Patent Publication Number: US-2023160460-A1

Title: Transmission structure and working vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to a transmission structure including a hydromechanical transmission structure (HMT structure) that has a hydrostatic transmission (HST) and a planetary gear mechanism, and a working vehicle provided with the transmission structure. 
     BACKGROUND ART 
     The HMT structure containing a combination of the HST and the planetary gear mechanism has been suitably used for a traveling system transmission path of working vehicles, such as a combine and a tractor, for example. Further, various configurations for expanding the vehicle speed variable range in the working vehicle provided with the HMT structure have also been proposed. 
     For example, Japanese Patent No. 5822761 (hereinafter referred to as Patent Document 1) discloses a combine in which the HMT structure and a multistage speed change structure having three speed change stages of a low speed stage, an intermediate speed stage, and a high speed stage are disposed in series in a traveling system transmission path, whereby the vehicle speed variable range is extended. 
     However, the configuration described in Patent Document 1 assumes that a speed change operation of the multistage speed change structure is performed in advance before starting the traveling of a vehicle, and thus, when the speed change operation of the multistage speed change structure is performed during the vehicle traveling, the following inconveniences arise. 
     This point is described taking, as an example, a case where the HMT structure is operated to increase the traveling vehicle speed in a state where the multistage speed change structure is engaged with the low speed stage, and, when the traveling vehicle speed reaches a predetermined vehicle speed, the multistage speed change structure is speed-changed from the low speed stage to the intermediate speed stage. 
     In this case, in a stage where an output of the HMT structure reaches the maximum speed or around the maximum speed in the low speed stage engagement state of the multistage speed change structure, the multistage speed change structure is shifted from the low speed stage to the intermediate speed stage while the output of the HMT structure is maintained at the maximum speed or around the maximum speed, which causes a significant vehicle speed change in speed changing, so that the ride comfort reduces and an excessive load is applied to the traveling system transmission path. 
     With respect to this point, Japanese Patent No. 4162328 (hereinafter referred to as Patent Document 2) proposes a working vehicle transmission in which the HMT structure and the multistage speed change structure are disposed in series in the traveling system transmission path and which can suppress a vehicle speed change to prevent the application of an excessive load to the traveling system transmission path even when the multistage speed change structure is speed-changed during the vehicle traveling. 
     In detail, the transmission described in Patent Document 2 is provided with the HMT structure having the HST and the planetary gear mechanism, the multistage speed change structure speed-changing the output of the HMT structure in multiple stages, and a lock-up mechanism. 
     The HST has a pump inputting rotation power from a driving source, a motor fluidly driven by the pump, and an output adjustment member varying the capacity of at least one of the pump and the motor (for example, pump), in which the output adjustment member operates according to the operation amount of a speed change operation member which is manually operated, so that the rotational speed of the motor continuously changes in response to the operation. 
     The planetary gear mechanism is configured to synthesize rotation power from the HST input into a sun gear and rotation power from a driving source input into the carrier, and output the synthesized rotation power from an internal gear toward the multistage speed change structure. 
     The lock-up mechanism is configured to synchronously rotate the carrier and the internal gear only during a speed change period of the multistage speed change structure. 
     The speed change operation of the transmission described in Patent Document 2 is described taking a case where the multistage speed change structure is accelerated from a first speed stage to a second speed stage as an example. 
     When the speed change operation member is operated in a acceleration direction within the first speed stage operation range, the output adjustment member is moved in a direction of changing the speed from a first HST speed (for example, reverse rotation side maximum speed) to a second HST speed (for example, normal rotation side maximum speed). 
     Then, when the speed change operation member is operated to a boundary position between the first speed stage operation range and a second speed stage operation range, the output adjustment member is operated to a second HST speed position (for example, normal rotation side maximum tilted position), so that an HST output is brought into the second HST speed (for example, normal rotation side maximum speed). 
     This state is the maximum speed output state of the HMT structure in a first speed stage engagement state of the multistage speed change structure. 
     When the speed change operation member is operated to the second speed stage operation range beyond the boundary position between the first speed stage operation range and the second speed stage operation range, the speed of the multistage speed change structure is accelerated from the first speed stage to the second speed stage in response to the operation. 
     In the speed change period of the multistage speed change structure, the internal gear and the carrier are coupled by the lock mechanism to be synchronously rotated as described above. 
     Thus, the rotation power synchronized with the rotation power from the driving source input into the carrier is transmitted to the multistage speed change structure to which the rotation power from the internal gear is input. 
     Meanwhile, in the speed change period of the multistage speed change structure, the output adjustment member is brought into a free state where the connection with the speed change operation member is canceled. Therefore, a motor shaft and the sun gear which are operatively coupled with each other are rotated at a rotational speed (hereinafter referred to as “speed change period rotational speed”) defined by the rotational speed of the internal gear and the carrier which are coupled by the lock mechanism to be synchronously rotated with the rotation power from the driving source. 
     Thus, the output adjustment member is returned from the second HST speed position (for example, normal rotation side maximum tilted position) to a position where an HST output corresponding to the speed change period rotational speed of the sun gear is developed (hereinafter referred to as “speed change period reference position”) in a direction toward the first HST speed position. 
     Thereafter, when the speed change operation member is operated in the acceleration direction within the second speed stage operation range, the output adjustment member is operated from the speed change period reference position toward the second HST speed position, so that the rotational speed of the motor shaft is accelerated. 
     Thus, the rotational speed of the sun gear rotationally driven by the HST output from the motor shaft is accelerated, so that the rotational speed of the internal gear is accelerated. 
     As described above, in the transmission described in Patent Document  2 , the rotation power input into the sun gear in the speed change operation of the multistage speed change structure is reduced from the second HST speed (for example, normal rotation side maximum speed) to the speed change period rotational speed. 
     Although the transmission described in Patent Document 2 having such a configuration is useful in the point that the change width of the traveling vehicle speed in the speed change of the multistage speed change structure can be suppressed as compared with the configuration described in Patent Document 1, a certain large degree of speed change has still remained in the traveling vehicle speed in the speed change of the multistage speed change structure. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-described conventional technique. It is a first object of the present invention to provide a transmission structure having an HST and a planetary gear mechanism and a speed change output shaft operatively driven by an output portion of the planetary gear mechanism and capable of extending the speed change width of the speed change output shaft without causing a rapid rotational speed change in the speed change output shaft. 
     Moreover, it is a second object of the present invention to provide a working vehicle provided with the transmission structure. 
     In order to achieve the first object, a first aspect of the present invention provides a transmission structure including: an HST continuously changing rotation power operatively input into a pump shaft from a driving source to rotation power at least between a first HST speed and a second HST speed according to an operation position of an output adjustment member, and then outputting the changed rotation power as an HST power from a motor shaft; a planetary gear mechanism having first to third elements, in which the third element functions as an input portion of the HST output; a speed change output shaft; an input side first transmission mechanism capable of operatively transmitting the rotation power of the driving source to the first element at an input side first speed change ratio; an input side second transmission mechanism capable of operatively transmitting the rotation power of the driving source to the second element at an input side second speed change ratio; input side first and second clutch mechanisms engaging/disengaging power transmission of the input side first and second transmission mechanisms, respectively; an output side first transmission mechanism capable of operatively transmitting the rotation power of the second element to the speed change output shaft at an output side first speed change ratio; an output side second transmission mechanism capable of operatively transmitting the rotation power of the first element to the speed change output shaft at an output side second speed change ratio; output side first and second clutch mechanisms engaging/disengaging power transmission of the output side first and second transmission mechanisms, respectively; a speed change operation member; an HST sensor directly or indirectly detecting a speed change state of the HST; an output sensor directly or indirectly detecting rotational speed of the speed change output shaft; and a control device controlling operations of the output adjustment member, the input side first and second clutch mechanisms, and the output side first and second clutch mechanisms. In the first aspect of the present invention, based on detection signals of the HST sensor and the output sensor, while the control device develops a first transmission state where the first element is functioned as an input portion of reference power operatively transmitted from the driving source and the second element is functioned as an output portion of synthetic rotation power by bringing the input side and output side first clutch mechanisms into an engagement state and bringing the input side and output side second clutch mechanisms into a disengagement state in a low speed state where the rotational speed of the speed change output shaft is less than a predetermined switching speed, the control device operates the output adjustment member so that the HST output is speed-changed from the first HST speed toward the second HST speed in response to an acceleration operation of the speed change operation member, and meanwhile, while the control device develops a second transmission state where the first element is functioned as the output portion and the second element is functioned as the input portion of the reference power by bringing the input side and output side first clutch mechanisms into the disengagement state and bringing the input side and output side second clutch mechanisms into the engagement state in a high speed state where the rotational speed of the speed change output shaft is equal to or higher than the switching speed, the control device operates the output adjustment member so that the HST output is speed-changed from the second HST speed toward the first HST speed in response to the acceleration operation of the speed change operation member. The input side first and second speed change ratios are set so that rotational speed of the second element when the HST output is set to the second HST speed in the first transmission state and rotational speed of the second element by rotation power transmitted through the input side second transmission mechanism in the second transmission state are same and so that rotational speed of the first element when the HST output is set to the second HST speed in the second transmission state and rotational speed of the first element by rotation power transmitted through the input side first transmission mechanism in the first transmission state are same. The output side first and second speed change ratios are set so that rotational speed developed in the speed change output shaft when the HST output is set to the second HST speed is same in the first and second transmission states. 
     The transmission structure according to the first aspect can develop the first transmission state where the speed change output shaft is increased to the switching speed as the HST output is speed-changed from the first HST speed to the second HST speed and the second transmission state where the speed change output shaft is increased from the switching speed as the HST output is speed-changed from the second HST speed to the first HST speed to thereby expand the speed changeable range (speed change region) of the speed change output shaft, and further can effectively prevent or reduce the rotational speed difference in the speed change output shaft at the timing of switching between the first and second transmission states. 
     In order to achieve the first object, a second aspect of the present invention provides a transmission structure including an HST continuously changing rotation power operatively input into a pump shaft from a driving source to rotation power at least between a first HST speed and a second HST speed according to an operation position of an output adjustment member, and then outputting the changed rotation power as an HST output from a motor shaft; a planetary gear mechanism having first to third elements, in which the third element functions as an input portion of the HST output; a speed change output shaft; an input side first transmission mechanism capable of operatively transmitting the rotation power of the driving source to the first element at an input side first speed change ratio; an input side second transmission mechanism capable of operatively transmitting the rotation power of the driving source to the second element at an input side second speed change ratio; input side first and second clutch mechanisms engaging/disengaging power transmission of the input side first and second transmission mechanisms, respectively; output side first and second clutch mechanisms engaging/disengaging power transmission from the second element and the first element, respectively, to the speed change output shaft; a speed change operation member; an HST sensor directly or indirectly detecting a speed change state of the HST; an output sensor directly or indirectly detecting rotational speed of the speed change output shaft; and a control device controlling operations of the output adjustment member, the input side first and second clutch mechanisms, and the output side first and second clutch mechanisms, wherein based on detection signals of the HST sensor and the output sensor, while the control device develops a first transmission state where the first element is functioned as an input portion of reference power operatively transmitted from the driving source and the second element is functioned as an output portion of synthetic rotation power by bringing the input side and output side first clutch mechanisms into an engagement state and bringing the input side and output side second clutch mechanisms into a disengagement state in a low speed state where the rotational speed of the speed change output shaft is less than a predetermined switching speed, the control device operates the output adjustment member so that the HST output is speed-changed from the first HST speed toward the second HST speed in response to an acceleration operation of the speed change operation member and meanwhile, while the control device develops a second transmission state where the first element is functioned as the output portion and the second element is functioned as the input portion of reference power by bringing the input side and output side first clutch mechanisms into the disengagement state and bringing the input side and output side second clutch mechanisms into the engagement state in a high speed state where the rotational speed of the speed change output shaft is equal to or higher than the switching speed, the control device operates the output adjustment member so that the HST output is speed-changed from the second HST speed toward the first HST speed in response to the acceleration operation of the speed change operation member, and the input side first and second speed change ratios are set so that rotational speed of the second element when the HST output is set to the second HST speed in the first transmission state and rotational speed of the second element by rotation power transmitted through the input side second transmission mechanism in the second transmission state are same and so that rotational speed of the first element when the HST output is set to the second HST speed in the second transmission state and rotational speed of the first element by rotation power transmitted through the input side first transmission mechanism in the first transmission state are same. 
     The transmission structure according to the second aspect can develop the first transmission state where the speed change output shaft is increased to the switching speed as the HST output is speed-changed from the first HST speed to the second HST speed and the second transmission state where the speed change output shaft is increased from the switching speed as the HST output is speed-changed from the second HST speed to the first HST speed to thereby expand the speed changeable range (speed change region) of the speed change output shaft, and further can effectively prevent or reduce the rotational speed difference in the speed change output shaft at the timing of switching between the first and second transmission states. 
     In the second aspect, the control device preferably may operate, in switching between the first and second transmission states, the output adjustment member so that rotational speed developed in the speed change output shaft in a transmission state after the switching coincides with or approaches rotational speed developed in the speed change output shaft in a transmission state before the switching. 
     In any one of the various configurations according to the first and second aspects, in a switching transition stage between the first and second transmission states, a double transmission state preferably may be developed in which both the input side first and second clutch mechanisms are brought into the engagement state and both the output side first and second clutch mechanisms are brought into the engagement state. 
     In one embodiment capable of developing the double transmission state, at least one of an input side clutch unit formed by the input side first and second clutch mechanisms and an output side clutch unit formed by the output side first and second clutch mechanisms is configured as a dog clutch type. 
     The clutch unit of the dog clutch type has a slider supported by a corresponding rotation shaft so as not to be relatively rotatable and so as to be movable in an axial direction and first and second recess-projection engagement portions on one side and another side, respectively, in the axial direction. 
     When the slider is located at a first position on the one side in the axial direction, the first recess-projection engagement portion is engaged with a corresponding recess-projection engagement portion while the second recess-projection engagement portion is not engaged with a corresponding recess-projection engagement portion, whereby the slider brings only the first clutch mechanism into the engagement state, when the slider is located at a second position on the another side in the axial direction, the second recess-projection engagement portion is engaged with a corresponding recess-projection engagement portion while the first recess-projection engagement portion is not engaged with a corresponding recess-projection engagement portion, whereby the slider brings only the second clutch mechanism into the engagement state, and when the slider is located at an intermediate position between the first and second positions with respect to the axial direction, both the first and second recess-projection engagement portions are engaged with corresponding recess-projection engagement portions, whereby the slider brings both first and second clutch mechanisms into the engagement state. 
     The input side first transmission mechanism may have an input side first driving gear relatively rotatably supported by a main driving shaft operatively coupled with the driving source and an input side first driven gear operatively coupled with the input side first driven gear and made relatively unrotatable to the first element, and the input side second transmission mechanism may have an input side second driving gear relatively rotatably supported by the main driving shaft and an input side second driven gear operatively coupled with the input side second driving gear and made relatively unrotatable to the second element. 
     In this embodiment, the input side clutch unit may be configured as the dog clutch type and having an input side slider as the slider. 
     The input side slider is supported by the main driving shaft between the input side first and second driving gears so as not to be relatively rotatable and so as to be movable in the axial direction, when located at the first position, the first recess-projection engagement portion is engaged with a recess-projection engagement portion of the input side first driving gear while the second recess-projection engagement portion is not engaged with a recess-projection engagement portion of the input side second driving gear, whereby the input side slider brings only the input side first clutch mechanism into the engagement state, when located at the second position, the second recess-projection engagement portion is engaged with the recess-projection engagement portion of the input side second driving gear while the first recess-projection engagement portion is not engaged with the recess-projection engagement portion of the input side first driving gear, whereby the input side slider brings only the input side second clutch mechanism into the engagement state, and, when located at an intermediate position, the first and second recess-projection engagement portions are engaged with the recess-projection engagement portions of the input side first and second driving gears, respectively, whereby the input side slider brings both the first and second clutch mechanisms into the engagement state. 
     The transmission structure according to the first aspect may further include 
     a speed change intermediate shaft coupled with the second element so as not to be relatively rotatable around an axis, and the first element may be relatively rotatably supported by the speed change intermediate shaft. 
     In this case, the output side first transmission mechanism has an output side first driving gear supported by the speed change intermediate shaft so as not to be relatively rotatable and an output side first driven gear operatively coupled with the output side first driving gear and relatively rotatably supported by the speed change output shaft. The output side second transmission mechanism has an output side second driving gear coupled with the first element so as not to be relatively rotatable and an output side second driven gear operatively coupled with the output side second driving gear and relatively rotatably supported by the speed change output shaft. The output side first and second clutch mechanisms have recess-projection engagement portions provided in the output side first and second driven gears and an output side slider supported between the output side first and second driven gears by the speed change output shaft so as not to be relatively rotatable and so as to be movable in an axial direction and provided with first and second recess-projection engagement portions on one side and another side, respectively, in the axial direction. 
     When the output side slider is located at a first position on the one side in the axial direction, the first recess-projection engagement portion is engaged with a recess-projection engagement portion of the output side first driven gear while the second recess-projection engagement portion is not engaged with a recess-projection engagement portion of the output side second driven gear, whereby the output side slider brings only the output side first clutch mechanism into the engagement state, when the output side slider is located at a second position on the another side in the axial direction, the second recess-projection engagement portion is engaged with the recess-projection engagement portion of the output side second driven gear while the first recess-projection engagement portion is not engaged with the recess-projection engagement portion of the output side first driven gear, whereby the output side slider brings only the output side second clutch mechanism into the engagement state, and, when the output side slider is located at an intermediate position between the first direction and the second direction in the axial direction, the first and second recess-projection engagement portions are engaged with the recess-projection engagement portions of the output side first and second driven gears, respectively, whereby the output side slider brings both the output side first and second clutch mechanisms into the engagement state. 
     In another embodiment capable of developing the double transmission state, at least one of an input side clutch unit formed by the input side first and second clutch mechanisms and an output side clutch unit formed by the output side first and second clutch mechanisms may be configured as a hydraulic friction plate type developing a clutch engagement state by receiving pressure oil supply. 
     The transmission structure according to this embodiment is further provided with a pressure oil supply line receiving pressure oil supply from a hydraulic source, first and second supply/discharge lines supplying/discharging pressure oil to the first and second clutch mechanisms, respectively, in the clutch units of the hydraulic friction plate type, first and second electromagnetic valves which are interposed in the first and second supply/discharge lines, respectively, and which can take a discharge position where a corresponding supply/discharge line is drained and a supply position where a corresponding supply/discharge line is fluid-connected to the pressure oil supply line, and a clutch engagement detection unit detecting an engagement state of the first and second clutch mechanisms in the clutch units of the hydraulic friction plate type. 
     The control device locates the first electromagnetic valve at the supply position and locates the second electromagnetic valve at the discharge position to develop the first transmission state in the low speed state where the rotational speed of the speed change output shaft is less than the switching speed, while locating the first electromagnetic valve at the discharge position and locating the second electromagnetic valve at the supply position to develop the second transmission state in the high speed state where the rotational speed of the speed change output shaft is equal to or higher than the switching speed. Also, the control device moves the electromagnetic valve located at the discharge position at time before the switching from the discharge position to the supply position while maintaining the electromagnetic valve located at the supply position at the time before the switching at the supply position in the switching between the first and second transmission states, and then moves the electromagnetic valve located at the supply position at the time before the switching from the supply position to the discharge position after passage of predetermined time from time when recognizing that the clutch mechanism to which pressure oil is supplied through the electromagnetic valve, a position of which is moved from the discharge position to the supply position is brought into the engagement state based on a signal from the clutch engagement detection unit. 
     Preferably, the first and second electromagnetic valves may be configured as proportional electromagnetic valves configured to receive hydraulic pressure of corresponding supply/discharge lines as pilot pressure to thereby maintain the hydraulic pressure of the corresponding supply/discharge lines in a state where a position signal from the control device to the supply position is input at an engagement hydraulic pressure. 
     In the first and second aspects of the present invention, the input side first and second clutch mechanisms may be configured as a hydraulic friction plate type developing a clutch engagement state by receiving pressure oil supply. 
     In this case, the transmission structure is provided with a pressure oil supply line receiving pressure oil supply from a hydraulic source, input side first and second supply/discharge lines supplying/discharging pressure oil to the input side first and second clutch mechanisms, respectively, input side first and second electromagnetic valves which are interposed in the input side first and second supply/discharge lines, respectively, and which can take a discharge position where a corresponding supply/discharge line is drained and a supply position where a corresponding supply/discharge line is fluid-connected to the pressure oil supply line, and a clutch engagement detection unit detecting an engagement state of the input side first and second clutch mechanisms. 
     The control device locates the input side first electromagnetic valve at the supply position and locates the input side second electromagnetic valve at the discharge position in the low speed state where the rotational speed of the speed change output shaft is less than the switching speed, while locating the input side first electromagnetic valve at the discharge position and locating the input side second electromagnetic valve at the supply position in the high speed state where the rotational speed of the speed change output shaft is equal to or higher than the switching speed. Also, the control device moves the electromagnetic valve located at the discharge position at time before the switching from the discharge position to the supply position while maintaining the electromagnetic valve located at the supply position at the time before the switching at the supply position in the switching between the first and second transmission states, and then moves the electromagnetic valve located at the supply position at the time before the switching from the supply position to the discharge position when recognizing that the clutch mechanism to which pressure oil is supplied through the electromagnetic valve, a position of which is moved from the discharge position to the supply position, is brought into a sliding engagement state based on a signal from the clutch engagement detection unit. 
     In the first and second aspects of the present invention, the output side first and second clutch mechanisms may be configured as a hydraulic friction plate type developing a clutch engagement state by receiving pressure oil supply. 
     In this case, the transmission structure is provided with a pressure oil supply line receiving pressure oil supply from a hydraulic source, output side first and second supply/discharge lines supplying/discharging pressure oil to the output side first and second clutch mechanisms, respectively, output side first and second electromagnetic valves which are interposed in the output side first and second supply/discharge lines, respectively, and which can take a discharge position where a corresponding supply/discharge line is drained and a supply position where a corresponding supply/discharge line is flued-connected to the pressure oil supply line, and a clutch engagement detection unit detecting an engagement state of the output side first and second clutch mechanisms. 
     The control device locates the output side first electromagnetic valve at the supply position and locates the output side second electromagnetic valve at the discharge position in the low speed state where the rotational speed of the speed change output shaft is less than the switching speed, while locating the output side first electromagnetic valve at the discharge position and locating the output side second electromagnetic valve at the supply position in the high speed state where the rotational speed of the speed change output shaft is equal to or higher than the switching speed. Also, the control device moves, in switching between the low speed state and the high speed state, the electromagnetic valve located at the discharge position at time before the switching from the discharge position to the supply position while maintaining the electromagnetic valve located at the supply position at the time before the switching at the supply position, and then moves the electromagnetic valve located at the supply position at the time before the switching from the supply position to the discharge position when recognizing that the clutch mechanism to which pressure oil is supplied through the electromagnetic valve, a position of which is moved from the discharge position to the supply position, is brought into a sliding engagement state based on a signal from the clutch engagement detection unit. 
     In the first and second aspect of the present invention, the input side second clutch mechanism and the output side second clutch mechanism preferably may be configured as friction plate clutch mechanisms. 
     In more preferable configuration, all of the input side first clutch mechanism and the output side first clutch mechanism are configured as friction plate clutch mechanisms. 
     The transmission structure according to the present invention may include a pressure oil supply line, an upstream side of which is fluid-connected to a hydraulic source, a drain line, a first supply/discharge line supplying/discharging pressure oil to the input side and output side first clutch mechanisms, a second supply/discharge line supplying/discharging pressure oil to the input side and output side second clutch mechanisms, and a switching valve, a position of which is controlled by the control device. 
     The switching valve is configured to be able to take a first position where the pressure oil supply/discharge line is fluid-connected to the first supply/discharge line and the second supply/discharge line is fluid-connected to the drain line and a second position where the first supply/discharge line is fluid-connected to the drain line and the pressure oil supply/discharge line is fluid-connected to the second supply/discharge line. The input side first and second clutch mechanisms and the output side first and second clutch mechanisms are configured as a hydraulic type engaging power transmission of a corresponding transmission mechanism by receiving pressure oil supply. 
     Also, in order to achieve the first object, a third aspect of the present invention provides a transmission structure interposed in a traveling system transmission path of a working vehicle including an HST continuously changing rotation power operatively input into a pump shaft from a driving source to rotation power at least between a first HST speed and a second HST speed according to an operation position of an output adjustment member, and then outputting the changed rotation power as an HST output from a motor shaft; a planetary gear mechanism having first to third elements, in which the third element functions as an input portion of the HST output; a speed change output shaft; input side first and second transmission mechanisms capable of operatively transmitting the rotation power of the driving source to the first and second elements, respectively; input side and second clutch mechanisms engaging/disengaging power transmission of the input side first and second transmission mechanisms, respectively; output side first and second clutch mechanisms engaging/disengaging power transmission from the second element and the first element, respectively, to the speed change output shaft; a speed change operation member; an HST sensor directly or indirectly detecting a speed change state of the HST; an output sensor directly or indirectly detecting rotational speed of the speed change output shaft; and a control device controlling operations of the output adjustment member, the input side first and second clutch mechanisms, and the output side first and second clutch mechanisms, wherein at least one of an input side clutch unit formed by the input side first and second clutch mechanisms and an output side clutch unit formed by the output side first and second clutch mechanisms is configured as a hydraulic friction plate type developing a clutch engagement state by receiving pressure oil supply, the transmission structure is further provided with a pressure oil supply line receiving pressure oil supply from a hydraulic source, first and second supply/discharge lines supplying/discharging pressure oil to the first and second clutch mechanisms, respectively, in the clutch units of the hydraulic friction plate type, first and second electromagnetic valves which are interposed in the first and second supply/discharge lines, respectively, and which can take a discharge position where a corresponding supply/discharge line is drained and a supply position where a corresponding supply/discharge line is fluid-connected to the pressure oil supply line, and a clutch engagement detection unit detecting an engagement state of the first and second clutch mechanisms in the clutch units of the hydraulic friction plate type, wherein based on detection signals of the HST sensor and the output sensor, in a low speed state where the rotational speed of the speed change output shaft is less than a predetermined switching speed, while the control device develops a first transmission state where the first element is functioned as an input portion of reference power operatively transmitted from the driving source and the second element is functioned as an output portion of synthetic rotation power by bringing the input side and output side first clutch mechanisms into an engagement state and bringing the input side and output side second clutch mechanisms into a disengagement state, the control device operates the output adjustment member so that the HST output is speed-changed from the first HST speed toward the second HST speed in response to an acceleration operation of the speed change operation member and meanwhile, in a high speed state where the rotational speed of the speed change output shaft is equal to or higher than the switching speed, while the control device develops a second transmission state where the first element is functioned as the output portion and the second element is functioned as the input portion of reference power by bringing the input side and output side first clutch mechanisms into the disengagement state and bringing the input side and output side second clutch mechanisms into the engagement state, the control device operates the output adjustment member so that the HST output is speed-changed from the second HST speed toward the first HST speed in response to the acceleration operation of the speed change operation member, further, in switching between the first and second transmission states, while maintaining one of the first and second electromagnetic valves located at the supply positions at time before the switching at the supply positions, the control device moves another one of the first and second electromagnetic valves located at the discharge positions at the time before the switching from the discharge position to the supply position, and then, when recognizing that the clutch mechanism to which pressure oil is supplied through the other electromagnetic valve is brought into a sliding engagement state based on a signal from the clutch engagement detection unit, the control device moves the one electromagnetic valve from the supply position to the discharge position to thereby switch engagement/disengagement of the first and second clutch mechanisms in the hydraulic friction plate clutch units. 
     The transmission structure according to the third aspect makes it possible to increase a degree of freedom for design to thereby enhance flexibility in designing device, since it is not needed to strictly set the speed change ratios of the input side second transmission mechanism and the output side transmission mechanisms for preventing the rotational speed difference in the speed change output shaft in shifting between the first and second transmission states. The transmission structure according to the third aspect makes it also possible to suppress unintentional disengagement and reduction of power transmission in sifting clutch mechanisms under traveling with heavy load. 
     Also, in order to achieve the first object, a fourth aspect of the present invention provides a transmission structure including an HST continuously changing rotation power operatively input into a pump shaft from a driving source to rotation power at least between first HST speed and second HST speed according to an operation position of an output adjustment member, and then outputting the changed rotation power as an HST output from a motor shaft; a planetary gear mechanism having first to third elements, in which the third element functions as an input portion of the HST output; a speed change output shaft; input side first and second transmission mechanisms capable of operatively transmitting the rotation power of the driving source to the first element and the second element, respectively; input side first and second clutch mechanisms engaging/disengaging power transmission of the input side first and second transmission mechanisms, respectively; forward movement first and second transmission mechanisms capable of operatively transmitting rotation power of the second element and the first element, respectively, to the speed change output shaft in a normal rotation state; a reverse movement transmission mechanism capable of operatively transmitting the rotation power of the second element to the speed change output shaft in a reverse rotation state; a forward movement first clutch mechanism, a forward movement second clutch mechanism, and a reverse movement clutch mechanism engaging/disengaging power transmission of the forward movement first transmission mechanism, the forward movement second transmission mechanism, and the reverse movement transmission mechanism, respectively; a speed change operation member; an HST sensor directly or indirectly detecting a speed change state of the HST; an output sensor directly or indirectly detecting rotational speed of the speed change output shaft; and a control device controlling operations of the output adjust member, the input side first clutch mechanism, the input side second clutch mechanism, the forward movement first clutch mechanism, the forward movement second clutch mechanism, and the reverse movement clutch mechanism, wherein, 
     based on detection signals of the HST sensor and the output sensor, in a low speed state where the rotational speed of the speed change output shaft is from zero speed to speed less than switching speed in a forward movement direction, while the control device develops a forward movement first transmission state where the input side first clutch mechanism and the forward movement first clutch mechanism are brought into an engagement state, the control device operates the output adjustment member so that the HST output is speed-changed from the first HST speed toward the second HST speed in response to a forward movement side acceleration operation of the speed change operation member, in a high speed state where the rotational speed of the speed change output shaft is equal to or higher than the switching speed in the forward movement direction, while the control device develops a forward movement second transmission state where the input side second clutch mechanism and the forward movement second clutch mechanism are brought into the engagement state, the control device operates the output adjustment member so that the HST output is speed-changed from the first HST speed toward the second HST speed in response to the forward movement side acceleration operation of the speed change operation member, and, in a reverse movement transmission state where the rotational speed of the speed change output shaft is changed from the zero speed to the reverse movement side, while the control device develops a reverse movement transmission state where the input side first clutch mechanism and the reverse movement clutch mechanism are brought into the engagement state, the control device operates the output adjustment member so that the HST output is speed-changed from the first HST speed toward the second HST speed in response to a reverse movement side acceleration operation of the speed change operation member. 
     The transmission structure according to the fourth aspect makes it possible to develop the forward movement first transmission state where the rotational speed of the speed change output shaft is increased in forward movement direction until the switching speed as the HST output is speed-changed from the first HST speed to the second HST speed, the forward movement second transmission state where the rotational speed of the speed change output shaft is increased in forward movement direction from the switching speed as the HST output is speed-changed from the second HST speed to the first HST speed and the reverse movement transmission state where the rotational speed of the speed change output shaft is increased in reverse movement direction as the HST output is speed-changed from the first HST speed to the second HST speed to thereby expand the speed change range of the speed change output shaft, and further can effectively prevent or reduce the rotational speed difference in the speed change output shaft in switching between the forward movement first and second transmission states and between the forward movement first transmission state and the reverse movement transmission state. 
     In the fourth aspect, the input side first transmission mechanism may operatively transmit the rotation power of the driving source to the first element at an input side first speed change ratio, and the input side second transmission mechanism may operatively transmit the rotation power of the driving source to the second element at an input side second speed change ratio. 
     In this case, the input side first and second speed change ratios are preferably set so that rotational speed of the second element when the HST output is set to the second HST speed in the forward movement first transmission state and rotational speed of the second element by rotation power transmitted through the input side second transmission mechanism in the forward movement second transmission state are same and so that rotational speed of the first element when the HST output is set to the second HST speed in the forward movement second transmission state and rotational speed of the first element by rotation power transmitted through the input side first transmission mechanism in the forward movement first transmission state are same. 
     In the fourth aspect, the forward movement first transmission mechanism may operatively transmit the rotation power of the second element to the speed change output shaft at a forward movement first speed change ratio and the forward movement second transmission mechanism may operatively transmit the rotation power of the first element to the speed change output shaft at a forward movement second speed change ratio. 
     In this case, the forward movement first and second speed change ratios are preferably set so that rotational speed developed in the speed change output shaft when the HST output is set to the second HST speed is same in the first and second transmission states. 
     In the fourth aspect, the HST and the planetary gear mechanism are preferably set so that the rotational speed of the second element becomes the zero speed when the HST output is set to the first HST speed in the engagement state of the input side first clutch mechanism. 
     Also, in order to achieve the first object, a fifth aspect of the present invention provides a transmission mechanism including an HST continuously changing rotation power operatively input into a pump shaft from a driving source to rotation power at least between a first HST speed and a second HST speed according to an operation position of an output adjustment member, and then outputting the changed rotation power as an HST output from a motor shaft; a planetary gear mechanism having first to third elements, in which the third element functions as an input portion of the HST output; a speed change output shaft; an input side first transmission mechanism capable of operatively transmitting the rotation power of the driving source to the first element at an input side first speed change ratio; an input side second transmission mechanism capable of operatively transmitting the rotation power of the driving source to the second element at an input side second speed change ratio; input side first and second clutch mechanisms engaging/disengaging power transmission of the input side first and second transmission mechanisms, respectively; an output side first transmission mechanism capable of operatively transmitting the rotation power of the second element to the speed change output shaft at an output side first speed change ratio; an output side second transmission mechanism capable of operatively transmitting the rotation power of the first element to the speed change output shaft at an output side second speed change ratio; an output side third transmission mechanism capable of operatively transmitting the rotation power of the first element to the speed change output shaft at an output side third speed change ratio rotating the speed change output shaft at speed higher than the output side second speed change ratio; output side first to third clutch mechanisms engaging/disengaging power transmission of the output side first to third transmission mechanisms, respectively; a speed change operation member; an HST sensor directly or indirectly detecting a speed change state of the HST; an output sensor directly or indirectly detecting rotational speed of the speed change output shaft; and a control device controlling operations of the output adjustment member, the input side first and second clutch mechanisms, and the output side first to third clutch mechanism, wherein, based on detection signals of the HST sensor and the output sensor, in a low speed state where the rotational speed of the speed change output shaft is less than a first switching speed, while the control device develops a first transmission state where the first element is functioned as an input portion of reference power operatively transmitted from the driving source and the second element is functioned as an output portion of synthetic rotation power by bringing the output side first clutch mechanism into an engagement state and bringing other output side clutch mechanisms into a disengagement state while bringing the input side first clutch mechanism into the engagement state and bringing the input side second clutch mechanism into the disengagement state, the control device operates the output adjustment member so that the HST output is speed-changed from the first HST speed toward the second HST speed in response to an acceleration operation of the speed change operation member, in an intermediate speed state where the rotational speed of the speed change output shaft is equal to or higher than the first switching speed and less than a second switching speed, while the control device develops a second transmission state where the second element is functioned as the input portion of reference power and the rotation power of the first element is operatively transmitted to the speed change output shaft at the output side second speed change ratio by bringing the output side second clutch mechanism into the engagement state and bringing other output side clutch mechanisms into the disengagement state while bringing the input side first clutch mechanisms into the disengagement state and bringing the input side second clutch mechanism into the engagement state, the control device operates the output adjustment member so that the HST output is speed-changed from the second HST speed toward the first HST speed in response to the acceleration operation of the speed change operation member, and, in a high speed state where the rotational speed of the speed change output shaft is equal to or higher than the second switching speed, while the control device develops a third transmission state where the second element is functioned as the input portion of reference power and the rotation power of the first element is operatively transmitted to the speed change output shaft at the output side third speed change ratio by bringing the output side third clutch mechanism into the engagement state and bringing other output side clutch mechanisms into the disengagement state while bringing the input side first clutch mechanism into the disengagement state and bringing the input side second clutch mechanism into the engagement state, the control device operates the output adjustment member so that the HST output is speed-changed from the second HST speed toward the first HST speed in response to the acceleration operation of the speed change operation member, and meanwhile the control device operates the output adjustment member in switching between the second and third transmission states so that rotational speed developed in the speed change output shaft in a transmission state after the switching coincides with or approaches rotational speed developed in the speed change output shaft in a transmission state before the switching, and the input side first and second speed change ratios are set so that rotational speed of the second element when the HST output is set to the second HST speed in the first transmission state and rotational speed of the second element by rotation power transmitted through the input side second transmission mechanism in the second transmission state are same and so that rotational speed of the first element when the HST output is set to the second HST speed in the second transmission state and rotational speed of the first element by rotation power transmitted through the input side first transmission mechanism in the first transmission state are same. 
     The transmission structure according to the fifth aspect makes it possible to develop the first transmission state where the rotational speed of the speed change output shaft is increased until the first switching speed as the HST output is speed-changed from the first HST speed to the second HST speed, the second transmission state where the rotational speed of the speed change output shaft is increased from the first switching speed until the second switching speed as the HST output is speed-changed from the second HST speed to the first HST speed and the third transmission state where the rotational speed of the speed change output shaft is increased from the second switching speed as the HST output is speed-changed from the side of the second HST speed to the side of the first HST speed to thereby expand the speed change range of the speed change output shaft, and further can effectively prevent or reduce the rotational speed difference in the speed change output shaft in switching between the first and second transmission states and between the second and third transmission states. 
     The transmission structure according to the fifth aspect may include a speed change intermediate shaft coupled with the second element so as not to be relatively rotatable around an axis, and a speed change transmission shaft externally inserted into the speed change intermediate shaft in a relatively rotatable manner and coupled with the first element so as not to be relatively rotatable. 
     In this case, the input side first transmission mechanism has an input side first driving gear relatively rotatably supported by a main driving shaft operatively coupled with the driving source and an input side first driven gear operatively coupled with the input side first driving gear and made relatively unrotatable to the speed change transmission shaft. The input side second transmission mechanism has an input side second driving gear relatively rotatably supported by the main driving shaft and an input side second driven gear operatively coupled with the input side second driving gear and made relatively unrotatable to the second element. The output side first transmission mechanism has an output side first driving gear supported by the speed change intermediate shaft so as not to be relatively rotatable and an output side first driven gear operatively coupled with the output side first driving gear and relatively rotatably supported by the speed change output shaft. The output side second transmission mechanism has an output side second driving gear supported by the speed change transmission shaft so as not to be relatively rotatable and an output side second driven gear operatively coupled with the output side second driving gear and relatively rotatably supported by the speed change output shaft. The output side third transmission mechanism has an output side third driving gear supported by the speed change transmission shaft so as not to be relatively rotatable and an output side third driven gear operatively coupled with the output third driving gear and relatively rotatably supported by the speed change output shaft. 
     The transmission structure according to the present invention may further include a traveling transmission shaft disposed on a downstream side in a transmission direction relative to the speed change output shaft, and a forward/reverse movement switching mechanism capable of switching a rotation direction of driving force in a forward movement direction and a reverse movement direction between the speed change output shaft and the traveling transmission shaft. 
     Also, in order to achieve the first object, a sixth aspect of the present invention provides a transmission mechanism including an HST continuously changing rotation power operatively input into a pump shaft from a driving source to rotation power at least between first HST speed and second HST speed according to an operation position of an output adjustment member, and then outputting the changed rotation power as an HST output from a motor shaft; a planetary gear mechanism having first to third elements, in which the third element functions as an input portion of the HST output; an input side first transmission mechanism capable of operatively transmitting the rotation power of the driving source to the first element at an input side first speed change ratio; an input side second transmission mechanism capable of operatively transmitting the rotation power of the driving source to the second element at an input side second speed change ratio; an input side first and second clutch mechanisms engaging/disengaging power transmission of the input side first and second transmission mechanisms, respectively; a speed change output shaft; a traveling transmission shaft disposed on a downstream side in a transmission direction relative to the speed change output shaft; a forward/reverse movement switching mechanism interposed in a transmission path from the speed change output shaft to the traveling transmission shaft and capable of switching the traveling transmission state between a forward movement transmission state of rotating the traveling transmission shaft in a forward movement direction and a reverse movement transmission state of rotating the traveling transmission shaft in a reverse movement direction; an output side first transmission mechanism capable of operatively transmitting the rotation power of the second element to the speed change output shaft at an output side first speed change ratio; an output side second transmission mechanism capable of operatively transmitting the rotation power of the first element to the speed change output shaft at an output side second speed change ratio; an output side third transmission mechanism which can operatively transmit the rotation power of the first element to the traveling transmission shaft as driving force in the forward movement direction and in which a speed change ratio is set so that rotational speed of the traveling transmission shaft when the rotation power of the first element is operatively transmitted to the traveling transmission shaft through the output side third transmission mechanism is higher than rotational speed of the traveling transmission shaft when the rotation power of the first element is operatively transmitted to the traveling transmission shaft through the output side second transmission mechanism and the forward and reverse movement change mechanism in the forward movement transmission state; output side first to third clutch mechanisms engaging/disengaging power transmission of the output side first to third transmission mechanisms, respectively; a speed change operation member; an HST sensor directly or indirectly detecting a speed change state of the HST; and a control device controlling operations of the output adjustment member, the input side first and second clutch mechanisms, and the output side first to third clutch mechanisms. 
     The transmission structure according to the sixth aspect makes it possible to expand the speed change range of the forward movement which is high in frequency of use while effectively preventing or reducing the rotational speed difference in the speed change output shaft in switching between the first and second transmission states and between the second and third transmission states. 
     In the sixth aspect, 
     the control device operates the output adjustment member so that the HST output is set to the first HST speed which makes a synthetic rotation power of the planetary gear mechanism zero when the speed change operation member is positioned at a zero speed position, 
     when the speed change operation member is operated in a forward movement side low speed range from the zero speed position to a forward movement side first switching speed position, while the control device develops a first transmission state where the first element is functioned as an input portion of reference power operatively transmitted from the driving source and the second element is functioned as an output portion of the synthetic rotation power by bringing the output side first clutch mechanism into an engagement state and bringing other output side clutch mechanisms into a disengagement state while bringing the input side first clutch mechanism into the engagement state and bringing the input side second clutch mechanism into the disengagement state, the control device brings the forward/reverse movement switching mechanism into the forward movement transmission state and the control device operates the output adjustment member so that the HST output is speed-changed from the side of the first HST speed toward the side of the second HST speed in response to an acceleration operation of the speed change operation member, 
     when the speed change operation member is operated in a forward movement side intermediate speed range from the forward movement side first switching speed position to a forward movement side second switching speed position, while the control device develops a second transmission state where the second element is functioned as the input portion of reference power and the rotation power of the first element is operatively transmitted to the speed change output shaft at the output side second speed change ratio by bringing the output side second clutch mechanism into the engagement state and bringing other output side clutch mechanisms into the disengagement state while bringing the input side first clutch mechanism into the disengagement state and bringing the input side second clutch mechanism into the engagement state, the control device brings the forward/reverse movement switching mechanism into the forward movement transmission state and the control device operates the output adjustment member so that the HST output is speed-changed from the side of the second HST speed toward the side of the first HST speed in response to an acceleration operation of the speed change operation member, 
     when the speed change operation member is operated in a forward movement side high speed range beyond the forward movement side second switching speed position, while the control device develops a third transmission state where the second element is functioned as the input portion of reference power and the rotation power of the first element is operatively transmitted to the traveling transmission shaft as driving force in the forward movement direction through the output side third transmission mechanism by bringing the output side third clutch mechanism into the engagement state and bringing other output side clutch mechanisms into the disengagement state while bringing the input side first clutch mechanism into the disengagement state and bringing the input side second clutch mechanism into the engagement state, the control device operates the output adjustment member so that the HST output is speed-changed from the side of the second HST speed toward the side of the first HST speed in response to the acceleration operation of the speed change operation member, 
     when the speed change operation member passes the forward movement side second switching speed position between the forward movement side intermediate speed range and the forward movement side high speed range, the control device operates the output adjustment member so that rotational speed of the traveling transmission shaft in a transmission state developed immediately after the passage coincides with or approaches rotational speed of the traveling transmission shaft in a transmission state developed immediately before the passage, 
     when the speed change operation member is operated in a reverse movement side low speed range from the zero speed position to a reverse movement side first switching speed position, while the control device develops the first transmission state, the control device brings the forward/reverse movement switching mechanism into the reverse movement transmission state and operates the output adjustment member so that the HST output is speed-changed from the side of the first HST speed toward the side of the second HST speed in response to the acceleration operation of the speed change operation member, and 
     when the speed change operation member is operated in a reverse movement side high speed range beyond the reverse movement side first switching speed position, while the control device develops the second transmission state, the control device brings the forward/reverse movement switching mechanism into the reverse movement transmission state and operates the output adjustment member so that the HST output is speed-changed from the side of the second HST speed toward the side of the first HST speed in response to the acceleration operation of the speed change operation member. 
     In the sixth aspect, the input side first and second speed change ratios are set so that rotational speed of the second element when the HST output is set to the second HST speed in the first transmission state and the rotational speed of the second element by rotation power transmitted through the input side second transmission mechanism in the second transmission state are same and so that rotational speed of the first element when the HST output is set to the second HST speed in the second transmission state and the rotational speed of the first element by rotation power transmitted through the input side first transmission mechanism in the first transmission state are same. 
     The transmission structure according to the sixth aspect may include a speed change intermediate shaft coupled with the second element so as not to be relatively rotatable around an axis. 
     In this case, the input side first transmission mechanism has an input side first driving gear relatively rotatably supported by a main driving shaft operatively coupled with the driving source and an input side first driven gear operatively coupled with the input side first driving gear and the first element in a state of being relatively rotatably supported by the speed change intermediate shaft. The input side second transmission mechanism has an input side second driving gear relatively rotatably supported by the main driving shaft and an input side second driven gear operatively coupled with the input side second driving gear in a state of being supported by the speed change intermediate shaft so as not to be relatively rotatable. The output side first transmission mechanism has an output side first driven gear operatively coupled with the input side second driven gear in a state of relatively rotatably supported by the speed change output shaft. The output side second transmission mechanism has an output side second driven gear operatively coupled with the input side first driven gear in a state of being relatively rotatably supported by the speed change output shaft. The output side third transmission mechanism has an output side third driven gear operatively coupled with one of the output side first and second driven gears in a state of being relatively rotatably supported by the traveling transmission shaft. 
     The input side first and second clutch mechanisms are supported by the main driving shaft so as to engage/disengage the input side first and second driving gears, respectively, with/from the main driving shaft, the output side first and second clutch mechanisms are supported by the speed change output shaft so as to engage/disengage the output side first and second driven gears, respectively, with/from the speed change output shaft, and the output side third clutch mechanism is supported by the traveling transmission shaft so as to engage/disengage the output side third driven gear with/from the traveling transmission shaft. 
     Preferably, the transmission structure according to the sixth aspect may further include a hollow housing body; a first bearing plate detachably coupled with the housing body; and a second bearing plate detachably coupled with the housing body at a position spaced from the first bearing plate in a longitudinal direction of the housing body. 
     In this case, the main driving shaft, the speed change intermediate shaft, the speed change output shaft, and the traveling transmission shaft are supported by the first and second bearing plates in a state of being parallel to one another. The input side first and second driving gears and the input side first and second clutch mechanisms are supported in a portion located in a partitioned space sandwiched between the first and second bearing plates of the main driving shaft in a state where the input side first and second clutch mechanisms are located between the input side first and second driving gears with respect to an axial direction of the main driving shaft. The the input side first and second driven gears are supported in a portion located in the partitioned space of the speed change intermediate shaft in a state of being located at same positions as positions of the input side first and second driving gears, respectively, with respect to the axial direction. The output side first and second driven gears and the output side first and second clutch mechanisms are supported in a portion located in the partitioned space of the speed change output shaft in a state where the output side first and second driven gears are located at same positions as positions of the input side second and first driven gears, respectively, with respect to the axial direction and the output side first and second clutch mechanisms are located between the input side first and second driven gears with respect to the axial direction. The output side third driven gear and the output side third clutch mechanism are supported in a portion located in the partitioned space of the traveling transmission shaft in a state where the output side third driven gear is located at a same position in the axial direction as a position of one of the output side first and second driven gears and the output side third clutch mechanism is located on a far side of one of the output side first and second driven gears from the output side first and second clutch mechanisms with respect to the axial direction. The forward/reverse movement switching mechanism is supported in a portion located outside the partitioned space of the speed change output shaft and the traveling transmission shaft. 
     In one example, the housing body has a front housing body and a rear housing body detachably connected in series. 
     In this case, the first bearing plate is detachably coupled with a boss portion provided in an inner surface of the front housing body near a rear opening of the front housing body, and the second bearing plate is detachably coupled with a boss portion provided in an inner surface of the rear housing body near a front opening of the rear housing body. 
     In the fifth and sixth aspects, the output side first and second speed change ratios may be set so that rotational speed developed in the speed change output shaft when the HST output is set to the second HST speed is same in the first and second transmission states. 
     Alternatively, the control device may be configured to operate the output adjustment member so that, in switching between the first and second transmission states, rotational speed developed in the speed change output shaft in a transmission state after the switching coincides with or approaches rotational speed developed in the speed change output shaft in a transmission state before the switching. 
     In any one of the above configurations according to the present invention, assuming that a rotation direction of the rotation power input into the pump shaft is a normal rotation direction, the HST outputs rotation power in one of normal and reverse directions as the HST output of the first HST speed and outputs rotation power in another one of the normal and reverse directions as the HST output of the second HST speed. 
     In any one of the above configurations according to the present invention, an internal gear, a carrier, and a sun gear of the planetary gear mechanism form the first, second, and third elements, respectively. 
     In order to achieve the second object, the present invention provides a working vehicle including a driving source; a driving wheel; and the transmission structure according to any one of the above configurations interposed in the traveling system transmission path reaching the driving wheel from the driving source, wherein switching speed of the speed change output shaft is set to speed higher than speed in a work speed range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a transmission schematic view of a working vehicle to which a transmission structure according to an embodiment 1 of the present invention is applied. 
         FIG.  2    is a hydraulic circuit diagram of the transmission structure according to the embodiment 1. 
         FIGS.  3 A and  3 B  are graphs illustrating the relationship between the traveling vehicle speed and the rotational speed of the HST output in the working vehicle to which the transmission structure according to the embodiment 1 is applied, and illustrate states where a sub speed change mechanism  240  provided in the transmission structure is engaged with a low speed stage and a high speed stage, respectively. 
         FIG.  4    is a hydraulic circuit diagram of a transmission structure according to an embodiment 2 of the present invention. 
         FIG.  5 A  and  FIG.  5 B  illustrate graphs illustrating the relationship between the traveling vehicle speed and the rotational speed of the HST output in a working vehicle to which the transmission structure  2  is applied, and illustrate states where the sub speed change mechanism is engaged with a low speed stage and a high speed stage, respectively. 
         FIGS.  6 A and  6 B  illustrate graphs illustrating the relationship between the traveling vehicle speed and the rotational speed of the HST output in a working vehicle to which a modification of the embodiment 2 is applied, and illustrate states where the sub speed change mechanism is engaged with the low speed stage and the high speed stage, respectively. 
         FIG.  7    is a hydraulic circuit diagram of a transmission structure  3  according to an embodiment 3 of the present invention. 
         FIG.  8    is hydraulic pressure waveform charts in switching from first to second transmission states in the transmission structure according the embodiment 3. 
         FIG.  9    is a hydraulic circuit diagram of a transmission structure according to an embodiment 4 of the present invention. 
         FIG.  10    is a partial cross sectional view of a vicinity of an input side clutch unit of the transmission structure according to the embodiment 4, and shows a state in which an input side slider is positioned at a first position so that an input side first clutch mechanism is engaged and an input side second clutch mechanism is disengaged. 
         FIG.  11    is a partial cross sectional view of the vicinity of the input side clutch unit shown in  FIG.  10   , and shows a state in which the input side slider is positioned at an intermediate position so that both the input side first and second clutch mechanism are engaged. 
         FIG.  12    is hydraulic pressure wave form charts in which the transmission structure according to the embodiment 4 is switched from a first transmission state to a second transmission states. 
         FIG.  13    is a hydraulic circuit diagram of a transmission structure according to a modification of the embodiments 3 or 4. 
         FIG.  14    is hydraulic pressure waveform charts in which the transmission structure according to the modification is switched from a first transmission state to a second transmission state. 
         FIG.  15    is hydraulic pressure waveform charts in which a transmission structure according to an embodiment 5 of the present invention is switched from a first transmission state to a second transmission state. 
         FIG.  16    is a hydraulic circuit diagram of a transmission structure according to an embodiment 6 of the present invention. 
         FIG.  17    is hydraulic pressure waveform charts in which the transmission structure according to the embodiment 6 is switched from a first transmission state to a second transmission state. 
         FIG.  18    is hydraulic pressure waveform charts in which a transmission structure according to a modification of the embodiment 5 or 6 is switched from a first transmission state to a second transmission state. 
         FIG.  19    is a transmission schematic view of a working vehicle to which a transmission structure according to an embodiment 7 of the present invention is applied. 
         FIG.  20    is a hydraulic circuit diagram of the transmission structure according to the embodiment 7. 
         FIGS.  21 A and  21 B  are graphs illustrating the relationship between the traveling vehicle speed and the HST output in the working vehicle to which the transmission structure according to the embodiment 7 is applied, and illustrate states where a sub speed change mechanism provided in the transmission structure is engaged with a low speed stage and a high speed stage, respectively. 
         FIG.  22    is a hydraulic circuit diagram of a transmission structure according to a modification of the embodiment 7. 
         FIG.  23    is a transmission schematic view of a working vehicle to which a transmission structure according to an embodiment 8 of the present invention is applied. 
         FIG.  24    is a partial vertical cross-sectional side view of the working vehicle shown in  FIG.  23   . 
         FIG.  25    is a graph illustrating the relationship between the traveling vehicle speed and the HST output in the working vehicle shown in  FIG.  23   . 
         FIG.  26    is a transmission schematic view of a working vehicle to which a transmission structure according to a modification of the embodiment 8 is applied. 
         FIG.  27    is a transmission schematic view of a working vehicle to which a transmission structure according to an embodiment 9 of the present invention is applied. 
         FIG.  28    is a partial vertical cross-sectional side view of the working vehicle shown in  FIG.  27   . 
         FIG.  29    is a graph illustrating the relationship between the traveling vehicle speed and the HST output in the working vehicle shown in  FIG.  27   . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     Hereinafter, one embodiment of a transmission structure according to the present invention is described with reference to the accompanying drawings. 
       FIG.  1    illustrates a transmission schematic view of a working vehicle  200  to which a transmission structure  1  according to this embodiment is applied. 
       FIG.  2    illustrates a hydraulic circuit diagram of the transmission structure  1 . 
     As illustrated in  FIG.  1    and  FIG.  2   , the working vehicle  200  is provided with a driving source  210 , driving wheels  220 , and the transmission structure  1  interposed in a traveling system transmission path reaching the driving wheels  220  from the driving source  210 . The reference numeral  210   a  in  FIG.  1    and  FIG.  2    designates a flywheel contained in the driving source  210 . 
     As illustrated in  FIG.  1    and  FIG.  2   , the transmission structure  1  is provided with a hydrostatic transmission (HST)  10 , a planetary gear mechanism  30  forming an HMT structure (hydromechanical transmission structure) in cooperation with the HST  10 , a speed change output shaft  45 , a speed change operation member  90 , such as a speed change lever, capable of detecting the operation position by an operation position sensor  92 , an HST sensor  95   a  directly or indirectly detecting the speed change state of the HST  10 , an output sensor  95   b  directly or indirectly detecting the rotational speed of the speed change output shaft  45 , and a control device  100 . 
     As illustrated in  FIG.  1    and  FIG.  2   , the HST  10  has a pump shaft  12  operatively rotationally driven by the driving source  210 , a hydraulic pump  14  supported by the pump shaft  12  so as not to be relatively rotatable, a hydraulic motor  18  fluid-connected to the hydraulic pump  14  through a pair of hydraulic oil lines  15  and hydraulically rotationally driven by the hydraulic pump  14 , a motor shaft  16  supporting the hydraulic motor  18  so as not to be relatively rotatable, and an output adjustment member  20  varying the capacity of at least one of the hydraulic pump  14  and the hydraulic motor  18 . 
     The HST  10  can continuously change the ratio of the rotational speed of the HST output to be output from the motor shaft  16  to the rotational speed of the power input into the pump shaft  12  (i.e., speed change ratio of the HST  10 ) according to the operation position of the output adjustment member  20 . 
     More specifically, when the rotational speed of the rotation power operatively input into the pump shaft  12  from the driving source  210  is set to a reference input speed, the HST  10  continuously changes the rotation power of the reference input speed to the rotation power at least between the first HST speed and the second HST speed according to the operation position of the output adjustment member  20 , and then outputs the changed rotation power from the motor shaft  16 . 
     In this embodiment, the pump shaft  12  is coupled with a main driving shaft  212  operatively coupled with the driving source  210  through a gear train  214  as illustrated in  FIG.  1   . 
     In this embodiment, the HST  10  is configured so that the rotation direction of the HST output can be switched between the normal rotation direction and the reverse rotation direction. 
     More specifically, the HST  10  is configured so that, in the case where the rotation direction of the reference input speed is set to the normal rotation direction, when the output adjustment member  20  is located at a first operation position, the rotation power of the first HST speed in which the rotation direction is set to one of the normal rotation direction and the reverse rotation direction (for example, reverse rotation direction) is output from the motor shaft  16  and, when the output adjustment member  20  is located at a second operation position, the rotation power of the second HST speed in which the rotation direction is set to the other one of the normal rotation direction and the reverse rotation direction (for example, normal rotation direction) is output from the motor shaft  16 . 
     In this case, when the output adjustment member  20  is located at a neutral position between the first and second operation positions, the rotational speed of the HST output becomes neutral speed (zero speed). 
     In this embodiment, the HST  10  has, as the output adjustment member  20 , a movable swash plate varying the capacity of the hydraulic pump  14  by being oscillated around an oscillation shaft and capable of being oscillated to one side and the other side around the oscillation shaft across the neutral position where the discharge amount of pressure oil discharged from the hydraulic pump  14  is set to zero as illustrated in  FIG.  1    and  FIG.  2   . 
     When the movable swash plate is located at the neutral position, the pressure oil is not discharged from the hydraulic pump  14 , so that the HST  10  is brought into a neutral state where the output of the hydraulic motor  18  is zero. 
     Then, when the movable swash plate is oscillated from the neutral position to the normal rotation side which is the one side around the oscillation shaft, the pressure oil is supplied to one of the pair of hydraulic oil lines  15  from the hydraulic pump  14 , so that the one hydraulic oil line  15  becomes a high-pressure side and the other operation line  15  becomes a low-pressure side. 
     Thus, the hydraulic motor  18  is rotationally driven in the normal rotation direction, so that the HST  10  brought into a normal rotation output state. 
     On the contrary, when the movable swash plate is oscillated from the neutral position to the reverse rotation side which is the other side around the oscillation shaft, the pressure oil is supplied to the other side of the pair of hydraulic oil lines  15  from the hydraulic pump  14 , so that the other hydraulic oil line  15  becomes a high-pressure side and the one hydraulic oil line  15  becomes a low-pressure side. 
     Thus, the hydraulic motor  18  is rotationally driven in the reverse rotation direction, so that the HST  10  is brought into a reverse rotation output state. 
     In the HST  10 , the capacity of the hydraulic motor  18  is fixed by the fixed swash plate. 
     As illustrated in  FIG.  2   , the output adjustment member  20  is operatively controlled by the control device  100  based on the operation of the speed change operation member  90 . 
     More specifically, the control device  100  operates the output adjustment member  20  through an actuator  110  based on the operation to the speed change operation member  90 . 
     The actuator  110  can take various configurations, such as an electric motor and a hydraulic servo mechanism, insofar as the operation is controlled by the control device  100 . 
     As illustrated in  FIG.  1    and  FIG.  2   , the planetary gear mechanism  30  has a sun gear  32 , a planetary gear  34  meshed with the sun gear  32 , an internal gear  36  meshed with the planetary gear  34 , and a carrier  38  supporting the planetary gear  34  so as to be rotatable around the axis and rotating around the axis of the sun gear  32  while interlocked with the revolution around the sun gear  32  of the planetary gear  34 , in which the sun gear  32 , the carrier  38 , and the internal gear  36  form three planetary elements. 
     A third element which is one of the three planetary elements is operatively coupled with the motor shaft  16  and the third element functions as a variable power input portion which inputs the HST output. 
     As illustrated in  FIG.  1    and  FIG.  2   , the sun gear  32  is set as the third element in this embodiment. 
     In this embodiment, the sun gear  32  is operatively coupled with the motor shaft  16  through a gear train  216 . 
     The transmission structure  1  according to this embodiment enables switching between a first transmission state where the first element is functioned as a reference power input portion inputting the reference rotation power from the driving source  210  and the second element is functioned as an output portion outputting synthetic rotation power, and a second transmission state where the first element is functioned as the output portion and the second element is functioned as the reference power input portion. 
     Specifically, as illustrated in  FIG.  1    and  FIG.  2   , the transmission structure  1  has an input side first transmission mechanism  50 ( 1 ) and an input side second transmission mechanism  50 ( 2 ) capable of operatively transmitting the rotation power of the driving source  210  to the first element and the second element, respectively, an input side first clutch mechanism  60 ( 1 ) and an input side second clutch mechanism  60 ( 2 ) engaging/disengaging the power transmission of the input side first transmission mechanism  50 ( 1 ) and the input side second transmission mechanism  50 ( 2 ), respectively, an output side first transmission mechanism  70 ( 1 ) and an output side second transmission mechanism  70 ( 2 ) capable of operatively transmitting the rotation power of the first element and the second element, respectively, to the speed change output shaft, and an output side first clutch mechanism  80 ( 1 ) and an output side second clutch mechanism  80 ( 2 ) engaging/disengaging the power transmission of the output side first transmission mechanism  70 ( 1 ) and the output side second transmission mechanism  70 ( 2 ), respectively. 
     In this embodiment, the internal gear  36  and the carrier  38  function as the first and second elements, respectively. 
     The input side first transmission mechanism  50 ( 1 ) is configured to be able to transmit the rotation power of the driving source  210  to the first element (the internal gear  36  in this embodiment) at an input side first speed change ratio. 
     In detail, as illustrated in  FIG.  1    and  FIG.  2   , the input side first transmission mechanism  50 ( 1 ) has an input side first driving gear  52 ( 1 ) relatively rotatably coupled with the main driving shaft  212  and an input side first driven gear  54 ( 1 ) meshed with the input side first driving gear  52 ( 1 ) and coupled with the first element. 
     The input side second transmission mechanism  50 ( 2 ) is configured to be able to transmit the rotation power of the driving source  210  to the second element (the carrier  38  in this embodiment) at an input side second speed change ratio. 
     In detail, as illustrated in  FIG.  1    and  FIG.  2   , the input side second transmission mechanism  50 ( 2 ) has an input side second driving gear  52 ( 2 ) relatively rotatably supported by the main driving shaft  212  and an input side second driven gear  54 ( 2 ) meshed with the input side second driving gear  52 ( 2 ) and coupled with the second element. 
     In this embodiment, the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) are configured as friction plate clutch mechanisms. 
     In detail, the input side first clutch mechanism  60 ( 1 ) has an input side clutch housing  62  supported by the main driving shaft  212  so as not to be relatively rotatable, an input side first friction plate group  64 ( 1 ) containing a first driving side friction plate supported by the input side clutch housing  62  so as not to be relatively rotatable and a first driven side friction plate supported by the input side first driving gear  52 ( 1 ) so as not to be relatively rotatable in a state of being opposed to the first driving side friction plate, and an input side first piston (not illustrated) causing the input side first friction plate group  64 ( 1 ) to be frictionally engaged. 
     The input side second clutch mechanism  60 ( 2 ) has the input side clutch housing  62 , an input side second friction plate group  64 ( 2 ) containing a second driving side friction plate supported by the input side clutch housing  62  so as not to be relatively rotatable and a second driven side friction plate supported by the input side second driving gear  52 ( 2 ) so as not to be relatively rotatable in a state of being opposed to the second driving side friction plate, and an input side second piston (not illustrated) causing the input side second friction plate group  64 ( 2 ) to be frictionally engaged. 
     The output side first transmission mechanism  70 ( 1 ) is configured to be able to transmit the rotation power of the second element to the speed change output shaft  45  at an output side first speed change ratio. 
     In detail, the transmission structure has a speed change intermediate shaft  43  disposed coaxially with the planetary gear mechanism  30  and coupled with one of the first and second elements so as not to be relatively rotatable around the axis. 
     In this embodiment, the speed change intermediate shaft  43  is coupled with the second element so as not to be relatively rotatable. 
     Then, the output side first transmission mechanism  70 ( 1 ) has an output side first driving gear  72 ( 1 ) supported by the speed change intermediate shaft  43  so as not to be relatively rotatable and an output side first driven gear  74 ( 1 ) meshed with the output side first driving gear  72 ( 1 ) and relatively rotatably supported by the speed change output shaft  45 . 
     The output side second transmission mechanism  70 ( 2 ) is configured to be able to transmit the rotation power of the first element to the speed change output shaft  45  at an output side second speed change ratio. 
     In detail, the output side second transmission mechanism  70 ( 2 ) has an output side second driving gear  72 ( 2 ) coupled with the first element and an output side second driven gear  74 ( 2 ) meshed with the output side second driving gear  72 ( 2 ) and relatively rotatably supported by the speed change output shaft  45 . 
     In this embodiment, the output side second driving gear  72 ( 2 ) is supported by the speed change intermediate shaft  43  so as to be relatively rotatable. 
     In this embodiment, the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) are configured as friction plate clutch mechanisms. 
     In detail, the output side first clutch mechanism  80 ( 1 ) has an output side clutch housing  82  supported by the speed change output shaft  45  so as not to be relatively rotatable, an output side first friction plate group  84 ( 1 ) containing a first driving side friction plate supported by the output side first driven gear  74 ( 1 ) so as not to be relatively rotatable and a first driven side friction plate supported by the output side clutch housing  82  so as not to be relatively rotatable in a state of being opposed to the first driving side friction plate, and an output side first piston (not illustrated) causing the output side first friction plate group to be frictionally engaged. 
     The output side second clutch mechanism  80 ( 2 ) has the output side clutch housing  82 , an output side second friction plate group  84 ( 2 ) containing a second driving side friction plate supported by the output side second driven gear  74 ( 2 ) so as not to be relatively rotatable and a second driven side friction plate supported by the output side clutch housing  82  so as not to be relatively rotatable in a state of being opposed to the second driving side friction plate, and an output side second piston (not illustrated) causing the output side second friction plate group to be frictionally engaged. 
     In the transmission structure  1  according to this embodiment, the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) and the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) are of a hydraulic type in which an engagement state is exhibited when it receives pressure oil supply. 
     In detail, as illustrated in  FIG.  2   , the transmission structure  1  further has a pressure oil supply line  155 , the upstream side of which is fluid-connected to a hydraulic source  150 , such as a hydraulic pump, a drain line  157 , a first supply/discharge line  160 ( 1 ) supplying/discharging pressure oil to the input side and output side first clutch mechanisms  60 ( 1 ) and  80 ( 1 ), a second supply/discharge line  160 ( 2 ) supplying/discharging pressure oil to the input side and output side second clutch mechanisms  60 ( 2 ) and  80 ( 2 ), and a switching valve  165 , the position of which is controlled by the control device  100 . 
     The reference numeral  156  in  FIG.  2    designates a relief valve setting the hydraulic pressure of the pressure oil supply line  155 . 
     The switching valve  165  is configured to be able to take a first position where the pressure oil supply/discharge line  155  is fluid-connected to the first supply/discharge line  160 ( 1 ) and the second supply/discharge line  160 ( 2 ) is fluid-connected to the drain line  157  and a second position where the first supply/discharge line  160 ( 1 ) is fluid-connected to the drain line  157  and the pressure oil supply/discharge line  155  is fluid-connected to the second supply/discharge line  160 ( 2 ). 
     As illustrated in  FIG.  1   , the transmission structure  1  according to this embodiment further has a traveling transmission shaft  235  disposed on the downstream side in the transmission direction relative to the speed change output shaft  45  and a forward/reverse movement switching mechanism  230  configured to be able to switch the rotation direction of the driving force between the forward movement direction and the reverse movement direction between the speed change output shaft  45  and the traveling transmission shaft  235 . 
     The forward/reverse movement switching mechanism  230  is configured so that the forward movement direction and the reverse movement direction is switched by the control device  100  in response to the operation to the forward movement side and the reverse movement side of the speed change operation member  90 , for example. 
     More specifically, when recognizing that the speed change operation member  90  is operated to the forward movement side, the control device  100  brings the forward/reverse movement switching mechanism  230  into a forward movement transmission state and, when recognizing that the speed change operation member  90  is operated to the reverse movement side, the control device  100  brings the forward/reverse movement switching mechanism  230  into a reverse movement transmission state. 
     As illustrated in  FIG.  1   , the transmission structure  1  according to this embodiment is further provided with a second traveling transmission shaft  245  disposed on the downstream side in the transmission direction relative to the traveling transmission shaft  235  and a sub speed change mechanism  240  capable of changing, in multiple stages, the rotational speed of the driving force in two stages of a high speed stage and a low speed stage between the traveling transmission shaft  235  and the second traveling transmission shaft  245 . 
     The sub speed change mechanism  240  is configured so that switching between a high speed transmission state and a low speed transmission state is performed through a mechanical link mechanism or by the control device in response to a manual operation to a sub speed change operation member (not illustrated), for example. 
     The working vehicle  200  has one pair of right and left main driving wheels as the driving wheels  220 . Therefore, the working vehicle  200  further has a pair of main driving axles  250  correspondingly driving the pair of main driving wheels and a differential mechanism  260  differentially transmitting the rotation power of the second traveling transmission shaft  245  to the pair of main driving axles  250  as illustrated in  FIG.  1   . 
     As illustrated in  FIG.  1   , the working vehicle  200  further has a traveling brake mechanism  255  selectively applying braking force to the main driving axle  250 , a differential lock mechanism  265  forcibly synchronously driving the pair of main driving axles  250  by the rotation power from the second traveling transmission shaft  245 , and a driving force take-out mechanism  270  for sub-driving wheels capable of selectively outputting the rotation power branched from the second traveling transmission shaft  245  toward the sub-driving wheels. 
     Moreover, the working vehicle  200  has a PTO shaft  280  outputting the rotation power to the outside and a PTO clutch mechanism  285  and a PTO multistage speed change mechanism  290  interposed in a PTO transmission path reaching the PTO shaft  280  from the driving source  210 . 
     Herein, the operation control of the HST  10 , the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ), and the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) by the control device  100  is described. 
       FIG.  3 A  and  FIG.  3 B  illustrate graphs illustrating the relationship between the traveling vehicle speed and the rotational speed of the HST output in the working vehicle  200 . 
       FIG.  3 A  and  FIG.  3 B  illustrates states where the sub speed change mechanism  240  is engaged with a low speed stage and a high speed stage, respectively. 
     The control device  100  produces the first transmission state where the first element (the internal gear  36  in this embodiment) is functioned as the reference power input portion which receives the reference power operatively transmitted from the driving source  210  and the second element (the carrier  38  in this embodiment) is functioned as an output portion of the synthetic rotation power by bringing the input side and output side first clutch mechanisms  60 ( 1 ) and  80 ( 1 ) into an engagement state and bringing the input side and output side second clutch mechanisms  60 ( 2 ) and  80 ( 2 ) into a disengagement state when the speed change operation member  90  is operated before the switching speed positions (i.e., in a low speed state where the rotational speed of the speed change output shaft  45  is less than a predetermined switching speed based on a detection signal of the output sensor  95   b  ). 
     The output sensor  95   b  may take various forms, such as a sensor detecting the rotational speed of the speed change output shaft  45  and a sensor detecting the rotational speed of the driving wheel  20  or the driving axle  250 , insofar as the rotational speed of the speed change output shaft  45  can be directly or indirectly recognized. 
     The low speed state where the rotational speed of the speed change output shaft  45  is less than the predetermined switching speed means that, in a case where the traveling vehicle speed is set as a reference, the vehicle speed is within the range of −a(L) to +a (L) when the sub speed change mechanism  240  is engaged with the low speed stage (see  FIG.  3 A ) and the vehicle speed is within the range of −a(H) to +a (H) when the sub speed change mechanism  240  is engaged with the high speed stage (see  FIG.  3 B ). 
     “+” and “−” of the traveling vehicle speed mean that the traveling directions of the working vehicle  200  are the forward movement direction and the reverse movement direction, respectively. 
     In the first transmission state, the control device  100  operates the output adjustment member  20  so that the HST output is speed-changed from the first HST speed (reverse rotation side predetermined speed in this embodiment) toward the second HST speed (normal rotation side predetermined speed in this embodiment) based on the HST sensor  95   a  in response to an acceleration operation of the speed change operation member  90 . 
     The HST sensor  95   a  may take various forms, such as a sensor detecting the rotational speed of the motor shaft  16  and a sensor detecting the operation position of the output adjustment member  20 , insofar as the output state of the HST  10  can be detected. 
     More specifically, when the speed change operation member  90  is located before the switching speed position, the control device  100  produces the first transmission state, and then, 
     (1) when the speed change operation member  90  is located at a zero speed position (vehicle stop position), the control device  100  locates the output adjustment member  20  at the first HST speed position (reverse rotation side predetermined speed position in this embodiment) where the HST output is set to the first HST speed, 
     (2) until the speed change operation member  90  reaches the switching speed position (i.e., until the rotational speed of the speed change output shaft  45  reaches the switching speed from the zero speed (in a case where the traveling vehicle speed in the working vehicle  200  of this embodiment is used as a reference, equivalent to the time until the traveling vehicle speed reaches the vehicle speed −a(L) (in reverse movement) from the zero speed and the time until the traveling vehicle speed reaches the vehicle speed +a(L) (in forward movement) from the zero speed when the sub speed change mechanisms  240  is in the low speed stage engagement state ( FIG.  3 A ) and equivalent to the time until the traveling vehicle speed reaches the vehicle speed −a(H) (in reverse movement) from the zero speed and the time until the traveling vehicle speed reaches the vehicle speed +a(H) (in forward movement) from the zero speed when the sub speed change mechanism  240  is in the high speed stage engagement state ( FIG.  3 B )), the control device  100  operates the output adjustment member  20  so that the HST output is speed-changed from the side of the first HST speed to the side of the second HST speed in response to the acceleration operation of the speed change operation member  90  (so that the output adjustment member  20  is moved from the reverse rotation side predetermined speed position side to a normal rotation side predetermined speed position side in this embodiment), and 
     (3) when the speed change operation member  90  is located at the switching speed position (i.e., when the rotational speed of the speed change output shaft  45  reaches the switching speed (in a case where the traveling vehicle speed in the working vehicle  200  of this embodiment is used as a reference, equivalent to the time when the traveling vehicle speed reaches the vehicle speed −a(L) (in reverse movement) and the time when the traveling vehicle speed reaches the vehicle speed +a(L) (in forward movement) when the sub speed change mechanisms  240  is in the low speed stage engagement state ( FIG.  3 A ) and equivalent to the time when the traveling vehicle speed reaches the vehicle speed −a(H) (in reverse movement) and the time when the traveling vehicle speed reaches the vehicle speed +a(H) (in forward movement) when the sub speed change mechanism  240  is in the high speed stage engagement state ( FIG.  3 B )), the control device  100  operates the output adjustment member  20  at the second HST speed position (in this embodiment, normal rotation side predetermined speed position) where the HST output is set to the second HST speed. 
     Furthermore, when recognizing that the speed change operation member  90  is operated to the high speed side beyond the switching speed position (i.e., recognizing that the rotational speed of the speed change output shaft  45  reaches a high speed state equal to or higher than the predetermined switching speed based on a detection signal of the output sensor  95   b ), the control device  100  produces a second transmission state where the first element is functioned as an output portion and the second element is functioned as the reference power input portion by bringing the input side and output side first clutch mechanisms  60 ( 1 ) and  80 ( 1 ) into the disengagement state and bringing the input side and output side second clutch mechanisms  60 ( 2 ) and  80 ( 2 ) into the engagement state. 
     The high speed state where the rotational speed of the speed change output shaft  45  is equal to or higher than the predetermined switching speed means a state where, in a case where the traveling vehicle speed is used as a reference, the traveling vehicle speed is higher than or equal to −a(L) (in reverse movement) and higher than or equal to +a(L) (in forward movement) when the sub speed change mechanism  240  is engaged with the low speed stage (see  FIG.  3 A ) and, the traveling vehicle speed is higher than or equal to −a(H) (in reverse movement) and higher than or equal to +a(H) (in forward movement) when the sub speed change mechanism  240  is engaged with the high speed stage (see  FIG.  3 B ). 
     In the second transmission state, the control device  100  operates the output adjustment member  20  so that the HST output is speed-changed from the second HST speed (normal rotation side predetermined speed in this embodiment) toward the first HST speed (the reverse rotation side predetermined speed in this embodiment) based on the HST sensor  95   a  in response to the acceleration operation of the speed change operation member  90 . 
     More specifically, when the speed change operation member  90  is operated to the forward movement high speed side relative to the switching speed position, the control device  100  produces the second transmission state, and then, 
     (1) when the speed change operation member  90  is located at the speed switching position, the control device  100  locates the output adjustment member  20  at the second HST speed position where the HST output is set to the second HST speed (normal rotation side predetermined speed position in this embodiment), 
     (2) when the speed change operation member  90  is located between the switching speed position and a forward movement maximum speed position (i.e., until the rotational speed of the speed change output shaft  45  reaches the maximum speed from the switching speed (in a case where the traveling vehicle speed in the working vehicle  200  of this embodiment is used as a reference, equivalent to the time until the traveling vehicle speed reaches the vehicle speed −b(L) from the vehicle speed −a(L) (in reverse movement) and the time until the traveling vehicle speed reaches the vehicle speed +b(L) from the vehicle speed +a(L) (in forward movement) when the sub speed change mechanism  240  is engaged with the low speed stage ( FIG.  3 A ) and equivalent to the time until the traveling vehicle speed reaches the vehicle speed −b(H) from the vehicle speed −a(H) (in reverse movement) and the time until the traveling vehicle speed reaches the vehicle speed +b(H) from the vehicle speed +a(H) (in forward movement) when the sub speed change mechanism  240  is in the high speed stage engagement state ( FIG.  3 B )), the control device  100  operates the output adjustment member  20  so that the HST output is speed-changed from the side of the second HST speed to the side of the first HST speed in response to the acceleration operation of the speed change operation member  90  (the output adjustment member  20  is moved from the normal rotation side predetermined speed position to the reverse rotation side predetermined speed position in this embodiment), and 
     (3) when the speed change operation member  90  is operated to the forward movement maximum speed position (i.e., when the rotational speed of the speed change output shaft  45  reaches the maximum speed (in a case where the traveling vehicle speed in the working vehicle  200  of this embodiment is used as a reference, equivalent to the time when the traveling vehicle speed reaches the vehicle speed −b(L) (in reverse movement) and the time when the traveling vehicle speed reaches the vehicle speed +b(L) (in forward movement) when the sub speed change mechanism  240  is in the low speed stage engagement state ( FIG.  3 A ) and equivalent to the time when the traveling vehicle speed reaches the vehicle speed −b(H) (in reverse movement) and the time when the traveling vehicle speed reaches the vehicle speed +b(H) (in forward movement) when the sub speed change mechanism  240  is in the high speed stage engagement state ( FIG.  3 B )), the control device  100  locates the output adjustment member  20  at the first HST speed position (the reverse rotation side predetermined speed position in this embodiment) where the HST output is set to the first HST speed. 
     Herein, in this embodiment, the input side first and second speed change ratios are set so that the rotational speed of the second element is the same in the interval of time between when the HST output is set to the second HST speed under the first transmission state where the second element functions as the output portion and when the second transmission state is realized where the second element functions as the reference power input portion that receives the reference power from the driving source  210  operatively transmitted through the input side second transmission mechanism  50 ( 2 ). The input side first and second speed ratios are also set so that the rotational speed of the first element is the same in the interval of time between when the HST output is set to the second HST speed under the second transmission state where the first element functions as the output portion and when the first transmission state is realized where the first element functions as the reference power input portion that receives the reference power from the driving source operatively transmitted through the input side first transmission mechanism  50 ( 1 ). 
     More specifically, in this embodiment, the input side first and second speed change ratios are set so that a rotational speed difference does not occur in the second element and the third element during the transition between the first transmission state (where the first element is functioned as the reference power input portion and the second element is functioned as the output portion) and the second transmission state (where the first element is functioned as the output portion and the second element is functioned as the reference power input portion). 
     Furthermore, in this embodiment, the output side first and second speed change ratios are set so that the rotational speed developed in the speed change output shaft  45  when the HST output is set to the second HST speed is same in the first and second transmission states. 
     More specifically, in this embodiment, the output side first and second speed change ratios are set so that a change does not occur in the rotational speed of the speed change output shaft  45 , i.e., the traveling vehicle speed, during the transition between the first and second transmission states. 
     According to the transmission structure  1  provided with such a configuration, as illustrated in  FIG.  3   , a continuous speed change can be achieved over the speed change range where the speed change output shaft  45  is accelerated by speed-changing the HST output from the first HST speed to the second HST speed (speed change range of 0 to −a and 0 to +a in a case where the traveling vehicle speed is used as a reference, which is hereinafter referred to as a low speed side speed change range) and the speed change range where the speed change output shaft  45  is accelerated by speed-changing the HST output from the second HST speed to the first HST speed (speed change range of −a to −b and +a to +b in a case where the traveling vehicle speed is used as a reference, which is hereinafter referred to as a high speed side speed change range). 
     Furthermore, in the switching between the low speed side speed change range (first transmission state) and the high speed side speed change range (second transmission state), a change in the operation position of the output adjustment member  20  of the HST  10  is not required and a change in the traveling vehicle speed is not caused. 
     Therefore, the switching can be smoothly performed without applying a load to constituent members of the traveling system transmission path in which the transmission structure  1  is interposed. 
     Moreover, the transmission structure  1  enables the switching without causing a speed difference between the low speed side speed change range (first transmission state) and the high speed side speed change range (second transmission state) without being provided with two or more the planetary gear mechanisms  30 , and thus enables the realization of good transmission efficiency. 
     More specifically, when two or more of the planetary gear mechanisms are provided, the switch between the transmission state in the low speed side speed change range and the transmission state in the high speed side speed change range is enabled without requiring a change in the operation position of the output adjustment member of the HST and without causing a traveling vehicle speed change. 
     However, the transmission efficiency of the planetary gear mechanism is poor, and thus good transmission efficiency cannot be obtained in the configuration provided with two or more of the planetary gear mechanisms. 
     In contrast thereto, the transmission structure  1  can obtain the above-described effect by simply being provided with the single planetary gear mechanism  30 . 
     Moreover, in the transmission structure  1  according to this embodiment, the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) and the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) are configured as the friction plate clutch mechanisms as described above. 
     According to such a configuration, the switching between the low speed side speed change range (first transmission state) and the high speed side speed change range (second transmission state) can be more smoothly performed. 
     Preferably, the switching speed of the speed change output shaft  45  serving as the target speed to start the switch between the first and second transmission states can be set to a speed higher than speed in the work speed range set in the working vehicle  200 . 
     More specifically, working vehicles, such as a tractor and a combine, perform heavy load work, such as tilling work, plowing work, tamping work, and reaping work, while traveling at low speed in many cases. 
     In general, in the working vehicles, the traveling vehicle speed in performing such heavy load work is set as the work speed range. Traveling vehicle speed of 0 to 8 km/h is usually set as the work speed range and, depending on the specification, traveling vehicle speed of 0 to 10 km/h is set as the work speed range. 
     Therefore, by setting the switching speed of the speed change output shaft  45  to be higher than the speed in the work speed range in a case where the traveling vehicle speed is used as a reference, it can be effectively prevented that the switching between the first and second transmission states is performed in the state where the heavy load work is performed. 
     Embodiment 2 
     Hereinafter, another embodiment of the transmission structure according to the present invention is described with reference to the accompanying drawings. 
       FIG.  4    illustrates a hydraulic circuit diagram of a transmission structure  2  according to this embodiment. 
     In the figure, the same components as those in Embodiment 1 described above are designated by the same reference numerals and a description thereof is omitted as appropriate. 
     The transmission structure  2  according to this embodiment is different from the transmission structure  1  according to Embodiment 1 in a point that the output side first and second transmission mechanisms  70 ( 1 ) and  70 ( 2 ) are deleted. 
     In this embodiment, the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) are provided to engage/disengage the power transmission from the second element (the carrier  38  in this embodiment) and the first element (the internal gear  36  in this embodiment), respectively, to the speed change output shaft  45 . 
       FIG.  5 A  and  FIG.  5 B  illustrate graphs illustrating the relationship between the traveling vehicle speed and the rotational speed of the HST output in a working vehicle to which the transmission structure  2  is applied. 
       FIG.  5 A  and  FIG.  5 B  illustrate states where the sub speed change mechanism  240  is engaged with a low speed stage and a high speed stage, respectively. 
     As illustrated in  FIG.  5 A  and  FIG.  5 B , in this embodiment, the control device  100  is configured to perform switching from the first transmission state to the second transmission state when the speed change operation member  90  is operated from the zero speed position to the first switching speed positions, i.e., when recognizing that the speed change output shaft  45  reaches a predetermined first switching speed in the first transmission state based on a detection signal of the output sensor  95   b  (in a case where the traveling vehicle speed is used as a reference, −a(L)( 1 ) (in reverse movement) or +a(L)( 1 ) (in forward movement) in the low speed stage engagement and −a(H)( 1 ) (in reverse movement) or +a(H)( 1 ) (in forward movement) in the high speed stage engagement). 
     Herein, when the HST output is set to the second HST speed in the first transmission state, the rotational speed of the first switching speed is developed in the speed change output shaft  45 . 
     More specifically, when the reference power from the driving source  210  is operatively input into the first element (the internal gear  36  in this embodiment), the HST output of the second HST speed is operatively input into the third element (the sun gear  32  in this embodiment), and the synthetic rotation power is output from the second element (the carrier  38  in this embodiment), the speed change output shaft  45  rotates at the first switching speed by the synthetic rotation power operatively transmitted from the second element. 
     At this time, the traveling vehicle speed is set to −a(L)( 1 ) (in reverse movement) or +a(L)( 1 ) (in forward movement) ( FIG.  5 A ) in the low speed stage engagement of the sub speed change mechanism  240  and is set to −a(H)( 1 ) (in reverse movement) or +a(H)( 1 ) (in forward movement) ( FIG.  5 B ) in the high speed stage engagement of the sub speed change mechanism  240 . 
     When the speed change operation member  90  is operated to the first switching speed positions (i.e., when the switching from the first transmission state to the second transmission state is performed due to the fact that the HST output is set to the second HST speed, so that the rotational speed of the speed change output shaft  45  reaches the first switching speed in the first transmission state), a state is set where the reference power from the driving source  210  is operatively input into the second element (the carrier  38  in this embodiment), the HST output of the second HST speed is operatively input into the third element (the sun gear  32  in this embodiment), and the synthetic rotation power is output from the first element (the internal gear  38  in this embodiment), so that the speed change output shaft  45  is rotated by the synthetic rotation power operatively transmitted from the first element. 
     At this time, the transmission structure  2  according to this embodiment does not have the output side first and second transmission mechanisms  70 ( 1 ) and  70 ( 2 ) as described above, and therefore the rotational speed of the speed change output shaft  45  changes from the first switching speed to a second switching speed. 
     The traveling vehicle speed when the rotational speed of the speed change output shaft  45  is the second switching speed becomes −a(L)( 2 ) (in reverse movement) or +a(L)( 2 ) (in forward movement) ( FIG.  5 A ) in the low speed stage engagement of the sub speed change mechanism  240 , and becomes −a(H)( 2 ) (in reverse movement) or +a(H)( 2 ) (in forward movement) ( FIG.  5 B ) in the high speed stage engagement of the sub speed change mechanism  240 . 
     More specifically, in the transmission structure  2  according to this embodiment, a rotational speed difference occurs in the speed change output shaft  45  in the switching between the first and second transmission states, so that the traveling vehicle speed changes. 
     However, the rotational speed difference is not so large, and therefore can be absorbed by components forming the traveling system transmission path. 
     In particular, in this embodiment, the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) and the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) are configured as the friction plate clutch mechanisms, and thus the rotational speed difference can be effectively absorbed by the friction plate clutch mechanisms. 
     In place thereof, the input side second clutch mechanism  60 ( 2 ) brought into the engagement state in the second transmission state out of the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) and the output side second clutch mechanism  80 ( 2 ) brought into the engagement state in the second transmission state out of the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) can be configured as the friction plate clutch mechanisms and the remaining clutch mechanisms  60 ( 1 ) and  80 ( 1 ) can be configured as the other forms, such as a dog clutch mechanism. 
     According to the transmission structure  2  having such a configuration, although a certain traveling speed difference occurs in the switching between the first and second transmission states, the structure can be simplified by the deletion of the output side first and second transmission mechanisms  70 ( 1 ) and  70 ( 2 ) as compared with Embodiment 1. 
       FIG.  6 A  and  FIG.  6 B  illustrate graphs illustrating the relationship between the traveling vehicle speed and the rotational speed of the HST output in a modification of this embodiment. 
       FIG.  6 A  and  FIG.  6 B  illustrate states where the sub speed change mechanism  240  is engaged with the low speed stage and the high speed stage, respectively. 
     In the modification, the control device  100  operates the output adjustment member  20  so that the switching speed in a transmission state after the switching coincides with or approaches the switching speed in a transmission state before the switching in the switching between the first and second transmission states. 
     More specifically, in a case where the switching from the first transmission state to the second transmission state is taken as an example, the control device  100  is configured to operate the output adjustment member  20  so that the switching speed (second switching speed) in the second transmission state which is a transmission state after the switching coincides with or approaches the switching speed (first switching speed) in the first transmission state which is the transmission state before the switching. 
     Such a modification can effectively prevent or reduce the occurrence of the rotational speed difference in the speed change output shaft  45  in the switching between the first and second transmission states, i.e. the occurrence of a traveling speed difference. 
     Embodiment 3 
     Hereinafter, a still another embodiment of the transmission structure according to the present invention is described with reference to the accompanying drawings. 
       FIG.  7    illustrates a hydraulic circuit diagram of a transmission structure  3  according to this embodiment. 
     In the figure, the same components as those in Embodiments 1 and 2 described above are designated by the same reference numerals and a description thereof is omitted as appropriate. 
     The transmission structure  3  according to this embodiment is different from the transmission structure  1  according to Embodiment 1 in a point that a double transmission state where both the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) are brought into the engagement state and both the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) are brought into the engagement state is developed in the switching transition stage of the first and second transmission states. 
     Specifically, the transmission structure  3  is different from the transmission  1  according to Embodiment 1 in the pressure oil supply/discharge configuration to the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) and the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ). 
     More specifically, as illustrated in  FIG.  7   , the transmission structure  3  is provided with the pressure oil supply line  155 , an input side first supply/discharge line  360 ( 1 ), an input side second supply/discharge line  360 ( 2 ), an output side first supply/discharge line  362 ( 1 ) and an output side second supply/discharge line  362 ( 2 ) supplying/discharging pressure oil to the input side first clutch mechanism  60 ( 1 ), the input side second clutch mechanism  60 ( 2 ), the output side first clutch mechanism  80 ( 1 ) and the output side second clutch mechanism  80 ( 2 ), respectively, an input side first electromagnetic valve  365 ( 1 ), an input side second electromagnetic valve  365 ( 2 ), an output side first electromagnetic valve  367 ( 1 ) and an output side second electromagnetic valve  367 ( 2 ) interposed between the pressure oil supply line  155  and the input side first supply/discharge line  360 ( 1 ), the input side second supply/discharge line  360 ( 2 ), the output side first supply/discharge line  362 ( 1 ) and the output side second supply/discharge line  362 ( 2 ), respectively, and an input side first pressure sensor  370 ( 1 ), an input side second pressure sensor  370 ( 2 ), an output side first pressure sensor  372 ( 1 ) and an output side second pressure sensor  372 ( 2 ) interposed in the input side first supply/discharge line  360 ( 1 ), the input side second supply/discharge line  360 ( 2 ), the output side first supply/discharge line  362 ( 1 ) and the output side second supply/discharge line  362 ( 2 ), respectively. 
     Each of the electromagnetic valves  365 ( 1 ),  365 ( 2 ),  367 ( 1 ) and  367 ( 2 ) is configured to be able to take a discharge position where the corresponding supply/discharge line  360 ( 1 ),  360 ( 2 ),  362 ( 1 ) and  362 ( 2 ) is drained and a supply position where the corresponding supply/discharge line  360 ( 1 ),  360 ( 2 ),  362 ( 1 ) and  362 ( 2 ) is fluid-connected to the pressure oil supply line  155 . 
     In this embodiment, each of the electromagnetic valves  365 ( 1 ),  365 ( 2 ),  367 ( 1 ) and  367 ( 2 ) is biased toward the discharge positions by a biasing member and located at the supply position against the pressing force of the biasing member when a control signal from the control device  100  is input. In this embodiment, as illustrated in  FIG.  7   , each of the electromagnetic valve  365 ( 1 ),  365 ( 2 ),  367 ( 1 ) and  367 ( 2 ) is configured as a proportional electromagnetic valve that receives the hydraulic pressure of the corresponding supply/discharge line  360 ( 1 ),  360 ( 2 ),  362 ( 1 ) and  362 ( 2 ) as pilot pressure to thereby maintain the hydraulic pressure of the corresponding supply/discharge line  360 ( 1 ),  360 ( 2 ),  362 ( 1 ) and  362 ( 2 ) at engagement hydraulic pressure in a state where a position signal to the supply position is input from the control device  100 . 
     The position control of the electromagnetic valves  365 ( 1 ),  365 ( 2 ),  367 ( 1 ) and  367 ( 2 ) by the control device  100  is described taking a case of the switching from the first transmission state to the second transmission state as an example. 
       FIG.  8    illustrates hydraulic pressure waveform charts of the supply/discharge lines  360 ( 1 ),  360 ( 2 ),  362 ( 1 ) and  362 ( 2 ) in the switching from the first transmission state to the second transmission state. 
     In a state where the speed change operation member  90  is located between the zero speed position and the switching speed position (i.e., in a low speed state where the rotational speed of the speed change output shaft  45  is less than the switching speed), the control device  100  locates the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) at the supply positions and locates the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ) at the discharge positions. 
     In this state, while the hydraulic pressure of the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) are released, so that the input side second clutch mechanism  60 ( 2 ) and the output side second clutch mechanism  80 ( 2 ) are brought into the disengagement state, the input side first supply/discharge line  360 ( 1 ) and the output side first supply/discharge line  362 ( 1 ) are maintained at the engagement hydraulic pressure set by the pilot pressure of the corresponding electromagnetic valves  365 ( 1 ) and  367 ( 1 ), so that the input side first clutch mechanism  60 ( 1 ) and the output side first clutch mechanism  80 ( 1 ) are brought into the engagement state. 
     Thus, the transmission structure  3  is brought into the first transmission state. 
     Meanwhile, in a state where the speed change operation member  90  is operated beyond the switching speed position (i.e., in a high speed state where the rotational speed of the speed change output shaft  45  is equal to or higher than the switching speed), the control device  100  locates the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) at the discharge positions and locates the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ) at the supply positions. 
     In this state, while the hydraulic pressure of the input side first supply/discharge line  360 ( 1 ) and the output side first supply/discharge line  362 ( 1 ) is released, so that the input side first clutch mechanism  60 ( 1 ) and the output side first clutch mechanism  80 ( 1 ) are brought into the disengagement state, the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) are maintained at the engagement hydraulic pressure set by the pilot pressure of the corresponding electromagnetic valves  365 ( 2 ) and  367 ( 2 ), so that the input side second clutch mechanism  60 ( 2 ) and the output side second clutch mechanism  80 ( 2 ) are brought into the engagement state. 
     Thus, the transmission structure  3  is brought into the second transmission state. 
     Herein, when recognizing that the speed change operation member  90  is operated to the switching speed position at time Ta (see  FIG.  8   ), (i.e., when recognizing that the rotational speed of the speed change output shaft  45  reaches the switching speed from the state where the rotational speed is less than the switching speed based on a signal from the output sensor  95   b ), the control device  100  moves the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ) located at the discharge positions at the time before the switching to the supply positions from the discharge positions while maintaining the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) located at the supply positions at the time before the switching of the transmission state (at the time of the first transmission state in this example) at the supply positions. 
     Thus, while the input side first supply/discharge line  360 ( 1 ) and the output side first supply/discharge line  362 ( 1 ) are maintained at the engagement hydraulic pressure, the hydraulic pressure of the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) increase to the engagement hydraulic pressure at time Tb, so that the double transmission state is developed. 
     Thereafter, the control device  100  moves the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) located at the supply positions at the time before the switching from the supply positions to the discharge positions when predetermined time (time Tc) has passed from the time (time Tb) when recognizing that the hydraulic pressure of the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) to which pressure oil is supplied through the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ) moved to the supply positions reaches the engagement hydraulic pressure based on the signals from the corresponding pressure sensors  370 ( 2 ) and  372 ( 2 ). 
     Thus, the second transmission state where, while the input side second clutch mechanism  60 ( 2 ) and the output side second clutch mechanism  80 ( 2 ) are brought into the engagement state, the input side first clutch mechanism  60 ( 1 ) and the output side first clutch mechanism  80 ( 1 ) are brought into the disengagement state is developed. 
     The transmission structure  3  according to this embodiment having such a configuration can effectively prevent the occurrence of a state where traveling driving force is not transmitted to the driving wheels  220  in the switching between the first and second transmission states. 
     This is particularly effective when the switching between the first and second transmission states occurs in the work traveling. 
     This embodiment is configured so that the engagement states of the corresponding friction plate clutch mechanisms are detected based on the pressure sensors  370 ( 1 ),  370 ( 2 ),  372 ( 1 ) and  372 ( 2 ). However, in place of the configuration, this embodiment can be configured so that the engagement states of the corresponding friction plate clutch mechanisms are detected based on other clutch engagement detection units detecting a supply current value, supply current time, and the like of the proportional electromagnetic valves  365 ( 1 ),  365 ( 2 ),  367 ( 1 ) and  367 ( 2 ). 
     Embodiment 4 
     Hereinafter, yet still another embodiment of the transmission structure according to the present invention is described with reference to the accompanying drawings. 
       FIG.  9    illustrates a hydraulic circuit diagram of a transmission structure  4  according to this embodiment. 
     In the figure, the same components as those in Embodiments 1 to 3 described above are designated by the same reference numerals and a description thereof is omitted as appropriate. 
     The transmission structure  4  according to this embodiment has an input side clutch unit  410  and an output side clutch unit  430  of a dog clutch type in place of the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) and the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) of the friction plate type as compared with the transmission structure  1  according to Embodiment 1. 
       FIG.  10    illustrates a partial cross sectional view of the input side clutch unit  410 . 
     As illustrated in  FIG.  10   , the input side clutch unit  410  has an input side slider  412  supported by a corresponding main driving shaft  212  so as not to be relatively rotatable and so as to be movable in the axial direction. 
     The input side slider  412  is disposed between the input side first and second driving gears  52 ( 1 ) and  52 ( 2 ) and has a first recess-projection engagement portion  412 ( 2 ) on one side in the axial direction close to the input side first driving gear  52 ( 1 ) and a second recess-projection engagement portion  412 ( 1 ) on the other side in the axial direction close to the input side second driving gear  52 ( 2 ). 
     The input side clutch unit  410  further has recess-projection engagement portions  414 ( 1 ) and  414 ( 2 ) formed in the input side first and second driving gears  52 ( 1 ) and  52 ( 2 ), respectively. 
     More specifically, when the input side slider  412  is located at a first position on the one side in the axial direction illustrated in  FIG.  10   , while the second recess-projection engagement portion  412 ( 2 ) is not engaged with the recess-projection engagement portion  414 ( 2 ) of the input side second driving gear  52 ( 2 ), the first recess-projection engagement portion  412 ( 1 ) is engaged with the recess-projection engagement portion  414 ( 1 ) of the input side first driving gear  52 ( 1 ), whereby the input side first driving gear  52 ( 1 ) is coupled with the main driving shaft  212 , so that the input side slider  412  brings only an input side first clutch mechanism formed by the first recess-projection engagement portion  412 ( 1 ) and the recess-projection engagement portion  414 ( 1 ) into the engagement state. 
     When the input side slider  412  is located at a second position on the other side in the axial direction, while the first recess-projection engagement portion  412 ( 1 ) is not engaged with the recess-projection engagement portion  414 ( 1 ) of the input side first driving gear  52 ( 1 ), the second recess-projection engagement portion  412 ( 2 ) is engaged with the recess-projection engagement portion  414 ( 2 ) of the input side second driving gear  52 ( 2 ), whereby the input side second driving gear  52 ( 2 ) is coupled with the main driving shaft  212 , so that the input side slider  412  brings only the input side second clutch mechanism formed by the second recess-projection engagement portion  412 ( 2 ) and the recess-projection engagement portion  414 ( 2 ) into the engagement state. 
     As illustrated in  FIG.  11   , when the input side slider  412  is located at an intermediate position between the first and second positions with respect to the axial direction, the first and second recess-projection engagement portions  412 ( 1 ) and  412 ( 2 ) are engaged with the recess-projection engagement portions  414 ( 1 ) and  414 ( 2 ) of the input side first and second driving gears  52 ( 1 ) and  52 ( 2 ), respectively, whereby the input side slider  412  brings both the input side first and second driving gears  52 ( 1 ) and  52 ( 2 ) are coupled with the main driving shaft  212 , so that both the input side first and second clutch mechanisms into the engagement state. 
     More specifically, when the input side slider  412  moves between the first position where the first transmission state is developed and the second position where the second transmission state is developed, the input side slider  412  certainly passes the intermediate position where both the input side first and second clutch mechanisms are brought into the engagement state. 
     The transmission structure  4  according to this embodiment having such a configuration can also effectively prevent the generation of the state where the traveling driving force is not transmitted to the driving wheels in the switching between the first and second transmission states. 
     The output side clutch unit  430  has substantially the same configuration as that of the input side clutch unit  410 . 
     More specifically, the output side clutch unit  430  has a recess-projection engagement portion (not illustrated) formed in each of the output side first and second driven gears  74 ( 1 ) and  74 ( 2 ) and an output side slider  432  supported by the corresponding speed change output shaft  45  so as not to be relatively rotatable and so as to be movable in the axial direction between the output side first and second driven gears  74 ( 1 ) and  74 ( 2 ) with respect to the axial direction. 
     The output side slider  432  has a first recess-projection engagement portion (not illustrated) on one side in the axial direction close to the output side first driven gear  74 ( 1 ) and a second recess-projection engagement portion (not illustrated) on the other side in the axial direction close to the output side second driven gear  74 ( 2 ). 
     When the output side slider  432  is located at the first position on the one side in the axial direction, while the second recess-projection engagement portion is not engaged with a recess-projection engagement portion of the output side second driven gear  74 ( 2 ), the first recess-projection engagement portion is engaged with a recess-projection engagement portion of the output side first driven gear  74 ( 1 ), whereby the output side first driven gear  74 ( 1 ) is coupled with the speed change output shaft  45 , so that the output side slider  432  brings only an output side first clutch mechanism formed by the first recess-projection engagement portion and the recess-projection engagement portion of the output side first driven gear  74 ( 1 ) into the engagement state. 
     When the output side slider  432  is located at the second position on the other side in the axial direction, while the first recess-projection engagement portion is not engaged with the recess-projection engagement portion of the output side first driven gear  74 ( 1 ), the second recess-projection engagement portion is engaged with the recess-projection engagement portion of the output side second driven gear  74 ( 2 ), whereby the output side second driven gear  74 ( 2 ) is coupled with the speed change output shaft  45 , so that the output side slider  432  brings only an output side second clutch mechanism formed by the second recess-projection engagement portion and the recess-projection engagement portion of the output side second driven gear  74 ( 2 ) into the engagement state. 
     Furthermore, when the output side slider  432  is located at an intermediate position between the first and second positions with respect to the axial direction, the first and second recess-projection engagement portions are engaged with the recess-projection engagement portions of the output side first and second driven gears  74 ( 1 ) and  74 ( 2 ), respectively, whereby both the output side first and second driven gears  74 ( 1 ) and  74 ( 2 ) are coupled with the speed change output shaft  45 , so that the input side slider  432  brings both the output side first and second clutch mechanisms into the engagement state. 
     The transmission structure  4  according to this embodiment has a hydraulic driving mechanism as a pressing mechanism for the input side slider  412  and the output side slider  432 . 
     The hydraulic driving mechanism is provided with the pressure oil supply line  155 , the drain line  157 , an input side first oil chamber  450 ( 1 ) pressing the input side slider  412  toward the first position by pressure oil to be supplied, an input side second oil chamber  450 ( 2 ) pressing the input side slider  412  toward the second position by pressure oil to be supplied, an output side first oil chamber  452 ( 1 ) pressing the output side slider  432  toward the first position by pressure oil to be supplied, an output side second oil chamber  452 ( 2 ) pressing the output side slider  432  toward the second position by pressure oil to be supplied, a first supply/discharge line  460 ( 1 ) supplying/discharging pressure oil to the input side first oil chamber  450 ( 1 ) and the output side first oil chamber  452 ( 1 ), a second supply/discharge line  460 ( 2 ) supplying/discharging pressure oil to the input side second oil chamber  450 ( 2 ) and the output side second oil chamber  452 ( 2 ), and an electromagnetic valve  465 , the position of which is control by the control device  100 . 
     In the figure, the reference numeral  414  designates a biasing member pressing the input side slider  412  toward one side in the axial direction (first position in the example illustrated in the figure) and the reference numeral  434  designates a biasing member pressing the output side slider  432  toward one side in the axial direction (first position in the example illustrated in the figure). 
     The electromagnetic valve  465  is configured to be able to take a first position where the pressure oil supply/discharge line  155  is fluid-connected to the first supply/discharge line  460 ( 1 ) and the second supply/discharge line  460 ( 2 ) is fluid-connected to the drain line  157  and a second position where the first supply/discharge line  460 ( 1 ) is fluid-connected to the drain line  157  and the pressure oil supply/discharge line  155  is fluid-connected to the second supply/discharge line  460 ( 2 ). 
       FIG.  12    illustrates hydraulic pressure wave form charts of the first and second supply/discharge lines  460 ( 1 ) and  460 ( 2 ) in the switching from the first transmission state to the second transmission state. 
     The control device  100  locates the electromagnetic valve  465  at the first position in a state where the speed change operation member  90  is located between the zero speed position and the switching speed position (i.e., in a low speed state where the rotational speed of the speed change output shaft  45  is less than the switching speed). 
     In this state, the hydraulic pressure of the second supply/discharge line  460 ( 2 ) is released and pressure oil is supplied to the first supply/discharge line  460 ( 1 ), and thereby the input side slider  412  and the output side slider  432  are located at the first position. 
     Thus, while the input side second clutch mechanism and the output side second clutch mechanism are brought into the disengagement state, the input side first clutch mechanism and the output side first clutch mechanism are brought into the engagement state, so that the transmission structure  4  is brought into the first transmission state. 
     In a state where the speed change operation member  90  exceeds the switching speed position (in the high speed state where the rotational speed of the speed change output shaft  45  is equal to or higher than the switching speed), the control device  100  locates the electromagnetic valve  465  at the second position. 
     In this state, the hydraulic of the first supply/discharge line  460 ( 1 ) is released and pressure oil is supplied to the second supply/discharge line  460 ( 2 ), and thereby the input side slider  412  and the output side slider  432  are located at the second position. 
     Thus, while the input side first clutch mechanism and the output side first clutch mechanism are brought into the disengagement state, the input side second clutch mechanism and the output side second clutch mechanism are brought into the engagement state, so that the transmission structure is brought into the second transmission state. 
     Herein, when recognizing that the speed change operation member  90  is operated from the zero speed position side to the switching speed position at time Ta (see  FIG.  12   ) (i.e., when recognizing that the rotational speed of the speed change output shaft  45  reaches the switching speed from the state where the rotational speed is less than the switching speed based on a signal from the output sensor  95   b ), the control device  100  moves the electromagnetic valve  465  from the first position to the second position. 
     Thus, the input side slider  412  and the output side slider  432  are moved toward the second position from the first position where the input side slider  412  and the output side slider  432  are located at the time Ta, and then reach the second position at the time Tb. Then, the double transmission state is developed at the intermediate position in the middle of the movement. 
     Although both the input side clutch unit and the output side clutch unit are configured as the friction plate type in Embodiment 3 described above and both the input side clutch unit and the output side clutch unit are configured as the dog clutch type in Embodiment 4, the present invention is not limited to such configurations. 
     More specifically, one of the input side clutch unit and the output side clutch unit can be configured as the friction plate type and the other side can be configured as the dog clutch type. 
       FIG.  13    illustrates a hydraulic circuit diagram of a transmission structure  5  according to a modification in which the input side clutch unit is configured as the dog clutch type and the output side clutch unit is configured as the friction plate type. 
       FIG.  14    illustrate a hydraulic pressure waveform charts in the transmission structure  5  in the switching from the first transmission state to the second transmission state. 
     It is a matter of course that the configuration relating to the double transmission structure in Embodiment 3 and 4 described above is also applicable to Embodiment 2 described above. 
     Embodiment 5 
     Hereinafter, further yet still another embodiment of the transmission structure according to the present invention is described with reference to the accompanying drawings. 
     The transmission structure according to this embodiment is different from the transmission structure  3  according to Embodiment 3 described above only in the point that the position control timing of the electromagnetic valves  365 ( 1 ),  365 ( 2 ),  367 ( 1 ) and  367 ( 2 ) by the control device  100  in the switching between the first and second transmission states is changed. 
       FIG.  15    illustrates hydraulic pressure waveform charts of the supply/discharge lines  360 ( 1 ),  360 ( 2 ),  362 ( 1 ) and  362 ( 2 ) in the switching from the first transmission state to the second transmission state. 
     In the state where the speed change operation member  90  is located before the switching speed position (i.e., in a low speed state where the rotational speed of the speed change output shaft  45  is less than the switching speed), the control device  100  performs the same position control as that of Embodiment 3 described above to the electromagnetic valves  365 ( 1 ),  365 ( 2 ),  367 ( 1 ) and  367 ( 2 ). 
     More specifically, the control device  100  locates the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) at the supply position and locates the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ) at the discharge positions. 
     In this state, while the hydraulic pressure of the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) is released, so that the input side second clutch mechanism  60 ( 2 ) and the output side second clutch mechanism  80 ( 2 ) are brought into the disengagement state, the input side first supply/discharge line  360 ( 1 ) and the output side first supply/discharge line  362 ( 1 ) are maintained at the engagement hydraulic pressure set by the pilot pressure of the corresponding electromagnetic valves  365 ( 1 ) and  367 ( 1 ), so that the input side first clutch mechanism  60 ( 1 ) and the output side first clutch mechanism  80 ( 1 ) are brought into the engagement state as illustrated in  FIG.  15   . 
     Thus, the transmission structure is brought into the first transmission state. 
     Also in a state where the speed change operation member  90  exceeds the switching speed position (i.e., also in the high speed state where the rotational speed of the speed change output shaft  45  is equal to or higher than the switching speed), the control device  100  performs the same position control as that of Embodiment 3 described above to the electromagnetic valves  365 ( 1 ),  365 ( 2 ),  367 ( 1 ) and  367 ( 2 ). 
     More specifically, the control device  100  locates the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) at the discharge positions and locates the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ) at the supply positions. 
     In this state, while the hydraulic pressure of the input side first supply/discharge line  360 ( 1 ) and the output side first supply/discharge line  362 ( 1 ) is released, so that the input side first clutch mechanism  60 ( 1 ) and the output side first clutch mechanism  80 ( 1 ) are brought into the disengagement state, the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) are maintained at the engagement hydraulic pressure set by the pilot pressure of the corresponding electromagnetic valves  365 ( 2 ) and  367 ( 2 ), so that the input side second clutch mechanism  60 ( 2 ) and the output side second clutch mechanism  80 ( 2 ) are brought into the engagement state as illustrated in  FIG.  15   . 
     Thus, the transmission structure  3  is brought into the second transmission state. 
     Meanwhile, in the switching between the first and second transmission states, the control device  100  performs position control different from that of Embodiment 3 described above to the electromagnetic valves  365 ( 1 ),  365 ( 2 ),  367 ( 1 ) and  367 ( 2 ). 
     More specifically, when recognizing that the speed change operation member  90  is operated from the zero speed position side to the switching speed position at time Ta in  FIG.  15   . (i.e., when recognizing that the rotational speed of the speed change output shaft  45  reaches the switching speed from a state where the rotational speed is less than the switching speed based on a signal from the output sensor  95   b ), the control device  100  moves the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ) located at the discharge positions at the time before the switching to the supply positions from the discharge positions while maintaining the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) located at the supply positions at the time before switching the transmission state (at the time of the first transmission state in this example) at the supply positions. 
     Thus, while the input side first supply/discharge line  360 ( 1 ) and the output side first supply/discharge line  362 ( 1 ) are maintained at the engagement hydraulic pressure, the hydraulic pressure of the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) gradually increase to reach the engagement hydraulic pressure at time Tb. 
     Herein, when recognizing that the hydraulic pressure of the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) to which pressure oil is supplied through the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ), the positions of which are moved to the supply positions from the discharge positions, reaches switching hydraulic pressure P less than the engagement hydraulic pressure based on signals from the corresponding pressure sensors  370 ( 2 ) and  372 ( 2 ), the control device  100  moves the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) located at the supply positions at the time before the switching from the supply positions to the discharge positions. 
     The switching hydraulic pressure P is a hydraulic pressure at which the friction plate group of the corresponding clutch mechanism is brought into a sliding engagement state of performing power transmission while sliding. 
     The transmission structure according to this embodiment having such a configuration can prevent or reduce the generation of the state where the traveling driving force is not transmitted to the driving wheels  220  in the switching between the first and second transmission states as much as possible and further can effectively prevent or reduce a switching shock which may occur in the switching between the first and second transmission states. 
     More specifically, the input side first and second speed change ratios are set so that the rotational speed of the second element when the HST output is set to the second HST speed in the first transmission state and the rotational speed of the second element by the rotation power transmitted through the input side second transmission mechanism  50 ( 2 ) in the second transmission state are the same and so that the rotational speed of the first element when the HST output is set to the second HST speed in the second transmission state and the rotational speed of the first element by the rotation power transmitted through the input side first transmission mechanism  50 ( 1 ) in the first transmission state are the same. 
     Therefore, a rotational speed difference theoretically does not occur in the first element and/or the second element in the switching between the first and second transmission states. 
     However, a rotational speed difference may occur in the first element and/or the second element in the switching between the first and second transmission states due to a manufacturing error and the like. 
     With respect to this point, in this embodiment, even if a rotational speed difference occurs in the first element and/or the second element in the switching between the first and second transmission states, immediately before one of the clutch mechanisms (the input side second clutch mechanism  60 ( 2 ) in the example of  FIG.  15   ), to which pressure oil is supplied through the electromagnetic valve (the input side second electromagnetic valve  365 ( 2 ) in the example of  FIG.  15   ), the position of which is moved from the disengagement position to the supply position in the switching between the first and second transmission states, is brought into a perfect engagement state by way of processes in which the friction plate group of the clutch mechanism is gradually friction-engaged while sliding, and then the hydraulic pressure of the one clutch mechanism reaches the engagement hydraulic pressure, the hydraulic pressure of the other one of the clutch mechanisms (the input side first clutch mechanism  60 ( 1 ) in the example of  FIG.  15   ) brought into the engagement state before the switching between the first and second transmission states is released from the engagement hydraulic pressure. 
     Therefore, the generation of the state where the traveling driving force is not transmitted to the driving wheels  220  in the switching between the first and second transmission states can be prevented or reduced as much as possible and a damage on a transmission system due to the switching shock or the double transmission state which may occur in the switching between the first and second transmission states can be effectively prevented or reduced. 
     Moreover, the output side first and second speed change ratios are set so that the rotational speed developed in the speed change output shaft  45  when the HST output is set to the second HST speed is same in the first and second transmission states. 
     Therefore, a rotational speed difference theoretically does not occur in the speed change output shaft  45  in the switching between the first and second transmission states. 
     However, a rotational speed difference occurs in the speed change output shaft  45  in the switching between the first and second transmission states due to a manufacturing error and the like in some cases. 
     With respect to this point, in this embodiment, even if a rotational speed difference occurs in the speed change output shaft  45  in the switching between the first and second transmission states, immediately before one of the clutch mechanisms (the output side second clutch mechanism  80 ( 2 ) in the example of  FIG.  15   ), to which pressure oil is supplied through the electromagnetic valve (the output side second electromagnetic valve  367 ( 2 ) in the example of  FIG.  15   ), the position of which is moved from the disengagement position to the supply position in the switching between the first and second transmission states, is brought into a perfect engagement state by way of processes in which the friction plate group of the clutch mechanism is gradually friction-engaged while sliding, and then the hydraulic pressure of the one clutch mechanism reaches the engagement hydraulic pressure, the hydraulic pressure of the other one of the clutch mechanisms (the output side first clutch mechanism  80 ( 1 ) in the example of  FIG.  15   ) brought into the engagement state before the switching between the first and second transmission states is released from the engagement hydraulic pressure. 
     Therefore, the generation of the state where the traveling driving force is not transmitted to the driving wheels  220  in the switching between the first and second transmission states can be prevented or reduced as much as possible and a damage on the transmission system due to the switching shock or the double transmission state which may occur in the switching between the first and second transmission states can be effectively prevented or reduced. 
     The configuration in which, immediately before one of the clutch mechanisms is brought into a perfect engagement state from a disengagement state by way of processes in which the electromagnetic valves (the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ) in the example of  FIG.  15   ) located at the discharge positions at the time before the switching are moved from the discharge positions to the supply positions while maintaining the electromagnetic valves (the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) in the example of  FIG.  15   ) located at the supply positions at the time before the switching at the supply positions in the switching between the first and second transmission states, and then the hydraulic pressure of the supply/discharge lines (the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) in the example of  FIG.  15   ) to which pressure oil is supplied through the electromagnetic valves, the positions of which are moved from the discharge positions to the supply positions, reaches the engagement hydraulic pressure, the electromagnetic valves (the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) in the example of  FIG.  15   ) located at the supply positions at the time before the switching are moved from the supply positions to the discharge positions can be applied to only one of the input side clutch unit formed by the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) and the output side clutch unit formed by the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) in which a rotational speed difference may occur due to a manufacturing error and the like and the dog clutch type illustrated in  FIG.  10    described above can be adopted to the other clutch unit when the other clutch unit is free from the possibility or has less possibility, whereby a cost reduction can be achieved. 
     Embodiment 6 
     Hereinafter, further yet still another embodiment of the transmission structure according to the present invention is described with reference to the accompanying drawings. 
       FIG.  16    illustrates a hydraulic circuit diagram of a transmission structure  6  according to this embodiment. 
     In the figure, the same components as those in Embodiments described above are designated by the same reference numerals and a description thereof is omitted as appropriate. 
     In the transmission structure  1  according to Embodiment 1, the input side first speed change ratio of the input side first transmission mechanism  50 ( 1 ) and the input side second speed change ratio of the input side second transmission mechanism  50 ( 2 ) are set so that the rotational speed of the second element when the HST output is set to the second HST speed in the first transmission state and the rotational speed of the second element by the rotation power transmitted through the input side second transmission mechanism  50 ( 2 ) in the second transmission state are the same and so that the rotational speed of the first element when the HST output is set to the second HST speed in the second transmission state and the rotational speed of the first element by the rotation power transmitted through the input side first transmission mechanism  50 ( 1 ) in the first transmission state are the same, and further the output side first speed change ratio of the output side first transmission mechanism  70 ( 1 ) and the output side second speed change ratio of the output side second transmission mechanism  70 ( 2 ) are set so that the rotational speed developed in the speed change output shaft  45  when the HST output is set to the second HST speed is same in the first and second transmission states. 
     According to the transmission structure  1  according to Embodiment 1 described above, a rotational speed difference theoretically does not occur in the first element and/or the second element and the speed change output shaft  45  in the switching between the first and second transmission states. 
     However, the input side first and second speed change ratios cannot be set to the ideal set values described above due to the number of gear teeth configuring the input side first and second transmission mechanisms  50 ( 1 ) and  50 ( 2 ) in some cases. 
     In such a case, a rotational speed difference occurs in the first element and/or a rotational speed difference occurs in the second element in the switching between the first and second transmission states. 
     Similarly, the output side first and second speed change ratios cannot be set to the ideal set values described above due to the number of gear teeth configuring the output side first and second transmission mechanisms  70 ( 1 ) and  70 ( 2 ) in some cases. 
     In such a case, a rotational speed difference occurs in the speed change output shaft  45  in the switching between the first and second transmission states. 
     In view of this point, the transmission structure  6  according to this embodiment is configured so that, when a rotational speed difference occurs in the first element and/or the second element and the speed change output shaft  45  in the switching between the first and second transmission states, a damage on the transmission system due to the switching shock or the double transmission state resulting from the rotational speed difference can be prevented or reduced as much as possible. 
     Specifically, as illustrated in  FIG.  16   , the transmission structure  6  according to this embodiment has input side first and second transmission mechanisms  650 ( 1 ) and  650 ( 2 ) in place of the input side first and second transmission mechanisms  50 ( 1 ) and  50 ( 2 ) and output side first and second transmission mechanisms  670 ( 1 ) and  670 ( 2 ) in place of the output side first and second transmission mechanisms  70 ( 1 ) and  70 ( 2 ) as compared with the transmission structure  1  according to Embodiment 1 described above. 
     As illustrated in  FIG.  16   , the input side first transmission mechanism  650 ( 1 ) has an input side first driving gear  652 ( 1 ) relatively rotatably coupled with the main driving shaft  212  and an input side first driven gear  654 ( 1 ) meshed with the input side first driving gear  652 ( 1 ) and coupled with the first element. 
     The input side second transmission mechanism  650 ( 2 ) has an input side second driving gear  652 ( 2 ) relatively rotatably supported by the main driving shaft  212  and an input side second driven gear  654 ( 2 ) meshed with the input side second driving gear  652 ( 2 ) and coupled with the second element. 
     The output side first transmission mechanism  670 ( 1 ) has an output side first driving gear  672 ( 1 ) supported by the speed change intermediate shaft  43  so as not to be relatively rotatable and an output side first driven gear  674 ( 1 ) meshed with the output side first driving gear  672 ( 1 ) and relatively rotatably supported by the speed change output shaft  45 . 
     The output side second transmission mechanism  670 ( 2 ) has an output side second driving gear  672 ( 2 ) coupled with the first element and an output side second driven gear  674 ( 2 ) meshed with the output side second driving gear  672 ( 2 ) and relatively rotatably supported by the speed change output shaft  45 . 
       FIG.  17    illustrates hydraulic pressure waveform charts of the supply/discharge lines  360 ( 1 ),  360 ( 2 ),  362 ( 1 ) and  362 ( 2 ) in the switching from the first transmission state to the second transmission state. 
     In this embodiment, the control device  100  performs the same position control as that in Embodiment 5 described above to the electromagnetic valves  365 ( 1 ),  365 ( 2 ),  367 ( 1 ) and  367 ( 2 ) in the switching between the first and second transmission states. 
     More specifically, when recognizing that the rotational speed of the speed change output shaft  45  reaches the switching speed from the state where the rotational speed is less than the switching speed based on a signal from the output sensor  95   b  at time Ta in  FIG.  17   , the control device  100  moves the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ) located at the discharge positions at the time before the switching from the discharge positions to the supply positions while maintaining the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) located at the supply positions at the time before the switching the transmission state (at the time of the first transmission state in this example) at the supply positions. 
     Thus, as illustrated in  FIG.  17   , the hydraulic pressure of the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) gradually increases to reach the engagement hydraulic pressure at time Tb while the input side first supply/discharge line  360 ( 1 ) and the output side first supply/discharge line  362 ( 1 ) are maintained at the engagement hydraulic pressure. 
     Herein, when recognizing that the hydraulic pressure of the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) to which pressure oil is supplied through the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ), the positions of which are moved to the supply positions from the discharge positions, reaches the switching hydraulic pressure P less than the engagement hydraulic pressure based on signals from the corresponding pressure sensors  370 ( 2 ) and  372 ( 2 ), the control device  100  moves the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) located at the supply positions at the time before the switching from the supply positions to the discharge positions. 
     According to the transmission structure  6  having such a configuration, even if a rotational speed difference occurs in the first element and/or the second element and the speed change output shaft  45  in the switching between the first and second transmission states, immediately before the clutch mechanisms are brought into a perfect engagement state from a disengagement state by way of processes in which the friction plate groups of the clutch mechanisms (the input side second clutch mechanism  60 ( 2 ) and the output side second clutch mechanism  80 ( 2 ) in the example of  FIG.  17   ) to which pressure oil is supplied through the electromagnetic valves (the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ) in the example of  FIG.  17   ), the positions of which are moved from the disengagement positions to the supply positions in the switching between the first and second transmission states, are gradually friction-engaged while sliding, and then the hydraulic pressure of the clutch mechanisms reaches the engagement hydraulic pressure, the clutch mechanisms (the input side first clutch mechanism  60 ( 1 ) and the output side first clutch mechanism  80 ( 1 ) in the example of  FIG.  17   ) brought into the engagement state before the switching between the first and second transmission states are released from the engagement hydraulic pressure. 
     Therefore, the generation of the state where the traveling driving force is not transmitted to the driving wheels  220  in the switching between the first and second transmission states can be prevented or reduced as much as possible and the switching shock or a damage on the transmission system which may occur in the switching between the first and second transmission states can be effectively prevented or reduced. 
     In this embodiment, the configuration where, when recognizing that, while one electromagnetic valve located at the supply position at the time before the switching is maintained at the supply position, the other electromagnetic valve located at the discharge position at the time before the switching is moved from the discharge position to the supply position in the switching between the first and second transmission states, and then the hydraulic pressure of a supply/discharge line to which pressure oil is supplied through the other electromagnetic valve reaches the switching hydraulic pressure P lower than the engagement hydraulic pressure based on a signal from the corresponding pressure sensor, the one electromagnetic valve is moved from the supply position to the discharge position is applied to both the input side clutch unit formed by the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) and the output side clutch unit formed by the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ). However, it is a matter of course that the present invention is not limited to such an aspect, and the above-described configuration can be applied to only one of the input side clutch unit and the output side clutch unit in which a rotational speed difference occurs due to the setting of the number of gear teeth configuring the transmission mechanisms and the like and the dog clutch type illustrated in  FIG.  10    described above can be adopted to the other clutch unit when the other clutch unit is free from the possibility, whereby a cost reduction can be achieved. 
     Moreover, this embodiment is also applicable to the transmission structure  2  according to Embodiment 2. 
     In Embodiments 5 and 6 described above, the hydraulic pressure of the clutch mechanism brought into the engagement state at the time before the switching (hereinafter referred to as “engaged clutch mechanism before the switching”) is lowered at a substantially fixed rate to be released from the engagement hydraulic pressure in the switching between the first and second transmission states. In place of the configuration, as illustrates in  FIG.  18   , it is possible to lower the hydraulic pressure of the engaged clutch mechanism before the switching at a substantially fixed rate in response to the fact that the hydraulic pressure of the clutch mechanism brought into the disengagement state at the time before the switching (hereafter referred to as “disengaged clutch mechanism before the switching) reaches the switching hydraulic pressure P and then lower the hydraulic pressure of the engaged clutch mechanism before the switching to the release hydraulic pressure at once at the time when the hydraulic pressure of the disengaged clutch mechanism before the switching reaches the engagement hydraulic pressure, whereby unnecessary sliding transmission state time can be reduced to improve durability of the friction plate. 
     Also in Embodiments 5 and 6 described above, in place of the configuration of detecting the engagement state of the corresponding friction plate clutch mechanisms by the pressure sensors  370 ( 1 ),  370 ( 2 ),  372 ( 1 ) and  372 ( 2 ), a configuration of detecting the engagement state of the corresponding friction plate clutch mechanisms by other clutch engagement detection units detecting a supply current value, supply current time, and the like of the proportional electromagnetic valves  365 ( 1 ),  365 ( 2 ),  367 ( 1 ) and  367 ( 2 ) can also be adopted. 
     Embodiment 7 
     Hereinafter, further yet still another embodiment of the transmission structure according to the present invention is described with reference to the accompanying drawings. 
       FIG.  19    illustrates a transmission schematic view of a working vehicle  202  to which a transmission structure  7  according to this embodiment is applied. 
       FIG.  20    illustrates a hydraulic circuit diagram of the transmission structure  7  according to this embodiment. 
     In the figure, the same components as those in Embodiments described above are designated by the same reference numerals and a description thereof is omitted as appropriate. 
     The transmission structure  1  according to Embodiment 1 is configured so that normal and reverse switching of driving force is performed by the forward/reverse movement switching mechanism  230  disposed on the downstream side in the transmission direction relative to the speed change output shaft  45 . 
     More specifically, as illustrated in  FIG.  1   , the transmission structure  1  according to Embodiment 1 has the forward/reverse movement switching mechanism  230  switching the rotation direction of the driving force between the forward movement direction and the reverse movement direction between the speed change output shaft  45  and the traveling transmission shaft  235  operatively rotationally driven by the rotation power of the speed change output shaft  45 . 
     In contrast thereto, the transmission structure  7  according to this embodiment is configured to be able to switch the rotation direction of the driving force transmitted to the speed change output shaft  45  between the normal direction and the reverse direction. 
     Specifically, as illustrated in  FIG.  19   , the transmission structure  7  has the HST  10 , the planetary gear mechanism  30 , the input side first and second transmission mechanisms  50 ( 1 ) and  50 ( 2 ), the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ), the output side first transmission mechanism  70 ( 1 ) (forward movement first transmission mechanism) capable of operatively transmitting the rotation power of the second element to the speed change output shaft  45  in the normal rotation state, the output side second transmission mechanism  70 ( 2 ) (forward movement second transmission mechanism) capable of operatively transmitting the rotation power of the first element to the speed change output shaft  45  in the normal rotation state, a reverse movement transmission mechanism  70 (R) capable of operatively transmitting the rotation power of the second element to the speed change output shaft  45  in the reverse rotation state, the output side first clutch mechanism  80 ( 1 ) (forward movement first clutch mechanism), the output side second clutch mechanism  80 ( 2 ), and a reverse movement clutch mechanism  80 (R) engaging/disengaging the power transmission of the output side first transmission mechanism  70 ( 1 ) (forward movement first transmission mechanism), the output side second transmission mechanism  70 ( 2 ) (forward movement second transmission mechanism), and the reverse movement transmission mechanism  70 (R), respectively, the speed change operation member  90 , the HST sensor  95   a,  the output sensor  95   b,  and the control device  100 . 
     The reverse movement transmission mechanism  70 (R) has a reverse movement driving gear  72 (R) supported by the speed change intermediate shaft  43  so as not to be relatively rotatable, a reverse movement driven gear  74 (R) relatively rotatably supported by the speed change output shaft  45 , and a reverse movement idle gear  73 (R) meshed with the reverse movement driving gear  72 (R) and the reverse movement driven gear  74 (R). 
     The reverse movement clutch mechanism  80 (R) has a reverse movement clutch housing  82 (R) supported by the speed change output shaft  45  so as not to be relatively rotatable, a reverse movement friction plate group  84 (R) containing a reverse movement driving side friction plate supported by the reverse movement driven gear  74 (R) so as not to be relatively rotatable and a reverse movement driven side friction plate supported by the reverse movement clutch housing  82 (R) so as not to be relatively rotatable in a state of being opposed to the reverse movement driving side friction plate, and a reverse movement piston (not illustrated) frictionally-engaging the reverse movement friction plate group  84 (R). 
     The reference numeral  105  in  FIG.  19    designates an electromagnetic valve unit containing the input side first electromagnetic valve  365 ( 1 ) and the like. 
     The reference numeral  242  in  FIG.  19    is a sub speed change mechanism containing the friction plate clutch mechanism and is provided in place of the sub speed change mechanism  240  containing the dog clutch type clutch mechanism in the transmission structure  1  according to Embodiment 1. 
     The control device  100 
     develops a forward movement first transmission state where the input side first clutch mechanism  60 ( 1 ) and the output side first clutch mechanism  80 ( 1 ) are brought into the engagement state in a state where the speed change operation member  90  is located between the zero speed position and the switching speed position (i.e., in a low speed state where the rotational speed of the speed change output shaft  45  is from the zero speed to speed less than the switching speed in the forward movement direction based on detection signals of the HST sensor  95   a  and the output sensor  95   b ) develops a forward movement second transmission state where the input side second clutch mechanism  60 ( 2 ) and the output side second clutch mechanism  80 ( 2 ) are brought into the engagement state in a state where the speed change operation member  90  is operated beyond the switching speed position (i.e., in a high speed state where the rotational speed of the speed change output shaft  45  is equal to or higher than the switching speed in the forward movement direction), and   develops the reverse movement transmission state where the input side first clutch mechanism  60 ( 1 ) and the reverse movement clutch mechanism  80 (R) are brought into the engagement state in a state where the speed change operation member  90  is operated from the zero speed position to the reverse movement side (i.e., in a reverse movement transmission state where the rotational speed of the speed change output shaft  45  changes from the zero speed to the reverse movement side).   

       FIGS.  21 A and  21 B  illustrate graphs illustrating the relationship between the traveling vehicle speed and the HST output in the working vehicle  202  to which the transmission structure  7  according to this embodiment is applied. 
       FIGS.  21 A and  21 B  illustrate graphs in states where the sub speed change mechanism  242  is engaged with a low speed stage and a high speed stage, respectively. 
     As illustrated in  FIGS.  21 A and  21 B , the control device  100  operates the output adjustment member  20  so that the HST output is speed-changed from the first HST speed toward the second HST speed in response to a forward movement side acceleration operation of the speed change operation member  90  in the forward movement first transmission state. 
     More specifically, when the speed change operation member  90  is operated between the zero speed position and the forward movement side switching speed position, the control device  100  develops the forward movement first transmission state, and then operates the output adjustment member  20  so that the HST output is speed-changed from the side of the first HST speed to the side of the second HST speed as the acceleration operation of the speed change operation member  90  is performed from the zero speed position side to the forward movement side switching speed position. 
     In the forward movement second transmission state, the control device  100  operates the output adjustment member  20  so that the HST output is speed-changed from the second HST speed toward the first HST speed in response to the forward movement side acceleration operation of the speed change operation member  90 . 
     More specifically, when recognizing that the speed change operation member  90  is operated from the zero speed position side to the forward movement side switching speed position, the control device  100  performs switching from the forward movement first transmission state to the forward movement second transmission state, and develops the forward movement second transmission state when the speed change operation member  90  is located on the forward movement high speed side relative to the forward movement side switching speed position, and then operates the output adjustment member  20  so that the HST output is speed-changed from the second HST speed toward the first HST speed in response to the forward movement side acceleration operation of the speed change operation member  90 . 
     In the reverse movement transmission state, the control device  100  operates the output adjustment member  20  so that the HST output is speed-changed from the first HST speed toward the second HST speed in response to the reverse movement side acceleration operation of the speed change operation member  90 . 
     More specifically, when the speed change operation member  90  is operated from the zero speed position to the reverse movement side, the control device  100  develops the reverse movement transmission state, and then operates the output adjustment member  20  so that the HST output is speed-changed from the side of the first HST speed to the side of the second HST speed as a reverse movement acceleration operation of the speed change operation member  90  is performed. 
     As illustrated in  FIG.  21   , this embodiment is also configured so that a speed difference does not occur in the traveling speed (i.e., the speed change output shaft  45 ) in the switching between the forward movement side first and second transmission states in the same manner as in Embodiment 1. 
     Specifically, the speed change ratio (input side first speed change ratio) of the input side first transmission mechanism  50 ( 1 ) and the speed change ratio (input side second speed change ratio) of the input side second transmission mechanism  50 ( 2 ) are set so that the rotational speed of the second element when the HST output is set to the second HST speed in the forward movement first transmission state and the rotational speed of the second element by the rotation power transmitted through the input side second transmission mechanism  50 ( 2 ) in the forward movement second transmission state are the same and so that the rotational speed of the first element when the HST output is set to the second HST speed in the forward movement second transmission state and the rotational speed of the first element by the rotation power transmitted through the input side first transmission mechanism  50 ( 1 ) in the forward movement first transmission state are the same. 
     The speed change ratio (forward movement first speed change ratio) of the output side first transmission mechanism  70 ( 1 ) (forward movement first transmission mechanism) and the speed change ratio (forward movement second speed change ratio) of the output side second transmission mechanism  70 ( 2 ) (forward movement second transmission mechanism) are set so that the rotational speed developed in the speed change output shaft  45  when the HST output is set to the second HST speed is same in the first and second transmission states. 
     According to such a configuration, the occurrence of a rotational speed difference in the speed change output shaft  45  in the switching between the forward movement side first and second transmission states, i.e., the occurrence of a traveling speed difference, can be effectively prevented or reduced. 
     The HST  10  and the planetary gear mechanism  30  are set so that, when the HST output is set to the first HST speed in the engagement state of the input side first clutch mechanism  60 ( 1 ), the rotational speed of the second element becomes the zero speed. 
     According to such a configuration, the forward/reverse movement switching of a vehicle can be smoothly performed. In particular, the configuration is effective in the case of a working vehicle performing work frequently requiring forward/reverse movement switching. 
     Next, the pressure oil supply/discharge configuration of the transmission structure  7  is described. 
     As illustrated in  FIG.  20   , the transmission structure  7  has the pressure oil supply line  155 , the input side first supply/discharge line  360 ( 1 ), the input side second supply/discharge line  360 ( 2 ), the output side first supply/discharge line  362 ( 1 ) (forward movement side first supply/discharge line), the output side second supply/discharge line  362 ( 2 ) (forward movement side second supply/discharge line), a reverse movement supply/discharge line  364  supplying/discharging pressure oil to the reverse movement clutch mechanism  80 (R), a forward movement supply line  310 (F), a reverse movement supply line  310 (R), the input side first electromagnetic valve  365 ( 1 ), the input side second electromagnetic valve  365 ( 2 ), the output side first electromagnetic valve  367 ( 1 ), and the output side second electromagnetic valve  367 ( 2 ), the input side first pressure sensor  370 ( 1 ) and the input side second pressure sensor  370 ( 2 ), the output side first pressure sensor  372 ( 1 ) and the output side second pressure sensor  372 ( 2 ), a check valve  320  interposed in the input side first supply/discharge line  360 ( 1 ), a pilot valve  330  interposed in the reverse movement supply line  310 (R), the drain line  157 , and a forward/reverse movement switching electromagnetic valve  300 . 
     In this embodiment, the input side first electromagnetic valve  365 ( 1 ), the input side second electromagnetic valve  365 ( 2 ), the output side first electromagnetic valve  367 ( 1 ) and the output side second electromagnetic valve  367 ( 2 ) are interposed between the forward movement supply line  310 (F) and the input side first supply/discharge line  360 ( 1 ), the input side second supply/discharge line  360 ( 2 ), the output side first supply/discharge line  362 ( 1 ) (forward movement side first supply/discharge line) and the output side second supply/discharge line  362 ( 2 ) (forward movement side second supply/discharge line), respectively, and configured to drain the corresponding supply/discharge lines  360 ( 1 ),  360 ( 2 ),  362 ( 1 ) and  362 ( 2 ) when located at the discharge positions and meanwhile, fluid-connect the corresponding supply/discharge lines  360 ( 1 ),  360 ( 2 ),  362 ( 1 ) and  362 ( 2 ) to the forward movement supply line  310 (F) when located at the supply positions. 
     The position of the forward/reverse movement switching electromagnetic valve  300  is controlled by the control device  100  so as to be able to take a forward movement position F where the forward movement supply line  310 (F) is fluid-connected to the pressure oil supply line  155  and the reverse movement supply line  310 (R) is fluid-connected to the drain line  157 , a reverse movement position R where the reverse movement supply line  310 (R) is fluid-connected to the pressure oil supply line  155  and the forward movement supply line  310 (F) is fluid-connected to the drain line  157 , and a neutral position N where the pressure oil supply line  155 , the forward movement supply line  310 (F), and the reverse movement supply line  310 (R) are fluid-connected to the drain line  157 . 
     The check valve  320  is interposed in the input side first supply/discharge line  360 ( 1 ) to, while permitting that pressure oil supplied from the forward movement supply line  310 (F) through the input side first electromagnetic valve  365 ( 1 ) to flow toward the input side first clutch mechanism  60 ( 1 ) in a pressure oil supply direction, prevent the flow in a pressure oil discharge direction opposite thereto. 
     In the reverse movement supply line  310 (R), the upstream side close to the hydraulic source  150  is fluid-connected to a secondary side of the forward/reverse movement switching electromagnetic valve  300  and the downstream side on the side opposite to the hydraulic source  150  is fluid-connected to the input side first supply/discharge line  360 ( 1 ) on the downstream side in the pressure oil supply direction relative to the check valve  320 . 
     The pilot valve  330  is configured to be able to selectively take a communication position where the reverse movement supply line  310 (R) is made to communicate and a check position where, while the flow of the pressure oil of the reverse movement supply line  310 (R) in the pressure oil supply direction is permitted, the reverse flow is prevented. 
     The pilot valve  330  is configured to use the hydraulic pressure of the forward movement supply line  310 (F) as pilot pressure while being energized toward the communication position by a biasing member  332  and to be located at the check position against the pressing force of the biasing member  332  when pressure oil is supplied to the forward movement supply line  310 (F). 
     In the reverse movement supply/discharge line  364 , the upstream side is fluid-connected to the reverse movement supply line  310 (R) on the upstream side in the pressure oil supply direction relative to the pilot valve  330  and the downstream side is fluid-connected to the reverse movement clutch mechanism  80 (R). 
     The pressure oil supply/discharge configuration of the transmission structure  7  operates as follows. 
     When the speed change operation member  90  is located at the zero speed position, the control device  100  locates the forward/reverse movement switching electromagnetic valve  300  at the neutral position. 
     In this state, the input side first supply/discharge line  360 ( 1 ), the input side second supply/discharge line  360 ( 2 ), the output side first supply/discharge line  362 ( 1 ), the output side second supply/discharge line  362 ( 2 ), and the reverse movement supply/discharge line  364  are all opened and all the clutch mechanisms  60 ( 1 ),  60 ( 2 ),  80 ( 1 ),  80 ( 2 ), and  80 (R) are brought into the disengagement state, so that power is not transmitted to the speed change output shaft  45 . 
     When the speed change operation member  90  is operated to the forward movement side, the control device  100  locates the forward/reverse movement switching electromagnetic valve  300  at the forward movement position F before the HST  10  outputs the first HST speed. 
     Thus, the reverse movement supply line  310 (R) is fluid-connected to the drain line  157  and the forward movement supply line  310 (F) is fluid-connected the pressure oil supply line  155 . 
     At this time, the pilot valve  330  is located at the check position by the hydraulic pressure of the supply line in forward  310  (F). Therefore, the input side first supply/discharge line  360 ( 1 ) is brought into a state where the hydraulic pressure is held. 
     When the speed change operation member  90  is operated from the zero speed position to the forward movement side switching speed position, the control device  100  locates the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) at the supply positions. 
     Thus, pressure oil flows into the input side first supply/discharge line  360 ( 1 ) and the output side first supply/discharge line  362 ( 1 ) from the forward movement supply line  310 (F), so that the forward movement first transmission state where the input side first clutch mechanism  60 ( 1 ) and the output side first clutch mechanism  80 ( 1 ) are brought into the engagement state is developed. 
     At this time, the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) are drained by the corresponding electromagnetic valves  365 ( 2 ) and  367 ( 2 ) and the reverse movement supply/discharge line  364  is drained through the reverse movement supply line  310 (R) and the forward/reverse movement switching electromagnetic valve  300 . 
     When the speed change operation member  90  is operated to the forward movement high speed side beyond the forward movement side switching speed position, the control device  100  locates the input side second electromagnetic valve  365 ( 2 ) and the output side second electromagnetic valve  367 ( 2 ) at the supply positions while locating the input side first electromagnetic valve  365 ( 1 ) and the output side first electromagnetic valve  367 ( 1 ) at the discharge positions before the HST  10  outputs the second HST speed. 
     Thus, pressure oil flows into the input side second supply/discharge line  360 ( 2 ) and the output side second supply/discharge line  362 ( 2 ) from the forward movement supply line  310 (F), so that the forward movement second transmission state is developed where the input side second clutch mechanism  60 ( 2 ) and the output side second clutch mechanism  80 ( 2 ) are brought into the engagement state. 
     At this time, the input side first supply/discharge line  360 ( 1 ) and the output side first supply/discharge line  362 ( 1 ) are drained through the corresponding electromagnetic valves  365 ( 1 ) and  367 ( 1 ) and the reverse movement supply/discharge line  364  is drained through the reverse movement supply line  310 (R) and the forward/reverse movement switching electromagnetic valve  300 . 
     When the speed change operation member  90  is operated to the reverse movement side, the control device  100  locates the forward/reverse movement switching electromagnetic valve  300  at the reverse movement position R before the HST  10  outputs the first HST speed. 
     Thus, the forward movement supply line  310 (F) is fluid-connected to the drain line  157  and the reverse movement supply line  310 (R) is fluid-connected to the pressure oil supply line  155 . 
     At this time, the input side first electromagnetic valve  365 ( 1 ), the input side second electromagnetic valve  365 ( 2 ), the output side first electromagnetic valve  367 ( 1 ), and the output side second electromagnetic valve  367 ( 2 ) are all located at the discharge positions. 
     Therefore, the input side second supply/discharge line  360 ( 2 ), the output side first supply/discharge line  362 ( 1 ), and the output side second supply/discharge line  362 ( 2 ) are opened by the corresponding electromagnetic valves  365 ( 2 ),  367 ( 1 ), and  367 ( 2 ), respectively. 
     Meanwhile, the input side first supply/discharge line  360 ( 1 ) is fluid-connected to the reverse movement supply line  310 (R) on the downstream side in the pressure oil supply direction relative to the check valve  320 . 
     Therefore, although the input side first electromagnetic valve  365 ( 1 ) is located at the discharge position, pressure oil is supplied to the input side first supply/discharge line  360 ( 1 ) through the reverse movement supply line  310 (R), so that the input side first clutch mechanism  60 ( 1 ) is brought into the engagement state. 
     At this time, the forward movement supply line  310 (F) is opened, and therefore the pilot valve  330  using the hydraulic pressure of the forward movement supply line  310 (F) as pilot pressure is located at the communication position by the pressing force of the biasing member  332 . 
     Therefore, pressure oil is effectively supplied to the input side first supply/discharge line  360 ( 1 ) through the reverse movement supply line  310 (R). 
     As described above, the reverse movement supply/discharge line  364  is fluid-connected to the reverse movement supply line  310 (R) on the upstream side in the pressure oil supply direction relative to the pilot valve  300  and receives the pressure oil supply from the reverse movement supply line  310 (R). 
     Thus, the reverse movement transmission state is developed where the input side first clutch mechanism  60 ( 1 ) is brought into the engagement state and the reverse movement clutch mechanism  80 (R) is brought into the engagement state. 
     With respect to the switching control timing of the input side first electromagnetic valve  365 ( 1 ) and the input side second electromagnetic valve  365 ( 2 ) and the switching control timing of the output side first electromagnetic valve  367 ( 1 ) and the output side second electromagnetic valve  367 ( 2 ) in the switching between the forward movement side first and second transmission states, various embodiments, such as Embodiment 3 described above, Embodiment 5 described above, and Embodiment 6 described above, are applicable. 
     As illustrated in  FIG.  20   , in this embodiment, the input side first clutch mechanism  60 ( 1 ), the input side second clutch mechanism  60 ( 2 ), the output side first clutch mechanism  80 ( 1 ), the output side second clutch mechanism  80 ( 2 ), and the reverse movement clutch mechanism  80 (R) are all configured as a hydraulic friction plate type. 
     In place of the configuration, at least one of the input side clutch unit formed by the input side first and second clutch mechanisms and the output side clutch unit formed by the output side first and second clutch mechanisms can be configured as the dog clutch type in Embodiment 4 described above. 
       FIG.  22    illustrates a hydraulic circuit diagram of a transmission structure  7 B according to a modification of this embodiment. 
     In the figure, the same members as those in Embodiments described above are designated by the same reference numerals and a description thereof is omitted as appropriate. 
     The transmission structure  7 B has the input side clutch unit  410  of the dog clutch type in place of the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) of the friction plate type as compared with the transmission structure  7  according to this embodiment. 
     It is a matter of course that the output side clutch unit  430  of the dog clutch type can be provided in place of the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) and the clutch mechanism of the dog clutch type can also be provided in place of the reverse movement clutch mechanism  80 (R). 
     Embodiment 8 
     Hereinafter, further yet still another embodiment of the transmission structure according to the present invention is described with reference to the accompanying drawings. 
       FIG.  23    illustrates a transmission schematic view of a working vehicle  203  to which a transmission structure  8  according to this embodiment is applied. 
       FIG.  24    illustrates a partial vertical cross-sectional side view of the working vehicle  203 . 
     In the figures, the same components as those in Embodiments described above are designated by the same reference numerals and a description thereof is omitted as appropriate. 
     As illustrated in  FIG.  23   , the transmission structure  8  according to this embodiment is further provided with an output side third transmission mechanism  70 ( 3 ) and an output side third clutch mechanism  80 ( 3 ) as compared with the transmission structure  1  according to Embodiment 1 described above. 
     More specifically, the transmission structure  8  is provided with the HST  10 , the planetary gear mechanism  30 , the speed change output shaft  45 , the input side first and second transmission mechanisms  50 ( 1 ) and  50 ( 2 ), the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ), the output side first to third transmission mechanisms  70 ( 1 ) to  70 ( 3 ), the output side first to third clutch mechanisms  80 ( 1 ) to  80 ( 3 ), the speed change operation member  90 , the HST sensor  95   a,  the output sensor  95   b,  and the control device  100 . 
     In this embodiment, the clutch mechanisms  60 ( 1 ),  60 ( 2 ), and  80 ( 1 ) to  80 ( 3 ) are configured as hydraulic friction plate clutch units. 
     The output side third transmission mechanism  70 ( 3 ) is configured to be able to transmit the rotation power of the first element (the internal gear  36  in this embodiment) to the speed change output shaft  45  at an output side third speed change ratio where the speed change output shaft  45  is rotated at rotational speed higher than the rotational speed at the output side second speed change ratio of the output side second transmission mechanism  70 ( 2 ). 
     The output side third clutch mechanism  80 ( 3 ) is configured to engage/disengage the power of the output side third transmission mechanism  70 ( 3 ). 
       FIG.  25    illustrates a graph illustrating the relationship between the traveling vehicle speed and the HST output in the working vehicle  203  to which the transmission structure  8  according to this embodiment is applied. 
     As illustrated in  FIG.  25   , in this embodiment, the control device  100  develops a first transmission state where, while the input side first clutch mechanism  60 ( 1 ) is brought into the engagement state and the input side second clutch mechanism  60 ( 2 ) is brought into the disengagement state, the output side first clutch mechanism  80 ( 1 ) is brought into the engagement state and the remaining output side second and third clutch mechanisms  80 ( 2 ) and  80 ( 3 ) are brought into the disengagement state in a state where the speed change operation member  90  is located between the zero speed position and the first switching speed position (i.e., in a low speed state where the rotational speed of the speed change output shaft  45  is from the zero speed to speed less than the first switching speed based on detection signals of the HST sensor  95   a  and the output sensor  95   b ),
     develops a second transmission state where, while the input side first clutch mechanism  60 ( 1 ) is brought into the disengagement state and the input side second clutch mechanism  60 ( 2 ) is brought into the engagement state, the output side second clutch mechanism  80 ( 2 ) is brought into the engagement state and the remaining output side first and third clutch mechanisms  80 ( 1 ) and  80 ( 3 ) are brought into the disengagement state in a state where the speed change operation member  90  is located between the first switching speed position and the second switching speed position. (i.e., in the intermediate speed state where the rotational speed of the speed change output shaft  45  is from the first switching speed to the second switching speed based on detection signals of the HST sensor  95   a  and the output sensor  95   b ), and   develops a third transmission state where, while the input side first clutch mechanism  60 ( 1 ) is brought into the disengagement state and the input side second clutch mechanism  60 ( 2 ) is brought into the engagement state, the output side third clutch mechanism  80 ( 3 ) is brought into the engagement state and the remaining output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) are brought into the disengagement state in a state where the speed change operation member  90  is operated beyond the second switching speed position (i.e., in a high speed state where the rotational speed of the speed change output shaft  45  exceeds the second switching speed based on detection signals of the HST sensor  95   a  and the output sensor  95   b ).   

     As illustrated in  FIG.  23   , the transmission structure  8  has the forward/reverse movement switching mechanism  230  interposed between the speed change output shaft  45  and the traveling transmission shaft  235 , in which the output of the forward/reverse movement switching mechanism  230  is operatively transmitted to the traveling transmission shaft  235 . 
     Then, the control device  100  brings the forward/reverse movement switching mechanism  230  into a forward movement transmission state and a reverse movement transmission state in response to an operation to the forward movement side and the reverse movement side of the speed change operation member  90 , respectively. 
     More specifically, in the working vehicle  203 , when the speed change operation member  90  is operated between the zero speed position and a forward movement side first switching speed position, the forward movement first transmission state is developed, and, when the speed change operation member  90  is located at the forward movement side first switching speed position, the traveling transmission shaft  235  rotates at rotational speed setting the traveling vehicle speed to +a. 
     When the speed change operation member  90  is operated between the forward movement side first switching speed position and a forward movement side second switching speed position, the forward movement second transmission state is developed and, when the speed change operation member  90  is located at the forward movement side second switching speed position, the traveling transmission shaft  235  rotates at rotational speed setting the traveling vehicle speed to +b. 
     Then, when the speed change operation member  90  is operated beyond the forward movement side second switching speed position, a forward movement third transmission state is developed and, when the speed change operation member  90  is located at a forward movement side maximum speed position, the traveling transmission shaft  235  rotates at rotational speed setting the traveling vehicle speed to +c. 
     Similarly, when the speed change operation member  90  is operated between the zero speed position and a reverse movement side first switching speed position, a reverse first transmission state is developed and, when the speed change operation member  90  is located at the reverse movement side first switching speed position, the traveling transmission shaft  235  rotates at rotational speed setting the traveling vehicle speed to −a. 
     When the speed change operation member  90  is operated between the reverse movement side first switching speed position and a reverse movement side second switching speed position, a reverse movement second transmission state is developed and, when the speed change operation member  90  is located at the reverse movement side second switching speed position, the traveling transmission shaft  235  rotates at rotational speed setting the traveling vehicle speed to −b. 
     Then, when the speed change operation member  90  is operated beyond the reverse movement side second switching speed position, a reverse movement third transmission state is developed and, when the speed change operation member  90  is located at a reverse movement side maximum speed position, the traveling transmission shaft  235  rotates at rotational speed setting the traveling vehicle speed to −c. 
     In the same manner as in Embodiment 1 described above, the input side first speed change ratio of the input side first transmission mechanism  50 ( 1 ) and the input side second speed change ratio of the input side second transmission mechanism  50 ( 2 ) are set so that the rotational speed of the second element (the carrier  38  in this embodiment) when the HST output is set to the second HST speed in the first transmission state and the rotational speed of the second element by the rotation power transmitted through the input side second transmission mechanism  50 ( 2 ) in the second transmission state are the same and so that the rotational speed of the first element (the internal gear  36  in this embodiment) when the HST output is set to the second HST speed in the second transmission state and the rotational speed of the first element by the rotation power transmitted through the input side first transmission mechanism  50 ( 2 ) in the first transmission state are the same. 
     As illustrated in  FIG.  25   , the output side first speed change ratio of the output side first transmission mechanism  70 ( 1 ) and the output side second speed change ratio of the output side second transmission mechanism  70 ( 2 ) are set so that the rotational speed developed in the speed change output shaft  45  when the HST output is set to the second HST speed is same in the first and second transmission states. 
     As illustrated in  FIG.  25   , the transmission structure  8  according to this embodiment is configured so that the control device  100  operates the output adjustment member  20  so that the rotational speed developed in the speed change output shaft  45  in a transmission state after the switching coincides with or approaches the rotational speed developed in the speed change output shaft  45  in a transmission state before the switching in the switching between the second and third transmission states. 
     More specifically, when the speed change operation member  90  is operated in the acceleration direction under the second transmission state to reach the second switching speed position (when the rotational speed of the speed change output shaft  45  reaches the second switching speed), while the control device  100  shifts the output side second clutch mechanism  80 ( 2 ) from the engagement state to the disengagement state and shifts the output side third clutch mechanism  80 ( 3 ) from the disengagement state to the engagement state, the control device  100  operates the output adjustment member  20  so that the output of the HST  10  is speed-changed from the rotational speed (first HST speed) rotating the speed change output shaft  45  at the second switching speed under the second transmission state to the rotational speed (third HST speed of  FIG.  25   ) rotating the speed change output shaft  45  at the second switching speed or speed around the second switching speed under the third transmission state. 
     When the speed change operation member  90  is operated in a deceleration direction under the third transmission state to reach the second switching speed position (when the rotational speed of the speed change output shaft  45  reaches the second switching speed), while the control device  100  shifts the output side third clutch mechanism  80 ( 3 ) to the disengagement state from the engagement state and shifts the output side second clutch mechanism  80 ( 2 ) to the engagement state from an disengagement state, the control device  100  operates the output adjustment member  20  so that the output of the HST  10  is speed-changed from the rotational speed (third HST speed) rotating the speed change output shaft  45  at second switching speed under the third transmission state to the rotational speed (first HST speed) rotating the speed change output shaft  45  at the second switching speed or speed around the second switching speed under the second transmission state. 
     The transmission structure  8  having such a configuration can extend the speed changeable range (speed change region) while obtaining the same effects as those of the transmission structure according to Embodiment 1 described above. 
     In this embodiment, the output side first and second speed change ratios are set so that the rotational speed developed in the speed change output shaft  45  when the HST output is set to the second HST speed is same in the first and second transmission states as described above. However, in place of the configuration, a configuration may be acceptable in which the control device  100  operates the output adjustment member  20  so that, in the switching between the first and second transmission states, the rotational speed developed in the speed change output shaft  45  in a transmission state after the switching coincides with or approaches the rotational speed developed in the speed change output shaft  45  in the transmission state before the switching. 
     As illustrated in  FIG.  23    and  FIG.  24   , the transmission structure  8  according to this embodiment has a speed change transmission shaft  44  externally inserted in a relatively rotatable manner into the speed change intermediate shaft  43  coupled with the second element (the carrier  38  in this embodiment) so as not to be relatively rotatable around the axis. 
     The speed change transmission shaft  44  is configured to be coupled with the first element (the internal gear  36  in this embodiment) so as not to be relatively rotatable around the axis, the input side first transmission mechanism  50 ( 1 ) is configured to operatively transmit the rotation power of the main driving shaft  212  to the first element through the speed change transmission shaft  44 , and the output side first and second transmission mechanisms  70 ( 1 ) and  70 ( 2 ) are configured to operatively transmit the rotation power of the first element to the speed change output shaft  45  through the speed change transmission shaft  44 . 
     In detail, as illustrated in  FIG.  23    and  FIG.  24   , the input side first driven gear  54 ( 1 ) of the input side first transmission mechanism  50 ( 1 ) is supported by the speed change transmission shaft  44  so as not to be relatively rotatable in a state of being operatively coupled with the input side first driving gear  52 ( 1 ) relatively rotatably supported by the main driving shaft  212  in this embodiment. 
     The output side second driving gear  72 ( 2 ) of the output side second transmission mechanism  70 ( 2 ) is supported by the speed change transmission shaft  44  so as not to be relatively rotatable in a state of being operatively coupled with the output side second driven gear  74 ( 2 ). 
     The output side third transmission mechanism  70 ( 3 ) has an output side third driving gear  72 ( 3 ) supported by the speed change transmission shaft  44  so as not to be relatively rotatable and an output side third driven gear  74 ( 3 ) operatively coupled with the output side third driving gear  72 ( 3 ) and relatively rotatably supported by the speed change output shaft  45 . 
     The input side second transmission mechanism  50 ( 2 ) has the input side second driving gear  52 ( 2 ) relatively rotatably supported by the main driving shaft  212  and the input side first driven gear  54 ( 2 ) operatively coupled with the input side second driving gear  52 ( 2 ) and made relatively unrotatable to the second element in the same manner as in Embodiment 1 described above. 
     As illustrated in  FIG.  24   , the transmission structure  8  has a variable input shaft  31  supporting the third element (the sun gear  32  in this embodiment) functioning as the variable power input portion so as not to be relatively rotatable around the axis and the input side first driven gear  54 ( 2 ) is relatively rotatably supported by the variable input shaft  31 . 
     The variable input shaft  31  supports a driven gear  216   b  of the gear train  216  operatively transmitting the rotation power of the motor shaft  16  to the third element (the sun gear  32 ) so as not to be relatively rotatable. 
     In this embodiment, the output side third clutch mechanism  80 ( 3 ) has an output side clutch housing  83  supported by the speed change output shaft  45  so as not to be relatively rotatable, an output side third friction plate group  84 ( 3 ) containing a third driving side friction plate supported by the output side third driven gear  74 ( 3 ) so as not be relatively rotatable and a third driven side friction plate supported by the output side clutch housing  83  so as not to be relatively rotatable in a state of being opposed to the third driving side friction plate, and an output side third piston (not illustrated) frictionally engaging the output side third friction plate group  84 ( 3 ). 
     As illustrated in  FIG.  24   , the transmission structure  8  according to this embodiment is housed in a housing structure  500  in the working vehicle  203 . 
     The housing structure  500  has a front housing  510  and a rear housing  550  coupled in series. 
     The front housing  510  has a hollow front housing body  512 , a front supporting wall  514  and a second supporting wall  518  extended radially inward from the inner surface at an intermediate position in the longitudinal direction of the front housing body  512 , and a front bearing plate  516  detachably coupled with a boss portion formed to project radially inward from the inner surface near a rear opening of the front housing body  512 . 
     The rear housing  550  has a hollow rear housing body  552  detachably coupled with the front housing body  512 , a rear bearing plate  554  detachably coupled with a boss portion formed to project radially inward from the inner surface near a front opening of the rear housing body  552 , and a rear supporting wall  556  extended radially inward from the inner surface at an intermediate position in the longitudinal direction of the rear housing body  552 . 
     In such a configuration, the main driving shaft  212  is supported by the front supporting wall  514 , the front bearing plate  516 , and the rear bearing plate  554  so as to be rotatable around the axis and the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) are supported by the main driving shaft  212  in the front housing body  512 . A relay cylinder portion  516   a  for supplying/discharging pressure oil to/from the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) is integrally formed in the front bearing plate  516  fitted onto the main driving shaft  212 . To the relay cylinder portion  516   a,  the input side first and second supply/discharge lines  360 ( 1 ) and  360 ( 2 ) illustrated in  FIG.  16    described above are connected using a means, such as a pipe. 
     The variable input shaft  31  is configured as a hollow shaft integrally formed in a rotation center portion of the driven gear  216   b  and the front end side is supported by the front supporting wall  514  so as to be rotatable around the axis. 
     In a sun gear shaft  32   a  integrally having the sun gear  32  on the rear end side, the front end side is inserted into the rear end side of the variable input shaft  31  to be spline-coupled therewith and the intermediate portion in the axial direction relatively rotatably supports the input side second driven gear  54 ( 2 ). 
     The speed change intermediate shaft  43  is disposed coaxially with the variable input shaft  31  and the sun gear shaft  32   a.  The speed change intermediate shaft  43  integrally has the carrier  38  on the front end side and the carrier  38  is coupled with the input side second driven gear  54 ( 2 ) through a bolt. Thus, the front end side of the speed change intermediate shaft  43  is supported by the front bearing plate  516  through the sun gear shaft  32   a  and the speed change input shaft  31  so as to be rotatable around the axis and the rear end side of the speed change intermediate shaft  43  is supported by the rear bearing plate  554  so as to be rotatable around the axis. 
     The hollow speed change transmission shaft  44  externally inserted into the speed change intermediate shaft  43  in a relatively rotatable manner integrally has the internal gear  36  in a front end portion, the front end side thereof is supported by the front supporting wall  514  through the speed change intermediate shaft  43 , and the rear end side thereof is supported by the front bearing plate  516 . Then, the output side third driving gear  72 ( 3 ), the input side first driven gear  54 ( 1 ), and the output side second driving gear  72 ( 2 ) are spline-fitted onto the outer periphery of an intermediate portion reaching the front bearing plate  516  from the internal gear  36  of the speed change transmission shaft  44 . 
     The speed change output shaft  45  is supported by the second supporting wall  518 , the rear bearing plate  554 , and the rear supporting wall  556  so as to be rotatable around the axis. The first traveling transmission shaft  235  is supported by the rear bearing plate  554  and the rear supporting wall  556  so as to be rotatable around the axis. 
     The speed change output shaft  45  supports the output side third clutch mechanism  80 ( 3 ) on the front end side, supports the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) in an intermediate portion, and supports the clutch mechanism of the forward/reverse movement switching mechanism  230  on the rear end side, respectively. The hydraulic friction plate clutch unit is used also for the clutch mechanism of the forward/reverse movement switching mechanism  230 . A relay cylinder portion  556   a  for supplying/discharging pressure oil to/from the five clutch mechanisms arranged on the speed change output shaft  45  is mounted on the rear supporting wall  556  and fitted to a rear end portion of the speed change output shaft  45 . 
     In a rear half portion not illustrated of the rear housing  550 , the differential mechanism  260 , the PTO clutch mechanism  285 , and the PTO multistage speed change mechanism  290  are housed. 
     In this embodiment, although the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) and the output side first to third clutch mechanisms  80 ( 1 ) to  80 ( 3 ) are all configured as the friction plate type, some or all thereof can be configured as the dog clutch type. 
       FIG.  26    illustrates a transmission schematic view of a working vehicle  203  to which a transmission structure  8 B according to a modification of this embodiment provided with the input side clutch unit  410  of the dog clutch type in place of the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) of the friction plate type is applied. 
     Embodiment 9 
     Hereinafter, further yet still another embodiment of the transmission structure according to the present invention is described with reference to the accompanying drawings. 
       FIG.  27    illustrates a transmission schematic view of a working vehicle  205  to which the transmission structure  9  according to this embodiment is applied. 
       FIG.  28    illustrates a partial vertical cross-sectional side view of the working vehicle  205 . 
     In the figures, the same components as those in Embodiments described above are designated by the same reference numerals and a description thereof is omitted as appropriate. 
     The transmission structure  9  according to this embodiment is common to the transmission structure  8  according to Embodiment 8 described above in the point of having the output side first to third clutch mechanisms  80 ( 1 ) to  80 ( 3 ). 
     Meanwhile, the transmission structure  9  is different from the transmission structure  8  according to Embodiment 8 described above in the following point. 
     More specifically, the transmission structure  8  according to Embodiment 8 described above is configured so that the rotation power is transmitted to the traveling transmission shaft  235  through the forward/reverse movement switching mechanism  230  in all the first to third transmission states developed according to the engagement states of the output side first to third clutch mechanisms  80 ( 1 ) to  80 ( 3 ). 
     In contrast thereto, the transmission structure  9  according to this embodiment is configured so that, while the rotation power is transmitted to the traveling transmission shaft  235  through the forward/reverse movement switching mechanism  230  in the first and second transmission states developed according to the engagement states of the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ), the rotation power in the forward movement direction is transmitted to the traveling transmission shaft  235  without via the forward/reverse movement switching mechanism  230  in the third transmission state developed in the engagement state of the output side third clutch mechanism  80 ( 3 ). 
     In detail, as illustrated in  FIG.  27   , the transmission structure  9  is provided with the HST  10  and the planetary gear mechanism  30 , the input side first transmission mechanism  750 ( 1 ) capable of operatively transmitting the rotation power of the driving source  210  to the first element (the internal gear  36  in this embodiment) at the input side first speed change ratio and the input side second transmission mechanism  750 ( 2 ) capable of operatively transmitting the rotation power of the driving source  210  to the second element (the carrier  38  in this embodiment) at the input side second speed change ratio, the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ), the speed change output shaft  45  and the traveling transmission shaft  235 , the forward/reverse movement switching mechanism  230 , an output side first transmission mechanism  770 ( 1 ) capable of operatively transmitting the rotation power of the second element at the output side first speed change ratio to the speed change output shaft  45 , an output side second transmission mechanism  770 ( 2 ) capable of operatively transmitting the rotation power of the first element at the output side second speed change ratio to the speed change output shaft  45 , an output side third transmission mechanism  770 ( 3 ) capable of operatively transmitting the rotation power of the first element as the driving force in the forward movement direction to the traveling transmission shaft  235 , the output side first to third clutch mechanisms  80 ( 1 ) to  80 ( 3 ), the speed change operation member  90 , the HST sensor  95   a,  and the control device  100 . 
     As illustrated in  FIG.  27    and  FIG.  28   , the transmission structure  9  further has the speed change intermediate shaft  43  coupled with the second element (the carrier  38  in this embodiment) so as not to be relatively rotatable around the axis. 
     Moreover, the transmission structure  9  has an output sensor  95   b  directly or indirectly detecting the rotational speed of the traveling transmission shaft  235 . 
     The input side first transmission mechanism  750 ( 1 ) has an input side first driving gear  752 ( 1 ) relatively rotatably supported by the main driving shaft  212  operatively coupled with the driving source  210  and an input side first driven gear  754 ( 1 ) operatively coupled with the input side first driving gear  752 ( 1 ) and the first element (the internal gear  36  in this embodiment) in a state of being relatively rotatably supported by the speed change intermediate shaft  43 . 
     The input side second transmission mechanism  750 ( 2 ) has an input side second driving gear  752 ( 2 ) relatively rotatably supported by the main driving shaft  212  and an input side second driven gear  754 ( 2 ) operatively coupled with the input side second driving gear  752 ( 2 ) in a state of being supported by the speed change intermediate shaft  43  so as not to be relatively rotatable, in which the rotation power of the main driving shaft  212  is operatively transmitted to the second element (the carrier  38  in this embodiment) through the speed change intermediate shaft  43 . 
     In this case, as illustrated in  FIG.  27    and  FIG.  28   , the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) are supported by the main driving shaft  212  so as to engage/disengage the input side first and second driving gears  752 ( 1 ) and  752 ( 2 ), respectively, with/from the main driving shaft  212 . 
     More specifically, in this embodiment, the input side first clutch mechanism  60 ( 1 ) is configured to have the input side clutch housing  62  supported by the main driving shaft  212  so as not to be relatively rotatable, an input side first friction plate group  64 ( 1 ) containing a first driving side friction plate supported by the input side clutch housing  62  so as not to be relatively rotatable and a first driven side friction plate supported by the input side first driving gear  752 ( 1 ) so as not to be relatively rotatable in a state of being opposed to the first driving side friction plate, and an input side first piston (not illustrated) frictionally engaging the input side first friction plate group  64 ( 1 ). 
     The input side second clutch mechanism  60 ( 2 ) is configured to have the input side clutch housing  62 , an input side second friction plate group  64 ( 2 ) containing a second driving side friction plate supported by the input side clutch housing  62  so as not to be relatively rotatable and a second driven side friction plate supported by the input side second driving gear  752 ( 2 ) so as not to be relatively rotatable in a state of being opposed to the second driving side friction plate, and an input side second piston (not illustrated) frictionally engaging the input side second friction plate group  64 ( 2 ). 
     In this embodiment, the output side first transmission mechanism  770 ( 1 ) is configured to be able to operatively transmit the rotation power of the second element to the speed change output shaft  45  utilizing the input side second driven gear  754 ( 2 ) in the input side second transmission mechanism  750 ( 2 ). 
     In detail, as illustrated in  FIG.  27    and  FIG.  28   , the output side first transmission mechanism  770 ( 1 ) has an output side first driven gear  774 ( 1 ) operatively coupled with the input side second driven gear  754 ( 2 ) in a state of being relatively rotatably supported by the speed change output shaft  45 . 
     The output side second transmission mechanism  770 ( 2 ) is configured to be able to operatively transmit the rotation power of the first element to the speed change output shaft  45  utilizing the input side first driven gear  754 ( 2 ) in the input side first transmission mechanism  750 ( 1 ). 
     In detail, as illustrated in  FIG.  27    and  FIG.  28   , the output side second transmission mechanism  770 ( 2 ) has an output side second driven gear  774 ( 2 ) operatively coupled with the input side first driven gear  754 ( 1 ) in a state of being relatively rotatably supported by the speed change output shaft  45 . 
     In this case, as illustrated in  FIG.  27    and  FIG.  28   , the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) are supported by the speed change output shaft  45  so as to engage/disengage the output side first and second driven gears  774 ( 1 ) and  774 ( 2 ), respectively, with/from the speed change output shaft  45 . 
     More specifically, in this embodiment, the output side first clutch mechanism  80 ( 1 ) is configured to have an output side clutch housing  82  supported by the speed change output shaft  45  so as not to be relatively rotatable, an output side first friction plate group  84 ( 1 ) containing a first driving side friction plate supported by the output side first driven gear  774 ( 1 ) so as not to be relatively rotatable and a first driven side friction plate supported by the output side clutch housing  82  so as not to be relatively rotatable in a state of being opposed to the first driving side friction plate, and an output side first piston (not illustrated) frictionally engaging the output side first friction plate group  84 ( 1 ). 
     The output side second clutch mechanism  80 ( 2 ) is configured to have the output side clutch housing  82 , an output side second friction plate group  84 ( 2 ) containing a second driving side friction plate supported by the output side second driven gear  774 ( 2 ) so as not to be relatively rotatable and a second driven side friction plate supported by the output side clutch housing  82  so as not to be relatively rotatable in a state of being opposed to the second driving side friction plate, and an output side second piston (not illustrated) frictionally engaging the output side second friction plate group  84 ( 2 ). 
     In the output side third transmission mechanism  770 ( 3 ), the speed change ratio is set so that the rotational speed of the traveling transmission shaft  235  at the timing when the rotation power of the first element is operatively transmitted to the traveling transmission shaft  235  through the output side third transmission mechanism  770 ( 3 ) is higher than the rotational speed of the traveling transmission shaft  235  at the timing when the rotation power of the first element is operatively transmitted to the traveling transmission shaft  235  through the output side second transmission mechanism  770 ( 2 ) and the forward/reverse movement switching mechanism  230  in the forward movement transmission state. 
     In this embodiment, the output side third transmission mechanism  770 ( 3 ) is configured to be able to operatively transmit the rotation power of the first element to the traveling transmission shaft  235  utilizing the output side second driven gear  774 ( 2 ) in the output side second transmission mechanism  770 ( 2 ). 
     In detail, as illustrated in  FIG.  27    and  FIG.  28   , the output side third transmission mechanism  770 ( 3 ) has an output side third driven gear  774 ( 3 ) operatively coupled with the output side second driven gear  774 ( 2 ) in a state of relatively rotatably being supported by the traveling transmission shaft  235 . 
     In this case, as illustrated in  FIG.  27    and  FIG.  28   , the output side third clutch mechanism  80 ( 3 ) is supported by the traveling transmission shaft  235  so as to engage/disengage the output side third driven gear  774 ( 3 ) with/from the traveling transmission shaft  235 . 
     More specifically, in this embodiment, the output side third clutch mechanism  80 ( 3 ) is configured to have the output side clutch housing  83  supported by the traveling transmission shaft  235  so as not to be relatively rotatable, an output side third friction plate group  84 ( 3 ) containing a third driving side friction plate supported by the output side third driven gear  774 ( 3 ) so as not to be relatively rotatable and a third driven side friction plate supported by the output side clutch housing  83  so as not to be relatively rotatable in a state of being opposed to the third driving side friction plate, and an output side third piston (not illustrated) frictionally engaging the output side third friction plate group  84 ( 3 ). 
     The output side third driven gear  774 ( 3 ) can also be operatively coupled with the output side first driven gear  774 ( 1 ) in place of the output side second driven gear  774 ( 2 ). 
     More specifically, the output side third transmission mechanism  770 ( 3 ) can also be modified so as to operatively transmit the rotation power of the first element to the traveling transmission shaft  235  utilizing the output side first driven gear  774 ( 1 ) in the output side first transmission mechanism  770 ( 1 ). 
       FIG.  29    illustrates a graph illustrating the relationship between the traveling vehicle speed and the HST output in the working vehicle  205  to which the transmission structure  9  according to this embodiment is applied. 
     As illustrated in  FIG.  29   , the control device  100  in this embodiment
     operates the output adjustment member  20  so that the HST output is set to the first HST speed setting the synthetic rotation power of the planetary gear mechanism  30  to zero in response to an operation to the zero speed position of the speed change operation member  90 ,   when the speed change operation member  90  is operated in a forward movement side low speed range between the zero speed position and the forward movement side first switching speed position, while the control device  100  develops a first transmission state where the first element is functioned as the reference power input portion operatively transmitted from the driving source  210  and the second element is functioned as the output portion of synthetic rotation power, so that the synthetic rotation power output from the second element is operatively transmitted to the speed change output shaft  45  by bringing the output side first clutch mechanism  80 ( 1 ) into the engagement state and bringing the other output side clutch mechanisms  80 ( 2 ) and  80 ( 3 ) into the disengagement state while bringing the input side first clutch mechanism  60 ( 1 ) into the engagement state and bringing the input side second clutch mechanism  60 ( 2 ) into the disengagement state, the control device  100  brings the forward/reverse movement switching mechanism  230  into the forward movement transmission state and the control device  100  operates the output adjustment member  20  so that the HST output is speed-changed from the side of the first HST speed toward the side of the second HST speed in response to the acceleration operation of the speed change operation member  90 ,   when the speed change operation member  90  is operated in a forward movement side intermediate speed range between the forward movement side first switching speed position and the forward movement side second switching speed position, while the control device develops a second transmission state where the second element is functioned as the reference power input portion and the first element is functioned as the output portion of synthetic rotation power, so that the synthetic rotation power output from the first element is operatively transmitted to the speed change output shaft  45  by bringing the output side second clutch mechanism  80 ( 2 ) into the engagement state and bringing the other output side clutch mechanisms  80 ( 1 ) and  80 ( 3 ) into the disengagement state while bringing the input side first clutch mechanism  60 ( 1 ) into the disengagement state and bringing the input side second clutch mechanism  60 ( 2 ) into the engagement state, the control device  100  brings the forward/reverse movement switching mechanism  230  into the forward movement transmission state and the control device  100  operates the output adjustment member  20  so that the HST output is speed-changed from the side of the second HST speed toward the side of the first HST speed in response to the acceleration operation of the speed change operation member  90 ,   when the speed change operation member  90  is operated in a forward movement side high speed range beyond the forward movement side second switching speed position, while the control device  100  develops a third transmission state where the second element is functioned as the reference power input portion and the first element is functioned as the output unit of synthetic rotation power, so that the synthetic rotation power output from the first element is operatively transmitted to the traveling transmission shaft  235  as driving force in the forward movement direction through the output side third transmission mechanism  770 ( 3 ) by bringing the output side third clutch mechanism  80 ( 3 ) into the engagement state and bringing the other output side clutch mechanisms  80 ( 1 ) and  80 ( 3 ) into the disengagement state while bringing the input side first clutch mechanism  60 ( 1 ) into the disengagement state and bringing the input side second clutch mechanism  60 ( 2 ) into the engagement state, the control device  100  operates the output adjustment member  20  so that the HST output is speed-changed from the side of the second HST speed toward the side of the first HST speed in response to the acceleration operation of the speed change operation member  90 ,   when the speed change operation member  90  passes a forward movement side second switching speed position between the forward movement side intermediate speed range and the forward movement side high speed range, the control device  100  operates the output adjustment member  20  so that the rotational speed of the traveling transmission shaft  235  in a transmission state developed immediately after the passage coincides with or approaches the rotational speed of the traveling transmission shaft  235  in a transmission state developed immediately before the passage,   when the speed change operation member  90  is operated in a reverse movement side low speed range between the zero speed and a reverse movement side first switching speed position, while the control device  100  develops the first transmission state, the control device brings the forward/reverse movement switching mechanism  230  into the reverse movement transmission state and the control device  100  operates the output adjustment member  20  so that the HST output is speed-changed from the side of the first HST speed toward the side of the second HST speed in response to the acceleration operation of the speed change operation member  90 , and   when the speed change operation member  90  is operated in a reverse movement side high speed range beyond the reverse movement side first switching speed position, while the control device  100  develops the second transmission state, the control device  100  brings the forward/reverse movement switching mechanism  230  into the reverse movement transmission state and the control device  100  operates the output adjustment member  20  so that the HST output is speed-changed from the side of the second HST speed toward the side of the first HST speed in response to the acceleration operation of the speed change operation member  90 .   

     In the same manner as in Embodiment 1, the input side first speed change ratio of the input side first transmission mechanism  750 ( 1 ) and the second speed change ratio of the input side second transmission mechanism  750 ( 2 ) are set so that the rotational speed of the second element (the carrier  38  in this embodiment) when the HST output is set to the second HST speed in the first transmission state and the rotational speed of the second element by the rotation power transmitted through the input side second transmission mechanism  750 ( 2 ) in the second transmission state are the same and so that the rotational speed of the first element (the internal gear  36  in this embodiment) when the HST output is set to the second HST speed in the second transmission state and the rotational speed of the first element by the rotation power transmitted through the input side first transmission mechanism  750 ( 1 ) in the first transmission state are the same. 
     As illustrated in  FIG.  29   , the output side first speed change ratio of the output side first transmission mechanism  770 ( 1 ) and the output side second speed change ratio of the output side second transmission mechanism  770 ( 2 ) are set so that the rotational speed developed in the speed change output shaft  45  when the HST output is set to the second HST speed is same in the first and second transmission states. 
     Then, as described above, when the speed change operation member  90  passes the forward movement side second switching speed position between the forward movement side intermediate speed range and the forward movement side high speed range, the control device  100  operates the output adjustment member  20  so that the rotational speed of the traveling transmission shaft  235  in a transmission state developed immediately after the passage coincides with or approaches the rotational speed of the traveling transmission shaft  235  in a transmission state developed before the passage. 
     More specifically, when the speed change operation member  90  is operated in the acceleration direction in the forward movement side intermediate speed range (second transmission state), passes the forward movement side second switching speed position, and then enters the forward movement side high speed range, while the control device  100  shifts the output side second clutch mechanism  80 ( 2 ) from the engagement state to the disengagement state and shifts the output side third clutch mechanism  80 ( 3 ) shifted from the disengagement state to the engagement state to perform switching from the second transmission state to the third transmission state, the control device  100  operates the output adjustment member  20  so that the output of the HST  10  is speed-changed from the rotational speed (first HST speed) setting the traveling vehicle speed to +b under the second transmission state to the rotational speed (third HST speed) setting the traveling vehicle speed to +b or speed therearound under the third transmission state. 
     When the speed change operation member  90  is operated in the deceleration direction in the forward movement side high speed range (third transmission state), passes the forward movement side second switching speed position, and then enters the forward movement side intermediate speed range, while the control device  100  shifts the output side third clutch mechanism  80 ( 3 ) from the engagement state to the disengagement state and shifts the output side second clutch mechanism  80 ( 2 ) from the disengagement state to the engagement state to perform switching from the third transmission state to the second transmission state, the control device  100  operates the output adjustment member  20  so that the output of the HST  10  is speed-changed from the rotational speed (third HST speed) setting the traveling vehicle speed to +b under the third transmission state to the rotational speed (first HST speed) setting the traveling vehicle speed to +b or speed therearound under the second transmission state. 
     The transmission structure  9  having such a configuration can further extend the speed changeable range (speed change region) on the forward movement side while obtaining the same effects as those of the transmission structure  1  according to Embodiment 1. 
     In this embodiment, the output side first and second speed change ratios are set so that the traveling vehicle speed when the HST output is set to the second HST speed is same in the first and second transmission states as described above. However, in place of the setting, a configuration may be acceptable in which the control device  100  operates the output adjustment member  20  so that, in the switching between the first and second transmission states, the traveling vehicle speed in a transmission state after the switching coincides with or approaches the traveling vehicle speed in the transmission state after the switching. 
     The transmission structure  9  according to this embodiment is housed in a housing structure  500 B in the working vehicle  205 . 
     As illustrated in  FIG.  28   , the housing structure  500 B is provided with a hollow housing body  505 B, a first bearing plate  516 B detachably coupled with the hollow housing body  505 B, and a second bearing plate  554 B detachably coupled with the housing body  505 B at a position spaced from the first bearing plate  516 B in the longitudinal direction of the housing body  505 B and forming a partitioned space S between the first bearing plates  516 B and the second bearing plate  554 B. 
     In this embodiment, the housing body  505 B has a front housing body  510 B and a rear housing body  550 B detachably connected in series. 
     The first bearing plate  516 B is detachably coupled with a boss portion  511  provided in the inner surface of the front housing body  510 B near a rear opening of the front housing body  510 B and the second bearing plate  554 B is detachably coupled with a boss portion  551  provided in the inner surface of the rear housing body  550 B near a front opening of the rear housing body  550 B. 
     As illustrated in  FIG.  28   , the main driving shaft  212 , the speed change intermediate shaft  43 , the speed change output shaft  45 , and the traveling transmission shaft  235  are supported by the first and second bearing plates  516 B and  554 B in a state of being parallel to one another and disposed along the longitudinal direction of the housing body  505 B. 
     The input side first and second driving gears  752 ( 1 ) and  752 ( 2 ) and the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) are supported in a portion located in the partitioned space S of the main driving shafts  212  in a state where the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) are located between the input side first and second driving gears  752 ( 1 ) and  752 ( 2 ) with respect to the axial direction of the main driving shaft  212 . 
     The input side first and second driven gears  754 ( 1 ) and  754 ( 2 ) are supported in a portion located in the partitioned space S of the speed change intermediate shaft  43  in a state of being located at the same positions as those of the input side first and second driving gears  752 ( 1 ) and  752 ( 2 ), respectively, with respect to the axial direction. 
     The output side first and second driven gears  774 ( 1 ) and  774 ( 2 ) and the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) are supported in a portion located in the partitioned space S of the speed change output shaft  45  in a state where the output side first and second driven gears  774 ( 1 ) and  774 ( 2 ) are located at the same positions as those of the input side second and first driven gears  754 ( 2 ) and  754 ( 1 ), respectively, with respect to the axial direction and the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) are located between the input side first and second driven gears  774 ( 1 ) and  774 ( 2 ) with respect to the axial direction. 
     The output side third driven gear  774 ( 3 ) and the output side third clutch mechanism  80 ( 3 ) are supported in a portion located in the partitioned space S of the traveling transmission shaft  235  in a state where the output side third driven gear  774 ( 3 ) is located at the same position in the axial direction as that of the output side second driven gear  774 ( 2 ) and the output side third clutch mechanism  80 ( 3 ) is located on the far side of the output side second driven gear  774 ( 2 ) from the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) with respect to the axial direction. 
     The forward/reverse movement switching mechanism  230  is supported in a portion located outside the partitioned space S of the speed change output shaft  45  and the traveling transmission shaft  235 . 
     In detail, as illustrated in  FIG.  27    and  FIG.  28   , the forward/reverse movement switching mechanism  230  has a forward movement side gear train  710 F containing a forward movement side driving gear  711 F supported by the speed change output shaft  44  and a forward movement side driven gear  712 F supported by the traveling transmission shaft  235  and meshed with the forward movement side driving gear  711 F, a reverse movement side gear train  710 R containing a reverse movement side driving gear  231 R supported by the speed change output shaft  45  and a reverse movement side driven gear  712 R supported by the traveling transmission shaft  235  and meshed with the reverse movement side driving gear  711 R through an idle gear  713  (see  FIG.  27   ), a forward movement side clutch mechanism  720 F engaging/disengaging the power transmission in the forward direction from the speed change output shaft  45  to the traveling transmission shaft  235  through the forward movement side gear train  710 F, and a reverse movement side clutch mechanism  720 R engaging/disengaging the power transmission in the reverse movement direction from the speed change output shaft  45  to the traveling transmission shaft  235  through the reverse movement side gear train  710 R. 
     As illustrated in  FIG.  28   , in this embodiment, the forward movement side driving gear  711 F is supported in a rear portion extended rearward relative to the second bearing plate  554 B of the speed change output shaft  45  so as not to be relatively rotatable and the reverse movement side driving gear  711 R is supported in a rear portion of the speed change output shaft  45  so as not to be relatively rotatable at a position spaced from the forward movement side driving gear  711 F in the axial direction. 
     In this embodiment, the speed change output shaft  45  has a first speed change output shaft  45   a  located on the front side and a second speed change output shaft  45   b  located on the rear side and coupled coaxially with the first speed change output shaft  45   a  so as not to be relatively rotatable around the axis, in which the second speed change output shaft  45   b  forms the rear portion of the speed change output shaft  45 . 
     The forward movement side driven gear  712 F is relatively rotatably supported in a rear portion extended rearward relative to the second bearing plate  554 B of the traveling transmission shaft  235  at the same position as that of the forward movement side driving gear  711 F with respect to the axial direction. 
     The reverse movement side driven gear  712 R is relatively rotatably supported in a rear portion of the traveling transmission shaft  235  at the same position as that of the reverse movement side driving gear  711 R with respect to the axial direction. 
     Then, the forward movement side and reverse movement side clutch mechanisms  720 F and  720 R are supported in the rear portion of the traveling transmission shaft  235  in a state of being located between the forward movement side driven gear  712 F and the reverse movement side driven gear  712 R with respect to the axial direction. 
     In this embodiment, the traveling transmission shaft  235  has a first traveling transmission shaft  235   a  located on the front side and a second traveling transmission shaft  235   b  located on the rear side and coupled coaxially with the first traveling transmission shaft  235   a  so as not to be relatively rotatable around the axis, in which the second traveling transmission shaft  235   b  forms the rear portion of the traveling transmission shaft  235 . 
     By having such a housing structure, the transmission structure  9  according to this embodiment can effectively achieve a reduction in the number of used gears and the size. 
     As described above, in this embodiment, the output side third driven gear  774 ( 3 ) is located at the same position as that of the output side second driven gear  774 ( 2 ) with respect to the axial direction and is meshed with the output side second driven gear  774 ( 2 ). 
     In place of the configuration, the output side third driven gear  774 ( 3 ) may be located at the same position as that of the output side first driven gear  774 ( 1 ) with respect to the axial direction and meshed with the output side first driven gear  774 ( 1 ). In this case, the output side third clutch mechanism  80 ( 3 ) is supported in a portion located in the partitioned space S of the traveling transmission shaft  235  in a state of being located on the far side of the output side first driven gear  774 ( 1 ) from the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) with respect to the axial direction. 
     As illustrated in  FIG.  28   , in this embodiment, a rotary joint  68  for supplying/discharging hydraulic oil to the input side first and second clutch mechanisms  60 ( 1 ) and  60 ( 2 ) is provided in the main driving shaft  212  and a bearing portion of the main driving shaft  212  in the second bearing plate  554 B. 
     Moreover, a rotary joint  88  for supplying/discharging hydraulic oil to the output side first and second clutch mechanisms  80 ( 1 ) and  80 ( 2 ) is provided in the speed change output shaft  45  and a bearing portion of the speed change output shaft  45  in the first bearing plate  516 B. 
     Furthermore, a rotary joint  238  for supplying/discharging hydraulic oil to/from the output side third clutch mechanism  80 ( 3 ), the forward movement side clutch mechanism  720 F, and the reverse movement side clutch mechanism  720 R is provided in the traveling transmission shaft  235  and a relay cylinder portion  555  mounted in the second bearing plate  554 B.