Abstract:
Differential apparatus which includes input and output members rotatable relative to each other, a clutch mechanism for interconnecting them, an actuator and a cam mechanism. The clutch mechanism includes first and second clutch members rotating with the input and output members, respectively. The actuator limits rotation of the second clutch member relative to the input member to angularly displace the second clutch member relative to the output member. The cam mechanism is provided between the second clutch member and the output member, and includes first and second cam faces rotating with the second clutch member and the output member, respectively. When the actuator operates, these cam faces cooperate to axially displace the second clutch member away from the output member, whereby the second clutch member is axially displaced to engage with the first clutch member.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a differential apparatus provided with a differential limiting function or a differential locking function. 
     2. Description of the Related Art 
     The Japanese Patent Application Laid-open No. 50(1975)-20331 discloses a differential apparatus with a differential limiting function including a cone clutch, a cam mechanism, an actuator, a stop mechanism and the like. 
     In the differential apparatus, the differential operation is mechanically limited by the actuator, the stop mechanism and the like. Therefore, a wheel needs to spin at high speed to lock the differential operation, whereby the differential operation cannot be controlled while the vehicle is moving. Specifically, the differential operation cannot be locked or the lock cannot be released in dependence on conditions of vehicle speed, steering, a road surface, or the like. 
     The actuator and the stop mechanism are composed of a number of parts, such as balances and weights, coil springs of three types, or the like, which are difficult to adjust weight or spring constants thereof, causing unstable motion and variation of products. 
     Generally, a fluid pressure actuator, such as a pneumatic actuator and a hydraulic actuator or an actuator using an electromagnet, is employed to control the differential operation in dependence on conditions of vehicle speed, steering, and the road surface. Particularly, the fluid pressure actuator is employed because it is capable of generating a large differential limiting force required to lock the differential operation. 
     However, the fluid pressure actuator needs a pressure source (pump) and a pressure line (piping). Modification or alteration of a casing or a vehicle body for accommodating the differential apparatus is required to make space for arrangement of the pressure source and line. Moreover, it is difficult to unitize, package or modularize the differential apparatus, the pressure source, the pressure line, and the like. 
     The fluid pressure actuator has low reliability because of possible leak from each portion of the pressure source and line, which causes loss of function. For preventing the leak, seals need to be enforced, thus leading to cost increase. 
     Meanwhile, in the case of the actuator using an electromagnet, it is difficult to obtain enough differential limiting force to lock the differential operation. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a differential apparatus using an electromagnet, in which enough differential limiting force can be obtained to lock differential operation. 
     Another object of the present invention is to provide a unitized differential apparatus having a differential limiting function, making modification and alteration of a casing or a vehicle body minor or unnecessary. 
     An aspect of the present invention is a differential apparatus comprising: a rotary input member; a rotary output member within the input member, rotatable relative to the input member; a clutch mechanism for interconnecting the input member and the output member, the clutch mechanism including a first clutch member rotatable together with the input member and a second clutch member rotatable together with the output member, the second clutch member to be axially displaced to engage with the first clutch member; an actuator for limiting rotation of the second clutch member relative to the input member to angularly displace the second clutch member relative to the output member; and a cam mechanism provided between the second clutch member and the output member for engaging the first and second clutch members, the cam mechanism including a first cam face to be rotated together with the second clutch member and a second cam face to be rotated together with the output member, wherein the first and second cam faces cooperate to axially displace the second clutch member away from the output member, as the second clutch member is angularly displaced relative to the output member by the actuator, whereby the second clutch member is axially displaced to engage with the first clutch member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the accompanying drawings wherein: 
     FIG. 1 a  is a sectional view of a differential apparatus according to a first embodiment of the present invention. 
     FIG. 1 b  is an explanatory view of a cam mechanism of the differential apparatus in FIG. 1 a.    
     FIG. 2 is an enlarged sectional view showing a clutch mechanism of a differential apparatus according to a second embodiment of the present invention. 
     FIG. 3 is an enlarged sectional view showing a clutch mechanism of a differential apparatus according to a third embodiment of the present invention. 
     FIG. 4 is a sectional view of a differential apparatus according to a fourth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be explained below with reference to the drawings, wherein like members are designated by like reference characters, and members without reference characters are not illustrated in the drawings. Left/right in the following description corresponds to left/right in FIGS. 1 a  to  3 , respectively. 
     First Embodiment 
     As shown in FIG. 1 a , a differential apparatus  1  (a first embodiment of the present invention) is composed of a differential case  3  (input member), a bevel gear type differential mechanism  5  (output member), an actuator  8  including a frictional force generation mechanism  7  and a transmission mechanism  9 , a cone clutch  11  (clutch mechanism), a cam  13  (cam mechanism), a return spring  15 , a controller, and the like. 
     The differential case  3  includes a casing body  19  in the right side of FIG. 1 a  and a cover  17  fixed on a left opening of the casing body  19  with bolts. The differential case  3  is arranged within a differential carrier. 
     A boss portion  21  of the cover  17  and a boss portion  23  of the casing body  19  are individually supported on the differential carrier with bearings. An oil reservoir is formed within the differential carrier. 
     On the differential case  3 , a ring bevel gear is fixed with bolts to be engaged with an output bevel gear of a propeller shaft of a power train. The propeller shaft is connected to a transmission. A driving force from an engine is transmitted to the differential case  3  via the transmission and the propeller shaft to rotate the differential case  3 . 
     The differential mechanism  5  within the differential case  3 , which is rotatable relative to the differential case  3 , is composed of pinion shafts  25 , pinion gears  27 , left and right side gears  29  and  31 , and the like. 
     Each pinion shaft  25  is engaged with a through hole  33  at both ends thereof provided in the casing  19 , and fixed to the casing  19  with a spring pin. Each pinion gear  27  is rotatably supported on the pinion shaft  25 . The left side gear  29  and the right side gear  31  are engaged with the pinion gear  27  on the left and right thereof, respectively. 
     Between the casing body  19  and each pinion gear  27 , a spherical washer  35  is interposed. The spherical washer  35  receives a centrifugal force of the pinion gear  27  when the differential case  3  is rotating and a reaction force which is applied to the pinion gear  27  by engagement of the left and the right side gears  29  and  31 . 
     A hub portion  37  of the left side gear  29  is rotatably supported by a bearing portion  39  of the cover  17  and connected to a left wheel through an axle spline-connected to the hub portion  37 . A hub portion  41  of the right side gear  31  is rotatably supported by a bearing portion  43  of the casing body  19  and connected to a right wheel through an axle spline-connected to the hub portion  41 . 
     Between the hub portion  37  of the left side gear  29  and the boss portion  21  of the cover  17 , and between the hub portion  41  of the right side gear  31  and the boss portion  23  of the casing body  19 , thrust washers  45  are individually arranged. The thrust washers  45  receive reaction forces applied to the left and right side gears  29  and  31  by engagement of the pinion gears  27  and the left and the right side gears  29  and  31 , respectively. 
     The driving force from the engine is distributed to the left and the right side gears  29  and  31  from the differential case  3  via the pinion shafts  25  and the pinion gears  27 , and transmitted to the left and the right wheels via the axles thereof. 
     For example, when a difference of drive resistance occurs between the left and the right wheels while a vehicle is moving on a rough road, each pinion gear  27  rotates about an axis of the pinion shaft  25 , and thus the driving force from the engine is distributed to the left and right sides. 
     The actuator  8  is composed of the frictional force generation mechanism  7 , the transmission mechanism  9 , and the like. 
     The frictional force generation mechanism  7  is composed of a right ring gear  47  (armature) made of a magnetic material, a friction clutch  49  (pilot clutch) formed between the right side surface of the right ring gear  47  and the inner surface of the right side wall of the casing body  19 , an electromagnet  51 , and the like. 
     The right ring gear  47  Is supported on the outer circumferential surface of the hub portion  41  of the right side gear  31  so as to be displaceable in an axial direction and rotatable relative to the hub portion  41 , and is centered. 
     A core  53  of the electromagnet  51  is connected to the differential carrier through a support member, and fixed thereto so as not to rotate. Moreover, the core  53  is supported on the outer circumferential surface of the boss portion  23  with a bearing interposed therebetween and is centered. The inner diameter of the core  53  is smaller than the outer diameter of the right ring gear  47 , and projections of the core  53  and the right ring gear  47  in the axial direction are overlapped each other. A moderate air gap is provided between a left side face of the core  53  and the outer surface of the right side wall of the casing body  19  in the vicinity thereof. A lead wire of the electromagnet  51  is drawn out of the differential carrier through a grommet and connected to a battery on the vehicle. 
     The core  53 , the right side wall of the casing body  19 , and the right ring gear  47  constitutes a magnetic path of the electromagnet  51 . The right side wall of the casing body  19  is radially divided into an inner wall and an outer wall by a ring  55  made of stainless steel of a non-magnetic material. The ring  55  is embedded in the casing body  19  at a radial position corresponding to the electromagnet  51 . The ring  55  prevents a short circuit of magnetic flux on the magnetic path. 
     The transmission mechanism  9  is composed of a gear portion  47   a  (first gear portion) formed in the right ring gear  47 , a gear portion  57   a  (second gear portion) formed in a left ring gear (clutch member)  57 , left and right pinion gears  61  and  63 , small-diameter shafts (shaft member)  59  connecting the left and right pinion gears  61  and  63 , and the like. 
     The left ring gear  57  is supported on the outer circumferential surface of the hub portion  37  of the left side gear  29  so as to be movable in the axial direction and angularly displaceable, and is centered. The left ring gear  57  can rotate together with the left side gear  29  and rotates relative to the cover  17 . The left ring gear  57  is the same as the right ring gear  47  in diameter. 
     The shafts  59  are arranged along the axial direction between the pinion gears  27  outside the left and right side gears  29  and  31  in the radial direction within the casing body  19 . Both ends of each shaft  59  are rotatably supported by left and right bearing portions  65  and  67 , which are formed in the cover  17  and the right side wall of the casing body  19 , respectively. 
     The right pinion gear (first pinion gear)  61  is formed on the right end side of the shaft  59  and engaged with the right ring gear  47 . The left pinion gear (second pinion gear)  63  is formed on the left end side of the shaft  59  and engaged with the left ring gear  57 . 
     The left and right pinion gears  61  and  63  and the left and right ring gears  47  and  57  are spur gears in order that an engagement reaction force is not generated in the axial direction when the pinion gears  61  and  63  and the ring gears  47  and  57  rotate in engagement with each other. 
     Therefore, the rotation of the left ring gear  57  is transmitted to the right ring gear  47  at equal speed via the left pinion gear  63 , the shafts  55 , and the right pinion gear  61 . 
     The cone clutch  11  is operated to interconnect the cover  17  and the left ring gear  57 . The cone clutch  11  includes a cone portion  69  integrally formed on a left sidewall of the left ring gear  57  and a conical friction surface portion  70  increasing in diameter toward the right. The friction surface portion  70  is provided on the inner surface of the sidewall of the cover  17  and rotates with the cover  17 . 
     The cam  13  is provided between a right side surface of the left ring gear  57  and a left side surface of the left side gear  29 . As shown in FIG. 1 b , the cam  13  includes a cam face  13   a  rotating together with the hub portion  37  of the left side gear  29  and a cam face  13   b  rotating together with the left ring gear  57  and engaging with the cam face  13   a . These cam faces  13   a  and  13   b  are inclined in the circumferential direction at a certain distance from the rotation axis. If differential torque is applied between the left side gear  29  and the left ring gear  57  by the actuator  8 , a circumferential direction force F 1  acts on the cam faces  13   a  and  13   b , and then slip restrained by the engaging faces thereof is produced to generate a thrust force F 2  in the axial direction, which is called a cam thrust force. The left side gear  29  and the left ring gear  57  are displaced in the axial direction so as to separate from each other by this thrust force F 2 . 
     The cone clutch  11  and the cam  13  are arranged on the left side of the differential mechanism  5  in the axial direction, and the friction clutch  49  and the electromagnet  51  are arranged on the right side of the differential mechanism  5  in the axial direction, which are substantially symmetrically arranged. 
     The return spring  15  is arranged between the left ring gear  57  and a snap ring  71  attached on the outer circumferential surface of the hub portion  37  of the left side gear  29 . The return spring  15  presses the left ring gear  57  rightward, that is, in a direction of releasing coupling of the cone clutch  11 , or in a direction of engaging the cam  13 . 
     The controller detects the vehicle moving in a curve from detection signals of a vehicle speed, a steering angle, a lateral gravity, and the like. The controller excites the electromagnet  51 , controls an exciting current, stops excitation, and so on in dependence on road conditions. 
     When the electromagnet  51  is excited, a magnetic flux loop  73  is formed in the above described magnetic path, and the right ring gear  47  is attracted rightward to be pressed against the casing body  19 . Accordingly, the friction clutch  49  is engaged to generate a frictional force. The frictional force brakes rotation of the right ring gear  47  relative to the casing body  19 . Furthermore, the braking force is transmitted to the left ring gear  57  via the right ring gear  47 , the right pinion gear  61 , the shafts  59 , and the left pinion gear  63  of the transmission mechanism  9  to brake rotation of the left ring gear  57  relative to the cover  17 . 
     If differential rotation is generated within the differential mechanism  5  in this state, relative angular displacement is generated between the left ring gear  57  and the left side gear  29 , and thus differential torque is applied to the cam  13  provided therebetween. The differential torque causes the cam  13  to generate the cam thrust force to displace the left ring gear  57  and the cone portion  69  thereof leftward in the axial direction against the return spring  15 . Accordingly, the cone portion  69  is engaged with the friction surface portion  70  to engage the cone clutch  11  and a differential limiting force is generated. 
     As described above, the cam  13  is actuated using the differential torque of the left side gear  29  to amplify the engaging force of the cone clutch  11 , so that a large differential limiting force can be obtained. Furthermore, a wedge effect (self-lock function) by an angle of the conical friction surface of the cone clutch  11  amplifies the differential limiting force. Therefore, the electromagnet  51  can be employed, obtaining the differential limiting force enough to lock the differential operation. 
     When the differential limiting force thus obtained is larger than differential lock torque of the differential mechanism  5 , the differential operation is locked. When the differential limiting force is smaller than the differential lock torque, the cam thrust force of the cam  13  is increased or decreased in dependence on variation in the differential torque, so that a differential limiting function similar to that of a torque sensitive type can be obtained. 
     Furthermore, if slip of the friction clutch  49  is adjusted by controlling the exciting current of the electromagnet  51 , the braking force by the frictional force, differential torque, and the cam thrust force of the cam  13  vary, and the differential limiting force can be freely controlled. 
     When the excitation of the electromagnet  51  is stopped, the friction clutch  49  is disengaged, and the cam thrust force of the cam  13  disappears. Then, the left ring gear  57  is returned rightward by the pressing force of the return spring  15 , and the cone clutch  11  is disengaged. Accordingly, the differential rotation of the differential mechanism  5  becomes free. 
     Note that, even if the friction clutch  49  is activated by the excitation/non-excitation and the current control of the electromagnet  51  as described above, a moderate air gap is always maintained between the outer surface of the right sidewall of the casing body  19  and the electromagnet  51 . The air gap prevents the rotation of the casing body  19  from being interfered by contacting the electromagnet  51  and receiving sliding resistance. 
     When the friction clutch  49  and the cone clutch  11  are disengaged as described above, the left ring gear  57 , the shafts  59 , the left and right pinion gears  61  and  63 , and the right ring gear  47  rotate in conjunction with the rotation of the left side gear  29  to generate relative rotation (sliding rotation) between the right ring gear  47  and the right side gear  31 . 
     The thrust washer  45  on the left end of the left side gear  29  receives a reaction force which is applied to the left side gear  29  by engagement of the pinion gears  27  and the left side gear  29  and resists the leftward movement of the left side gear  29  relative to the casing body  19 . The return spring  15  presses the left ring gear  57  rightward relative to the left side gear  29 . Therefore, a moderate gap is secured within the cone clutch  11 , thus preventing the cone clutch  11  from being inadvertently engaged and generating the differential limiting force. 
     The controller excites the electromagnet  51 , controls the exciting current, and stops the excitation at arbitrary timing to perform differential lock, adjustment of the differential limiting force, release of the lock, and the like in dependence on conditions of a vehicle speed, steering, a road surface, or the like. Such operations improve a starting ability, an acceleration ability, a turning ability, a steerability, stability, an off-road ability of the vehicle and the like. 
     The differential case  3  is provided with an opening, and on the inner circumferential surfaces of the boss portions  21  and  23 , spiral oil grooves are formed. 
     The lower half of the differential apparatus  1  is immersed in oil of the oil reservoir. In accordance with the rotation of the differential case  3  and the ring gears thereof, the oil flows into/out of the differential case  3  through the opening and the spiral oil grooves, and sufficiently lubricates and cools the engaging portions of the gears  27 ,  29 , and  31  (the differential mechanism  5 ), the sliding portions between the outer circumferential surfaces of the pinion shafts  25  and the pinion gears  27 , the thrust washers  45  and  45 , the spherical washer  35 , the support portions  39  and  43  of the left and right side gears  29  and  31 , the friction clutch  49 , the engaging portions of the gears  47 ,  61 ,  63 , and  57  of the transmission mechanism  9 , the sliding portions between the ring gears  47  and  57  and the hub portions  41  and  37  of the side gears  29  and  31 , the cam  13 , the cone clutch  11 , and so on. 
     Moreover, the electromagnet  51  is cooled by the oil which is splashed over by the rotation of the differential case  3  and the ring gears as well as the lower half thereof is immersed in the oil reservoir. Accordingly, the capability (magnetic force) thereof is stabilized, so that the function of the friction clutch  49  and the differential limiting force of the cone clutch  11  are stabilized. 
     In the differential apparatus  1  structured as described above, the pressure source, the pressure line and the space for arrangement thereof become unnecessary unlike the differential apparatus using the fluid pressure actuator. Accordingly, the differential apparatus becomes simple in structure and compact, thus enhancing mountability on the vehicle. Moreover, installation of the pressure line becomes unnecessary, so that assembly of the device is facilitated and the assembly costs are reduced. 
     Moreover, function is not lost by pressure leak, so that high reliability can be obtained. It becomes unnecessary to enforce the seals of the pressure line for preventing leak, which saves the costs. 
     The differential apparatus  1  and the electromagnet  51  can be easily unitized, packaged, or modularized. Accordingly, the impact to the differential carrier and the vehicle body is minimized and the modification and the alteration thereof become minor or unnecessary. 
     The frictional force generation mechanism  7  (friction clutch  49 ) for generating frictional force between the differential case  3  and the right ring gear  47  is a pilot clutch for activating the cam  13 . As described above, the cone clutch  11  for the differential limiting force is engaged by the cam  13 . Accordingly, the electromagnet  51  only needs a magnetic force in amount enough to engage the friction clutch  49 , and the electromagnet  51  is not required to have a particularly large amount of magnetic force (excitation power). 
     Therefore, the electromagnet  51  becomes small and lightweight, and accordingly the differential apparatus  1  is made to be small and lightweight. Furthermore, burdens on the in-vehicle battery and an alternator for charge of the battery are reduced, thus reducing fuel consumption of the engine which drives the alternator. 
     The cone clutch  11  and the cam  13  are arranged on the left side of the differential mechanism  5  and the friction clutch  49  and the electromagnet  51  are arranged on the right side of the differential mechanism  5 . Accordingly, the deferential device  1  is balanced in weight with respect to the differential center, and factors of generating vibration can be restrained. Moreover, the burden on the bearing supporting the differential apparatus  1  on the differential carrier is reduced, thus enhancing durability. 
     With respect to the dimensional center of the differential mechanism  5 , the differential mechanism  5  is balanced in the axial direction, so that the axles connected to the side gears  29  and  31  can be designed to have equal dimensions. Therefore, it is possible to share the axles and reduce the costs. Moreover, since the modification and the alteration of the differential carrier or the vehicle body for compensating imbalance in the lengths of the left and right axles and the weight are avoided, the costs are further reduced. 
     Since the cone clutch  11  can generate a large differential limiting force with a comparatively small area of the friction surface, the differential apparatus  1  is structured to be compact. 
     Since the cone clutch  11  has a friction surface of a small area, drag torque owing to oil viscosity is small. Therefore, the differential limit and lock are easily released, thus maintaining good steerability without the remaining differential limit torque and keeping the engine fuel-efficient. 
     Moreover, if the gear ratio of the right pinion gear  61 , the right ring gear  47 , the left pinion gear  63 , and the left ring gear  57  is changed, for example, the diameter of the right pinion gear  61  is designed to be larger than that of the left pinion gear  63  and the diameter of the right ring gear  47  is designed to be smaller than that of the left ring gear  57 , the braking force of the friction clutch  49  is amplified to be transmitted to the left ring gear  57 . Accordingly, the capacity of the electromagnet  51  can be reduced. 
     Furthermore, if the right pinion gear  61  and the ring gear  47 , or the left pinion gear  63  and the left ring gear  57  are helical gears, the positions thereof in the axial direction can be controlled by utilizing the engagement reaction force generated in rotation of the right pinion gear  61  and the ring gear  47 , or the left pinion gear  63  and the left ring gear  57  in engagement with each other. 
     Second Embodiment 
     As shown in FIG. 2, a differential apparatus  101  (a second embodiment of the present invention) is composed of a differential case  3 , a bevel gear type differential mechanism  5 , an actuator  8  including a frictional force generation mechanism  7  and a transmission mechanism  9 , a multiple plate clutch  103  (clutch mechanism), a cam  13 , a return spring  15 , a controller, and the like. 
     The differential apparatus  101  has a structure in which the cone clutch  11  in the differential apparatus  1  of the first embodiment is substituted with the multiple plate clutch  103 . 
     Next, description will be made on differences from the differential apparatus  1  of the first embodiment. 
     The multiple plate clutch  103  is provided between the inner surface of the sidewall of the cover  17  and the left side surface of the left ring gear  57 . In the radially inside portion of the left ring gear  57 , a hub portion  109  is formed extending leftward in the axial direction along the outer circumferential surface of the hub portion  37  of the left side gear  29 . Outer plates  105  are connected to a spline portion  107  so as to be slidable in the axial direction, the spline portion  107  being formed on the inner circumferential surface of the cover  17 . Inner plates  111  are connected to a spline portion  113  so as to be slidable in the axial direction, the spline portion  113  being formed on the outer circumferential surface of the hub portion  109  of the left ring gear  57 . 
     When the electromagnet  51  is excited, the right ring gear  47  is attracted rightward to engage the friction clutch  49 . The frictional force thereof brakes rotation of the left ring gear  57  via the transmission mechanism  9 . If differential rotation is generated in the differential mechanism  5  in such a state, the cam  13  is activated by differential torque between the left ring gear  57  and the left side gear  29  to engage the multiple plate clutch  103 . 
     When the differential limiting force thus obtained is larger than the differential lock torque of the differential mechanism  5 , the differential operation is locked. When the differential limiting force is smaller than the differential lock torque, a differential limiting force similar to that of a torque sensitive type can be obtained by the cam thrust force of the cam  13 . 
     When the slip of the friction clutch  49  is adjusted by controlling the exciting current of the electromagnet  51 , the differential limiting force can be freely controlled. 
     When the excitation of the electromagnet  51  is stopped, the multiple plate clutch  103  is disengaged by the pressing force of the return spring  15 , and the differential rotation of the differential mechanism  5  becomes free. 
     The multiple plate clutch  103  generates differential limiting force enough to lock the differential operation because of the wide area of the friction surfaces between a number of plates  105  and  111 . 
     Moreover, in the multiple plate clutch  103 , the differential limiting force can be easily adjusted by changing the number of plates  105  and  111 . Therefore, the differential apparatus  101  can be applied to the wide range of different vehicle types. 
     Third Embodiment 
     As shown in FIG. 3, a differential apparatus  201  (a third embodiment of the present invention) is composed of a differential case  3 , a bevel gear type differential mechanism  5 , an actuator  8  including a frictional force generation mechanism  7  and a transmission mechanism  9 , a multiple plate clutch  203  (clutch mechanism), a cam  13 , a return spring  15 , a controller, and the like. 
     Next, description will be made on differences from the differential apparatus  1  of the first embodiment. 
     The multiple plate clutch  203  includes inner plates  211  and the outer plates  105 , which are provided between the inner surface of the sidewall of the cover  17  and the left side surface of the left ring gear  57 , and the left ring gear  57  as a pressing member for pressing the inner and outer plates  211  and  105  for displacement in the axial direction to engage the inner and outer plates  211  and  105 . In the radially inside portion of the left ring gear  57 , a hub portion  209  is formed. The hub portion  209  extends rightward in the axial direction along the outer circumferential surface of the hub portion  37  of the left side gear  29 . The outer plates  105  are connected to the spline portion  107 , which is formed on the inner circumferential surface of the cover  17 , so as to be slidable in the axial direction. The inner plates  211  are connected to a spline portion  213 , which is formed on the outer circumferential surface of the hub portion  37  of the left side gear  29 , so as to be slidable in the axial direction. 
     The snap ring  71  is attached to the outer circumferential surface of the hub portion  37  of the left side gear  29  on the right side of the multiple plate clutch  203  and stops the return spring  15 . 
     When the multiple plate clutch  203  is engaged by excitation of the electromagnet  51 , the differential limiting force thereof is transmitted directly to the left side gear  29  via the inner plate  211  of the multiple plate clutch  203 , without passing through the cam  13 . Accordingly, the capacity of the cam  13  can be reduced. 
     Fourth Embodiment 
     As shown in FIG. 4, a differential apparatus  301  (a fourth embodiment of the present invention) is composed of a differential case  3 , a bevel gear type differential mechanism  5 , an armature  303 , a cam  305  (cam mechanism), a multiple plate pilot clutch  309  (clutch mechanism), a return spring  307 , an electromagnet  51  (actuator), a controller, and the like. 
     Next, description will be made on differences from the differential apparatus  1  of the first embodiment. 
     The differential case  3  includes the cover  17 , the casing body  19  of a non-magnetic material, and a rotor  311  of a magnetic material. The cover  17  is fixed on the left opening of the casing body  19  with bolts. The rotor  311  is welded on a right opening of the casing body  19  to constitute the right side wall of the differential case  3 . 
     The differential case  3  is arranged within the differential carrier. The boss portion  23  of the cover  17  and a boss portion  313  of the rotor  311  are supported by the differential carrier with bearings interposed therebetween. The oil reservoir is formed within the differential carrier. 
     The hub portion  37  of the left side gear  29  is supported by the support portion  39  of the cover  17  and connected to the left wheel via the axle spline-connected to the hub portion  37 . The hub portion  41  of the right side gear  31  is connected to the right wheel via the axle spline-connected to the hub portion  41 . 
     Between the hub portion  41  of the right side gear  31  and the boss portion  313  of the rotor  311 , a thrust washer  45  is arranged. The thrust washer  45  receives a reaction force which is applied to the right side gear  31  by engagement of the right side gear  31  and the pinion gears  27 . 
     The armature  303  is composed of a disk-shaped flange portion  349  and a hub portion  347  integrally formed so as to extend rightward in the axial direction from the radially inside portion of the flange portion  349  along the outer circumferential surface of the hub portion  41  of the right side gear  31 . The armature  303  is supported on the outer circumferential surface of the hub portion  41  of the right side gear  31 . 
     The cam  305  is provided between the left surface of the flange portion  349  of the armature  303  and the right side surface of the right side gear  31 . The return spring  307  is provided on the outer circumferential surface of the hub portion  41  of the right side gear  31  between a left end surface of the boss portion  313  of the rotor  311  and a right end portion of a hub portion  347  of the armature  303 . The return spring  307  presses the armature  303  leftward with respect to the differential case  3 , that is, in the direction of engaging the cam  305 . 
     The pilot clutch  309  is provided between the left side surface of the rotor  311  and the flange portion  349  of the armature  303  inside the casing body  19 . Outer plates  351  thereof are connected to a spline portion  353  so as to be slidable in the axial direction, the spline portion  353  being formed on the inner circumferential surface of the casing body  19 . Inner plates  355  thereof are connected to a spline portion  357  so as to be slidable in the axial direction, the spline portion  357  being formed on the outer circumferential surface of the hub portion  347  of the armature  303 . 
     The core  53  of the electromagnet  51  is supported on the rotor  311  with a bearing interposed therebetween and is centered. The inner diameter of the core  53  is smaller than the outer diameter of the flange portion  349  of the armature  303 , and the projections of the core  53  and the armature  303  in the axial direction are overlapped each other. A moderate air gap is provided between the left side surface of the core  53  and the right side surface of the rotor  311  in the vicinity thereof. 
     The core  53 , the rotor  311 , the pilot clutch  309 , and the armature  303  constitute a magnetic path of the electromagnet  51 . 
     The rotor  311  is radially divided into an inner wall and an outer wall by the ring  55  of stainless steel as a non-magnetic material. The ring  55  is embedded in the rotor  311  at the radial position corresponding to the electromagnet  51 . Each of plates  351  and  355  of the pilot clutch  309  is circumferentially provided with notches  359  at a plurality of positions and bridge portions between the notches  359  in a radial position corresponding to the ring  55 . The bridges connect the radially inside and the outside of each of the plates  351  and  355 . The ring  55  and the notches  359  prevent a short circuit of magnetic flux on the magnetic path. 
     When the electromagnet  51  is excited, a magnetic flux loop  373  is formed on the above described magnetic path, and the armature  303  is attracted and displaced rightward in the axial direction. The armature  303  presses and engages the pilot clutch  309  between the armature  303  and the rotor  311  to generate pilot torque (frictional force). Therefore, the relative rotation of the armature  303  with respect to the differential case  3  is limited. 
     When differential rotation is generated in the differential mechanism  5  in the state where the pilot torque is generated, relative angular displacement is generated between the armature  303 , which is connected to the differential case  3  via the pilot clutch  309 , and the right side gear  31 , and the differential torque is applied to the cam  305  provided therebetween. With the differential torque, the cam  305  generates the cam thrust force and moves the armature  303  rightward against the return spring  307 . The engaging force of the pilot clutch  309  is thus amplified. 
     The above described self-lock function of the pilot clutch  309  by the cam  305  allows a large differential limiting force to be obtained. When the differential limiting force thus obtained exceeds the differential lock torque of the differential mechanism  5 , the differential operation is locked. As described above, the actuator using the electromagnet  51  can generate enough differential limiting force to lock the differential operation. 
     When the differential limiting force is smaller than the differential lock torque, the differential limiting force of the torque sensitive type can be obtained by the cam thrust force of the cam  305 , which varies in dependence on variation in the differential torque. 
     Furthermore, if slip of the pilot clutch  309  is adjusted by controlling the exciting current of the electromagnet  51 , the pilot torque of the pilot clutch  309  and the cam thrust force of the cam  305  vary, and the differential limiting force can be freely controlled. 
     When the excitation of the electromagnet  51  is stopped, the armature  303  is returned leftward by the pressing force of the return spring  307 , and the pilot clutch  309  is disengaged. Accordingly, the pilot torque and the cam thrust force of the cam  305  disappear, and the differential rotation of the differential mechanism  5  becomes free. 
     In the embodiment, as described above, the pilot clutch  309  serves as a main clutch for locking the differential operation with the engaging force amplified by the cam  305 . In other words, the clutch mechanism serves as the pilot clutch and the clutch for limiting the differential operation. 
     Moreover, the thrust washer  45  on the right end of the hub portion  41  of the right side gear  31  receives a reaction force which is applied to the right side gear  31  by engagement of the right side gear  31  and the pinion gears  27 , and resists the rightward movement of the right side gear  31  relative to the rotor  311 . The return spring  307  presses the armature  303  leftward relative to the rotor  311 . Therefore, a moderate gap is secured between the armature  303  and the pilot clutch  309 , thus preventing the pilot clutch  309  from being inadvertently engaged and generating the differential limiting force. 
     The differential case  3  is provided with an opening, and spiral oil grooves are formed on the inner circumferential surfaces of the boss portions  21  and  313 . 
     The lower half of the differential apparatus  301  is immersed in the oil of the oil reservoir. In accordance with the rotation of the differential case  3 , the oil flows into/out of the differential case  3  through the opening and the spiral oil grooves, and sufficiently lubricates and cools the engaging portions of the gears  27 ,  29 , and  31 , the sliding portions between the outer circumferential surfaces of the pinion shafts  25  and the pinion gears  27 , the thrust washer  45 , the spherical washer  35 , the support portion  39  of the left side gear  29 , the sliding portion between the hub portion  347  of the armature  303  and the hub portion  41  of the right side gear  31 , the cam  305 , the pilot clutch  309 , the both ends of the return spring  307 , and so on. 
     Moreover, the electromagnet  51  is cooled by the oil which is splashed over by the rotation of the differential case  3  and the ring gears thereof, thus stabilizing the capability (magnetic force) thereof. Accordingly, the operating function of the pilot clutch  309  is stabilized. 
     The invention may be practiced or embodied in still other ways without departing from the spirit or essential character thereof. For instance, the engine in the present invention can be an electric motor converting electric energy into torque. 
     The clutch mechanism for use in the differential apparatus may be a clutch mechanism of another type. For example, the clutch mechanism used in the differential apparatus of the fourth embodiment may be a cone clutch. As shown in the third embodiment, the multiple plate clutch having the inner plate attached to the side gear may be used. 
     The differential mechanism is not limited to the bevel gear differential mechanism, but may be a planetary gear type differential mechanism, a differential mechanism including a pair of output side gears connected to a pinion gear, which is accommodated in a housing hollow of the differential case so as to be freely slidable and rotatable, a differential mechanism using a worm gear, and so on. 
     Moreover, the cam mechanism may be a ball cam, which is composed of a curved surface rotating together with the side gear, a curved surface rotating together with the ring gear or the armature, and a ball interposed therebetween. The ball cam displaces the curved surfaces so as to separate from each other in the direction of the rotation axis when relative angular displacement between the surfaces is generated. 
     The differential apparatus can be used as any one of a front differential (differential apparatus for distributing the driving force from the motor into the left and the right front wheels), a rear differential (differential apparatus for distributing the driving force from the engine into the left and the right rear wheels), and a center differential (differential apparatus for distributing the driving force from the engine into the front wheels and the rear wheels). 
     The preferred embodiments described herein are therefore illustrative and not restrictive, the scope of the invention being indicated by the claims and all variations which come within the meaning of claims are intended to be embraced therein. 
     The present disclosure relates to subject matters contained in Japanese Patent Application No. 2001-397602, filed on Dec. 27, 2001, and Japanese Patent Application No. 2002-33043, filed on Feb. 8, 2002, the disclosure of which are expressly incorporated herein by reference in its entirety.