Fluid Power
Control Power
CVHS
Fluid Power
Fluid power systems are used to transmit power
from one point to another
They are used in many transportation devices
example: brake system on a car
2 types of Fluid Power Systems:
1) Hydraulic systems are those that use a liquid, such
as oil as the transmitting medium
2) Pneumatic systems use air or gas as the medium
Hydraulic & Pneumatic
Hydraulic fluids can not be compressed
Pneumatic fluids are considered gases and are
compressible
Hydraulic fluids must have complete hydraulic
circuits. Return lines must be used, and a reservoir
is needed to hold the extra fluid
(closed
system)
Pneumatic fluids do not need complete circuits. Air
can be dumped into the atmosphere
(open system)
Force & Pressure
Force is defined as “the pushing or
pulling action of one object upon
another”
force usually causes an object to move
usually measured in pounds
Pressure is defined as “a force acting
upon an area”
measured commonly in psi
Types of Pressure
Pressure per square inch-absolute (psia)
• Measure of pressure of a fluid starting at zero
atmospheres and working upward to a
maximum pressure
• An atmosphere is a unit of pressure in which
14.7 psi of pressure is considered absolute at
sea level
Pressure per square inch-gage (psig)
• Measures the pressure of a fluid above or
below the surrounding atmosphere
Pascal’s Law
“A pressure applied to a confined fluid is
transmitted undiminished to every portion of
the surface of the containing vessel”
Also states: “Pressure on a fluid is equal to
the force applied divided by the area”
• P=F/A
– where:
– P = pressure in psi
– F = force applied in pounds
– A = area to which the force is applied
Applying Pascal’s Law
Given:
• Pressure = 200 psi
• Force = 100 lbs
Determine the Area to which the Force is
Applied
•P=F/A
•A=F/P
• A = 200 lbs / 100 psi
• A = 2 in2
Boyle’s Law
“The volume of a gas varies inversely with
the pressure applied to it, provided the
temperature of the gas remains constant”
This means that as the volume of a gas is reduced
such as in an engine when the piston compresses
air - the pressure is increased
If the volume was halved, the pressure would be
doubled
• Note: remember hydraulic fluids cannot be compressed,
but pneumatic fluids can
Charles Law
“As the temperature of a gas increases,
the volume of the gas increases
proportionally, if the pressure is kept
constant
This means that as the temperature of a
gas goes up, so will the volume of that gas
If the temperature is doubled, then the
volume is also doubled (unconfined gas)
Measuring Pressure
Bourdon Gage
• Can read either pneumatic or hydraulic
fluid pressure
• Tube bends out with pressure, hook end is
connected to the end of the pressure needle
(mounted on a pivot)
Manometer
• Used to read pneumatic pressures and vacuum
readings
• U shaped tube with a fluid inside (fluid varies
dependent upon application - i.e.: mercury
used for high pressure)
Fluid Characteristics
Viscosity - the fluidity or thickness of the liquid at a
specific temperature
• Measured by a device known as a Saybolt universal
viscosimeter - (SUS) Saybolt universal seconds test
Viscosity Index - a measure of how much the viscosity
of a fluid changes with a given temperature
• Usually, the higher the VI, the smaller the relative change in
viscosity with temperature
Pour Point - the temperature at which the fluid ceases
to flow
SAE Ratings
SAE - Society of Automotive Engineers
ratings in viscosity
they use the Saybolt universal seconds test (SUS)
10W-30
• rating with the W after it (10) means it was tested at 0oF
• if no W after (30), it was tested at 210oF
– thicker fluids have higher SAE ratings
– sometimes a third rating is mentioned, the middle, - tested
at 150oF
Fluid Power Pumps
Provide the pressure needed for a fluid power
system to operate
Hydraulic types include:
Gear Pumps
Centrifugal Pumps
Reciprocating Pumps
Pneumatic are called air compressors
Use pistons to compress air (reciprocating)
May be single or multi stage
Usually send air to a receiver (tank)
Fluid Power Conductors
Provide a path for
the pressurized fluid to travel
Includes hoses (flexible)
and lines (ridged)
Must be properly sized
to suit system requirements
Control Devices
Valves control fluid pressures, flow rates
and directions
Include:
Pressure Relief Valves
Flow Control Valves
Pressure Control Valves (regulator)
Directional Control Valves
Fluid Power Actuators
These devices put the pressurized fluid
to work
Cylinders are devices that use pressure
to move in or out (linear)
Single Acting
Double Acting
Motors are devices that use pressure to
rotate (spin)
Fluid Power storage
Pneumatic: use a receiver (pressure
tank) to store large volumes of
pressurized air
Hydraulic systems may use an
accumulator to store a small
amount of pressurized liquid
(acts like a capacitor)
Fluid Power Components & Example
Circuithttp://www.patchn.com/tutr-hyd.htm
Assignment
Chapter 9 Questions
1-10
23-34