| 摘要 |
<p>A tunnel knife shield has a support frame, and a plurality of knife positioned side-by-side on the support frame. Each of the knives is associated with a double-acting hydraulic ram, the rams being used for advancing the knives, and for causing the support frame to follow up the advance of the knives. Hydraulic control means are provided for controlling the extension and retraction of the rams. Each ram has first and second working chambers, pressurization of which is effective to extend and retract that ram. The hydraulic control means comprises a plurality of control valves, each of which is associated with a respective hydraulic ram. Each control valve is provided with a first input constituted by a separate first hydraulic line. Each control valve is also provided with a second input, which is constituted by a common second hydraulic line. The common second hydraulic line is fed with hydraulic fluid by all of the first hydraulic lines. Thus, when a given control valve is in a first operating position, the first working chamber of the associated ram is connected to the common second hydraulic line; and, when that valve is in a second operating position, the second working chamber of that ram is connected to the first hydraulic line associated with the control valve.</p> |
