mounted (fig. 9-54), or self-propelled wheel mounted
These excavators are hydraulic powered and consist
of three structures: the revolving unit, the travel base,
and the attachment.
The revolving unit rests and revolves on a turntable
and is normally a rectangular steel deck that carries the
engine, the pumps, the attachments, the controls, and the
operators cab. The center of rotation is usually forward
of the center of the revolving unit that places a major
part of the revolving unit weight at the rear. This serves
to counterbalance the weight and pull of the backhoe
when performing excavation operations.
The swing axis is centered in the travel unit, so the
rear edge of the revolving unit overhangs. This overhang
must be accounted for when the revolving unit is rotated
from side to side to avoid hitting personnel, equipment,
The operators cab is either mounted to the right or
left of the boom and is the location of controls, gauges,
and warning lights for all phases of operation. Some
units may have fixed or removable front and side
windows and a roof window that is helpful in watching
out for and avoiding wires and tree branches. These
windows should be cleaned during the prestart operation
and anytime an amount of dust and dirt that has
accumulated on the window obstructs your vision.
The excavator travel unit may be track (crawler)
mounted, truck mounted, or self-propelled wheel
mounted. Of the three, the most common mounting is
Track frames are single or double beams welded to
the outer ends of the dead axles in the car body. The car
body is a massive frame that includes the turntable and
the dead axles or cross members that transmit its weight
to the track flames. The track may be the link-shoe
construction that is made up of a number of identical
shoes cut and drilled at their ends, so they can be
fastened together by pins (fig. 9-56). Wedge-shaped
projections are cast into the upper surfaces of the shoes
to provide a grip for the drive sprockets and to keep the
tracks centered on the idler and rollers.
Figure 9-56.Link-shoe construction.
Figure 9-57.Track links and bolt-on shoes.
The other type of track is the roller chain with
bolt-on shoes. Each linked pair is fastened together with
a bushing at one end. A pin goes through the bushing
and holds the overlapping ends of the next pair of links
(fig. 9-57). The track is assembled on a hydraulic press
that is able to force the slightly oversized pins and
bushings into the links that very seldom work apart in
service. The pins turn easily in the bushing, providing
the necessary hinge action.
The propel, traction, or travel drive may come from
a pair of live axles set across the center of the car body
or a pair of reversible hydraulic motors fastened to the
The revolving unit is carried on a turntable fastened
to a truck chassis. Some units may have an engine
mounted in the revolving unit to provide power for the
upper unit controls and a engine mounted in the truck to
be used for traveling. Some truck-mounted units may
only have one engine used to power both the revolving
unit and truck.
The truck-mounted excavator can ordinary swing in
a full 360-degree rotation, but with most attachments, it
can work through only 270 degrees because of
interference presented by the cab and the truck front.