| 摘要 |
A security screen having two rams to operate the screen which are located against the panel of the screen, the rams being connected at one of their ends substantially symmetrically relating to the centre of the screen at or adjacent the top thereof and, at their other ends, substantially symmetrically relative to the screen to a structure beneath or beside the screen, the connections being such as to permit at least a degree of rotation of the rams relative to the panel and the structure, the arrangement being such that, as the rams are extended, the outward component of forces of the rams are substantially equal and opposite, the resultant being an upward component which can drive the screen upwardly rapidly. We also provide a brake assembly for a security screen system whereby gas displaced by the movement of the ram piston can be passed to atmosphere by an arrangement including a valve which is held to its seat directly or indirectly by gas at a pressure substantially equal to that applied to the ram whereby the screen can move its full distance even under reduced pressure to the underside of the ram. We also provide for a security screen system a quick opening pneumatic valve having three passages therein, one of which is common and in contact with a reservoir, the second being connected to a source of gas whereby the reservoir can be charged and the third being an outlet passage whereby gas can pass from the reservoir, the second and third passages having valve seats and a valve member which can selectively close one of the passages and normally being biassed to one of the valve seats. |
