Pneumatic and hydraulic actuation systems: Pneumatic and hydraulic systems actuating systems. Classifications of Valves: Pressure relief valves, Pressure regulating / reducing valves Cylinders and rotary actuators. DCV & FCV: Principle & construction details. Types of sliding spool valve & solenoid operated. Symbols of hydraulic elements, components of hydraulic system, functions of various units of hydraulic system. Design of simple hydraulic circuits for various applications

1. Department of Mechanical Engineering
JSS Academy of Technical Education, Bangalore-560060
MECHATRONICS
(Course Code:15ME753)

2. MECHATRONICS
CHAPTER 8: Pneumatic Actuation systems

3. TEXT BOOKS
• Mechatronics Electronic control system in Mechanical and Electrical Engineering, W Bolton,
Pearson Education, 1st Ed., 2005.
REFERENCE BOOKS:
• Mechatronics by HMT Ltd. - Tata McGraw-Hill, 1st Edition, 2000
Further Reference:
National Programme on Technology Enhanced Learning (NPTEL)
https://nptel.ac.in/courses/112103174/1 by Dr. S. N. Joshi (IITG)

4. Module 5
• Pneumatic and hydraulic actuation systems: Pneumatic and hydraulic systems actuating systems.
• Classifications of Valves: Pressure relief valves, Pressure regulating / reducing valves
• Cylinders and rotary actuators.
• DCV & FCV: Principle & construction details.
• Types of sliding spool valve & solenoid operated.
• Symbols of hydraulic elements, components of hydraulic system, functions of various units of
hydraulic system.
• Design of simple hydraulic circuits for various applications.

5. • Understand concepts and working of Pneumatic and hydraulic actuation systems.
• Understand concepts and working of direction control valves and flow control valves.
Learning Objectives

6. Actuation systems are the elements of control systems, responsible for transforming the output of a
microprocessor or control system into a controlling action on a machine or device.
Example:
• An electrical output from the controller which has to be transformed into a linear motion to move
a load.
• An electrical output from the controller has to be transformed into an action which controls the
amount of liquid passing / flowing through a pipe.
• Pneumatics is the term used when compressed air is used
• Hydraulics when a liquid, typically oil.
Introduction
Actuation systems

7. • In hydraulic system, pressurized oil is provided by a
pump driven by an electric motor.
• The pump pumps oil from a sump through a non-return
valve and an accumulator to the system, from which it
returns to the sump.
• Figure illustrates the arrangement.
• A pressure-relief valve is included, being to release the
pressure if it rises above a safe level.
• The non-return valve is to prevent the oil being back
driven to the pump.
• The accumulator is to smooth out any short-term
fluctuations in the output oil pressure.
Hydraulic systems
Hydraulic power supply

8. • An accumulator is just a container in which the oil is held under
pressure against an external force as shown in Figure.
• The most commonly used form is a gas pressurized and involves gas
within a bladder in the chamber containing the hydraulic fluid;
• An older type involved a spring-loaded piston.
• If the oil pressure rises then the bladder contracts, increases the
volume the oil can occupy and so reduces the pressure.
• If the oil pressure falls, the bladder expands to reduce the volume
occupied by the oil and so increases its pressure.
Hydraulic systems

9. Positive displacement pumps: Rotary
1. Gear pump
2. Vane pump
3. Piston pump.
Classification of Hydraulic pump
Hydraulic pumps
Positive displacement pump
Non Positive displacement pump, E.g.: Centrifugal Pump
Positive displacement pumps: Reciprocating
1. Plunger
2. Piston
3. Diaphragm
4. Circumferential Piston

10. Classification of Hydraulic pump
Non Positive displacement pump, E.g.: Centrifugal Pump

11. Difference between Non positive and positive displacement pump
1. Positive displacement pump
A positive displacement pump provides a constant flow at fixed speed, regardless of changes in
pressure.
2. Non Positive displacement pump
• A non-positive displacement pump’s output flow rate can change in response to the pressure on
the outlet.
• Use Newton’s first law of motion to move fluid against the system resistance.
• Their flow output is reduced as the system resistance is increased.

12. Difference between Non positive and positive displacement pump
Positive Displacement Pumps Non-Positive Displacement Pumps
Pressure
Work in high pressure applications, up to
800 bar
Used for low pressure application, maximum
pressure of 18 to 20 bar
Efficiency
Efficiency increases with increasing
pressure
Efficiency peaks at best-efficiency-point.
At higher or lower pressures, efficiency
decreases
Viscosity
Efficiency increases with increasing
viscosity
Efficiency decreases with increasing viscosity
due to frictional losses inside the pump
Performance Flow is constant with changing pressure Flow varies with changing pressure

13. Gear pump
The gear pump consists of two close-meshing gear wheels which
rotate in opposite directions as shown in fig.
Fluid is forced through the pump, it becomes trapped between the
rotating gear teeth and the housing and is transferred from the inlet
port to be discharged at the outlet port.

14. Gear pump
Advantages:
• Low cost, robust & work smoothly
• Operate well at pressures around 210 bar.
• Pump with higher speeds up to 3000-6000 rpm.
• The maximum flow capacity is about 0.5 m3/min.
Disadvantages:
• Gears gradually wear down the housing and/or main bushings,
reducing the volumetric efficiency of the pump.
Applications:
• Used to pump Petrochemicals, chemicals, resins, adhesives.
• Chemical additive & polymer metering.
• Chemical mixing and blending.
• Food: Chocolate, fillers, sugar, vegetable fats and oils

15. Vane pump
• The vane pump has spring-loaded sliding vanes slotted in a driven
rotor as shown in fig.
• As the rotor rotates, the vanes follow the contours of the casing.
• This results in fluid becoming trapped between successive vanes
and the casing and transported round from the inlet port to outlet
port.
• As the area between the vanes decreases on the outlet side and
increases on the inlet side of the pump, oil is drawn in through the
supply port and expelled through the outlet as the vane cartridge
rotates due to the change in area.

16. Vane pump
Advantages:
• Handles thin liquids at relatively higher pressure.
• Can run dry for short periods.
• Develops good vacuum.
• Compensates for wear through Vane extension.
Disadvantages:
• Complex housing and many parts.
• Not suitable for high pressures & Viscosity
• Not good with abrasives.
• Have fixed clearance.
Applications: Pumping Refrigerants,
• Aerosol and propellants, LPG cylinder filling, Aqueous
solution, solvents, Bulk transfer of LPG & NH3.

17. Piston pump
• Piston pumps used in hydraulics can take a number of
forms.
• In radial piston pump shown in fig. a cylinder block rotates
round the stationary cam and this causes hollow pistons,
with spring return, to move in and out.
• This results in, the fluid is drawn in from the inlet port and
transported round for ejection from the discharge port.

18. Piston pump
• In Axial piston pump shown in fig. has pistons which
move axially rather than radially.
• Pistons are arranged axially in a rotating cylinder
block and made to move by contact with the swash
plate.
• This plate is at an angle to the drive shaft.
• As the shaft rotates they move the pistons so that air
is sucked in when a piston is opposite the inlet port
and expelled when it is opposite the discharge port.
Applications: Pumps can be used like an automotive air
conditioning compressor, a separate pump, otherwise a
hydraulic motor.

19. • In a pneumatic power supply shown in fig. an electric
motor drives an air compressor.
• The air inlet to the compressor is likely to be filtered and
via a silencer to reduce the noise level.
• A pressure-relief valve provides protection against the
pressure in the system rising above a safe level.
• Since the temp. increases there is a cooling system.
• To remove contamination and water from the air, filter with
a water trap is provided.
• An air receiver increases the volume of air in the system
and smoothes out any short-term pressure fluctuations.
Pneumatic systems
Pneumatic power supply

20. Pneumatic pumps
1. Single-acting, single stage, vertical, reciprocating compressor
2. rotary vane compressor
3. Screw compressor.
Classification of Pneumatic pump

21. • Air intake stroke: The descending piston causes air to be sucked into the
chamber through the spring-loaded inlet valve and when the piston starts to
rise again, the trapped air forces the inlet valve to close and so becomes
compressed.
• When the air pressure has increased sufficiently, the spring-loaded outlet
valve opens and the compressed air flows out of the system.
• After the piston has reaches the TDC, it then begins to descend and the
cycle repeats.
• Compressor is termed single-acting because one pulse of air is produced
per stroke; Also, compressor goes directly from atmospheric pressure to the
required pressure in a single operation.
• In double-acting compressors, produce pulses of air on both the up and
down strokes of the piston.
Single-acting, single stage, vertical, reciprocating compressor

22. • For the production of compressed air at more than a few bars, two or
more stages are generally used.
• Normally two stages are used for pressures up to about 10 to 15 bar and
more stages for higher pressures.
Single-acting, single stage, vertical, reciprocating compressor

23. • In Rotary Vane compressor, a rotor mounted eccentrically in a cylindrical
chamber as shown in fig.
• The rotor has blades, the vanes, which are free to slide in radial slots with
rotation causing the vanes to be driven outwards against the walls of the
cylinder.
• As the rotor rotates, air is trapped in pockets formed by the vanes and as the
rotor rotates so the pockets become smaller and the air is compressed.
• Compressed packets of air are thus discharged from the discharge port.
Rotary Vane Compressor
• Single-stage, rotary vane compressors are used for pressures up to 800 kPa,
with flow rates of the order of 0.3 m3/min to 30 m3/min.

24. • The Screw compressor has two intermeshing rotary screws which rotate in
opposite directions.
• As the screws rotate, air is drawn into the casing through the inlet port and
into the space between the screws.
• This trapped air moves along the length of the screws and compressed as
the space becomes progressively smaller, emerging from the discharge
port.
Screw Compressor
• The single-stage, rotary screw compressors used for pressures up to about
1000 kPa with flow rates of between 1.4 m3 /min and 60 m3/min.

25. MECHATRONICS
DCV & FCVDirectional and Flow control valves

26. Valves
• Valves are used with hydraulic and pneumatic systems to direct and regulate the fluid flow.
• There are basically two forms of valve;
1. The finite position valves
2. The infinite position valves.
• The finite position valves are ones where the action is just to allow or block fluid flow, to switch
actuators on or off. Also, used for directional control to switch the flow from one path to another.
• The infinite position valves are able to control flow between fully on and fully of.
• Used to control varying actuator forces or the rate of fluid flow for a process control situation.

27. Valves
• Pneumatic and hydraulic systems use directional control valves to direct the flow of fluid through
a system / Start, stop and change the direction of fluid flow
• They are not intended to vary the rate of flow of fluid but are either completely open or completely
closed, i.e. on/off devices.
• Such on/off valves are widely used to develop sequenced control systems.
• They might be activated to switch the fluid flow direction by means of mechanical, electrical or
fluid pressure signals.
Directional control valves (DCV)

28. Valves
Directional control valve are classified as;
Directional control valves
Construction
• Spool valve
• Poppet valve
No. of Ports
• Two way valve
• Three way valve
• Four way valve
No. of Switching position
• Two position
• Three position
Actuating Mechanism
• Manual
• Mechanical
• Hydraulic
• Pneumatic
• Solenoid

29. Valves
Spool Valve
Directional control valves
• Spool valves can be used in both hydraulics or Pneumatics and their job is to control the
flow direction of the energy source by combining or switching the paths through which the
oil or air can travel.

30. Directional control valves
Spool Valve
• Spool valve is a cylinder inside a sealed outer case.

31. Directional control valves - Spool Valve
• In (a) the air supply is connected to port 1 and port 3 is closed. Thus the device connected to port 2
can be pressurised.
• When the spool is moved to the left (b) the air supply is cut off and port 2 is connected to port 3. Port
3 is a vent to the atmosphere and so the air pressure in the system attached to port 2 is vented.
• Thus, movement of the spool has allowed the air firstly to flow into the system and then be reversed
and flow out of the system.
(b)(a)

32. Valves
Directional control valves
Spool Valve
• Rotary spool valve.
• Rotary spool valves have a rotating spool which, when it rotates, opens and closes ports in a
similar way.

33. DCV - Poppet valve
• The valve is normally in the closed condition, being no connection between port 1 to which the pressure
supply is connected and port 2 to which the system is connected.
• In poppet valves, balls, discs or cones are used in conjunction with valve seats to control the flow.
• In the fig. a ball is shown.
• When the push-button is pressed, the ball is pushed out of its seat and flow occurs as a result of port 1 being
connected to port 2.
• When the button is released, the spring forces the ball back up against its seat and so closes off the flow.

34. Valve symbols
• The symbol used for a control valve consists of a square for each of its switching positions.
• For poppet valve there are two positions; one with the button not pressed and one with it pressed.
• Thus, a two-position valve will have two squares, a three-position valve three squares.

35. Valve symbols
• In (a) Arrow-headed lines are used to indicate the directions of flow in each of the positions, with
blocked-off lines indicating closed flow lines (b).
• In (c) the initial position of the valve has the connections to the ports shown;
• In (c) the valve has four ports.
• Ports are labelled by a number or a letter according to their function.
• The ports are labelled 1 (or P) for pressure supply, 3 (or T) for hydraulic return port, 3 or 5 (or R or
S) for pneumatic exhaust ports, and 2 or 5 (or B or A) for output ports.
(a) Flow path (b) flow shut-off (c) initial

36. Valve Actuation symbols
• The above fig. (a) shows examples of some of the symbols which are used to indicate the
various ways the valves can be actuated.
• More than one of these symbols might be used with the valve symbol.
Valve actuation symbols

37. Valve symbols
• The above fig. (a) shows examples of some of the symbols which are used to indicate the
various ways the valves can be actuated.
• More than one of these symbols might be used with the valve symbol.
Valve actuation symbols

38. Valve Symbols
1. P or 1 = Ports for pressure supply.
2. T or 3 = For hydraulic return port.
3. 3 or 5 or R or S = For pneumatic exhaust ports.
4. 2 or 5 or B or A = For output ports.
1. P or 1 = Pressure line.
2. T or 3 = Return line.
3. A & B = Cylinder line.

39. Valve Symbols

40. Valve Symbols
• The number of position and flow boxes makes a valve
symbol, indicating the number of positions the valve has.
• Flow is indicated by the arrows in each box.
• The arrows represent the flow paths the valve has when
it is in that position.

41. Valve Symbols
• In three position valve, the center position shows the flow path when neither actuator is active, and
the springs are holding the valve in the center position.
• Here, the center box indicates that there will be no air flow unless one of the two actuators is active.
3 Position, Double Solenoid Actuated,
Spring Return Valve

42. Valve Symbols
Ports
• The number of ports is shown by the number of end points in a given box.
• Count only the ports in one flow box per symbol.
• The other boxes just show different states of the same valve.
• In the above example, there are a total of 5 ports.
• Sometimes a port (usually an exhaust port) goes directly to atmosphere and there is no
mechanical means for attachment of silencers.
• To indicate this (in some flow diagrams), ports with attachment capability will have a short line
extending beyond the box (as shown on ports 1, 2, & 4), while the ports you cannot attach to will
not have the external line segment (ports 3 & 5 in this example).

43. Directional Control Valve symbols
Symbol Designation Description
2/2 way valve
Position
Port
(2 ports and 2
switching position)
• Two closed ports in normally closed and flow during
actuated position
3/2 valve
(3 ports and 2
switching position)
• In the first position flow takes place to the cylinder.
• In the second position flow takes out of the cylinder to
the exhaust (single acting cylinder)

44. Directional Control Valve symbols
Symbol Designation Description
4/2 way valve
(4 ports and 2
switching position)
• For double acting cylinder all the ports are open.
4/3 valve
(4 ports and 3
switching position)
• Two position and one closed normal position.

45. Directional Control Valve symbols
Symbol Designation Description
5/2 way valve • Two open positions with two exhaust ports.
3/4 way valve
• 3 position, 4 way, 4 ported, Center Closed

46. Valve symbols
• As an illustration, (b) shows the symbol for the two-port, two-position poppet valve.
• A two-port, two-position valve described as a 2/2 valve.
• The first number indicates the number of ports and the second number the number of positions.
• The valve actuation is by a push-button and a spring.
Valve actuation symbols
2/2 Valve
2 ports/2 way

47. Valve symbols
2/2 Valve 2 ports / 2 way/position

48. Solenoid-operated spool valve
• The fig. shows solenoid-operated spool valve and its symbol.
• The valve is actuated by a current passing through a solenoid
and returned to its original position by a spring.
3/2 Valve

49. 3/2 Valve
• The 3/2 valve is used to control items such as single acting cylinders which have a single input.
• The input to the cylinder is connected to port 2, the air supply to port 1 and port 3 is allowed to
exhaust to atmosphere.
• The number 3 signifies that the valve has three ports, whilst the number 2 signifies that the valve
has 2 directions or states.

50. 3/2 Valve

51. 3/4 Valve
• Three position four way (3/4) valves are used in double-acting cylinders to perform advance, hold and
return operation to the piston. Fig. show three position four way valves.
• These valves have three switching positions. Also, have a variety of possible flow path configurations but
have identical flow path configuration.
• When the spool is actuated, port A and B are connected with both the ports P and T.
• In this case, valve is not active because all the ports are open to each other.
• The fluid flows to the tank at atmospheric pressure. In this position work cannot be done by any part of the
system.
Three position four way valve: P to B and A to T

52. 3/4 Valve
• When left end (port B) is actuated, the port P is connected with ports B and T is connected with
port A as shown in Fig.
• Similarly, when the right end is actuated the port P is connected to A and working port B is
connected to port T.
Three position four way valve: P to A and B to TThree position four way valve: P to B and A to T

53. 3/4 Valve - Closed center position
• In closed center, all user ports (port A and port B) are closed. Therefore, these ports are hydraulically
locked and the actuator cannot be moved by the external load.
• The pumped fluid flows through the relief valve.
• The pump works under the high pressure condition, which wastes the pump power, leads to wear of the
pump parts.
• The fluid temperature also rises due to heat generation by the pump, leading to the oxidation and viscosity
drop of the fluid, in turn reduces the pump life.
Three position four way valve: Closed center

54. 3/4 Valve - Tandem centered valve
• In this configuration, the working ports A and B are blocked.
• The pump port P is connected to the tank port T.
• Tandem center results in the locked actuator.
• However, pump to tank flow takes place at the atmospheric temperature.
• This kind of configuration can be used when the load is needed to hold.
Three position four way valve: Tandem center

55. 3/4 Valve – Regenerative center
• Regenerative means the flow is generated from the system itself.
• Regenerative center is used when the actuator movement in one direction requires two different
speeds. For example, the half-length of the stroke requires fast movement during no-load
condition and remaining half-length requires slow motion during load conditions.
• The regenerative center saves the pump power.
Three position four way valve: Tandem center

56. • The fig. shows the symbol for a 4/2 valve.
• 2 working port (A, B), 1 pressure port and 1 tank connection.
Control delivery
• Normal position – flow from P to B and A to T
• Actuated position – flow from P to A and B to T.
4/2 Valve

57. • The 5/2 valve is used to control items such as double acting cylinders which have two inputs.
• The inputs to the cylinder are connected to ports 2 and 4, the air supply to port 1.
• Ports 3 and 5 are allowed to exhaust to atmosphere.
• The number 5 signifies that the valve has five ports, whilst the number 2 signifies that the valve has
2 directions or states
5/2 Valve

58. Application of Pneumatic systems - Lift system
• In lift, Two push-button 2/2 valves are used.
• When the button on the up valve is pressed, the load is lifted.
• When the button on the down valve is pressed, the load is lowered.
• In pneumatic systems an open arrow is used to indicate a vent to the atmosphere.

59. Pilot - operated valves
• The force required to move the ball or shuttle in a valve can often be too large for manual or
solenoid operation. To overcome this problem a pilot-operated system is used where one valve
is used to control a second valve.

60. Pilot - operated valves
• The pilot valve is small capacity, can be operated manually or by a solenoid.
• Used to allow the main valve to be operated by the system pressure.
• The pilot pressure line is indicated by dashes.
• The pilot and main valves can be operated by two separate valves but they are often combined
in a single housing.

61. Directional valves
• Figure shows a simple directional valve and its symbol.
• Free flow can only occur in one direction through the valve: which results in the ball being
pressed against the spring.
• Flow in the other direction is blocked by the spring forcing the ball against its seat.

62. Classification based on Actuation
1. Manual Actuation: The spool is operated manually. Manual actuators are hand lever, push
button, pedals etc.
2. Mechanical Actuation: Using mechanical elements such as roller and cam, roller and plunger,
rack and pinion.
3. Solenoid Actuation: Also known as electrical actuation. The energized solenoid coil creates a
magnetic force which pulls the armature into the coil. This movement of armature controls the
spool position.
Solenoid Actuation

63. Classification based on Actuation
4. Hydraulic Actuation: Known as Pilot-actuated valve.
In this type of actuation, the hydraulic pressure is directly applied on the spool. The pilot port is
located on one end of the valve. Fluid entering from pilot port operates against the piston and forces
the spool to move forward. The needle valve is used to control the speed of the actuation.

64. Classification based on Actuation
5. Pneumatic Actuation: Operated by applying compressed air against a piston at either end of the
valve spool.
The construction of the system is similar to the hydraulic actuation as shown in fig.
The only difference would be the actuation medium. I.e. compressed air.

65. MECHATRONICS
FCVFlow control valves

66. • In practice, the speed of actuator is very crucial in terms of required output.
• The speed of actuator can be controlled by regulating the fluid flow.
• A flow control valve can regulate the flow or pressure of the fluid.
• The fluid flow is controlled by varying area of the valve opening through which fluid passes.
• The fluid flow can be decreased by reducing the area of the valve opening and can be
increased by increasing the area of the valve opening.
Flow control valves

67. • A very common example of the flow control valve is the
household tap.
• The fig. shows the schematic diagram of a flow control valve.
• The pressure adjustment screw varies the fluid flow area in
the pipe to control the discharge rate.
• The pressure drop across the valve may keep on fluctuating.
• Generally, the hydraulic systems have a pressure
compensating pump.
• The inlet pressure remains almost constant but the outlet
pressure keeps on fluctuating depending on the external load.
• It creates fluctuating pressure drop, Hence, the ordinary flow
control valve will not be able to
• maintain a constant fluid flow.
Flow control valves

68. • A pressure compensated flow control valve maintains the
constant flow throughout the movement of a spool, which
shifts its position depending on the pressure.
• FCV are also be affected by temperature changes (the
viscosity of the fluid changes with temperature)
• The advanced flow control valves often have the temperature
compensation.
• The temperature compensation is achieved by the thermal
expansion of a rod, which compensates for the increased
coefficient of discharge due to decreasing viscosity with
temperature.
Flow control valves

69. Classification - Flow control valves
• The flow control valves work on applying a variable restriction in the flow path.
• Based on the construction; mainly four types
1. Plug valve
2. Butterfly valve
3. Ball valve
4. Balanced valve

70. Classification - Flow control valves
• Schematic of plug or glove valve is shown in Fig.
• This valve has a plug, can be adjusted in vertical
direction by setting flow adjustment screw.
• The adjustment of plug alters the orifice size between
plug and valve seat.
• Thus adjustment of plug controls the fluid flow.
E.g. Stopcock: Used in laboratory glassware.
1. Plug valve / Glove valve

71. Classification - Flow control valves
• A butterfly valve is shown in Fig.
• It consists of a disc which can rotate inside the pipe.
• The angle of disc determines the restriction.
• Butterfly valve can be made to any size.
• The disc is always present in the flow, therefore a pressure drop
is induced regardless of the valve position.
• Widely used to control the flow of gas, Also, have different
pressure ranges and applications.
2. Butterfly valve

72. Classification - Flow control valves
• Lowest Pressure rating: Resilient butterfly valve.
• The high performance butterfly valves have a slight offset in the
way the disc is positioned. It increases its sealing ability and
decreases the wear.
• High-pressure systems: Triple offset butterfly valve.
• It has higher risk of leakage on the shut-off position and suffer from
the dynamic torque effect.
• Butterfly valves are favored because of their lower cost and lighter
weight.
2. Butterfly valve

73. Classification - Flow control valves
• Here the flow control valve uses a ball rotated inside a
machined seat. The ball has a through hole as shown in Fig.
• It has very less leakage in its shut off condition.
• Applications: Widely used in industries, because of versatility,
support high pressures (up to 1000 bar) and temperatures (up
to 250°C). Easy to repair and operate.
• Excellent choice for shutoff applications, Durable.
• They do not offer fine control which may be necessary in
throttling applications.
3. Ball valve

74. • Schematic of a balanced valve is shown in fig.
• It comprises of two plugs and two seats.
• The opposite flow gives little dynamic reaction onto the actuator shaft, results in the negligible dynamic torque
effect.
• However, leakage is more in these kind of valves because the manufacturing tolerance can cause one plug to
seat before the other.
• Applications: The pressure-balanced valves are used in the houses. They provide water at nearly constant
temperature to a shower or bathtub despite of pressure fluctuations in either the hot or cold supply lines.
4. Balanced valve

75. Pressure relief valves

76. Pressure Relief valves
• The pressure relief valves are used to protect the hydraulic components from excessive
pressure.
• This is one of the most important components of a hydraulic system and is essentially required
for safe operation of the system.
• Its primary function is to limit the system pressure within a specified range.
• It is normally a closed type and it opens when the pressure exceeds a specified maximum value
by diverting pump flow back to the tank.

77. Pressure Relief valves
1. Direct type of relief valve
2. Unloading Valve
3. Sequence valve
4. Counterbalance Valve
5. Pressure Reducing Valve
Classification of Pressure relief valves

78. Pressure Relief valves
• Schematic of direct pressure relief valve is shown in fig.
• This valves has two ports; one is connected to the pump and another is
connected to the tank. It consists of a spring chamber where poppet is placed
with a spring force.
• Generally, the spring is adjustable to set the maximum pressure limit of the
system.
• The poppet is held in position by combined effect of spring force and dead
weight of spool.
• As the pressure exceeds this combined force, the poppet raises and excess
fluid bypassed to the reservoir (tank). The poppet again reseats as the
pressure drops below the pre-set value.
• A drain is also provided in the control chamber. It sends the fluid collected due
to small leakage to the tank and thereby prevents the failure of the valve.
1. Direct type of relief valve

79. Pressure Relief valves
• The valve consists of a control chamber with an adjustable spring which
pushes the spool down.
• The valve has two ports: one is connected to the tank and another is
connected to the pump.
• The valve is operated by movement of the spool.
• Normally, the valve is closed and the tank port is also closed.
• These valves are used to permit a pump to operate at the minimum load.
• It works on the same principle as direct control valve that the pump
delivery is diverted to the tank when sufficient pilot pressure is applied to
move the spool.
2. Unloading valve

80. Pressure Relief valves
• The pilot pressure maintains a static pressure to hold the valve opened.
• The pilot pressure holds the valve open until the pump delivery is needed.
• As the pressure is needed in the hydraulic circuit; the pilot pressure is
relaxed and the spool moves down due to the self weight and the spring
force.
• The drain is provided to remove the leaked oil collected in the control
chamber to prevent the valve failure.
• The unloading valve reduces the heat buildup due to fluid discharge at a
preset pressure value.
2. Unloading valve

81. Pressure Relief valves
• The primary function of this valve is to divert flow in a predetermined
sequence.
• It is used to operate the cycle of a machine automatically.
• A sequence valve may be of direct-pilot or remote-pilot operated type.
• Its construction is similar to the direct relief valve.
• It has two ports; one main port connecting the main pressure line and
another port (secondary port) is connected to the secondary circuit.
• The secondary port is usually closed by the spool.
• The pressure on the spool works against the spring force.
• When the pressure exceeds the preset value of the spring; the spool lifts
and the fluid flows from the primary port to the secondary port.
• For remote operation; the passage used for the direct operation is closed
and a separate pressure source for the spool operation is provided in the
remote operation mode.
3. Sequence valve

82. Pressure Relief valves
• The schematic of counterbalance valve is shown in Fig.
• Used to maintain the back pressure and to prevent a load from failing.
• The counterbalance valves can be used as breaking valves for decelerating
heavy loads.
• Counterbalance valves work on the principle that the fluid is trapped under
pressure until pilot pressure overcomes the pre-set value of spring force.
• Fluid is then allowed to escape, letting the load to descend under control.
• The valve is normally closed until it is acted upon by a remote pilot pressure
source. Therefore, a lower spring force is sufficient for the valve operation
at the lower pilot pressure and hence the power consumption reduces, pump
life increases and the fluid temperature decreases.
4. Counterbalance valve

83. Pressure Relief valves
• Applications: used in vertical presses, lift trucks, loaders and other
machine tools where position or hold suspended loads are important.
4. Counterbalance valve

84. Pressure Relief valves
• Sometimes a part of the system may need a lower pressure. This is possible
by using pressure reducing valve as shown in Fig.
• These valves are used to limit the outlet pressure.
• Generally, used for the operation of branch circuits where the pressure may
vary from the main hydraulic pressure lines.
• These are open type valve and have a spring chamber with an adjustable
spring, a movable spool as shown in fig.
• A drain is provided to return the leaked fluid in the spring (control) chamber.
• A free flow passage is provided from inlet port to the outlet port until a signal
from the outlet port tends to throttle the passage through the valve.
• The pilot pressure opposes the spring force and when both are balanced,
the downstream is controlled at the pressure setting.
4. Pressure reducing valve

85. Pressure Relief valves
• When the pressure in the reduced pressure line exceeds the valve setting,
the spool moves to reduce the flow passage area by compressing the
spring.
• It can be seen from the fig. if the spring force is more, the valve opens wider
and if the controlled pressure has greater force, the valves moves towards
the spring and throttles the flow.
4. Pressure reducing valve

86. Cylinders and Rotary Actuators

87. Cylinders and Rotary Actuators
• The hydraulic or pneumatic cylinder is an example of a linear actuator.
• The principles and form are the same for both hydraulic and pneumatic versions, differences
being a matter of size as a consequence of the higher pressures used with hydraulics.
• The cylinder consists of a cylindrical tube along which a piston/ram can slide.
There are two basic types
1. Single acting cylinders
2. Double-acting cylinders
Cylinders

88. Cylinders and Rotary Actuators
• The term single acting refers, when the control pressure is applied to just one side of the piston, a spring
being used to provide the opposition to the movement of the piston.
• The other side of the piston is open to the atmosphere.
• The fluid is applied to one side of the piston at a gauge pressure p with the other side being at atmospheric
pressure and produces a force on the piston of pA,( where A is the area of the piston).
Single Acting Cylinders

89. Cylinders and Rotary Actuators
• When a current passes through the solenoid, the valve switches position and pressure is applied to move
the piston along the cylinder.
• When the current through the solenoid ceases, the valve reverts to its initial position and the air is vented
from the cylinder.
• As a consequence the spring returns the piston back along the cylinder.
Single Acting Cylinders

90. Cylinders and Rotary Actuators
• The term ‘double acting’ is refers, when the control pressures are applied to each side of the piston (Fig)
• A difference in pressure between the two sides then results in motion of the piston, the piston being able
to move in either direction along the cylinder as a result of high-pressure signals.
• For the double-acting cylinder shown in Fig, current through one solenoid causes the piston to move in
one direction with current through the other solenoid reversing the direction of motion
Double Acting Cylinders

91. Cylinders and Rotary Actuators
• A linear cylinder can, with suitable mechanical linkages, be used to produce rotary movement through
angles less than 360°, Fig (a) illustrates such an arrangement.
• Another alternative is a semi-rotary actuator involving a vane Fig (b).
• A pressure difference between the two ports causes the vane to rotate and so give a shaft rotation which
is a measure of the pressure difference. Depending on the pressures, so the vane can be rotated
clockwise or anti-clockwise.
Rotary Actuators

92. Cylinders and Rotary Actuators
• For rotation through angles greater than 360° a pneumatic motor can be used. one such form is the vane
motor Fig (c).
• An eccentric rotor has slots in which vanes are forced outwards against the walls of the cylinder by the
rotation.
• The vanes divide the chamber into separate compartments which increase in size from the inlet port round
to the exhaust port. The air entering such a compartment exerts a force on a vane and causes the rotor to
rotate. The motor can be made to reverse its direction of rotation by using a different inlet port.
Rotary Actuators

93. End