Transmission structure and working vehicle

A transmission structure interposed in a traveling system transmission path from a driving source until a traveling device of a vehicle, the transmission structure comprising:

• an Hydrostatic Transmission (HST) configured to continuously change rotation power operatively input from the driving source to rotation power at least between a first HST speed and a second HST speed;
• a planetary gear mechanism having first, second and third elements, in which the third element functions as an input portion of the HST output;
• an input side first transmission mechanism operatively coupled with the first element and capable of inputting the rotation power from the driving source;
• an output side first transmission mechanism operatively coupled with the second element and capable of outputting the rotation power to the traveling device;
• an input side second transmission mechanism operatively coupled with the second element and capable of inputting the rotation power from the driving source;
• output side second and third transmission mechanisms operatively coupled with the first element and capable of outputting the rotation power to the traveling device;
• a speed change operation member capable of setting the traveling speed of the vehicle;
• and
• a control device performing operation control of the HST, the input side first and second transmission mechanisms, and the output side first to third transmission mechanisms upon receipt of a signal from the speed change operation member,
• wherein the output side first and second transmission mechanisms have output side first and second speed change ratios, respectively, and the output side third transmission mechanism has an output side third speed change ratio higher than the output side second speed change ratio,
• wherein when the speed change operation member is positioned at a stop position, the control device controls the HST so that the HST output has the first HST speed that brings the planetary gear mechanism into a synthetic rotation power zero speed state while developing a first transmission state where the first element functions as an input portion of the rotation power from the driving source and the second element functions as an output portion of outputting rotation power to the traveling device by bringing the input side first transmission mechanism and the output side first transmission mechanism into an engagement state and bringing the input side second transmission mechanism and the output side second and third transmission mechanisms into a disengagement state,
• wherein when the speed change operation member is operated in a range from the stop position to a first traveling speed position, the control device operates the HST 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 while developing the first transmission state,
• wherein when the speed change operation member is operated in a range from the first traveling speed position to a second traveling speed position, the control device operates the HST so that the HST output is speed-changed from a side on the second HST speed toward a side on the first HST speed in response to an acceleration operation of the speed change operation member while developing a second transmission state where the first element functions as the output portion of outputting rotation power to the traveling device and the second element functions as the input portion of the rotation power from the driving source by bringing the input side first transmission mechanism and the output side first and third transmission mechanisms into a disengagement state and bringing the input side second transmission mechanism and the output side second transmission mechanism into an engagement state,
• wherein when the speed change operation member is operated in a range between the second traveling speed position and a third traveling speed position, the control device operates the HST so that the HST output is speed-changed from the side on the second HST speed toward the side on the first HST speed in response to an acceleration operation of the speed change operation member while developing a third transmission state where the first element functions as the output portion of outputting rotation power to the traveling device and the second element functions as the input portion of the rotation power from the driving source by bringing the input side first transmission mechanism into a disengagement state, bringing the input side second transmission mechanism into an engagement state, bringing the output side third transmission mechanism into an engagement state and bringing the remaining output side transmission mechanisms into a disengagement state,
• wherein speed change ratios of the input side first and second transmission mechanisms are set so that when the HST output has the second HST speed, the rotational speeds developed in the second elements in the first and second transmission states substantially coincides with each other and the rotational speeds developed in the first elements in the first and second transmission states substantially coincides with each other, and
• wherein in switching between the second and third transmission states, the control device operates the HST so that rotational speed developed in the traveling device in a transmission state after the switching substantially coincides with rotational speed developed in the traveling device in a transmission state before the switching.