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Third Year Millwright > Hydraulics > Flashcards

Hydraulics Flashcards

1
Q

What must be in every hydraulic system?

A

Maximum pressure relief valve

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2
Q

What are some advantages of hydraulics

A

It has infinitely variable speed, torque, and force control. It has highly accurate control of movement. It has very little loss from friction

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3
Q

What are some disadvantages of

A

They are expensive, and heavy. There is also a high-pressure fluid hazard, possible fire hazard and leaks are messy and difficult to control

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4
Q

Describe pascals law

A

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

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5
Q

How was pressure created

A

It is created by resistance to flow

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6
Q

What is gauge, and atmospheric pressure

A

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

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7
Q

What is the pressure area called on the side of the piston that has the rod?

A

The annular area

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8
Q

What are the two types of cylinders?

A

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

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9
Q

How was flow produced?

A

Flow is produced either buy a pump, or by a pressure drop across an orifice

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10
Q

What three things does every hydraulic system have?

A

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

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11
Q

Hydraulic cylinders can either be __________ or ___________ cylinders

A

Extending or retracting

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12
Q

Describe a 2-1 cylinder

A

When the cap end or piston area is two times the area than on the rod side, or annular area

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13
Q

What does a pump produce?

A

It only produces flow. Pressure is produced by a resistance to flow

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14
Q

What is the flow triangle?

A

V/t(flow rate)=S(speed)Times A(area)

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15
Q

What effect does a smaller pipe size have on a fluid?

A

If a pipe is changed with one that is smaller, the speed will increase.

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16
Q

What is the recommended maximum pressure line velocity or speed ?

A

7-20 feet per second

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17
Q

Describe laminar flow

A

When the fluid moves through the system smoothly in parallel lines.

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18
Q

Describe turbulent flow

A

When the fluid is disturbed by various conditions, and the fluid swirls.

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19
Q

Why is laminar flow good?

A

Least amount of friction, least amount of pressure, energy loss, least mount of wear, reduces possibility of cavitation or aeration

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20
Q

What is cavitation?

A

It is the violent collapse of trapped vapor bubbles of whatever fluid you are moving.

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21
Q

How is cavitation recognizable?

A

Buy a rattling sound and confirmed by the abnormally low pressure at the pump inlet

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22
Q

What are the effects of cavitation?

A

Rapid erosion of metal surfaces, severe pump damage, erratic operation of valves and actuators

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23
Q

What are some causes of cavitation?

A

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

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24
Q

What is the output unit of a hydraulic system called?

A

It is called an actuator. He could be a hydraulic cylinder or a hydraulic motor

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25
Q

What are the components you have to work on in the hydraulic system?

A

The reservoir, a pump, pipes tubes and hoses, valves that control direction speed and pressure, and actuator that does the work

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26
Q

Hydraulics are based on what principal?

A

Pascals law

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27
Q

What is the greatest waste of power in a hydraulic system?

A

Maximum pressure relief valve

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28
Q

What is the maximum pump inlet velocity?

A

2-4 feet per second

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29
Q

What is the reason for slower speeds in hydraulic system?

A

Too high of speeds will create:

  • wasted energy
  • increase friction
  • wear out components
  • shock components
  • create heat
  • cause turbulent flow
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30
Q

What is Bernoulli’s principle in regards to velocity/pressure

A

When the pressure increases, the velocity decreases.

When the velocity increases, the pressure decreases

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31
Q

What is energy?

A

Energy is the capacity to do work. Work=force X distance

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32
Q

What is power?

A

Power is the rate at which work is done. Power=work / time

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33
Q

What is power efficiency?

A

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

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34
Q

What is volumetric efficiency?

A

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

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35
Q

What is the main difference between an open loop circuit and a closed loop circuit?

A

An open loop circuit the return line does not connect directly to the pump inlet

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36
Q

What is the volume required in the reservoir in an open loop system?

A

The volume is three times the pump output

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37
Q

What is the reservoir capacity of a close loop system?

A

Three quarters the volume of the charge pump capacity

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38
Q

Why is hydraulic fluid used rather than water?

A

Oil is better at lubricating and ceiling. It’s better that he control and it is more versatile and doesn’t freeze

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39
Q

What does a diagonal arrow on a hydraulic symbol always indicate?

A

It indicates the component is variable or adjustable in its output

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40
Q

What is the problem with the metered in system?

A

They can have lack of speed control. The motor can run away on you

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41
Q

What are cross port relief valves used for?

A

They are used as a type of cushion valve for start up and to avoid shock on system

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42
Q

How can directionals control valves be operated?

A

They can be operated manually electrically hydraulically or pneumatically

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43
Q

What is a valve that is normally open?

A

A pressure reducing valve

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44
Q

What can happen in certain areas of the circuit when metering out?

A

You can get an area of pressure intensification

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45
Q

What is the fluid law that applies to stationary fluid and is the foundation of all hydraulic therory?

A

Pascals law

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46
Q

Doubling the inside diameter of the pipe will?

A

Quadruple the inside cross-sectional area

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47
Q

A high resistance to flow creates?

A

A pressure and heat increase

48
Q

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?

A

20 seconds

49
Q

What is the greatest waste of power in a hydraulic system?

A

Maximum pressure relief valve

50
Q

The volumetric efficiency of a hydraulic system has decreased by 40%. What is changed at the actuator?

A

The actuator decreases in speed

51
Q

What is flow created by in the hydraulic system?

A

The pump

52
Q

What are some functions of the charge pump in a hydrostatic system

A

Recirculates case drain flow, maintains a positive pressure at main pump inlet, circulate oil through filter and cooler when required

53
Q

What can volume volumetric efficiency of a pump help determine?

A

Pump wear

54
Q

In an open loop circuit would the return line be pressurized or return to reservoir at nominal pressure?

A

Return to tank at nominal pressure

55
Q

What are some problems with metering in?

A

Works for opposing loads only, it cannot control the speed of retraction

56
Q

What does a bleed off flow control do?

A

It bleeds of a set amount of flow before it gets to cylinder, it is returned to tank at no pressure

57
Q

Describe hydraulic motors

A

They require an external drain, they use cross port relief valves to protect motor from shock loading, are often bi-rotational

58
Q

What is a benefit of that externally piloted counter balance valve?

A

The externally piloted line greatly increases the maximum downward force capability

59
Q

What are the three things valves control in a hydraulic system?

A

All valves control pressure, directional control, or flow control

60
Q

What is cracking pressure?

A

I can pressure is the pressure required to just open the valve and that the first drop of fluid past the seat

61
Q

What is full flow pressure?

A

Full flow pressure is the pressure required to hold the valve open during full flow

62
Q

What is pressure over ride?

A

What is the difference between full flow pressure in the cracking pressure

63
Q

Why do you want to minimize the pressure over ride?

A

It has greater efficiency

64
Q

Describe a pilot operated relief valve

A

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

65
Q

What do unloading valves usually referred to in a circuit?

A

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

66
Q

What is in unloading valve always unloading?

A

It is unloading the pump

67
Q

Why are brake valves used on the discharge of hydraulic motors?

A

They prevent over speeding when an over running load is applied, they prevent excessive pressure build up when decelerating or stopping a load

68
Q

What type of spools can be used in a directional control valve

A

Rotary spool, sliding spool, or poppet

69
Q

What is the symbol for a pneumatically operated directional control valve?

A

And inward facing triangle that is not colored in

70
Q

What does a hydraulically operated directional control valve symbol look like?

A

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

71
Q

What is the symbol for a solenoid controlled pilot operated directional control valve

A

The solenoid symbol is a diagonal line

72
Q

What is the difference and symbols between a spring centered in the spring offset directional control valve?

A

Spring centered will have the spring symbol on both sides of the valve spring offset will only have it on one side

73
Q

What are the four types of spool center designs?

A

Open Center, close Center, tandem center and float center

74
Q

What are the different types of check valves?

A

In-line check valve, right angled check valves, pilot to open check valve and pilot to close check valve

75
Q

What is a pilot open check valve similar to?

A

It is similar to a counterbalance valve. The valve ratio must be greater than the cylinder ratio

76
Q

What purpose is a pilot to close check valve

A

It is used like an auto bleed off

77
Q

What is the pressure compensated flow control also known as?

A

Hydrostat

78
Q

How do you get speed control in both directions?

A

Install a flow control without a bypass

79
Q

What are the two types of hydraulic actuators?

A

Either linear or rotary. A linear actuator would be a cylinder and a rotary actuator would be a motor

80
Q

When an actuator is moving, the pressure on a hydraulic system is what?

A

The load pressure

81
Q

What is different about a piloted pressure relief valve?

A

It is a two-stage valve. It reduces the pressure override and has a balance piston

82
Q

What is in advantage disadvantage of a direct acting pressure relief valve?

A

It’s advantage is it is quick acting. Disadvantage is it has poor efficiency because of the high-pressure override.

83
Q

What is the difference in circuit application between and unloading valve in the maximum pressure relief valve?

A

Unload valve dumps at minimal pressure

84
Q

What is meant by the term normally open?

A

That the passage is an open state when there’s no action on the valve

85
Q

If you want to stop locking actuator in position anywhere long stroke what type of center would you use?

A

A closed center or tandem center

86
Q

Name for ways to control a directional control valve

A

Manually hydraulically pneumatically or solenoid (electrically)

87
Q

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?

A

You must like to check valve with an area ratio greater in the area ratio of the cylinder

88
Q

What is more efficiant than an in-line check valve?

A

A right angle check valve

89
Q

The problem could your system suffer from an in-line check valve?

A

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

90
Q

What is mounted first and what is mounted last in a stack valve?

A

The maximum pressure relief valve is mounted first, and the directional control valve is mounted last

91
Q

What are the two types of cartridge valves? Which one is classed as a throwaway item?

A

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

92
Q

How does a proportional valve differ from a regular directional control valve?

A

The proportional valve will change direction and give flow control through metering notches. This spool can move to an infinite number of positions

93
Q

What are the five functions of an amplifier card?

A

Dead band compensation, Gain , dither signal, ramp, and the feedback system

94
Q

What is the most efficient feedback system?

A

Closed outer loop because it gives feedback directly from the motor

95
Q

Explain dither signal

A

AC current is applied to the solenoid to rapidly move the cylinder back-and-forth to reduce hang up

96
Q

Explain the gain function

A

The amount that the voltage signal is increased from input to output

97
Q

What determines if a pressure valve needs an externally drained spring chamber?

A

If there is pressure downstream from the valve

98
Q

Name three ways a directional control valve can be controlled?

A

Rotary valves, sliding spool valves, poppet valves

99
Q

What would the balance piston in a pilot operated pressure relief valve have?

A

A small hole through the piston land

100
Q

When comparing the direct acting control valve to a pilot operated pressure?control valve the directed acting valve does what

A

It is faster acting

101
Q

What does a float centered directional control valve allow a hydraulic motor to do?

A

Coast to a stop

102
Q

What directional control valve control loop provides the most accurate flow control?

A

Closed outer loop

103
Q

Which pressure control valve is internally and externally piloted

A

A brake valve. It is always used with a hydraulic motor

104
Q

What prevents a runaway motor in a system?

A

The external pilot of The brake valve

105
Q

How are stack valves supplied and drained?

A

Through the sub plate

106
Q

How does a proportional directional control valve control the speed of an actuator?

A

Through metering notches

107
Q

What is the main drawback to a metering out flow control

A

Pressure intensification

108
Q

What does each port house to seal the connections between one valve in the next in a stack valve?

A

O ring

109
Q

In a cartridge valve what is used for low flow and what is used for high flow?

A

Screw in type is used for low flow in the slip in type is used for high flow

110
Q

What is especially important on a cartridge valve system?

A

Cleanliness is very important. Always use new O-rings, and flush each port before use

111
Q

What are some advantages of a proportional valve?

A

They offer a simple design, controlled electrically. Greater accuracy than conventional valves. More rugged and dirt tolerant than servo valves.

112
Q

How do metering notches control flow?

A

The smoothly and accurately control the size of the port opening

113
Q

What are the three types of feedback to a system?

A

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

114
Q

What are the three types of proportional pressure control valves?

A

Nozzle, plate, and poppet

115
Q

What does the servo valve always have?

A

The torque motor. It is depicted by a circle with three arrows around it

116
Q

How far does the Armature tilt on a flapper nozzle?

A

It can move up to 3° when the coils are energized

Third Year Millwright (12 decks)

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