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1. Fluid Mechanics-II
Teacher/Instructor: Engr. Muhammad Sumair
B.Sc. Mechanical Engineering (UET Lahore 2014-2018)
M.Sc. Thermal Power Engineering (UET Lahore 2018-2020)

2. Hydraulic Circuit Design & Analysis
• A hydraulic circuit is a group of components such as pumps, actuators,
control valves, and conductors so arranged that they would perform a
useful task. When analyzing or designing a hydraulic circuit, the
following three important considerations must be taken into
account:
1. Safety of operation,
2. Performance of desired function, and
3. Efficiency of operation
• Hydraulic circuits are developed through the use of graphical symbols
for all components. Before hydraulic circuits can be understood, it is
necessary to know these fluid power symbols. American National
Standards Institute (ANSI) provide a set of symbols used to design and
analyze hydraulic circuits.

3. Control of Single Acting Hydraulic Cylinder
• Control of Single Acting Hydraulic Cylinder: Figure 1 shows how a
3/2 (three way, two positions)lever-actuated, directional control valve
(DCV) can be used to control the operation of a single-acting cylinder.
In the spring offset mode, full pump flow goes to the tank via the
pressure relief valve. In this mode, the spring in the rod end of the
cylinder retracts the piston as oil from the blank end drains back to the
tank. When the position of the valve is turned into its left envelope,
pump flow extends the cylinder. Once it is fully extended, deactivation
of the DCV occurs which allows the cylinder to retract as the DCV
shifts into its spring offset mode (right envelope).

4. Control of Single Acting Hydraulic Cylinder
(Cont’d)
Figure 1: Control of single acting hydraulic cylinder

5. Control of Single Acting Hydraulic Cylinder
(Cont’d)
Figure 2: Various symbols with their explanation used in Fig.1

6. Control of Double Acting Hydraulic Cylinder
• Figure 3 shows a circuit used to control a double-acting hydraulic
cylinder using 4/3 (four way, three positions) DCV. The operation is
described as follows:
1. When the four-way valve is in its spring-centered position (tandem
design), the cylinder is hydraulically locked. Also, the pump is
unloaded back to the tank at essentially atmospheric pressure.
2. When the four-way valve is actuated into the flow path
configuration of the left envelope, the cylinder is extended against
its load force Fload as oil flows from port P through port A. Also, oil
in the rod end of the cylinder is free to flow back to the tank via the
four-way valve from port B through port T. Note that the cylinder
could not extend if this oil were not allowed to leave the rod end of
the cylinder.

7. Control of Double Acting Hydraulic Cylinder
(Cont’d)
3. When the four-way valve is deactivated, the spring-centered envelope
prevails, and the cylinder is once again hydraulically locked.
4. When the four-way 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.
5. 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. In any event, the system is protected
from any cylinder overloads.
6. The check valve prevents the load (if it becomes excessive) from
retracting the cylinder while it is being extended using the left envelope
flow path configuration.

8. Control of Double Acting Hydraulic Cylinder
(Cont’d)
Figure 3: Control of double acting hydraulic cylinder
using 4/3 DCV

9. Sealing Devices
• Oil leakage, located anywhere in a hydraulic system, reduces efficiency and
increases power losses. Internal leakage does not result in loss of fluid
from the system because the fluid returns to the reservoir. Most hydraulic
components possess clearances that permit a small amount of internal
leakage. This leakage increases as component clearances between mating
parts increase due to wear. If the entire system leakage becomes large
enough, most of the pump's output is bypassed, and the actuators will not
operate properly.
• External leakage represents a loss of fluid from the system. In addition, it
might represent a safety hazard. Improperly assembled pipe fittings is the
most common cause of external leakage. Overtightened fittings may
become damaged, or vibration can cause properly tightened fittings to
become loose. Shaft seals on pumps and cylinders may become damaged
due to misalignment or excessive pressure.

10. Sealing Devices (Cont’d)
• Seals are used in hydraulic systems to prevent excessive internal and
external leakage and to keep out contamination. Seals can be of the
positive or non-positive type and can be designed for static or dynamic
applications.
• Positive seals do not allow any leakage whatsoever (external or
internal). Non-positive seals (such as the clearance used to provide a
lubricating film between a valve spool and its housing bore) permit a
small amount of internal leakage.
• Static seals are used between mating parts that do not move relative to
each other. Figure 4 shows some typical examples which include
flange gaskets and seals. Notice that these seals are compressed
between two rigidly connected parts. They represent a relatively
simple and non-wearing joint, which should be trouble-free if properly
assembled.

11. Sealing Devices (Cont’d)
Figure 4: Static seal flange applications

12. Sealing Devices (Cont’d)
• Dynamic seals 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.
• The following represent the most widely used types of seal
configurations:
1. O-rings
2. Compression packings (V- and U-shapes)
3. Piston cup packings
4. Piston rings
5. Wiper rings

13. Sealing Devices (Cont’d)
• The O-ring is one of the most widely used seals for hydraulic systems.
It is a molded, synthetic rubber seal that has a round cross section in
its free state. See Fig. 5 for several different-sized O-rings, which can
be used for most static and dynamic conditions. These O-ring seals
give effective sealing through a wide range of pressures, temperatures,
and movements.
Figure 5: Different sized O-rings

14. Sealing Devices (Cont’d)
• As illustrated in Fig. 6, an O-ring is installed in an annular groove
machined into one of the mating parts. When pressure is applied, the
O-ring is forced against a third surface to create a positive seal. As a
result, the O-ring is capable of sealing against high pressures.
• However, O-rings are not generally suited for sealing rotating shafts or
where vibration is a problem. At very high pressures, the O-ring may
extrude into the clearance space between mating parts, as illustrated in
Fig. 7. This is unacceptable in dynamic application because of the
rapid resulting seal wear. This extrusion is prevented by installing a
backup ring, as shown in Fig. 7. If the pressure is applied in both
directions, a backup ring must be installed on both sides of the O-ring.

15. Sealing Devices (Cont’d)
Figure 6: O-ring operation

16. Sealing Devices (Cont’d)
Figure 7: Back-up ring for prevention of
extrusion

17. Filters and Strainers
• The hydraulic systems must provide high accuracy. This requires highly
precision-machined components. The worst enemy of a precision-made
hydraulic component is contamination of the fluid. Essentially,
contamination is any foreign material in the fluid that results in detrimental
operation of any component of the system. Contamination may be in the
form of a liquid, gas or solid and can be caused by any of the following:
1. Built into system during component maintenance and assembly:
Contaminants here include metal chips, bits of pipe threads, tubing burrs,
shreds of plastic tape, welding beads, bits of hose, and dirt.
2. Generated within system during operation: During the operation of a
hydraulic system, many sources of contamination exist. They include
moisture due to water condensation inside the reservoir, entrained gases, scale
caused by rust, bits of worn seal materials, particles of metal due to wear, and
sludges and varnishes due to oxidation of the oil.

18. Filters and Strainers (Cont’d)
3. Introduced into system from external environment: The main source
of contamination here is due to the use of dirty maintenance equipment
such as funnels, rags, and tools. Disassembled components should be
washed using clean hydraulic fluid before assembly. Any oil added to
the system should be free of contaminants and poured from clean
containers.

19. Filters and Strainers (Cont’d)
• Both filters and strainers are devices for trapping contaminants from a fluid.
The key difference is in the size of the particles which they remove.
Strainers typically remove larger particles that are visible in a liquid or gas,
while filters remove contaminants that are often so small, they cannot be
seen with the naked eye.
• Specifically, a filter is a device whose primary function is to retain the
insoluble contaminants from a fluid using some porous medium.
• Basically, a strainer is a coarse filter. Strainers are constructed of a wire
screen that rarely contains openings less than 100 mesh (U.S. Sieve No.).
The screen is wrapped around a metal frame. A 100-mesh screen has
openings of 0.0059 in., and thus a strainer removes only the larger particles.
Observe that the lower the mesh number, the coarser the screen is. Because
strainers have low-pressure drops, they are usually installed in the pump
suction line to remove contaminants large enough to damage the pump.

20. Filters and Strainers (Cont’d)
• A pressure gage is normally installed in the suction line between the
pump and strainer to indicate the condition of the strainer. A drop in
pressure indicates that the strainer is becoming clogged. This can
starve the pump, resulting in cavitation and increased pump noise.
• A filter can consist of materials in addition to a screen. Particle sizes
removed by filters are measured in micrometers (or microns). The
smallest sized particle that can be removed by a filter is 1 µm (one-
millionth of a meter, or 0.000039 in. The smallest-sized particle that
can normally be removed by a strainer is 0.0059. in. or approximately
150 µm.

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