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Heavy Equipment Tech > 190303Fa > Flashcards

190303Fa Flashcards

1
Q

How do hydraulic valves assert their control over the system?

A
  1. Controlling pressure
  2. regulating the amount of oil that flows
  3. determining the path of oil flow
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2
Q

What are the three categories of hydraulic valves?

A
  1. pressure control valves
  2. flow control valves
  3. directional control valves
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3
Q

What are hydraulic valves rated for?

A

Maximum pressure capability, as well as the pressure drop they create

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4
Q

What are the two classifications of pressure controls?

A
  1. Normally open
  2. Normally closed
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5
Q

Pressure control valves perform what tasks?

A

Limit or regulate pressure
create a pressure condition required for control cause actuators to operate in order

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6
Q

What two forces must be balanced in a pressure control valve?

A

Spring force vs. hydraulic force (pressure)

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7
Q

How can spring force be adjusted in a pressure control valve?

A

With an adjustment screw (mechanical mean)

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8
Q

True of False: A normally open pressure control valve allows flow from inlet to outlet?

A

True

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9
Q

Where does a drain valve allow oil to flow to on a pressure control valve?

A

Usually to tank/reservoir

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10
Q

What does the drain port prevent on the spring side of a pressure control valve?

A

Hydrostatic lock

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11
Q

True or False: A normally closed pressure control valve allows oil flow from inlet to outlet port?

A

False

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12
Q

What is the constant movement in a valve called?

A

Valve Modulation

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13
Q

What is the stiffness of a spring called?

A

Spring Constant

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14
Q

What is the pressure called, when oil just begins to flow to the outlet port?

A

Cracking Pressure

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15
Q

The difference between cracking and full flow pressure is called?

A

Pressure override

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16
Q

Out of the three styles of direct acting pressure control valves which style can handle the least amount of flow?

A

The ball type (11 L/min)

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17
Q

Why does the guided poppet type of direct acting pressure control valve have holes drilled in the poppet?

A

allow the oil to flow from inlet to outlet port, limits max flow capacity of the valve

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18
Q

What does excessive override do?

A

Wastes power and created unwanted heat

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19
Q

What is the greatest disadvantage of direct acting pressure controls?

A

a high amount of pressure override is unavoidable when passing high flow rates

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20
Q

What is another name for a pilot operated pressure control valve?

A

Compound pressure control

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21
Q

The main poppet is held closed by a light spring or strong spring?

A

Light spring

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22
Q

What happens when cracking pressure is achieved in a pilot operated pressure control valve?

A

forces the pilot poppet off its seat

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23
Q

When the main poppet is open is oil flow allowed to go to the outlet ports?

A

Yes

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24
Q

What is used to adjust the pressure setting in a pilot operated pressure control valve?

A

Pressure adjusting screw

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25
Q

Is the override pressure of a pilot operated valve high or lower than the direct acting valve?

A

override pressure is lower

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26
Q

What are some advantages of pilot operated pressure control valves?

A
  • Less prone to Chatter
  • operate at a much wider pressure range
  • less override pressure
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27
Q

Do the majority of flow travel pass the pilot poppet or main poppet in a pilot operated valve?

A

Pass the main poppet

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28
Q

Are pressure relief valves normally open, closed or either?

A

Normally CLOSED

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29
Q

Does the outlet of a pressure relief valve return to tank?

A

YES

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30
Q

What are two common applications of relief valves?

A

System relief and Circuit relief

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31
Q

What is a system relief valve connected to?

A

In parallel with the pump outlet line

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32
Q

With the system relief valve cracked what happens to the actuators?

A
  • Flow is diverted
  • Slow speeds
  • excessive heat
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33
Q

What is the main difference between the two types of pressure relief valve applications?

A

Location in system and basic construction

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34
Q

What are three types of hydraulic accumulators?

A

Weighted, Spring-Loaded or gas-charged

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35
Q

What are three styles for gas-charged accumulators?

A

Bladder, Piston or Diaphram

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36
Q

In a gas-charged accumulators what is used as the “gas”?
Hint: Oxygen, compressed air or nitrogen gas (dry)

A

Nitrogen gas (dry) N²

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37
Q

In an gas-charged accumulator the hydraulic oil can mix with N²?

A

No, unless it is faulty

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38
Q

What will be the symptom if the gas-charged accumulator is faulty?

A

You will not see aeration or any sign of the leaked N² because it will go to tank and leave
No shock load absorption - broken pins, spools, control valves
Poor operation

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39
Q

Are spring accumulators holding a constant pressure?

A

No, the spring will be compressed depending on oil

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40
Q

What are some functions of an accumulator?

A

Energy storage
Shock absorption
Gradual pressure build up
Constant pressure maintenance

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41
Q

What are the 5 basic pressure control valve application? Bonus 2 extra

A

Pressure relief valves
Unloaded valves
Pressure reducing valves
Sequence Valves
Backpressure valves
Bonus: Counter balance valve
Brake valve line lock

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42
Q

In a pressure relief valve, is the pilot pressure connected in parallel and upstream of the valve?

A

Yes

43
Q

System relief valves are usually connected in parallel where in a system?

A

Usually at pump outlet

44
Q

What is the pro to using a pilot operated system relief valve?

A

Little pressure override and giving FULL FLOW

45
Q

What does a circuit relief valve protect?

A

A circuit or branch in a system

46
Q

Are all relief valve springs adjustable?

A

Not all, some need to be replaced to get a different pressure

47
Q

Are circuit relief valves usually direct acting?

A

Yes, Low flow

48
Q

When adjusting system relief valve should the engine be at full throttle or idle?

A

Full throttle and stall actuator

49
Q

What are two ways to adjust the system relief valve if it is adjustable?

A

Screw or add/remove Shims

50
Q

What are we adjusting in a circuit relief valve?

A

Cracking pressure

51
Q

When adjusting a circuit relief valve what should the engine be doing?

A

Low idle, Minimum flow

52
Q

What are some faults that can happen in a relief valve?

A

Poppet stuck open
Poppet seized closed
broken spring
damaged o-rings
contamination

53
Q

When replacing a relief valve what should be done?

A

Relieve all hydraulic pressure
Cleanliness
Replace O-ring seals
Torque to spec
Back off relief valve before initial start up then adjust

54
Q

Unloader valve are more energy efficient than a relief valve?

A

Yes

55
Q

Are unloader valves normally open?

A

No

56
Q

Where is the outlet port on an unloader valve connected to?

A

Tank/Reservoir

57
Q

What are the two application versions of a unloaded valve?

A

Closed center or demand type

58
Q

Closed centre hydraulic systems with an unloader valve can use a fixed displacement pump?

A

Yes

59
Q

What does a check valve in an unloader valve provide?

A

It holds high pressure downstream (accumulator, control valve)

60
Q

Pressure reducing valves are normally open?

A

Yes

61
Q

Why does the spring cavity need to have an external drain?

A

High pressure on both sides would not let the spring internally drain

62
Q

In a pressure reducing valve is the pilot connected to the inlet or outlet of the valve? (upstream or downstream)

A

The pilot is sensing downstream from the valve

63
Q

In a sequence valve the pilot is sensing from upstream or downstream from valve?

A

Pilot is sensing upstream (input) from valve

64
Q

Is a pressure reducing valve is it normally open or closed?

A

Normally open

65
Q

Where is the outlet connected to on a pressure reducing valve?

A

Outlet is connected to the circuit

66
Q

Can a pressure reducing valve be “remotely” controlled?

A

Yes, connect into port near drain

67
Q

Is a sequence valve normally closed?

A

Yes

68
Q

Why is a drain line needed in a sequence valve?

A

To allow internal leakage to return to tank/reservoir

69
Q

A check valve is required for rapid return of reversible actuators in a sequence valve, correct?

A

Yes

70
Q

Is pilot pressure in a sequence valve sensed from upstream?

A

Yes

71
Q

What is a sequence valve used for?

A

Cause actions in a hydraulic system to occur in a certain order,
Maintain a predetermined minimum pressure in the primary hydraulic function while the secondary function is in operation

72
Q

What allows rapid return of reversible actuators in a sequence valve?

A

Check valve

73
Q

Does a sequence valve need to have a reversible check valve in the valve body?

A

No doesn’t have to be in the valve, has to be a separate check valve (cartridge)

74
Q

What is used with a one way sequence valve?

A
  • One way motor
  • Flow must only go in one direction
  • From pump to directional control valve
75
Q

If pressure is controlled in one direction in a sequence valve what happens in the opposite direction if a check valve is installed?

A

Freeflow in “reverse” direction

76
Q

What are three backpressure valves?

A

Counterbalance, brake valve and lock valve

77
Q

What does a counterbalance valve control when lowering a vertical load?

A

Freefall, restricts or prevents

78
Q

How are sequence valves adjusted?

A

By installing a pressure gauge in the primary circuit and adjusting the valve spring force until proper primary circuit pressure is achieved just prior to secondary function movement

79
Q

What characteristics are shared by all backpressure valves?

A
  • Normally closed
  • Pilot pressure to open valve varies.
  • connected in series in the actuator return line
  • check valve is required to allow reversible actuators to return
80
Q

What changes the amount of pressure intensification?

A

Amount of restriction in the return line
ratio of base end and piston end areas
force increase created by the load on the rod

81
Q

Why should you be careful when installing or adjusting valves installed on the rod end of a double acting cylinder?

A

Pressure intensification, can exceed max system pressure

82
Q

What is the purpose of a brake valve?

A

Prevent load from overspeeding the motor
Provide a controlled deceleration of load

83
Q

What makes a lock valve easier to identify?

A

Double ball check valve

84
Q

What does a lock valve do if the system is suspending a load and a hydraulic line ruptures (fails)?

A

The lock valve will Hold

85
Q

Whats another name for the lock valve?

A

Pilot operated check valves

86
Q

What are the functions of a pilot operated check valves (lock valve)?

A

Hold a suspended load in the even of a hydraulic line ruptures
Prevent the load from drifting due to leakage in the directional control valve
Prevent the load from overspeeding the actuator

87
Q

What MUST a counter balance valve have?

A

Check valve - reverse

88
Q

Is a counter balance valve normally closed?

A

Yes

89
Q

In a system using a counterbalance valve; if the operator lets go of the control valve and it returns to its normal position, what will happen to the “load”?

A

If the control valve returns to normal position with no flow going to the “cylinder”, the load will hold if the counterbalance valve is operating correctly

90
Q

What is the purpose of the check valve in a counterbalance valve?

A

Allow free flow to reverse through the valve, for example when lifting the load

91
Q

What is the visible difference between a counterbalance valve and a sequence valve?

A

On a sequence valve the spring is externally drained and a counterbalance valve spring is internally drained

92
Q

How is the speed of an actuator determined?

A

Internal capacity of the actuator
input flow rate supplied to the actuator

93
Q

What is the easiest way to control actuator speed?

A

Varying flow rate

94
Q

What is the common component in a hydraulic flow control valve?

A

Orifice

95
Q

What is a difference in pressure called?

A

Pressure drop

96
Q

Where is pressure always highest in relation to an orifice?

A

Upstream of the orifice

97
Q

What happens with pressure downstream from an orifice?

A

It lowers

98
Q

Where is the lowest pressure found? In regards to an orifice

A

Right in the orifice

99
Q

How is the amount of flow through an orifice determined?

A

Orifice size and shape
pressure drop across the orifice
Viscosity of the oil

100
Q

What type of flow control is used to have accurate control of actuator speed?

A

Pressure compensated flow control

101
Q

What can be added to a non-compensated flow control to make it pressure compensated?

A

Compensator

102
Q

Where is excess flow directed to in a bypass type pressure compensated flow control valve?

A

To reservoir

103
Q

Meter in circuit controls the speed of cylinder extensions by controlling _______?

A

Controlling the amount of oil flowing into the base end of the cylinder

Heavy Equipment Tech (25 decks)

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