HYDRAULICS & PNEUMATICS

HYDRAULICS & PNEUMATICS

Presented by: Dr. Abootorabi

IntroductionComponents

1

Introduction

Three basic methods of transmitting power:

Electrical

Mechanical

Fluid power

In practice, most applications actually use combination of

the three methods to achieve the most efficient overall

systems. 2

Introduction

We have to understand the features of each method in

order to get the best result.

For example, fluid systems can transmit power more

economically over greater distance compare to

mechanical systems. But fluid systems are restricted

to shorter distances compared to electrical systems.

3

Definition

Hydraulics is the

science of forces and

movements transmitted

by means of liquids.

4

Comparison of power system

5

Comparision of power system

6

Applications of hydraulic power

Machine-tool construction is a

typical area of application of

hydraulics. With modern CNC

machine tools, the tools and

workpieces are clamped by

hydraulic means. Feed motions

and the spindle drive can also

be hydraulically powered.

7

Machine tools

Press with elevated reservoir

This is an application in which extremely

high forces are required.

A special feature is the elevated reservoir,

which utilizes the static pressure in the

pressure medium.

8


Mobile hydraulics: Excavator

On this hydraulic

excavator, not only all

working movements

(linear drives) but also

the propulsion of the

vehicle (rotary drive) are

hydraulically powered.

The primary drive of the

excavator is an internal-

combustion engine.9


Fluid power is divided into two: hydraulic system

(using oils) and pneumatic system (using compressed

air).

Each system has its own advantages and drawbacks.

There are many factors to consider to choose a

suitable system.

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Fluid power

Advantages of fluid power can be summarized as

follows:

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Ease and accuracy of control

Multiplication of force

Constant force or torque

Simplicity, safety, economy

Hydraulic oils are messy and leakage is impossible to

eliminate completely.

Hydraulic lines can burst and might result in injuring

people and damaging surrounding objects.

Prolonged exposure to loud noise can damage hearing.

Most hydraulic oils can cause fires if there is a leakage.

Compressive air for pneumatic systems can be dangerous

if the pressure is too high.

12

Drawbacks of fluid power

Structure of a hydraulic system

This simplified block

diagram shows the division

of hydraulic systems into a

signal control section and a

hydraulic power section.

This signal control section is

used to activate the valves

in the power control

section.

13

Hydraulic power sectionThe diagram of the hydraulic power

section is complemented in this case by a circuit diagram to allow correlation of the various function groups; the power supply section contains the hydraulic pump and drive motor and the components for the preparation of the hydraulic fluid. The power control section consists of the various valves used to provide control and regulate the flow rate, pressure and direction of the hydraulic fluid. The drive section consists of cylinders or hydraulic motors, depending on the application in question. 14

Components of a fluid power system: Hydraulic system

A hydraulic system has six basic components:

A tank to hold the hydraulic oil

A pump to force the oil through the system

An electric motor or other power source to drive the

pump

Valves to control oil direction, pressure and flow rate

An actuator to convert the pressure of the oil into

mechanical force or torque to do useful work

Piping to carry the oil from one location to another 15

Components of a fluid power system: Hydraulic system

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• A tank (reservoir) to hold the hydraulic oil (A)• An electronic motor or other power source to drive the pump (B)• A pump to force the oil through the system (C)• Valves to control oil direction, pressure and flow rate (D-G)• An actuator to convert the pressure of the oil into mechanical force or torque to do useful work (H)• Piping to carry the oil from one location to another

Components of a fluid power system: Pneumatic

system

A pneumatic system also has six basic components:

An air tank to store a certain volume of compressed air

A compressor to compress the air coming from the atmosphere

An electric motor or other prime mover to drive the compressor

Valves to control air direction, pressure, and flow rate.

Actuators, which are similar in operation to hydraulic actuators

Piping to carry the pressurized air from one location to another

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Components of a fluid power system: Pneumatic

systemA pneumatic system also has six basic components:

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Primary functions of a hydraulic fluidTransmit power

Lubricate moving parts

Seal clearance between mating parts

Dissipate heat

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Hydraulic fluid In order to be safe, hydraulic fluids must also be

changed periodically.

The frequency of changing depends on the fluid as well

the operating conditions.

Advice from laboratory analysis could be sought to

determine when the fluid should be changed.

20

Fluids: liquids and gasesA liquid is a fluid which has a definite volume

independent of the shape of its container.

A liquid is considered to be incompressible so that its

volume does not change with pressure changes.

This is only approximation but the change in volume

due to pressure change is quite small that it is ignored

for most engineering purposes.

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Fluids: liquids and gasesA gas is a fluid which is compressible.

In addition, its volume will vary to fill the vessel

containing it.

A gas is greatly influenced by the pressure to which it is

subjected.

If the pressure increases, the volume decreases, and

vice versa.

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Fluids: liquids and gasesAir is the only gas commonly used in fluid power

systems because it is inexpensive and readily

available.

Air has the following desirable features:

1. Fire resistant

2. Not messy

3. Can be released back to atmosphere

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Disadvantages of airDue its compressibility, air cannot be used in an application

requiring accurate positioning or rigid holding.

Because air is compressible, it tends to be sluggish.

Air can be corrosive since it contains oxygen (about 21%) and

water.

A lubricant must be added to air to lubricate valves and actuators.

High pressure air (greater than 250 psi = 17 atm) is typically not

used due to the explosive dangers.

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Hydrostatic pressureHydrostatic pressure is the pressure

created above a certain level within

a liquid as a result of the weight of

the liquid mass. Hydrostatic

pressure is not dependent on the

shape of the vessel concerned but

only on the height and density of

the column of liquid.

Hydrostatic pressure can generally

be ignored for the purpose of

studying hydraulics.

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Pressure propagation

If a force F acts on an area A

of an enclosed liquid, a

pressure p is produced

which acts throughout the

liquid (Pascal's Law).

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Power transmission If a force F1 is applied to an area

A1 of a liquid, a pressure p

results. If, as in this case, the

pressure acts on a larger surface

A2, then a larger counter-force F2

must be maintained. If A2 is three

times as large as A1, then F2 will

also be three times as large as F1.

Hydraulic power transmission is

comparable to the mechanical law

of levers. 27

Displacement transmission

If the input piston of the

hydraulic press travels a

distance s1, a volume of fluid

will be displaced. This same

volume displaces the output

piston by the distance s2. If

the area of this piston is

larger than that of the input

piston, the distance s2 will be

shorter than s1.28

Pressure transferThe fluid pressure p1 exerts a force F1 on the

surface A1 which is transferred via the piston rod

to the small piston. The force F1 thus acts on the

surface A2 and produces the fluid pressure p2 .

Since the piston area A2 is smaller than the

piston area A1, the pressure p2 must be larger

than the pressure p1.

The pressure-transfer (pressure-intensification)

effect is put to practical use in

pneumatic/hydraulic pressure intensifiers and

also in purely hydraulic systems when extremely

high pressures are required which a pump

cannot deliver. 29

Types of flowA distinction is made between laminar

flow and turbulent flow. In the case of

laminar flow, the hydraulic fluid moves

through the pipe in ordered cylindrical

layers. If the flow velocity of the

hydraulic fluid rises above a critical

speed, the fluid particles at the center

of the pipe break away to the side, and

turbulence results.

Turbulent flow should be avoided in

hydraulic circuits by ensuring they are

adequate sized.30

Types of flowLaminar

Turbulent

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CavitationMotion energy is required for an

increase in the flow velocity of the oil at

a restriction. This motion energy is

derived from the pressure energy. If the

vacuum which results is smaller than -

0.3 bar, air dissolved in the oil is

precipitated out. When the pressure

rises again due to a reduction in speed,

the oil bursts into the gas bubbles.

Cavitation is a significant factor in

hydraulic systems as a cause of wear in

devices and connections. 32

CavitationLocal pressure peaks occur during cavitation. This causes the

erosion of small particles from the wall of the pipe immediately after

the reduced cross-section, leading to material fatigue and often also

to fractures. This effect is accompanied by considerable noise.

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Components of a hydraulic system

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Components of a hydraulic system Power supply section:

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Components of a hydraulic system Hydraulic fluid:

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Valves:

Components of a hydraulic system Types of valves:

1. Directional control valves:

39

Components of a hydraulic system 2. Pressure valves:

00

40

Components of a hydraulic system 3. Flow control valves:

41

Components of a hydraulic system 4. Non-return valves:

42

Components of a hydraulic system Cylinders (linear actuators):

43

1. Single-acting Cylinders:

Components of a hydraulic system Cylinders (linear actuators):

2. Double-acting cylinders:

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Components of a hydraulic system Motors (rotary actuators):

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Tank (reservoir) for a hydraulic system:

The function of a tank is to store the fluid used. However, it serves functions other than storage and is actually a working part of the system.

A hydraulic reservoir or tank has the following functions:Stores the hydraulic fluid of the system, including some

reserveProtects the stored fluid from outside contaminationProvides means to check the amount of fluid in the systemProvides means to add or change the fluidCools the fluid as it returns from the actuators, and Removes contaminants such as water, dirt, pieces of

metal, or chemicals from the fluid

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Tank (reservoir) of a hydraulic system47


Tank (reservoir) of a hydraulic system48


Components of a hydraulic system Most tanks are of welded construction with supports for

mounting for easy access to the drain plug and also to

permit cooling air to circulate underneath.

A tank must be totally enclosed and should have a filtered

air breather to screen out particles from the surrounding air.

The fluid that flows in the hydraulic system must be

cleaned. Contaminant are screened out using a strainer and

a filter. Some reservoirs have magnetic plugs to trap iron

and steel particles carried by the fluid.

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A strainer blocks the relatively large solid particles

from entering the system. It is attached to the pump

inlet line and may immersed in the oil near the bottom

of the tank. Particles stuck to the strainer are cleaned

off later and the strainer is ready for reuse.

A filter is used to remove smaller particles by

absorbing them. Fluid is allowed to flow through but

fine particles are trapped and absorbed. When the

filter becomes clogged, it is replaced by a new one.

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Strainers and Filters:

Strainers are constructed of a fine wire screen that usually

has openings more than 150 micron or μm.

A strainer only moves the larger particles.

The condition of the strainer can be monitored by installing a

pressure gage between the pump and the strainer. A pressure

drop shown by the gage indicated that the strainer is

becoming clogged. If the strainer is not cleaned, the pump

can be starved, resulting in cavitations and increased pump

noise. 51


Basically, filters and strainers are similar. However, the size of

particles that can be removed by a strainer is normally

greater than 150 μm. On the other hand, a filter can remove

much smaller particles, down to 1 μm.

Even particles as small as 1 μm can produce a damping effect

on hydraulic systems and can also accelerate oil

deterioration.

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The end.

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HYDRAULICS & PNEUMATICS

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