Abstract:
The differential includes a differential housing ( 7 A,  7 B). A torque transmission member ( 5 A,  5 B) is supported to a differential housing ( 7 A,  7 B) for rotating relative to the differential housing. A clutch system ( 13 A,  13 B) is configured to interconnect between the torque transmission member ( 5 A,  5 B) and the differential housing ( 7 A,  7 B) for transmitting a drive torque therebetween.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a differential and a differential system adapted for a four-wheel drive vehicle and, more specifically, to ones adapted for a vehicle mutually switchable between four-wheel drive and two-wheel drive.  
         [0002]     Conventionally, a drive force transmission with a differential of Japanese Patent Application Laid-Open Publications No. 3-118233 and NO. 3-292437 are known.  
         [0003]     The drive force transmission is located to a rear wheel drive system in a four-wheel drive system. The transmission has a differential with a rotatable differential housing. The transmission includes a ring gear member for the transmission of a drive force to the casing.  
       SUMMARY OF THE INVENTION  
       [0004]     The ring gear member, however, is rotatably supported to another member such as a shaft, not the differential housing. During two-wheel drive, the sliding of the ring gear member on the differential housing or the shaft results in sliding resistance. This causes seizing and galling to generate drive resistance as drag torque for the reduction of the fuel cost of the engine.  
         [0005]     It therefore is an object of the present invention to provide a differential and a differential system in which drive resistance reduces remarkably.  
         [0006]     To achieve the object, a first aspect of the invention provides a differential. The differential includes a differential housing. The differential includes a torque transmission member supported to the differential housing for rotating relative to the differential housing. The differential includes a clutch system configured to interconnect between the torque transmission member and the differential housing for transmitting a drive torque therebetween.  
         [0007]     The differential includes a non-limited slip differential and a limited slip differential (LSD). The LSD includes a corn-clutch type, a multiplate-clutch type, or a parallel-axis type.  
         [0008]     Preferably, the differential further includes a support member located between the torque transmission member and the differential housing. The support member supports the torque transmission member to the differential housing for rotation.  
         [0009]     The support member includes a bearing, a roller, and a ball. The bearing includes a ball bearing and a slide bearing.  
         [0010]     Preferably, the support member and the clutch system are axially arranged each other.  
         [0011]     Preferably, the torque transmission member has a gear located in radial alignment with the support member.  
         [0012]     Preferably, the clutch system includes a first clutch provided between the torque transmission member and the differential housing. The clutch system includes an actuator for operating the first clutch. The first clutch is located axially between support member and the actuator.  
         [0013]     The first clutch includes a dog clutch, and a friction clutch. The actuator includes a electromagnet type, and hydraulic type.  
         [0014]     Preferably, the support member supports at least two points of the torque transmission member.  
         [0015]     Preferably, the torque transmission member axially has an end. The actuator is located at the end. The first clutch is located axially back from the end.  
         [0016]     Preferably, the support member is located in alignment with the clutch system.  
         [0017]     Preferably, the actuator includes a second clutch for transmitting a drive torque from the torque transmission member. The actuator includes a converter provided between the first and second clutches for converting a drive torque to a thrust force and for engaging the first clutch.  
         [0018]     Preferably, the actuator further includes an electromagnet system for engaging the second clutch.  
         [0019]     Preferably, the electromagnetic system includes a core. The electromagnetic system includes a rotor located between the core and the second clutch for magnetically conducting therebetween. The rotor is supported on the differential housing.  
         [0020]     Preferably, the converter includes a cam mechanism configured to be operated by the second clutch.  
         [0021]     Preferably, the second clutch includes first clutch plates connected the torque transmission member, the first clutch plates being spaced each other. The second clutch includes second clutch plates connected to the converter. Respective second clutch plates are slidably interposed between respective first clutch plates.  
         [0022]     Preferably, the first clutch plates are spaced radially from the converter.  
         [0023]     Preferably, the second clutch plates are spaced radially from the torque transmission member.  
         [0024]     Preferably, the electromagnet system includes an armature configured to be attracted for pressing and engaging with the second clutch. The armature is spaced radially from the torque transmission member.  
         [0025]     Preferably, the rotor has openings each extending within an angular range. The openings are angularly spaced from each other and are located radially inward of a coil of the electromagnet system.  
         [0026]     Preferably, the openings face a core of the electromagnet system.  
         [0027]     Preferably, the support member includes bearings arranged in axial alignment with each other.  
         [0028]     A second aspect of the invention provides a differential system. The system includes a transmission mechanism for transmitting a drive torque. The system includes a differential. The system includes a torque transmission member supported to the differential for rotating relative to the differential. The system includes a clutch system configured to interconnect between the torque transmission member and the differential for transmitting a drive torque between the transmission mechanism and the differential. 
     
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS  
       [0029]     These and other features, aspects, and advantage of the present invention will be better understood with reference to the following description, appended claims, and accompanying drawings where:  
         [0030]      FIG. 1  is a schematic view of a drive train including a differential system according to a first embodiment of the invention, where A 1  shows the axis;  
         [0031]      FIG. 2  is a sectional view of a differential system of  FIG. 1 , where a differential is sectioned along two directions at a right angle from an axis;  
         [0032]      FIG. 3  is an enlarged view of a primary part of a differential of  FIG. 2 , where R 1  shows the radial direction and A 2  shows the axis direction;  
         [0033]      FIG. 4  is an elevational view of a rotor viewed from an arrow A;  
         [0034]      FIG. 5  is a sectional view of a differential according to a second embodiment of the invention, where a differential is sectioned along two directions at a right angle from an axis; and  
         [0035]      FIG. 6  is a sectional view of a differential according to a third embodiment of the invention, where a differential is sectioned along two directions at a right angle from an axis. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]     An embodiment of the invention will be explained with reference to drawings.  
         [0037]     First Embodiment  
         [0038]     As shown in  FIG. 1 , the embodiment has a differential system mounted on, for example, a hybrid automobile with a four-wheel drive system. The four wheel drive system has a front-wheel drive system and a rear-wheel drive system. Engine  2101  as a primary drive source drives front wheels  2113 ,  2115 . Electrical motor  2129  drives rear wheels  2125 ,  2127 . The differential system includes a rear differential  1 A mounted to a rear-wheel drive system for distributing drive torque to left and right rear wheels  2115 ,  2127 . The differential system includes a reduction mechanism  3  connected to rear differential  1 A.  
         [0039]     The front-wheel drive system has engine  2101  and transmission  2103  linked to each other. Front differential  2107  distributes the drive force from engine  2101  to left and right front wheels  2113  and  2115 . Front shafts  2109  and  2111  link the front wheels  2113 ,  2115  and front differential  2107 .  
         [0040]     The rear-wheel drive system has motor  2131  linked to reduction mechanism  3 . Reduction mechanism  3  connects with ring gear  5 A. Ring gear  5 A transmits drive force to rear differential  1 A. Rear differential  1 A and ring gear  5 A have clutch system  13 A for the connection and disconnection therebetween. Clutch system  13 A has clutch  49 ; and actuators  47 ,  51 ,  53  to operate the engagement and disengagement of the clutch  49 .  
         [0041]     A control system has sensor  2135  for detecting a drive state and generating a signal. The signal causes controller  2133  to generate a control signal. The control signal causes motor  2129  to be driven. Battery  2131  supplies power to motor  2129 .  
         [0042]     During normal drive, engine  2101  drives front wheels  2113 ,  2115 . As necessary, during, for example, starting, acceleration, or racing of front wheels, the driving of motor  2129  causes the auxiliary driving of rear wheels  2125 ,  2127 .  
         [0043]     The width direction of the view of the vehicle using rear differential  1 A in  FIG. 1  corresponds with the view in  FIGS. 2 and 3 . The members without reference characters omitted in Figs.  
         [0044]      FIG. 2  shows rear differential  1 A and reduction mechanism  3 .  
         [0045]     Rear differential  1 A and reduction mechanism  3  are housed in casing  15 . Casing  15  includes: gear casing  15   a  housing reduction mechanism  3 ; differential casing  15   b  housing rear differential  1 A; and cover  15   c  fixed to the gear casing  15   a  and the differential casing  15   b  for sealing. Casing  15  has an oil sealed therein, with its internal portion forming an oil reservoir.  
         [0046]     Reduction mechanism  3  is constituted with two-stepped sets of reduction gears. In addition, reduction mechanism  3  is preferably adaptable to three-stepped sets of reduction gears. Respective sets of reduction gears are constituted with respective small-sized input and large-sized output reduction gears. Reduction mechanism  3  reduces the rotation of motor  2129  in two steps, causing the amplification of torque for the rotation of ring gear  5 A.  
         [0047]     First and second shafts  311 ,  319  are arranged in rows in gear casing  15   a . Respective sets of transmission gears are composed of a spur gear.  
         [0048]     Cylindrical first shaft  311  is rotatably supported to gear casing  15   a , using ball bearing  312 . One end  311   a  of first shaft  311  is connected to the output shaft of motor  2129  as an auxiliary drive force for the rear-wheel drive. The first-stepped set of reduction gears has input reduction gear  313  formed around the other end  311   b  of first shaft  311 .  
         [0049]     Cylindrical second shaft  319  is rotatably supported to cover  15   c , with one end using ball bearing  325  and to gear casing  15   a , with the other end using roller bearing  327 . Second shaft  319  is fixed to annular output reduction gear  317 . Reduction gear  317  extends radially outwardly from second shaft  319 . Reduction gears  313 ,  317  are meshed with each other to reduce the rotational speed of first shaft  311  to be transmitted to second shaft  319 .  
         [0050]     The second stepped set of reduction gears has input reduction gear  321  formed on second shaft  319 . The output gear is ring gear  5 A fixed to clutch housing  23  by welding.  
         [0051]     As shown in  FIG. 5 , casing  15  preferably has an opening  29  provided at the left end of second shaft  319 . Mounted to opening  29  is a cover  30  for preventing a foreign material from entering or the leakage of an oil.  
         [0052]     Rear differential  1  has differential housing  7 A located coaxially with and radially inward of ring gear  5 A. Ring gear  5 A and differential housing  7 A have two ball bearings  9  interposed therebetween. Ring gear  5 A is supported to differential housing  7 A for relative rotation. Located between differential housing  7 A and clutch housing  23  is clutch system  13 A. Differential housing  7 A has bevel-type differential mechanism  11  located inside thereof.  
         [0053]     The gear part  5 Aa of ring gear  5 A, ball bearings  9 , and differential housing  7 A are located coaxially and axially overlapping each other.  
         [0054]     Gear part  5 Aa and ball bearings  9  are arranged in radial alignment with each other, overlapping each other at an axial position. Ball bearings  9  are fixed to ring gear  5 A and differential housing  7 A, while preferably being mounted to one or both of them, using a spacer.  
         [0055]     Left and right drive shafts  2121 ,  2123  pass through the respective bosses  75 ,  77  of differential housing  7 A, the inner peripheries of which are provided with spiral oil channels  79 ,  81 . Differential housing  7 A has opening  83  corresponding with primary clutch  49 . Clutch housing  23  has opening  85  therethrough.  
         [0056]     Differential mechanism  11  between the left and right wheels is constituted with pinion shaft  31 , pinion gear  33 , and left and right side gears  35 ,  37 .  
         [0057]     Pinion shafts  31  are arranged radially to the axis of differential housing  7 A. Respective pinion shafts  31  have ends linked to differential housing  7 A. Ring  50  engages pinion shaft  31  and is fixed by a snap ring. The ring  50  stops the rotation or displacement of pinion shaft  31 .  
         [0058]     Pinion gears  33  are rotatably supported on pinion shafts  31 . Differential housing  7 A and pinion gears  33  have spherical washers  41  interposed therebetween, which receive a centrifugal force from pinion gears  33  and interlocking reactive force from side gears  35 ,  37 .  
         [0059]     Side gears  35 ,  37  are meshed with pinion gears  33 , respectively. Respective side gears  35 ,  37  and differential housing  7 A have thrust washers  43  interposed therebetween, for receiving interlocking reactive force from respective side gears  35 ,  37 .  
         [0060]     Side gears  35 ,  37  are spline linked to left and right drive shaft  2121 ,  2123  respectively. Respective drive shafts pass outward through casing  15 , linking with respective left and right rear wheels, using joints.  
         [0061]     Between respective drive shaft  2121 ,  2123 , and the joints or casing  15 , oil seals  45  for the preventing an oil form leaking out are interposed.  
         [0062]     The drive force of the motor for the rotation of ring gear  5 , as described later, is transmitted to differential housing  7 A via clutch system  13 A. The rotation of differential housing  7 A is distributed to respective side gears  35 ,  37 , using pinion gears  33 . In addition, the transmission of the rotation of the drive shafts to the left and right wheels causes vehicle to be in four-wheel drive. This remarkably improves the escape and run property on bad roads, the starting, and the acceleration property, and the stability of the vehicle body.  
         [0063]     When a difference of the drive resistance of the left and right wheels occurs on a bad road, the rotation of pinion gears  33  distribute the drive force of the motor to the left and right wheels.  
         [0064]     Clutch system  13 A, as shown in  FIG. 3 , is constituted with electromagnet  47  as an operator, multiplate-type primary clutch  49  as a first clutch, pilot clutch  51 A as a second clutch, ball cam  53  as a converter, return spring  55 , and controller  2133 .  
         [0065]     Electromagnet  47 , primary clutch  49 , pilot clutch  51 A, ball cam  53 , and return spring  55  are located coaxially with differential housing  7 A. Primary clutch  49  and ball bearings  9  are arranged in axial alignment with each other.  
         [0066]     Core  57  of electromagnet  47  is fixed to casing  15 , with its lead wire being drawn outside and being connected to battery  2131  and controller  2133  mounted on the vehicle.  
         [0067]     The left end of differential housing  7 A is supported to cover  15   c , using ball bearing  59 . The right end is supported to core  57  (casing  15   b ), using ball bearing  59 . Differential housing  7 A is rotatable relative to electromagnet  47  and casing  15 .  
         [0068]     Rotor  61 A, made of a magnetic material, fixed on the outer periphery of the right boss  77  of the differential housing, using snap ring  177 , thus being axially positioned. Rotor  61 A serves as the right wall of housing  23 .  
         [0069]     Primary clutch  49  is located on the right of ball bearings  9  and between clutch housing  23  and differential housing  7 A. Primary clutch  49  has inner plates  49   a  and outer plates  49   b  which are slid against each other for frictional clutch. Inner plates  49   a  are spline linked to differential housing  7 A. Inner plates  49   a  extend radially outward from differential housing  7 A, being axially spaced each other at a distance therebetween. Outer plates  49   b  are spline linked to clutch housing  23 . Outer plates  49   b  extend radially inward, being interposed between inner plates  49   a.    
         [0070]     Pilot clutch  51 A is located between clutch housing  23  and cam ring  65 . Pilot clutch  51 A has inner plate  51 Aa and outer plates  51 Ab to be slid against each other for frictional clutch. Inner plates  51 Aa are spline linked to cam ring  65 . Inner plates  51 A extend radially outward from cam ring  65 , being spaced at a predetermined distance. Outer plates  51 Ab are spline linked to clutch housing  23 . Outer plates  51 Ab extend radially inward from housing  23 , being interposed between inner plates  51 Aa.  
         [0071]     Ball cam  53  is interposed between cam ring  65  and pressure plate  67 . Pressure plate  67  spline links to differential housing  7 A, thus being axially movable. As described below, pressure plate  67  receives the cam thrust force of ball cam  53  to press down primary clutch  49 .  
         [0072]     Interposed between rotor  61 A and cam ring  65  is thrust bearing  69  which receives the cam reactive force of ball cam  53 .  
         [0073]     Return spring  55  is interposed between pressure plate  67  and differential housing  7 A, biasing pressure plate  67  against the pressure force of primary clutch  49 .  
         [0074]     Ring-shaped armature  73 A is located between pressure plate  67  and pilot clutch  51 A for axial movement. The inner periphery of armature  73 A centers around stepped part  94  of pressure plate  67 .  
         [0075]     Rotor  61 A, inner and outer plates  51 Aa,  51 Ab of pilot clutch  51 A, and armature  73 A constitute the magnetic path of electromagnet  47 . When electromagnet  47  is excited, magnetic loop  95  is generated through the magnetic path.  
         [0076]     Provided between rotor  61 A and core  57  of electromagnet  47  are air gaps  97 ,  99  at a spacing forming a part of the magnetic path.  
         [0077]     Rotor  61 A, as shown in  FIG. 4 , has six arced openings  105  within an angular range θ and with equal radial spacing between the radial outer portions  101  and inner portions  103  as two separate magnetic paths. Provided between respective openings  105  are bridges  107  joining outer portions  101  and inner portions  103  each other, thus constituting a bridge structure.  
         [0078]     The openings  105 , or the magnetic resistance of air inside openings  105 , magnetically insulates between outer portion  101  and inner portion  103 . This prevents a short in the magnetic path.  
         [0079]     Due to the improvements in prevention from a short in the magnetic path, bridges  107  each have axial recesses formed on both sides thereof, being axially thin, as shown in  FIG. 3 .  
         [0080]     In addition, rotor  61 A has six arced openings  105  within ah angular range θ and at equal angular spacing, formed radially inward of magnetic loop  95 .  
         [heading-0081]     Formed between respective openings  203 , are bridges  205  joining openings  203  to each other.  
         [0082]     The arrangement of the six openings  203  of rotor  61 A in a circular shape causes the outer portion formed with magnetic loop  95  and the inner portion supported on boss  75  of differential housing  7 A to be magnetically insulated due to the magnetic resistance of air in openings  203 . This prevents the leakage of magnetic force (flux) from magnetic loop  95  to inner portion  207 , being different from the conventional art.  
         [0083]     Specifically, the embodiment has portions  203  as a generation base of the leakage of magnetic flux, provided closest to rotor  61 A and core  57 , thus effectively preventing a shorting of the magnetic path.  
         [0084]     Openings  203  of rotor  61 A as an oil path allow the oil in the oil reservoir of the casing to flow in and out therethrough. This improves the lubricant and cooling functions of pilot clutch  51 A, thrust bearing  69 , and ball cam  53 .  
         [0085]     Specifically, an oil is effectively provided to pilot clutch  51 A, radially outside respective openings  203 , under centrifugal force, thus stabilizing the sliding resistance between outer plates  51 Ab and inner plates  51 Aa.  
         [0086]     In the rear differential  1 A, openings  203  prevent magnetic shorts in rotor  61 A, by preventing magnetic leakage from magnetic loop  95 , thus remarkably improving the magnetic efficiency of electromagnet  47 . This reduces the load of the battery and improves the fuel cost of the engine.  
         [0087]     In the rear differential  1 A, as described above, oil path openings  203  improve the lubricant function of pilot clutch  51 A. The stabilization of the cam thrust force of ball cam  65 , resulting from the engagement torque of pilot clutch  51 A, remarkably improves the control accuracy of the engagement torque of primary clutch  49  (the engine drive force to be transmitted to the rear wheels) and the durability of pilot clutch  51 A.  
         [0088]     The inner peripheries  51  Ab 1  of outer plates  51 Ab and cam ring  53  are spaced from each other to define a space  115  therebetween. The outer peripheries  51 Aa 1  of inner plates  51 Aa and housing  23  are spaced from each other to define a space  117  therebetween. Housing  23  and the outer periphery  73 Aa of armature  73 A are spaced from each other to define space  119  therebetween. Respective spaces  115 ,  117 ,  119  also contribute the prevention of the short in the magnetic path.  
         [0089]     The lower portion of housing  23  is immersed in the oil reservoir provided to the casing. The oil flows from spaces  115 ,  117 ,  119  to pilot clutch  51 A, the slide portion of armature  73 A and pressure plate  67 , ball cam  53 , thrust bearing  69 , primary clutch  49 , and ball bearings  9 , thus lubricating them.  
         [0090]     The oil flows in differential housing  7 A through spiral oil channels  79 ,  81 , with the rotation of the casing. The oil lubricates and cools the meshing portion of respective gears and spherical washers  41 . The oil receives centrifugal force to flow through the openings to primary clutch  49 . The oil lubricates and cools primary clutch  49 , ball bearings  9 , ball cam  53 , pilot clutch  51 A, and thrust bearing  69 . The oil flows out of space  115 ,  117 ,  119  and opening  85  to return to the oil reservoir.  
         [0091]     Ball bearings  9  are lubricated and cooled by the oil splash caused the rotation of ring gear  5 A.  
         [0092]     Coil  87  of electromagnet  47  is cooled by an oil, its property being stabilized. The heat of coil  87  heats the oil in the oil reservoir, the peripheral pilot clutch  51 A and ball cam  53 .  
         [0093]     The controller conducts the excitation of electromagnet  47 , the control of excited electric current, and the stop of exciting (demagnetization). The exciting and the stop of exciting cause the motor to be rotated and stopped, respectively.  
         [0094]     The rotation of electric motor  2129  causes electromagnet  47  to be excited. When electromagnet  47  is excited, armature  73 A is attracted to press against and engage with pilot clutch  51 A.  
         [0095]     When pilot clutch  51 A is engaged, pilot clutch  51 A applies the drive force of motor  2129  to ball cam  53  via cam ring  65  and pressure plate  67 . While amplifying the drive force, ball cam  53  converts the drive force into a cam thrust force, for the pressing and engaging of primary clutch  49 , using pressure plate  67 .  
         [0096]     When clutch system  13  is engaged, as described above, the rotation of ring gear  5  is transmitted to differential housing  7 A. The differential mechanism  11  distributes the rotation to the left and right wheels, causing the vehicle to be in four-wheel drive.  
         [0097]     When the excited current is controlled, the change of the slide of pilot clutch  51 A causes the change of the cam thrust force of ball cam  53 , thus controlling the drive force to be transmitted to the rear wheels.  
         [0098]     The control of the drive force, for example, during turning, significantly improves the turning property and stability.  
         [0099]     When electromagnet  47  is demagnetized, the disengagement of pilot clutch  51 A causes the disappearance of the cam thrust force of ball cam  53 . The biasing force of return spring  55  returns pressure plate rightwardly, causing primary clutch  49  to be disengaged. The disengagement of clutch system  13 A causes the vehicle to be in two wheel drive with the front wheel drive, using the engine.  
         [0100]     At this time, controller  2133 , as described above, stops the rotation of electric motor  2129 .  
         [0101]     When the vehicle starts, controller  2133  causes motor  2129  to be rotated and clutch system  13 A to be engaged for four-wheel drive. The drive force of the engine and electric motor reinforce the drive force, improving starting and acceleration properties.  
         [0102]     When the speed of vehicle reaches a predetermined value such as 20 km/h, rendering the drive force of the electric motor unnecessary, controller  2133  stops the rotation of motor  2129 . This causes clutch system  13 A to be disengaged, thus putting the vehicle in two-wheel drive.  
         [0103]     The controller causes the vehicle to be in four-wheel drive when climbing a slope. This reinforces the drive force of the vehicle.  
         [0104]     If roll back phenomenon, that is the skidding of front wheels causing the backward movement of the vehicle, occurs when climbing of slope, the controller stops the rotation of the electric motor, thus disengaging clutch system  13 A.  
         [0105]     The disengagement of clutch system  13 A causes the rear wheels to be in drag rotation, and the electric motor  2129  to separate from the rear wheels. The motor is released from forced rotation due to the rotation of the rear wheels (positive rotation during forward movement drive or reverse rotation during roll back).  
         [0106]     When, without relation to a predetermined speed after the aforementioned starting, drive torque during drive is enlarged, the engagement of clutch system  13 A due to the rotation of the electric motor further improves the drivability over a step or a recess and the acceleration property of the vehicle.  
         [0107]     According to the embodiment, rear differential  1 A, as described above, has ball bearings  9  interposed between ring gear  5 A and differential housing  7 A. Thus, during two-wheel drive where clutch system  13 A, ring gear  5 A and differential housing  7 A do not directly contact, allowing rotational resistance to be remarkably small.  
         [0108]     In the embodiment, clutch system  13 A is located between the inner periphery of ring gear  5 A and the outer periphery of differential housing  7 A. The left and right drive shafts are supported only by differential housing  7 A. Thus, in contrast to the conventional art, there is no necessity for the left and right drive shafts to be supported by ring gear  5 A, and the absence of slide-contact between them in two-wheel drive allows a rotational resistance to be significantly smaller.  
         [0109]     The overlapping of the gear part  5 Aa of ring gear  5 A and ball bearings  9  at an axial position allows the interlocking reactive force of ring gear  5 A to be supported on ball bearings  9 . This prevents any galling and seizing of ring gear  5 A and differential housing  7 A.  
         [0110]     The absence of slide-contact between ring gear  5 A and differential housing  7 A and at the supporting portion of the drive shafts prevents galling and seizing.  
         [0111]     Thus, ring gear  5 A and differential housing  7 A do not interlock due to galling and seizing, and the separating function of the rear wheels during two-wheel drive is ensured. This prevents the lowering of fuel-cost resulting from drive resistance due to the drag rotation of the rear-wheel drive system and restricts galling and seizing around the drive shafts. Thus, this improves the differential function of differential mechanism  11  and the turning and steering properties of the vehicle.  
         [0112]     Galling and seizing do not generate around the drive shafts. In contrast to the conventional art, a specifically high level of an oil in the casing (casing  15 ) is unnecessary, thus minimizing the amount of a sealed oil.  
         [0113]     Even if oil seal  45  is damaged, oil does not leak out, thus retaining the advantage even in a failure mode.  
         [0114]     The reduction of the amount of an oil allows the lightening of reduction mechanism  3  and rear differential  1 A, thus resulting in lower production cost.  
         [0115]     The small rotational resistance between ring gear  5 A and differential housing  7 A, the absence of a slide resistance around the drive shafts, and the small amount of an oil as a rotational resistance (agitating resistance) of each rotational member allow drag torque to be significantly small. This improves fuel-cost and turn properties due to the drag torque.  
         [0116]     In the embodiment, the four-wheel drive vehicle, using the electric motor as an auxiliary drive force, does not lock due to galling and seizing. If, during two-wheel drive or when climbing a slope, roll-back phenomenon occurs, clutch system  13 A allows the electric motor to be securely separated from the rear wheels. This electromotive force prevents the application of a large load to the battery, the alternator, or the elements of the control circuit.  
         [0117]     Thus, these functions are maintained and durability improves remarkably.  
         [0118]     The absence of locking causes the rotation of the rear wheels not to force the rotation of the electric motor. This reduces the load, the temperature rise applied to the coil at a rotor or a magnetic field and the load applied to the bearings. This remarkably improves the durability of the electric motor.  
         [0119]     In the brush-type electric motor, the improvement of the durability of a brush reduces the number of replacement of the brush. This reduces a maintenance cost remarkably.  
         [0120]     Without drag torque causing the electric motor to be mechanically rotated, the battery, alternator, and a circuit elements are protected, and the durability of the electric motor is improved.  
         [0121]     In the embodiment, rear differential  1 A has clutch system  13 A on the outer periphery of differential housing  7 A. In contrast to the conventional art where the outer periphery of an outer casing (boss) and the inner periphery of an inner casing have a multiplate-clutch located therebetween, the enlargement of the size and torque volume of clutch system  13 A allows the transmission of large torque.  
         [0122]     The arrangement of ball bearings  9  and clutch system  13 A in axial alignment with each other allows them to be small-sized. Thus, the interference of casing  15  with the second shaft  319  of reduction mechanism  3  is prevented, thus improving the equipability of a vehicle and enlarging the load clearance of a vehicle body.  
         [0123]     The dimensional allowance due to the small-sization allows clutch system  13 A to be further large-sized, thus enlarging its torque volume.  
         [0124]     The large-sization of clutch system  13 A reduces the load to be applied to the frictional face due to the enlargement of torque at the identical volume, thus improving durability.  
         [0125]     The amplification of the pressing force against primary clutch  49  by ball cam  53  causes primary clutch  49  to obtain a sufficient clutch volume even at a small-size and a light weight. This allows a sufficient drive force to be transmitted to rear wheels.  
         [0126]     The provision of ball cam  53  for the amplification of the pressing force of primary clutch  49  allows the clutch system to be small-sized, compared to one of identical volume without the amplifying mechanism. This results in an even more compact rear differential  1 A, thus improving the equipability of the vehicle.  
         [0127]     The primary clutch  49  is warmed due to the heat of electromagnet  47  (coil  87 ). This, when clutch system  13 A is disengaged, allows for the reduction of the drag torque of the rear wheels generated due to the viscosity of an oil at a low temperature. This reduces the loss in the drive force of the engine, thus improving fuel cost.  
         [0128]     The multi-plate type primary clutch  49  and pilot clutch  51 A are employed in clutch system  13 A. This prevents the generation of a ratchet sound from the dog clutch allowing for a high silent property, and a release from shock and shock sound during engagement and disengagement.  
         [0129]     Clutch system  13 A using multi-plate type primary clutch  49  and pilot clutch  51 A does not require the synchronization of the rotation during engagement and disengagement. The lack of necessity of a synchronization mechanism allows rear differential  1 A to be light and compact at a low production cost.  
         [0130]     Rear differential  1 A, as described above, has rotor  61 A supported by differential housing  7 A located inside of but not being supported by housing  23 .  
         [0131]     Ball cam  53 , as shown in  FIG. 1 , is located close to a rotational axis. The distance L2 from the functional point receiving its cam thrust force to the support point (fulcrum) of the rotor is significantly shorter than the conventional one. This allows the torque generated by the cam thrust force to be reduced.  
         [0132]     Thus, the necessary strength of rotor  61 A is small, allowing lightening.  
         [0133]     The small load of rotor  61 A allows openings  105  and bridges  107  between radial outer and inner parts  101 ,  103  to be alternately formed as a bridge structure. The result causes rotor  61   a  to be one piece structure, thus, in contrast to the three-piece structure of the conventional rotor, allowing lightening at a low production cost.  
         [0134]     The space  115  between outer plates  51 Ab of pilot clutch  51 A and cam ring  65 , the space  117  between inner plates  51 Aa and clutch housing  23 , and the space  119  between armature  73 A and clutch housing  23  cause the magnetic loss of electromagnet  47  to be small and the attractive force of armature  73 A to be strong. This improves the operational response of clutch system  13 A.  
         [0135]     In accordance with the small loss of magnetic force, electromagnet  47  becomes small-sized, thus improving the fuel cost of the engine.  
         [0136]     The spaces  115 ,  117 ,  119  as oil passages improve the lubricating and cooling properties of pilot clutch  51 A, ball cam  53 , and primary clutch  49 .  
         [0137]     Armature  73 A and clutch housing  23  have space  119  provided therebetween, reducing the leakage of magnetic force toward clutch housing  23 . This allows the omission of a leakage preventing member of magnetic flux such as a non-magnetic member welded to a differential housing (for the prevention of the leakage of magnetic flux) according to the conventional art. Thus, the structure of the housing is simplified and costs are kept low.  
         [0138]     The aforementioned embodiment shows an example adapted to the differential of the four-wheel drive vehicle which is constituted with the engine as a primary drive source and the electric motor as an auxiliary drive source. However, without being limited to the adapted example, the differential of the invention is also preferably employed, to the drive wheels of four-wheel drive vehicle with an engine as a drive force, which are separate during two-wheel drive.  
         [0139]     In this case, this obtains the similar benefits except for the ones of the electric motor.  
         [0140]     In the embodiment, ring gear  5 A, differential housing  7 A, ball bearings  9  are overlapped at an axial position. However, a partial overlap of them at an axial position would obtain a similar function.  
         [0141]     The bearing  9  employs a ball bearing as an example, and, without being limited to this, preferably uses a sliding bearing.  
         [0142]     The operating mechanism of the pilot clutch, without being limited to the electromagnet, preferably employs a fluid-hydraulic actuator such as a oil-hydraulic actuator or an electric motor.  
         [0143]     The main and pilot clutches preferably employ a multi-plate clutch or, for example, a single-plate clutch or corn clutch as a frictional clutch. They are preferably either of a wet or dry type.  
         [0144]     The multi-plate and single-plate clutches employ a steal, a carbon, or a paper as a clutch plate.  
         [0145]     The differential mechanism, without being limited a bevel type, employs, for example, a planetary gear type, a worm gear type, or a differential mechanism where a pinion gear, housed slidably in the housing opening of a differential housing, connects output side gears.  
         [0146]     The differential of the invention, without limiting the constitution (F.R.D) where the clutch system of the embodiment connects or disconnects a drive force, is preferably adapted to the constitution (L.S.D.) where a clutch mechanism limits differential motion.  
         [0147]     In the L.S.D., an internal rotational member as a differential rotational member such as side gears and the arrangement of the primary clutch between a torque transmission member and an internal rotational member obtains a differential limiting function for limiting a differential motion of a differential mechanism. The supporting of the rotor on the internal rotational member allows for adaptation of the invention.  
         [0148]     The differential of the embodiment is preferably employed to a front differential adapted to the four-wheel drive vehicle where front wheels separate from a drive source during two-wheel drive.  
         [0149]     Second Embodiment  
         [0150]     As shown in  FIG. 5 , spring pins  39  fasten pinion shaft  31  to differential housing  7 A. In rear differential  1 B, the right end of differential housing  7 A has a rotor  61 B of magnetic material as a side wall. The rotor  6 B is spline linked to clutch housing  23 , being axially positioned by snap ring  63  fixed to the inner periphery of housing  23 .  
         [0151]     Rotor  61 B constitutes part of the magnetic circuit of electromagnet  47 . Rotor  61 B and core  57  have air gap G 1  at a predetermined width as a part of the magnetic circuit, provided therebetween. Rotor  61 B has ring  71  of stainless steel (non-magnetic material) which magnetically breaks off between the radial outer and inner portions, thus preventing magnetic short circuit.  
         [0152]     Pressure plate  67  and pilot clutch  51 B have an axially movable armature  73 B provided therebetween.  
         [0153]     Third Embodiment  
         [0154]     As shown in  FIG. 6 , rear differential  1 C is constituted with: housing  150  (torque transmission member); differential housing  7 B located radially inward of the housing  150 ; bevel gear type differential mechanism  11 ; clutch system  13 A; rotor  61 A constituting a part of system  13 A.  
         [0155]     Rear differential  1 C is housed in casing  15 . Casing  15  has an oil reservoir therein.  
         [0156]     Housing  150  is constituted with ring gear  5 B and clutch housing  23 . Clutch housing  23  is press manufactured, being welded to ring gear  5 B.  
         [0157]     Ring gear  5 B is supported to differential housing  7 B, using large-sized and small-sized ball bearings  217 ,  219 . Ring gear  5 B has helical gear  5 Ba to be meshed with, for example, the mating helical gear connected to the propeller shaft of the rear wheels.  
         [0158]     Housing  150  transmits a torque from ring gear  5 B, resulting in a floating structure, which is released from the supporting function of a member.  
         [0159]     Ring gear  5 B gives axial rightward interlocking thrust force to housing  150  due to its helix angle during a forward drive of the vehicle, while giving axial leftward interlocking thrust force during the backward drive.  
         [0160]     Outer race  221  of ball bearing  217  is positioned axially leftward on the stepped part  223  of ring gear  5 B. Inner race  225  is positioned axially rightward on stepped part  228  of differential housing  7 B.  
         [0161]     Outer race  229  of ball bearing  229  is positioned radially and rightwardly on stepped part  231  of ring gear  5 B. Inner race  233  thereof is positioned on snap ring  237  mounted to left boss  235  of differential housing  7 B.  
         [0162]     Snap ring  237  has an adequate strength for a sufficient positioning function and for self-destruction upon receiving more than a predetermined thrust force.  
         [0163]     The left boss  275  of differential housing  7 B is supported to casing  15 , using ball bearing  59 . Right boss  277  is supported to casing  15 , using ball bearing  59  and core  57 .  
         [0164]     Primary clutch  49  is interposed between housing  150  (member  23 ) and differential housing  7 B. Outer plates  49   b  thereof are linked to spline  281  provided on the inner periphery of clutch housing  23 . Inner plates  49   a  thereof are linked to spline  285  provided on the outer periphery of differential housing  7 B.  
         [0165]     Pilot clutch  51 A is interposed between clutch hosing  23  and cam ring  65 . Outer plates  51 Ab thereof are linked to spline  281  of clutch housing  23 . Inner plates  51 Aa thereof are linked to spline  291  provided to the outer periphery of cam ring  65 .  
         [0166]     Spline  281  is manufactured when clutch housing  23  is manufactured, passing through clutch housing  23  and reaching its right end.  
         [0167]     Rotor  61 A and pilot clutch  51 A have washer  109  interposed therebetween for the improvement of the abutting of pilot clutch  51 A against rotor  61 A formed with opening  105 . Washer  109  is mounted to rotor  61 A, with its three claws being bent in recess  113  formed to the outer periphery of rotor  61 A.  
         [0168]     When, for example, a gear box or a bearing is seized between the engine and rear differential  1 B, the drive rotation of the rear wheels causes ring gears  5 B of housing  150  to be rotated, leading the mating helical gear.  
         [0169]     In this state, the direction of the torque, to be transmitted between ring gear  5 B and the mating helical gear, is identical to one of rear drive. As mentioned above, the meshing of the helical gears generates thrust force for the movement of housing  150  leftwardly.  
         [0170]     As mentioned above, snap ring  237  for positioning of ball bearing  219  is adjusted at an adequate strength. The receiving of the thrust force through ball bearing  219  causes the destruction of snap ring  237 , the leftward movement of housing  150 . The movement causes outerplates  51 Ab to be separated from spline  281  of clutch housing  281 .  
         [0171]     When outer plates  51 Ab is separated from spline  281 , similar to the disengagement of pilot clutch  51 A, the disappearance of the cam thrust force of ball cam  53  causes primary clutch  49  to be disengaged, thus separating the rear wheels.  
         [0172]     Thus, even when seizing occurs in the engine when in four-wheel drive, the rear wheels are automatically separated. The receiving of the rotation of the rear wheels does not deteriorate the damaged seizing portion, thus improving a failure mode.  
         [0173]     When clutch system  13 A is disengaged (two-wheel drive mode), inner plates  51 Aa of pilot clutch  51 A, pressure plate  67 , armature  73 A, cam ring  65  (ball cam  53 ), thrust bearing  69 , and rotor  61 A rotate together with differential housing  7 B. Pilot clutch  51 A and outer plate  51 Ab rotate together with housing  150 .  
         [0174]     With the constitution, when outer plates  51 Ab are located facing armature  73 A, during two-wheel drive, the drive force is transmitted from outer plates  51 Ab to armature  73 A due to the friction therebetween. This causes the rear wheels to be dragged, thus lowering a fuel cost due to energy loss. The rear differential  1 C, however, is arranged of facing inner plates  51 Aa and armature  73 A. No transmission of drive force due to friction prevents the drag of the rear wheels or the lowering of fuel cost.  
         [0175]     If rotor  61 A is supported by housing  150 , during a two-wheel drive, the rotation force of cam ring  65  of differential housing  7 B relative to rotor  61 A of housing  150  is applied to thrust bearing  69 , thus reducing durability. However, in rear differential  1 C, in which rotor  61 A is supported to differential housing  7 B, thrust bearing  69  is released from the relative rotation, thus preventing the lowering of durability.  
         [0176]     The support of rotor  61 A by differential housing  7 B causes housing  150  and clutch housing  23  to be separated from each other. Without the necessity of supporting rotor  61 A, housing  150  is also released from the supporting of a member located inside thereof, thus allowing reduction of strength and lightening.  
         [0177]     Housing  150 , released from the role of supporting member, becomes a floating structure. This reduces the need for a manufacture accuracy, thus allowing clutch housing  23  to be press manufactured.  
         [0178]     Thus, comparing to the conventional art in which a differential housing is cut-manufactured in high accuracy after forging or molding, rear differential  1 C becomes remarkably light and costs remarkably low.  
         [0179]     The entire contents of Japanese Patent Applications P2000-211544 (filed Jul. 12, 2000), P2000-319911 (filed Oct. 19, 2000), and P2001-74746(filed Mar. 15, 2001) are incorporated herein by reference.  
         [0180]     While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.