ME 6021 - HYDRAULICS AND PNEUMATICS / UNIT II - HYDRAULIC SYSTEM AND COMPONENTS

ME6021 –
Hydraulics &
Pneumatics
Unit II - Hydraulic System
and Components
Kinathukadavu, Coimbatore.
Prepared By:
Santhosh Kumar.B, M.E.,
Asst.Prof/Dept. of Mech. Engg.

Syllabus.
UNIT II HYDRAULIC SYSTEM AND COMPONENTS
Sources of Hydraulic power: Pumping Theory – Pump Classification-
Construction, Working, Design, Advantages, Disadvantages, Performance, Selection
criterion of Linear, Rotary- Fixed and Variable displacement pumps, Hydraulic
Actuators: Cylinders – Types and construction, Hydraulic motors Control
Components: Direction control, Flow control and Pressure control valves- Types,
Construction and Operation- Applications – Types of actuation. Accessories:
Reservoirs, Accumulators, Intensifiers, Pressure Switches- Applications- Fluid Power
ANSI Symbol.
Hours Required : 13
19-Jul-17B.Santhosh Kumar, M.E., - A.P/Mech - Sri Eshwar College of Engg. - CBE.

Hydraulic Pump.
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.
A pump is a device that moves fluids (liquids or gases), or sometimes
slurries, by mechanical action.

Pumping Theory.
The basic operating principle that moves fluid through a pump is similar
in all pumps.
– Enlarging the volume of a chamber allows fluid to enter the pump
– Reducing the chamber volume moves fluid to the system.
– Inlet and discharge valves or ports control fluid movement through the pump.

Pump Classifications.
There are two broad classifications of pumps as identified by the fluid
power industry. They are described as follows.
– Hydrodynamic or non-positive pumps
• They are used for low-pressure, high-volume flow applications.
• Normally their maximum pressure capacity is limited to 250-300 psi.
– Hydrostatic or positive pumps (Gear, vane, piston pumps)
• High pressure capability (up to 10,000 psi or higher)
• Small compact size
• High volumetric efficiency

Pump Classifications. (Cont…)
A positive displacement pump makes a fluid move by trapping a fixed
amount and forcing (displacing) that trapped volume into the discharge pipe.
Some positive displacement pumps use an expanding cavity on the suction side
and a decreasing cavity on the discharge side.
A non-positive-displacement pump. produces a continuous flow.
However, because it does not provide a positive internal seal against slippage,
its output varies considerably as pressure varies. Centrifugal and
propeller pumps are examples of non-positive-displacement pumps.
.

Centrifugal Pump.
Centrifugal pumps are a sub-class of dynamic axisymmetric work-
absorbing turbomachinery, Centrifugal pumps are used to transport fluids by the
conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow.
The rotational energy typically comes from an engine or electric motor. The
fluid enters the pump impeller along or near to the rotating axis and is accelerated
by the impeller, flowing radially outward into a diffuser or volute chamber (casing),
from where it exits.
Common uses include water, sewage, petroleum and petrochemical pumping.
Energy Transfer: the transfer of energy from the shaft to the impellor and from the
impeller to water
Centrifugal Force: the force used to throw the water from the impeller.

Centrifugal Pump. (Cont…)
Three different configurations:
End Suction Centrifugal. – Center of the suction line is centered on the
impeller eye.
Split case pumps. – Volute Case is split horizontally.
Vertical Turbines. – Primarily mounted with a Vertical Shaft.

Axial Flow (Propeller) Pump.
An axial-flow pump, or AFP, is a common type of pump that essentially
consists of a propeller (an axial impeller) in a pipe.
In axial flow centrifugal pumps the rotor is a propeller. Fluid flows parallel
to the axis as illustrated in Figure. Diffusion vanes are located in the discharge
port of the pump to eliminate the rotational velocity of the fluid imparted by
the propeller.
There is also the axial flow centrifugal pump which uses a curved
propeller-shaped impeller, whereas the impeller on a radial flow centrifugal
pump looks more like a fan. Axial flow pumps move fluid by drawing fluid into
their axis and using the impeller to send fluid out on the other side of the pump

Gear Pump.
A gear pump (Fixed Displacement only by Geometrical Necessity) uses
the meshing of gears to pump fluid by displacement. They are one of the most
common types of pumps for hydraulic fluid power applications. Gear pumps are
also widely used in chemical installations to pump high viscosity fluids.
Types:
Internal (Gerotor) Gear Pump.
External Gear Pump.
Lobe Pump.
Screw Pump.

Vane Pump.
A rotary vane pump is a positive-displacement pump that consists
of vanes mounted to a rotor that rotates inside of 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.
Types:
Balanced Vane Pump. ( Fixed Displacement only)
Unbalanced Vane Pump. (Fixed or Variable Displacement)

Piston Pump.
Piston pumps (Fixed or Variable Displacement) and plunger pumps use a
mechanism (typically rotational) to create a reciprocating motion along an axis,
which then builds pressure in a cylinder or working barrel to force gas or fluid
through the pump. The pressure in the chamber actuates the valves at both the
suction and discharge points.
Types:
Radial Design.
Axial Design.

Pump Performance.
1. Volumetric efficiency (ᾐvol ):
volumetric efficiency indicates the amount of leakage that takes place
within the pump. This involves considerations such as manufacturing
tolerances and flexing of the pump casing under design pressure operating
conditions:
Volumetric efficiencies typically run from 80% to 90% for gear pumps, 82%
to 92% for vane pumps, and 90% to 98% for piston pumps.
actual flow rate produced by pump
100
flow rate pump should produce
u
theoretical
  
100A
T
Q
Q


Pump Performance. (Cont…)
2. Mechanical efficiency (ᾐmech):
Mechanical efficiency indicates the amount of energy losses that occur
due to reason other than leakage. This includes friction in bearings and
between other mating parts.
It also includes energy losses due to fluid turbulence. Mechanical
efficiencies typically run from 90% to 95%.
theoretical powe required to operate pump
power delivered to pump
pump output power assuming no leakage
m
m
actual
or
input power delivered to pump




100r
m
PQ
TN
  
/1714
100
/ 63,000
T
m
PQ
TN
  

3. Overall efficiency (ᾐoverall):
The overall efficiency considers all energy losses and is defined
mathematically as follows:
volumetric effeciency mechanical efficieancy
efficiency=
100
overall

1
1
/1714100
100
100 100 / 63,000
o m A
o
Q PQ
Q TN
 
   

PUMP
TYPE
PRESSURE
RATING
(PSI)
SPEED
RATING
(RPM)
OVERALL
EFFICIENCY
(PER CENT)
HP
PER LB
RATIO
FLOW
CAPACITY
(GPM)
COST (DOL
LARS PER HP)
EXTERNAL GEAR
INTERNAL GEAR
VANE
AXIAL PISTON
RADIAL PISTON
2000-3000
500-2000
1000-2000
2000-12000
3000-12000
1200-2500
1200-2500
1200-1800
1200-3000
1200-1800
80-90
70-85
80-95
90-98
85-95
2
2
2
4
3
1-150
1-200
1-80
1-200
1-200
4-8
4-8
6-30
6-50
5-35

Pump Selection.
– Select the actuator (hydraulic cylinder or motor) that is appropriate based on the
loads encountered.
– Determine the flow-rate requirements. This involves the calculation of the flow rate
necessary to drive the actuator to move the load through a specified distance within
a given time limit.
– Determine the pump speed and select the prime mover.
– Pump type based on the Application & Cost.
– Select the system pressure. This ties in with the actuator size and the magnitude of
the resistive force produced by the external load on the system.
– Select the reservoir and associated plumbing, including piping, valving, hydraulic
cylinders, and motors and other miscellaneous components.

Hydraulic Actuators.
– An actuator is a component of a machine that is responsible for moving or controlling a mechanism
or system.
– A hydraulic actuator is used for converting hydraulic energy into mechanical energy.
– A hydraulic actuator consists of cylinder or fluid motor that uses hydraulic power to facilitate
mechanical operation. The mechanical motion gives an output in terms of linear, rotatory
or oscillatory motion. As liquids are nearly impossible to compress, a hydraulic actuator can exert a
large force. The drawback of this approach is its limited acceleration.
Types: Linear actuators (also called ‘hydraulic cylinders’).
Rotary actuators (also called ‘hydraulic motors’).

Hydraulic Cylinders.
The hydraulic cylinder is used to convert fluid power into linear mechanical
force and motion.
Applications of hydraulic cylinders.
– The hydraulic cylinders are basically used for performing work such as pushing, pulling,
tilting, and pressing in a variety of engineering applications such as in material handling
equipment, machine tools, construction equipment, and automobiles.
Different types of hydraulic cylinders are
– single acting cylinders.
– Double acting cylinder.
– Telescoping cylinders.

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.
The design of a hydraulic motor and a hydraulic pump are very similar. For
this reason, some hydraulic pumps with fixed displacement volumes may also
be used as hydraulic motors.
A hydraulic motor works the other way round as it converts hydraulic
energy into mechanical energy: a rotating shaft.

Control Components .
One of the most important functions in any fluid power system is control. If
control components are not properly selected, the entire system will fail to deliver
the required output.
Elements for the control of energy and other control in fluid power system are
generally called “Valves”.
The selection of these control components not only involves the type, but also
the size, the actuating method and remote control capability. There are 3 basic
types of valves.
1. Directional control valves.
2. Pressure control valves.
3. Flow control valves.

Directional Control Valve.
Directional control valves control start/stop, directions, and
acceleration/deceleration of hydraulic cylinders and motors. They can be used
in a various applications, and a wide range of products is available.
They can be categorized into three types: spool, poppet, and ball.
The spool type can be either a sliding type or a rotary type. The former is
the most popular for pressure balancing and high capacity.
The poppet type offers excellent leak-tight capability (zero leak) for its
poppet-seat contact.
The ball type is an alternative for the poppet; a ball is used instead of a
poppet.

Classifications of DC Valve.
According to number of working ports :
Two- way valves
Three – way valves
Four- way valves.
According to number of Switching position:
Two – position
Three – position
According to type of construction:
Poppet valves
Spool valves

Designation of DC Valve.
– The designation of the directional control valve refers to the number of
working ports and the number of switching positions.
– A valve with 2 service ports and 2 switching positions is designated as 2 / 2 way
valve.
– A valve with 3 service ports and 2 position is designated as 2 / 3 way valve.
– A valve with 4 service ports and 2 position is designated as 2 / 4 valve.
– A valve with 4 Service ports and 3 Switching position is designated as 3 / 4 way
valve.

2/2 DC Valve.
– The simplest type of directional control valve is a check valve which is a two way
valve because it contains two ports.
– These valves are also called as on-off valves because
– They allow the fluid flow in only in one direction and the valve is normally
closed.
– In Fig, the port P is blocked by the action of spring as the valve is unactuated
(absence of hand force). Hence the flow from port P to A is blocked. When
actuated ( Presence of hand force ) the valve is opened, thereby connecting
port P to A.

2/3 DC Valve.
– A directional control valve primary function is alternatively to pressurize and exhaust one
working port is called three-way valve. Generally, these valves are used to operate single-
acting cylinders.
– These valves are normally closed valves ( i.e. the pump port is blocked when the valve is not
operating ). The three-way valve ports are inlet from the pump, working ports , and exhaust to
tank.
– Spool position 1: When the valve is actuated, the spool moves towards left . In this position
flow from pump enters the valve port P and flows out through the port A as shown by the
straight- through line and arrow ( fig a). In this position, port T is blocked by the spool.
– Spool position 2: When the valve is un-actuated by the absence of hand force, the valve
assumes this position by the action of spring In this position, port P is blocked by the spool.
Flow from the actuator can go to the tank from A to T as shown by straight – through line and
arrow.

2/4 DC Valve.
– These valves are also used to operate double acting cylinder. These valves are
also called as impulse valve as 2 / 4 DCV has only two switching positions, i.e it
has no mid position.
– These valves are used to reciprocate or hold and actuating cylinder in one
position.
– They are used on machines where fast reciprocation cycles are needed.
– Since the valve actuator moves
– such a short distance to operate the valve from one position to the other, this
design is used for punching, stamping and for other machines needing fast
action. Fig (a) and (b) shows the two position of 2 / 4 DCV

3/4 DC Valve.
– These type of DCV consists of three switching position.
– Most three- position valves have a variety of possible flow path configurations,
but has identical flow path configuration in the actuated position and different
spring centered flow paths.
– It should be noted that a three-position valve is used whenever it is necessary
to stop or hold a actuator at some intermediate position within its stroke range,
or when multiple circuit or functions must be accomplished from one hydraulic
power source.

3/4 DC Valve. (Cont…)
Types:
 Open center 3 / 4 DCV
 Closed Center 3 / 4 DCV
 Tandem centered 3 /4 DCV
 Regenerative Center 3 / 4 DCV
 Floating Center 3 / 4 DCV

3/4 DC Valve. – Open Centered.
– open- center circuit, all ports are open to each other in the center position.
– When the valve is in open center position, the pump flow is directed to the tank
at atmospheric pressure.
– At the same time the actuator can be moved freely by applying an external
force. Open center valve help to prevent heat buildup, and no work can be done
by any part of the system.

3/4 DC Valve. – Close Centered.
– The working of the valve is similar to open center DCV and in actuated position
the port connection is identical.
– In closed center DCV all ports are closed to each other. Hence the actuator
connected to ports A and B is hydraulically locked and cannot be moved by an
external force.
– In this position the pump flow must go over the relief valve when flow is not
being used for any other parts of the circuit.
– This promotes oil oxidation , viscosity drop, which further raises the wear of
parts and increased leakage.

3/4 DC Valve. – Tandem Centered.
– In the center configuration, the working ports A and B are blocked , and the
pump port P is connected to tank port T.
– The tandem center also results in a locked actuator. However, it also unloads
the pump at atmospheric pressure.
– The application of this design may be to hold a cylinder or fluid motor under
load or to permit the pump flow to be connected to a series of valves for
multiple circuitry.

3/4 DC Valve. – Regenerative.
– This is another type of common center configuration used in hydraulic circuits.
Regenerative means the flow generated from the system itself.
– Regenerative center is used whenever the actuator movement in one direction
requires two different speeds eg., Part of length of extending stroke of actuator
requiring fast movement during no load condition and remaining length slow
controlled motion.
– During fast extending the DCV is un-actuated thereby by spring forces it comes
to the mid position. This center saves on additional pump capacity required.

3/4 DC Valve. – Regenerative. (Cont…)
– Fig. shows double acting single rod cylinder connected to 3 / 4 DCV having
regenerative center.
– The piston area(A) is more than ring area (B) . When the cylinder is actuated by
this type of DCV , in the center configuration, the pump flow is directed towards
A, since the piston area is more it offers least resistance( fluid flows always in
the path of least resistance ) and hence the fluid flows towards A.
– The return flow from B also joins the pump flow, which increased the piston
speed due to increased flow.

3/4 DC Valve. – Floating Centered.
– In this position the pump port is blocked and the two working ports A and B are
connected to tank port T.
– Since the working ports A and B are connected to tank T, the actuator can be
moved freely without any external force and hence the name floating.

2/2 DC Valve. (Poppet design).
– It is essentially a check valve as it allows free flow of fluid only in one direction
(P to A) as the valve is opened hydraulically and hence the pump Port P is
connected to port A.
– In the other direction the valve is closed by the ball poppet ( note the fluid
pressure from A pushes the ball to its seat) and hence the flow from the port A
is blocked

Actuation Mechanisms.
– DC Valves may be actuated by a variety of methods. Actuation is the method of
moving the valve element from one position to another.
– Types:
– Manually operated.
– Mechanical Actuated.
– Solenoid-actuated.
– Hydraulic actuated.
– Pneumatic actuated.

Actuation Mechanisms. ( Cont…)
Manually operated.
– A manually actuated DCV uses muscle power to actuate the spool. Manual
actuators are hand lever , push button, pedals.

Mechanical Actuated.
– The DCV spool can be actuated mechanically, by roller and cam, roller and
plunger. The spool end contains the roller and the plunger or cam can be
attached to the actuator (cylinder).
– When the cylinder reaches a specific position the DCV is actuated. The roller
tappet connected to the spool is pushed in by a cam or plunger and presses on
the spool to shift it either to right or left reversing the direction of flow to the
cylinder.

Solenoid Actuated.
– When the electric coil (solenoid) is energized, it creates a magnetic force that
pulls the armature into the coil.
– This caused the armature to push on the spool rod to move the spool of the
valve
– The advantage of a solenoid lies with in its less switching time

Actuation Mechanisms. ( Contd…)
Hydraulic Actuated. (Pilot Actuated)
– The hydraulic pressure may directly used on the end face of the spool . The
pilot ports are located on the valve ends.
– Fluid in the Y end (right end ,not shown in the figure) is passed through the
adjustable needle valve and exhausted back to tank. The amount of fluid bled
through the needle valve controls how fast the valve will shift

Actuation Mechanisms. ( Contd…)
Pneumatic Actuated.
– When air is introduced through the left end passage ( X), its pressure pushes
against the piston to shift the spool to the right.
– Removal of this left end air supply and introduction of air through the right end
passage (Y) causes the spool to shift to the left
– Unshaded arrow represent pneumatic signal .

Pressure Control Valve.
– These are the units ensuring the control of pressure.
– A throttling orifice is present in the valve and by variation of orifice, the pressure level
can be controlled or at a particular pressure, a switching action can be influenced.
– Basically one differentiates between pressure regulating and pressure switching valves.
– Pressure regulation valves are for maintaining a constant pressure in a system. Pressure
switching valves, apart from a definite control function they also perform a switching
action.
– Such valves not only provide a switching signal, as in the case of pressure switches, but
also operate themselves as a DCV type of switching within the hydraulic system.

Pressure Control Valve. (Cont…)
Opening and Closing Pressure Difference:
– The minimum pressure at which the valve action starts is called as the opening or
cracking pressure. The difference between the cracking pressure (commencement of
flow) and the pressure obtained at maximum flow ( normal flow without change of
spring force ) is referred as the “opening pressure difference”.
– Similarly the difference between the pressure corresponding to nominal flow and
no flow during closing of the valve is referred as “closing pressure difference”. This is
larger than the opening due to the flow forces acting in the opening direction as also the
hysterisis in the spring.

Pressure Control Valve. (Cont…)
Classifications of PC Valve
– Pressure Relief valve
– Unloading Valve
– Sequence valve
– Counterbalance Valve
– Pressure Reducing Valve

Pressure Relief Valve.
– Its primary function is to limit the system pressure.
– It is normally a closed valve whose function is to limit the pressure to a specified
maximum value by diverting pump flow back to the tank.
– The poppet is held in position by spring force plus the dead weight of spool.
– When pressure exceeds this force, the poppet is forced off its seat and excess fluid in the
system is bypassed back to the reservoir

Unloading Valve.
– A unloading valve is used to permit a pump to operate at minimum load.
– It operates on the principle that pump delivery is diverted to the tank, when sufficient
pilot pressure is applied to move the spool against the spring force.
– The valve is held open by pilot pressure until the pump delivery is again needed by the
circuit.
– The pilot fluid applied to move the spool upwards becomes a static system.
– In other words, it merely pushes the spool upward and maintains a static pressure to
hold it open. When the pilot pressure is relaxed, the spool is moved down by the spring,
and flow is diverted through the valve into the circuit.

Sequence Valve.
– A sequence valve’s primary function is to divert flow in a predetermined sequence.
– A sequence valve may be direct or remote pilot- operated. These valves are used to control
the operational cycle of a machine automatically.
– It consists of 2 ports, one main port connecting the main line and other (secondary port)
connected to the secondary circuit. Usually the secondary port is closed by the spool.
– The pressure is effective on the end of the spool. This pressure will urge the spool against the
spring force and at the preset value of the spring it allows a passage from the primary to the
secondary port.
– For remote operation it is necessary to close the passage used for direct operation by plugging
and provide a separate pressure source as required for the operation of the spool in the
remote operation mode.

Counterbalance Valve.
– A Counterbalance valve is used to maintain back pressure to prevent a load from failing.
– Application in vertical presses, lift trucks, loaders and other machine tool that must position
or hold suspended loads. valve is used to maintain back pressure to prevent a load from
failing.
– It has two ports, one connected to load and the other to the tank A small opening connecting
the tank is provided in the control chamber to drain the oil that may collected due to leakage,
thereby preventing the failure of valve.
– Counterbalance valve acts on the principle that fluid is trapped under pressure until pilot
pressure.
– counterbalance valve is normally closed valve and will remain closed until acted upon by a
remote pilot pressure source. Therefore, a much lower spring force is sufficient to allow the
valve to operate at a lesser pilot pressure.

Pressure Reducing Valve.
– Pressure reducing valve is used to limit its outlet pressure. Reducing valves are used for
the operation of branch circuits, where pressure may vary from the main system
pressures.
– it can be visualized that if the spring has greater force, the valves open wider and if the
controlled pressure has greater force, the valves moves towards the spring and throttles
the flow.

Flow Control Valve.
– Flow Control Valves are used to regulate the rate of flow to the actuators.
– Control of flow is extremely important because the speed of the hydraulic actuator
depends on the rate of flow of the fluid.
– Types:
– Non Pressure Compensated FC Valve.
– Pressure Compensated FC Valve.

Non Pr. Compensated FC Valve.
– Non-pressure-compensated flow-control valves are used when the system pressure is
relatively constant and motoring speeds are not too critical.
– Basic Principle is the rate of flow through an orifice depends on the pressure drop across
it.
– Disadvantages: The inlet pressure is the pressure from the pump that remains constant.
Therefore, the variation in pressure occurs at the outlet that is defined by the work load.
This implies that the flow rate depends on the work load. Hence, the speed of the piston
cannot be defined accurately using non-pressure-compensated flow-control valves when
the working load varies.

Pr. Compensated FC Valve.
– Pressure-compensated flow-control valves overcome the difficulty caused by non-
pressure-compensated valves by changing the size of the orifice in relation to the
changes in the system pressure.
– If the pressure drop across the valve increases, that is, the upstream pressure increases
relative to the downstream pressure, the compensator spool moves to the right against
the force of the spring. This reduces the flow that in turn reduces the pressure drop and
tries to attain an equilibrium position as far as the flow is concerned.

Fluid Power Accessories.
– Reservoirs
– Accumulators
– Intensifiers
– Pressure Switches

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.
– The purpose of the hydraulic reservoir is to hold a volume of fluid, transfer heat
from the system, allow solid contaminants to settle and facilitate the release of
air and moisture from the fluid. The hydraulic pump transmits mechanical
energy into hydraulic energy.

Hydraulic Accumulators.
– A hydraulic accumulator is a pressure storage reservoir in which a non -
compressible hydraulic fluid is held under pressure that is applied by an external source.
The external source can be a spring, a raised weight, or a compressed gas.
Types
– Raised weight accumulator
– Compressed-gas accumulator
Bladder type accumulator
Diaphragm type accumulator
Piston type accumulator
Metal bellow type accumulator
– Spring accumulator.

Raised Weight Accumulators.
– A raised weight accumulator consists of a vertical cylinder containing fluid to
the hydraulic line.
– The cylinder is closed by a piston on which a series of weights are placed that
exert a downward force on the piston and thereby energizes the fluid in the
cylinder.
– Gravity acts on the weight to pressurize the hydraulic system fluid, thus storing
energy.

Spring Accumulators.
– It uses the energy stored in springs to create a constant force on the liquid
contained in an adjacent ram assembly.
– The load characteristics of a spring are such that the energy storage depends on
the force required to compress s spring.
– The free (uncompressed) length of a spring represents zero energy storage.
– As a spring is compressed to the maximum installed length, high 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.

Compressed Gas Accumulators.
– It is widely used accumulator in present scenario.
– It is popularly known as “hydro-pneumatic accumulator”. It apply force to the
liquid by using a compressed gas that acts as the spring.
– It uses inert gas (nitrogen) under pressure that provides the compressive force
on fluid.
– Oxygen is not used because oxygen and oil can form an explosive mixture when
combined under pressure
– As the volume of the compressed gas changes the pressure of the gas, and
pressure of the fluid, changes inversely.

C. Gas Accumulators. (Cont…)
BLADDER TYPE ACCUMULATOR
– A bladder accumulator consists of seamless high-pressure cylinder with an internal
elastomeric bladder with pressurized nitrogen on it and hydraulic fluid on the
other(external) side.
– The accumulator is charged with nitrogen through a valve installed on the top. The
accumulator will be pre-charged to nominal pressure when the pumps are not
operating.
– The maximum flow rate of the accumulator is controlled by the opening orifice and
the pressure difference across the opening.
– Bladder material widely used are epichlorohydric rubber(ECO) and Acrylonitrile
butadiene rubber (NBR).

DIAPHRAGM TYPE ACCUMULATOR
– A similar to bladder type, expect an elastomeric diaphragm is used in place of a bag.
– This would typically reduce the usable volume of the accumulator, so the diaphragm
accumulator may not have volume capacity of a bladder accumulator.
– Diaphragm accumulator may be spherical or cylindrical.
– The main difference with bladder accumulator is an increased maximum
compression ratio of approximately 8:1
– It is low weight, compact design and good for shock applications (good response
characteristics)

PISTON TYPE ACCUMULATOR
– This accumulator consists of a cylinder assembly, a piston assembly, and two
end-cap assemblies.
– An accumulator contains a free-floating piston with liquid on one side of the
piston and pre-charged air or nitrogen on the other side.
– An increase of liquid volume decreases the gas volume and increases gas
pressure, which provides a work potential when the liquid is allowed to dis-
charged.

METAL BELLOW ACCUMULATOR
– The metal bellows accumulator is similar to bladder type, expect the elastic is
replaced by a hermitically sealed welded metal bellows.
– Fluid may be internal or external to the bellows.
– It is used when a fast response time is not critical, yet reliability is important.
– Metal bellow types are pre-charged by supplier and then permanently sealed
leading to a maintenance free accumulator.

Pressure Intensifier.
– A hydraulic intensifier is a hydraulic machine for transforming hydraulic power at
low pressure into a reduced volume at higher pressure.
– A hydraulic intensifier is a device which is used to increase the intensity of pressure
of any hydraulic fluid or water, with the help of the hydraulic energy available from a
huge quantity of water or hydraulic fluid at a low pressure.
– In most of the hydraulic machinery used, the usual pressure of 80 to 100-psi may
not be sufficient to operate certain spool valves and other mechanisms.
– To cater to the need for a high pressure requirement for a comparatively short
period of time, pumps and accessories are definitely not the solution.
– The hydraulic intensifiers which can increase the pressure from 100 psi to 40,000
psi, using small volumes of fluid.

Pressure Intensifier. (Cont…)
Types:
There are different types based on the medium of hydraulic fluids used and the number
of strokes used to intensify to the desired pressure. They are
– Single-Stroke.
– Differential Cylinder Intensifiers.
– Oil-Oil Intensifiers.
– Air-Air Intensifiers.
– Oil-Air Intensifiers.
Recent developments are so vast that huge pressures are achieved by using
combinations of the above types.

Pressure Switch.
– A pressure switch is a form of switch that closes an electrical contact when a
certain set fluid pressure has been reached on its input. The switch may be
designed to make contact either on pressure rise or on pressure fall.
– Pressure switches are widely used in industry to automatically supervise and
control systems that use pressurized fluids.
– Another type of pressure switch detects mechanical force. For example, a
pressure-sensitive mat is used to automatically open doors on commercial
buildings. Such sensors are also used in security alarm applications such as
pressure sensitive floors.

Pressure Switch. (Cont…)
– A pressure switch for sensing fluid pressure contains a capsule, bellows, Bourdon
tube, diaphragm or piston element that deforms or displaces proportionally to the
applied pressure.
– The resulting motion is applied, either directly or through amplifying levers, to a set
of switch contacts.
– Since pressure may be changing slowly and contacts should operate quickly, some
kind of over-center mechanism such as a miniature snap-action switch is used to
ensure quick operation of the contacts.
– Hydraulic pressure switches have various uses in automobiles, for example, to warn
if the engine´s oil pressure falls below a safe level, or to control automatic
transmission torque converter lock-up.

ANSI Symbol. (Cont…)
– ANSI or may refer to: American National Standards Institute, a private
nonprofit organization that oversees the development of voluntary consensus
standards. Area of Natural and Scientific Interest.

Thank You…

ME 6021 - HYDRAULICS AND PNEUMATICS / UNIT II - HYDRAULIC SYSTEM AND COMPONENTS

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