Abstract:
A control apparatus ( 64 ) for a transmission. The transmission includes first and second transmission lines ( 7 8 ), ( 10, 11 ) disposed in parallel between an upstream transmission shaft ( 2 ) for receiving power from an engine ( 1 ) and a downstream transmission shaft ( 4 ) for transmitting the power to a traveling unit ( 53, 54 ); a hydraulic multiple disc transmission clutch ( 6 ) disposed transmission-wise upstream or downstream of the first and second transmission lines, the power of the upstream transmission shaft being transmitted via one of the first and second transmission lines to the downstream transmission shaft; a first gear change-speed mechanism ( 13 ) disposed between either the upstream transmission shaft or the downstream transmission shaft and the first transmission line, the first gear change-speed mechanism having a plurality of speed positions; a first actuator ( 35 ) for operating the first gear change-speed mechanism; a first friction clutch ( 9 ) incorporated in the first transmission line; a second gear change-speed mechanism ( 15 ) disposed between either the upstream transmission shaft or the downstream transmission shaft and the second transmission line, the second gear change-speed mechanism having a plurality of speed positions; a second actuator ( 37 ) for operating the second gear change-speed mechanism; and a second friction clutch ( 12 ) incorporated in the second transmission line. When the power is being transmitted via the first transmission line, a first controlling means operates the second gear change-speed mechanism into a predetermined speed position by means of the second actuator and operates the first friction clutch from a transmitting state to a non-transmitting state and simultaneously operates the second friction clutch from a non-transmitting state to a transmitting state, thereby to realize a progressive shifting of the transmission clutch from a transmitting state to a semi-transmitting state.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a control apparatus for a work-vehicle transmission having a transmission clutch and a gear change-speed mechanism. This control apparatus is operable, in response to an instruction for change speed, to automatically operate the transmission clutch into a non-transmitting state to operate the gear change-speed mechanism by means of an actuator and subsequently to automatically operate the transmission clutch back to a transmitting state.  
           [0003]    2. Description of the Related Art  
           [0004]    An example of the work-vehicle transmission to be controlled by the above-described control apparatus is disclosed by Japanese Patent Application “Kokai” No.: Hei. 6-313478. In this, a gear change-speed mechanism (denoted with a mark A in FIG. 1) to be operated by actuators (denoted with marks T 1 , T 2  in FIG. 1) and a hydraulic multiple disc clutch (denoted with numeral  19  in FIG. 1) are arranged in series. With this, in response to a change-speed instruction from the control apparatus, the transmission clutch is automatically operated into the non-transmitting state and the gear change-speed mechanism is operated by the actuators. Upon completion of this operation of the gear change-speed mechanism by the actuators, the transmission clutch is automatically and gradually operated into the transmitting state.  
           [0005]    A work vehicle is often subjected to a large load such as when the vehicle runs on a soft ground surface providing a significant running resistance to the vehicle or when the vehicle tows a cart mounting load therein. Hence, when the transmission clutch is operated into the non-transmitting state, the power transmission from the engine to a traveling unit of the vehicle is broken at this timing, whereby the traveling speed of the vehicle may be reduced suddenly due to the traveling load. Then, under this condition when the traveling speed of the vehicle has been reduced significantly with completion of the operation of the gear change-speed mechanism by the actuator, if the transmission clutch is operated back to the transmitting state thereby to connect the power of the engine to the traveling unit, this will result in sudden acceleration of the vehicle to the previous traveling speed before the speed reduction, thus giving a significant shock to the operator.  
           [0006]    In this respect, according to the construction disclosed by the above document, there is provided an auxiliary transmission clutch (denoted with mark E in FIG. 1) capable of transmitting the power from the gear change-speed mechanism to the downstream side with bypassing the (main) transmission clutch. With this construction, when the transmission clutch is operated into the non-transmitting state as described above, the auxiliary transmission clutch is operated from its non-transmitting state to its transmitting state, whereby the power from the gear change-speed mechanism is transmitted via this auxiliary transmission clutch to the downstream side, thus restricting reduction in the traveling speed of the vehicle due to the traveling load. Then, when the transmission clutch is operated back to the transmitting stage after completion of the operation of the gear change-speed mechanism by the actuator, the auxiliary transmission clutch is operated from the transmitting state to the non-transmitting state. In this way, the construction functions to restrict occurrence of sudden acceleration of the vehicle to the previous high traveling speed, thus restricting occurrence of shock associated therewith.  
           [0007]    In the case of the above-described construction disclosed by the gazette, while the operation of the gear change-speed mechanism is going on with the transmission clutch being at its non-transmitting stage, the power from the gear change-speed mechanism is transmitted to the downstream via the auxiliary transmission clutch. In this, the transmission ratio (the transmission ratio of the gear shown on the left end of a gear reduction mechanism shown in FIG. 1 of the gazette) of the power being transmitted via the auxiliary transmission clutch remains substantially fixed. With this, if a change-speed operation is effected at a higher speed range than the transmission ratio of the power transmitted via the auxiliary transmission clutch (e.g. when the transmission ratios provided by speed positions before and after the change-speed operation are higher than the transmission ratio of the power transmitted via the auxiliary transmission clutch), such change-speed operation result in change from the condition of the power being transmitted at the transmission ratio provided by the high speed position to the condition of the power being transmitted at the transmission ratio at the lower speed provided by the auxiliary transmission clutch. Hence, such operation will again cause a shock.  
         SUMMARY OF THE INVENTION  
         [0008]    A primary object of the present invention is to provide a control apparatus for a work-vehicle transmission capable of reducing a change-speed shock when the transmission clutch is operated, in response to a change-speed instruction to automatically operate the transmission clutch into the non-transmitting state to operate the gear change-speed mechanism by means of an actuator and subsequently operate the transmission clutch back to the transmitting state.  
           [0009]    The typical work-vehicle transmission to which the invention is applied, includes:  
           [0010]    first and second transmission lines disposed in parallel between an upstream transmission shaft for receiving power from an engine and a downstream transmission shaft for transmitting the power to a traveling unit;  
           [0011]    a hydraulic multiple disc transmission dutch disposed transmission-wise upstream or downstream of the first and second transmission lines, the power of the upstream transmission shaft being transmitted via one of the first and second transmission lines to the downstream transmission shaft;  
           [0012]    a first gear change-speed mechanism disposed between either the upstream transmission shaft or the downstream transmission shaft and the first transmission line, the first gear change-speed mechanism having a plurality of speed positions;  
           [0013]    a first actuator for operating the first gear change-speed mechanism;  
           [0014]    a first friction clutch incorporated in the first transmission line;  
           [0015]    a second gear change-speed mechanism disposed between either the upstream transmission shaft or the downstream transmission shaft and the second transmission line, the second gear change-speed mechanism having a plurality of speed positions;  
           [0016]    a second actuator for operating the second gear change-speed mechanism; and  
           [0017]    a second friction clutch incorporated in the second transmission line.  
           [0018]    For such work-vehicle transmission as described above, a control apparatus according to the invention comprises first controlling means and second controlling means. When the power is being transmitted via the first transmission line, the first controlling means operates the second gear change-speed mechanism into a predetermined speed position by means of the second actuator and operates the first friction clutch from a transmitting state to a non-transmitting state and simultaneously operates the second friction clutch from a non-transmitting state to a transmitting state, thereby to realize a progressive shifting of the transmission clutch from a transmitting state to a semi-transmitting state. When the power is being transmitted via the second transmission line, the second controlling means operates the first gear change-speed mechanism into a predetermined speed position by means of the first actuator and operates the first friction clutch from the non-transmitting state to the transmitting state and simultaneously operates the second friction clutch from the transmitting state to the non-transmitting state, thereby to realize a progressive shifting of the transmission clutch from the transmitting state to the semi-transmitting state.  
           [0019]    Thanks to the control apparatus having the above-described construction, during a change-speed operation, the second friction clutch is operated from the non-transmitting state to the transmitting state and at the same time the first friction clutch is operated from the transmitting state to the non-transmitting state, or conversely, the second friction clutch is operated from the transmitting state to the non-transmitting state while the first friction clutch is operated from the non-transmitting state to the transmitting state. In whichever case, there is realized a condition (“dual-transmitting condition” hereinafter) in which the power of the upstream transmission shaft is provided in distribution to both the first and second transmission lines and then the distributed powers from the first and second transmission lines are provided as combined together to the downstream transmission line. Hence, even if a torque variation occurs in this dual-transmitting condition, such torque variation will be effectively absorbed by slipping of the transmission clutch under its semi-transmitting state, so that the power may be transmitted with reduced torque variation to the traveling unit. That is to say, when a change-speed operation is taking place, the dual-transmitting condition is realized and the power with reduced or minimized torque variation can be transmitted to the traveling unit. As a result, it has become possible to restrict, during a change-speed operation, occurrence of reduction in the traveling speed of the work vehicle due to the traveling load. So that, it has become possible to avoid the sudden reduction in the traveling speed of the vehicle due to traveling load and the resultant shock in association of the subsequent operation of the transmission clutch to its transmitting state under such speed reduced condition. In this transmission, the first and second gear change-speed mechanisms each has a plurality of speed positions. Hence, in realizing the dual-transmitting condition, in case the power is being transmitted via the first transmission line (i.e. when the first gear change-speed mechanism is set at a predetermined speed position and the first friction clutch is under is transmitting state), the second gear change-speed mechanism can be operated to an appropriate speed position selected from the plurality of speed positions available. On the other hand, in case the power is being transmitted via the second transmission line (i.e. when the second gear change-speed mechanism is set at a predetermined speed position and the second friction clutch is under its transmitting state), the first gear change-speed mechanism can be operated to an appropriate speed position selected from the plurality of speed positions available.  
           [0020]    Consequently, it has become possible to avoid also the above-described further case of shock occurrence which results from shifting from the condition where the power is transmitted at a transmission ratio provided by a high speed position to the condition where the power is transmitted at a transmission ratio provided by a low speed position and then back to the previous condition at the transmission ratio provided by the high speed position.  
           [0021]    According to one preferred embodiment of the present invention, the control apparatus further comprises acceleration detecting means for detecting acceleration of the vehicle and the transmission clutch is operated from the semi-transmitting state to the transmitting state in such a manner that the acceleration of the vehicle may have a predetermined characteristics when the transmission clutch is progressively operated from the semi-transmitting state to the transmitting state by the first and second controlling means. With this feature, there is achieved a smooth acceleration (or deceleration) from the traveling speed of the vehicle corresponding to the speed position of the first transmission line (or the second transmission line) to the traveling speed of the vehicle corresponding to the speed position of the second transmission line (or the first transmission line).  
           [0022]    According to a further preferred embodiment of the invention, a working pressure of the transmission clutch under its semi-transmitting state is variable. With this, the working pressure of the transmission clutch under the semi-transmitting state may be set to an appropriate value, in accordance with particular conditions of the work site (e.g. the softness or hardness of the ground or presence/absence of undulations on the ground, etc.), a traveling load being applied to the vehicle, a particular condition of an implement mounted on the vehicle, etc. Consequently, by appropriately setting the working pressure of the transmission clutch under its semi-transmitting state, when the power with reduced torque variation is transmitted via this transmission clutch under the semi-transmitting state to the traveling unit during a change-speed operation, this power transmitted to the traveling unit may be appropriately set, depending on the various conditions described above.  
           [0023]    According to a still further preferred embodiment of the present invention, the traveling load to the vehicle is detected and the working pressure of the transmission clutch under its semi-transmitting state is increased in response to increase in the detected traveling load. In the above condition when the power with reduced torque variation is being transmitted to the traveling unit via the transmission clutch under the semi-transmitting state during a change-speed operation, it is expected that the reduction in the traveling speed of the vehicle will be significant if the traveling load is large. Then, by increasing the working pressure of the transmission clutch under the semi-transmitting state, when the transmission clutch is operated subsequently from the transmitting state to the semi-transmitting state and then progressively to the transmitting state, the transmission clutch can reach the transmitting state quickly. Therefore, in the case of large traveling load, the transmission clutch can reach the transmitting state to complete the change-speed operation before the traveling speed of the vehicle is reduced significantly.  
           [0024]    As other preferred constructions to be provided for the detection of traveling load, it is also possible to detect a difference between a revolution of the engine under zero load condition and a current revolution of the engine and detect the traveling load based on the revolution difference or to detect a reduction ratio in the traveling speed of the vehicle at the time of start of a change-speed operation and detect the traveling load based on the detected reduction ratio in the traveling speed of the vehicle.  
           [0025]    With a work vehicle, in general, the traveling load applied to the vehicle body will be greater in a high-speed working run than in a low-speed working run (for instance, in the case of an agricultural tractor, the traveling load applied to its vehicle body will be greater in a leveling work run (high-speed working run) in which the vehicle travels with a plow connected thereto for leveling raised earth than in a plowing work run (low-speed working run) in which the vehicle travels with a rotary plow implement connected thereto). For this reason, according to a preferred embodiment of the invention, the working pressure of the transmission clutch under the semi-transmitting state in a high-speed working run is set higher than that in a low-speed working run. As a result, in the case of the high-speed working run, the transmission clutch can reach the transmitting state quickly to complete a change-speed operation before the traveling speed of the vehicle is reduced significantly.  
           [0026]    Further, with such work vehicle as above, when an implement is connected to its vehicle body, in general, the work is carried out by the implement while the vehicle travels forward. And, the vehicle travels reverse with the implement being lifted off the ground surface (i.e. a condition for not effecting a work by the implement). For this reason, the traveling load applied to the vehicle will be greater in the case of the forward run than the reverse run. In view of this, according to a preferred embodiment of the invention, the working pressure of the transmission clutch under the semi-transmitting state is set higher for the forward run than the reverse run. As a result, in the case of the forward run, the transmission clutch can reach the transmitting state quickly to complete a change-speed operation before the traveling speed of the vehicle is reduced significantly.  
           [0027]    Also, with such work vehicle as above, in general, the traveling load applied to the vehicle body will be greater during a working run than a road run (i.e. in a road run, the vehicle travels on a well-conditioned road such as a paved road. Whereas, in the working run, the vehicle has to travel generally on an unpaved road with a lot of surface unevenness, hence, a greater traveling load will be applied to the vehicle during a working run than a road run). In view of this, according to a preferred embodiment of the invention, the working pressure of the transmission clutch under the semi-transmitting state is set higher for the working run than the road run. As a result, in the case of the working run, the transmission clutch can reach the transmitting state quickly to complete a change-speed operation before the traveling speed of the vehicle is reduced significantly.  
           [0028]    According to a still further preferred embodiment of the invention, there is provided an alternate control mode, in which the first controlling means and the second controlling means are activated alternately of each other from a previous speed position prior to the issuance of the change-speed instruction to a target speed position instructed by the change-speed instruction, thereby to realize the target speed position instructed by the change-speed instruction. With this feature, the above-described dual-transmitting condition and the power transmission with minimized torque variation will be realized in repetition in the course of shifting from the previous speed position prior to issuance of the change-speed instruction to the target speed position instructed by the change-speed instruction, so that the change-speed operation from the previous speed position prior to issuance of the change-speed instruction to the target speed position instructed by the change-speed instruction may take place smoothly and shocklessly.  
           [0029]    Incidentally, when this alternate control mode is used, it may take a relatively long time until the completion of the shift from previous speed position prior to issuance of the change-speed instruction to the target speed position instructed by the change-speed instruction. In view of this, according to a further preferred embodiment of the invention, there is provided a first skip change-speed mode, in which there is provided an intermediate speed position substantially midway between previous speed position prior to issuance of the change-speed instruction to the target speed position instructed by the change-speed instruction, and one of the first and second controlling means is activated for realizing shift from the previous speed position prior to issuance of the change-speed instruction to the intermediate speed position and the other of the first and second controlling means is activated for realizing subsequent shift form the intermediate speed position to the target speed position instructed by the change-speed instruction.  
           [0030]    According to a further embodiment of the invention, there is provided a second skip change-speed mode, in which there is provided an intermediate position slightly offset from the target speed position instructed by the change-speed instruction toward the previous speed position prior to the issuance of the change-speed instruction, and one of the first and second controlling means is activated for realizing shift from the previous speed position prior to issuance of the change-speed instruction to the intermediate speed position and the other of the first and second controlling means is activated for realizing subsequent shift form the intermediate speed position to the target speed position instructed by the change-speed instruction.  
           [0031]    When either the first skip change-speed mode or the second skip change-speed mode is used, rather than alternately acting the first and second controlling means for the shifting from the previous speed position prior to issuance of the change-speed instruction to the target speed position instructed by the change-speed instruction, the shifting from the previous speed position to the target speed position is effected in two steps of first shifting from the previous speed position prior to issuance of the change-speed instruction to the intermediate speed position and then from this intermediate speed position to the target speed position instructed by the change-speed instruction. As a result, while restricting the change-speed shock, the period required for the shifting from the previous speed position prior to issuance of the change-speed instruction to the target speed position instructed by the change-speed instruction may be relatively short.  
           [0032]    According to a still further preferred embodiment of the invention, there are provided a basic control mode and an alternate control mode one of which can be selected. According to a still further embodiment of the invention, there are provided a basic control mode and a skip change-speed mode one of which can be selected. In the basic control mode, upon issuance of a change-speed instruction, the transmission clutch is operated to the non-transmitting state and the first and second change-speed mechanism are operated to the change-speed position instructed by the change-speed instruction by means of the first and second actuators respectively, thereby to operate the transmission clutch to the transmitting state progressively. In the alternate control mode, the first and second controlling means are alternately actuated until completion of shifting from the previous speed position prior to the issuance of the change-speed instruction to the target speed position instructed by the change-speed instruction, thereby to realize the shifting to the target speed position instructed by the change-speed instruction. In the skip change-speed mode, in which there is provided an intermediate speed position substantially midway between the previous speed position prior to issuance of the change-speed instruction to the target speed position instructed by the change-speed instruction, and one of the first and second controlling means is activated for realizing shift from the previous speed position prior to issuance of the change-speed instruction to the intermediate speed position and the other of the first and second controlling means is activated for realizing subsequent shift form the intermediate speed position to the target speed position instructed by the change-speed instruction.  
           [0033]    The basic mode described above is a change-speed control mode which places priority on the speediness of the change-speed operation, rather than on the prevention of change-speed shock. On the other hand, in the alternate control mode, the above-described dual-transmitting condition and the power transmission with minimized torque variation will be realized in repetition in the course of shifting from the previous speed position prior to issuance of the change-speed instruction to the target speed position instructed by the change-speed instruction. So, this is a change-speed control mode for realizing shockless shift from the previous speed position prior to issuance of the change-speed instruction to the target speed position instructed by the change-speed instruction. Further, in the skip change-speed mode, in the realization of the dual-transmitting state and the power transmission with reduced torque variation to the traveling unit, the time required for shifting from the previous speed position prior to issuance of the change-speed instruction to the target speed position instructed by the change-speed instruction is shorter, in comparison with the alternate control mode. Hence, this is a mode designed to achieve both speediness of change-speed operation and low change-speed shock at one time.  
           [0034]    Preferably, manual selecting means is provided for allowing manual selection between the basic control mode and the alternate control mode or between the basic control mode and the skip change-speed mode. With this, an operator may advantageously make an appropriate selection, based on his/her own judgment, between the basic control mode and the alternate control mode or between the basic control mode and the skip change-speed mode.  
           [0035]    For instance, when the vehicle travels on a paved road, the traveling load applied to the vehicle is relatively small. Whereas, when the vehicle travels on a ground surface of uneven work site or a muddy site, the traveling load applied to the vehicle will be relatively large. Further, when the vehicle tows an empty cart, the traveling load to the vehicle is relatively small. Whereas, when the vehicle tows a cart mounting a heavy load, the traveling load to the vehicle is relatively large. In these, in the case of small load (towing load), the basic control mode is appropriate for controlling a change-speed operation. Conversely, in the case of large load (towing load), the reduction in the traveling speed of the vehicle associated with a change-speed operation will be significant. For a change-speed control in this case, the alternate control mode or the skip change-speed mode is appropriate. Advantageously, when the traveling load (towing load) applied to the vehicle is below a predetermined value, the basic mode is automatically selected, whereas, when the traveling load (towing load) exceeds the predetermined value, the alternative control mode or the skip change-speed mode is automatically selected.  
           [0036]    When a ground-work implement is connected to the vehicle body, in order to maintain the towing load applied to the vehicle body from this ground-work implement at a predetermined value, a draft operation for lifting up and down the ground-work implement relative to the vehicle body is sometimes effected. In this case, since a large towing load is applied from the ground-work implement to the vehicle body during the draft operation, the alternate control mode or the skip change-speed mode is appropriate. And, advantageously, the selection of this mode is automatically effected.  
           [0037]    When the vehicle is traveling at a high speed, the inertia of the vehicle body is relatively large. Hence, the reduction in the traveling speed of the vehicle associated with a change-speed operation will be relatively small. Hence, the basic control mode is appropriate. Conversely, when the vehicle is traveling at a low speed, the inertia of the vehicle body is relatively small. Hence, the reduction in the traveling speed of the vehicle associated with a change-speed operation will be relatively large. Hence, the alternate control mode or the skip change-speed mode is appropriate. Therefore, advantageously, when the traveling speed of the vehicle is high, the basic mode is automatically selected, whereas, when the traveling speed of the vehicle is low, the alternative control mode or the skip change-speed mode is automatically selected.  
           [0038]    When the ground-work implement is connected to the vehicle body, if the altitude of this ground-work implement relative to the vehicle body is higher than a predetermined value, it may be judged that the vehicle is engaged in e.g. a road run not effecting any work by the ground-work implement. Therefore, if the altitude of this ground-work implement relative to the vehicle body is higher than a predetermined value, it may be judged that the reduction in the traveling speed of the vehicle associated with a change-speed operation will be relatively small. Hence, for a change-speed control in this condition, the basic control mode is appropriate.  
           [0039]    On the other hand, if the altitude of this ground-work implement relative to the vehicle body is lower than the predetermined value, it may be judged that the vehicle is engaged in e.g. a working run effecting a work by the ground-work implement. Therefore, if the altitude of this ground-work implement relative to the vehicle body is lower than the predetermined value, it may be judged that the reduction in the traveling speed of the vehicle associated with a change-speed operation will be relatively large. Hence, for a change-speed control in this condition, the alternate control mode or the skip change-speed mode is appropriate.  
           [0040]    Another type of work-vehicle transmission to which the present invention may be applied includes a forward/reverse switchover mechanism disposed transmission-wise downstream of the first and second transmission lines and having a forward traveling clutch and a reverse traveling clutch both of which comprise a hydraulic multiple disc type. In the case of this type of transmission, the above-described functions of the transmission clutch in the foregoing transmission may be assigned to this forward/reverse switchover mechanism.  
           [0041]    Further and other features and advantages of the invention will become apparent from the following detailed description of preferred embodiments thereof with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0042]    [0042]FIG. 1 is a schematic view showing a construction of a transmission to which the present invention is to be applied,  
         [0043]    [0043]FIG. 2 is a diagram showing input and output devices for a control apparatus of the invention,  
         [0044]    [0044]FIG. 3 is a view showing a change-speed operation from a first speed position to a second speed position in a first change-speed mode,  
         [0045]    [0045]FIG. 4 is a view showing a change-speed operation from a first speed position to a second speed position in a second change-speed mode,  
         [0046]    [0046]FIG. 5 is a flowchart illustrating flow of the second change-speed mode,  
         [0047]    [0047]FIG. 6 is a flowchart illustrating flow of a third change-speed mode,  
         [0048]    [0048]FIG. 7 is a table showing conditions of shifters and first and second friction clutches at first through eighth speed positions,  
         [0049]    [0049]FIG. 8 is a flowchart illustrating flow for setting a working pressure,  
         [0050]    [0050]FIG. 9 is a flowchart illustrating flow of the second change-speed mode in which the working pressure is not set for each change-speed operation,  
         [0051]    [0051]FIG. 10 is a flowchart illustrating flow of the third change-speed mode in which the working pressure is not set for each change-speed operation,  
         [0052]    [0052]FIG. 11 is a schematic view showing a construction of a further transmission to which the present invention is to be applied,  
         [0053]    [0053]FIG. 12 is a schematic view showing a construction of a still further transmission to which the present invention is to be applied, and  
         [0054]    [0054]FIG. 13 is a schematic view showing a construction of a still further transmission to which the present invention is to be applied. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0055]    [1] 
         [0056]    [0056]FIG. 1 shows a traveling transmission line of a four-wheel-drive agricultural tractor as an example of a work vehicle. In this, power of an engine  1  is transmitted to a transmission shaft  2  and to PTO shaft  3 . On the transmission shaft  2 , tubular transmission shafts  4 ,  5  are mounted to be rotatable relative to each other and a hydraulic multiple-disc friction type transmission clutch  6  is interposed between the transmission shafts  4 ,  5 . In response to supply of a working fluid thereto, the transmission clutch  6  is operated to a power transmitting state. In response to discharge of the working fluid therefrom, the clutch  6  is operated to a power non-transmitting state.  
         [0057]    As shown in FIG. 1, in parallel to the transmission shafts  2 ,  4 , a first main transmission shaft  7  and a first auxiliary transmission shaft  8  are disposed. Between these first main and auxiliary transmission shafts  7 ,  8 , a first friction clutch  9  is disposed. In parallel also to the transmission shafts  2 ,  4 , a second main transmission shaft  10  and a second auxiliary transmission shaft  11  are disposed. Between these second main and auxiliary transmission shafts  10 ,  11 , a second friction clutch  12  is disposed. Each of the first and second friction clutches  9 ,  12  is a hydraulic multiple-disc friction type and is operated to the transmitting state in response to supply of the working fluid thereto and to the non-transmitting state in response to discharge of the working fluid therefrom.  
         [0058]    As shown in FIG. 1, between the transmission shaft  2  and the first main transmission shaft  7 , a first gear change-speed mechanism  13  of a synchromesh type is provided. Further, between the transmission shaft  2  and the second main transmission shaft  10 , a second gear change-speed mechanism  15  of a synchromesh type is provided. The transmission shaft  2  fixedly mounts thereon a first gear  17 , a second gear  18 , a third gear  19  and a fourth gear  20 . The first gear  17  and the third gear  19  mesh with a low-speed gear  21  and a high-speed gear  22  rotatably mounted on the first main transmission shaft  7 . And a shifter  23  is splined on the first main transmission shaft  7  to be rotatable therewith and slidable relative thereto. These arrangements together constitute the first gear change-speed mechanism  13 . The second main transmission shaft  10  rotatably mounts a low-speed gear  24  and a high-speed gear  25  which mesh with the second gear  18  and the fourth gear  20 . And, a shifter  26  is splined on the second main transmission shaft  10  to be rotatable therewith and sidable relative thereto. These arrangements together constitute the second gear change-speed mechanism  15 .  
         [0059]    As also shown in FIG. 1, between the transmission shaft  4  and the first auxiliary transmission shaft  8 , there is interposed a first auxiliary gear change-speed mechanism  14  of the synchromesh type. Between the transmission shaft  4  and the second auxiliary transmission shaft  11 , there is interposed a second auxiliary gear change-speed mechanism  16  of the synchromesh type. The first auxiliary transmission shaft  8  rotatably mounts a low-speed gear  29  and a high-speed gear  30  which mesh with the low-speed gear  27  and the high-speed gear  28 . And, a shifter  31  is splined on the first auxiliary transmission shaft  8  to be rotatable therewith and sidable relative thereto. These arrangements together constitute the first auxiliary gear change-speed mechanism  14 . The second auxiliary transmission shaft  11  rotatably mounts a low-speed gear  32  and a high-speed gear  33  which mesh with the low-speed gear  27  and the high-speed gear  28 . And, a shifter  34  is splined on the second auxiliary transmission shaft  11  to be rotatable therewith and slidable relative thereto. These arrangements together constitute the second auxiliary gear change-speed mechanism  16 .  
         [0060]    With the above-described constructions, as described later in section [3], there are obtained a condition (the transmitting state of the first friction clutch  9 ) in which the power of the transmission shaft  2  is transmitted via the first main and auxiliary transmission shafts  7 ,  8  to the transmission shaft  4  and a further condition (the transmitting state of the second friction clutch  12 ) in which the power of the transmission shaft  2  is transmitted via the second main and auxiliary transmission shafts  10 ,  11  to the transmission shaft  4   
         [0061]    As shown in FIG. 1, in the condition (the transmitting state of the first friction clutch  9 ) in which the power of the transmission shaft  2  is transmitted via the first main and auxiliary transmission shafts  7 ,  8  to the transmission shaft  4 , the power of the transmission shaft  2  is changed in four speeds (first speed position, third speed position, fifth speed position and seventh speed position to be described later) via the first gear change-speed mechanism  13 , the first main transmission shaft  7 , the first friction clutch  9 , the first auxiliary transmission shaft  8  and the first auxiliary gear change-speed mechanism  14  and transmitted to the transmission shaft  4 .  
         [0062]    As also shown in FIG. 1, in the further condition (the transmitting state of the second friction clutch  12 ) in which the power of the transmission shaft  2  is transmitted via the second main and auxiliary transmission shafts  10 ,  11  to the transmission shaft  4 , the power of the transmission shaft  2  is changed in four speeds (second speed position, fourth speed position, sixth speed position and eighth speed position to be described later) via the second gear change-speed mechanism  15 , the second main transmission shaft  10 , the second friction clutch  12 , the second auxiliary transmission shaft  11  and the second auxiliary gear change-speed mechanism  16  and transmitted to the transmission shaft  4 .  
         [0063]    [2] 
         [0064]    As shown in FIG. 1, on downstream of the transmission shaft  5 , there is disposed a tubular transmission shaft  43 . Between the transmission shaft  5  and the transmission shaft  43 , there is interposed a forward/reverse switchover mechanism  44  of the synchromesh type. The transmission shaft  43  is rotatably mounted on a transmission shaft  45 . And, between the transmission shaft  43  and the transmission shaft  45 , there is interposed an auxiliary change-speed mechanism  46  of the synchromesh type. Further, a front-wheel transmission shaft  55  for transmitting the power to front wheels  53  is provided. And, between the transmission shaft  45  and the front-wheel transmission shaft  55 , there is provided a front-wheel change-speed mechanism  56  of the hydraulic clutch type.  
         [0065]    As also shown in FIG. 1, the transmission shaft  43  fixedly mounts a forward gear  47  and a reverse gear  48 . The forward gear  47  meshes with a forward gear  49  rotatably mounted on the transmission shaft  5  and the reverse gear  48  meshes via an intermediate gear  51  with a reverse gear  50  rotatably mounted on the transmission shaft  5 . A shifter  52  is splined is on the transmission shaft  5  to be rotatable therewith and slidable relative thereto. These arrangements together constitute the forward/reverse switchover mechanism  44 . As shown in FIG. 2, the shifter  52  is mechanically linked with a forward/reverse lever  57 . Then, by operating the forward/reverse lever  57  to a forward position F or a reverse position R thereby to slide the shifter  52  to mesh with the forward gear  49  or the reverse gear  50 , the forward/reverse switchover mechanism  44  is operated.  
         [0066]    As shown in FIG. 1, the auxiliary change-speed mechanism  46  is operable, by sliding operations of the two shifters, into a high-speed position H, a first low-speed position L 1  and a second low-speed position L 2  (the second low-speed position L 2  provides a lower speed than the first low-speed position L 1 ). As shown in FIG. 2, there is provided an auxiliary change-speed lever  66  for sliding the tow shifters. Then, the front-wheel change-speed mechanism  56  is operable into a standard condition in which the front wheels  53  and rear wheels  54  are driven at a same speed and an accelerated condition in which the front wheels  53  are driven at a higher speed than the rear wheels  54 .  
         [0067]    With the above-described constructions, in the straight traveling condition, the power of the transmission shaft  5  is transmitted via the forward/reverse switchover mechanism  44 , the auxiliary change-speed mechanism  46 , the transmission shaft  45  and a rear-wheel differential mechanism  58  to the rear wheels  54 . And, the power of the auxiliary change-speed mechanism  46  is transmitted via the front-wheel change-speed mechanism  56  under the standard condition, the front-wheel transmission shaft  55  and a front-wheel differential mechanism  59  to the front wheels  53 . When the front wheels  53  is steered to the right or to the left from the straight traveling position thereof, the front-wheel change-speed mechanism  56  is switched over from the standard condition to the accelerated condition, whereby the front wheels  53  are driven at a higher speed than the rear wheels  54 , so that the vehicle can smoothly make a small turn.  
         [0068]    [3] 
         [0069]    As shown in FIG. 1 and FIG. 2, the first gear change-speed mechanism  13  includes a first actuator  35  of a double-acting hydraulic cylinder type for sliding the shifter  23  and a control valve  39  for supplying and discharging the working fluid to and from the first actuator  35 . The first auxiliary gear change-speed mechanism  14  includes a first auxiliary actuator  36  of a double-acting hydraulic cylinder type for sliding the shifter  31  and a control valve  40  for supplying and discharging the working fluid to and from the first auxiliary actuator  36 . The first actuator  35  is operable into a low-speed position L for meshing the shifter  23  with the low-speed gear  21 , a high-speed position H for meshing the shifter  23  with the high-speed gear  22  and a neutral position N. The first auxiliary actuator  36  is operable into a low-speed position L for meshing the shifter  31  with the low-speed gear  29  and a high-speed position H for meshing the shifter  31  with the high-speed gear  30 .  
         [0070]    As shown also in FIG. 1 and FIG. 2, the second gear change-speed mechanism  15  includes a second actuator  37  of a double-acting hydraulic cylinder type for sliding the shifter  26  and a control valve  41  for supplying and discharging the working fluid to and from the second actuator  37 . The second auxiliary gear change-speed mechanism  16  includes a second auxiliary actuator  38  of a double-acting hydraulic cylinder type for sliding the shifter  34  and a control valve  42  for supplying and discharging the working fluid to and from the second auxiliary actuator  38 . The second actuator  37  is operable into a low-speed position L for meshing the shifter  26  with the low-speed gear  24 , a high-speed position H for meshing the shifter  26  with the high-speed gear  25  and a neutral position N. The second auxiliary actuator  38  is operable into a low-speed position L for meshing the shifter  34  with the low-speed gear  32  and a high-speed position H for meshing the shifter  34  with the high-speed gear  33 .  
         [0071]    As shown in FIG. 2, there are provided a control valve  60  of an electromagnetic proportional reducing valve type for supplying and discharging the working fluid to and from the transmission clutch  6 , a control valve  61  of an electromagnetic proportional reducing valve type for supplying and discharging the working fluid to and from the first friction clutch  9 , and a control valve  62  of an electromagnetic proportional reducing valve type for supplying and discharging the working fluid to and from the second friction clutch  12 .  
         [0072]    With the above-described constructions, as shown in FIG. 1 and FIG. 7, in the condition (the transmitting state of the first friction clutch  9 ) in which the power of the transmission shaft  2  is transmitted via the first main and auxiliary transmission shafts  7 ,  8  to the transmission shaft  4 , the first speed position is realized with the shifter  31  at its low-speed position L and the shifter  23  at its low-speed position L. The third speed position is realized with the shifter  31  at the low-speed position L and the shifter  23  at the high-speed position H. The fifth speed position is realized with the shifter  31  at the high-speed position H and the shifter  23  at the low-speed position L. The seventh speed position is realized with the shifter  31  at the high-speed position H and the shifter  23  at the high-speed position H. In the first and third speed positions described above, the second friction clutch  12  is operated to the non-transmitting state and the shifter  26  is at its neutral position N and the shifter  34  is at its low-speed position L. In the fifth and seventh speed positions described above, the second friction clutch  12  is operated to the non-transmitting state and the shifter  26  is at its neutral position N and the shifter  34  is at its high-speed position H.  
         [0073]    As shown in FIG. 1 and FIG. 7, in the condition (the transmitting state of the second friction clutch  12 ) in which the power of the transmission shaft  2  is transmitted via the second main and auxiliary transmission shafts  10 ,  11  to the transmission shaft  4 , the second speed position is realized with the shifter  34  at its low-speed position L and the shifter  26  at its low-speed position L. The fourth speed position is realized with the shifter  34  at the low-speed position L and the shifter  26  at the high-speed position H. The sixth speed position is realized with the shifter  34  at the high-speed position H and the shifter  26  at the low-speed position L. The eighth speed position is realized with the shifter  34  at the high-speed position H and the shifter  26  at the high-speed position H. In the second and fourth speed positions described above, the first friction clutch  9  is operated to the non-transmitting state and the shifter  23  is at its neutral position N and the shifter  31  is at its low-speed position L. In the sixth and eighth speed positions described above, the first friction clutch  9  is operated to the non-transmitting state and the shifter  23  is at its neutral position N and the shifter  31  is at its high-speed position H.  
         [0074]    [4]  
         [0075]    Next, control scheme of the change-speed operations will be described.  
         [0076]    This agricultural tractor provides four change-speed modes of a first change-speed mode, a second change-speed mode, a third change-speed mode and a fourth change-speed mode. One of these first, second, third and fourth change-speed modes may be selected by an operator by operating a setting switch  65  (see FIG. 2).  
         [0077]    As shown in FIG. 2, there are provided a change-speed lever  63  operable into one of the first through eighth speed positions and the setting switch  65 . And, the operated positions of the change-speed lever  63 , an auxiliary change-speed lever  66 , the forward/reverse lever  57  and the setting switch  65  are all inputted to a controller  64 . As shown in FIGS. 1 and 2, the controller  64  receives also inputs of detection values of a revolution sensor  67  for detecting a revolution of the engine  2  and a further revolution sensor  68  for detecting a revolution of the rotary power transmitted to the rear-wheel differential mechanism  58 .  
         [0078]    With the above, based on the operated conditions of the change-speed lever  63 , an auxiliary change-speed lever  66 , the forward/reverse lever  57  and the setting switch  65 , the detection values from the revolution sensors  67 ,  68  and also on the mode selected from the first through fourth change-speed modes, the controller  64  actuates the control valves  39 ,  40 ,  41 ,  42 ,  60 ,  61  and  62  thereby to operate the first actuator  35 , the first auxiliary actuator  36 , the second actuator  37 , and the second auxiliary actuator  38 , thereby to operate the transmission clutch  6 , the first friction clutch  9 , and the second friction clutch  12  into the respective transmitting or non-transmitting states thereof  
         [0079]    [First Change-Speed Mode (Standard Control Mode)] 
         [0080]    The First change-speed mode will be described First.  
         [0081]    In this first change-speed mode, if the change-speed lever  63  is operated from a certain operational position (speed position) to another operational position (speed position), the change-speed operation from the operational position (speed position) prior to the operation of the change-speed lever  63  to the further operational position (speed position) into which the change-speed lever  63  has been operated is effected at one time. Specifically, in this first change-speed mode, change-speed operations are carried out in the manners described below in accordance with the operational positions (speed positions) of the change-speed lever  63 . And, in this mode, the operated positions of the auxiliary change-speed lever  66  and the forward/reverse lever  57  and the detection values from the revolution sensors  67 ,  68  are not considered.  
         [0082]    As shown in FIG. 3 and FIG. 7, for example, let us assume that the change-speed lever  63  is operated from the first speed position to the second speed position. When the change-speed lever  63  is at the first speed position, if the shifter  31  is at the low-speed position L, the shifter  23  is at the low-speed position L, the transmission clutch  6  and the first friction clutch  9  are under the transmitting states with a working pressure P 1  and if the shifter  34  is at the low-speed position L and the shifter  26  is at the neutral position N, the second friction clutch  12  is operated to the non-transmitting state with a working pressure P 0 .  
         [0083]    As shown in FIG. 3, when the change-speed lever  63  is operated from the first speed position to the second speed position (timing T 11 ), the working pressure of the transmission clutch  6  and the first friction clutch  9  is immediately reduced to the working pressure P 0  so that the clutches are operated to the non-transmitting stages (see a solid line A 3  and a dotted chain line A 1 ). At the same time, the shifter  23  is operated from the low-speed position L to the neutral position N and the shifter  26  is operated from the neutral position N to the low-speed position N (the shifters  31 ,  34  are maintained at the low-speed positions L).  
         [0084]    As shown in FIG. 3, when the shifter  23  is operated from the low-speed position L to the neutral position N and the shifter  26  is operated from the neutral position N to the low-speed position L (timing T 12 ), the working pressure of the transmission clutch  6  is gradually increased from the working pressure P 0  to the working pressure P 1  (see a solid line A 3 ) and the transmission clutch  6  is operated to the transmitting stage (timing T 13 ). In the above-described manner, the change-speed operation is completed. In this case, a time period T 1  required for one change-speed operation (from the timing T 11  to the timing T 13 ) is relatively short.  
         [0085]    For instance, if the change-speed lever  63  is operated from the second speed position to the first speed position, as shown in FIG. 7, the shifters  23 ,  26  are operated (the shifters  31 ,  34  are maintained at the low-speed positions L), and in addition, in FIG. 3, the states of the first and second friction clutches  9 ,  12  are reversed, so that the first friction clutch  9  is operated from the non-transmitting state to the transmitting state, whereas the second friction clutch  12  is operated from the transmitting state to the non-transmitting state. And, as illustrated by the solid line A 3  in FIG. 3, the pressure reducing operation and the pressure increasing operation of the transmission clutch  6  are effected.  
         [0086]    For instance, if the change-speed lever  63  is operated form the first speed position to the third speed position, as shown in FIG. 7, the shifter  23  is operated. But, the shifter  26  is maintained at the neutral position N (the shifters  31  and  34  are maintained at the low-speed positions L). So that, the first friction clutch  9  is maintained at the transmitting state and the second friction clutch  12  is maintained at the non-transmitting state. And, as illustrated by the solid line A 3  in FIG. 3, the pressure reducing operation and the pressure increasing operation of the transmission clutch  6  are effected.  
         [0087]    For instance, if the change-speed lever  63  is operated form the second speed position to the fourth speed position, as shown in FIG. 7, the shifter  26  is operated. But, the shifter  23  is maintained at the neutral position N (the shifters  31  and  34  are maintained at the low-speed positions L). So that, the first friction clutch  9  is maintained at the non-transmitting state and the second friction clutch  12  is maintained at the transmitting state. And, as illustrated by the solid line A 3  in FIG. 3, the pressure reducing operation and the pressure increasing operation of the transmission clutch  6  are effected.  
         [0088]    [Second Change-Speed Mode (Alternate Control Mode)] 
         [0089]    [5] 
         [0090]    Next, a former half of the second change-speed mode will be described with reference to FIGS. 4, 5 and  7 .  
         [0091]    In this second change-speed mode, when the change-speed lever  63  is operated from a certain operational position (speed position) to another operational position (speed position), the change-speed operation from the previous operational position (speed position) prior to the operation of the change-speed lever  63  is effected in plurality of steps until reaching the operational position (speed position) in which the change-speed lever  63  has been operated.  
         [0092]    For instance, if the change-speed lever  63  is operated from the first speed position to the fifth speed position, a change-speed operation is effected from the first speed position to the second speed position, then another change-speed operation is effected from the second speed position to the third speed position, then still another change-speed operation is effected from the third speed position to the fourth speed position and still another change-speed operation is effected from the fourth speed position to the fifth speed position, finally. Similarly, if, e.g. the change-speed lever  63  is operated from the sixth speed position to the third speed position, a change-speed operation is effected from the sixth speed position to the fifth speed position, then another change-speed operation is effected from the fifth speed position to the fourth speed position and still another change-speed operation is effected from the fourth speed position to the third speed position, finally.  
         [0093]    For instance, when the change-speed lever  63  is at the first speed position (i.e. the condition in which the shifter  23  is at the low-speed position L, the shifter  26  is at the neutral position N, the shifters  31 ,  34  are at the low-speed positions L, the transmission clutch  6  and the first friction clutch  9  are at the transmitting states with the working pressure P 1 , and the second friction clutch  12  is at the non-transmitting state with the working pressure P 0 ), if the change-speed lever  63  is operated to the fifth speed position (step S 0 ) (timing T 21 ), as describe later in section [11], the working pressure P 2  is set (step S 1 ), and the shifter  26  is operated from the neutral position N to the low-speed position N (steps S 2 , S 3 ) (from timing T 21  to timing T 22 ). With this, there is realized a condition in which the shifters  23 ,  31  are at the first speed position and the shifters  26 ,  34  are at the second speed position. In this case, the shifter  34  is not operated to the high-speed position H, but maintained at the low-speed position L (pass step S 4 ) (see FIG. 7).  
         [0094]    When the shifter  26  is operated to the low-speed position (the condition of the second speed position) (step S 3 ) (timing T 22 ), the working pressure of the transmission clutch  6  is quickly reduced to the working pressure P 2  (midway between the working pressures P 0  and P 1 ), so that the clutch assumes a semi-transmitting state (step S 5 ) (timing T 22 ) (see the solid line A 3 ). Substantially simultaneously therewith, the working pressure of the second friction clutch  12  is quickly raised from the working pressure P 2 , so that the clutch is operated to the transmitting state (from timing T 22  to timing T 23 ) (see the solid line A 2 ) and the working pressure of the first friction clutch  9  is quickly reduced from the working pressure P 1 , so that this clutch is operated to the non-transmitting state (step S 6 ) (from timing T 22  to timing T 23 ) (see the dotted chain line A 1 ).  
         [0095]    With the above, there is realized a dual-transmitting condition in which the power under the condition of the shifters  23 ,  31  being at the first position is transmitted to the transmission shaft  4  and at the same time the further power under the condition of the shifters  26 ,  34  is transmitted also to the transmission shaft  4  to be combined with said power. Under this dual-transmitting condition, even if a torque variation occurs, this torque variation can be effectively absorbed by a certain amount of slipping of the transmission clutch  6  under its semi-transmitting state, so that the power with reduced torque variation may be transmitted to the front wheels  53  and the rear wheels  54 .  
         [0096]    If the second friction clutch  12  is operated to the transmitting state with the working pressure P 1  and the first friction clutch  9  is operated to the non-transmitting state with the working pressure P 0  (timing T 23 ), the shifter  23  is operated to the neutral position N (step S 7 ) (from timing T 23  to timing T 24 ). In this case, the shifter  31  is not operated to the high-speed position, but maintained at the low-speed position L (pass step S 8 ) (see FIG. 7). When the shifter  23  is operated to the neutral position N (timing T 24 ), the working pressure of the transmission clutch  6  is gradually raised from the working pressure P 2  to the working pressure P 1 , so the clutch is operated to the transmitting state (step S 9 ) (from timing T 24  to timing T 25 ). In the above-described manner, the change-speed operation from the first speed position to the second speed position is completed.  
         [0097]    At step S 9 , the revolution (traveling speed of the vehicle) of the power transmitted to the rear-wheel differential mechanism  58  is detected by the revolution sensor  68  and inputted to the controller  64 , in which the detection value from the revolution sensor  68  undergoes a differential operation to obtain an acceleration of the vehicle. Based on this, the working pressure of the transmission clutch  6  is progressively raised from the working pressure P 2  to the working pressure P 1  in such a manner as to maintain the acceleration of the vehicle constant (predetermined characteristics) (or to vary the acceleration according to a predetermined characteristics) (from timing T 24  to timing T 25 ). In this case, a jerk value may be obtained by the differential processing of the vehicle acceleration and based on this jerk value, the working pressure of the transmission clutch  6  is progressively raised from the working pressure P 2  to the working pressure P 1  so as to render the clutch into the transmitting state.  
         [0098]    [6] 
         [0099]    Next, the latter half of the second change-speed mode will be described with reference to FIGS. 4, 5 and  7 .  
         [0100]    Upon completion of the change-speed operation from the first speed position to the second speed position as described in the foregoing section [5], the process proceeds from step S 16  to step S 1 , so as to set the working pressure P 2  as described later in section [11]. And, as the process proceeds from step S 2  to step S 10 , the shifters  23 ,  31  are operated to the third speed positions with the shifters  26 ,  34  being at the second speed positions (step S 10 ). In this case, the shifter  31  is not operated to the high-speed position H, but maintained at the low-speed position L (pass step S 11 ) (see FIG. 7). Then, the working pressure of the transmission clutch  6  is quickly reduced to the working pressure P 2  (intermediate pressure between the working pressures P 0 , P 1 ), so that the clutch assumes the semi-transmitting state (step S 12 ). Substantially simultaneously therewith, the working pressure of the first friction clutch  9  is quickly raised from the working pressure P 0 , so that the clutch is operated into the transmitting state and the working pressure of the second friction clutch  12  is quickly reduced from the working pressure P 1 , so that this clutch is operated into the non-transmitting state (step S 13 ).  
         [0101]    With this, there is realized a dual-transmitting condition in which the power provided under the condition of the shifters  26 ,  34  being at the second speed positions is transmitted to the transmission shaft  4  and at the same time, the power provided under the condition of the shifters  23 ,  31  at the third speed positions is transmitted also to the transmission shaft  4  to be combined therewith. Under this dual-transmitting condition, even if a torque variation occurs, this torque variation can be effectively absorbed by slipping of the transmission clutch  6  under its semi-transmitting state, so that the power with reduced torque variation may be transmitted to the front wheels  53  and the rear wheels  54 .  
         [0102]    When the first friction clutch  9  is operated to the transmitting state with the working pressure P 1  and the second friction clutch  12  is operated to the non-transmitting state with the working pressure P 0 , the shifter  26  is operated to the neutral position N (step S 14 ). In this case, the shifter  34  is not operated to the high-speed position H, but maintained at the low-speed position L (pass step S 15 ) (see FIG. 7). When the shifter  26  is operated to the neutral position N, the working pressure of the transmission clutch  6  is progressively raised from the working pressure P 2  to the working pressure P 1 , so that the clutch is operated into the transmitting state (step S 9 ). In this way, the change-speed operation from the second speed position to the third speed position is complete.  
         [0103]    In this case, as described in the foregoing section [5], the working pressure of the transmission clutch  6  is progressively raised from the working pressure P 2  to the working pressure P 1  in such a manner as to maintain the acceleration of the vehicle constant (predetermined characteristics) (or to vary the acceleration according to a predetermined characteristics).  
         [0104]    Upon completion of the change-speed operation from the second speed position to the third speed position described above, then, a further change-speed operation from the third speed position to the fourth speed position with the shifters  23 ,  31  being at the third speed positions and the shifters  26 ,  34  being at the fourth speed positions will be effected according to the process described in the foregoing section [5] and steps S 1  through S 9  is effected (in this case, the shifters  34 ,  31  are not operated to the high-speed positions H, but maintained at the low-speed positions L (pass steps S 4 , S 8 ) (see FIG. 7).  
         [0105]    Next, a still further change-speed operation from the fourth speed position to the fifth speed position with the shifters  26 ,  34  being at the fourth speed positions and the shifters  23 ,  31  being at the fifth speed positions will be effected according to the process described in this section [6] and steps S 1 , S 2 , S 10 -S 15  and S 9  (in this case, at step S 11 , the shifter  31  is operated from the low-speed position L to the high-speed position H, and at step S 15 , the shifter  34  is operated from the low-speed position L to the high-speed position H) (see FIG. 7).  
         [0106]    With the above-described repetition of the change-speed operations, the process reaches the operated position (speed position) of the change-speed lever  63 . With this, the change-speed operation is completed.  
         [0107]    [Third Change-Speed Mode (First Skip Change-Speed Mode)] 
         [0108]    [7] 
         [0109]    Next, a former half of the third change-speed mode will be described with reference to FIG. 6 and FIG. 7.  
         [0110]    In this third change-speed mode, when the change-speed lever  63  is operated from a certain operational position (speed position) to another operational position (speed position), there is set a first intermediate speed position (or first and second intermediate speed positions) of a transmission ratio which is substantially in the middle of the operational position (speed position) prior to the operation of the change-speed lever  63  and the operational position (speed position) into which the lever  63  has been operated. Then, a change-speed operation is effected first from the operational position (speed position) prior to the operation of the change-speed lever  63  to the first intermediate speed position and then a further change-speed operation is effected from this first intermediate speed position to the further operational position (speed position) into which the lever  63  has been operated (Alternatively, a change-speed operation is effected first from the operational position (speed position) prior to the operation of the change-speed lever  63  to the first intermediate speed position, then a further change-speed operation is effected from this first intermediate speed position to the second intermediate speed position and then a still further change-speed operation is effected from the second intermediate speed position to the further operational position (speed position) into which the lever  63  has been operated.).  
         [0111]    If the change-speed lever  63  is operated from a certain operational position (speed position) to another adjacent higher or lower speed position (e.g. if the lever  63  is operated from the first speed position to the second speed position or from the eighth speed position to the seventh speed position, etc.), the process proceeds from steps S 21 , S 22 , S 23  to steps S 24 , S 25  to set the working pressure P 2  as described later in section [11], to effect the change-speed operation to the operated position (speed position) of the change-speed lever  63 . In this case, if the previous operational position (speed position) prior to the operation of the change-speed lever  63  is one of the first, third, fifth or seventh speed position, the change-speed operation will be effected according to the process of the foregoing section [5] and steps S 3  through S 9 . Whereas, if the previous operational position (speed position) prior to the operation of the change-speed lever  63  is one of the second, fourth, sixth or eighth speed position, the change-speed operation will be effected according to the process of the foregoing section [6] and steps S 10  through S 15  and S 9 .  
         [0112]    If the change-speed lever  63  is operated from one of the first, third, fifth and seventh speed position to another of these first, third, fifth and seventh speed position (steps S 21 , S 22 ), between the previous operational position (speed position) prior to the operation of the change-speed lever  63  and the further operational position (speed position) into which the lever  63  has been operated, from the second, fourth, sixth and eighth speed positions, there is selectively set a first intermediate speed position providing a transmission ratio substantially in the middle thereof (step S 26 ). For instance, if the change-speed lever  63  is operated from the first speed position to the seventh speed position, the fourth speed position is set as the first intermediate speed position.  
         [0113]    Similarly, if the change-speed lever  63  is operated from one of the second, fourth, sixth and eighth speed positions to another of these the second, fourth, sixth and eighth speed positions (steps S 21 , S 22 ), between the previous operational position (speed position) prior to the operation of the change-speed lever  63  and the further operational position (speed position) into which the lever  63  has been operated, from the first, third, fifth and seventh speed position, there is selectively set a first intermediate speed position providing a transmission ratio substantially in the middle thereof (step S 26 ). For instance, if the change-speed lever  63  is operated from the eighth speed position to the second speed position, the fifth speed position is set as the first intermediate speed position.  
         [0114]    If the change-speed lever  63  is operated from one of the first, third, fifth and seventh speed positions to one of the second, fourth, sixth and eighth speed positions (excluding the above-described case in which the change-speed lever  63  is operated from a certain operational position (speed position) to another adjacent higher or lower speed position) (steps S 21 , S 22 ), between the previous operational position (speed position) prior to the operation of the change-speed lever  63  and the further operational position (speed position) into which the lever  63  has been operated, from the second, fourth, sixth and eighth speed positions, there is selected set a first intermediate speed position providing a transmission ratio substantially in the middle thereof and further from the first, third, fifth and seventh speed position, there is selectively set a second intermediate speed position providing a transmission ratio substantially in the middle thereof (step S 29 ). In this case, the second intermediate speed position is closer to the further operational position (speed position) into which the change-speed lever  63  has been operated than the first intermediate speed position is. For instance, if the change-speed lever  63  is operated from the first speed position to the sixth speed position, the third speed position will be selectively set as the first intermediate speed position and the fourth speed position will be selectively set as the second intermediate speed position. Further, if the change-speed lever  63  is operated from the first speed position to the eighth speed position, the fourth speed position will be selectively set as the first intermediate speed position and the fifth speed position will be selectively set as the second intermediate speed position.  
         [0115]    If the change-speed lever  63  is operated from one of the second, fourth, sixth and eighth speed positions to one of the first, third, fifth and seventh speed positions (excluding the above-described case in which the change-speed lever  63  is operated from a certain operational position (speed position) to another adjacent higher or lower speed position) (steps S 21 , S 22 ), between the previous operational position (speed position) prior to the operation of the change-speed lever  63  and the further operational position (speed position) into which the lever  63  has been operated, from the first, third, fifth and seventh speed positions, there is selected set a first intermediate speed position providing a transmission ratio substantially in the middle thereof and further from the second, fourth, sixth and eighth speed positions, there is selectively set a second intermediate speed position providing a transmission ratio substantially in the middle thereof (step S 29 ). In this case, the second intermediate speed position is closer to the further operational position (speed position) into which the change-speed lever  63  has been operated than the first intermediate speed position is. For instance, if the change-speed lever  63  is operated from the eighth speed position to the first speed position, the fifth speed position will be selectively set as the first intermediate speed position and the fourth speed position will be selectively set as the second intermediate speed position.  
         [0116]    [8] 
         [0117]    Next, the latter half of the third change-speed mode will be described with reference to FIGS. 6 and 7.  
         [0118]    As described in the foregoing section [7], when the change-speed lever  63  is operated to set the first intermediate speed position (the first and second intermediate speed positions), the working pressure P 2  is set as described in section [11] to be described later (steps S 24 , S 25 ).  
         [0119]    As described in the foregoing section [7], under the condition of the first intermediate speed position being set (step S 26 ), the change-speed operation from the previous operational position (speed) prior to the operation of the change-speed lever  63  to the first intermediate speed position will be effected (step S 27 ). In this case, if the previous operational position (speed position) prior to the operation of the change-speed lever  63  is one of the first, third, fifth or seventh speed position, the change-speed operation will be effected according to the process of the foregoing section [5] and steps S 3  through S 9 . Whereas, if the previous operational position (speed position) prior to the operation of the change-speed lever  63  is one of the second, fourth, sixth or eighth speed position, the change-speed operation will be effected according to the process of the foregoing section [6] and steps S 10  through S 15  and S 9 .  
         [0120]    Next, as the working pressure P 2  is set as described in the section [11] described later (step S 28 ), and the change-speed operation from the first intermediate speed position to the further operational position (speed position) into which the change-speed lever  63  has been operated will be effected (step S 34 ). In this case, if the first intermediate speed position is one of the first, third, fifth or seventh speed position, the change-speed operation will be effected according to the process of the foregoing section [5] and steps S 3  through S 9 . Whereas, if the first intermediate speed position is one of the second, fourth, sixth or eighth speed position, the change-speed operation will be effected according to the process of the foregoing section [6] and steps S 10  through S 15  and S 9 .  
         [0121]    As described in the foregoing section [7], under the condition of the first intermediate speed position and the second intermediate speed position being set (step S 29 ), the change-speed operation from the previous operational position (speed) prior to the operation of the change-speed lever  63  to the first intermediate speed position will be effected (step S 30 ). In this case, if the previous operational position (speed position) prior to the operation of the change-speed lever  63  is one of the first, third, fifth or seventh speed position, the change-speed operation will be effected according to the process of the foregoing section [5] and steps S 3  through S 9 . Whereas, if the previous operational position (speed position) prior to the operation of the change-speed lever  63  is one of the second, fourth, sixth or eighth speed position, the change-speed operation will be effected according to the process of the foregoing section [6] and steps S 10  through S 15  and S 9 .  
         [0122]    Next, as the working pressure P 2  is set as described in the section [11] described later (step S 31 ), and the change-speed operation from the first intermediate speed position to the second intermediate speed position will be effected (step S 32 ). In this case, if the first intermediate speed position is one of the first, third, fifth or seventh speed position, the change-speed operation will be effected according to the process of the foregoing section [5] and steps S 3  through S 9 . Whereas, if the first intermediate speed position is one of the second, fourth, sixth or eighth speed position, the change-speed operation will be effected according to the process of the foregoing section [6] and steps S 10  through S 15  and S 9 .  
         [0123]    Next, as the working pressure P 2  is set as described in the section [11] described later (step S 33 ), and the change-speed operation from the second intermediate speed position to the further operational position (speed position) into which the change-speed lever  63  has been operated will be effected (step S 34 ). In this case, if the second intermediate speed position is one of the first, third, fifth or seventh speed position, the change-speed operation will be effected according to the process of the foregoing section [5] and steps S 3  through S 9 . Whereas, if the second intermediate speed position is one of the second, fourth, sixth or eighth speed position, the change-speed operation will be effected according to the process of the foregoing section [6] and steps S 10  through S 15  and S 9 .  
         [0124]    [Fourth Change-Speed Mode (Second Skip Change-Speed Mode)] 
         [0125]    [9] 
         [0126]    In this fourth change-speed mode, like the third change-speed mode described in the foregoing sections [7], [8], a first intermediate speed position (or first and second intermediate speed positions) is set. However, this first intermediate speed position (or the first and second intermediate speed positions) is different from that of the third change-speed mode as described below.  
         [0127]    In the fourth change-speed mode, when the change-speed lever  63  is operated from a certain operational position (speed position) to another operational position (speed position), the first intermediate speed position (or the first and second intermediate speed positions) is (are) set slightly closer to the previous operational position (speed position) prior to the operation of the change-speed lever  63  than to the further operational position (speed position) into which the lever  63  has been operated. And, from the previous operational position (speed position) prior to the operation of the change-speed lever  63 , the setting operation of the working pressure P 2  described later in section [11] and a change-speed operation to the first intermediate speed position will be effected. And, from this first intermediate speed position, the setting operation of the working pressure P 2  described later in section [11] and a change-speed operation to the second intermediated speed position will be effected. Then, from this second intermediate speed position, the setting operation of the working pressure P 2  described later in section [11] and a change-speed operation to the further operational position (speed position) into which the lever  63  has been operated will be effected.  
         [0128]    If the change-speed lever  63  is operated from a certain operational position (speed position) to another adjacent higher or lower speed position (e.g. if the lever  63  is operated from the first speed position to the second speed position or from the eighth speed position to the seventh speed position, etc.), like the third change-speed mode (see the foregoing section [7]), the setting operation of the working pressure P 2  described later in section [11] and a change-speed operation to the further operational position (speed position) into which the change-speed lever  63  has been operated will be effected. In this case, if the previous operational position (speed position) prior to the operation of the change-speed lever  63  is one of the first, third, fifth or seventh speed position, the change-speed operation will be effected according to the process of the foregoing section [5] and steps S 3  through S 9 . Whereas, if the previous operational position (speed position) prior to the operation of the change-speed lever  63  is one of the second, fourth, sixth or eighth speed position, the change-speed operation will be effected according to the process of the foregoing section [6] and steps S 10  through S 15  and S 9 .  
         [0129]    If the change-speed lever  63  is operated from one of the first, third, fifth and seventh speed position to another of these first, third, fifth and seventh speed position, from the second, fourth, sixth and eighth speed positions, there is selectively set a first intermediate speed position slightly closer to the previous operational position (speed position) prior to the operation of the change-speed lever  63  than to the further operational position (speed position) into which the lever  63  has been operated. For instance, if the change-speed lever  63  is operated from the first speed position to the seventh speed position, the sixth speed position is set as the first intermediate speed position.  
         [0130]    Similarly, if the change-speed lever  63  is operated from one of the second, fourth, sixth and eighth speed positions to another of these second, fourth, sixth and eighth speed positions, from the first, third, fifth and seventh speed position, there is selectively set a first intermediate speed position slightly closer to the previous operational position (speed position) prior to the operation of the change-speed lever  63  than to the further operational position (speed position) into which the lever  63  has been operated. For instance, if the change-speed lever  63  is operated from the eighth speed position to the second speed position, the third speed position is set as the first intermediate speed position.  
         [0131]    If the change-speed lever  63  is operated from one of the first, third, fifth and seventh speed positions to one of the second, fourth, sixth and eighth speed positions (excluding the above-described case in which the change-speed lever  63  is operated from a certain operational position (speed position) to another adjacent higher or lower speed position), from the second, fourth, sixth and eighth speed positions, there is selected set first intermediate speed position slightly closer to the previous operational position (speed position) prior to the operation of the change-speed lever  63  than to the further operational position (speed position) into which the lever  63  has been operated, and further from the first, third, fifth and seventh speed position, there is selectively set a second intermediate speed position slightly closer to the previous operational position (speed position) prior to the operation of the change-speed lever  63  than to the further operational position (speed position) into which the lever  63  has been operated. In this case, the second intermediate speed position is closer to the further operational position (speed position) into which the change-speed lever  63  has been operated than the first intermediate speed position is. For instance, if the change-speed lever  63  is operated from the first speed position to the eighth speed position, the sixth speed position will be selectively set as the first intermediate speed position and the seventh speed position will be selectively set as the second intermediate speed position.  
         [0132]    If the change-speed lever  63  is operated from one of the second, fourth, sixth and eighth speed positions to one of the first, third, fifth and seventh speed positions (excluding the above-described case in which the change-speed lever  63  is operated from a certain operational position (speed position) to another adjacent higher or lower speed position), from the first, third, fifth and seventh speed positions, there is selected set first intermediate speed position slightly closer to the previous operational position (speed position) prior to the operation of the change-speed lever  63  than to the further operational position (speed position) into which the lever  63  has been operated, and further from the second, fourth, sixth and eighth speed positions, there is selectively set a second intermediate speed position slightly closer to the previous operational position (speed position) prior to the operation of the change-speed lever  63  than to the further operational position (speed position) into which the lever  63  has been operated. In this case, the second intermediate speed position is closer to the further operational position (speed position) into which the change-speed lever  63  has been operated than the first intermediate speed position is. For instance, if the change-speed lever  63  is operated from the eighth speed position to the first speed position, the third speed position will be selectively set as the first intermediate speed position and the second speed position will be selectively set as the second intermediate speed position.  
         [0133]    As described above, with the setting of the first intermediate speed position (or the first and second intermediate speed positions), like the third change-speed mode described in the foregoing section [8] and illustrated in FIG. 6, the setting operation of the working pressure P 2  described later in section [11] and a change-speed operation to the first intermediate speed position will be effected. And, from this first intermediate speed position, the setting operation of the working pressure P 2  described later in section [11], a change-speed operation to the operated position (speed position) of the change-speed lever  63  will be effected. And, from the previous position (speed position) prior to the operation of the change-speed lever  63 , the setting operation of the working pressure P 2  described later in section [11] and a change-speed operation to the second intermediate speed position will be effected. And, from this second intermediate speed position, the setting operation of the working pressure P 2  described later in section [11] and a change-speed operation to the operated position (speed position) of the change-speed lever  63  will be effected.  
         [0134]    [10] 
         [0135]    Next, there will be described a case when the forward/reverse lever  57  is operated.  
         [0136]    As described in the foregoing section [2], in the forward/reverse switchover mechanism  44 , as shown in FIG. 2, the forward/reverse lever  57  is mechanically linked to the shifter  52 . And, the operated position of the forward/reverse lever  57  is inputted to the controller  64 . The shifter is slid to a forward drive position F or a reverse drive position R by operating the forward/reverse lever  57 .  
         [0137]    With this, when an operation for operating the forward/reverse lever  57  from the forward drive position F to the reverse drive position R (or from the reverse drive position R to the forward drive position F) is initiated, like the latter half (of the first change-speed mode) described in the foregoing section [4], the working pressure of the transmission clutch  6  is automatically reduced as shown by the solid line A 3  in FIG. 3, so that the clutch is operated into the non-transmitting state. And, when the forward/reverse lever  57  is operated to the reverse drive position R (or the forward drive position F), the working pressure of the transmission clutch  6  is automatically raised progressively, so that the clutch is operated into the transmitting state.  
         [0138]    For this transmission clutch  6  alone, there is provided a clutch pedal (not shown) which can be manually operated. And, the operated position of this clutch pedal is inputted to the controller  64 . With this, when the clutch pedal is depressed by a foot, the controller  64  and the control valve  60  operate the transmission clutch  6  into its non-transmitting state. When the clutch pedal is released, the controller  64  and the control valve  60  operate the transmission clutch  6  into its transmitting state.  
         [0139]    [11] 
         [0140]    Next, the setting operations of the working pressure P 2  at the step S 1  of the foregoing sections [5] through [9] and FIG. 5, and the steps S 24 , S 25 , S 28 , S 31 , S 33  in FIG. 6 will be described with reference to FIG. 8.  
         [0141]    As shown in step S 0  in FIG. 5 and steps S 22 , S 23  in FIG. 6, when the operation of the change-speed lever  63  is started, the revolution of the engine  1  at this very moment is detected by the revolution sensor  67  and this detected revolution (current revolution of the engine  1 ) is compared with a revolution of the engine  1  under zero-load condition, thereby to obtain a revolution difference N (step S 41 ).  
         [0142]    Then, based on this revolution difference N, the working pressure P 2  is set. Specifically, if the revolution difference N is large, this is interpreted as a large load being applied to the vehicle. Hence, the working pressure P 2  will be set to a relatively high value. If the revolution difference N is small, this is interpreted as a small load being applied to the vehicle. Hence, the working pressure P 2  will be set to a relatively low value. (step S 42 ). In the case of an agricultural tractor, in general, a desired traveling speed of the vehicle is obtained by operating the change-speed lever  63  and the auxiliary change-speed lever  66  with an accelerator lever (not shown) being set to its full-open position. Therefore, the above-described revolution of the engine under the zero lead condition means a revolution of the engine  1  under the condition in which the accelerator lever is operated to the full-open position.  
         [0143]    The operated position (speed position) of the auxiliary change-speed lever  66  is detected. And, if it is detected that the auxiliary change-speed lever  66  is set to the high-speed position H, this is interpreted as the road run of the vehicle (step S 43 ). Then, the working pressure P 2  set at step S 42  is adjusted to a slightly lower speed side (step S 44 ). Conversely, if it is detected that the auxiliary change-speed lever  66  is set at the first or second low-speed position L 1 , L 2  , this is interpreted as the working run of the vehicle (step S 43 ). Then, the working pressure P 2  set at step S 42  is adjusted to a slightly higher speed side (step S 45 ).  
         [0144]    When the auxiliary change-speed lever  66  is set at the high-speed position H (steps S 43 , S 44 ), if the forward/reverse lever  57  is set at the forward drive position F (step S 46 ), the working pressure P 2  set at step S 42  is adjusted to a slightly higher speed side (step S 47 ). Whereas, if the forward/reverse lever  57  is set at the reverse drive position R (step S 46 ), the working pressure P 2  set at step S 42  is adjusted to a slightly lower speed side (step S 48 ).  
         [0145]    When the auxiliary change-speed lever  66  is set at the first or second low-speed position L 1 , L 2  (steps S 43 , S 45 ), if the forward/reverse lever  57  is set at the forward drive position F (step S 49 ), then, the operated position (speed position) of the change-speed lever  63  is detected (step S 50 ). Then, based on this detected operated position of the change-speed lever  63 , the working pressure P 2  set at step S 42  is adjusted (step S 51 ). In this case, the higher position is detected at step S 51  as the operated position (speed position) of the change-speed lever  63 , the higher side the working pressure P 2  set at step S 45  will be adjusted to. Conversely, the lower position is detected at step S 51  as the operated position (speed position) of the change-speed lever  63 , the lower side the working pressure P 2  set at step S 45  will be adjusted to.  
         [0146]    When the auxiliary change-speed lever  66  is set at the first or second low-speed position L 1 , L 2  (steps S 43 , S 45 ), if the forward/reverse lever  57  is set at the reverse drive position R (step S 49 ), then, the working pressure P 2  set at step S 45  will be adjusted to a slightly lower side (step S 52 ).  
         [0147]    As described above, the working pressure P 2  is set, in accordance with the revolution difference N between the revolution of the engine  1  under the zero load condition and a detected revolution of the engine  1  (current revolution of the engine  1 ), the operated position (speed position) of the change-speed lever  63 , the operated position (speed position) of the auxiliary change-speed lever  66  and the operated position of the forward/reverse lever  57 . Therefore, the working pressures P 2  set respectively at step S 1  of FIG. 5, and at steps S 24 , S 25 , S 28 , S 31 , S 33  of FIG. 6 will be slightly different from each other.  
         [0148]    [Modified Constructions of the Invention] 
         [0149]    [A-1] 
         [0150]    In place of steps S 41 , S 42  in FIG. 8, upon an operation of the change-speed lever  63 , a resultant reduction ratio in the traveling speed of the vehicle at this very moment may be calculated based on the detection value from the revolution sensor  68  and the working pressure P 2  may be set based on this reduction ratio in the traveling speed of the vehicle. In this case, if the reduction ratio in the traveling speed of the vehicle is large, this is interpreted as a large load being applied to the vehicle, so that the working pressure P 2  will be set to a higher value. Conversely, if the reduction ratio in the traveling speed of the vehicle is small, this is interpreted as a small load being applied to the vehicle, so that the working pressure P 2  will be set to a lower value.  
         [0151]    [A-2] 
         [0152]    The steps S 41  through S 52  of FIG. 8 may be omitted. Instead, a manually operable dial switch (not shown) may be provided for allowing an operator to manually set the working pressure P 2 . Like this case, when the working pressure P 2  is not calculated and set for each change-speed operation, the flowcharts of FIGS. 5 and 6 described above will be modified as flowcharts of FIGS. 9 and 10, respectively.  
         [0153]    [A-3] 
         [0154]    As shown in step S 9  of FIG. 5, in the case of the construction for progressively raising the working pressure of the transmission clutch  6  from the working pressure P 2  to the working pressure P 1  for operating the clutch into the transmitting state in such a manner as to maintain the acceleration of the vehicle constant (predetermined characteristics) (or to vary it according to the predetermined characteristics), a manually operable dial switch (not shown) may be provided for allowing an operator to manually set or adjust the acceleration of the vehicle (predetermined characteristics of the acceleration of the vehicle) to be maintained constant.  
         [0155]    [A-4] 
         [0156]    In the first change-speed mode described above, when the change-speed lever  63  is operated from one of the first, third, fifth and seventh speed positions to one of the second, fourth, sixth and eighth speed positions, like the second change-speed mode, the change-speed mode may be effected according to steps S 3  through S 9  of FIG. 5. Similarly, when the change-speed lever  63  is operated from one of the second, fourth, sixth and eighth speed positions to one of the first, third, fifth and seventh speed positions, like the second change-speed mode, the change-speed mode may be effected according to steps S 10  through S 15  and S 9  of FIG. 5.  
         [0157]    [B-1] 
         [0158]    The setting switch  65  may be omitted and the construction may be modified as follows.  
         [0159]    A revolution sensor (not shown) for detecting a revolution of the engine  1  is provided for obtaining a difference between the detected revolution of the engine  1  and a revolution of the engine  1  under zero load condition. In the case of an agricultural tractor, in general, a desired traveling speed of the vehicle is obtained by operating the change-speed lever  63  and the auxiliary change-speed lever (not shown) with an accelerator lever (not shown) being set to its full-open position. Therefore, the above-described revolution of the engine under the zero lead condition means a revolution of the engine  1  under the condition in which the accelerator lever is operated to the full-open position.  
         [0160]    Then, if the revolution difference is below a predetermined value, this is interpreted as a small traveling load being applied to the vehicle, so that the first change-speed mode is automatically selected. Conversely, if the revolution difference is above the predetermined value, this is interpreted as a large traveling load being applied to the vehicle, so that the second change-speed mode is automatically selected. In this case, the construction may be further modified such that the third change-speed mode or the fourth change-speed mode is automatically selected instead of the second change-speed mode.  
         [0161]    [B-2] 
         [0162]    The setting switch  65  may be omitted and the construction may be further modified as follows.  
         [0163]    A hitch (not shown) is provided at the rear of the vehicle body for connecting the implement, the cart or the like for towing it. And, a towing load sensor (not shown) is provided for detecting a towing load applied to the hitch.  
         [0164]    Then, if the towing load is below a predetermined value, the first change-speed mode is automatically selected. Conversely, if the towing load is above the predetermined value, the second change-speed mode is automatically selected. In this case, the construction may be further modified such that the third change-speed mode or the fourth change-speed mode is automatically selected instead of the second change-speed mode.  
         [0165]    [B-3] 
         [0166]    The setting switch  65  may be omitted and the construction may be further modified as follows.  
         [0167]    With an agricultural tractor, to a top link (not shown) and a lower link (not shown) provided at the rear of the vehicle body, a rotary plow (not shown) (an example of the ground-work implement) may be connected for effecting a plowing work or a plow (not shown) (another example of the ground-work implement) may be connected for effecting a drafting work for leveling the ground surface. As the plowing work and the drafting work differ in the modes of the operations, when an operator manually operates an operation switch (not shown), the plowing work mode (rotary plow) or the drafting work mode (plow) will be selected.  
         [0168]    Then, in the case of the plowing work mode, the plowing depth of the rotary plow implement will be detected. And, the rotary plow implement will be automatically lifted up or down relative to the vehicle body so as to maintain the plowing depth constant. In this case, the towing load applied to the vehicle will be relatively small (i.e. the rotation of the rotary pawls of the rotary plow implement tends to propel the vehicle body forward).  
         [0169]    On the other hand, in the case of the drafting work mode, the towing load applied to the lower link will be detected. Then, the plow implement will be automatically lifted up or down relative to the vehicle body so as to maintain the towing load constant. In this case, the towing load applied to the vehicle (lower link) will be relatively large.  
         [0170]    Then, the construction may be provided as follow. Namely, with an operation of a work mode switch, if a disengaged condition for disabling both the plowing work mode and the drafting work mode or the plowing work mode is selected, the first change-speed mode will be automatically selected. Whereas, if the drafting work mode is selected with an operation of the work mode switch, the section change-speed mode will be automatically selected. In this case, the construction may be further modified such that the third change-speed mode or the fourth change-speed mode is automatically selected instead of the second change-speed mode.  
         [0171]    [B-4] 
         [0172]    The setting switch  65  may be omitted and the construction may be further modified as follows.  
         [0173]    In this case, if the auxiliary change-speed lever for operating the auxiliary change-speed mechanism  46  shown in FIG. 1 is operated to the high-speed position, this is interpreted as the traveling-speed of the vehicle being high. Then, the first change-speed mode will be automatically selected. Conversely, if the auxiliary change-speed lever is operated to the middle-speed or low-speed position, this is interpreted as the traveling speed of the vehicle being low. Then, the second change-speed mode will be automatically selected. In this case, the construction may be further modified such that the third change-speed mode or the fourth change-speed mode is automatically selected instead of the second change-speed mode.  
         [0174]    [B-5] 
         [0175]    The setting switch  65  may be omitted and the construction may be further modified as follows.  
         [0176]    The agricultural tractor may include a lift arm (not shown) pivotable up and down by means of a hydraulic cylinder, thereby to lift up and down the lower link (in the plowing work mode and the drafting work mode described above, the rotary plow implement and the plow implement are lifted up and down by the lift arm).  
         [0177]    As described hereinbefore, in case the ground-work implement such as the rotary plow or the plow is connected to the top link and the lower link, when no ground-work is to be effected such as in the case of the road run, the ground-work implement will be lifted up far away from the ground relative to the vehicle body. In this case, it may be judged that the traveling load applied to the vehicle is relatively small.  
         [0178]    Conversely, if the ground-work implement is significantly lowered relative to the vehicle body, this often means that the vehicle is to effect a work by the ground-work implement. In this case, it may be judged that the traveling load applied to the vehicle is relatively large.  
         [0179]    Then, the construction may be modified as follows. Namely, the height or altitude of the ground-work implement relative to the vehicle body is detected (e.g. a vertical angle of the lift arm or the lower link relative to the vehicle body is detected as the height of the ground-work implement relative to the vehicle body). If the height of the ground-work implement relative to the vehicle body is above a predetermined value, the first change-speed mode is automatically selected. Conversely, if the height of the ground-work implement relative to the vehicle body is below the predetermined value, the second change-speed mode is automatically selected. In this case, the construction may be further modified such that the third change-speed mode or the fourth change-speed mode is automatically selected instead of the second change-speed mode.  
         [0180]    [D-1] 
         [0181]    In place of the construction shown in FIG. 1, the transmission to which the present invention is applied may be modified as shown in FIG. 11.  
         [0182]    As shown in FIG. 11, in this construction, the transmission clutch  6  (see FIG. 1) is omitted, and the transmission shaft  4  and the transmission shaft  5  are connected. A transmission shaft  43  fixedly mounts a forward gear  47  and a reverse gear  48 . The forward gear  47  meshes with a forward gear  49  rotatably mounted on the transmission shaft  5  an the reverse ger  48  meshes via an intermediate gear  51  with a reverse gear  50  rotatably mounted on the transmission shaft  5 . Between the forward gear  49  and the transmission shaft  5 , there is provided a hydraulic multiple-disc friction type forward clutch  66 . Between the reverse gear  50  and the transmission shaft  5 , there is provided a hydraulic multiple-disc friction type reverse clutch  67 . These together constitute a forward/reverse switchover mechanism  44 . Like the transmission clutch  6 , the forward and reverse clutches  66 ,  67  each is operable into a transmitting state in response to supply of a working fluid thereto and operable into a non-transmitting state in response to discharge of the working fluid therefrom. In this construction, the shifter  52  shown in FIG. 1 is omitted. And, the operated position of the forward/reverse lever  57  is inputted to the controller  64 .  
         [0183]    With the above, when the forward/reverse lever  57  is operated to the forward drive position F, the forward clutch  66  is operated into the transmitting state and the reverse clutch  67  is operated into the non-transmitting state. On the other hand, when the forward/reverse lever  57  is operated to the reverse drive position R, the reverse clutch  67  is operated into the transmitting state and the forward clutch  66  is operated into the non-transmitting state.  
         [0184]    Under the condition of the forward/reverse lever  57  being set at the forward drive position F (i.e. the condition of the forward clutch  66  being under the transmitting state and the reverse dutch  67  being under the non-transmitting state), instead of the transmission clutch  6 , the forward clutch  66  is operated. Under the condition of the forward/reverse lever  57  being set at the reverse drive position R (i.e. the condition of the reverse clutch  67  being under the transmitting state and the forward clutch  66  being under the non-transmitting state), instead of the transmission clutch  6 , the reverse clutch  67  is operated.  
         [0185]    For the forward and reverse clutches  66 ,  67 , there is provided a clutch pedal (not shown) which can be manually operated by being stepped on. With this, under the condition of the forward/reverse lever  57  being at the forward drive position F (i.e. the condition of the forward clutch  66  being under the transmitting state and the reverse clutch  67  being under the non-transmitting state), if the clutch pedal is stepped on, the forward clutch  66  is operated into the non-transmitting state. And, when the clutch pedal is released, the forward clutch  66  is operated back into the transmitting state. Under the condition of the forward/reverse lever  57  being at the reverse drive position R (i.e. the condition of the reverse clutch  67  being under the transmitting state and the forward clutch  66  being under the non-transmitting state), if the clutch pedal is stepped on, the reverse clutch  67  is operated into the non-transmitting state. And, when the clutch pedal is released, the reverse clutch  67  is operated back into the transmitting state.  
         [0186]    Further, a first gear change-speed mechanism  13  may be interposed between a first auxiliary transmission shaft  8  and the transmission shaft  4 . And, a first auxiliary gear change-speed mechanism  14  may be interposed between a main transmission shaft  7  and the transmission shaft  2 . And, a second gear change-speed mechanism  15  may be interposed between a second auxiliary transmission shaft  11  and the transmission shaft  4  and a second auxiliary gear change-speed mechanism  16  may be interposed between a second main transmission shaft  10  and the transmission shaft  2 .  
         [0187]    [D-2] 
         [0188]    In place of the constructions shown in FIGS. 1 and 11, the transmission to which the present invention is applied may be modified as shown in FIG. 12.  
         [0189]    [0189]FIG. 12 also shows a traveling transmission system for an four-wheel-drive agricultural tractor as an example of the work vehicle. The power of the engine  1  is transmitted to a transmission shaft  105  and transmitted also via a multiple-disc type PTO clutch  66 , a PTO change-speed mechanism  67  and a transmission shaft  68  to a PTO shaft  3 . The PTO change-speed mechanism  67  is operable, by a sliding operation of a shifter, into a high-speed position, a middle-speed position or a low-speed position.  
         [0190]    As shown in FIG. 12, tubular transmission shafts  102 ,  4  are rotatably mounted on the transmission shaft  105 . And, between the transmission shaft  105  and the transmission shaft  102 , there is provided a forward/reverse switchover mechanism  144  of the synchromesh type. The transmission shaft  105  rotatably mounts a forward gear  149  and the transmission shaft  102  rotatably mounts a reverse gear  150 . The forward gear  149  meshes with a transmission gear  147  fixedly mounted on a transmission shaft  143 . The reverse gear  150  meshes via an intermediate gear  151  with a transmission gear  148  fixedly mounted on the transmission shaft  143 . A shifter  152  is splined on the transmission shaft  102  to be rotatable therewith and sidably relative thereto. These together constitute the forward/reverse switchover mechanism  144 .  
         [0191]    As shown in FIG. 12, between the transmission shaft  5  and the forward gear  149 , there is provided a hydraulic multiple-disc friction type transmission clutch  106 . This transmission clutch  106  is operable into a transmitting state in response to supply of a working fluid thereto and operable into a non-transmitting state in response to discharge of the working fluid therefrom.  
         [0192]    As shown in FIG. 12, in the forward/reverse switchover mechanism  144 , a forward/reverse lever  57  (see FIG. 2) is mechanically linked to the shifter  152 . Then, by operating the forward/reverse lever  57  to the forward drive position F or the reverse drive position R, the shifter  152  is slid to mesh with the forward gear  149  and the reverse gear  150 . When the shifter  152  is meshed with the forward gear  149 , the power of the transmission clutch  106  is transmitted in the forward drive condition directly to the transmission shaft  102 . When the shifter  152  is meshed with the reverse gear  150 , the power of the transmission shaft  106  is transmitted via the forward gear  149 , the transmission gear  147 , the transmission shaft  143 , the transmission gear  148 , the intermediate gear  151  and the reverse gear  150  to the transmission shaft  102  in the forward drive condition.  
         [0193]    As shown in FIG. 12, in parallel with the transmission shafts  102 ,  4 , a first main transmission shaft  7  and a first auxiliary transmission shaft  8  are disposed. Between these first main and auxiliary transmission shafts  7 ,  8 , there is provided a first friction clutch  9 . In parallel with the transmission shafts  102 ,  4 , a second main transmission shaft  10  and a second auxiliary transmission shaft  11  are disposed. Between these second main and auxiliary transmission shafts  10 ,  11 , there is provided a second friction clutch  12 . Each of these first and second friction clutches  9 ,  12  is a multiple-disc friction type clutch, which is operable into a transmitting state in response to supply of a working fluid thereto and operable into a non-transmitting state in response to discharge of the working fluid therefrom.  
         [0194]    As also shown in FIG. 12, between the transmission shaft  102  and the first main transmission shaft  7 , there is provided a first gear change-speed mechanism  13  of the synchromesh type. And, between the transmission shaft  102  and the second main transmission shaft  10 , there is provided a second gear change-speed mechanism  15  of the synchromesh type. The transmission shaft fixedly mounts a first gear  17 , a second gear  18 , a third gear  19  and a fourth gear  20 . The first main transmission shaft  7  rotatably mounts a low-speed gear  21  and a high-speed gear  22 , which mesh with the first gear  17  and the third gear  19 . A shifter  23  is splined to the first main transmission shaft  7  to be rotatable therewith and slidable relative thereto. These together constitute the first gear change-speed mechanism  13 . The second main transmission shaft  10  rotatably mounts a low-speed gear  24  and a high-speed gear  25 , which mesh with the second gear  18  and the fourth gear  20 . A shifter  26  is splined to the second main transmission shaft  10  to be rotatable therewith and slidable relative thereto. These together constitute the second gear change-speed mechanism  15 .  
         [0195]    As also shown in FIG. 12, between the transmission shaft  4  and the first auxiliary transmission shaft  8 , there is provided a first auxiliary gear change-speed mechanism  14  of the synchromesh type. And, between the transmission shaft  4  and the second auxiliary transmission shaft  11 , there is provided a second auxiliary gear change-speed mechanism  16  of the synchromesh type. The transmission shaft  4  fixedly mounts a low-speed gear  27  and a high-speed gear  28 . The first auxiliary transmission shaft  8  rotatably mounts a low-speed gear  29  and a high-speed gear  30 , which mesh with the low-speed gear  27  and the high-speed gear  28 . A shifter  31  is splined to the first auxiliary transmission shaft  8  to be rotatable therewith and slidable relative thereto. These together constitute the first auxiliary gear change-speed mechanism  14 . The second auxiliary transmission shaft  11  rotatably mounts a low-speed gear  32  and a high-speed gear  33 , which mesh with the low-speed gear  27  and the high-speed gear  28 . A shifter  34  is splined to the second auxiliary transmission shaft  11  to be rotatable therewith and slidable relative thereto. These together constitute the second auxiliary gear change-speed mechanism  16 .  
         [0196]    With the above-described construction, there are realized a condition in which the power of the transmission shaft  102  is transmitted via the first main and auxiliary transmission shafts  7 ,  8  to the transmission shaft  4  (the transmitting state of the first friction clutch  9 ) and a further condition in which the power of the transmission shaft  102  is transmitted via the second main and auxiliary transmission shafts  10 ,  11  to the transmission shaft  4  (the transmitting state of the second friction clutch  12 ).  
         [0197]    As shown in FIG. 12, in the case of the condition in which the power of the transmission shaft  102  is transmitted via the first main and auxiliary transmission shafts  7 ,  8  to the transmission shaft  4  (the transmitting state of the first friction clutch  9 ), the power of the transmission shaft  102  is transmitted via the first gear change-speed mechanism  13 , the first main transmission shaft  7 , the first friction clutch  9 , the first auxiliary transmission shaft  8  and the first auxiliary gear change-speed mechanism  14  to the transmission shaft  4  in four speeds (the first speed position, the third speed position, the fifth speed position and the seventh speed position).  
         [0198]    As shown in FIG. 12, in the case of the further condition in which the power of the transmission shaft  102  is transmitted via the second main and auxiliary transmission shafts  10 ,  11  to the transmission shaft  4  (the transmitting state of the second friction clutch  12 ), the power of the transmission shaft  102  is transmitted via the second gear change-speed mechanism  15 , the second main transmission shaft  10 , the second friction clutch  12 , the second auxiliary transmission shaft  11  and the second auxiliary gear change-speed mechanism  16  to the transmission shaft  4  in four speeds (the second speed position, the fourth speed position, the sixth speed position and the eighth speed position).  
         [0199]    For this transmission system too, the control apparatus functions in the same manners as described hereinbefore.  
         [0200]    [D-3] 
         [0201]    A variation of the transmission system of FIG. 12 is shown in FIG. 13.  
         [0202]    In this transmission system shown in FIG. 13, the transmission clutch  6  (see FIG. 12) is omitted. The transmission shaft  102  fixedly mounts a forward gear  149 . The transmission shaft  105  rotatably mounts a reverse gear  150 . The forward gear  149  meshes with a transmission gear  147  fixedly mounted on the transmission shaft  143 . The reverse gear  150  meshes via an intermediate gear  151  with a transmission gear  148  fixedly mounted on the transmission shaft  143 . Between the transmission shaft  105  and the transmission shaft  102 , there is provided a hydraulic multiple-disc friction type forward clutch  144   a . Between the transmission shaft  105  and the reverse gear  150 , there is provided a hydraulic multiple-disc friction type reverse clutch  144   b . These together constitute a forward/reverse switchover mechanism  144 . Each of the forward and reverse clutches  144   a ,  144   b  is operable into a transmitting state in response to supply of a working fluid thereto and operable into a non-transmitting state in response to discharge of the working fluid therefrom. In this construction, the shifter  52  shown in FIG. 12 is omitted. And, the operated position of the forward/reverse lever  57  is inputted to the controller  64 .  
         [0203]    When the forward/reverse lever  57  is operated to the forward drive position F, the forward clutch  144   a  is operated into the transmitting state and the reverse clutch  144   b  is operated into the non-transmitting state. With this, the power of the transmission shaft  105  is transmitted via the forward clutch  144   a  in the forward drive condition directly to the transmission shaft  102 . On the other hand, when the forward/reverse lever  57  is operated to the reverse drive position R, the reverse clutch  144   b  is operated into the transmitting state and the forward clutch  144   a  is operated into the non-transmitting state. With this, the power of the transmission shaft  105  is transmitted via the reverse clutch  144   b , the reverse gear  150 , the intermediate gear  151 , the transmission gear  148 , the transmission shaft  143 , the transmission gear  147  and the forward gear  149  to the transmission shaft  102  in the forward drive condition.  
         [0204]    Under the condition of the forward/reverse lever  57  being set at the forward drive position F (i.e. the condition of the forward clutch  144   a  being under the transmitting state and the reverse clutch  144   b  being under the non-transmitting state), instead of the transmission clutch  6 , the forward clutch  144   a  is operated. Under the condition of the forward/reverse lever  57  being set at the reverse drive position R (i.e. the condition of the reverse clutch  144   b  being under the transmitting state and the forward clutch  144   a  being under the non-transmitting state), instead of the transmission clutch  6 , the reverse clutch  144   b  is operated.  
         [0205]    For the forward and reverse clutches  144   a ,  144   b , there is provided a clutch pedal (not shown) which can be manually operated by being stepped on. With this, under the condition of the forward/reverse lever  57  being at the forward drive position F (i.e. the condition of the forward clutch  144   a  being under the transmitting state and the reverse clutch  144   b  being under the non-transmitting state), if the clutch pedal is stepped on, the forward clutch  144   a  is operated into the non-transmitting state. And, when the clutch pedal is released, the forward clutch  144   a  is operated back into the transmitting state. Under the condition of the forward/reverse lever  57  being at the reverse drive position R (i.e. the condition of the reverse clutch  144   b  being under the transmitting state and the forward clutch  144   a  being under the non-transmitting state), if the clutch pedal is stepped on, the reverse clutch  144   b  is operated into the non-transmitting state. And, when the clutch pedal is released, the reverse clutch  144   b  is operated back into the transmitting state.  
         [0206]    [E] 
         [0207]    The present invention may be applied to a work vehicle having right and left crawler traveling units, in place of the front wheels  53  and the rear wheels  54 .