From backyard log splitters to the huge machines you see on
construction sites, hydraulic equipment is amazing in its
strength and agility! On any construction site you see
hydraulically-operated machinery in the form of bulldozers, backhoes,
fork lifts and cranes.
Hydraulics operate the control surfaces on any large airplane.
You see hydraulics at car service centers lifting the cars so
that mechanics can work underneath them, and many elevators
are hydraulically-operated using the same technique. Even the
your car use hydraulics!
In this edition of HowStuffWorks,
you will learn about the basic principles that hydraulic
systems use to do their work, and then we'll examine several
different pieces of hydraulic machinery found on a
construction site. You will be amazed at the power and
versatility available with hydraulics!
The Basic Idea The basic idea behind any
hydraulic system is very simple: Force that is applied at
one point is transmitted to another point using an
incompressible fluid. The fluid is almost always an oil of
some sort. The force is almost always multiplied in the
process. The picture below shows the simplest possible
A Simple hydraulic system consisting of two pistons
and an oil-filled pipe connecting them. Click on the red arrow
to see the animation.
Air in the
It is important
that a hydraulic system contains no air bubbles. You may
have heard about the need to "bleed the air out of the
brake lines" of you car. If there is an air bubble in
the system, then the force applied to the first piston
gets used compressing the air in the bubble rather than
moving the second piston, which has a big effect on the
efficiency of the
drawing, two pistons (red) fit into two glass cylinders filled
with oil (light blue) and connected to one another with an
oil-filled pipe. If you apply a downward force to one piston
(the left one in this drawing), then the force is transmitted
to the second piston through the oil in the pipe. Since oil is
incompressible, the efficiency is very good -- almost all of
the applied force appears at the second piston. The great
thing about hydraulic systems is that the pipe connecting the
two cylinders can be any length and shape, allowing it to
snake through all sorts of things separating the two pistons.
The pipe can also fork, so that one master cylinder can
drive more than one slave cylinder if desired.
The neat thing about hydraulic systems is that it is very
easy to add force multiplication (or division) to the system.
If you have read How a Block and
Tackle Works or How Gears
Work, then you know that trading force for distance
is very common in mechanical systems. In a hydraulic system,
all you do is change the size of one piston and cylinder
relative to the other, as shown here:
Hydraulic multiplication. The piston on the right
has a surface area nine times greater than the piston on the
left. When force is applied to the left piston, it will move
nine units for every one unit that the right piston moves, and
the force is multiplied by nine on the right-hand piston.
Click the red arrow to see the animation.
To determine the multiplication factor, start by
looking at the size of the pistons. Assume that the piston on
the left is 2 inches in diameter (1-inch radius), while the
piston on the right is 6 inches in diameter (3-inch radius).
The area of the two pistons is Pi * r2. The area of the left piston is
therefore 3.14, while the area of the piston on the right is
28.26. The piston on the right is 9 times larger than the
piston on the left. What that means is that any force applied
to the left-hand piston will appear 9 times greater on the
right-hand piston. So if you apply a 100-pound downward force
to the left piston, a 900-pound upward force will appear on
the right. The only catch is that you will have to depress the
left piston 9 inches to raise the right piston 1 inch.
The brakes in
your car are a good example of a basic piston-driven hydraulic
system. When you depress the brake pedal in your car, it is
pushing on the piston in the brake's master
cylinder. Four slave pistons, one at each wheel, actuate
to press the brake pads against the brake rotor to stop the
car. (Actually, in almost all cars on the road today two
master cylinders are driving two slave cylinders each. That
way if one of the master cylinders has a problem or springs a
leak, you can still stop the car.)
In most other hydraulic systems, hydraulic cylinders and
pistons are connected through valves to a pump supplying
high-pressure oil. You'll learn about these systems in the
How Log Splitters Work The simplest
hydraulic device that you find in common use today is the
log splitter. It contains all of the basic components
of a hydraulic machine:
An engine, normally a small four-stroke
gasoline engine, provides the power for the system. The
engine is attached to a hydraulic oil pump.
The hydraulic oil pump creates a stream of
high-pressure oil, which runs to a valve.
The valve lets the operator actuate the
hydraulic cylinder to split a log.
There is also a tank to hold the hydraulic oil
that feeds the pump and usually a filter to keep the
The major components of a log splitter
are shown below:
In cross section, the splitter's important hydraulic parts
look like this:
High-pressure oil from the pump is shown in light
blue, and low-pressure oil returning to the tank is shown in
yellow. Click the button to activate the
In the figure above you can see how the valve can apply
both forward and backward pressure to the piston. The valve
used here, by the way, is referred to as a "spool valve"
because of its resemblance to a spool from a spool of thread.
Let's look at some of the specifics of these components to
see how a real hydraulic system works. If you take a trip down
to your local building supply center or a place like Northern
Tool and Equipment and look at the log splitters, you will
find that a typical backyard log splitter has:
A 5-horsepower gasoline engine
A two-stage hydraulic oil pump rated at a maximum of 11
gallons per minute (3 gpm at 2,500 psi)
A 4-inch-diameter, 24-inch-long hydraulic cylinder
A rated splitting force of 20 tons
A 3.5-gallon hydraulic oil tank
pump is an ingenious time-saver. The pump actually
contains two pumping sections and an internal pressure-sensing
valve that cuts over between the two. One section of the pump
generates the maximum gpm flow rate at a lower pressure. It is
used, for example, to draw the piston back out of a log after
the log has been split. Drawing the piston back into the
cylinder takes very little force and you want it to happen
quickly, so you want the highest possible flow rate at low
pressure. When pushing the piston into a log, however, you
want the highest possible pressure in order to generate the
maximum splitting force. The flow rate isn't a big concern, so
the pump switches to its "high pressure, lower volume" stage
to split the log.
One thing you can see is that the advertised "20-ton
splitting force" is generous. A 4-inch piston has an area of
12.56 square inches. If the pump generates a maximum pressure
of 3,000 pounds per square inch (psi), the total pressure
available is 37,680 pounds, or about 2,320 pounds shy of 20
tons. Oh well...
Another thing you can determine is the cycle time of the
piston. To move a 4-inch-diameter piston 24 inches, you need
3.14 * 22 * 24 = 301
cubic inches of oil. A gallon of oil is about 231 cubic
inches, so you have to pump almost 1.5 gallons of oil to move
the piston 24 inches in one direction. That's a fair amount of
oil to pump -- think about that the next time you watch how
quickly a hydraulic backhoe
is able to move! In our log splitter, the maximum flow rate is
11 gallons per minute. That means that it will take 10 or so
seconds to draw the piston back after the log is split, and it
may take almost 30 seconds to push the piston through a tough
log (because the flow rate is lower at high pressures).
From this discussion you can see that just to fill the
cylinder with oil, you need at least 1.5 gallons of hydraulic
oil in the system. You can also see that one side of the
cylinder has a larger capacity than the other side, because
one side has the piston shaft taking up space and the other
doesn't. Therefore, big hydraulic machines usually have:
Large appetites for hydraulic oil (100 gallons is
not uncommon if there are six or eight large hydraulic
cylinders used to operate the machine.)
Large external reservoirs to hold the difference
in the volume of oil displaced by the two sides of any
Now that you understand the basics of a simple hydraulic
system, we can look at some really interesting equipment!
Large Hydraulic Machines One of the best
places to get up close and personal with large hydraulic
machines is at a construction site. The thing that is most
amazing about these machines is their sheer size. For example,
here is a medium-size shovel/excavator:
This machine weighs just over 28 tons, but if you watch the
video of it you will see that it can be quite swift in its
actions. The bucket can scoop out more than a cubic meter of
dirt, which weighs in at approximately 1 to 1.5 tons, and move
it around with no difficulty at all. Moving a person around is
To have this sort of agility, this particular shovel uses
an 8.3-liter diesel
engine capable of generating 340 horsepower.
The engine is connected to a pair of pumps that can generate
140 gallons per minute at 4,500 psi. The hydraulic pistons
have 5.5-inch-diameter faces with 4-inch-diameter shafts. In
addition, there is one hydraulic motor for each track and a
hydraulic motor that can spin the cab and arm at 11 rpm.
You can see from the picture that the arm has a pair of
pistons working in unison at the "shoulder" -- one at the
"elbow" and then one to rotate the bucket. These pistons,
along with the two track motors and the rotating motor, are
all controlled by two joy sticks and four pedals in the cab:
These controls send electrical signals to an
electrically-operated valve block located next to the pump:
From the valve block, high-pressure hydraulic lines make
their way to the cylinders:
Tracks The tracks are
interesting. If you look at the tracks on any piece of large
machinery, you will find that there is a hydraulic
motor at one end, a free-spinning toothed wheel at
the other, and then a set of rollers for the track to
move over, as shown here:
Shovel Here are the
specifications for the Halla HE280LC shovel:
Skid/Loaders Another common piece of
equipment at any construction site is the skid/loader
(also known generically as a "Bobcat" because that was the
name given by the manufacturer that first produced them):
Skid/loaders have three pairs of pistons:
The first pair raises and lowers the bucket:
The second pair rotates the bucket to dump its contents.
The third pair splits the bucket so you can use it to
grab and pick up things (such as logs):
There are also hydraulic motors on the four wheels. If you
watch the MPEG
video you will be able to see these pistons and motors in
Dump Trucks A dump truck isn't much more
complicated than the log splitter we saw previously. Dump
trucks typically have one cylinder or a pair of cylinders that
lift the bed. The only unique thing about these cylinders is
that they often telescope, giving them a very long
range of motion.
Check out the MPEG videos on the next page to watch some
large hydraulic machines in action!
MPEG Videos These short videos show you
three different pieces of hydraulic equipment in operation.
They take a while to download, especially over a 28.8K modem
-- you will want to save them to a temporary spot on your hard
disk so you can play them more than once.