Dragline Excavators
Dragline excavators are typically used for digging open cuts, drainage
ditches, canals, sand, and gravel pits, where the material is to be moved
20 to 1,000 ft (6 to 305 m) before dumping. They cannot handle rock
unless the rock is blasted. Since they are provided with long booms and
mounted on turntables, permitting them to swing through a full circle,
these excavators can deposit material directly on the spoil bank farther
from the point of excavation than any other type of machine. Whereas a
shovel stands below the level of the material it is digging, a dragline
excavator stands above and can be used to excavate material under
water.
Figure 10.4.30 shows a self-contained dragline mounted on crawler
treads. The drive is almost exclusively gasoline in the small sizes and
diesel, diesel-electric, or electric, frequently with Ward Leonard control,
in the large sizes. The boom a is pivoted at its lower end to the
turntable, the outer end being supported by cables b. so that it can be
raised or lowered to the desired angle. The scraper bucket c is supported
Fig. 10.4.30 Dragline excavator.
by cable d, which is attached to a bail on the bucket, passes over a
sheave at the head of the boom, and is made fast to the engine. A second
cable e is attached to the front of the bucket and made fast to the second
drum of the engine. The bucket is dropped and dragged along the surface
of the material by cable e until filled. It is then hoisted by cable d,
drawn back to its dumping position, e being kept tight until the dumping
point is reached, when e is slacked, allowing the bucket to dump by
gravity. After the bucket is filled, the boom is swung to the dumping
position while the bucket is being hauled out. A good operator can
throw the bucket 10 to 40 ft (3 to 12 m) beyond the end of the boom,
depending on the size of machine and the working conditions. The
depth of the cut varies from 12 to 75 ft (3.7 to 23 m), again depending
on the size of machine and the working conditions. With the smaller
machines and under favorable conditions, two or even three trips per
minute are possible; but with the largest machines, even one trip per
minute may not be attained. The more common sizes are for handing 3⁄4-
to 4-yd3 (0.6- to 3-m3) buckets with boom lengths up to 100 or 125 ft
(30 to 38 m), but machines have been built to handle an 8-yd3 (6-m3)
bucket with a boom length of 200 ft (60 m). The same machine can handle a 12-yd3 (9-m3) bucket with the boom shortened to 165 ft
(50 m).
Stackline Cableways
Used widely in sand-and-gravel plants, the slackline cableway employs
an open-ended dragline bucket suspended from a carrier (Fig. 10.4.31)
which runs upon a track cable. It will dig, elevate, and convey materials
in one continuous operation.
Fig. 10.4.31 Slackline-cable bucket and trolley.
Figure 10.4.32 shows a typical slackline-cableway operation. The
bucket and carrier is a; b is the track cable, inclined to return the bucket
and carrier by gravity; c is a tension cable for raising or lowering the
track cable; d is the load cable; and e is a power unit with two friction
drums having variable speeds. A mast or tower f is used to support guide
and tension blocks at the high end of the track cable; a movable tail
tower g supports the lower end of the track cable. The bucket is raised
and lowered by tensioning or slacking off the track cable. The bucket is
loaded, after lowering, by pulling on the load cable. The loaded bucket,
after raising, is conveyed at high speed to the dumping point and is
Fig. 10.4.32 Slackline-cable plant. (Sauerman.)
returned at a still higher speed by gravity to the digging point. The
cableway can be operated in radial lines from a mast or in parallel lines
between two moving towers. It will not dig rock unless the rock is
blasted. The depth of digging may vary from 5 to 100 ft.

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