Engineering Mechanics

Engineering Mechanics
Case Studies
Hydraulics and Pneumatics
Braking Systems
Lifting Devices
Aeronautical Engineering
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Introduction to Fluid Mechanics
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.

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.

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

Archimedes Principle
Principles of Fluid Mechanics
In 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.

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.

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.

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.

Pascal's Principle
So 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 area
Thus the total force or thrust on a surface is the area of the surface, times a pressure exerted on that
surface
F=pxA
Basic unit of pressure is the Pascal (Pa)
Pascal’s Principle F1 = F2
A1 A2
F2 = F1 x F2
A1

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.

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.

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.

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.

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 thewheel cylinder back and the fluid flows
back into the master cylinder.

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

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 partial application.

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.

Case Study: Innovation in Braking Systems
Anti-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
 .
Advantages
 Effective way to prevent crashing due to the sensors detecting lockup thus reducing hydraulic pressure at the wheel
Disadvantages
 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

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.

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.

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.

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.

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

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.

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.

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

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.

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

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.

Engineering Mechanics

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