Abstract:
The purpose of the invention is to enable a crawler tractor to brake and stop certainly without applying a load to an engine and a driving mechanism, and without complicated operation. An operation link links a conic linkage ( 53 ) which links a steering wheel ( 7 ) with a main speed change lever ( 55 ) of the crawler tractor, and with an operation mechanism of a brake pedal ( 54 ). A neutral return mechanism for the operation link is provided. The operation link neutral return mechanism comprises a hook-like cam lever ( 61 ). When the brake pedal is depressed, the main speed change lever is moved to its neutral position before braking force occurs, and then, the brake is actuated. Accordingly, engine brake is used effectively, and the brake can be miniaturized.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a crawler tractor used for agricultural work and the like. More particularly, it relates to a driving mechanism, an HST and a brake mechanism concerning speed change, traveling and steering of the crawler tractor. 
     2. Related Art 
     Conventionally, there is a well-known agricultural working vehicle (crawler tractor) provided with a hydraulic stepless transmission for traveling and a hydraulic stepless transmission for turning as one typical hydraulic-driving agricultural working machine having left and right independent crawler type traveling devices. 
     It is important for improving operativity of the crawler tractor to ensure transmission/isolation of driving force and braking. Therefore, a crawler tractor is provided with a neutral-position holding mechanism for ensuring neutral location of the HST for traveling and the HST for turning. 
     As described in the Japanese Patent Laid Open Gazette Hei. 8-242601, for enhancing certainty of cutting off transmission of driving force to crawler traveling devices at the time of stopping the vehicle, with respect to some crawler tractors, an additional clutch device is disposed on a driving power train and a hydraulic circuit is provided for controlling the clutches. With respect to other crawler tractors, a neutral return mechanism for a steering device is disposed in the vicinity of the HST for turning and a steering shaft connected to the steering device is connected to the HST for turning through an universal joint or the like so as to transmit steering operation. 
     With regard to the above-mentioned hydraulic circuit, there is a crawler tractor constructed such that an oil cooler for cooling pressure oil supplied into a transmission casing is disposed in front of a radiator and the oil cooler is cooled by cooling wind generated from a fan driven by an engine. Also, as shown in the Japanese Patent Laid Open Gazette Hei. 10-54462, there is a crawler tractor such that pressure oil supplied from a charge pump is cooled by an oil cooler and then the fluid is supplied to each HST. 
     As disclosed in the Japanese Patent Laid Open Gazette Hei. 8-280206, for example, there is a well-known construction for connecting a sub control valve for controlling a working machine connected to a crawler tractor, wherein the sub control valve is detachable, and when the sub control valve is attached, hydraulic ports are connected with respective ports of the sub control valve through hydraulic pipings, and when the sub control valve is not attached, metal is disposed so as to cover an upper surface of a cylinder casing and the ports are connected mutually through oil passages provided within the metal. 
     However, with regard to many of the conventional crawler tractors, an inscribed gear is used as one of gears constituting a planetary gearing of a differential mechanism for independent left and right crawler type traveling devices. Accordingly, the planetary gearing becomes diametrically large. 
     Besides, for providing a steering brake to the left and right axles, left and right brake mechanisms are required, and they are large-sized so as to brake a large torque. Such a mechanism results in not only restriction of design of the crawler tractor but also increase of full length of a linkage, thereby hindering improvement of operativity at the time of braking. 
     Furthermore, conventional mechanisms for holding the HST for traveling and the HST for turning in neutral and a conventional mechanism for holding a steering device in neutral are complicated in construction and require much effort for assembling thereof and the like, thereby hindering reduction of costs for their production. 
     Besides, a conventional main circuit of the HST for traveling is not provided with a valve for releasing pressure from the HST for traveling. Accordingly, pressure is accumulated in the HST for traveling, thereby making an output shaft hard to rotate, requiring a large power for starting thereof, and hindering smooth change of speed or gears. 
     With regard to cooling of pressure oil, if an oil cooler for cooling pressure oil used for a transmission is disposed alongside of a capacitor for an air-conditioner and a radiator of an engine, cooling efficiency may fall. Furthermore, maintenance work may become complicated. 
     With regard to the conventional construction of hydraulic piping, it takes a long time for attaching the piping. Every when the oil pressure is selectively extracted to either front or rear side, it is necessary to close hydraulic ports on an unused side. 
     SUMMARY OF THE INVENTION 
     In a crawler tractor according to the invention, power from an engine is transmitted into a transmission casing disposed at a rear portion of the vehicle body and varied in speed, and a traveling power take-off section transmits the speed-varied power to a traveling device. The traveling power take-off section is disposed in a lower portion of the transmission casing, and a brake mechanism is provided in the traveling power take-off section. Accordingly, a space in the transmission casing can be used effectively. Furthermore, the brake mechanism is disposed near a brake pedal. 
     In a crawler tractor according to the invention, power from an engine is transmitted into a transmission casing through a hydrostatic stepless transmission for traveling, and a traveling power take-off section transmits the power varied in speed in the transmission casing to a traveling device. When a brake pedal is depressed, a swash plate of the hydrostatic stepless transmission for traveling returns to a neutral position and then a parking brake provided in the traveling power take-off section is actuated. Accordingly, the capacity of the brake may be small and the brake device may be compacted. 
     In a crawler tractor disposed in a crawler tractor according to the invention, power from an engine is transmitted to a sub transmission in a transmission casing, and a traveling power take-off section transmits power to a traveling device. A braking device is provided in the traveling power take-off section in a lower portion of the transmission casing and in the vicinity of the sub transmission. Therefore, a linkage between the brake device and the transmission can be simplified. 
     According to the invention, a conic linkage links a steering device with a main speed change lever, and an operation link is provided for connecting a brake pedal operation mechanism with the conic linkage. The operation link is provided with a neutral return mechanism, which is disposed in a lower portion of a section incorporating the conic linkage. The neutral return mechanism is disposed on the way of the linkage connecting the main speed change lever with the steering wheel, thereby being simplified. A play of the linkage is equally shared between the main speed change lever and the steering wheel so as to improve their operativity. 
     According to the invention, a conic linkage links a steering device with a main speed change lever, an operation link is provided for connecting a brake pedal operation mechanism with the conic linkage, and a neutral return mechanism for the operation link is provided in the operation link. The neutral return mechanism for the operation link comprises a hook-like cam. Accordingly, the neutral return mechanism is simplified and facilitates for easy adjustment of the neutral return mechanism. Such a simple and easy neutral return mechanism improves operativity and durability thereof. 
     According to the invention, a conic linkage links a steering device with a main speed change lever, an operation link is provided for connecting a brake pedal operation mechanism with the conic linkage, and a neutral return mechanism for the operation link is provided in the operation link. A hook-like cam is arranged on the way of a link between the conic linkage and the main speed change lever. Therefore, a play of the linkage is not disposed eccentrically, thereby reducing an error in a mechanism for operation thereof. 
     According to the invention, a conic linkage links a steering device with a main speed change lever, an operation link is provided for connecting a brake pedal operation mechanism with the conic linkage, and a neutral return mechanism for the operation link is provided in the operation link. When an operation for braking is performed, a brake begins to work after a swash plate control arm of an HST is positioned in the vicinity of its neutral position. Therefore, when the brake operation is performed, a brake caused by the HST is applied and then a brake is actuated so as to stop. Accordingly, the vehicle is always braked when the brake operation is performed. The time lug between the two brake systems improves a feeling of braking operation. 
     According to the invention, an operation link links a main speed lever operation mechanism with a brake pedal operation mechanism, and a neutral return mechanism for the operation link is provided in the operation link. A servo valve is disposed between the brake pedal operation mechanism and a swash plate control arm of an HST. Operation of the brake pedal is transmitted to the swash plate control arm of a HST through the servo valve tardily. Therefore, the feeling in operation is improved and a hard brake is prevented. 
     According to the invention, in a steering mechanism, a conic linkage links a steering device with a main speed change lever, and a rotation restriction cam is provided in a lower portion of a steering shaft connected to a steering wheel. The cam is provided with a portion for abutting against a neutral-returning member and with a portion for abutting against a rotation-restricting stopper. Therefore, a space in an upper portion of a steering column is efficiently usable. 
     According to the invention, in a steering mechanism, a conic linkage links a steering device with a main speed change lever, and a cam is formed to gradually expand in its radial direction as the cam goes apart from a neutral position thereof. The cam is provided at an end thereof with a portion for abutting against a stopper for restriction of rotation. Therefore, a mechanism for neutral-returning of the steering device is simplified, and the cam is also simplified. 
     According to the invention, a steering mechanism comprises a conic linkage linking a steering device with a main speed change lever. A stopper and a neutral-returning member arranged symmetrically with respect to an axial center of a cam. Therefore, the single stopper is provided to the cam so as easily to restrict rotational degree of the cam. Also, the mechanism for restricting rotation of the cam is enhanced in durability. 
     In a crawler tractor according to the invention, power form an engine is transmitted to an HST for traveling and speed change operation is done by the HST for traveling. A mechanism for releasing residual pressure comprises a valve for releasing residual pressure which is provided to a main circuit of the HST for traveling. Accordingly, pressure is released from the HST for traveling so as to facilitate easy actuation of an output shaft for traveling and easy restart of an engine. Also, the gear change of a sub transmission is done smoothly. 
     In a crawler tractor according to the invention, power form an engine is transmitted to an HST for traveling and speed change operation is done by the HST for traveling. A mechanism for releasing residual pressure comprises a valve for releasing residual pressure which is provided to a main circuit of the HST for traveling. The valve for releasing residual pressure is interlockingly connected with a brake pedal. Therefore, an engine is easily started and load applied on the engine and a drive mechanism is lightened. 
     In a crawler tractor according to the invention, power form an engine is transmitted to an HST for traveling and speed change operation is done by the HST for traveling. A mechanism for releasing residual pressure comprises a valve for releasing residual pressure which is provided to a main circuit of the HST for traveling. The valve for releasing residual pressure is an electro-magnetic valve linked with a key switch. Accordingly, an additional operation for releasing the residual pressure is unnecessary. When the starter actuates, pressure in the HST is always low so as to improve the start of the engine. 
     In a crawler tractor according to the invention, power form an engine is transmitted to an HST for traveling, speed change operation is done by the HST for traveling, the power from the engine is also transmitted to an HST for turning, and steering operation is done by transmitting output of the HST for turning to a differential mechanism. A clutch is disposed between the HST for turning and a transmission shaft for transmitting driving force to the differential mechanism. Such a simple construction is provided for shutting off driving force from the HST for turning. Therefore, loss of driving force at the time of stopping the vehicle can be reduced, thereby improving fuel economy of the crawler tractor. 
     In a crawler tractor according to the invention, power form an engine is transmitted to an HST for traveling, speed change operation is done by the HST for traveling, the power from the engine is also transmitted to an HST for turning, and steering operation is done by transmitting output of the HST for turning to a differential mechanism. An input shaft of the HST for turning projects from a casing of the HST for turning, and a pulley is equipped on the input shaft. By such a simple construction, a PTO section is provided at a front portion of the vehicle body. Therefore, manufacturing costs is reduced, and the PTO section is compacted, so as to improve the flexibility of design. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective front view of a crawler tractor; 
         FIG. 2  is an entire side view of the crawler tractor; 
         FIG. 3  is a side view showing arrangement of an engine, HSTs and a transmission casing; 
         FIG. 4  is a sectional side view showing a construction of the rear transmission casing; 
         FIG. 5  is a sectional side view showing a construction of a brake mechanism; 
         FIG. 6  is a sectional plan view showing the construction of the brake mechanism; 
         FIG. 7  is a side view showing a frame construction of the crawler tractor; 
         FIG. 8  is a plan view showing the frame construction of the crawler tractor; 
         FIG. 9  is a side view showing a connection construction of the engine and the rear transmission casing; 
         FIG. 10  is a front view showing the connection construction of the engine and the rear transmission casing; 
         FIG. 11  is a front view showing a construction of the rear transmission casing and a connection frame; 
         FIG. 12  is a plan view showing another frame construction of the crawler tractor; 
         FIG. 13  is a front view showing a connection construction of the connection frame and a center support frame; 
         FIG. 14  is a plan view showing a construction of an attaching linkage; 
         FIG. 15  is a side view showing the construction of the attaching linkage; 
         FIG. 16  is a side view showing an anchored state of a top link; 
         FIG. 17  is a side view showing a construction of hydraulic piping from a sub control valve; 
         FIG. 18  is a plan view showing the construction of hydraulic piping from the sub control valve; 
         FIG. 19  is a schematic diagram showing a connection construction of the sub control valve; 
         FIG. 20  is a schematic diagram showing a construction of electromagnetic switching valves; 
         FIG. 21  is a rear view showing an arrangement of a fuel tank and a pressure oil tank; 
         FIG. 22  is a plan view showing the arrangement of the fuel tank and the pressure oil tank; 
         FIG. 23  is a side view showing a construction of an auxiliary base disposed in the vicinity of the fuel tank; 
         FIG. 24  is a bottom view showing a construction of the auxiliary base; 
         FIG. 25  is a drawing showing a construction of a hydraulic circuit; 
         FIG. 26  is a side view showing a construction of hydraulic piping; 
         FIG. 27  is a plan view showing the construction of the hydraulic piping; 
         FIG. 28  is a side view showing an arrangement of a capacitor for air-conditioner and a radiator; 
         FIG. 29  is a sectional plan view showing a construction of a seal member; 
         FIG. 30  is a side view showing an arrangement of an oil cooler and the pressure oil tank; 
         FIG. 31  is a plan view showing another embodiment about arrangement of the oil cooler; 
         FIG. 32  is a side view showing a construction of a front part of the crawler tractor; 
         FIG. 33  is a sectional side view showing a construction of a HST for turning; 
         FIG. 34  is a sectional side view showing a construction between the HST for turning and a differential mechanism; 
         FIG. 35  is a plan view showing a construction of the HST for turning; 
         FIG. 36  is a side view showing a construction of a front PTO section; 
         FIG. 37  is a side view showing control devices of the crawler tractor; 
         FIG. 38  is a side view showing a construction of a conic linkage; 
         FIG. 39  is a sectional rear view showing a construction of the conic linkage; 
         FIG. 40  is a side view showing an actuation construction of a main speed change lever; 
         FIG. 41  is a side view showing an actuation construction of a brake pedal; 
         FIG. 42  is a front view showing the actuation construction of the brake pedal; 
         FIG. 43  is a drawing showing an interlocking construction of the brake pedal and the main speed change lever; 
         FIG. 44  is a side view showing a construction of the brake pedal and a cam lever; 
         FIG. 45  is a side view showing a construction of a rod connected to the brake mechanism; 
         FIG. 46  is a schematic diagram showing a brake mechanism using a servo valve; 
         FIG. 47  is a front view showing a construction of a steering box; 
         FIG. 48  is a sectional side view showing a construction of the steering box; 
         FIG. 49  is a plan view showing a construction of a neutral return mechanism of steering; 
         FIG. 50  is a plan view showing a construction of a switch disposed in a cam box; 
         FIG. 51  is a side view showing an arrangement of a residual pressure release valve; 
         FIG. 52  is a hydraulic circuit diagram showing a connection construction of the residual pressure release valve; 
         FIG. 53  is a schematic diagram showing an example of the actuation construction of the residual pressure release valve; 
         FIG. 54  is a schematic diagram showing a construction of the residual pressure release valve; 
         FIG. 55  is a sectional plan view showing a construction of the differential mechanism, and 
         FIG. 56  is a schematic diagram showing rotation action of a pressure plate. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Best Mode for Carrying out the Invention 
     The invention will be described in detail according to attached drawings. 
     First, description will be given of a general construction of a crawler tractor as an embodiment having a steering device according to the invention. 
     As shown in  FIGS. 1 and 2 , an engine  3  is arranged above front portions of crawler type traveling devices  1 . A rear transmission casing  5  is arranged above rear portions of the device  1 . The engine  3  is covered with a bonnet  4  and fixed at a lower portion thereof between left and right main frames  6 . 
     A steering column  2  is disposed behind the bonnet  4 , and a steering wheel  7  is arranged above the steering column  2 . A seat  8  is disposed behind the steering wheel  7 , and a step  18  is arranged between lower portions of the steering column  2  and the seat  8 , thereby constructing a driver&#39;s compartment, which is covered with a cabin  9 . 
     The vehicle is provided at the rear end thereof with a three point linkage  10  for supporting various kinds of work machines. 
     The crawler type traveling devices  1  are supported by respective crawler frames  15 . The crawler frames  15  have front end portions, to which a front transmission casing  16  supporting driving sprockets  11  is fixed. Each crawler frame  15  rotatably supports an idler  12  at a rear end portion thereof, and rollers  13  between the drive sprocket  11  and the idler  12 . A crawler belt  14  is looped over the drive sprocket  11 , the idler  12  and the rollers  13 . 
     Description will now be given of a power transmission system of the crawler tractor in accordance with  FIGS. 3 and 4 . 
     A clutch casing  19  is attached to the rear portion of the engine  3 . A damped output shaft  42  is extended rearward from the clutch casing  19  and connected to an input shaft of a hydraulic stepless transmission for traveling (hereafter, referred to as an HST for traveling)  22  attached to the front portion of the rear transmission casing  5 . 
     The rear transmission casing  5  contains a sub transmission  41 , a PTO transmission  43  and others. Driving force from the HST for traveling  22  is transmitted to the sub transmission  41 . The PTO transmission  43  is connected to the input shaft of the traveling HST  22  through a clutch  44 . A lower front portion of the rear transmission casing  5  serves as a traveling power take-off section in which an output shaft  102  is disposed. 
     In front of the engine  3  is disposed the front transmission casing  16  having a hydraulic stepless transmission for turning (hereafter, referred to as an HST for turning)  20  attached to the front surface thereof. The front transmission casing  16  is supported by the front portion of the main frames  6  so as to be arranged in the front portion of the vehicle body. 
     The HST for turning  20  is driven by driving force from the engine  3 . The drive sprockets  11  are driven by resultant output force of output force from the HST for turning  20  and output force from the above-mentioned sub transmission  41 . 
     The output rotation of the HST for traveling  22  on its motor side is varied by the sub transmission  41 , and then inputted to a differential gearing through a transmission shaft  45 . 
     The differential gearing will now be described in accordance with  FIG. 55 . 
     In the rear transmission casing  5 , the output shaft  102  transmits power from the engine  3  through a universal joint and the transmission shaft  45  to an input shaft  319  of the differential gearing in a differential casing  330 . Driving force of the input shaft  319  is transferred to an input shaft  343  through bevel gears  341  and  342 . 
     The rotational output of the output shaft  343  is divided into left and right planetary gear mechanisms  340 L and  340 R constituting the differential gearing. The left and right planetary gear mechanisms  340 L and  340 R comprise sun gears  344 L and  344 R, planetary gears  345 L and  345 R, carriers  346 L,  346 R,  348 L and  348 R, output gears  347 L and  347 R, etc. 
     The rotation of the input shaft  343  drives the sun gears  344 L and  344 R fixed on the left and right ends of the input shaft  343  in the same direction and at the same speed. Each of the planetary gears  345 L is shaped into two gears  360 L and  361 L stuck to each other on a common rotational axis. The planetary gears  345 R are formed substantially similarly with the planetary gears  345 L. The gears  360 L and  360 R engage with the respective sun gears  344 L and  344 R, and the gears  361 L and  361 R with the respective output gears  347 L and  347 R. 
     Each of the planetary gears  345 L is rotatably supported at one end thereof by a carrier  346 L rotatably provided on the input shaft  343 , and at the other end thereof by a carrier  348 L rotatably provided on a drive output shaft  349 L. The planetary gears  345 L are rotatably supported between the carriers  346 L and  348 L, and revolved together with the carriers  146 L and  148 L around the rotational axis of the input shaft  343  and the drive output shafts  349 L and  349 R. 
     The assembly of planetary gears  345 R is substantially similar with that of planetary gears  345 L. 
     A gear  362 L is formed on the outer peripheral surface of the carrier  346 L and engages with a gear  354 L. A divisional shaft  353 L penetrates the gear  354 L at an intermediate portion thereof. A bevel gear  352 L is fitted onto one end of the delivery shaft  353 L and engages with a bevel gear  325  fitted on a differential transmission shaft  224 . 
     Similarly with the carrier  346 L, the carrier  346 R is formed on the outer peripheral surface thereof with a gear  362 R, which engages with a gear  354 R. A divisional shaft  353 R penetrates the gear  354 R at an intermediate portion thereof. A bevel gear  352 R is fitted onto one end of the divisional shaft  353 R and also engages with the bevel gear  325 . 
     With regard to the above construction, it is assumed that the steering wheel  7  is kept in its neutral position. In this case, the HST for turning  20  is stationary so that the bevel gear  325  fixed on the differential transmission shaft  224 , and the bevel gears  352 L and  352 R and the gears  354 L and  354 R fixed on the respective divisional shafts  353 L and  353 R remain stationary, thereby braking the left and right carriers  346 L and  346 R engaging with the respective gears  354 L and  354 R. Accordingly, the carriers  146 L and  146 R remain substantially stationary without rotating around the input shaft  343 . 
     In this situation, rotation of the sun gears  344 L and  344 R is transferred into the planetary gears  345 L and  345 R rotatably supported on the stationary carriers  346 L and  348 L. 
     Therefore, the output gears  347 L and  347 R engaging with the gears  361 L and  361 R of the planetary gears  345 L and  345 R are rotated so as to rotate the left and right drive output shafts  349 L and  349 R. 
     Namely, while the steering wheel  7  remains in its neutral position, the differential gearing receives driving force from the engine  3  through only the transmission casing  5  and the input shaft  319 , thereby rotating the left and right drive output shafts  349 L and  349 R at the same speed in the same direction. 
     On the other hand, when the steering wheel  7  is rotated for turning of the vehicle, the differential transmission shaft  224  of the HST for turning  20  is rotated in correspondence to the rotational degree of the steering wheel  7 . 
     At this time, the bevel gear  325  is rotated so as to rotate the bevel gears  352 L and  352 R fixed on the divisional shafts  353 L and  353 R at the same speed in opposite directions through the bevel gear  325 . 
     Accordingly, the left and right carriers  3146 L and  346 R engaging with gears  354 L and  354 R are rotated on the outer periphery of the input shaft  343  at the same speed in opposite directions. The planetary gears  345 L are rotated integrally with the carriers  346 L and  348 L, and the planetary gears  345 R integrally with the carriers  346 R and  348 R, so that the planetary gears  345 L and  345 R revolve around the input shaft  343  at the same speed in opposite directions. 
     If either the planetary gears  345 L or  345 R are rotated so that their relative rotation to the corresponding carrier  346 L or  346 R is opposite to their relative rotation to the input shaft  343 , the sum of the two relative rotational speeds results in the rotational speed of the corresponding output gear  349 L or  349 R. If the two relative rotations are performed in the same direction, the remainder of subtraction between the two relative rotational speeds results in the rotational speed of the corresponding output gear  349 L or  349 R. 
     Namely, the resultant output of output from the engine  3  through the transmission casing  5  and output from the engine  3  through the HST for turning  20  causes a difference between the rotational speeds of the left and right drive output shafts  349 L and  349 R, thereby generating difference of rotation between the drive sprockets  11  of the left and right crawler type traveling devices, whereby the traveling vehicle turns left or right. 
     With regard to the HST for traveling  22 , an output shaft  101  of the HST for traveling  22  is driven by the rotation of the input shaft of the HST for traveling  22 . The drive rate and rotational direction of the output shaft  101  is controlled by controlling angle of a swash plate of a hydraulic pump of the input shaft side. 
     Driving force of the output shaft  101  is transferred to the sub transmission  41 , varied in speed by the sub transmission  41 , and transferred to the output shaft  102  in the traveling power take-off section. The above-mentioned transmission shaft  45  is connected to the output shaft  102  in the traveling power take-off section. 
     A brake mechanism  74  is constructed in the vicinity of the output shaft  102  so as to brake the output shaft  102 . The output shaft  102  is connected to a sun gear for driving the drive sprockets  11  through the transmission shaft  45 . Braking the output shaft  102  results in braking the drive sprockets  11 . 
     The lower portion of the transmission casing below the sub transmission  41  therein serves as the power take-off section in which the brake mechanism  74  is provided. The HST for traveling  22  is arranged in front of the sub transmission  41 , and the brake mechanism  74  is disposed under the HST for traveling  22 . 
     The arrangement of the brake mechanism  74  serving as a parking brake in the lower portion of the rear transmission casing  5  is available for the brake mechanism  74  to approach a later-discussed brake pedal  54 , so that a linkage, etc. therebetween becomes simple and it becomes so short as to reduce its friction loss and clearance, thereby facilitating operavility of the vehicle. 
     Furthermore, due to the structure peculiar to the crawler tractor, an advantageously large space is reserved under the cabin between the HST for traveling  22  and the engine  3 . Specifically, the space may be left as it is so as to facilitate for maintenance. Otherwise, any additional equipment such as a mechanism or a hydraulic valve for controlling a working machine may be disposed in the space without changing the appearance of the vehicle. 
     Description will be given on the brake mechanism  74  in accordance with  FIGS. 5 ,  6  and  56 . 
     The output shaft  102  is rotatably held by a transmission casing  107  and a brake cover  108  through a bearing, and a gear  102   a  is fixedly fitted on a rear end of the output shaft  102 . The gear  102   a  engages with a gear of the sub transmission  41  so that driving force from the sub transmission  41  is transferred to the output shaft  102 . The brake cover  108  is arranged in front of the brake mechanism  74  and fitted to the transmission casing  107  so as to cover the brake mechanism  74 . 
     A pressure plate  103 , and alternate multiple brake plates  105  and friction plates  104  are arranged around the output shaft  102 . 
     A front portion of the pressure plate  103  is fitted to an inside portion of the brake cover  108  so as to be restricted in its sliding direction by the brake cover  108 . A spring  110 , which is anchored at one of its ends to the brake cover  108 , is connected to the pressure plate  103  so as to bias the pressure plate  103  forwardly. 
     A ball  109  is disposed between the pressure plate  103  and the brake cover  108 , and engaged in a slot  103   b  formed by the pressure plate  103  and the brake cover  108 . 
     In the pressure plate  13 , the slot  103   b  is formed like an arc centered on the output shaft  102 . The depth of the slot  103   b  contacting with the ball  109  is reduced according to rotation of the pressure plate  103 . 
     Accordingly, as shown in  FIG. 56 , by depressing the later-discussed brake pedal  54 , the linkage mechanism is operated so that an actuation arm  113  is rotated centering around a steering input shaft  112  projecting from the actuation arm  113 . A notch is provided on an end of the steering input shaft  112 . When viewed in the axial direction of the steering input shaft  112 , the notch is semicircular, and a contact surface  112   a  is formed as a chord of the semicircular shape so as to abut against a contact surface  103   a  formed on the pressure plate  103 . When the steering input shaft  112  is rotated, an edge of the contact surface  112   a  close to the output shaft  102  is moved downward so as to push down the contact surface  103   a  of the pressure plate  103 , thereby rotating the pressure plate  103  centering around the output shaft  102 . Accordingly, the slot  103   b  formed in the pressure plate  103  moves so as to gradually move the ball  109  from the deep portion to the shallow portion in the slot  103   b . The ball  109  is engaged in the slot  103   b  so as not to move relative to the brake cover  108 , and the pressure plate  103  is slidable in the axial direction of the output shaft  102  toward/away from the brake cover  108 . As a result, the pressure plate  103  rotates and slides in the direction to press the friction plates  104  against the brake plates  105 . 
     The friction plates  104  and the brake plates  105  are disposed alternately behind the pressure plate  103 . The peripheral portions of the friction plates  104  engage with the transmission casing  107 , and the brake plates  105  with the output shaft  102 . 
     The friction plates  104  are not rotatable relative to the transmission casing  107  but are slidable in the longitudinal direction of the transmission casing  107 . On the other hand, the brake plates  105  are not rotatable relative to the output shaft  102  but are slidable in the longitudinal direction of the transmission casing  107 . 
     The friction plates  104  and the brake plates  105  are disposed between the pressure plate  103  and a rib of the transmission casing  107 . By sliding the pressure plate  103  backward with respect to the transmission casing  107 , the friction plates  104  are pressed against the brake plates  105 , thereby generating force for braking the output shaft  102 . 
     Accordingly, the braking force is applied on the output shaft  102  so as to actuate as a parking brake. 
     Since the brake mechanism  74  is disposed in the traveling power take-off section, i.e., the lower portion of the rear transmission casing  5 , the distance between the brake pedal  54  and the brake mechanism  74  is so short as to provide a compact linkage therebetween facilitating for its simple design and operativity. 
     Next, description will be given of a frame construction of the crawler tractor in accordance with  FIGS. 7 and 8 . 
     The main frames  6  are disposed in the front portion of the vehicle body, and the engine  3  is disposed on the main frames  6 . The HST for turning  20  is disposed between the main frames  6 . The front transmission casing  16  is connected to the main frames  6 , and the drive sprockets  11  are disposed on outsides of the front transmission casing  16 . 
     The front ends of the crawler frames  15  are connected to the outer surfaces of the front transmission casing  16 . The crawler frames  15  are also connected to the clutch casing  19  with frames  254  through support frames  251 . Furthermore, the rear ends of the crawler frames  15  are connected to the rear transmission casing  5  through rear support frames  252 . 
     The frames  254  cover the left and right sides of the HST for traveling  22  and the transmission shaft  45 . Side frames  253  are disposed on the respective outer surfaces of the frames  254 , and connect the support frames  251  with the rear support frames  252 . 
     Next, description will be given of the construction of the frames  254  according to  FIGS. 9 to 11 . 
     The frames  254 , which are C-like shaped when viewed in front, connect the clutch casing  19  and the rear transmission casing  5  to each other. As the above mentioned, the engine  3  is laid on the main frames  6 , and the clutch casing  19  is attached to the rear portion of the engine  3 . 
     The frames  254  are connected to the left and right surfaces of the clutch casing  19 , and the rear transmission casing  5  is connected to the rear portions of the frames  254 . 
     Since the frames  254  connect the clutch housing  19  and the rear transmission casing  5  on their left and right sides, the engine  3  is integrated with the rear transmission casing  5  through the frames  254 . 
     Due to the above-mentioned frame construction, the HST for traveling  22  is provided with a vertical opening which is advantageous for maintainability and cooling effect. For example, with regard to this embodiment, by removing a cover of a floor step in the cabin, the HST for traveling  22  can be maintained from its upper side. Since the frames  254  is clamped to the rear transmission casing  5  and the clutch casing  19  with bolts, the frames  254  can be removed easily by removing the bolts. 
     Further, this frame construction has sufficient strength against torsion and vertical weighting in spite of its lightweight. 
     Furthermore, as an embodiment shown in  FIGS. 12 and 13 , the frames  254  may be connected at the rear portions thereof to the respective crawler frames  15  through center support frames  255 , thereby enhancing frame rigidity of the crawler tractor. 
     With regard to this embodiment, the support frame  251 , the center support frame  255  and the rear support frame  252  are arranged at substantially regular intervals so as to enhance their rigidity for supporting the crawler frame  15 . 
     Next, description will be given of a construction for holding the top link in the linkage  10  for lifting a working machine in accordance with  FIGS. 14 to 16 . 
     The top link  261  is rotatably supported above the rear portion of the rear transmission casing  5  so that the rear end of the top link  261  is vertically rotatable. A stopper  264  is disposed above the supported portion of the top link  261 , and a hook  263  is rotatably supported on a side surface of the stopper  264 . The hook  263  is open upwardly backward. 
     A handle  262  is disposed on the upper surface of the top link  261 , and is longitudinally rotatable relative to the top link  261 . 
     When a work machine is removed from the linkage  10 , the top link  261  is raised to be held. 
     The handle  26  is hooked on the hook  263  for holding the raised top link  261 . Namely, the top link  261  is rotated upwardly, and then the handle  262  is pushed down ahead and engaged with the hook  263 . 
     At this time, a rotation fulcrum  263   b  of the hook  263 , a rotation fulcrum  262   b  of the handle  262 , and a point  265  where the handle  262  is engaged with the hook  263  are aligned on a straight line. Also, the rotational axis of the hook  263  and the rotational axis of the handle  263  are parallel. 
     Accordingly, when the top link  261  is rotated vertically by a shock in traveling of the vehicle or another, the hook  263  and the handle  262  are rotated, and the line, which passes along the rotation fulcrum  263   b , the engaging point  265  and the rotation fulcrum  262   b , is bent. Namely, the shock is absorbed and distributed by rotation of the handle  262  and the hook  263 . 
     Since the hook  263  is provided in the vicinity of the stopper  264 , the rotational degree of the hook  264  is restricted by the stopper  264 . Accordingly, the top link  261  is prevented from interfering with circumferential parts and being damaged. Furthermore, since a portion of the stopper  263  for abutting against the hook  264  is disposed above the hook  264 , parts disposed above the stopper  264  are protected from the hook  263 . 
     When the handle  262  is going to be removed from the hook  263 , the top link  261  is rotated upwardly so that the hook  263  abuts against the stopper  264 . Then, by rotating the top link  261  further upwardly, the handle  262  is slid forwardly against the hook  263 . Accordingly, the handle  262  is disengaged from the hook  263 . 
     Then, the hook  263  is rotated downwardly, and the handle  262  is pushed down rearwardly. The top link  261  is moved down so as to be released from its held state. 
     A disc spring or the like is disposed on the rotational axis of the hook  263  so as to press the hook  263  perpendicularly to the rotational direction of the hook  263 , so as to generate friction force for holding the rotated hook  263  at the place. 
     Therefore, even if the top link  261  is rotated upwardly by a shock in traveling of the vehicle or another and the hook  263  is disengaged from the handle  262 , the hook  263  is held in its upwardly rotated place. Accordingly, when the top link  261  is rotated downwardly, the hook  263  and the handle  262  are engaged again. 
     Namely, unless the hook  263  is rotated downwardly, the engaging state of the top link  261  is sustained. 
     Next, description will be given of hydraulic piping construction in accordance with  FIGS. 17 to 20 . 
     The crawler tractor is provided with a hydraulic pump for supplying pressure oil to the working machine connected to the front or rear portion of the vehicle body. Pressure oil is supplied from the hydraulic pump to front and rear connection ports (specifically, a front hydraulic port  285 , and couplers  275  and  276 ) through a sub control valve (hereafter, referred to as a SCV)  277 . 
     The SCV  277  is disposed sidewise from the rear transmission casing  5 . With regard to this embodiment, there are two hydraulic systems directly controlled by the SCV  277 . The systems are a first system  400  comprising pipings  280  and  281 , and a second system  401  comprising pipings  282  and  283 . 
     As shown in  FIG. 20 , the first system  400  is connected to an electromagnetic switching valve  278 , and is constructed so as to selectively supply pressure oil to either a front first system  402  or a rear first system  403 . The front first system  402  comprises pipings  271  and  272 . The front first system  402  is connected at one end thereof to the electromagnetic switching valve  278 , and at the other end thereof to the front hydraulic port  285 . The rear first system  403  comprises pipings  406  and  407 . The rear first system  403  is connected at one end thereof to the electromagnetic switching valve  278 , and at the other end thereof to the coupler  275 . 
     According to the above construction, two hydraulic systems: one for a front-loaded working machine; and the other for a rear-loaded working machine, are selectively controlled by operation of the SCV  277  for pressure oil supply to the first system  400  combined with switching operation of the electromagnetic switching valve  278 . 
     On the other hand, the hydraulic route construction on the downstream of the second system  401  is substantially similar with the hydraulic circuit construction on the downstream of the above-mentioned first system  400 . 
     The second system  401  is connected to an electromagnetic switching valve  279 , and is constructed so as to selectively supply pressure oil to either a front second system  404  or a rear second system  405 . The front second system  404  comprises pipings  273  and  274 . The front second system  404  is connected at one end thereof to the electromagnetic switching valve  279 , and at the other end thereof to a front hydraulic port  285 . The rear second system  405  comprises pipings  408  and  409 . The rear second system  405  is connected at one end thereof to the electromagnetic switching valve  279 , and at the other end thereof to the coupler  276 . 
     According to the above construction, two hydraulic systems: one for a front-loaded working machine; and the other for a rear-loaded working machine, are selectively controlled by operation of the SVC  277  for pressure oil supply to the second system  401  combined with switching operation of the electromagnetic switching valve  279 . 
     Namely, the maximum number of controllable hydraulic circuit systems by operative combination of the SCV  277  with the electromagnetic switching valves  278  and  279  is four, that is, the maximum two systems for a front working machine and the maximum two systems for a rear working machine. 
     In this embodiment, the two electromagnetic switching valves  278  and  279  are provided on the downstream of the SCV  277 . Alternatively, such an electromagnetic switching valve may be added so as to increase the number of controllable hydraulic systems by the single SCV  277 . 
     Next, description will be given of an arrangement of a fuel tank and an oil tank in accordance with  FIGS. 21 and 22 . 
     Rear fenders  203  are disposed on left and right sides of the lower rear portion of the cabin  9  so as to cover the lower rear side of the cabin  9 . A fuel tank  192  is disposed in the left rear fender  203 . A pressure oil tank  215  for HST is disposed in the right rear fender  203 . An oil cooler  195  is disposed in front of the pressure oil tank  215 . 
     A refueling opening of the fuel tank  192  is disposed outward for facilitating fuel-supply to the fuel tank  192 . 
     An auxiliary base on which an oil feeding tank and others are mounted will be described in accordance with  FIGS. 23 and 24 . 
     An auxiliary base  197  is disposed in the lower portion of the fender  203  with the fuel tank  192  therein. The auxiliary base  197  is horizontally rotatably supported at a front portion thereof on a pivot  199 . A release lever  198  is attached to the rear portion of the auxiliary base  197 , and is engaged with the auxiliary base  197  when the auxiliary base  197  is stored. 
     A hook is provided on the release lever  198 . The hook is engaged with a stopper  196  fixed to the vehicle body so as to keep the auxiliary base  197  in the stored state. 
     When the auxiliary base  197  is going to be used, the lever  198  is pulled so as to disengage the hook from the stopper  196 . Then, the auxiliary base  197  is pulled out rotating around the pivot  199 . 
     Since the auxiliary base  197  receives frictional force only from the pivot  199 , it can be pulled out smoothly. Accordingly, expansion of the auxiliary base  197  can be done easily. Furthermore, the auxiliary base  197  may be constructed compact. 
     A refueling opening  201  is provided in the upper portion of the fender  203  on the left side of the vehicle body, and fuel is supplied to the fuel tank  192  through the refueling opening  201 . The auxiliary base  197  is disposed rearward from the refueling opening  201  so that the front portion of the auxiliary base  197  is disposed in the vicinity of the refueling opening  201  in the longitudinal direction. 
     Accordingly, as shown in  FIG. 23 , when an oil-feeding tank  200  is placed on the auxiliary base  197 , an opening of the oil-feeding tank  200  is positioned in the vicinity of the refueling opening  201  so as to be easily inserted into the refueling opening  201 . 
     Next, description will be given of the construction of hydraulic circuits of the crawler tractor in accordance with  FIGS. 25 to 27 . 
     Hydraulic circuits of the crawler tractor are broadly divided into two circuits. One is a circuit for driving the transmission and the working machine, and controlling the attitude of the crawler tractor. The other is a circuit for actuating the HST for turning  20  and the HST for travelling  22 . Hydraulic pumps  182  and  191  for supplying pressure oil (lubricating oil) to these hydraulic circuits are actuated by driving force of the engine  3 . 
     Pressure oil (lubricating oil) drawn up by the hydraulic pump  191  from the bottom of the rear transmission casing  5  through a piping  363 , a filter  364  and a piping  365  is supplied to a clutch actuation circuit of a transmission  204  in the rear transmission casing  5  through a piping  366 , and further supplied to a flow dividing valve  208 . A level control mechanism  205  and a lift control mechanism  207  are connected to the flow dividing valve  208  so as to be supplied with pressure oil therefrom. A lift actuating mechanism  206  and the oil cooler  195  are connected to the downstream of the lift control mechanism  207  so as to be supplied with pressure oil. Pressure oil cooled by the oil cooler  195  is returned to the rear transmission casing  5 . 
     On the other hand, pressure oil drawn up by the hydraulic pump  182  from the pressure oil tank  215  through a piping  370 , a filter  371  and a piping  372  is branched at a branch point  216  through a piping  374  and a filter  375 . A part of the pressure oil is supplied from the branch point  216  to the HST for traveling  22  through a piping  376 . Return pressure oil of the HST for traveling  22  is returned to the pressure oil tank  215  through a common returning piping  212 . 
     The remainder of the pressure oil is supplied from the branch point  216  to the HST for turning  20  through a piping  377 . Return pressure oil of the HST for turning  20  is cooled by an oil cooler  214  provided in the front portion of the crawler tractor, and then is returned to the pressure oil tank  215  through a piping  378  and the common returning piping  212 . 
     According to this construction, it is not necessary to provide separate oil coolers for cooling the HST for turning  20  and the HST for traveling  22 , thereby reducing weight of the crawler tractor. Also, the oil passages is constructed so simply as to reduce cost of the crawler tractor. 
     A bypass switch  209  comprising relief valves and others is provided in a return pressure oil passage from the HST for turning  20  in parallel to the oil cooler  214 . 
     The circuit through the bypass switch  209  bypasses the oil cooler  214 . If the oil cooler  214  is clogged, the flux of pressure oil supplied to the HST for turning  20  is reduced so as to spoil a good performance of the crawler tractor in turning. Therefore, in the case that the oil cooler  214  is clogged or pressure oil have such low temperature as to require no further cooling, pressure oil is returned to the pressure oil tank  215  through the bypass circuit bypassing the oil cooler  214 , thereby constantly keeping the good performance of the crawler tractor in turning. 
     Using the clogging oil cooler  214  for a long time is not desirable in the viewpoint of durability of the HST for turning  20  and the HST for traveling  22 . Therefore, a sensor or the like is preferably provided to the bypass switch  209  so as to easily detect the condition of the bypass switch  209 . 
     With regard to this embodiment, the oil cooler  214  is arranged in the front portion of the crawler tractor, and the oil cooler  195  is arranged in the rear fender  203  disposed in the rear portion of the crawler tractor, but the construction is in no way limited to this. They may be interchanged or changed in location in consideration of heat radiated therefrom when they are cooled. 
     A construction in the vicinity of the oil cooler  214  of this embodiment will be described in accordance with  FIGS. 28 to 31 . 
     A capacitor for air-conditioner  124  is disposed behind the oil cooler  214 , and a radiator  125  is disposed behind the capacitor for air-conditioner  124 . 
     A fan (not shown) is disposed behind the radiator  125  so as to take in air from the front and exhaust it to back. 
     A seal member  126  is equipped between the radiator  125  and the capacitor for air-conditioner  124 . The seal member  126  is equipped on the circumferential portion of the radiator  125  and abuts against the circumferential portion of the capacitor for air-conditioner  124 . Accordingly, the capacitor for air-conditioner  124  is sealed with the contact portion of the radiator  125  so as to prevent air between the radiator  125  and the capacitor for air-conditioner  124  from flowing out, so that quantity of air flowing into the capacitor for air-conditioner  124  is increased to enhance cooling efficiency of the capacitor for air-conditioner  124 . 
     The seal member  126  comprises a tubular contact portion  126   b  and a sectionally C-like attached portion  126   c . Accordingly, the contact portion  126   b  is easily deformable when pushed, and has high adherence. The seal member  126  may be made of elastic material such as gum so as to facilitate for attachment and detachment of the attached portion  126   c . Easy removable weather strip may be used as the seal member  126 . 
     As mentioned above, the seal member  126  enhances adherence of the radiator  125  with the capacitor for air-conditioner  124 . The long-term used seal member  126  is easily detached so that dust may be easily cleared out therefrom. Accordingly, the maintainability is improved, overheat is prevented, and lives of the parts are prolonged. 
     Next, description will be given of arrangement of the oil cooler  195  of this embodiment. 
     As mentioned above, the oil cooler  195  is disposed in front of the pressure oil tank  215 , in the rear fender  203 . Since the lower portion of the rear fender  203  is opened, airflow is generated in the rear fender  203  by the movement of the crawler so as to prevent the temperature therein from easily arising. Since the oil cooler  195  is disposed in the lower portion of the rear fender  203 , air is supplied from the outside to the oil cooler  195 . The pressure oil tank  215  arranged in front of the oil cooler  195  protects the oil cooler  195  from splashes of mud. 
     An electric fan attached to the oil cooler  195  takes in the air from outside of the vehicle body, and exhausts it into the inside of the vehicle body, thereby preventing noise of the fan from easily leaking out. 
     A switch is disposed in the transmission casing so as to control the fan. The switch is turned on when oil temperature rises, and turned off when oil temperature falls. 
     Since the oil cooler  195  is disposed near the rear transmission casing  5  so as to facilitate for shortening pipings thereto and for its compact design. 
     The oil cooler  195  arranged as mentioned above has enhanced cooling effect so as to improve durability of the fan of the oil cooler  195  and save energy required for its cooling. The rear fender  203  covering the oil cooler  195  and the pressure oil tank  215  keeps good appearance and reduces noise. 
     A sufficiently large space is ensured in the rear fender  203  so as to enhance the flexibility of arrangement of the oil cooler  195 . Accordingly, as shown in  FIG. 31 , the air-exhaust side of the oil cooler  195  is directed to the pressure oil tank  215 . 
     By directing the air exhausted from the oil cooler  195  to the pressure oil tank  215 , the pressure oil tank  215  is cooled by the exhausted air. 
     Description will now be given on the connection construction between the HST for turning and the front transmission casing according to  FIGS. 32 to 34 . 
     The engine  3  is laid on the main frames  6 . An input shaft  221  of the front transmission casing  16  is disposed in front of the engine  3 . The HST for turning  20  is disposed in front of the front transmission casing  16 . 
     Driving force of the engine  3  is inputted into the front transmission casing  16  through the input shaft  221 , and transferred to the differential gear mechanism in the front transmission casing  16  again through the HST for turning  20 . 
     The driving force inputted to the input shaft  221  is transmitted to an input shaft  222  of the HST for turning  20  through the transmission in the front transmission casing  16 . The input shaft  222  drives a hydraulic pump of the HST for turning  20 , and an output shaft  223  is driven by a hydraulic motor of the HST for turning  20 . 
     The output shaft  223  of the HST for turning  20  is connected to the differential transmission shaft  224 , which transfers driving force to the left and right planetary gear mechanisms  340 L and  340 R constituting the differential gear unit in the front transmission casing  16 . 
     Spline grooves are formed on the rear end of the output shaft  223  and the front end of the differential transmission shaft  224 , so that a boss  225  may be fitted on the output shaft  223  and the differential transmission shaft  224 . 
     The boss  225  is slidable longitudinally on the rear end of the output shaft  223  and the front end of the differential transmission shaft  224 . In the case that the boss  225  is engaged with both the output shaft  223  and the differential transmission shaft  224 , driving force of the output shaft  223  is transferred to the differential transmission shaft  224 . In the state that the boss  225  is engaged with only the differential transmission shaft  224 , output of the output shaft  223  is not transferred to the differential transmission shaft  224 . 
     On the rear peripheral portion of the boss  225  is formed a recess  226 . A fork  227  fixed to a piston  228  is engaged at one end thereof in the recess  226 . 
     The piston  228  is disposed in the longitudinal direction of the vehicle body, and inserted at one end thereof into a cylinder part  230  provided in the front transmission casing  16 . A spring is engaged with the piston  228 , and abuts against the fork  227  and the front transmission casing  16  so as to bias the piston  228  forward. 
     The piston  228  is inserted at a front end thereof into the cylinder part  230 , to which an oil passage  229  is connected, so that charge pressure is transferred to the cylinder part  230  through the oil passage  229 . 
     When oil pressure in the cylinder part  230  increases and the force applied to the piston  228  becomes larger than the bias force of the spring, the piston is slid backward. Then, the fork  227  connected to the piston  228  makes the boss  225  slide backward, thereby shutting off the transfer of driving force from the output shaft  223  to the differential transmission shaft  224 . 
     When oil pressure in the cylinder part  230  decreases, the fork  227  connected to the piston  228  makes the boss  225  slide forward, thereby transferring driving force from the output shaft  223  to the differential transmission shaft  224 . 
     Since the clutch mechanism which performs on-off operation of transfer of driving force is provided between the HST for turning  20  and the actuation mechanism, and the fork connected to the clutch is slid by oil pressure, the on-off operation of driving force of the HST for turning  20  can be performed with simple construction. Furthermore, since the fork  227  is slid by the piston using charge pressure, the slide mechanism for the fork can be constructed easily. In addition, charge pressure can be used easily by connecting the hydraulic passage  229  to a hydraulic circuit supplied with charge pressure. 
     Next, description will be given of the construction for driving a front PTO shaft. 
     As shown in  FIGS. 35 and 36 , the input shaft  222  of the HST for turning  20  is inserted into a pulley  232  so that the pulley  232  is rotated integrally with the input shaft  222 . A casing of the HST for turning  20  is projected in the extension direction of the input shaft  222  around the input shaft  222 . The pulley  232  is fitted to the projecting part of the casing of the HST for turning  20  through a bearing. Namely, the pulley  232  is supported by the casing of the HST for turning  20  so as not to stress the input shaft  222 . 
     A belt  237  is looped over the pulley  232  so as to transfer driving force of the pulley  232  to a front PTO mechanism  234 . In the front PTO mechanism  234 , the belt  237  is looped over a pulley  235 . The pulley  235  is connected to a front PTO shaft  236  so as to construct the front PTO shaft  236  to be rotated integrally with the pulley  235 . The construction for transferring driving force to the front PTO mechanism  234  through the belt  237  is simple, and flexibility of its design is improved. 
     The front PTO mechanism  234  is arranged under the input shaft  222  so as to bypass the frames  6 , thereby being connected to the PTO shaft  236 . 
     In this regard, the input shaft  222  of the HST for turning  20  projects so as to be attached to the pulley  232 , thereby taking off power for driving a working machine equipped on a front portion of the working vehicle. 
     According to the above-mentioned construction, a PTO section with a simple construction is provided at the front portion of the vehicle body, thereby facilitating maintainability and assembly thereof. 
     Next, description will be given of operative construction for turning, main speed change, and brake. 
     Referring to  FIG. 37 , the steering wheel  7  is connected to a steering box  52 , so that operation of the steering wheel  7  is transferred to a conic linkage  53  through the steering box  52 . Also, a main speed change lever  55  is connected to the conic linkage  53  through another linkage. A brake pedal  54  is disposed in the vicinity of the conic linkage  53 . 
     Operation of the steering wheel  7  and operation of the main speed change lever  55  are inputted into the conic linkage  53 . 
     In the conic linkage  53 , joint portions are moved along a bottom surface and side surfaces of conic shapes. 
     A rod for straight traveling and a rod for turning are located at one ends thereof to serve as respective peaks of two large and small conic shapes while they are located at the other ends thereof on the common bottom circle of the conic shapes with the phase difference of 90 degrees. Therefore, the peaks of the conic shapes for straight traveling and for turning are moved in association with location of arms for operating the HST for traveling  22  and the HST for turning  20 . 
     It is constructed so that the rotational angle of the main speed change lever  55  is proportional to the inclination of the bottom surface of the conic shapes and the rotational angle of the steering wheel  7  is proportional to the rotational angle of the bottom surface of the conic shapes. 
     In a stationary state, even if the steering wheel  7  is rotated left or right, constant distances from the peaks to the bottom circle of the conic shapes are kept so as to prevent the operation arms of the HST for traveling  22  and the HST for turning  20  from moving. Accordingly, the hydraulic motors of the HST for traveling  22  and the HST for turning  20  are not driven, so that the vehicle remains stationary. 
     Next, when the main speed change lever  55  is pushed down to its advancing direction, the operation arm of the HST for traveling is rotated to its advancing side. However, since the end of the rod for turning is located on the rotational center line of the bottom surface of the conic shape, the operation arm of the HST for turning  20  remains stationary regardless of the inclination of the bottom surface, whereby the vehicle travels straight. 
     If the steering wheel  7  is turned from this state, the operation arm of the HST for traveling  22  is moved to its slowing down direction, and the operation arm of the HST for turning  20  is moved. Namely, as the steering wheel  7  is turned, the crawler tractor turns while being automatically decelerated. 
     In the backing state, when the steering wheel  7  is turned, the operation arm of the HST for turning  20  is moved to the direction opposite to that in advancing. The operation of the HST for turning  20  is reversed depending on whether it is done in advancing or backing, thereby canceling the reverse steering phenomenon. 
     As mentioned above, operation of the steering wheel  7  and operation of the main speed change lever  55  are linked mutually by the conic linkage  53 . In this embodiment, furthermore, operation of the brake pedal  54  is linked with operation of the main speed change lever  55  or rotation of the operation arm of the HST for traveling  22 . 
     Description will now be given of the conic linkage  53  according to  FIGS. 38 and 39 . 
     A steering input shaft  112  is connected to the above-mentioned steering wheel  7  through a universal joint, so that the steering input shaft  112  is connected at a lower end thereof through the universal joint to a rocking member  117  of a converting mechanism for bringing the turning direction of the vehicle into correspondence to speed change in advancing and backing. 
     The rocking member  117  is formed in a substantially conic shape, and is supported at a middle portion thereof by a bracket provided on an end of a rocking shaft  118  through a bearing. The rocking member  117  is rotated around the steering input shaft  112  and tilted around the rocking shaft  118  in connection with the rotation of the above-mentioned steering wheel  7 . The rocking shaft  118  is rotatably supported by an inner surface of a column through a bearing so as to arrange the axis of the rocking shaft  118  laterally horizontally. 
     An arm part  133  is extended laterally from the rocking member  117  so as to be connected at an end thereof to a connection member  119 . A steering link  115  is connected to one end of the connection member  119  through a universal joint or the like, and a traveling link  111  is connected to the other end of the connection member  119  through a joint. 
     When the steering wheel  7  is positioned in its straight traveling position, the joint for connecting the steering link  115  with the connection member  119  is positioned on extension of the axial center of a bracket provided on an end of the above-mentioned rocking member  117 . The rocking shaft  118  is arranged in the lateral direction to intersect extension of the above-mentioned axial center of the bracket when it is set in neutral. 
     Also, the steering link  115  is connected at a lower end thereof to an arm engaged with a shaft  89 , so that, by rotating the arm, a rod  106  is operated so as to operate the speed change arm of the HST for turning  20 . 
     The travel link  111  is connected to the other end of the connection member  119  disposed on the rocking member  117  through a joint. This joint is arranged in the position at an angle of 90 degrees from the above-mentioned joint connecting the steering link  115  with the connection member  119  with respect to the axial center of the bracket  147  on the end of the above-mentioned rocking member  117 . 
     Then, the lower end of the travel link  111  is connected to an arm engaging at one end thereof with a shaft  116 . A rod  127  is connected at one end thereof to an arm connected to the shaft  116 , and at the other end thereof to one end of an arm fitted on the shaft  89 . 
     The lower end of the above-mentioned steering link  115  and the lower end of the travel link  111  are arranged on extension of the axial center of the bracket  147  on the end of the rocking member  117 . 
     An arm  120  is projected backward from the bracket  147  of the above-mentioned rocking shaft  118 . A main speed change link  114  is connected at an upper end thereof to an end of the arm  120 , and at a lower end thereof to the arm engaged on the shaft  89 . Accordingly, the main speed change link  114  is connected to the HST for traveling  22 . 
     Next, description will be given of a linkage of the main speed change lever  55  according to  FIG. 40 . 
     The main speed change lever  55  is rotatably pivotally provided at a lower end thereof on a pivot  90 , and a forwardly projecting arm is fixed to the lower end of the main speed change lever  55 . The arm is connected at an end thereof with an upper end of a rod  81 . The rod  81  is connected at a lower end thereof with an arm rotatably supported on a shaft  82 . 
     The arm supported by the shaft  82  is rotated integrally with an arm  83 . Accordingly, by rotating the main speed lever  55 , the arm  83  is rotated longitudinally. 
     A rod  91  is connected at a rear end thereof to the lower end of the arm  83 , and at a front end thereof to an arm  84  fixed to the shaft  89 . The arm  84  is rotatable longitudinally around the shaft  89 , and is rotated integrally with an arm  84   b  whose one end is engaged on the shaft  89  similarly. An arm  85  is connected to the arm  84   b . The shaft  89  is connected to the above-mentioned conic linkage  53 . 
     The rod  85  connects an arm  86  with the arm  84   b , so that the arm  86  is rotatably supported at the lower end thereof. A plate  87  is disposed longitudinally and is connected to the arm  86 . An arm  88  of the HST  22  is connected to an end of the plate  87 . 
     Due to such a linkage, the HST for traveling  22  is operated by operating the main speed change lever  55 . 
     Next, description will be given of a linkage mechanism of the brake according to  FIGS. 41 and 42 . 
     The brake pedal  54  is connected to the above-mentioned brake mechanism  74  through a linkage so that the brake mechanism  74  is operated by the brake pedal  54 , whereby a parking brake is actuated. 
     The brake pedal  54  is fixed on a pivot  62 , and the pivot  62  is rotatably supported on an upper portion of a front column. The pivot  62  is constructed to be rotated together with the brake pedal  54  by depressing the brake pedal  54 . Arms  63  and  64  are fixed to the pivot  62 , and rods  65  and  66  are fixed to ends of the arms  63  and  64 , respectively. 
     The rods  65  and  66  are disposed longitudinally, and a cam lever  61  is connected to the lower end of the rod  66 . The cam lever  61  is rotatably supported on a pivot fixed on a side surface of a box which contains the conic linkage  53 , and is rotated by vertical sliding of the rod  66 . 
     An arm  68  is connected to a lower end of the above-mentioned rod  65 , and the arm  68  is rotatably supported on a pivot  67 . An arm  69  rotated integrally with the arm  68  is fixed on the arm  68 , and a rod  70  is connected to an end of the arm  69 . 
     The rod  70  is disposed in the longitudinal direction of the vehicle body, and an L-like shaped arm  72  rotatably supported on a pivot  71  is connected at one end thereof to the rear end of the rod  70 . 
     A vertical rod  73  is connected to the other end of the L-like shaped arm  72 , and an actuation arm  113  of the brake mechanism  74  is connected to the lower end of the rod  73 . 
     Accordingly, by depressing the brake pedal  54 , the actuation arm  113  is rotated, thereby actuating the brake mechanism  74 . 
     At this time, since the brake mechanism  74  is located near the brake pedal  54  (namely, around the output shaft  102  projected from the rear transmission casing  5  under the cabin  9 ), the link for the brake is shortened in its full length. As a result, the linkage mechanism of the brake mechanism  74  may be designed simply, and operativity of the brake mechanism  74  is improved. 
     In addition, a parking brake lever  75  is provided on the inside of the brake pedal  54  so as to anchor the depressed brake pedal  54  (in the state for applying brake). 
     By depressing the brake pedal  54  and rotating the parking brake lever  75  upward, the brake pedal  54  is kept in its depressed state, thereby applying the parking brake. The parking brake is released by rotating the parking brake lever  75  downward. 
     Next, description will be given of the linkage between the brake pedal  54  and the main speed change lever  55  according to  FIG. 40 . 
     As mentioned above, the shaft  89  connected to the conic linkage  53  is rotated integrally with the main speed change lever  55 . An arm  92  is fixed at one end thereof to the shaft  89 . By rotating the main speed change lever  55 , the arm  92  is rotated, and in connection with it, the main speed change lever  55  is rotated. 
     A projection  93  is provided on a tip of the arm  92  so as to project in the direction of the rotational axis of the arm  92 . 
     The projection  93  is positioned on the inside of a cam part of the above-mentioned cam lever  61  so that the projection  93  may abut against the inner surface of the cam part of the cam lever  61  by rotating the cam lever  61 . 
     By depressing the brake pedal  54 , the cam lever  61  is rotated, and the arm  92  is returned to its predetermined position by the cam lever  61 . The predetermined position where the arm  92  is returned by the cam lever  61  corresponds to the neutral position of the main speed change lever  55 . Therefore, by depressing the brake pedal  54 , the main speed change lever  55  is returned to its neutral position, and the brake mechanism  74  is actuated. 
     For stopping a crawler tractor without applying too much load onto driving parts and brakes, it is desirable that a main speed change lever is moved to its neutral position and then the brake is actuated. However, according to this embodiment, by only depressing the braking pedal  54 , the HST for traveling  22  returns to its neutral position and the brake mechanism  74  is actuated. 
     Also, both braking force of the HST for traveling  22  (engine brake) and braking force of the brake mechanism  74  are available as braking force for stopping the vehicle. The brake mechanism  74  is so constructed as to apply braking force after the HST for traveling  22  is braked, thereby being smaller than the conventional brake mechanism. 
     Furthermore, by adjusting actuation timings of the two brakes, abrasion of the parking brake in the brake mechanism  74  can be prevented. 
     The shaft  89  is connected to the main speed change link  114  and the steering rod  115  in the conic linkage  53 , as shown in  FIGS. 39 and 41 . The shaft  89  is a member to which main speed change operation and steering operation are transferred. A mechanism for keeping the HST for turning  20  and the HST for traveling  22  in neutral is provided on the shaft  89 . Namely, by returning an arm connected to the shaft  89  to its neutral position, the conic linkage  53  is returned to its neutral position. 
     Also, as shown in  FIG. 43 , a mechanism for neutral-setting of the operation linkage comprising the cam lever  61  and the arm  92  is arranged in a lower portion of a chamber containing the conic linkage  53 , and is positioned in the middle of the linkage connecting the main speed change lever  55  with the steering wheel  7 . Accordingly, the main speed change lever  55  can be connected with the steering wheel  7  easily, and plays of the main speed change lever  55  and the steering wheel  7  can be adjusted easily. 
     More detailed description will be given of the mechanism for neutral-setting of the operation linkage comprising the cam lever  61  and the arm  92  according to  FIGS. 44 and 45 . 
     The cam lever  61  is formed in a hook-like shape, and a cam is formed in the hook. The cam shape formed in the cam lever  61  is broad in its lower portion and narrow in its upper portion. 
       FIG. 44(   a ) illustrates the cam lever  61  and the arm  92  in the neutral state of the brake pedal  54 ;  FIG. 44(   b ) illustrates the same in the state that the brake pedal is depressed half, and  FIG. 44(   c ) illustrates the same in the state that the parking brake is applied. 
     As shown in  FIG. 44(   a ), when the brake pedal  54  is in its neutral state, the projection  93  does not abut against the cam lever  61 , so that the arm  92  is free from motion of the cam lever  61 . Accordingly, the actuation state of the conic linkage  53  is not restricted. 
     Then, as shown in  FIG. 44(   b ), when the brake pedal  54  is depressed half, the arm  92  is returned to its neutral position according to the cam shape of the cam lever  61 , and the conic linkage  53  is also put into its neutral state. However, at this time, the brake mechanism  74  as the parking brake remains actuation-free. 
     By further depressing the brake pedal  54 , as shown in  FIG. 44(   c ), the brake mechanism  74  actuates. 
     A portion of cam groove in the cam lever  61  for neutral location of the arm  92  is elongated so that the brake pedal  54  can be depressed while the projection  93  is located in its neutral position. 
     Next, description will be given of a construction of the rod  73  for connecting the brake mechanism  74  with the brake pedal  54 . 
     By depressing the brake pedal  54 , the rod  65  is slid upward so as to rotate the arms  68  and  69 . Accordingly, the rod  70  is slid forwardly so as to rotate the arm  72  and slide the rod  73  upward. 
     A shock absorbing mechanism  73   b  is provided on the rod  73 . The shock absorbing mechanism  73   b  is constructed by connecting upper and lower portions of the rod  73  through an elastic member such as a spring. According to an example shown in  FIG. 45 , a spring is wound around a lower portion of the rod  73 , and a spring is inserted into an upper cylindrical portion the rod  73 . 
     The springs are contracted when the upper portion of the rod  73  is slid upward. When contraction of the springs reaches a fixed degree, the lower portion of the rod  73  is raised upward, thereby actuating the braking mechanism  74 . 
     Accordingly, the brake is actuated after the conic linkage  53  is returned to neutral, so that load applied to the brake mechanism  74  is reduced, thereby improving the durability of the brake mechanism  74 . 
     Since the shock absorbing mechanism  73   b  is provided on the rod  73 , when the actuation arm  113  of the brake mechanism  74  are raised upward completely, the brake mechanism  74  can be actuated by a substantially constant force without raising the actuation arm  113  any more. In addition, by providing the shock absorbing mechanism  73   b  on the rod  73 , it is not necessary to provide another shock absorbing mechanism on the rod  65  or the like, and the arm  68  and the brake pedal  54  can be connected mutually through a simple rod. 
     Next, description will be given of the construction for avoiding harsh brake according to an example shown in  FIG. 46 . 
     According to the example shown in  FIG. 46 , a swash plate control arm of the HST for traveling  22  is connected to the brake pedal  54  through a servo valve  134 . Accordingly, the brake works gradually when the brake pedal  54  is operated. Therefore, the operativity is improved and hard brake is prevented. 
     The brake pedal  54  is connected to the arm  68  through the rod  65 . The arm  68  is rotated integrally with the arm  69  and an arm  135 . 
     The arm  69  is connected with the brake mechanism  74  as mentioned above, and the arm  135  is connected with the arm of the HST for traveling  22  through the servo valve  134 . Also, the arm of the HST for traveling  22  is connected to the arm  84   b , and the arm  84   b  is rotated with rotation of the main speed change lever  55 . 
     When the brake pedal  54  is depressed, the servo valve  134  is pulled forward by the arm  135 . As mentioned above, the brake mechanism  74  does not cause braking force unless the brake pedal  54  is depressed to some degree, and the servo valve  134  is actuated when the arm of the HST for traveling  22  is returned to its neutral position. Accordingly, when the arm of the HST for traveling  22  is returned to its neutral position gradually by the servo valve  134 , the conic linkage  53  and the main speed change lever  55  are returned to their neutral positions along with the arm. 
     Then, the arm of the HST for traveling  22  is set in its neutral position so as to apply an engine brake, thereby braking the crawler tractor to some degree, and then, the brake mechanism  74  is actuated so as to apply a sure parking brake to the crawler tractor. 
     Accordingly, harsh brake is prevented so as to improve operativity of the crawler tractor. 
     Next, description will be given of a construction of the steering box  52  according to  FIGS. 47 to 50 . 
     The steering box  52  is disposed between the steering wheel  7  and the conic linkage  53  so as to transmit operation of the steering wheel  7  to the conic linkage  53 . 
     A steering shaft  151  is rotatably disposed in the steering box  52 , and a cam box  163  is provided on the lower part of the steering box  52 . The steering shaft  151  projects at an upper portion thereof upward from the steering box  52  and connected to the steering wheel  7 . The steering shaft  151  projects at a lower portion of the steering shaft  151  into the cam box  163 . 
     A gear  152  is formed on a lower end of the steering shaft  151  so as to engage with a cam gear  150 . 
     A steering transmission shaft  164  is fixed on the rotation center of the cam gear  150 , and connected at a lower end thereof to the steering input shaft  112  of the conic linkage  53  through a universal joint or the like. 
     According to the above-mentioned construction, when the steering wheel  7  is rotated, the steering shaft  151  is rotated, and then the steering transmission shaft  164  is rotated with the cam gear  150 . Therefore, the steering input shaft  112  connected to the steering transmission shaft  164  is rotated. 
     A cam mechanism for restricting rotation of the steering wheel  7  and returning the steering wheel  7  to the neutral position is built in the cam box  163 . 
     A cam  153  is equipped on a lower surface of the cam gear  150  fixed to the steering transmission shaft  164  so as to be rotated integrally with the cam gear  150 . A support base  161  is provided in the lower part of the cam box  163 , and a stopper  156  for restricting rotation of the cam  153  is provided upright on the support base  161 . 
     A neutral return mechanism  166  is provided symmetrically with the stopper  156  with respect to the rotational center of the cam  153 . The neutral return mechanism  166  comprises a roller  157 , an arm  158  and a spring for biasing the arm  158  toward the cam  153 . 
     The arm  158  is rotatably supported at one end thereof by an upright shaft provided on the support base  161 , and the roller  157  is rotatably supported on the arm  158 . Since the arm  158  is biased toward the cam  153 , the roller  157  is pressed against the cam  153 . The stopper  156  and the roller  157  as a neutral returning member are disposed symmetrically with respect to the axial center of the cam  153 . 
     The cam  153  is provided with a contact portion  154  for abutting against the stopper  156  and with a neutral portion  155  for abutting against the roller  157  in neutral. The contact portion  154  is formed on a side surface of the cam  153  furthest apart from the rotational center of the cam  153 , and the neutral portion  155  nearest to the rotational center of the cam  153 . The contact portion  154  and the neutral portion  155  are connected mutually through a smooth curve line, so that the cam  153  is formed symmetrically with respect to a line joining the rotational center of the cam  153  and the neutral portion  155 . The cam  153  is formed to gradually expand in its radial direction as it goes apart from its neutral position, and the cam  153  is formed at one end thereof with the contact portion  154  for abutting against the stopper for restricting rotation thereof. 
     When the cam  153  constructed as mentioned above is rotated from its neutral position, the cam  153  pushes away the roller  157  against biasing force of the spring. Since the roller  157  is biased toward the cam  153 , when the steering wheel  7  is released from an operator&#39;s hands, the cam  153  is rotated so as to return to its neutral position. 
     The cam  153  is rotatable unless the contact portion  154  abuts against the stopper  156 , so that rotation of the cam  153  is restricted based on the positions of the stopper  156  and the contact portion  154 . 
     A plate  159  is fixed at one end thereof on a lower surface of the arm  158 . The plate  159  is extended downward, and a bolt and a nut are equipped on the lower end of the plate  159  in the rotation direction of the arm  158 . 
     A switch base  160  is attached to the support base  161  toward the above-mentioned plate  159 . On an end of the switch base  160  is disposed a switch  171 , which is brought into contact with the bolt on the plate  159  so as to be turned on or off. 
     Namely, by rotating the steering wheel  7 , the cam  153  is rotated, and the arm  153  is rotated so as to change a distance between the switch  171  and the above-mentioned bolt. The distance between the switch  171  and the bolt becomes the shortest when the cam  153  is in its neutral position, and becomes the longest when the contact portion  154  of the cam  153  abuts against the stopper  156 . According to this construction, the switch  171  detects the neutral location of the cam  153 . 
     Next, description will be given of a mechanism for releasing residual pressure of the HST for traveling according to  FIGS. 41 ,  42  and  51  to  53 . 
     By depressing the clutch pedal  54 , as mentioned above, the arm of the HST for traveling  22  is set in its neutral position, and the brake mechanism  74  is actuated after the crawler tractor is braked to some degree by engine brake of the neutral HST for traveling  22 . At this time, the HST for traveling  22  is stopped when one of a pair of passages which mutually connect a motor  22   a  and a variable type pump  22   b  in the HST for traveling  22  is hydraulically pressured higher than the other. Namely, each plunger of the motor  22   a  and the variable type pump  22   b  is stressed at one end in its sliding direction by an oil pressure difference (residual pressure) between the above-mentioned pair of passages so as to cause a load at the time of starting the engine. Therefore, it is desirable to release the residual pressure in the pair of passages connecting the motor  22   a  to the variable type pump  22   b  (namely, to cancel the pressure difference therebetween). 
     When the brake pedal  54  is depressed, the arm  63  is rotated integrally with the brake pedal  54  around the pivot  62 , and the rod  65  pivotally attached to an end of the arm  63  is moved upward. At this time, the arm  68  pivotally connected at an end thereof to the lower end of the rod  65  is rotated together with an arm  176  around the pivot  67 , and a rod  177  pivotally connected to a rear end of the arm  176  is moved downward. By moving the rod  177  downward, a solenoid in a residual pressure release valve  178  is pushed in against biasing force of the spring, so as to bring pipings  180  and  181  into mutual fluidal communication for releasing the residual pressure. 
     The residual pressure release valve  178  is disposed on a rear surface of the clutch casing  19  and in front of the HST for traveling  22 , and the pipings  180  and  181  are respectively connected to the pair of passages which mutually connect the motor  22   a  and the variable type pump  22   b  in the HST for traveling  22 . 
     Due to the above construction, by depressing the brake pedal  54 , the pair of passages which connect the motor  22   a  and the variable type pump  22   b  are connected mutually so as to cancel the oil pressure difference (release the residual pressure). When the depressed brake pedal  54  is released, the above-mentioned members from the brake pedal  54  to the solenoid in the residual pressure release valve  178  are operated in the opposite direction so as to cut off the connection of the pipings  180  and  181  for releasing residual pressure, so that the main circuit of the HST for traveling  22  may be actuated. 
     Next, description will be given of another valve for releasing the residual pressure. 
     With regard to an example shown in  FIG. 53 , an electromagnetic valve  188  serving as a residual pressure release valve is operated by the brake pedal  54 . 
     A linkage  184  is connected to the brake pedal  54  so as to put on/off a switch  185  connected to the electromagnetic valve  188 . When the switch  185  is turned on, the electromagnetic valve  188  is slid so as to mutually connect a pair of bypassing passages  186  and  187  connected to the motor  22   a  and the variable type pump  22   b.    
     When the switch  185  is turned off, the electromagnetic valve  188  cuts off the connection between the passages  186  and  187 . Accordingly, when the brake pedal  54  is not depressed, the main circuit of the HST for traveling  22  is not bypassed. 
     The linkage  184  connected to the brake pedal  54  for releasing residual pressure in the HST for traveling  22  has such a simple structure as to operate the switch  185 . 
     Therefore, the start of the engine is facilitated because the brake pedal is always depressed in starting operation of the engine, and residual pressure is released with an easy construction. 
     Alternatively, as shown in  FIG. 54 , the residual pressure release valve may be operated by a key switch. 
     A key switch  189  has an electric circuit, which is turned on/off by a key  190  inserted thereinto and rotated. 
     The electromagnetic valve  188  serving as a residual pressure release valve is connected to the key switch  189 . The key  190  is inserted into the key switch  189  and rotated so as to connect the key switch  189  to the electromagnetic valve  188 . 
     The key switch  189  serves as a starter switch of the crawler tractor, so that, when the starter switch is actuated, the electromagnetic valve  188  is operated. 
     Accordingly, when the starter is actuated, the pressures in the HST for traveling  22  and the HST for turning  20  are always low so as to facilitate for start of the engine. Also, hard actuation for releasing residual pressure becomes unnecessary. 
     The above-mentioned residual pressure release valve  178 , which is connected to the main circuits of the HST for traveling  22  and the HST for turning  20 , may be disposed in a housing integrally with the HSTs. Due to this, the residual pressure of the HST for traveling  22  and the HST for turning  20  is released so as to easily rotate the output shaft for traveling, thereby facilitating for restarting of the engine. 
     INDUSTRIAL APPLICABILITY 
     As mentioned above, the crawler tractor according to the invention is suitable to be used as an agricultural working machine. Especially, it is usable in such a case as to travel on a muddy or hilly ground, that is hard for a tractor having normal drive wheels to work, or as to require a large driving force for using working machines.