Hydraulics & pneumatics (AU) Unit-II

By,
Mr.Vimal raja M
Assistant Professor
Dept.of.Mech.Engg.,
KIT-CBE.
HYDRAULICS AND PNEUMATICS

OBJECTIVES: L T P C
3 0 0 3
 To provide student with knowledge on the application of fluid power in process,
construction and manufacturing industries.
 To provide students with an understanding of the fluids and components utilized in
modern industrial fluid power system.
 To develop a measurable degree of competence in the design, construction and
operation of fluid power circuits.

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.

HYDRAULIC ACTUATORS
 Hydraulic cylinders (also called linear actuators) extend and retract a piston rod to
a push or pull force to drive the external load along a straight-line path.

HYDRAULIC ACTUATORS
 Hydraulic motors (also called rotary actuators) rotate a shaft to provide a torque to
drive the load along a rotary path.
 Pumps perform the function of adding energy to the fluid (on the fluid) of a
hydraulic system for transmission to some output location.
 Hydraulic cylinders and hydraulic motors do just the opposite. They extract
energy from the fluid (by the fluid) and convert it to mechanical energy to perform
useful work.

LINEAR HYDRAULIC ACTUATORS (Hydraulic Cylinders)

SINGLE ACTING CYLINDER:
 The simplest type of hydraulic cylinder is the single-acting design.
 It consists of a piston inside a cylindrical housing called a barrel. Attached to one
end of the piston is a rod, which extends outside one end of the cylinder (rod end).
 At the other end (blank end) is a port for the entrance and exit of oil.
 A single-acting cylinder can exert a force in only the extending direction as fluid
from the pump enters the blank end of the cylinder.
 Single acting cylinders do not retract hydraulically. Retraction is accomplished by
using gravity or by the inclusion of a compression spring in the rod end.

DOUBLE ACTING CYLINDER:
 In this double-acting cylinder design, the barrel is made of seamless steel tubing,
honed to a fine finish on the inside.
 The piston, which is made of ductile iron, contains U-cup packings to seal against
leakage between the piston and barrel.
 The ports are located in the end caps, which are secured to the barrel by tie rods.

DOUBLE ROD CYLINDER:
 In a double-rod cylinder, the rod extends out of the cylinder at both ends. For such
a cylinder, the words extend and retract have no meaning.
 Since the force and speed are the same for either end, this type of cylinder is
typically used when the same task is to be performed at either end.
 Since each end contains the same size rod, the velocity of the piston is the same
for both strokes.

TANDEM CYLINDER:
 Tandem cylinder is used in applications where a large amount of force is required
from a small diameter cylinder.
 Pressure is applied to both pistons, resulting in increased force because of the
larger area.
 The drawback is that these cylinders must be longer than a standard cylinder of
larger flow rate than a standard cylinder to achieve an equal speed because flow must
go to both pistons.

TELESCOPIC CYLINDER (Double Acting):
 The telescopic cylinder contains multiple cylinders that slide inside each other.
 They are used where long work strokes are required but the full retraction length
must be minimized.
 One application for a telescopic cylinder is the high-lift fork truck.

CYLINDER CUSHIONING:

CYLINDER CUSHIONING:
 Double-acting cylinders sometimes contain cylinder cushions at the ends of the
cylinder to slow the piston down near the ends of the stroke. This prevents excessive
impact when the piston is stopped by the end caps.
 As shown, deceleration starts when the tapered plunger enters the opening in the
cap. This restricts the exhaust flow from the barrel to the port.
 During the last small portion of the stroke, the oil must exhaust through an
adjustable opening.
 The cushion design also incorporates a check valve to allow free flow to the
piston during direction reversal.

CYLINDER MOUNTING: illustrated in Fig.
This permits versatility in the anchoring of cylinders. The rod ends are usually
threaded so that they can be attached directly to the load, a clevis a coke, or some other
mating device.
The following benefits are obtained
1. Free range of mounting positions
2. Reduced cylinder binding and side loading
3. Allowance for universal swivel
4. Reduced bearing and tube wear
5. Elimination of piston blow-by caused by misalignment

CYLINDER MOUNTING:

DIRECTION CONTROL VALVES
CHECK VALVE (One directional flow control valve):

 Simplest type of direction control valve is a check valve.
 It is a two-way valve because it contains two ports.
 The purpose of a check valve is to permit free flow in one direction and prevent
any flow in the opposite direction.
 Fig showing the internal operation of a poppet check valve. A poppet is a
specially shaped plug element held onto a seat by a spring.
 Fluid flows through the valve in the space between the seat and poppet.
 A light spring holds the poppet in the closed position. In the free-flow direction,
the fluid pressure overcomes the spring force at about 5 psi.
 If flow is attempted in the opposite direction, the fluid pressure pushes the poppet
(along with the spring force) in the closed position. Therefore, no flow is permitted.

 The higher the pressure, the greater will be the force pushing the poppet against
its seat. Thus, increased pressure will not result in any tendency to allow flow in the
no-flow direction.
 Fig shows the graphic symbol of a check valve along with its no flow and free-
flow directions.
 Graphic symbols, which clearly show the function of hydraulic components (but
without the details provided in schematic drawings), are used when drawing hydraulic
circuit diagrams.
 Note that a check valve is analogous to a diode in electric circuits.

PILOT OPERATED CHECK VALVE

PILOT OPERATED CHECK VALVE
 A second type of check valve is the pilot-operated check valve.
 This type of check valve always permits free flow in one direction but permits
flow in the normally blocked opposite direction only if pilot pressure is applied at the
pilot pressure port of the valve.
 In the design, the check valve poppet has the pilot piston attached to the threaded
poppet stem by a nut.
 The light spring holds the poppet seated in a no flow condition by pushing against
the pilot piston.
 The purpose of the separate drain port is to prevent oil from creating a pressure
buildup on the bottom of the piston.
 The dashed line represents the pilot pressure line connected to the pilot pressure
port of the valve.
 Pilot check valves are frequently used for locking hydraulic cylinders in position.

3/2 VALVE

DIRECTION CONTROL VALVES:
3/2 VALVE
 Three-way directional control valves, which contain three ports are typically of
the spool design rather than poppet design.
 A spool is a circular shaft containing lands that are large diameter sections
machined to slide in a very close fitting bore of the valve body. The radial clearance
between the land and bore is usually less than 0.001 in. The grooves between the lands
provide the flow paths between ports.
 These valves are designed to operate with two or three unique positions of the
spool. The spool can be positioned manually, mechanically, by using pilot pressure, or
by using electrical solenoids.
 Figure shows the flow paths through a three-way valve that uses two positions of
the spool. Such a valve is called a three-way, two-position directional control valve.
 The flow paths are shown by two schematic drawings (one for each spool
position) as well as by a graphic symbol (containing two side-by-side rectangles). In
discussing the operation of these valves, the rectangles are called as "envelopes."

4/2 VALVE

4/2 VALVE
 Figure shows the flow paths through a four-way, two-position directional control
valve.
 Observe that fluid entering the valve at the pump port can be directed to either
outlet port A or B.
 The following is a description of the flow paths through this four-way valve:
1. Spool Position 1: Flow can go from P to A and B to T.
2. Spool Position 2: Flow can go from P to B and A to T.

4/3 VALVE

4/3 VALVE
 Figure shows the flow paths through a four-way, three-position directional control
valve.
 They are mainly 4 types:
1. Manually Actuated Valves
2. Mechanically Actuated Valves
3. Pilot-Actuated Valves
4. Solenoid-Actuated Valves

4/3 VALVE (Cont…)
1. Manually Actuated Valves :
 Figure shows a cutaway of a four-way valve. It is manually actuated.
 Since the spool is spring loaded at both ends, it is a spring centered three-position
directional control valve, Thus, when the valve is unactuated (no hand force on lever),
the valve will assume its center position due to the balancing opposing spring forces.
 The graphic symbol of this four-way valve also shown.
 In the graphic symbol that the ports are labelled on the center envelope, which
represents the flow path configuration. Also the spring and lever actuation symbols
used at the ends of the right and left envelopes. These imply a spring-centered,
manually actuated valve.
 It should be noted that a three-position valve is used when it is necessary to stop
or hold a hydraulic actuator at some intermediate position within its entire stroke
range.

4/3 VALVE (Cont…)
1. Mechanically Actuated Valve :
 Figure shows a four-way, spring offset valve that is mechanically rather than
manually actuated.
 This is depicted in the cutaway view, with the spool end containing a roller that is
typically actuated by a cam-type mechanism.
 Note that the graphic symbol is the same except that actuation is depicted as
being mechanical (the circle represents the cam-driven roller) rather than manual.

4/3 VALVE (Cont…)
1. Pilot Actuated Valve :
 Directional control valves can also be shifted by applying air/oil pressure against
a piston at either end of the valve spool.
 As shown, springs (located at both ends of the spool) push against centering
washers to center the spool when no air/oil is applied.
 When air/oil is introduced through the left end passage, its pressure pushes
against the piston to shift the spool to the right. Removal of this left end air/oil supply
and introduction of air through the right end passage causes the spool to shift to the
left. Therefore, this is a four-way, three-position, spring-centered, air pilot-actuated
directional control valve.
 In the graphic symbol, the dashed lines represent pilot pressure lines.

4/3 VALVE (Cont…)
1. Solenoid Actuated Valve :
 A very common way to actuate a spool valve is by using a solenoid as illustrated
in Figure.
 When the electric coil (solenoid) is energized, it creates a magnetic force that
pulls the armature into the coil. This causes the armature to push on the push pin to
move the spool of the valve.
 Solenoids are actuators that are bolted to the valve housing as shown.
 Like mechanical or pilot actuators, solenoids work against a push pin which is
sealed to prevent external leakage of oil.
 There are two types of solenoid designs used to dissipate the heat created by the
electric current flowing in the wire of the coil.
1. Air gap solenoids 2. Wet(oil) pin solenoids

FLOW CONTROL VALVES
Pressure Relief Valve (PRV):

FLOW CONTROL VALVES
Compound Pressure Relief Valve :

COMPOUND PRESSURE RELIEF VALVE: (Pilot Operated)
 A compound pressure relief valve (which is normally closed) is one that operates
in two stages. As shown in Figure, the pilot stage is located in the upper valve body
and contains a pressure-limiting pop pet that is held against a seat by an adjustable
spring.
The lower body contains the port connections. Diversion of the fuel pump flow is
accomplished by the balanced piston in the lower body.
 The operation is as follows: In normal operation, the balanced piston is in
hydraulic balance. Pressure at the inlet port acts under the piston and also on its top
because an orifice is drilled through the large land.
 For pressures less than the valve setting, the piston is held on its seat by a light
spring. As soon as pressure reaches the setting of the adjustable spring, the poppet is
forced off its seat.
 This limits the pressure in the upper chamber. The restricted flow through the
orifice and into the upper chamber results in an increase in pressure in the lower
chamber.

COMPOUND PRESSURE RELIEF VALVE :
 This causes an unbalance in hydraulic forces, which tends to raise the piston off
its seat. When the pressure difference between the upper and lower chambers reaches
approximately 20 psi, the large piston lifts off its seat to permit flow directly to the
tank.
 If the flow increases through the valve, the piston lifts farther off its seat.
However, this compresses only the light spring, and hence very little override occurs.
 Compound relief valves may be remotely operated by using the outlet port from
the chamber above the piston.
 For example, this chamber can be vented to the tank via a solenoid directional
control valve.
 When this valve vents the pressure relief valve to the tank, the 20-psi pressure in
the bottom chamber overcomes the light spring and unloads the pump to the tank.

FLOW CONTROL VALVES
Pressure Regulating Valve : (Pressure Reducing Valve)

FLOW CONTROL VALVES
Pressure Regulating Valve : (Pressure Reducing Valve)
 A second type of pressure control valve is the pressure-reducing valve.
 This type of 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. Figure illustrates the operation of a pressure-reducing valve that uses
a spring-loaded spool to control the down stream pressure.
 If downstream pressure is below the valve setting, fluid will flow freely from the
inlet to the outlet.
 Note that there is an internal passageway from the outlet, which transmits outlet
pressure to the spool end opposite the spring.
 When the outlet (downstream) pressure increases to the valve setting, the spool
moves to the right to partially block the outlet port.

FLOW CONTROL VALVES
Pressure Regulating Valve : (Pressure Reducing Valve) (Cont…)
 Just enough flow is passed to the outlet to maintain its preset pressure level.
 If the valve closes completely, leakage past the spool could cause downstream
pressure to build up above the valve setting.
 This is prevented from occurring because a continuous bleed to the tank is
permitted via a separate drain line to the tank.
 Figure also provides the graphic symbol for a pressure-reducing valve. Observe
that the symbol shows that the spring cavity has a drain to the tank.

FLOW CONTROL VALVES
Unloading Valve :

FLOW CONTROL VALVES
Unloading Valve:
 An additional pressure control device is the unloading valve.
 This valve is used to permit a pump to build pressure to an adjustable pressure
setting and then allow it to discharge oil to the tank at essentially zero pressure as long
as pilot pressure is maintained on the valve from a remote source.
 Hence, the pump has essentially no load and is therefore developing a minimum
amount of power.
 This is the case in spite of the fact that the pump is delivering a full pump flow
because the pres sure is practically zero.
 This is not the same with a pressure relief valve because the pump is delivering
fuel pump flow at the pressure relief valve setting and thus is operating at maximum
power conditions.
 Figure shows a schematic of an unloading valve used to unload the pump
connected to port A when the pressure at port X is maintained at the value that satisfies
the valve setting.

FLOW CONTROL VALVES
Unloading Valve: (Cont…)
 The high-flow poppet is controlled by the spring-loaded ball and the pressure
applied to port X. Flow entering at port A is blocked by the poppet at low pressures. The
pressure signal from A passes through the orifice in the main poppet to the topside area
and on to the ball.
 There is no flow through these sections of the valve until the pressure rises to the
maximum permitted by the adjustable set spring-loaded ball. When that occurs the
poppet lifts and flow goes from port A to port B, which is typically connected to the tank.
 The pressure signal to port X (sustained by another part of the system) acts against
the solid control piston and forces the ball farther off the seat. This causes the topside
press sure on the main poppet to go to a very low value and allows flow from A to B with
a very low pressure drop as long as signal pressure at X is maintained.
 The ball reseats, and the main poppet closes with a snap action when the pressure at
X falls to approximately 90% of the maximum pressure setting of the spring-loaded ball.
 Figure also includes the graphic symbol of an unloading valve.

FLOW CONTROL VALVES
Sequence Valve:

FLOW CONTROL VALVES
Sequence Valve :
 The sequence valve, which is designed to cause a hydraulic system to operate in a
pressure sequence. After the components connected to port A have reached the adjusted
pressure of the sequence valve, the valve passes fluid through port B to do additional
work in a different portion of the system.
 The high-flow poppet of the sequence valve is con trolled by the spring-loaded
cone. Flow entering at port A is blocked by the pop pet at low pressures. The pressure
signal at A passes through orifices to the topside of the poppet and to the cone.
 There is no flow through these sections until the pressure rises at A to the
maximum permitted by the adjustably set spring-loaded cone. When the pressure at A
reaches that value, the main poppet lifts, passing flow to port B.
 It maintains the adjusted pressure at port A until the pressure at B rises to the
same value. A small pilot flow (about 1/4 gpm) goes through the control pis ton and
past the pilot cone to the external drain at this time.

FLOW CONTROL VALVES
Sequence Valve :
 When the pressure at B rises to the pressure at A, the control piston seats and
prevents further pilot flow loss.
 The main poppet opens fully and allows the pressure at A and B to rise to higher
values together. Flow may go either way at this time.
 The spring cavity of the control cone drains externally from port Y, generally to
the tank. This sequence valve may be remotely controlled from vent port X.
 Figure also includes the graphic symbol for a sequence valve.
 The pilot line can come from anywhere in the circuit and not just from directly
upstream, as shown.

FLOW CONTROL VALVES
Needle Valve:

FLOW CONTROL VALVES
Ball Valve:

FLOW CONTROL VALVES
Butterfly Valve:

FLOW CONTROL VALVES
Counter Balance Valve:

FLOW CONTROL VALVES
Counter Balance Valve :
 The purpose of a counterbalance valve is to maintain control of a vertical hydraulic
cylinder to prevent it from descending due to the weight of its external load.
 As shown in Figure, the primary port of this valve is connected to the bottom of the
cylinder, and the secondary port is connected to a directional control valve (DCV).The
pressure setting of the counterbalance valve is somewhat higher than is necessary to
prevent the cylinder load from falling due to its weight.
 As shown in Figure, when pump flow is directed (via the DCV) to the top of the
cylinder, the cylinder piston is pushed downward. This causes pressure at the primary
port to increase to a value above the pressure setting of the counterbalance valve and
thus raise the spool of the CBV.
 This then opens a flow path through the counterbalance valve for discharge through
the secondary port to the DCV and back to the tank. When raising the cylinder, an
integral check valve opens to allow free flow for retracting the cylinder.
 Figure also gives the graphic symbol for a counterbalance valve.

PRESSURE CONTROL VALVES
Pressure Compensated Flow Control Valve:

FLOW CONTROL VALVES
 If the load on an actuator changes significantly, system pressure will change
appreciably.
 Thus, the flow-rate through a non-pressure-compensated valve will change for the
same flow-rate setting. Figure illustrates the operation of a pressure compensated
valve.
 This design incorporates a hydrostat that maintains a constant 20-psi differential
across the throttle, which is an orifice whose area can be adjusted by an external knob
setting.
 The orifice area setting determines the flow-rate to be controlled. The hydrostat is
held normally open by a light spring. However, it starts to close as inlet pressure
increases and overcomes the light spring force.
 This closes the opening through the hydrostatic and thereby blocks of all flow in
excess of the throttle setting. As a result, the only oil that will pass through the valve is
the amount that 20 psi can force through the throttle.

FLOW CONTROL VALVES
 Flow exceeding this amount can be used by other parts of the circuit or return to
the tank via the pressure relief valve.
 Also included in Figure is the graphic symbol for a pressure-compensated flow
control valve.

ELECTRO HYDRAULIC SERVO VALVE :

ELECTRO-HYDRAULIC SERVO VALVE :
 Figure gives a cutaway view of an electrohydraulic servo valve.
 This servo valve is an electrically controlled, proportional metering valve suitable
for a variety of mobile vehicles and industrial control applications such as earth-
moving vehicles, articulated arm devices, cargo-handling cranes, lift trucks, logging
equipment, farm machinery, steel mill controls, utility construction, fire trucks, and
servicing vehicles.
 The torque motor includes:
1. Coils 2. Pole pieces 3. Magnets and 4. An armature.
 The armature is supported for limited movement by a flexure tube. The flexure
tube also provides a fluid seal between the hydraulic and electromagnetic portions of
the valve.
 The flapper attaches to the center of the armature and extends down, inside the
flexure tube.

ELECTRO-HYDRAULIC SERVO VALVE (Cont..):
 A nozzle is located on each side of the flapper so that flapper motion varies the
nozzle openings.
 Inlet-pressurized hydraulic fluid is filtered and then supplied to each nozzle
through one of the two inlet orifices located at the ends of the filter.
 Differential pressure between the ends of the spool is varied by flapper motion
between the nozzles.
 The four-way valve spool directs the flow from the supply pressure port to either
of the two outlet-to-load ports in an amount proportional to spool displacement.
 The spool contains flow metering slots in the control lands that are uncovered by
spool motion.
 Spool movement deflects a feedback wire that applies a torque to the
armature/flapper.
 Electrical current in the torque motor coils causes either clockwise or counter-
clockwise torque on the armature.

ELECTRO-HYDRAULIC SERVO VALVE (Cont…) :
 This torque displaces the flapper between the two nozzles. The differential nozzle
flow moves the spool to either the right or left.
 The spool continues to move until the feedback torque counteracts the
electromagnetic torque.
 At this point the armature/flapper is returned to center, so the spool stops and
remains displaced until the electrical input changes to a new level.
 Therefore, valve spool position is proportional to the electrical signal.
 The actual outlet flow from the valve to the external load will depend on the load
pressure.
Rated flow is achieved with either a +100% or -100% electrical signal, at which
point the actual amount of rated flow depends on the valve pressure drop (inlet pressure
minus load pressure).

PROPORTIOANL VALVE :

PROPORTIONAL VALVE :
 Proportional control valves, which are also called electro-hydraulic proportional
valves, are similar to electro-hydraulic servo valves in that they both are electrically
controlled.
 However there are a number of differences between these two types of valves. For
example servo valves are used in closed-loop systems whereas pro proportional valves
are used in open-loop systems.
 In servo valves, electrical current in a torque motor coil causes either clockwise or
counter-clockwise torque on an armature to control the movement of the valve spool.
 On the other hand, a proportional valve uses a solenoid that produces a force
proportional to the current in its coils.
 Thus, by controlling the current in the solenoid coil, the position of the spring-
loaded spool can also be controlled. This means that unlike a standard so solenoid
valve, a proportional valve can provide both directional and flow control capability in
a single valve.

Hydraulics & pneumatics (AU) Unit-II

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Hydraulics & pneumatics (AU) Unit-II