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UNIT IV

PNEUMATIC SYSTEMS AND COMPONENTS

Introduction

The majority of pneumatic systems using compressed air as

the working medium. The compressor compresses the atmospheric air

into the compressed air and supplies the necessary quantity of air

with required pressure. Before the compressed air is being supplied to

other circuit components, it should be conditioned so as to achieve

better and safe operations. For this purpose, fluid conditioners such as

filters, regulators, lubricators, mufflers and air dryers are used.

Components of pneumatic system:

1. Compressor 5. Actuators

2. Air tank 6. Fluid conditioners

3. Prime mover 7. Piping4. Control valves

Air Compressor:

It is a device used to compress air from a low inlet pressure to

higher desired pressure. The pressure of the air is increased by

decreasing the volume as per law of perfect gas.

Types of Compressor:

1. Positive displacement type

a. Reciprocating Compressor

1. Piston type Compressor 2. Diaphragm type Compressor

3. Labyrinth Compressor

b. Rotary Type Compressor

1. Screw Compressor

2. Vane Compressor

3. Lobe Compressor

4. Liquid ring Compressor

5. Gear Compressor

2. Dynamic Type

a. Radial Flow Compressor

b. Axial Flow Compressor

Piston Type Reciprocating Compressor:

Piston compressors are the most commonly used compressors

in the fluid power industry. The construction and working of a piston-

type reciprocating compressor is very much similar to that of an

internal combustion (IC) engine. It consists of a cylinder, cylinder

head, piston with piston rings, inlet and outlet valves, connecting rod,

crank, crankshaft, bearings, etc. The arrangement of a basic single

cylinder compressor is illustrated in Figure.



First a prime mover, mostly an electric motor, is used to drive the

compressor unit. The electric motor supplies the rotary motion to the

crank shaft, which in turn converted into reciprocating motion of

piston through the crank and connecting rod arrangement.

Inlet stroke: During the downward motion of the piston the pressure

inside the cylinder falls below the atmospheric pressure and the inlet

valve is opened due to the pressure difference. The air is drawn into

the cylinder until the piston reaches the bottom of the stroke.

Outlet stroke: As the piston starts moving upwards the inlet valve is

close, and the pressure starts increasing continuously until the

pressure inside the cylinder is above the pressure of the delivery side

which is connected to the receiver. Then the outlet valve opens and

air is delivered during the remaining upward motion of the piston to

the receiver.

Advantages:

1. Piston type compressors are available in wide range of

capacity and pressure.

2. The overall efficiency of piston compressors is high when

compared to other compressors.3. Very high air pressure and air volume flow rate can be

obtained by using the multistage compressors.

4. Better mechanical balance can be achieved with multi-stage

compressors.

Screw Compressor:

Screw compressors are used in many applications where medium

pressures < 10 bar and medium volumes of air (up to 5000 m3/hr) are

required. The construction and operation of a screw compressor is

very similar to a hydraulic screw pump. A typical screw lobe



compressor having unsymmetrical profile of screw rotors is show in

Figure. It consists of two screws--one with convex and the other with

concave contour, generally called male and female rotor respectively.

Also a minimal clearance is maintained between the two

intermeshing rotating screws.

As the screws rotate, air is sucked into the housing through the inlet

port. The sucked air is trapped between the screws and carried along

to the outlet port.

Advantages:1. Simplicity.

2. Fewer moving parts rotating at a constant speed.

3. Steady delivery of air without pressure pulses.

Rotary Vane Compressor:

The rotary vane type compressors are used in applications

where low-pressure and low volume is needed. For example, they are

used for instrument and other laboratory-type air needs.

The construction and operation of a rotary vane compressor is very

much similar to a hydraulic vane pump. A typical sliding-vane-type



rotary compressor is shown Figure. It consists of a rotor located

eccentrically in a cylindrical outer casing. The rotor carries a set of

spring-loaded vanes in the slots of the rotor. The air at atmospheric

pressure is entrapped between two vanes. As the rotor rotates, the

entrapped air is compressed between the vanes and then discharged

through a port to the receiver.

Advantages:

1. Rotary vane compressors are pulse free and therefore can be

used without a receiver if needed.

2. They are smaller in size and lighter in weight.

3. They can work at high speed.

Graphical Symbols of Compressor:

Fluid Conditioners:

The atmospheric air that is compressed in the compressor is

obviously not clean because the atmospheric air contains many

contaminants such as smoke, dirt, water vapor, etc. This

contaminated air may lead to excessive wear and failure of pneumatic

components. The system performance and accuracy depend mainly

on the supply of clean, dry and contamination-free compressed air.

Therefore fluid conditioners are used to condition the compressed air

before leaving into various pneumatic components.

Elements of Fluid Conditioners:

1. Filters, 2. Regulators 3. Lubricators4. Mufflers 5. Air dryers.

In these, the first three units together are called FRL (Filter-

Regulator-Lubricator) unit or service unit

Air Filter:

The function of air filters is to remove all foreign matter and

allow dry, clean air to flow without restriction to the regulator and

then on to the lubricator. Filters are available in wide ranges starting

from a fine mesh wire cloth (which only strains out heavier foreign



particles) to elements made of synthetic materials (which are

designed to remove very small particles), Usually in-line filter

elements can remove contaminants in the 5 to 50 µm range.

It consists of the filter cartridge, deflector, plastic bowl, baffle, water

drain valves etc. The air to be filtered is allowed downward with a

swirling motion that forces the moisture and the heavier particles to

fall down. The deflector used in the filter mechanically separates the

contaminants before they pass through the cartridge filter. The filter

cartridge provides a random zigzag passage for the air flow. This type

of air flow arrests the solid particles in the cartridge passage. The

water vapor gets condensed inside the filter and is collected at the

bottom of the filter bowl. Also heavier foreign particles that are

separated from the air are collected at the bottom of the bowl. Then

the accumulated water and other solid particles at the bottom of the

filter bowl are drained off with the use of an on-off drain valve

located at the bottom of the filter bowl.

Air pressure Regulator:

The function of the air pressure regulator is to regulate the

pressure of the incoming compressed air so as to achieve the desired

air pressure at a steady condition. The compressed air leaving the

compressor should be properly prepared before it goes into the

circuit. The air should have the proper operating pressure for the

circuit. Improper fluctuating pressure level in the piping system can

adversely affect the operating characteristics of the system



components such as valves, cylinders, etc. Therefore, air pressure

regulators are fitted to ensure the constant supply pressure

irrespective of the pressure fluctuations in the compressor unit. For

example, the line from the compressor may carry a pressure of l0 bar,

the air pressure regulator can reduce this pressure to 0 bar to any

point between the full line pressure and zero pressure. Thus the air

pressure regulators act as pressure guards by preventing pressure

surges or drops from entering the air circuits .

Types:

1. Diaphragm-type regulator

2. Piston-type regulator.

The construction and operation of a typical diaphragm-type air

pressure regulator is illustrated in Figure.

It consists of diaphragm, valve, main and dampening springs. Usually

the diaphragm is made of oil resistance synthetic rubber with a nylon

cloth reinforcements. The diaphragm allows the proper amount of

movement for opening and closing at the valve seat. When the

adjusting screw is in the fully retracted position, the valve is closed.

When the adjusting screw is turned to compress the adjusting and

dampening springs, the valve is opened. Thus the air is allowed from

inlet port to the outlet port. The pressure of the outlet air depends

upon the size of the valve opening that is maintained. This is

determined by the compression of the adjustable spring. Higher the

spring compression, more will be the amount of opening and hence



more the pressure and vice versa. The vent-holes are provided to let

out the undesirable excessive outlet pressure, if any, into the

atmosphere. The dampening spring is provided to act as a dampening

device needed to stabilize the pressure.

Air Lubricator:

The function of an air lubricator is to add a controlled amount

of oil with air to ensure proper lubrication of internal moving parts of

pneumatic components. The lubricator adds the lubrication oil in the

form of a fine mist to reduce the friction and wear of the moving

parts of pneumatic components such as valves, packing used in air

cylinders, etc. At the same time excessive lubrication is also

undesirable. Excessive lubrication may result (i) malfunction

components, (ii) increased environmental problems, and (iii) seizingof components after prolonged downtime. Generally a good-quality,

light-grade spindle oil is used in pneumatic systems. Sometimes, a

mixture of 50% kerosene and 50% SAE 30 oil is also used as

lubricant.

The construction and operation of a typical force-feed type air

lubricator is illustrated in Figure. Its operation is similar to the

principle of simple carburetor used in the petrol engines to obtain air-

fuel mixture. As the air to be lubricated enters into the inlet pipe, the

venturi ring located in the pipe increases its velocity of low. It causes

a local reduction in the upper chamber. This pressure differential

between upper and lower chambers causes suction of lubrication oil



from the oil reservoir to the upper chamber. Now the oil in the form

of mist is sprayed in the air stream and the air-oil mixture is obtained.

This air-oil mixture is forced to swirl as it leaves the central cylinder

causing more oil particles to be spread out of the air stream. The

amount of oil dropping into the upper chamber can be controlled by a

needle valve.

FRL Unit:

In most pneumatic systems, the compressed air is first filtered

and then regulated to the specific pressure and made to pass through a

lubricator for lubricating the oil. Thus usually a filter, regulator, and

lubricator are placed in the inlet line to each air circuit. These may be

installed as separate units, but more often they are used in the form of

a combined unit. The combination of filter, regular, and lubricator is

often labeled as FRL unit or service unit.

Mufflers or Silencers: The function of muffler (also known as

pneumatic exhaust silencer) is to control the noise caused by a rapidly

exhausting air-stream flowing into the atmosphere. Noise created by

air exhausting from an air system not only cause nervous tension and

dissatisfaction among the operators, but also results in mental fatigue,

lack of concentration, and inefficiency. This exhaust noises can be

greatly reduced by installing a muffler at each pneumatic exhaust

port.



Quick Exhaust Valve:

A quick exhaust valve is a typical shuttle valve. The quick (or

fast) exhaust valve is used to exhaust the cylinder air to the

atmosphere quickly. It is basically used with spring return single-

acting pneumatic cylinders to increase the piston speed of cylinders.

The higher speed of piston in a cylinder is possible by reducing the

resistance to flow of the exhausting air during motion of the cylinder.

The resistance can be reduced by expelling the exhausting air to the

atmosphere quickly by using a special valve. That's why this valve is

known as a quick exhaust valve.

The construction and operation of a typical quick exhaust valve is

shown in Figure. It consists of a movable disc and three ports-an inlet port (P), and exhaust port (R), and a cylinder port (A). Its working

principle is very much similar to that of a shuttle valve. When the air

flowing to the cylinder from the DC valve is applied at port P, then

the flexible ring covers the exhaust port R, whereby the compressed

air passes from port P to the cylinder through port A. But the return

air from the cylinder pushes the flexible ring to cover the inlet port P,

Where by the exhaust air immediately expelled to the atmosphere.

Thus the resistance to piston movement is reduced considerably and

the speed of the piston in the cylinder is accelerated proportionately.Time Delay Valve:

The time delay valve consists of an in built air reservoir, an in

built non return flow control valve and a pilot controlled spring return

3/2 DC valve. This valve is used in the pneumatic system to initiate a

delayed signal. When the compressed air is supplied to port ‘P’ of the

valve, it prevents the air from flowing to port ‘A’ from ‘P’, as this is

blocked by the spring actuated spool.



Air is accumulated in the in built reservoir of the valve from

the pilot control port ‘Z’, the control passage of the same being

controlled by the needle of the in built throttle valve. Pressure starts

building up here. When the pressure needed to push the spool in builtup in the reservoir, the pilot spool of the 3/2 direction control valve

shifts, thus opening port ‘P’ of the main valve to A and closing ‘R’.

The time required to build up pressure in the reservoir is the amount

of delay time offered by the time delay valve and with further

increase of pressure, the in built check valve opens. The air from the

reservoir gets exhausted and the spool returns to its original position.

PNEUMATIC CYLINDERS

1. Single

acting

2. Double

acting

3. Cushion end 4. Tandem

cylinder

5. Dual

cylinder

6. Double

Rod

7. Telescoping 8. Turn

cylinder



UNIT V

DESIGN OF PNEUMATIC CIRCUITS

SERVO SYSTEMS AND PROPORTIONAL CONTROLS:

In the fluid power systems, especially in hydraulic systems,

fluid power designers and engineers are continuously trying to

provide accurate positioning of the load with improved control on

speed, force, and other parameters. In this regard, there has been

tremendous development in the field of control technology. Among

them, servo and proportional controls are being increasingly used in

various fluid power applications. These servo and proportional

controls can be found in various fields like production, material

handling, aviation, shipping, robotics, etc. Also in the automatic

control of many machine functions, there are requirements such asthe exceptionally high degree of accuracy in acceleration, velocity,

and positioning. These requirements can be met by servo control

systems.

Servo System:

A system in which a small input force is capable to control a

larger output force is called as a servo system. A servo control

systems is one in which a comparatively large amount of power is

controlled by small impulses or command signals and any errors are

corrected by feedback signals. As stated in the above definition, aservo system should provide both signal amplification and automatic

correction of any deviation that may take place between the output

quantity and quantity set by the command signal. Basically most of

the servo systems are closed-loop systems.

Mechanical Hydraulic Servo Valve:

In this design, a small input force shifts the spool of the servo

valve to the right by the specified amount. The oil flows through the

port P, retracting the hydraulic cylinder to the right. The feedback

link is connected to the rod of the piston. So the action of the

feedback link shifts the sliding sleeve to the right until it blocks the

hydraulic cylinder. Thus, a given input motion produces a specific

and a controlled amount of output motion. This type of valve is used

in hydraulic power steering system of automobiles and

other transportation type vehicles.



Electro Hydraulic Servo Valve:The electro hydraulic servo valve operates due to an electrical

signal given to its torque motor which positions the spool of the

directional control valve. A torque motor is a low displacement

electric motor. Movement of the armature is proportional to the direct

current applied to the windings of the motor. The signal to the torque

motor comes from an electrical device such as a potentiometer. The

signal from the potentiometer is electrically amplified to drive the

torque motor. The torque motor actuates the servo valve. The

hydraulic flow output of the servo valve powers an actuator. The

actuator in turn drives the load. The velocity or position of the load is

fed back in electrical form to the input of the servo valve by a

feedback device.



The feedback signal IS compared to the command input signal

and the difference between the two signals is sent to the torque motor

as an error signal. This produces the correction in the velocity or

position of the torque motor until it matches with the desired value.

At this point, the error signal to the torque motor becomes zero.

Electro hydraulic systems use low power electrical signals (I W) for

controlling the movements of large power hydraulic pistons (7640 W

or more). The typical applications are aircraft controls and numerical

control machines.

Single Stage Servo Valve:

In a single stage servo valve, the armature of the motor is

connected directly to one end of the valve spool. With equal currents

flowing through the two coils, the armature remains centered.Increasing the current in one coil and reducing it in another causes

the armature to move proportional to the change in current. Thus, the

spool also shifts by a distance proportional to change in current.

Two Stage Servo Valve:

The most commonly used servo valves are the two stage units.

It can handle large flow at high pressure with a high sensitivity tocontrol changes. This valve has sliding spools in both pilot and main

stages. A command signal from the servo amplifier is directed to the

two coils of the permanent magnet torque motor. A differential

current is established in the coil which deflects the armature by an

amount proportional to the command. The deflection of the armature

is mechanically transmitted to the pilot spool by means of a stiff

connecting wire. Thus, mechanical displacement of the pilot valve

spool is directly proportional to the command received by the torque

motor. The direction of movement is determined by the torque motor

coil having the larger current. When the pilot spool moves to the left,



a flow of oil is metered to the end N of the main spool. The control

pressure acting continuously on area (M) is acting now at both the

ends. Since the effective area of the left hand end of the spool is twice

that of the right hand end (due to the presence of a rod on the right

hand end of the spool), the main spool shifts towards the right. The

supply pressure is to be directed to port (A) to actuate the hydraulic

cylinder in a direction, proportional to the electrical signal.

A signal from the servo amplifier, resulting in a pilot spool

movement to the right, Will permit the control pressure acting on area

(M) to move only the main spool to the left because area N is now

connected to the drain. The main supply pressure will be directed into

port (B) and will move the hydraulic cylinder in the opposite

direction. Again the amount of movement is proportional to the

electrical command. The valve feedback linkage mechanically links

the main spool and the pilot spool sleeve. So any movement of the

main spool is feedback through the linkage to act on the pilot spoolsleeve. The sleeve follows the pilot spool to the new position until the

control pressure is closed off. Thus, the servo valve provides an

extremely accurate flow modulation for fast and precise control of

position, velocity and acceleration of an actuator.

Flapper Type Servo Valve:

In this type of valve, the sliding spool is actuated by a pressure

difference at the two ends. Normally, control pressure is equal at both

ends of the spool. A controlled amount of fluid continuously flows

through the orifice passages to the nozzles against a 'flapper' valve



which is attached to the armature. When a signal to the

electromagnetic coil moves the armature, the flapper moves towards

one of the nozzles. The balance of flow is changed throughout the

orifices and it causes the pressure to increase at one end of the spool

and decrease at the other, The spool then moves until the pressure

difference is balanced by the tension of the spool springs, Internal

feedback is provided by means of a mechanical linkage from the

spool to the flapper.

The spool moves in proportion to the movement of the flapper valve,

which in turn is proportional to the input current. Therefore, the

volume of fluid passing through the valve IS also proportional to the

input current. Electro hydraulic servos of this type require fluids

which are continuously filtered to a high standard of cleanliness,

usually 10 microns absolute.

Jet Pipe Servo Valve:A jet pipe servo valve is similar to flapper type valve. It

consists of a jet pipe, a tube with an orifice end. Fluid from the jet is

directed to two pipes connected to the ends of the spool. When the jet

pipe is in center position, the flow in both the pipes is equal. So the

pressures at both ends of the spool are equal and the spool is held in

center by springs. The torque motor can deflect the jet pipe in either

direction, an amount proportional to the signal It receives, When the

jet is deflected, the flow in the pipes are unequal. This causes a

pressure differential at the spool and the spool shifts against one of its



centering springs. The movement of the spool ceases, when it exactly balances the electrical signal. Now, the Jet pipe is again centrally

located because the spool is linked mechanically to the jet pipe.

Proportional Valves:

Conventional solenoid operated direction control valve has

digital control systems, i.e, either fully open or, when the solenoid is

energized, fully closed. This 'bang-bang' operation gives rise to flow

and pressure surges in the hydraulic circuit with all the resultant

problems. If the valve can be gradually closed or opened as amanually operated gate valve, it results in a gradual transition

between fully opened and fully closed conditions. For this,

proportional valves are used. The proportional valve has a DC

solenoid. The force exerted by the armature of the solenoid is

proportional to the current flowing through it and independent of the

armature movement over the working range of the solenoid.

Control of Proportional Valves

Force control: The electrical control of the proportional valve

normally uses a variable current rather than a variable voltage. If a

voltage control system is adopted any variation in coil resistance

caused by a temperature change will result in a change of current,

although the voltage remains fixed. This causes a change in force.

This problem is eliminated by using a current control system. The

spool in the proportional valve is acted upon by a spring at one end

and a proportional solenoid at the other end. Thus, it is possible to

control the force on the spool electrically and the orifice size can be

varied in accordance to the control current. The flow from the valve

is proportional to the current flowing through the solenoid.



Using notched spool or overlap spool in the proportional valve gives

better control of the flow rate as the orifice is progressively opened.

Due to difficulties in manufacturing zero lap spool, i.e. one in whichthe land on the spool is exactly the same length as the port in the

valve body, overlapped spools are used in proportional spool valves.

This means that the spool has to move a distance equal to the overlap

before any flow occurs through the valve. This gives rise to a 'dead

zone' in the valve characteristic.

Propositional Pressure Relief Valves:

In conventional pressure relief valves, a compression spring is

used to control the pressure at which the valve operates. This spring

is replaced by a DC solenoid in the case of a proportional valve. In

this, the proportional solenoid exerts a force on the poppet keeping

the valve closed until the hydraulic pressure at port P overcomes this

force and opens the valve. The force exerted by the proportional

solenoid has an upper limit owing to the physical size limitations. So

to increase the operating pressure of the valve the size of the orifice

in the valve is decreased and vice versa.

The operating pressures of the valve will depend on the

current in the solenoid and the quantity of fluid flowing through the

valve.



Proportional Pressure Reducing Valve:

This operates in a manner similar to a conventional pressure

regulating valve, the control spring being replaced by a proportional

solenoid. However, when the solenoid is not energized, the

proportional valve is closed unlike the conventional pressure

reducing valve which is normally open. When the solenoid is

energized, it will move the spool to the right. The control orifice A

will open and allow fluid to flow to the Output port X. As the

aperture of orifice A increases, the aperture of orifice B will decrease.

The pressure at the control output X is dependent upon the

openings of control orifices A and B. Let the supply pressure be P 1The pressure drops across the control orifices A and B are P A and P Brespectively and the output pressure is Px. If the control orifice B is

fully closed, then Px will be equal to the supply pressure P 1 The

output pressure is applied to the right end of the spool and if this is

greater than the equivalent pressure exerted by the proportional

solenoid, the spool will improve to the left. This increases the

opening of orifice B and reduces the opening of orifice A, therebyreducing the output pressure. For equilibrium P x a = F. The output

pressure is proportional to the current flowing in the proportional

solenoid. There will always be a flow to the tank from this type of

valve if the output pressure Px is less than the supply pressure P 1 It, is

essential that there is no back pressure in the tank line if the valve is

to function correctly.



Proportional Direction Control Valve:

The pressure output from proportional pressure reducing

valves are directed to move the spool of the main valve against the

control spring. Energizing solenoid 1 causes pressure to be applied to

pilot port X, moving the spool to the right against the control spring.

The movement of the spool will be proportional to the pressure

applied to the pilot port X and hence to the current in solenoid 1. As

the main spool lands are notched, a movement to the right will

progressively open the flow paths from P to B and A to T. De-

energizing solenoid 1 will depressurize spring chamber C and the

control spring will centralize the spool. Similarly solenoid 2 controls

the flow paths P to A and B to T.

Comparison of Proportional and Servo Valve:

Proportional Valve Servo Valve

Proportional valves use relatively

high power electronics to drive

proportional solenoids

Servo valves use low power

current driven torque motor

It typically respond to a control

signal in the range of ±10V

It typically require a control

signal in the range of ±100mADo not have any position feed

back within the valve

It use an internal mechanical feed

back system

Better dirt tolerance economical

and satisfactory alternative for

servo valves

Used where fast response and

accurate controls are required



FLUIDIC ELEMENT:

In the early sixties, a new type of pneumatic control elements

called Fluidic Element or Fluid Logic Element was developed. The

biggest advantage of these elements over all the other forms of

control elements is that they have a minimum number of mechanical

moving parts. Because of this, these elements are also known as ‘non-

moving logic controllers.

The various advantages of fluidic elements

1. No wear and tear of elements

2. No actuating force needed

3. Very little space needed for mounting

4. Quite insensitive to temperature, vibration, shock, electric

noise and radiation

The entire development of fluidic technology is based on the wallattachment phenomenon, which was first discovered by the

Rumanian engineer Henri Coanda in 1932. This phenomenon is

frequently called ‘Coanda effect’.

A free jet of air is emitted into a confined region or orifice at avelocity high enough to produce turbulent flow. The free jet of air

will continue in a given direction, pulling in with it then available air

from its surroundings as it leaves the orifice. If there is greater

availability of this entraining air from one side, a small vortex area is

created near the nozzle exit. This low pressure area then tend to

attract the free jet, distorting it and pulling it towards the wall,

because the atmospheric pressure on the other side forces the jet to

cling to the surface. This free jet attachment continues until a small

air supply is fed to the low pressure area, thus relieving the attraction



of the jet to the wall. When this signal is injected, the free jet then

detaches itself from the wall and resumes its normal uninterrupted

flow path.

The various fluidic elements are briefly explained here.

Bi-stable Flip-flop:

In this air flow is going from the input Ps down through the

O1 channel or the O2 channel depending upon the signal from either

C1 or C2.

A truth table tells how a particular device behaves. Number 0

means OFF and number 1 means ON for all the devices. Therefore

when control signal C1 is ON and control signal C2 is OFF the outputis at O1. If C1 is then turned OFF the device stays in its first stable and

the output is at O1. If C2 is ON and control signal C1 is OFF the

output is at O2. Removing signal C2 leaves the device in its second

stable state with the output at O2. Thus the flip-flop has two stable

states when control signals are OFF. Initially the basic flip-flop has

its power supply pressure Ps turned ON and neither control signal has

been turned ON. Otherwise both the control signals C1 and C2 should

be ON simultaneously. Under these conditions, the output flow

would split because no low pressure bubble can be sustained on either



wall. Thus with C1 and C2 ON the flip-flop does not produce any

useful signal.

Preference Flip-Flop:

In some applications, specific output is necessary when the

power supply is turned ON and all the control signals are OFF. For

these applications, a flip flop with a start up preference is used. When

control signals are OFF, the output is ON at O1 and OFF at O2.

OR / NOR Gate Device:

This element has outputs corresponding with two conditionsOR - Pressure at one or any combinations of the control port

NOR - Pressure at none of the control ports

The OR/NOR gate is designed in such a manner that flow will always

be in the NOR signal when no control signal is present of course Ps is

present. The O2 port represents the NOR output and the O1 port



represents the OR output. Either C1 or C3 signal must be present to

get an output at the C1 port. But neither C1 nor C3 should be present to

get an output at the O2 port.

AND / NAND Gate Device:

Another element is the AND/NAND gate, which is similar to

the NOR gate, except that the NOR gate is used to determine when

none of the control signal is present, whereas the AND gate is used to

determine when all the control signals are present.

Both the C1 and C3 control signals are present to get an output at the

O1 port. The absence of either or both signals will result in the stable

output at the O2 port.

MICRO ELECTRONIC CONTROL OF FLUID POWER:

In a typical industrial environment there are a number of

applications where a partial sequence of operations repeats itself.

Automation of these operations can be effectively carried out using

low-cost automation techniques which employs fluid power system.

Until recently, the sequence control of fluid power has been

performed by electro mechanical means. They employ relays,

counters, switches etc. This hardwired systems tend to be expensive

and pose serious limitation if found necessary to change the machinesequence. Moreover majority of applications demand precise control

of speed, force and Position. This is achieved using electrically

modulated valves (proportional valves and servos) which adjust the

flow of pressure in response to a continuously varying input voltage

or input current. Here a conventional analog closed loop electronic

control is used. But where there is a complex interaction of two

variables or where a non-linear operation is to be performed, this

complicates the electronics.



There is now an increase in the use of Programmable Logic

Controller (PLC) and Microprocessor (µP), to overcome the,

above mentioned limitations. A programmable logic controller uses a

program to link together a number of input and output devices to

produce a desired sequence of operations. Moreover the program can

easily be changed for the new jobs. A microprocessor as a closed

loop controller appears to have many attractions as complex linear or

non-linear functions can easily be introduced into the control loop.

Microprocessor is also involved in sequential operations. It is felt that

the combination of microelectronics to supply the 'brain' and fluid

power to supply the 'brawn' is the most effective way of exploiting

the benefits of the fluid power system. There is enormous potential

for controlling fluid power elements in sophisticated applications

such as CNC machines and Robotics as well as in applications likevehicles, heavy equipments, aerospace etc. This gives some

indication of the range of application for the combination of

microelectronics with fluid power.

PLC CONSTRUCTION:

A PLC can be defined as a digital electronic device that uses a

programmable memory to store instructions such as logic,

sequencing, timing, counting and arithmetic to control machines or

processes. It is a software based instrument and hence it can be programmed using an easy-to-learn programming language.

The three basic elements of PLC are

1. Central processing unit (CPU) with an associated memory

2. Input modules

3. Output module

Central Processing Unit:

The CPU receives input signals from the various input

modules and based on the programs stored in the memory, decides on



the appropriate signals, which it transmits to the respective output

modules.

Memory:

In choosing a PLC, the available memory capacity plays an

important role. The different memory types used in PLC of both

volatile and non-volatile type are given below.

1. RAM (Random access memory) - Volatile to - program

development

2. Read/Write memory-Non-volatile

3. ROM (Read only Memory ) – Non-volatile to store execution

program

4. PROM (Programmable read only memory)

5. EPROM (Erasable PROM )

6. EEPROM (Electrically erasable PROM)I / O Module:

The input and output modules provide the necessary

interfacing between the PLC and the system. The input modules

translate the incoming signals to 5V DC, on which the PLC can work.

The output modules on the other hand translate the low current 5V

DC signals coming from the PLC to high voltage current signals,

required to actuate the various devices of the fluid power circuit. The

I/O modules are available for different voltages such as 115V AC,

230V AC, 24V AC, 24V DC and 5V DC etc. They are usually packaged in units of 4.8 or even more.

INSTALLATION, MAINTENANCE AND

TROUBLESHOOTING OF FLUID POWER SYSTEM:

Installation of Hydraulic Systems:

The location where the machine is to be installed must be

clean and all the construction work, painting etc, should be finished

before installation commences. The order in which the installation is

undertaken depends upon the machine and the site. Each section of

the machine must be sealed to prevent ingression of dirt before

starting on another section.

Installation of Pumps:

• The installation of pumps with respect to its reservoir is very

important because it affects the performance of the pump. The

higher the pump is mounted above the fluid level, the greater

is the resistance faced by the pump while drawing fluid into it.

So generally the height of the pump inlet above the reservoir

should be as minimum as possible.



• The pump should be mounted on a strong foundation so that

the alignment is maintained. They should be located such that

they can be easily removed for repairs or replacement

•

Entry of air into the pump should be prevented because thiswill reduce the life of the pump and may lead to airlocks in

the system. Hence the pipe connections should be airtight.

• For the ranges of operating pressures, temperatures and

speeds manufacture’s recommendations should be always

followed. The maximum temperature of the fluid in the

system should normally be not more than 60°C

• The lubrication of the internal moving parts is very important.

Since the lubrication of the bearings and other internal parts is

done by the fluid present in the pump, initial start up of the

pump should not be attempted unless the pump case has been

filled with the fluid.

Installation of Cylinders:

• The hydraulic cylinder should be installed using proper

mountings. Improper mounting is likely to damage the

packing very soon.

• During installation particular attention must be paid to

cleanliness

• The cylinders must be installed free of tension and in

particular free of radial forces, to prevent functional faults and

premature wear

• Eccentric loading should be minimized because it is not only

harmful to the piston rod and rod bearing but can also cause

problems with cushioning, piston rod, piston and cylinder

tube.

• Piston rod seal protection is important wherever process heat

may be encountered. High temperature seals are commercially

available. Use of heat shields will protect the fluid in the

cylinder as well as the seals.Common faults in Hydraulic Systems:

• Reduced speed of travel of machine tool elements

• Slow response to control

• Excessive loss of system pressure

• Excessive leakage in the system

• Rise in the oil temperature

• Non uniform movement of tables, carriage especially at low

feed rates

• Increased noise in the system



• No supply or less supply from pump

• Cavitations of seal failure

• High rate of seal failure

•Poor oil life

• High degree of contamination level of system medium

Various faults, Causes and Remedies in Fluid Power System:

Trouble Causes Remedies

Pump

delivering

insufficient or

no oil

1. Wrong direction of

shaft

2. Pump shaft turning too

slowly to prime itself

3. Clogged strainer or suction pipe line

4. Strainer capacity

insufficient

1. Must be reversed

immediately to prevent

seizure and breakage of

parts due to lack of oil

2. Check minimum speedrecommendation and

momentarily increase rpm,

to rectify

3. Clean strainer or

suction pipe line.

Pump

developing

unstable or

zero pressure

1. Pump not delivering oil

for any of the above

reasons

2. Relief valve setting notenough

3. Clogged orifice of the

relief valve

1. Apply the above

remedies

2. Correct valve setting by

using pressure gauge3. Overhaul and clean

relief valve

Pump making

noise

1.Misalignment of pump

and prime mover

2. Strainer capacity

insufficient

3. Air leak at pump's

suction pipe joints or fromshaft packing of the pump

4. Air remains in pump

casing

1.Check and rectify

2. Replace with a strainer

whose capacity is more

than twice the maximum

flow rate

3. Pour oil on suspected joints while listening for

change in sound. If sound

stops, tighten the joint

4. Eliminate air through

the air breather



Faulty or

incomplete

shifting of

DCV

1. Worn out control

linkage, shift pin, etc.

2. Insufficient pilot

pressure

3. Burned out solenoid

4. Worn spring centering

1. Check and repair

2. Check and rectify

3. Check and replace

4. Check and replace

Motors

Turning in

wrong

Direction

Incorrect piping between

control valve and fluid

motor

Check circuits to

determine correct piping

Absence of

proper speedand torque

1. System overload, relief

valve adjustment not sethigh enough

2. Relief valve sticking

open

3. Free recirculation of oil

to reservoir

4. Driven mechanism

binding because of mis-

alignment

1. Check required system

pressure and reset relief valve

2. Inspect and overhaul

relief valve set correctly

3. Identify the exact point

of fault and rectify

4. Remove fluid motor

and check the torque

required for drive shaft

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