Engineering MechanicsCase Studies
Hydraulics and Pneumatics
Braking SystemsLifting Devices
Aeronautical Engineering
The term fluid applies to both liquids and gases. Fluid mechanics is the study of gases and liquids, their physical behaviour, and their role in engineering systems.
Introduction to Fluid Mechanics
Fluids are: Shapeless and do not resist being sheared When a force is exerted on fluid the pressure increases, whereas the
force is directional the pressure is omnidirectional ( exerted in all directions)
Viscous (Oil has a high viscosity whilst water has a low viscosity) Oil has a higher viscosity when cold. As the temperature increases the
viscosity becomes lower so the oil becomes thinner Subject to turbulence when force is applied
There are two types of fluids
Hydraulic fluids are: Incompressible ( when a pressure is exerted no volumetric change
occurs). Oil is often used as a hydraulic fluid.
Pneumatic fluids are: Gases can be compressed. An example is Liquid Petroleum Gas
(LPG). This is pressurised into a gas tank to be sored as a liquid. When released it turns back to a gas.
Introduction to Fluid Mechanics
Advantages of hydraulic systems include: Appropriate method of power transmission over long distances
(Example: trucks use hydraulic power instead of fuel) Good flexibility Variable speed control Safe and reliable
Disadvantages: Need to be in a confined space Fire hazard Leaks can pose a safety hazard or environmental hazard Oil filtration must be maintained
Introduction to Fluid Mechanics
Archimedes PrincipleIn 212 B.C., the Greek scientist Archimedes discovered the following principle: When an object is completely or partly immersed in a fluid it experiences a force thrusting it up. The force (upthrust on object) is equal to the weight of the fluid displaced by the object.
Principles of Fluid Mechanics
Archimedes Principle cont.…d If the density of the object is greater than that of the fluid, the object
will sink. If the density of the object is equal to that of the fluid, the object will
neither sink or float. If the density of the object is less than that of the fluid, the object will
float.
Principles of Fluid Mechanics
Pascal's Principle cont.…d
Pascals principle states that pressure exerted anywhere in a confined fluid is transmitted equally in all directions throughout the fluid.
A good example of this is when two pistons are fitted into two glass cylinders filled with oil and connected to one another with an oil filled pipe. If you applied a downward force on one of the pistons then the force is transmitted to the second piston through the oil in the pipe. Since oil is incompressible, efficiency is very good so most of the applied force appears at the second piston.
Principles of Fluid Mechanics
Pascals Principle cont.…d
Therefore the application of a force (F1) in a cylinder of cross sectional area (A1), an equal pressure will be transmitted to the other piston and cylinder, of area (A2), causing a thrust or force in this piston, of magnitude F2.
If A2 is very large compared to A1 a comparatively smaller force applied to the smaller piston can overcome a large resistance acting on the larger piston. Additionally, this can apply to a number of
different cylinders and pistons attached to the sealed system.
Principles of Fluid Mechanics
Pascal's PrincipleSo we see that Pascals principle states that pressure exerted anywhere in a confined fluid is transmitted equally in all directions throughout the fluid.
What is meant by pressure?Pressure is force per unit areaThus the total force or thrust on a surface is the area of the surface, times a pressure exerted on that surfaceF=pxABasic unit of pressure is the Pascal (Pa)Pascal’s Principle F1 = F2
A1 A2
F2 = F1 x F2
A1
Principles of Fluid Mechanics
Case Study: Braking Systems in Private Vehicles
Brakes are the most important feature of any modern vehicle.
A typical modern vehicle weighs around 1.4 tonnes, has a 3.5 litre engine, and accelerates from 0 to 100 kph in approximately 10 seconds.
To do this it has a sophisticated engine, transmission and drive line system. This system has thousands of parts and takes up nearly half the vehicles weight. In contrast the braking system of a car has only approximately 200 parts weighing less than 40 kilos and has to be able to stop the vehicle from 100kph to 0 in 3 to 5 seconds.
Principles of Fluid Mechanics
We all know that a car slows down and stops when we apply brakes. How does this happen?
How does the force exerted on the foot pedal stop or slow down a car?
How does it multiply the force enough to stop something as big as a car?
The basic idea behind any hydraulic system is very simple. The force applied at one point is transmitted to another point (as stated by Pascal's principle) using an incompressible fluid, generally oil. Most brake systems multiply the force in the process.
The advantages of hydraulic systems are the pipe connecting the two cylinders can be of any length and shape allowing to choose any path separating the two pistons and the force applied is multiplied.
Principles of Fluid Mechanics
Here you can see the hydraulic
brake system of a car.
It consists of a pipeline containing fluid.
One end of which is connected
to the master cylinder fitted with
a piston attached to the foot pedal.
The other end of the pipeline is
connected to the wheel cylinder
which has two steel caliper pistons
on either side of it. Attached to the
pistons is the brake drum and within
the brake drum is the brake shoes.
The area of cross-section of
the wheel cylinder is greater
than the area of the cross-section
of the master cylinder
Principles of Fluid Mechanics
Let us see what
happens when brakes are applied.
When the brakes are applied
the foot pedal is pushed exerting
pressure on the fluid in
the master cylinder.
Principles of Fluid Mechanics
This pressure is transmitted equally and
undiminished throughout the fluid and to
the pistons of the wheel cylinder.
This pushes the pistons outwards forcing
the brake shoes to press against the
rim of the wheel due to which the motion
retards. On releasing the pressure
on the pedal the return spring
forces the pistons of the
wheel cylinder back and the
fluid flows back into the master cylinder.
Principles of Fluid Mechanics
Case Study: Air Brake System used in Trains (Pneumatic System)
The air brake is the standard, fail-safe, train brake used by railways all over the world It is based on the simple physical properties of compressed
air A moving train has kinetic energy which needs to be removed
in order for it to stop. The majority of trains still use the compressed air braking
system. These systems are known as air brakes or pneumatic brakes
Principles of Fluid Mechanics
Air Brake System used in Trains (Pneumatic System) cont.…d
The force of the air pushes blocks
or pads onto the train wheels.
The compressed air is fed through
the train by a brake pipe.
Varying the level of air pressure
in the pipe causes change in the
state of the brake on each vehicle.
The driver can apply the brake,
release it or hold it on after
a partial application.
Principles of Fluid Mechanics
Air Brake System used in Trains (Pneumatic System) cont.…d When the driver places the brake valve in the application position this
causes air pressure in the brake pipe to escape. This loss of pressure is detected by the slide vale in the triple valve Due to the loss of pressure on one side, the brake side, one side of the
valve has fallen causing the auxiliary reservoir pressure to push the valve towards the right so that the feed groove over the valve closes.
This in turn causes the connection between the brake cylinder and the exhaust to be closed
The connection between the auxiliary reservoir and the brake cylinder has become open.
Auxiliary air feeds through into the brake cylinder This air forces the piston to move against the spring putting pressure on
the brake blocks which then are applied to the wheels. Air will still pass through the reservoir to the brake cylinder until the
pressure in both equalises.
Principles of Fluid Mechanics
Case Study: Innovation in Braking SystemsAnti-Lock Braking system (ABS) Anti- lock braking(ABS) systems first came about around the 1920’s
when it was applied to the concept of an automatic override system for aircraft brakes.
ABS was primarily used up until the 1950’s for aircraft braking technology Car manufactures started to experiment with ABS in the 1960’s however
it became an expensive project which was soon abandoned In the 70’s saw the addition of computer-controlled sensors which led to
the revival of ABS for safety purposes.Advantages Effective way to prevent crashing due to the sensors detecting lockup
thus reducing hydraulic pressure at the wheelDisadvantages Debate on whether the driver should have full control of the car and not
rely on a braking system that could fail Drivers tend to drive aggressively knowing they have the ABS to rely on
Principles of Fluid Mechanics
Innovation in Braking Systems
Anti-Lock Braking system (ABS)
The existing hydraulic braking
system which consists of the
master cylinder, calipers,
wheel cylinders, pads, shoes
and associated connecting valves,
line and hoses has the ABS system
incorporated into the car as well.
The computer receives a signal from
the individual sensors which are
located at each wheel
It compares the speed of each
wheel with the other wheels
If the comparison indicates wheel
lock up is present signals are sent
to valves and actuators which raise or
lower the hydraulic pressure to each
wheel which corrects the skid.
Principles of Fluid Mechanics
Innovation in Braking Systems cont...d
Anti-Lock Braking system (ABS)
This process is produced thousands of times per second enabling maximum stopping ability under any condition
All of these actions go unnoticed by the driver unless warnings lights are shown signalling failure of the braking system.
When the driver applies the brakes and ABS kicks the driver will feel a shudder or vibration. This is normal, however the driver tends to ease of the brakes. The driver should carry on applying the brakes which will eventually stop the car skidding.
Principles of Fluid Mechanics
Case Study: Fluid Mechanics in Lifting Devices
Prior to the introduction of the hydraulic jack in 1851 by Richard Dudgeon, screw jacks were being used. Screw jacks took more time and effort to raise the desired object.
Scissor screw jacks are usually used to lift a car to change a flat tyre
The bottom of the jack rests on the ground while the top fits under the body of a car. A screw is inserted in the center of the scissor system and is turned to the right to raise the jack and lift the car. After the tire is replaced, the screw is turned to the left to lower the car back to the ground.
Principles of Fluid Mechanics
Case Study: Fluid Mechanics in Lifting Devices
Hydraulic Bottle Jacks are extremely adaptable since they can be placed in restricted spaces and provide good leverage.
They have a longer handle as compared to rest of the hydraulic jacks and push up against a lever that gives a lift to the main lift arm.
With their use, it is possible to give a greater lift per stroke.
They are extensively used in the construction of buildings and repairing the foundation of houses.
It has also been found to be very useful in search and rescue operations.
Principles of Fluid Mechanics
Case Study: Fluid Mechanics in Lifting Devices
Hydraulic jacks have revolutionised the way we lift heavy objects and are widely used all across the globe.
They make our life much more comfortable than it was before.
These jacks have outweighed conventional screw jacks that were in use at some point of time.
They have two cylinders which are joined together and are filled with a fluid usually oil.
The hydraulic jack works on the principle of Pascal's law
Principles of Fluid Mechanics
Case Study: Fluid Mechanics in Lifting Devices
The jack basically consists of two cylinders, one small, one large.
The two cylinders are each filled with oil, and there is a passage between them. Inside each cylinder is a piston.
The oil in the jack is a liquid, so it’s incompressible.
When you push down on the jack’s lever, you create a force, F1, on the small piston.
This then creates equal pressure in the oil under both the small and large pistons.
Principles of Fluid Mechanics
Case Study: Fluid Mechanics in Lifting Devices
We know that pressure is force divided by area p = F
A
In the diagram the large piston is going to lift the weight of the car. Because the large piston has a greater surface area than the small piston, the fluid in the large cylinder will create a much larger force to push against the weight of the car hence lifting it off the ground.
Principles of Fluid Mechanics
Case Study: Hydraulic Systems in Aeronautical Engineering
Hydraulics are used for different aircraft applications. Brakes Landing gear Flight control Flaps Speed brakes Nose wheel tillers
Hydraulic Fluid
Superior hydraulic fluid should be:
Incompressible
Flows with minimal friction
Has strong lubricating properties
Resistant to foaming
Maintain properties at high temperatures
Should never be mixed
Flammable at 5606°C
Principles of Fluid Mechanics
Case Study: Hydraulic Systems in Aeronautical Engineering
System Components
Hydraulic pumps are usually engine or electrically driven gear type pumps that provide system pressure
Large aircraft will have more than one interconnected hydraulic systems with backup pumps in case of failure
Hydraulic motors utilise hydraulic pressure to provide mechanical power to flaps or landing gear
Hydraulic cylinders use pistons to translate hydraulic pressure into linear mechanical movement for brakes
Hydraulic lines deliver hydraulic power from pump to motor or actuator
Pressure gauge supplies the pilot with system pressure information.
Principles of Fluid Mechanics
Case Study: Hydraulic Systems in Aeronautical Engineering
Valves direct the flow of hydraulic fluid and control and regulate pressure
Actuators convert hydraulic pressure to move components to a desired position, also helps maintain a constant pressure within the system. Absorbs the shocks due to rapid pressure variations
Reservoir store adequate hydraulic fluid fro system
Standpipe is designed into the reservoir to guard against system leakage.
The diagram represents a
hydraulic landing gear system
in a aeroplane
Principles of Fluid Mechanics
Case Study: Hydraulic Systems in Aeronautical Engineering
Landing gear
The aircraft landing gear is a combination of mechanical structure, pneumatics (air springs) and hydraulic damping.
A good landing gear design reduces the loads produced into the airframe during landing and take-off.
Principles of Fluid Mechanics
Metcalf, P & Metcalf, R. (2009) Excel Senior High School Engineering Studies: Your Step –By-Step Guide To Exam Success. Sydney. Pascal Press.
EDUC6505 Engineering Education Studies 2 Notes
B.O.S (2009) NSW HSC Online Engineering Studies Syllabus. Retrieved February 8th 2012, from www.hsc.csu.edu.au/engineering_studies
Schlenker, B.R., & McKern, D. (1979) IntroductionTo Engineering Mechanics (2nd ed).Sydney, Jacaranda Wiley Ltd.
PBR (2005) Hydraulic Brake Systems Guide. Retrieved 21st May 2012 fromhttp://www.pbr.com.au/technical/documents/hydraulicbrakesystemsguide.pdf
Selkirk College (2008). Hydraulic Landing Gear. Retrieved 18th May 2012 from http://selair.selkirk.ca/Training/systems/power-point/AVIA%20140/11HYDRAULICS%20AND%20LANDING%20GEAR.pdf
TAFE NSW (2002) Engineering Studies Preliminary Stage 6 Course: Braking Systems . Retrieved 16th May 2012 from http://www.picnicpt-h.schools.nsw.edu.au/Faculty_Webs/Industrial%20Arts/Engineering/Braking%20systems.pdf
Prepared by Yanake Tennant SID No: 3159851
References