Consists of the basic concepts of a Hydraulic System used in industries along with the working of its elements used with well-labeled diagrams and also it has information about the Hydraulic Power Pack and its circuit diagram.

1. Concepts of
Hydraulics System
Prepared by - Ruzaan Karanjia

2. TABLE OF CONTENT
Chapter Number Chapter Particulars
Abstract
1 Introduction
1.1 Working Principle
1.1.1 Major Parts and Components
2 Classification and Working of Hydraulic System's Elements
2.1 Hydraulic Power Generating Elements - PUMPS
2.1.1 Classification of Pumps
2.1.2 Fixed Displacement Pumps
2.1.2.1 Gear Pumps
2.1.2.1A Internal Gear Pump
2.1.2.1B External Gear Pump
2.1.3 Variable Displacement Pump
2.1.3.1 Vane Pump
2.1.3.2 Piston Pump
2.1.3.2A Axial Piston Pump
2.1.3.2B Radial Piston Pump
2.2 Hydraulic Power Controlling Elements - VALVES
2.2.1 Classification of Valves
2.2.1.1 Pressure Control Valve
2.1.1.2 Pressure Relief Valve

3. 2.1.1.3 Pressure Reducing Valve
2.1.1.4 Pressure Sequencing Valve
2.1.1.5 Counterbalancing Valve
2.2.2 Flow Control Valve
2.2.2.1 Diaphragm Flow Control Valve
2.2.2.2 Throttle Flow Control Valve
2.2.3 Direction Control Valve
2.2.3.1 Solenoid Operated Directional Control Valve
2.2.3.2 Number of Positions (Symbol) Based Directional
Control Valve
2.3 Hydraulic Power Utilising Elements - CYLINDERS and
MOTORS
2.3.1 Hydraulic Cylinders
2.3.1.1 Single acting vs Double acting
2.3.2 Hydraulic Motors
2.4 Hydraulic Power Conveying Elements - HOSES, PIPES and
FITTINGS
2.5 Hydraulic Accessories - ACCUMULATORS and HEAT
EXCHANGERS
2.5.1 Accumulator
2.5.2 Heat Exchanger
3 Hydraulic Power Pack
4 Hydraulic System Circuit Diagram
5 References

4. ABSTRACT
Hydraulics is the study of fluids whether in motion or at rest. Hydrodynamics is the study of
fluids in motion, and hydrostatics considers the properties of fluids in static equilibrium
(motionless).
Specifically, Hydraulics is defined in three different concepts which are
1) Applied Science, 2) Fluid Mechanics and 3) Fluid Power
Hydraulics is a topic in applied science and engineering dealing with the mechanical
properties of Liquids.
Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the
engineering uses of fluid properties.
In fluid power, hydraulics is used for the generation, control, and transmission of power by
the use of pressurised liquids.
Concepts from these fields will be used as necessary to understand the operation of
hydraulic devices
1. INTRODUCTION
Specifically, the use of hydraulics (or pressurised liquids) for power transmission will be
considered here. Power transmission is the result of the force of a confined liquid. The
confined liquid merely transmits the force generated by the power supply; the flow
contributes to the other component of work, i.e., displacement.
The amount of work accomplished depends on the overall force and the overall distance to
which it is applied. The power supply may be an electric motor, gasoline engine, or hand
power. Although the liquid must flow to cause motion, its velocity is usually sufficiently low
so as to have only a small kinetic energy component relative to the overall work
accomplished. Some common systems that use hydraulics are hand-operated hydraulic jacks
and presses, power steering and brakes on vehicles.
Hydraulic systems offer many advantages, including a high level of flexibility due to their
compact size per given level of power, the use of small forces to control large forces, their
relatively simple and economical design and operation, and self-lubricating components.
Energy is easily transferred by fluid under pressure instead of cumbersome systems of gears
and chains or pulleys and belts. Vibration is usually minimal in hydraulic systems.
Safe operation of hydraulic systems is important as the high pressure involved is potentially
dangerous. A failure of the system such as the accidental release of the system's oil may lead
to catastrophe, such as when weights fall suddenly.
Fig. 1

5. 1.1 Working Principle
The hydraulic system works on the principle of Pascal's law which says that the pressure in
an enclosed fluid is uniform in all the directions.
Pascal's law is illustrated in the figure.
Fig. 1.1 (a) Fig. 1.1 (b)
The Pressure given by fluid is given by the division of Force and Area of cross-section.
Pressure = Force / Area
1.1.1 Major Parts and Components
Major Parts and Components used in a Hydraulic Systems are as follows:-
a. Hydraulic Power Generating Elements – Pumps.
b. Hydraulic Power Controlling Elements – Valves.
c. Hydraulic Power Utilising Elements – Cylinders and Motors
d. Hydraulic Power Conveying Elements – Hoses, pipes and Fittings.
e. Hydraulic Accessories – Accumulators and Heat Exchangers.
Fig 1.1.1
Above are the images of the types of the components which are used in hydraulic systems.

6. 2. CLASSIFICATION AND WORKING OF HYDRAULIC SYSTEM’S ELEMENTS
2.1 Hydraulic Power Generating Elements - PUMPS
A hydraulic pump is a mechanical source of power that converts mechanical power into
hydraulic energy (hydrostatic energy i.e. flow, pressure). It generates flow with enough
power to overcome pressure induced by the load at the pump outlet. When a hydraulic
pump operates, it creates a vacuum at the pump inlet, which forces liquid from the reservoir
into the inlet line to the pump and by mechanical action delivers this liquid to the pump
outlet and forces it into the hydraulic system.
2.1.1 Classification of Pumps
PUMPS
↓
↓ ↓
Fixed Displacement Variable Displacement
FIXED DISPLACEMENT PUMPS
↓
↓ ↓ ↓
Gear Pump Vane Pump Piston Pump
VARIABLE DISPLACEMENT PUMPS
↓
↓ ↓
Gear Pump Piston Pump
GEAR PUMPS (FIXED AND VARIABLE DISPLACEMENT TYPE)
↓
↓ ↓
Internal Gear Pump External Gear Pump
PISTON PUMPS (FIXED AND VARIABLE DISPLACEMENT TYPE)
↓
↓ ↓
Axial Piston Pump Radial piston Pump

7. 2.1.2 Fixed Displacement Pumps
Fixed Displacement Pumps :-
A hydraulic pump that cannot be adjusted to increase or decrease the amount of liquid that
is moved in one pump cycle.
It is often used in applications with low pressure hydraulic ratings.
It is of three types :- Gear (Internal and External), Vane and Piston (Axial and Radial)
2.1.2.1 Gear Pumps
Gear pumps (with external teeth) (fixed displacement) are simple and economical pumps.
The swept volume or displacement of gear pumps for hydraulics will be between about 1 to
200 millilitres. They have the lowest volumetric efficiency of all three basic pump types
(gear, vane and piston pumps)These pumps create pressure through the meshing of the gear
teeth, which forces fluid around the gears to pressurise the outlet side.
Some gear pumps can be quite noisy, compared to other types, but modern gear pumps are
highly reliable and much quieter than older models. This is in part due to designs
incorporating split gears, helical gear teeth and higher precision/quality tooth profiles that
mesh and unmesh more smoothly, reducing pressure ripple and related detrimental
problems.
2.1.2.1.A Internal Gear Pumps
Internal Gear Pumps consist of an internal and external gear arrangement where internal
gear is called rotor and external gear is called idler or pinion gear.
It also has a crescent / spacer portion which is stationary and is used for compressing or
pressurising the fluid.
External gear is the driving gear and connected with the motor while the internal gear is a
driven gear and for the power transmission they both are in meshing with each other and
the direction of rotation of both the gears is the same, meshing of the gears will only
happen when the flowing fluid comes between the portion of inlet and outlet.
When the external and internal gear rotates in anticlockwise direction, so the meshing is
done and there is no clearance, but near the suction port the clearance between the teeths
of external and internal gear increases which leads to the low pressure zone (-ve pressure)
so suction happens.
Now, after the entering of fluid when it moves ahead it comes towards the crescent / spacer
portion which compresses our fluid and in between the clearance of the gears the
compressed fluid is supplied ahead.
Now at the time of discharge, the clearance between the gears decreases, which develops
high pressure and we get high pressurised discharge fluid because of meshing.
Fig. 2.1.2.1.A

8. 2.1.2.1.B External Gear Pumps
External Gear Pumps consist of an internal and external gear arrangement where internal
gear is called rotor and external gear is called idler or pinion gear.
It also has a casing, casing seal, bushing, suction and discharge port, mounting flange, seal
and drive shaft which is connected with the motor.
Rotor is connected to the drive shaft and drive shaft is connected to the motor, as motor
rotates it drives / rotates the rotor and with the rotor our idler is in mesh, so idler also gets
rotated
On both the sides of gear, either bearings or the bush are provided.
Rotation of both the gears are in opposite directions i.e if one rotates in clockwise direction
then other rotates in anticlockwise direction.
At the suction port, both gears move in different directions causing expansion of fluid and
that fluid is pushed up or moves up in between the gear cavity and pump casing.
And at the discharge port, the compression is happening because due to the meshing of
gears the fluid is forced out from a very narrow space and high pressure is formed.
And also there is no back flow in this type of pump because of very less space between the
teeths and the casing.
Fig. 2.1.2.1.B
2.1.3 Variable Displacement Pumps
Variable Displacement Pumps :-
A hydraulic pump that converts mechanical energy to hydraulic energy as in this the amount
of fluid pumps per revolution of the input can be varied while the pump is running.
It is of two types :- Vane and Piston (Axial and Radial)
2.1.3.1 Vane Pump
A rotary vane pump is a positive-displacement pump that consists of vanes mounted to a
rotor that rotates inside a cavity. In some cases these vanes can have variable length and/or
be tensioned to maintain contact with the walls as the pump rotates. A critical element in
vane pump design is how the vanes are pushed into contact with the pump housing, and
how the vane tips are machined at this very point. Several types of "lip" designs are used,
and the main objective is to provide a tight seal between the inside of the housing and the
vane, and at the same time to minimise wear and metal-to-metal contact. Forcing the vane
out of the rotating centre and towards the pump housing is accomplished using
spring-loaded vanes, or more traditionally, vanes loaded hydrodynamically.

9. In this type of pump the shaft is connected to motor as the motor rotates our shaft will also
rotate so as the rotor which is attached to the shaft will also rotate as with this complete
rotation process our vanes will start to show the up and down motion to maintain contact
with the pump housing with the help of springs and one more important thing that the shaft
is eccentrically placed which causes vanes to come in contact with the housing properly and
also it creates a crescent cavity, now when the rotor rotates it creates vacuum at the inlet
port and therefore the fluid is pumped into the pump inlet then fluid enters in the gap
between the vanes and moves towards the opposite side of the crescent cavity where it gets
squeezed through the discharge holes of the cam as the vanes approaches the crescent
cavity the fluid exits through the discharge port.
Fig. 2.1.3.1
2.1.3.2 Piston Pumps
The piston pump can be defined as it is a positive displacement pump. These pumps use a
piston, diaphragm, or otherwise plunger for moving liquids. These pumps use check valves
as the input and output valves.
They are of two types :-
1. Axial Piston Pump 2. Radial Piston Pump
2.1.3.2.A Axial Piston Pump
An Axial piston pump is a pump that has a number of pistons in a circular array within a
cylinder block.
They are of two types
a. Swash Plate b. Bent Axis
In both cases, the angle of the swash plate or the bent axis mechanism determines the
effective stroke of the piston and angle is given for the purpose of suction and discharge.
a. Swash Plate - Swash plate mechanism is a displacement type mechanism, the
displacement pistons of which are arranged axially to the drive shaft, the reaction
force of piston is carried by the swash plate, the distance travelled by the piston in its
pore in the cylinder block depends on the angle of the swash plate on the respective
side of the piston.
The components of a swash plate mechanism are :-
Flange, Shaft seal, Bearings, Swash plate, Drive shaft, Piston, Cylinder block, Housing
and Port plate.

10. When the shaft is rotated, it will rotate the cylinder block and with that the piston
mounted on the swash plate will also gets rotated and will show up and down
motion, so when the piston reaches to the point where the angle is more of the plate
then the travel of piston will be more in its pore of the cylinder block and it will suck
the oil and then the other half of the rotation the piston reaches at the point where
the angle is less so the travel of the piston is also less and there the piston will be
pushed due to the less space and then the fluid will be discharged with high pressure
Fig . 2.1.3.2.A1
b. Bent Axis - In bent axis piston pump the cylinder block turns with the shaft at an
enough set angle, the piston rods are attached to the drive shaft flanged by the ball
joints.
The components of a swash plate mechanism are :-
Flange, Shaft seal, Retaining rings, Housing, Bearings, Drive shaft, Drive flange, Piston
Axis, Cylinder block, Spherical sliding surface and Port plate.
Inside the cylinder block there is piston, Piston is attached to the lever which is linked
With the drive shaft .
Now, when the shaft is rotated the rotatory in which the cylinder block is there starts
to rotate which allows the pistons to show up and down movement in in the cylinder
block which is set at a particular angle of 30 - 40 degree with the shaft because of the
set angle and locked lever,
Now, suppose the rotatory completes the half rotation,
the piston will move up at less angle with the lever which causes the suction and the
fluid enters. Then after this the rotatory completes the other half of rotation in which
piston will be at the down position as the angle will be more with that of the lever
that causes the liquid to be pressurised by the pistons down movement and from the
discharge port we will get the pressurised fluid.
Fig. 2.1.3.2.A2

11. 2.1.3.2.B Radial Piston Pump
Radial Piston Pump is a form of hydraulic pump. The working pistons extend in a radial
direction symmetrically around the drive shaft, in contrast to the axial piston pump.
It has an eccentric cam follower unit (bearings / rings) to press the piston upward or to
maintain the contact of piston with the housing of our radial piston pump with the spring
arrangement other than this it also has NRV type check valve on the piston head to ensure
the single flow of fluid i.e the fluid enters through the inlet only and discharged from the
outlet only.
Pentagonal eccentric cam is mounted on the drive shaft and the cam will rotate in such a
manner that it will move towards the piston and will push the piston in the cylinder block
and discharge the fluid.
As the cam moves away the spring helps in retracting the piston and causes an intake stroke.
Fig. 2.1.3.2.B
*Now, the thing that comes to our mind is how these pumps can be variable displacement
pumps?
Answer to this is as follows : Piston pumps can be made variable-displacement by inserting
springs inline with the pistons. The displacement is not positively controlled, but decreases
as back-pressure increases, in this we can have a set amount weight attached to in the outlet
line with the check valve which will be attached to the swash plate in this mechanism when
the outlet pressure gets increased with respect to that of set weight then it will pressure the
swash plate to move and hence the angle of swash plate can be changed.
Other than this we can have a screw / bolt mechanism which will be attached to the swash
plate so that by the process of tightening and loosening of the screw we can easily change
the angle of the swash plate.
In axial piston pumps, by varying the stroke length the displacement can be varied . In bent
axis type, by varying the angle between the cylinder block and drive shaft centerline, the
stroke length is varied. In swash plate type by varying the swash plate angle the stroke
length can be varied.*
By this we can attain the variable displacement in the fixed displacement type piston pumps.

12. 2.2 Hydraulic Power Controlling Elements - VALVES
Hydraulic valves are mechanical devices that are used to regulate the flow of fluid within a
hydraulic circuit or system. They can be used to completely close a line, to redirect
pressurised fluid or to control the level of flow to a certain area.
So a hydraulic valve is just a device that opens and closes to allow the flow that will move
actuators and loads. It sounds simple, but there are various techniques used in hydraulics to
allow this to occur. Valves can be mechanically operated (by handle, knob or cam), electric
solenoid-operated, or pilot-operated (air or hydraulic pressure actuates the valve). Some
valves use the pressure of the circuit’s fluid to actuate themselves, like with relief valves.
Valves can also be actuated with cables, levers, plungers, torque motors and so forth.
Hydraulic valves are used in hydraulic power packs to direct the fluid to and from the
cylinder. Hydraulic valves can be used to control the direction and amount of fluid power in a
circuit. So these valves would do it by controlling the pressure and the flow rate in various
sections of the circuit.
2.2.1 Classification of Valves
VALVES
↓
↓ ↓ ↓
Pressure Control Flow Control Direction Control
PRESSURE CONTROL VALVE
↓
↓ ↓ ↓ ↓
Relief Reducing Sequencing Counter balancing
FLOW CONTROL VALVE
↓
↓ ↓
Diaphragm Throttle
DIRECTION CONTROL VALVE
↓
↓ ↓ ↓
Solenoid Inline check Pilot operated check

13. 2.2.1.1 Pressure Control Valve
Pressure Control Valves :-
Valves in a hydraulic system that are used to control the pressure of the fluid flowing
through a pipe.
It is of four types :-
Pressure Relief, Pressure Reducing, Pressure Sequencing and Counterbalancing valves.
2.2.1.2 Pressure Relief Valve
Functioning of a pressure relief valve is similar to the relief valve (whistle) of a pressure
cooker, if pressure inside the system increases above the preset level then this valve opens
to release the working fluid back to the reservoir tank, so the system pressure comes to
normal, It is a close system type valve opens only when it is operated.
The components of a pressure relief valve are as follows :-
Adjusting screw, two supporting plates on either side of the spring and a conical poppet
which is mounted inside of the valve’s body.
When the pressure at the inlet increases above the preset limit, the conical poppet moves
inside against the spring force and creates a passage for working fluid to flow from the
outlet.
The outlet of the relief valve is connected to the reservoir in the fluid power system, so that
working fluid flows to the tank again.
Once the pressure comes to normal the conical poppet sits back on its mean position,
closing the passage from inlet to outlet.
Fig. 2.2.1.1

14. 2.2.1.3 Pressure Reducing Valve
A pressure relief valve is a type of safety valve used to control or limit the pressure in a
system; pressure might otherwise build up and create a process upset, instrument or
equipment failure, or fire.
Pressure reducing valves are used to set the pressure of the hydraulic circuit, below that of
the main circuit.
It reduces the pressure of oil and supplies it to the system, it is used to maintain and control
constant reduced pressure in the system.
This outlet is connected to the system (different from relief valve), it is an open type valve, it
limits the outlet pressure.
The components of a pressure reducing valve are as follows :-
Adjusting screw, a spring and a conical poppet fitted to the diaphragm.
Conical poppet is connected to the spring and spring is connected to the screw where screw
is used to adjust the tension on spring and we are able to set the pressure of the valve.
If the pressure at the outlet increases, the diaphragm deflects upward, due to which the
conical poppet will move upward and will close the passage of the working fluid thus flow
reduces and pressure reduced to normal.
Once the pressure comes to normal the conical poppet sits back on its mean position,
closing the passage from inlet to outlet.
Fig. 2.2.1.2

15. 2.2.1.4 Pressure Sequencing Valve
A sequence valve is a pressure-operated, normally closed, poppet or spool valve that opens
at an adjustable set pressure. Some designs use a spring acting directly on the spool or
poppet, others are pilot-operated. A sequence valve always has an external drain port to
keep from trapping leakage oil.
A sequence valve is used to perform two operations one by one i.e it follows the sequence.
There are times when two or more actuators, operating in a parallel circuit, must move in
sequence.
This setup assures the first actuator has reached a specific location before the next
operation commences.
If there is no safety concern of product damage, if the first actuator does not complete its
cycle before the second starts, a sequence valve can be a simple way to control the actuator
actions.
The components of a pressure sequencing valve are as follows :-
Adjusting screw, a spring and a conical poppet which is mounted inside the valve body.
It also has 1 inlet port and 2 outlet ports.
When the working fluid is supplied to inlet port of sequence valve, it flows directly to outlet
port (1), hence cylinder 1 extends, now at some point the pressure will increase in cylinder 1
and will give back pressure to the conical poppet of sequencing valve and it lifts off from the
seat and allows fluid to flow in cylinder 2, therefore sequencing is achieved between the two
cylinders.
Sequencing valves can allow the working fluid in reverse direction from port (1) but it does
not allow reverse flow from port (2), retraction of cylinder 1 is possible but for cylinder 2 it
cannot be done, so for this to come in action a check valve is essential for reverse flow of
fluid from cylinder 2 to the tank.
In between the outlet (1) and (2) check valve is provided, when the retraction in cylinder 2 is
going on at that time oil is coming in outlet (2) from there it is supplied to the outlet 1 from
there it will go to the tank.
Fig. 2.2.1.3

16. 2.2.1.5 Counter Balancing Valve
A counterbalance, or load-holding, valve is a mechanism typically located near the actuator
that uses hydraulic pressure to keep a load from moving. Generally, these valves have three
related functions: support, control and safety.
When required for load support, these valves prevent a hydraulic actuator under load from
drifting in position. When lowering loads, the counterbalance controls the rate or speed of
motion of the load on the actuator.
Counterbalance valves are hydraulic devices that function using this basic principle: fluid can
freely flow through a check valve into the actuator, and reverse flow will be blocked using a
relief valve until a pre-set pressure is reached that is set based on the system pressure and
load capability. This pressure is higher than the system pressure when the load is applied
and allows the fluid to flow in the opposite direction and the actuator to function. When
pressure is removed, the valve goes below this set value, closes, and the load holds its place.
The preset pressure to the pilot port will determine the direction the load can move. To lift a
load, the valve allows free flow through the check valve, so the cylinder can extend. When
fluid flows to the rod end of the cylinder, this pressure will pilot open the valve, so you can
lower the load. This pressure will decrease if the load starts to run away, and the
counterbalance valve will adjust to match the cylinder speed to the pump flow.
In this illustration, the lines are connected to the hydraulic cylinder and feed the hydraulic
fluid to drive the cylinder in extension or retraction. Fluid supplied to the lower end of the
cylinder provides force to drive the piston to extend the rod and position the crane boom.
Fluid supplied above the piston to the cylinder rod end retracts the piston and rod and
lowers the crane boom. The hydraulic system raises and lowers the crane boom to position
the load over the location the load is to be lifted or lowered. If the hydraulic system has a
failure, the boom will descend and the load would land on whatever it is elevated over,
causing injury and damage to people and property.
Fig. 2.2.1.4

17. 2.2.2 Flow Control Valve
Flow Control Valves :-
Flow control valves are used to regulate the flow rate and pressure of liquids or gases
through a pipeline. The purpose of a flow control valve is to regulate the flow rate in a
specific portion of a hydraulic circuit. In hydraulic systems, they’re used to control the flow
rate to motors and cylinders, thereby regulating the speed of those components. Hydraulic
flow control valves also control the rate of energy transfer at a given pressure. This is based
on the physics concept surrounding work, energy, and power:
Actuator force x distance travelled = work done on load
The energy transfer must be equal to the total work done. Because the actuator speed
determines the rate of energy transfer, speed is a function of the flow rate.
2.2.2.1 Diaphragm Flow Control Valve
Diaphragm valves are characterised by a flexible disc that contacts a seat at the top of the
valve body and forms a seal. The diaphragm is flexible and pressure-responsive; it transmits
force to open, close, or control a valve. While diaphragm valves are related to pinch valves,
they use an elastomeric diaphragm rather than an elastomeric liner in the valve body. The
elastomeric diaphragm is attached to a compressor and separates the flow stream from the
closure element. Diaphragm valves are ideal for handling corrosive, erosive, dirty services.
Fig. 2.2.2.1
2.2.2.2 Throttle Flow Control Valve
The picture below is the axial orifice type throttle valve. The inlet port and outlet port P and
T are drilled on the valve house, there is a axial triangle throttle orifice hole is created on the
top of valve poppet (part no.: 3), the fluid flows into P oil ports and flows out from Toil port
through triangle throttle orifice hole to actuators or to oil tank. By regulating the adjustable
knob to move the valve poppet position axially, which is able to achieve fluid flow rate
adjustment by adjusting the cross sectional area of the throttle port.
Fig. 2.2.2.2

18. 2.2.3 Direction Control Valve
Directional control valves (DCVs) are one of the most fundamental parts of hydraulic and
pneumatic systems. DCVs allow fluid flow (hydraulic oil, water or air) into different paths
from one or more sources. DCVs will usually consist of a spool inside a cylinder which is
mechanically or electrically actuated. The position of the spool restricts or permits flow, thus
it controls the fluid flow.
The spool (sliding type) consists of lands and grooves. The lands block oil flow through the
valve body. The grooves allow oil or gas to flow around the spool and through the valve
body. There are two fundamental positions of directional control valves, namely the normal
position where the valve returns on removal of actuating force and the other is the working
position which is position of a valve when actuating force is applied. There is another class of
valves with 3 or more positions that can be spring centred with 2 working positions and a
normal position.
Directional control valves can be classified according to:
● number of ports
● number of positions
● actuating methods
● type of spool
2.2.3.1 Solenoid Operated Directional Control Valve
Solenoid Type actuation method They are widely used in the hydraulics industry. These
valves make use of electromechanical solenoids for sliding of the spool. Because simple
application of electrical power provides control, these valves are used extensively. However,
electrical solenoids cannot generate large forces unless supplied with large amounts of
electrical power. Heat generation poses a threat to extended use of these valves when
energised over time. Many have a limited duty cycle. This makes their direct acting use
commonly limited to low actuating forces. Often, a low power solenoid valve is used to
operate a small hydraulic valve (called the pilot) that starts a flow of fluid that drives a larger
hydraulic valve that requires more force.
Fig. 2.2.3.1

19. 2.2.3.2 Number of positions (Symbol) based Directional Control Valve
Two-way two-position directional control valve
Gate valve is an example of 2W/2P directional control valve which either turns on or off the
flow in normal or working positions depending on need of application. Here the arrow
indicates that fluid flow is taking place whereas the other position shows cut-off position.
Four-way two-position directional control valve
The 4/2 valve has four connections to it and two valve positions. Normally, one port is open
to flow from the pump.
Four-way three-position directional control valve
It has one way for pump (P), one for reservoir (R) or tank (T) and two for the inlet to the
actuator. And it has 3 positions: one normal, one cross way, and one straight way.
Fig. 2.2.3.1

20. 2.3 Hydraulic Power Utilising Elements - CYLINDERS and MOTORS
2.3.1 Hydraulic Cylinders
A hydraulic cylinder (also called a linear hydraulic motor) is a mechanical actuator that is
used to give a unidirectional force through a unidirectional stroke. It has many applications,
notably in construction equipment (engineering vehicles), manufacturing machinery,
elevators, and civil engineering.
Hydraulic cylinders get their power from pressurised hydraulic fluid, which is typically oil.
The hydraulic cylinder consists of a cylinder barrel, in which a piston connected to a piston
rod moves back and forth. The barrel is closed on one end by the cylinder bottom (also
called the cap) and the other end by the cylinder head (also called the gland) where the
piston rod comes out of the cylinder. The piston has sliding rings and seals. The piston
divides the inside of the cylinder into two chambers, the bottom chamber (cap end) and the
piston rod side chamber (rod end/head-end).
Flanges, trunnions, clevises, and lugs are common cylinder mounting options. The piston rod
also has mounting attachments to connect the cylinder to the object or machine component
that it is pushing or pulling.
A hydraulic cylinder is the actuator or "motor" side of this system. The "generator" side of
the hydraulic system is the hydraulic pump which delivers a fixed or regulated flow of oil to
the hydraulic cylinder, to move the piston. There are three types of pump widely used:
hydraulic hand pump, hydraulic air pump, and hydraulic electric pump.[1]
The piston pushes
the oil in the other chamber back to the reservoir. If we assume that the oil enters from the
cap end, during extension stroke, and the oil pressure in the rod end/head end is
approximately zero, the force F on the piston rod equals the pressure P in the cylinder times
the piston area A.
A hydraulic cylinder has the following parts:
Cylinder barrel, Cylinder base or cap. Cylinder head, Piston, Piston rod, Seal gland and Seals
2.3.1.1 Single acting vs. double acting
Single-acting cylinders are economical and the simplest design. Hydraulic fluid enters
through a port at one end of the cylinder, which extends the rod by means of area
difference. An external force, internal retraction spring or gravity returns the piston rod.
Double acting cylinders have a port at each end or side of the piston, supplied with hydraulic
fluid for both the retraction and extension.
Fig. 2.3.1.1

21. 2.3.2 Hydraulic Motors
A hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into
torque and angular displacement (rotation). The hydraulic motor is the rotary counterpart of
the hydraulic cylinder as a linear actuator. Most broadly, the category of devices called
hydraulic motors has sometimes included those that run on hydropower (namely, water
engines and water motors) but in today's terminology the name usually refers more
specifically to motors that use hydraulic fluid as part of closed hydraulic circuits in modern
hydraulic machinery.
Conceptually, a hydraulic motor should be interchangeable with a hydraulic pump because it
performs the opposite function - similar to the way a DC electric motor is theoretically
interchangeable with a DC electrical generator. However, many hydraulic pumps cannot be
used as hydraulic motors because they cannot be backdriven. Also, a hydraulic motor is
usually designed for working pressure at both sides of the motor, whereas most hydraulic
pumps rely on low pressure provided from the reservoir at the input side and would leak
fluid when abused as a motor
These are of 5 types:-
Gear, vane, gerotor, axial plunger and radial piston type
Gear motors are used in simple rotating systems. Their benefits include low initial cost, high
rpm, higher tolerance to contamination, and durability. Gear motor failures are generally
less catastrophic.
The gerotor motor is in essence a rotor with N-1 teeth, rotating off centre in a rotor/stator
with N teeth. Pressurised fluid is guided into the assembly using a (usually) axially placed
plate-type distributor valve. Several different designs exist, such as the Geroller (internal or
external rollers) and Nichols motors. Typically, the Gerotor motors are low-to-medium speed
and medium-to-high torque.
Fig. 2.3.2

22. 2.4 Hydraulic Power Conveying Elements - HOSES, PIPES and FITTINGS
Hoses, tubing and fittings are the critical elements of all hydraulic systems. They transmit
fluid from the pump to valves, actuators and motors, and generate the force and motion to
make the system work. The importance of selecting the correct hose, tubing and coupling is
what allows a processing system to be repeatable and reliable, while reducing or even
eliminating costly downtime. The correct sizes, materials and configurations are what ensure
system dependability. Proper selection of the hose or tubing is crucial. But not matching it to
the compatible fitting that is specific to the application will only increase the chances of
system failure.
For hose and tubing, first understand the compatibility of the fluid that is to be transferred
with the material of the hose or tube and its required pressure. Consider the media or
material that is to be transferred, the chemical resistance of the hose or tubing, and the
working pressure and temperature. Select hose and tubing that meets the required ratings
for standard operating pressure, burst test and impulse life. Proper hose and tubing
selection lowers cost of ownership and avoids downtime and unscheduled maintenance,
which ultimately maximises uptime and improves ROI of the system.
For the compatible fitting, as with hose and tubing, there are a number of important factors
to consider, including:
● Attachment (i.e., a crimped hydraulic fitting for a hose, and a compression fitting for
tubing)
● Fitting configuration (straight, elbow, tee, etc.)
● Flow
● Compatible material of hose or tubing
● Size of hose or tubing (in some cases consider wall thickness)
● Vibration
● Working pressure (maximum PSI)
Additionally, consider whether an elastomeric seal is to be used, such as an O-ring or gasket.
Critical components in O-ring face seal fittings and most flange assemblies are an
elastomeric seal. The O-ring material selection is dependent on the factors mentioned
above, particularly chemical compatibility of the media being transferred and system
pressure.
Fig. 2.4

23. 2.5 Hydraulic Accessories - ACCUMULATORS and HEAT EXCHANGERS
2.5.1 Accumulators
A hydraulic accumulator is a pressure storage reservoir in which an incompressible hydraulic
fluid is held under pressure that is applied by an external source of mechanical energy. The
external source can be an engine, a spring, a raised weight, or a compressed gas. An
accumulator enables a hydraulic system to cope with extremes of demand using a less
powerful pump, to respond more quickly to a temporary demand, and to smooth out
pulsations. It is a type of energy storage device.
A compressed gas accumulator consists of a cylinder with two chambers that are separated
by an elastic diaphragm, a totally enclosed bladder, or a floating piston. One chamber
contains the fluid and is connected to the hydraulic line. The other chamber contains an
inert gas (typically nitrogen), usually under pressure, that provides the compressive force on
the hydraulic fluid. Inert gas is used because oxygen and oil can form an explosive mixture
when combined under high pressure. As the volume of the compressed gas changes, the
pressure of the gas (and the pressure on the fluid) changes inversely.
For low pressure water system use the water usually fills a rubber bladder within the tank
(pictured), preventing contact with the tank which would otherwise need to be corrosion
resistant. Units designed for high-pressure applications such as hydraulic systems are usually
pre-charged to a very high pressure (approaching the system operating pressure) and are
designed to prevent the bladder or membrane being damaged by this internal pressure
when the system pressure is low. For bladder types this generally requires the bladder to be
filled with the gas so that when system pressure is zero the bladder is fully expanded rather
than being crushed by the gas charge. To prevent the bladder being forced out of the device
when the system pressure is low there is typically either an anti-extrusion plate attached to
the bladder that presses against and seals the entrance, or a spring-loaded plate on the
entrance that closes when the bladder presses against it.
It is possible to increase the gas volume of the accumulator by coupling a gas bottle to the
gas side of the accumulator. For the same swing in system pressure this will result in a larger
portion of the accumulator volume being used. If the pressure does not vary over a very
wide range this can be a cost effective way to reduce the size of the accumulator needed. If
the accumulator is not of the piston type, care must be taken that the bladder or membrane
will not be damaged in any expected over-pressure situation; many bladder-type
accumulators cannot tolerate the bladder being crushed under pressure.
Fig. 2.5.1

24. 2.5.2 Heat Exchangers
ONE of the critical conditioning requirements of hydraulic fluid is that it is maintained at an
optimal operating temperature. As oil temperature drops, the viscosity of the fluid increases,
making it more difficult to pump, creating higher pressure drop and increasing the chance of
cavitation. As oil temperature increases, the viscosity of the fluid decreases, which reduces
lubricity, increases oxidation rate and can cause the fluid to varnish.
Hydraulic systems use heat exchangers to control oil temperature—and therefore
viscosity—within an optimal range, where the fluid has the best combination of properties
useful to the components of the hydraulic system. Although a few hydraulic machines can
make do without external cooling, such as small, low-duty or load-sensing systems, most
require a device to keep oil in its ideal temperature range. This is where heat exchangers
come in.
What a heat exchanger does is self-explanatory. It will use a fluid such as water or air to
transfer heat into or away from hydraulic liquid; very simple. However, the nature in which
heat exchangers transfer heat can vary vastly. Liquid-to-air and liquid-to-liquid are the two
primary types of heat exchangers, and you can imagine they can use air and water,
respectively, to remove heat from a hydraulic system.
Liquid-to-air coolers transfer the heat from the hydraulic fluid through radiation and
convection. The simplest liquid-to-air coolers are radiators that count on the thermal
difference between the hydraulic fluid and the ambient air. The rate in which a heat is
removed from the oil is factored only by the temperature difference between the air and the
oil (higher differential means more cooling) and by the existence of airflow (which is
sometimes likely in a mobile application).
The basic tube and fin cooler is the most economical method of cooling hydraulic fluid, but is
for light duty applications, such as low duty cycle or low horsepower applications. They are
often very small, such as the type used in a vehicle’s transmission fluid cooler, but in
hydraulic applications, they can be sometimes paired with light duty fans to improve
efficiency. This type of cooler is small and light enough to be attached to the back of an
electric motor to take advantage of the motor’s cooling fan. They’re constructed by forming
a copper tube into a snaked web, and then aluminium or copper fins are added to surround
the tubes. Heat energy is imparted from the oil, to the tube wall, to the fins and then to the
air.
Fig. 2.5.2

25. 3. HYDRAULIC POWER PACK
A hydraulic power pack is a stand-alone assembly consisting of a drive motor, hydraulic
pump and hydraulic fluid tank. The drive motor is connected to the hydraulic pump via a
shaft and drives this. The motor and pump can be installed on the tank, as well as inside the
tank in oil. The tank size, withdrawal, feed and critical flow speeds depend on the task and
application, and must be configured accordingly. The extracted volume is determined in
accordance with the size of the consumers and thus the overall need for fluid in the
hydraulic system. The warming of the fluid, as well as potential leakages must be taken into
consideration here. The critical flow speed occurs at the smallest diameter. The pump is
driven by the electric drive and converts the electrical energy into hydraulic energy. This is
also referred to as hydraulic pressure being created and a flow rate being provided. Using
connections enables this to be used for drive-in machines.
Further components can also be installed directly on the power pack:
● Valves
● Filter
● Oil cooler
● Pressure-limiting valve
● Sensor technology
Depending on the design, a distinction is made between standard hydraulic power packs, oil
immersed hydraulic power packs and compact hydraulic power packs (also mini power
packs). In the case of oil immersed hydraulic power packs, the electric drive is integrated
directly in the tank (Submerged motor). This enables particularly small exterior dimensions
to be achieved for the power pack. As the electric drive is in the hydraulic oil, the lost heat
from the motor is transferred to the oil. If the switch-on duration is too long, this could
result in an impermissibly high oil temperature. In the case of a compact hydraulic power
pack, the electric drive is mostly flange-mounted on the outside of the tank.
Fig. 3

26. 4. HYDRAULIC SYSTEM CIRCUIT DIAGRAM
Basic Hydraulic Circuit Diagrams :-
Fig. 4 a
Fig. 4 b

27. 5. REFERENCES
1. Afanasyev V V (1968) Variations of the effective areas of diaphragms. In:
Aizerman M A (ed) Pneumatic and hydraulic control systems. Pergamon Press,
Oxford London Edinburgh New York,
2. Barker H F (1976) Design of a state observer for improving the response of a
linear pneumatic servo-actuator. Proc Int Conf on Hydraulics, Pneumatics and
Fluidics in Control and Automation, Toronto,
3. Beater P (2000) Modelling and digital simulation of hydraulic systems in design
and engineering education using Modelica and HyLib. Proc Modelica Workshop
2000, Lund, pp
4. Howe R E (2004) Five myths of pneumatic motion control. Hydraulics and
pneumatics 36(9):
5. Jelali M, Kroll A (2002) Hydraulic servo-systems – modelling, identification and
control. Springer, London Berlin Heidelberg New York
6. https://www.machinerylubrication.com/Read/277/hydraulic-systems-fluid
7. http://www.pressmaster-hydraulic-presses.com/news/12-fascinating-facts-about-hyd
raulics-81.aspx
8. 2. http://www.rkmachinery.ca/news/10-fascinating-facts-about-hydraulics-76.aspx
9. http://www.bbc.co.uk/schools/gcsebitesize/science/triple_aqa/using_physics_make_
things_work/hydraulics/revision/3/
10. http://en.m.wikipedia.org/wiki/Hydraulic_brake