Hydaulic Priciple Alberta Moduals Flashcards
Hydraulic systems do what?
Transmit power
The output unit in a hydraulic system is referred to as a?
Actuator
Cylinder or hydraulic motor
How does a hydraulic system transmit power?
Pressurized liquid
What are the components you must work with in a hydraulic system?
-Reservoir
-Pump that produces flow
-Various pipes, tubes, and hoses
-Valves that control the direction, speed and pressure of the fluid
-actuator that does the work
How much does hydraulic oil compress when the pressure is at 1000psi?
1/2%
Fluid in 8’pipe will compress 1/2” at 1000psi before it will lift a cylinder
Pressure conversion chart
Describe a 2-1 cylinder?
When the cross section of the rod occupies exactly half the area of the piston on the rod end side.
How is flow expressed?
Flow is expressed in either velocity(speed) feet/min or feet/second
Or
Flow rate in gallons/min or cubic inches/min
When calculating flow what must you always do?
Calculate every thing into the proper units first.
What is the 231 trick?
To convert gallons/minute to cubic inches a minute multiply 231 to you number of us gallons.
231 x (# of us gallons) = cubic inches/min
How do you find the area of a circle?
.7854 X Diameter squared
A regenerative circuit is designed to provide equal speed and force when the cylinder extends and retracts. To achieve this you need a _______ cylinder.
How does it work?
2-1 cylinder
Why can reducing the pipe size in a hydraulic system be bad?
Because it can drastically increase the speed of fluid which waste a lot of energy, Robs pressure and wears out components
What are the recommended maximum velocities in a hydraulic system?
The amount of friction in laminar flow _______ as velocity increases?
Increases
Laminar flow Vs turbulent flow
(Study chart)
Conditions for laminar and turbulent flow
(See chart)
How is cavitation recognizable?
Immediately recognizable by rattling sound and confirmed by abnormally low pressure at the pump inlet
What are the effects of cavitation?
- very rapid erosion of metal surfaces
- severe pump damage
- generation of high frequency damage
- flow reduction
- erratic operation of valves and actuators
- accelerated fluid oxidation
- interference with lubrication of the components
What are the causes of cavitation?
Although very similar characteristics what is the difference between aeration and cavitation?
Cavitation is caused by the violent collapse of gas bubbles. The cause of cavitation originate from gasses being dissolved in hydraulic fluid
Aeration occurs when air becomes mixed with the fluid. The process begins with site leaking into the system through inadequate sealing or air becoming entrapped on the system during maintenance
Causes of aeration?
Pascals law applies only to fluid at rest. What principle applys when the fluid starts to move?
Bernoulli’s principle notes that pressures in the system vary with the velocity of the flowing fluids.
As velocity of fluid increases the pressure decreases. When pressure increases the velocity decreases.
Bernoulli’s principle does not take _______ into account?
Energy losses due to friction
In the real world you must add losses caused by friction
What is the common imperial unit of work?
Foot pound
A force of one foot pound is applied to move an object a distance of one foot.
Describe power?
The rate at which work is done
If you move an object very quickly over a distance of one foot you use more power than if you move the same object with the same force over the same distance slowly .
1hp = 33000 foot pounds/second how many foot pounds/minute?
Divide by 60
550 foot pounds per minute
What are the two types of efficiency in a hydraulic system?
Power efficiency
And
Volumetric efficiency
If efficiency is low consider what two possibilities?
System has been maintained properly
Then
Check how the system was designed.
Example, is flow always flowing through relief valve
What is volumetric efficiency?
A measure of how much fluid an actuator or pump discharges in comparison to the amount of fluid intake.
What is the difference between an open loop system and a closed loop system?
An open loop system the return line does not connect directly to the pump inlet.
A closed loop system the return lines hook directly to the pump
What is the purpose of a charge pump?
To return the fluid from the case drains back to the inlet of the Main pump. The us known as a hydrostatic drive (closed loop)
What are the different symbols for conductors?
What is a tube symbol?
What is the hose symbol?
What does the open tank reservoir indicate?
The reservoir is vented to atmosphere. Allowing for atmospheric pressure to push the fluid into the pump inlet where there is partial vacuum
Why is a inlet filter installed? And is it a coarse or fine filter?
To protect the pump.
It is a course filter to remove large particles or small objects that may damage the pump.
If the filter is to fine it will restrict flow to the pump
Why is the return line filter so important?
It captures any wear debris generated in the system or contaminants that may have been introduced when new fluid was installed.
Very fine filter, does not restrict flow as much because the fluid is at its hottest point.
What is the purpose of the pump in a hydraulic system?
To make fluid flow. It is drive by the prime mover (electric motor for stationary and internal combustion engine for mobile systems)
Where is the maximum pressure relief valve always located?
Directly after the pump and is the first valve in the system.
When you set the pressure on the pressure relief valve, you are setting the ____________ the cylinder exerts?
Maximum
How are you directional control valves controlled?
Manually, electrically, hydraulically or pneumatically
What is a follow control used for?
To control the speed in which the actuator operates.
What does the diagonal arrow on a hydraulic symbol always indicate?
The component is variable (adjustable)
What are the two types of actuators in a hydraulic system?
Force applied by a cylinder (straight line)
Torque applied by a rotary motion (hydraulic motor)
What is the purpose of a pressure reducing valve?
When a system has more then one actuator and one of the actuators requires a lower pressure a pressure reducing valve may be used.
When looking at a diagram with a pressure reducing valve what is the small dotted line from the spring to a reservoir?
It represents an external drain line that prevents pressure build up on top of the valve element.
What is the purpose of a check valve?
To allow flow in only one direction
What is the purpose of a sequence valve?
When have more then one actuator in a system doing two different things. The sequence valve stops the flow to the second cylinder but then releases the flow as soon as the first cylinder reaches the end of its stroke.
Where should the sequence valve be installed?
So that the inlet of the valve is connected to the first cylinder and the outlet to the second cylinder.
How can sequence valves be recognized on drawings?
- normally closed valve
- the valve inlet connects to the first cylinder to be operated in the circuit
- the valves pilot line connects to the inlet side of the valve
- the valve uses reverse flow bypass
- the valve has a external drain that must connect to the tank to depressurize the spring chamber
When do you need a metre in flow control?
Used to control the fluid into the cylinder and how fast it extends. Or you can hook the outlet of the flow control to the port at the rod end which allows you to meter the flow and control how fast the cylinder retracts.
What does a meter-out flow control do?
Control the flow of fluid from the rod end so the reactive load cannot run away.
What does a bleed off flow control do?
Can be used to ensure no flow goes over the relief valve when flow to the cylinder is reduced.
What is more efficient bleed-off flow control or meter-in/meter-out flow control?
Bleed-off flow control but cannot be used in certain applications
What is the purpose of a counter balance valve?
A pressure control valve that is utilized to control the movement of a vertical load. If you just used the DCV the load could move faster the hydraulic oil entering the actuator.
Internally sensed counterbalance valves are adjusted so that the static pressure from the load alone is insufficient to cause opening of the valve. How much higher of pressure is required
10 to 30 %
Why would you use a deceleration valve?
When you want the cylinder to extend at full speed for part of the stroke then slow down to a set speed at the end of its travel.
What is the difference between tubing and pipe?
Tubing has a smoother finish, thinner walls, and comes in smaller sizes. It is sized by its outside diameter.
What are the advantages of tubing?
Lighter weight
Smoother internal finish
Long radius bends
Bend-ability
Smooth internal connections
Is it ok to use copper tube with hydraulics? Why or why not?
No because it accelerates the oxidation of the oil and work hardens and cracks.
What are the wall thickness designations for plumbing tube?
K - thick wall
L - medium wall
M - thin wall
What are the designations for oxygen tube?
K - thick wall
L - medium wall
Specially cleaned to avoid spontaneous combustion
Outside diameter is measure by _______ of an inch and wall thickness is measured in _______ thousands of an inch?
Fractions of an inch
Thousands of an inch
When should you not use flare fittings with tubing?
When it will be exposed to vibration
What are the two standard flare angles?
37 degree
45 degree
How can you tell a properly tightened
Flare fitting?
Can undo it by hand after it is loosened with a wrench.
What is the difference between working pressure and bursting pressure of a hose?
Busting pressure is only for testing purposes. Always match the working pressure with the maximum operating pressure of the system.
Hose size is based on which diameter, inside or outside?
Inside diameter and is in 1/16th of an inch
What are the functions of hydraulic oil, other than transmitting power?
- Lubricates moving parts through out the system
- helps seal clearances between moving parts
- Carries heat away from the components
- flushes contaminants away from the components.
Hydraulic fluids must be:
- Compatible with seals and hoses
- resist foaming
- separate air and water
- maintain its viscosity over a wide temperature range
- compress as little as possible under load
- be fire resistant in some applications
What is the most common fluid in hydraulic systems?
Petroleum based:
-low cost
- readily available
- have good lubrication
- relatively
What is the baffle for in a tank?
A plate with a hole in the bottom welded into the tank. It’s purpose is to slow the fluid down before getting to the inlet. But also:
- promotes cooling
- promotes settling of contaminants
- it allows air to escape from the fluid to reduce foaming
Why is the temperature of the fluid in the tank so important?
If the fluid heats up past it’s maximum temp point it oxidizes rapidly.
What does the breather vent on the tank do?
Prevents air pressure from becoming too low and causing cavitation and from becoming too high and causing aeration
How many microns should your breather be?
3 micron
How many microns should your inlet filter be? And why?
100 microns
It’s only to collect big particles from the tank. It must not be to fine that it restricts the flow to the pump causing it to cavitate
Symbol for cooler/heater
Symbol for water cooler
What are the three types of reservoirs?
Open
Pressurized
Closed - circuit
What is the volume for large reservoirs?
2 to 3 times pump volume
What is the volume for small reservoir closed loop system?
Volume = 3/4 charge pump outlet
What are the contamination sources?
- new oil being added to the system
- built in
- built in during assembly
- ingressed (dirty cylinder rod)
- generated (wear debris)
What is a good way of getting new oil into the system?
Filter cart
This removes and contaminates that may be in the barrel of new oil
What are some measures to reduces ingressed contamination?
- filter new oil as it is put into the system
- use a good quality breather cap
- change rod seals before end of service
- change pump and motor shaft seals before end of service
- keep cylinder rods and fittings clean
- keep all fittings tight and leave free
- keep hydraulic system and the surrounding are clean
- thoroughly clean pipe joints and the surroundings area before you disassemble
What are the particle sizes that cause damage to hydraulic systems?
Above 6-14 microns
Why can 4 micron particles be bad?
These particles can cause parts to stick when they accumulate in the clearance between moving parts
Filter with bypass check valve symbol?
When do you want to change the filter with an indicator?
What do the colours mean?
As soon as it hits the yellow
Green - clean element
Yellow - partial bypass
Red - complete bypass
Which filter do you a absolutely need in your hydraulic system?
Return line filter
Strainer
And should have an inlet high pressure filter
What are the advantages of an off line filter?
- change filters without shutting system down
- filter flow rate and type of filter can be changed without modifying system piping
- pre filter fluid before start up
- off-line filter can be located where it is easy to service
What are the oxidation by-products and their effects?
Gum and varnish - sticky valves
Sludge - blocked pilot line passages
Acids - corrosion of parts
What is the main cause of oxidation?
Heat
For every 10 degrees C above 60 degrees C the oxidation rate doubles
What is the best way to find a pin hole in a hydraulic system?
With a piece of paper.
A tiny pinhole can pierce skin causing fluid to enter blood stream.
When air is introduced into the system where to the air bubbles collapse?
When they reach the high pressure area
What is the difference between positive seals and non-positive seals?
Positive seals have zero leakage
Non positive seals have controlled leakage
What is the difference between static seals and dynamic seals?
Static seals provide seal agains stationary objects
Dynamic seals seal against moving surfaces
Where are metal - metal seals used?
High pressure applications in which the pressure is steady.
What are the steps when installing metal seals?
- inspect surfaces for any imperfections
- thoroughly wash the housing
- assemble and snug up bolts evenly
- torque the bolts to a third of the recommended torque in the proper sequence
- tighten to 2/3rds the recommended values in the proper sequence then to full torque
What percentage of a static o ring must protrude above their groove?
About 10 percent
When are back up rings needed?
When you are using a soft o-ring to prevent it from being extruded.
What is the typical amount of controlled leakage in a metal to metal seal?
5 to 15 percent
Shaft seals can either be high pressure lip seals or ______\?
Mechanical seals
When should metallic piston seals not be used?
When creep cannot be tolerated
What is a accumulator?
A vessel that stores pressurized fluid
What are the uses of a accumulator?
- Maintain pressure in the system
- to absorb shock
- even out pulsations
- protect against overdressed due to thermal expansion
