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HYDRAULIC POWER
Suggested Answers
Activity 1
Visit the following web site to read a statement of Archimedes’ principle http://www.infoplease.com/ce6/sci/A0804583.html 
and explain how a submarine floating on the surface can be made to dive.
Submarines on the surface displace as much water as their own mass and so they float. They have tanks within their structure that can be filled, or emptied, of water from the ocean. If the submarine is to dive then its weight must be increased beyond that which enables it to float. This is achieved by flooding the tanks with water and expelling the air. To surface, the water is pumped out of the tanks, increasing the buoyancy, and therefore allowing the submarine to surface.
Activity 2
Visit the following web site and use Pascal’s principle to explain how hydraulic brakes work in a motor vehicle http://webphysics.davidson.edu/physlet_resources/bu_semester1/c23_pressure_pascal.html .
The simplest hydraulic disc brake system for a motor car comprises a small diameter 'master' cylinder and piston. The piston is connected to the brake pedal. Pipes are distributed from the master cylinder to each of the four wheels. At each wheel a 'slave' cylinder is fixed to the suspension, and its piston is connected to the disc brake pads. The slave cylinder is usually larger in diameter than the master cylinder. The system is full of hydraulic fluid. When the driver presses on the brake pedal the master piston tries to compress the fluid in the master cylinder. The pressure developed in the fluid is transmitted equally throughout the fluid. At the 'slave' cylinders the fluid pressure developed at the master cylinder acts on each of the slave pistons. As the slave pistons are free to move, they push the brake pads onto the disc brake causing it to slow down and stop the vehicle.
Activity 4
If the mass on the accumulator piston was 102 tonne, and the piston diameter was 508 mm, calculate the water pressure.
P = F/A
= 4 x F / π x d2
= 4 x 102000 x 9.8 / π x 5082
= 4.93 MPa.
Activity 5
Analyse a typical lift system and describe some of its limitations.
Some limitations of the direct acting lift system include:
- The number of floors serviced by the lift is limited to the length of the piston, and the depth of hole for the cylinder.
- The load is limited. The longer the piston, the less the load
- Speed is limited to the maximum flow rate of the water in the supply piping. This was typically 50 mm diameter.
Activity 6
If this lift serviced three floors at 3 m each, the piston was 127 mm in diameter and the pressure 5 MPa. What is the maximum load that can be carried?
F = P x A
F = P x π x d2 / 4
= 5 x 106 x π x 0.1272 / 4
= 63 kN
Activity 7
Why was grey cast iron the main material used for the production of cylinders for hydraulic systems, and why were the cylinder walls relatively thick?
- Grey cast iron was relatively cheap at the end of the nineteenth century compared to steel.
- Grey cast iron melts at 1130°C, steel at about 1500°, and is therefore cheaper to melt.
- Grey cast iron flows easily from the ladle to the mould, and it fills complex mould shapes easily.
- The shrinkage rate of cast iron is not excessive.
- Grey cast iron contains small flakes of graphite in the microstructure. These act as lubricants during machining, making grey cast iron easy to machine.
- Grey cast iron dampens vibration well.
- The tensile strength of grey cast iron is very low compared to that of steel. Therefore thick cylinder walls were necessary to resist the applied stresses.
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