Heavy rains spawned by Hurricane
Danny flooded many parts of Charlotte, NC. Flood waters
rose so quickly in Little Sugar Creek that the underpinnings
of a train trestle collapsed, bringing a diesel
locomotive crashing down into the creek. As you might
imagine, a 412,000-pound (186,880-kg) locomotive is not easily
lifted. After the waters receded, emergency teams brought in
three large hydraulic truck cranes -- one 500-ton, one 300-ton
and one 175-ton -- to lift the train out of the creek bed and
back onto the train tracks.
Hydraulic truck cranes can lift thousands of
pounds using the simple concept of transmitting forces
from point to point through a
Hydraulic cranes are very simple by design but can perform
Herculean tasks that would otherwise seem impossible. In a
matter of minutes, these machines are able to raise multi-ton
beams on highways, heavy equipment in factories and even lift
beachfront houses onto pilings. Hydraulic truck cranes are
also used to lift killer
whales like Shamu out of water tanks when places like Sea
World ship the whales to new destinations.
When watching a hydraulic truck crane in action, it's hard
to believe just how much weight it's moving because it deals
with these multi-ton objects with relative ease. Hydraulic
truck cranes vary in lifting power. It's easy to tell how much
a particular hydraulic truck crane can lift just by the name
of it: A 40-ton crane can lift 40 tons (80,000 lb or 36,287
In this edition of HowStuffWorks,
you will learn how these cranes are able to lift thousands of
pounds using hydraulics, and we'll climb into the cab to show
you just how these machines are operated.
It's All About Hydraulics If you've read How
Hydraulic Machines Work, you know that the hydraulic crane
is based on a simple concept -- the transmission of forces
from point to point through a fluid. Most hydraulic machines
use some sort of incompressible fluid, which is a fluid
that is at its maximum density. Oil
is the most commonly used incompressible fluid for hydraulic
machines, including hydraulic cranes. In a simple hydraulic
system, when a piston pushes down on the oil, the oil
transmits all of the original force to another piston, which
is driven up.
In a simple hydraulic system, when one piston is
pushed down, another piston is pushed up. Click on the arrow
for a demo.
A hydraulic pump creates the pressure that moves the
pistons. Pressure in a hydraulic system is created by one of
two types of hydraulic pumps:
Variable-displacement pump - Click
here to learn more about variable-displacement pumps.
Some basic equipment on a
hydraulic truck crane:
Boom - The large arm mainly responsible for
Counterweights - Multi-ton weights placed
on the back of the cab to prevent the crane from
tipping during lifts
Two-gear pump - Hydraulic pump system that
uses two rotating gears to pressurize oil
Jib - Lattice structure that extends out of
Outriggers - Supports that keep the crane
Rotex gear - Large gear under the cab that
allows the boom to be rotated
Load Moment Indicator - Array of lights
located in the cab just above the operator's eye
level; flashes if crane's lifting limits are reached
Most hydraulic truck cranes use two-gear pumps that
have a pair of inter-meshing gears to
pressurize the hydraulic oil. When pressure needs to increase,
the operator pushes the foot throttle to run the pump faster.
In a gear pump, the only way to get high pressure is to run
the engine at full power.
A 70-ton hydraulic truck crane uses a 12.7-L diesel
engine that generates up to 365 horsepower.
The engine is connected to three two-gear pumps, including:
Main pump - This pump operates the piston
rod that raises and lowers the boom, as well as
the hydraulic telescoping sections that extend the boom. The
main pump is able to generate 3,500 pounds per square inch
(psi) of pressure. It generates more pressure than the other
two pumps because it is responsible for moving much more
Pilot pressure counterweight pump - A hydraulic
truck crane uses counterweights on the back of the cab to
keep it from tipping over. These are added and removed by a
hydraulic lift that has its own pump. The counterweight gear
pump can generate 1,400 psi.
Steering/outrigger pump - One pump controls the
steering and the outriggers. The outriggers are used to
stabilize the truck during lifting operations. Because
steering and outrigger operation are not performed
simultaneously, they run off of the same pump. This pump
generates 1,600 psi.
In the next section, you'll see how the hydraulic system
acts on the other parts of the hydraulic truck crane.
Parts of a Hydraulic Truck Crane Although
everything on the truck begins and ends with the hydraulic
system, there's more to a hydraulic truck crane than the
hydraulics. There are many components involved in lifting a
load. Here are the basic parts of every hydraulic truck crane:
The most recognizable part of any crane is the boom.
This is the steel arm of the crane that holds the load. Rising
up from just behind the operator's cab, the boom is the
essential piece of a crane, allowing the machine to raise
loads to heights of several dozen feet.
Most hydraulic truck cranes have a boom that has several
telescoping sections. For instance, a 70-ton Link-Belt
hydraulic truck crane has a boom with three telescoping
sections. This particular boom has a length of 127 feet (38.7
meters). Some booms are equipped with a jib, which is
the lattice structure attached to the end of the boom. On the
70-ton hydraulic truck crane, the jib is 67 feet (20.4 m)
long, giving the crane a total length of 194 feet (59.1 m). As
the load is lifted, the sections telescope out to the desired
Reinforced-steel cable lines run from a winch just behind
the operator's cab, extending up and over the the boom and
jib. Each line is capable of holding a maximum load of 14,000
pounds (6,350 kg). So, a 70-ton hydraulic truck can use up to
10 cable lines for a total of 140,000 pounds (63,503 kg), or
70 tons. The lines run up the boom and jib and attach to a
285-pound (129 kg) metal ball that keeps the lines pulled taut
when no load is attached to the hook.
The large metal ball attached to the hook
keeps the cable taut when there is no load on the
To maneuver the load, the boom has to be able to move right
and left, as well as up and down. Underneath the operator's
cab is a Rotex gear on a turntable bearing that turns
at 2 revolutions per minute (rpm). It is driven by a
bidirectional, hydraulic motor mounted on the cab and housed
in a metal cover to prevent injuries. The rotation is
controlled by a foot-operated, hydraulic pedal in the cab.
The large gear under the cab is the Rotex
gear, which allows the cab to swivel and move the boom
from side to
Hydraulic truck cranes are used to lift heavy loads to tall
heights, and it's important that the truck be completely
stable during the lifting operation. The tires don't
offer the stability needed, so the truck employs
outriggers that act as balances to keep the crane from
leaning too much to one side or the other. The outriggers use
hydraulics to lift the entire truck, tires and all, off the
ground. The outriggers are comprised of the beam, which is the
leg of the outrigger, and the pad, which is the foot.
Sometimes, "floats" are placed under the pad to dissipate the
force of the crane and the load over concrete or pavement.
Floats are usually wood planks that are lined up to create a
base that is larger than the pad itself.
The outriggers keep the crane balanced during
a lift. The inset shows all outriggers
The outriggers are only one mechanism used to balance the
crane during lifting operations. There are also detachable
counterweights that can be placed on the back of the crane on
the underside of the cab. These counterweights prevent the
crane from tipping forward during operation. The amount of
counterweight needed for a particular lift is determined by
the weight of the load, the radius of the boom and the boom's
angle during operation. The 70-ton Link-Belt truck crane has
counterweights that come in 4,000-pound (1,814-kg) sections.
Counterweights are only used during lifts; they have to be
removed before the truck can be driven.
In the Operator's Seat In the previous two
sections, you learned how the hydraulics and other pieces of
equipment on the hydraulic truck crane work. All of this
equipment is controlled by the operator inside the cab, which
is located on top of the deck. Crane operators use several
control mechanisms to raise and lower the boom, rotate the cab
and boom, wind and unwind the winch and control other
The crane is operated by hydraulic joysticks
The 70-ton Link-Belt hydraulic truck crane has two basic
types of controls for maneuvering a load:
Joysticks - There are two joysticks in the cab.
One controls left-to-right movement of the boom, and the
other controls forward and aft movement.
Foot pedals - These pedals are responsible for
retracting and extending the telescoping sections of the
boom. They also control the amount of pressure being
generated by the pump.
Joy sticks and foot pedals are connected to hydraulic hoses
that connect various hydraulic rams to spool valves.
The spool valve is connected to the hydraulic pump via a third
hose that is placed between the two hoses that run from the
spool valve to the hydraulic ram. When a joystick is pushed in
one direction, it causes the valve to shut off one of the
hydraulic hoses leading to the ram and open the other. Which
way the joystick is pushed determines whether the piston in
the hydraulic ram slides in or out.
The spool-valve system lets the crane operator
control the hydraulic pistons.
Prior to any lift, the operator enters data into a computer
inside the cab, including the weight of the object to be
lifted and the height to which it is to be lifted. This
computer serves as the operator's backup, warning the operator
if the crane is being pushed beyond its capability. Using a
binder of charts in the cab, the operator also determines the
angle of lift and the radius of the boom. Once all of this is
entered, the computer can track the progress of the lift and
warn the driver if the crane is nearing its limitations. If
the boom is lifted too high for the load amount, a series of
lights just above the inside of the front window will begin to
light up. These lights are the warning lights for the Load
Moment Indicator (LMI).
If the operator tries to lift a load too
high, the Load Moment Indicator will light
There are at least two other people needed to perform a
lift properly, including the oiler and the
signalman. The oiler is responsible for making sure
that all of the crane's parts are in place and secured prior
to any lift. He or she also acts as a spotter during a lift to
ensure that the lift is being performed properly. The
signalman, as the name suggests, gives hand signals to the
operator during the lift to make sure the load is being
maneuvered correctly. Click
here to see some of the hand signals used during a lift.
Hydraulic truck cranes provide brute strength to move
objects, machines and even large animals that would otherwise
be very difficult to budge. Using a very simple principle of
hydraulics, these machines move thousands of pounds with
relative ease, making them an essential component of most
construction projects and a great example of the power of