Hydraulic circuit for large crane

A hydraulic circuit for a large crane having a counterweight carriage including driving wheels and detachably connectable to the rear end of a swivel unit of the crane thereof, comprising:

• a first variable displacement pump;
• a second variable displacement pump;
• a first hydraulic motor for driving said swivel unit of the crane for swivel motion;
• a first control valve for controlling the flow of working fluid from said first variable displacement pump to said first hydraulic motor;
• a second control valve for controlling operation of said first control valve and for applying a signal pressure;
• a first flow control circuit for controlling a discharge rate of said first variable displacement pump according to said signal pressure applied thereto from said second control valve;
• a second hydraulic motor for driving said driving wheels of the counterweight carriage;
• a second flow control circuit for controlling a discharge rate of said second variable displacement pump according to said signal pressure from said second control valve;
• a hydraulic circuit for joining the flow of the working fluid discharged from said second variable displacement pump with the flow of working fluid discharged from said first variable displacement pump when the counterweight carriage is disconnected from the swivel unit;
• a controller for controlling the discharge rate of said first variable displacement pump;
• and
• changeover means provided in said first flow control circuit for selectively changing the state of said first flow control circuit between a state for applying the signal pressure provided by said second control valve to said controller for controlling the discharge rate of said first variable displacement pump and a state for applying a second signal pressure produced by reducing the former signal pressure at a reduction ratio to the said controller for controlling the discharge rate of said first variable displacement pump.