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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

Chapter 13
  • Hydraulics

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.

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

  • 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

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.

  • 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
  • Hydrodynamics is the science of moving liquids to
    transmit energy.
  • We can define hydrostatics and hydrodynamics as
  • Hydrostatics low fluid movement with high system
  • Hydrodynamics high fluid velocity with lower
    system pressures

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

  • 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
  • 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
  • F P x A

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

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

  • A manometer is a single tube arranged in a
    U-shape used to measure very small pressure
  • 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
  • A gauge therefore reads zero when exposed to the
  • To avoid confusing absolute pressure with gauge
  • Absolute pressure is expressed as psia.
  • Gauge pressure is usually expressed as psi or

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)

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
  • 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 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
  • 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
  • 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
  • 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 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

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.

Bernoullis 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 cross-currents in
    a hydraulic circuit, resulting in friction losses
    and pressure drops.

Types of Hydraulic Systems
  • Hydraulic systems can be grouped into two main
  • 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
  • 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.

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

  • 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
  • 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
  • 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
  • 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

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
  • Three types of gear pumps are used
  • External gear
  • Internal gear
  • Rotor gear

External-gear Pumps
  • Two intermeshing gears are close-fitted within a
  • 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

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
  • 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

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.

Vane Pumps
  • Vane pumps are also used extensively in hydraulic
  • Truck power-assisted steering systems use vane
  • 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
  • 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
  • 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

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
  • A rotating cylinder with piston bores machined
    into it rides against an inclined plate.
  • The pistons are arranged parallel with the pump
  • 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 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
  • 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
  • Pilot valves
  • Pilot valves may be controlled mechanically,
    hydraulically, or electrically.

  • 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
  • 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
  • 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
  • An alternative to one double-acting vane cylinder
    for this application would be a pair of opposing

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
  • 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

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
  • 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.

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
  • Tapered threads
  • O-rings
  • Nipple and seat (flair)
  • When making hydraulic connections, ensure that
    the coupler fittings are compatible with each
  • 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

  • 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
  • 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
  • Tubing should be manufactured from cold-drawn
    steel if used in moderate-to-high-pressure
  • When used in low-pressure circuits, copper or
    aluminum tubing may be used.

Flared Fittings
  • 45 degrees (SAE standard -- Society of Automotive
  • 37 degrees (JIC standard -- Joint Industry
  • Inverted flare
  • A 45-degree flare is formed inside of the
  • 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.

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

Flareless Fittings
  • Ferrule fittings
  • Consist of a body, a compression nut, and a
  • 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
  • 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
  • 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
    todays 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
  • Resist breakdown over long periods of time
  • Protect circuit components against rust and
  • 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 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.

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
  • 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
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