| 摘要 |
<p>PURPOSE: To improve workability and operability by branching a second pilot oil passage for pressurizing a direction change-over valve for breaker to a neutral side from a first pilot oil passage for changing over a direction change- over valve for boom when the boom is operated to a descent side. CONSTITUTION: When a boom is lowered while a breaker is operating, pilot pressure oil is supplied to a pilot port 17c on the operation side from a pilot valve 20 for breaker to position a direction change-over valve 17 for breaker at an operation position. Moreover, a pressure switch 21 is closed when it detects the supply of pilot pressure oil to the pilot port l7c on the operation side. When an operation lever for boom is operated in this condition, a valve 16D on a descent side is changed over to change over a direction change-over valve 13 for boom to a descent side, and pilot pressure oil is supplied to a second pilot oil passage P. Since an open-close selector valve 22 is opened, the direction change-over valve 17 for breaker is pressurized to a neutral position side. Consequently, the direction change-over valve 17 for breaker restricts a pressure oil supply passage to a breaker piston 9.</p> |
