NONLINEARITIES IN CONTROL SYSTEM

• In most control systems we can not avoid the presence of certain types
of nonlinearities in control system.
These can be classified as static or dynamic.
• A system for which there is a nonlinear relationship between input and
output, that does not involve a differential equation is called a static
non linearity.
• On the other hand, the input and output may be related through a non
linear differential equation. Such a system is called a dynamic non
linearity.

1
 NONLINEARITIES IN CONTROL SYSTEM

BACKLASH

• It is also called as lash or play, is a clearance or lost motion in a

mechanism caused by gaps between the parts.
• It can be defined as "the maximum distance or angle through which
any part of a mechanical system may be moved in one direction
without applying appreciable force or motion to the next part in
mechanical sequence", and is a mechanical form of dead band.

2
 NONLINEARITIES IN CONTROL SYSTEM

BACKLASH
• An example, in the context of gears and gear trains, is the amount of
clearance between mated gear teeth. It can be seen when the
direction of movement is reversed and the slack or lost motion is
taken up before the reversal of motion is complete.

3
 NONLINEARITIES IN CONTROL SYSTEM

BACKLASH

• In applications requiring precise positioning, backlash can create

uncertainty and inaccuracy.
• When running open-loop or using only motor position to determine
linear position, backlash introduces inaccuracy when changing
direction. In addition, when a system is at rest, backlash can allow
free play and unwanted motion leading to vibration, noise and
diminished performance.

4
 NONLINEARITIES IN CONTROL SYSTEM
FRICTION
• Anything which opposes the relative motion of the body is called
friction. It is a kind of non linearity present in the system. The
common example in an electric motor in which we find coulomb
friction drag due to the rubbing contact between the brushes and the
commutator.

5
 NONLINEARITIES IN CONTROL SYSTEM

FRICTION
Friction may be of three types:
• Static Friction : The static friction acts on the body when the body is
at rest.
• Dynamic Friction: Dynamic friction acts on the body when there is a
relative motion between the surface and the body.
• Limiting Friction: It is defined as the maximum value of friction that
acts on the body when it is at rest.

6
 NONLINEARITIES IN CONTROL SYSTEM

FRICTION
Dynamic friction can also be classified as
(a) Sliding friction
(b) Rolling friction.
• Sliding friction acts when two bodies slides over each other while
rolling acts when the bodies rolls over another body.
In mechanical system we have two types of friction namely
(a) Viscous friction
(b) Static friction.
7
 NONLINEARITIES IN CONTROL SYSTEM
DEAD BAND
• This type of non linearity is shown by various electrical devices like
motors, DC servo motors, actuators etc.
• Dead zone nonlinearities refer to a condition in which output
becomes zero when the input crosses certain limiting value.

8
 NONLINEARITIES IN CONTROL SYSTEM

HYSTERESIS
• Hysteresis is a common phenomenon in physical systems and occurs when the
system's output depends not only on its present inputs but also on past inputs
(history).
• Types of Hysteresis
1. Systems with rate-dependent hysteresis have a memory of recent inputs that
fades with time - if the input stops changing and we wait long enough, the output
will (eventually) reach the same value for that particular input.
2. Systems with rate-independent hysteresis retain a permanent memory of certain
input patterns and even the steady state value of the output depends on the nature
of the input history.
9
NONLINEARITIES IN CONTROL SYSTEM

HYSTERESIS

10
NONLINEARITIES IN CONTROL SYSTEM

HYSTERESIS

11
 NONLINEARITIES IN CONTROL SYSTEM

HYSTERESIS
• Hysteresis is expressed as the percent difference in the rated current
required to give the same output when approached from higher and
lower inputs.

• For servo valves, it is typically 1–2%. To overcome the problem of

hysteresis, some controllers are designed so that the set point is
always approached from the lower side.

12
 HYDRAULIC CIRCUITS

• A hydraulic circuit is a group of components such as pumps,

actuators, control valves, conductors and fittings arranged to perform
useful work.

• There are three important considerations in designing a hydraulic

circuit:

1.Safety of machine and personnel in the event of power failures.

2.Performance of given operation with minimum losses.
3.Cost of the component used in the circuit.

•
13
 Control of a
Single-Acting
Hydraulic
Cylinder

14
 HYDRAULIC CIRCUITS
Control of a Single-Acting Hydraulic Cylinder
• Figure shows that the control of a single-acting, spring return
cylinder using a three-way two-position manually actuated, spring
offset direction-control valve (DCV).

• In the spring offset mode, full pump flow goes to the tank through
the pressure-relief valve (PRV).

• The spring in the rod end of the cylinder retracts the piston as the oil
from the blank end drains back into the tank.

• When the valve is manually actuated into its next position, pump
flow extends the cylinder. After full extension, pump flow goes
through the relief valve.
15
 Control of a
Double-Acting
Hydraulic
Cylinder

16
 HYDRAULIC CIRCUITS
Control of a Double-Acting Hydraulic Cylinder
• The control of a double-acting hydraulic cylinder is described as
follows:

1. When the 4/3 valve is in its neutral position, the cylinder is

hydraulically locked and the pump is unloaded back to the tank.

2. When the 4/3 valve is actuated into the flow path, the cylinder is
extended against its load as oil flows from port P through port A. Oil
in the rod end of the cylinder is free to flow back to the tank through
the four-way valve from port B through port T.

17
 HYDRAULIC CIRCUITS
Control of a Double-Acting Hydraulic Cylinder
3. When the 4/3 valve is actuated into the right-envelope
configuration, the cylinder retracts as oil flows from port P through
port B. Oil in the blank end is returned to the tank via the flow path
from port A to port T.

• At the ends of the stroke, there is no system demand for oil. Thus,
the pump flow goes through the relief valve at its pressure level
setting unless the four-way valve is deactivated.

18
 Regenerative
cylinder circuit

19
 HYDRAULIC CIRCUITS
Regenerative Cylinder Circuit
• A regenerative circuit is used to speed up the extending speed of a
double-acting cylinder.

• The pipelines to both ends of the hydraulic cylinder are connected in

parallel and one of the ports of the 4/3 valve is blocked by simply
screwing a thread plug into the port opening.

• When the DCV is shifted in to its left-mode, the oil flows from the
pump to the blind end of the cylinder and hence cylinder extends.

20
 HYDRAULIC CIRCUITS
Regenerative Cylinder Circuit
• During retraction stroke, the 4/3 valve is configured to the right
mode. During this stroke, the pump flow bypasses the DCV and
enters the rod end of the cylinder.

• Oil from the blank end then drains back to the tank through the DCV.

• The speed of extension is greater than that for a regular double-

acting cylinder because the flow from the rod end regenerates with
the pump flow QP to provide a total flow rate QT.

21
 HYDRAULIC CIRCUITS
Regenerative Cylinder Circuit
Expression for the Cylinder Extending Speed

22
 HYDRAULIC CIRCUITS
Regenerative Cylinder Circuit
Expression for the Cylinder Extending Speed

• In general, the greater the ratio of the piston area to rod area, the greater is
the ratio of the extending speed to retraction speed
23
 HYDRAULIC CIRCUITS
Regenerative Cylinder Circuit
Load-Carrying Capacity During Extension

• The load-carrying capacity of a regenerative cylinder during

extension is less than that obtained from a regular double-acting
cylinder.
• The load-carrying capacity Fload-extension for a regenerative cylinder
during extension equals pressure times the piston rod area. This is
because system pressure acts on both sides of the piston during
extension. Then

• Thus, we do not obtain more power from the regenerative cylinder

during extension because the extension speed is increased at the
24
expense of reduced load-carrying capacity.
 HYDRAULIC CIRCUITS
Cylinder Synchronization Circuits
• In industry, there are instances when a large mass must be moved,
and it is not feasible to move it with just one cylinder.

• In such cases we use two or more cylinders to prevent a moment or

moments that might distort and damage the load.

• These cylinders must be synchronized. There are two ways that can
be used to synchronize cylinders: Parallel and series

25
 HYDRAULIC CIRCUITS
Cylinder Synchronization Circuits
Cylinders in Parallel

• When the two cylinders are identical, the loads on the cylinders are
identical, and then extension and retraction are synchronized.

• If the loads are not identical, the cylinder with smaller load extends
first. Thus, the two cylinders are not synchronized.

• Practically, no two cylinders are identical, because of

packing(seals)friction differences.

26
 Cylinder
Synchronization
Circuits

(Cylinders in
Parallel)

27
 HYDRAULIC CIRCUITS
Cylinder Synchronization Circuits
Cylinders in Series

• During the extending stroke of cylinders, fluid from the pump is

delivered to the blank end of cylinder 1.

• As cylinder 1 extends, fluid from its rod end is delivered to the blank
end of cylinder 2 causing the extension of cylinder 2.

• As cylinder 2 extends, fluid from its rod end reaches the tank. For
two cylinders to be synchronized, the piston area of cylinder 2 must
be equal to the difference between the areas of piston and rod for
cylinder 1.
28
 Cylinder
Synchronization
Circuits

(Cylinders in
Series)

29
 HYDRAULIC CIRCUITS
High-Low Circuit
• Many systems require a high volume at low pressure for rapid
movement of a vise or tool, then low volume at high pressure for
clamping or feeding. This can be accomplished by a high-low circuit
using two pumps.

• During rapid traverse, both pumps supply the system. When pressure
rises during clamping or feed, the large-volume main pump unloads,
and the small pump maintains pressure.

• Output flow of the small pump is low enough to prevent heating of

the oil. Instead of pilot operation, the unloading valve can be
solenoid controlled and actuated by a pressure switch.

30
High-Low Circuit

31
 HYDRAULIC CIRCUITS
Pressure Intensifier Circuits
• Pressure or fatigue-testing machines often require high pressure for
long periods of time. Other circuits might need a small volume of
high-pressure fluid for a short period while most of the cycle only
needs low pressure.

• Other machines can use air cylinders to manipulate a part but need
very high pressure to perform one operation.

• Some manufacturers make high-pressure rotary pumps — rated up

to approximately 10,000 psi — but these pumps are expensive and
may heat the fluid. Another choice for low-volume/high-pressure
circuits is an intensifier.

32
 HYDRAULIC CIRCUITS
Pressure Intensifier Circuits
• When a circuit calls for a small volume of high-pressure oil or air, an
intensifier can be used — sometimes called a booster. Most cylinder
manufacturers build air- or hydraulic-powered intensifiers.

• An air-oil intensifier consists of two pistons with different diameters.

The large-area air piston pushes a small-area hydraulic ram against
trapped oil.

33
 HYDRAULIC CIRCUITS
Pressure Intensifier Circuits
• The difference between the two areas gives high-pressure capability
at the small ram. This capability is indicated by the area ratio.

• If the air piston has a 5-in. diameter and the oil piston has a 1-in.
diameter, the area ratio is 25:1.

• With this area ratio, 80 psi acting on the air piston produces 2000 psi
at the hydraulic piston.

34
 Air-oil
Intensifier
Circuit

35
 HYDRAULIC CIRCUITS
Pressure Intensifier Circuits
Air-oil intensifier Circuit

• Extension: when the first 4/2 DCV (valve 1) is shifted to left mode,
the air from the reservoir flows to the approach tank. In the
approach tank the air forces the oil to the blind end of the cylinder
through the bottom of the intensifier. Now the cylinder extends.

• Useful work: when the cylinder experiences its load, the second 4/2
DCV (valve 2) is actuated to the left mode. This valve position sends
air to the top end of the intensifier. Now the intensifier moves down,
and piston of the intensifier blocks path of oil from the approach
tank. Now the cylinder receives high pressure oil at the blind end to
perform the useful work.
36
 HYDRAULIC CIRCUITS
Pressure Intensifier Circuits
Air-oil intensifier Circuit

• Retraction: When the valve 2 is released (shifted to right mode) the

air flow from the reservoir is blocked. The air from the top end of the
intensifier is vented to the atmosphere. This completes the high
pressure portion of the cycle.
• When the valve 1 is released (shifted to right mode) the air flow is
diverted to the return tank and also the air from the approach tank is
vented. The diverted air flow pushes the oil to the rod end of the
cylinder. This causes the cylinder to retract. The oil from the piston
end of the cylinder is diverted back to the approach tank through the
bottom end of the intensifier. This completes the entire cycle of
operation.
37
 HYDRAULIC CIRCUITS
Speed Control of a Hydraulic Cylinder
• In hydraulic operations, it is necessary to control the speed of the
actuator so as to control the force, power, timing and other factors of
the operation.

• Actuator speed control is achieved by controlling the rate of flow into

or out of the cylinder.

• Speed control by controlling the rate of flow into the cylinder is

called meter-in control.

• Speed control by controlling the rate of flow out of the cylinder is

called meter-out control.
38
Meter-in Circuit

39
 HYDRAULIC CIRCUITS
Speed Control of a Hydraulic Cylinder
METER-IN CIRCUIT

• The inlet flow into the cylinder is controlled using a flow-control

valve.

• In the return stroke, however, the fluid can bypass the needle valve
and flow through the check valve and hence the return speed is not
controlled.

• This implies that the extending speed of the cylinder is controlled

whereas the retracing speed is not.

40
Meter-out
Circuit

41
 HYDRAULIC CIRCUITS

Speed Control of a Hydraulic Cylinder

METER-OUT CIRCUIT

• When the cylinder extends, the flow coming from the pump into the
cylinder is not controlled directly.

• However, the flow out of the cylinder is controlled using the flow-
control valve (metering orifice).

• On the other hand, when the cylinder retracts, the flow passes
through the check valve unopposed, bypassing the needle valve.
Thus, only the speed during the extend stroke is controlled.

42
 HYDRAULIC CIRCUITS
Bleed-off Circuit
• Compared to meter-in and meter-out circuits, a bleed-off circuit is
less commonly used.

• In this type of flow control, an additional line is run through a flow-

control valve back to the tank.

• To slow down the actuator, some of the flow is bled-off through the
flow-control valve into the tank before it reaches the actuator.

• This reduces the flow into the actuator, thereby reducing the speed
of the extend stroke.

• Figure shows a bleed-off circuit with extend stroke control. 43

 HYDRAULIC CIRCUITS
Bleed-off Circuit
• The main difference between a bleed-off circuit and a meter-
in/meter-out circuit is that in a bleed-off circuit, opening the flow-
control valve decreases the speed of the actuator, whereas in the
case of a meter-in/meter-out circuit, it is the other way around.

• Figure shows a bleed-off circuit with extend stroke control.

44
Bleed-off Circuit

(a) Bleed-off for

both
directions

(b) bleed-off for

inlet to the
cylinder or
motor.

45
Speed Control
of a Hydraulic
Motor

46
 HYDRAULIC CIRCUITS
Speed Control of a Hydraulic Motor
• It uses a pressure-compensated FCV.
• In a spring-centered position of the four-way valve, the motor is
hydraulically blocked.
• When the valve is actuated to the left envelope, the motor rotates in
one direction. Its speed can be varied by adjusting the throttle of the
FCV. Thus, the speed can be infinitely varied and the excess oil goes
through the PRV.
• When the valve is deactivated, the motor stops suddenly and
becomes locked.
• When the right envelope is in operation, the motor turns in the
opposite direction. The PRV provides overload protection if, for
example, the motor experiences an excessive torque load.
•
47
 HYDRAULIC CIRCUITS
Fail-safe circuit
• Fail-safe circuits are those designed to prevent injury to the operator
or damage to the equipment.

• In general, they prevent the system from accidentally falling on an

operator and also prevent overloading of the system.

• Two fail-safe circuits:

✓Protection from inadvertent cylinder extension

✓Fail-safe overload protection.

48
 HYDRAULIC CIRCUITS
Fail-safe circuit
Protection from inadvertent cylinder extension

• Figure shows a fail-safe circuit that is designed to prevent the

cylinder from accidentally falling in the event when a hydraulic line
ruptures or a person inadvertently operates the manual override on
the pilot-actuated DCV when the pump is not working.

49
Fail-safe circuit

Protection from
inadvertent
cylinder
extension

50
 HYDRAULIC CIRCUITS
Fail-safe circuit
Protection from inadvertent cylinder extension
• To lower the cylinder, pilot pressure from the blank end of piston
must pilot open the check valve to allow oil to return through the
DCV to the tank.

• This happens when the push button is actuated to permit the pilot
pressure actuation of DCV or when the DCV is directly manually
actuated when the pump operates.

• The pilot-operated DCV allows free flow in the opposite direction to

retract the cylinder when this DCV returns to its offset mode.

51
 HYDRAULIC CIRCUITS
Counterbalance valve circuit

• A counterbalance valve is applied to create a back pressure or

cushioning pressure on the underside of a vertically moving piston to
prevent the suspended load from free falling because of gravity while
it is still being lowered.

52
 Counter-
balance valve
circuit

53
 HYDRAULIC CIRCUITS
Counterbalance valve circuit
Valve Operation (Lowering)
• The pressure setting on the counterbalance valve is set slightly higher
than the pressure required to prevent the load from free falling. Due
to this back pressure in line A, the actuator piston must force down
when the load is being lowered.

• This causes the pressure in line A to increase, which raises the spring-
opposed spool, thus providing a flow path to discharge the exhaust
flow from line A to the DCV and then to the tank. The spring-
controlled discharge orifice maintains back pressure in line A during
the entire downward piston stroke.

54
 HYDRAULIC CIRCUITS
Counterbalance valve circuit
Valve Operation (Lifting)

• As the valve is normally closed, flow in the reverse direction (from

port B to port A) cannot occur without a reverse free-flow check
valve.

• When the load is raised again, the internal check valve opens to
permit flow for the retraction of the actuator.

55
 HYDRAULIC CIRCUITS
Counterbalance valve circuit
Valve Operation (Suspension)

• When the valve is held in suspension, the valve remains closed.

• Therefore, its pressure setting must be slightly higher than the
pressure caused by the load.
• Spool valves tend to leak internally under pressure.
• This makes it advisable to use a pilot-operated check valve in
addition to the counterbalance valve if a load must be held in
suspension for a prolonged time.

56
 ACCUMULATOR

• A hydraulic accumulator is a device that stores the potential energy

of an incompressible fluid held under pressure by an external source
against some dynamic force.

• The stored potential energy in the accumulator is a quick secondary

source of fluid power capable of doing useful work.

57
 ACCUMULATOR
Functions of Accumulator

• Energy storage
• Pulsation absorption
• Shock cushioning
• Supplementing pump flow
• Maintaining pressure
• Fluid dispensing

58
 ACCUMULATOR
Types of Accumulator

Weight-loaded or gravity accumulator

• It is a vertically mounted cylinder with a large weight. When the

hydraulic fluid is pumped into it, the weight is raised.

• The weight applies a force on the piston that generates a pressure on

the fluid side of piston.

• The advantage of this type of accumulator over other types is that it

applies a constant pressure on the fluid throughout its range of
motion. The main disadvantage is its extremely large size and heavy
weight. This makes it unsuitable for mobile application.
59
 ACCUMULATOR
Types of Accumulator

Weight-loaded or gravity accumulator

60
 ACCUMULATOR
Types of Accumulator

Spring-loaded accumulator

• Spring-loaded accumulator stores energy in the form of a

compressed spring.

• A hydraulic fluid is pumped into the accumulator, causing the piston

to move up and compress the spring. The compressed spring then
applies a force on the piston that exerts a pressure on the hydraulic
fluid.

• This type of accumulator delivers only a small volume of oil at

relatively low pressure.
61
 ACCUMULATOR
Types of Accumulator

Spring-loaded accumulator

62
 ACCUMULATOR
Types of Accumulator

Gas-loaded accumulator

Here the force is applied to the oil using compressed air.

A gas accumulator can be very large and is often used with water or
high water-based fluids using air as a gas charge.

Typical application is on water turbines to absorb pressure surges

owing to valve closure and on ram pumps to smooth out the delivery
flow.

63
 ACCUMULATOR
Types of Accumulator

Gas-loaded accumulator

64
 ACCUMULATOR
Types of Accumulator

Gas-loaded accumulator

There are two types of gas-loaded accumulators:

• Non-separator-type accumulator: Here the oil and gas are not

separated. Hence, they are always placed vertically.

• Separator-type accumulator: Here the oil and gas are separated by

an element. Based on the type of element used to separate the oil
and gas, they are classified as follows:

65
 ACCUMULATOR
Types of Accumulator

Gas-loaded accumulator (Piston-type)

• It consists of a cylinder with a freely floating piston with proper seals.

Its operation begins by charging the gas chamber with a gas
(nitrogen) under a pre-determined pressure. This causes the free
sliding piston to move down.
• Once the accumulator is pre-charged, a hydraulic fluid can be
pumped into the hydraulic fluid port. As the fluid enters the
accumulator, it causes the piston to slide up, thereby compressing
the gas that increases its pressure and this pressure is then applied
to the hydraulic fluid through the piston.

66
 ACCUMULATOR
Types of Accumulator

Gas-loaded accumulator (Piston-type)

67
 ACCUMULATOR
Types of Accumulator

Gas-loaded accumulator (Diaphragm-type)

• In this type, the hydraulic fluid and nitrogen gas are separated by a
synthetic rubber diaphragm.

• The advantage of a diaphragm accumulator over a piston

accumulator is that it has no sliding surface that requires lubrication
and can therefore be used with fluids having poor lubricating
qualities.

• It is less sensitive to contamination due to lack of any close-fitting

components.
68
 ACCUMULATOR
Types of Accumulator

Gas-loaded accumulator (Diaphragm-type)

69
 ACCUMULATOR
Types of Accumulator

Gas-loaded accumulator (Bladder-type)

• Here the gas and the hydraulic fluid are separated by a synthetic
rubber bladder.

• The bladder is filled with nitrogen until the designed pre-charge

pressure is achieved.

• The hydraulic fluid is then pumped into the accumulator, thereby

compressing the gas and increasing the pressure in the accumulator.

70
 ACCUMULATOR
Types of Accumulator

Gas-loaded accumulator (Bladder-type)

71
 ACCUMULATOR SIZING

• Accumulators are manufactured to a variety of pressure ratings and

the one chosen should be rated for a pressure more than the
maximum system working pressure.

72
73
74
75
76
77
 ACCESSORIES USED IN FLUID POWER SYSTEMS

Hydraulic Seals
• Seals are used in hydraulic systems to prevent excessive internal and
external leakage and to keep out contamination.

Various functions of seals include the following:

1. They prevent leakage – both internal and external.

2. They prevent dust and other particles from entering into the system.
3. They maintain pressure.
4. They enhance the service life and reliability of the hydraulic system.

78
 ACCESSORIES USED IN FLUID POWER SYSTEMS

Hydraulic Seals
Classification of Hydraulic Seals

• According to the method of sealing:

1. Positive sealing: A positive seal prevents even a minute amount of

oil from getting past. A positive seal does not allow any leakage
whatsoever (external or internal).
2. Non-positive sealing: A non-positive seal allows a small amount of
internal leakage, such as the clearance of the piston to provide a
lubrication film.

79
 ACCESSORIES USED IN FLUID POWER SYSTEMS

Hydraulic Seals
Classification of Hydraulic Seals

• According to the relative motion existing between the seals and

other parts:

1. Static seals: These are used between mating parts that do not
move relative to one another.

2. Dynamic seals: These are assembled between mating parts that

move relative to each other. Hence, dynamic seals are subject to
wear because one of the mating parts rubs against the seal.

80
 ACCESSORIES USED IN FLUID POWER SYSTEMS

Hydraulic Seals
Classification of Hydraulic Seals

• According to geometrical cross-section:

1. O-ring seal: It is a molded synthetic rubber seal that has a round

cross-section.

81
 ACCESSORIES USED IN FLUID POWER SYSTEMS

Hydraulic Seals
Classification of Hydraulic Seals

• According to geometrical cross-section:

2. V-ring seal and U-ring seal: V- and U-ring seals are compression-
type seals used in virtually in all types of reciprocating motion
applications.

82
 ACCESSORIES USED IN FLUID POWER SYSTEMS

Hydraulic Seals
Classification of Hydraulic Seals

• According to geometrical cross-section:

3. T-ring seal: T-ring seal is a dynamic seal that is extensively used to

seal cylinder-pistons, piston rods and other reciprocating parts.

83
 ACCESSORIES USED IN FLUID POWER SYSTEMS

Hydraulic Seals
Classification of Hydraulic Seals

• According to the type of seal material used

Plastic and synthetic rubber materials are used and must compatible
with hydraulic fluid.

1. Neoprene (chloroprene).
2. Buna-N.
3. Silicone (Teflon).
4. Tetrafluoroethylene.
5. Viton.
84
 ACCESSORIES USED IN FLUID POWER SYSTEMS

Reservoirs
1. To provide a chamber in which any change in the volume of fluid in a
hydraulic circuit can be accommodated.
2. To provide a filling point for the system.
3. To serve as a storage space for the hydraulic fluid used in the system.
4. It is used as the location where the fluid is conditioned.
5. To provide a volume of fluid which is relatively stationery to allow
entrained air to separate out and heavy contaminants to settle.
6. It is a place where the entrained air picked up by the oil is allowed to
escape.
7. To accomplish the dissipation of heat by its proper design and to
provide a radiating and convective surface to allow the fluid to cool.

85
ACCESSORIES USED IN FLUID POWER SYSTEMS

Reservoirs

86
 ACCESSORIES USED IN FLUID POWER SYSTEMS

Reservoirs
Features of a Hydraulic Reservoir

1. Filler cap (breather cap): It should be airtight when closed but may
contain the air vent that filters air entering the reservoir to provide a
gravity push for proper oil flow.
2. Oil level gauge: It shows the level of oil in the reservoir without
having to open the reservoir.
3. Baffle plate: It is located lengthwise through the center of the tank
and is two-third the height of the oil level. It is used to separate the
outlet to the pump from the return line. This ensures a circuitous
flow instead of the same fluid being recirculated. The baffle prevents
local turbulence in the tank and allows foreign material to settle, get
rid of entrapped air and increase heat dissipation.
87
 ACCESSORIES USED IN FLUID POWER SYSTEMS

Reservoirs
Features of a Hydraulic Reservoir

4.Suction and return lines: They are designed to enter the

reservoir at points where air turbulence is the least. They can
enter the reservoir at the top or at the sides, but their ends
should be near the bottom of the tank. If the return line is
above the oil level, the returning oil can foam and draw in air.
5.Intake filter: It is usually a screen that is attached to the suction
pipe to filter the hydraulic oil.
6.Drain plug: It allows all oil to be drained from the reservoir.
Some drain plugs are magnetic to help remove metal chips from
the oil.
7.Strainers and filters: Strainers and filters are designed to
remove foreign particles from the hydraulic fluid.
88
 ACCESSORIES USED IN FLUID POWER SYSTEMS
Reservoirs

Types of Reservoirs

• Non-pressurized: The reservoir may be vented to atmosphere using

an air filter or a separating diaphragm.

• Pressurized: A pressurized reservoir usually operates between 0.35

and 1.4 bar and has to be provided with some method of pressure
control; this may be a small air compressor maintaining a set charge
pressure.

89
 ACCESSORIES USED IN FLUID POWER SYSTEMS
Filters and Strainers

Filters

• Filters are used to pick up smaller contaminant particles because

they are able to accumulate them better than a strainer.

• The filter elements are held in position by springs or other retaining

devices. Because the filter element is not capable of being cleaned,
that is, when the filter becomes dirty, it is discarded and replaced by
a new one.

• Particle sizes removed by filters are measured in microns.

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Filters and Strainers

Strainers

• A strainer is a coarse filter.

• A strainer is constructed of a fine wire mesh screen or of screening

consisting of a specially processed wire of varying thickness wrapped
around metal frames.

• It does not provide as fine a screening action as filters do, but offers
less resistance to flow and is used in pump suction lines where
pressure drop must be kept to a minimum.

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Types of Filters

1. According to the filtering methods:

• Mechanical filters: This type normally contains a metal or cloth

screen or a series of metal disks separated by thin spacers.
Mechanical filters are capable of removing only relatively coarse
particles from the fluid.

• Absorption filters: These filters are porous and permeable

materials such as paper, wood pulp, diatomaceous earth, cloth,
cellulose and asbestos. Paper filters are impregnated with a resin to
provide added strength. In this type of filters, the particles are
actually absorbed as the fluid permeates the material. Hence,
these filters are used for extremely small particle filtration.
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Types of Filters

1. According to the filtering methods:

• Adsorbent filters: Adsorption is a surface phenomenon and refers to

the tendency of particles to cling to the surface of the filters. Thus,
the capacity of such a filter depends on the amount of surface area
available. Adsorbent materials used include activated clay and
chemically treated paper.

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Types of Filters

2. According to the size of pores in the material:

• Surface filters: These are nothing but simple screens used to clean oil
passing through their pores. The screen thickness is very thin and
dirty unwanted particles are collected at the top surface of the
screen when the oil passes, for example, strainer.

• Depth filters: These contain a thick-walled filter medium through

which the oil is made to flow and the undesirable foreign particles
are retained. Much finer particles are arrested and the capacity is
much higher than surface filters.

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Types of Filters

3. According to the location of filters:

• Intake or inline filters (suction strainers): These are provided first

before the pump to protect the pump against contaminations in the
oil .

• Pressure line filters (high-pressure filters): These are placed

immediately after the pump to protect valves and actuators and can
be a finer and smaller mesh.

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Types of Filters

3. According to the location of filters:

Suction filter Pressure filter

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Types of Filters

3. According to the location of filters:

• Return line filters (low-pressure filters): These filters filter the oil
returning from the pressure-relief valve or from the system, that is,
the actuator to the tank. They are generally placed just before the
tank.

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Types of Filters

3. According to the location of filters:

Return line Filter

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Types of Filters

4. Depending on the amount of oil filtered by a filter:

• Full flow filters: In this type, complete oil is filtered. Full flow of oil
must enter the filter element at its inlet and must be expelled
through the outlet after crossing the filter element fully.

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Types of Filters

4. Depending on the amount of oil filtered by a filter:

• Full flow filters

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Types of Filters

4. Depending on the amount of oil filtered by a filter:

• Proportional filters (bypass filters): In some hydraulic system

applications, only a portion of oil is passed through the filter instead
of entire volume and the main flow is directly passed without
filtration through a restricted passage.

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Heat Exchangers

• Significant sources of heat include the pump, pressure-relief valves

and flow control valves.

• Heat can cause the hydraulic fluid temperature to exceed its normal
operating range of 35–70oC.

• Excessive temperature hastens the oxidation of the hydraulic oil and

causes it to become too thin.

• This promotes deterioration of seals and packing and accelerates

wear between closely fitting parts of hydraulic components of valves,
pumps and actuators.
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Heat Exchangers

• There are two types of heat exchangers: liquid-to-liquid and liquid-

to-air type.

• Liquid-to-liquid types are of shell and tube construction consisting of

a bundle of small tubes held inside a shell. The coolant flows through
the small tubes, while the hydraulic oil passes around and between
these tubes.

• Liquid-to-air heat exchangers transfer heat from the hydraulic fluid to

the atmosphere, just like an automobile radiator. The air passes over-
finned tubes made of either copper or aluminum in which the hot
hydraulic fluid circulates.

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• To perform the functions and make the hydraulic system work

efficiently, the hydraulic fluid must be clean and should possess
certain properties.

• A hydraulic system is fairly easy to maintain.

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The following is a list of most common causes of hydraulic system breakdown:

• Clogged or dirty oil filters.

• Inadequate supply of oil in the reservoir.
• Leaking seals.
• Loose inlet lines that cause the pump to take in air.
• Incorrect type of oil.
• Excessive oil temperature.
• Excessive oil pressure.
It is important for the total system to provide easy access to components requiring
periodic inspection such as filters, strainers, sight gauges, drain and fill plugs, flow
meters, and pressure and temperature gauges.
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The Importance of Cleanliness

• Cleanliness is the first requirement when it comes to servicing hydraulic systems.
Keep dirt and other contaminants out of the system. Small particles can score
valves, seize pumps, clog orifices and cause expensive repair jobs.

How to keep the hydraulic system clean?

• Keep the oil clean.
• Keep the system clean.
• Keep your work area clean.
• Be careful when you change or add oil.

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Importance of Oil and Filter Changes

• Some contaminants get into the system anyway.

• Good hydraulic oils hold these contaminants in suspension and filters collect
them as the oil passes through.

• Good hydraulic oil contains many additives that work to keep contaminants
from damaging or plugging the system.

• However, these additives lose their effectiveness after a period of time.

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• The oil should be changed at the recommended intervals to make

sure that the additives do their job.

• The system filters can absorb only a limited amount of dirt particles
and other contaminants from the oil.

• After that the filters stop working. At this point, the filters should be
cleaned or replaced with new ones so that the cleaning process can
be maintained.

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Draining the System

• Periodic draining of the entire hydraulic system is very important.

• This is the only positive way to completely remove contaminants,

oxidized fluid and other substances from the system.

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Cleaning and Flushing the System

• In some hydraulic systems, there might be deposits left in the system.

• It is advisable to clean and flush the system after draining the oil out.
After draining the system, clean any sediment from the reservoir and
clean or replace the filter elements.

• Be sure to clean or replace the system filters before refilling the

system.

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Filling the System

• Before filling the system, be sure the area around the filler cap is
clean.

• Fill the reservoir to the specified level with the recommended

hydraulic oil.

• Use only clean oil and funnels or containers, and then be sure to
replace the filler cap before operating the equipment.

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Preventing Leaks
• There are internal and external leakages.

• Internal leakage does not result in actual loss of oil but it does reduce
the efficiency of the system.

• External leakage does result in direct loss of oil and can have other
undesirable effects as well.

• A hydraulic system should always be monitored for leaks and

remedial actions should be taken immediately.
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Preventing Overheating
• Heat causes hydraulic oil to breakdown faster and lose its
effectiveness. This is why cooling of the oil is needed.

• On high-pressure, high-speed circuits, oil coolers are needed to

dissipate the extra heat.

• To help prevent overheating, keep the oil at the proper level; clean
dirt and mud from lines, reservoirs and coolers; check for dented and
kinks lines; and keep relief valves adjusted properly.

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Problems Caused By Gases in Hydraulic Fluids

Gases can be present in a hydraulic fluid (or any other fluid) in three
ways: free air, entrained gas and dissolved air.
a) Free Air
• Air can exist in a free pocket located at some high point of a hydraulic
system
• This free air either existed in the system when it was initially filled or
was formed due to air bubbles in the hydraulic fluid rising into the
free pocket.
• Free air can cause the hydraulic fluid to possess a much lower
stiffness (bulk modulus), resulting in a spongy and unstable operation
of hydraulic actuators.
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Problems Caused By Gases in Hydraulic Fluids

b) Entrained gas
• Entrained gas (gas bubbles within the hydraulic fluid) is created when
the pressure drops below the vapor pressure of the hydraulic fluid.

• Entrained gas can cause cavitation problems in pumps and valves.

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Problems Caused By Gases in Hydraulic Fluids

c) Dissolved Air
• Dissolved air is in the solution and thus cannot be seen and does not
add to the volume of the hydraulic fluid.

• A hydraulic fluid, as received at atmospheric pressure, typically

contains about 6% of dissolved air by volume that increases to 10%
when pumped.

• Dissolved air creates no problem in hydraulic systems as long as the

air remains dissolved.
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