EXCAVATING MACHINERY USED IN LAND DEAINAGE. 7 



The vertical spuds of various makes are more nearly alike. The 

 rear spud is always of the vertical type and is used to keep the stern 

 of the boat from swinging from side to side as the dredge is operated. 

 The spuds are usually raised and lowered by steel cables connected 

 with the engine. On large machines they are sometimes operated by 

 means of a steam cylinder placed in front of each spud, with a movable 

 clamp or shoe encircling the spud and attached to the piston of the 

 cylinder; this method is, however, wasteful of steam and expensive. 

 Sometimes compressed air is used to aid in releasing the foot of the 

 spud from the mud; less power is thus required to raise the spud. 

 All types of spuds must be equipped with a strong locking device; 

 they must also be so designed that little time is lost in raising or low- 

 ering them. A dipper dredge with vertical spuds is illustrated in 

 Plate I, figure 2. 



BOOM. 



The boom is built either of steel or of wood. In the former case it is 

 made of standard structural sections strongly riveted together. If of 

 wood, it is generally of the " fish-bellied " shape. Some types of long 

 booms are of the open or "knee" build, with a solid filler at the lower 

 end and the chords sprung over posts and cross bulkheads (Plate I, 

 fig. 1). This construction reduces the wind pressure when swinging. 

 Practice has taught that the length of boom must bear a definite 

 relation to the width of the hull. Even on a large dredge it is not 

 advisable to have a boom longer than 80 to 90 feet, although the 

 manufacturers will build them 100 feet long if desired. Large 

 dredges with long booms are much slower in operating than are the 

 smaller-sized dredges. The same number of men are required in 

 either case. 



The lower end of the boom is pivoted. The upper and outer end is 

 connected to the yoke at the top of the A-frame by means of adjust- 

 able wire cables. A sheave placed at the outer end of the boom 

 carries the cable leading from the dipper through the fair-lead sheaves 

 at the lower end of the boom and thence to the hoisting drum. 



On the early type of dipper dredge, chains were used for hoisting 

 and backing. These were hard to install and would break without 

 warning. Steel cable has entirely replaced the old chain since it is 

 less expensive, easier to install, clean, and noiseless; also its weak- 

 ening, due to wear, is more apparent and accidents are therefore less 

 likely. 



DIPPER AND DIPPER HANDLE. 



The dipper handle which carries the dipper at its lower end is made 

 either of steel or of wood. On its under side is a cog rack which moves 

 over pinions mounted on the upper side of the boom. It must be 

 made of sufficient stiffness to prevent bending when the dipper is being 

 filled. 



