To drive a hydraulic motor without wastefully consuming the operating horse power of a hydraulic pump.
In this hydraulic driving device for driving a hydraulic motor 43 in response to an arbitrary operating position input from an operating level 1, a hydraulic control circuit 23B receives the pressure of the hydraulic oil for driving the hydraulic motor 43 detected by pressure guages 47, 48 and the pressure of the hydraulic oil supplied from a hydraulic pump 42 to a direction switching valve 25 and detected by a pressure guage 49, and the hydraulic control circuit 23B outputs the current to a solenoid relief valve 46 and changes the set pressure of the solenoid relief valve 46 so as to change the set pressure of a main relief valve 45, and controls the pressure of the hydraulic oil supplied from the hydraulic pump 42 so as to be higher by the predetermined pressure than the pressure of the hydraulic oil for driving the hydraulic motor 43.
SHIMIZU NOBUAKI
ASANO YOJI
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