  1 ; a control device  110  for controlling the differential gear  1 ; right and left front wheels  103 ,  104  and right and left rear wheels  105 ,  106 ; and right and left drive shafts  107 ,  108 . The differential gear  1  receives a driving force of the engine  101  via a ring gear  109 , the driving force being changed in speed by the transmission  102 , and distributes the driving force to the right and left drive shafts  107 ,  108  as a pair of output shafts. The control device  110  includes an ECU  113 . The control device  110  supplies an exciting current to an electromagnetic coil  60  (illustrated in  FIG. 2 ) of the differential gear  1  via a cable  111 . Further, the control device  110  receives a detection signal from a position sensor (illustrated in  FIG. 2 ) of the differential gear  1  through a signal wire  112 . 
     Further, a lock mode selecting switch  120  operated by a driver of the vehicle  100  is connected to the control device  110 . When the lock mode selecting switch  120  enters an ON state, a relative rotation between the right and left drive shafts  107 ,  108  is restricted. At the time of traveling on a rough road such as a mud road, for example, the driver turns on the lock mode selecting switch  120 . Hereby, even if one of the right and left front wheels  103 ,  104  is stuck in the mud, it is possible to travel by a driving force transmitted to the other wheel. 
     The following describes details of a configuration and an operation of the differential gear  1  into which a dog clutch is incorporated, according to the embodiment of the present invention, and a control consent of the control device  110 . Note that, in the following description, a right side and a left side of  FIG. 2  may be just referred to as the “right side” and the “left side” for convenience, but the “right side” and the “left side” herein do not necessarily limit right and left in a vehicle width direction in a state where the differential gear  1  is provided in the vehicle  100 . 
     The differential gear  1  includes: a differential case  2  to which the ring gear  109  (illustrated in  FIG. 1 ) is fixed; a first side gear  31  and a second side gear  32  stored in the differential case  2 ; a plurality of (five in the present embodiment) of pinion gear sets  40  each configured such that a first pinion gear  41  and a second pinion gear  42  are meshed with each other; a clutch member  5  that can transmit a torque between the differential case  2  and the first side gear  31 ; a position sensor  9  serving as a position detecting portion for detecting a position of the clutch member  5 ; and a pressing mechanism  10  for giving a pushing pressure to the clutch member  5 . The differential case  2 , the first side gear  31 , the clutch member  5 , the position sensor  9 , and the pressing mechanism  10  constitute a dog clutch  11 . 
     The first side gear  31  is placed on the right side and the second side gear  32  is placed on the left side. The first side gear  31  and the second side gear  32  have a cylindrical shape. A spline portion  310  to which the drive shaft  108  is connected in a relatively non-rotatable manner is formed on an inner peripheral surface of the first side gear  31 , and a spline portion  320  to which the drive shaft  107  is connected in a relatively non-rotatable manner is formed on an inner peripheral surface of the second side gear  32 . 
     The differential case  2 , the first side gear  31 , and the second side gear  32  are placed rotatable relative to each other around a common rotation axis O. Hereinafter, a direction parallel to the rotation axis O is referred to as an axial direction. 
     A plurality of holding holes  20  for rotatably holding the first pinion gear  41  and the second pinion gear  42  of the pinion gear sets  40  is formed in the differential case  2 . The first pinion gear  41  and the second pinion gear  42  revolve around the rotation axis O and are also rotatable in their corresponding holding holes  20  with their respective central axes being taken as their rotating axes. 
     The first side gear  31  and the second side gear  32  have a common outer diameter, and gear wheel portions  311 ,  321  constituted by a plurality of helical teeth are formed on respective outer peripheral surfaces thereof. A center washer  81  is placed between the first side gear  31  and the second side gear  32 . Further, a side washer  82  is placed on the right side of the first side gear  31 , and a side washer  83  is placed on the left side of the second side gear  32 . 
     The first pinion gear  41  integrally includes a long gear wheel portion  411 , a short gear wheel portion  412 , and a connection portion  413  for connecting the long gear wheel portion  411  to the short gear wheel portion  412  in the axial direction. Similarly, the second pinion gear  42  integrally includes a long gear wheel portion  421 , a short gear wheel portion  422 , and a connection portion  423  for connecting the long gear wheel portion  421  to the short gear wheel portion  422  in the axial direction. 
     The first pinion gear  41  is configured such that: the long gear wheel portion  411  meshes with the gear wheel portion  311  of the first side gear  31  and the short gear wheel portion  422  of the second pinion gear  42 ; and the short gear wheel portion  412  meshes with the long gear wheel portion  421  of the second pinion gear  42 . The second pinion gear  42  is configured such that: the long gear wheel portion  421  meshes with the gear wheel portion  321  of the second side gear  32  and the short gear wheel portion  412  of the first pinion gear  41 ; and the short gear wheel portion  422  meshes with the long gear wheel portion  411  of the first pinion gear  41 . Note that  FIG. 3  does not illustrate the helical teeth of these gear wheel portions. 
     When the first side gear  31  and the second side gear  32  rotate at the same speed, the first pinion gear  41  and the second pinion gear  42  revolve together with the differential case  2  without rotating in respective holding holes  20 . Further, when the first side gear  31  and the second side gear  32  have different rotation speeds at the time of turning of the vehicle  100  and the like, for example, the first pinion gear  41  and the second pinion gear  42  revolve while rotating in respective holding holes  20 . Hereby, a driving force input into the differential case  2  is distributed while a differential action between the first side gear  31  and the second side gear  32  is allowed. Note that the differential case  2  is an example of a “first rotational member” of the present invention, and the first side gear  31  is an example of a “second rotational member” of the present invention. 
     The clutch member  5  is movable in the axial direction between a connecting position where the differential case  2  is connected to the first side gear  31  in a relatively non-rotatable manner and a non-connecting position where the differential case  2  and the first side gear  31  are allowed to rotate relative to each other.  FIG. 5A  illustrates a state where the clutch member  5  is placed at the non-connecting position, and  FIG. 5B  illustrates a state where the clutch member  5  is placed at the connecting position. 
     When the clutch member  5  is placed at the connecting position, a differential action between the differential case  2  and the first side gear  31  is restricted, so that the first pinion gear  41  and the second pinion gear  42  cannot rotate and the differential action between the differential case  2  and the second side gear  32  is also restricted. The clutch member  5  is biased toward the non-connecting position by a return spring  84  placed between the clutch member  5  and the first side gear  31 . 
     The pressing mechanism  10  includes an electromagnet  6  for generating an electromagnetic force, and a plunger  7  moved in the axial direction by a magnetic force of the electromagnet  6  so as to press and move the clutch member  5  in the axial direction. The electromagnet  6  includes a cylindrical electromagnetic coil  60 , and a yoke  61  serving as a magnetic path of a magnetic flux generated by current application to the electromagnetic coil  60 . The electromagnetic coil  60  generates a magnetic force to move the clutch member  5  in the axial direction from the non-connecting position toward the connecting position. 
     The yoke  61  includes: an inner ring portion  611  opposed to an inner peripheral surface of the electromagnetic coil  60 ; an outer ring portion  612  opposed to an outer peripheral surface of the electromagnetic coil  60 ; and first and second axial end portions  613 ,  614  opposed to respective axial end surfaces of the electromagnetic coil  60 . The first axial end portion  613  is opposed to a left end portion of the electromagnetic coil  60 , and the second axial end portion  614  is opposed to a right end portion of the electromagnetic coil  60 . In the present embodiment, the yoke  61  is constituted by an inner member  62  having the inner ring portion  611  and the first axial end portion  613 , and an outer member  63  having the outer ring portion  612  and the second axial end portion  614 . The inner member  62  and the outer member  63  are integrated by welding. 
     A discontinuous portion  611   a  where the magnetic path of the magnetic flux of the electromagnetic coil  60  becomes discontinuous is formed in the inner ring portion  611  of the yoke  61  along a circumferential direction. In the present embodiment, an axial length of the inner ring portion  611  of the yoke  61  is shorter than an axial length of the outer ring portion  612 , and a gap formed between an axial end portion of the inner ring portion  611  and the second axial end portion  614  serves as the discontinuous portion  611   a.    
     Further, a cut  611   b  formed in a radial direction is formed on an inner peripheral surface of the inner ring portion  611  on a first axial-end-portion- 613  side relative to the discontinuous portion  611   a . Outer peripheral ends of a plurality of (three in the present embodiment) flabellate fixing plates  85  made of a nonmagnetic material are fitted to the cut  611   b . In  FIG. 3 , two fixing plates  85  among them are illustrated. The fixing plate  85  is fixed to the differential case  2  by a pin  86 . An axial position of the yoke  61  relative to the differential case  2  is fixed such that the fixing plates  85  are fitted to the cut  611   b.    
     The plunger  7  includes an annular magnetic material core  70  made of a soft magnetic material, and a pressing member  71  made of a nonmagnetic material and configured to move in the axial direction integrally with the magnetic material core  70  so as to press the clutch member  5 . The magnetic material core  70  is axially opposed to at least one end portion out of both end portions of the yoke  61  across the discontinuous portion  611   a . In the present embodiment, part of an outer peripheral side of the magnetic material core  70  is opposed to an end portion of the inner ring portion  611  of the yoke  61  on a second-axial-end-portion- 614  side. 
     More specifically, an inclined surface  70   a  inclined relative to the axial direction is formed in a part of the outer peripheral side of the left end portion of the magnetic material core  70 , and an inclined surface  611   c  inclined relative to the axial direction so as to be parallel to the inclined surface  70   a  of the magnetic material core  70  is formed in an axial end portion of the inner ring portion  611  of the yoke  61  on a discontinuous-portion- 611   a  side. The inclined surface  70   a  of the magnetic material core  70  is axially opposed to the inclined surface  611   c  of the inner ring portion  611  of the yoke  61 . Further, the outer peripheral surface  70   b  of the magnetic material core  70  is opposed to an end portion of the second axial end portion  614  of the yoke  61  on the inner peripheral side. 
     The pressing member  71  includes: an annular plate portion  711  opposed to an axial end surface of the magnetic material core  70 ; a cylindrical plate portion  712  opposed to an inner peripheral surface of the magnetic material core  70 ; and a plurality of (three in the present embodiment) extending portions  713  axially extended from the cylindrical plate portion  712  and abutting with an axial end surface (a distal surface  53   b  of the after-mentioned engageable portion  53 ) of the clutch member  5  so as to press the clutch member  5 . 
     The differential case  2  includes: a first case member  21  and a second case member  22  fixed to each other by a plurality of screws  200 ; and a plurality of (three in the present invention) pillar-shaped guide members  23  fixed to the first case member  21  so as to axially guide the plunger  7 . The plunger  7  is movable in the axial direction relative to the differential case  2  by being guided by the guide members  23 . 
     The guide member  23  is a nonmagnetic material made of austenitic stainless steel or aluminum, for example, and integrally includes a columnar shaft portion  231  and a falling-off prevention portion  232  provided in one end of the shaft portion  231 , as illustrated in  FIGS. 5A and 5B . Insertion holes  7   a  through which the shaft portions  231  of the guide members  23  are passed are formed at a plurality of (three in the present embodiment) places in the plunger  7 . The insertion holes  7   a  extend in the axial direction so as to axially penetrate through the magnetic material core  70  and the pressing member  71 . 
     The shaft portion  231  of the guide member  23  has an outside diameter slightly smaller than an inside diameter of the insertion hole  7   a  of the plunger  7 , and a longitudinal direction along its central axis is parallel to the rotation axis O. The falling-off prevention portion  232  has a discoid shape having an outside diameter larger than the inside diameter of the insertion hole  7   a  of the plunger  7 , and abuts with an end of the plunger  7  on an opposite side to the clutch member  5  so as to prevent the plunger  7  from falling off. 
     As illustrated in  FIG. 2 , the position sensor  9  includes: a body portion  90  supported by a bracket (not shown) fixed to a vehicle body; and a movable portion  91  movable relative to the body portion  90  in a reciprocating manner in parallel to the rotation axis O. The movable portion  91  includes a roller  911  making contact with the annular plate portion  711  of the pressing member  71 , and a support portion  912  pivotally supporting the roller  911 . The roller  911  rotates along with a rotation of the plunger  7 . Further, the movable portion  91  moves in the axial direction together with the plunger  7  and the clutch member  5  while maintaining the contact between the roller  911  and the pressing member  71 . 
     A detection signal of the position sensor  9  is sent to the control device  110  through the signal wire  112 . In the present embodiment, when the clutch member  5  is placed at the non-connecting position, the detection signal of the position sensor  9  is in an OFF state, and when the clutch member  5  moves from the non-connecting position to the connecting position, the detection signal of the position sensor  9  is switched from OFF to ON. The control device  110  detects, by the position sensor  9 , that the clutch member  5  has moved to the connecting position. Note that the position sensor  9  may be a linear scale that can continuously measure an axial position of the clutch member  5 . In this case, when a moving amount from an initial position (a position at the time when no current is applied to the electromagnetic coil  60 ) of the clutch member  5 , the moving amount being measured by the position sensor  9 , exceeds a predetermined value, the control device  110  detects that the clutch member  5  has moved to the connecting position. 
     The first case member  21  integrally includes: a cylindrical portion  211  having a cylindrical shape and holding the plurality of pinion gear sets  40  rotatably; a bottom portion  212  extending inwardly from one end of the cylindrical portion  211 ; and a flange portion  213  butted against the second case member  22 . An annular recess  210  to which the electromagnet  6  is mounted is formed at a corner between the cylindrical portion  211  and the bottom portion  212 . 
     The first side gear  31  and the second side gear  32  are placed inside the cylindrical portion  211 . Further, the first case member  21  is made of metal having a magnetic permeability lower than the yoke  61 , and the ring gear  109  (illustrated in  FIG. 1 ) is fixed to the flange portion  213 . The differential case  2  rotates around the rotation axis O by a driving force transmitted from the ring gear  109 . The ring gear  109  is mounted to the differential case  2  from a bottom-portion- 212  side of the first case member  21 . At this time, the electromagnet  6  is stored in the annular recess  210 . Since an outside diameter of the electromagnet  6  is equal to an outside diameter of the cylindrical portion  211  of the first case member  21 , the ring gear  109  can be mounted with the electromagnet  6  being fixed. 
     As illustrated in  FIG. 3 , in the bottom portion  212  of the first case member  21 , a plurality of press-fitting holes  212   a  into which one ends of the shaft portions  231  of the guide members  23  are press-fitted, and a plurality of insertion holes  212   b  through which the extending portions  713  of the pressing members  71  are passed are formed. The insertion hole  212   b  axially penetrates through the bottom portion  212 . In the present embodiment, three press-fitting holes  212   a  and three insertion holes  212   b  are formed at regular intervals in a circumferential direction of the bottom portion  212 .  FIG. 3  illustrates two press-fitting holes  212   a  and one insertion hole  212   b  among them. 
     When a current is applied to the electromagnetic coil  60 , a magnetic flux is generated in a magnetic path G indicated by a broken line in  FIG. 5B , and the plunger  7  is drawn to the inner ring portion  611  so that the inclined surface  70   a  of the magnetic material core  70  approaches the inclined surface  611   c  of the inner ring portion  611  of the yoke  61 . Hereby, the magnetic material core  70  receives a magnetic force so that a tip end of the extending portion  713  of the pressing member  71  abuts with an axial end surface of the clutch member  5  so as to press the clutch member  5 . 
     The clutch member  5  is placed inside the yoke  61  because an outermost diameter (a diameter of an outermost part) of the clutch member  5  is smaller than an inside diameter (a minimum diameter of the inner ring portion  611 ) of the yoke  61 . Further, as illustrated in  FIGS. 4A and 4B , the clutch member  5  integrally includes: a circular plate portion  51  having an annular disk shape and including a plurality of bowl-shaped recessed portions  510  formed on one axial end surface  51   a ; a meshing portion  52  formed on the other axial end surface  51   b  of the circular plate portion  51 , the other axial end surface  51   b  being axially opposed to the first side gear  31 ; and engageable portions  53  having a trapezoidal pillar shape and formed so as to axially project from the one axial end surface  51   a  of the circular plate portion  51 . 
     The circular plate portion  51  is placed on a radially inner side of the annular recess  210  where the electromagnet  6  is mounted. The one axial end surface  51   a  of the circular plate portion  51  is axially opposed to the bottom portion  212  of the first case member  21 . The engageable portion  53  is partially inserted into the insertion hole  212   b  formed in the bottom portion  212  of the first case member  21 . A plurality of meshing teeth  521  projecting in the axial direction is formed in the meshing portion  52 . The plurality of meshing teeth  521  is formed in a part, on the outer peripheral side, of the other axial end surface  51   b  of the circular plate portion  51 , and the axial end surface  51   b  provided on an inner side relative to the meshing portion  52  is formed as a flat receiver surface that abuts with the return spring  84  so as to receive a biasing force toward the non-connecting position. 
     The first side gear  31  is configured such that a plurality of meshing teeth  313  meshing with the plurality of meshing teeth  521  of the clutch member  5  is formed in an annular wall portion  312  provided in a projecting manner on the outer peripheral side relative to the gear wheel portion  311 . 
     When the clutch member  5  is pressed by the plunger  7  and moved in the axial direction, the plurality of meshing teeth  521  of the meshing portion  52  meshes with the plurality of meshing teeth  313  of the first side gear  31 . That is, when the clutch member  5  moves toward the first side gear  31 , the clutch member  5  and the first side gear  31  are connected to each other in a relatively non-rotatable manner by meshing between the plurality of meshing teeth  521 ,  313 . 
     In the first case member  21 , an engaged portion to which the engageable portion  53  of the clutch member  5  circumferentially engages is constituted by the insertion hole  212   b . The engageable portion  53  of the clutch member  5  includes an abutting surface  53   a  that abuts with an inner surface  212   c  (see  FIG. 3 ) of the insertion hole  212   b  so as to receive a torque from the first case member  21 . 
     Further, the distal surface  53   b  of the engageable portion  53  is formed as a pressed surface with which the tip end of the extending portion  713  of the pressing member  71  abuts. When a current is applied to the electromagnetic coil  60 , the plunger  7  presses the clutch member  5  toward an annular-wall-portion- 312  side of the first side gear  31  such that the extending portion  713  of the pressing member  71  abuts with the distal surface  53   b  of the engageable portion  53 . 
     An inner surface  510   a  of the bowl-shaped recessed portion  510  is formed as a cam surface to generate an axial cam thrust by a relative rotation with respect to the first case member  21 . In other words, in the clutch member  5 , a part of an opposed surface (one axial end surface  51   a ) of the circular plate portion  51  to the bottom portion  212  of the first case member  21  is formed as a cam surface. 
     As illustrated in  FIG. 2 , a projection  212   d  that abuts with the inner surface  510   a  of the bowl-shaped recessed portion  510  is provided in the bottom portion  212  of the first case member  21  so as to project in the axial direction. In the present embodiment, the projection  212   d  is constituted by a sphere  24  fixed to the bottom portion  212 . The sphere  24  is partially stored in an axial recess  212   e  provided in the bottom portion  212 , so as to be held by the first case member  21 . Note that the projection  212   d  may be formed integrally as a part of the bottom portion  212 . Even in this case, it is desirable that a tip end of the projection  212   d  be spherical. 
     The insertion hole  212   b  of the bottom portion  212  has a circumferential width wider than a circumferential width of the engageable portion  53  of the clutch member  5 , and a relative rotation between the differential case  2  and the clutch member  5  is restricted within a predetermined angle range corresponding to a difference between the circumferential width of the insertion hole  212   b  and the circumferential width of the engageable portion  53 . The inner surface  510   a  of the bowl-shaped recessed portion  510  is formed in the clutch member  5  over an angle range larger than this predetermined angle range. Hereby, even if the clutch member  5  rotates relative to the differential case  2 , the tip end of the projection  212   d  (the sphere  24 ) is always stored in the bowl-shaped recessed portion  510  so as to be axially opposed to the inner surface  510   a.    
     The projection  212   d  of the bottom portion  212  of the first case member  21  and the bowl-shaped recessed portion  510  of the circular plate portion  51  of the clutch member  5  constitute a cam mechanism  12  for generating an axial thrust to separate the clutch member  5  from the bottom portion  212 . Referring now to  FIG. 6A to 6C , an operation of the cam mechanism  12  will be described below. 
       FIGS. 6A to 6C  are explanatory views schematically illustrating the operation of the cam mechanism  12  in the dog clutch  11  with a circumferential section of the clutch member  5 , the bottom portion  212  of the first case member  21 , and the annular wall portion  312  of the first side gear  31 . In  FIGS. 6A and 6B , a rotation direction of the first side gear  31  relative to the differential case  2  (the first case member  21 ) is indicated by an arrow A. 
     As illustrated in  FIG. 6A , the inner surface  510   a  of the bowl-shaped recessed portion  510  is constituted by a first inclined surface  510   b  inclined toward one side in a circumferential direction of the clutch member  5 , and a second inclined surface  510   c  inclined toward the other side in the circumferential direction of the clutch member  5 . An inclination angle of the first inclined surface  510   b  to the circumferential direction of the clutch member  5  is the same as an inclination angle of the second inclined surface  510   c.    
     The meshing tooth  521  of the clutch member  5  and the meshing tooth  313  of the first side gear  31  both have a trapezoidal section. A plurality of recessed portions  313   a  fitted to the meshing teeth  521  of the clutch member  5  is formed each between the meshing teeth  313  adjacent to each other in the circumferential direction. A tooth flank  521   a  of the meshing tooth  521  of the clutch member  5  and a tooth flank  313   b  of the meshing tooth  313  of the first side gear  31  are diagonally inclined relative to the circumferential direction (the rotation direction) of the clutch member  5  and the first side gear  31 . Hereby, due to a torque transmitted between the differential case  2  and the first side gear  31 , a meshing reaction force for pressing the clutch member  5  toward the non-connecting position side is generated. 
     When an inclination angle (a cam angle) of the first inclined surface  510   b  and the second inclined surface  510   c  in the bowl-shaped recessed portion  510  of the clutch member  5  is assumed α, an inclination angle of the tooth flank  521   a  of the meshing tooth  521  relative to the circumferential direction of the clutch member  5  is assumed β, and an inclination angle of the tooth flank  313   b  of the meshing tooth  313  relative to the circumferential direction of the first side gear  31  is assumed γ, β=γ is established, and α is smaller than β and γ. Hereby, when the cam mechanism  12  is operated and the meshing teeth  521  of the clutch member  5  mesh with the meshing teeth  313  of the first side gear  31 , a cam thrust of the cam mechanism  12  becomes larger than a meshing reaction force of the meshing teeth  521 ,  313 , so that the clutch member  5  is not pushed back toward the bottom portion  212  of the first case member  21  by the meshing reaction force. 
     When no current is applied to the electromagnetic coil  60 , the clutch member  5  is pressed against the bottom portion  212  of the first case member  21  by a biasing force of the return spring  84 . This state is illustrated in  FIG. 6A . As illustrated in  FIG. 6A , the projection  212   d  of the bottom portion  212  abuts with a backmost part of the bowl-shaped recessed portion  510 , and the meshing teeth  521  of the clutch member  5  do not mesh with the meshing teeth  313  of the first side gear  31 . In this state, the differential case  2  is rotatable relative to the first side gear  31 , so a torque input into the differential case  2  is distributed while a differential action between the first side gear  31  and the second side gear  32  is allowed. Further, the detection signal of the position sensor  9  is in the OFF state. 
     When the lock mode selecting switch  120  is switched by a driver from the OFF state to the ON state, the control device  110  supplies, to the electromagnetic coil  60 , an exciting current to move the clutch member  5  from the non-connecting position to the connecting position. When the exciting current is supplied to the electromagnetic coil  60 , the pressing member  71  of the plunger  7  presses the clutch member  5  due to its magnetic force, and after that, the cam mechanism  12  operates so that the clutch member  5  meshes with the first side gear  31 .  FIG. 6B  illustrates a state at the time when the meshing starts, and  FIG. 6C  illustrates a state where the meshing is completed. 
     As illustrated in  FIG. 6B , when a current is applied to the electromagnetic coil  60  and the clutch member  5  is pressed by the pressing member  71  of the plunger  7 , respective tip ends of the meshing teeth  521  of the clutch member  5  and the meshing teeth  313  of the first side gear  31  mesh with each other. At this time, the detection signal of the position sensor  9  is switched from the OFF state to the ON state. Note that, in a case where a relative rotation speed between the differential case  2  and the first side gear  31  is fast, even if the clutch member  5  is pressed by the plunger  7 , the meshing teeth  521  of the clutch member  5  may not mesh with the meshing teeth  313  of the first side gear  31  soon. A mounted position of the position sensor  9  is adjusted so that the detection signal of the position sensor  9  is not turned ON in such a state. 
     Due to the meshing between the meshing teeth  521  of the clutch member  5  and the meshing teeth  313  of the first side gear  31 , the clutch member  5  rotates following the first side gear  31  so as to rotate relative to the differential case  2 , so that the projection  212   d  of the bottom portion  212  slides on the first inclined surface  510   b  or the second inclined surface  510   c  of the bowl-shaped recessed portion  510 .  FIG. 6B  illustrates a case where the projection  212   d  of the bottom portion  212  slides on the first inclined surface  510   b  of the bowl-shaped recessed portion  510 . Due to this sliding, a part with which the projection  212   d  of the bottom portion  212  abuts gradually moves to a shallow part of the bowl-shaped recessed portion  510 , so that the clutch member  5  moves toward the first side gear  31  by a cam thrust. Hereby, a depth of the meshing between the meshing teeth  521  of the clutch member  5  and the meshing teeth  313  of the first side gear  31  (a distance of an axial overlap between the meshing teeth  521 ,  313 ) d 1  is gradually deepened. 
     A relative rotation of the clutch member  5  to the differential case  2  is restricted such that the abutting surface  53   a  of the engageable portion  53  of the clutch member  5  makes contact with the inner surface  212   c  of the insertion hole  212   b  in the first case member  21 . That is, as illustrated in  FIG. 6C , when the abutting surface  53   a  of the engageable portion  53  of the clutch member  5  abuts with the inner surface  212   c  of the insertion hole  212   b , the relative rotation of the clutch member  5  to the differential case  2  stops, so that the axial movement of the clutch member  5  to the differential case  2  also stops. 
     At this time, a gap S 1  with an axial dimension of d 2  is formed between a bottom face  313   c  of the recessed portion  313   a  between the meshing teeth  313  of the first side gear  31  and a distal surface  521   b  of the meshing tooth  521  of the clutch member  5 , as illustrated in  FIG. 6C . That is, even if the clutch member  5  rotates relative to the differential case  2 , the meshing teeth  521  of the clutch member  5  are not butted against the annular wall portion  312  of the first side gear  31 , so that the clutch member  5  does not directly press the first side gear  31  in the axial direction due to the cam thrust of the cam mechanism  12 . Further, a gap S 2  is also formed between a distal surface  313   d  of the meshing tooth  313  of the first side gear  31  and the circular plate portion  51  of the clutch member  5 . 
     In a state where the meshing between the meshing teeth  521  of the clutch member  5  and the meshing teeth  313  of the first side gear  31  is completed, the engageable portion  53  of the clutch member  5  engages with the insertion hole  212   b  of the first case member  21  so that the relative rotation between the differential case  2  and the clutch member  5  is restricted, and due to the meshing between the meshing teeth  521  of the clutch member  5  and the meshing teeth  313  of the first side gear  31 , a relative rotation between the clutch member  5  and the first side gear  31  is restricted. Hereby, a relative rotation between the differential case  2  and the first side gear  31  is restricted, and a torque is transmitted from the differential case  2  to the first side gear  31  via the clutch member  5 . 
     As such, when the clutch member  5  moves in a direction to mesh with the first side gear  31 , the depth of the meshing with the first side gear  31  is deepened by a cam thrust and then the engageable portion  53  of the clutch member  5  engages with the insertion hole  212   b  of the first case member  21 , so that the clutch member  5  receives a torque from the differential case  2 . 
     Further, a differential action between the differential case  2  and the first side gear  31  is restricted, so that the first pinion gear  41  and the second pinion gear  42  cannot rotate and a differential action between the differential case  2  and the second side gear  32  is also restricted, thereby resulting in that a torque is transmitted to the second side gear  32  from the differential case  2  via the first pinion gear  41  and the second pinion gear  42 . 
     As illustrated in  FIG. 6C , when a cam thrust of the cam mechanism  12  is assumed Fc, a pressing force of the plunger  7  due to current application to the electromagnetic coil  60  is assumed Fp, a meshing reaction force between the meshing teeth  521  of the clutch member  5  and the meshing teeth  313  of the first side gear  31  is assumed Fd, and a biasing force of the return spring  84  is assumed Fr, if Fp&gt;Fr is established, it is possible to shift from the state illustrated in  FIG. 6A  to the state illustrated in  FIG. 6B . After that, the meshing between the meshing teeth  521  of the clutch member  5  and the meshing teeth  313  of the first side gear  31  is completed due to the cam thrust Fc of the cam mechanism  12 . 
     When the meshing teeth  521  of the clutch member  5  mesh with the meshing teeth  313  of the first side gear  31 , the meshing reaction force Fd is generated, but since the relationship of α&lt;β, γ is established as has been described above, the cam thrust Fc is larger than the meshing reaction force Fd. A condition to maintain the meshing between the meshing teeth  521  of the clutch member  5  and the meshing teeth  313  of the first side gear  31  is Fd+Fr&lt;Fc+Fp. 
     Further, when the current application to the electromagnetic coil  60  is stopped, the clutch member  5  returns to the non-connecting position illustrated in  FIG. 6A  by the meshing reaction force Fd and the biasing force Fr of the return spring  84 . A condition for this is Fd+Fr&gt;Fc. That is, the inclination angle α of the first inclined surface  510   b  and the second inclined surface  510   c  in the bowl-shaped recessed portion  510  of the clutch member  5 , the inclination angle β of the tooth flank  521   a  of the meshing tooth  521  of the clutch member  5 , the inclination angle γ of the tooth flank  313   b  of the meshing tooth  313  of the first side gear  31 , a magnetic force of the electromagnet  6 , and a spring constant of the return spring  84  are set so as to satisfy an inequality of Fd+Fr&lt;Fc+Fp and an inequality of Fd+Fr&gt;Fc. 
       FIG. 7  is a time chart illustrating an example of change of a supply current to the electromagnetic coil  60 , a position of the clutch member  5 , and a detection signal of the position sensor  9  when the lock mode selecting switch  120  is switched on/off. 
     In the time chart of  FIG. 7 , an initial position P 0  indicates a position of the clutch member  5  at the time when the annular plate portion  711  of the pressing member  71  is butted against the falling-off prevention portion  232  of the guide member  23 , and a first position P 1  indicates a position where the meshing teeth  521  of the clutch member  5  starts to mesh with the meshing teeth  313  of the first side gear  31 . Further, a second position P 2  indicates a position of the clutch member  5  at the time when the abutting surface  53   a  of the engageable portion  53  of the clutch member  5  abuts with the inner surface  212   c  of the insertion hole  212   b , namely, a position where the clutch member  5  moves closest to the first side gear  31 . 
     The meshing teeth  521  of the clutch member  5  mesh with the meshing teeth  313  of the first side gear  31  between the first position P 1  and the second position P 2 . A part between the initial position P 0  and the first position P 1  is the non-connecting position where the clutch member  5  does not mesh with the first side gear  31 , and a part between the first position P 1  and the second position P 2  is the connecting position where the clutch member  5  mesh with the first side gear  31 . 
     When the lock mode selecting switch  120  is switched from OFF to ON at a time t 1 , the control device  110  supplies an exciting current to the electromagnetic coil  60  promptly. The exciting current at this time is a driving current I 2  necessary to move the plunger  7  with the clutch member  5 . The clutch member  5  moves in the axial direction from the initial position P 0  after the time t 1 . 
     When the clutch member  5  reaches the first position P 1  at a time t 2 , the detection signal of the position sensor  9  is switched from OFF to ON. When the control device  110  detects that the clutch member  5  has moved to the connecting position by the change in the detection signal of the position sensor  9 , the control device  110  reduces the exciting current to the electromagnetic coil  60  promptly. Here, “promptly” indicates that the reduction of the exciting current is started within 0.1 seconds, for example, without waiting until a timer value exceeds a predetermined value after a timer is started, for example. The control device  110  reduces the exciting current to the electromagnetic coil  60  before the second position P 2  of the clutch member  5  (on a first-position-P 1  side) at the latest. 
     In the present embodiment, when the control device  110  detects, by the position sensor  9 , that the clutch member  5  has moved to the connecting position, the control device  110  reduces the supply current to the electromagnetic coil  60  from the driving current I 2  to a holding current I 1 . The holding current I 1  is a current which is smaller than the driving current I 2  and which has a current value that can maintain a state where the clutch member  5  is placed at the connecting position. When the clutch member  5  reaches the first position P 1 , if there is a relative rotation between the clutch member  5  and the differential case  2 , the cam mechanism  12  can generate a cam thrust. On this account, even if the supply current to the electromagnetic coil  60  is reduced promptly, the clutch member  5  moves to the second position P 2  by a cam thrust of the cam mechanism  12 . That is, in the present embodiment, when it is determined that the cam mechanism  12  generates a cam thrust, the control device  110  reduces the supply current to the electromagnetic coil  60 . 
     When the control device  110  detects, by the position sensor  9 , that the clutch member  5  has moved to the connecting position, the control device  110  gradually reduces the supply current to the electromagnetic coil  60  from the driving current I 2  to the holding current I 1 . Here, “gradually” indicates that the supply current to the electromagnetic coil  60  is slowly reduced at a rate of change that is slower than a rise of the current at the time when the lock mode selecting switch  120  is switched from OFF to ON. In the present embodiment, the supply current to the electromagnetic coil  60  reaches the holding current I 1  at a time t 4 , which is after a time (a time t 3 ) when the clutch member  5  reaches the second position P 2 . Note that the supply current to the electromagnetic coil  60  may reach the holding current I 1  before the clutch member  5  reaches the second position P 2 . 
     In order to gradually reduce the supply current to the electromagnetic coil  60 , the control device  110  includes current adjusting means having a switching element such as a transistor that is switched by a PWM control, for example. The control device  110  gradually reduces the supply current to the electromagnetic coil  60  by gradually lowering a duty ratio of the switching element. Note that  FIG. 7  illustrates a case where the supply current to the electromagnetic coil  60  is linearly reduced at a constant ratio, but this is not the only option, and the rate of change of the current may be increased over time or may be decreased. 
     When the lock mode selecting switch  120  is switched from ON to OFF at a time t 5 , the control device  110  sets the supply current to the electromagnetic coil  60  to zero. Hereby, the clutch member  5  returns to the initial position P 0  from the second position P 2  through the first position P 1 . When the clutch member  5  moves closer to the initial position P 0  than the first position P 1  at a time t 6 , the detection signal of the position sensor  9  is switched from ON to OFF. 
     Note that, in a case where the lock mode selecting switch  120  is in the ON state, when the detection signal of the position sensor  9  is switched from ON to OFF because the right front wheel  103  runs onto a curb stone or the like and the meshing reaction force Fd temporarily increases to disengage the meshing teeth  521  of the clutch member  5  from the meshing teeth  313  of the first side gear  31 , the control device  110  sets the supply current to the electromagnetic coil  60  to the driving current I 2  so that the meshing teeth  521  of the clutch member  5  mesh with the meshing teeth  313  of the first side gear  31 . 
     A main operation/working-effect to be provided by the present embodiment described above is as follows. 
     When the position sensor  9  detects that the clutch member  5  has moved to the connecting position, the control device  110  immediately reduces the supply current to the electromagnetic coil  60 . Accordingly, it is possible to reduce power consumption of the electromagnetic coil  60  and also restrain heat generation in comparison with a case where the supply current to the electromagnetic coil  60  is reduced after a predetermined time after it is detected that the clutch member  5  has moved to the connecting position, for example. 
     When the position sensor  9  detects that the clutch member  5  has moved to the connecting position, the control device  110  gradually reduces the supply current to the electromagnetic coil  60 . Accordingly, even in a case where the clutch member  5  temporarily moves back toward the non-connecting position side due to a collision between the meshing teeth  521  of the clutch member  5  and the meshing teeth  313  of the first side gear  31 , it is possible to mesh them with each other immediately by a pressing force of the plunger  7 . 
     The control device  110  reduces the supply current to the electromagnetic coil  60  before the position (the second position P 2 ) where the clutch member  5  moves closest to the first side gear  31 . Accordingly, it is possible to restrain power consumption and heat generation of the electromagnetic coil  60  in comparison with a case where the supply current to the electromagnetic coil  60  is reduced after the axial movement of the clutch member  5  is completed. 
     The tooth flank  521   a  of the meshing tooth  521  of the clutch member  5  is inclined so as to generate the meshing reaction force Fd for pressing the clutch member  5  toward the non-connecting position side due to a torque transmitted between the differential case  2  and the first side gear  31 . Accordingly, when the supply current to the electromagnetic coil  60  is stopped, the clutch member  5  moves to the non-connecting position immediately. 
     Since the dog clutch  11  includes the cam mechanism  12 , even if the control device  110  reduces the supply current to the electromagnetic coil  60  after a cam thrust of the cam mechanism  12  is generated, the clutch member  5  moves, due to the cam thrust, to a position where the clutch member  5  moves closest to the first side gear  31 . On this account, when the control device  110  reduces the supply current to the electromagnetic coil  60  after the cam thrust of the cam mechanism  12  is generated, it is possible to restrain power consumption and heat generation of the electromagnetic coil  60  and to surely mesh the meshing teeth  521 ,  313  with each other. 
     The cam thrust Fe of the cam mechanism  12  is larger than the meshing reaction force Fd due to the meshing between the meshing teeth  521  of the clutch member  5  and the meshing teeth  313  of the first side gear  31 . Accordingly, a magnetic force generated by the electromagnetic coil  60  should be a magnetic force to such an extent that the plunger  7  and the clutch member  5  can be moved against the biasing force Fr of the return spring  84 . On this account, in comparison with a case where the cam thrust Fc is smaller than the meshing reaction force Fd, for example, it is possible to restrain power consumption and heat generation of the electromagnetic coil  60 . 
     The present invention has been described based on the above embodiment, but the present invention is not limited to this embodiment, and various modifications can be made within a range which does not deviate from the gist of the present invention. For example, the above embodiment deals with a case where the present invention is applied to a parallel-axis differential gear in which respective rotating axes of a pair of side gears (the first side gear  31  and the second side gear  32 ) and a pair of pinion gears (the first pinion gear  41  and the second pinion gear  42 ) are parallel to each other. However, the present invention is not limited to this, and the present invention is also applicable to a differential gear configured such that a pair of side gears and a pair of pinion gears mesh with each other with their gear axes being at right angles.