301C: Hydraulic Pump Service

Question 1

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

Answer 1

Variable displacement pumps

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Question 2

A reversible hydraulic pump is able to:

Answer 2

Reverse the direction of oil flow through the pump

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Question 3

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

Answer 3

False

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Question 4

Why do manufacturers of gear pumps with aluminum housings allow the gears to cut a gear path into the pump housing during break-in?

Answer 4

Gear track
Provides low gear tip clearance and provides high volumetric efficiency.

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Question 5

What is the function of the trap in a gear pump pressure plate?

Answer 5

• Provides an escape path for oil trapped between the gear teeth as they come into mesh.
• Helps control noise

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Question 6

Why is some internal leakage necessary within all hydraulic pumps?

Answer 6

Internal leakage lubricates moving pump parts.

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Question 7

Why is oil pressure from the pumping chambers of an axial piston pump connected to the piston shoes? (2)

Answer 7

• Lubricates the contact areas between the swash plate, shoe, and piston.
• Balance the piston by offsetting the force generated by the pressure in the pumping chamber.

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Question 8

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

Answer 8

Internal leakage

Question 9

List three general guidelines that should be observed when disassembling a hydraulic pump.

Answer 9

• Mark pump housings so they can be returned to their proper location during assembly.
• Determine the correct direction of driveshaft rotation for unidirectional pumps so that the pump can be returned to the proper rotation on assembly.
• Keep mated components (such as gear sets, pistons and barrels) together so that they can be returned to their original positions.

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Question 10

The most common cause of pitted erosion on the internal surfaces of a hydraulic pump is:

Answer 10

Cavitation

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Question 11

List three reasons that could cause a hydraulic pump to lack lubrication.

Answer 11

• Using improper hydraulic oil for the application and climatic conditions
• Low oil level which causes the pump inlet to become uncovered
• Severe aeration

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Question 12

What four tasks should be performed when a hydraulic pump is installed to ensure that the system is clean enough to ensure a good pump service life?

Answer 12

• Hydraulic oil may need to be changed or filtered to remove contamination
• Filters must be serviced
• Reservoir must be thoroughly cleaned
• Complete system may have to be flushed

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Question 13

Why should hydraulic system pressure controls be backed off prior to starting a machine in which the pump has been replaced?

Answer 13

Prevents overloading the pump in the event that the pressure controls have been misadjusted prior to pump replacement.

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Question 14

Pump cavitation failure is most commonly caused by: (3)

Answer 14

• Using improper hydraulic oil
• Restrictions in the pump inlet line
• Excessive pump driveshaft speed

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Question 15

Evidence of a lack of lubrication in a hydraulic pump is shown by: (4)

Answer 15

• Heavy wear of surfaces of moving parts
• Blue colouring from overheating
• Smearing and metal transfer between contacting surface
• Pump seizure

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Question 16

How are multiple axial pump configurations commonly driven?

Answer 16

With a common driveshaft

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Question 17

On external gear hydraulic pumps, the gears come into mesh at the inlet side. (T/F)

Answer 17

False
The gears come into mesh at the outlet side

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Question 18

When dissasembling an external gear pump, why should the gear teeth be non-destructively marked before removal?

Answer 18

• Gears are non-hunting and need to be returned to their origional position durring assembly
• Non-destructively marked so the pressure plates do not get scored

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Question 19

When dissasembling an external gear pump, you notice two check valves in the housing. What does this tell you?

Answer 19

Bi-directional pump

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Question 20

When inspecting an external gear pump housing, what side of the housing would you expect to see more wear?

Answer 20

Inlet side, due to the unbalanced design of the pump

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Question 21

When inspecting the pressure plate in an external gear pump, you notice a cut into the outer edges as shown by “C”. What is this for?

Answer 21

Chamfer
Partially offsets the thrust loads on the gear set and provides better balance and higher pressure capability.

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