| 摘要 |
<p>PROBLEM TO BE SOLVED: To accomplish low fuel consumption, an improvement in working efficiency during combined control, and an improvement in the workability and operability of work, which is attended with difficult boom operations, such as work for scraping up rubble and slope tamping work so as to control supply/ discharge of pressure oil to/from a boom hydraulic cylinder. SOLUTION: A hydraulic control circuit of the boom hydraulic cylinder 8 is provided with the first switching valve 19, which makes a control valve 13 for a boom held in a neutral position N, a communicating oil passage E, which makes a head-side oil chamber 8a communicate with a rod-side oil chamber 8b, an opening/closing valve 18, which opens/closes the oil passage E, and a pilot operation check valve 20 enabling free switching between a unidirectional state of inhibiting an oil flow in a direction opposite to the direction of the allowed oil flow from the oil chamber 8a to the oil chamber 8b and a bidirectional state of allowing the bidirectional oil flow.</p> |
