GATES AND VALVES 



153 



motion begins. It then drops somewhat until the valve has 

 opened about one-fourth, after which it takes comparatively 

 little power to complete the opening, as the pressure on the valve 

 is then comparatively small. When closing the valve, friction 

 only needs to be overcome in starting and there is no pressure on 



Remote Control Panel 



Open 



Limit Switch 



FIG. 79. A. C. One-station Remote Control Equipment. 



the valve until it has begun to close. The torque is therefore 

 not very high until the valve is about three-fourths closed, after 

 which the pressure causes the torque to increase rapidly. At the 

 end of the closing cycle the torque does not, however, reach the 

 value it did during the period of starting. 



Valve motors are, therefore, generally rated for maximum 



