Stepless speed change structure

A stepless speed change structure including a hydrostatic transmission and a speed change operating member, wherein the hydrostatic transmission includes a variable displacement hydraulic pump so configured that a volume of a pump body changes in accordance with an operation of a pump volume adjusting member, and a variable displacement hydraulic motor so configured that a volume of a motor body changes in accordance with an operation of a motor volume adjusting member, and wherein the speed change operating member is configured to change a speed change state of the hydrostatic transmission, the stepless speed change structure comprising:

• a pump operation piston that is movable bidirectionally in first and second slide directions on one side and the other side in an axial direction, and that is directly or indirectly engaged with an operation end portion of the pump volume adjusting member such that movements of the pump operation piston in the first and second slide directions cause the pump volume adjusting member to be operated in first and second operating directions, respectively;
• a neutral spring mechanism that holds the pump operation piston at a neutral position when no external force is added to the pump operation piston and generates a biasing force toward the neutral position when the pump operation piston is moved in the first and second slide directions from the neutral position;
• first and second slide oil chambers that are so configured that pressure oil, which has been supplied thereinto, presses and moves the pump operation piston in the first and second slide directions, respectively, against the biasing force of the neutral spring mechanism;
• a motor operation piston that is movable bidirectionally in a smaller volume direction on one side and a larger volume direction on the other side in an axial direction, and that is directly or indirectly engaged with an operation end portion of the motor volume adjusting member such that movements of the motor operation piston in the smaller volume direction and the larger volume direction cause the motor volume adjusting member to be operated in a smaller volume direction and a larger volume direction, respectively;
• a larger volume operation spring that biases the motor operation piston in the larger volume direction;
• a smaller volume operating oil chamber that is so configured that pressure oil, which has been supplied thereinto, presses and moves the motor operation piston in the smaller volume direction against the biasing force of the larger volume operation spring;
• a first pressure control valve that commonly switches supplying and discharging of pressure oil to and from the first slide oil chamber and the smaller volume operating oil chamber in accordance with the operation of the speed change operating member;
• and
• a second pressure control valve that switches supplying and discharging of pressure oil to and from the second slide oil chamber in accordance with the operation of the speed change operating member;
• wherein biasing forces of the neutral spring mechanism and the larger volume operation spring are so set that, after pressure oil within the first slide oil chamber causes the pump operation piston to move in the first slide direction so that the neutral spring mechanism is brought into a predetermined retained-elastic state, pressure oil within the smaller volume operating oil chamber causes the motor operation piston to start moving in the smaller volume direction while compressing the larger volume operation spring.