| 摘要 |
<p>A valve actuator has an electric motor 50 driving a nut 20 on a valve stem 18, via gear train 58-66 and a combined centrifugal clutch and torque-limiting device 56, to move the valve and compress spring 28. When a disc 36 on valve stem 18 abuts stationary shims 42, the torque limiting device disconnects motor shaft 54 from a drive shaft 57. The valve is held open against the closing force of the spring by a one-way clutch 100, which is locked by a pin (110, Fig 6) held in a groove (104) by a solenoid 106 against the action of a further spring. When power fails, the solenoid releases the pin and the valve returns to its fail-safe condition under the action of spring 26. The return motion of the valve and gear train is damped by means of a centrifugal brake (116, 118, Fig 6, 300, 302, Fig 14). The centrifugal clutch prevents the 36 torque limiter from operating under the high torque associated with starting of the motor. <IMAGE></p> |
