94 



DAMS AND HEADWORKS 



operated by an electrically driven crane running along the bridge, 

 which forms the top of the dam. 



For smaller installations a much simpler structure can, of 

 course, be used, such as an ordinary hand-operated sluice gate. 

 (See section on " Gates and Valves.") 



A good example of the enormous size to which sliding gates 

 with roller guides can be built is that of the Gatun spillway of the 

 Panama Canal, as. shown in Figs. 45 and 46. Each of these 

 gates has a height of 19 feet and an over-all length of 47 feet. The 



FIG. 45. Gatun Spillway, Panama, Showing Spillway Gates. 



operating machinery is designed to raise or lower the gates in 

 approximately ten minutes. It consists essentially of two 

 counterweights, one at each end of the gate, which practically 

 balance the weight of the gate, so that the machine has to over- 

 come only the resistance to movement, of the gate due to the 

 water pressure. These counterweights are connected to the 

 gate by a screw and chain, the screw being moved vertically by 

 means of a worm nut, which is motor driven by a worm. The 

 two screws at the gate ends are driven simultaneously through a 

 driving shaft which is provided with a worm at each end for 



