| 摘要 |
The present invention relates to a boom cylinder control circuit for a construction machine including a boom cylinder (1) with an ascending-side chamber (1a) and a descending-side chamber (1b). The boom cylinder control circuit comprises a primary floating valve (30) and a secondary floating valve (40). Primary and secondary input ports (31)(32) respectively connected to the descending-side chamber (1b) and the ascending-side chamber (1a) of the boom cylinder are provided on one side of the primary floating valve, and a primary output port (33) and a secondary output port (34) connected to a drain tank (T) are provided on the other side. Therefore, the primary floating valve is switched to selectively connect the primary and secondary input ports (31)(32) to the respective primary and secondary output ports (33)(34) according to an operation signal from a floating selection operation part (10) (110). The second floating valve (40) is connected to the primary output port (33) of the primary floating valve (30) on one side and to the drain tank (T) on the other side and switched to selectively connect the primary output port (33) to the drain tank (T) according to an operation signal from the floating select operation part (10)(110). |
