EXCAVATING MACHINERY USED IN LAND DRAINAGE. 5 



engines as suitable for operating machines which have such constantly 

 changing loads as is the case with dipper dredges. 



Owing to the constant jar and pound on the hull the vertical engine 

 is not so well adapted to excavating machinery as the horizontal 

 type. On a large dredge an independent engine unit is used for 

 each of the operations of swinging, hoisting,- and handling spuds. 

 The hoisting and swinging engines are generally of the horizontal, 

 double-cylinder type and must be self-contained on a cast-iron or 

 structural-steel bed plate. Steam engines are generally designated 

 by the dimensions of their cylinders rather than by the horsepower 

 they develop. 



Owing to the cost of fuel, the expense of transporting it to the 

 dredge, and the impure and muddy water that must be used in some 

 cases, the size and type of boiler must be selected with great care. 

 The boiler commonly used is the locomotive type with either open 

 or water bottom. Vertical boilers have been used in dredges of the 

 smallest sizes, but are not economical in the consumption of fuel. 

 The grate area of the locomotive- type boilers is ordinarily less than 

 that for the same size of Scotch marine boiler. The return-flue 

 Scotch marine boiler is used on many dredges and meets with great 

 favor. The earlier boilers were designed for a pressure of 100 pounds. 

 Later this was increased to 150 pounds and the boiler was worked at 

 100 pounds or more pressure. The size of boiler should be at least 

 25 per cent greater than that theoretically required to operate the 

 engines. Owing to the foul character of the water that must often 

 be used, the boiler should have two separate and distinct boiler feeds, 

 either injectors or pumps. A great saving of fuel can be effected by 

 covering the boiler and steam pipes with asbestos. Either wood or 

 coal is used for fuel. 



A-FRAME . 



The A-frame is a tower composed of timber or steel members 

 securely anchored on the deck of the hull near the front and joined at 

 the top by a cast-steel head or yoke. (See PI. I.) The A-frame 

 may have either two or four legs. In the latter case the two front 

 or main legs are set in a vertical plane. If only two legs are used 

 they are inclined slightly forward. The A-frame must be strongly 

 guyed and held rigidly hi position, as the severe stresses from the 

 outer and loaded end of the boom are carried by the top of this 

 tower. Failure of any part of the A-frame may result in serious 

 damage to the dredge and even in loss of life. The height is governed 

 by the required elevation of the end of the boom, which in turn is 

 determined by the depth of excavation and the distance at which the 

 excavated material must be placed. On the top of the head block is 

 a large pin on which the yoke revolves, this latter being a short beam 

 to the ends of which are attached the cables which support the outer 

 end of the boom. 



