| 摘要 |
<p>An actuator for moving the stem (9) of a gate valve or the like is provided with hydraulic means (15,17) for driving the stem (9) axially inward and a volute spring (7) for returning the stem axially outward upon loss or other removal of the control hydraulic pressure. A ball screw (49) and ball nut (54) assembly rotatable by a disengageable drive shaft (63) is operable to manually drive the stem (9) inward during periods of insufficient control pressure.</p> |
