Hydraulic control device for controlling a boom-swing frame combined motion in an excavator

A hydraulic control device for controlling a boom-swing frame combined motion in an excavator, comprising first and second fluid pumps (P1, P2), a swing control spool (13) and a boom second-speed control spool (11B) respectively provided on parallel lines (16, 17) for receiving a hydraulic flow from the first pump (P1) via the parallel lines (16, 17), a boom first-speed control spool (11A) for receiving a hydraulic flow from the second pump (P2), a swing motor (1) actuated by the hydraulic flow supplied from the first pump (P1) through the swing control spool (13), a boom cylinder (3) actuated by the hydraulic flows supplied from the first and second pumps (P1, P2) through the boom second-speed control spool (11B) and the boom first-speed control spool (11A), a pilot control valve (24) for providing a pilot pressure to displace the swing control spool (13), the boom first-speed control spool (11A) and the boom second-speed control spool (11B), and a confluence line (25) for merging the hydraulic flow from the boom second-speed control spool (11B) with that from the boom first-speed control spool (11A), characterized by further comprising:

• a swing priority valve (30) provided on the confluence line (25) and having a variable orifice (31) for variably reducing the hydraulic flow supplied to the boom cylinder (3) through the confluence line (25);
• and
• a swing priority control line (27) interconnecting a pressure receiving part of the swing priority valve (30) and a swing control pilot pressure signal line of the pilot control valve (24) in such a manner that an aperture area of the variable orifice (31) of the swing priority valve (30) is reduced in proportion to the magnitude of a swing control pilot pressure of the pilot control valve (24).