Hydraulics Flashcards
What must be in every hydraulic system?
Maximum pressure relief valve
What are some advantages of hydraulics
It has infinitely variable speed, torque, and force control. It has highly accurate control of movement. It has very little loss from friction
What are some disadvantages of
They are expensive, and heavy. There is also a high-pressure fluid hazard, possible fire hazard and leaks are messy and difficult to control
Describe pascals law
It is undiminished, equal in all directions, at right angles to every point on the inside surface of the container. Pascals law only applies when fluid is not moving
How was pressure created
It is created by resistance to flow
What is gauge, and atmospheric pressure
Absolute pressure gauges have atmospheric pressure reading on them at 14.7 psi. Gauge pressure is set at zero and does not factor in atmospheric pressure
What is the pressure area called on the side of the piston that has the rod?
The annular area
What are the two types of cylinders?
A differential cylinder any nondifferential cylinder. The differential cylinder has the rod on one side and a non-differential has the same size rod on both sides of the cylinder
How was flow produced?
Flow is produced either buy a pump, or by a pressure drop across an orifice
What three things does every hydraulic system have?
A power supply, control devices, output.
Power supply – prime movers pump reservoirs are motors
Control devices – directional control pressure control flow control
Output – actuators rotary or linear and loads
Hydraulic cylinders can either be __________ or ___________ cylinders
Extending or retracting
Describe a 2-1 cylinder
When the cap end or piston area is two times the area than on the rod side, or annular area
What does a pump produce?
It only produces flow. Pressure is produced by a resistance to flow
What is the flow triangle?
V/t(flow rate)=S(speed)Times A(area)
What effect does a smaller pipe size have on a fluid?
If a pipe is changed with one that is smaller, the speed will increase.
What is the recommended maximum pressure line velocity or speed ?
7-20 feet per second
Describe laminar flow
When the fluid moves through the system smoothly in parallel lines.
Describe turbulent flow
When the fluid is disturbed by various conditions, and the fluid swirls.
Why is laminar flow good?
Least amount of friction, least amount of pressure, energy loss, least mount of wear, reduces possibility of cavitation or aeration
What is cavitation?
It is the violent collapse of trapped vapor bubbles of whatever fluid you are moving.
How is cavitation recognizable?
Buy a rattling sound and confirmed by the abnormally low pressure at the pump inlet
What are the effects of cavitation?
Rapid erosion of metal surfaces, severe pump damage, erratic operation of valves and actuators
What are some causes of cavitation?
Clogged inlet filters, if the pump is mounted to high above the reservoir, when the fluid is too sick, clogged breathers, if the inlet pipes are too small, if there is turbulence. All of these conditions create low-pressure at Inlet
What is the output unit of a hydraulic system called?
It is called an actuator. He could be a hydraulic cylinder or a hydraulic motor
What are the components you have to work on in the hydraulic system?
The reservoir, a pump, pipes tubes and hoses, valves that control direction speed and pressure, and actuator that does the work
Hydraulics are based on what principal?
Pascals law
What is the greatest waste of power in a hydraulic system?
Maximum pressure relief valve
What is the maximum pump inlet velocity?
2-4 feet per second
What is the reason for slower speeds in hydraulic system?
Too high of speeds will create:
- wasted energy
- increase friction
- wear out components
- shock components
- create heat
- cause turbulent flow
What is Bernoulli’s principle in regards to velocity/pressure
When the pressure increases, the velocity decreases.
When the velocity increases, the pressure decreases
What is energy?
Energy is the capacity to do work. Work=force X distance
What is power?
Power is the rate at which work is done. Power=work / time
What is power efficiency?
Power efficiency tells how much power is obtained from the actuator compared to the power you put into the system through the prime mover. It is expressed as a percentage
What is volumetric efficiency?
It measures how much fluid and actuator or pump discharge is in comparison to the amount of fluid intake. It is expressed as
volume out / volume in percentage
10 gpm in 8 gpm out= 80% volumetric efficiency
What is the main difference between an open loop circuit and a closed loop circuit?
An open loop circuit the return line does not connect directly to the pump inlet
What is the volume required in the reservoir in an open loop system?
The volume is three times the pump output
What is the reservoir capacity of a close loop system?
Three quarters the volume of the charge pump capacity
Why is hydraulic fluid used rather than water?
Oil is better at lubricating and ceiling. It’s better that he control and it is more versatile and doesn’t freeze
What does a diagonal arrow on a hydraulic symbol always indicate?
It indicates the component is variable or adjustable in its output
What is the problem with the metered in system?
They can have lack of speed control. The motor can run away on you
What are cross port relief valves used for?
They are used as a type of cushion valve for start up and to avoid shock on system
How can directionals control valves be operated?
They can be operated manually electrically hydraulically or pneumatically
What is a valve that is normally open?
A pressure reducing valve
What can happen in certain areas of the circuit when metering out?
You can get an area of pressure intensification
What is the fluid law that applies to stationary fluid and is the foundation of all hydraulic therory?
Pascals law
Doubling the inside diameter of the pipe will?
Quadruple the inside cross-sectional area
A high resistance to flow creates?
A pressure and heat increase
If a double acting cylinder with an area ratio of 3 to 1 Fully extends in one minute, how long would it take to retract?
20 seconds
What is the greatest waste of power in a hydraulic system?
Maximum pressure relief valve
The volumetric efficiency of a hydraulic system has decreased by 40%. What is changed at the actuator?
The actuator decreases in speed
What is flow created by in the hydraulic system?
The pump
What are some functions of the charge pump in a hydrostatic system
Recirculates case drain flow, maintains a positive pressure at main pump inlet, circulate oil through filter and cooler when required
What can volume volumetric efficiency of a pump help determine?
Pump wear
In an open loop circuit would the return line be pressurized or return to reservoir at nominal pressure?
Return to tank at nominal pressure
What are some problems with metering in?
Works for opposing loads only, it cannot control the speed of retraction
What does a bleed off flow control do?
It bleeds of a set amount of flow before it gets to cylinder, it is returned to tank at no pressure
Describe hydraulic motors
They require an external drain, they use cross port relief valves to protect motor from shock loading, are often bi-rotational
What is a benefit of that externally piloted counter balance valve?
The externally piloted line greatly increases the maximum downward force capability
What are the three things valves control in a hydraulic system?
All valves control pressure, directional control, or flow control
What is cracking pressure?
I can pressure is the pressure required to just open the valve and that the first drop of fluid past the seat
What is full flow pressure?
Full flow pressure is the pressure required to hold the valve open during full flow
What is pressure over ride?
What is the difference between full flow pressure in the cracking pressure
Why do you want to minimize the pressure over ride?
It has greater efficiency
Describe a pilot operated relief valve
It has a pilot passage to the pilot stage which is full pressure. There’s a balance piston and pop at unit, a 20 PSI late spring. It is considered a two-stage relief valve. It minimizes the pressure override to be just the 20 PSI above the working pressure
What do unloading valves usually referred to in a circuit?
They refer to systems that use an accumulator which maintains pressure in the system. When there is no demand on the system the normally open valve closes to return fluid back to tank at minimal pressure
What is in unloading valve always unloading?
It is unloading the pump
Why are brake valves used on the discharge of hydraulic motors?
They prevent over speeding when an over running load is applied, they prevent excessive pressure build up when decelerating or stopping a load
What type of spools can be used in a directional control valve
Rotary spool, sliding spool, or poppet
What is the symbol for a pneumatically operated directional control valve?
And inward facing triangle that is not colored in
What does a hydraulically operated directional control valve symbol look like?
And inward facing triangle that is colored in if it is internally supplied. If it is pilot or remotely operated it will have a dashed pilot line coming to the triangles
What is the symbol for a solenoid controlled pilot operated directional control valve
The solenoid symbol is a diagonal line
What is the difference and symbols between a spring centered in the spring offset directional control valve?
Spring centered will have the spring symbol on both sides of the valve spring offset will only have it on one side
What are the four types of spool center designs?
Open Center, close Center, tandem center and float center
What are the different types of check valves?
In-line check valve, right angled check valves, pilot to open check valve and pilot to close check valve
What is a pilot open check valve similar to?
It is similar to a counterbalance valve. The valve ratio must be greater than the cylinder ratio
What purpose is a pilot to close check valve
It is used like an auto bleed off
What is the pressure compensated flow control also known as?
Hydrostat
How do you get speed control in both directions?
Install a flow control without a bypass
What are the two types of hydraulic actuators?
Either linear or rotary. A linear actuator would be a cylinder and a rotary actuator would be a motor
When an actuator is moving, the pressure on a hydraulic system is what?
The load pressure
What is different about a piloted pressure relief valve?
It is a two-stage valve. It reduces the pressure override and has a balance piston
What is in advantage disadvantage of a direct acting pressure relief valve?
It’s advantage is it is quick acting. Disadvantage is it has poor efficiency because of the high-pressure override.
What is the difference in circuit application between and unloading valve in the maximum pressure relief valve?
Unload valve dumps at minimal pressure
What is meant by the term normally open?
That the passage is an open state when there’s no action on the valve
If you want to stop locking actuator in position anywhere long stroke what type of center would you use?
A closed center or tandem center
Name for ways to control a directional control valve
Manually hydraulically pneumatically or solenoid (electrically)
If you’re using a pilot open check valve to lock a cylinder in place, what must be maintained between the pilot open check valve and the cylinder?
You must like to check valve with an area ratio greater in the area ratio of the cylinder
What is more efficiant than an in-line check valve?
A right angle check valve
The problem could your system suffer from an in-line check valve?
And in-line check valve causes a considerable pressure drop because the pop it is directly in the way of the flow and creates turbulent flow
What is mounted first and what is mounted last in a stack valve?
The maximum pressure relief valve is mounted first, and the directional control valve is mounted last
What are the two types of cartridge valves? Which one is classed as a throwaway item?
There is the screw in type in the slip in type. The screw in type is considered as a throwaway because the threaded portion is damaged when it’s under compression
How does a proportional valve differ from a regular directional control valve?
The proportional valve will change direction and give flow control through metering notches. This spool can move to an infinite number of positions
What are the five functions of an amplifier card?
Dead band compensation, Gain , dither signal, ramp, and the feedback system
What is the most efficient feedback system?
Closed outer loop because it gives feedback directly from the motor
Explain dither signal
AC current is applied to the solenoid to rapidly move the cylinder back-and-forth to reduce hang up
Explain the gain function
The amount that the voltage signal is increased from input to output
What determines if a pressure valve needs an externally drained spring chamber?
If there is pressure downstream from the valve
Name three ways a directional control valve can be controlled?
Rotary valves, sliding spool valves, poppet valves
What would the balance piston in a pilot operated pressure relief valve have?
A small hole through the piston land
When comparing the direct acting control valve to a pilot operated pressure?control valve the directed acting valve does what
It is faster acting
What does a float centered directional control valve allow a hydraulic motor to do?
Coast to a stop
What directional control valve control loop provides the most accurate flow control?
Closed outer loop
Which pressure control valve is internally and externally piloted
A brake valve. It is always used with a hydraulic motor
What prevents a runaway motor in a system?
The external pilot of The brake valve
How are stack valves supplied and drained?
Through the sub plate
How does a proportional directional control valve control the speed of an actuator?
Through metering notches
What is the main drawback to a metering out flow control
Pressure intensification
What does each port house to seal the connections between one valve in the next in a stack valve?
O ring
In a cartridge valve what is used for low flow and what is used for high flow?
Screw in type is used for low flow in the slip in type is used for high flow
What is especially important on a cartridge valve system?
Cleanliness is very important. Always use new O-rings, and flush each port before use
What are some advantages of a proportional valve?
They offer a simple design, controlled electrically. Greater accuracy than conventional valves. More rugged and dirt tolerant than servo valves.
How do metering notches control flow?
The smoothly and accurately control the size of the port opening
What are the three types of feedback to a system?
Open loop system, which provides no feedback. Closed inner loop system which provides feedback only from the valve. And closed outer loop system which provides feedback directly from The actuator. The closed outer loop system is the most efficient and the greatest accuracy
What are the three types of proportional pressure control valves?
Nozzle, plate, and poppet
What does the servo valve always have?
The torque motor. It is depicted by a circle with three arrows around it
How far does the Armature tilt on a flapper nozzle?
It can move up to 3° when the coils are energized
