Abstract:
A multi-wheel-drive vehicle has at least six wheels, a transmission with a first brake, and a transaxle device for the front drive wheels. The transaxle device includes a drive axle, an input shaft perpendicular to the drive axle for receiving power from the transmission, a drive train connecting the drive axle to the input shaft, a second brake, and a clutch device on the input shaft. The transaxle device may include a pair of drive axles connected by a differential unit. The clutch device can selectively isolate the drive axles from the rotation of the input shaft. Further, the clutch device is engaged when the first brake is applied. Additionally, the first and second brakes may be connected such that their operation may be synchronized.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a front transaxle device of a multi-wheel-drive vehicle. 
     2. Background Art 
     Conventionally, a multi-wheel-drive vehicle wherein four or more wheels are driven is known. 
     In this multi-wheel-drive vehicle, transaxle devices for supporting axles are disposed corresponding to the positions of the axles. For example, a rear transaxle device for supporting rear axles is disposed at a rear portion of the vehicle, and a front transaxle device for supporting front axles is disposed at a front portion of the vehicle. In a structure where six or more wheels are driven, a middle transaxle device for supporting middle axles is disposed at a longitudinally intermediate portion of the vehicle. 
     Furthermore, a transmission which transmits the power from a prime mover (e.g., an engine) is provided. By transmitting the power from the transmission to each of the transaxle devices, the wheels are driven through each of the axles. 
     In comparison with a two-wheel-drive structure, the above-mentioned multi-wheel-drive structure is more useful in that its driving performance over a bad road is good, and plenty of power is available for climbing a hill. Thus, this structure has come to be widely adopted by various kinds of vehicles such as automobiles, agricultural trucks, and the like. 
     Now, further improvement of such a multi-wheel-drive vehicle in terms of its driving performance over bad roads, cost-saving, maintainability, etc., is increasingly desired given the increasing popularity of such vehicles. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the invention is to provide a front transaxle device which gives improved braking performance to a multi-wheel-drive vehicle so as to improve further the driving performance of the vehicle over bad roads. Another object of the invention is to provide the front transaxle device with a simple structure so as to reduce manufacturing costs and enhance the maintainability thereof. 
     According to the present invention, a front transaxle device provided to a multi-wheel-drive vehicle comprises an input shaft for receiving power, a pair of left and right front axles supported in the front transaxle device, a differential connecting the left and right front axles in a differential manner, a pinion shaft, a clutch device which engages the pinion shaft with and disengages the pinion shaft from the input shaft, a rotary object interposed between the differential and the pinion shaft, and a brake device which brakes the rotary object. Therefore, the braking performance is improved and the vehicle&#39;s braking distance can be shortened. Thus, a multi-wheel-drive vehicle, which can run smoothly on a bad road and enhance fuel economy, may be available. Furthermore, by operating the clutch device, it is easy to select between the mode wherein the power is transmitted to the front wheels supported by the front transaxle device and the mode wherein the power is not transmitted to the front wheels. Thus, by linking the clutch device with operating means, a vehicle which can be put between 4-wheel-drive mode and 6-wheel-drive mode (for example) is available. Additionally, because the clutch device is disposed between the input shaft and the pinion shaft and the brake device is disposed at the rotary object, the two devices are separated and can avoid interfering with each other, thereby reducing the complexity of the layout. 
     The brake device comprises a piston which can be moved hydraulically, friction objects which engage with each other by the force of the piston, and a mechanism which maintains a constant stroke of the piston to engage the friction objects regardless of any abrasive reduction of the friction objects. Therefore, in spite of abrasive reduction of friction objects in the brake device, it is unnecessary to adjust the stroke of the piston to keep a good braking response of the brake device, thereby reducing the need for maintenance. 
     The rotary object is a middle shaft disposed between the pinion shaft and the differential and supported parallel to a rotational axis of the differential, and the middle shaft is engaged with the differential through a spur gear. Therefore, the parts of the brake device are arranged along and detached from the middle shaft parallel to the rotational axis of the differential. Thus, installation and removal of the brake device is easy, thereby resulting in good maintainability. Furthermore, because the middle shaft is connected with the differential through the spur gear, realignment using a shim and the like, which is necessary in a structure having the middle shaft connected with the differential through bevel gears, is not necessary. Such alignment can be eliminated. 
     A front transaxle device is provided to a multi-wheel-drive vehicle which has six or more wheels, wherein a pair of foremost wheels of the vehicle are supported and can be driven. A transmission provided to the vehicle is connected with the front transaxle device through a clutch device which is engaged when a brake operating means provided to the vehicle is operated to brake. Therefore, when the brake operating means is operated to its braking position by the linkage between the brake operating means and the clutch device, braking force is also transmitted to the pair of foremost wheels. Thus, the vehicle&#39;s braking distance at high speed can be shortened. Additionally, the front transaxle device can be bypassed when the brake device is not being operated, thereby enhancing fuel economy. 
     Other and further objects, features, and advantages of the invention will appear more fully from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
     FIG. 1 is a schematic diagram of a driving transmission system of a multi-wheel-drive vehicle including a front transaxle device of the present invention; 
     FIG. 2 is a horizontally sectional view of the front transaxle device; 
     FIG. 3 is an expanded horizontally sectional view of the front transaxle device, showing an automatic gap alignment mechanism, wherein a piston is located at its original brake-released position; 
     FIG. 4 is a sectional view of the same showing the state that the piston is moved at a stroke of length A from the state shown in FIG. 3, and friction discs are engaged with each other; 
     FIG. 5 is a sectional view of the same showing the state that the piston is moved at a stroke of length A from its original brake-released position when the friction discs are worn away; 
     FIG. 6 is a sectional view of the same showing the state that the piston is moved at a stroke of length B from the state shown in FIG. 5, and the friction discs are engaged with each other; 
     FIG. 7 is a sectional view of the same showing the state that the piston is returned at a stroke of length A from the state shown in FIG. 6 to its new brakeleased released position; 
     FIG. 8 is a horizontally sectional view of a modification of the front transaxle device wherein the brake device is disposed onto a pinion shaft; 
     FIG. 9 is a hydraulic circuit diagram of a control system for controlling front and rear brake devices; 
     FIG. 10 is a hydraulic circuit diagram of a control system for controlling the front and rear brake devices according to another embodiment; and 
     FIG. 11 is a diagram of the embodiment shown in FIG. 10, showing the state that a brake pedal is depressed and a clutch device linked with the brake pedal is engaged. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a multi-wheel-drive vehicle  1  comprises a front transaxle device  10  disposed at its front portion, a middle transaxle device  16  disposed at its longitudinally intermediate portion, and a rear transaxle device  4  disposed at its rear portion. The front transaxle device  10  includes a pair of left and right front axles  11 , the middle transaxle device  16  includes a pair of left and right middle axles  25 , and the rear transaxle device  4  includes a pair of left and right rear axles  8 . Each of above-mentioned front, middle, and rear axles  11 ,  25  and  8  supports each of front wheels  12 , middle wheels  26 , and rear wheels  9 , respectively, at their outer ends. 
     A front brake device  100  which serves as a first braking device is provided to the front transaxle device  10 , and rear brake devices  22  which serve as a second braking device are provided to the rear transaxle device  4 . 
     The front wheels  12  are steerable, i.e., rotatable leftward and rightward according to manipulation of a steering operating device (not shown). 
     A transmission  13  is provided in the rear transaxle device  4 . The power from an engine  3  installed in the body of the vehicle is transferred to the transmission  13  and changes rotational speed. Then, the power is used to drive the left and right rear wheels  9  through the rear axles  8 , and also, it is transferred to the middle transaxle device  16  so as to drive the middle wheels  26  through the middle axles  25 . Thus, the vehicle moves forward and backward by the driving of the rear wheels  9  and the middle wheels  26 , i.e., in 4-wheel-drive. 
     Alternatively, the power from the transmission  13  may be transferred to the front wheels  12  so as to drive all six wheels  9 ,  12  and  26 , thereby enabling the vehicle to be put in 6-wheel-drive. This structure will be described later. 
     A structure of the rear transaxle device  4  will now be described. 
     The rear transaxle device  4  comprises a rear axle housing  31  which houses the transmission  13  together with the rear axles  8 . An input shaft  5  of the transmission  13  is connected to an output shaft  6  of the engine  3  through a belt-type automatically continuous variable transmission (hereafter “CVT”)  7  comprising split pulleys and a belt. 
     The transmission  13  comprises a torque sensor  34  and a speed-changing gear mechanism  35 . The torque sensor  34  detects torque, which is applied on the wheels as load, and translates the torque into an output signal. The speed-changing gear mechanism  35  is operated by manipulating a speed-changing operating device like a lever or a pedal (not shown) disposed outside the rear axle housing  31 . 
     The rear axle housing  31  also houses a differential  32  interposed between the speed-changing gear mechanism  35  and the pair of left and right rear axles  8 . The differential  32  connects the left and right rear axles  8  differentially with each other. The differential  32  is provided with a differential locking mechanism  33  in the rear axle housing  31 . The differential locking mechanism  33  is linked with a differential-locking device like a lever or a pedal (not shown) disposed outside the rear axle housing  31  so as to lock the differential  32 . A power take-off casing  15  is fixed on a side portion of the rear axle housing  31 . The power take-off casing  15  is provided therein with a power output section from which power is transferred to the middle transaxle device  16  and the front transaxle device  10 . 
     The above-mentioned input shaft  5  is supported laterally in the rear axle housing  31  and projects outwardly from either the left or right sides thereof. A follower split pulley  36  is provided on the outwardly projecting portion of the input shaft  5 , which serves as an input section receiving the power from the engine  3 . The output part of the CVT  7  is formed by this follower pulley  36 . The CVT  7  is normally formed such that the speed reduction ratio is automatically steplessly reduced according to the increase of rotary speed of the engine  3 . 
     In the rear axle housing  31 , a main shaft  37  is provided coaxially with the input shaft  5 . The main shaft  37  and the input shaft  5  are connected with each other through above-mentioned torque sensor  34 . The torque sensor  34  detects various type resistances such as rolling resistance, air resistance, acceleration resistance, and grade resistance generated from each of the driven wheels, and outputs detection signals into a controller (not shown). The controller adjusts the degree of opening of a throttle valve of the engine  3  corresponding to the detection signals, thereby serving as a torque sensing governor. 
     In the rear axle housing  31 , a counter shaft  41  is disposed parallel to the main shaft  37 . The speed-changing gear mechanism  35  is provided between both shafts  37  and  41 . 
     The speed-changing gear mechanism  35  comprises a plurality of (in this embodiment, two) drive gears fixed on the main shaft  37  to rotate together with the main shaft  37 , and a plurality of (in this embodiment, two) transmission gears supported rotatably on the counter shaft  41  to engage with the respective drive gears on the main shaft  37 , thereby providing various (in this embodiment, two, i.e., high and low) gear ratios. In order to reverse the rotational direction of the counter shaft  41  while the main shaft  37  is rotated in a fixed direction, the speed-changing gear mechanism  35  also comprises a driving reverse gear fixed on the main shaft  37 , a reverse gear supported rotatably on the counter shaft  41 , and an idle gear through which both the reverse gears on the shafts  37  and  41  engage with each other. 
     A gear-changing clutch slider  47  is axially slidably but not relatively rotatably fitted onto the counter shaft  41  through a spline. By sliding the gear-changing clutch slider  47 , one gear is selected from among the two transmission gears and the reverse gear on the counter shaft  41  to engage with the counter shaft  41  through the gear-changing clutch slider  47 . This selection brings the counter shaft  41  into a high-speed regularly directed rotation, a low-speed regularly directed rotation, or a reversely directed rotation depending upon which gear is chosen. Also, the gear-changing clutch slider  47  can be located at its neutral position where it engages with none of the gears. The gear-changing clutch slider  47  is linked with the above-mentioned speed-changing device (not shown). 
     The counter shaft  41  is fixedly provided thereon with an output gear  51  adjacent to one of its ends, thereby transmitting the rotation of the counter shaft  41  to the above-mentioned differential  32 . 
     The differential  32  generally uses bevel gears to connect the left and right rear axles  8  in a differential manner. An input gear  53  is disposed on a differential casing, which houses the bevel gears, so as to engage with the output gear  51 . The differential locking mechanism  33  is disposed around one of the axles  8  so as to engage the differential casing with and disengage the differential casing from the axle  8  according to operation of the differential locking lever (not shown). When the differential casing engages with the axle  8 , both the axles  8  are locked together, i.e., the differential  32  is locked. 
     The rear brake devices  22  are provided respectively on the pair of left and right rear axles  8  so as to apply brake force onto both rear axles  8  according to the operation of a later-discussed brake pedal. 
     One end of the counter shaft  41  extends toward one of the left or right sides into the power take-off case  15 , and a bevel gear  62  is fixed onto its end portion. An output shaft  63  is supported in the longitudinal direction of the vehicle and perpendicularly to the counter shaft  41  in the power take-off case  15 . A bevel gear  64  is fixed onto the output shaft  63  and engages with the bevel gear  62 . 
     The output shaft  63  projects forward from the power take-offcase  15 , and connects to a transmission shaft  87  of the middle transaxle device  16  through a drive shaft  17 . 
     Next, the middle transaxle device  16  will be described. 
     The transmission shaft  87  is supported in the longitudinal direction of the vehicle, and its rear end projects rearward so as to receive driving force from the rear transaxle device  4 . The transmission shaft  87  also projects forward from the middle transaxle device  16 , thereby forming an output section for the front transaxle device  10 . 
     A middle-axle drive gear  86  is fixed onto the transmission shaft  87 , and a middle shaft  83  is rotatably supported parallel to the transmission shaft  87 . An intervention gear  84  is fixed onto one end of the middle shaft  83  so as to engage with the middle-axle drive gear  86 , and a bevel gear  85  is provided onto the other end of the middle shaft  83 . The bevel gear  85  engages with an input bevel gear  90  of a differential  89  which differentially connects the left and right middle axles  25  with each other. 
     Next, the structure of the front transaxle device  10  will be described in accordance with FIGS. 1 and 2. 
     In the front transaxle device  10 , an input shaft  14  is rotatably supported by a housing  88 , and connects with the transmission shaft  87  of the middle transaxle device  16  through a propeller shaft  18 , universal joints, and the like. 
     In the housing  88 , a pinion shaft  95  is disposed forward of the input shaft  14  and supported coaxially with the input shaft  14 . A bevel gear  97  is fixed onto one end portion of the pinion shaft  95 . The input shaft  14  is notched on its periphery so as to form splines, and a front clutch slider  96  is axially slidably but not relatively rotatably disposed around the splines. The pinion shaft  95  is also notched on its periphery so as to form splines, thereby being engaged with or disengaged from the front clutch slider  96 . A detent mechanism  21  is formed in the input shaft  14  to define positions of the front clutch slider  96 , i.e., an engage position where the front clutch slider  96  engages with the pinion shaft  95 , and a disengage position where the front clutch slider  96  disengages from the pinion shaft  95 . 
     This clutch device  140  is interlocked with a later-discussed drive mode changing lever  130  through a linkage. 
     In the housing  88  of the front transaxle device  10 , a differential  99  is provided onto the left and right front axles  11  so as to differentially connect the front axles  11  with each other. The differential  99  is constructed similarly to the differential  89  of the middle transaxle device  16 . As shown in FIG. 2, the differential  99  comprises a hollow differential casing  45 , a pinion shaft  46 , pinions  49 , and differential side gears  48 . The differential casing  45  is disposed coaxially with the front axles  11  and rotatably supported by the housing  88 . The pinion shaft  46  is disposed in the differential casing  45  so as to be integrally rotatable with the differential casing  45 . The pinions  49  are disposed oppositely to each other and rotatably supported on the pinion shaft  46 . Each of the differential side gears  48  is fixed onto an inner end of each of the front axles  11  so as to engage with both the pinions  49 . 
     An input gear  98 , which is a spur gear to receive driving force for the differential  99 , is fixed onto the differential casing  45 . 
     Next, description will be given on a middle shaft  92  serving as a rotary object which intervenes between the differential  99  and the pinion shaft  95 . 
     The middle shaft  92  is disposed parallel to a rotational axis of the differential  99  (that is, a rotational axis of the differential casing  45 ). A bevel gear  93  is fixed onto the middle shaft  92 , and is engaged with a bevel gear  97  fixedly provided on the pinion shaft  95 . 
     The midway portion of the middle shaft  92  is notched on its periphery to form a reduction gear  91  as a spur gear. The reduction gear  91  is engaged with the input gear  98  of the differential  99 . 
     The middle shaft  92  projects outwardly from the housing  88 . A brake casing  115  is fixedly provided onto the outside of the housing  88  so as to cover the projecting end portion of the middle shaft  92 . A front brake device  100  as a multi-disc type brake is set up around the projecting end portion of the middle shaft  92  between the brake casing  115  and the housing  88 . 
     In the front brake device  100 , first friction discs  110  are axially slidably but not relatively rotatably provided onto the middle shaft  92 . Second friction discs  111  are slidably but not relatively rotatably engaged with the housing  88  of the front transaxle device  10 . Each of the first friction discs  110  and each of the second friction discs  111  are arranged alternately. A pressure member  113  is provided slidably and coaxially to the middle shaft  92  for pressuring the multi-layered friction discs  110  and  111  against a receiving surface  112  formed at an inner wall of the housing  88 . A piston  114  is provided integrally with the pressure member  113  through a bolt  116 . 
     The brake casing  115  projects outwardly and coaxially to the middle shaft  92  so as to form a cylindrical portion. The piston  114  is slidably fitted in the cylindrical portion. Hydraulic fluid is to be tightly supplied into a fluid chamber of the cylindrical portion of the brake casing  115  which is formed between an utmost end surface of the cylindrical portion and the piston  114 . By the hydraulic pressure of the fluid supplied into the fluid chamber, the piston  114  slides integrally with the pressure member  113  so as to press the friction discs  110  and  111  against one another, thereby braking the middle shaft  92 . 
     As shown in FIG.  3  and others, there is formed a substantially ring-shaped gap between an end surface of the piston  114  and the pressure member  113  along the inner peripheral surface of the brake casing  115 . In the gap are arranged a return spring  71 , a collar  72 , and a friction ring  73 , which constitute an automatic gap alignment mechanism  70  to keep a constant stroke of the piston  114  for the braking operation regardless of abrasive reduction of the friction discs  110  and  111 . 
     The return spring  71  is a ring-shaped spring, which is semicircular in its radial section. The major portion of the spring  71  is inserted into a ring-like groove  74 , which is formed on an end surface of the piston  114  around the middle shaft  92  so as to face toward the discs  110  and  111 . An apex portion of the spring  71  in its sectionally semicircular shape projects toward the discs  110  and  111  so as to abut against the collar  72 . Thus, the spring  71  is sandwiched between the piston  114  and the collar  72 . The collar  72  is slidable on the inner peripheral surface of the cylindrical portion of the brake casing  115 . The friction ring  73  has outward biasing force in the radial direction and is fitted to an inner peripheral face of the brake casing  115 . Therefore, the friction ring  73  is slidable on the inner peripheral surface of the cylindrical portion of the brake casing  115  against frictional resistance between the friction ring  73  and the inner peripheral face of the brake casing  115 . This friction resistance applied onto the friction ring  73  is larger than the spring force of the return spring  71  and smaller than the hydraulic pressure applied on the piston  114 . 
     Referring to FIG. 3, the friction discs  110  and  111  are new, i.e., they are not worn. The total clearance between the friction discs  110  and  111  is of a length A. Therefore, a stroke of length A is required for the piston  114  to bring the friction discs  110  and  111  into contact with one another. An original amount of hydraulic fluid is filled in the fluid chamber so that the utmost end of the piston  114  is located at an original brake-released position P. At this time, the return spring  71  expands so as to generate a gap of the length A between the end surface of the piston  114  and the collar  72 . The retaining ring  73  is sandwiched between the collar  72  and the pressure member  113 . 
     For the braking operation of the front brake device  100 , hydraulic fluid is supplied into the fluid chamber in the brake casing  115  so as to push the pressure member  113  toward friction discs  110  and  111 . As shown in FIG. 4, when the piston  114  is moved at a stroke of length A, the friction discs  110  and  111  are brought into engagement so that the middle shaft  92  starts to be braked. During this stroke of the piston  114 , the return spring  71  is compressed between the collar  72  and the piston  114  so as to absorb the pressure force of the piston  114 , thereby maintaining the positions of the collar  72  and the friction ring  73 . Therefore, the gap of the length A between the piston  114  and the collar  72  is diminished, and a gap of the length A is generated between the friction ring  73  and the pressure member  113 . 
     For releasing the middle shaft  92  from its brake condition shown in FIG. 4, fluid is drained from the fluid chamber in the cylindrical portion of the brake casing  115  so that the spring  71  returns to its expanded condition, thereby locating the piston  114  at the original brake-releasing position P. The pressure member  113  follows the piston  114 , thereby disengaging the friction discs  110  and  111 . Consequently, the front brake device  100  returns to the state as shown in FIG.  3 . 
     Description will be given on the action of the automatic gap alignment mechanism  70  corresponding to the abrasive reduction of the friction discs  110  and  111  in accordance with FIGS. 5 to  7 . 
     Referring to FIG. 5, friction discs  110 ′ and  111 ′ are abraded versions of friction discs  10  and  111 . The total abrasive reduction of the discs  110 ′ and  111 ′ in the axial direction of the middle shaft  92  is of a length B. Therefore, even if the same amount of fluid as that in the situation of FIG. 4 is supplied so as to move the piston  114  at a stroke of length A from its original brake-released position P, the friction discs  110 ′ and  111 ′ are still disengaged. To bring the discs  110 ′ and  111 ′ into engagement, the piston  114  requires an additional stroke of length B. In other words, the piston  114  at the original brake-released position P requires a stroke of lengths A+B for braking. 
     However, in the situation as shown in FIG. 5, the pressure member  113  is allowed to further move because of the additional clearance among the friction discs  110 ′ and  111 ′ generated by their abrasion. Also, the collar  72  abuts against the end surface of the piston  114  because of the compression of the spring  71 . Therefore, as shown in FIG. 6, increased fluid is supplied so that the piston  114  is completely moved together with the pressure member  113  at the stroke of length A+B from its original brake-released position P. During the movement of the piston  114  and the pressure member  113 , the end surface of the piston  114  pushes the collar  72  together with the friction ring  73  against the friction resistance between the friction ring  73  and the brake casing  115 . Therefore, the collar  72  and the friction ring  73  are shifted from their original positions as shown in FIGS. 3 and 4. 
     Referring to FIG. 7, when the hydraulic pressure on the piston  114  is released, the friction ring  73  remains at its new position shifted from its original position by its frictional resistance and the spring  71  expands between the collar  72  and the piston  114 . Therefore, the piston  114  retreats only a stroke of length A by the expansion of the spring  71 . The pressure member  113  follows the retreating of the piston  114 , thereby disengaging the friction discs  110 ′ and  111 ′. Consequently, a new brake-released position Q of the utmost end of the piston  114  is shifted from its original brake-release position P. The required stroke of the piston  114  in addition to the stroke of length A in the next braking operation of the front brake device  100  is just as much as the new abrasive reduction of the discs  110  and  111 . 
     Thus, on every braking action of the piston  114 , the friction ring  73  is shifted so as to counter the additional clearance caused by the abrasion of the friction discs  110 ′ and  111 ′, thereby shifting the brake-release position of the piston  114  toward the discs  110 ′ and  111 ′. Strictly speaking, the required stroke of the piston  114  in every braking operation is of the length A+B. However, in each braking operation, the additional stroke of length B as much corresponding to the abrasive reduction of the friction discs  110 ′ and  111 ′ is extremely small, thereby being able to be ignored in measurement. Therefore, it may be said that the stroke of the piston  114  required for every braking operation is substantially of the length A. In this meaning, the stroke of the piston  114  required for braking is kept constant regardless of the abrasive reduction of the friction discs  110 ′ and  111 ′. Consequently, the swift response of the front brake device  100  can be maintained for a long period of use. 
     Referring to FIG. 8, in the front transaxle device  10 ′, a brake device  100 ′ is disposed at the pinion shaft  95 ′, instead of the front brake device  100  disposed at the middle shaft  92 . Description will be given on this structure. 
     In the brake device  100 ′, first friction discs  110  are fit onto the pinion shaft  95 ′ such that the first friction discs  110  cannot rotate with respect to the pinion shaft  95 ′. Second friction discs  111  are engaged with the housing  88 ′. Each of the first friction discs  110  and each of the second friction discs  111  are arranged alternately. The piston  119 ′ is provided to press the friction discs  110  and  111 . The piston  119 ′ is formed into a ring-shape, and is fluid-tightly fitted with a groove formed at an inner wall of the housing  88 ′ such that the piston  119 ′ can be displaced in parallel with the pinion shaft  95 ′. An oil path  121  is formed at the groove so as to apply hydraulic force onto one end face of the piston  119 ′, thereby operating the piston  119 ′ hydraulically. The oil path  121  is connected to an oil hydraulic circuit  120  which will be described below. 
     In this structure, the piston  119 ′ is driven by the oil supplied from the oil hydraulic circuit  120  in such a direction as to project and to press the friction discs  110  and  111 , thereby braking the pinion shaft  95 ′ by friction. 
     Next, the structure in the multi-wheel-drive vehicle to operate the front and rear brake devices  100  and  22  for braking by manipulation of the above-mentioned brake pedal will be described in accordance with FIG.  9 . 
     The brake pedal  19  constituting the brake operating means in the present embodiment is connected with the rear brake devices  22  and the front brake device  100  through the oil hydraulic circuit  120  shown in FIG.  9 . The oil hydraulic circuit  120  comprises a master cylinder  101  to discharge oil for the brake devices  22  and  100 , an oil tank  102  for supplying oil to the master cylinder  101 , a filter  103  for removing impurities from the oil, an oil path  105  for leading oil from the master cylinder  101  to the front and rear brake devices  100  and  22 , and the like. 
     The brake pedal  19  is supported rotatably, and an end of a rod  106  is connected to the midway portion of the brake pedal  19 . The other end of the rod  106  is fixed on a piston  107  disposed in the master cylinder  101 . A biasing spring  108 , which also serves as a recovering spring for the brake pedal  19 , is disposed in the master cylinder  101 . 
     The filter  103  and a manual valve  104  are disposed at a midway portion of the circuit for supplying oil from the oil tank  102  into the master cylinder  101 . The manual valve  104  is interlocked with the rod  106  such that the manual valve  104  opens the circuit when the brake pedal  19  is not depressed, and that the manual valve  104  is switched by the rod  106  and shuts the circuit when the brake pedal  19  is depressed thereby preventing oil from back-flowing in the circuit when the rod  106  pushes the piston  107 . 
     In this structure, when an operator depresses the brake pedal  19 , the piston  107  is pushed through the rod  106 , and the master cylinder  101  discharges the oil. 
     The discharged oil is led into the oil path  105  and is divided into two branches. The oil in one branch runs to each of the rear brake devices  22 , thereby applying braking force onto the rear axles  8 . The braking force is transmitted to the middle axles  25  connected to the rear axles  8  through the drive shaft  17  and the like, thereby also braking the middle axles  25 . The oil in the other branch is led into the front brake device  100  to make the piston  114  in the front brake device  100  press against friction discs  110  and  111 , thereby applying braking force onto the front axles  11  through the middle shaft  92 . 
     Description will be given on the structure in the multi-wheel-drive vehicle to transmit the power from the engine  3  to the wheels  9 ,  12 , and  26 . 
     As shown in FIG. 1, the transmission  13  provided in the rear transaxle device  4  transmits the power from the engine  3  to the rear axles  8  to drive the rear wheels  9 , and also transmits the power to the middle transaxle device  16  through the drive shaft  17  to drive the middle wheels  26  through the middle axles  25 . 
     In other words, the power from the transmission  13  branches to the rear axles  8  and the middle axles  25 , thereby constantly driving the rear wheels  9  and the middle wheels  26  (four wheels in total). 
     Furthermore, the power, which is led from the engine  3  into the middle transaxle device  16 , drives the input shaft  14  in the front transaxle device  10  constantly through the transmission shaft  18 . 
     The earlier-discussed clutch device  140  is disposed at the input shaft  14 . As shown in FIG. 9, the drive mode changing lever  130  is provided at the appropriate portion of the vehicle to operate the clutch device  140 , and the drive mode changing lever  130  is shiftable among a 4-wheel-drive position and a 6-wheel-drive position (two positions in total). The drive mode changing lever  130  is linked with the front clutch slider  96  in the clutch device  140  such that the clutch device  140  is engaged when the drive mode changing lever  130  is located at its 6-wheel-drive position (as shown by ‘6WD’ position in FIG. 9) and that the clutch device  140  is disengaged when the drive mode changing lever  130  is located at its 4-wheel-drive position (as shown by ‘4WD’ position in FIG.  9 ). 
     Therefore, when the drive mode changing lever  130  is located at its 6-wheel-drive position, the clutch device  140  is engaged to drive the front transaxle device  10  such that the front wheels  12  are driven through the front axles  11 . Because the four wheels of the middle wheels  26  and the rear wheels  9  are driven as described above at this time, the vehicle is put into 6-wheel-drive mode and all six wheels are driven. 
     On the other hand, when the drive mode changing lever  130  is located at its 4-wheel-drive position, the clutch device  140  is disengaged and the power from the transmission  13  is shut off such that the front wheels  12  are not driven. In this case, the vehicle is put into 4-wheel-drive mode and only the middle wheels  26  and the rear wheels  9 , four wheels in total, are driven. 
     The above-mentioned structure is an example and other embodiments may be given. For instance, instead of the structure where an oil hydraulic circuit  120  is used, a structure which will be described below may apply. 
     An oil hydraulic circuit  120 ′ shown in FIG. 10, which is used in this modification, is of the structure that a manual valve  150  which is switchable among two positions is provided at the midway of a path for supplying oil of the master cylinder  101  for the front brake device  100  in the oil hydraulic circuit  120 ′ in the brake system. 
     A brake mode changing lever  155  serving as a brake mode changing means is provided at the operator&#39;s section in the vehicle, and the manual valve  150  is interlocked with the brake mode changing lever  155 . 
     The brake mode changing lever  155  is shiftable according to an operator&#39;s manipulation between a front-rear-brake position FRb and a rear-brake position Rb. 
     When the brake mode changing lever  155  is located at its front-rear-brake position FRb, the manual valve  150  is opened. Thus, when the brake pedal  19  is depressed, oil from the master cylinder  101  is supplied into both the rear brake devices  22  and front brake device  100 . In this case, the vehicle is put into front-rear-brake mode wherein the rear and front brake devices  22  and  100  are put into action. 
     On the other hand, when the brake mode changing lever  155  is located at its rear-brake position Rb, the manual valve  150  is closed. Thus, when the brake pedal  19  is depressed, oil from the master cylinder  101  is not supplied into the front brake device  100 , but into the rear brake devices  22 . In this case, the vehicle is put into the rear-brake mode wherein only the rear brake devices  22  are put into action. 
     Furthermore, the brake pedal  19  is linked with above-mentioned drive mode changing lever  130  through a linkage so as to make the drive mode changing lever  130  located at its 6-wheel-drive position when the brake pedal  19  is depressed. 
     The action of the above structure will be described. When the brake mode changing lever  155  is located at its rear-brake position Rb and the drive mode changing lever  130  is located at its 4-wheel-drive position 4WD, and when the brake pedal  19  is depressed, the manual valve  150  is closed and only the rear brake devices  22  are put into action. However, because the drive mode changing lever  130  is switched into its 6-wheel-drive position 6WD at the time when the brake pedal  19  is depressed and the clutch device  140  linked with the drive mode changing lever  130  is engaged, the braking force which the rear brake devices  22  apply onto the rear axles  8  and the middle axles  25  is also transmitted to the front axles  11  through the propeller shaft  18  and the like, thereby also braking the front axles  11 . 
     Therefore, though the power from the engine  3  is transmitted to only the rear axles  8  and the middle axles  25  such that four wheels are driven, braking force generated by only rear brake devices  22  is applied onto not only the rear axles  8  and the middle axles  25  but also the front axles  11  such that all the six wheels can be braked. 
     In this structure, changing among 4-wheel-drive mode and 6-wheel-drive mode as the occasion arises is easy by engaging and disengaging the clutch device  140  by shifting the drive mode changing lever  130 . 
     If the vehicle is put into the 4-wheel-drive mode and the brake pedal  19  is depressed in the rear-brake mode, braking force generated by the rear brake devices  22  is transmitted to the front axles  11  by the linkage between the brake pedal  19  and the clutch device  140 . Though the front brake device  100  in the front transaxle device  10  is out of action, not only the rear wheels  9  and the middle wheels  26  but also the front wheels  12  are contributing to the braking of the vehicle. Thus, by putting the vehicle into the rear-brake mode, while good braking performance can be maintained, abrasion of the front brake device  100  can be prevented. 
     Of course, the vehicle can be put into the front-rear-brake mode which is effective when strong braking force is frequently desirable. In this mode, the front wheels  12  are braked by the front brake device  100  and the rear wheels  9  and the middle wheels  26  are braked by the rear brake devices  22 . Good braking performance is achieved by applying braking force onto the rear wheels  9 , the middle wheels  26 , and the front wheels  12  (all six wheels), and the rear brake devices  22  are protected from overload, such that heating and abrasion can be minimized. 
     Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form may be changed in the details of construction and the combination and arrangement of parts may be resorted without departing from the spirit and the scope of the invention as hereinafter claimed. 
     For example, the front transaxle device in the present invention can apply not only to a six-wheel-drive vehicle as described in above embodiment but also to a multi-wheel-drive vehicle wherein eight or more wheels are driven.