| 摘要 |
A bistable slide valve 10 (FIG. 1) has a slide member 12 moved between operating positions by short control fluid pulses and has a safety shut-off for supply pressure failure in the form of safety spring 38 opposed by valve supply liquid in line 22 acting on a safety piston 37. Slide member 12 has a through-aperture 29 connecting supply and function lines 22, 23 when the valve is open and venting the function line via slide chamber 12 to return line 24 when the slide member is displaced. Safety shut off, less dependent upon indeterminate frictional resistance of the slide member, is achieved by a stronger safety spring and greater liquid-derived restraining force so that the frictional resistance force is a small proportion of the total restraining force, with smaller variations in residual pressure at which the safety means operates. To prevent a low friction slide member from being displaced by external shocks a holding piston 46 in cylinder 47 receives supply liquid by port 49 when the slide member is in the valve open position but vented directly into the slide chamber when the slide member is moved. The holding force exerted by the piston is less than the normal valve force of the control pulse and direct venting into the slide chamber ensures no residual holding pressure re-opens the valve after the control pulse has ended.
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