Directional control valves (DCVs) direct hydraulic fluid flow and are categorized by the number of ports and positions of the internal spool. Common types include 2-way 2-position and 4-way 3-position valves. DCVs are usually actuated electrically via solenoids or hydraulically. Check valves allow fluid flow in one direction only and are used to hold pressure or for safety. Pilot-operated check valves are remotely controlled by a directional valve via a pilot pressure line.

1. DIRECTIONAL CONTROL VALVE
• The primary function of a DC valve is to direct
or prevent fluid flow to specific piping or system
actuator.
• These valves usually consists of spool inside it
which is electrically/hydraulically controlled.
• There are two fundamental positions of these
valves :
1. Working Position
2. Normal Position

2. According to no. of valve ports and spool position
DC valve can be categorized into following :
1. 2-way, 2-position
2. 3-way, 2-position
3. 4-way, 2-position
4. 4-way, 3-position and many more…

3. Common Abbreviation used in Hydraulics
P =Pressure line
T =Tank Line
A,B =System Actuator line
X =Pilot Line
Y =Drain Line

4. Spool Position

5. 2 - Way, 2 – Position DC Valve

6. 3 –Way, 2 - Position DC Valve

7. 4 –Way, 2 –Position DC Valve

8. 4 –Way, 3 –Position DC Valve

9. Electrically actuated DC Valves are actuated
by means of solenoid. They can be single/double
actuated. When these solenoids are
the DC valves are shifted to Working
solenoid
energized
position.
Along with solenoid, there are spring
actuators which are used to shift the spool of a DC
valve to normal position when the solenoid is de-
energized.

10. Single solenoid, 2 –
position, Spring Offset

11. Double Solenoid, 2 - Position

12. Double Solenoid, 3 –Position,
Spring Centre

13. 1. Continuous line –Flow line
2. Dashed line –Pilot , drain
3. Spring
4. Flow Restriction
5. Single Acting Cylinder
6. Double Acting Cylinder
7. 2-way, 2-position DC valve (NC)
8. 2-way, 2-position DCvalve (NO)
9. 3-way, 2-Position DCvalve(NO)
10. 3-way, 2-Position DCvalve (NC)
CommonHydraulic Symbols

14. 11. 4-Way, 2-position DC valve
12. 4-way, 3-position DC valve
13. Solenoid actuated
14. Hydraulic Actuated
15. Check Valve
16. Pilot operated Check Valve
17. Pressure Relief valve

15. It is an electro-mechanical device that take
electrical energy and produces a linear force by the use
of magnetism. It is basically a winding of wire around a
metal core.
SOLENOID ACTUATED DC VALVES

16. When the solenoid is energized, the air gap is
closed quickly and a force is developed in the direction
of the valve spool.

17. SOLENOID CONTROLLED, PILOT-OPERATED
DCV
Although a valve could be shifted directly by the
force of a solenoid, large flow DCVs are most often
shifted using fluid at system pressure. Larger flow
valves demand larger shifting forces and it is no longer
practical to use a solenoid there.

18. CHECK VALVE
• A check valve, non-return valve or one-way
valve is a valve that normally allows fluid to flow
through it in only one direction.
• Check valves are two-port valves, meaning they
have two openings in the body, one for fluid to enter
and the other for fluid to leave.
• An important concept in check valves is the
cracking pressure which is the minimum upstream
pressure at which the valve will operate.

20. PILOT OPERATED CHECK VALVE
• This type of check valves are remotely operated
through any one of the directional valves.
• This is controlled by a pilot pressure line
which is indeed controlled by a directional control
valve.

21. Use of Circuit Symbols
Circuit symbols are used for components to enable clear representation of
hydraulic systems in diagrams
A symbol identifies a component and its function
The symbols in this course are based on DIN ISO 1219
The most important symbols are dealt with in this basic hydraulics course.
3. Hydraulic Components Symbols
3.1 Transfer of energy and condition of the pressure medium
Introduction
These symbols are
used in circuit
diagrams for energy
transfer components
and condition of
pressure medium

22. 3. Hydraulic Components Symbols
Transfer of energy and condition of the pressure medium

23. Hydraulic Components Symbols
Hydraulic pump Hydraulic motor Type of Fluid
Pumps and Motors
Measuring Devices

24. Squares with arrows inside are used
for DCV symbol
Number of squares indicate number
of switching positions
Arrows within squares indicate flow
direction and how the ports are
interconnected in various switching
positions
P, T, R, A, B, L designations or labels
denote ports (A, B, C, D, E, …. Is
sometimes used)
The rest position is defined as the
position automatically assumed by
the valve in the absence of an
actuating force.
Hydraulic Components Symbols
Direction Control Valves (DCV)

25. Direction Control Valves (DCV) – Normally closed/Normally open - 2/2way and 3/2way
Direction Control Valves (DCV) – 4/3 way mid position closed or pump recirculation
3. Hydraulic Components Symbols

26. • Direction Control Valves (DCV) – Method of Actuation
3. Hydraulic Components Symbols

27. Square used for symbol
Ports labels used: P, T or A,B
Position of valve inside
square indicates whether
valve is normally open or
normally closed
Are either fixed or adjustable
pressure setting
Divided into Pressure relief
valve and pressure regulator
Pressure Valves
Fixed or Adjustable ,Example: Pressure Relief
valve
Hydraulic Components Symbols

28. 3.6 Flow Control Valves
• Provide means to adjust
speed of the drive
component
3.7 Non-return valves and shut-off valves
Hydraulic Components Symbols

29. 3.8 Hydraulic actuators 3.9 Non-return valves and shut-off valves
Single acting cylinders
Cylinder
Cushioning
3. Hydraulic Components Symbols

30. 3.10 Combination of Devices
If several devices are brought together in a single housing, the symbols for the individual
devices are placed into a box made up of broken lines from which the connections are ready
for plugging
Example:
3. Hydraulic Components Symbols

31. 4. Hydraulic Circuit Illustration
Circuit with 4/3-way valve -
pump by-pass (re-
circulating)
31
• Circuit with 3/2-way
valve
31
• Circuit with 4/2 –way
valve
31

32. X: Pilot line
A: I/P line to the checkvalve
B : O/P line from the check valve
• The main application of check valves is to hold
the pressure of a load and for safety purposes.

33. PROPORTIONAL VALVE
• The proportional DCV is actuated by means of
an electrical control signal. The control signal
influences the flow rate and flow direction.
• It is exactly the same as the solenoid valve
except that solenoid valve acts as ON/OFF switch
whereas this proportional solenoid valve can achieve
each and every point in between thus varying the flow
rate.
• Different flow rate is achieved by varying the i/p
current signal.

34. •The difference between a solenoid and a proportional
solenoid valve is in the construction of their spool.

35. • The Servo valve has a hydraulic pressure inlet and an
electrical input for the torque motor. The input current controls the
flapper position. The flapper position controls the pressure in rod
side or piston side of the cylinder. So, a current (+ or -) will position
the flapper, leading to a delta pressure on the servo, which cause
the servo to move in one direction or the other. Movement of the
servo ports hydraulic pressure to one side of the actuator or the
other, while porting the opposite side of the actuator to return.
• Flapper position is controlled by the electromagnetic torque
motor. A torque motor consists of two permanent magnets with a
coil winding attached to a magnetically permeable armature. The
armature is part of the flapper piece. When a current is applied to
the coils, magnetic flux acting on the ends of the armature is
developed. The direction of the magnetic flux (force) depends on the
sign (direction) of the current.

36. • The magnetic flux will cause the armature tips to be
attracted to the ends of the permanent magnets (current direction
determines which magnetic pole is attracting and which one is
repelling). This magnetic force creates an applied torque on the
flapper assembly, which is proportional to applied current.
• As the applied current is increased, the armature and
flapper will rotate. As the flapper moves closer to one nozzle, the flow
area through this nozzle is decreased while the flow area through
the other nozzle increases.
• In the above the figure the flapper nozzle consists of the
flapper, two inlet orifices (O1 and O2), two outlet nozzles (n1 and n2),
nozzle backpressure nozzle (n3) and usually a feedback spring. As
described above, the torque motor positions the flapper, which in
turns controls the flow through the nozzles. When the flapper is in
the neutral position, the nozzle flow areas are equal and the
pressures Pn1 and Pn2 are equal. When the flow areas and inlet
nozzle pressures are equal, the flow forces through each nozzle keep
the flapper centered in the neutral position.

37. • As the flapper moves towards one of the nozzles, the outlet
flow area is reduced for this nozzle. Outlet flow area increases for
the other nozzle. For example, looking at Figure let the flapper
move towards the n1 nozzle. This will reduce the outlet flow area
and the pressure Pn1 will increase. At the same time, the outlet
flow area at the n2 nozzle will increase and the pressure Pn2 will
decrease. A delta pressure ΔP = Pn1 – Pn2 will occur across the pilot
spool piston and the pilot spool will displace to the right. High
pressure fluid will then flow to the PA actuator chamber while the
PB actuator chamber is ported to return.

38. • Servo valves are normally used when accurate position
and force control is required.
• The main advantage of a servo valve is that a low power
electrical signal can be used to accurately position an actuator or
motor.
• The disadvantage is complexity and cost which results
from a component consisting of many detail parts manufactured
to very tight tolerances. Therefore, servo valves should only be
used when accurate position (or rate) control is required.

39. SERVO VALVE CONNECTOR

40. PIN FUNCTION VOLTAGE /
CURRENT RATING
A SUPPLY VOLTAGE 24 V DC
B GROUND
C ENABLE INPUT < 6. 5 VDCDISABLED
D COMMAND SIGNAL I/P -10 to +10 mA OR
E 4 – 20 mA
F COMMAND SIGNAL O/P 4 – 20 mA
G P.E.

41. SERVO CONNECTOR
24 v dc
A B C D E
G CONNECTOR
(4 – 20 mA)
X Y
POTENTIOMETER

42. In No Load Condition
When the wiper is at ‘X’ end :
Current across DE will be, 24/970 = 24 mA (Apprx. )
When the wiper is at ‘Y’ end :
Current across DE will be 0 mA.