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Terms in this set (70)
you require a hydraulic cylinder that has an extremely long stroke length but must have a relatively small retracted length, you should choose a
the torque hydrostatic drive consists of a
variable displacement pump and a fixed displacement motor
Fixed displacement pump and a fixed displacement motor
which of the following pumps can be variable displacement
unbalanced vane, axial piston, and radial piston pumps
consider a typical double acting cylinder. the cylinder must extend and retract while under the same load. Where and when will the max pressure occur inside the cylinder occur?
in the rod end when extracting
the maximum return flow rate experienced by a hydraulic system during the operation of a standard double-acting cylinder
is the return flow from blind end of the cylinder during retraction
T/F the level of filtration required for a system or component is specified using the ISO cleanliness code
T/F Consider a hydraulic motor connected to a hydraulic pump. Decreasing the displacement of the pump will slow the motor down
t/f Consider a hydraulic motor connected to a hydraulic pump. Decreasing the displacement of the motor will slow the motor down
T/F If the input horsepower(hydraulic) to a motor is 40hp and the output horsepower of the motor is 50hp the motor has a mechanical efficiency of .80
T/F The "size" of a pump refers to its displacement
T/F Bernoulli's equation does not account for energy lost due to friction
T/F Variable flow is a concern for double rod cylinders
T/F Hydraulic pumps generate pressure
T/F Volumetric efficiency decreases with increasing pressure
T/F The power supply segment of a hydraulic system consists of th prime mover, which could be an electric motor or vehicle motor
T/F The theoretical torque output calculated for a hydraulic motor is less than the actual torque it will produce
Fluid flows through the pipe below at a constant flow rate. T/F
a) the pressure is higher at point 1
b) the flow rate is higher at point 2
c) the velocity of the fluid is higher at point 1
T/F Fluid A and Fluid B will have the same viscosity at room temperature but Fluid B has a lower viscosity index. At 140F the fluid B will have a higher viscosity than fluid A
State Pascal's law
Pressure confined in an area will be the same all around the inside
Explain the difference between hydrodynamic and hydrostatic systems
Hydrostatic systems transmit force through a confined fluid under pressure and in hydrodynamic systems they use the fluid velocity or kinetic energy as the mode of power transmission
Of the three types of pumps discussed in class, which type can withstand the highest pressure and why?
Piston pumps, they have tight fitting parts
Explain the purpose of cushioning a hydraulic cylinder
Cushioning allows for a slowed down retraction speed just before it comes to a complete stop. This lessens the amount of force when the pieces collide.
Pressure drops below the vapor pressure of the hydraulic fluid causing the fluid to boil
Air leaks into the system
Causes of cavitation
Clogged or restricted strainer
High fluid viscosity
Low fluid temperature
Clogged reservoir breather
Pump inlet line too small
Pump too far above reservoir
Pump too far from reservoir
Excessive pump RPM
Too many bends in pump inlet line
Collapsed hose on pump inlet line
Restriction on pump inlet line
Failure of supercharge pump
Causes of aeration
Low reservoir fluid level
Defective pump shaft seal
Return line above fluid level
Improper baffling in the reservoir
Loose fitting on pump inlet line
Defective deal on pump inline line
Incorrect reservoir design
Porous hose in pump inlet line
How is displacement adjusted in a variable-displacement axial piston pump?
The swash plate angle can be adjusted. This essentially changes the stroke length of the piston which changes the volume of fluid pumped per revolution of the pump shaft.
How can displacement be varied in an unbalanced vane pump?
By changing the eccentricity between the cam surface and the rotor.
Why are centrifugal pumps not often used in fluid power systems
Large gaps between the impeller and housing do not stop fluid from re-entering the pump if the outlet pressure is high. This means the pump flow is greatly reduced if the resistance (pressure) of the circuit is high, which is undesirable since fluid power systems operate at high pressures.
List several advantages of hydraulic systems
constant force and torque
multiplication of force
What is the difference between the viscosity and the viscosity index
viscosity is a fluid's resistance to flow or thickness. The index is an indication of viscosity change with temperature
how are headlines affected by piping that is undersized?
The lower the diameter of the pipe the higher the velocity of flow thus a higher head loss and vice versa.
what is meant by the terms elevation head, pressure head, and velocity head
- as the relative potential energy in terms of an elevation
- the height of a fluid column that can produce a static pressure
-velocity of a fluid expressed in terms of the head on static pressure required to produce that velocity
why is the viscosity of a hydraulic fluid important
It's very important because it affects the performance and efficiency of a complete system as well as we
T/F To minimize pressure losses the K factor of a valve should be made as small as possible.
What is the effect of increasing a motor's displacement on its speed and torque?
As the displacement increases the speed decreases and the torque increases. The speed is decreased due to the motor requiring more fluid to turn in each revolution.
Why does a hydraulic motor deliver less torque than it theoretically should?
Friction in a hydraulic motor the actual torque delivered is less than the theoretical value
What is a hydrostatic transmission?
It's a system that comprises of a hydraulic pump, a hydraulic motor, and suitable valves/pipes. It receives the rotary power and transmits the energy to some load.
If a hydraulic motor is pressure compensated, what is the effect of an increase in the working load?
The torque will increase when the working load is increasing, but the speed will remain constant
3 Most common applications of fluid power
hydraulic fluids perform the following
-media through which power is transferred
- lubricate components
-carry heat away from components and back to the reservoir where it can be transferred to the environment
-seal clearances between mating parts
desireable properties of hydraulic fluids
-compatibility with system materials
-high degree of incompressibility
-good heat-transfer capability
lubrication provides a
cushion between mating components
viscosity decreases with
increasing temperature ( fluid becomes thinner)
5 types of hydraulic fluids commonly used
-petroleum based oil
-vegetable based fluids
energy loss due to friction becomes
Types of pumps
Two types-radial and axial
two types -external and internal
External gear pumps advantages
High speed and pressure
Variety of build materials
External gear pumps disadvantages
Bushings wear in liquid areas
Fixed end clearances
Gear pump advantages
Smooth / Pulseless flow
Slightly more horsepower for size
Most tolerant of contamination (of the 3 pump types discussed)
Gear pump disadvantages
Least efficient of the 3 pump types discussed
Limited sizes available
Low to moderate pressure ratings
To modify the displacement of gear pumps you would need to
Replace existing gears with gears of different dimensions
Can gear pumps be variable displacement?
Variable Displacement Unbalanced Vane pumps
Displacement adjusted by changing the eccentricity between the rotor and cam surface.
Vane pump advantages
Work with low viscosity fluids at high pressures
Dry run OK for short periods of time
Vane pump disadvantages
low pressure application only
500-2000psi for unbalanced
Up to 4000 psi for balanced
Positive displacement pumps are very sensitive to contamination
Due to close fitting parts.
Leads to increased leakage, inefficiency, and possible failure
Single acting cylinder
Acts under pressure in one direction and returns automatically when pressure is released.
Used when a large stroke length is needed as well as a very short retracted length
Double rod cylinder
Has a rod extending from both ends
Used for applications where work can be done by both ends
Example power steering
Can withstand high side loads because each rod has a set of bearings
Will differential flow be a concern for double rod cylinders?
Used in applications where large amounts of force are required from a small cylinder.
Increasing the displacement of a motor
decreases the motor speed and increases the motor torque
gear,vane, and piston
Most tolerant of contamination
Lowest pressure ratings (of the three types discussed)
Only fixed displacement
why it is called a "balanced" motor?
No net bearing force on the shaft increased max operating pressure and drive speed
The displacement in variable displacement motors can be changed on the fly
Reducing displacement means less fluid needed per revolution (higher speed), but also results in reduced torque output.
Motors are very sensitive to contamination
due to close fitting parts
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