Hydraulic Principles MW Manual (bc) Flashcards
What is gage pressure?
Measured using atmospheric pressure as starting reference point.
Psig or kPa gauge
What is absolute pressure?
When pressure is measured using a total vacuum(absolute 0 pressure) as the starting reference point.
Psia or kPa abs
What is the difference between absolute zero and gage zero pressure?
14.7 psi or 101.325 kPA
How do you covert gage pressure to absolute pressure?
Gage pressure + 14.7 (atmospheric pressure)
What is the symbol for mercury?
Hg
What are the force-area-pressure mathematical relationships?
F=AxP
A=F/P
P=F/A
How much force does a 1 kilogram mass equal?
9.8 newtons
Describe pascals law?
When pressure is applied to a confined fluid the pressure is transmitted equally throughout that fluid, and is exerted perpendicular to every surface of the container.
How is pressure created in a hydraulic system?
Pressure is created whenever there is a resistance to fluid flow. The resistance will occurs when:
-A load is placed on linear and rotary actuators (cylinders and motors)
- an orifice in a line to control flow
- fluid moves through pipes, hoses and valves causing friction
What is the purpose of a hydraulic pump?
To produce fluid flow
What is velocity?
Velocity = distance/time
How much does 1 imperial gallon equal in cubic inches?
How much does 1 US gallon equal in cubic inches?
277 cu in
232 cu in
How is velocity measured?
Meters per second (m/s)
Feet per second (ft/s)
What is the equation to convert flow rate to velocity? Metric and imperial
Metric
Velocity = flow rate x 1666.7/area
Imperial
Velocity = flow rate/3.117 x area
Why are long bends better then abrupt 90’s in piping?
Turbulent flow is created from abrupt corners
Define friction?
The resistance to relative motion between two bodies
True or false?
The friction in laminar flow is usually proportional to the velocity?
True
What are the main causes of friction in a hydraulic system?
Excessive length of lines
Excessive velocity because the lines are to small
Excessive number of bends or fittings
Sustained flow at high pressure
What type of energy is is created from fluid moving through the lines?
Kinetic energy from the weight and velocity of the fluid
Potential energy from the pressure
Heat energy cause by friction
Fluid leaving the pump has kinetic energy and pressure energy. When fluid goes through a restriction the _________ energy is increased and the _______ energy is decreased.
Kinetic
Pressure
What does bernoullis principle state?
If the flow rate is constant, the sum of the kinetic energy and the potential energy at various points in the system is constant. Therefore whenever the velocity(kinetic energy) of a fluid increases, the pressure(potential energy) decreases.
What do hydraulic actuators do?
Convert hydraulic energy into mechanical energy.
Cylinder create linear motion
Motors create rotary motion
Describe a cylinder?
A linear actuator
Single or double acting
Tubular housing containing a piston, and sealing
One or more rods attached to the piston
Cap on one end
Head on the other end
A gland fits inside the head for the rod to pass through
How are cylinder usually sized?
Piston diameter
Piston rod diameter
Stroke length
What is the difference between single and double acting cylinders?
Single acting cylinders applies force or is pressurized in one direction only. And uses external force such as gravity, spring, or a small diameter auxiliary piston to return it
Double acting cylinders have hydraulic fluid delivered under pressure to both sides of the piston to produce force in either direction
Why is a double ended piston rod more balanced than a single piston rod cylinder?
The double ended piston rod has a rod on both sides, displacing equal amount of fluid on each side.
What is the purpose of a cusion in a cylinder?
Slow down the piston as it approaches the end of its travel
What are the symbols for the cusions?
How is cushion created?
By slowing the flow of oil being discharged.
What is the purpose of a tandem cylinder?
Allows for higher forces at the rod end without an increase in fluid pressure or cylinder diameter
What is the purpose of a retracting cylinder?
Permits a working stoke much longer than its retracted length
Cylinder force triangle
F
P A
Cylinder speed triangle
V
S A triangle
S = speed of the rod end
V = volume delivered
A = area of the effective piston surface
What is the difference between hydraulic pumps and motors?
Pumps drive the hydraulic fluid and motors are driven by the hydraulic fluid.
What types of hydraulic motors are there?
Gear, screw, vane, or piston.
What is a fixed capacity hydraulic motor?
When any variations must be controlled independently of the motor it is considered to be fixed capacity. They run at a constant rate with a predetermined flow.
What is a variable capacity hydraulic motor?
When variations can be done in the motor it is a variable capacity motor. Regardless of the flow rate they can be altered.
What are the symbols for fixed capacity motors?
One direction flow
Two direction flow
What are the symbols for variable capacity hydraulic motors?
One direction
Two direction
Are gear motors fixed or variable capacity motors?
Fixed.
Are screw Motors fixed or variable capacity?
Fixed,
Operate quit and free of vibration
What is different about a vane Motor vs a vane pump?
Because there is no centrifugal force in a cane motor springs are needed to ouch the vanes against the casing.
They can be balanced or unbalanced fixed or variable capacity
Rotary piston motors are either ________ or axial pistons?
Radial
Where does the oil enter a radial piston motor?
Through the cylinder block in the centre forcing the piston outward against the reaction ring.
During the first half rotation the piston extends the pressurized fluid enter the motor
During the second half rotation the pistons contract forcing the fluid into the reservoir line.
The cylinder block is connected to the output or driveshaft
Describe axial piston motors?
Pressurized fluid enters the valve plate forcing the pistons towards the swash plate. The angle of the swash plate determines the capacity of the motor.
During one half of the cycle pressurized fluid enters the motor as the pistons travel to their full extent.
During the other half of the cycle fluid is exhausted into the reservoir line.
The drive shaft is connected to the swash plate
What is the symbol for oscillating motors (rotary actuators)?
Also known as limited rotation motor.
What is a rotary motor designed to do?
Give a shaft rotation of less than 360 degrees. Usually consist of one vane. When fluid enters one port it pushed the vane and when the fluid enters the other port it pushes the vane back in the opposite direction.
What should be installed to help the vane from hitting the housing in a oscillating motor?
External mechanical stops
How are hydraulic motors rated?
By their displacement(size), pressure, torque, and speed.
What is displacement?
The amount of liquid handled on one rotation of the motor.
Usually measured in cubic centimetres per revolution. Cm3/rev
Cubic inches per revolution. In3/rev
Pressure requirements for hydraulic motors varies with the size of the motors ________?
Displacement
The larger the displacement of the mother the less pressure required to produce the given torque
Torque output is a function of the system pressure and the hydraulic motor _________?
Displacement
It is expressed in newton-meters (n.m)
Inch pounds per 100psi (in.lb/100psi)
How is mechanical efficiency expressed?
As a percentage
Speed (S) is expressed in _______?
How is it determined?
Rpm
Speed is determined by the flow rate rate or volume per unit of time delivered (V) divided by the motors displacement or area (A)
Effects of changes on a hydraulic motor?
What does DCV mean?
Direction control valve?
What is the difference between single position and multiple position DCV’s
Single position DCV maintains a unidirectional flow pattern. (Check valve)
Multiple position DCV’s start, stop, or change direction flow to or from hydraulic actuators.
The symbol for a multi direction DCV what does each square or envelope represent?
Position and flow pattern inside the valve body. Two or more envelopes joined together indicate that the valve can be used with two or more flow patterns though it.
two position valves indicates the ports on the preferred or starting position, either normally opened or _______?
Closed
Three position valves indicate the ports on the ________ position. The lower two ports on the symbol are labeled (P) and (T). What are the top ports lettered?
Neutral
A and B
What are the symbols for different flow paths?
DCV’s are identified numerically. What does the first number indicate? And what does the second number indicate?
First: number of ways the fluid can flow. This is usually also number of ports (excluding pilot ports)
Second number: number of distinct positions.
How are DCV valves classified?
Spool type: sliding or rotary
Nominal size: nominal pipe connection or recommended maximum volume supply through the valve
Maximum allowable pressure
Port connection: threaded NPT or NPS
Flanged or flat face
Spools within the DCV valves may be sliding or __________?
Rotary
Manual control symbols
Mechanical control symbols
Hydraulic and pneumatic control
Electric control symbol
DCV’s maybe non-throttling or________?
Throttling
Normally opened valves _______ fluid to pass thought until the valve is activated. Normally closed valves ________ the flow of fluid until the valve is activated?
Allow
Block
Normally open/normally closed symbols
Three way two position valve symbols
(3/2)
What is the number designation for a 4 way 3 position valve?
4/3
Can an actuator move in either direction under power using a four ways DCV?
Yes
What is happening in this picture?
4 way 3 position valve
Normally closed centre
Piston is locked
What are the 5 most common valve centre flow patterns?
Closed
Open
Tandem
Float
Regenerative
Each centre flow pattern has a definite effect on?
Actuator control or position
The fluid flow from the pump (heat build up and HP demand)
Closed centre
All ports are blocked off in neutral. Actuator is locked and cannot move. Fluid must go through the relief valve at maximum pressure. This requires maximum power and wastes energy due to heat.
Open centre
All ports are connect in neutral. The actuator moves in the direction of any external forces. The flow of the pump is back to tank with minimum power demand. There is no pressure and only a low heat rise.
Tandem centre
The actuator ports are blocked off, flow from the pump is back to the tank. This centre provides a hydraulic lock to hold actuator in position. Fluid can flow back to tank, this requires minimal power and generates small amount of heat.
Float centre
Ports A and B are connected to the tank, and the pump is blocked off. This centre allows the actuator to coast to a stop or be moved manually without coming to a stop.
Fluid must go through a relief valve at maximum pressure generating heat and maximum power
Regenerative centre
Ports A and B are connected to the pump, this centre maintains constant pressure to both ports of the actuator. This allows for very rapid start of the actuator. Fluid must go through a relief valve at maximum pressure, generating heat and maximum pressure
What are the common styles of check valves?
Ball or poppet
What orientation are check valves available in?
In-line or right angle mounting
What are the symbols for free check valves and spring loaded check valve?
Free flow check valves must be orientated correctly to work properly. Is this the same for spring loaded check valves?
No spring check valves use a light spring force of approximately 5psi to hold valve seated. Can be oriented any direction
When some reverse flow is needed in a system what check valve would be used? And what is the symbol?
Restricted flow or pilot operated
When is a pilot controlled check valve to open (pilot-to-open) used?
Commonly used to lock cylinders in place until the main DVC shifts
Describe pressure relief valves?
Valves used to control hydraulic pressure in all or part of the system. May be normally closed or normally open valves.
When do normally closed hydraulic relief valves open?
When pressure reaches the set limit.
When do normally open hydraulic relief valves close?
When pressure reaches a set limit the valve closed
What are the types of relief valves?
Direct acting
Pilot operated
Unloading
Pressure reducing
Sequence
Counter balance
Brake
What are direct acting relief valves commonly used for?
Frequently used as safety valves to prevent damage from high surge pressure.
Normally closed
When is a pilot operated relief valve used?
When are large amount of fluid is to be relieved under a small pressure differential.
Normally closed
When are unloading valves used?
When systems use accumulators and a continuous flow of fluid may not be needed. The unloading valve returns pump output to the reservoir after the required system pressure has been reached.
Normally closed and installed in the pump outlet with a T.
When is a pressure reducing valve needed?
When a secondary circuit operates operates efficiently at a lower pressure than the relief valve setting, a pressure reducing valve is used to reduce this pressure.
Normally open and is held open by an adjustable spring
Why is a sequence valve used?
When hydraulic have more than one cylinder, it is often necessary to work the cylinders in a specific order. This can be done by hand, electrical control or by the sequence valve
Normally closed, two way valve
Why is a counter balance Valve used?
This valve is used to maintain a set pressure in part of a circuit. The controlled pressure is required to keep a weight such as the platen on a press from falling or to keep a rotating load from running away. Installed on the exhaust side of the actuator.
Normally closed
Why is a brake valve used?
Commonly found with a motor. It stops it’s rotation as the DCV shift to it’s centre position.
Normally closed
How does a flow control valve regulate the amount of flow to the actuator?
Controls the volume of hydraulic fluid entering or leaving the actuator. In systems using a fixed capacity pump the regulation is by flow control or flow metering valve.
What are the two ways flow controls control the amount of flow?
An orifice in the line for fixed control
A throttle valve for adjustable control
What are the different types of flow control valves?
Throttle valve
Non compensating
Fixed pressure-compensating
Adjustable, pressure-compensating
Pressure-and-temperature compensating and flow divining
What are the different types of throttle valves ?
Gate, glove, plug, ball, and needle valves.
Simplist and most finely adjustable is the needle valve. These can also be used as shut off valves.
Why is a throttle valve considered non compensating?
Because they do not compensate for any variation in pressure or flow in the system. As the pressure increases the flow through the valve increases.
Fixed (non-adjustable) no compensating valve
Adjustable (non-compensating valve)
What is the difference between pressure compensating flow control valves and non compensating control valves?
Pressure compensating valves maintains a constant rate of pressure flow through the valve regardless of the downside pressure.
What are the two types of pressure compensating valves?
Fixed pressure
Adjustable
Why would a pressure temperature compensating flow control valve be needed?
As the temperature of hydraulic fluid changes so does it’s viscosity. When the fluid warms up more can go through the orifice. Use of pressure temperature flow controls can help maintain the required about of flow.
When are flow dividing valves used?
When hydraulic systems have more the. One circuit, flow dividing valves control the amount of fluid to each Circuit.
What are the three possible stages of electro-hydraulic control?
Electical signal converting to mechanical movement.
Mechanical movement to a piloting stage
Piloting stage to the main valve
Solenoid with one winding
Solenoid with two windings
Describe solenoid-controlled pilot operated valves?
A combination of a small solenoid-controlled pilot valve and a pilot-operated main valve.
What is a proportional solenoid?
Conventional solenoids of a simple on off function. A proportional solenoid allows the operator to vary the position of the plunger by varying the amount of current going to the solenoid.
Do hydraulic systems use dynamic pumps or positive displacement pumps?
Positive displacement
What are the common fixed capacity pumps?
Gear, vane, or piston pumps
What the common variable capacity pumps?
Vane or piston
Does a pump produce pressure?
No
A pump creates flow and system pressure is created by a resistance to flow.
As pressure increases the flow rate______?
Decreases
What is the advantage of below the reservoir pump mounting?
Atmospheric pressure help push the liquid into the suction side of the pump.
Above reservoir the pump must be able to create enough vacuum or pressure drop yo overcome:
The weight and friction of the liquid
The height from the liquid level to the pumps centre line.
What is a major cause of failure in a in a hydraulic system component? And what are the ways this can be mitigated?
Contamination.
Magnetic plugs, strainers, and filters are used to catch the contaminants.
What is the difference between strainers and filters?
Strainers are considered course filters removing larger solids
Filters remove fine contaminants
What are the two types of filters?
Proportional filters
Full flow filters
What are the different filter elements?
Mechanical metal filters: remove larger solid particles but do not remove water or very fine particles.
Absorbent filters: remove fine particles, water and water soluble impurities.
Adsorbent (active) filters: remove impurities by both mechanical and chemical means. Remove all solid particles and insoluble sludge, water and soluble and oxidized material.
Caution…. Adsorbent filters may also remove most additives used to inhibit hydraulic fluids
How solids measured?
What is the smallest Particle that can be seen with 20-20 vision?
Microns
40 microns
What is the difference between nominal and absolute filters?
Nominal filters can stop most of its rated micron size particles.
Absolute rating means it will stop all particles of its micron size from entering the system.
What are the three positions of the hydraulic system can the filter be mounted?
Inlet side
Pressure side
Return line side
Which placement of filters is the best?
Why?
The return-side position is usually best because the fluid is usually warm and at it’s lowest viscosity. Does not protect parts which can form or be introduced into the tank while filling.
What are the symbols for the reservoirs?
What is the volume of fluid in a reservoir equal to?
Two to three times the rated pump delivery for one minute.
What is the purpose of a baffle plate in a hydraulic reservoir?
Prevents a direct flow of fluid from the return line to the suction line. This slows the oil movement and allows trapped air to escape and foreign materials to sink to the bottom.
How far should the suction and return lines be from the bottom of the hydraulic tank?
11/2 to 2 times the pipe diameter from the bottom of the tank.
What is the purpose of a heat exchanger in a hydraulic system?
To ensure correct operating temperatures of the hydraulic fluid.
What are the two different types of coolers in a hydraulic system?
Water cooler and air cooler.
How can heat be added to the hydraulic system to create the right viscosity for operation?
Electric immersion heaters
By passing steam or hot water through a pipe mounted in the tank
By starting up the hydraulic system and allowing fluid to flow through the maximum pressure relief valve
When using accumulators what are the two values that must be known?
The amount of oil to be added before the gas charge
The recommended pressure of the gas charge.(pre-charge is a percentage of operating systems maximum pressure.
Before working on a accumulator-loaded system you must:?
Isolate the accumulator with a shut off valve
Or
Discharge or drain the accumulator back to the tank.
Accumulators may be:
Weight loaded
Spring loaded
Or
____________?
Pneumatic or gas-charged
What gas law does a gas-charged accumulator use?
Boyle’s Law.
At constant temperature, the volume(V) of a gas varies inversely to the absolute pressure (P)
What are the different types of gas-charged accumulators:
Non separated
Piston
Bladder
And
________?
Diaphragm
Compressed air is usually used in accumulators for low pressure systems . When is dry nitrogen used?
Medium to high pressure systems
Nitrogen is used because it is inert and will not react with oil.
Are non separated accumulator used on high or low pressure systems?
Low
When would a pressure intensifier in a hydraulic system be used?
When extremely high pressure is needed but cannot be achieved with the system components.
