fUKINGTOK.] 
DKED(^IN(J. 
179 
parts of the dredging machinery where there is severe wear and tear, 
has been in most eases successful. 
The design of the upper and lower tumblers has been modi lied and 
all parts subject to wear are now easily replaceable. In earlier instal- 
lations the tumblers were of such design and construction that the 
wearing out of one part sometimes necessitated the loss of the whole. 
Some trouble has been caused by the working loose of the " wearing 
plates." Tumblers with more numerous faces are now also used. In 
early practice the square upper tumbler was supposed to be superior 
to the one with more faces, it being assumed that the square tumbler 
was more capable of firmly holding the buckets and preventing the 
danger of its revolving without moving the bucket chain. The hex- 
agonal upper tumbler is now in successful operation, and it is safe to 
predict that heptagonal and possibly octagonal upper tumblers will be 
Fig. 34.— Angles for upper tumbler. 
used, particularly on the smaller dredges. The objection that they 
will not hold is not good. Even if true, an arrangement making the 
i relation of bucket to tumbler that of chain to sprocket would solve the 
difficulty. The use of the heptagonal and octagonal upper tumblers 
introduces a new set of problems, involving larger tumblers and shaft 
ing, different arrangement of chute to hopper, and modified forms of 
save-alls. These modifications are necessary because of the resulting 
longer period of discharge of the bucket. It is probable, however, 
that saving in wear of the bucket pins and bushings due to lesser deflec- 
tion in turning tumblers of more numerous faces will compensate for 
the extra expense involved in construction. 
Fig. 34, above, shows the relative size and different deflection angles 
made by the buckets in turning the square, pentagonal, hexagonal, 
