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## Hydraulics

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### Hydraulic pumps convert mechanical energy into hydraulic potential. Summary (2 of 3) Valves manage flow and direction through a hydraulic circuit. – PowerPoint PPT presentation

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Title: Hydraulics

1
Chapter 13
• Hydraulics

2
Objectives (1 of 2)
• 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.

3
Objectives (2 of 2)
• Outline the properties of hydraulic fluids.
• Identify graphic symbols.
• Interpret a hydraulic schematic.
• Perform maintenance procedures on truck hydraulic
systems.

4
Hydraulics
• The term hydraulics is used to specifically
describe fluid power circuits that use
liquidsespecially formulated oilsin confined
circuits to transmit force or motion.
• Hydraulic circuits
• Hydraulic brakes
• Power steering systems
• Automatic transmissions
• Fuel systems
• Wet-line kits for dump trucks
• Torque converters
• Lift gates

5
Pascals 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.

6
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

7
Atmospheric Pressure
• 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).

8
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 P x A

9
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 _at_ 60 F

10
Torricellis Tube
• Evangelista Torricelli (16081647) 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
Torricellis tube would support 29.92 inches of
mercury.

11
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

12
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.

13
Hydraulic Levers (1 of 2)
• Hydraulic levers can be used to demonstrate
Pascals law
• Pressure equals force divided by the sectional
area on which it acts.
• (PF\A)
• Force equals pressure multiplied by area.
• ( F P x A)

14
Hydraulic Levers (2 of 2)
• 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.

15
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).

16
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
• 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.

17
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.

18
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.

19
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

20
Bernoullis Principle (1 of 2)
• Bernoullis 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.

21
Bernoullis Principle (2 of 2)
22
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 cross-currents in
a hydraulic circuit, resulting in friction losses
and pressure drops.

23
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.

24
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 open-center hydraulic system on
a truck is power-assisted steering.

25
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.

26
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.

27
Hydraulic Components
• Reservoirs
• Accumulators
• Pumps
• Valves
• Actuators
• Hydraulic motors
• Conductors and connectors
• Hydraulic fluids

28
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

29
• The gas and hydraulic oil occupy the same chamber
but are separated by a piston, diaphragm, or
• 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.

30
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
• Lift pumps
• Power steering pumps
• Transmission pumps
• Lube pumps

31
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.

32
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

33
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.

34
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
crescent-shaped 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.

35
Rotor-gear Pumps
• 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.

36
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

37
Balanced Vane Pumps
• 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.

38
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
acting on the discharge side of the rotor and
none on the inlet side because the inlet oil is
under little or no pressure.

39
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
• 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.

40
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.

41
Axial Piston Pumps
• 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.

42
• 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

43
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

44
Directional Control Valves (1 of 3)
45
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

46
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.

47
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.

48
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.

49
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.

50
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.

51
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.

52
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.

53
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.

54
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

55
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.

56
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)

57
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.

58
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 crimp-type or
reusable fittings.

59
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.

60
Assembling Hose Fittings
• Sealing fittings
• Fittings can be sealed to couplers using the
following
• O-rings
• Nipple and seat (flair)
• When making hydraulic connections, ensure that
the coupler fittings are compatible with each
other.
• 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

61
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.

62
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
• Use two wrenches when tightening unions to avoid
twisting hose.
• Never over-torque hydraulic fittings.

63
Shop Talk
• When tightening the fittings on a pair of
hydraulic couplers, always use two wrenches to
avoid twisting hoses or damaging adapters.

64
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.

65
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.

66
Caution
• Never attempt to cross-couple SAE and JIC
fittings.
• The result will be to damage both.

67
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.

68
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
• Double poppet.
• Sleeve and poppet
• Sliding seal
• Double rotating ball

69
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
todays hydraulic circuits because they have
wider temperature operating ranges and offer
greater longevity.

70
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

71
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.

72
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.

73
Summary (1 of 3)
• Fundamental hydraulic principles include Pascals
Law, Bernoullis 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.

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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.

75
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.