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HET3 Hydraulics 190301 > 190301c > Flashcards

Flashcards in 190301c Deck (21):

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

variable displacement pumps.


A pump termed reversible is able to:

reverse the direction of oil flow through the pump


As pump outlet pressure increases, the pump's internal leakage decreases.T/F



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

To provide good volumetric efficiency by keeping gear tip clearance to a minimum.


State the function of the trap in a gear pump pressure plate.

The trap provides an escape path for oil trapped between the gear teeth as they come
into mesh.


Why is some internal leakage necessary within all hydraulic pumps?

Internal leakage lubricates moving pump parts.


List two (2) reasons why oil pressure from the pumping chambers of an axial piston
pump is connected to the piston shoes.

a) To lubricate the ball and socket joint between the piston and the shoe as well as
the contact area between the shoe and the swash plate.
b) Balance the piston by offsetting the force generated by the pressure in the
pumping chamber.


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

internal leakage.


List three (3) general guidelines that should be observed when disassembling a
hydraulic pump.

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


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



List three (3) reasons that could cause a hydraulic pump to lack lubrication.

a) using improper hydraulic oil for the application and climatic conditions
b) low reservoir oil level which causes the pump inlet to become uncovered
c) severe aeration


List four (4) tasks that should be performed when a hydraulic pump is installed to
ensure that the system is clean enough to ensure good pump service life:

a) The hydraulic oil may need to be changed or filtered to remove contamination.
b) Hydraulic filters must be serviced.
c) The hydraulic reservoir must be thoroughly cleaned.
d) The complete system may have to be flushed.


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

Pressure controls are backed off to prevent overloading the pump in the event that the
pressure controls have been misadjusted prior to pump replacement.


Pump Cavitation failure is most commonly caused by?

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


Evidence of a lack of lubrication is shown by

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


How are most multiple pump configurations driven?

With a common inlet (driveshaft)


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

False; The gears must come into mesh at the outlet.


What is very important before disassembling a pump?

You must determine the direction of rotation of the shaft. Often marked with arrows.


On a vane pump there is a larger inlet than an outlet. T/F



What is the function of the pressure plate?

They keep the internal clearances to a minimum, limiting the internal leakage.


What should you consider when examining a vane pump?

Condition of parts.
Type and condition of oils and filters.
Operating conditions at time of failure.
Severity of job application. ( machine too small for job)
Amount of time pump has been in service.
Previous failures and repairs.