MEO Class IV - GEK

Pump
Variable Delivery Pump Exam Guide

This exam guide cont ains m odel quest ions sim ilar t o t he ones you would answer in t he writ t en
exam inat ion. Our aim is t o equip you on im port ant subj ect areas. Answers provided are in t he form of cues
and bullet ed list t o enable easy recollect ion.

Qu e st ion s

1 a. With reference to positive displacement type pumps, describe two methods that will
reduce unacceptable fluctuations in the output pressure.

b. Sketch and describe the operation of a radial, rotary, unidirectional, reversible pump
suitable for hydraulic power systems.

2 a. With reference to positive displacement piston type pumps, explain how unacceptable
fluctuations in the output pressure may be avoided.

b. Sketch and describe a radial, rotary, unidirectional, reversible flow pump suitable for
steering gear applications.

3 a. Describe the type of pump that would be installed in a hydraulic system for deck
machinery.

b. State its advantages compared to Hele-Shaw pump.

4 a. Sketch and describe a swash plate pump.

b. State two design features of a swash plate pump that enables it to be smaller in size than
radial pump designs for a given duty.

c. Explain why hydraulic pumps are usually provided with an odd number of cylinders.

5 a. Explain the possible causes of contamination in a hydraulic system.

b. Describe the likely results of contamination.

c. Explain how contamination can be monitored.

 MEO Class IV - GEK
Pump
Variable Delivery Pump Exam Guide

Q 1. a.
With reference to positive displacement type pumps, describe two methods
that will reduce unacceptable fluctuations in the output pressure.

A Unacceptable fluctuations in the output pressure may be avoided by using

accumulators. They are of two types:

z Piston type accumulator

z Bag type accumulator

Nitrogen

Upward
movement
Sealing arrangement
Nitrogen
bag
Piston
Downward
movement Liquid

Liquid inlet
Liquid inlet

Bag type accumulator Piston type accumulator

As the pressure in the hydraulic system rises above the pre-charge pressure, oil enters
and displaces the piston or rubber bag. The piston or rubber bag moves upwards,
increasing the gas pressure and decreasing its volume. Oil continues to enter the
accumulator until the system reaches maximum pressure

As the hydraulic pressure falls, the oil discharges from the accumulator. Thus the
unacceptable fluctuations in the system are avoided.
 MEO Class IV - GEK
Pump
Variable Delivery Pump Exam Guide

Q 1. b.
Sketch and describe the operation of a radial, rotary, unidirectional,
reversible pump suitable for hydraulic power systems.

A Hele-Shaw pump is a radial rotary unidirectional reversible pump.

Co n st r u ct io n

Gudgeon pin
Cylinder body

Plunger

Slipper
Slipper path
Annular ring Floating ring
Actuating spindle

Roller bearing
Casing

Top port

Oil block
Hele-Shaw pump

The diagram shows the construction and operation of this type of pump which is normally
driven by a constant speed electric motor.

Gudgeon pin
Slipper
Top port
Floating
Fixed shaft
Actuating or control arm
Cylinder bore
Piston

Bottom port
 MEO Class IV - GEK
Pump
Variable Delivery Pump Exam Guide

Co n st r u ct io n :

The pistons are fitted in a row of radial cylinders and the outer end of each piston has
a gudgeon pin, which connects the slippers to the piston. The slippers are free to oscillate
on their gudgeon pins and fit into circular grooves in the circular floating ring. This floating
ring is free to rotate being mounted on ball bearings. Ball bearings are housed on the
guide blocks.

W o r k in g :

The movement of the floating ring by the actuating control spindle from the central
position causes pistons to reciprocate in the radial cylinders. Hence, pumping action takes
place. The direction of the pumping depends upon whether the movement is to the left or
right of the central or neutral position.

The action of the pump is shown above.

Direction of rotation Direction of rotation

Discharge port Discharge port

A B
Suction port Suction port

Center line Center line of Center line Center line of

of shaft fished ring of shaft fished ring

In no flow position, the central body and the bearing locating the path of the piston rod
ends are coaxial. This means that the whole system rotates together, with no relative
motion of the pistons in the cylinder.

In the flow position, the outer bearing is laterally displaced by the control rod. Therefore
the centres of the central body and the centres of the bearing are no longer coaxial. This
means that the piston rod ends are constrained to follow the outer bearing as they
rotate, which means that a relative reciprocating motion of the pistons occurs within the
cylinders. Piston withdraws oil through suction port in first half of the revolution and
discharges through the discharge port in next half of the revolution.

The rate of discharge is controlled by axial displacement of the outer bearing with respect
to the central body.
 MEO Class IV - GEK
Pump
Variable Delivery Pump Exam Guide

By moving the control rod laterally in the opposite direction, both the direction and
quantity of flow can be altered.

Ad v a n t a g e :

In reversing the direction of flow of fluid, the pump moves from maximum delivery to zero
delivery in one direction and then through zero to maximum in the opposite direction.
The build up in fluid pressure takes place without any shock load on pipelines, supplying
fluid to the hydraulic unit.

Q 2. a.
With reference to positive displacement piston type pumps, explain how
unacceptable fluctuations in the output pressure may be avoided.

A Refer answer 1 a.

Q 2. b.
Sketch and describe a radial, rotary, unidirectional, reversible flow pump
suitable for steering gear applications.

A Refer answer 1 b.
 MEO Class IV - GEK
Pump
Variable Delivery Pump Exam Guide

Q 3. a.
Describe the type of pump that would be installed in a hydraulic system for
deck machinery.

A Variable Stroke Reversible Swash Plate Pump

Swash plate Piston Valve plate

Cylinder
Slipper
Oil delivery/suctio

Shaft
Control
rod

Cylinder block keyed to shaft

Swash plate pump

This pump is also known as variable stroke gear pump. It runs in flooded condition, the
make up tank being above the level of pump so that all the working parts are immersed in
oil. It is driven by a constant speed electric motor. The volume and direction of the oil flow
are controlled by means of a stroke control lever.

Slipper pads bear against the swash plate face and the pistons are driven in and out
axially for each revolution of the rotor. For one direction of tilt, ports on one side of the
horizontal centre line become suction and on the other side become discharge. For the
opposite direction of the tilt, the direction of flow is reversed. The quantity of discharge
depends on the angle of tilt. In the mid position no relative movement exists between the
piston and end plate and no pumping action takes place.
 MEO Class IV - GEK
Pump
Variable Delivery Pump Exam Guide

Q 3. b.
State its advantages compared to Hele-Shaw pump.

A The Swash plate pump or Variable Stroke Gear pump is stated to have some advantages
over the Hele-Shaw.

z The centre of gravity of the Hele-Shaw pistons is at a relatively larger distance from
the centre of rotation resulting in large centrifugal forces. But variable stroke gear
pistons have a centre of gravity close to the centre of rotation creating relatively
small centrifugal forces. This means that the variable stroke gear system can run at
much higher speeds despite being much smaller and can do the same work as the
Hele-Shaw.

z Due to centrifugal forces, the wear on the variable stroke gear pump pistons can
be greater than that of radial type pistons. There is a small clearance between the
valve and cylinder blocks when running off load. When the unit comes on stroke the
hydraulic pressure forces the two faces together.

Q 4. a.
Sketch and describe a swash plate pump.

A Refer answer 3 a.

Q 4. b.
State two design features of a swash plate pump that enables it to be
smaller in size than radial pump designs for a given duty.

A Refer answer 3 b.

Q 4. c.
Explain why hydraulic pumps are usually provided with an odd number of
cylinders.

A Variable stroke gear pumps and Hele-Shaw pumps have an odd number of cylinders.
Calculation shows that this gives better hydrodynamic balancing and a better starting
torque when used in a pump driving a hydraulic motor.
 MEO Class IV - GEK
Pump
Variable Delivery Pump Exam Guide

Q 5. a.
Explain the possible causes of contamination in a hydraulic system.

A The possible sources are:

z Inadequate preparation or accidental contamination during construction of circuit pipe

runs such as welding

z Ingress of debris through the tank filler cap

z Leaving the system exposed after dismantling the system

z Self generated debris from action of pumping and valve mechanisms

Q 5. b.
Describe the likely results of contamination.

A Hydraulic system should be maintained clean at all times. Contamination should be

avoided by all means. If hydraulic oil is contaminated then it might result in one of the
following:

z Choking of control valve nozzles and orifices - It will result in erratic behaviour of the
system.

z Choking of filters - It will prevent the flow of hydraulic oil. No flow means no
pressure. Hence performance of the system drops.

z Chances of hydraulic oil loosing its desired properties - Due to contamination the
properties of oil such as viscosity gets affected. It will cause severe back pressure
on the hydraulic pumps and overload the electric motor. The system also becomes
sluggish.

z Damage to hydraulic machinery clearances - Contaminants might be abrasive, which

might damage the working clearances. If the clearances exceed the rated value, then
pressure build up will be insufficient, leading to drop in performance.
 MEO Class IV - GEK
Pump
Variable Delivery Pump Exam Guide

Q 5. c.
Explain how contamination can be monitored.

A By keeping a close watch on pressure drop across the filter.

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