Transcript Unit 8 - Intro to Pascal's Law - Hydraulics

SYSTEMS AND CONTROL
Electrical and Electronics
Hydraulics and Pneumatics
Combining Systems
Work to be covered this term:
 Electrical and electronic systems
 Pneumatics and Hydraulics
 Pascal’s law and MA
 Combining systems
What is a hydraulic system?
 It is a system that transmits energy
through the use of fluid pressure by
obeying Pascal’s law.
 The major function of a hydraulic fluid
is to provide energy transmission
through the system which enables work
and motion to be accomplished.
PASCAL’S LAW
 The pressure applied to a confined
liquid is transmitted, without loss,
throughout the entire liquid and the
walls of the container
Application of Pascal’s Law
 It wasn’t until the beginning of the industrial
revolution when a British mechanic named Joseph
Bramah applied the principle of Pascal’s law in
the development of the first hydraulic press. In
1795, he patented his hydraulic press, known as
the Bramah press.
 Bramah figured that if a small force on a small
area would create a proportionally larger force on
a larger area, the only limit to the force that a
machine can exert is the area to which the
pressure is applied.
Simple Hydraulic system
Simple hydraulic system
Examples of Hydraulic systems
 Hydraulic jacks and many other
technological advancements such as
automobile brakes and dental chairs
work on the basis of Pascal's Principle,
named for Blaise Pascal, who lived in
the seventeenth century.
Hydraulic System
Components
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The major components that make up a hydraulic system are the reservoir, pump, valve(s) and
actuator(s) (motor, cylinder, etc.).
Reservoir
The purpose of the hydraulic reservoir is to hold a volume of fluid, transfer heat from the system,
allow solid contaminants to settle and facilitate the release of air and moisture from the fluid.
Pump
The hydraulic pump transmits mechanical energy into hydraulic energy. This is done by the
movement of fluid which is the transmission medium. There are several types of hydraulic pumps
including gear, vane and piston. All of these pumps have different subtypes intended for specific
applications such as a bent-axis piston pump or a variable displacement vane pump. All hydraulic
pumps work on the same principle, which is to displace fluid volume against a resistant load or
pressure.
Valves
Hydraulic valves are used in a system to start, stop and direct fluid flow. Hydraulic valves are
made up of poppets or spools and can be actuated by means of pneumatic, hydraulic, electrical,
manual or mechanical means.
Actuators
Hydraulic actuators are the end result of Pascal’s law. This is where the hydraulic energy is
converted back to mechanical energy. This can be done through use of a hydraulic cylinder which
converts hydraulic energy into linear motion and work, or a hydraulic motor which converts
hydraulic energy into rotary motion and work. As with hydraulic pumps, hydraulic cylinders and
hydraulic motors have several different subtypes, each intended for specific design applications.
PASCAL’S LAW
 The pressure applied to a confined
liquid is transmitted, without loss,
throughout the entire liquid and the
walls of the container
PASCAL’S LAW
Pascal's law means that when there is
an increase in pressure at any point in a
confined fluid, there is an equal increase
at every other point in the container.
The Basic Idea
 The basic idea behind any hydraulic system is
very simple:
 Force that is applied at one point is transmitted to
another point using an incompressible fluid.
 The fluid is almost always an oil of some sort.
 The force is almost always multiplied in the
process.
 The picture below shows the simplest
possible hydraulic system:
How does a hydraulic jack illustrate
the law?
Pascal’s law/principle contd.
 Basically, the principle states that the pressure
in a closed container is the same at all points.
 Pressure is described mathematically by a
Force divided by Area.
 Therefore if you have two cylinders connected
together, a small one and a large one, and apply
a small Force to the small cylinder, this would
result in a given pressure.
 By Pascal's Principle, this pressure would be
the same in the larger cylinder, but since the
larger cylinder has more area, the force emitted
by the second cylinder would be greater.
Pascal’s law/principle contd.
 This is represented by rearranging the
pressure formula P = F/A, to F = PA.
 The pressure stayed the same in the second
cylinder, but Area was increased, resulting in
a larger Force.
 The greater the differences in the areas of
the cylinders, the greater the potential force
output of the big cylinder.
 A hydraulic jack is simply two cylinders
connected as described above.
How does a hydraulic jack illustrate
the law?
How does a hydraulic jack work?
 http://www.hyjacks.net/animation.ht
m
How does a hydraulic jack illustrate
Pascal’s law?
 What can you say
about the pressure
on piston 1 and
piston 2?
F1
P1 
A1
•But, by Pascal’s Law,
and
P1  P2
F2
P2 
A2
How does a hydraulic jack
illustrate Pascal’s law?
 An enclosed fluid under pressure
exerts that pressure throughout its
volume and against any surface
containing it.
 That's called 'Pascal's Principle', and
allows a hydraulic lift to generate large
amounts of FORCE from the application
of a small FORCE.
How does a hydraulic jack
illustrate Pascal’s law?
 Assume a small piston (one square
metre area) applies a weight of 1N to a
confined hydraulic fluid.
 That provides a pressure of 1 N per
square metre throughout the fluid.
 If another larger piston with an area of
10 square metres is in contact with the
fluid, that piston will feel a force of
1N/square metre x 10 square metres =
10 N/square metre.
http://www.physlink.com/educatio
n/askexperts/ae526.cfm
 Explain the diagram!
Explanation
 In the example, when the smaller piston
moves a distance of 10 inches it
displaces 10 cubic inch
(163,8millilitres) of fluid.
 That 10 cubic inch displaced at the 10
square inch (0.000645 square metres)
piston moves it only 1 inch (2.54cm), so
a small force and larger distance have
been exchanged for a large force
through a smaller distance.
Hydraulic multiplication
 To determine the multiplication factor in
the figure above, start by looking at the
size of the pistons.
 Assume that the piston on the left is 2
inches in diameter, while the piston on
the right is 6 inches in diameter.
To do:
1. Calculate the area of the two pistons. (1 inch
= 2.54 cm).
2. How do the two areas compare is size?
3. What does this imply i.t.o. the force applied to
the left-hand piston?
4. If you applied a 100-pound downward force to
the left piston, what will the upward force be
on the right piston? (First convert pounds into
kg and then work out the force in Newtons).
5. What is the catch?
 So we can apply 1 lbs. to the small piston
and get 10 lbs. of force to lift a heavy
object with the large piston.
 Is this 'getting something for nothing'?
 Unfortunately, no. Just as a lever provides
more force near the fulcrum in exchange
for more distance further away, the
hydraulic lift merely converts work (force x
distance) at the smaller piston for the
SAME work at the larger one.
Example
 The pistons of a hydraulic press have
a radii of 2cm and 12cm.
a) What force must be applied to the smaller
piston to exert a force of 5000N on the
larger piston?
b) What is the pressure on each piston?
c) What is the actual mechanical advantage?
Homework
A hydraulic press has an input cylinder 2
inches in diameter and an output cylinder
6 inches in diameter.
Assuming 100% efficiency, find the force
exerted by the output piston when a
force of 10 pounds is applied to the input
piston.