GATE STRUCTURES FOR IRRIGATION CANALS. 9 



GATE STEMS AND LIFTING DEVICES. 



For very small gates using wooden shutters, the simple stem with 

 staggered holes is recommended. (See PL I, c, d, e.) 



For small iron shutters and wooden shutters up to 5 feet wide 

 and 5 feet high a screw stem and wheel lift are recommended, for 

 the reasons that they permit of adjustment to any desired point and 

 are not controlled by the position of holes, notches, or other stepped 

 devices (PL I, g). They exert a steady and gradual pull on the 

 shutter, and do not jerk the gate to pieces. They may be used to 

 force the shutter down as well as up, but it is recommended that 

 they be placed in a pipe to prevent buckling if they are longer than 

 twice the height of the shutter. The disadvantages are that the 

 threads become bruised and corroded; that a screw can not jerk the 

 gate where silt piles up against it when closed temporarily; that 

 they are slow for large gates ; and that the usual position is such that 

 they require great physical effort to start and operate as compared 

 with long-lever lifts, where a man simply may hang his weight on 

 the end of the lever rather than use muscular force in turning the 

 wheel. 



For large sliding wooden gates a double stem with lever lift is 

 recommended wherever they are known. The general construction 

 of this type is shown in Plate I, /. This type is especially adapted 

 to situations where a jerk may be necessary to start a gate. The 

 lever may be used to force the gate either up or down. The dis- 

 advantages mentioned, which can be corrected easily, are that the 

 stem crowds away from the fulcrum under the action of the lever; 

 that a pawl is not reliable in holding the gate up ; and that the lever 

 is easily lost. By simply running the stem through a cast frame 

 with a roller behind the stem, the latter may be held firmly up to 

 the fulcrum. An even simpler method is to bolt a heavy timber to 

 the head beam with a space for the stem between the timbers, as 

 shown in Plate VI, figure 2. The pawl is used in the Arkansas Valley 

 to hold this type of lift, and seems to give satisfaction. The lever 

 may be chained to the structure. The double-stem lift does not 

 have any complicated unit in its makeup, and all the necessary parts 

 that go into the construction can be purchased at any hardware store. 

 For this reason a broken piece can be replaced in a short time. A 

 winch is recommended as a powerful hoist, but the great disad- 

 vantage is that it exerts tension only on the cable and can not be 

 used to force down a gate. This practically precludes its use for any 

 type but the radial gate, which has so little friction that it will close 

 of its own weight where hung true and settling does not occur. 



The rack-and-pinion lift is used a great deal throughout northern 

 Colorado for gates of all sizes except the very smallest delivery gates. 

 45613°— Bull. 115—14 — -2 



