70 A TREATISE ON DREDGES AND DREDGING 



when buckets and chains would always fall alternately on the same 

 faces and consequently would wear out unevenly. The lower 

 tumbler is made as large as possible with a polygonal cross-section 

 of six or seven sides. 



The endless chains are built up of links of soft untempered steel; 

 the links are connected together by cast-steel bolts, the bolt holes 

 being lined with soft steel rings in order to be easily renewed when 

 worn out, thus preventing the bars of the links from wearing. 



Steel buckets of different shapes and capacity are attached to 

 every second link of the two parallel endless chains. The buckets 

 are generally made of two different shapes, either prismoidal or 

 as round as possible. The lower bucket of prismoidal form is not very 

 satisfactory, especially in clayey soils, when the material tends to 

 adhere to the corners and bottom; but the buckets should not be 

 very round either, since the lower part must be flat in order to easily 

 slide on the upper side of the ladder and revolve smoothly around 

 the tumblers. All the buckets are made of steel, reinforced at their 

 cutting edge. Buckets used on loose soil are reinforced with a 

 ring riveted to the edge of the bucket, but those used in the excava- 

 tion of rock or very hard soil are reinforced by strong projecting 

 steel teeth riveted to the bucket so as to be easily replaced when 

 worn out. The capacity of the buckets working through loose soil 

 varies between J and 1 cu.yd., while that of the buckets used on 

 rock varies between 7 and 13 cu.ft. To extend the usefulness of 

 the machinery every ladder or elevator dredge should be provided 

 with two sets of buckets, thus making the machine available in 

 any material. 



The tower of the ladder dredge may be constructed either of 

 iron or wood. Dredges with iron towers are lighter, of less draft 

 and stiffer than those provided with wooden towers, besides, the 

 machinery runs smoother and with less vibration. Yet many dredges, 

 especially those employed on narrow rivers and canals, are even 

 today constructed with wooden towers. When wood is employed in 

 the construction of the tower, its upper part is usually reinforced 

 with packing pieces at the points where shafting causes heavy strains 

 and vibrations. Yet in spite of perfect adjust mcnl working at the 

 beginning, owing to the elasticity of the material and through 

 friction, the joints rack and the whole packing becomes loosened. 



Tower dredges are classified as low- and high-tower machines. 

 The former are those with towers not more than 20 or 25 ft. 



