Transcript Hydraulic Systems

Hydraulic Systems
• The term hydraulic is derived from greek
words Hydra and aulos means pipe.
• Hydraulic systems are defined as fluid based
systems using liquids as a transmission
medium.
• Oil is the most commonly used medium in
these systems.
• Hence hydraulic system may also be defined
as a system that uses oil.
• Petroleum Based Oils : The petroleum based
oils used in hydraulic systems are Pennsylvania
or paraffin-base oils.
Elements of a hydraulic system
• It consists of a tank, pump, pressure, regulator,
control valve and cylinder-piston
arrangement.The hydraulic fluid/oil is stored in a
tank.
• Before allowing the oil to enter in the system, it is
filtered by the filter because dirt in the oil may
cause the oil to stick the valve, seal failure.
• Filters are porous materials like paper, cellulose
and cotton.
• The pump pumps this filtered oil .
• A hydraulic pump takes oil from a tank and
delivers it to the rest of the hydraulic system.
• During this process, the pump raises the oil
pressure to the desired level.
• This hydraulic pump is driven by ac induction
motor or internal combustion engine.
• The function of pressure regulator is to limit
the oil pressure to design limits.
• The control-valve operates the cylinder piston
as per load requirements.
ACCESSORIES USED IN HYDRAULIC
SYSTEMS
Actuators
Thus the force is applied by the piston depends upon the
area and the applied pressure. And cylinder may be single
acting or double acting.
Rotary Actuators:
Rotary actuators are the hydraulic and pneumatic equivalent of electric motors. They
include gear and vane motors.
Figure (a) shows the diagram of a gear motor. Here the fluid enters at high pressure
from the inlet that pressurizes the top chamber. This pressure is applied to the
gears, which results in rotating the gears. Figure (b) shows the diagram of a vane
motor. In this type of motor, the oil enters at high pressure through the inlet,
pushes the vane to rotate the rotor and passes through the outlet.
Control Valve:
Both hydraulic and pneumatic systems require control valves to direct and regulate
the flow of fluid/air from pump/compressor to different load devices in the
system. The operating principles and details of hydraulic and pneumatic valve
devices are same. The only difference is operating pressure and type of sealing
required. There are two types of valve as discussed below:
(i)
Infinite position valve. These valves take discrete positions like open or close to
modulate the flow or pressure.
(ii) Finite position valve. These valves have discrete positions like open or close and
allow flow of fluid/air.
Inlet and outlet connections of a valve are called ports. The finite position valves are
designated by x/y, where x is number of ports and y represents number of
positions, e.g. a 4/3 valve has 4 ports and 3 positions.
Figure (a), represents OFF position of a valve in which P and T are connected
and hence the system pressure is vented to tank.
Figure (b), the output port B is pressurized and the port A is vented to tank.
Figure (c), the output port A is pressurized and the port B is vented to tank.
This type of valve is useful in double acting cylinders.
Different types of control valves are Poppet, Spool and Rotary.
Poppet valve. Figure (a) shows diagram and symbol of a 2/2 poppet valve. It has two
positions as follows:
(i) When the push button is pressed, the ball pushes out from the seat and fluid
flows from P to A.
(ii) As push button is released, the spring and fluid pressure causes the valve to
close. The valve may be actuated by spring, plunger, solenoid, push button, level
and roller arrangements.
Spool Valve. This is a common type of directional control valve. In this type of valve a
spool moves horizontally within the valve body to control the flow. Figure (b) shows
a spool valve
Rotary Valve. A rotary valve consists of a rotating spool, which aligns with the holes in
valve body to provide desired operation figure (c) shows a rotary valve.
PNEUMATIC SYSTEMS:
The term pneumatic is derived from Greek word pneumn, which means
wind or breath. Gas based systems are called pneumatic systems.
These systems use compressed air or gas.
Figure shows arrangement of a pneumatic system
A pneumatic system uses compressed air as medium. It consists of
elements like compressor, reservoir, control valve, cylinder-piston and
motor. Air in any pneumatic system must be clean and dry to reduce
wear and avoid maintenance
The function of compressor is to provide pressurized air. It increases air
pressure by reducing its volume. This compressed air is processed
through air treatment unit. If there is any humidity in the air, it gets
drier and lower the dew point. The volume of compressed air is
stored in reservoir. A pressure sensitive switch is attached to the
reservoir that activates the compressor through on-off control and
motor. The control valve operates the cylinder-piston as per the load
requirements.
COMPARISON BETWEEN HYDRAULIC AND PNEUMATIC
SYSTEMS:
HYDRAULIC SYSTEM
PNEUMATIC SYSTEM
It uses fluid or oil as a medium
It uses compressor air
As oil us used, this system is self-lubricated
Oil is to be added in the compressed air for
lubrication of sliding part
Metallic pipes like copper are used in this
system
Rubber piper and hosepipes can be used to
carry compressed air
Used fluid or oil is recollected in the sump,
filtered and reused in the system
The residue air in not used
It is fire-hazardous system and causes
accidents due to slippage of oil
It is non-hazardous system
Efficient close control over the speed can be
obtained
No close control over the speed is possible
It is complicated and has high cost
It is simple and has low cost
Actuators used in the system operate smoothly Actuators used in the system operate suddenly
with precise control and without jerks
without control on jerks
Control valve sizing:
The term control valve sizing is used for the procedure
of determining the correct size of valve body. The
equation relating the rate of flow of liquid Q
through a wide open valve to its size is:
Where Av is the valve flow coefficient,
is the
pressure drop across the valve and p the density of
fluid. This equation is sometimes written, with the
quantities in SI units, as
• Determine the valve size for a valve that is
required to control the flow of water when
the maximum flow required is 0.012m3/s and
the permissible pressure drop across the valve
at this flow rate is 300kPa.
•
A cylindrical pipe having diameter of 0.75m contains water flowing at the speed of 1.2m/s. Find the
volume flow rate of mass flow rate of water if the density of water is 1000kg/m3 .
Solution:
d=0.75m
v=1.2m/s
ρ=1000
Qv=Av
Qv = π 2 .1.2
Qv=0.530
Qm= ρQv
= (1000)(0.530)
Qm= 5300kg/sec
Voltage sensitivity of Peizoelectric
Crystal
• Q. A barium titanate crystal has charge density
of 150pC/N and the di-electric constant of
1.25*10^-8F/m.Find the voltage sensitivity
and relative permittivity of the crystal.
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Sq=150 pC/N
Є = 1.25*10^-8
Sv = ?
Єr=?
Sv=Sq/Є = V/N
Єr=Є/Є₀
Q-For the barrium titanate crystal in the previous
example if the thickness of the crystal is 0.8mm
and its effective area is 1.25 mm2 then find the
voltage across the crystal when the force of 50N
is applied.
t = 0.8mm
A = 1.25mm2
V=?
F= 50 N
V = sq.t/Є.A