Abstract:
A power transfer apparatus including a clutch having a biasing device for biasing the clutch piston in a direction in which the clutch discs engage with the clutch plates, a transmission brake disposed in line in an axial direction of the clutch, an actuator disposed in line in an axial direction of the transmission brake for disengaging the clutch at the same time of applying the transmission brake, and a planetary carrier sub-assembly disposed in line in the axial direction of the clutch, wherein the power transfer apparatus provided between an input shaft and an output shaft which are accommodated within a casing for selectively changing the speed of the output shaft.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a power transfer apparatus for secondary or coupled driving wheels of a four-wheel drive vehicle. 
   2. Description of the Related Art 
   In conventional four-wheel drive vehicles, in turning a corner of a small turning radius at low or middle speed in a four-wheel driving mode, there is caused a difference in wheel speed between front and rear wheels which is attributed to a difference in turning radius between the front and rear wheels, resulting in the occurrence of a tight corner braking phenomenon. 
   Front and rear wheels driving systems disclosed in JP-B-7-61779 and JP-B-7-64219 are known as related arts for solving the problem of tight corner braking phenomenon. 
   In the front and rear wheels driving systems disclosed in the Japanese Examined Patent Publications, an average wheel speed of secondary driving wheels relative to an average wheel speed of primary driving wheels is adjusted by providing a transfer (a speed increasing system) between the primary and secondary driving wheels. 
   In this transmission, by switching on and off a direct connective clutch and a transmission clutch, there occurs a switch between a direct connecting mode in which the average wheel speed of the primary driving wheels and the average wheel speed of the secondary driving wheels are almost equalized and a speed increasing mode in which the average wheel speed of the secondary driving wheels is made larger than the average wheel speed of the primary driving wheels. 
   In this front and rear wheels driving system, when turning a small corner in the four-wheel driving mode, the occurrence of tight corner braking phenomenon is prevented by bringing the secondary driving wheels in the speed increasing mode by the transmission. 
   Incidentally, in the aforesaid transfer (the speed increasing apparatus) for four-wheel drive vehicles, at least two hydraulic or electromagnetic actuators are required as a power source for operating the direct connective clutch and the transmission clutch, and this leads to a problem that the transmission itself is made larger in size and hence heavier in weight. 
   Furthermore, in this transmission, since constituent components are assembled piece by piece in the assembling processes of the transmission, the number of processes is increased and hence the productivity is deteriorated. In addition, since there are many items needing adjustments such as clearance and spring load, the productivity is also deteriorated. 
   SUMMARY OF THE INVENTION 
   Consequently, an object of the present invention is to provide power transfer apparatus that can be made smaller in size and lighter in weight overall. 
   According to the first aspect of the present invention, there is provided a power transfer apparatus provided between an input shaft and an output shaft for selectively changing the speed of the output shaft relative to the speed of the input shaft. 
   The power transfer apparatus system includes a clutch for directly connecting the input shaft to the output shaft, a transmission brake serially disposed in an axial direction of the clutch, an actuator serially disposed in an axial direction of the transmission brake for disengaging the clutch at the same time of activating the transmission brake and a planetary carrier sub-assembly serially disposed in the axial direction of the clutch. 
   The clutch has a clutch inner hub fixed to the input shaft, a clutch guide, a plurality of clutch discs attached to the clutch inner hub, a plurality of clutch plates attached to the clutch guide so as to be disposed alternately with the clutch discs, a clutch piston and a biasing unit for biasing the clutch piston in a direction in which the clutch discs engage with the clutch plates. 
   The transmission brake has a brake inner hub coupled with the clutch piston at one end thereof, a plurality of brake discs attached to the brake inner hub and a plurality of brake plates attached to the casing so as to be disposed alternately with the brake discs. 
   The planetary carrier sub-assembly has a planetary carrier rotatably disposed around the input shaft and the output shaft and coupled with the clutch guide, a first pinion gear rotatably carried on the planetary carrier, a second pinion gear having the number of teeth which is different from that of the first pinion gear, a first sun gear fixed to the input shaft and meshing with the first pinion gear and a second sun gear fixed to the output shaft and meshing with the second pinion gear. 
   According to the power transfer apparatus of the first aspect of the present invention, since the single actuator can be used to disengage the clutch at the same time of activating the transmission brake, it is possible to provide a power transfer apparatus which is made smaller in size and light in weight. In addition, since the clutch and the transmission brake are serially disposed in the axial direction, the outside diameter of the power transfer apparatus can be made small. 
   Furthermore, when the actuator is activated, since the engagement of the clutch is gradually released while the transmission brake is gradually activated, the transfer of power from the input shaft to the output shaft is never interrupted, and the occurrence of a gearshift shock can be prevented. 
   Note that in the above mentioned power transfer apparatus, the first and second pinion gears may be formed integrally. 
   According to the second aspect of the present invention, the power transfer apparatus wherein the clutch further has a one-way clutch interposed between the clutch inner hub and the clutch guide. 
   Thus, since the one-way clutch is interposed between the clutch inner hub and the clutch guide, a load generated when the input shaft is connected to the output shaft can be partly borne by the one-way clutch, and therefore the load capacity of the clutch can be reduced. 
   According to the third aspect of the present invention, there is provided a power transfer apparatus as set forth in the first aspect of the present invention, wherein the clutch piston is disposed within a space defined between the clutch guide and a radial outer side of the clutch plates so as to be extend axially. 
   According to the construction, even if a power transfer member such as the clutch inner hub is disposed on the inner diameter side, the (on and off) control of the clutch can be implemented on the outside diameter side. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a power train of a four-wheel drive vehicle which is suitable for application of a power transfer apparatus according to the present invention; 
       FIG. 2  is a cross-sectional view of the power transfer apparatus (transmission) and a rear differential according to an embodiment of the present invention; 
       FIG. 3  is an enlarged cross-sectional view of the power transfer apparatus according to the embodiment of the present invention; 
       FIG. 4  is a front view of a clutch guide; 
       FIG. 5  is a cross-sectional view taken along the line  5 — 5  in  FIG. 4 ; 
       FIG. 6  is a rear view of the clutch guide; 
       FIG. 7  is a front view of a clutch piston; 
       FIG. 8  is a cross-sectional view taken long the line  8 — 8  in  FIG. 7 ; 
       FIG. 9  is a front view of a one-way clutch; 
       FIG. 10  is a cross-sectional view taken along the line  10 — 10  in  FIG. 9  showing a state in which the one-way clutch is mounted on a clutch inner hub; 
       FIG. 11  is a cross-sectional view taken along the line  11 — 11  in  FIG. 3 ; 
       FIG. 12  is a cross-sectional view taken along the line  12 — 12  in  FIG. 3 ; 
       FIG. 13  is a front view of the brake inner hub; 
       FIG. 14  is a cross-sectional view taken along the line  14 — 14  in  FIG. 13 ; 
       FIG. 15  is a cross-sectional view showing a process for assembly of an oil pump driving pin; 
       FIG. 16  is a cross-sectional view showing a process for assembly of an oil pump sub-assembly; 
       FIG. 17  is a cross-sectional view showing a process for assembly of a planetary carrier sub-assembly; 
       FIG. 18  is a cross-sectional view showing a process for assembly of a direct connective clutch sub-assembly; 
       FIG. 19  is a cross-sectional view showing processes for inserting an input shaft and measuring N 1 , N 2 ; 
       FIG. 20  is a cross-sectional view showing a process for measuring dimensions S 1 , S 2  of a front case sub-assembly; 
       FIG. 21  is a cross-sectional view showing a process for assembly of the front case sub-assembly; 
       FIG. 22  is a cross-sectional view showing a process for assembly of a companion flange; and 
       FIG. 23  is an explanatory view showing a state in which a four-wheel drive vehicle is turning left. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , there is shown a schematic view of a power train for a wheel-drive vehicle built based on a front-engine, front-drive (FF) vehicle to which a speed increasing apparatus (a power transfer apparatus) according to the present invention. 
   What should be noticed here is that the present invention is not limited to a four-wheel drive vehicle which is built based on the FF vehicle but may be applied to a four-wheel drive vehicle built based on a rear-engine, rear-drive (RR) vehicle or a front-engine, rear-drive (FR) vehicle. 
   As shown in  FIG. 1 , a power train according to an embodiment according to the present invention mainly includes a front differential  6  and a rear differential  12  and a power transfer apparatus of the present invention. The power or drive from an engine  2  disposed at the front of the vehicle is transmitted from an output shaft  4  a of a transmission  4  to the front differential  6 . A power transfer apparatus or a speed increasing apparatus (a transmission)  10  according to the present invention to which the power so transmitted to the front differential  6  is then transmitted via a propeller shaft  8 . The power from the speed increasing apparatus is  10  transmitted to the rear differential  12 . 
   The front differential  6  has a conventionally known construction in which power from the output shaft  4  a of the transmission  4  is transmitted to left and right front drive axles  20 ,  22  via a plurality of gears  14  within a differential case  6  a and output shafts  16 ,  18 , whereby left and right front wheels are driven, respectively. 
   As will be described later on, the rear differential  12  includes a pair of planetary gear sets and a pair of electromagnetic actuators adapted for controlling the application of multi-plate brake mechanisms respectively, and left and right rear wheels are driven by virtue of power transmitted to left and rear wheel drive axles  24 ,  26  by controlling the electromagnetic actuators. 
     FIG. 2  shows a cross-sectional view of the speed increasing apparatus  10  of the present invention and the rear differential  12  disposed on a downstream side of the speed increasing apparatus  10 . The speed increasing apparatus  10  includes an input shaft  30  rotatably mounted in a casing  28  and an output shaft (hypoid pinion shaft)  32 . 
   The speed increasing apparatus  10  includes further an oil pump sub-assembly  34 , a planetary carrier sub-assembly  38 , a (directly connective) clutch sub-assembly  40 , and a transmission brake  42 . 
   The rear differential  12  disposed on the downstream side of the transmission  10  has a hypoid pinion gear  44  formed on a distal end of the hypoid pinion shaft  32 . 
   The hypoid pinion gear  44  meshes with a hypoid ring gear  48 , and power from the hypoid ring gear  48  is inputted into ring gears of a pair of left and right planetary gear sets  50 A,  50 B. 
   Sun gears of the planetary gear sets  50 A,  50 B are rotatably mounted around the left rear axle  24  and the right rear axle  26 , respectively. Planetary carriers of the planetary gear sets  50 A,  50 B are fixed to the left rear axle  24 , and the right rear axle  26 , respectively. A planet gear carried on the planetary carrier meshes with the sun gear and the ring gear. 
   The left and right planetary gear sets  50 A,  50 B are connected, respectively, to brake mechanisms  51  each provided for variably controlling the torque of the sun gear. The brake mechanism  51  includes a wet multi-plate brake  52  and an electromagnetic actuator  56  for actuating the multi-plate brake  52 . 
   Brake plates of the wet multi-plate brake  52  are fixed to a casing  54 , and brake discs thereof are fixed to the sun gear of the planetary gear set  50 A,  50 B. 
   The electromagnetic actuator  56  is made up of a core (yoke)  58 , an electromagnetic coil  60  inserted into the core  58 , an armature  62  and a piston  64  connected to the armature  62 . 
   When current is applied to the electromagnetic coil  60 , the armature  62  is attracted to the core  58  by the coil  60  to thereby generate a thrust. The piston  64  integrally connected to the armature  62  is caused to press against the multi-plate brake  52  by virtue of the thrust so generated, whereby a brake torque is generated. 
   As this occurs, the sun gears of the planetary gear sets  50 A,  50 B are fixed relative to the casing  54 , respectively, and a driving force of the hypoid pinion shaft  32  is transmitted to the left and right rear axles  24 ,  26  via the ring gears, planet gears and planetary carriers of the planetary gear sets  50 A,  50 B. 
   Output torques to the left and right rear axles  24 ,  26  can be variably controlled by varying current applied to the electromagnetic coil  60 . 
   Next, referring to  FIG. 3 , the construction of the speed increasing apparatus  10  will be described in detail. The casing  28  of the speed increasing apparatus  10  is fixed to the casing  54  of the rear differential  12  with a bolt  66 . 
   The oil pump sub-assembly  34  includes a base  68 , an oil pump body  70  and a cover  72 . The cover  72  is fixed to the oil pump body  70  with a bolt  76 . The oil pump sub-assembly  34  is fixed to the casing  54  of the rear differential  12  with a bolt  74 . 
   The oil pump sub-assembly  34  is made up of a trochoidal pump, and the oil pump body  70  has an outer rotor having internal teeth and an inner rotor having external teeth. An oil pump driving pin  36  is fitted in the inner rotor. 
   The planetary carrier sub-assembly  38  includes a planetary carrier  78  rotatably mounted around the input shaft  30  and the output shaft  32  via bearings  80 . 
   The planetary carrier  78  has a shaft  82 , and a small-diameter pinion gear (a first pinion gear)  84  and a large-diameter pinion gear (a second pinion gear)  86  which are both integrally formed around the shaft  82  are rotatably mounted on the planetary carrier  78 . 
   The small-diameter pinion gear  84  meshes with a first sun gear  88  which is fixed to the input shaft  30  with splines  90 ,  92 , whereas the large-diameter pinion gear  86  meshes with a second sun gear  94  which is fixed to the output shaft  32  with splines  96 ,  98 . 
   The direct connective clutch sub-assembly  40  includes a clutch guide  104  which is fixed to the planetary carrier  78  with splines  100 ,  102 .  FIG. 4  is a front view of the clutch guide  104 ,  FIG. 5  is a cross-sectional view taken along the line  5 — 5  in  FIG. 4 , and  FIG. 6  is a rear view of the clutch guide  104 . 
   As shown best in  FIG. 5 , the clutch guide  104  has an outer circumferential clutch guide  104   a , a ring  104   b  welded to the outer circumferential clutch guide  104   a  and an inner circumferential clutch guide  104   c  fixed to the outer circumferential clutch guide  104   a . The inner circumferential clutch guide  104   c  has splines  102 . 
   As shown best in  FIG. 6 , the clutch guide  104  has six protruding portions  122  which protrude in radial direction. Recesses  124  are disposed between the protruding portions  122  and the ring  104   b.    
   Referring to  FIG. 3  again, the direct connective clutch sub-assembly  40  has a clutch inner hub  106  that is fixed to the input shaft  30  with splines  108 ,  110 . A plurality of clutch discs  112  are mounted on an outer circumferential portion of the clutch inner hub  106  so as not to rotate relative to the clutch inner hub  106  but to move in an axial direction of the same. 
   Furthermore, a plurality of clutch plates  114  are mounted on the clutch guide  104  so as not to rotate but to move in the axial direction, and are disposed such that the clutch plates  114  alternate with the clutch discs  112 . 
   A clutch piston  116  is disposed within a space defined between the clutch guide  104  and a radial out side of the clutch discs  112  so as to be extended in the axial direction. As shown in  FIGS. 7 and 8 , the clutch piston  116  has six protruding portions  116   a  each of which extends in the axial direction. 
   Each protruding portion  116   a  has a shoulder  117  on each side of the protruding portion  116  in the vicinity of a distal end thereof. A coil spring  118  is interposed between the clutch guide  104  and the clutch piston  116  for biasing the clutch piston  116  in a direction in which the clutch discs  112  and the clutch plates  114  are brought into engagement with each other. 
   A one-way clutch  120  is interposed between the clutch inner hub  106  and the clutch guide  104  of the direct connective clutch sub-assembly  40 .  FIG. 9  is a front view of the one-way clutch  120 , and  FIG. 10  is a cross-sectional view taken along the line  10 — 10  in FIG.  9 . 
   An outer ring  126  of the one-way clutch  120  is fixed to the clutch guide  104 , and an inner ring  128  thereof is fixed to the clutch inner hub  106 . 
   As shown in  FIG. 9 , the outer ring  126  of the one-way clutch  120  has a plurality of projections  130 , and a recess  132  is imposed between a pair of adjacent projections  130 . 
   The one-way clutch  120  is such as to transmit a torque in one direction when the rotational speed of the input shaft  30  is equal to or larger than the rotational speed of the clutch guide  104  which is located on the output side. 
   Referring to  FIG. 11 , a cross-sectional view taken along the line  11 — 11  in  FIG. 3  is shown. The axially protruding portions  116   a  of the clutch piston  116  are inserted into between the clutch guide  104  and the outer ring  126  of the one-way clutch  120 . 
   Furthermore, the protruding portions  122  of the clutch guide  104  fit in between the pairs of projections  130  on the outer ring  126  of the one-way clutch  120 , respectively, and torque is transmitted from the outer ring  126  of the one-way clutch  120  to the clutch guide  104  at these fitting portions. 
   Referring to  FIG. 12 , a cross-sectional view taken along the line  12 — 12  in  FIG. 3  is shown. The axially protruding portions  116   a  of the clutch piston  116  are inserted into between the clutch plates  114  and the clutch guide  104 . 
   The clutch plate  114  has a plurality of projections  114   a  which are formed on an outer circumferential side thereof, and the protruding portions  122  of the clutch guide  104  fit in between pairs of adjacent projections  114   a , respectively, whereby the clutch plate  114  is mounted on the clutch guide  104  so as not to rotate but to move in the axial direction. 
   Referring back to  FIG. 3 , again, reference numeral  42  denotes a transmission brake, and an end of a brake inner hub  136  of the transmission brake  42  is in engagement with the clutch piston  116 .  FIG. 13  shows a front view of the brake inner hub  136 , and  FIG. 14  shows a cross-sectional view of the brake inner hub  136  taken along the line  14 — 14  in FIG.  13 . 
   As shown in  FIG. 13 , the brake inner hub  136  has six holes  136   a  which are spaced apart from each other in a circumferential direction. The axially protruding portions  116   a  of the clutch piston  116  shown in  FIG. 8  are inserted into the holes  136   a  of the brake inner hub  136 , respectively, whereby the brake inner hub  136  is restricted from moving in an axially rightward direction when the brake inner hub  136  is brought into abutment with the shoulders  117  of the clutch piston  116 . 
   A plurality of brake discs  138  are mounted on the brake inner hub  136  so as not to rotate relative to the clutch inner hub  136  but to move in the axial direction of the same. Furthermore, a plurality of brake plates  140  are mounted on the casing  28  so as not to rotate but to move in the axial direction and are disposed such that the brake plates  140  alternate with the brake discs  138 . An end plate  144  is interposed between the brake inner hub  136  and the rightmost brake disc  138 . 
   Reference numeral  148  denotes a hydraulic piston functioning as an actuator, which activates the transmission brake  42  when moved rightward by virtue of an oil pressure introduced into an oil pressure chamber  152 . The oil pressure so introduced into the oil pressure chamber  152  is supplied from the oil pump sub-assembly  34 . The hydraulic piston  148  is normally biased by a coil spring  150  in a direction in which the application of the transmission brake  42  is released. 
   A companion flange  154  is fixed to the input shaft  30  by means of splines  156 ,  158 . The companion flange  154  is coupled with the propeller shaft  8  shown in FIG.  1 . 
   Next, referring to  FIGS. 15  to  22 , an assembling procedure of the speed increasing apparatus  10  according to the present invention which has been described heretofore will be described. Firstly, as shown in  FIG. 15 , the oil pump driving pin  36  is assembled to the hypoid pinion shaft (the output shaft)  32  of the rear differential  12 , which has not yet been assembled into the casing  54 . 
   Next, as shown in  FIG. 16 , the oil pump sub-assembly  34  is assembled to the rear differential  12 , and the bolt  74  is tightened. As this occurs, the oil pump driving pin  36  fits in the inner rotor which is incorporated in the oil pump main body  70 . 
   Thus, the function of the oil pump itself can be verified by constituting a primary complete body in which the base  68 , the oil pump body  70  and the cover  72  are assembled and installed together with the other associated components to be incorporated as the oil pump sub-assembly  34 . In addition, since the assembled state of the oil pump is maintained during a transportation, the oil pump sub-assembly  34  so assembled as the primary complete body is also effective when assembling oil pumps at a location which is far away from the final assembly line. 
   Next, as shown in  FIG. 17 , the planetary carrier sub-assembly  38  is assembled to a bearing supporting portion  73  (refer to  FIG. 16 ) disposed on the oil pump cover  72 . As this occurs, the splines  98  of the second sun gear  94  and the splines  96  of the hypoid pinion gear shaft  32  come to fit together. 
   Thus, the components to be incorporated such as the pinion gears  82 ,  86 , the sun gears  88 ,  94  and the bearings  80  are fixed by a circlip  81  after the components have been assembled in place onto the planetary carrier  78 , whereby the planetary carrier sub-assembly  38  can be completed as the primary complete body. 
   Consequently, the meshing conditions between the pinion gears  84 ,  86  and the sun gears  88 ,  94  and the thrust clearance can be verified on the planetary carrier sub-assembly  38 . In addition, since the assembled condition of the planetary carrier sub-assembly  38  is maintained during transportation, the planetary carrier sub-assembly  38  thus assembled as the primary complete body is also effective when assembling oil pumps at a location which is far away from the final assembly line. 
   Next, as shown in  FIG. 18 , the direct connective clutch sub-assembly  40  is assembled to splines  100  (refer to  FIG. 17 ) of the planetary carrier  78 . Furthermore, the transmission brake  42  is assembled to the direct connective clutch sub-assembly  40 . 
   Namely, the brake inner hub  136  of the transmission brake  42  is brought into engagement with the piston  116  of the direct connective clutch sub-assembly  40 , and the brake discs and the brake plates are mounted on the brake inner hub  136  in such a manner that the discs and the plates alternate with each other. 
   The clutch torque (which is determined by a set load of the coil spring  118  ) of the (direct connective) clutch  40  needs to be controlled at a certain value. To make this happen, while the clutch discs  112  and the clutch plates  114  need to be installed by selecting thicknesses thereof, performing this process during assembling processes of the whole rear differential  12  leads to a deterioration in working efficiency of the total assembly operation. 
   However, in this embodiment, since the clutch discs  112  and the clutch plates  114  are assembled as a part of the direct connective clutch sub-assembly  40 , the aforesaid process can be separated from the assembling processes of the whole rear differential  12 . In addition, since the assembled condition of the direct connective clutch sub-assembly  40  can be maintained during transportation, this arrangement is also effective when attempting to assemble rear differentials at a location which is far away from the final assembly line. 
   Next, as shown in  FIG. 19 , the input shaft  30  is inserted. As this occurs, the input shaft  30  fits in place in the first sun gear  88  at the splines  90 ,  92 . In addition, the input shaft  30  also fits in place in the clutch inner hub  106  of the direct connective clutch sub-assembly  40  at the splines  108 ,  110 . 
   In this condition, heights N 1 , N 2  in  FIG. 19  are measured. Namely, the height N 1  from the casing  54  to a shim  142  of the rear differential  12  and the height N 2  to the brake plate  140  at the uppermost end of the transmission brake  42  are measured. 
   Next, as shown in  FIG. 20 , dimensions S 1 , S 2  of two locations of a front case sub-assembly  27  which is assembled in a separate process are measured. The front case sub-assembly  27  includes a casing  28 , bearings  146  which rotatably bear the input shaft  30 , the hydraulic piston  148  of the transmission brake  42  and the coil spring  150 . 
   Thickness are selected for the shim  142  and the brake end plate  144  with a view to set a specified clearance from a difference between N 1  and N 2  which were measured in the previous process. 
   The transmission hydraulic piston  148  is incorporated in the front case sub-assembly  27 , and in order to set the clearance of the transmission brake, the dimension S 2  from a mating surface between the casing  28  of the transfer apparatus  10  and the casing  54  of the rear differential  12  to an end surface of the hydraulic piston  148  needs to be measured. 
   Performing this measuring process during the assembling processes of the whole rear differential  12  leads to the deterioration in working efficiency of the total assembly operation. However, since the front case sub-assembly according to the embodiment is not affected by dimensions of the peripheral components, this measuring process can be separated from the assembling processes of the whole rear differential  12 . 
   Next, as shown in  FIG. 21 , the front case sub-assembly  27  is assembled. An inner race of the bearing  146  is press fitted over the input shaft  30 , and the casing  28  is fastened to the casing  54  of the rear differential  12  with screws  66 . As this occurs, the brake clearance of the transmission brake  42  and the axial clearance of the respective components incorporated in the casing  28  become specified values. 
   Lastly, as shown in  FIG. 22 , the companion flange  154  is assembled onto the input shaft  30 . Namely, the companion flange  154  is fixed onto the input shaft  30  by fitting the splines  156 ,  158  together. 
   The operation of the speed increasing apparatus  10  and the rear differential  12  according to the embodiment that have been described heretofore will be described below. 
   The transmission brake  42  is in an OFF mode in which no oil pressure is introduced to the oil chamber  152  of the hydraulic piston  148 , the direct connective clutch  40  is engaged by virtue of the biasing force of the coil spring  118 . 
   Thus, the input shaft  30  and the planetary carrier  78  are connected together via the direct connective clutch  40  and the one-way clutch  120 , whereby the planetary carrier  78  encompassing the pinion gears  84 ,  86  and the first and second sun gears  88 ,  94  rotate together. Namely, these constituent components rotate as a block or unit. 
   As this occurs, the pinion gears  84 ,  86  do not rotate on their axes but rotate together with the input shaft  30  and the output shaft  32 . Namely, power inputted from the companion flange  154  is outputted to the output shaft (hypoid pinion shaft)  32  as it is. 
   In the event that the left and right electromagnetic coils  60  of the rear differential  12  are switched off in this direct connecting mode, since the respective brake mechanisms  51  are not activated, the respective sun gears of the planetary gear sets  50 A,  50 B idly rotate around the left and right rear axles  24 ,  26 . 
   Consequently, the driving force (torque) of the hypoid pinion gear  32  is not transmitted to the left and right rear axles  24 ,  26  at all. In this case, the rear wheels spin, and all the driving force is directed to the front wheels, whereby the vehicle operates as a two-wheel drive vehicle. 
   In the event that a predetermined amount of current is conducted to the left and right electromagnetic coils  60  so that the left and right multi-plate brakes  52  are fully applied via the pistons  64 , the sun gears of the planetary gear sets  50 A,  50 B are fixed to the casing  54 , respectively. 
   Thus, the driving force of the hypoid pinion shaft  32  is transmitted to the left and right rear axles  24 ,  26  via the ring gears of the planetary gear sets  50 A,  50 B, the planet gears and the planet carriers. 
   Consequently, the driving force of the input shaft  30  is equally divided and is then transmitted to the left and right rear axles  24 ,  26 . As a result, the four-wheel drive vehicle is put in the four-wheel drive mode to thereby be allowed to drive straight ahead. 
   On the other hand, when turning a corner having a small turning radius in the four-wheel drive mode in low and middle speed ranges, an oil pressure is introduced to the oil chamber  152  of the transmission brake  42  so as to push the hydraulic piston  148  in the rightward direction to thereby activate the transmission brake  42 . 
   At the same time as this occurs, the brake inner hub  136  of the transmission brake  42  pushes the clutch piston  116  of the direct connective clutch  40  in the rightward direction against the biasing force of the coil spring  118 , so that the engagement of the direct connective clutch  40  is released. 
   By this operation, the clutch guide  104  is fixed to the casing  28  via the transmission brake  42 , and the planetary carrier  78  coupled with the clutch guide  104  is then fixed to the casing  28 . 
   Even with the planetary carrier  78  being fixed to the casing  28 , the small-diameter pinion gear  84  and the large-diameter pinion gear  86  which are held within the planetary carrier  78  can still rotate, and in this condition, the planetary carrier sub-assembly  38  part becomes a gear train having a certain gear ratio, whereby a change in speed is established between the input shaft  30  and the output shaft (hypoid pinion shaft)  32 . 
   Here, setting the number of teeth (N 1 ) of the sun gear  88 , the number of teeth (N 2 ) of the small-diameter pinion gear  84 , the number of teeth (N 3 ) of the large-diameter pinion gear  86  and the number of teeth (N 4 ) of the sun gear  94  to establish the following relationship among them, an increase in speed is established between the input shaft  30  and the output shaft  32 . 
                 N1   N2     ·     N3   N4       &gt;   1.0           [     Equation   ⁢           ⁢     No   .           ⁢   1       ]             
 
   In this embodiment, the numbers of teeth of the respective pinion gears  84 ,  86  and the first and second sun gears  88 ,  94  are set so that an increased speed ratio becomes 1.07. 
   Assume that the vehicle turns left as shown in  FIG. 23  in a state in which the rotational speed of the output shaft (the hypoid pinion shaft)  32  is made larger than that of the input shaft  30 . As this occurs, more current is conducted to the right-hand side electromagnetic coil  60  than to the left-hand side electromagnetic coil  60  in the rear differential, so that the right-hand side brake mechanism  51  is applied more strongly than the left-hand side brake mechanism  51 . 
   This allows the driving force of the hypoid pinion shaft  32  to be distributed more to the right rear axle  26 , and since this allows, in turn, the driving torque of the rear outer wheel of the turning vehicle to become larger than the driving torque of the rear inner wheel thereof, as indicated by an arrow  4  in  FIG. 23 , the turning performance in, for example, the low to middle speed ranges can be enhanced. 
   In addition, on the contrary, the driving torque of the rear inner wheel of the turning vehicle is allowed to be made larger than the driving force of the rear outer wheel thereof, whereby a required running stability can be obtained in a high speed range. 
   Thus, by controlling the values of current conducted to the left and right electromagnetic coils  60 , the driving force of the input shaft  30  can arbitrarily be distributed to the left and right rear axles  24 ,  26  in the direct connecting mode or by increasing the rotational speed thereof by the speed increasing apparatus  10 , whereby an optimum turning control and/or easy escape from a trap in the muddy road can be attained. 
   A switch from the direct connecting mode to the speed increasing mode will be controlled as below. A threshold for the steering effort or steering angle is set relative to the vehicle speed, and the speed increasing apparatus  10  is controlled so as to be put in the speed increasing mode when the steering effort or steering angle exceeds the threshold so set. 
   In addition, the rear differential  12  will be controlled as below. Values of current that is conducted to the electromagnetic coils  60  relative to the steering effort or steering angle are set in advance as a map. 
   By using this, the values of current that is conducted to the left and right electromagnetic coils  60  are controlled based on the turning angle and the steering effort or steering angle, the driving torque of the rear outer wheel of the turning vehicle is controlled so as to become larger than the driving torque of the rear inner wheel thereof. 
   While there has been described in connection with the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the present invention, and it is aimed, therefore, to cover in the appended claims all such changes and modifications as fall within the true spirit and scope of the present invention. 
   According to the first aspect of the present invention, since only the single actuator can be used to disengage the clutch while activating the transmission brake in a simultaneous fashion, it is possible to provide the power transfer apparatus that is smaller in size and lighter in weight. In addition, since the clutch and the transmission brake are serially disposed in the axial direction, the outside diameter of the power transfer apparatus can be made small. 
   Furthermore, since when the actuator is activated, the engagement of the clutch is gradually released while the transmission brake is gradually applied, there is caused no interruption in transmitting power from the input shaft to the output shaft, thereby making it possible to prevent the occurrence of a shock when a change in speed takes place. 
   According to the second aspect of the present invention, since the one-way clutch is interposed between the clutch inner hub and the clutch guide, the load generated when the input shaft and the output shaft are directly connected to each other can be partially borne by the one-way clutch, thereby making it possible to reduce the load capacity of the clutch. 
   According to the third aspect of the present invention, even if the power transmission member such as the clutch inner hub is disposed on the inner diameter side, the control (On/Off) of the clutch can be implemented on the outside diameter side.