Unit-3.pdf mechanical enginnering hydraulics

SUBJECT PRESENTATION: UNIT – 3
HYDARULIC AND PNEUMATIC CONTROL DESIGN
COURSE TYPE & L T P: OPEN ELECTIVE [3 – 1 – 0]
SEMSTER & SUBJECT CODE: 4TH & ME6039
COURSE CREDIT: 04
TOPIC: HYDRAULIC VALVES
(DIRECTIONAL, PRESSURE, FLOW AND PROPORTIONAL CONTROL VALVES)
Instructor
Dr. Gurjeet Singh
MECHANICAL ENGINEERING DEPARTMENT
PUNJAB ENGINEERING COLLEGE
DEEMED TO BE UNIVERSITY
CHANDIGARH-160012

Unit – 3: Hydraulic Valves
(Directional, Pressure and Flow Control Valves)
 Introduction to Directional Control Valve and Its Classifications
 Actuating Devices, Check Valves, Shuttle Valves
 Detailed Discussion on Directional Control Valves
 Applications of Directional Control Valves
 Mounting of Directional Control Valves
 Materials of Directional Control Valves
 Pressure and Flow Control Valves

Learning Objectives
Directional Control Valves
 Upon completion of this chapter, the student should be able to:
 List different types of valves used in fluid power.
 Explain various classifications of directional control valves.
 Describe the working and construction of various direction control valves.
 Identify the graphic symbols for various types of direction control valves.
 Explain the different applications of direction control valves.
 Explain the working principle of solenoid-actuated valves.
 Evaluate the performance of hydraulic systems using direction control
valves

Introduction
 One of the most important considerations in any fluid power system is
control. If control components are not properly selected, the entire system
does not function as required. In fluid power, controlling elements are called
valves.
 There are three types of valves:
 Directional control valves (DCVs): They determine the path through which
a fluid transverses a given circuit.
 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.
 Flow control valves: Shock absorbers are hydraulic devices designed to
smooth out pressure surges and to dampen hydraulic shock.

Introduction
 In addition, the fluid flow rate must be controlled in various lines of
a hydraulic circuit. For example, the control of actuator speeds can
be accomplished through use of flow control valves.
 Non-compensated flow control valves are used where precise speed
control is not required because the flow rate varies with pressure
drop across a flow control valve.
 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.

Types of Directional Control Valves {L15,L16,L17}
 Based on Fluid Path:
1. Check Valve, Shuttle Valve
2. Two /Three / Four Way Valve
3. Pilot Operated DCV
 Based on Design Characteristics:
1. Internal mechanism, Number of Switching Positions (2/3)
2. Number of Connecting Ports / Ways, Actuation Method.
 Based on Control Method:
1. Direct Control: Spool Valve, Indirect Control: Solenoid valve
 Based on Construction of Internal Moving Parts
1. Rotary Spool Valve
2. Sliding Spool Valve

Types of Pressure Control Valves {L18, L19}
1. Pressure Relief Valve
 Simple PRV
 Compound PRV
2. Pressure Reducing Valve
3. Unloading Valve
 Direct Unloading Valve
 Pilot Operated Unloading Valve
4. Counterbalance Valve
5. Pressure Sequence Valve

Types of Flow Control Valves {L20, L21}
 Non Pressure Compensated Valve
 Needle Type Flow Control Valve
 Needle Type Integrated Check Valve
2. Pressure Compensated Valve
3. Speed Control Circuit
 Meter-In Circuit
 Meter-Out Circuit
 Bleed Off Circuit
I. For Both Directions
II. For Inlet to Cylinder or Motor

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
system. A DCV does this by changing the position of internal movable parts.
 To be more specific, a DCV is mainly required for the following purposes:
1. To start, stop, accelerate, decelerate and change the direction of motion of a
hydraulic actuator.
2. 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.
3. To vent the relief valve by either electrical or mechanical control.
4. To isolate certain branch of a circuit.

 Classification of DCVs based Fluid Path
 Based on fluid path, DCVs can be classified as follows:
 Check valves.
 Shuttle valves.
 Two-way valves.
 Three-way valves.
 Four-way valves.
 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.

Classification of DCVs based on the Control Method
 Direct controlled DCV:A valve is actuated directly on the valve spool. This is
suitable for small-sized valves.
 Indirect controlled DCV:A valve is actuated by a pilot line or using a solenoid or by
the combination of electrohydraulic and electro-pneumatic means. The use of
solenoid reduces the size of the valve. This is suitable for large-sized valves.
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. The spool is fitted with precision into the body bore through
the longitudinal axis of the valve body. The lands of the spool divide this bore into a
series of separate chambers. The ports of the valve body lead into these chambers
and the position of the spool determines the nature of inter-connection between the
ports.

 The switching position, flow direction, and port for different
configurations is represented in Table 1.1. Two-way, three-way,
four-way and five-way representation is shown in Table 1.2.

Symbolic Notations for Directional Control Valves
This Table shows:
1. switching position,
2. flow direction, and
3. Port for different configurations

This Table shows Two-way, three-way, four-way and five-way DCVs

1. Each different switching position is shown by a square.
2. Flow directions are indicated by arrows.
3. Blocked ports are shown by horizontal lines.
4. Ports are shown in an appropriate flow direction with line arrows

Actuating Devices
 Direction control valves may be actuated by a variety of methods.
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. Graphical symbols of such combinations are given in Table.
 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.

Actuating Devices
 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 creates a magnetic force 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.

Unit-3.pdf mechanical enginnering hydraulics

Check Valve
 The simplest DCV is a check valve. A check valve allows flow in one
direction, but blocks the flow in the opposite direction.
 It is a two-way valve because it contains two ports.
 Figure shows the graphical symbol of a check valve along with its no-
flow and free-flow directions.
 In the same figure, a light spring holds the ball against the valve seat.
Flow coming into the inlet pushes the ball off the seat against the light
force of the spring and continues to the outlet.
 A very low pressure is required to hold the valve open in this direction.
If the flow tries to enter from the opposite direction, the pressure pushes
the ball against the seat and the flow cannot pass through.

Ball-Type Check Valve.
Poppet Check Valve:
(a) Open Position
(b) Closed position

Raising and lowering large weights using Check Valve
(a Extend cylinder; (b) hold cylinder; (c) return cylinder.

Advantages of a Poppet Valve
 Virtually zero leakage in closed position.
 Poppet elements do not stick even when left under pressure for long periods.
 Fast, consistent response time: typically 15 millisecond.
Disadvantages of a Poppet Valve
 Axial pressure balance is impossible and considerable force may be needed
to open the poppet against the flow at a high pressure. This limits valves
that have direct mechanical actuation to low flow duties.
 Generally individual poppets are required for each flow path that
significantly increases the complexity of multi-port valves.
 Lapping and super finishing of valves add cost.

Pilot-Operated check Valve
 A pilot-operated valve along with its symbol is shown in Fig. above. This type of
check valve always permits free flow in one direction but permits flow in the
normally blocked opposite direction only if the pilot pressure is applied at the
pilot pressure point of the valve.
 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
build-up at the bottom of the piston.
 The dashed line in the graphical symbol represents the pilot pressure line
connected to the pilot pressure port of the valve.
 Pilot check valves are used for locking hydraulic cylinders in position.

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. Figure shows the
operation of a shuttle valve.
 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.
 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. Graphical
symbol of shuttle valve is shown in Fig.

Shuttle Valve: (a) Flow from left to outlet and (b) flow from right to outlet
Graphical Symbol of
Shuttle Valve

Use of shuttle valves to control
single-acting cylinders (forward).
Use of shuttle valves to control
single-acting cylinders (return).

2/2 DCV normally opened.
(a) Ports A and P are
connected when force is not
applied (valve unactuated).
(b) Ports A and P are not
connected when force is
applied.
Two-way–two-position
normally closed DCV. (a)
Ports A and P are not
connected when force is not
(b) Ports A and P are
connected when force is
applied (valve actuated).

Three-Way Direction Control
 3/2-Way DCV (Normally Closed)
 Three-way valves either block or allow flow from an inlet to an
outlet. They also allow the outlet to flow back to the tank when the
pump is blocked, while a two-way valve does not. A three-way valve
has three ports, namely, a pressure inlet (P), an outlet to the
system(A) and a return to the tank (T).
 Figure shows the operation of a 3/2-way valve normally closed. In its
normal position, the valve is held in position by a spring as shown in
Fig. In the normal position, the pressure port P is blocked and outlet
A is connected to the tank. In the actuated position shown in Fig. The
pressure port is connected to the tank and the tank port is blocked.

3/2-way DCV
(a) Ports A and T are connected when force is not
(b) Ports A and P are connected when force is
applied (valve actuated).

3/2-way DCV
(a) Ports A and P are connected when
force is not applied (valve
unactuated).
(b) Ports A and T are connected when
force is applied (valve actuated).

Application of 3/2 DCV valve–filling and draining a vessel:
(a) hold; (b) fill; (b) drain.

 Four-way DCVs are capable of controlling double-acting cylinders and
bidirectional motors.
 Figure shows the operation of a typical 4/2 DCV. A four-way has four ports
labeled P,T,A and B.
 P is the pressure inlet and T is the return to the tank; A and B are outlets to
the system.
 In the normal position, pump flow is sent to outlet B. Outlet A is connected
to the tank.
 In the actuated position, the pump flow is sent to port A and port B
connected to tank T. In four-way DCVs, two flows of the fluids are
controlled at the same time, while two-way and three-way DCVs control
only one flow at a time.
 Figure shows the complete graphic symbol for a four-way two-piston DCV.

Application of 4/2-way valve – control of single-acting cylinder:
(a) return; (b) extend.

Application of 4/2-way valve – control of a
bi-directional motor.

Solenoid-Actuated Valve
 A spool-type DCV can be actuated using a solenoid as shown 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.
 Like mechanical or pilot actuators, solenoids work against a push pin, which in turn
actuates a spool. There are two types of solenoid designs used to dissipate the heat
developed in electric current flowing in the coil. The first type dissipates the heat
into surrounding air and is referred to as an “air gap solenoid.”
 In the second type “wet pin solenoid,” the push pin contains an internal passage
way that allows the tank port oil to communicate between the housing of the valve
and the housing of the solenoid. Wet pin solenoids do a better job in dissipating heat
because the cool oil represents a good heat sink to absorb heat from the solenoid.
 As the oil circulates, the heat is carried into the hydraulic system where it can be
easily dealt with.

Applications of Pilot-Operated Valve to Control the Table of a Surface Grinder
 Figure shows the application of a pilot-operated DCV where the actuation of a
double-acting cylinder is used to reciprocate the table of a surface grinder.
 The table is fitted with adjustable stops as shown in the figure. The pilot valve is
a DCV that is actuated by a push button. During the operation when stop S1
hits push button B1, the pilot valve sends a pilot signal to the main valve to shift
the configuration shown in the right envelope of the main valve.
 This actuates the double-acting cylinder to extend. At the end of the extension,
stroke S2 hits push button B2, which causes the pilot signal directions to be
reversed. Due to this change in the pilot signal direction, the main valve moves
to the configuration shown in the left envelope of the main valve.
 This in turn actuates the double-acting cylinder to retract. Thus, a pilot valve
controls a main valve and the main valve used to control the double-acting
cylinder.

Application of Pilot – Operated DCVs.

Regenerative Circuit (Position 1, 2 & 3).

Pressure Control Valve
 Hydraulic energy is produced as long as the prime mover (usually an
electric motor) drives the pump, and hydraulic pressure develops by
resistance to pump flow.
 Hence, the hydraulic system suffers damage if the pump flow is not stopped
or off loaded (recirculate) back to the tank during non-action periods of the
circuit. Non-action periods arise from stalling an actuator, or by reaching
the end of the stroke or the circuit sequence, or during the time-delay
periods of the circuit sequence.
 In order to avoid hydraulic system damage, power wastage and overheating
of the hydraulic fluid, circuit designers use a variety of cleverly designed
systems to control maximum system pressure and pump flow during non-
action periods.

Pressure Control Valve
 Pressure-control valves are used in hydraulic systems to control actuator
force (force = pressure × area) and to determine and select pressure levels
at which certain machine operations must occur. 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.
 Unloading system pressure.
 Assisting sequential operation of actuators in a circuit with pressure
control.
 Any other pressure-related function by virtue of pressure control.
 Reducing or stepping down pressure levels from the main circuit to a lower
pressure in a sub-circuit.

SPRINGS
1 KG
2 KG
3 KG
We know that under increasing
force springs deform more.

SIMPLE PRESSURE RELIEF VALVES & ITS SYMBOLIC NOTATION

SIMPLE & COMPOUND PRESSURE RELIEF VALVES

PRESSURE REDUCING VALVE & ITS SYMBOLIC NOTATION

UNLOADING VALVE & ITS SYMBOLIC NOTATION

COUNTERBALANCE VALVE & ITS SYMBOLIC NOTATION

PRESSURE SEQUENCE VALVE & ITS SYMBOLIC NOTATION

Introduction To Flow Control Valves
 Flow-control valves, as the name suggests, control the rate of flow of a fluid
through a hydraulic circuit.
 Flow-control valves accurately limit the fluid volume rate from fixed
displacement pump to or from branch circuits.
 Their function is to provide velocity control of linear actuators, or speed
control of rotary actuators.
 Typical application 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 on a priority basis.

Introduction To Flow Control Valves
 Typically, fixed displacement pumps are sized to supply maximum system
volume flow rate demands.
 For industrial applications feeding two or more branch circuits from one
pressurized manifold source, an oversupply of fluid in any circuit operated
by itself is virtually assured.
 Mobile applications that supply branch circuits, such as the power steering
and front end loader from one pump pose a similar situation.
 If left unrestricted, branch circuits receiving an oversupply of fluid would
operate at greater than specified velocity, increasing the likelihood of
damage to work, hydraulic system and operator.

Functions of Flow-Control Valves
 Regulate the speed of linear and rotary actuators
 Regulate the power available to sub-circuits by controlling
flow to them
 Proportionally divide or regulate pump flow to various
branches of circuit
Classification of Flow-Control Valves
 Flow-control valves can be classified as follows:
 Non-pressure compensated.
 Pressure compensated.

Simple Restrictor Type Flow Control Valves

 Non-pressure-compensated flow-control valves are used when the system pressure
is relatively constant and motoring speeds are not too critical.
 The operating principle behind these valves is that the flow through an orifice
remains constant if the pressure drop across it remains the same.
 In other words, the rate of flow through an orifice depends on the pressure drop
across it.
 The disadvantage of these valves is discussed below. 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.
 This is an extremely important problem to be addressed in hydraulic circuits where
the load and pressure vary constantly.

Non Pressure Compensated Needle Type Flow Control Valve.
(a) Fully closed; (b) partially opened; (c) fully opened.

 Schematic diagram of non-pressure-compensated needle-type flow-control valve is
shown in Fig.
 It is the simplest type of flow-control valve. It consists of a screw (and needle) inside
a tube-like structure.
 It has an adjustable orifice that can be used to reduce the flow in a circuit. The size
of the orifice is adjusted by turning the adjustment screw that raises or lowers the
needle.
 For a given opening position, a needle valve behaves as an orifice. Usually, charts
are available that allow quick determination of the controlled flow rate for given
valve settings and pressure drops.
 Sometimes needle valves come with an integrated check valve for controlling the
flow in one direction only.
 The check valve permits easy flow in the opposite direction without any restrictions.
As shown in Fig. only the flow from A to B is controlled using the needle.
 In the other direction (B to A), the check valve permits unrestricted fluid flow.

Flow Control Valve with an integrated check 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.
 This is accomplished through a spring-loaded compensator spool
that reduces the size of the orifice when pressure drop increases.
 Once the valve is set, the pressure compensator acts to keep the
pressure drop nearly constant. It works on a kind of feedback
mechanism from the outlet pressure. This keeps the flow through the
orifice nearly constant.

 Schematic diagram of a pressure compensated flow-control valve is
shown in Fig. and its graphical symbol in Fig.
 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 biases 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 an equilibrium position as far as the flow is concerned.
 In the static condition, the hydraulic forces hold the compensator
spool in balance, but the bias spring forces it to the far right, thus
holding the compensator orifice fully open.
 In the flow condition, any pressure drop less than the bias spring
force does not affect the fully open compensator orifice, but any
pressure drop greater than the bias spring force reduces the
compensator orifice.

 Any change in pressure on either side of the control orifice, without a
corresponding pressure change on the opposite side of the control orifice,
moves the compensator spool.
 Thus, a fixed differential across the control orifice is maintained at all times.
It blocks all flow in excess of the throttle setting. As a result, flow exceeding
the preset amount can be used by other parts of the circuit or return to the
tank via a pressure-relief valve.
 Performance of flow-control valve is also affected by temperature changes
which changes the viscosity of the fluid. Therefore, often flow-control valves
have temperature compensation.
 Graphical symbol for pressure and temperature compensated flow-control
valve is shown in Fig.

Sectional view of a pressure-
compensated flow-control
valve.

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