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I ns t i t ut eofManagement & Techni calSt udi es


IMTS (ISO 9001-2008 Internationally Certified) FLUID MECHANICS & PNEUMATICS






Introduction , Objectives,Basic components of Hydraulic system,


Law,Properties of Hydraulic Fluids,Hydraulic Pumps,Pump Types,Piston Pumps,Gear Pumps,Vane Pumps,Summary



HYDRAULIC ACTUATORS Introduction ,Objectives,Linear Actuators,Hydraulic Cylinder Types,Single Acting Cylinder,Double





Actuators,Gear Motor,Vane Motor,Piston Motor,Power and efficiency,Summary



HYDRAULIC SYSTEM CONTROL COMPONENTS Introduction ,Objectives,Direction Control Valves,One way valves,Shuttle valves,Two way valves,Three way valves,Four way valves,Valve Actuation Types,Pressure Control Valves,Pilot operated pressure control valve,Pressure relief valve,Pressure reducing


valve,Sequencing Valve,Flow Control Valves,Needle valve,Pressure compensated Flow control valve,Cushioned Cylinders,Flow Dividers,Hydraulic graphic symbols,Summary












exchangers,Filters,Instrumentation and Measurement,,Pressure gauges,Temperature gauges,Flow





circuits,Counter balance circuit,Sequence circuit,Speed control circuits,Intensifier circuits,Summary



PNEUMATICS Introduction






pneumatics,Compressors,Piston compressor,Rotary vane compressor,Rotary screw compressor,Vacuum




actuators,FRL unit,Electronic control of fluid power,Solenoid and servo valves,Summary











Basic components of Hydraulic system


Pascal’s Law


Properties of Hydraulic Fluids


Hydraulic Pumps


Pump Types


Piston Pumps


Gear Pumps


Vane Pumps







1:0 INTRODUCTION Fluid power is defined as the generation, control, and transmission of power by using compressed fluids (either liquids or gases). This power is used to provide force and motion to mechanisms which maybe in the form of pushing, pulling, rotating, regulating, or driving. Fluid power includes hydraulics, which involves liquids, and pneumatics, which involves gases. In Hydraulic system, enclosed water (or) oil, can be used to convey energy from one location to another. In Greek, hydra means water. In Pneumatic system, enclosed gas (normally compressed air) is used to transfer energy from one location to another). In Greek, Pneumatic means wind.

Hydraulic Principles The governing principles in a hydraulic system are: 

All liquids are incompressible and can be used for power transmission.

Any load to be lifted offers resistance to flow of liquid. This resistance to flow is called pressure.

If the capacity of the pump is more, then it pumps out more liquid then the speed of the hydraulic actuator will be more.

If the force developed in the hydraulic cylinder is more than the external load, then the actuator lifts the external load. If the force developed in the hydraulic cylinder is less than the external load, then the actuator will not lift the external load. The load carrying capacity of the hydraulic system independent of quantity of Oil flowing.

In the operation of a hydraulic system, the liquid chooses the path of least resistance. If one path is connected to the hydraulic actuator to lift the load and the other path is connected to the reservoir then the liquid will choose the path of least resistance (i.e. reservoir path) and flows back into the reservoir, without choosing the path that offers higher resistance i.e. lifting the load.

1:1 OBJECTIVES After studying this lesson, you should be able to: 

Understand the concept of fluid power transmission, advantages and applications

Identify examples of common industrial applications of hydraulics

Identify the components of typical hydraulic systems

Understand the different types of pumps used and their working principles.





1. A reservoir tank – For storing oil. 2. A pump – To force the oil through the system and it is known as heart of the system. 3. An electric motor – Prime mover which drives the pump. 4. Valves to control oil direction, pressure, and flow rate. 5. An actuator – To move or lift the load. 6. Piping – To connect all the system components for transfer of oil.

Control valves Actuator


Electric motor


Coupling Oil Reservoir

Fig 1.1 Basic components of a hydraulic system Incase of pneumatic system instead of pump we have compressor and no reservoir was used there in the circuit since the air after doing work in the actuator is released back to atmosphere. Advantages of Fluid Power 

Simple in construction

Motion transmission without the slack

Less wear and tear of system components

Parts can be conveniently located at different places

Force transmission over considerable distances with minor loss.

Forces conveyed up and down or around corners without complicated mechanisms.

Very large forces can be obtained from smaller input force and can be controlled accurately

Systems provide smooth, flexible, uniform action without vibration, and is unaffected by variation of load.

System is protected against breakdown or strain due to over load because of automatic pressure release.

Widely variable motions in both rotary and straight-line transmission of power.

Automatic control of system is possible.

Economical operation.

Disadvantages 

Hydraulic fluids are messy.




All fluid power systems are susceptible to damage by dirt or contamination.

Piping failure due to either overpressure or mechanical stress is always possible.

The fire or explosion hazard due to leaking hydraulic oils.

Applications of Fluid Power 

Vehicle drives in Defense, Agricultural and Civil applications

Earth moving machineries


Aerospace Applications-Landing Wheels etc

Hydraulic Hoists

Automation actuators

Machine tool drives

Flight Simulators- For training pilots

Motion Simulators- For vibration seismic testing

1:3 Pascal’s law Pascal’s law states that the pressure applied anywhere to a confined liquid is transmitted equally in all directions. Consider a closed container filled with oil having a smaller piston on one end and larger on the other side as shown in the fig1.2. When a small force F is applied to the liquid by a smaller piston, it transmits this force equally to the other end were larger area piston lifts the larger load W. Let 2

Area of smaller piston = a m and that of larger piston as A m


Pressure on smaller piston side = Pressure on larger piston side F/a = W/A

(Pressure = Force/Area)

Now Load lifted W = F/a x A

Applied force F

Load lifted W






Liquid is used in hydraulic systems to transfer energy and produce required force at the hydraulic actuators. In olden days, water was used. But water has many disadvantages like high freezing point of 0°C, its corrosive properties and poor lubrication.

The important functions of hydraulic fluids are 

Power transmission



Heat dissipation

Some of the fluid characteristics are discussed below: Density or Mass Density ( ρ) 3

It is defined as the mass of a substance per unit volume. SI Unit is kg/m . ρ = Mass (m)/Volume(V) Specific Weight or Weight Density ( w) 3

It is defined as the weight of a substance per unit volume. SI Unit is N/m . w = Weight / Volume = (m x g)/V = m/V x g

(Weight = Mass(m) x g)

Therefore, w = ρ g Specific Gravity (S) It is the ratio of specific weight (density) of liquid to the Specific Weight (density) of water at 40º C. S = Density of liquid / Density of water Specific Weight of water = 9810 N/m


Specific Volume It defined as the volume of fluid per unit weight. It is the reciprocal of density. 3

SI unit is m /kg.

Viscosity 

It is that property of a fluid by virtue of which it offers resistance to the movement of one layer of fluid over an adjacent layer i.e. resistance to flow of oil.

The absolute viscosity (μ) of a fluid measures its resistance to flow under an applied shear stress.

Units for viscosity are N-s/m in S.I. System and dyne sec/cm in C.G.S. System also known as



poise designated by P 

The kinematic viscosity (γ) is the ratio of the absolute viscosity to the density: γ = μ/ ρ




Viscosity of liquids in general, decreases with increasing temperature.

Bulk modulus (K) 

It is defined as the ration between change in pressure to volumetric strain. Volumetric strain is the change in volume divided by the original volume. Therefore, K = dp/ (-dV/V)

Negative sign for dV indicates the volume decreases as pressure increases. Typical values of Bulk Modulus are: 9




K = 2.05 x 10 N/m for water K = 1.62 x 10 N/m for oil. Importance of Hydraulic Fluid properties 

Liquid is used in hydraulic systems to transfer energy and produce required force at the hydraulic actuators. In olden days, water was used. But water has many disadvantages like high freezing point of 0°C, its corrosive properties and poor lubrication. So hydraulic liquids are designed specifically for hydraulic circuits.

Sealing and lubrication are most important properties of hydraulic fluids. Moving parts in values rely on fine machining of spools and body to form the seal in conjunction with the fluid.

In spite of fine machining, still some burrs, irregularities present on the surface. The hydraulic liquid is required to pass between the two surfaces, holding them apart to reduce friction and prevent metal to metal contact which causes premature wear.

The temperature of hydraulic fluid tends to rise with the work done - heat to be dissipated. The liquid should be able to transfer heat from where it is generated. Normally the heat is generated in valves, actuators, frictional losses in pipes. Ultimately, the hydraulic fluid should not be affected by the temperature changes.

The hydraulic fluid should not cause deterioration of components. For example, water is causing rust.

Oxidation leads to deterioration of components. The products of oxidation are acidic in nature which causes corrosion. Hence the hydraulic fluid should be chemically stable and it should not suffer from oxidation. The rapid rise of temperature strongly influences the rate of oxidation which should be controlled.

Petroleum based oil is the most commonly used hydraulic fluid. It is used for lubrication, reduce foaming and inhibit rust. With proper addition of additives, it can be used as hydraulic fluids. But it is highly inflammable. If the leakage is not properly prevented, use of petroleum oil will be a dangerous one.

For fire resistant fluid, an oil and water emulsion is most commonly used. The cost of such fluids is cheaper. But their lubricating properties are poor. This oil and water emulsion has a tendency




to form rust and to foam but these properties can be changed by suitable additions. Water-glycol mix is also another non-inflammable fluid. 

Viscosity is the most important property of the hydraulic fluid. Low viscosity fluid flows easily and wastes little energy and leakage is more. A viscous fluid flows very slowly. It seals well, but is sluggish and wastes lot of energy. So moderate viscosity is needed for the hydraulic fluids.


The reliability of a hydraulic system is strongly influenced by the condition of the fluid. Presence of dirt, products of oxidation, poor lubrication ability will cause rapid wear and failure which should be avoided.

1:5 HYDRAULIC PUMPS Hydraulic pumps are used to pump out the liquid from the reservoir to the hydraulic actuator through a set of valves. A pump converts mechanical energy into hydraulic energy. The mechanical energy is given to the pump by an electric motor. Due to mechanical action, the pump creates a partial vacuum at its inlet. This makes the atmospheric pressure to force the liquid through the inlet line and into the pump. The pump then pushes the liquid into the hydraulic system. Pumps do not pump pressure, instead they produce fluid flow. The resistance to the flow, produced by the hydraulic system determines the pressure. If a pump has its discharge line open to atmosphere, there will be low discharge pressure, because there is no resistance to flow. If the discharge line is blocked then we have theoretically infinite resistance to flow. Then the pressure will rise until some component breaks and the pressure is relieved.

1:5:1 PUMP TYPES The pumps are classified into (i) Positive displacement pumps and (ii) Hydrodynamic (or) Nonpositive displacement pumps.

Hydrodynamic (or) Non-positive displacement pumps : These pumps are used for transporting fluids from one location to another and generally used for low pressure, high-volume flow applications, since they are not capable of withstanding high pressures. The centrifugal pumps and axial flow pumps are the examples of non-positive displacement pumps. These pumps provide smooth flow. But the output flow rate is reduced when the resistance to flow is increased.

Positive displacement pumps




Positive displacement pumps have their internal working elements with a very close fit together so that there is very little leakage (or) slippage between them. This type of pumps ejects a fixed quantity of liquid into the hydraulic system for each revolution of the pump shaft. Advantages of this type is, 

High pressure capability

Small and compact size

High volumetric efficiency

Great flexibility of performance- i.e. pumps can operate over a wide range of pressure requirements and speed ranges.

Positive-displacement displacement.





further classified




or variable

pump delivers the same discharge amount of fluid on each

cycle. The variable-displacement pump is the discharge quantity per cycle can be varied with the help of an internal controlling element.

Positive displacement pumps are classified into,  Gear Pumps  Vane Pumps  Piston Pumps Gear Pumps are further classified as (i) External Gear Pump, (ii) Internal Gear Pump,(iii) Lobe Pump, (iv) Screw Pump and (v) Gerotor Pump

Vane Pumps are further classified as (i) Unbalanced Vane Pump, (ii) Balanced Vane Pump and (iii) Variable displacement Vane Pump

Piston Pumps are further classified as (i) Radial Piston Pump,(ii) Piston Pump with stationary cam and rotating block, (iii) Axial Pump with Swash Plate, and (iv) Bent axis Pump


A piston pump is similar to reciprocating engine. It can draw in liquid when retracts in cylinder and discharge it when it extends. There are two main types of piston pumps 1. Axial piston pumps (a) Swash plate type (b) Bent axis type. 2. Radial piston pump. In axial pump, the piston is moving parallel to the axis of the cylinder block. In radial pump, the piston is moving radially in the cylinder block.




Axial piston pump - swash plate type

Fig 1.3 Axial piston pump - swash plate type In this pump, number of pistons are arranged in a rotating cylinder barrel assembly. The pistons stroked by a fixed angled plate which is known as swash plate. Each piston has contact with the swash plate by rotating shoe plate linked to it. When the cylinder rotates, the pistons reciprocate because the piston shoes follow the angled surface of the swash plate. Pump capacity can be changed by altering the angle of the swash plate. If we increase the angle, the capacity will be increased. If the swash plate is vertical, then the capacity will be zero and even the flow can be reversed by changing the angle of swash plate. So the angle of tilt of the swash plate determines the piston stroke and hence determines the pump discharge. During one half of the rotation, the piston moves out of the cylinder barrel and generates an increasing volume. In the other half of the rotation the piston moves into the cylinder barrel and generates a decreasing volume. This reciprocating motion draws fluid in and pumps it out. The maximum angle of swash plate is about 25ยบ to 30ยบ from the vertical line.

Axial piston pump - bent axis type This type of pump does not have a tilting cam plate as the in-line pump does. Instead, the cylinder block axis is kept at an inclination ( maximum 30ยบ) to the drive shaft axis. The cylinder block is rotated with the drive shaft by a universal link at the intersection of the drive shaft and the cylinder block shaft. As the pistons are hinged at the bend by ball and socket joints while rotating about the axis they also reciprocate. This reciprocating motion draws fluid in and pumps it out. In order to vary the pump displacement, the cylinder block and valve plate are mounted in a yoke and the entire cylinder block assembly is swung in an arc around a pair of mounting pintles attached to the pump housing.




Fig 1.4 Axial piston pump- bent axis type Radial piston pump In a radial piston pump, the pistons are arranged like wheel spokes in a short cylindrical block which houses the reaction ring of hardened steel against which the piston heads press. A drive shaft, which is inside a circular housing, rotates a cylinder block.

This pump has a stationary pintle contains

the inlet and outlet ports acts as a valve. The cylinder block is attached to the drive shaft as it rotates centrifugal force slings the pistons, which follow a circular housing. Housing’s centerline is offset from a cylinder block’s centerline. The amount of eccentricity between the two determines a piston stroke and, therefore, a pump’s displacement. Controls can be applied to change housing’s location and thereby vary a pump’s delivery from zero to maximum.

Fig 1.5 Radial piston pump 1:5:3 GEAR PUMPS

Gear pumps are compact, relatively inexpensive, and have few moving parts. The gear pumps are used at pressures up to 150 bar and capacities of around 675 litres/mm. The volumetric efficiency




(ratio of actual discharge to the theoretical discharge) of gear pump is generally at 90% which is the lowest of the three types of pumps (Gear, vane and piston pumps). The performance of gear pump is limited by leakage and the ability of the pump to withstand the differential pressure between inlet and outlet ports.

External gear pump The simplest type of positive displacement pump is external gear pump. It consists of just two closely externally meshing gears which rotate. This pump creates flow by carrying liquid between the teeth of two meshing gears. Here one gear is connected to the drive shaft which is connected to the motor known as driver gear. Another gear is meshing with the driver gear. When the teeth come out of mesh, a vacuum is created so that the liquid is drawn into the suction side. Liquid is trapped in between the outer teeth and the pump housing and transferred from inlet side to outlet side where the gear teeth once gain meshing with each other and the liquid is discharged to the system.

Fig 1.6 External gear pump The displacement of the gear pump is determined by volume of fluid between each pair of teeth, number of teeth, and speed of rotation. The gears in the gear pump can be of spur gear, helical gear and herringbone gear. The spur gears create noise at high speeds. The helical gears reduce noise and provide smooth operation but these gear pumps are limited to low pressure applications since they develops excessive side thrust. Internal gear pump




Fig 1.7 Internal gear pump

The internal gear pump consists of an internal gear, external gear, a crescent shaped seal and an external housing. The drive gear is attached directly to the drive shaft of the pump and is placed eccentrically in relation to the internal gear. The two gears mesh on one side of the pump, between the



and discharge






of the chamber, a crescent-

shaped plate is fitted and fills the gap between the two gears. When the power is given to any one of the gears, the motion of gears draws liquid from the storage tank and forces it around both sides of the crescent seal. The crescent acts as a seal between the suction side and delivery side ports. When the teeth mesh on the opposite side of the crescent seal, the liquid is forced to enter the outlet port of the pump. 1:5:4


The leakage in a gear pump through the small gaps between the teeth and also between teeth and pump housing is reduced in the vane pump. The vane pump reduces this leakage by using spring loaded vanes.

Unbalanced Vane pump The vane pump consists of a cylindrical rotor, vanes, cam ring and an external housing. The rotor contains radial slots and is connected to the drive shaft and rotates inside a cam ring. Vanes are sliding in the slots of the rotor and they are carried around by the rotor. These vanes are kept in continuous contact with the cam surface by centrifugal force. The rotor is offset within the pump housing i.e. There is an eccentricity between the centre of rotor and centre of cam rings.




Fig 1.8 Unbalanced vane pump If the eccentricity is zero, there is no flow. During one-half revolution of the rotor, the volume increases between the rotor and cam ring which causes a reduction of pressure i.e., vacuum. This vacuum causes the fluid to flow through the inlet port. When the rotor rotates, through second half revolution, the cam ring surface pushes the vanes back into their slots and the trapped volume is reduced thereby the trapped liquid is ejected through the outlet port.

Balanced vane pump A balanced vane pump has two lobes on the cam surface on opposite sides of the shaft. The cam surface, instead of being circular, is elliptical, so that each vane makes two strokes on each revolution of the shaft.

Fig 1.9 Balanced vane pump This pump has two intake parts and two outlet ports diametrically opposite to each other. Thus, the pressure ports are opposite to each other and a complete hydraulic balance is achieved. Since the cam ring is an elliptical one, it forms two separate pumping chambers on opposite sides of the rotor. So the side load produced by one chamber is exactly balanced by an equal side load from the other chamber. Thus, the bearing loads from internal pressure are zero and it permits the higher operating pressures. Balanced vane pumps have much improved service lives than unbalanced vane pumps.




Balanced vane pumps, due to its symmetrical construction, are difficult to design as a variable displacement pumps.

1:6 SUMMARY Today, hydraulic power is used to operate many different tools and mechanisms like hydraulic jack, hydraulic brakes, power steering etc. Automation of industrial activities are necessitated the development of fluid power devices almost for all its applications. It is better to understand the principles involved in fluid power and the working of components of the system as we are going to face them in our day to day activities.









Linear Actuators

2:2:1 Hydraulic Cylinder Types 2:2:2 Single Acting Cylinder 2:2:3 Double acting Cylinder 2:2:4 Telescopic Cylinder 2:2:5 Tandem Cylinder 2:3

Rotary Actuators

2:3:1 Gear Motor 2:3:2 Vane Motor 2:3:3 Piston Motor 2:4

Power and efficiency




2:0 INTRODUCTION Pumps convert mechanical input of motor into pressure energy of fluid where as. Hydraulic actuators convert the pressure energy of fluid into mechanical output to perform useful work. Fluid power is transmitted through either linear (or) rotary motion. Linear motion is obtained by using linear actuators called hydraulic cylinders. Rotary motion is obtained by using rotary actuators called hydraulic motors. 2:1 OBJECTIVES After studying this lesson, you should be able to: 

Understand the how actuators work in a hydraulic system

Identify common industrial applications of hydraulics actuators

Identify the components of typical hydraulic actuators

Understand the different types of actuators used and their working principles.

2:2 LINEAR ACTUATORS The linear actuators are converting fluid power to linear, or straight line, force and motion. It has a cylinder barrel and a ram or piston operating within it. Cylinder barrel is fixed and the ram or piston is attached to the mechanism to be operated, or the piston or ram may be fixed and the cylinder barrel connected to the mechanism to be operated. Actuating cylinders for pneumatic and hydraulic systems are similar in design and operation. For pneumatic cylinder applications light weight alloy materials are used for the construction of components. 2:2:1 HYDRAULIC CYLINDER TYPES There are two main types of hydraulic cylinders. 1. Single acting cylinder 2. Double acting cylinder




2:2:2 SINGLE ACTING HYDRAULIC CYLINDER: In this type the oil pressure acts on only one side of the piston to extend the cylinder, and during retraction force generated by gravity or a spring returns the piston rod to its original state. The following are the important components of single acting cylinder, 1. Cylinder body (or) barrel 2. Two end cover plates 3. Piston or plunger 4. Piston rod 5. U - cup seal 6. O - ring 7. Bush to guide the piston

Gravity return cylinder

2.1 Gravity return ram type single acting cylinder A hydraulic jack for vehicles represents a common application of a single-acting, gravityreturn cylinder. The cylinder housing has only one port for oil entry as well as exit. The applied force in only one direction since the fluid is directed into the cylinder on one side displaces the ram and forces it outward, lifting the object placed on it. Since there is no separate return line for retracting the ram by fluid power, when fluid pressure is released, the weight of the object forces the ram back into the cylinder. This in turn forces the fluid back to the reservoir.




Spring return Cylinder: The most common type of single-acting cylinder uses a return spring. In this type the return stroke is actuated by a spring. Simplest type of linear actuator is the single acting hydraulic cylinder. It consists of a piston inside a cylinder body called a barrel. The prison rod is attached to the one end of the piston. The piston rod extends out during extension and goes inside cylinder during retraction. Inlet port is provided at the other end of the cylinder. In this device, the pressurized liquid is admitted through only one side. So this cylinder will produce work in only one direction. In this version, pressurized fluid enters the cap end of the cylinder to extend the piston rod. When fluid is allowed to flow out of the cap end, the return spring exerts force on the piston rod to retract it. The end covers are fitted to the body by using four cover screws (or) tie rods.

2.2 Spring return piston type single acting cylinder 2:2:3 DOUBLE ACTING CYLINDERS In this type the pressurized liquid is admitted in both sides of the piston alternately. Work is performed during forward motion as well as backward motion of the piston. The important components of double acting cylinder are 1. Two end cover plates (or) two end caps with port connections (i) Base Cap

(ii) Bearing cap

2. Cylinder barrel 3. Piston 4. Piston rod




2.3 Double acting piston type cylinder The stroke of the piston and piston rod assembly in either direction is produced by fluid pressure. The two fluid ports, one near each end of the cylinder getting the oil supply alternatively as inlet and outlet ports, depending on the direction of flow from the directional control valve. Since there is a difference in the effective working areas on the two sides of the piston the cylinder carries the greater load during the extension stroke. To slow an action and prevent shock at the end of a piston stroke, some actuating cylinders are constructed with a cushioning device at either or both ends of a cylinder. This cushion is usually a metering device built into a cylinder to restrict the flow at an outlet port, thereby slowing down the motion of a piston.

2:2:4 TELESCOPIC CYLINDERS Telescoping cylinders contain five or more sets of tubing, or stages that nest inside one another. Each stage is equipped with seals and bearing surfaces to act as both a cylinder barrel and piston rod. Available for extensions exceeding 15 ft, most are used on mobile applications where available mounting space is limited. This special type of cylinders are used when longer working strokes are required compared with the standard cylinder length. A telescopic cylinder can be a single- or double acting type. In this cylinder, a series of rams are nested in a telescoping assembly. Except for the smallest ram, each ram is hollow and serves as cylinder housing for the next smaller ram. A ram assembly is contained in a main cylinder housing, which also provides the fluid ports. Although an assembly requires a small space with all of the rams retracted, a telescoping action of an assembly provides a relatively long stroke when the rams are extended. When the oil is admitted into port A the ram1 moves out first and gets fully extended, next the ram 2 moves out and then lastly the smallest ram 3 moves out to get the required longer




working stroke. At the same time the oil trapped at the backside of rams goes to the reservoir through port B.

2.4 Telescopic type cylinder During retraction the oil pressure will be supplied to the port B which causes the retraction of rams simultaneously to their initial position.


2.5 Tandem Cylinder

These are designed for applications where high force must be generated within a narrow radial space where substantial axial length is available. A tandem cylinder functions as two single rod-end cylinders connected in line with each piston inter-connected to a common rod as well as a second rod which extends through the rod-end cap. Each piston chamber is double acting to produce much higher forces without an increase in fluid pressure or bore diameter.




Cylinder mounting

The best way to support a cylinder is along its centre line. The mounting bolts are subjected to simple shear (or) simple tensile forces. Cylinder mounting styles, which generally provide:

1. Straight-line fixed mounts that absorb force on the centerline of the cylinder Ex. Cylinder tie rod mounts, Flange mounts and Center line lug mounts, Cap mounts 2. Straight-line fixed mounts that do not absorb force on the centerline of the cylinder Ex. Side mounted cylinders, Side lug mounts, flush mounting, side lugs, end lugs, and end angle mounts. 3. Pivot force transfer with pivot mounts which absorb force on the centerline of the cylinder and allow the cylinder to change alignment in one plane. Ex. Fixed clevis mounts or Trunnion pivot mounts


2:3 ROTARY ACTUATORS A fluid power motor is a device that converts fluid power energy to rotary motion and force. The function of a hydraulic motor is opposite that of a pump i.e. instead of pushing on the fluid as pumps do, the motors are pushed upon by the fluid. A motor, which is connected to load, is actuated by the flow of pressurized oil so that motion or torque, or both, are conveyed to the work.

However, the design and operation of fluid power motors are very similar to pumps. Motors have many uses in fluid power systems. In hydraulic power drives, pumps and motors are combined with suitable lines and valves to form hydraulic transmissions. Although most fluid power motors are capable of providing rotary motion in either direction. The principal ratings of a motor are torque, pressure, and displacement. Torque and pressure ratings indicate how much load a motor can handle. Displacement indicates how much flow is required for a specified drive speed and is expressed in cubic inches per revolutions, the same as pump IMTSINSTITUTE.COM



displacement. Displacement is the amount of oil that must be pumped into a motor to turn it one revolution. Most motors are fixed-displacement; however, variable-displacement piston motors are in use, mainly in hydrostatic drives. Hydraulic motors develop torque and produce continuous rotary motion. There are three basic types of hydraulic motors, 1. Gear motor 2. Vane motor 3. Piston motor


2.6 Gear motor In this hydraulic motor both gears are driven gears but only one is connected to the output shaft. As fluid under pressure enters chamber A, it takes the path of least resistance and flows around the inside surface of the housing, forcing the gears to rotate as indicated. The flow continues through the outlet port to the return. This rotary motion of the gears is transmitted through the attached shaft to the work unit. Hence, there is an imbalance of forces on the gears resulting in rotation. Leakage occurs in the gear motors at low speed. So these gear motors are used for medium speed and low torque applications.





2.7 Vane motor

The construction of vane motor is similar to vane pump. In vane motor, leakage is less than the gear motor at lower speeds. Side loading occurs on the shaft of a single vane motor like the vane pump. These forces can be balanced by using a dual design similar to the balanced vane pump. In a vane pump, vanes are held out by the rotational speed where as in vane motor the vanes are held out by fluid pressure since the rotational speed is low here. Flow from the pump enters the inlet, forces the rotor and vanes to rotate, and passes out through the outlet. Motor rotation causes the output shaft to rotate. Since no centrifugal force exists until the motor begins to rotate, something, usually springs, must be used to initially hold the vanes against the casing contour. However, springs usually are not necessary in vane-type pumps because a drive shaft initially supplies centrifugal force to ensure vane-to-casing contact. 2:3:3 PISTON MOTOR




2.8 Axial piston swash plate type motor Piston type motors can be in-line-axis or bent-axis types. Inline-Axis swash plate piston type motors are almost identical to the pumps. They are built-in, fixed- and variabledisplacement models in several sizes. Torque is developed by a pressure drop through a motor. Pressure exerts a force on the ends of the pistons, which is translated into shaft rotation. Shaft rotation of most models can be reversed anytime by reversing the flow direction. Oil from a pump is forced into the cylinder bores through a motor’s inlet port. Force on the pistons at this point pushes them against a swash plate. They can move only by sliding along a swash plate to a point further away from a cylinder’s barrel, which causes it to rotate. The barrel is then splined to a shaft so that it must turn.

2.8 Axial piston bent axis type motor Both of the axial-piston motors described in this section may be operated in either direction. The direction of rotation is controlled by the direction of fluid flow to the valve plate. The direction of flow may be instantly reversed without damage to the motor.


The two vane rotary actuators known as limited rotation cylinders consists of two vanes connected to drive shaft and supply ports in the casing for inlet and outlet. The angle of rotation is limited to 100º and torque carrying capacity was about 80 KNm. The pair of inlet and exhaust ports is connected together for both vanes together. The oil pressure applied to the inlet




ports causes the turning of output shaft in the clockwise direction and at the same time the oil from the outlet ports goes to the reservoir. For the counter clock wise turning the oil supply is reversed by using a directional control valve. This type of actuators is used for mechanical function involving restricted rotation.

2.9 Two vane rotary actuator


2:3:5 RACK-AND-PINION ROTARY ACTUATORS The rack-and-pinion-type actuators, also referred to as limited rotation cylinders, of the single or multiple, bidirectional piston are used for turning, positioning, steering, opening and closing, swinging, or any other mechanical function involving restricted rotation. The actuator consists of a body and two reciprocating pistons with an integral rack for rotating the shaft mounted in roller or journal bearings. The shaft and bearings are located in a central position and are enclosed with a bearing cap. The pistons, one on each side of the rack, are enclosed in cylinders machined or sleeved into the body. The body is enclosed with end caps and static seals to prevent external leakage of pressurized fluid




2.10 rack and pinion rotary actuator

2:4 POWER AND EFFICIENCY Fluid power (F.P) = Q x ΔP Watts Q is the flow rate m3/sec and ΔP is pressure difference between inlet and out let pressure in N/m2. The output power is the shaft power (S.P) = 2πNT = ωT Watts N is speed in rev/sec and T is torque in Nm and ω in rad/sec. The output power is reduced due to friction and leakage of fluid. This gives the overall efficiency defined as the ration between shaft power to fluid power, i.e. Overall efficiency = Output / Input = Shaft power / Fluid power Volumetric efficiency The high pressure liquid may leak into the low pressure side without doing any work resulting in reduced flow rate. Volumetric efficiency of motor = Ideal flow rate / Actual flow rate Speed and Torque The relationship between speed and flow rate is Q = VD x N m3/sec VD Nominal volumetric displacement of motor m3/rev and N is the speed of motor in rev/sec


FLUID MECHANICS & PNEUMATICS For 100% efficient motor


Shaft power = Fluid power 2πNT = Q x ΔP

Now the torque T =

(Q x ΔP) / 2πN


Specifications The hydraulic motors are specified by the following parameters, 

Torque output required

Speed range required

Cycle time of operation

Working environment

Displacement and

Operating pressure.

2:5 SUMMARY In any hydraulic circuit the output power obtained from the actuators as either linear motion or rotary motion. It is necessary to study the basic theory behind various types of actuators and also their working, and then only it is possible for us to select the better type to suit the required applications. Only after the selection of actuator other system components are decided to complete the hydraulic circuit design.









Direction Control Valves


One way valves


Shuttle valves


Two way valves


Three way valves


Four way valves


Valve Actuation Types


Pressure Control Valves


Pilot operated pressure control valve


Pressure relief valve


Pressure reducing valve


Sequencing Valve


Flow Control Valves


Needle valve


Pressure compensated Flow control valve


Cushioned Cylinders


Flow Dividers


Hydraulic graphic symbols






3:0 INTRODUCTION Fluid power is controlled primarily through the use of control devices called valves. Hydraulic systems require control valves to direct and regulate the flow of fluid from pump to hydraulic cylinders or hydraulic motors. The selection of these control devices depends on the type, size, actuating technique and remote control capability.

There are three basic types of control valves. 

Direction control valves – To determine the path of the fluid through which it should travel within a given circuit. The control of fluid path is carried out by check valves, shuttle valves and two way, three way and four way direction control valves

Pressure control valves - to protect the hydraulic system against over pressure. This over pressure may occur due to a gradual buildup as fluid demand decreases (or) due to sudden surge as valve close. The buildup of pressure is controlled by pressure relief, pressure reducing, sequence, unloading and counter balance valves.

Flow control valves - To control the fluid flow in hydraulic circuits. The control of actuator speeds depends on the flow rates. So to control the actuator speed, the flow rate should be controlled by using flow control valves.

There are some practical differences between the hydraulic and pneumatic direction control valves, even though the principle of operation is the same. But the pressure and the flow control valves for both hydraulic and pneumatic systems are same. 3:1 OBJECTIVES After studying this lesson, you should be able to: 

Understand the importance of direction control valves

Identify common types of pressure control valves

Identify the components of and working of flow control valves

Understand the types of flow dividers used and their working principles.

3:2 DIRECTION CONTROL VALVES Direction control valves are used to control the direction of the flow to the actuator from the pump. Directional control valves may be classified by the type of control, the number of ports in the valve housing, and the specific function of the valve. The most common method is by the type of valve seat element used in the construction of the valve.






valve seat

elements are the ball, cone or sleeve, poppet, rotary spool, and sliding spool. The following are the important types of direction control valves in the hydraulic system.



Check Valve

Shuttle valve

Two way valve

Three way valve

Four way valve


3:2:1 Check valve: The simplest type of one direction flow valve is check valve. Check valve is a one way valve because it permits flow in only one direction and prevents any flow in the opposite direction. The most common type of check valve, installed in fluid-power systems, uses either a ball or cone (poppet) for the seating element.

Fig 3.1 Check valves The spring holds the poppet in the closed position. When the fluid attains the required pressure, it overcomes the spring force, the spring is compressed the poppet is moving right and free flow occurs from left to right. If the flow is attempted in the opposite direction, the fluid pressure along with the spring force pushes the poppet in closed position. Hence, no flow is occurred in opposite direction. The graphic symbol shows the function of the check valve. Pilot operated check valve




Fig 3.2 Pilot operated check valve The pilot operated check valve always permits free flow in one direction but permits flow in the normally blocked opposite direction only if pilot pressure punches the pilot piston downward. Irk this construction, the pilot piston is attached (or) integral part of the valve poppet. The spring holds the poppet seated in a no flow condition by pushing against the piston. The pilot operated check valves are frequently used to lock the hydraulic cylinders in position.

3:2:2 SHUTTLE VALVE In certain fluid power systems, the supply of fluid to a subsystem must be from more than one source to meet system requirements. It allows a system to operate from either of two fluid power sources. It is also known as a double check valve. It is mostly used in pneumatic device and is rarely used in hydraulic circuits. If the pressure is applied through port X, the ball is moved to the right blocking the port Y, and the ports X and A are connected. Similarly, when the pressure is applied through port Y, the ball is moved to the left blocking the port X, and the ports Y and A are connected.

Fig 3.3 Shuttle valve Since the ball is shuttled to one side (or) the other side of the valve depending on which side of the ball has the greater pressure, it is known as shuttle valve. This shuttle valve is used for safety purpose in the event that the main pump can no longer provide hydraulic power to operate emergency




devices, the shuttle valve will shift to allow fluid to flow from a secondary backup pump. The shuttle may be one of four types: (1) sliding plunger, (2) spring-loaded piston, (3) spring-loaded ball, or (4) spring-loaded poppet. In shuttle valves designed with a spring, the shuttle is normally held against the alternate system inlet port by the spring. 3:2:3 TWO WAY VALVE It contains and controls two flow control ports namely an inlet port and an outlet port. As the spool is moved front and back, it either allows fluid to flow through the valve or prevents flow. In the open position, the fluid enters the inlet port, flows around the shaft of the spool, and through the outlet port. The spool cannot move because the forces are equal there.

Fig 3.4 Two way valve In the closed position, one of the pistons of the spool simply blocks the inlet port, thus preventing flow through the valve.

3:2:3 THREE WAY VALVE Three-way valves contain a pressure port, a cylinder port, and a return or exhaust port. The three-way directional control valve is designed to operate an actuating unit in one direction; it permits either the load on the actuating unit or a spring to return the unit to its original position. It normally has two spool positions, either pressure-to-output or output-to-exhaust; however, a third position that seals the output port from either pressure or exhaust is available. Three-way valves are normally used to control single-acting cylinders or fluid motors.




Fig 3.5 Three way valve 3:2:4 FOUR WAY VALVE Most actuating devices require system pressure for operation in either direction.

It is one of

the most widely used directional control valves in fluid power systems. The typical four-way directional control valve has four ports: a pressure port, a return or exhaust port, and two cylinder or working ports. The pressure port is connected to the main system pressure line and the return line is connected to the reservoir in hydraulic systems. In pneumatic systems the return port is usually vented to the atmosphere. The two cylinder ports are connected by lines to the actuating units. Spool designs are mostly used like rotary four way valve. When the rotor is rotating inside the valve body, it connects (or) closes the passages with the ports A (or) B (or) P (or) T, to provide four flow paths. In the first position, the pressure port P is connected to the port A and the port B is connected to the tank T. In the 2nd position centered position, all four ports are blocked. In the third position, the pressure port P is connected to port B and the port A is connected to tank T. 4/2 DCV

4/3 DCV

Fig 3.6 Four way valves A three-position directional control valve incorporates a neutral or center position which designates the circuit as open or closed, depending on the interconnection of the P and T ports, and designates the type of work application depending on the configuration of the A and B ports. The four most common types of three-position valves are: open type, closed type, flow type, and tandem type.





Manual actuation- uses the muscle power or spring force to actuate the spool. Common manual actuators include levers, push button, pedals, etc.

Mechanical Actuation- various mechanical devices are used to control a DCV. Eg. Roller, Plunger etc. a very common mechanical actuator is a plunger.

Pneumatic actuation-in order to have a high force of actuation and eliminate mechanical control DCV’s can be actuated using oil or air pressure

Hydraulic Actuation- The hydraulic pressure may act directly on the end face of the spool

Electrical actuation- Electrically actuated valves use a solenoid to operate the valve pool. The solenoid can be either AC or DC.

Electro-Hydraulic / Electro-Pneumatic actuation- This is a combination of electric and hydraulic or pneumatic control method.

Fig 3.7 Valve actuation types




3.3 PRESSURE CONTROL VALVES The safe and efficient operation of fluid power systems, system components, and related equipment requires a means of controlling pressure. There are many types of automatic pressure control valves. Some of them merely provide an escape for pressure that exceeds a set pressure; some only reduce the pressure to a lower pressure system or subsystem; and some keep the pressure in a system within a required range.

3:3:1 SIMPLE PRESSURE RELIEF VALVE The pressure control valves are used in hydraulic circuits to maintain desired pressure in various parts of the circuits.

Fig 3.8 Simple pressure relief valve It is also employed as a backup device when the main pressure control device fails. The simple relief valve has a poppet is held seated inside the valve by a heavy spring. When the system pressure reaches enough high pressure, the poppet is forced off its seat. This allows the flow through the outlet to tank as long as the high pressure level is maintained. The adjusting screw cap is used to vary the spring force and thus to vary the cracking pressure at which the valve begins to open. If the hydraulic system obtain maximum pressure (cracking pressure), then all the pump flow will return back to the tank through the relief valve. The pressure relief valve protects the hydraulic system against any overloads. If a pressure compensated vane pump is used, then pressure relief valve is not needed. The main function of a pressure relief valve is to control the force or torque produced by hydraulic actuators. Pilot operated pressure control valve It follows that simple relief valves have a tendency to open and close rapidly as they “hunt� above and below the set pressure, causing pressure pulsations and undesirable vibrations and producing a noisy chatter. Compound relief valves use the principles of operation of simple relief




valves for one stage of their action—that of the pilot valve. Provision is made to limit the amount of fluid that the pilot valve must handle, and thereby avoid the weaknesses of simple relief valves.

Fig 3.9 Compound pressure relief valve . Passage C is used to keep the piston in hydraulic balance when the valve's inlet pressure is less than its setting (diagram A). The valve setting is determined by an adjusted thrust of spring (3) against poppet (4). When pressure at the valve’s inlet reaches the valve’s setting, pressure in passage D also rises to overcome the thrust of spring (3). When flow through passage C creates a sufficient pressure drop to overcome the thrust of spring (2), the piston is raised off its seat (diagram B).This allows flow to pass through the discharge port to the reservoir and prevents further rise in pressure.

3:3:2 SIMPLE PRESSURE REDUCING VALVE Pressure-reducing valves provide a steady pressure into a system that operates at a lower pressure than the supply system. A reducing valve can normally be set for any desired downstream pressure within the design limits of the valve. Once the valve is set, the reduced pressure will be maintained regardless of changes in supply pressure (as long as the supply pressure is at least as high as the reduced pressure desired) and regardless of the system load, providing the load does not exceed the design capacity of the reducer.




Fig 3.10 Pressure reducing valve This valve consists primarily of a pressure-compensated spool valve that meters the flow into the branch outlet to control the pressure at a preset value. The pressure set point is provided by an adjustable relief valve (1). When the branch circuit pressure falls below the set point, the spring forces the valve open allowing full pressure flow. As branch pressure builds up until the relief valve (1) opens. Flow through the relief valve reduces the pressure in the spring chamber (2) causing the valve to move upward and meter the flow. Upward valve travel continues until the outlet pressure reaches the set point, and again the pressure balances the control valve. These valves will accurately control branch circuit pressures regardless of flow.

3:3:3 SEQUENCE VALVE A sequence valve is placed in a hydraulic system to delay the operation of one portion of that system until another portion of the same system has functioned. The opening pressure is obtained by adjusting the tension of the spring that normally holds the piston in the closed position. (Note that the top part of the piston has a larger diameter than the lower part.) Fluid enters the valve through the inlet port, flows around the lower part of the piston and exits the outlet port, where it flows to the primary (first) unit to be operated. This fluid pressure also acts against the lower surface of the piston. When the primary actuating unit completes its operation, pressure in the line to the actuating unit increases sufficiently to overcome the force of the spring, and the piston rises. The valve is then in the open position. The fluid entering the valve takes the path of least resistance and flows to the secondary unit. A drain passage is provided to allow any fluid leaking past the piston to flow from the top of the valve. In hydraulic systems, this drain line is usually connected to the main return line.




Fig 3.11 Sequence valve

3:4 FLOW CONTROL VALVES Flow control valves are used to control the speed of the hydraulic actuators by adjusting the flow rate to these actuators. Control of fluids is very much necessary since the speed of the machine elements depends on the rate of flow of the pressurized hydraulic fluid. These valves may be fixed orifice valve, adjustable needle valve, globe valve, gate valve, non-compensated valve (or) compensated valves. The flow control valves could be either 1. Throttle valves or Flow restrictors which are pressure dependent. 2. FCV which are pressure independent. 3:4:1 NEEDLE VALVES Needle valves are used to provide fine control of flow in small diameter piping. As the name implies, the needle valves have sharp pointed conical disc and matching seat. Needle valves are normally made up of steel bar. The needle valves are also used as a stop valve in hydraulic circuit to shut off the flow of fluid from one part of a circuit to another part. These valves can be adjusted to control the rate of fluid flow. The stem of this flow control valve has several color rings and number knob permits the reading of a given valve opening. To determine the flow rate for given valve settings and pressure drops, charts are available. A locknut is provided to prevent unwanted changes in flow.

Fig 3.12 Needle valve




3:4:2 PRESSURE COMPENSATED FLOW CONTROL VALVES This valve is used to main the flow rate constant to compensate for the variations in pressure. Usually an orifice setting is fixed and to maintain flow control, a constant ΔP across the orifice is obtained.

Fig 3.13 Pressure Compensated Flow control valve Assume PL increases, therefore discharge Q decreases temporarily at throttle. Therefore Q in at variable orifice is > Q


Therefore, PI increases As well ΔP across piston forces the variable orifice

open causing PI to increase until original balance on ΔP is achieved. Assume initially load is zero, i.e. pump is off. The piston is pushed down by the spring creating an orifice for the pump. As fluid is pumped through because of the restrictor throttle pin, the chamber pressure increases and If this P I x A is equal to the spring force, the desired ΔP across the throttle has been reached results in Q D. But if the pump attempts to put more Q than QD, PI rises above the desired value and forces piston upward restricting the inlet flow. Since flow is restricted at the inlet Pinlet will rise. A balance occurs when QP = QR + QV where QR flows through a relief valve. Now if PL a larger orifice and, hence, less restriction; PI thus raises unit ΔP balance is achieved.

3:5 CUSHIONED CYLINDERS To slow an action and prevent shock at the end of a piston stroke, some actuating cylinders are constructed with a cushioning device at either or both ends of a cylinder. This cushion is usually a metering device built into a cylinder to restrict the flow at an outlet port, thereby slowing down the motion of a piston.




Fig 3.14 Cushioned cylinders 3:6 FLOW DIVIDERS It is defined as a valve which divides a single flow into two or more prescribed flows regardless of the load pressures at the outlet ports. This flow divider valve is a form of pressure-compensated flow control valve which received one input flow and splits it into two output flows. The valve can deliver equal flow rates in each stream or a predetermined ratio of flow rates. Flow dividers operate over a narrow bandwidth rather than at one set point. Thus, there will be a variation of flow rates in the secondary branches, and precise synchronization cannot be achieved with a flow divider valve alone. These hydraulic flow dividers provide many useful functions from a single pump source: 

Synchronized operation of multiple cylinders or fluid motors.

Proportional division of pump output among several circuits.

Intensified pressure when pressure higher than pump capacity is needed

There are two common types of flow-divider valves are rotary and sliding-spool.


BALANCED SPOOL FLOW DIVIDER The key component of this type of flow divider is the spool. Spool has passage drilled down the

center. Fluid enters spool and splits to flow along passage in both directions. Orifices provided at both ends of passage, if they are same size, flow divider is designed for equally dividing the input flow. When flow through both orifices is same, pressure drop is same at both ends, and spool is in force balance. If pressure at left port is lower than at right port, fluid entering the passage takes the path of least resistance and flows to left port. Higher flow at left orifice produces higher pressure drop at the orifice. Greater pressure on the upstream side of left orifice creates a force imbalance on spool and shifts it to the left. Spool moves closer to end plate and partially blocks the orifice. End of the spool moves towards the end plate. Spool moves until it finds the position where flow is equal in both directions, i.e. the spool will oscillate and eventually stop at the position where P1 = P2 and equal flow to the ports will be achieved.




Fig 3.15 Balanced spool flow divider


ROTARY FLOW DIVIDER A rotary flow divider consists of several hydraulic motors connected together in parallel by a

common shaft. One output fluid stream is split into as many output streams as there are motor sections in the flow divider. Since all motor sections turn at the same speed, output stream flow rates are proportional to the sum of displacements of the motor sections. Rotary flow dividers usually have larger capacities. The pressure drop across each motor section is relatively small because no energy is delivered to an external load, as is usually the case with a hydraulic motor. However, the designer cannot overlook pressure intensification generated by a rotary flow divider. If for any reason the load pressure in one or more branches drops to some lower level or to zero, full differential pressure will be felt across the motor sections in the particular branch or branches. The section thus pressurized will act as hydraulic motors and drive the remaining sections as pumps. This results in an elevated or intensified pressure in these circuit branches. Caution must be exercised in applying rotary flow dividers to minimize the potential for pressure intensification. Rotary flow dividers can also integrate multiple branch return flows into a single return flow.

Fig 3.16 Symbol of Rotary flow divider





Graphical symbols are used throughout the world in the design, operation and maintenance of the hydraulic and pneumatic systems.

Similar to the electrical circuit diagram, hydraulic, and pneumatic circuits are made with graphic symbols to understand the working of a particular system.

Knowledge of hydraulic and pneumatic symbols is an absolute necessity for an engineer to read and understand the hydraulic and pneumatic circuit diagram and other important drawings of fluid power systems.

A schematic diagram is a 'road map' of the hydraulic system and to a technician skilled in reading and interpreting hydraulic symbols, is a valuable aid in identifying possible causes of a problem. This can save a lot of time and money when troubleshooting hydraulic problems.

The important symbols used in hydraulic and pneumatic systems are shown in the following table. These symbols conform to the American National standards Institute (ANSI) and ISO.




Line, Pilot (For Control)

Pressure Compensated Units

Line, Enclosure Outline

Temperature Cause or Effect

Direction of Flow Hydraulic

Reservoir Vented

Direction of Flow Pneumatic

Reservoir Pressurized

Lines Crossing

Line, to Reservoir Above Fluid Level

Lines Joining

Line, to Reservoir Below Fluid Level





Line with Fixed Restriction

Station, Testing, Measurement or Power Take-Off

Line, Flexible

Vented Manifold




PUMPS Fixed Displacement


VALVES Sequence valve

MISCELLANEOUS COMPONENTS Variable Displacement Electric Motor


MISCELLANEOUS COMPONENTS Pressure reducing Direction of Shaft valve Rotation

HYDRAULIC Accumulator, MOTORS Spring Loaded


Shuttle valve Pressure Indicator

Hydraulic Motor Fixed Accumulator, Displacement Gas Charged

Flow Control, Temperature Indicator Adjustable Non compensated

Hydraulic Motor Variable -Heater Displacement

FCV, Adjustable (Temp. and Pressure Pressure intensifier compensated) VALVES ACTUATION METHODS DIRECTION CONTROL VALVES Manual Two Position Two Way Push Button

CYLINDERS Cylinder, single acting Cooler


Cylinder, Double Acting Single End Rod Temperature Controller Cylinder, Double Acting Double End Rod Filter, Strainer Cylinder, Double Acting Adjustable Cushion

Two Position Three wayLever Push-Pull

Pilot Pressure - Remote Cylinder, Double Acting Supply Differential Piston VALVES Component Enclosure Check valve

Two PedalPosition or Treadle Four Way SYMBOL

Mechanical Three Position Four Solenoid way

On-Off (Manual Shut-Off) Pressure Switch

Servo Valves Capable of Infinite Positioning

Pressure Relief valve








3:8 SUMMARY In any hydraulic circuit valves are the main control element that decides the working of actuators and other components. It is important to know their types and working to select the proper valve to suit the specific applications. The directional control valve is the component that starts, stops, and changes the direction of the fluid flowing through a hydraulic system. Hydraulic systems have devices to protect against excessive pressure. These are called pressure relief valves. The valves are adjustable and are set to open at a point slightly above maximum system pressure. When this occurs, the fluid is returned to the system reservoir. Flow control valves are used to regulate the volume of oil supplied to different areas of hydraulic systems.











Diaphragm accumulator


Spring loaded accumulator


Weight loaded accumulator


Hydraulic Reservoirs


Heat exchangers




Instrumentation and Measurement


Pressure gauges


Temperature gauges


Flow meters


Conduits and Fittings








Hydraulic circuits


Counter balance circuit


Sequence circuit


Speed control circuits


Intensifier circuits






4:0 INTRODUCTION Fluid power systems must have a sufficient and continuous supply of uncontaminated fluid to operate efficiently and the fluid must be kept free of all foreign matter. There are different types of hydraulic accumulators, Hydraulic reservoirs, strainers and filters, and other accessories installed in fluid power systems for proper functioning. A number of circuits are used frequently in fluid power systems to perform useful functions. Metering circuits offer precise control of actuator speed without a lot of complicated electronics and other circuits are designed for safety, sequencing of operations, and for controlling force, torque, and position. 4:1 Objectives After studying this lesson, you should be able to: 

Understand the importance of accumulators.

Familiarize with the hydraulic system component accessories.

Identify the instrumentation and measuring components of systems.

Understand the types of hydraulic circuits with their applications.

4:2 HYDRAULIC ACCUMULATORS Hydraulic accumulators are used to store the hydraulic fluid under pressure and release pressurized fluid to the system on demand. The potential energy is stored in the accumulator and act as a secondary (or) auxiliary power source to do useful work whenever required by the system. Accumulators improve the system efficiency by reducing the pump requirement. A hydraulic accumulator is a device that stores the potential energy of an incompressible fluid held under pressure by an external source against some dynamic force. The dynamic force can come three different sources: Gravity, Mechanical Springs, and Compressed gases. The stored potential energy in the accumulator is a quick secondary source of fluid power capable of doing useful work as required by the system.

Accumulators are classified as 

Dead weight (or) gravity type

Spring loaded type and

Gas loaded type

Gas loaded separator type accumulators are very much widely used in the hydraulic circuits and further classified as: 1. Piston type 2. Diaphragm type

3. Bladder type




4:2:1 DIAPHRAGM ACCUMULATOR It consists of two hollow, hemispherical metal sections bolted together at the center. One of the halves has a fitting to attach the unit to the hydraulic system and the other half is provided with an air valve for charging the unit with compressed air or nitrogen. Mounted between the two halves is a synthetic rubber diaphragm that divides the accumulator into two sections. The accumulator is initially charged with air through the air valve forces the diaphragm upward against the inner surface of the upper section of the accumulator. When the system pressure is down and when additional oil is required in the hydraulic circuit, the gas expands, the oil from the accumulator is forced into the system to make up the pressure drop in the system. A shutoff button, which is secured at the base of the diaphragm, covers the inlet of the line connection when the diaphragm is fully stretched. Because of its small weight- to volume ratio, it is used exclusively for mobile applications. The restriction is on the deflection of the diaphragm.

Fig. 4.1 Diaphragm accumulator 4:2:2 SPRING LOADED ACCUMULATOR A spring loaded accumulator is similar to the weight – loaded type except that the piston is preloaded with a spring which is the source of energy that acts against the piston, forcing the fluid into the hydraulic system. The pressure generated by this type of accumulator depends on the size and preloading of the spring. In addition, the pressure exerted on the fluid is not a constant. The spring- loaded accumulator typically delivers a relatively small volume of oil at low pressures. Thus, they tend to be heavy and large for high- pressure, large –volume systems. This type of accumulator should not be used for applications requiring high cycle rates because the spring will fatigue and lose its elasticity. The load characteristics of a spring are such that the energy storage depends on the force required to compress the spring. Uncompressed length of a spring represents zero energy storage. As a spring is compressed to the maximum installed length, a minimum pressure value of the liquid in a ram assembly is established.




As liquid under pressure enters the ram cylinder, causing a spring to compress, the pressure on the liquid will rise because of the increased loading required to compress the spring.

Fig. 4.2 Spring loaded accumulator 4:2:2 WEIGHT LOADED ACCUMULATOR This type consists of a vertical, heavy- wall steel cylinder, which incorporates a piston with packing to pressure leakage. A dead weight is attached to the top of the piston. The force of gravity of the dead weight provides the potential energy in the accumulator.

This type of accumulator creates a

constant fluid pressure throughout the full volume output of the unit regardless of the rate and quantity of output. The main disadvantage of this type of accumulator is extremely large size and heavy weight which makes it unsuitable for mobile equipment.

Fig. 4.3 Weight loaded accumulator




Applications of a hydraulic accumulator 

Dampens hydraulic shocks which may develop when pressure surges occur in hydraulic systems.

Adds to the output of a pump during peak load operation of the system, making it possible to use a pump of much smaller capacity than would otherwise be required.

Absorbs the increases in fluid volume caused by increases in temperature.

Acts as a source of fluid pressure for starting aircraft auxiliary power units

Assists in emergency operations.

Pressure Intensifiers (or) Boosters It gives an output pressure higher than the input pressure and used in hydraulic circuit to increase value above the pump discharge pressure. Pressure intensifiers consist of cylinders of different diameters with pistons that are connected by a common rod. In double piston-cylinder devices, fluid is pumped into the larger cylinder and then expelled from the smaller cylinder at a higher pressure. To maximize efficiency, the larger piston is fitted with an o-ring. The smaller piston is lapped to a close fit. Typically, pressure intensifiers that produce higher pressures are built with heavier cylinders and pistons. Though not designed with high volumetric capacities, pressure intensifiers provide a simple and economical way to raise fluid pressures. For low pressure input is given by pneumatic pressure to the larger piston side which is transformed into a higher hydraulic pressure on the smaller piston side.

Fig. 4.4 Air oil intensifier 4:3 HYDRAULIC RESERVOIRS




The hydraulic reservoir is a container for holding the fluid required to supply the system, including a reserve to cover any losses from minor leakage and evaporation. The reservoir can be designed to provide space for fluid expansion, permit air entrained in the fluid to escape, and to help cool the fluid. Filling reservoirs to the top during servicing leaves no space for expansion. Most reservoirs are designed with the rim at the filler neck below the top of the reservoir to prevent overfilling. Some means of checking the fluid level is usually provided on a reservoir. This may be a glass or plastic sight gage, a tube, or a dipstick. Hydraulic reservoirs are either vented to the atmosphere or closed to the atmosphere and pressurized. Vented Reservoir A vented reservoir is one that is open to atmospheric pressure through a vent line. Because atmospheric pressure and gravity are the forces which cause the fluid to flow to the pump, a vented reservoir is mounted at the highest point in the hydraulic system. Air is drawn into and exhausted from the reservoir through a vent line. A filter is usually installed in the vent line to prevent foreign material from being taken into the system. Pressurized Reservoir A pressurized reservoir is sealed from the atmosphere. This reservoir is pressurized either by engine bleed air or by hydraulic pressure produced within the hydraulic system itself. Pressurized reservoirs are used on aircraft intended for high altitude flight, where atmospheric pressure is not enough to cause fluid flow to the pump. Simple reservoir construction A reservoir should have capacity of three to four times the volumetric flow rate of the pump for most of the hydraulic system where average demands are expected. Welded steel plates are used to construct the reservoir. The bottom plate contains a drain plug at its lowest point to drain out the oil when required. Air breather cap and fitter are also included to allow the tank to breathe as oil level changes due to system demand requirements. By this arrangement, the tank is always vented to the atmosphere.

Fig. 4.5 Hydraulic reservoir




The electric motor and pump can be installed at its top surface. The return line should enter the reservoir on the side of the baffle plate that is opposite from pump suction line. The pump suction strainer should be well below the normal oil level in the reservoir. A sight glass is also included to permit visual check of the fluid level. A baffle plate is provided to separate the pump inlet line from the return line in order to prevent the same fluid from recirculation continuously within the tank. 4:4 HEAT EXCHANGERS In larger hydraulic systems additional cooling is required with the help of heat exchangers known as hydraulic coolers. Both air cooling as well as water cooling are done in hydraulic system. Water cooling Water cooling is most common in hydraulic systems with the help of shell and tube heat exchanger. This is the counter flow type in which the cooling water flows in the opposite direction. This is fitted in the return line to the tank. The oil to be cooled is flowing through the tubes and the water is circulated over the tubes to absorb the heat of oil.

Fig. 4.6 Shell and tube heat exchanger Air cooling It is similar to automobile indicator but has very high pressure rating. The fan blows air through the radiator matrix. But air cooling is noisy and it occupies more space than a water cooler. But it does not have the contamination from leakage inside a water cooler.

Fig. 4.7Air cooling heat exchanger




4:5 FILTERS AND STRAINERS Filters and strainers are used to remove the dirt and contaminants from the fluid. Filters are used to remove very fine particles (in microns) where strainers are used to remove larger particles only. Filters return the insoluble contaminants from fluid by means of some porous medium. A strainer known as coarse filter is constructed by means of wire screen. Since drops, they are usually installed in the pump






suction. Dirt in a hydraulic system causes contamination

of fluid and results in sticking of valves, failure of seals and premature wear. Even small particle of 20 micron size is enough to contaminate the fluid. Contamination is in the form of a liquid, gas or solid. The metal chips, bits of pipe threads, tubing burrs, pipe dope, sheds of plastic tape, bits of seal material, welding beads, bits of hose and dirt cause the contamination during component maintenance and assembly. The moisture due to water condensation inside the reservoir, entrained gases, scale caused by rust, bits of worn seal materials, particles of metal due to wear, and sludge’s and varnishes due to oxidation of the oil cause the contamination during operation of a hydraulic system. To remove iron and steel particles from the fluid, a magnetic plug is installed in some reservoir. But the filters and strainers are playing important role for making the fluid clean. Filtering methods There are three methods to filter the fluid in the hydraulic system. 

Mechanical filtering method

Absorbent filtering method

Adsorbent filtering method

Mechanical filters contain closely woven metal screens or discs. They generally remove only fairly coarse particles.

Absorbent inactive filters, such as cotton, wood pulp, yarn, cloth, or resin, remove much smaller particles; some remove water and water-soluble contaminants. The elements often are treated to make them sticky to attract the contaminants found in hydraulic oil.

Absorbent active materials, such as charcoal and Fuller's Earth (a claylike material of very fine particles used in the purification of mineral or vegetable-base oils), are not recommended for hydraulic systems.

The three basic types of filter elements are surface, edge, and depth. 

A surface-type element is made of closely woven fabric or treated paper. Oil flows through the pores of the filter material, and the contaminants are stopped.

An edge-type filter is made up of paper or metal discs; oil flows through the spaces between the discs. The fineness of the filtration is determined by the closeness of the discs.

A depth-type element is made up of thick layers of cotton, felt, or other fibers.




Position of Filters Filters are located in a reservoir, a pressure line, a return line, or in any other location where necessary. 

Inlet line filters part the pump by being installed in between the pump and reservoir. These filters must be designed to give a low pressure drop.

Pressure line filters are installed after the pump to protect valves and actuators and can be finer and smaller. They must be designed to withstand the maximum system pressure without damage.

Return line filters part of the reservoir by being installed in between the actuator and reservoir. Oil after doing work in the actuator returns to the reservoir through return line just it the reservoir passes through the filter.

4:6 INSTRUMENTATION AND MEASUREMENT For safe and efficient operation, fluid power systems are designed to operate at a specific pressure and temperature ranges. Most fluid power systems are provided with pressure gauges and thermometers for measuring and indicating the pressure and/or the temperature in the system. In addition to this flow meters are also used for measuring the flow quantity of oil in specific locations.

4:6:1 PRESSURE GAUGES Most of the pressure gauges use a Bourdon-tube as a measuring element that senses pressure and converts the pressure to displacement. Since the Bourdon-tube displacement is a function of the pressure applied, it may be mechanically amplified and indicated by a pointer. Thus, the pointer position indirectly indicates pressure The Bourdon-tube gauge is available in various tube shapes namely curved or C-shaped, helical, and spiral. The size, shape, and material of the tube depend on the pressure range and the type of gauge desired.




Fig. 4.8 Pressure gauge bourdon tube mechanism 4:6:2 TEMPERATURE GAUGES Temperature is the degree of hotness or coldness of a substance measured on a definite scale. Temperature is measured when a thermometer, is brought into contact with the medium being measured. All temperature-measuring instruments use some change in a material to indicate temperature. Some of the effects that are used to indicate temperature are changes in physical properties and changes in physical dimensions. One of the more important physical properties used in temperature- measuring instruments is the change in the length of a material in the form of expansion and contraction. Dial thermometer Distant-reading dial thermometers are used when the indicating portion of the instrument must be placed at a distance from where the temperature


being measured.

It has a long

capillary tube which separates the sensing bulb from the Bourdon tube and dial. The thermometers are filled with fluid (liquid or gas) at some temperature and sealed. Almost the entire volume of the fluid is in the sensing bulb. As the temperature of the bulb changes, the volume of the fluid tries to change. Since the volume is constant, a pressure change occurs within the thermometer. This increase in pressure causes the Bourdon tube to straighten out which actuates the system of levers and gears, causing the thermometer pointer to move over the dial and register temperature.




Fig. 4.9 Temperature gauge with bourdon tube mechanism 4:6:3 FLOW METERS Measuring flow depends on the quantities, flow rates, and types of liquid involved. All flow meters are made to measure specific liquids and must be used only for the purpose for which they were made. It is usually not bi-directional and acts as a check valve blocking flow in the reverse direction. The main components consist of: a metering cone, a magnetic piston which is held in the no-flow position by a tempered spring. Fluid first enters the device, flowing around the metering cone, putting pressure on the magnetic piston and spring. As flow increases in the system, the magnetic piston begins to compress the spring, indicating the flow rate on the graduated scale.

Fig. 4.10

Flow meter

4:7 CONDUITS AND FITTINGS The control and application of fluid power would be impossible without suitable means of transferring the fluid between the reservoir, the power source, and the points of application. Fluid lines are used to transfer the fluid, and fittings are used to connect the lines to the power source and the points of application. The three types of lines used in fluid power systems are pipe (rigid), tubing (semi rigid) and hose (flexible). Fluid lines are selected considering the type of fluid, the required system pressure, and the location of the system. Some type of connector or fitting must be provided to attach the lines to the components of the system and to connect sections of line to each




other. There are many different types of connectors and fittings provided for this purpose namely threaded connectors, flange connectors,


connectors, Flared connectors



connectors. The type of connector or fitting required for a specific system depends mainly on the type of fluid line (pipe, tubing, or flexible hose) used in the system and also the type of fluid medium and the maximum operating pressure of the system.


PIPES Piping is used economically in larger-sized hydraulic systems where large flow is carried. It is

particularly suited for long, permanent straight lines. Piping is taper-threaded on its outer diameter into a tapped hole or fitting. However, it cannot be bent. Instead, fittings are used wherever a joint is required. This results in additional costs and an increased chance of leakage. Sizes of pipe are listed by the nominal inner diameter and the wall thickness. The material,

Inner diameter,




are the three primary considerations in the selection of lines for a particular fluid power system. The pipes used in fluid power systems are commonly made from steel, copper, brass, aluminum, and stainless steel. Fluid power systems are designed as compactly as possible, to keep the connecting lines short. Every section of line should be anchored securely in one or more places so that neither the weight of the line nor the effects of vibration are carried on the joint to minimize stress throughout the system. Lines should normally be kept as short and free of bends as possible.


TUBING Tubing differs from pipe in its size classification and is designated by its actual outer

diameter. The two types of tubing used for hydraulic lines are seamless and electric welded. Seamless tubing is made in larger sizes than tubing that is electric welded. Seamless tubing is flared and fitted with threaded compression fittings. Tubing bends easily, so fewer pieces and fittings are required. Tubing can be cut and flared and fitted in the field. Generally, tubing makes a neater, less costly, lower-maintenance system with fewer flow restrictions and less chances of leakage. Knowing the flow, type of fluid, fluid velocity and system pressure will help determine the type of tubing to use. A system’s pressure determines the thickness of the various tubing walls. Tubing above 1/2 inch outer diameter usually is installed with either flange fittings with metal or pressure seals or with welded joints. If joints are welded, they should be stress-relieved.

4:7:3 SEALS It is used to prevent the oil leakage in a hydraulic system. A seal is any gasket, packing, seal ring, or other part designed specifically for sealing. Oil leakage reduces the efficiency and increases power losses. Most of the hydraulic components have clearances which permit small amount of internal leakage .When the clearance between the mating parts increase due to wear, the leakage increases. If the leakage increases, then the part of the pump’s output is wasted and the actuators will not operate




properly. To prevent this leakage, seals are used. Also, the seals are used to keep out contaminated particles. Seals are designed as positive or non-positive type and can be designed for static and dynamic applications. 

Positive type seals will not allow any leakage.

Non positive type seals will allow clearance to provide a lubricating oil film between the mating parts and hence they will permit a small amount of internal leakage.

Static seals are assembled between the mating parts which are not moving relative to each other. Flange gaskets and seals are the examples of this type and they are compressed between two rigidly connected (or fastened) parts.

Dynamic seals are assembled between mating parts which move relative to each other. Hence, the dynamic seals are subject to wear fast because one of the mating parts rubs against the seal.



include Teflon, synthetic rubber, cork, leather, metal and asbestos. Fluid power seals

are usually typed according to their shape or design. These types include T-seals, V-rings, O-rings, and U-cups O-rings An O-ring is a positive seal that is used in static and dynamic applications. It has replaced the flat gasket on hydraulic equipment. When being installed, an O-ring is squeezed at the top and bottom in its groove and against the mating part. It is capable of sealing very high pressure. Pressure forces the seal against the side of its groove, and the result is a positive seal on three sides. Dynamic applications of an O-ring are usually limited to reciprocating parts that have relatively short motion. T- Seals This seal is reinforced with back-up rings on each side. A T-ring seal is used in reciprocating dynamic applications, particularly on cylinder pistons and around piston rods.

V-rings It is the part of the packing set that does the sealing. It has a cross section resembling the letter V.



V-rings Fig. 4.10 Types of seals





4:8: HYDRAULIC CIRCUITS A Hydraulic circuit is a group of components such as pumps, actuators, and control valves so arranged that they will perform a useful task. When analyzing or designing a hydraulic circuit, the following three important considerations must be taken into account: 

Safety of operation

Performance of desired function

Efficiency of operation

It is very important for the fluid power designer to have a working knowledge of components and how they operate in a circuit. Hydraulic circuits are developed through the use of graphical symbols for all components. 4:8:1 COUNTER BALANCE VALVE CIRCUITS The following circuit shows the use of a counter balance valve to keep a cylinder in the upward vertically while the pump is idling and the DCV is in its center position. The cylinder supports a load W and the load is held in that position. This is done by the counter balance valve set for a force slightly above the external load W. When the load is to be lowered, the fluid is forced to the blank end of the piston through DCV. The increased pressure causes the counter balance valve to open in order to lower the load. A check valve is provided in the circuit to allow free flow for raising the load.




4.11 counterbalance valve circuit During the downward movement of the cylinder the counterbalance valve is set to open at slightly above the pressure required to hold the piston up ( check valve does not permit flow in this direction ). The control signal for the counterbalance valve can be obtained from the blank end or rod end of the cylinder. This pressure is less and hence usually it has to be derived from blank end. This permits the cylinder to be forced downward when pressure is applied on the top. The check valve is used to lift the cylinder up as the counterbalance valve is closed in this direction. 4:8:2 SEQUENCE VALVE CIRCUIT A sequence valve causes operations in a hydraulic circuit sequentially. In the following circuit two sequence valves are used to control the sequence of operations of two double-acting cylinders. When the DCV is shifted into its left envelope position, the left cylinder extends completely, and only when the left cylinder pressure reaches the pressure setting of sequence valve, the valve opens and then the right cylinder extends. If the DCV is then shifted into its right envelope position, the right cylinder retracts fully, and then the left cylinder retracts.

4.12 Sequence valve Hence

application circuit







controlled by the sequence valves. The spring centered position of the DCV locks both cylinders in place. The application of this circuit is in press circuit. For example, the left cylinder the clamping cylinder could extend and clamp a work piece. Then the right cylinder the punching cylinder extends to punch a hole in the work piece. The right cylinder then retracts the punch, and then the left cylinder retracts to de-clamp the work piece for removal. Obviously these machining operations must occur in the proper sequence as established by the sequence valves in the circuit.




4:8:3 SPEED CONTROL CIRCUITS The speed of the hydraulic cylinder is determined by the fluid flow rate and the piston area. These dimensions are generally taken as standard fixed one and so speed is controlled by only adjusting the flow to the hydraulic cylinder. There are three speed control circuits which are given below: 

Meter - in circuit – Primary control

Meter - out circuit – Secondary control

Bleed - off circuit – By pass control

Meter in circuit In this type of speed control, the flow control valve is placed between the pump and the actuator. Thereby, it controls the amount of fluid going into the actuator. When the directional control valve is actuated to the left envelope position, oil flows through the flow control valve to extend the cylinder. The extending speed of the cylinder depends on the setting (percent of full opening position) of the flow control valve. When the directional control valve is actuated to the right envelope position, the cylinder retracts as oil flows from the cylinder to the oil tank through the check valve as well as the flow control valve. Milling cutters and drills passing through the work piece often tend to drag the entire tool unit forward. Considerable force is required to push the tool during the cutting cycle. Metering and controlling the flow of fluid into the blank end of the cylinder prevents this break through condition from affecting the speed of drill feed. During retraction stroke, the fluid is forced through integral check valve to the tank and results in a repaid travel of the tool to the starting position and hence become ready for the next cycle.

4.13 Meter in circuit Meter out circuit In this type of speed control, the flow control valve is placed between the actuator and the tank. Thereby, it controls the amount of fluid going out of the actuator. If a weight pulling downward on the




piston rod of a vertical cylinder results in sudden dropping of load pulling the piston rod down if a meter-in system is used even if the flow control valve is completely closed. Thus, the meter-out system is generally preferred over the meter-in type. One drawback of a meter-out system is the possibility of excessive pressure buildup in the rod end of the cylinder while it is extending. This is due to the magnitude of back pressure that the flow control valve can create depending on its nearness to being fully closed as well as the size of the external load and the piston-to-rod area ratio of the cylinder. In addition an excessive pressure buildup in the rod end of the cylinder results in a large pressure drop across the flow control valve. This will produce the undesirable effect of a high heat generation rate with a resulting increase in oil temperature.

4.14 Meter out circuit Bleed off circuit Bleed off circuit is used to control the flow of fluid in both directions of flow (or) on a specific line and limits speed in only one direction of the cylinder travel. The bleed off circuit is shown here.




4.15 Bleed off circuit Here the compensated flow control valve is connected in the pressure line. Hence it can control the flow of fluid in both directions of the y travel. If the compensated flow control valve is connected to the cylinder line (i.e. above the DCV), then it can control the speed of extension stroke (or) retraction stroke. Bleed off circuits are widely used in broaching machines, shaping machines, planning machines, and similar types of machines where a fairly large quantity of fluid is used. For large flows, the degree of accuracy of bleed off circuits is adequate. But, this circuit may not be sensitive enough to compensate for very small flows such as those encountered in precise boring operations. 4:8:4 INTENSIFIER CIRCUITS Pressure intensifier are widely used for punching and other press applications were larger force is required to complete the operation. These are positioned close to the actuators to reduce the length of high pressure transmission lines. The circuit uses a pilot operated check valve and sequence valve. If the direction control valve is shifted to the right envelope mode by the solenoid valve forces the pump oil into the blank end through the pilot check valve which extends the cylinder. When the actuator experiences the load resistance the pressure in the line increases and opens the sequence valve. Now the oil is supplied to the actuator from the intensifier at high pressures to perform the required punching operation also blocks the pilot check valve pressure line.

4.16 Intensifier circuit




If the direction control valve is shifted to the left envelope mode by the solenoid valve forces the pump oil into the rod end through the pilot check valve which retracts the cylinder. Now the oil in the blank end returns to the reservoir through the pilot check valve by avoiding the high pressure intensifier line. Pressure intensifier are widely used for punching, piercing, riveting and also in pneumatic circuits for clamping of jobs at higher pressures.

4:9 SUMMARY In order to design any hydraulic circuit we should have details of hydraulic accessories and their types and working to use them in the proper way to suit the specific applications. The use of fluid under pressure to transmit power and to control intricate motions is relatively modern and has had its greatest development in the past two or three decades. Industrial hydraulics is necessary it can move rapidly in one part of its length and slowly in another. No other medium combines the same degree of positiveness, accuracy, and flexibility, maintaining the ability to transmit a maximum of power in a minimum of bulk and weight.









Basic principles


Difference between hydraulics and pneumatics




Piston compressor


Rotary vane compressor


Rotary screw compressor


Vacuum pumps


Pneumatic actuators


Pneumatic cylinders


Rotary actuators


FRL unit


Electronic control of fluid power


Solenoid and servo valves






5:0 INTRODUCTION Pneumatics is used extensively in industry as well as in many everyday applications. It has many distinct advantages in terms of energy consumption, cost and safety. Many of the principles of hydraulics apply to pneumatics. For example, Pascal’s law applies to gases as well as liquids. One of the most common uses of pneumatic power is in the operation of pneumatic tools. Pneumatics is also used a lot in industry in production lines and processing plants and used to do lots of different jobs such as moving, holding or shaping objects. Pneumatic systems make use of compressed air which is made available using compressors and energy stored in this compressed air to do things. 5:1 OBJECTIVES After studying this lesson, you should be able to: 

Understand the basic principles governing pneumatics

Describe the operation of pneumatic systems

Understand of the benefits of pneumatics as well as the obvious limitations.

Understand the principles behind electronic controls for fluid power systems.

5:2 BASIC PRINCIPLES Pneumatics is concerned with the use of compressible fluids to provide power or to control power. It must be included in any study of fluid power because of the increasing use of air in industrial plants.

Properties of Air Air is a mixture of gases and is generally considered to contain 21 % oxygen and 79 % nitrogen. This is an approximation since air also contains a number of minor constituents, among them argon and oxides of carbon and nitrogen. Depending on the humidity, air may contain up to 4 % water vapor at any one time and place. Air has mass and is acted upon by gravity so that it exhibits the property of weight. Air is the standard for determining gas gravity and, like water, is assigned a specific gravity of 1. Absolute Pressure and Temperature All calculations concerning volume and pressure changes of gases must be performed using absolute pressure and temperature. Absolute pressure was defined as a pressure measured above a point of zero pressure. Atmospheric pressure is always absolute, while those pressures that are measured against a reference point of atmospheric pressure are called gauge pressures. Since pressure is produced by molecular energy and motion, a more precise definition for absolute pressure would be any pressure measured with reference to a point of no molecular motion or energy. Such a point is known as absolute zero, and is referred to by both absolute temperature and pressure.

The conversion of reading pressure to absolute pressure is





Where, Pabs: absolute pressure, N/m absolute 2

Pg: gauge pressure, N/m gauge Patm: atmospheric pressure The Gas Laws The gas laws were theoretically derived on the basis of a perfect gas. No perfect gas exists; however, air and most diatomic gases have subsequently been found to obey these laws so closely that these laws can be used, without modification, for calculations. Boyle’s law states that if the temperature of a gas is held constant, the volume varies inversely with the absolute pressure of the gas.

Charles’ law states that if the pressure on a confined gas id held constant, the volume of the gas is directly proportional to the absolute temperature.

Gay-Lussac’s law states that if the volume of a confined gas is held constant, the pressure exerted by the gas is directly proportional to the absolute temperature.

A combination of these three laws, which is appropriately termed the combined gas laws or general gas law:


Pneumatics is used a lot in factories, production lines and processing plants. It can be used to do lots of different jobs such as moving, holding or shaping objects which requires lesser force compared to hydraulics.


FLUID MECHANICS & PNEUMATICS Pneumatics The working medium used is compressed air Operating pressures up to 12 bars in industrial applications Mostly tubes are used for transmission of air through the system components Transmission of air for long distances easier through the system components Energy consumption is low when compared with hydraulics Not affected by temperature changes in the environment Contamination is less Pneumatic systems are easy to install and flexible Pneumatic systems are very safe compared to other systems. If air leaks, it escapes and no safety problems. Air is available everywhere at free of cost.

68 Hydraulics The working medium used is hydraulic oil Operating pressures up to 400 bars in industrial applications Pipes are used for transmission of oil through the system components Difficult to transmit high pressure oil for long distances Energy consumption is high when compared with pneumatics Sensitive to temperature changes in the environment More possibilities for oil contamination Hydraulic systems are less flexible than the pneumatic systems are relatively difficult to install. Hydraulic systems are to be designed with safety devices. If oil leaks, it lead to severe safety concerns. Hydraulic system has to be designed considering the availability and cost of oil to be used.

5:4 COMPRESSORS A compressor is a machine that compresses air or another type of gas from a low inlet pressure (usually atmospheric) to a higher desired pressure level. This is accomplished by reducing the volume of the gas. Air compressors are utilized to raise the pressure of the given volume of air. An air compressor operates by taking in a fixed volume of surrounding air (free air), and compresses this air into a smaller volume, which (by Boyle’s law) increases the air’s pressure, and then discharges it into a closed container or system. When air is compressed, work is done and it is part of this work of compressing that is manifested as heat in the compressed air. Since the heat energy is a part of the internal energy of the compressed gas, less work input is required on a given volume of gas if this heat energy is removed. Therefore most compressors are cooled either by water or air. Cooling reduces the potential energy of the compressed gas, but the loss is insignificant since the air in the receiver and the lines would lose this heat by natural cooling anyway. Cooling also helps to prevent excessive wear on the moving parts. Air compressors are available in many configurations and will operate over a wide range of flow rates and pressures. All air compressor designs are based on four basic principles: 



Compressor displacement and volumetric efficiency

Specific energy consumption

Staging 

Compressors are staged thereby reducing the temperature rise and improving the compression efficiency. The temperature of the air leaving each stage is cooled prior to entering the next stage. This cooling process is called inter cooling.




Volumetric efficiency also increases with multi-stage compression since the pressure ratio over the first stage will be decreased.

Inter-cooling 

A heat exchanger or intercooler is used to cool the compressed air between stages. The intercooler may be either air-cooled or liquid (water)-cooled.

Perfect inter cooling is achieved when the air temperature leaving the intercooler is identical to the air temperature entering the compressor.

Minimum compressor power consumption can be achieved with perfect inter cooling when the pressure ratio across all stages is the same.

Perfect inter cooling is difficult to achieve in actual practice since the cost of either the heat exchanger(s) or the cooling water is prohibitive.

Selection of the most efficient number of compressor stages is determined by the actual operating pressure and desired compressor efficiency.

Compressor displacement and volumetric efficiency 

Theoretically, the capacity of the compressor is equal to the swept volume or amount of air drawn into the compression area.

Actual compressor capacity will be reduced by pressure drop on the intake side, preheating of the intake air, internal and external air leakage and expansion of air in the clearance volume.

Volumetric efficiency is the ratio of compressor capacity to compressor displacement.

Specific energy consumption 

The specific energy consumption of a compressor is defined as the shaft input power per unit of compressor capacity.

The usefulness of using compresses air as a power source is as: 


Pressure is transmitted undiminished in all direction throughout the system

Low cost

The best solutions for the jig and fixture systems, automation lines, pick and place in electronics industry.

However, a typical maximum pressure for the pneumatic systems is 7 to 10 bars. This indicates that the pneumatic systems aren't suitable for the heavy duty in terms of load.

Compressors are classified as:




Fig 5.2 Types of compressors 5:4:1 PISTON COMPRESSORS In this type of compressor a cylinder bore encloses a moving piston. As the crankshaft of the compressor rotate, the piston moves within the cylinder, similar to the piston in a car engine. As the piston is pulled down, the volume increases, creating a lower atmospheric pressure in the piston chamber. This difference in pressure causes air to enter via the inlet valve. As the piston is forced upwards the volume of air reduces. The air pressure therefore increases. Eventually the pressure forces the outlet valve to open. Single Acting Type This reciprocating air compressor utilizes automotive type pistons, connecting rods and crankshaft. All compression takes place on the top of the piston. Single-acting air compressors are available in either single-stage or multi-stage compression or can either be air-cooled or liquid (water)-cooled. Single-stage and two-stage are the most common reciprocating compressors utilized in (7 bar) plant air applications.

Typical pressure, flow and horsepower ratings are shown below. Single Stage type-

Pressure range - From 1.7 to 8.6 bar 3

Flow rating – From 0.11 to 0.57 m /min




Power rating – From 0.75 to 3.73 kW Two- Stage type-

Discharge pressure- From 7 to 17.2 bar 3

Flow rating - From 0.23 to 2.97 m /min Power rating – From 1 to 22 kW

Fig 5.2 Two stage single acting piston compressor

The reciprocating air compressor uses a number of automatic spring loaded valves in each cylinder that open only when the proper differential pressure exists across the valve. Inlet valves open when the pressure in the cylinder is slightly below the intake pressure. Discharge valves open when the pressure in the cylinder is slightly above the discharge pressure. A compressor is considered to be single stage when the entire compression is accomplished with a single cylinder or a group of cylinders in parallel. Many applications involve conditions beyond the practical capability of a single compression stage. Too great a compression ration (absolute discharge pressure/absolute intake pressure) may cause excessive discharge temperature or other design problems. For practical purposes most plant air reciprocating air compressors over 100 horsepower are built as multi-stage units in which two or more steps of compression are grouped in series. The air is normally cooled between the stages to reduce the temperature and volume entering the following stage. Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated configurations, may be packaged, and provide a wide range of pressure and capacity selections Double Acting This type of reciprocating air compressor utilizes a double-acting piston (i.e. compression takes place on both sides of the piston), piston rod, crosshead, connecting rod and crankshaft. Double-acting compressors are available in single and multi-cylinder and single and multi-stage configurations. Singlecylinder, single-stage and double-cylinder, two-stage configurations are the most common types found in




industrial applications. Most double-acting air compressors are available in either water-cooled or aircooled. 5:4:2 ROTARY VANE COMPRESSOR

Fig 5.3 Rotary vane compressor Rotary vane compressors consist of a rotor with a number of blades inserted in radial slots in the rotor. The rotor is mounted offset in a larger housing which can be circular or a more complex shape. As the rotor turns, blades slide in and out of the slots keeping contact with the outer wall of the housing. Thus, a series of decreasing volumes is created by the rotating blades. The air inlet is placed where the volume of the compression chamber is greatest, the outlet where the volume is smallest. Consequently, as the vanes turn, the space between them is reduced. This reduction in volume compresses the air as it travels from the inlet to the outlet. Rotary Vane compressors are with suitable port connections may be used as a compressor or a vacuum pump. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. A rotary vane compressor is well suited to electric motor drive and is significantly quieter in operation than the equivalent piston compressor.

5:4:3 ROTARY SCREW COMPRESSOR Rotary air compressors are positive displacement compressors. The most common rotary air compressor is the single stage helical or spiral lobe oil flooded screw air compressor. These compressors consist of two rotors within a casing where the rotors compress the air internally. There are no valves. Precise positioning of the screw is essential for its performance. Oil provides a seal between the rotating screws as well as lubricating the parts and cooling the air. The oil is then separated from the air before it enters the system.




Fig 5.3 Rotary Screw Compressor These units are basically oil cooled (with air cooled or water cooled oil coolers) where the oil seals the internal clearances. Since the cooling takes place right inside the compressor, the working parts never experience extreme operating temperatures. The rotary compressor, therefore, is a continuous duty, air cooled or water cooled compressor package. Because of the simple design and few wearing parts, rotary screw air compressors are easy to maintain, operate and provide great installation flexibility. Rotary air compressors can be installed on any surface that will support the static weight. Rotary Screw Compressors are the most widely applied industrial compressors in the 40 (30kW) to 500 hp (373 kW) range. They are available in both lubricated and oil-free configurations. The popularity of rotary compressors is due to the relatively simple design, ease of installation, low routine maintenance requirements, ease of maintenance, long operating life and affordable cost. Selection of compressors Selection of the air compressor is only the first step in designing an efficient and reliable compressed air system. The air exiting the compressor is saturated with moisture and will have compressor lubricants (lubricated compressors only). Other chemicals that may have been drawn into the compressor intake may also be present. This contamination is harmful to many processes, pneumatic tools, instruments and equipment. Air purification equipment, filters, air dryers, breathing air purifiers, monitoring equipment, used alone or in combination will remove these contaminants. Selection and purchase of the compressor and necessary purification equipment can be easily done based on Standards. 

System flow rate demand


Standby capacity for emergences


Future air requirement

Air capacity rating of compressors Air compressors are generally rated in terms of volume of free air delivered at




actual atmospheric conditions. The equation that allows for this calculation is

5:4:4 VACUUM PUMPS A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. A partial vacuum, or rough vacuum, can be created using a positive displacement pump that transports a gas load from an inlet port to an outlet (exhaust) port. Because of their mechanical limitations, such pumps can only achieve a low vacuum. To achieve a higher vacuum, other techniques must then be used, typically in series (usually following an initial fast pump down with a positive displacement pump). These will be connected with suitable devices to perform material handling, clamping, sealing and vacuum forming operations. In material handling it is used for lifting glass plates, sheet metals, paper and flooring materials. The weight lifted will be based on the available vacuum.

5:5 PNEUMATIC ACTUATORS Actuating cylinders for pneumatic and hydraulic systems are similar in design and operation. For pneumatic cylinder applications light weight alloy materials are used for the construction of components. A pneumatic cylinder converts the energy of the compressed air into linear motion which extends or retracts the piston rod. Pneumatic Rotary actuators convert the energy into a rotary motion. Most of them are vane air motor. In Pneumatic semi-rotary actuators limited rotary motion can be achieved by using a rack and pinion into a linear actuator or as seen opposite by a vane mechanism within the body of the cylinder. 5:5:1 PNEUMATIC CYLINDERS There are two main types of pneumatic cylinders, 1. Single acting cylinder- In this type the air pressure acts on only one side of the piston to extend the cylinder, and during retraction force generated by gravity or a spring returns the piston rod to its original state. 2. Double acting cylinder - In this type the compressed air is admitted in both sides of the piston alternately. Work is performed during forward motion as well as backward motion of the piston. .




Fig 5.4 Pneumatic cylinders 5:5:2 PNEUMATIC ROTARY ACTUATORS The purpose of a rotary motor is to convert fluid power into shaft power by forcing the shaft to rotate. Pressure is converted into torque and flow rate is converted into speed. In other words, the faster you push the fluid through the motor, the faster it goes and the harder it is to turn the shaft, the higher the pressure needed to make it go round. The basic symbol is a circle with a white triangular arrow pointing inwards indicating the direction of flow. These may be operated as one way rotation or two ways rotation.

Fig 5.4 Pneumatic rotary actuators symbol Pneumatic semi-rotary actuators

Fig 5.5 Pneumatic semi rotary actuators symbol There is a special motor called a rotary actuator that is designed for slow rotation of less than one revolution but with a large torque. These actuators are commonly used on robotic devices. Pneumatic actuators are used for simple pick and place operations and for opening and closing pipeline valves.




5:6 FRL UNIT FRLs are used in compressed air systems (pneumatic) in various industries for different applications. Filter-Regulator-Lubricator's (FRL) are available in different configurations: •

individual components

combination filter-regulator

two-unit modular combinations

three-unit modular combinations

Filters removes solid contaminants, such as dirt or metal particles, from a liquid or gas (air); or that separates one liquid from another or a liquid from a gas. Regulators govern line pressure changes and deliver a constant outlet pressure to the downstream air components. Lubricators provide a constant oilair mixture to lubricate downstream air equipment for reduced friction and wear. Air Filters The air needs to be filtered to be free of moisture and contamination. Air filter is used to do this job. The filter elements remove the particles and moisture as small as 5 microns.

Fig 5.6 Air filter Air pressure regulator The pressure regulator is used to adjust the desired pressure for the pneumatic system. This use a piston to sense downstream pressure fluctuations. The piston, in turn, works against a set spring pressure. As the pressure downstream drops it is sensed by the diaphragm and the popper valve opens. This adjusts the position of the poppet valve, which limits the downstream pressure to the pre-set valve.




Fig 5.7 Pressure regulator Air lubricator A lubricator ensures proper lubrication of internal moving parts pneumatics components. The proportional increase in oil mist by an increase of air flow is achieved by the spring loaded poppet assembly. As the flow increases and the valve opens, the area is increased and a pressure differential created.

Fig 5.8 Lubricator Pneumatic Silencer To decrease the noise of air in the outlet of valves, a silencer can be used. They are made from the porous plastic or bronze. Some of them are equipped with a control flow valve to control velocity of flow in the outlet of valves as well.




Fig 5.9 Pneumatic silencer

Pneumatic Valves 

The pneumatic valves are similar to hydraulic valves in terms of physical concepts. They are categorized into three classes:

Direction control valves – To determine the path of the fluid through which it should travel within a given circuit. The control of fluid path is carried out by check valves, shuttle valves and two way, three way and four way direction control valves

Pressure control valves – pressure regulator to protect the system against over pressure.

Flow control valves - The control of actuator speeds depends on the flow rates. So to control the actuator speed, the flow rate should be controlled by using flow control valves.

The same symbols as Hydraulic components are used for the pneumatic components. However, it is necessary to use an unfilled arrow marker. 2/2 Pneumatic Valve

3/2 Pneumatic Valve

4/2 Pneumatic Valve

5/2 Pneumatic Valve

5/3 Pneumatic Valve




5.10 Direction control valves All pneumatic direction control valves possess logic characteristics. To obtain logic functions, the valves can be combined appropriately. Actuating systems in the pneumatic valves are same mechanisms that are used to activate the DCV’s. The most common actuating systems are Solenoid and Manual control. 5:7 Microelectronic Control of Fluid Power In many control systems, an electrical output from the controller may be used to operate a solenoid valve and its action used to produce a pneumatic or hydraulic signal to actuate some device.

Solenoid Valve A solenoid valve is an electromechanical valve for use with liquid or gas controlled by running or stopping an electric current through a solenoid, which is a coil of wire, thus changing the state of the valve. When a current is applied to the solenoid, the shuttle moves to the right. This changes the ports through which the air flows. Before the solenoid is activated, the air pressure from the supply enters the valve and exits through the output port. When the solenoid is activated, the air supply is switched off and the output port is connected to exhaust port. When the solenoid current is switched off, the spring returns the valve to its original position. Thus, for the form of valve shown, the current to the solenoid switches the air pressure to the output port from on to off.

5.11 Solenoid valve




Solenoid valves are the most frequently used control elements in fluidics. Their tasks are to shut off, release, dose, distribute or mix fluids. Solenoids offer fast and safe switching, high reliability, long service life, good medium compatibility of the materials used, low control power and compact design. Besides the plunger-type actuator which is used most frequently, pivoted-armature actuators and rocker actuators are also used.

Servo Valves

Servo valves are is widely used in closed loop electro-hydraulic control. Closed loop means the operating conditions are constantly being monitored using a feedback device and the difference between the desired result and the actual result are constantly being compared- and corrected for. The Two-Stage Flapper-nozzle valve is what is most commonly used in the industry today. The electrical current creates a magnetic force twisting the torque motor. This moves the flapper that creates a pressure difference on the two ends of the spool. The spool moves and this bends the feedback spring, which pulls the flapper back to center. Therefore the more current applied to the torque motor the more the feedback spring will bend and the further the spool will move. Any contamination in the fluid will increase the amount of wear between the ball at the end of the feedback spring and the spool that will reduce the accuracy of the valve. The speed of the fluid across the nozzles can reach supersonic velocities. Any contamination in the fluid will also wear the nozzles and that will affect the null condition.

5.12 Servo valve PLC - Programmable Logic controller 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 I. Central processing unit (CPU) with an associated memory 2. Input modules 3. Output modules

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. RAM (Random access memory), ROM (Read only memory), PROM (Programmable read only memory), EPROM (Erasable PROM), EEPROM (Electrically erasable PROM). Programming the PLC PLC is programmed by means of a programming device. The programming device is usually detachable from the PLC and it can be shared between the different controllers. Different devices ranging from simple teach pendent type devices to special programmed keyboards and CRT displays are adopted. Most of the programming methods used today for PLCs are based on the ladder logic diagram. There are various approaches for entering the program into the PLC. 1. Ladder diagram based 2. Low-level language based on Boolean expressions 3. Functional blocks 4. High-level languages PLC Operation In the PLC, the program steps defined by the ladder diagrams are executed simultaneously and continuously. First the inputs to the PLC are sampled by the processor and the contents are stored in




memory. Next the control program is executed. The input values stored in memory are used in the control logic calculations to determine the value of the output. Finally the outputs are updated to agree with the calculated values. This cycle consisting of reading inputs, executing the control program and revising the outputs is referred to as a scan.

5:8 SUMMARY Pneumatic systems are widely used in almost all industries. Implementation of a sequence of actions by a full pneumatic circuit is widely used in industries. Many industries like automotive, food, chemical and so on need to use a full pneumatic system due to the hazardous area in the site and limitation of use of the electricity in the system. In the contemporary industrial world, fluid power, particularly the hydraulics branch of it, is a magic world for energy transmission. The application of fluid power is causing many positive changes in the world around us. The application of hydraulic control and drive systems has resulted in new designs and improved efficiency for machines and installations.





Answer the following questions 1. The fluid power system when loaded excessively, stall without damage. 2. Balanced vane pump can be designed only as fixed displacement pump.


3. What are the functions of hydraulic oil?


Answer the following questions 1. What is the effect of increasing the oil flow rate in the system? 2. Write down the specification of any hydraulic pump? 3. Volumetric efficiency piston pumps have higher than the other pump types. TRUE / FALSE

UNIT QUESTIONS 1. Define fluid power and give its merits and demerits. 2. Briefly explain the pascal’s law.


FLUID MECHANICS & PNEUMATICS 3. Discuss the properties of hydraulic fluids. 4. How pumps are classified? 5. Explain the working of axial piston pump with a neat sketch. 6. Explain the working of external gear pump with a neat sketch. 7. Explain the working of internal gear pump with a neat sketch 8. Explain the working of unbalanced vane pump with a neat sketch. 9. Explain the working of balanced vane pump with a neat sketch. .

ANSWERS OF SELF ASSESSMENT QUESTIONS: I: 1. True. 2. Heat description, lubrication and seating. 3. Actuator speed increases. II: 1. Yes. 2. Flow rate (Q), generating performance (P), and speed (N). 3. True.



Answer the following questions 1. The main two types of hydraulic actuators are ………………and…………….. 2. What is a double acting cylinder? 3. What are the specifications of a hydraulic cylinder?






Answer the following questions 1. In case of vane motor springs are used to hold the vanes initially against the caring contour TRUE / FALSE 2. In a hydraulic motor rapid change of direction of rotation is made easier. 3. What are the specifications of a hydraulic motor?

UNIT QUESTIONS 10. Define fluid power and give its merits and demerits. 11. Explain the working of gravity return single acting cylinder with a neat sketch. 12. Explain the working of spring return single acting cylinder with a neat sketch. 13. Explain the working of double acting cylinder with a neat sketch. 14. Explain the working of telescopic cylinder with a neat sketch. 15. Explain the working of gear motor with a neat sketch 16. Explain the working of vane motor with a neat sketch. 17. Explain the working of piston motor with a neat sketch. 18. Explain the working of rack and pinion rotary actuator with a neat sketch. 19. Define the terms overall efficiency and volumetric efficiency.

ANSWERS OF SELF ASSESSMENT QUESTIONS: I: 1. Hydraulic cylinder of hydraulic motor. 2. Oil pressure acting on both sides of Piston. 3. Cylinder bore diameter, piston rod diameter and stroke length. II: 1. True 2. Yes 3. Torque, speed, displacement and pressure.







Answer the following questions 1. Directional control valves used for changing the direction of motion of hydraulic actuator YES / NO 2. What the check valve?

3. What are the ways in which the valve spool is made to move inside the valve body in a DCV?


Answer the following questions 1. The two types of flow control valves are ……………..and…………………….. 2. Pressure reliving valve maintains reduced pressures in specific location of hydraulic systems. TRUE / FALSE

3. What is the cushioned cylinder?

UNIT QUESTIONS 20. What are the different types of valves used in hydraulic systems? 21. Explain the working of check valve with a neat sketch. 22. Explain the working of shuttle valve with a neat sketch. 23. Explain the working of pilot operated pressure relief valve with a neat sketch. 24. Explain the working of sequence valve with a neat sketch. 25. Explain the working of pressure compensated flow control valve with a neat sketch.




26. What are cushioned cylinders? Explain with a neat sketch. 27. Explain the working of balanced spool flow divider with a neat sketch. 28. Explain the working of rotary flow divider with a neat sketch.

ANSWERS OF SELF ASSESSMENT QUESTIONS: I: 1. Yes. 2. It is a directional control valve which permits oil flow id along one direction. 3. Manual operation, pilot operation, solenoid operation.

II: 1. Non compensated FCV and pressure compensated flow control valves. 2. True. 3. In which the speed of movement piston in the cylinder is reduced nearly at the end of strokes of the cylinder. .



Answer the following questions 1. Dead weight accumulators are not suitable for mobile application.


2. The capacity of hydraulic reservoir is normally 3 to 4 times the pump discharge (Q) TRUE / FALSE 3. What ate the factors to be considered for selection of seals?





Answer the following questions 1. Name the speed control circuits. 2. Seals are used in fluid power system components to prevent leakages.


3. Pressure intensifier is used when a greater force is needed for a relatively smaller distance. TRUE/ FALSE

UNIT QUESTIONS 29. What are the different types of accumulators? 30. Explain the working of diaphragm type accumulator with a neat sketch. 31. Explain the working of spring loaded type accumulator with a neat sketch. 32. Explain the working of dead weight accumulator with a neat sketch. 33. What are heat exchangers? Explain their importance. 34. Explain the working of pressure gauge with a neat sketch. 35. What are flow meters? Explain. 36. Discuss the different types of seals used in hydraulic systems. 37. Explain the working of counter balance circuit with a neat sketch. 38. Explain the working of sequence valve application circuit with a neat sketch. 39. What are the types of speed control circuits? Explain. 40. Explain the working of intensifier application circuit with a neat sketch.





I: 1. Yes 2. True 3. Flow quantity, Speed, Operating pressure, temperature and compatibility . II: 1. Meter in, meter out, and bleed off 2. Yes 3. True. UNIT-V


Answer the following questions 1. What are the specifications of a compressor? 2. Intercooler is used in two stage compressors to cool the compressed air after first stage of compressors.


3. Rotary air compressors are positive displacement compressors. TRUE/FALSE


Answer the following questions 1. Solenoid valves are extensively used in electro pneumatic circuits 2. What is a servo valve? 3. Give the applications of electronic control of fluid power

UNIT QUESTIONS 41. What are the differences between hydraulic and pneumatic systems?



FLUID MECHANICS & PNEUMATICS 42. How the compressors are classified? 43. Explain the working of two stage piston compressor with a neat sketch. 44. Explain the working of rotary vane compressor with a neat sketch. 45. Explain the working of rotary screw compressor with a neat sketch. 46. What are vacuum pumps? Explain. 47. Explain the working of double acting pneumatic cylinder with a neat sketch. 48. Explain the working of gear motor with a neat sketch. 49. What is FRL? Explain. 50. Discuss about electronic control of fluid power. ANSWERS OF SELF ASSESSMENT QUESTIONS: I: 1. Capacity of air intake, volumetric efficiency, horse power and reservoir capacity. 2. Yes 3. True. . II: 1. Yes 2. Valves are capable of receiving a feed back and adjusting the output to the desired level. 6. CNS Machines, robotics etc.

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IMTS Mechanical Eng. (Fluid mechanics & pneumatic)  

IMTS Mechanical Eng. (Fluid mechanics & pneumatic)

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