PNEUMATIC AND HYDRAULIC EQUIPMENT

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MODULE TITLE : APPLICATIONS OF PNEUMATICS AND

HYDRAULICS

TOPIC TITLE : PNEUMATIC AND HYDRAULIC EQUIPMENT

LESSON 5 : HYDRAULIC PUMPS

APH - 2 - 5

Published by Teesside University Open Learning (Engineering)

School of Science & Engineering

Teesside University

Tees Valley, UK

TS1 3BA

+44 (0)1642 342740

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INTRODUCTION ________________________________________________________________________________________

Usually, the most important and expensive item in a hydraulic system is the

pump. In this lesson we deal with the design features of commonly used

hydraulic pumps and their operating principles.

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YOUR AIMS ________________________________________________________________________________________

On completion of this lesson you should be able to:

• understand why power hydraulic systems use only positive

displacement pumps

• explain the operating principles of the most commonly-used

hydraulic pumps

• differentiate between the balanced and unbalanced designs of pumps

• understand the term 'volumetric efficiency' as applied to hydraulic

pumps

• describe the effects of cavitation and aeration on the operation of a

hydraulic pump.

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HYDRAULIC PUMPS ________________________________________________________________________________________

At the heart of every hydraulic system is the hydraulic pump; its function in

the system is to convert the mechanical energy, supplied by the prime mover,

into hydraulic energy.

The pump forces the oil to flow in the system. This flow causes a rise in the

pressure due to the resistance to flow within the system.

Nearly all of the pumps used in hydrostatic hydraulic systems are of the

positive displacement type. Displacement is the volume of liquid transferred

in one revolution. This type of pump will provide a definite amount of fluid

per cycle; its output, neglecting leakage losses, is independent of outlet

pressure. This makes it ideally suited for use in hydraulic power transmission

systems.

Other types of pump, such as centrifugal and axial flow pumps, are not used to

power hydraulic systems because their output is very sensitive to resistance to

flow. This would lead to erratic operation of the actuator; as a consequence

their use in power hydraulic systems is limited to cooling or circulating duties.

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POSITIVE DISPLACEMENT PUMPS ________________________________________________________________________________________

There are three main groups of positive displacement pumps used to power

hydraulic systems, they are:

• gear pumps (fixed displacement)

• vane pumps (fixed or variable displacement)

• piston pumps (fixed or variable displacement).

Gear pumps are probably the most commonly used pump for hydraulic power

as they are relatively cheap, robust and cause few problems. The external

gear design is illustrated in FIGURE 1 along with its symbolic representation

to BS 2917.

FIG. 1 External Gear Pump

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In this type of pump, a pair of close-tolerance gears mesh together with

minimum clearance. One gear is coupled to the system prime-mover and, as it

rotates, drives the other gear in the opposite direction. As the teeth unmesh at

the pump inlet a partial vacuum is created, which causes the fluid from the

reservoir to enter the pump. The fluid is then carried around the housing in the

spaces created between the teeth and the casing. When the fluid reaches the

pump outlet, the oil is forced out of the pump by the action of the teeth

reengaging. The high and low pressure sides of the pump are separated by the

close fit of the meshing teeth.

This type of pump is an unbalanced design, because of the pressure differential

which will build up between the inlet and outlet ports of the pump. As the

system pressure builds up at the outlet of the pump, high forces generated act

on the gears, forcing them against the housing. This imposes an increased load

on the bearings and results in increased friction and wear. It is very important,

therefore, that maximum operating pressures are not exceeded: higher

pressures could lead to a significant reduction in pump performance and useful

working life. This type of pump is not normally used for systems requiring

pressures greater than 200 bar.

Another design of gear pump is the internal gear pump, one type of which is

the crescent type, shown in FIGURE 2. This pump is also an unbalanced

design, but can be used for pressures up to 300 bar in certain cases. It can be

seen from the illustration that the inlet and outlet ports are larger than the

external gear pump: this reduces fluid flow velocity, making the pump quieter

in operation.

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FIG. 2 Internal Gear Pump

In the internal gear pump, the pinion is coupled to the prime mover and drives

the outer ring (external gear). A partial vacuum is created by the unmeshing of

the pinion and the gear ring. Atmospheric pressure acting on the fluid in the

reservoir pushes the oil into the pump. The fluid is carried in the cavities

formed by the unmeshing of the teeth either side of the crescent seal and

delivered to the pump outlet. The inlet and outlet are separated by the close

tolerances between the pinion, the gear ring and the housing.

Discharge

Internal ring-gear

Crescent

Suction

Drive gear

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With increasing use, the clearances between the outlet of the pump and its inlet will

increase. What effect do you think this will have on the pump's performance?

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Increased clearance will allow a greater amount of oil to leak back from the outlet side of

the pump to the low pressure inlet side. This will have the effect of reducing the pump's

effective output and its volumetric efficiency.

VOLUMETRIC EFFICIENCY

Theoretically, a pump displaces a fixed volume of fluid for each revolution,

equal to the volume of fluid in each pumping chamber multiplied by the

number of chambers. In practice this is not achieved, because internal leakage,

from the outlet to inlet of the pump, will always result in reduced output. The

volumetric efficiency of the pump is defined as the actual output of the pump

divided by its theoretical output.

As output pressure rises, volumetric efficiency decreases due to increased

internal leakage; therefore a pump's volumetric efficiency should always be

determined under load.

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VANE PUMPS

Vane pumps may be either fixed delivery or variable volume units. Their

principle of operation is very similar to the rotary vane compressor, dealt with

earlier in this unit, and is illustrated in FIGURE 3.

FIG. 3 Vane Pump

The slotted rotor is connected to the drive shaft and is rotated inside the cam

ring by the prime mover. Vanes are fitted into the rotor slots and follow the

inner surface of the cam ring as the rotor turns. Normally, a rotational speed of

Housing Cam ring

Vanes

Pumping chambers

Inlet Outlet

Rotor

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about 600 rpm is required to throw the vanes out against the ring, where they

are held in contact by centrifugal force and pump outlet pressure. Pumping

chambers are formed in the spaces by the vanes enclosed, rotor, cam ring and

side plates.

As the chambers are created at the pump inlet by the turning of the rotor, a

partial vacuum is created which allows fluid from the reservoir to enter the

pump. This fluid is carried around in these pumping chambers to the outlet,

where it is forced to leave the pump as these chambers reduce in volume due to

the eccentricity of the rotor in the cam ring.

Is this design of pump pressure-balanced or not?

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This design is not pressure balanced and therefore its maximum working pressure is limited.

Fixed capacity vane pumps are, however, normally of the pressure-balanced design as

illustrated in FIGURE 4 opposite.

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FIG. 4 Pressure Balanced Vane Pump

In this design the cam ring is elliptical, rather then circular, which permits two

pairs of ports diametrically opposed to each other. This means that the

pressure forces on the rotor cancel each other out, preventing side loading of

the drive shaft and bearings, giving longer pump life, and allowing the pump to

operate at higher pressures.

Vane pumps can also be manufactured to give a variable output. They work

on the same principle as the unbalanced design of vane pump with fixed

capacity, except that the eccentricity of the cam ring is adjustable, which gives

control over pump displacement.

Outlet

Inlet

Cam ring

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FIGURE 5 illustrates the principle of the variable-capacity vane pump. In this

design of pump it can be seen that, when unloaded, the cam ring is pushed over

to its maximum displacement position by the action of the pressure control

spring. As pressure in the system and pump outlet increases to a preset level, it

acts on the control piston to compress the control spring moving the cam ring

to reduce its eccentricity and so decrease the pump's output flow. When the

pressure in the system drops, the spring will move the cam ring to increase the

eccentricity, and the pump output will increase accordingly. This type of pump

will only supply sufficient flow into the system to maintain the system at the

required pressure. The preset system pressure can be altered by adjusting the

tension on the control spring.

FIG. 5 Variable Capacity Vane Pump

Centre line f i

Centre line f

Maximum volume

stop screw

Pivot point

Eccentricity

1. When pressure is great enough to overcome compensator spring force, the ring shifts to decrease eccentricity

2. Adjustment of compensator spring determines pressure at which ring shifts

Outlet

Inlet

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What advantage, if any, do you think is to be gained from using a variable capacity

pump of this type?

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The use of a variable-capacity pump with this type of control will save energy, due to its

ability to reduce its output to meet the demands of the system; this will also result in less

heat being generated.

Vane pumps cover the low to medium-high volume flow ranges with operating

pressures up to 200 bar. They are reliable, efficient and easy to maintain, and

are available in multiple pump units. Vane pumps have a low noise level and

will have a long trouble free life, provided the system fluid is kept in a suitable

condition with regard to cleanliness and lubricity.

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PISTON PUMPS

Piston pumps give high performance, deliver high pressure, and are capable of

operating at up to 650 bar with efficiencies of up to 95%. They operate on the

principle that a piston reciprocating in a bore will draw in fluid as it is retracted

and expel it on the forward stroke.

They are available in either fixed or variable displacement modes and three

design types:

• axial piston or swash plate pump

• bent axis pump

• radial piston pump.

Axial Piston Pumps with Swash Plate

FIGURE 6 shows the operating principle of the axial piston pump with a

swash plate. In this design the pump driveshaft rotates the cylinder block past

a stationary angled plate called a swash plate. This causes the pistons mounted

in the rotating cylinder block to reciprocate as they follow the inclination of

the plate. As the pistons are being withdrawn from their cylinders, the cylinder

block will be passing the inlet section of the stationary port plate, allowing

fluid to be drawn into the pumping chamber. As the piston is being pushed

back into the cylinder due to its rotation around the swash plate, fluid will be

discharged from the pump through the outlet section of the port plate.

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FIG. 6 Axial Piston Pump

The displacement of the pump per revolution is dependent upon the bore and

stroke length of the pistons and their number. In the fixed displacement design

the angle of the swash plate which controls the length of piston stroke is

rigidly fixed.

In the variable capacity design the output of the pump is controlled by altering

the swash plate angle, as shown in FIGURE 7. The maximum swash plate

angle results in the longest stroke and maximum displacement; zero swash

plate angle result in zero displacement.

Alteration of the swash plate angle can be achieved manually by using a screw,

or hydraulically using a cylinder and valve arrangement.

Swash plate (non-rotating)

Pistons

Pump body

Inlet and outlet ports

Valve plate slots

Valve plate (kidney port plate) view on AA'

Piston block (rotating)

Valve plate (non-rotating)

Shoe plate (rotating)

Piston stroke

Angle of swash plate

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FIG. 7 Variable Delivery Axial Piston Pump

What would be the effect upon the output of a variable capacity pump, if the swash

plate angle was reduced from its maximum output position through zero degrees to a

negative value?

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This would have the effect of reversing the output from the pump. The pump would now

draw fluid in through what was previously its discharge port and expel it through its

previous inlet port. The unit now effectively becomes a variable capacity, dual-direction

pump.

Stroke length No stroke

Maximum swash plate angle

(maximum displacement)

Decreased swash plate angle

(reduced displacement)

Zero swash plate angle

(zero displacement)

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Bent Axis Piston Pumps

In the bent axis design of piston pump, the cylinder block containing the

pistons is rotated at an offset angle to the driveshaft. The pistons are attached

to the driveshaft flange by ball joints and rotation of the cylinder block at an

offset angle causes the pistons to be reciprocated in their bores.

FIGURE 8 helps explain the principle of operation of a bent axis pump.

FIG. 8 Bent Axis Piston Pump

C B A

To inlet

To outlet

Piston stroke

Piston is withdrawing

from bore at inlet

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The rotating group consists of a drive shaft (A), a universal joint (B),

connecting rods (C), pistons (D) and a cylinder block (E). The non-rotating

valve plate (F) directs the fluid to and from the cylinder block during operation

of the pump.

The cylinder block (E) and drive shaft (A) rotate in unison with alignment

maintained by the universal linkage (B). The angle between the axis of the

cylinder block and the drive shaft, causes the pistons (D) to reciprocate during

rotation of the cylinder block.

During the first 180° of one revolution the piston is being withdrawn from the

cylinder and fluid is being drawn into the chamber created, via the inlet port of

the stationary valve plate.

During the second 180° of one revolution the piston is being forced back into

the cylinder and the fluid is discharged from the chamber and the pump via the

outlet port of the stationary port plate.

The stroke of the pistons is controlled by the angle of offset (α) between the cylinder block and the driveshaft: this is normally limited to a maximum of

30°. If the offset angle is reduced then this will reduce the stroke length of the

piston and reduce the pump flow output. See FIGURE 9 opposite.

The output from this pump can also be reversed if the cylinder block (E) is

moved across centre to a reverse angle.

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FIG. 9 Variable Delivery Bent Axis Pump

Why do you think it is necessary to have a hydraulic pump that is capable of pumping

in both directions?

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No piston stroke

No angle

No flow

Reduced angle

Reduced flow

Redu ced s

troke

Maximum angle

Maximum flow

Ma xim

um str

oke

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Most hydraulic systems are termed open-loop systems, because fluid is drawn from a

reservoir and, after a circuit of the system, is returned to the reservoir.

However, in certain systems, known as closed-loop systems, the fluid is fed from the pump

outlet into a hydraulic motor inlet; the exhaust from the motor is then directed back into the

inlet of the pump. Provision is made in the system to make up any shortage due to leakage

of fluid in the system and to cool and clean the fluid. If one direction of rotation of the

hydraulic motor is required, then a single-direction pump is all that is required; but if

reverse driving is required also, then a dual-direction pump is needed to drive the system.

FIGURE 10 shows a symbolic representation of the different types of pump

and the principle of a dual-direction closed-loop drive.

FIG. 10

Radial Piston Pumps

In a radial piston pump, as the name suggests, the pumping pistons are

arranged radially from the centre of the pump axis, instead of being aligned to

the drive shaft axis as in the axial piston pumps already discussed.

A typical design can be seen in FIGURE 11. The cylinder block rotates about

a central pintle valve which houses inlet and outlet ports. The piston's slipper

pads locate and follow a cam ring similar to that in an unbalanced vane pump.

Hydraulic closed loop system (simplified diagram)

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The outward stroke of the piston draws in fluid through the inlet of the pintle

valve; the inward stroke of the piston discharges fluid through the outlet.

FIG. 11 Radial Piston Pump

The pumping stroke of the pistons is controlled by the degree of eccentricity

between the rotor (cylinder block) and the cam ring. In the variable-

displacement design of this pump, the cam ring can be moved to change the

piston stroke and therefore pump output.

This type of pump is able to run at high speeds up to 5000 rev min–1, mainly

due to its good cylinder-filling characteristics and relatively low piston

velocity.

Inlet

Outlet

Pintle

Pistons

Reaction ring

Cylinder block

Centre line Cylinder block centre line

Case

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Volumetric efficiency may be up to 95%, and pressures up to 670 bar are

possible. Piston pumps are regarded as the ultimate for hydraulic systems,

because of their high performance. This is due to the high precision

engineering that goes into the manufacture of such a pump, with very close

clearances and finely machined surfaces. For this reason, system cleanliness

and good quality hydraulic fluid are essential to maintain the pump's high

operational efficiency and long service life.

PUMP INTERNAL LEAKAGE

In all hydraulic pumps, there will be some internal leakage of fluid from the

high pressure zones to the areas of lower pressure. This is due to the fact that,

to allow components to move in relation to each other, there must be clearance

between their mating surfaces. Fluid will therefore leak between the pump

components. A certain amount of leakage is also necessary to lubricate and

cool the pump components as they operate. However, as the pump becomes

worn due to use, the clearances and hence the amount of leakage increase,

which will reduce the effective output of the pump. It may then appear that a

hydraulic pump is operating perfectly in supplying fluid to a system; but the

amount of fluid being supplied may in fact only be 30% of that required by the

system. The machine may still function and generate the force or torque at the

hydraulic actuator, but the actuation speed will be greatly reduced.

Excessive internal leakage will also cause problems inside the pump. If the

internal leaking fluid is not removed, it will increase external leakage from the

pump. Gear and vane pumps are usually designed to remove this leakage

volume of oil through the components of the pump. In piston designs,

however, it must be removed externally, by a pipeline connecting the top of the

pump casing to the system reservoir. The size of this pipeline should be such

that it imposes very little back pressure in the pump casing.

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Most piston pump manufacturers will provide data about case drainage,

quoting allowable back pressure and typical leakage at the rated operating

pressure. If the leakage rate is monitored and results compared this

information can be used to ascertain the condition of the pump.

What effect do you think operating pressure, temperature and fluid viscosity will have

on internal pump leakage?

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An increase in operating pressure and temperature, and a reduction in fluid viscosity will all

result in increased internal leakage and reduced pump efficiency.

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CAVITATION AND AERATION

As well as pump performance being reduced due to wear of components, pump

components will also be damaged by the consequences of cavitation and

aeration of the fluid. Cavitation results from the pressure in the inlet port

being reduced to a level that causes the fluid to vaporize. The bubbles

produced are carried through the pump to the high pressure side where they

collapse or implode suddenly. The pressure of the implosion has the effect of

driving the fluid into the metal leading to surface erosion, and can cause pump

failure after a short period of time. The collapse of the bubbles is accompanied

by a characteristic high frequency 'pinking' sound.

To prevent cavitation it is therefore essential to limit the vacuum at the pump

inlet. Given below are some guidelines to help reduce cavitation:

• a reservoir raised above the pump inlet will provide a higher pressure

there

• keep the length of the inlet pipe short

• use a large diameter intake line to reduce fluid velocity and hence friction

pressure drop

• employ a fluid of the correct specification

• do not overspeed the pump

• keep inlet filters clean and correctly sized.

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Aeration is caused by air being dissolved in the fluid, and causes similar

problems to cavitation. Bubbles in the air/oil mixture give rise to pump noise

and erosion of the pump's internal surfaces.

When air reaches the high pressure side of the pump, it is compressed, but

rarely collapses, and is carried into the system, causing poor control of the

actuator, foaming of the fluid, and overheating.

The following steps should be taken to avoid aeration:

• maintain good joints on the intake line

• keep the oil level in the reservoir above the pump intake

• maintain a good shaft seal on the pump.

It is very important that any noise emanating from the pump be investigated:

failure to do so may result in rapid degradation of the pump's performance, and

may lead to catastrophic failure of the unit.

This is the end of the text relating to hydraulic pumps; now answer the Self-

Assessment Questions on the next page.

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SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. Why are centrifugal pumps not used to power hydraulic systems?

2. Which types of hydraulic pump are available in variable-displacement

models?

3. Explain the principle of the balanced design of vane pump.

4. How is the volumetric efficiency of a positive-displacement pump

determined?

5. Why is it necessary to supply external drains to some types of hydraulic

pump?

6. Explain the effect of cavitation in hydraulic pumps and list four possible

causes of this undesirable condition.

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NOTES ________________________________________________________________________________________

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ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. Centrifugal pumps are not used to power hydraulic systems because their

flow output is dramatically reduced as system resistance is increased.

This will result in erratic and unreliable movement of the hydraulic

actuator.

2. Hydraulic pumps that are available in variable displacement designs are

the:

• unbalanced vane pump

• axial piston swash plate pump

• bent axis piston pump

• radial piston pump.

3. The balanced design of vane pump has two outlet ports which are

positioned diametrically opposite to each other. The pump also has two

inlet ports arranged in the same manner. This has the effect of balancing

the pressure forces from one port against the force generated from the

port opposite. This will effectively cancel out any side loading on the

driveshaft and bearings.

4. The volumetric efficiency of a positive displacement pump is determined

by dividing the pump's actual displacement by its theor etical

displacement. As this volumetric efficiency will vary with operating

pressure, the measurement of actual displacement should always take

place when the pump is loaded.

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5. It is necessary to provide external drains to those pump types that cannot

remove their internal leakage by an internal drain. If this leakage volume

is not removed from the pump, it will cause the pump casing to be

pressurised and seals to be damaged.

6. Pump cavitation is a result of the inlet pressure of the pump reducing to

such a level that it causes the system fluid to vapourise. The resulting

bubbles are carried through the pump to the high-pressure zone where

they implode on metallic surfaces causing erosion.

To prevent cavitation the following precautions can be taken:

• provide a positive pressure at the inlet by using a raised reservoir

• reduce the length of the pump suction line to a minimum

• use a suction line of adequate diameter

• use a fluid of the correct density

• do not run the pump at excessive speed

• keep pump suction filters clean and correctly sized.

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SUMMARY ________________________________________________________________________________________

In this lesson on hydraulic pumps we have dealt with the different designs of

positive displacement pump that are most commonly used to power hydraulic

systems.

We have discussed the reasons for the use of positive displacement pumps to

power hydraulic systems, in preference to dynamic types, and dealt in detail

with the operating principles of gear, vane and piston pumps. We identified

those vane and gear pumps that are of balanced design and those that are not,

and discussed the advantages to be gained from balanced design pumps.

In the latter part of the lesson we dealt with variable-displacement pumps and

those that can be made with a dual direction of output.

Finally we looked at the effects of cavitation and aeration, and the means by

which they can be avoided in a hydraulic system.

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