Page 150 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

3.44 THE WORK

CRAWLER TRACTORS

Crawler machines do not bog down as readily as wheeled ones, but can do an even more thorough
job of it.
A crawler tractor may sink in mud too soft to support it, or it may dig itself in while pulling a
load, or both. If the tracks are allowed to spin, the grousers act as buckets on a ditcher, digging
soil from underneath and piling it behind. On soft ground they can work down rapidly this way,
until the frame parts of the tractor are resting on the ground, or on a stump or other object its nor-
mal clearance would have taken it over. When the weight of the tractor rests thus on the frame,
the tracks churn helplessly in air or loose mud.

Pulling Out. If outside power is available, the machine may be pulled out by a line attached to
its drawbar, front pull hook, or other hold. It should be pulled straight forward or backward, if
possible, with its own power being used also. The drawbar, or a dozer blade, can take almost any
pull, but use of a front pull hook may pull the engine out of some models.
Poling Out. If no outside power is available, and the stuck tractor has no dozer, winch, or other
helpful equipment, the first thing to try is digging a shallow ditch in front of (or behind) the tracks,
a foot or two wider than the tractor. In this put a green sapling, or strong board as long as the ditch,
pressing or wedging it tightly against the tracks, which should then be turned slowly so as to pull
the stick or board underneath. When it is well under, press in another stick, and perhaps more.
Their effect will be to lift the tractor and restore the weight to the tracks, provide a wide base for
support and traction, and cut off or uproot obstacles under the tractor. They are almost certain to
get the machine out if it will pull them under.
If the tractor has flat shoes or grousers that will not grip the size of pole available, and no bolt-
on cleats can be obtained, or if there is an aversion to digging ditches; planks or heavy angle irons
may be drilled and bolted to track shoes on both sides. The effect is then positive, but it is neces-
sary to unbolt and remove them as they come up at the other end. Poles or logs may be fastened
to the shoes by loops of cable and used in the same manner. Short sticks should not be used unless
absolutely necessary, as they do not afford nearly as much lift or traction as long ones, and they
are liable to turn underneath and jam things. See Fig. 3.35.
These stratagems are equally effective at moving the tractor forward or backward, but reverse
gear often has less power than low, and backward movement under difficult conditions is a severe
strain on the tracks.
Cable Ramps. Another system is to fasten a cable to each track, or the two ends of a single cable
to the tracks, perhaps by passing it from the outside through a hole in the shoe, and catching it
inside with a loop and clamps. These cables should be stretched parallel or nearly so, straight
ahead of the machine, to anchors of some sort. When the tracks are turned to move forward, they
will advance on top of the cables, which will prevent them from spinning and provide a tightrope
ramp on which they can climb. This technique should be used with caution in reverse, as the strain
on the track may break it. See Fig. 3.36.
Chains may be bolted to the track and used instead of cables, but they are much heavier for the
same strength and are seldom long enough for the job.
The tractor may also be jacked up on planks, in the same manner as a truck, and the hole filled
in or bridged.

Winching Out. If the tractor is equipped with a towing winch, the cable may be fastened to an
anchor behind it, and the machine will come out of the mud as the cable winds in. This should be
done with caution if the presence of a stump or large stone underneath is suspected, as it might
force a track off, or do some other damage. Risk can be reduced by putting blocks behind the tracks
so that it will move up as well as back, or by anchoring the cable at a height, as in a large tree.