1asas2 - chap 1 fluid pwr (v02)(2011-10-01)

Fluid Power Systems

1. Introduction to Fluid Power Systems

Aircraft Structures & Systems II

Fluid 1-1

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Three Major Topics

• Fluid Power System

– Hydraulic

– Pneumatic

• Environmental

Control System

– Air Conditioning

– Cabin Pressurisation

– Oxygen

• Auxiliary System

– Fire

– Icing

– Rain

– Water & Waste

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Week

No.

Lecture Topic

(Chapter No.)

Tutorial

No.

Lab

No. Remarks

1 Aircraft Fluid Power Systems (1) 16/4 Topic 1 & 2

- No Lab

2 Aircraft Fluid Power Systems (1) 23/4 Topic 3

- 1

(even)


1 1

(odd)


- 2

(even)


- 2

(odd)

6 Aircraft Fluid Power Systems (1) 21/5 Topic 6 & 7

2 3

(even)

7 Aircraft Environmental Control Systems (2)

Topic 1 & 2 28/5 -

3

(odd) Quiz 1

8 COMMON TEST WEEK (04/6)

9 TERM BREAK

10 TERM BREAK Fluid 1-3

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Topic 2 25/6 -

4

(even)


Topic 3 02/7 3

4

(odd)


Topic 3 & 4 09/7 -

5

(even)


Topic 4 16/7 -

5

(odd)

15 Aircraft Auxiliary Systems (3) 23/7 Topic 1

4 6

(even)

16 Aircraft Auxiliary Systems (3) 30/7 Topic 2

- 6

(odd)

17 Aircraft Auxiliary Systems (3) 06/8 Topic 3 & 4

5 - Quiz 2

EXAM STUDY WEEK / REVISION

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S/No Assessment Mode Percentage

1. Quiz 1 & 2 5%

2. Tutorials (total of 5) 10%

3. Practicals 20%

4. Common Test 25%

5. Examination 40%

Total 100%

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Aircraft Fluid Power

System

Course: Diploma in Aerospace

Tech

Section

: AT Centre: N.A.

Module: Aircraft Structures and Systems 2 ID/Cat.: 005829 /

1ASAS2

Document Title: Module Notes and Slides (Chapter 1 – Aircraft Fluid Power Systems)

Remarks: (V02)(2011-10-01)

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Topics to be covered

1. Introduction to Fluid Power

2. Hydraulic Basics

3. Hydraulic Plumbing

4. Hydraulic Components

5. Aircraft Hydraulic system

6. Pneumatic

7. Maintenance

Fluid Power Systems


Fluid power systems are mechanical

systems in which a moving fluid

performs work. This fluid may be either

a compressible gas or an

incompressible liquid. Systems that

use compressible gases are called

pneumatic systems, while those that

use incompressible liquids are called

hydraulic systems.

Hydraulic and pneumatic systems in aircraft provide a means for the

operation of large aircraft components. The operation of landing gear,

flaps, flight controls, and other components is largely accomplished with

hydraulic power. Pneumatic systems are used in some aircraft designs

to perform the same type of operations performed by hydraulic systems.

However, the majority of aircraft that have pneumatic systems use them

only as backup systems for the operation of hydraulic components when

the hydraulic system has failed.

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Hydraulics is used in a wide range of flight controls, ranging from the

traditional hydro-mechanical to electro-mechanical fly-by-wire systems;

from smart actuators incorporating closed-loop electronics to fiber optic

equipment that “flies-by-light”.

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Application of Hydraulics in the Aircraft

• Primary Flight Controls

– Elevators, Rudders, Ailerons, Canards

• Secondary Flight Controls

– Flaps, Slats, Spoilers, Airbrakes

• Utilities

– Landing Gear

– Gear steering

– Aerial Refueling probes

– Cargo doors / ramps

– Hoists

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A fluid power system is much like an electrical system. It must have a

source of power, a means of transmitting this power, and finally some

type of device to use the power.

An open hydraulic or pneumatic system such as that used by

hydroelectric power plants or windmills works well for the production of

electrical energy, but has no practical application to airborne systems.

To apply fluid power to aircraft systems, we must enclose the fluid, move

it through a system of rigid lines and flexible hoses, and put its energy to

use in various types of actuators and motors.

Among the first hydraulic systems

used on airplanes was the hydraulic

brake.

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Hydraulics Applications

• Energy Source

– Pump

• “Transporting” Energy

– Fluid is the medium used to transfer the energy from

the source to the area of interest

• Application

– Convert fluid energy into mechanical

workdone, e.g. turning shaft (torque), push an

object (linear displacement)

Battery

Cables

Motor,

soleniod

Electrical

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4. Hydraulic System Components.ppt

4.%20Hydraulic%20System%20Components.ppt

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HYDRAULIC ELECTRICAL MECHANICAL THERMALPRESSURE VOLTAGE FORCE TEMPERATURE

FLOW CURRENT VELOCITY HEAT TRANSFER

FLOW RESTRICTION RESISTANCE FRICTION THERMAL RESISTANCE

Between Fluid power, Electrical, Mechanical &

Thermal Engineering

Potential Difference in Fluid Power P = ½ v2 k

where P is the Pressure drop between 2 points in the same

circuit, k is the pressure drop coefficient, is fluid density, and v is

the fluid velocity.

Potential Difference in Elect Power V = I R

Fluid 1-13

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Between Hydraulic & Electrical

In Series

Fluid Power k = k1+ k2+ …+kn

v = v1= v2=…= vn

Elect Power R = R1+ R2+ R3

I = I1= I2=…= In

Fluid 1-14

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Between Hydraulic & Electrical

In Parallel

Fluid Power 1/k = 1/k1+ 1/k2+ ---+1/kn

P = P1= P2=…= Pn

Elect Power 1/R = 1/R1+ 1/R2+ 1/R3

V = V1= V2=…= Vn

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All hydraulic systems must have

a fluid that is capable of flowing

through the system with a

minimum of friction.

There must be a reservoir to

hold enough fluid to actuate all

of the components.

A pump is needed to move the

fluid against the opposition of

the system, and

there must be actuators to

convert the pressure of the fluid

into a mechanical force to

perform work.

There must be flow control

valves to direct the fluid to the

correct component, and

pressure control valves to

maintain the correct pressure

within the system. Fluid 1-16

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Pressure

Relief

Valve

Selector

(Directional

Control Valve)

Actuator

(Cylinder)

Pump

Reservoir

Electrical

motor Power Pack

Valve

Hydraulic Actuators

A typical hydraulic schematic diagram in graphical symbols:

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Opening.ppt

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2. Basic Principles of Hydraulics

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Basic Hydraulic


System

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Outline

• Theory

– Law of Conservation of Energy

– Pascal’s Law

– Bernoulli’s Principle

– Boyle’s law

– Charles’ Law

• Pros and Cons

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The theory of hydraulics is simply the application of basic fluid

mechanics principles, such as

• Law of Conservation of Energy

• Pascal’s Law

• Bernoulli’s Principle

Law of Conservation of Energy

We can neither create nor destroy energy, but the same amount of energy

can only be transformed from one form to another.

Energy in a fluid power system may be in one of the two forms: potential

or kinetic. Potential energy in a fluid power system is expressed in the

pressure of the fluid. Kinetic energy is expressed in the velocity of the

moving fluid.

• Power = (Force Distance) Time

= Force Velocity

= Volume flow rate Pressure Fluid 2-3

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The terms area, pressure, force, stroke, and volume are mathematically

related. This relationship establishes the foundation upon which

hydraulic systems are based.

Consider the relationship of force (F), pressure (P), and area (A). If any

two of these factors are known, it is possible to calculate the third.

10 lb 10 lb

1 in2 4 in2

10 lb acting on 1 in2 area will result

in a pressure of 10 lb/1 in2 = 10 psi

10 lb acting on 4 in2 area will result

in a pressure of 10 lb/4 in2 = 2.5 psi

Fluid 2-4

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Liquids are regarded as being incompressible. This means that the

volume of a given quantity of liquid will remain constant even though it is

subjected to high pressure.

Similarly, a given output volume from a hydraulic pump will provide an

equal volume of fluid at the actuator operating unit.

Because of the incompressibility nature of a liquid, safeguards must be

provided in hydraulic systems to allow for the expansion and contraction

of fluid as temperature changes.

Pascal’s Law

When pressure is applied to a confined liquid, this pressure is

transmitted equally in all directions and at right angles to any surface

containing the fluid.

Consequently, a force acting on a small area can transmit a pressure

which then acts on a large area and thereby creating a larger force.

This law holds under static conditions and when the force of gravity is

not taken into consideration.

Fluid 2-5

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Fluid 2-6

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A_Pascal.swf

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10 lb

2. 10 lb acting on a stopper of

area 1 in2 …… 1. A vessel filled with

incompressible fluid

3. …. will result in 10 lb of force acting at

right angle on every 1 in2 of the vessel

wall i.e. a pressure of 10 lb/1in2 = 10 psi

1. Hence, if a pressure of 10 lb/in2

(or 10 psi) acts in the space

behind a cylinder piston…

3. Hence, a pressure of 10 psi

produces 200 lb of force.

2. …. And the piston has an area of 20 in2 , the force

on the piston is 10 lb/ in2 x 20 in2 = 200 lb

Fluid 2-7

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10 L 1 L

One of the principal advantages of hydraulics is the fact that force can be

multiplied to almost any degree by the proper application of hydraulic

pressure.

1. 10 lb acting over an

area of 1 in2 ……

2. will result in a

pressure of

10 lb/in2 being

transmitted equally

throughout the fluid

3. … which will be able to

balance a force (on the area

of 10 in2 ) whose magnitude

is F = P x A = 10 lb/ in2 x 10 in2

= 100 lb

1. A 10 lb force here … 2. … can be balanced with a

100 lb force

Fluid 2-8

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1 in2

10 in2 10 in

1 in

3. Hence during the same time that the small piston moves 10 in,

the large piston moves only 1 in. The speed is also decreased.

1. Moving the small piston 10 in

displaced 10 in3 of fluid…

2. 10 cm3 of fluid displaced by small piston

will be transmitted to this side. The big

piston will move up by 10 in3 10 in2 = 1 in.

By the Law of Conservation of Energy, the multiplication of force is

accomplished at the expense of distance and speed respectively, so that

energy and power remains unchanged from small piston to big piston.

Energy = Force Distance moved Power = Force Speed

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Fluid 2-10

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A_HeadPressure.swf

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Fluid 2-12

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A_Force_Conversions.swf

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Bernoulli’s Principle

Between any two points on a streamline in steady, inviscid,

incompressible flow, the Bernoulli equation can be applied in the form

TpzVp streamline a along constant 2

21

The sum of the static pressure ( ), dynamic pressure ( ), and

hydrostatic pressure ( ), is termed the total pressure .

Tpp 2

21 V

z

The Bernoulli equation is also an energy equation for an invisicd,

incompressible, steady flow ie. the sum of the various energies (= total

energy) of the fluid remains constant as the fluid flows from one point to

another along a streamline.

Here, an useful interpretation of the Bernoulli equation is introduced with

the concept of hydraulic grade line (HGL) and the energy line (EL).

The Bernoulli equation is a statement that the TOTAL pressure remains

constant along a streamline.

Fluid 2-13

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The concept of “head” is introduced by dividing each term in the

previous equation by the specific weight, , to express the Bernoulli

equation in the following form: g

Hzg

Vp streamline a along constant

2

2

Each of the terms in the above equation has the units of length and

represents a certain type of head.

- pressure head; - velocity head; - elevation head

p

g

V

2

2

z

The Bernoulli equation states that the sum of the pressure head, the

velocity head, and the elevation head is constant along a streamline.

This constant is called the total head, . H

Fluid 2-14

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The energy line is a line that represents the total head available to the

fluid. It remains constant along the streamline if the assumptions of the

Bernoulli equation are valid. The sum of the pressure head and elevation

head is given by the hydraulic grade line.

- Static pressure tap

connected to

piezometer tube

- Pitot-static tube

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Fluid 2-16

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A_boyle.swf

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PUMP

0 psi

0 psi

Relief valve set at

500 psi

10 in2

Pump produces flow. Pressure is the result of resistance to flow. The

resistance may result from either a load on the actuator, or a restriction in

the flow path.

NO LOAD ON CYLINDER

- Pressure gauge registered 0 psi when there is no load

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A_charles.swf

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400 psi

0 psi

10 in2

LOAD = 4000 lb



the flow path.

Load of 4000 lb will result in a pressure

P = F/A = 4000 lb / 10 in2 = 400 psi.

Pressure at outlet remains at 0 psi as

there is no resistance to flow

4000 lb LOAD ON CYLINDER

- Pressure gauge registered 400 psi when there is a 4000 lb load

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500 psi

0 psi

10 in2

LOAD = 5000 lb



the flow path.

As cylinder load increases, inlet pressure increases …

… until CYLINDER LOAD = 5000 lb

If load exceeds 5000 lb, the piston stops

moving to the right. Pressure relief valve

opens to direct excess

flow back to the reservoir

Fluid 2-20

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Two points concerning a “real” hydraulic system should be noted:

1. There will be a loss of power as energy is changed from one form to

another. This loss is usually in the form of heat and sound.

2. There will be a certain continuous loss of hydraulic fluid from the high

pressure side of the system to the low pressure side via drain lines

forming a so-called leakage flow back to the reservoir.

Electrical/

Thermal

power

Mechanical

power Hydraulic

power

Mechanical

power

Power

Transmission Hydraulic

power

Hydraulic

Control

and

Regulating

Units

Operating

Elements to

be actuated

USER

Hyd. Cylinder

Hyd. Motor

Hydraulic

Pump

DRIVE

Electric Motor /

I.C. Engine

Fluid 2-21

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The advantages of hydraulic systems over other methods of power

transmissions are:

• able to produce high forces (torques) with compact size ie. High

power to weight ratio.

• automatic force adaptation

• movement from standstill under load possible

• stepless change of speed, torque, force, etc. can be achieved simply

• simple overload protection

• can control both extremes of fast and slow actuator speed with just a

relatively simple circuit

• ease of shock absorption during actuator reversals, starting and

stopping

• power from one prime mover (power pack) can be transmitted at the

same time to several actuators

Fluid 2-22

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Opening.ppt

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• Large power amplification

• High power to weight ratio

• Less hazardous than electrical in fuel environment

• Hydraulic leak is normally safe

• No EMI as in electrical system

Fluid 2-23

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The Disadvantages of hydraulic systems over other methods of power

transmissions are:

• Leaks

• Hot

• May be flammable

• Dirty (in leaked area)

• Slipping hazard (in leaked area)

• Sensitive to contamination

• Difficult to use in low power application

Fluid 2-24

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Opening.ppt

Opening.ppt

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Summary

• Theory

– Law of Conservation of Energy

– Pascal’s Law

– Bernoulli’s Principle

– Boyle’s law

– Charles’ Law

• Pros and Cons

Fluid 2-25

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3. Hydraulic Fluids and Plumbing Components

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Hydraulic Fluids &

Plumbing Components

Aircraft Fluid Power System

Fluid 3- 1

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Outline

• Hydraulic Fluid

• Fluid Lines

• Fittings

• Hoses

• Quick-Disconnect Couplings

• Seals

Fluid 3- 2

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HYDRAULIC FLUIDS

Hydraulic fluids serve as the medium for the transmission of pressure

and energy in a hydraulic system.

They also act as a lubricating medium, thereby reducing the friction

between moving parts and carry away some of the heat.

Hydraulic fluids perform various tasks:

• transmit hydraulic energy or motion

• provide internal lubrication of hydraulic components

• provide an effective pneumatic seal inside a hydraulic component

• prevent corrosion of internal parts of hydraulic components

• help to dissipate heat generated in the hydraulic system

Fluid 3- 3

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Considerations for using Hydraulic fluids

• Fluid Properties

– Viscosity

– Chemical stability

– Flammability

– Flash Point

– Fire Point

– Lubrication

– Operating

Temperature &

Pressure

– Compressibility

• Others

– Standardization

– Availability

– Safety

– Environment

4

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There are three principal types of hydraulic fluids:

• Vegetable-base fluids

• Mineral-base fluids

• Phosphate ester-base fluids

Vegetable-base fluid (MIL-H-7644)

• composed essentially of castor oil and alcohol

• dyed blue or blue-green for identification

• natural rubber seals are used with this type of fluid

• not used in modern aircraft hydraulic systems but may still be found

in some brake systems of older aircraft

• considered obsolete by many industries

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Mineral-base fluid (MIL-H-5606)

• most commonly used type of hydraulic fluids today

• a high-quality petroleum oil colored in red

• used in many systems, especially where the fire hazard is

comparatively low

• has good lubricating characteristics and chemical stability

• contain additives to inhibit foaming and prevent corrosion

• exhibit little viscosity change with temperature

• main disadvantage is that it is flammable!

• seals used are of synthetic rubber, leather or metal composition

• a mineral-base, synthetic hydrocarbon fluid called Braco 882

conforming to MIL-H-83282 is used extensively by the military in place

of Mil-H-5606 because of its increased fire-resistance properties

Fluid 3- 6

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Hydrocarbon-base fluid (MIL-H-83282)

• MIL-H-83282 replaces MIL-H-5606 which was the principal hydraulic

fluid used in naval aircraft.

• MIL-H-83282 has a synthetic hydrocarbon base and contains

additives to provide the required viscosity and anti-wear

characteristics.

• The oxidation inhibiter was included to reduce the amount of

oxidation that occurs in petroleum-based fluids when they are

subjected to high pressure and high temperature, and to minimize

corrosion of metal parts due to oxidation and resulting acids.

• The temperature range is between –40°F to +275°F. At lower than

-40°F, the fluid becomes too viscous (like peanut butter).

• Flash point and fire point of MIL-H-83282, which is fire resistant,

exceeds that of MIL-H-5606 by more than 200°F.

Fluid 3- 7

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Phosphate ester-base fluid (MIL-H-8446)

• a synthetic fluid formulated to be extremely fire resistant

• not to be confused with fire-proof, phosphate ester fluids can still

burn when stretched beyond certain limits

• utilized in many modern transport aircraft for its high safety – aircraft

hydraulic systems are highly pressurized and any leak in the system

usually takes the form of a combustible spray and cause fire hazard

• Continual development of more advanced aircraft resulted in

continual modification of fluid specification – Type I, II, III and IV fluid.

• Typical examples of latest Type IV fluids are Skydrol LD-4 and Skydrol

500B-4, which are colored light-purple.

• exhibit a wide range of operating temperatures, from somewhere

around -65oF (-54oC) to over 225oF (107oC).

• Seals, gaskets, and hoses used with this type of fluid are made of

specialized materials: butyl synthetic rubber or Teflon fluorocarbon

resin Fluid 3- 8

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Comparison of Hydraulic Fluids

Characteristic Units MIL-H5606 MIL-H-83282 Skydrol LD

Density lb/in3 (60F) 0.0316 0.0304 0.0361

Viscosity centistokes

-65 F 2275 12,230 1130

-40 F 487 2100 320

100 F 21 23 11.2

140 F 13.5 14.6

212 F 5 3.6 3.9

Pour Point F -75 -80 <-80

Flash Point F 205 435 340

Fire Point F 250 480 360

Autoignition F 470 640 >1000

Bulk Modulus

ksi (at 3000 psi &

76 F) 266 greater than 150 275

Toxicity low low high

9

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Hydraulic fluids are required to have the following properties:

• virtually incompressible

• adequate viscosity largely unaffected by temperature changes

• maintain fluidity at low temperatures

• maintain good oxidation stability at high temperatures

• sustainable under severe operating conditions and durations

• provide sufficient measure of corrosion prevention to internal parts of

the hydraulic components, including seals and packings

• provide sufficient elements to control foaming

• provide sufficient degree of fire resistance

• do not cause undue harm to man and environment

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Hydraulic fluids have four primary enemies: water, air, heat and dirt.

• Water

Water mixed with phosphate ester-based hydraulic fluids will produce

acids, which will cause corrosion problems for the system. Even with

mineral-based fluids, water can also lead to corrosion of system parts.

• Air

Air in the system can hinder the performance of hydraulic system

components due to the eventual formation of sizeable air bubbles that

may block the flow of hydraulic fluid in the system.

• Heat

Heat degrades the performance of hydraulic fluids as it will cause

chemical decomposition of the fluids over time.

• Dirt

Dirt in the fluid reduces component effectiveness because it degrades

the lubrication property of the fluid and impedes proper functioning of

the equipment, leading to component and even system failure.

Fluid 3- 11

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FLUID LINES, FITTINGS AND SEALS

Fluid Lines

Aircraft hydraulic systems require fluid lines for transmission of

hydraulic fluid to the various hydraulic components.

Fluid lines are made of rigid, semi-rigid or flexible tubes, depending on

the application.

A tube is defined as a hollow object, long in relation to its cross-section,

with a uniform wall thickness. Tubes used for fluid lines are usually

round in cross-section.

Typical pressure of the fluid in fluid lines of an aircraft hydraulic system

varies between 3000 to 5000 psi. Such high-pressure fluid lines are made

in a variety of materials, including aluminum alloy, stainless steel, and

reinforced flexible hoses.

A rigid fluid line would be one that is NOT normally bent to shape or

flared. Directional changes and connections are made by the use of

threaded fittings. Fluid 3- 12

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A fitting is a device used to

connect two or more fluid lines,

and is made from a variety of

materials for many different

applications.

Semi-rigid fluid lines are bent and formed to shape and have a relatively

thin wall thickness in comparison to rigid lines. Various types of fittings

are used to make connections between semi-rigid tubes.

Flexible fluid lines are made from rubber or synthetic materials and are

usually called hoses. Depending upon the pressure they are designed to

carry, hoses may have reinforcing material wrapped around them. Like

other types of fluid lines, various types of fittings are also used to attach

hoses to each other or to other components.

Fluid 3- 13

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Rigid Fluid Lines

The wall thickness

increases with the schedule

number. As the schedule

number increases, the

pressure carried by the pipe

can be greater, but the fluid-

carrying capability of the

pipe will be reduced due to

a smaller inner diameter.

The term pipe refers to a rigid fluid line that may be made in standardized

combinations of outside diameter (OD) and wall thickness.

The various combinations of diameter and wall thickness are specified as

schedule numbers and have been standardized by ANSI (American

National Standards Institute). Pipe is specified by its nominal diameter

(¼ in, ½ in, 1 in, etc) and the desired schedule number. A pipe’s nominal

diameter cannot be obtained by direct measurement.

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Pipes are joined by fittings that use threads cut in the wall of the pipe. To

provide a fluid-tight seal, the threads are tapered, unlike machine-screw

threads. As two parts are screwed together, the taper will cause a very

tight metal-to-metal seal.

The pipe shown has external

threads with the taper occurring on

the crests. For internal threads, the

taper is reversed and affects the

thread roots.

Large-scale use of pipe on aircraft is impractical because of weight.

However, certain aircraft components use pipe threads. This requires the

use of special fittings with pipe threads that connect the aircraft

component to the rest of the hydraulic system.

Pipes are usually installed in low-

pressure hydraulic or pneumatic

systems and are mostly made of

5052-0 aluminum alloy.

Fluid 3- 15

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An example showing an externally threaded

pipe connected to an elbow fitting with

corresponding internal pipe threads.

Fluid 3- 16

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http://upload.wikimedia.org/wikipedia/commons/b/b1/Malefemalepipe.jpg

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An example showing an externally threaded

pipe connected to elbow fittings whenever

adjustment is needed to align the plumbing.

Fluid 3- 17

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Semi-rigid fluid lines are usually referred to as tubes or tubing.

Unlike pipes, tubes can be bent at various locations along their lengths.

Tubes used for fluid lines are sized according to their outside diameter,

which is given in increments of sixteenths of an inch, . The wall

thickness of these metal lines are relatively standard with less variations.

Often, the tube may be referred to as a or tube rather

than by its fractional dimensions.

Tube fittings also use dash numbers to indicated their size, eg. A -8 fitting

is sized to fit an or tube.

Tubes are made from several metals. High pressure hydraulic tubings

usually use 2024-T aluminum alloy or annealed stainless steel tubing.

In any case, only the materials authorized by the manufacturer, or

approved substitutes, may be used on a specific aircraft system.

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Semi-rigid Fluid Lines

Fluid 3- 18

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Semi-rigid pipe lines around the hydraulic pack

Fluid 3- 19

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Because of the thin wall thickness of semi-rigid tubes, threads usually

cannot be cut in the tube. Special fittings have been designed to allow

tubes to be connected to other tubes as well as to aircraft system

components.

These fittings may be classified as flared, flare-less, swaged, soldered, or

brazed. Many of the fittings used for these connections are standard

parts and carry AN, AND, or MS specification numbers.

Flared fittings required a 37o flare to be formed on the end of the tube.

The flare of the tube matches a cone on the fitting.

The fitting is threaded with

standard machine-screw threads.

A special nut and a sleeve are used

to pull the flare into contact with

the cone and form a fluid-tight

metal-to-metal seal.

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Flared fittings are made to AN or MS standards. Flared fittings made

prior to WWII were made to an AC standard. Hence AC fittings may still

be found on older aircraft. The AN fitting has a space on each side of the

threads, while the AC does not.

AC fittings found in a system may be replaced with AN fittings.

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The basic components of a flared connection are the AN818 nut, the

AN818 sleeve, and one of a number of fittings with a cone to match the

tube’s flare.

Fittings commonly used are classified as tube fittings, bulkhead fittings,

universal fittings and pipe-to-tube (or pipe-to-AN) fittings.

Fittings are specified by AN or MS numbers that identify the function of

the fitting.

The fitting is sized by the OD of the tube it is

used with, which is expressed in of an

inch. An AN818 fitting for a tube would

have a designation of AN818-4, and for a

tube it would be AN818-12.

Aluminum-alloy fittings are indicated by a D

before the dash. The letter C indicates the

fitting is made form corrosion-resistant steel

(CRES). No letter before the dash number

indicates that the material is steel.

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Typical AN tube fittings for aircraft

tubing installation.

The use of angled fitting allows

rotation of the fitting to any

desired orientation before

tightening. This allows proper

tube and component alignment.

It is often necessary to have fuel,

oil, and other tubes pass through

structural portions (bulkheads) of

an aircraft. This requires a fitting

to the bulkhead. Two fittings of

this type are shown. A

corresponding nut would be used

to fasten the fitting to a bulkhead.

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Different Fitting Examples

24

AN844 ELBOW – NPT Pipe thread

to Hose, 45º

AN834 TEE, Flared Tube,

Bulkhead And Universal

AN919 REDUCER, External

Thread

MS21900 ADAPTOR, Flareless

tube to AN Flared Tube

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The term universal applies to fittings used to connect a component with

internal machine-screw threads to a tube or hose. This type of fitting is

used in installations that require a certain degree of flexibility.

Machine screws have straight threads and will not provide a fluid-tight

seal. A rubber O-ring gasket (eg. AN6290) is required to prevent fluid

leakage and is installed with a universal nut (eg. AN6289).

They may be used with either stationary or moving units. When used

with moving units, such as hydraulic actuating cylinders, they are often

called banjo fittings.

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banjo fittings

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Many aircraft components use internal pipe threads for connections.

This requires the use of a special fitting called a pipe-to-AN nipple.

The fluid-tight seal is provided by pipe threads

for the component and a metal-to-metal flare

for the tube.

Various designs of pipe-to-AN fittings are

available, such as 45 and 90o elbows and tees

with one leg having pipe threads.

The use of pipe-to-AN fittings eliminates the need for rubber seals and

nuts. However, directional positioning of the line from the component is

not as versatile as with a universal fitting.

Flared fittings are usually made of aluminum, steel, or stainless steel,

with the fitting being the same material as the tube on which it is used.

An exception is the use of steel fittings in higher temperature areas, such

as in an engine compartment.

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High-pressure fluid lines are made of material which is too hard to form

satisfactory flares. Flareless fittings, made to MS Standards, are

designed to eliminate the need for a flare on the tube.

The flareless tube fitting consists

of three units: a body, a sleeve, and

a nut.

The body of the fitting has a

counterbored shoulder against

which the end of the tube rests.

The counterbore has a cone angle

of 24o which, upon assembly,

causes the cutting edge of the

sleeve to cut into the outside of the

tube.

Tightening of the nut forces the

sleeve to form the metal-to-metal

seal. Body of flareless fittings have no

flare cone nor space between the

threads and the end of the fitting. Fluid 3- 28

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Calculation of Pipe Size

When liquid flows through a pipe without changing its flowing condition

ie. steady flow, its flow rate remains the same irrespective of change in

sectional area of pipe.

A1

A2

v1

A3

v2

v3

332211, vAvAvArateflowQ

• Volume flow rate, Q = A x v where Q = flow rate (m3/s)

A = flow cross section (m2)

• Mass flow rate, m = x Q v = flow velocity (m/s)

= density of fluid (kg/m3)

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Example

Calculate the diameter of a pipe for a hydraulic pump of delivery flow rate

of 60 l/min at a velocity of 2 m/s.

Solution :

m0252.041054A

pipeofdiameter,D

D4

A

m1052

10

v

QA

2v

1060

1

10

16060Q

vAQ

4

2

24

sm

sm3

sm

sm3

smin

lm

3minl

minl

3

33

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Plumbing lines are marked with colored tapes and symbols to identify its

contents. The most commonly used symbols are shown as follows:

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Flexible Fluid Lines

Because of the need for flexibility in many areas of airplane construction,

it is often necessary to employ hoses instead of semi-rigid tubings for

the transmission of pressurized fluids and gases.

For many years most hoses used on aircraft were made of rubber to one

of three MIL-H specifications. These were considered the “universal

standard” for aircraft.

Eventually, synthetic materials such as Teflon and several elastomers

have proven to be superior in many ways to rubber hoses.

Flexible hose for use in aircraft systems is manufactured under four

different pressure categories:

• low-pressure – maximum 300 psi (2068 kPa)

• medium-pressure – 300 to 1500 psi (2068 to 10342 kPa)

• high-pressure – 1500 to 3000 psi (10342 to 20685 kPa)

• extra-high pressure – 3000 to 6000 psi (20865 to 41370 kPa)

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Low-pressure aircraft hose

(max. pressure 300 psi)

This hose conforms to

specification MIL-H-5593.

The inner tube of the hose consists of synthetic rubber with a braided-

cotton reinforcement. The outer cover is synthetic rubber.

Low-pressure hose is seldom used for aircraft hydraulic systems but

used mainly for instrument air or vacuum systems, automatic pilots, and

instruments where the maximum pressure of the hose is not exceeded.

Lightweight plastic or rubber tubing has replaced MIL-H-5593 hose in

newer installations.

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This type of hose has a synthetic rubber tube with one layer of braided

cotton and one layer of stainless-steel braid for reinforcement. All of this

is encased in a synthetic rubber-impregnated cotton braid.

MIL-H-8794 medium-pressure hose is approved for aircraft hydraulic

(mineral-based), pneumatic, coolant, fuel, and oil systems.

Many hoses currently being manufactured for medium pressure range

use inner tubes made of tetrafluoroethylene (TFE or Teflon), which can be

used for practically all fluids that may be encountered on an airplane.

Medium-pressure Teflon hose is produced under spec. MIL-H-27267.

Medium-pressure aircraft hose




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High-pressure aircraft hose




This type of hose has a seamless inner tube made of synthetic rubber

and either two or three high-tensile strength, carbon-steel wire braid

reinforcements, depending on the size of the hose.

Application of this type of hose, except for handling high pressures, is

similar to the MIL-H-8794 hose.

Another type of high-pressure hose manufactured under specification

MIL-H-38360 uses a Teflon inner tube. Such hose can be used for most of

the fluids employed in aircraft systems.

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Teflon hose can be used for practically all fluids that may be encountered

on an airplane. It is unaffected by any fuel, petroleum, or synthetic-based

oils, alcohol, coolants or solvents commonly used in airplane.

Telflon is non-aging, chemically inert, and physically stable, and it can

withstand relatively high temperatures up to 400oF (204oC) without

compromising its high operating strength.

Medium-pressure Teflon hose is covered with a stainless-steel braid, and

high-pressure Teflon hose has several layers of spiral wound stainless

steel wire and one or more layers of stainless steel braid.

One problem with Teflon hose, however, is the tendency of taking a

permanent set after it has been in service. Such hose MUST NOT be

straightened out if it is temporarily removed from aircraft.

Medium-pressure Teflon hose High-pressure Teflon hose

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In certain areas of an aircraft, it is

advisable to protect hoses from heat and

wear.

For these purposes, protective sleeves

of various types have been developed.

Fire sleeves are installed on hose in

areas where high temperatures exist,

such as in engine compartments.

Abrasion sleeves are used where the

hose may rub against parts of the

aircraft.

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A hose is sized in accordance with the size of a tube with equivalent

fluid-carrying capacities. Hence, a ½-in hose will carry the same amount

of fluid as a ½-in tube.

The hose uses the same dash number to indicate size as that used on a

comparable-size tube. Thus, a ½-in hose will be identified as a -8 hose

(eg. MIL-H-8794-8 indicates a ½-in medium-pressure hose).

Care must be taken in the selection of hose for a particular application.

Hose is selected on the basis of size, pressure rating, temperature rating,

and material. If markings on the hose are not legible or are missing, then

the hose should not be installed on aircraft.

Hose-end fittings may be of a permanent, factory-assembled design, or

they may be designed to be removed and reused on new hose.

Hose fittings are made in a variety of configurations, such as straight,

45o, and 90o. They are made to mate with either flared tube fittings (AN

and MS types) or flareless fittings (MS types).

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Straight

45o

90o

Permanent hose fitting

NIPPLE

Fitting for extra-high-pressure hose

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Quick-Disconnect Couplings

The purpose of quick-disconnect couplings is to save time in the removal

and replacement of components, to prevent the loss of fluid, and to

protect the fluid from contamination. Typical uses are in fuel, oil,

hydraulic and pneumatic systems.

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Hose assemblies, ie. the hose and the installed fittings, are usually

pressure tested to a pressure of at least twice the maximum operating

pressure.

The burst pressure is usually required to be at least four times the

maximum operating pressure.

Synthetic rubber hose and hose assemblies should be stored in a dark,

cool, dry area and be protected from circulating air, sunlight, fuel, oil,

water, and dust.

Storage life of synthetic rubber hose normally does not exceed five years

from the cure date stenciled on the hose. Storage life for hose

assemblies, ie. hoses installed with fittings, does not exceed four years.

Nevertheless, storage life among various hoses and hose assemblies

may vary and it is necessary to consult manufacturer’s instructions to

determine the exact condition for any particular type of hose.

Hoses must be checked regularly for cracks, cuts, abrasions, soft spots,

and any other indication of deterioration.

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Rigid and Semi-rigid lines and fittings should be inspected carefully at

regular intervals for leaks, damage, loose mountings, cracks, scratches,

dents, and other damage.

If damage to a line is local, it is permissible to cut out the damaged

section and insert a new section with approved fittings. Care must be

taken in this case that no foreign material enters the line during the repair

operation.

The following defects are not acceptable for metal lines:

• cracked flare

• scratches or nicks greater in depth than 10% of the tube wall thickness,

or found in the heel of a bend

• a dent of more than 20% of the tube diameter or found in the heel of a

bend

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Inspection of Fluid-Line Systems

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Shapes of typical hydraulic seals,

such as the standard O-ring seal,

chevron (V-ring) seal, universal

gasket, and crush washer.

Various operating devices of a hydraulic system require tubing, fittings,

seals, couplings, and hoses to transmit fluid pressure from unit to unit.

To prevent leakage of hydraulic fluid and thereby the loss of system

pressure, various types of seals are employed.

The two most common types of seals in use on aircraft are:

• gasket between two stationary surfaces

• packing between two surfaces that move in relation to each other

High-Pressure Seals

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The O-ring seal is probably the most common type used for sealing

pistons and rods because it is effective in both directions.

A typical O-ring installation

Generally, the O-ring seal requires no adjustment when it is installed.

However, care must be exercised to prevent scratching or cutting the seal

on threads or sharp corners. Also, make certain that the O-ring is not

installed in a twisted condition.

The O-ring seal should not be used alone in a system where the pressure

is greater than 1500 psi. If the hydraulic pressure is too great, the ring

can become pinched between the moving part and stationary part of the

unit, thus damaging the ring and destroying the seal. Fluid 3- 44

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O-ring seal, pinched condition

When the O-ring is used in higher-pressure systems, backup rings are

installed. Backup rings are used in conjunction with seals subjected to

high pressure.

Their purpose is to prevent the high pressures from pinching the seal

between the moving and stationary parts, such as between a piston and

cylinder wall.

Backup rings give additional strength to the seal and are usually made of

Teflon.

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If pressure is exerted on the seal in one direction only, the backup ring

should be placed on the side away from the pressure. Where the

pressure is exerted alternately in each direction, backup rings are

installed on both sides of the O-ring. Fluid 3- 46

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Summary

• Hydraulic Fluid

– Considerations, Types, Good properties

– 4 enemies of Hydraulic fluids

• Fluid Lines

– rigid, semi-rigid and flexible tubes

• Fittings

– AN, AC and MS fittings

• Hoses

– Low, Medium, High Pressure hoses

– Telfon Pressure hoses

• Seals

– Gasket, Packing, O-rings

47

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4. Hydraulic System Components

Hydraulic Components


System

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Hydraulic Filters

Hydraulic Reservoirs

Heat Exchangers

Hydraulic Pumps

Accumulators

Hydraulic Valves

Hydraulic Actuators

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../../Practicals/Lab 4.ppt

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4. Hydraulic System Components Hydraulic Symbols

Hydraulic Work Line

Hydraulic Pilot Line

Accumulator

Filter

Pressure

Gauge

Orifice

Pressure

relief valve

Check valve

Pump

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4. Hydraulic System Components Hydraulic Symbols

4/2 way selector

valve, spring

returned, solenoid

on other way

4/3 way selector

valve, with

spring centered

Single Acting Cylinder

Single Acting Cylinder,

Spring returned

Double Acting

Differential Cylinder

Double Action Cylinder,

with piston rods on both

ends

Directional Selector

Valve

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4. Hydraulic System Components Hydraulic Filters

Symbol of Filter

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One of the important requirements for hydraulic fluid is its cleanness.

In hydraulic systems, filters are used to remove any particles that may

enter and contaminate the hydraulic fluid.

These particles may enter the system when it is being serviced or during

wear of operating components.

The extremely small clearances between components in many hydraulic

pumps and valves make effective filtering extremely important. If these

contaminants were allowed to remain in the circulating fluid, they could

damage the seals and cylinder walls, causing internal leakage and

prevent components such as check valves from seating properly.

The best way to ensure that hydraulic components have good protection

is to place a filter immediately upstream of the components. Hence,

filters are normally found at the inlet and outlet of the reservoir, and the

pump outlet.

Commonly used filters are of the micronic type and porous metal type.

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A micronic filter contains a treated

paper element folded into pleats to

increase its surface area to trap

particles in the fluid as the fluid

flows through the element.

This pleated paper micronic

element is wrapped around a

spring steel wire coil to prevent its

collapsing.

The micron filters can be designed

to filter out particles as small as

3 microns (or 3 m)

These filters usually have a built-in

bypass valve that opens to allow

the fluid to bypass the element if it

should become clogged.

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Example of a Micronic Element

Fluid 4-8

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A filter is rated in the size of particles it will remove, and these sizes are

measured in microns. 1 micron is one millionth of a meter ie. 1 x 10-6 m.

The unaided human eye can see something as small as 40 microns, and

our white blood cells are about 25 mircons.

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If the element clogs, there will

be a large pressure drop across

the bypass poppet valve and

the inlet fluid will force the valve

off its seat.

Fluid will then bypass the

clogged element and flow

through the system unfiltered.

When this pressure gradient

across the filter is large enough

to unseat the bypass valve, the

red differential pressure

indicator button pops up to

inform maintenance that the

filter element is clogged and

needs to be replaced.

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A porous metal filter have

elements made of stainless steel

wire woven into a mesh and

wrapped around a wire frame.

Unlike in micronic filters where

the elements are throw-away type

and cannot be cleaned and

reused, these wire mesh filters

may be cleaned for subsequent

reuse.

Porous metal filters can trap

particles as small as 5 microns

(5 m) in size.

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A special 2-stage filter is used in the return line for some large aircraft

hydraulic systems in place of the standard single-element unit. This type

of filter allows the use of an extremely fine element at low flow rates

without causing an excessively high pressure drop.

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4. Hydraulic System Components Hydraulic Reservoirs

Messier-Bugatti

P/N C24891020

Airbus A330/340

Courtesy of Parker-Hannifin

Symbol of Reservoir

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A hydraulic reservoir is a tank or container designed to store sufficient

hydraulic fluid for all conditions of operation.

Hydraulic fluid reservoirs store hydraulic fluid when it is not in use in a

system, and they also provide sufficient fluid to make up for normal

losses of fluid by leakage past seals. When the maximum amount of fluid

is being used in the system, the reservoir must still have a reserve

adequate to meet all requirements.

Hydraulic reservoirs vary in complexity from nothing but a can with a

vent hose on top of the cap and an outlet at the bottom, to sophisticated

designs incorporating filters, quantity indicators, and pressure relief

systems.

Replenishment of hydraulic fluid is normally accomplished by adding

fluid directly to the reservoir through a filter opening.

Reservoirs in hydraulic systems that require a reserve of fluid for the

emergency operation of landing gear, flaps, etc. are equipped with

standpipes. During normal operation, fluid is drawn through the

standpipe. When system fluid is lost, emergency fluid is drawn from the

bottom of the tank. Fluid 4-14

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Hydraulic power system using an

engine-driven pump and a hand

pump as a backup.

Fluid 4-15

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Reservoirs can be broken down into two basic types: in-line and integral,

and these can be further classified as pressurized and unpressurized.

In-line reservoirs are reservoirs that exist as separate, stand-alone

components in the hydraulic system. This is the most common type of

reservoir and can be pressurized or unpressurized.

Unpressurized reservoirs are normally used in aircraft flying at lower

altitudes, such as below 15000 ft, or whose hydraulic systems are limited

to those associated with ground operations, such as brakes.

Pressurized reservoirs are commonly found in aircraft designed for high-

altitude flight where atmospheric pressure is low.

At sea level, the 14.7 psi of atmosphere provides the force to push the

fluid from the reservoir to the pump. As altitude increases, atmospheric

pressure decreases.

With little or no pressure on the fluid, it tends to foam, causing air

bubbles to form in the low part of the system. Unless some means of

pressurization is provided, the pump will be starved for fluid.

Fluid 4-16

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Therefore, to provide a continuous supply of fluid to the pumps, the

reservoir is pressurized.

Along with providing a positive feed (pressure) to the hydraulic pumps,

a pressurized reservoir reduces or eliminates the foaming of hydraulic

fluid when it returns to the reservoir.

The reservoir may be pressurized

by spring pressure, air pressure, or

hydraulic pressure. The desired

pressure to be maintained ranges

from approximately 10 to 90 psi.

Turbine engine bleed air or a

venturi-type aspirator can be used

to pressurize a reservoir with air.

The use of air pressure acting

directly on the fluid eliminates the

need for any elaborate chambering

of the reservoir. A reservoir pressurized by spring and

hydraulic pressure

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Hydraulic pressure can be

used to pressurize the

reservoir in a manner similar to

that used to assist the spring in

pressurizing the reservoir.

Hydraulic pressure entering the

pressure port acts on the

pressurization piston to cause

the reservoir piston to move

downward, which pressurizes

the reservoir.

The depressurization valve is

used to equalize the pressure

in the two piston areas during

servicing.

A reservoir pressurized by hydraulic pressure

Fluid 4-18

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A reservoir in the same assembly

as the fluid pump

Reservoirs that combine with

hydraulic pumps are called Intergral

reservoirs.

These types of reservoirs are often

found in small aircraft where the

compact arrangement of this type of

mechanism is desirable.

An example of this is the brake master

cylinder used with many light-aircraft

systems. The upper portion of the

assembly serves as the reservoir and

the lower portion serves as the pump

to operate the brake.

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4. Hydraulic System Components Heat Exchangers

Symbol of

Heat Exchanger

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Because of the high pressures involved in many hydraulic systems and

the high rates of fluid flow, hydraulic fluid becomes heated as the

subsystems are operated.

For this reason, it is often necessary to provide cooling for the fluid.

The heat exchanger is a heat radiator much similar in design to an oil

cooler for an engine. Fluid 4-21

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The heat exchanger is equipped with a temperature-operated bypass

valve to increase the fluid flow through the cooling element as

temperature rises.

Heat exchangers are often installed in the return lines to cool the

hydraulic fluid before it enters the reservoir.

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The temperature-operated bypass valve in the heat exchanger controls

the volume of return fluid circulating through the fluid cooler.

As fluid temperatures rise, the bypass valve starts to close, porting more

return fluid through the hydraulic cooler.

At high fluid temperatures, the bypass valve is fully closed, porting all

return fluid through the cooler. Fluid 4-23

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Cooling of the hydraulic fluid in the heat exchanger may be provided by

different means. Some heat exchangers are installed in aircraft fuel cells

and so the cooling is achieved by transferring heat to the fuel. Other heat

exchangers utilize air to cool the fluid, with ram air used during flight and

engine bleed air used when aircraft is on ground.

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4. Hydraulic System Components Hydraulic Pumps

Symbol of Pump

Axial Piston pump

Vane pump

Gear pump

Fluid 4-25

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The function of the hydraulic pump is to move fluid through the system,

and in so doing converts mechanical force into hydraulic pressure.

In a hydraulic system, the pump is regarded as the source of hydraulic

power. However, the pump itself needs to be driven by an engine, an

electric motor or even by hand.

There are two basic types of pumps: constant-displacement and variable-

displacement.

• A constant-displacement pump moves a specific volume of fluid each

time its shaft turns. It must have some form of pressure regulator or

pressure relief valve in the system to prevent the building of

excessive pressure which will eventually rupture a line or even damage

the pump itself.

• A variable-displacement pump does not move a constant amount of

fluid each revolution, but only the amount the system will accept. By

varying the pump output, the system pressure can be maintained

within the desired range without the use of regulators or relief valves.

Such pumps can turn without fluid being forced into the system.

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Hand Pumps

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Hand Pumps

Hand pumps are used as back-ups in aircraft hydraulic systems to supply

fluid under pressure to sub-systems such as landing gear, flaps and to

charge system accumulators.

A single-acting hand pump

Single-acting pumps move fluid only

on one stroke of the piston, while

double-acting pump move it on both

strokes. Most hand pumps used in

aircraft are of the double-acting type

because of their greater efficiency.

A double-acting hand pump

Fluid 4-28

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Gear Pump

One of the most generally used types of constant-displacement pumps

for medium-pressure hydraulic systems. Gear pumps are rugged and

dependable and are relatively inexpensive to manufacture.

One of the two gears is driven by the

power source, which could be an engine

drive or an electric-motor drive. The other

gear is meshed with and driven by the

first gear.

As the gears rotate in the direction

shown, fluid enters the IN port to the

gears, where it is trapped between the

gear teeth and carried around the pump

case to the OUT port.

The fluid cannot flow between the gears because of their closely meshed

design; therefore, it is forced out through the OUT port.

Fluid 4-29

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Gear Pump

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Vane Pump

Another constant-displacement pump which consists of a slotted rotor

located off-center within the cylinder of the pump body with rectangular

vanes free to move radially in each slot. The vanes are held against the

wall of the sleeve by a spacer.

As the rotor turns in the direction

shown, the volume between the

vanes on the inlet side increases,

while the volume between the

vanes on the outlet side decreases.

This change in volume draws fluid

into the pump through the inlet port

and forces it out through the outlet

port.

Vane pumps are used more for

moving fuel and air than they are

for moving hydraulic fluid.

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Gerotor Pump

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Multiple-Piston Pump

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One of the most widely used hydraulic pumps for modern aircraft is the

piston pump, which can be either of fixed or variable-delivery type.

The axis of rotation for the cylinder

block is at an angle to the axis of

rotation of the drive shaft.

This cause the pistons to move in

and out of the cylinders as rotation

occurs.

The pistons on one side of the

cylinder block are moving outward,

thus increasing the volume of the

cylinder spaces to induce suction;

On the other side of the cylinder

block the pistons are moving inward,

thus forcing the hydraulic fluid out.

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A fixed-delivery piston pump A variable-delivery piston pump

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Fixed-delivery means that the pump will deliver a fixed amount of fluid at

a given number of rpm.

A variable-delivery pump is designed so the alignment of the rotational

axis of the cylinder block can be changed as desired to vary the volume

of fluid being delivered at a given rpm.

By changing the angle of the

rotational axis, the stroke of the

pistons is decreased or increased;

the volume of fluid pumped during

each stroke will be correspondingly

reduced or increased.

If the axis of the cylinder block is

parallel to the axis of the drive shaft,

no fluid will be delivered as there is

no lateral movement of the pistons

within their respective cylinders.

Fluid 4-39

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Cam-Type, Variable Delivery Pump

By changing the angle of the rotational axis, the stroke of the pistons is

decreased or increased; the volume of fluid pumped during each stroke

will be correspondingly reduced or increased.

If the axis of the cylinder block is parallel to the axis of the drive shaft,

no fluid will be delivered as there is no lateral movement of the pistons

within their respective cylinders.

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It operates on the same principle as the multiple-piston pump except the

drive shaft is parallel to the pistons.

The movement of the pistons necessary

to create a pumping action is caused by

a cam plate, also known as a yoke.

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As the cam rotates, the elevated portion pushes the pistons into the

cylinders, causing them to eject the fluid into the OUT port. The pistons

on the descending portion of the cam draw fluid from the IN port of the

pump.

The angle of the cam or wobble plate, as established by the pressure-

controlling device, determines the amount of fluid expelled during each

stroke of the pistons, thus providing the variable delivery required.

In this way, just enough fluid is pumped into the hydraulic system to

maintain system pressure at the level corresponding to the setting of the

internal pressure-controlling device.

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Ram Air Turbines

A method used on many aircraft to power a pump in the event of engine

and electrical system failure is the utilization of a ram air turbine (RAT).

The RAT installation typically consists

of an air turbine, a speed-governing

device, and a variable-delivery pump.

The RAT can usually be deployed

manually or may automatically deploy

in the event of engine failure.

The RAT does not require a source of

power other than that provided by the

passage of air over the turbine due to

the forward movement of the aircraft.

Once deployed, the RAT remains

extended for the duration of the flight

and cannot be restowed without

maintenance action on the ground. Fluid 4-43

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Constant Displacement vs. Variable Displacement Pumps

Displacement (or Delivery) is the volume of fluid pushed out by the pump

for every revolution that the pump shaft turns.

pp vnQ where = flowrate (l/min)

= pump rotational speed (rev/min)

= pump displacement (l/rev)

Q

pn

pv

Constant (ie. fixed) displacement pumps will produce a constant flowrate

at a certain speed of pump rotation.

Variable displacement pumps can produce a variable flowrate even when

the pump is operating at a fixed rotation speed.

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Analysis of a Simple Constant Displacement Pump

Q1

Q2

Tp

np

1

M

Tp,th

np

2

Let p : the pump volumetric displacement [litre/rev]

np : the pump speed or the speed of the prime mover [rev/min]

Q1 : the pump suction flowrate or theoretical delivery at point (1) [litre/min ]

Q2 : the actual pump delivery at point (2) [litre/min]

Tp : the actual input torque at the pump shaft required to drive the pump [Nm]

Tp,th : the theoretical input torque to drive the pump [Nm]

Pp : the pressure rise across the pump [bar]

Pp = P2 P1

Point 1 and 2 are the pump inlet and outlet

respectively.

As the pump converts MECHANICAL WORK

into HYDRAULIC ENERGY, the input power

to the pump will be mechanical and the

output power will be hydraulic.

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Analysis of a Simple Constant Displacement Pump

Theoretical pump delivery, = pump displacement x pump speed 1Q

pp1 nvQ

In reality, the actual pump delivery , will be less than the theoretical flow

rate as a result of leakage and slippage. 1Q2Q

Hence, the volumetric efficiency of the

pump is defined as

%100Q

Q

1

2pv

Loss of flow as a result of pump leakages

through case drain is computed as

21L QQQ

Q1

Q2

Tp

np

1

M

Tp,th

np

2

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Example 1

A hydraulic gear pump that has a volumetric efficiency of 95% is

operating at a speed of 1200 rpm.

a) Calculate the pump’s displacement in cm3/rev if it delivers an output

flowrate of 45 l/min.

b) Calculate the new output flowrate in l/min if the prime mover is

now rotating at 800 rpm.

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Power required to drive the Pump

Due to frictional losses across the couplings, the actual torque TP from

the prime mover at (a) is actually greater than the ideal torque Tp,th

required at (b) to drive the pump shaft, ie. TP > Tp,th

Tp

np

1

M

Tp,th

np

2

(a) (b)

Power = Force Velocity

= Force 2r Angular Velocity (eg. rpm)

= 2 Torque Angular Velocity

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Mechanical Input Power at (a) is :

Mechanical Power required at (b) is :

pp Tn2

th,pp Tn2

Therefore, the mechanical efficiency of the pump, which is defined as the

ratio of the Mechanical Power at (b) to the Mechanical Power at (a), is

%100T

T

%100Tnπ2

Tnπ2

%100(a)atPowerMechanical

(b)atPowerMechanicalη

p

th,p

pp

p,thp

pm

Tp

np

1

M

Tp,th

np

2

(a) (b)

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Now let us consider the pump.

Hydraulic Power available at (1) :


1p QP

2p QP

The energy from the prime mover is used to convert

low pressure inlet flow to higher pressure outlet

flow. In cases where the inlet pressure, P1, is non-

zero, the pressure rise which is ∆Pp = P2 – P1 should

be taken in account instead of P2.

We assume Mechanical Power from the theoretical torque at (b) is 100%

converted to the Hydraulic Power at the theoretical flowrate at (1), ie.

Mechanical Power at (b) = Hydraulic Power at (1)

Tp

np

1

M

Tp,th

np

2

(a) (b)

Unless otherwise stated, the inlet pressure is usually assumed to be zero

since its magnitude is small compared to the system working pressure.

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%100Tπ2

vP

%100Tnπ2

vnP

%100Tnπ2

QP

%100)a(atPowerMechanical

)1(atPowerHydraulic

%100(a)atPowerMechanical

(b)atPowerMechanicalη

p

pp

pp

ppp

pp

1p

pm

Tp

np

1

M

Tp,th

np

2

(a) (b)

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Now the overall efficiency of the pump is defined as the ratio of the

Output Hydraulic Power at (2) to the Input Mechanical Power at (a). Thus,

%100Tnπ2

QP

%100(a)atPowerMechanicalInput

)2(atPowerHydraulicOutputη

pp

2p

po

Tp

np

1

M

Tp,th

np

2

(a) (b)

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%100T

T

Q

Q

Tn2

Tn2

QP

QP

(a)atPowerMechanicalInput

)b(atPowerMechanical

)1(atPowerHydraulic

)2(atPowerHydraulicOutput

p

th,p

1

2

pp

th,pp

1p

2p

pmpv

po

EfficiencyMechanicalEfficiencyVolumetric

Alternatively,

Tp

np

1

M

Tp,th

np

2

(a) (b)

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Example 2

Figure shows a hydraulic piston pump driven by an ac motor. The pump

is able to deliver 60 litres/min when the ac motor is rotating at 1200 rpm.

The operating pressure is 120 bar and the pump volumetric and

mechanical efficiencies are rated at 90% and 85% respectively. Calculate

a) the pump displacement in cm3/rev.

b) the mechanical power in kW provided by the ac motor.

c) the torque in Nm generated by the ac motor to operate the pump

Q1

M

Q2np, Tp

Q1

M

Q2np, Tp

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Pump Speed

The speed of the pump is the speed of its prime mover. The flowrate

delivered by a pump varies directly with this speed.

Pump manufacturer catalogue will usually indicate the minimum and the

maximum operating speeds.

In general, the lower the speed, the longer the pump life due to less wear

and tear.

Type of hydraulic fluid and fluid contamination

Pumps are designed to operate with a particular viscosity of fluid.

Other types of fluids with different viscosities may affect the required

lubrication of the pump mechanism.

Certain pumps are more tolerant of fluid contamination than others.

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Pump Rating

Pumps are rated according to:

• Maximum allowable operating pressure (psi, kPa, or bar)

• Flowrate (litres/min, m3/s) at maximum drive speed of the pump (rpm)

• Displacement (cm3/rev)

• Mechanical Efficiency, Volumetric Efficiency, and Overall Efficiency –

presented graphically on charts as these parameters are not constant

throughout the operating range of the pump.

The main factors affecting the selection of pumps

1. Maximum operating pressure

2. Maximum delivery

3. Pump efficiencies

4. Pump speed

5. Types of control

6. Noise

7. Size and Weight

8. Cost

9. Maintenance and Spares

10. Type of fluid and contamination

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Pump Selection

The table summarizes the characteristics of various pump types:

Pump

type

Output Maximum operating

pressure

Other characteristics

Vane • Constant

displacement

• Relatively large

volume of fluid

Low pressure

(P 300 psi)

Reliable, efficient, easy

to maintain, less dirt

tolerant.

Gear • Constant

displacement

• Medium volume of

fluid

Medium pressure

(100 P 1500 psi)

Durable, dirt tolerant,

noisy

Piston • Constant

displacement

or

Variable

displacement

• Low to High

volume of fluid

High pressure

(P 2500 psi)

High efficiency, not dirt

tolerant

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4. Hydraulic System Components Accumulators

Symbol of

Accumulator

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Diaphragm Accumulator Bladder Accumulator Piston Accumulator

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An accumulator is basically a chamber for storing hydraulic fluid under

system pressure.

Functions of an accumulator:

• It dampens pressure surges caused by the operation of an actuator.

• It can aid or supplement the system pump when several units are

operating at the same time and the demand for hydraulic fluid is thus

beyond the pump’s capacity.

• It can also store power for limited operation of a component if the

pump is not operating.

• Finally, it can supply fluid under pressure to make up for small system

leaks. Such leaks would cause the system to cycle continuously

between high and low pressures.

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All accumulators consist of a high-strength container divided by some

form of movable partition into two sections or compartments.

One compartment is connected to the hydraulic pressure manifold, and

the other compartment is filled with either compressed air or nitrogen.

There are three types of accumulators commonly found in aircraft

hydraulic systems:

• diaphragm type

• bladder type

• piston type

Diaphragm-Type Accumulator

It consists of a metal sphere separated by a synthetic-rubber diaphragm.

The sphere is constructed in two parts, which are joined by means of

screw threads. At the bottom of the sphere is an air valve, and at the top

is a fitting for the hydraulic line. A screen is placed at the fluid outlet

inside the sphere to prevent the diaphragm from being pressed into the

fluid outlet.

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During operation, the air chamber is

preloaded, or charged, with air pressure

which is approximately one-third of the

maximum system pressure.

As soon as a very small amount of

hydraulic fluid is introduced to the fluid

side of the accumulator, the system

pressure gauge will show the pressure in

the air chamber. This provides a means for

checking the air charge in the accumulator.

Some aircraft hydraulic systems monitor

the system hydraulic pressure by indicating

the pressure on the air side of the

accumulator.

Hence, in such installations, when the system pressure is zero, the gauge

for the system indicates the accumulator air pressure. As soon as the

system pressure is greater than the air charge, the air is compressed to

the value of the system pressure. Fluid 4-62

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Bladder-Type Accumulator

It usually consists of a metal sphere in which a bladder is installed to

separate the air and the hydraulic fluid.

The bladder serves as the air chamber,

and the space outside the bladder

contains the hydraulic fluid.

The air valve is at the bottom of the

sphere; the fluid port at the top.

Initially, the bladder is charged with air

pressure according to the reading

specified in the aircraft manual.

When fluid is forced into the

accumulator, the bladder collapses to

the extent necessary to make space for

the fluid, depending upon the fluid

pressure.

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Piston-Type Accumulator

It consists of a cylinder with a free piston

inside to separate the air from the hydraulic

fluid.

The piston is equipped with seals that

effectively prevent the air from leaking into

the fluid chamber and vice versa.

Many modern hydraulic systems employ

piston-type accumulators because they

require less space than an equivalent

spherical accumulator.

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Diaphragm Accumulator Bladder Accumulator Piston Accumulator

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Flow Control Valves

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4. Hydraulic System Components Hydraulic Valves

Pressure Control Valves

Automatic-Operating Control Valves

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4. Hydraulic System Components Flow-Control Valves

Flow-Control Valves

Flow-control valves, also known as selector valves, are used to direct the

flow of hydraulic fluid to or from a component and achieve the desired

operation.

These valves fall into one of four general types:

• Rotary

• Poppet

• Spool or Piston

• Open-center

These valves may be positioned by hand (ie. pilot-operated), by an

electrical or electronic control device, by hydraulic pressure, or by

pneumatic pressure.

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Rotary Valve

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Rotary Valve

When a rotary four-two-way valve is in the position shown in (A), the fluid

will flow from the valve at the top port and will cause the actuating

cylinder to be extended. When the valve is rotated 900, the fluid to and

from the cylinder will be in the opposite direction, and the cylinder will

retract.

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Poppet Valve

Diagram shows the valve operating a landing gear system. In this valve,

individual poppet valves are used to open and close the ports to change

the direction of fluid flow. The valves are operated by cam lobes on cam

rod C.

NC – Neutral Chamber

RC – Return Chamber

PC – Pressure Chamber

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Poppet Valve

Some poppet valve assemblies are arranged with valves in a radial

position, and they are opened and closed by means of a rotary cam unit.

The results are the same in any case.

Poppet valves are also manufactured with electric controls, and the

individual valves are opened and closed by means of solenoids.

Spool or Piston Selector Valve

There is no fluid flow when the valve is in the OFF position; therefore, the

valve must be used in a system where a pressure relief valve or variable

delivery pump is employed.

Otherwise a high pressure would build up and cause excessive wear or

other damage to the pressure pump.

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Spool or Piston Selector Valve

P R

P R

Flow-Control Valves

Spool-type

selector valve

Piston-type

selector valve

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Open-Center Valve

Like other forms of selector valve, the open-center selector valve

provides a means of directing hydraulic fluid under pressure to one end

of an actuating cylinder and of simultaneously directing fluid from the

opposite end of the cylinder to the return line.

One advantage of the open-center selector valve is that the valve

automatically returns to neutral when the actuating cylinder reaches the

end of its stroke. Thus, the fluid output of the pump is directed through

this valve to the reservoir when the valve is in neutral (“center”) position.

Flow-Control Valves

Sliding piston in one of

two operating positions

Ports B and D are

connected to the

actuator

Fluid 4-73

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Sliding piston in the

neutral position

Open-center

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Open-Center Valve

When the actuator has reached the end of its stroke, fluid pressure

accumulates and opens the internal check valve which admits fluid

pressure to the end of the sliding piston.

When this happens, the pressure then forces the sliding piston back to

the neutral position, aided by the action of the spring in the roller

mechanism. The lever is thus locked in the neutral position. This action

takes place for either position of the valve.

Flow-Control Valves

Sliding piston in one of

two operating positions

Ports B and D are

connected to the

actuator

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4. Hydraulic System Components Pressure-Control Valves

Pressure-Control Valves

Numerous devices have been designed to control pressure in hydraulic

systems. Among these devices are:

• Pressure switch

• Pressure regulator

• Pressure relief valve

• Pressure-reducing valve

Pressure switch

Electrically operated pressure switches are used in hydraulic systems

with electrically driven pumps to maintain system pressure within set

limits.

The pressure switch will open the electrical circuit to the pump motor

when system pressure has build up to the correct pressure settings,

causing the pump to stop. As pressure drops to a lower value, the

pressure switch closes the circuit to start the pump operating again.

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Pressure switch

The pressure switch transforms

the hydraulic pressure into an

electrical signal.

Piston ➀ is supported by a

spring plate ② and acts

against the continuous force of

the compression spring ③.

Spring plate ② itself acts on an

electrical micro-switch ④. The

plate will connect or disconnect

the switch based on pressure

differential across the plate.

Pressure switches are also used in hydraulic systems to control the

operation of warning and protective devices. The switch may turn on a

light to warn the pilot of insufficient or over- pressure, or it may turn off a

pump to prevent reservoir fluid being drained off through a broken line.

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Pressure Regulator

A pressure regulator is designed to maintain a certain range of pressures

within a hydraulic system.

Continuous pressure on the pump increases wear and the possibility of

failure. Therefore the pressure regulator is used to relieve the pressure

on the system pump when it is not needed for operating a unit in the

system.

Hence pressure regulators are also called

unloading valves because they unload the

pump when hydraulic pressure is not

required for operation of subsystems such

as landing gear, flaps or flight controls.

In this example of an unloading valve, the

pump is operating through the valve to

supply fluid for charging an accumulator

and to supply fluid pressure for operating

units downstream of the system.

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Pressure Regulator

Unloading valve directing fluid

from pump to accumulator and

subsystems

from

Pump

to Accumulator

and Subsystems

Unloading valve in the

“kicked-out” position

Compression

Spring

Pilot

Valve

Plunger

Directional

Spool

Unloading

Spool

to Reservoir

When pressure in the accumulator builds up

to the maximum level the system can deliver,

the same pressure is exerted on the plunger.

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Pressure Regulator


“kicked-in” position


“kicked-out” position

Compression

Spring

Pilot

Valve

Plunger

Directional

Spool

Unloading

Spool

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Pressure Relief Valve

The function of a pressure relief valve is to limit the maximum safe

pressure that can be developed in a hydraulic system. In other words, it

acts as a safety valve.

During operation, the relief valve remains closed

unless the maximum pressure in the system exceeds

that for which the valve is set. When this happens,

the valve will open to allow the fluid to flow through a

return line to the reservoir.

The pressure at which the relief valve lifts is called

the cracking pressure.

Relief valves are used to control the maximum

system pressure and to control the operating

pressure in various parts of a subsystem.

A relief valve known as the wing-flap overload valve

is placed in the DOWN line of wing-flap subsystem to

prevent lowering of the flaps at too high an airspeed.

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Thermal Relief Valve

A thermal relief valve is similar to a pressure relief valve; except such

valves are installed in parts of the hydraulic system where fluid pressure

is trapped and may need to be relieved because of the increase in

pressure caused by higher temperatures.

A likely source of higher temperatures can be the warm air on the ground

that can result in hydraulic fluid expansion in subsystems such as the

landing gear system or flap-control system when the selector valves for

these systems are set to the neutral (or OFF) position because the

airplane has landed.

Such fluid expansion could cause damage unless thermal relief valves

are incorporated in the affected subsystems.

Similar to the pressure relief valve, the thermal relief valves are also

adjusted to pressure settings that are above those required for the

operations of the subsystems so that these valves do not interfere with

the normal system operations.

Fluid 4-81

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Pressure-Reducing Valve

Some parts of the hydraulic system may require a lower operating

pressure than the normal system pressure, such as to prevent

overloading some structures.

A pressure-reducing valve will reduce system pressure to the desired

level, at the same time relieve thermal expansion in the section of the

system that it isolates.

Diagram shows how a

pressure-reducing valve is

positioned to bring about a

lower pressure for operation of

the actuating cylinder.

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• Fluid under system pressure enters

the system pressure port, where it

opens the poppet and flow out the

reduced-pressure port E.

• As the fluid going out port E builds

pressure, hydraulic force is

transmitted back through the hollow

portion of the poppet and exerts a

force on the reservoir return valve B.

• When this force overcomes spring

force at A, the reservoir return valve is

pushed to the right, allowing the

poppet spring to seat the poppet. This

prevents system fluid from going out

the reduced-pressure port E to build

up a further pressure.

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• The inlet pressure has no effect on

the poppet itself because the areas on

each end of the poppet exposed to the

pressure are the same; therefore, the

forces exerted on the poppet are

balanced.

• The pressure exerts an unbalanced

force only on the area of the reservoir

return valve.

• In actual operation, this pressure-

reducing valve will close when the

desired pressure is reached. When the

actuating is in operation under

reduced pressure, the valve will vary

its opening to meter the fluid at the

speed required to maintain the desired

pressure. Excess pressure will be

release via the reservoir return port.

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Debooster Valve

Another type of pressure-reducing

valve is a debooster valve used in an

aircraft brake system to reduce system

pressure; in addition to reducing

pressure it will provide for a higher

volume of fluid flow to the brakes for

rapid application of braking forces.

A debooster valve consists of two

pistons of different sizes contained

inside a cylinder housing machined for

each piston. Movement of one piston

will cause the other to move. The two

pistons will be capable of developing

pressures in inverse proportion to their

areas. Hence, the extra fluid that is

discharged will cause rapid application

of the brakes.

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4. Hydraulic System Components Automatic-Operating Control Valves


An automatic-operating control valve is a flow control valve that is

designed to operate WITHOUT being positioned or activated by any

external intervention.

These valves are located in line with the system flow and perform

operations such as prevent or restrict flow in a line, allow flow at the

proper time, and transfer hydraulic power between independent pressure

systems.

Orifice or Restrictor Valve

An orifice is merely an opening, passage, or hole. A restrictor can be

described as an orifice or similar to an orifice.

The size of a fixed orifice must remain constant, whereas a variable

orifice or restrictor permits adjustment in its opening size so that its

effect can be altered to meet changing system requirements.

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The purpose of an orifice is to limit the rate of flow of the fluid in a

hydraulic line. In limiting the rate of flow, the orifice causes the

mechanism being operated by the system to move more slowly.

An orifice of this construction may be placed in a hydraulic line between

a selector valve and an actuating cylinder to slow the rate of movement

of the cylinder.

When fluid flows through the central passage, it will limit the rate of flow.


A fixed orificeA fixed orifice

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In a variable restrictor, there are two horizontal ports and a vertical,

adjustable needle valve. The size of the passage through which the

hydraulic fluid must flow may be adjusted by screwing the needle valve

in or out. In essence, the variable restrictor functions like a water tap.

This feature of the passage being able to be varied in size distinguishes

the variable restrictor from the simple fixed orifice as shown previously.

A variable restrictor


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Check Valve

The function of the check valve is to allow free flow in one direction while

preventing fluid flow in the opposite direction.

Fluid pressure at port A will tend to push the ball off its seat and allow the

fluid to flow through the valve. When the pressure is applied at port B,

the ball will hold firmly on its seat and prevent the flow of fluid.

Check valves are used as individual units in hydraulic systems, as well

as being used as internal components of more complex valves and

devices to control the flow of fluid within the unit in a given direction.

There is usually an arrow on the body of the check valve to indicate the

direction of free fluid flow.


A ball check valve

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An orifice check valve

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Orifice Check Valve

An orifice check valve, also known as a one-way restrictor, is designed to

provide free flow of hydraulic fluid in one direction and restricted flow in

the opposite direction.

When the valve is on its seat, fluid flow can occur only through the center

orifice, but when the fluid flow is in the opposite direction, the valve

moves off its seat and there is free flow of the hydraulic fluid through

multiple openings.

Orifice check valves are commonly found in the UP line of landing gear

systems to counter the effect of gravity when lowering the landing gear.

They are also used to restrict the DOWN movement of flaps in flight

controls to counter the constant aerodynamic forces tending to raise the

flaps to a streamlined position. Fluid 4-91

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Metering Check Valve

A metering check valve serves the same

function as an orifice check valve, ie. to restrict

flow in one direction, except it is adjustable

whereas an orifice check valve is not.

The metering pin is adjusted to hold the ball

slightly off its seat. When fluid enters port B, it

forces the ball away from its seat and then

flows out through port A to the actuating

cylinder unrestricted.

When the flow of fluid is reversed, fluid

entering from the actuating cylinder flows

through the tiny opening between the ball and

its seat, thus restricting the flow. A metering

check valve

By adjusting the metering pin, the rate at which the fluid can return from

the actuating cylinder is controlled, because the position of the metering

pin changes the width of the opening between the ball and its seat.

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Hydraulic Fuse

The hydraulic fuse is a device designed

to seal off a broken hydraulic line and

prevent excessive loss of fluid.

Fluid flows from A to B as long as the

spring holds the piston away from any of

the internal holes in the valve body.

If a break should occur in a line beyond

the fuse, system pressure on side B will

drop and the pressure at side A will force

the piston over to cover the holes in the

valve body and stop all flow of fluid.

The fuse remains closed only as long as a

substantial pressure differential exists.

If the pressure differential decreases to a

certain predetermined level, the spring

will unseat the piston thus permitting

normal flow to resume. Fluid 4-93

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Sequence Valve

A sequence valve is sometimes

called a timing valve because it

helps to ensure certain hydraulic

operations are executed in the

proper design sequence.

The sequence valve is essentially

a bypass check valve that is

automatically operated.

There is a free flow of hydraulic

fluid from port A to port B, but the

reverse flow from port B to port A

is prevented unless the ball is

unseated by depressing the

plunger.

Sequence valve in

closed position

Sequence valve in

open position

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Sequence Valve

A common example of the use of this valve is in a landing gear system,

where the landing-gear doors must be opened before the gear is

extended, and the gear must be retracted before the doors are closed.

Landing gear system with sequence valves

LDG extended

LDG retracted

LDG Door closed

LDG Door opened

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Priority Valve

A priority valve is a sequence valve that is operated by hydraulic

pressure rather than by mechanical contact, ie. no plunger is used.

This valve is used to allow one actuator to operate and complete its

operation before allowing a second component to operate. This gives the

first component a priority over the second, thus resulting in the name

“priority valve”.

When actuators A and B are both

selected at the same time, pressure is

available at the intersection of the lines.

The priority valve in the line for

actuator B requires full-system

pressure be applied to it in order to

open so fluid can flow to actuator B.

Since fluid is flowing directly to

actuator A, the system pressure is

below the operating value of the valve. Fluid 4-96

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Priority Valve

Once actuator A has stopped

moving, system pressure will build

up at the end of the actuator stroke.

VIEW B: When the system pressure

is up to near the full value, the

poppet has enough pressure applied

to it to offseat the spool valve and

allow fluid to flow to actuator B.

VIEW C: When fluid flows in the

opposite direction, the seat shifts to

the left and a free flow of fluid

occurs.

Besides being used for sequencing,

priority valves are also used to give

one component priority over another

component in unrelated operations.

POPPET SEAT SPOOL

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Shuttle Valve

Quite frequently in hydraulic systems it is necessary to provide

alternative or emergency sources of power with which to operate critical

parts of the system. An example is the landing-gear system in the case

of hydraulic pump failure.

The shuttle valve is designed, in such situations, to provide a means of

disconnecting the normal source of hydraulic power and connecting the

emergency source of power. Sometimes the emergency power source is

provided by an emergency hand pump and sometimes by a volume of

compressed air or gas stored in a high-pressure air bottle.

DOWN UP

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Shuttle Valve

Port 1 of the valve is the normal entrance for hydraulic fluid from the

pressure system. Port 2 is the outlet, such as leading to the DOWN line

of the landing-gear actuating cylinder.

In view A, the valve is in the normal position with free passage of fluid

from port 1 to port 2.

With sufficient pressure drop across the shuttle valve due to loss of

system pressure supplied to port 1, the emergency pressure is able to

force the valve to the left blocking off the hydraulic line connecting to

port 1, and opens port 2 to port 3.

If the main system

pressure fails such as

during the extension

of the landing gear,

emergency pressure

is applied to port 3, as

shown in view B.

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4. Hydraulic System Components Hydraulic Actuators

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Hydraulic actuators are devices for converting hydraulic pressure to

mechanical motion or work. In essence, they are the opposite of pumps.

The most commonly utilized actuator is the actuating cylinder; however,

servo actuators and hydraulic motors are also employed for special

applications where modified motion is required.

Actuating cylinders are used for direct and positive movement such as

retracting and extending landing gear and the extension and retraction of

wing flaps, spoilers and slats.

Servo actuators are employed in situations where accurately controlled

intermediate positions of units are required. The servo unit feeds back

position information to the pilot’s control, thus making it possible for the

pilot to select any control position required.

The servo actuator is used to move large control surfaces such as the

rudder, elevator, and ailerons. Servo units are also used to aid the pilot

in the operation of cyclic and collective pitch controls in a helicopter.

Fluid 4-101

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Actuating Cylinders

Actuating cylinders, each consisting of a cylinder and piston, use

pressure in the fluid to produce straight-line motion, hence they are also

known as linear actuators.

The cylinder is usually attached to the aircraft structure, with the piston

attached to the component being moved.

The design of actuating cylinders is determined by the functions that

they are to perform. There are three basic types of actuating cylinders:

• single-acting

• double-acting unbalanced

• double-acting balanced

Fluid 4-102

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Single-Acting Cylinder

Hydraulic pressure is applied to one side of the piston to provide force in

one direction only. When hydraulic pressure is removed from the piston,

a return spring moves the piston to its start position.

The cylinders in hydraulic brakes are good examples of single-acting

cylinders. Hydraulic pressure moves the pistons out to apply the brakes,

but when the pedal is released, springs pull the shoes away from the

drum and move the pistons back into the cylinder.

Fluid 4-103

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Double-Acting Cylinder

Double-acting actuating cylinder is designed so hydraulic pressure can

be applied to both sides of the piston. Thus, the cylinder can provide

force in either direction.

Double-Acting Unbalanced

The fluid entering the UP port acts on the entire area of the piston, while

the fluid entering the DOWN port acts only on that portion of the piston

not covered by the actuating rod.

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Double-Acting Balanced

Some applications require the same amount of force in both directions of

piston motion. This results in the use of a balanced actuator.

Double-acting unbalanced actuators are normally used for raising and

lowering the landing gear, wing flaps, spoilers, etc.

It is due to this difference in effective piston areas between the 2

chambers in the cylinder that a much greater force is produced to raise

the landing gear than is used to lower it. Hence, the term “unbalanced”.

Fluid 4-105

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Cushioned Actuator

In a cushioned actuator, the piston starts its motion slowly, accelerates to

full speed, and then is cushioned at the end of its motion.

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Cushioned Actuator

Fluid enters the actuator through the GEAR-DOWN port, and it must flow

around the metering rod to move the piston out of the cylinder. As soon

as the piston travels far enough to remove the metering rod from the

orifice, the fluid flow increases and moves the piston out at its full speed.

As the piston near the end of its travel, the piston head contacts the

poppet and compresses the poppet spring to bring the piston to a

smooth stop at the end of its travel.

When the selector is placed in the GEAR-UP position, fluid enters the

GEAR-UP port and moves the piston rapidly until the metering pin enters

the orifice. The travel is then slowed until it reaches the full-up position.

Fluid 4-107

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Internal-Lock-Type Actuator

Released Position

Locked Position

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Internal-Lock-Type Actuator

The internal-lock actuator allows the hydraulically operated mechanism

to be locked in one of the extreme positions without the use of an

external locking device.

The hollow portion at the rear of the lock rod piston can be used for the

attachment of a cable operated control so that the actuator can be

unlocked manually during an emergency or ground operation.

This type of actuator can be used in many hydraulically operated

subsystems and is ideal for hydraulically operated doors.

Fluid 4-109

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Rotary Actuators

One of the simplest forms of a rotary actuator is the rack-and-pinion type

that is used in the popular high-performance, single-engine Cessna

aircraft for retracting the main landing gear.

The piston has a rack of teeth cut in its shaft.

These teeth mesh with those in a pinion gear

that rotates as the piston moves in or out.

Rotation of the pinion shaft subsequently

raises or lowers the landing gear.

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Douglas C-47 Skytrain

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Rotary Actuators

If a continuous rotational force is required, a hydraulic motor may be

used. Note that the motor is almost identical to the multiple-piston

hydraulic pump described previously.

When hydraulic fluid pressure is

applied to the inlet, it forces the

pistons downward in the cylinders.

Since the cylinder block axis is at an

angle to the axis of the output shaft,

and both assemblies are connected

by a universal joint at the center, the

pistons will cause the shaft to rotate

as they move down the cylinder bore.

As each piston reaches the bottom of the bore, the cylinder is rotated

past the inlet slot in the valve plate and on to the outlet slot. By now, the

piston is moved back up the bore, and the hydraulic fluid discharged to

the return line. Fluid 4-112

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Rotary Actuators

Multi-Piston Hydraulic Motor

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Rotary Actuators

An example of a Cam-Type Hydraulic Motor

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Rotary Actuators

A close-up view of Cam-Type Hydraulic Motor

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Rotary Actuators

The rpm of the hydraulic motor depends upon hydraulic fluid pressure

and the load on the motor.

A hydraulic motor has the advantage over a constant-speed

electric motor of being able to operate through a wide range of speeds

from 0 rpm to the maximum for the particular motor.

Although variable-speed electric motors can provide some flexibility in

the rate of actuation or turning; however, they lose efficiency as speed

decreases.

Fluid 4-116

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Rotary Actuators

Where less torque is needed, a

vane-type motor may be used.

Diagram shows a balanced vane-

type motor, in which pressure is

directed to vanes on opposite sides

of the rotor to balance the load on

the shaft.

Fluid under pressure enters the

inlet chambers of the motor and

pushes the vanes around to the

outlet chambers.

The vanes are free to slide back and forth in the slots of the rotor as the

rotor rotates under the application of hydraulic pressure. Centrifugal

force holds the vanes against the wall of the interior chamber, also

known as the sleeve.

Fluid 4-117

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An example of a Vane-Type Hydraulic Motor

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Linear Actuator (Cylinder) Calculations

The output force and the speed developed by an actuator piston during

its extension and retraction strokes can be calculated as follows:

1) During Extension

A

Bore Area Pb

a

Rod Area

Q q

P

vE, Extension Speed

2) During Retraction

Pb a

Rod Area A

Bore Area

q Q

P

vR, Retraction Speed

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During Extension

Extension force, FE , is computed by

FE = (P A) [Pb (A a)] Ff

where FE = Extension force [ N ]

Ff = Friction force within the cylinder [ N ]

P = System pressure [ N/m2 ]

Pb = Back pressure [ N/m2 ]

A = Piston bore area [ m2 ] on which pressure P is acting.

a = Piston rod area [ m2 ]

A – a = Annulus area [ m2 ] on which back pressure Pb is acting

A

Bore Area Pb

a

Rod Area

Q q

P

vE, Extension Speed

Fluid 4-120

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During Extension

In many instances, when the cylinder extends

• the bore-end pressure, P, is the system pressure

• the rod-end pressure, Pb, which is the back pressure, is zero

• the frictional force is negligible

A

Bore Area Pb

a

Rod Area

Q q

P

vE, Extension Speed

Hence, Extension force,

Extension speed,

Output flowrate,

APFE

A

QvE

aAvq E

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During Retraction

Retraction force, FR , is computed by

FR = [P (A a)] (Pb A) Ff

where FR = Retraction force [ N ]

Ff = Friction force within the cylinder [ N ]

P = System pressure [ N/m2 ]

Pb = Back pressure [ N/m2 ]

A = Piston bore area [ m2 ] on which pressure P is acting.

a = Piston rod area [ m2 ]

A – a = Annulus area [ m2 ] on which back pressure Pb is acting

Pb a

Rod Area A

Bore Area

q Q

P


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During Retraction

Likewise in many instances, when the cylinder retracts

• the rod-end pressure, P, is the system pressure

• the bore-end pressure, Pb, which is the back pressure, is zero

• the frictional force is negligible

Hence, Retraction force,

Retraction speed,

Output flowrate,

aAPFR

aA

QvR

Avq R

Pb a

Rod Area A

Bore Area

q Q

P


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Worked Example

A hydraulic cylinder which has a bore diameter of 40mm and rod

diameter of 15mm is driven by a pump of output delivery 15 l/min. The

system pressure is set at 100 bar. If the cylinder and the pump is

connected in the configuration as shown, determine at the cylinder

piston,

a) the extension speed [ m/min ]

b) the retraction speed [ m/min ]

c) the extension force [ N ], and

d) the retraction force [ N ]

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Basic Motor Calculations

Motor displacement is the theoretical amount of fluid required to push

the motor through one revolution.

Like their pump counterparts, hydraulic motors are available as either

fixed or variable-displacement type.

Fixed-displacement

motor

Variable-displacement

motor

mm vnQ where = flowrate (l/min)

= motor rotational speed (rev/min)

= motor displacement (l/rev)

Q

mn

mv

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Q4

Q3

Tmnm

4

Tm,th nm3

Load

Let m : the motor volumetric displacement [litre/rev]

nm : the motor speed or the rotational speed of the load [rev/min]

Q3 : the flowrate entering the motor at point (3) [litre/min ]

Q4 : the flowrate leaving the motor at point (4) [litre/min]

Tm : the actual output torque to drive the load [Nm]

Tm,th : the theoretical output torque developed at the motor shaft [Nm]

Pm : the pressure drop across the motor [bar]

Pm = P3 P4

Fluid 4-126

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Q4

Q3

Tmnm

4

Tm,th nm3

Load

The volumetric efficiency is defined as

%100Q

Q

3

4mv

Loss of flow to leakage through case

drains of motor is computed as

43L QQQ


Points (3) and (4) are the motor’s inlet and outlet respectively. Since the

motor converts HYDRAULIC ENERGY into MECHANICAL WORK, the

input power to the motor will be hydraulic while the output power is

mechanical.

Because of internal leakage and slippage within the motor, the

input flowrate Q3 > output flowrate Q4. Hence, not all fluid flowing into

the motor will drive it. Thus, the actual flowrate driving the motor is Q4.

Fluid 4-127

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Power generated by the Motor

Due to frictional losses across the couplings, the actual torque Tm

generated by the hydraulic motor at (d) is therefore less than the ideal

torque Tm,th available at (c) at the motor shaft ie. Tm < Tm,th .

Q4

Q3

Tmnm

4

Tm,th nm3

Load

(c) (d)

Mechanical Power output at (d) is :

Mechanical Power available at (c) is :

mm Tn2

th,mm Tn2

Therefore, the mechanical efficiency of the hydraulic motor, is given as

%100T

T

%100Tnπ2

Tnπ2

%100(c)atPowerMechanical

(d)atPowerMechanicalη

th,m

m

th,mm

mm

mm

Fluid 4-128

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Q4

Q3

Tmnm

4

Tm,th nm3

Load

(c) (d)

Hydraulic Power input at (3) :


3m QP

4m QP

Pressure drop occurs across the motor when hydraulic energy is

expended to drive the load. In cases where the outlet pressure is non-

zero, the pressure drop which is ∆Pm = P3 – P4 should be used instead of

P3 for pump calculation.

Logically, the Hydraulic Power available at

outlet (4) should be converted to the

Mechanical Power at ideal torque at (c), ie.

Mechanical Power at (c) = Hydraulic Power at (4)

However, unless otherwise stated, the outlet

pressure P4 is usually assumed to be zero

since its magnitude is small compared to the

system working pressure P3.

Fluid 4-129

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Q4

Q3

Tmnm

4

Tm,th nm3

Load

(c) (d)

%100vP

Tπ2

%100vnP

Tnπ2

%100QP

Tn2

%100)4(atPowerHydraulic

(d)atPowerMechanical

%100(c)atPowerMechanical

(d)atPowerMechanicalη

mm

m

mmm

mm

4m

mm

mm

Fluid 4-130

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Now the overall efficiency of the motor is defined as the ratio of the

Output Mechanical Power at (d) to the Input Hydraulic Power at (3). Thus,

%100QP

Tnπ2

%100)3(atPowerHydraulicInput

(d)atPowerMechanicalOutputη

3m

mm

mo

Q4

Q3

Tmnm

4

Tm,th nm3

Load

(c) (d)

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%100T

T

Q

Q

Tn2

Tn2

QP

QP

(c)atPowerMechanical

)d(atPowerMechanicalOutput

)3(atPowerHydraulicInput

)4(atPowerHydraulic

th,m

m

3

4

th,mm

mm

3m

4m

mmmv

mo

EfficiencyMechanicalEfficiencyVolumetric

Alternatively,

Q4

Q3

Tmnm

4

Tm,th nm3

Load

(c) (d)

Fluid 4-132

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Worked Example

A fixed-displacement hydraulic motor of displacement 100 cm3/rev

operates at a speed of 150 rpm through a pressure drop of 100 bar.

Given that the volumetric and mechanical efficiencies are 95% and 90%

respectively, determine

a) the inlet flowrate Q3 supplied to the motor [ l/min ]

b) the leakage flow (case drain) [ l/min ]

c) the output mechanical power available at the motor shaft [ kW ], and

d) the output torque Tm [ Nm ]

Q4

Q3

Tmnm

4

Tm,th nm3

Load

(c) (d)

Q4

Q3

Tmnm

4

Tm,th nm3

Load

(c) (d)

Fluid 4-133

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Summary

• Filters

– Micronic and Porous types

• Reservoir

– Standpipe

– Pressurised/ In-Line/ Unpressurised

– Heat Exchanger

• Pump

– Hand, Gear, Vane, Georotor, Multi-Piston (fixed and variable)

– Pump Calculations

– Pump Selection

Fluid 4-134

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Summary

• Accumulator

– Diaphragm / Bladder / Piston

• Flow Control Vales

– DCV

• Pressure Control Valves

– Pressure switch, Pressure regulator, Thermal Relief Valve,

Pressure Reducing Valve, Debooster valve

• Auto Operating Valves

– Check Valve, Sequence Valve, Priority Valve, Fuse, Orifice

Fluid 4-135

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Summary

• Actuators

– Cylinders

• Single Acting, Double Acting, Cushioned, Internal-Lock

– Motor

• Hydraulic Caluculations

Fluid 4-136

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5. Hydraulic Systems for Aircraft

Aircraft Hydraulic System


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Outline

• Types of Aircraft Hydraulic Systems

• Aircraft Hydraulic Schematic Daigrams

• More on Wide-Body Aircraft Hydraulic System

– Hydraulic Power Configuration

– Normal Operations

– Loss of Hydraulic Power

– Primary and Secondary Flight Control System

Fluid 5-2

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Reservoir

Every aircraft hydraulic system has two major parts or sections, the

power section and the actuating section(s).

The power section provides for fluid flow, regulates and limits pressure,

and carries fluid to the various selector valves in the system.

The actuating sections are the sections containing the various operating

units, such as the wing flaps, flight controls, landing gear, brakes, boost

systems, and steering mechanisms.

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Fluid 5-4

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BAe SYSTEMS Hawk 200

Hydraulic Systems

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The power section may either be an open or closed system using an

engine-driven pump or a pump driven by an electric motor.

An OPEN system means there is a free path for fluid flow back to the

reservoir until one of the selector valves is actuated and directs fluid flow

to the corresponding actuator unit. Fluid flow is then directed to the

actuators (cylinders, motors, etc.) and pressures will build sufficiently to

move the actuating units.

Basic Open-Center System

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Open Systems

An open system, also known as open-center system, is one having fluid

flow but no appreciable pressure in the system whenever the actuating

mechanisms are idle. Fluid circulates from the reservoir, through the

pump, through the open valves, and back to the reservoir.

Selector valves (open-center valves) in an open system are always

connected in series with each other. Fluid is allowed free passage

through each selector valve and back to the reservoir until one of the

selector valves is positioned to operate a mechanism. Fluid is then

directed to the affected actuator, and pressure is allowed to buildup in

the systems.

Fluid 5-7

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Open Systems

Open systems develop no pressure only until a mechanism is being

operated; the pressure is then metered by the selector valve and limited

by a relief valve. Each subsystem has an individual relief valve that limits

the maximum pressure allowed in the subsystem.

The main advantage of the open system is its simplicity, and the main

disadvantage is that only ONE subsystem can be operated at a time.

Open systems are mostly found only on light, general aviation aircraft.

Transport aircraft usually operate several subsystems at the same time,

thus require more complex systems.

Fluid 5-8

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An open hydraulic system

Fluid 5-9

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Closed Systems

A closed system, or closed-center system, is one that directs fluid flow to

the main system manifold and builds up pressure (ie. stores fluid under

pressure) in that portion of the system leading to all the selector valves.

There are two basic types of closed systems:

• One has a constant-volume pump and a pressure regulator to control

the pressure at a working range and to “unload” the pump when there

is no flow requirement and pressure builds up in the system manifold.

• The other utilizes a variable-volume pump to direct flow to the

system manifold, similar in concept to the constant-volume system.

The difference is that the output of the variable-volume pump is

controlled by an internal control valve which reduces the pump flow to

zero when no units are operating in the system and sufficient

pressure has build up in the manifold.

The pump may be driven by either an engine or by an electric motor. The

means of pressure control and the means of driving the pump may vary,

but all closed systems are basically the same.

Fluid 5-10

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Closed Systems

Unlike in an open system, any number of subsystems may be

incorporated in a closed system. However, the subsystems differ from

those in the open system in that the selector valves are arranged

in parallel rather than in series.

When the subsystems are not in use, pressure is relieved from the

continuously operating pump by means of the pressure regulator (also

known as the cut-out or unloading valve) after the accumulator is

charged to the correct level.

system

manifold

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Closed Systems

When a selector valve is positioned to operate a subsystem, pressure

first comes from the accumulator for operation; when accumulator

pressure has dropped to a predetermined level, the pressure regulator

kicks in and directs pump flow to the operating system.

The pump directs flow into the system to increase the system pressure

until the maximum setting of the regulator is reached again, at which

point the flow is kicked-out. In this way, the system pressure is always

maintained between the kick-out and kick-in settings of the regulator.

A system relief valve is

installed to safeguard the

system in the event the

pressure regulator should fail.

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Closed Systems

Multiple power pumps are used in many hydraulic closed systems.

Normally they are used in multi-engine aircraft where they can be driven

by separate engines. This assures continuous hydraulic system

operation in the event of an engine failure or the failure of one of the

pumps.

Diagram shows how dual power

pumps are connected into the

system.

Both pumps supply into the same

system by combining their volume

output into a common pressure

manifold.

The pumps may be either the

constant-volume type or the

variable-volume type.

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A closed hydraulic system

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An OPEN hydraulic system

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An OPEN hydraulic system

Engine Driven

PumpM

Pressure

Gage

Accumulator

Reservoir

Unloading

Valve

4/3 Way Selector Valve

Hydraulic Cylinder

4/3 Way Selector Valve 4/3 Way Selector Valve

Hydraulic Cylinder Hydraulic Cylinder

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A CLOSED hydraulic system

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Engine Driven

PumpM

Pressure

Gage

Accumulator

Reservoir

Unloading

Valve


Hydraulic Cylinder



System Manifold

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Engine Driven

PumpM

Pressure

Gage

Accumulator

Reservoir

Unloading

Valve


Hydraulic Cylinder



System Manifold

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Triple Redundancy

There are three independent hydraulic systems. Each hydraulic system

has two or more pumps that operate from different pneumatic,

mechanical, or electrical power sources.

Each hydraulic system can independently operate the flight controls for

safe flight and landing.

Pump Operation on Demand

Normally, one or two pumps in each hydraulic system operate

continuously. The other pumps operate only when there is a hydraulic

demand. This increases pump life, efficiency, and reliability.

Ram Air Turbine

If all normal pressure sources become unavailable during flight, the ram

air turbine supplies an emergency source of hydraulic power for the

primary flight controls.

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The three hydraulic systems operate

independently at 3,000 psi nominal

pressure. The systems are named

LEFT (L), CENTER (C) and RIGHT (R)

based on the location of their main

components. Each system has its

own reservoir, pumps and filters.

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Accessory

Gear Box

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LEFT System

The left system has an engine-driven pump (EDP) and an alternating-

current motor pump (ACMP). The right AC bus supplies power to the

ACMP.

The LEFT system supplies power to the flight controls and the left thrust

reverser.

RIGHT System

The right system also has an EDP and an ACMP. The left AC bus supplies

power to the ACMP.

The RIGHT system supplies power to the flight controls, the normal main

gear brakes, and the right thrust reverser.

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CENTER System

The CENTER system has two ACMPs, two air-driven pumps (ADPs) and a

ram air turbine (RAT) pump. The left and right AC buses supply power to

the ACMPs. Pneumatic power from the two engines or from the auxiliary

power unit (APU) operates the ADPs.

The CENTER system supplies power for these functions:

• Flight controls

• Leading edge slats

• Trailing edge flaps

• Alternate and reserve main gear brakes

• Normal and Reserve nose gear steering and nose gear actuation

• Normal main gear actuation and steering

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CENTER System

The RAT deploys automatically during flight when any of these following

conditions occur:

• Both engines are shut down

• Both AC buses are not powered

• All three hydraulic system pressures are low

Ram air turns the turbine which then drives a piston pump on the RAT to

provide required hydraulic power to the primary flight controls.

The RAT can only be retracted on the ground.

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Primary and Demand Pumps

The PRIMARY pumps are the EDPs in the LEFT and RIGHT systems, and

the ACMPs in the CENTER system. These pumps operate continuously.

The DEMAND pumps are the ACMPs for the LEFT and RIGHT systems,

and the ADPs for the CENTER system. These pumps normally operate

only during heavy system demands.

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Controls and Indications

The hydraulic pump controls and indication lights are on the P5 overhead

panel.

Pump Manual Controls

Pump controls on the Hydraulic/RAT panel permit manual control of the

hydraulic systems.

The primary pump switches have ON and OFF positions. Primary pumps

are normally ON. The demand pump selectors may be set to OFF, AUTO,

or ON. To permit automatic pump control, demand pumps are normally

set to AUTO.

RAT Manual Control

The RAT deploy switch, on the upper part of the hydraulic/RAT panel,

permits the flight crew to manually deploy the ram air turbine.

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Indicating Lights

Each pump has an amber fault light which shows a pump overheat or low

pressure condition. The RAT switch has a green light which shows high

RAT output pressure and an amber light which shows the RAT is

unlocked, ie deployed.

Engine Fire Switches

When operated, the engine fire switches shut off hydraulic fluid supply to

the EDPs. The engine fire switches are on the P8 aft aisle stand panel.

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An Aircraft Landing Gear System

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Boeing 777 Flight Control Systems

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Primary Flight Control System

The primary flight control system (PFCS) is a modern, three-axis, fly-by-

wire system. The fly-by-wire design permits a more efficient structural

design. Some benefits of this fly-by-wire design are increased fuel

economy, and smaller vertical fin and horizontal stabilizer.

The PFCS supplies manual and automatic airplane control and envelope

protection in all three axes. There is stability augmentation in the roll,

pitch, and yaw axes.

The PFCS calculates commands to move the control surfaces with

sensor inputs from these components:

• Control wheels

• Control column

• Rudder pedals

• Speedbrake lever

• Pitch trim switches

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Primary Flight Control System

The following control surfaces are used for roll control:

• Two ailerons

• Two flaperons

• Fourteen spoilers

The following control surfaces are used for pitch control:

• Two elevators

• A single moveable horizontal stabilizer

The following control surfaces are used for yaw control:

• A single tabbed rudder

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Primary Flight Control System (PFCS) Operational Overview

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PFCS Operational Overview

Manual Operation

Position transducers change the flight crew commands of the control

wheels, the control columns, the rudder pedals and the speedbrake lever

to analog electrical signals.

These signals go to the actuator control electronics (ACEs). The ACEs

change the signal to digital format and send them to the primary flight

computers (PFCs).

The PFCs communicate with the airplane systems through the three flight

controls ARINC 629 buses. In addition to the command signals from the

ACEs, the PFCs also receive data from the AIMS, ADIRU and SAARU.

These signals are airspeed, attitude and inertial reference data.

The PFCs calculate the flight control commands based on control laws,

stability augmentation and envelope protection. The digital command

signals from the PFCs then go to the ACEs.

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Manual Operation

The ACEs change these command signals to analog format and send

them to the power control units (PCUs). One, two or three PCUs operate

each control surface.

The PCUs contain a hydraulic actuator, an electro-hydraulic servo valve,

and position feedback transducers.

A servo-control unit

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Manual Operation

The servo valve causes the hydraulic actuator to move the control

surface. The actuator position transducer sends a position feedback

signal to the ACEs. After conversion to digital format, the ACEs send the

signal to the PFCs.

The ACEs stop the PCU command when the position feedback signal

equals the commanded position.

Some of the PCUs have differential pressure transducers to measure the

force from the PCU. The PFC uses this pressure data to equalize the

force of all PCUs acting on a control surface.

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Autopilot Operation

The PFCs receive autopilot commands from the three autopilot flight

director computers (AFDCs).

The PFCs calculate the flight control commands in the same manner as

for manual operation. In addition, the PFCs supply the backdrive signals

to the backdrive actuators through the AFDCs.

The backdrive actuators move the control wheels, control columns and

rudder pedals in synchronization with the autopilot commands. The

movement of the flight deck controls supplies visual feedback of

autopilot control to the flight crews.

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Boeing 777 High Lift Surfaces

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High Lift Control System

The high lift control system (HLCS) supplies increased lift at lower

speeds for takeoff and landing.

High lift surfaces include one inboard and one outboard trailing edge flap

on each wing. There are seven leading edge slats and one Krueger flap

on each wing.

Trailing Edge Flaps

The trailing edge flaps have an inboard double-slotted flap and an

outboard single-slotted flap on each wing. The flaps have seven

positions: UP, 1, 5, 15, 20, 25, and 30. The takeoff setting is at 5, 15, 20.

The landing setting is at 25 or 30. The flaps retract at settings 1 and UP.

Hydraulic or electric motors on the flap power drive unit (PDU) turn the

flap torque tubes. The torque tubes operate the flap transmission

assemblies. The flap transmission assemblies use a ballscrew and

gimbal to extend and retract the flaps.

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Leading Edge Slats

The leading edge slat system has seven slats and one Krueger flap on

each wing. The Krueger flap seals the gap between the engine strut and

the inboard slat.

The slats have three positions:

• Cruise (retracted)

• Takeoff (sealed)

• Landing (gapped)

The Krueger flap has two positions: retracted and deployed.

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Leading Edge Slats

Hydraulic or electric motors on the slat power drive unit (PDU) turn the

slat torque tubes. The torque tubes drive the slat rotary actuators. The

rotary actuators extend and retract the slats with a rack and pinion drive.

Flap/Slat Electronic Units

Two identical and interchangeable flap/slat electronic units (FSEUs), in

the main equipment center, process the high lift commands.

Flap Position Sensing

There are two position sensors on each side of the flap PDU. These

sensors supply the flap position to the FSEUs for control and monitoring.

Slat Position Sensing

There are two position sensors at each end of the slat torque tubes.

These sensors supply the slat position to the FSEUs for closed loop

control and monitoring.

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High Lift Control System (HLCS) Operational Overview

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HLCS Operational Overview

The high lift control system (HLCS) extends and retracts the trailing edge

and leading edge devices.

The HLCS operates in three modes:

• Primary

• Secondary

• Alternate

Primary Mode

The primary mode has a fly-by-wire closed loop control and operates

hydraulically. The pilot controls the HLCS with the flap lever on the

control stand. The lever has seven detents corresponding to the different

flap position settings. Four sensors transmit the flap lever position to the

two FSEUs.

The FSEUs receive and transmit data on the systems ARINC 629 buses.

Other airplane systems supply airspeed and hydraulic data through these

buses for the high lift protection functions.

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Primary Mode

The FSEUs control solenoids in the primary control valves.

These valves, in turn, control the hydraulic power to the hydraulic motors

on the flap and slat PDUs. These motors operate the flap and slat

mechanisms.

The FSEUs also operate the autoslat priority valve for autoslat extension

when the airplane is near a stall condition.

Secondary Mode

If the FSEUs find a fault in the primary mode, they switch to the

secondary mode. The secondary mode operates electrically, but the pilot

control is the same as in the primary mode.

The FSEUs control the secondary/alternate control relays. These relays

engage clutches and supply electrical power to electric motors on the

flap and slat PDUs. The electric motors move the flap and slat

mechanisms. Fluid 5-52

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Alternate Mode

The alternate mode is independent of the FSEUs and uses electrical

power to move the flaps and slats.

The pilot selects the alternate mode with the alternate flaps arm switch.

The pilot then selects extend or retract using the alternate flaps selector.

These switches are on the control stand, outboard of the flap lever.

These switches control the secondary/alternate control relays for the

flaps and slats in the same way as the secondary mode.

The alternate mode uses the flap and slat limit switches to limit the flaps

to 20o and the slats to the sealed position.

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Summary

• Type of Aircraft Hydraulic Systems

– Open system

– Closed system

• Schematic Diagrams

• Aircraft Hydraulic Power Selection

• Flight Control System

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6. Pneumatic System Components

Pneumatic System


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Outline

• Features

• High / Medium / Low Pressure System

• Relief Valve

• Control Valve

• Check Valve

• Restrictor

• Filter

• Moisture Separator

• Shuttle Valve

2

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Modern aircraft uses pneumatic (compressed air) systems for a variety of

purposes. Some use pneumatics rather than hydraulics for the operation

of landing gear, flaps, brakes, cargo doors, and other forms of

mechanical actuation.

Other aircraft using hydraulics for these major functions rely on

pneumatic systems for emergency sources of pressure in the event of a

failure in any of the hydraulically actuated subsystems.

Still other aircraft use pneumatics only for de-icing and for the operation

of various flight instruments.

The principle of operation for a pneumatic power system is the same as

that of a hydraulic power system, with one important exception:

• The air in a pneumatic system is compressible; therefore, the pressure

in the system can reduce gradually from the maximum system

pressure to zero pressure.

• In the hydraulic system, as soon as the accumulator fluid has been

used and the pump is not operating, the pressure in the system

quickly drops from accumulator pressure to zero pressure.

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In the pneumatic system, the entire system (including the air-storage

bottles) can act to store air pressure.

In the hydraulic system, the only pressurized fluid storage is in the

accumulator, and this pressure is supplied by the compressed air or gas

in the air chamber of the accumulator.

Some of the advantages of using compressed air over hydraulic or

electrical systems are:

• Air is universally available in an inexhaustible supply.

• Pneumatic system components are reasonably simple and lightweight.

• Compressed air, as a fluid, is lightweight and there is no need for

return lines. After the compressed air has served its purpose, it can be

dumped overboard, which saves tubing, fittings, and valves.

• The system is relatively free from temperature problems.

• There is no fire hazard, and the danger of explosion is minimized by

careful design and operations.

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The air in a pneumatic system must be kept clean by means of filters and

at the same time also kept free from moisture and oil droplets or vapor.

For this reason, liquid separators and chemical air driers are

incorporated in pneumatic systems. Moisture in a pneumatic system

may freeze in the low temperatures encountered at high altitudes,

resulting in serious system malfunctions.

Pneumatic systems are often compared to hydraulic systems, but such

comparisons can only hold true in general terms. Pneumatic systems do

not utilize reservoirs, hand pumps, or accumulators. Nevertheless,

similarities with their hydraulic counterparts do exist in some pneumatic

components.

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The type of unit used to provide pressurized air for pneumatic systems is

determined by the particular system’s air pressure requirements.

High Pressure Systems

For high-pressure systems, air is usually stored in

metal bottles at pressures ranging from 1,000 to 3,000

psi depending on the particular system.

This type of air bottle has two valves, one of which is a

charging valve. The other valve is a control or shutoff

valve.

Although the high pressure storage cylinder is light in

weight, it has a definite disadvantage – system cannot

be recharged in flight and therefore operation is limited

by the small supply of bottled air.

This arrangement cannot be used for continuous

operations of systems such as landing gear or brakes,

unless other air-pressurizing units are added to the

aircraft to ensure continuous supply of pressurized air. Fluid 6-6

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On some aircraft, permanently installed air

compressors are added to recharge air

bottles whenever pressurized air has been

discharged from the bottles for operating

subsystems.

Several types of compressors are used for

this purpose. Some have two stages of

compression, while others have more.

Diagram shows a simplified schematic of a

2-stage compressor: the pressure of the

incoming air is boosted first by cylinder

No. 1 and again by cylinder No. 2.

Some source of power, such as an electric

motor or aircraft engine, operates the

drive shaft. As the shaft turns, it drives

the pistons in and out of their cylinders.

Air Compressor

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When piston No. 1 is moved to the right, the volume in cylinder No. 1

becomes larger, and outside air is drawn through the filter and check

valve into the cylinder.

As the drive shaft continues to turn, it reverses the direction of piston

movement. Piston No. 1 is now moved to the left deeper into its cylinder,

forcing air through the pressure line and into cylinder No. 2. At the same

time, piston No. 2 is moved to the left out of its cylinder so that incoming

air can be received in cylinder No. 2.

Because cylinder No. 2 is smaller than

cylinder No. 1; thus piston No. 1 gives the

air its first stage of compression when fitting

the air into cylinder No. 2.

The second stage occurs as piston No. 2

moves deeper into its cylinder, further

forcing high-pressure air to flow through the

pressure line and into the air storage bottle.

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Air Driven Compressor

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Medium Pressure Systems

A medium pressure pneumatic system (100 – 150 psi) usually does not

include an air bottle. Instead, it generally takes bleed air from the aircraft

engine compressor section.

Engine bleed air will first be routed to a pressure-controlling unit and

then to the operating units.

Some jet aircraft use compressor bleed air from the engines to provide a

relatively large volume of compressed air at a low pressure

• to heat the leading edge of the wing and prevent the formation of ice

(anti-icing),

• to provide air for starting engines on the ground (engine start-up),

• for pressurizing and controlling the temperature of air in the aircraft

interior cabin (aircraft air-conditioning and pressurization).

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Medium Pressure Systems

Schematic diagram of a typical

aircraft air-conditioning system

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Low Pressure Systems

Low-pressure pneumatic systems are used in many aircraft to provide

the necessary compressed air to drive their gyro instruments.

In modern aircraft, primary gyros are electrically driven. Hence, the low-

pressure pneumatic system serves as a backup power source to drive

the primary gyros.

In some design, the gyro instrumentation has primary and back-up gyros.

The primary gyros are electrically driven while the back-up gyros are

pneumatically driven.

Turbine-powered aircraft bleed some of the air from the engine

compressor, regulate and filter it, and then direct the low-pressure air

over the gyros.

Aircraft with reciprocating engines use air pumps (ie. compressors)

driven by electric motors or by the aircraft engine to provide the

necessary air flow for the gyros. Similarly, this air is regulated and

filtered before it is ready for the instrument.

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Relief Valves

Relief valves are used in pneumatic systems to prevent damage. They

act as pressure-limiting units and prevent excessive pressures from

bursting lines and blowing out seals.

At normal pressures, a spring holds

the valve closed, and air remains in the

pressure line.

If pressure grows too high, the force it

creates on the disk overcomes spring

tension and opens the relief valve.

Then, excess air flows through the

valve and is exhausted as surplus air

into the atmosphere. The valve

remains open until the pressure drops

to normal.

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Control Valves

Control valves are also a necessary part of a typical pneumatic system.

Diagram illustrates how a valve is used to control emergency air brakes.

The control valve consists of a 3-port housing, two poppet valves, and a

control lever with two lobes.

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Control Valves

In figure (A), the control valve is shown in the “off” position. A spring

holds the left poppet closed so that the compressed air entering the

pressure port cannot flow to the brakes.

In figure (B), the control valve has been placed in the “on” position. One

lobe of the lever now holds the left poppet open, and a spring closes the

right poppet. Compressed air now flows around the opened left poppet,

through a drilled passage, and into a chamber below the right poppet.

Since the right poppet is closed thus effectively sealing off the vent port,

the high-pressure air can now only flow out of the brake port and into the

brake line to apply the brakes.

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Check Valves

Check valves are used in both hydraulic and pneumatic systems.

Diagram illustrates a flap-type pneumatic check valve. Air enters the left

port of the check valve, against the action of a light spring, forcing the

check valve open and allowing air to flow out the right port.

But if air enters from the right, air pressure closes the valve, preventing a

flow of air out the left port. Thus, a pneumatic check valve is a

1-direction flow control valve.

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Restrictors

Restrictors are a type of control valve used in pneumatic systems.

Diagram shows an orifice-type restrictor with a large inlet port and a

small outlet port. The small outlet port reduces the rate of airflow and the

speed of operation of an actuating unit.

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Variable Restrictors

Another type of speed-regulating unit is the variable restrictor.

It contains an adjustable needle valve, which has threads around the top

and a point on the lower end.

Depending on the direction turned, the

needle valve moves the sharp point

either into or out of a small opening to

decrease or increase respectively the

size of the opening.

Since air entering the inlet port must

pass through this opening before

reaching the outlet port, this

adjustment will determine the rate of

airflow through the restrictor.

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Filters

Like their hydraulic counterparts, pneumatic

systems are protected against dirt by means of

various types of filters.

A micronic filter consists of a housing with two

ports, a replaceable filter cartridge, and a relief

valve.

Air enters from the inlet and circulates around

the cellulose cartridge, then flows to the center

of the cartridge before exiting through the

outlet port.

If the cartridge becomes clogged with dirt,

pressure rises in the filter. This build-up

pressure then forces the relief valve open and

allows unfiltered air to flow out directly through

the outlet port, bypassing the clogged filter.

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Filters

A screen-type filter is similar to the micronic

filter but contains a permanent wire screen

instead of a replaceable cartridge.

In the screen filter, a handle extends

through the top of the housing and can be

used to clean the screen by rotating it

against metal scrapers.

The basic construction of these filters is the

same as those used in hydraulic systems.

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Desiccant / Moisture Separator

In a pneumatic system it is of the utmost

importance that the air in the system be

completely dry.

Moisture in the system can cause freezing of

operating units, interfere with the normal

operation of valves, pumps, etc., and cause

corrosion.

For this reason, moisture separators are

used in pneumatic systems to remove

approximately 98% of any moisture and/or

oil that may pass from the compressor.

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A moisture separator, as shown, collects the

water that is in the air on a baffle. The water

collected is then caused to settle at the

bottom of the air chamber against a closed

drain valve.

When the inlet pressure to the separator

drops below a preset value, the drain valve

opens and all of the accumulated water is

blown overboard.

An electric heater built into the base of the

separator unit prevents the water from

freezing.

To function properly, the moisture separator

must be mounted vertically with the

overboard drain at the bottom.

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After the air leaves the moisture separator, it must pass through a

desiccant, or chemical dryer, to remove the last traces of moisture.

This unit, installed downstream of the mechanical moisture separator,

consists of a tubular housing with an inlet and outlet port and contains a

desiccant cartridge.

Air is directed through this replaceable cartridge, and any moisture that

the separator has failed to removed will be absorbed by a dehydrating

agent (Mil-D-3716) contained inside the cartridge.

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Shuttle Valves

Shuttle valves are installed to allow pneumatic systems to be operated

from a ground source.

When the pressure from the external ground source is higher than that of

the compressor as it is when the engine is not running, the shuttle slides

over and isolates the compressor. The pneumatic systems may then be

operated from the ground source.

Shuttle valves may also be used to

provide an emergency pneumatic

backup for hydraulically operated

landing gear or brake systems.

Fluid 6-28

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Pneumatic%20System%20Layout.ppt

Pneumatic System Layout.ppt

Fluid Power Systems


Summary

• Features

– Advantages

– Difference from hydraulic

– Problems with Pneumatic

• High / Medium / Low Pressure System

• Relief Valve

• Control Valve

• Check Valve

• Restrictor

• Filter

• Moisture Separator

• Shuttle Valve

29

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7. Fluid Power System Maintenance Practices

Maintenance Practice


System

Fluid 7-1

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Servicing Inspection

Troubleshooting Maintenance

Fluid 7-2

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Servicing

• Performed at intervals specified by the manufacturer.

• When servicing a hydraulic reservoir, make certain the correct type

of fluid is used by checking the fluid color and smell for verification.

• When air has entered the system, it is necessary to fill the reservoir and

then purge the air from the system by operating all subsystems through

several cycles until all sounds of air in the system are eliminated.

• When cleaning filters or replacing filter elements, be sure there is no

pressure on the filter. Check system pressure to locate any leaks in the

filter assembly after it has been cleaned or replaced.

• Hydraulic fluid leaks are readily apparent because of the appearance of

fluid. Large pneumatic leaks are indicated by the sound of escaping air

and the feel of an airflow. Small pneumatic leaks can be located by the

use of a soap-and-water solution.

• Fluid containers should never be left open to the air longer than necessary

to reduce the possibility of contamination.

Fluid 7-3

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Fluid Power Systems


Inspection

• Check for leakage, worn or damaged tubing or hoses, wear of

moving parts, security of mounting for all units, safetying,

and any other conditions specified by the manufacturer. An

operational check of all subsystems is also part of inspection.

• Leakage from any stationary connection in a system is NOT permitted. A

small amount of fluid seepage may be permitted on moving units such as

actuator piston rods and rotating shafts. A thin film of fluid in these areas

indicates the seals are being properly lubricated.

• Any hydraulic tubings and flexible hoses must not be nicked, cut, dented,

collapsed, or twisted beyond approved limits set by the manufacturer.

• All connections and fittings associated with moving units should be

examined in an unloaded condition for play evidencing wear.

• Accumulators should be checked for leakage, air or gas preload, and

position. Accumulators installed in upright positions should be mounted

with the air chamber upward to remove gravity effects on the fluid inside.

Fluid 7-4

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Fluid Power Systems


Inspection

• During operational check, the entire system and each

subsystem should be checked for smooth operation, unusual

noises, and speed of operation for each unit.

• The pressure section of the system should be checked with no subsystems

to see that pressure holds for the required time without the pump

supplying the system.

• System pressure should be checked during operation of each subsystem

to see that the pump maintain the required pressure.

• When inspection of hydraulic filters indicates that the fluid is

contaminated, flushing the system is necessary.

This is accomplished by connecting the system to a

ground hydraulic test stand and pumping clean,

filtered fluid through the system and operating all

subsystems until no further signs of contamination

are found upon re-inspection of the filters.

Fluid 7-5

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Fluid Power Systems


Troubleshooting

• Refer to the troubleshooting information furnished by the

manufacturer as various systems and sub-systems vary in

complexity.

• Lack of pressure in a system is a common occurrence, especially when the

unit has not been properly maintained over a substantial period of time.

- any condition that permits free flow back to the reservoir or overboard

can lead to the lack of pressure in the system.

- can be caused by a sheared pump shaft, defective relief valve or

pressure regulator or unloading valve stuck in the “kicked-out” position,

lack of hydraulic fluid in the system, check valve installed backward, etc.

• One technique used in troubleshooting is the method of isolation.

This requires the suspect sub-system to be isolated while operating the

system, and only one sub-system can be isolated at any one time. An

example is when a system operates satisfactorily with a ground test unit

but not with the system pump, then the pump should be examined.

Fluid 7-6

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Fluid Power Systems


Troubleshooting

• If a system fails to hold pressure in the pressure section, the likely cause is

a defective

(1) pressure regulator (or unloading valve)

(2) accumulator

(3) relief valve

(4) check valve Fluid 7-7

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Fluid Power Systems


Troubleshooting

• If the pump fails to keep pressure up during operation of a

sub-system, the pump may be worn or one of the pressure-

control units may be leaking.

• High pressure in a system may be caused by a defective or improperly

adjusted pressure regulator or unloading valve, or by an obstruction in a

line or control unit.

• Another common occurrence is the generation of unusual noise in the

hydraulic system, such as banging and chattering.

This may be caused by air or contamination in the system. It can also be

due to a faulty pressure regulator, another pressure-control unit, or a lack

of proper accumulator action, ie. a faulty accumulator.

• Presence of air in a hydraulic system may cause the actuators to produce a

spongy response. This problem occurs because the air contracts and

expands as pressure is applied and released, thus creating the spongy

feeling and reducing the positive pressure that should be available for

actuator operation.

Fluid 7-8

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Maintenance

• Usually carried out in approved repair facilities; however,

replacement of seals and packings may be performed from

time to time in the field.

• When a unit is disassembled, all O-ring and chevron seals should be

removed and replaced with new seals. The new seals must be of the same

material as the original; otherwise they must have the necessary approval

from the manufacturer to replace the original.

• No seal should be installed unless it is positively identified as the correct

part and its shelf life has not expired.

• When installing seals, care must be exercised to ensure that the seal is not

scratched, cut, or otherwise damaged. Sharp-edged metal tools should not

be used to stretch and force seals into place.

Seals

Fluid 7-9

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Fluid Power Systems


Maintenance

Hydraulic Units and Tubings

• All openings in hydraulic (and pneumatic) systems should

be capped or plugged to prevent contamination of the

system. Dirt particles quickly cause hydraulic units to

become inoperative and may cause severe damage.

• Every hydraulic unit has some specification (eg. NAS 1638) on the

acceptable level of fluid cleanliness for its proper operations. If there is

any question regarding the cleanliness of the fluid, do not use it.

• Exercise care when handling hydraulic fluid of phosphate ester type. Such

type contains a powerful solvent characteristic that can soften or dissolve

many types of paints, lacquers, and enamels.

• When any of such type of fluid is spilled, it should immediately be removed

and the area washed.

• Never allow hydraulic fluids of different types to become mixed. Mixed

fluid will render a hydraulic system useless Fluid 7-10

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Maintenance

Flexible hoses

• When installing flexible hose, always hold the metal sleeve of the internal

hose fitting with a wrench while the nut is being tightened to the external

fitting. This is to avoid twisting and damaging the hose.

• Proper torque must always be applied in connecting fittings to prevent the

fittings from coming loose and causing leaks during operations, especially

since they are subjected to vibration and often associated with moving

parts of the system.

Torquing

• Proper torque settings are often indicated in the manufacturer’s manual.

Too much torque will damage metal and seals, and too little torque will

result in leaks and loose parts.

• When installing flexible hoses, do not change the shape of

the hose from its original setting – secure with an

appropriate clamp or cable-tie to hold the shape.

Fluid 7-11

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Summary

• Servicing

– Reservoir

– Filters

• Inspection

– Tubing and Hoses

• Troubleshooting

– Lack of Pressure

– Method of Isolation

– Presence of Air

• Maintenance

– Tubing and Hoses

– Torque Value

Fluid 7-12

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1asas2 - chap 1 fluid pwr (v02)(2011-10-01)

Documents

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