Chapter 13
Hydraulics
Objectives (1 of 2)
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Explain fundamental hydraulic principles.
Apply the laws of hydraulics.
Calculate force, pressure, and area.
Describe the function of pumps, valves,
actuators, and motors.
• Describe the construction of hydraulic
conductors and couplers.
Objectives (2 of 2)
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Outline the properties of hydraulic fluids.
Identify graphic symbols.
Interpret a hydraulic schematic.
Perform maintenance procedures on truck
hydraulic systems.
Hydraulics
• The term hydraulics is used to specifically describe
fluid power circuits that use liquids—especially
formulated oils—in confined circuits to transmit
force or motion.
• Hydraulic circuits:
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Hydraulic brakes
Power steering systems
Automatic transmissions
Fuel systems
Wet-line kits for dump trucks
Torque converters
Lift gates
Pascal’s Law
• Pressure applied to a confined liquid is
transmitted undiminished in all directions and
acts with equal force on all equal areas, at
right angles to those areas.
Fundamentals
• Hydrostatics is the science of transmitting force by
pushing on a confined liquid.
– In a hydrostatic system, transfer of energy takes place
because a confined liquid is subject to pressure.
• Hydrodynamics is the science of moving liquids to
transmit energy.
• We can define hydrostatics and hydrodynamics as
follows:
– Hydrostatics: low fluid movement with high system
pressures
– Hydrodynamics: high fluid velocity with lower system
pressures
Atmospheric Pressure
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A column of air measuring 1 square
inch extending 50 miles into the sky
would weigh 14.7 pounds at sea
level.
If we stood on a high mountain, the
column of air would measure less
than 50 miles and the result would
be a lower weight of air in the
column.
Similarly, if we were below sea
level, in a mine for instance, the
weight of air would be greater in the
column.
In North America, we sometimes
use the term atm (short for
atmosphere) to describe a unit of
measurement of atmospheric
pressure.
Europeans use the unit bar (short
for barometric pressure).
Force
• Force is push or pull effort.
– The weight of one object placed upon another exerts force on it
proportional to its weight.
– If the objects were glued to each other and we lifted the upper
one, a pull force would be exerted by the lower object
proportional to its weight.
• Force does not always result in any work done.
– If you were to push on the rear of a parked transport truck, you
could apply a lot of force, but that effort would be unlikely to
result in any movement of the truck.
• The formula for force (F) is calculated by multiplying pressure (P)
by the area (A) it acts on.
– F=PxA
Pressure Scales
• There are a number of different pressure scales used today but
all are based on atmospheric pressure. One unit of atmosphere
is the equivalent of atmospheric pressure and it can be
expressed in all these ways:
– 1 atm = 1 bar (European)
= 14.7 psia
= 29.920 Hg (inches of mercury)
= 101.3 kPa (metric)
• However, each of the above values is not precisely equivalent to
the others:
1atm = 1.0192 bar
1 bar = 29.530 Hg = 14.503 psia
10 Hg = 13.60 H2O @ 60° F
Torricelli’s Tube
• Evangelista Torricelli (1608–
1647) discovered the concept
of atmospheric pressure.
• He inverted a tube filled with
mercury into a bowl of the
liquid and then observed that
the column of mercury in the
tube fell until atmospheric
pressure acting on the surface
balanced against the vacuum
created in the tube.
• At sea level, vacuum in the
column in Torricelli’s tube
would support 29.92 inches of
mercury.
Manometer
• A manometer is a single
tube arranged in a U-shape
used to measure very small
pressure values.
• It may be filled to the zero
on the calibration scale with
either water H2O) or
mercury (Hg), depending on
the pressure range desired.
• A manometer can measure
either push or pull on the
fluid column. Examples:
– Crankcase pressure
– Exhaust backpressure
– Air inlet restriction
Absolute Pressure
• Absolute pressure uses a scale in which the zero
point is a complete absence of pressure.
• Gauge pressure has as its zero point atmospheric
pressure.
• A gauge therefore reads zero when exposed to the
atmosphere.
• To avoid confusing absolute pressure with gauge
pressure
– Absolute pressure is expressed as: psia.
– Gauge pressure is usually expressed as: psi or psig.
Hydraulic Levers (1 of 2)
• Hydraulic levers can be
used to demonstrate
Pascal’s law:
– Pressure equals force
divided by the
sectional area on
which it acts.
– (P=F\A)
– Force equals pressure
multiplied by area.
– ( F = P x A)
Hydraulic Levers (2 of 2)
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One of the cylinders has a sectional
area of 1sq.” and the other 50 sq.”
Applying a force of 2 lbs. on the
piston in the smaller cylinder would
lift a weight of 100 lbs.
Applying a force of 2 lbs. on the
piston in the smaller cylinder
produces a circuit pressure of 2 psi.
The circuit potential is 2 psi and
because this acts on a sectional
area of 50 sq.”, it can raise 100 lbs.
If a force of 10 lbs. was to be
applied to the smaller piston, the
resulting circuit pressure would be
10 psi and the circuit would have
the potential to raise a weight of
500 lbs.
Flow
• Flow is the term we use to describe the movement
of a hydraulic fluid through a circuit.
• Flow occurs when there is a difference in pressure
between two points.
• In a hydraulic circuit, flow is created by a device
such as a pump.
– A pump exerts push effort on a fluid.
• Flow rate is the volume or mass of fluid passing
through a conductor over a given unit of time.
– An example would be gallons per minute (gpm).
Flow Rate and Cylinder Speed
• Given an equal flow rate, a small cylinder will move
faster than a larger cylinder. If the objective is to
increase the speed at which a load moves, then:
– Decrease the size (sectional area) of the cylinder.
– Increase the flow to the cylinder (gpm).
• The opposite would also be true, so if the objective
were to slow the speed at which a load moves,
then:
– Increase the size (sectional area) of the cylinder.
– Decrease the flow to the cylinder (gpm).
• Therefore, the speed of a cylinder is proportional to
the flow to which it is subject and inversely
proportional to the piston area.
Pressure Drop
• In a confined hydraulic circuit, whenever there is
flow, a pressure drop results.
• Again, the opposite applies. Whenever there is a
difference in pressure, there must be flow.
• Should the pressure difference be too great to
establish equilibrium, there would be continuous
flow.
• In a flowing hydraulic circuit, pressure is always
highest upstream and lowest downstream. This is
why we use the term pressure drop.
– A pressure drop always occurs downstream from a
restriction in a circuit.
Flow Restrictions
• Pressure drop will occur whenever there is a
restriction to flow.
• A restriction in a circuit may be unintended
(such as a collapsed line) or intended (such
as a restrictive orifice).
• The smaller the line or passage through
which the hydraulic fluid is forced, the greater
the pressure drop.
• The energy lost due to a pressure drop is
converted to heat energy.
Work
• Work occurs when effort or force produces
an observable result.
• In a hydraulic circuit, this means moving a
load.
• To produce work in a hydraulic circuit, there
must be flow.
• Work is measured in units of force multiplied
by distance, for example, in pound-feet.
– Work = Force x Distance
Bernoulli’s Principle (1 of 2)
• Bernoulli’s Principle states that if flow in a circuit is
constant, then the sum of the pressure and kinetic
energy must also be constant.
– Pressure x Velocity IN = Pressure x Velocity OUT
• When fluid is forced through areas of different
diameters, fluid velocity changes accordingly.
– For example, fluid flow through a large pipe will be
slow until the large pipe reduces to a smaller pipe;
then the fluid velocity will increase.
Bernoulli’s Principle (2 of 2)
Laminar Flow
• Flow of a hydraulic medium through a circuit
should be as streamlined as possible.
• Streamlined flow is known as laminar flow.
• Laminar flow is required to minimize friction.
• Changes in section, sharp turns, and high
flow speeds can cause turbulence and crosscurrents in a hydraulic circuit, resulting in
friction losses and pressure drops.
Types of Hydraulic Systems
• Hydraulic systems can be grouped into two
main categories:
– Open-center systems
– Closed-center systems
• The primary difference between open-center
and closed-center systems has to do with
what happens to the hydraulic oil in the circuit
after it leaves the pump.
Open-center Systems
• In an open-center system,
the pump runs constantly
and oil circulates within the
system continuously.
• An open-center valve
manages flow through the
circuit. When this valve is in
its neutral position, fluid
returns to the reservoir.
• An example of an opencenter hydraulic system on
a truck is power-assisted
steering.
Closed-center Systems
• In a closed-center system, the pump can be
“rested” during operation whenever flow is not
required to operate an actuator.
• The control valve blocks flow from the pump when it
is in its “closed” or neutral position.
• A closed-center system requires the use of either a
variable displacement pump or proportioning control
valves.
• Closed-center systems have many uses on
agricultural and industrial equipment, but on trucks,
they would be used on garbage packers and front
bucket forks.
Calculating Force
• In hydraulics, force is the product of pressure
multiplied by area.
– Force = Pressure x Area
• For instance, if a fluid pressure of 100 psi
acts on a piston sectional area of 50 square
inches it means that 100 pounds of pressure
acts on each square inch of the total
sectional area of the piston. The linear force
in this example can be calculated as follows:
– Force = 100 psi x 50 sq. in. = 5000 lbs.
Hydraulic Components
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Reservoirs
Accumulators
Pumps
Valves
Actuators
Hydraulic motors
Conductors and connectors
Hydraulic fluids
Reservoirs
• A reservoir in a hydraulic system has the following roles:
– Stores hydraulic oil
– Helps keep oil clean and free of air
– Acts as a heat exchanger to help cool the oil
• A reservoir is typically equipped with:
• Filler cap
• Oil-level gauge or dipstick
• Outlet and return lines
• Baffle(s)
• Intake filter
• Oil filter
• Drain plug
Gas-loaded Accumulators
• The gas and hydraulic oil
occupy the same chamber but
are separated by a piston,
diaphragm, or bladder.
• When circuit pressure rises,
incoming oil to the chamber
compresses the gas.
• When circuit pressure drops
off, the gas in the chamber
expands, forcing oil out into
the circuit.
• Most gas-loaded accumulators
are pre-charged with the
compressed gas that enables
their operation.
Fixed-Displacement Pumps
• A fixed-displacement pump will move the same
amount of oil per revolution with the result that the
volume picked up by the pump at its inlet equals the
volume discharged to its outlet per revolution.
• This means that pump speed determines how much
hydraulic oil is moved.
• Fixed-displacement pumps are commonly used for
applications such as:
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Lift pumps
Power steering pumps
Transmission pumps
Lube pumps
Variable-displacement Pumps
• Variable-displacement pumps are positive
displacement pumps designed to vary the
volume of oil they move each cycle even
when they are run at the same speed.
• They use an internal control mechanism to
vary the output of oil— usually with the
objective of maintaining a constant pressure
value and reducing flow when demand for oil
is minimal.
Gear Pumps
• Gear pumps are widely used in mobile
hydraulics because of their simplicity.
• They are also widely used to move fuel
through diesel fuel subsystems and as
engine lube oil pumps.
• Three types of gear pumps are used:
– External gear
– Internal gear
– Rotor gear
External-gear Pumps
• Two intermeshing gears are
close-fitted within a housing.
• One of the gears is a drive
shaft and this drives the
second gear because they
are in mesh.
• As the gears rotate, oil from
the inlet is trapped between
the teeth and the housing,
and is carried around the
housing and forced from the
outlet.
Internal-gear Pumps
• A spur gear rotates within an
annular internal gear, meshing
on one side of it.
• Both gears are divided on the
other side by a crescentshaped separator.
• When an external gear is in
mesh with an internal gear,
they both turn in the same
direction of rotation.
• As the gear teeth come out of
mesh, oil from the inlet is
trapped between the teeth and
the separator and is carried to
the outlet and expelled.
Rotor-gear Pumps
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A rotor-gear pump is a variation of
the internal-gear pump.
An internal rotor with external lobes
rotates within an outer rotor ring
with internal lobes.
No separator is used.
The internal rotor is driven within
the outer rotor ring. The internal
rotor has one less lobe than the
outer rotor ring, with the result that
only one lobe is fully engaged to the
rotor ring at any given moment of
operation.
As the lobes on the internal rotor
ride on the lobes on the outer ring,
oil becomes entrapped: as the
assembly rotates, oil is forced out of
the discharge port.
Vane Pumps
• Vane pumps are also used extensively in
hydraulic circuits.
– Truck power-assisted steering systems use
vane pumps.
• A slotted rotor fitted with sliding vanes rotates
within a stationary liner known as a cam ring.
There are two types:
– Balanced
– Unbalanced
Balanced Vane Pumps
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As the rotor rotates, centrifugal
force moves the vanes outward.
Fluid is trapped between the
crescent-shaped “chambers”
formed between vanes.
The size of these chambers are
continually expanding and
contracting as the rotor turns.
Oil from the inlet is trapped in the
space between two vanes.
As the rotor continues to turn, the
chamber contracts until it is aligned
with the outlet and the oil is
expelled.
This action repeats itself twice per
revolution because there are a pair
of inlet ports and a pair of discharge
ports.
Unbalanced Vane Pumps
• This has the same principle
as the balanced version,
with the exception that the
operating cycle only occurs
once per revolution because
it has only one inlet and one
outlet port.
• The disadvantage of the
unbalanced vane pump is
the radial load caused by
high pressure that is acting
on the discharge side of the
rotor and none on the inlet
side because the inlet oil is
under little or no pressure.
Piston Pumps
• There are a wide variety of piston pumps, beginning
with the most simple and including some of the
more complex pumps used in hydraulic circuits.
• There are three general types of piston pump:
– Plunger pumps
– Axial piston
– Radial piston
• Plunger-type pumps are seldom found on hydraulic
circuits, but the latter two are used on systems that
demand high flow and high-pressure performance.
Plunger Pumps
• A bicycle pump is an example of a plunger
pump as are the fuel hand-priming pumps
used on many diesel fuel systems.
• A plunger reciprocates within a stationary
barrel. Fluid to be pumped is drawn into the
pump chamber formed in the barrel on the
outward stroke of the plunger.
• This fluid is then discharged on the inboard
stroke of the plunger.
Axial Piston Pumps
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A rotating cylinder with piston bores
machined into it rides against an
inclined plate.
The pistons are arranged parallel
with the pump drive.
The base of each piston rides
against a tilted plate known as a
swashplate or wobble plate which
does not rotate.
They provide a method for
controlling the tilt angle of the
swashplate.
Fluid is charged to each pump
element as the piston is drawn to
the bottom of its travel.
As the cylinder head rotates, the
piston follows the tilt of the
swashplate and is driven upward
forcing fluid out of the discharge
port.
Radial Piston Pumps
• Radial piston pumps
are capable of high
pressures, high
speeds, high volumes,
and variable
displacement.
However, they cannot
reverse flow.
• Radial piston pumps
operate in two ways:
– Rotating cam
– Rotating piston
Valves
• Valves are used to manage flow and
pressure in hydraulic circuits.
• There are three basic types of valves used in
hydraulic circuits.
– Pressure control
– Directional control
– Volume (flow) control
Directional Control Valves (1 of 3)
Directional Control Valves (2 of 3)
• Directional control valves direct the flow of oil
through a hydraulic circuit. They include:
– Check valves
– Rotary valves
– Spool valves
– Pilot valves
Directional Control Valves (3 of 3)
• Check valves
– A check valve uses a spring-loaded poppet. It permits flow in
one direction and prevents flow in the other.
• Rotary valves
– A rotary spool turns to open and close oil passages. Rotary
valves are commonly used as pilots for other valves in systems
with multiple sub-circuits.
• Spool valves
– A sliding spool within a valve body to open and close hydraulic
circuits. Spool valves are used extensively in hydraulic systems
and automatic transmissions.
• Pilot valves
– Pilot valves may be controlled mechanically, hydraulically, or
electrically.
Actuators
• Hydraulic actuators convert the fluid power
from the pump into mechanical work.
• A hydraulic cylinder is a linear actuator.
• A hydraulic motor is a rotary actuator.
Single-acting Cylinders
• Hydraulic pressure is applied to only one side of the piston.
• Single-acting cylinders may be either:
– Outward-actuated: When an outward-actuated cylinder has
hydraulic pressure applied to it, the piston and rod are forced
outward to lift the load. When the oil pressure is relieved, the
weight of the load forces the piston and rod back into the
cylinder.
– Inward-actuated: When an inward-actuated cylinder has
hydraulic pressure applied to it, the rod is pulled inward into the
cylinder.
• One side of a single-acting cylinder is dry. The dry side must be
vented so that when oil pressure on the pressure side is relieved,
air is allowed to enter, preventing a vacuum.
• A ram is a single-acting cylinder in which the rod serves as the
piston.
Double-acting Cylinders
• Double-acting cylinders provide force in both directions.
• Pressure is applied to one side of the piston to either extend or
retract the cylinder; the oil on the opposite side returns to the
reservoir.
• Double-acting cylinders may be balanced or unbalanced.
– Balanced double-acting cylinder
• The piston rod extends through the piston head on both
sides, giving an equal surface area on which hydraulic
pressure can act.
– Unbalanced double-acting cylinder
• A piston rod is located on one side of the piston. There is
more surface area on the side without the rod because the
rod occupies part of the space on the other side.
Vane-type Cylinders
• Vane-type cylinders may be found in some
much older hydraulic systems.
• A vane-type cylinder provides rotary motion.
• Double-acting vane-type cylinders can be
used in applications such as backhoes
because they enable a boom and bucket to
swing rapidly from trench to pile.
– An alternative to one double-acting vane
cylinder for this application would be a pair of
opposing cylinders.
Hydraulic Motors (1 of 2)
• The function of hydraulic motors is the
opposite of hydraulic pumps:
– Pump
• It draws in oil and displaces it, converting
mechanical force into fluid force.
– Motor
• Oil under pressure is forced in and spilled out,
converting fluid force into mechanical force.
Hydraulic Motors (2 of 2)
• There are three categories of hydraulic
motors:
– Gear motors
– Vane motors
– Piston motors
• All hydraulic motors rotate, driven by
incoming hydraulic oil under pressure.
Gear Motors
• External gear
– An external-gear motor is
driven by pressurized
hydraulic oil forced into the
pump inlet, which acts on a
pair of intermeshing gears,
turning them away from
the inlet, with the oil
passing between the
external gear teeth and the
pump housing.
• Internal gear
– An internal-gear motor is
similar to an internal-gear
pump. The motor drive
shaft is connected to the
inner rotor.
Conductors and Connectors
• The hydraulic fluid has to be conveyed to
various components.
• Mobile hydraulic equipment uses hoses as
hydraulic conductors because they:
– Allow for movement and flexing
– Absorb vibrations
– Sustain pressure spikes
– Enable easy routing and connection on
chassis
Hydraulic Hoses (1 of 2)
• The size of any hydraulic hose is determined by its inside
diameter.
• This is sometimes indicated as dash size in 1⁄16-inch
increments.
• Each dash number indicates 1⁄16 inch,
– a #4 dash hose would be equivalent to 4⁄16 inch or 1⁄4 inch.
• Dash size Nominal diameter
– # 4 = 1⁄4 inch
– # 6 = 3⁄8 inch
– # 8 = 1⁄2 inch
– # 10 = 5⁄8 inch
– # 12 = 3⁄4 inch
• A large-diameter internal hose has to be stronger to sustain the
working pressures of a hydraulic circuit.
Hydraulic Hoses (2 of 2)
• Another consideration for hose selection is
that the hose must be compatible with the
hydraulic fluid used in the system.
• There are four general types of hoses used in
hydraulic circuits:
– Fabric braid
– Single-wire braid
– Multiple-wire braid (up to 6 wire braid)
– Multiple-spiral wire (up to 6 wire spirals)
Couplers (Connectors)
• Hydraulic hose couplers (also known as connectors
and fittings) are made of steel, stainless steel,
brass, or fiber composites.
• Hose couplers or fittings can either be reusable or
permanent.
• Hose fittings are installed at the hose ends and the
mating end consists of either a nipple (male fit) or
socket (female fit).
• Adapters are separate from the hose and are used
to couple hoses to other components such as
valves, actuators, or pumps.
Permanent Hose Fittings
• These fittings are
crimped or swaged
onto the hose.
• When a hose fitted with
permanent hose fittings
fails, the hose must be
replaced either as an
assembly or one must
be made up using
stock hose cut to length
fitted with either crimptype or reusable
fittings.
Reusable Hose Fittings
• Reusable fittings are common in truck shops
because a hose assembly station, some stock
hose, and an assortment of reusable fittings can
replace many of the hundreds of different types of
hose used on various OEM truck chassis.
• When a hose with reusable fittings wears out, the
fittings can be removed and assembled onto new
stock hose.
• Reusable fittings are usually screwed onto hose,
although some types of low-pressure hose may use
press fits.
Assembling Hose Fittings
• Sealing fittings
– Fittings can be sealed to couplers using the following:
• Tapered threads
• O-rings
• Nipple and seat (flair)
• When making hydraulic connections, ensure that the coupler
fittings are compatible with each other.
• Adapters
– Adapters are separate from the hose assembly. They have the
following functions:
• To couple a hose fitting to a component
• To connect hydraulic lines in a circuit
• To act as a reducer in a circuit
• To connect a pair of hoses on either side of a bulkhead
Caution
• Separation of a fitting and hose at high
pressure can be dangerous!
• Never reuse a suspect fitting and observe the
manufacturer assembly procedure to the
letter.
Coupling Guidelines
• When making hydraulic connections, the following guidelines
should be observed:
– Torque the fitting on the hose, not the hose on the fittings.
– Couple male ends before female ends.
– Ensure the sealing method of each fitting to be coupled is the
same.
– Use 45- and 90-degree elbows to improve hose routing.
– Use hydraulic pipe seal compound only on the male threads,
and on thread-seal unions.
– Use two wrenches when tightening unions to avoid twisting
hose.
– Never over-torque hydraulic fittings.
Shop Talk
• When tightening the fittings on a pair of
hydraulic couplers, always use two wrenches
to avoid twisting hoses or damaging
adapters.
Pipes and Tubes
• Pipes used in hydraulic circuits are generally made
from cold-drawn, seamless mild steel.
• They should never be galvanized because the zinc
can flake off and plug up hydraulic circuits.
• Tubing can also be used.
– It has the advantage of being able to sustain some
flex— hence its use in vehicle brake systems.
– Tubing should be manufactured from cold-drawn steel
if used in moderate-to-high-pressure circuits.
– When used in low-pressure circuits, copper or
aluminum tubing may be used.
Flared Fittings
• 45 degrees (SAE standard -- Society of Automotive Engineers)
• 37 degrees (JIC standard -- Joint Industry Committee)
• Inverted flare
– A 45-degree flare is formed inside of the fitting.
• Two-piece flare
– A tapered nut aligns and seals the flared end of the tube.
• Three-piece flare
– A three-piece flare fitting consists of a body, sleeve, and nut and
fits over the tube. The sleeve free-floats, permitting clearance
between the nut and tube and aligning the fitting. When
tightened, the sleeve is locked without imparting twist to the
flared tube.
• Self-flaring
– These fittings use a wedge-type sleeve that, as the sleeve is
tightened, is forced into the tube end, spreading it into a flare.
Caution
• Never attempt to cross-couple SAE and JIC
fittings.
• The result will be to damage both.
Flareless Fittings
• Ferrule fittings
– Consist of a body, a compression nut, and a ferrule.
– A wedge-shaped ferrule is compressed into the fitting body by
the compression nut, creating a seal between the tube and the
body.
• Compression fittings
– These are used with thin-walled tubing and are sealed by
crimping the end of the tube to form a seal.
• O-ring fittings
– The principle is similar to ferrule-type fittings except that a
compressible rubber compound O-ring replaces the ferrule.
– As the fitting nut is torqued, the O-ring is compressed, forming a
seal between the tube and the fitting body.
– Several different types of O-rings are used, including round
section, square section, D-section, and steel-backed.
Quick-release Couplers
• When hydraulic lines have to be frequently
connected and disconnected, a quick release
coupler is used.
• A quick coupler is a self-sealing device that shuts
off flow when disconnected.
• Quick-release couplers consist of a male and
female coupler. There are four types:
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Double poppet.
Sleeve and poppet
Sliding seal
Double rotating ball
Hydraulic Fluids (1 of 2)
• Hydraulic fluids used in truck hydraulic systems
may be:
– Specialty hydraulic oils
– Engine oil
– Transmission oil
• Always check when adding to or replacing hydraulic
oil.
• Synthetic hydraulic oils are commonly used in
today’s hydraulic circuits because they have wider
temperature operating ranges and offer greater
longevity.
Hydraulic Fluids (2 of 2)
• Hydraulic oils must:
– Act as hydraulic media to transmit force
– Lubricate the moving components in a hydraulic
circuit
– Resist breakdown over long periods of time
– Protect circuit components against rust and corrosion
– Resist foaming
– Maintain a relatively constant viscosity over a wide
temperature range
– Resist combining with contaminants such as air,
water, and particulates
– Conduct heat
Safe Practice (1 of 2)
• Truck hydraulic circuits are designed to run at high
pressures and support high loads. It is essential
that you work safely around chassis hydraulic
equipment. Some basic rules:
– Never work under any device that is only supported
by hydraulics.
– A raised dump box or chassis hoist must be
mechanically supported before you work under it.
– Just as when using a floor jack, you must use some
means of mechanically supporting any raised
equipment or components.
Safe Practice (2 of 2)
• Hydraulic circuit components can retain high
residual pressures.
• The system does not have to be active for
this to be a hazard.
• Ensure that pressures are relieved
throughout the circuit before opening it up.
• Crack hydraulic line nuts slowly and be sure
to wear both safety glasses and gloves.
Summary (1 of 3)
• Fundamental hydraulic principles include
Pascal’s Law, Bernoulli’s Principle, and how
force, pressure, and sectional area are used
in hydraulic circuits to produce outcomes.
• A typical simple hydraulic circuit consists of a
reservoir, pump, valves, actuators,
conductors, and connectors.
• Hydraulic pumps convert mechanical energy
into hydraulic potential.
Summary (2 of 3)
• Valves manage flow and direction through a
hydraulic circuit.
• Actuators such as hydraulic cylinders and
motors convert hydraulic potential into
mechanical movement.
• Hydraulic oil is used to store and transmit
hydraulic energy through a hydraulic system.
• ANSI and ISO graphic symbols are used to
represent hydraulic components and
connectors in hydraulic schematics.
Summary (3 of 3)
• Maintenance procedures on truck hydraulic
systems begin with ensuring the system is clean
both inside and outside the circuit.
• Routine replacement of hydraulic fluid, sometimes
accompanied by system flushing, is recommended
to minimize system malfunctions and downtime.
• Hydraulic circuit testers are used to analyze
hydraulic circuit performance.
• A hydraulic tester consists of flow gauge, pressure
gauge, temperature gauge, and gate valve.