HYDRAULICS AND PNEUMATICS (UNIT 4)

By,
KANNA M
Assistant Professor
Dept.of.Mech.Engg.,
KIT-CBE.
HYDRAULICS AND PNEUMATICS

OBJECTIVES: L T P C
3 0 0 3
 To provide student with knowledge on the application of fluid power in process,
construction and manufacturing industries.
 To provide students with an understanding of the fluids and components utilized in
modern industrial fluid power system.
 To develop a measurable degree of competence in the design, construction and
operation of fluid power circuits.

UNIT IV
PNEUMATIC AND ELECTRO PNEUMATIC SYSTEMS
Properties of air – Perfect Gas Laws – Compressor – Filters, Regulator, Lubricator, Muffler, Air
control Valves, Quick Exhaust Valves, Pneumatic actuators, Design of Pneumatic circuit – Cascade
method – Electro Pneumatic System – Elements – Ladder diagram – Problems, Introduction to
fluidics and pneumatic logic circuits.

INTRODUCTION:
 Pneumatic systems use pressurized gases to transmit and control power.
 As the name implies pneumatic systems typically use air (rather than some other
gas) as the fluid medium, because air is a safe, Low-cost, and readily available fluid.
 It is particularly safe in environments where an electrical spark could ignite
leaks from system components.
 There are several reasons for considering the use of pneumatic systems in stead of
hydraulic systems.
 Liquids exhibit greater inertia than do gases.
 Therefore, in hydraulic systems the weight of oil is a potential problem when
accelerating and decelerating actuators and when suddenly opening and closing valves.
 Liquids also exhibit greater viscosity than do gases. This results in larger
frictional pressure and power losses.

INTRODUCTION:
 Hydraulic systems require special reservoirs and no-leak system designs.
 Pneumatic systems use air that is exhausted directly back into the surrounding
environment. Generally speaking, pneumatic systems are less expensive than
hydraulic systems.
 However, because of the compressibility of air, it is impossible to obtain precise,
controlled actuator velocities with pneumatic systems.
 Also, precise positioning control is not obtainable.
 In applications where actuator travel is to be smooth and steady against a
variable load, the air exhaust from the actuator is normally metered.
 Hydraulics can be high-power systems, whereas pneumatics are confined to low-
power applications.
 Typical examples include stamping, drilling, hoisting, punching, clamping,
assembling, riveting, materials handling, and logic controlling operations.

PROPERTIES OF AIR:
 The earth is surrounded by a blanket of air-the atmosphere.
 Because air has weight, the atmosphere exerts a pressure at any point due to the
column of air above that point.
 The reference point is sea level, where the atmosphere exerts a pressure of 14.7
psia (101 kPa abs). For the region up to an altitude of 20,000 ft (6.1 km), the
relationship is nearly linear, with a drop in pressure of about 0.5 psi per 1000-ft change
in altitude (11 kPa per km).
 When making pneumatic circuit calculations, atmospheric pressure of 14.7 psia is
used as a standard. The corresponding standard specific weight value for air is 0.0752
lb/ft3 at 14.7 psia and 68°F (11.8 N/m3 at 101 kPa abs and 20°C).
 In a discussion of perfect gas laws, the density of a gas depends not only on its
pressure but also on its temperature.

PROPERTIES OF AIR:
 Air is not only readily compressible, but its volume will vary to fill the vessel
containing it because the air molecules have substantial internal energy and are at a
considerable distance from each other.
 This accounts for the sensitivity of density changes with respect to changes in
pressure and temperature.
 Free air is considered to be air at actual atmospheric conditions.
 Since at atmospheric pressure and day to day, the characteristics of free air vary
accordingly.
 Thus, when making pneumatic circuit calculations, the term standard air is used.
 Standard air is sea-level air having a temperature of 68°F, a pressure of
14.7 psia (20°C and 101 kPa abs), and a relative humidity of 36%.
 Absolute pressure (psia) = Gauge pressure (psig) + 14.7
 Absolute temperature (K) = temperature (°C) + 273

COMPRESSORS:
 In pneumatic systems, compressors are used to compress and supply the
necessary quantities of air.
 Compressors are typically of the piston, vane, or screw type.

COMPRESSORS:
 A compressor increases the pressure of a gas by reducing its volume as described
by the perfect gas laws.
 Pneumatic systems normally use a large centralized air compressor, which is
considered to be an infinite air source similar to an electrical system where you merely
plug into an electrical outlet for electricity.
 In this way, pressurized air can be piped from one source to various locations
through out an entire industrial plant.
 The compressed air is piped to each circuit through an air filter to remove
contaminants, which might harm the closely fitting parts of pneumatic components
such as valves and cylinders.
 The air then flows through a pressure regulator, which reduces the pressure to the
desired level for the particular circuit application.
 Because air is not a good lubricant, pneumatic systems require a lubricator to
inject a very fine mist of oil into the air discharging from the pressure regulator.

COMPRESSORS:
 This prevents wear of the closely fitting moving parts of pneumatic components.
 Free air from the atmosphere contains varying amounts of moisture.
 This moisture can be harmful in that it can wash away lubricants and thus cause
excessive wear and corrosion. Hence, in some applications, air dryers are needed to
remove this undesirable moisture.
 Since pneumatic systems exhaust directly into the atmosphere, they are capable of
generating excessive noise. Therefore, mufflers are mounted on exhaust ports of air
valves and actuators to reduce noise and prevent
 Operating personnel from possible injury resulting not only from exposure to
noise but also from high-speed airborne particles.

COMPRESSORS: PISTON TYPE

COMPRESSORS: PISTON TYPE
 Figure illustrates the design features of a piston-type compressor Such a design
contains pistons sealed with piston rings operating in precision bored close-fitting
cylinders. Note that the cylinders have air fins to help dissipate heat. Cooling is
necessary with compressors to dissipate the heat generate during compression.
 When air is compressed, it picks up heat as the molecules of air come closer
together and bounce off each other at faster and faster rates Excessive temperature can
damage the metal components as well as put power requirements, Portable and small
industrial compressors are normally air-cooled, whereas larger units must be water-
cooled.
 A single-piston compressor can provide pressure up to about 150 psi. Above 150
psi, the compression chamber size and heat of compression prevent efficient pumping
action. For compressors having more than one cylinder, staging can be used to improve
pumping efficiency. Staging means dividing the total pressure among two or more
cylinders by feeding the exhaust from one cylinder into the in let of the next..

COMPRESSORS: SCREW TYPE

COMPRESSORS: SCREW TYPE
 There is a current trend toward increased use of the rotary-type compressor due to
technological advances, which have produced stronger materials and better manufacturing
processes.
 Figure shows a cutaway view of a single-stage screw-type compressor, which is very
similar to a screw pump.
 Compression is accomplished by rolling the trapped air into a progressively smaller
volume as the screws rotate.
 Figure illustrates the unsymmetrical profile of the two rotors. The rotors turn freely, with
a carefully controlled clearance between both rotors and the housing, protected by a film of oil.
 Rotor wear will not occur, since metal-to-metal contact is eliminated. A precisely
measured amount of fil tered and cooled air is injected into the compression chamber, mixing
with the air as it is compressed.
 The oil lubricates the rotors, seals the rotor clearances for high compression efficiency,
and absorbs heat of compression resulting in low discharge air temperatures. Single stage
screw compressors are available with capacities up to 1450 cfm and pressures of 120 psi.

THE PERFECT GAS LAWS:
 Even though perfect gases do not exist, air behaves very closely to that predicted
by Boyle’s law, Charles law, Gay-Lussac’s Law and the general gas law for the
pressure and temperature ranges experienced by pneumatic systems.
 Boyle’s law
Boyle's law states that if the temperature of a given amount of gas is held
constant, the volume of the gas will change inversely with the absolute pressure of the
gas.
=

 Charles law
Charles law states that if the pressure on a given amount of gas is held
constant, the volume of the gas will change in direct proportion to the absolute
temperature.
=
 Gay-Lussac’s Law
Gay-Lussac's law states that if the volume of a given gas is held constant, the
pres sure exerted by the gas is directly proportional to its absolute temperature.
=

 General gas law :
Boyle's, Charles' and Gay-Lussac's laws can be combined into a single general
law, as defined by,
=

FLUID CONDITIONERS:
The purpose of fluid conditioners is to make air a more acceptable fluid
medium for the pneumatic system as well as operating personnel.
It includes filters, regulators, lubricators, mufflers and air dryers.
=

FLUID CONDITIONERS:
Filter :
=

FLUID CONDITIONERS : Filter
 The function of a filter is to remove contaminants from the air before it reaches
pneumatic components such as valves and actuators.
 Generally speaking, in-line filters contain filter elements that remove
contaminants in the 5- to 50-m range.
 Figure shows a cutaway view of a filter that uses 5- m cellulose felt, reusable,
surface-type elements.
 These elements have gaskets molded permanently to each end to prevent air
bypass and make element servicing foolproof.
 These elements have a large ratio of air to filter media and thus can hold an
astonishing amount of contamination on the surface without suffering significant
pressure loss.
 The baffling system used in these filters mechanically separates most of the
contaminants before they reach the filter element.
 In addition, a quiet zone prevents contaminants collected in the bowl from re-
entering the airstream.

FLUID CONDITIONERS:
Regulator :
=

FLUID CONDITIONERS : Regulator
 A constant pressure for a given pneumatic system, a pressure regulator is used.
 Airflow enters the regulator at A. Turning adjusting knob B clockwise (viewed
from knob end) compresses spring C, causing diaphragm D and main valve E to move,
allowing flow across the valve seat area.
 Pressure in the downstream area is sensed through aspirator tube F to the area H
above diaphragm D.
 As downstream pressure rises, it offsets the load of spring C Diaphragm D and
valve E move to close the valve against its seat, stopping airflow through the regulator.
 The holding pressure of spring C and downstream pressure H are in balance, at
reduced outlet pressure.
 Any airflow demand downstream, such as opening a valve, will cause the
downstream pressure to drop. Spring C will again push open valve E, repeating the
sequence in a modulating fashion to maintain the downstream pressure setting.

FLUID CONDITIONERS : Regulator
 A Fise in downstream pressure above the set pressure, will cause diaphragm D to
lift of the top of valve stem), thus relieving the excess pressure to the atmosphere
under knob B.
 When the downstream pressure returns to the set pressure, the diaphragm seats on
the valve stem, and the system is again in equilibrium.

FLUID CONDITIONERS:
Lubricator :
=

FLUID CONDITIONERS : Lubricator
 A lubricator ensures proper lubrication of internal moving parts of pneumatic
components.
 Figure illustrates the operation of a lubricator, which inserts every drop of oil
leaving the drip tube, as seen through the sight dome, directly into the airstream.
 These drops of oil are transformed into an oil mist prior to their being transported
downstream. This oil mist consists of both coarse and fine particles.
 The coarse particles may travel distances of 20 ft or more, while the fine particles
often reach distances as great as 300 ft from the lubricator source.
 These oil mist particles are created when a portion of the incoming air passes
through the center of the variable orifice and enters the mist generator, mixing with the
oil delivered by the drip tube.
 This air-oil mixture then rejoins any air that has bypassed the center of the
variable orifice and continues with that air toward its final destination.

FLUID CONDITIONERS : Lubricator
 Oil reaching the mist generator was first pushed up the siphon tube, past the
adjustment screw to the drip tube located within the sight dome.
 This is accomplished by diverting a small amount of air from the mainstream
through the bowl pressure control valve, into the bowl or reservoir.
 This valve is so located that it will close, shutting off the air supply to the bowl
when the fill plug is loosened or removed, permitting refilling of the bowl or reservoir
without shutting off the air supply line.
 On replacement of the fill plug, the bowl pressure control valve will open
automatically, causing the bowl to be pressurized once again and ready to supply
lubrication where it is needed.

FLUID CONDITIONERS:
Muffler :
=

FLUID CONDITIONERS:
Quick Exhaust Valve :
=

FLUID CONDITIONERS:
Air Dryer :
=

PNEUMATIC SINGLE ACTING CYLINDER :
=

CASCADE METHOD: (A+B+B-A-C+C-)
=

HYDRAULICS AND PNEUMATICS (UNIT 4)

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