How hydraulics is used with Elevators

1. Fluid Mechanics and Braking Systems 27/5/12
Fluid Mechanics in Braking Systems
Case studies, Part 1
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Definition of a fluid
A continuous, amorphous substance whose molecules move
freely past one another and that has the tendency to
assume the shape of its container; a liquid or gas.
http://www.thefreedictionary.com/fluid
Properties of fluids
• Unable to support shear stresses
• Take on the shape of the container
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Principles of Fluid Mechanics
• Hydrostatic pressure
• Pressure Principle
• Archimedes' Principles
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Hydrostatic Pressure
• When an object placed in a fluid, the fluid exerts a pressure over the entire surface of the
object
• The pressure acts at 90o to the surface of the object at any point
Pressure, P (Pa) = F = Force (N)
A Area (m2)
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Hydrostatic Pressure and Depth
Rule: The deeper an object is submerged, the greater the hydrostatic pressure
Hydrostatic Pressure Depth Relationship
Given the pressure at the surface, Po (in Pa), the pressure (in Pa) at depth, h ( in m) is:
P = Po + ρgh
where
-3
ρ = density of the fluid (a constant - kgm )
-2
g = acceleration due to gravity (ms )
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Additional References
http://celebrate.digitalbrain.com/celebrate/accounts/tak
acs/web/Hydrostatica/membr/
• http://www.youtube.com/watch?v=GdAuzpYLOsc
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Pascal's Principle
• Pressure applied to an enclosed liquid is transmitted undiminished to every point
in the fluid and to the walls of the container
• Commonly used in sealed hydraulic systems, e.g. automotive braking systems
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Pascal's Principle
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Additional References
• http://hyperphysics.phy-astr.gsu.edu/hbase/pasc.html
• http://en.wikipedia.org/wiki/Pascal's_law
• http://www.grc.nasa.gov/WWW/k-
12/WindTunnel/Activities/Pascals_principle.html
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Archimedes Principle
• When immersed in water an object will weigh less
than in air
• Water provides an upward force (buoyancy) that
partially counteracts the weight force
• If the upward force by the water on an object is
greater than the weight force, the object floats
Factors that effect buoyancy
• The density of the fluid
• The volume of the submerged object
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As Archimedes put it ....
A body wholly or partially
submerged in a fluid is buoyed up
This is independent of the
by a force equal to the weight of
composition or shape of the
the fluid displaced by the body
object
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Specific Gravity
Using Archimedes Principle, an object's specific gravity can be
calculated
Specific Gravity = Weight of object in air
Weight of object when submerged in water
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Additional References
• http://en.wikipedia.org/wiki/Buoyancy
• http://library.thinkquest.org/27948/archimede.html
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Use of Hydraulic Principles in Braking Systems
To find out about:
• the basic principles of a braking system, use the link
http://auto.howstuffworks.com/auto-parts/brakes/brake-types/brake.htm
• Drum brakes in cars, use the link
http://auto.howstuffworks.com/auto-parts/brakes/brake-types/drum-brake.htm
• Disk brakes in cars, use the link
http://auto.howstuffworks.com/auto-parts/brakes/brake-types/disc-brake.htm
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How hydraulic disk brakes work in cars
http://www.youtube.com/watch?v=LYetGatzWjM
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How hydraulic drum brakes work
http://www.youtube.com/watch?v=LX4mqzoe3BY
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Brakes used on Trains
• The braking systems on trains are based on a different "fluid" - namely
compressed air
• In 1869, George Westinghouse developed a braking system that used compressed
air to hold the brake shoe off the wheels of the carriages
• To apply the brake the air is released, (or the pressure fails), and the bakes are
immediately activated
• The method of braking is used with trains as a safety feature and is still used
today
For more information on train brakes see:
• http://en.wikipedia.org/wiki/Railway_air_brake
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http://www.railway-technical.com/airbra-app.gif
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Innovation in Braking Systems - ABS or Anti-lock
Braking System
Why was it developed?
• When a vehicle brakes suddenly, the wheels typically lock and the vehicle
skids
• A skidding vehicle has less traction than a turning wheel and as a result the
driver has less control of the vehicle
• If the wheels can be prevented from skidding, the driver will be able to:
• Stop faster
• Have some control over the vehicle when the brakes are applied
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ABS or Anti-lock Braking System
Components of ABS ?
• Speed sensors - located in wheels. Detect when the wheels are about to lock
• ABS Controlled Valve - located in the brake line. The valve has three possible
positions:
1. Open - pressure from the master cylinder is passed directly to the
brake
2. Closed - The brake line is closed off from the master cylinder.
Prevents increase in brake pressure if the brake is pressed harder
3. Release - some (not all) pressure is released from the brake
• Pump - Restores pressure in the brake line sfter the valve has been activated
• Controller - a computer that monitors the speed sensors and controls the
valves
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http://auto.howstuffworks.com/auto-
parts/brakes/brake-types/anti-lock-
brake4.htm
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How do the speed sensors work - Step 1
ABS speed sensors, mounted in the vehicle's wheel hubs constantly monitor the
rotation of each wheel to determine if the ABS needs to override manual braking
of the vehicle.
The speed sensors calculate
* revolutions of wheels as well as
* evaluating the continuity between all wheels.
If any differences are detected in rotation, the ABS is activated to control
braking.
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How do the speed sensors work - Step 2
Generating the Speed Sensor Signal
Speed sensors are made of:
A magnet wrapped in a coil and
A toothed sensor ring mounted around the CV joint hub.
When contact is made between the toothed ring and magnet, an electrical field is
given off forming a signal.
The signal includes measurements of the number of pulses per second created by
the electrical field existing between the magnet coil and sensor ring.
After conversion into digital form the signal is transmitted to the ABS
controller.
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How do the speed sensors work - Step 3
Sending Messages to the ABS Controller
This signal is passed along to the controller which counts the number of pulses.
This is used to determine wheel speed.
The ABS uses this data to determine whether it should intercede to control
braking
http://sciencealerts.blogspot.com.au/2011
/07/how-to-replace-abs-wheel-speed-
sensor.html
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ABS or Anti-lock Braking System
How does ABS work?
http://www.youtube.com/watch?v=ngKSirE7zJA
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Additional Resources
• http://en.wikipedia.org/wiki/Anti-lock_braking_system
• http://www.tc.gc.ca/eng/roadsafety/tp-tp13082-absind_e-127.htm
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Bibliography
Copeland, P. Engineering Studies, The Definitive Guide, Volume 1: The Preliminary
Course, 2002, Anno Domini 2000 Pty Ltd
TutorVista.com, Pascal's Law and Hydraulic Brake System,
http://www.youtube.com/watch?v=VxLTDtaRCZk, Accessed 18/5/12
KeepVid, http://keepvid.com/?url=http%3A%2F%2Fwww.youtube.com%2Fwatch%
3Fv%3DVxLTDtaRCZk, Accessed 18/5/12
The Free Dictionary by Farlex, http://www.thefreedictionary.com/fluid, Accessed
19/5/12, 2012, Farlex Inc
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Bibliography
Nice K., How Anti-Lock Brakes Work, http://auto.howstuffworks.com/auto-
parts/brakes/brake-types/anti-lock-brake.htm, Accessed 20/5/12
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End of Part 1
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