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HET3 > 301B: Hydraulic Pump Fundamentals > Flashcards

301B: Hydraulic Pump Fundamentals Flashcards

1
Q

All external gear pumps are made to operate regardless of direction of rotation. (T/F)

A

False.
Some are unidirectional and some are designed with check valves for bidirectional flow.

301B Pg. 19

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2
Q

What disadvantages do gear pumps have?

A
  • Fixed displacement
  • Limited in max pressure due to being unbalanced

301B Pg. 16

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3
Q

What are two disadvantages of vane pumps?

A
  • Expensive to purchase and repair
  • More susceptible to contamination damage

301B Pg. 21

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4
Q

On a vane type pump, what is the component that holds the vane blades?

A

Rotor

301B Pg. 24

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5
Q

When the internal volume of a hydraulic pump increases, the pressure:

A

Decreases

301B

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6
Q

Hydraulic oil is forced out of the pump when the volume of the pumping chamber is decreased. (T/F)

A

True

301B

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7
Q

Pumps that have a constant pumping chamber volume are known as:

A

Fixed displacement pumps

301B Pg. 7

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8
Q

Hydraulic pumps that are able to change the capacity of their pumping chambers are called:

A

Variable displacement pumps

301B Pg. 8

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9
Q

A pump termed reversible is able to:

A) Reverse the direction of oil flow through the pump
B) Reverse its direction of rotation
C) Deliver oil from the system back to the reservoir

A

A) Reverse the direction of oil flow through the pump

301B Pg. 11

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10
Q

Three categories of pump efficiency are:

A
  • Mechanical efficiency
  • Volumetric efficiency
  • Overall efficiency

301B Pg. 13

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11
Q

As pump outlet pressure increases, the pump’s internal leakage decreases. (T/F)

A

False
Known as Slip

301B Pg. 14

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12
Q

Overall pump efficiency:

a) is a comparison of pump input and output power.
b) accounts for both mechanical and flow losses.
c) is greater for piston than gear pumps.

A

All of the above responses are correct

301B Pg. 15

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13
Q

Why is some internal leakage necessary within all hydraulic pumps?

A

Internal leakage lubricates moving pump parts.

301B

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14
Q

The two factors that determine the amount of internal leakage in a pump are:

A
  • Pressure difference between the pump inlet and outlet ports
  • Amount of internal clearance / wear within the pump

301B Pg. 15

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15
Q

A unidirectional pump that is driven opposite to its intended direction will:

A

Have the flow through the pump reversed

301B

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16
Q

What is the main difference between the cam ring used in an unbalanced vane pump and the cam ring used in a balanced vane pump?

A

Unbalanced vane pump
- Circular cam ring
- Variable displacement
- Limited max pressure

Balanced vane pump
- Elliptical cam ring
- Fixed displacement
- Higher max pressure

301B Pg. 28

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17
Q

Pressure plates are used in vane pumps to control the clearance between the tip of the vane and the cam ring. (T/F)

A

False
Pressure plates are used in vane pumps to control rotor side clearance.

301B Pg. 29

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18
Q

A variable displacement vane pump changes its displacement by:

A

Changing the eccentricity between the rotor and cam ring centres.

301B Pg. 33

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19
Q

List the two forces that act on the vanes of a typical vane pump.

A
  • Centrifugal force, throwing the vanes outward
  • Outlet oil pressure acting on the underside of the vane

301B Pg. 26

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20
Q

Why must the force exerted by the vanes against the cam ring of a high performance vane pump be reduced?

A

High vane force will cause the film of lubricating oil between the vane tip and cam ring to be squeezed out resulting in metal-to-metal contact and very rapid wear.

301B Pg. 26

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21
Q

List two reasons why oil pressure from the pumping chambers of an axial piston pump is connected to the piston shoes.

A
  • Lubricate the ball and socket joint between the piston and the shoe and the contact area between the shoe and the swash plate
  • Balance the piston by offsetting the force generated by the pressure in the pumping chamber.

301B Pg. 44

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22
Q

The oil that gathers in the case of an axial piston pump comes from:

A

Internal leakage

301B Pg. 44

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23
Q

A pressure compensated axial piston pump of the pressure limiting type will move its swash plate to the zero displacement position when maximum system pressure is achieved. (T/F)

A

True

301B Pg. 46

24
Q

A hydraulic control system that copies the movement of the control lever is called a ____ system.

A

Servo

301B Pg. 49

25
Q

A variable displacement pump that varies its flow rate in relation to outlet pressure is called:

A

A pressure compensated pump

301B Pg. 46

26
Q

The direction of rotation on a unidirectional pump is often shown by:

A
  • An arrow in the casting of the pump housing
  • Inlet port larger than the outlet

301B

27
Q

You can change the direction of oil flow in a vane pump by simply changing the vanes. (T/F)

A

False
Depending on the pump make, you can disassemble the pump and change the orientation of the cam ring or rotor.

301B

28
Q

What are the three kinds of piston pumps?

A

Radial - Perpendicular to the drive shaft
Axial - Parallel to the drive shaft
Bent Axis - Angled to the drive shaft

301B Pg. 38

29
Q

Why are piston pumps more efficient than gear pumps?

A

Closer tolerances
Least amount of internal leakage

301B Pg. 38

30
Q

In a radial piston pump, what prevents high pressure oil from returning back into the pumping chamber?

A

A check valve on the outlet ports.

301B Pg. 39

31
Q

What are the two design styles of radial piston pumps?

A
  • Rotating cam
  • Rotating piston

301B Pg. 38

32
Q

In a radial piston pump, what assists with the variable displacement?

A

Excess oil flow returns to the pump housing case and builds pressure.
This pressure acts against the pistons and holds them off the cam. The more pressure in the case, the farther away from the cam they are held, resulting in a shorter pumping stroke and displacement.

301B Pg. 40

33
Q

What are some design characteristics of an axial piston pump?

A
  • Pistons are arranged so that the centreline is parallel with the pump driveshaft
  • Pump driveshaft is connected to and drives the cylinder barrel
  • Pistons are closely fitted into the cylinder barrel and slide on a swash plate
  • Swash plate angle determines stroke
  • Odd number of pistons to reduce pulsation
  • Can be fixed or variable displacement

301B Pg. 41

34
Q

What are the cylinder and barrel called in an axial piston pump?

A

The rotating group

301B Pg. 42

35
Q

In an axial piston pump, what holds the pistons against the swash plate?

A

Shoe plate

301B Pg. 42

36
Q

What is the formula to calculate the displacement of a gear pump?

A

Disp/Rev = 3W(2D-L)(L-D)

W = Width of gear
D = Diameter of gear
L = Length of bores in the housing

301B Pg. 4

37
Q

What is the formula to calculate the displacement of a vane pump?

A

Disp/Rev = 1.5W(L+D)(L-D)

W= Width of rotor
L= Inlet (Widest part of cam ring)
D= Outlet (Narrow part of cam ring)

301B Pg. 5

38
Q

What is mechanical efficiency as it relates to hydraulic pumps?

A

The amount of power required to overcome the internal friction of the pump.

Wasted power is converted to heat.

301B Pg. 13

39
Q

What is volumetric efficiency as it relates to hydraulic pumps?

A

How effectively a pump generates flow. Stated as a percentage.

VE = Actual pump flow / Theoretical pump flow

301B Pg. 14

40
Q

What is hydraulic pump slip?

A

Oil that leaks from the outlet side to the inlet side of the pump.

Increases as the pressure difference between the outlet and inlet increases and as the pump wears.

301B Pg. 14

41
Q

Oil that unintentionally leaks from the high pressure to low pressure side of a hydraulic pump is referred to as:

A

Slip

301B Pg. 14

42
Q

As a hydraulic pump’s operating pressure increase, flow rate:

A

Decreases

301B Pg. 15

43
Q

Explain hydraulic pump overall efficiency.

A

Compares input power to output power of the pump. Accounts for mechanical and volumetric efficiency.

Peaks at normal operating range and begins to drop as pressure increases to a maximum. This is due to internal leakage and mechanical losses cuased by internal loading.

301B Pg. 15

44
Q

What are three advantages of external gear pumps?

A
  • Least complex, with fewer moving parts
  • Less expensive
  • More tolerant to contamination

301B Pg. 16

45
Q

Looking into one of the ports while rotating the drive shaft on a simple hydraulic pump, you see gears coming into mesh. What type of pump are you looking at and what port are you looking into?

A

Outlet port of an external gear pump

301B Pg. 19

46
Q

What it meant when a hydraulic pump is said to be unbalanced?

A

The force created by outlet pressure acting on the surface of the gears is not counteracted on the inlet side of the pump.

301B Pg. 19

47
Q

What area of an external gear pump carries the oil from the inlet to the outlet side of the pump?

A

Gear tooth pocket

301B Pg. 17

48
Q

The amount of fluid flowing internally (wanted & unwanted) through a hydraulic pump is determined by what two factors?

A
  • Pressure difference between the inlet and outlet
  • Amount of internal clearances within the pump

301B Pg. 18

49
Q

What carries oil from the inlet to outlet ports in a vane pump?

A

Vane pockets

301B Pg. 21

50
Q

A variable displacement pump that reduces its flow rate in relation to outlet pressure is called a:

A

Pressure compensated pump

301B Pg. 33

51
Q

What does this symbol represent?

A

Fixed displacement unidirectional pump
Clockwise rotation

301B Pg. 35

52
Q

What does this symbol represent?

A

Unidirectional variable displacement pressure compensating pump.

301B Pg. 33

53
Q

What additonal component is used with an axial piston pump to maintain maximum system pressure?

A

Pressure Compensating Valve

301B Pg. 47

54
Q

What two pistons are used in a pressure compensating axial piston pump to control the swash plate angle in order to maintain a maximum system pressure?

A

Bias Piston
- Smaller of the two
- Works in conjunction with a spring
- Pushes the swash plate into max displacement position

Stroking Piston
- Larger of the two
- Pushes the swash plate into zero displacement position

301B Pg. 47

55
Q

A releif valve is needed in a pressure compensated axial pump in order to maintain maximum system pressure. (T/F)

A

False. No releif valve needed.
The pressure compensating valve maintains maximum system pressure with use of the Bias and Stroking pistons.

301B Pg. 47

56
Q

Why are bent axis piston pumps more reliable than axial pumps?

A

Do not have piston shoes sliding over a stationary swash plate.

301B Pg. 51

57
Q

In an axial piston pump, what component has two semicircle openings which is mated to the end of the cylinder barrel?

A

Valve plate

301B Pg. 53

HET3 (34 decks)

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