NOTE: A common rule of thumb is to never swing
or perform work with a revolving unit over the cab or
front of the truck.
Some truck-mounted units are equipped with
outriggers mounted on the rear that increase the stability
of the truck. These outriggers are normally hydraulically
actuated and controlled from the cab of the truck and
provide a much larger and more rigid base than tires.
The advantage of truck mounting over track
mounting is its capacity for rapid movement from one
job to another. The boom can be placed easily in the
boom rest for traveling and then driven down the road
at 25 to 35 miles per hour. This is better than the slow
laborious job of trailer loading, securing, hauling, and
unloading a track-mounted excavator.
The truck-mounted excavator suffers from a lack of
maneuverability compared to the track mounting,
because it requires a large area to turn around or to
sidestep. Additionally, an important weakness is the ease
with which it can get stuck. Constant care must be
exercised to keep away from soft ground during or after
it rains. Also, tire damage can occur when working in
garbage dumps or a rock quarry.
Self-Propelled Wheel Mounted
The self-propelled single-engine unit has a two-
range transmission, enabling it to travel between 3 and
28 miles per hour. Maneuverability on the job is subject
to the same limitations as the truck mounted, except the
short wheelbase, and in some models, four-wheel
steering allows it in tighter places.
The self-propelled model has front axle oscillation
lock levers. These levers are used to lock out the front
axle from oscillating up or down, holding the axle rigid
and level with the main chassis. The lock lever is used
to help stabilize the excavator when working over the
NOTE: When reading, make sure the oscillation
lock levers are up in the oscillate position, allowing the
axle freedom to oscillate up or down.
The self-propelled model has a set of outriggers
used to increase the stability of the unit. These outriggers
are hydraulically actuated and are controlled from the
cab and provide a much larger and more rigid base when
the revolving unit is placed in the working position.
When traveling, always check the travel route for
weight, height, and width limits, make sure the boom
and steering selector are placed in the travel position,
and the swing brake is engaged. Do not travel with the
boom over the side of the excavator, and if traveling off
of the road, do not travel faster than 5 miles per hour.
NOTE: After 2 hours of highway travel or every 50
miles, whichever occurs first, stop the machine to let the
tires cool for 1/2 hour. Heat damages the tires and can
cause tire failure.
All hydraulic excavator attachments are made of
three strong structural members, such as the boom, the
dipper stick, and the bucket (fig. 9-58). The structural
members are hinged to each other, and the boom is
hinged to the revolving unit. Movement at each hinge is
controlled by two-way hydraulic cylinders.
The boom is normally concave towards the ground
that allows space to pull the bucket closer to the
excavator, permits deeper digging without interference
from the travel unit, and enables the operator to see past
it more easily when it is raised. There are two holes for
connecting the boom cylinder rod eye to the boom (fig.
9-59). The top hole is for maximum digging depth, and
the bottom hole is for maximum dump height. Be sure
to read the operators manual for instructions on the
boom height-depth adjustment.
The outer end of the boom is usually prolonged into
a two-piece bracket, in which the dipper stick is held by
a heavy hinge pin or pins.
The dipper stick is usually one-piece, but some
models may hydraulically extend and retract by a
telescoping boom. The dipper stick hydraulic crowd
cylinder is either connected on the top or on the bottom
of the dipper, and the bucket and bucket dump arms are
connected at the end.
If the dipper stick hydraulic crowd cylinder is
mounted on the top, extending the cylinder forces the
bucket in towards the machine, known as crowding.
Retracting the cylinder forces the bucket outward,
known as extending. When the cylinder is mounted
underneath the boom, retracting the cylinder crowds the
dipper stick, and extending the cylinder extends the