Hydraulic actuators and control components

Hydraulics and Pneumatics
Prepared by
David Blessley S
AP/Mechanical Engg.,
Kamaraj College of Engineering and Technology,
Virudhunagar
Hydraulic Actuators and Control Components
David Blessley S AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar

• Actuator is a device that causes a machine or other device to operate.
• The primary function of hydraulic actuators is to convert the
potential energy, carried by fluid pressure and flow rate, to
mechanical power in the form of force and velocity to drive the
load.
• Hydraulic actuators can commonly be classified into two categories:
linear actuators (hydraulic cylinders) and rotary actuators (hydraulic
motors).
• Because of the significant dissimilarity in their structures, the
operating principles of linear and rotary actuators have some
fundamental differences.
Hydraulic Actuators

Types Of Hydraulic Actuator
Linear actuator (Hydraulic cylinder)-
 Provides motion in straight line.
 Linear displacement depends on stroke length.
 Usually referred to as cylinders, rams (single acting
cylinders) or jacks.
Rotary actuators (Hydraulic motors)-
 Produces continuous rotational motion.
 Pump shaft is rotated to generate flow.
 Motor shaft is caused to rotate by fluid being forced into
driving chambers.

Applications
Hydraulic jack.
 Hydraulic brake.
 Hydraulic ram.
 Used as sensor.
 Close loop velocity controlling.
 Highly precise positioning for heavy loads.

Hydraulic Cylinders
Single acting cylinders
Double acting cylinders
• Single acting cylinders
• As the name suggests, Single-acting
cylinders can exert a force in the extending
direction only
• Simplest in design
• Consists of a piston inside a cylindrical
barrel
• Single acting cylinders produce force in one direction by hydraulic pressure acting on
the piston
• The return of the piston is not done hydraulically. The retraction is done either by
gravity or by spring
• On one end of the piston is the piston rod which can reciprocate and at the opposite
end, there is a port for the entrance and exit of the oil

• Advantages
• Lower priced installation
• Simpler pipework and more compact
• In case of failure of supply they will return to their rest condition

• Double acting cylinder
• Powered by compressed fluids in both
the directions
• Difference in pressure between the two
sides results in motion of piston
• It consists of base cap, bearing cap,
cylinder barrel, piston and piston rod
• The end caps and piston are common
to all cylinders whereas the barrels and
piston rods need to be changed
according to the size of the cylinder
• During the extension stroke, the fluid enters through extend port and moves the piston
towards left and the fluid already present on the other side pushed out to the
reservoir through retract port
• During retraction stroke, the fluid is allowed to enter through retract port and the
piston moves towards right and the fluid present on the other side is pushed to the
cylinder through extend portDavid Blessley S AP/Mechanical Engg., Kamaraj College of

Application
• In scissor jack, to convert small linear motion of horizontal direction to larger linear
motion in vertical direction
• In elevators

• Cylinder Cushioning
• Cylinder end caps have to withstand shock
loads at extremes of piston travel.
• These loads arise not only from fluid
pressure but also from kinetic energy of
the moving parts
• If inertia is high enough at this point, the
cylinder may experience a shock that
could damage
• As per cushion arrangement, cylinders are classified into three types
• Non cushioned cylinder
• Fixed cushion cylinder
• Adjustable cushion cylinder
• The end of travel shock loads can be reduced with fixed cushions or valves built into
the end caps.

• Fixed cushion cylinder
• Small bore light duty cylinders have fixed cushions. These cylinders make use of synthetic
rubber buffers to give a fixed cushion effect
• The shock absorbent discs placed into the end covers the cushion impact of the piston
• Adjustable cushion cylinder
• An adjustable cushion slows down a cylinder’s piston movement just before reaching the
end
• Cushions are applied at one end or both ends of the cylinder
• The cushion consists of a throttle valve and a sleeve attached to the piston
• As the piston approaches the end, the cushion sleeve blocks the normal exit for the air
and forces it to pass through the throttle valve which restricts the flow progressively
retarding the piston movement
• The cushioning can be adjusted by controlling the throttle valve
• During retraction, the flow bypasses the throttle valve by means of a check valve within
the cushion seal of the cylinderDavid Blessley S AP/Mechanical Engg., Kamaraj College of

• Telescopic cylinders
• In some situations, it is required the stroke
length and a standard cylinder constructed
for the purpose is longer than the space
available for its fitment
• In such applications, it is common to look
for a telescopic cylinder which can provide
a longer stroke length as it can have more
than one stage of extension
• In telescopic cylinder, oil enters through the
outer cover of the cylinder and pushes
upon the first stage of the cylinder
• Then the oil entering the second stage
pushes upon the second stage of the
cylinder and makes the piston rod extend
further

• Thus the stroke length of the first stage gets added to the second stage to get a
total stroke length much more than a single stage cylinder
• On the other hand, the first and second stage tubes are within each other and the
total length required for the cylinder is normally less than the standard cylinder
• Generally most of the telescopic cylinders are single acting. i.e., extension by
pressurized oil and retraction by self weight

• Tandem cylinders
• Tandem cylinder has one cylinder cover with two pistons and other parts
• In tandem cylinders, the piston rod diameters are kept the same so that the
annulus areas also remain the same
• The cylinders have one piston connected to piston rods on both its sides
• Such cylinders are used in certain special purpose machines

Hydraulic Motors
• In simple, reverse of a hydraulic pump
• A hydraulic pump driven by a prime mover pumps out hydraulic oil
• Hydraulic oil is supplied to the motor through inlet port and this
hydraulic oil under pressure imparts a rotary motion to the internal
elements of the motor
• This internal element is connected to the shaft from which the rotary
motion is transmitted to any other driven member

Types of Hydraulic Motors

• Selection parameters
• Torque output required
• Speed range required
• Work cycle i.e., how long the motor is to operate
• Working atmosphere
Once the above mentioned factors have been taken care of, the final
selection of the motor depends on
• Displacement (cm3/revolution)
• Working pressure

• Gear motors
• Fall into the category of high speed low torque motors
• Its types are
• External gear motors
• Internal gear motors
External gear motors
• The gears are external to each other and mesh to produce movement
Both the gears are enveloped in a casing
• Gears and housing are made and machined with very fine tolerances such that no
leakage of hydraulic fluid occurs
• Hydraulic fluid under pressure enters through inlet and pushes or forces the idler
gear to rotate
• The meshing and rotation of this gear makes the other one to rotate and this driven
gear is connected to the output shaft

• Internal gear motors
• There is one meshing gear inside the other one
• The meshing of gears take place with the bigger
gear housing the smaller gear inside its hollow
space available
• Gears are specially designed in such a way that
the outer teeth surface of the inner gear is
meshing with the inner surface of the outer
gear
• Inner gear having one teeth less than the outer
gear and mounted a little eccentric to the outer
gear
• Oil gets entrapped between the spaces of the
teeth and pushes the teeth and the gear begins
to rotate
• The driven gear is connected to the inner gear
and the hydraulic drive is obtained

• Vane motors
• Vane motors have slotted rotor mounted on a
driveshaft which is driven by rotor
• Vanes closely fitted into the rotor slots move radially
to seal against the cam ring
• The ring has two major and two minor radial sections
to balance the pressure developed
• Pressurized fluid entering at the inlet ports move the
rotor counter clockwise
• The rotor transports the fluid to the ramp openings at
the outlet ports to return to tank
• If pressure is introduced at the outlet ports, it will turn
the motor clockwise
• Vane motors provide good operating efficiencies but not as high as those of piston
motors. However vane motors generally cost less than piston motors of
corresponding horsepower ratings
• Service life of vane motor is usually less than piston motorsDavid Blessley S AP/Mechanical Engg., Kamaraj College of

• Piston motors
• Falls under the category high torque and low speed
• Its types are
• Axial or inline piston
• Radial piston motors
• Bent axis motors

• Radial piston motors
• Having a cylinder barrel attached to the driven
shaft
• Barrel contains pistons that reciprocate in radial
bores
• Pressurized fluid flows through a pintle in the
center of the barrel to drive the piston outward
• The pistons push against the reaction ring and the
reaction forces rotate the barrel
• When the centerlines of the cylinder barrel and
housing coincide there is no fluid flow and
therefore the cylinder barrel stops
• Moving the slide past center reverses the direction
of motor rotation
• Radial piston motors provide high torque at relatively low shaft speeds and
excellent low speed operation with high efficiency

• Axial piston motors
• similar to principle in radial piston pump but motion is axial
• Inline piston motors generate torque through pressure exerted on the ends of pistons
that reciprocate in a cylinder block
• The motor driveshaft and cylinder block are centered on the same axis
• Pressure at the ends of the pistons causes reaction against a tilted swash plate and
rotates the cylinder block and motor shaft
• These motors are built in fixed and variable displacement models
• Efficiency characteristics similar to radial piston motors
• Swash plate angle determines motor displacement.

• Bent axis piston motors
• Develop torque through a reaction
to pressure on reciprocating pistons
• Here, cylinder block and driveshaft
are mounted at an angle to each
other and the reaction is against
the drive shaft flange
• Speed and torque change with
changes in the angle

Hydraulic components: Direction Control Valve, Flow
Control Valves and Pressure Control Valves
• Control components are devices used to control the flow direction,
flow rate and pressure in the hydraulic circuits
• Based on the function, control components are classified into
• Direction Control Valve (DCV)
• To change the direction of flow of oil
• Flow Control Valve
• To adjust the flow rate of oil thereby reducing the speed of the actuator
• Pressure Control Valve
• To control the pressure of the oil in the system

• Classification of Direction Control Valves
• Based on construction
• Poppet valve
• Sliding spool valve
• Rotary spool valve
• Based on number of ports
• Two way valves
• Three way valves
• Four way valves
• Based on mode of actuation
• Manually operated directional valves
• Mechanically operated directional valves
• Solenoid operated directional valves
• Pilot operated directional valves
Types of Direction Control Valves
• Check valves
• Position valves
• Shuttle valves

• Based on the modes of their configuration,
control valves are classified as
• Poppet valves
• Sliding spool valves
• Rotary spool valves
• Poppet valves
• Usually balls, discs or cones are used in
conjunction with valve seats to control the flow
• Normally the valve is in closed state, when the
push button is pressed, the ball is pushed away
from its seat and allows fluid flow from port A to
port P
• When the push button is released, spring and fluid
pressure forces the ball back to its seat and arrests
the flow.

• Sliding spool valves
• A spool can move horizontally within the valve body to control the flow
• Spool position 1 : the fluid supply is connected to port 1 and port 3 is in closed condition.
Thus it allows fluid to flow from port 1 to port 2
• Spool position 2 : when the spool moved to left, the fluid supply is cut off and port 2 is
connected to port 3 thus permits flow form port 2 to port 3

• Rotary spool valves
• Rotation of spool causes the ports to
open and close subsequently
• There are four ports R, P, A & B
• Position 1 : flow is permitted form port P
to port A and port R to port B
• Position 2 : flow is permitted from port P
to port B and port R to port A
• Position 3 : no flow

• Check valves
• Function is to allow flow in one direction and prevents flow in another direction
• Since it blocks reverse flow, also know as non return valves
• It is of two types
• Poppet operated
• Pilot operated
• Poppet operated
• Normally a spring is used to hold the poppet in closed
position
• When flows in normal direction, the liquid pressure acts
against the spring force to offset the seat and allows fluid
flow
• When the flow stops, the spring seats its position back by
spring action
• If fluid is attempted in opposite direction, the liquid pressure along with the spring force
pushes the poppet in closed position. Hence no flow is permitted in opposite directionDavid Blessley S AP/Mechanical Engg., Kamaraj College of

• Pilot operated check valve
• Allows flow in one direction but reverse flow
depends on pilot actuation
• i.e., reverse flow is permitted only when the
pilot pressure exceeds the spring force of the
poppet
• Flow in the normal direction from Port A to
Port B is achieved in usual manner. Reverse
flow is blocked as the fluid pushes the poppet
into the closed position
• To obtain reverse flow i.e., from port B to Port
A, a pilot pressure is applied through pilot port
which seats off the poppet from its position

• Position valves
• Two way, two position valves (2/2 valves)
• Three way, two position valves (3/2 valves)
• Four way, two position valves (4/2 valves)
• Four way, three position valves (4/3 valves)
• 2/2 DCV
• Position 1 : when the valve is in normal
position, spring and fluid pressure forces the
spool up therefore the flow is blocked
• Position 2 : when valve is actuated, the
pressure developed pushes the spool off its
seat and permits fluid flow from port P to
port A

• 3/2 DCV
• A typical 3/2 DCV has three ports pressure
supply port (A), output port (T) and exhaust
port (P)
• Position 1 : pressurized fluid flows from port A
to Port T to move the actuator and the port P
is in closed condition
• Position 2 : when the force is applied, the
spool moves to the left extreme and the fluid
from port T gets closed and allows flow from
port P to port A

• 4/2 DCV
• It has 4 ports and 2 positions
• Position 1 : fluid flows from Port A to port
A and port B to port P
• Position 2 : fluid flows from port T to port A
and port B to port T David Blessley S AP/Mechanical Engg., Kamaraj College of

• 4/3 DCV
• It has 4 ports and 3 positions
• Position 1 : fluid flows from port P to port A and port B to port T
• Position 2 : no flow; blocked
• Position 3 : fluid flows from port P to port B and port A to port TDavid Blessley S AP/Mechanical Engg., Kamaraj College of

• Shuttle valves
• Used when control is required from more than one power source
• As long as the pressure in port 2 is greater than the port 1, the shuttle piston closes
the left port
• When pressure in port 1 exceeds port 1, the piston moves to the right and closes it
and provides way for port 1

Pressure control valves
Used to control fluid pressure
• Its functions are to
• Limit maximum pressure
• Maintain desired pressure levels
• Unload system pressure
• Assist sequential pressure
• Its types are
• Pressure relief valve
• Pressure reducing valve

• Pressure relief valves
• Pressure relief valve protects the system from excessive fluid pressure over and
above the design pressure limit
• Normally located in between the pump and actuator
• Its types are
• Direct acting or simple pressure relief valve
• Pilot operated or compound pressure relief valve
• Direct acting or simple pressure relief valve
• Direct acting pressure relief valve has a ball or poppet or a
sliding spool against a spring
• It has one inlet port which is normally closed by spring
force
• When the inlet pressure overcomes the spring force, the
valve opens and the fluid flows back to the sump
• Thus the relief valve protects the other elements from
excessive pressure

• Compound or pilot operated pressure relief
valve
• The fluid passes from the inlet port through the
orifice to a control chamber which acts on the
main poppet to add to the spring force
• When the system pressure exceeds the setting
pressure of the main poppet, the poppet is
pushed from its seat towards left. This action
forces the pressurized fluid on the left to escape
through the centrally drilled drain hole, which
limits the pressure on the control chamber
• Due to this, the fluid cannot enter the control
chamber as the fluid leaves through drain hole.
Because of this the pressure on the right exceeds
the left and the main poppet moves to the left.
Thus it permits the fluid flow directly to the
reservoir from inlet port
• When the pressure falls below the setting
pressure, the poppet retracts back to its original
position againDavid Blessley S AP/Mechanical Engg., Kamaraj College of

• Pressure reducing valve
• A pressure reducing or regulating valve is used to supply a prescribed reduced
pressure in a circuit to maintain a constant pressure
• Its types
• Direct acting pressure reducing valve
• Pilot operated pressure reducing valve
Note: Pilot operated pressure reducing valve works the same principle as compound
pressure relief valve
• Direct acting pressure reducing valve
• It has a spring loaded spool to control outlet pressure
• When the main supply pressure is below valve setting
pressure, fluid will flow freely from inlet to outlet
• When the supply pressure exceeds the valve setting
pressure, the spool moves to the left and partially
blocks the outlet. Thus only enough flow passes the
outlet to maintain the pressure

• Sequence valve
• Used to control the fluid flow to ensure several operations in a particular
order of priority in the system
• When the system inlet pressure is within the preset valve pressure, the valve
allows free fluid flow
• When the system pressure exceeds the preset value, the spool moves up and
the flow is diverted to secondary port to operate secondary phase.

• Flow control valves
• Used to regulate the rate of fluid flow
• Its types are
• Non pressure compensated flow control valve
• Globe valve
• Needle valve
• Pressure compensated flow control valve

• Non pressure compensated flow control valve
• This type of flow control valves are used where pressure varies considerably
• Working principle of this valve is that, flow through the orifice will be constant if the
pressure drop remains constant
• Globe valve
• Flow rate can be changed by means of disk, plug or ball which nests against a seat
• Disk type globe valve also called butterfly valve consists of a rotating disc inside the
pipe. Its angle of rotation determines the flow rate
• Plug type globe valve uses tapered plug to control the flow rate by varying the
vertical plug position
• Ball type globe valve has a ball with a through hole which is rotating inside a
machined seat.

• Needle valve
• Needle valve has a pointed stem which can be adjusted to control the flow
rate
• Pressure compensated flow control valve
• It consists of two parts, throttle valves and pressure compensating spool
• The throttle valve has a orifice whose area can be adjusted by knob setting

Servo and proportional valves
• Servo valve
• A servo valve is a DC valve having infinite number of valve positions
• Usually servo valves are coupled with a feedback device
• It can control direction, flow, velocity, position etc.,
Types of Servo valve
 Hydromechanical servo valve
 Electrohydraulic servo valve
 Single stage servo valve
 Two stage servo valve
 Jet pipe servo valve
 Flapper jet servo valve

• Hydromechanical servo valve
• A small force applied on the main spool
shifts it towards right which allows oil to
enter through port P1 from inlet
• Oil from inlet port enters P1 and causes the
cylinder to retract towards right.
• As shown in fig. a feedback link is
connected to the piston rod and sliding
sleeve which shifts the sliding sleeve
towards right until it blocks off the flow to
Port P1
• Thus the given input motion has produced
a specific output motion and the output
motion is feedback to modify the input
through feedback line
Lap conditions
Closed centre servo valve : lands are larger than
ports
Open centre servo valve : lands are narrower
than port
Zero lap servo valve : both lands and edges are
made preciseDavid Blessley S AP/Mechanical Engg., Kamaraj College of

• Hydromechanical servo valve in
steering applications
• Input force is the turning of the steering
wheel
• Steering wheel movement permits the
sliding sleeve to allow oil from port to the
steering cylinder which in turn moves the
wheel through steering linkage
• Since the valve spool is attached to the
linkage, it also moves along with the
linkage
• When the valve spool has moved far
enough, it cuts off the flow to the cylinder
by blocking the port. Thus the motion of
the cylinder is stopped
Thus the given input force (Steering wheel movement) has produced a specific output
motion(wheel movement) and the output motion is feedback to modify the input
through feedback line David Blessley S AP/Mechanical Engg., Kamaraj College of

• Electrohydraulic servo valve
• The electrohydraulic servo valve operates due to
an electric signal to its torque motor which
positions the spool of the DC valve. In simple,
the electrical signal is used as an input signal to
control the hydraulic output
• Single stage electrohydraulic servo valve
• An electric signal is passed through the coils of a
torque motor
• The torque motor produces an armature
deflection depend on the input current
• Since the armature of the torque motor is
directly connected to the valve spool, the spool
also shifts by a distance proportional to the
change in current.

• Two stage electrohydraulic servo valve
• It consist of a main stage and a pilot stage
• The input signal produces an armature deflection
proportional to the input current
• The armature deflection is mechanically transmitted
to the pilot spool by means of a stiff wire
• If suppose the electrical signal causes the pilot spool
to shift towards left, the land X allows oil flow to end
B of the main spool which causes the main spool to
move towards right, which in turn pushes the sleeve
towards right
• The main valve stops moving when the hole in the
pilot sleeve exactly aligns with the land on the spool
(a feedback link connects the main spool and pilot
sliding sleeve. Thus any movement of the main spool is
fed back to the pilot spool sleeve)

• Jet pipe electrohydraulic servo valve
• A jet pipe is mechanically linked to the main
spool as in fig.
• The pilot pressure is applied to a jet pipe
which directs the flow into two pilot lines
• The amount of flow in these two pilot lines
vary depending on the input electrical control
signal i.e., the control signal diverts the jet
flow into two unequal flows
• The main spool moves towards less pressure
side. The spool movement stops when the jet
pipe is centrally located over the two pilot
pipes

• Flapper jet electrohydraulic servo valve
• A pilot pressure is applied to both the ends of
the main spool
• The sliding spool is actuated by pressure
difference between each end
• The pilot pressure lines are linked with two
opposing jet nozzles against a flapper valve
• The input signal produces and armature
deflection which in turn deflects the flapper
towards one of the jet nozzles causes the
pressure imbalance
• Now the main spool moves until the pressure is
balanced on both the ends

• Proportional valves
• Its types are
• Proportional pressure relief valve
• Proportional pressure reducing valve
• Proportional direction control valve
• Proportional pressure relief valve
• Works same as conventional pressure relief valve but springs are replaced by
proportional solenoid
• The input current to the solenoid energizes the coil and produces an armature
deflection proportional to the input current
• This armature deflection exerts a force on the poppet. The poppet keeps the valve
closed until the inlet pressure at port P overcomes the force by armature deflection
• When the inlet pressure overcomes this, the poppet valve opens and thus varies the
pressure of the valve

• Proportional pressure reducing valve
• Works same as conventional pressure
reducing valve but the spring is replaced
by proportional solenoid
• When the solenoid is energized, the
armature will deflect and move the spool
towards right.
• This action opens control orifice A and
allow flow from A to port X
• The outlet pressure of this valve depends
on the opening of control orifices A and B
which in turn depends on the input
current to solenoid
• Thus by proportionally varying the input
current, the operating pressures can be
reduced

• Proportional direction control valve
• When solenoid 1 is energized, the pressure is applied
to port X. This causes the spool to move towards right
against spring pressure
• It allows fluid flow from port P to port B and from port
A to T
• When the solenoid is deenergized, the control spring
centralizes the spool back to normal position
• Similarly, when solenoid 2 is energized, the pressure is
applied to port Y which causes spool to move towards
left against spring pressure.
• Now fluid flows from port p to A and port B to T. when
de-energizes, the spring centralizes the spool to
normal

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 entrapped 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.

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.

• 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 Symbols

Hydraulic actuators and control components

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