ME8694 -

Hydraulics
and
Pneumatics

Unit II - HYDRAULIC
ACTUATORS AND CONTROL
COMPONENTS
 Syllabus.
UNIT II HYDRAULIC ACTUATORS AND CONTROL COMPONENTS
Hydraulic Actuators: Cylinders – Types and construction, Application, Hydraulic
cushioning – Hydraulic motors - Control Components : Direction Control, Flow
control and pressure control valves – Types, Construction and Operation – Servo and
Proportional valves – Applications – Accessories : Reservoirs, Pressure Switches –
Applications – Fluid Power ANSI Symbols – Problems.

Hours Required : 9

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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.

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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.

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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

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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.
.

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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.

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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.

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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

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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.

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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)

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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.

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Pump Performance.
QA
1. Volumetric efficiency (ᾐvol ): QT
 100

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

u   100
theoretical flow rate pump should produce

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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
m  PQT /1714 PQr
actual power delivered to pump m   100 m   100
TN / 63,000 TN
or
pump output power assuming no leakage
m 
input power delivered to pump

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Pump Performance. (Cont…)

3. Overall efficiency (ᾐoverall):

The overall efficiency considers all energy losses and is defined
mathematically as follows:

volumetric effeciency  mechanical efficieancy

overall efficiency=
100

om QA 100 PQ1 /1714

o    100
100 Q1 100 TN / 63,000

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Pump Performance. (Cont…)

PRESSURE SPEED OVERALL HP FLOW

PUMP RATING RATING EFFICIENCY PER LB CAPACITY COST (DOL
TYPE (PSI) (RPM) (PER CENT) RATIO (GPM) LARS PER HP)

EXTERNAL GEAR 2000-3000 1200-2500 80-90 2 1-150 4-8

INTERNAL GEAR 500-2000 1200-2500 70-85 2 1-200 4-8

VANE 1000-2000 1200-1800 80-95 2 1-80 6-30

AXIAL PISTON 2000-12000 1200-3000 90-98 4 1-200 6-50

RADIAL PISTON 3000-12000 1200-1800 85-95 3 1-200 5-35

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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.

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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’).

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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.

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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.

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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.

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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.

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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

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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.

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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.

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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.

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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

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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.

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Actuation Mechanisms. ( Cont…)

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

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Actuation Mechanisms. ( Cont…)

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.

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Actuation Mechanisms. ( Cont…)

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

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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

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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 .

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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.

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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.

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

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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

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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.

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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.

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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.

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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.

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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.

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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).

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C. Gas Accumulators. (Cont…)

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)

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C. Gas Accumulators. (Cont…)

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.

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C. Gas Accumulators. (Cont…)

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.

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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.

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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.

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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.

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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.

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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.

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Thank You…

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