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33 Cards in this Set
- Front
- Back
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Describe the difference (with examples) between hydrodynamic and hydrostatic systems.
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Hydrodynamic systems transfer energy through the flow (movement) of the oil/medium. (Torque converter or waterwheel).
Hydrostatic systems transfer energy by controlling the force(s) created by fluid under pressure in a sealed system. (Hydraulic ram or motor.) P. 350 |
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State the laws/rules which govern the behaviour of pressurized fluid.
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1. Pressure is caused by resistance to flow.
2. Pressure of a confined liquid is always relative to applied force. 3. If there is no restriction, there is no pressure. p. 350 |
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Define force. How is the relationship between force, pressure and area expressed mathematically?
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Force: A push or pull effort - not work accomplished.
"Pascal's law": Force = Pressure x Area p. 351 |
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Define vacuum and head pressure.
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Vacuum is a condition of less than atmospheric pressure within a sealed vessel or area.
Head pressure is the pressure created by any forces acting downward on a body of fluid. p. 351 |
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What is the difference between gauge pressure and absolute pressure?
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Absolute pressure means 0 PSI = 0 PSI, gauge pressure means 0 PSI = atmospheric.
p. 352 |
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Describe the relationship between input force, output force, piston surface area and the distance a load will travel in a hydraulic circuit.
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Input forces are multiplied by larger output pistons, but travel will be diminished whereas the opposite is would be true for smaller output pistons.
p. 352/353 |
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Describe the relationship between conduit diameter and fluid velocity - assuming a constant flow rate.
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Fluid velocity will be inversely proportional to conduit diameter.
p. 354 |
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How are open and closed center systems different? Where would you find and example of each? What benefits and drawbacks does each system offer?
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Open center: oil returns to tank through the control valve when the control valve is not being operated. Found in truck power steering systems, circuits which employ a fixed displacement pump.
Closed center: Oil is dead ended when it reaches an unoperated control valve. A closed center circuit requires a variable displacement pump and/or a relief mechanism located between the pump and control valve. Found on garbage packers/front bucket forks/excavators. p. 355 |
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What are the 3 common functions of an accumulator?
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- Store pressurized fluid to be used in the event of a circuit failure.
- Dampen/absorb shocks and vibrations in the circuit. - Maintain consistent pressure. |
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What are the 3 most common types of accumulator and how do they work?
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Gas loaded, spring loaded, gravity/weight loaded.
The 'load' element and the oil/air are separated by a piston/diaphragm/bladder. As circuit pressure fluctuates the 'load' element compresses or expands. p. 356 |
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Where are gear pumps commonly found?
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Diesel fuel subsystems and engine oil pumps.
p. 357 |
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What are the 3 types of gear pumps?
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Internal gear, external gear, rotor gear pump.
p. 358 |
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Compare the differences, benefits and drawbacks of balanced and unbalanced vane pumps.
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Balanced vane pumps have and inlet and outlet on each side of the pump, whereas unbalanced vane pumps have only one inlet and one outlet.
Balanced vane pumps will have greater output and longer life. Unbalanced vane pump bearings tend to fail more frequently due to the excess pressure placed on the suction side of the bearing. p. 359/360 |
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How is the displacement of an axial piston pump varied?
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Changing swashplate angle increases or decreases each piston's effective stroke.
p. 360 |
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Describe how pumps are rated.
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Pumps are rated by flow (gallons or liters per minute/hour), maximum permissible backpressure, and RPM.
i.e 40 gpm @ 1800 rpm; 3500 psi max. p. 361/362 |
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What are some commons types of pump drive?
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PTO, engine aux, electrical motors.
p. 362 |
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Name and describe the 2 applications of bleed off and fixed orifice valves.
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Meter-in: controls flow (speed) of actuator.
Meter out: controls flow out of actuator. p. 364 |
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What is the most common type of directional control valve?
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Spool type.
p. 365 |
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What is the purpose of the lands and grooves in a spool valve?
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Lands - seal ports from each other.
Grooves - direct oil from one portion of the circuit to another by opening ports to each other. p. 365 |
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How is a ram different from a regular single acting cylinder?
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In a ram, the rod acts as the piston.
p. 366 |
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Explain the difference between balanced and unbalanced double acting cylinders.
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Balanced - rod is on one side of the piston only.
Unbalanced - rod extends on both sides of the piston. p. 366 |
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Where are fabric braid hoses commonly found?
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Low pressure side of diesel fuel system, suction or return lines
p. 369 |
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Where are single wire vs double wire/spiral wire hoses commonly found?
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Single wire - pressure side of diesel fuel systems
Multi/Spiral wire - high pressure hydraulic circuits. p. 370 |
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Why should hydraulic hard lines not be galvanized?
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Zinc may flake off and plug hydraulic orifices/valves.
p. 372 |
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Describe the difference in sealing between flared and non-flared fittings.
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Flared fittings depend on metal to metal crush to form a seal. Flareless fittings rely on compression to seal.
p. 374 |
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What are the flare angles for JIC and SAE fittings?
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JIC - 37 degrees
SAE - 45 degress p. 374 |
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What are 4 commons styles of flared fittings?
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Inverted flare, 2 piece flare, 3 piece flare, self flaring.
p. 374 |
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What are 4 commons types of flareless fittings?
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O-Ring (ORB), ORFS, ferrule, compression.
p. 374 |
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How are O-Ring (ORB) seals different from ferrule and ORFS?
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Oring/ORB uses an o-ring in place of a ferrule.
Relies on only 1 oring seal, whereas ORFS has 3 sealing surfaced. p.375 |
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What characteristics have made ORFS fittings more popular?
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- 3 separate seals
- Same size body/nut when switching between metric/imperial. - Rated to withstand a greater range of temperature and vibration. p. 375 |
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What are the 4 common types of hydraulic quick connects?
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- Double poppet, sleeve + poppet, sliding seal, double rotating ball.
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How does a double poppet connector differ from a sleeve + poppet?
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Double poppet has a self sealing poppet valve on each side of the connection; Sleeve + poppet has a poppet on one side and a tubular valve + seal on the other.
p. 376 |
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What are some of the functions hydraulic oil must perform?
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- Act as hydraulic media
- Lubricate moving parts - Resist breakdown over long periods - Protect against rust and corrosion - Resist foaming - Maintain a stable viscosity over a wide temperature range - Resist combining with contaminants - Conduct heat p. 377 |
