2. In fluid power, controlling elements are called valves.
There are three types of valves:
1.Directional control valves (DCVs): They determine the
path through which a fluid transverses a given circuit.
2.Pressure control valves: They protect the system against
overpressure, which may occur due to a sudden surge as
valves open or close or due to an increase in fluid demand.
3. Flow control valves: . the fluid flow rate must be
controlled in various lines of a hydraulic circuit.
It is important to know the primary function and operation of various
types of control components not only for good functioning of a system,
but also for discovering innovative methods to improve the fluid power
system for a given application.
3. DIRECTIONAL CONTROL VALVES
A valve is a device that receives an external signal
(mechanical, fluid pilot signal, electrical or electronics) to
release, stop or redirect the fluid that flows through it.
The function of a DCV is to control the direction of fluid flow
in any hydraulic / Pneumatic system
A DCV does this by changing the position of internal movable
parts.
A DCV is mainly required for the following purposes:
• To start, stop, accelerate, decelerate and change the direction of
motion of a hydraulic actuator.
• To permit the free flow from the pump to the reservoir at low
pressure when the pump’s delivery is not needed into the system.
•To vent the relief valve by either electrical or mechanical control.
4. Any valve contains ports that are external openings through
which a fluid can enter and exit via connecting pipelines.
The number of ports on a DCV is identified using the term
“way.” Thus, a valve with four ports is a four-way valve.
A DCV consists of a valve body or valve housing and a valve
mechanism usually mounted on a sub-plate.
The ports of a sub-plate are threaded to hold the tube fittings
which connect the valve to the fluid conductor lines.
The valve mechanism directs the fluid to selected output ports
or stops the fluid from passing through the valve.
5. i)Classification of DCVs based Fluid Path
• Check valves.
• Shuttle valves.
• Two-way valves.
• Three-way valves.
• Four-way valves. and Five way valves
ii)Classification of DCVs based on Design Characteristics
• An internal valve mechanism that directs the flow of fluid.
(such a mechanism can either be a poppet, a ball, a sliding
spool, a rotary plug or a rotary disk).
•Number of switching positions (usually 2 or 3).
• Number of connecting ports or ways.
• Method of valve actuation that causes the valve mechanism
to move into an alternate position.
6. iii)Classification of DCVs based on the Actuation Method
Mechanical Actuation
Manual Actuation,
pilot operated and solenoid
iv)Classification of DCVs based on the Construction of
Internal Moving Parts
•Rotary spool type: In this type, the spool is rotated to change
the direction of fluid. It has longitudinal grooves. The rotary
spools are usually manually operated.
• Sliding spool type: This consists of a specially shaped spool
and a means of positioning the spool.
11. Pilot operated Check valve
Free flow in one direction
Permits flow in other direction if pilot pressure
is ON
12.
13. Shuttle Valve
A shuttle valve allows two alternate flow sources to be
connected in a one-branch circuit.
The valve has two inlets P1 and P2 and one outlet A.
Outlet A receives flow from an inlet that is at a higher
pressure..
If the pressure at P1 is greater than that at P2, the ball slides to
the right and allows P1 to send flow to outlet A.
If the pressure at P2 is greater than that at P1, the ball slides to
the left and P2 supplies flow to outlet A .
One application for a shuttle valve is to have a primary pump
inlet P1 and a secondary pump inlet P2 connected to the
system outlet A The secondary pump acts as a backup,
supplying flow to the system if the primary pump loses
pressure.
14. A shuttle valve is called an “OR” valve because receiving a
pressure input signal from either P1 or P2 causes a pressure
output signal to be sent to A.
15. SPOOL
A cylindrical component on which film, thread, or
other flexible materials can be wound.
A modified spool used
in fluid power valves.
This spool at times
allows and blocks the
fluid flow between ports
26. ACTUATING DEVICES
Actuation is the method of moving the valve element from one position
to another. There are four basic methods of actuation: Manual,
mechanical, solenoid-operated and pilot-operated. Several combinations
of actuation are possible using these four basic methods.
Manually operated: In manually operated DCVs, the spool is shifted
manually by moving a handle pushing a button or stepping on a foot
pedal. When the handle is not operated, the spool returns to its original
position by means of a spring.
Mechanically operated: The spool is shifted by mechanical linkages
such as cam and rollers.
Solenoid operated: When an electric coil or a solenoid is energized, it
that pulls the armature into the coil. This causes the armature to push the
spool of the valve.
Pilot operated: A DCV can also be shifted by applying a pilot signal
(either hydraulic or pneumatic) against a piston at either end of the valve
spool. When pilot pressure is introduced, it pushes the piston to shift the
spool.
30. Pressure-control valves are used in hydraulic systems to
control actuator force (force = pressure × area) and to
maintain a safer pressure levels
Pressure controls are mainly used to perform the following
system functions:
Limiting maximum system pressure at a safe level.
Regulating/reducing pressure in certain portions of the
circuit.
Any pressure-related function
32. Pressure relief valve
• Pressure-relief valves limit the maximum
pressure in a hydraulic circuit by providing an
alternate path for fluid flow when the
pressure reaches a preset level.
• All circuits require a relief valve to protect
the system from excess pressure.
• In a hydraulic circuit, a relief valve opens and
bypasses fluid when pressure exceeds its
setting.
33. Cont..
• 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.
• A poppet is held seated inside the valve by a
heavy spring.
• When the system pressure reaches a high enough
value, the poppet is forced off its seat.
• This permits flow through the outlet to the tank as
long as this high pressure level is maintained.
• Note the external adjusting screw, which varies
spring force and, thus, the pressure at which the
valve begins to open (cracking pressure).
34.
35. The pressure that exists at full
pump flow can be substantially
greater than cracking pressure. The
pressure at full pump flow is the
pressure level that is specified
when referring to the pressure
setting of the valve. It is the
maximum pressure level permitted
by the relief valve.
36. Compound pressure relief valve
• It consists of two relief valve one is pilot relief
valve and other is main pressure relief valve
• It operates in two stages process
first the pilot relief valve opens then
main relief valve opens
37.
38. Cont..
• It consists of a pressure port connected to
pump and tank port connected to tank
• The main relief valve consists of a piston and a
stem, it has an orifice drilled to it
• The piston in the main relief valve is exposed
to equal force on both sides of the piston
• When a high pressure drop is induced across
the piston due to orifice, it causes the piston
to move upwards from the seat
39. Advantage over simple pressure relief valve :
For higher pressure settings the size of the
compound pressure valve is smaller than the
simple pressure relief valve
40.
41.
42.
43. Pressure-Reducing Valve
This valve (which is normally open) is used to maintain
reduced pressures in specified locations of hydraulic systems.
It is actuated by downstream pressure and tends to close as
this pressure reaches the valve setting.
It uses a spring-loaded spool to control the downstream
pressure.
If the downstream pressure is below the valve setting, the
fluid flows freely from the inlet to the outlet.
44.
45. When the outlet (downstream) pressure increases to the valve
setting, the spool moves to the right to partially block the
outlet port.
Just enough flow is passed to the outlet to maintain its preset
pressure level.
To prevent the outlet port from being complete closed, a
continuous bleed to the tank is permitted via a separate drain
line to the tank.
47. GRAPHICAL SYMBOL
A reducing valve is normally
open.
It reads the downstream
pressure.
It has an external drain.
This is represented by a line
connected from the valve
drain port to the tank.
The symbol shows that the
spring cavity has a drain to the
tank.
49. Cont..
• They are pressure-control devices that are used
to dump excess fluid to the tank at little or no
pressure.
• A common application is in high-low pressure
pump circuits where two pumps move an
actuator.
• The circuit then shifts to a single pump
providing a high pressure to perform work.
51. • The circuit that uses a high-pressure pump in conjunction with a low-
pressure pump.
• In a punching press, the hydraulic cylinder must extend rapidly over a
great distance with low-pressure, high-flow requirements. This rapid
extension of cylinder occurs under no external load (when the
punching tool approaches the sheet metal).
• But during punching operation for short motion, the pressure
requirements are high due to punching load. During this cylinder
travel, high-pressure, low-flow requirements are needed.
• When punching operation begins, the increased pressure opens the
unloading valve to unload the low-pressure pump.
• The check valve protects the low-pressure pump from high pressure,
which occurs during punching operation that occurs at the end of
cylinder extension.
52. Pressure Sequence Valve
A sequence valve is a pressure-control valve that is used to
force two actuators to operate in sequence. They are similar to
pressure-relief valves.
Instead of sending flow back to the tank, a sequence valve
allows flow to a branch circuit, when a preset pressure is
reached.
The check valve allows the sequence valve to be bypassed in
the reverse direction.
The sequence valve has an external drain line; therefore, a line
must be connected from the sequence valve’s drain port to the
tank.
55. • In this circuit, two cylinders are connected in parallel.
Without the sequence valve, these cylinders would extend
together as they are both unloaded.
• In order for this circuit to function properly, the clamp
cylinder must extend completely before the bending
cylinder begins to extend.
• The sequence valve accomplishes this by not allowing
flow into the bending cylinder branch of circuit until the
clamp cylinder has reached the end of its stroke.
• When the clamp cylinder extends completely, the pressure
rises and opens the sequence valve, thus allowing the
bending cylinder to extend.
• The sequence valve pressure must be set high enough so
that it opens only after the complete extension of the clamp
cylinder.
56. Counterbalance Valve
• These normally closed valves are primarily used on a
vertical cylinder to prevent it from falling due to gravity.
They are used to prevent a load from accelerating
uncontrollably.
• The valve primary port is connected to the cylinder rod
end and the secondary port to the directional control
valve. The pressure setting is slightly higher than that
required to keep the load from free-falling.
• When the pressurized fluid flows to the cylinder piston
end, the cylinder extends, increasing pressure in the rod
end and shifting the main spool in the counterbalance
valve.
57. • This creates a path that permits the fluid to
flow through the secondary port via the
directional control valve and to the reservoir.
As the load is raised, the integral check valve
opens to allow the cylinder to retract freely.
58.
59. Application of Counter Balance Valve
• To prevent acceleration of cylinder under
vertical load
60.
61.
62. Problems in Pressure relief valve
• A pressure relief valve contains a poppet with a
4.20 cm2 area on which system pressure acts.
During assembly a spring with spring constant
of 3200 N/ cm is installed in the valve to hold
the poppet against its seat. The adjustment
mechanism is then set so that the spring is
initially compressed 0.50 cm from its free
length condition. In order to pass full pump
flow through the valve at the PRV pressure
setting , the poppet must move 0.30 cm from
its fully closed position. Determine
64. Flow-control valves
• Control the rate of flow of a fluid to the actuators
• This provide speed control of linear actuators and rotary
actuators.
• Applications include regulating cutting tool speeds, spindle
speeds, surface grinder speeds, and the travel rate of
vertically supported loads moved upward and downward
by forklifts, and dump lifts.
• Flow-control valves also allow one fixed displacement
pump to supply two or more branch circuits fluid at
different flow rates
65. Functions of Flow-Control Valves
1. Regulate the speed of linear and rotary actuators
Velocity of piston (Vp) (m/s) =
2. Regulate the power available by controlling the flow to
them
Power (W) = Flow rate (m3/s) ×Pressure (N/m2)
Flow-control valves can be classified as follows:
1. Non-pressure compensated valve
2. Pressure compensated valve
68. • Non pressure compensated valve is used only
when the variation in the working load is very
less
• When the working load varies heavily then the
outlet pressure of the valve also increases,
non- pressure compensated valve does not
serve the purpose
• To overcome the difficulty we have
70. Cont…
• A pressure-compensated flow-control valve consists of a
main spool and a compensator spool.
• The adjustment knob controls the main spool’s position,
which controls the orifice size at the outlet.
• The upstream pressure is delivered to the valve by the pilot
line A. Similarly, the downstream pressure is ported to the
right side of the compensator spool through the pilot line B.
• The compensator spring pushes the spool so that it tends
toward the fully open position.
• 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 a normal flow
73. Flow rate from flow control valve
Q = Cv *√(
Δ𝑃
𝑆𝐺
)
Q – FLOW RATE (Lpm)
Cv – flow co.eff ( Lpm / √𝑃𝑎)
ΔP- pressure drop (Pa)
SG – specific gravity of oil
74. problem
• A needle valve is used to control the extending speed of the
hydraulic cylinder. The needle valve is placed in the outline of
the hydraulic cylinder. The following data are given
Desired cylinder speed – 25 cm/sec
Cylinder piston diameter – 5 cm
Cylinder rod diameter – 2.6 cm
Cylinder load – 4.5 kN
Specific gravity of oil – 0.90
PRV settings – 34 bar
Determine the flow coefficient of the needle valve
75. Servo Valves
• Thesevalveshaveinfinitelyvariablepositioningcapability
• Itcontrolsdirectionoffluidflowaswellastheamount
• Servovalvesarecoupledwithfeedbacksensingdeviceswhichallowforthe
veryaccuratecontrolofposition,velocityandaccelerationofanactuator
• Servovalvesareappropriateforsystemswithoneormoreofthefollowing
characteristics: high load stiffness (static or dynamic); high accuracy and
stability; precise positioning; fine control of velocity and acceleration
control.
76. Mechanical type servo valves
• Itisaforceamplifierusedforpositioningcontrol
• Inthisvalve,asmallinputforceshiftsthespoolto
therightbyaspecifiedamount
• Theoilthenflowsthroughportp1,thenretracting
thecylinder
• The feedback link shifts the sliding sleeve to the
right until it blocks off the flow to the hydraulic
cylinder
• Thusagiveninputmotionproducesaspecificand
controlledamountofoutputmotion.
• Suchasystemiscalled“closedloopsystem”
• Application :hydraulic power steering system of
automobilesandothertransportationvehicles
77.
78. Electrohydraulic servo valves
• It uses an electrical torque motor, a
double acting nozzle pilot stage, and a
slidingspoolsecondstage.
• It is an electrically controlled
,proportionalmeteringvalve suitable for
variety of mobile vehicles and industrial
control applications such as earth
moving vechicles,articulated arm
devices, cargo handling cranes, lift
trucks, logging equipment, farm
machinery, steel mill controls, utility
construction, fire trucks and servicing
vehicles.
79.
80. Electrohydraulic servo valves
• Torquemotorincludescoils,polepieces,magnetsandanarmature.
• The valve directs the flow from the supply pressure port to either of the
twooutletportsinanamountproportionaltospooldisplacement.
• The spool contains the flow metering slots in the control lands that are
uncoveredbyspoolmotion.
• Spool movement deflects a feedback wire that applies a torque to the
armature/flapper
• Electrical current in a torque motor coil causes either clockwise or
anticlockwise torque on an armature to control the movement of the
spool.
• The spool continues to move until the feedback torque counteracts the
electromagnetictorque
• Atthispointthearmature/flapperisreturnedtocenter,sothespoolstops
and remains displaced until the electrical input changes to a new
level,thereforevalvespoolpositionisproportionaltotheelectricalsignal.
86. Proportional control valves
• Also called electrohydraulic proportional valves-
electricallycontrolled
• Theseareusedinopenloopsystems.
• Here,itusesasolenoidthatproducesaforceproportional
tothecurrentinitscoils
• Thus by controlling the current in the solenoid coil the
positionofthespringloadedspoolcanalsobecontrolled.
• This can provide both directional and flow control
capabilityinasinglevalve
• Itiswidelyusedbecauseitalsocontrolsthepressure.
89. Pressure Intensifier / Booster
• During punching operation, it requires low pressure
for the initial stroke length
• high pressure oil is needed during the working
stroke(punching)for short period of time
• Some manufacturers make high-pressure rotary
pumps — rated up to approximately > 690 bar—
but these pumps are expensive and may heat the
fluid.
• Another choice for small-volume high-pressure
circuits is an intensifier.
91. Cont..
• When a circuit calls for a small volume of high-
pressure oil or air, consider using an intensifier —
sometimes called a booster.
• Diagram for pressure intensifier is shown below
92. Let
• P1 – air input pressure
• P2 – oil exit pressure
• d1 – large piston diameter
• d2 – smaller piston diameter
• Force acting will be same
• Pressure ratio or area ratio = P2 / P1
98. Problems in intensifier
• An air hydraulic intensifier is connected to a hydraulic
cylinder driving a 54 kN load. Diameter of hydraulic cylinder
piston is 38 mm. Following are the data for intensifier
air is supplied at 5 bar pressure
air piston diameter – 203 mm
oil piston diameter – 25 mm
intensifier stroke – 51 mm
intensifier cycle frequency – 1 stroke / sec
Determine
a. Movement of hydraulic cylinder per intensifier stroke
b. pressure developed from the oil piston
100. ACCUMULATOR
• An accumulator is device that stores hydraulic
potential energy by means of either gravity,
mechanical spring, or compressed gases.
• The stored potential energy in the
accumulator is a quick secondary source of
fluid power capable of doing useful work as
required by the system.
125. Notations
• Let
P1 – gas precharge pressure
P2 – maximum pressure
P3 – minimum pressure after discharging
V1 – initial volume during gas precharging
V2 – compressed volume of gas
V3 – expanded volume of gas after oil discharge
V2 – V3 = ?
Volume of oil enters the hydraulic cylinder
126. Problem
• What is the size of the accumulator required to
supply 5 litres of oil to the cylinder with an
allowable pressure of 210 bar to 105 bar. The
precharge pressure is 70 bar.
• Solve the problem by assuming
a. Charging and discharging to be isothermal
PV = c
a. Charging and discharging to be adiabatic
PVn = c
129. Conductors & Fittings
• steel tubes, plastic tubes Flexible hoses
• Selection of conductors depends on operating
pressure and flow rate
• It not a good idea to connect steel tubes to
the pump
130. Sealing Devices
• Efficiency and power loss will occur, if there is
any oil leakage in the system
• Due to wear between mating parts, clearance
increases results in more leakage
• To prevent excessive leakage, SEALS are used
131. Sealing
Most Widely used type of seals
• O-rings
• Compression packed (V and U- shape)
• Piston rings
• Wiper rings
