Our thanks to folks at the Bay Leaf Volunteer
Fire Department, located near Raleigh, NC, for their
help with this article.
Driving down the interstate,
you reach down to grab the road map from the passenger-side
floorboard. In an instant, you inadvertently swerve onto the
shoulder of the road, and your car flips as you attempt to
regain control. When your car comes to rest, you've got a
broken leg, your car is upside down and you're pinned
underneath the dashboard. In this type of situation, rescue
workers will use a set of tools commonly called the "Jaws
of Life" to cut away the car and get you out.
To extricate accident victims, firefighters
may make a relief cut to open up the vehicle's roof.
The Jaws of Life is actually a brand of tools that
is trademarked by the Hurst
Jaws of Life company, but the name is often used when
talking about other brands of rescue systems. The term "Jaws
of Life" refers to several types of piston-rod hydraulic tools
known as cutters, spreaders and rams,
which are used to pry open vehicles involved in accidents when
a victim may be trapped.
During emergencies, when a few wasted seconds can cost
lives, the Jaws of Life are brought in to remove victims from
the crashed vehicle. These devices are also used to extricate
victims from collapsed concrete and steel structures after earthquakes.
In this edition of HowStuffWorks,
you will learn how these simple hydraulic systems work, the
purpose of each device and how they're powered.
Simple Hydraulics If you've read How
Hydraulic Machines Work, you know that hydraulic equipment
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.
is the most commonly used incompressible fluid for hydraulic
machines. However, the Jaws of Life equipment uses a
phosphate-ester fluid, which is fire resistant and
electrically non-conductive. At a crash scene, this type of
synthetic fluid is favored over conventional oil.
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.
The Jaws of Life equipment is some of the most
unsophisticated hydraulic machinery, because there are very
few parts involved in making the devices work. In the cutter
and spreader, a portable engine pumps pressurized
hydraulic fluid into the piston cylinder through one of
two hose ports. A typical Jaws of Life machine uses about 1
quart of hydraulic fluid. An operator-controlled valve
switch controls which port the fluid enters through. If it
enters one port, the fluid forces the rod up and opens the
arms of the spreader or blades of the cutter. The operator can
then toggle the switch and cause the rod to retract, closing
the arms or blades.
A portable gasoline power unit sends
pressurized hydraulic fluid to the
In the next two of sections, we will take a closer look at
the types of equipment that are generally referred to as the
"Jaws of Life."
Spreaders Spreaders and cutters are probably
the two pieces of equipment that most people think about when
they hear about the Jaws of Life on a news report. The
powerful jaws of these machines can tear apart most vehicles
like cutting through a tin can. The spreader is used to
pull pieces of the structure apart, or it can be inserted into
the side of the vehicle to tear a section out. The
cutter, as the name suggests, is used to cut through
the vehicle like a pair of giant bolt cutters. The mechanics
of how these two devices work are very similar, and some Jaws
of Life equipment combine the cutter and spreader into one
A spreader consists of pincer-like, aluminum alloy
arms with tips made of heat-treated steel to
provide maximum strength for tearing into a vehicle or
building. There are spreaders of different sizes, so the
specifications differ as to how much spreading force the
equipment possesses or how much space can be opened up on a
vehicle. Let's look at the ML-32 Hurst Jaws of Life
spreader as an example. This particular spreader provides:
16,000 pounds (71 kiloNewtons) spreading force
14,400 pounds (64 kN) pulling force
32 inches (81.9 cm) opening distance
Other spreaders can provide more or less spreading and
pulling force. The body of the ML-32 spreader is made out of
aluminum alloy and the piston and piston rod are made from forged
alloy steel. When the portable engine is started, oil flows
through a set of hydraulic hoses into the hydraulic pump
inside the machine's housing. A typical power unit might be a
gasoline engine that operates at 5,000 pounds per square inch
(psi), although the pressure differs in different power units.
This type of engine can run on about 0.5 gallons (2 liters) of
for about 45 minutes to an hour.
This combination spreader/cutter has its arms
fully extended. Notice the cutter blades between the
To open the arms of the spreader, the operator slides a
valve switch that causes the hydraulic fluid to flow
from one hose into the cylinder, pushing the piston and rod
up. This rod is attached to linkages that are conjointly
attached to the spreader's arms. When the rod pushes up, it
causes the linkages to rotate, which opens the arms. To close
the arms, the operator moves the valve in the opposite
direction, which causes the hydraulic fluid to flow through a
The valve at the base of the spreader/cutter
controls the flow of the hydraulic
To use the spreader, a rescue worker inserts the closed
spreader arms into an opening in the vehicle or structure,
such as a door jamb. The spreader can also clamp down on a
structure to crush any material between its arms.
As you will see in the next section, cutters are very
similar to spreaders in how they operate.
Cutters Like spreaders, cutters have
a mouth that opens and closes. However, cutters are more like
big chompers that bite through metal and other vehicle
materials. If you've ever seen this device in action, you know
that it can snap a car-door post like a twig in a few seconds.
As the pressure comes down on the door post, the cutters just
snap right through it.
Cutters can be used to cut off the roof of a
car. Notice the notch above the car
Cutters typically have an aluminum-alloy housing with
forged, heat-treated steel blades. The piston and piston rod
are often made of heat-treated alloy steel. The cutters are
used to cut or shear through materials such as sheet metal and
plastic. Most often, they are used to cut through automobiles
and other vehicles to free trapped passengers. Like the
spreader, it can run off a gasoline-driven power unit. Jaws of
Life systems can also be powered electrically, pneumatically
Instead of arms, the cutter has curved, claw-like
extensions that come to a point. Just like in the
spreader, hydraulic fluid flows into a cylinder, placing
pressure on a piston. Depending on the side of the piston that
force is exerted on, the claws either open or close. When the
piston rod is raised, the claws open. As the piston rod
lowers, the claws of the cutter come together around a
structure, such as a car roof, and pinch through it.
Cutters come in different sizes, but let's look at the
Hurst Jaws of Life ML-40 model as an example. This
particular model gives the operator:
12,358 pounds (60 kN) cutting force at the blade center
22,455 pounds (99.9 kN) cutting force at the notch
4.25-inch (10.8-cm) cuts
If you understand the operation of the spreader and cutter,
the ram is going to seem about as complex as a pair of
scissors (if scissors had hydraulics, of course). Read the
next section to learn how a ram works.
Rams The ram is the most basic type
of hydraulic system: It's just a matter of using hydraulic
fluid to move a piston head inside a cylinder to extend and
retract a piston rod. If you look at some heavy construction
equipment, like a backhoe
loader, you'll notice that rams are used to control the
This cutaway shows the internal components of
In the image above, you can see that the piston rod that
extends outside the cylinder is actually moved by a piston
head inside the cylinder. There is fluid on both sides of this
piston head, fed by two different hoses. If the force is
greater on the blue side, the piston moves to the left; if it
is greater on the orange side, the piston moves to the right.
All you have to do to change the direction of force is stop
pumping oil to one side and start pumping it to the other.
A ram can be used to push a collapsed
dashboard forward to free a
The ram's function is to push apart sections of the car (or
other structure). For instance, a rescue worker can place a
ram on the door frame and extend the piston to push the
dashboard up, creating enough space to free a crash victim.
"This is what we call a dash bow up, where the
steering wheel or part of the dash has come down on the
patient. We would actually take that [ram] and roll that dash
forward," said David Price of the Bay Leaf Volunteer
Hydraulics play an important part in many of the machines
around us, but none may be as vital as the equipment known as
the "Jaws of Life." These devices have been called upon to
save thousands of lives in situations where a few seconds
could mean the difference between life and death.
For much more information, check out the links on the next