Patent ID: 8122714

Claim:
A variable hydraulic motor driving device, comprising: a hydraulic motor which is switched, by varying motor displacement, between a low-speed mode in which the motor rotates at low speed and a high-speed mode in which the motor rotates at high speed; a speed changing mechanism including a tilt piston which varies the motor displacement of the hydraulic motor, and a high-low speed switching valve which switches a state of a supply passage between an open state and a close state, the supply passage supplying pressure oil to a pressure chamber of the tilt piston; a speed signal pilot passage through which a speed signal is transmitted, in a form of a pilot pressure, to the high-low speed switching valve; a supply port and a discharge port of the hydraulic motor, the supply port and the discharge port communicating with a direction changeover valve which controls supply/discharge of pressure oil; a counterbalance valve disposed between the hydraulic motor and the direction changeover valve; and a relief valve which communicates with the supply port and the discharge port, and has a set pressure corresponding to the low-speed mode, wherein the hydraulic motor is switched to the high-speed mode when pressure oil is supplied to the pressure chamber of the tilt piston based on the transmission of the speed signal to the high-low speed switching valve, and the hydraulic motor is switched to the low-speed mode when pressure oil is discharged from the pressure chamber of the tilt piston, the variable hydraulic motor driving device further comprising a first braking purpose speed changing mechanism which operates so as to discharge pressure oil from the pressure chamber of the tilt piston when a pressure of pressure oil between the counterbalance valve and the direction changeover valve becomes equal to a tank pressure, the first braking purpose speed changing mechanism being provided in the variable hydraulic motor driving device.