Slewing-type working machine

A slewing-type working machine comprising:

• a base body;
• an upper slewing body mounted on the base body so as to be slewable;
• an attachment including an attachment main body attached to the upper slewing body and at least one optional device detachably attached to a front end portion of the attachment main body;
• a variable displacement hydraulic pump which discharges hydraulic fluid;
• a slewing motor which receives supply of the hydraulic fluid discharged from the hydraulic pump to operate so as to slew the upper slewing body;
• an optional device actuator which receives supply of the hydraulic fluid discharged from the hydraulic pump to operate so as to operate the optional device;
• a slewing operation member configured to be operated for slewing the upper slewing body;
• a slewing operation detection section which detects a slewing operation that is an operation of the slewing operation member;
• an optional device operation member configured to be operated to cause the optional device to operate;
• an optional device operation detection section which detects an operation of the optional device operation member;
• an optional device control valve interposed between the hydraulic pump and the optional device actuator and configured to open and close so as to change a flow rate of the hydraulic fluid supplied from the hydraulic pump to the optional device actuator, the optional device control valve being connected to the hydraulic pump and a tank;
• and
• an opening and closing motion control section which controls opening and closing motion of the optional device control valve,
• wherein the opening and closing motion control section controls the optional device control valve so as to reduce an opening degree of the optional device control valve only when an opening reduction condition set in advance for judging whether or not the opening degree of the optional device control valve is to be reduced is satisfied;
• the opening reduction condition includes a first opening reduction condition that the slewing operation detection section detects an operation amount equal to or more than a reference operation amount set in advance so that the reference operation amount is larger than a minimum operation amount of the slewing operation member for causing slewing motion of the upper slewing body and that the optional device operation detection section detects the operation of the optional device operation member, and
• the opening and closing motion control section controls the optional device control valve so as to reduce the opening degree of the optional device control valve when the first opening reduction condition is satisfied.