Working vehicle

In a working vehicle such as an agricultural tractor, a front wheel change speed device for transmitting drive to front wheels includes an equal speed clutch for driving the front wheels substantially at the same speed as rear wheels, and an accelerating clutch for driving the front wheels substantially at a higher speed than the rear wheels. In the front wheel change speed device of this invention, the equal speed clutch and accelerating clutch are arranged coaxially. The front wheel change speed device further includes a shift member axially shiftable for selectively operating the equal speed clutch and accelerating clutch, and an actuator for axially shifting the shift member.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to working vehicles, and more particularly to an
 improvement in a working vehicle having a front wheel change speed device
 for transmitting drive from an engine to front wheels in an equal speed
 mode for driving the front wheels and rear wheels at a substantially equal
 peripheral speed, and in an accelerating mode for driving the front wheels
 at a substantially faster peripheral speed than the rear wheels.
 2. Description of the Related Art
 A working vehicle having the above construction is disclosed in Japanese
 Patent Laying-Open Publication H5-162654, for example. This conventional
 vehicle has an equal speed clutch for transmitting substantially the same
 drive speed to front wheels as a peripheral speed of rear wheels, and an
 accelerating clutch for transmitting a faster drive speed to the front
 wheels than the peripheral speed of the rear wheels. When a steering angle
 of the front wheels is less than a predetermined value, the equal speed
 clutch remains engaged under the biasing force of a spring. When the
 steering angle of the front wheels reaches the predetermined value, a
 hydraulic selector valve is operated in response to the steering operation
 to engage the accelerating clutch by means of hydraulic drive to transmit
 high-speed drive to the front wheels.
 This type of working vehicle is constructed to reduce a turning radius by
 increasing drive speed of the front wheels automatically when the front
 wheels are steered by a large degree as when the vehicle makes a turn near
 a ridge during a farming operation. However, where, as in the conventional
 working vehicle, the equal speed clutch is maintained engaged by the
 biasing force of a spring, the spring used is relatively strong to secure
 the engaged state reliably. In order to accelerate the front wheels, an
 operation to engage the accelerating clutch must be executed against the
 biasing force of the spring. This operation requires strong drive, which
 has been a cause of an enlarged hydraulic system.
 SUMMARY OF THE INVENTION
 The object of this invention is to provide a working vehicle having an
 improved construction for operating clutches to switch front wheel driving
 modes.
 The above object is fulfilled, according to this invention, by a working
 vehicle comprising a front wheel change speed device having an equal speed
 clutch for transmitting drive to the front wheels in an equal speed drive
 state, an accelerating clutch disposed on a common axis with the equal
 speed clutch for transmitting drive to the front wheels in an accelerating
 state, a shift member shiftable along the axis to three positions for
 selectively operating the equal speed clutch and the accelerating clutch,
 and an actuator for shifting the shift member along the axis to the three
 positions.
 With this construction, the actuator drives the shift member for
 selectively engaging the equal speed clutch and accelerating clutch. Thus,
 the front wheel change speed device has a simple construction for
 operating the two clutches.
 In a preferred embodiment of this invention, the accelerating clutch is in
 the form of a friction type clutch. Thus, when drive is transmitted in
 acceleration to the front wheels, a sudden speed increase is checked to
 suppress shock. Further, the equal speed clutch is in the form of a claw
 type clutch operable to maintain a reliable, strong tractive force without
 slippage.
 Other features and advantages of this invention will be apparent from the
 following description of an embodiment to be taken with reference to the
 drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 An embodiment of this invention will described hereinafter with reference
 to the drawings.
 FIG. 1 shows an agricultural tractor which is one example of working
 vehicles. In the following description, the terms fore and aft direction
 and right and left direction are defined as the forward, rearward,
 rightward and leftward directions with reference to the agricultural
 tractor.
 The agricultural tractor includes a vehicle body with front wheels 1 and
 rear wheels 2 and having an engine 3 mounted in a front position thereof.
 Power of the engine 3 is transmitted through a main clutch 4 to a
 transmission case 5 disposed in a rearward position of the vehicle body. A
 pair of right and left lift arms 6 are connected to upper rear positions
 of the transmission case 5 to be vertically pivotable by a lift cylinder
 (not shown). The vehicle body includes a meter panel 7, a steering wheel 8
 and a driver's seat 9 arranged in a central region thereof. Further, a
 position lever 10 is disposed at the right side of the driver's seat 9 for
 controlling the lift arms 6. A main shift lever 11 is disposed at the left
 side of the driver's seat 9 for controlling a change speed device mounted
 in the transmission case 5.
 The change speed device has a hydraulic actuator (not shown) for
 controlling a synchromesh type change speed gearing. A hydraulic clutch is
 mounted in an intermediate position of a transmission shaft. In time of a
 shifting operation, the hydraulic clutch is disengaged, with pressure oil
 drained therefrom, by operation of the hydraulic actuator. After the
 shifting operation, pressure oil is supplied to the hydraulic clutch to
 engage the clutch. The main shift lever 11 is interlocked to a rotary
 valve (not shown) which controls the hydraulic actuator.
 As shown in FIGS. 1 and 2, a main clutch pedal 13 is disposed in a leftward
 region of a driver's platform, which is depressable to disengage the main
 clutch 4. A pair of right and left side brake pedals 14 are disposed in a
 rightward region of the driver's platform.
 The side brake pedals 14 are connected to side brakes 15, respectively. The
 side brakes 15 apply braking forces, independently of each other, to right
 and left transmission shafts (not shown) which transmit drive from the
 transmission case 5 to the right and left rear wheels 2. Operation of the
 steering wheel 8 is transmitted to a power steering mechanism (not shown)
 which drives a vertical rotary shaft 18. A pitman arm 19 is provided to be
 pivotable with rotation of the rotary shaft 18. The pitman arm 19 is
 interlocked to the right and left front wheels 1 (the interlocking
 structure not particularly described) to steer the front wheels 1.
 The pitman arm 19 has contact members 20 attached to right and left
 positions at a proximal end thereof to be pivotable with the pitman arm
 19. A body frame 21 supports pivotal members 22 to be pivotable about
 pivotal axes 22A extending vertically. When the pitman arm 19 swings in
 excess of a predetermined amount, the contact members 20 contact the
 pivotal members 22 to swing the pivotal members 22. The right and left
 pivotal members 22 are connected to right and left switch mechanisms 23
 through springs 24 and rods 25, respectively. The right and left selector
 or switch mechanisms 23 receive operating forces of the right and left
 side brake pedals 14 through rods 26, respectively. The right and left
 change mechanisms 23 are connected to the right and left side brakes 15
 through control arms 15A and rods 27, respectively. Each switch mechanism
 23 has a function to select the operating force from the side brake pedal
 14 or the operating force from the pivotal member 22, and transmits the
 selected operating force to the side brake 15 (a specific structure not
 particularly described). Thus, the switch mechanisms 23 are selectively
 operable in a mode for operating the right and left side brakes 15 in
 response to depression of the side brake pedals 14, and in a mode for
 operating one of the side brakes 15 lying inwardly of a vehicle turn in
 response to an operation of the steering wheel 8.
 The body frame 21 supports a potentiometer type steering sensor 28 having a
 control arm 28A connected to a proximal position of the pitman arm 19
 through a link 29. This construction is provided to measure a steering
 angle of the front wheels 1.
 As shown in FIG. 1, a front wheel change speed device A is disposed in a
 lower position of transmission case 5 for transmitting drive from the
 transmission case 5 to the front wheels 1. The front wheel change speed
 device A is operable, when the tractor makes a turn with a small radius,
 to provide an equal speed mode for driving the front wheels 1 and rear
 wheels 2 at a substantially equal peripheral speed, an accelerating mode
 for driving the front wheels 1 at a substantially faster peripheral speed
 than the rear wheels 2, and a two wheel drive mode for breaking power
 transmission to the front wheels 1. In the equal speed mode, the front
 wheels 1 are driven at substantially the same peripheral speed as the rear
 wheels 2. The term "substantially" includes a case where a difference
 between the peripheral speed of front wheels 1 and the peripheral speed of
 rear wheels 2 does not exceed 10% of the peripheral speed of rear wheels
 2. The difference should desirably be 5% or less. It is more preferable
 that the front wheels 1 are driven at a peripheral speed 2 to 3% faster
 than the rear wheels 2. In the accelerating mode, the front wheels 1 are
 driven at a "substantially" faster peripheral speed than the rear wheels
 2, which means that the front wheels 1 are driven at a peripheral speed at
 least 30% faster than the rear wheels 2. Preferably, the front wheels 1
 are driven, in this state, at a peripheral speed at least 50% faster than
 the rear wheels 2. Still more preferably, the front wheels 1 are driven at
 a peripheral speed 60 to 80% faster than the rear wheels 2.
 As shown in FIGS. 3 and 4, an intermediate shaft 31 extends in the fore and
 aft direction through a lower space inside the transmission case 5. A
 front wheel drive shaft 32 is rotatably supported parallel to the
 intermediate shaft 31. The intermediate shaft 31 has a first gear 35
 mounted thereon. The first gear 35 is meshed with a gear 34 which receives
 drive for driving the rear wheels 2. This gear 34 is relatively rotatably
 mounted on a power takeoff shaft 33 to receive the drive through a
 different gear not shown. The intermediate shaft 31 has, mounted to be
 rotatable therewith, a second gear 36 disposed adjacent the first gear 35,
 and a third gear 37 having a larger number of teeth than the second gear
 36. The front wheel drive shaft 32 has, mounted to be rotatable relative
 thereto, a fourth gear 38 meshed with the second gear 36, and a fifth gear
 39 meshed with the third gear 37 and having a smaller number of teeth than
 the fourth gear 38. The front wheel drive shaft 32 supports also an
 accelerating clutch B including a clutch case 40 rotatable with the fifth
 gear 39, a support member 41 disposed radially inwardly of the clutch case
 40 and splined to the front wheel drive shaft 32, and a plurality of
 friction disks 42 arranged between the clutch case 40 and support member
 41. Further, the front wheel drive shaft 32 supports an equal speed clutch
 C including clutch claws 38A formed on a side surface of the fourth gear
 38.
 A transmission line is formed to transmit drive from a forward end of front
 wheel drive shaft 32 to the front wheels 1 through a transmission shaft
 (not shown) mounted in a tubular case 44 shown in FIG. 1.
 A shift member 43 is axially slidably splined to the front wheel drive
 shaft 32 between the accelerating clutch B and equal speed clutch C. As
 shown in detail in FIG. 3, the shift member 43 has clutch claws 43A for
 engaging the clutch claws 38A of the fourth gear 38, and a tubular end 43B
 axially remote from the clutch claws 43A for contacting the friction disks
 42. A recess 43C is formed between the clutch claws 43A and end 43B for
 receiving a shifter 46 described later.
 The shift member 43 is slidable to a position (hereinafter called equal
 speed position S) to engage the clutch claws 43A with the clutch claws 38A
 of the fourth gear 38, thereby to establish an equal speed drive state for
 driving the front wheels 1 at substantially the same peripheral speed as
 the rear wheels 2.
 The shift member 43 is slidable also to a position (hereinafter called
 accelerating position U) to press the friction disks 42, thereby to
 establish an accelerating state for driving the front wheels 1 at a
 substantially faster peripheral speed than the rear wheels 2.
 Further, the shift member 43 is slidable to a position (hereinafter called
 neutral position N) between the equal speed position S and accelerating
 position U to establish a state for breaking power transmission to both
 the equal speed clutch C and accelerating clutch B.
 As shown in FIG. 4, a guide shaft 45 is disposed in a bulge portion of
 transmission case 5 to extend parallel to the front wheel drive shaft 32.
 The shifter 46 engaging the recess 43C of shift member 43 is provided to
 be movable as guided by the guide shaft 45. As shown in FIG. 4, the
 shifter 46 has a C-shaped end.
 The transmission case 5 has an opening 5A formed in a side wall of the
 bulge portion, and a lid-like member 47 for closing the opening 5A. In the
 region of opening 5A, a hydraulic cylinder D is disposed to acting as an
 actuator for driving the shifter 46. In this embodiment, the hydraulic
 cylinder D is attached to the lid-like member 47 detachably attached to
 the transmission case 5. Thus, as shown in FIG. 7, the hydraulic cylinder
 D is disposed in a region of the right surface of transmission case 5, and
 adjacent the front wheel drive shaft 32 connecting the front wheel change
 speed device A to the front wheels 1. The hydraulic cylinder D may of
 course be disposed in a region of the left surface of transmission case 5.
 As shown in FIG. 6, the hydraulic cylinder D includes a cylinder tube 49,
 and a piston 50 slidably mounted therein to act as an actuating member.
 The hydraulic cylinder D has an oil line 53A communicating with a
 large-diameter oil chamber 49A accommodating a neutral piston 51, an oil
 line 53B communicating with a small-diameter oil chamber 49B, and an oil
 line 53C communicating with an intermediate stepped portion 49C of the
 cylinder tube 49. The piston 50 has a pair of piston rods 50A and 50B
 formed thereon, with only one of the piston rods 50A projecting from the
 cylinder tube 49. The neutral piston 51, which is ring-shaped, is slidably
 mounted on the other piston rod 50B. The piston rod 50A has an engaging
 pin 52 projecting from one end thereof for engaging an engaging bore 46A
 of the shifter 46. Thus, drive is transmitted from the hydraulic cylinder
 D to the shift member 43 through the shifter 46.
 FIG. 9 shows a hydraulic system for operating the hydraulic cylinder D.
 Pressure oil is supplied to and drained from the oil lines 53A and 53B at
 opposite ends through electromagnetic valves (EMV) 53, respectively. Oil
 is only drained from the intermediate oil line 53C to a tank. With this
 hydraulic cylinder D, when pressure oil is supplied only to the
 small-diameter oil chamber 49B, the piston rod 50A is retracted as shown
 in FIG. 9 (A), to operate the shift member 43 to the accelerating position
 U. When pressure oil is supplied to the small-diameter oil chamber 49B and
 large-diameter oil chamber 49A at the same time as shown in FIG. 9 (B),
 the neutral piston 51 is operated to a position for contacting the stepped
 portion 49C of the cylinder tube 49. By operating the piston 50 to a
 position for contacting the neutral piston 51, the shift member 43 may be
 shifted to the neutral position N. When pressure oil is supplied only to
 the large-diameter oil chamber 49A, as shown in FIG. 9 (C), to move the
 neutral piston 51 to the position for contacting the stepped portion 49C,
 and thereafter apply a pressure to an end of piston rod 50B to project the
 piston rod 50A, the shift member 43 is shifted to the equal speed position
 S.
 The end of piston rod 50B has a smaller area for receiving the pressure of
 pressure oil than a portion of piston 50 for receiving the pressure of
 pressure oil supplied to the small-diameter oil chamber 49B. The neutral
 piston 51 has a larger area for receiving the pressure of pressure oil
 supplied to the large-diameter oil chamber 49A than the above portion of
 piston 50. Consequently, when pressure oil is supplied to both of the oil
 chambers 49A and 49B as noted above, the neutral piston 51 and piston 50
 are placed in pressure contact with each other to establish the neutral
 position N. Further, even when pressure oil is supplied under equal
 pressure to the oil chambers 49A and 49B, the shift member 43 is moved
 faster from neutral position N to equal speed position S than from neutral
 position N to accelerating position U.
 As shown in FIG. 4, the lid-like member 47 is connected to the transmission
 case 5 by tightening bolts 54 to close the opening 5A. The engaging pin 52
 is set to extend along the direction in which the lid-like member 47 is
 attached to and detached from the transmission case 5. Thus, the engaging
 pin 52 is movable into and out of engagement with the engaging bore 46A as
 the lid-like member 47 is attached to or detached from the transmission
 case 5.
 The lid-like member 47 has a plate 47A bolted thereto for covering an outer
 surface. The two electromagnetic valves 53 for controlling the hydraulic
 oil supplied to and drained from the hydraulic cylinder D are arranged on
 an inner surface of the plate 47A as shown in FIG. 6. A restrictor 56 is
 provided to be slidable in diametric directions of the first gear 35 into
 and out of engagement with the teeth thereof. As shown in FIGS. 4 and 5,
 the restrictor 56 acts as a parking brake for engaging the first gear 35
 to prevent rotation of the transmission system for driving the wheels 1
 and 2. The restrictor 56 has an engaging element 56A formed at an inward
 end thereof for engaging the teeth of the first gear 35, and a compression
 spring 57 mounted at an outward end thereof for biasing the restrictor 56
 in an engaging direction. A flange-like element 58 is provided at the
 outward end of the restrictor 56 for applying an operating force thereto
 in a disengaging direction. A change shaft 59 is rotatably supported by
 the lid-like member 47, with a change arm 60 attached to an outer end of
 change shaft 59. The change shaft 59 has a pair of first contact arms 61
 attached to a position thereof inside the lid-like member 47 to be
 pivotable with rotation of the change shaft 59 for contacting the
 flange-like element 58. A support shaft 62 extending parallel to the
 change shaft 59 has a pair of second contact arms 63 for contacting the
 flange-like element 58. The second contact arms 63 have a shaft 64
 contactable by cams 61A formed on the first contact arms 61.
 The change arm 60 is interlocked to the main shift lever 11 through a wire
 65. When the main shift lever 11 is operated to a parking position P, the
 wire 65 connected thereto allows the restrictor 56 to project under the
 biasing force of compression spring 57 to engage the first gear 35.
 Conversely, when the main shift lever 11 is operated away from the parking
 position P, the wire 65 imparts a pulling force to disengage the
 restrictor 56 from the first gear 35 (the connection between wire 65 and
 main shift lever 11 not particularly described). In time of disengaging
 the restrictor 56, the change arm 60 is swung by the pulling force of wire
 65. Simultaneously therewith, the change shaft 59 starts rotating, with
 distal ends of the first contact arms 61 spaced from the flange-like
 element 58. With this rotation, the shaft 64 of second contact arms 63
 contacts the cams 61A of first contact arms 61. This causes distal ends of
 second contact arms 63 to contact the flange-like element 58. Thus, the
 restrictor 56 is moved in the disengaging direction by a strong force due
 to a large arm ratio. After the restrictor 56 is disengaged, the first
 contact arms 61 contact the flange-like element 58 to retract the
 restrictor 56 through a large stroke toward the lid member 47.
 As shown in FIGS. 4 and 8, a support shaft 66 extending through and
 supported by the lid-like member 47 has a detecting arm 67 or first arm
 attached to an inward end thereof and engaged with the engaging pin 52 of
 the piston rod 50A. The support shaft 66 has an actuating arm 68, or
 second arm attached adjacent an outward end thereof and interlocked to an
 indicator 69 through a wire 70. The indicator 69 has a pointer 69A
 mechanically movable in response to the operative positions of the shifter
 47. Thus, the driver can visually recognize three different states, i.e.
 the state of driving the front wheels 1 at equal speed (4WD), the state of
 accelerating the front wheels 1 (acceleration) and the state of
 transmitting no drive to the front wheels 1 (2WD). The support shaft 66 is
 supported as embraced by a boss 47B formed on the lid-like member 47. As
 shown in FIG. 4, the detecting arm 67 is disposed between an end of boss
 47B and the piston rod 50A as seen in the fore and aft direction. The
 position of detecting arm 67 is stabilized through slidable contact with
 the boss 47B and piston rod 50A.
 As shown in FIG. 4, a magnetic toothed wheel 71 formed of steel is mounted
 at a rear end of the front wheel drive shaft 32 to be rotatable therewith.
 A pickup type vehicle speed sensor 72 is inserted through a bore formed in
 a wall surface of the transmission case 5 adjacent a rear surface of this
 toothed wheel 71. Thus, a rotating speed of the front wheel drive shaft 32
 is electrically measured. A control system is operable, when the front
 wheels 1 are in the state of acceleration, to perform an electrical
 process to reduce the vehicle speed measured by the sensor 72 to determine
 an accurate running speed. The vehicle speed sensor 72 transmits a signal
 through a cable (not shown) extending along an outer surface of the
 transmission case 5.
 As shown in FIG. 10, a controller 73 having a microprocessor receives
 signals from the steering sensor 28, vehicle speed sensor 72 and a running
 mode selecting switch 74, The controller 73 outputs signals to solenoids
 of the pair of electromagnetic valves 53.
 When the running mode selecting switch 74 selects a mode for allowing
 acceleration of the front wheels 1, and the steering sensor 28 detects the
 front wheels 1 being steered in excess of a predetermined amount, the
 electromagnetic valves 53 are driven to accelerate the front wheels 1 only
 if the running speed of the vehicle body measured by the vehicle speed
 sensor 72 is below a predetermined speed. Particularly when the selector
 mechanisms 23 described hereinbefore select, based on the operation of
 pitman arm 19, the mode for operating the side brake 15 lying inwardly of
 the turn, the inward side brake 15 is operated in response to the steering
 operation to reduce the turning radius. The front wheels 1 are not
 accelerated in making a turn when the running mode selecting switch 74 is
 operated to select the mode for maintaining the two wheel drive state or
 the mode for maintaining the four wheel drive state.
 In this tractor, as described above, switching may be made with the drive
 of the single hydraulic cylinder D, by setting the shift member to three
 positions, between the state of driving the front wheels 1 at
 substantially the same speed as the rear wheels 2, the state of driving
 the front wheels 1 substantially faster than the rear wheels 2, and the
 state of transmitting no drive to the front wheels 1. Thus, not only are a
 plurality of actuators unnecessary, but drive mode switching can be made
 reliably, and hardly any shock is produced in time of acceleration since
 the friction type accelerating clutch B is employed. When the equal speed
 drive is selected, the use of claw type equal speed clutch C maintains a
 strong tractive force without clutch slippage. Moreover, even when
 pressure oil is supplied under fixed pressure to the hydraulic cylinder D,
 the accelerating clutch B may be engaged slowly, and the equal speed
 clutch C engaged quickly. Thus, there is a still less chance of producing
 shock in time of acceleration. Switching is made to the equal speed drive
 without the inconvenience of having to accelerate the tractor after a
 slowdown.