Slewing control device for crane

A slewing control device for a crane for supplying a discharge oil from a pump through a direction selecting valve to a slewing motor and for controlling a rotational direction of said slewing motor, said slewing control device comprising:

• a main circuit including select means provided between said pump and said direction selecting valve, said select means selecting either an unload condition of said pump where a hydraulic oil in said main circuit is communicated with a tank or an on-load condition of said pump where communication between said main circuit and said tank is blocked;
• a first check valve provided between said direction selecting valve and a suction port of said slewing motor;
• a second check valve provided between said direction selecting valve and a discharge port of said slewing motor, said first and second check valves permitting flow of said hydraulic oil from said direction selecting valve to said slewing motor;
• a first electromagnetic proportional pressure control valve provided between said direction selecting valve and said suction port of said slewing motor in parallel with said first check valve;
• a second electromagnetic proportional pressure control valve provided between said direction selecting valve and said discharge port of said slewing motor in parallel with said second check valve, said first and second electromagnetic proportional pressure control valves controlling pressure of oil discharged from said slewing motor to said direction selecting valve;
• and
• control means for outputting a select signal to said select means to select said unload condition and also for outputting a pressure control signal to said second electromagnetic proportional pressure control valve between said direction selecting valve and said discharge port to control oil pressure on a discharge side of said slewing motor to stop rotation of said slewing motor.