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3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 1
Chapter three: Hydraulic system devices
31. Hydraulic pump
3.1.1. Introduction
A pump, which is the heart of a hydraulic system, converts mechanical energy in to hydraulic
energy. The mechanical energy is delivered to the pump via a prime mover such as an electric
motor. Due to mechanical action, the pump creates a partial vacuum at its inlet. This permits
atmospheric pressure to force the fluid through the inlet line into the pump. The pump then pushes
the fluid into the hydraulic system. There are two broad classifications of pumps as identified by
the fluid power industry.
1. Dynamic (non-positive displacement) pumps. This type is generally used for low-pressure,
high-volume flow applications. Because they are not capable of withstanding high pressures, they
are of little use in the fluid power field. Normally their maximum pressure capacity is limited to
250-300 psi. This type of pump is primarily used for transporting fluids from one location to
another. The two most common types of dynamic pumps are the centrifugal pump and the axial
flow propeller pumps.
2. Positive displacement pumps. This type is universally used for fluid power systems. As the
name implies a positive displacement pump ejects a fixed amount of fluid into the hydraulic system
per revolution of pump shaft rotation. Such a pump is capable overcoming the pressure resulting
from the mechanical loads on the system as well as the resistance to flow due to friction. These are
two features that are desired of fluid power pumps. These pumps have the following advantages
over non-positive displacement pumps:
a. High pressure capability (up to 12,000 psi)
b. Small, compact size
c. High volumetric efficiency throughout the design pressure range
d. Great flexibility of performance (can operate over a wide range of pressure requirements and
speed ranges)
There are three main types of positive displacement pumps: gear, vane and piston. Many variations
exist in the design of each of these main types of pumps. For example, vane and piston pumps can
be either fixed or variable displacement. A fixed displacement pump is one in which the amount of
fluid ejected per revolution (displacement) cannot be varied. In a variable displacement pump, the
displacement can be varied by changing the physical relationships of various pump elements. This
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 2
change in pump in displacement produces a change in pump flow output even though pump speed
remains constant.
It should be understood that pumps do not pump pressure. Instead they produce fluid flow. The
resistance to this flow, produced by the hydraulic system, is what determines the pressure. For
example, if the positive pump has its discharge line open to the atmosphere, there will be flow, but
there will be no discharge pressure above atmospheric because there is essentially no resistance to
flow. However, if the discharge line is blocked, then we have theoretically infinite resistance to
flow. Hence, there is no place for the fluid to go. The pressure will therefore rise until some
component breaks unless pressure relief is provided. This is the reason a pressure relief valve is
needed when a positive displacement pump is used. When the pressure reaches a set value, the relief
valve will open to allow flow back to the oil tank. Thus, a pressure relief valve determines the
maximum pressure level that the system will experience regardless of the magnitude of the load
resistance.
Some pumps are made with variable displacement, pressure compensation capability. Such pumps
are designed so that as system pressure builds up, they produce less flow. Finally at some
predetermined maximum pressure level, the flow output goes to zero due to zero displacement. This
prevents any additional pressure buildup. Pressure relief valves are not needed when pressure
compensated pumps are used. The hydraulic power developed by pumps is converted back into
mechanical power by hydraulic cylinders and motors, which produce the useful output. Figure 3.1.1
shows a backhoe loader which uses a variable displacement, pressure compensated, axial-piston
pump to provide optimum performance in both backhoe and loader operations. The backhoe portion
of the machine performs operations such as digging a trench. The front loader portion can then be
used to load a dump truck with the earth removed from the trench dug by the backhoe. The pump
delivers the desired flow to the hydraulic cylinders at the required pressure to fulfill implement
demands. At an operating of 2200 rpm, the pump produces a maximum flow of 43 gpm (163 Lpm)
at systems pressures of 3300 psi (22,700 kPa).
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 3
Figure 3.1.1 Performance of backhoe loader is enhanced with variable displacement, pressure
compensated, axial-piston pump. (Courtesy of Caterpillar Inc., Peoria, Illinois.)
3.1.2. Pumping theory
Pumps operate on the principle whereby a partial vacuum is created at the pump inlet due to the
internal operation of the pump. This allows atmospheric pressure to push the fluid out of the oil tank
(reservoir) and into the pump intake. The pump then mechanically pushes the fluid out the
discharge line. This type of operation can be visualized by referring to the simple piston pump of
figure 3.1.2. Note that this pump contains two ball check valves, which are described as follows:
• Check valve 1 is connected to the pump inlet line and allows fluid to enter the pump only at
this location.
• Check valve 2 is connected to the pump discharge line and allows fluid to leave the pump
only at this location.
As the piston is pulled to the left, a partial vacuum is generated in pump cavity 3, because the close
tolerance between the piston and cylinder (or the use of piston ring seals) prevents air inside cavity
4 from travelling in to cavity 3. This flow of air, if allowed to occur, would destroy the vacuum.
This vacuum holds the ball of check valve 2 against its seat (lower position) and allows atmospheric
pressure to push fluid from the reservoir into pump via check valve 1. This inlet flow occurs
because the force of the fluid pushes the ball of check valve 1 off its seat. When the piston is pushed
to the right, the fluid movement closes inlet valve 1 and opens outlet valve 2. The quantity of fluid,
displaced by the piston, is forcibly ejected out the discharge line leading to the hydraulic system.
The volume of oil displaced by the piston during the discharge stroke is called the displacement
volume of the pump.
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 4
Figure 3.1.2 Pumping action of a simple piston pump.
3.1.3. Pump classification
Dynamic pumps
The two most common types of dynamic pumps are the centrifugal (impeller) and axial (propeller)
pumps shown in figures 3.1.3. Although these pumps provide smooth continuous flow, their flow
output is reduced as circuit resistance is increased and thus are rarely used in fluid power systems.
In dynamic pumps there is a great deal of clearance between the rotating impeller or propeller and
the stationary housing. Thus as the resistance of the external system starts to increase, some of the
fluid slips back in to the clearance spaces, causing a reduction in the discharge flow rate. This
slippage is due to the fact that the fluid follows the path of least resistance. When the resistance of
the external system becomes infinitely large (for example, a valve is closed in the outlet line), the
pump will produce no flow. These pumps are typically used for low-pressure, high volume flow
applications. Also since there is a great deal of clearance between the rotating and stationary
elements, dynamic pumps are not self-priming unlike positive displacement pumps. This is because
the large clearance space does not permit a suction pressure to occur at the inlet port when the pump
is first turned on. Thus if the fluid is being pumped from a reservoir located below the pump,
priming is required. Priming is the prefilling of the pump housing and inlet pipe with fluid so that
the pump can initially draw in the fluid and pump it efficiently.
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 5
Figure 3.1.3. Dynamic (non-positive displacement) pumps. (Courtesy of Sperry Vickers, Sperry
Rand Corp., Troy, Michigan.)
Figure 3.1.4 (a) shows the construction features of centrifugal pump, the most commonly used type
of dynamic pump. The operation of a centrifugal pump is as follows. The fluid enters at the centre
of the impeller and picked up by the rotating impeller. As the fluid rotates with the impeller, the
centrifugal forces causes the fluid to move radially outward. This causes the fluid to flow through
the outlet discharge port of the housing. One of the interesting characteristics of a centrifugal pump
is its behavior when there is no demand for fluid. In such a case, there is no need for a pressure
relief valve to prevent pump damage. The tips of the impeller blades merely slosh through the fluid,
and the rotational speed maintains a fluid pressure corresponding to the centrifugal force
established. The fact that there is no positive internal seal against leakage is the reason that
centrifugal pump is not forced to produce flow against no demand. When the demand for the fluid
occurs (for example, the opening of the valve), the pressure delivers the fluid to the sources of the
demand. This is why centrifugal pumps are so desirable for pumping stations used for delivering
water to homes and factories. The demand for water may go to near zero during the evening and
reach a peak sometimes during the day time. The centrifugal pump can readily handle these large
changes in fluid demand.
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 6
Although dynamic pumps provide smooth continuous flow (when a demand exists), their output
flow rate is reduced as resistance to flow is increased. This is shown for centrifugal pumps in figure
3.4(b), where pumps pressure is plotted versus pump flow. The maximum pressure is called the
shutoff head because an external circuit valve is closed when shutoff the flow. As the external
resistance decreases due to the valve being opened, the flow increases at the expense of reduced
pressure.
Figure 3.1.4. Centrifugal pump. (a) Construction features. (b) Pressure versus flow curve
Positive displacement pumps
This type of pumps ejects a fixed quantity of fluid per revolution of the pump shaft. As a result,
pump output flow, neglecting changes in the small internal leakage, is constant and not dependent
on the system pressure. This makes them particularly well suited for fluid power systems.
However, positive displacement pumps must be protected against overpressure if the resistance to
flow becomes very large. This can happen if a value is completely closed and there is no physical
place for the fluid to go. The reason for this is that a positive displacement pumps continues to
ejects fluid (even though it has no place to go), causing an extremely rapid buildup in pressure as
the fluid is compressed. A pressure relief valve is used to protect pump against overpressure by
diverting pump flow back to the hydraulic tank, where the fluid is stored for system used.
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 7
Positive displacement pumps can be classified by the type of motion of internal elements. The
motion may be either rotary or reciprocating. Although these pumps come in a wide variety of
different designs, there are essentially three basic types:
1. Gear (fixed displacement only by geometrical necessity).
a. External gear pumps
b. Internal gear pumps
c. Lobe pumps
d. Screw pumps
2. Vane pumps
a. Unbalanced vane pumps (fixed or variable displacement)
b. Balanced vane pumps (fixed displacement only)
3. Piston pumps (fixed or variable displacement).
a. Axial design
b. Radial design
In addition, vane pumps can be of the balanced or unbalanced design. The unbalanced design can
have pressure compensation capability, which automatically protects the pump against
overpressure.
1. Gear pumps
External gear pump
Figure 3.1.5 illustrates the operation of an external gear pump, which develops flow by carrying
fluid between the teeth of two meshing gears. One of the gears is connected to a drive shaft
connected to the prime mover. The second gear is driven as it meshes with the driving gear. Oil
chambers are formed between the gear teeth, the pump housing and the sides wear plates. The
suction side is where teeth come out of mesh, and it is here that the volume expands, bringing about
a reduction in pressure below atmospheric pressure. Fluid is pushed into this void by the
atmospheric pressure because the oil supply tank is vented to the atmosphere. The discharge side is
where teeth go into mesh, and it is here that the volume decreases between mating teeth. Since the
pump has a positive internal seal against leakage the oil is positively ejected into the outlet port.
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 8
Figure 3.1.5. External gear pump operation. (Courtesy of Sperry Vickers, Sperry Rand Corp., Troy
Michigen.)
Volumetric displacement and theoretical flow rate
The following analysis permits us to evaluate the theoretical flow rate of a gear pump using
specified nomenclature:
=0D Out side diameter of gear teeth (in, m)
=iD Inside diameter of gear teeth (in, m)
=L Width of gear teeth (in, m)
=DV Displacement volume of pump (in3/rev, m3/rev)
=N Rpm of pump
=TQ Theoretical pump flow-rate
The volumetric displacement of a gear pump can be found by calculating the volume of a hollow
cylinder of outside diameter 0D and inside diameteriD , where the length of the cylinder is L .
There are actually two such cylinder volumes (because there are two gears) where oil could fill the
inside of the pump if there were no gear teeth. However one half of these two volumes is taken up
by the gear teeth of both gears. Thus the volumetric displacement can be represented by equation
(3.1).
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 9
( )LDDV iD22
04−= π
3.1.1
The theoretical flow-rate (in English units) is determined next:
( ) ( ) ( )min//min/ 33 revNrevinVinQ DT ∗=
Since ,2311 3ingal = we have
( ) ( ) ( )231
min//3 revNrevinVgpmQ D
T
∗= 3.1.2
Using metric units, we have
( ) ( ) ( )min//min/ 33 revNrevmVmQ DT ∗= 3.1.2M
Equations (3.1.2 and (3.1.2M) show the pump flow varies directly with speed as graphically
illustrated in figure 3.1.6 (a). Hence, the theoretical flow is constant at a given speed, as shown by
the solid line in figure 3.1.6 (b).
Figure 3.1.6. Positive displacement pumps Q versus N and P versus Q curves.
Volumetric efficiency
There must be a small clearance (about 0.001 in) between the teeth tip and pump housing. As a
result, some of the oil at discharge port can leak directly back toward the suction port. This means
that the actual flow-rate AQ is less than the theoretical flow rateTQ , which is based on volumetric
displacement and pump speed. This internal leakage, called pump slippage, is identified by the term
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 10
volumetric efficiency υη which equals about 90% for positive displacement pumps, operating at
design pressure:
T
A
Q
Q=υη 3.1.3
The higher the discharge pressure, the lower the volumetric efficiency because internal leakage
increases with pressure. This is shown by the dashed line in figure 3.1.6 (b). Pump manufacturers
usually specify volumetric efficiency at the pump rated pressure. The rated pressure of positive
displacement pump is that pressure below which no mechanical damage due to overpressure will
occur to the pump and the result will be a long reliable service life. Too high a pressure not only
produces excessive leakage but also can damage a pump by distorting the casing and overloading
the shaft bearings. This brings to mind once again the need for overpressure protection. High
pressures occur when a high resistance to flow is encountered such as actuator load or a closed
valve in the pump outlet line.
Figure 3.1.7 shows detailed features of an external gear pump. Also shown is the hydraulic symbol
used to represent fixed displacement pumps in hydraulic circuits. This external gear pump uses a
spur gear (teeth are parallel to the axis of the gear), which are noisy at relatively high speeds. To
reduce noise and provide smoother operation, helical gears (teeth inclined at a small angle to the
axis of the gear) are sometimes used. However, these helical gear pumps are limited to low-pressure
applications (below 200 psi) because they develop excessive end thrust due to the action of the
helical gears. Herringbone gears pumps eliminate this thrust action and thus can be used to develop
much higher pressure (up to 3000 psi). herringbone gears consist basically of two rows of helical
teeth cut into one gear. One of the rows of each gear is right-handed and the other is left-handed to
cancel out the axial thrust force. Herringbone gear pumps operate as smoothly as helical gear pumps
and provide greater flow rates with much less pulsating action.
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 11
Figure 3.1.7. Detailed features of an external gear pump. (Courtesy of Webster Electric Company,
Inc., subsidiary of STA-RITE Industries, Inc., Racine, Wisconsin)
Internal gear pump
Figure 3.1.8 illustrates the configuration and operation of the internal gear pump. The design
consists of an internal gear, a regular spur gear, a crescent-shaped seal and an external housing. As
power is applied to either gear, the motion of the gear draws fluid from the reservoir and forces it
around both sides of the crescent seal which acts a seal between the suction and discharges ports.
When the teeth mesh on the opposite to the crescent seal, the fluid is forced to enter the discharge
port of the pump. Figure 3.1.9 provides a cutaway view of an internal gear pump that contains its
own built-in safety relief value.
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 12
Figure 3.1.8. Operating of an internal gear pump. (Courtesy of Sperry Vickers, Sperry Rand Corp.,
Troy, Michigan.)
Figure 3.1.9. Cutaway view of an internal gear pump with built-in safety relief valve. (Courtesy of
Vicking Pump Division of Houdaile Industries Inc., Cedar Falls, Iowa.)
Lobe pump
Also in the general family of gear pumps is the lobe pump, which is illustrated in figure 3.1.10. this
pump operates in a fashion similar to the external gear pump. But unlike the external gear pump,
both lobes are driven externally so that they donot actually contact each other. Thus they are quieter
than other types of gear punps.
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 13
Figure 3.1.10. Operation of lobe pump. (Courtesy of Sperry Vickers, Sperry Rand Corp., Troy,
Michigan.)
Vane pumps
Basic design and operation
Figure illustrates the operation of a vane pump. The rotor, which contains radial slots, is splined to
the drive shaft and rotates inside a cam ring. Each slot contains a vane designed to mate with the
surface of the cam ring as the rotor turns. Centrifugal force keeps the vanes out against the surface
of the cam ring. During one-half revolution of rotor rotation, the volume increases between the
rotor and cam ring. The resulting volume expansion causes a reduction of pressure. This is the
suction process, which causes fluid to flow through the inlet port and fill the void. As the rotor
rotates through the second half revolution, the surface of the cam ring pushes the vanes back into
their slots and the trapped volume is reduced. This positively ejects the trapped fluid through the
discharge port.
Analysis of volumetric displacement
Careful observation of figure 3.1.11 will reveaal that there is an eccentricity between the centrline
of the rotor and the centerline of the cam ring. If the eccentricity is zero, there will be no flow. The
following analysis and nomenclature is applicable the vane pump:
=CD Diametere of cam ring (in, m)
=RD Diametere of Rotor (in, m)
=L Width of Rotor (in, m)
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 14
=DV Pump volumetric displacement (in3, m3)
=e Eccentricity (in, m)
=maxe Maximum possible eccentricity (in, m)
=maxDV Maximum possible volumetric displacement (in3, m3)
From geometry, we can find the maximum possible eccentricity:
2maxRC DD
e−
=
This maximum value of eccentricity produces a maximum volumetric displacement:
( )LDDV RCD22
max 4−= π
Noting that we have the difference between two squared terms yields
( )( )LDDDDV RCRCD −+=4max
π
Substituting the expression for maxe yields
( )( )LeDDV RCD maxmax 24
+= π
The actual volumetric displacement occurs when :max ee =
( )eLDDV RCD +=2max
π 3.1.4
Figure 3.1.11. Vane pump operation. (Courtesy of Sperry Vickers, Sperry Rand Corp., Troy,
Michigan.)
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 15
Piston pump
Fixed displacement piston pumps can be classified from the arrangement of the piston and drive
mechanism and include axial, radial, and in-line crankshaft driven pumps. Axial piston pumps are
further classified as in-line or bent axis drive piston pumps. Two in-line axial piston pump designs
are common: the swash plate design and the check valve design.
In-line axial piston pumps operate at pressures to 3,000 lbf/in2 with high volumetric and overall
efficiencies (above 96% and 85% respectively) because of the close fit between the reciprocating
pistons and the cylinder bores. While speeds in the 3,000-4,000 rpm range are nominal for industry,
some airborne applications are capable of speeds over 15,000 rpm. Component parts of the swash
plate design include the inlet and outlet ports, pump housing, valve plate, rotating group, shaft seal ,
bearings, and drive shaft (Fig. 3.1.12). The rotating group consists of the splined rotating cylinder
block and spherical washer, cylinder block spring and force transmitting pins, pistons supported by
shoes, swash plate, and retainer plate. Pumping action occurs as the rotating group causes the
pistons to reciprocate as they follow the angle of the stationary swash plate. As the pistons retract in
the cylinder bores, the rotating group passes over the kidney-shaped inlet port in the valve plate and
fluid is inducted into the cylinder bore. Further rotation causes the piston to extend into the cylinder
bore as the rotating group passes over the kidney-shaped outlet port in the valve plate and fluid is
expelled.
Figure 3.1.12 Fixed displacement swash plate piston pump (co urtesy of Sperry Vickers ).
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 16
Delivery from the pump (Q) is determined by the number of cylinders (n), the speed of the pump
(Nt), the bore (db) and the stroke (5). Stroke, in turn, is dependent on the angle of the swash plate.
For fixed displacement axial piston pumps, 30° is a common swash plate angle.
Fixed displacement bent axis pumps reciprocate the pistons in the rotating cylinder block through a
bevel gear mechanism. The axis is tilted near the junction of the drive shaft and cylinder block.
Displacement angles of 15°, 20°, 25°, 30°, and 40° are standard. These pumps operate at high
efficiencies and have received widespread application in the aerospace industry. Units range in sizes
from 2 to 45 lbf and flow rates of .25 to 150 gal /min at pressures to 5,000 lbf/in2. The speed range
is from 100 rpm to 12,000 rpm, depending on the size and application. Components include the
rotating group, valve plate, and antifriction bearing s. and aluminum case (Fig. 3.1.13).
Figure 3.1.13 Fixed displacement bent axis pump (courtesy of Volvo of America Corporation ).
Pump performance
Introduction
The performance of pump is primarily a function of the precision of its manufacture. Components
must be made to close tolerances, which must be maintained while the pump is operating under
design conditions. The maintenance of close tolerances is accomplished by designs that have
mechanical integrity and balanced pressures. Theoretically the ideal pump be one having zero
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 17
clearance between all matting parts. Although this is not feasible, working clearances should be as
small as possible while maintaining proper oil films for lubrication between rubbing parts.
Pump efficiencies
Pump manufacturers run tests to determine performance data for their various types of pumps. The
overall efficiency of a pump can be computed by comparing the hydraulic power out put of the
pump to the mechanical input power supplied by the prime mover. Overall efficiency can be broken
into two distinct components called volumetric efficiency and mechanical efficiency. These three
efficiencies are discussed below.
1. Volumetric efficiency (ηv). Volumetric efficiency indicates the amount of leakage that takes
place within the pump. This involves considerations such as manufacturing tolerances and
flexing of the pump casing under design pressure operating conditions:
T
Av Q
Q
produceshouldpumprateflowltheoretica
pumpbyproducedrateflowactual=
−−
=η 3.1.5
Volumetric efficiencies typically run from 80% to 90% for gear pumps, 82% to 92% for vane
pumps, and 90% to 98% for piston pumps. Note that when substituting efficiency values into
equations, decimal fraction values should be used instead of percentage values. For example, an
efficiency value of 90% would be represented by the value of 0.90.
2. Mechanical efficiency (ηm). Mechanical efficiency indicates the amount of energy losses that
occur for reason other than leakage. This indicates friction in bearings and between other
matting parts. It also includes energy losses due to fluid turbulence. Mechanical efficiencies
typically run from 90% to 95%.
pumptodeliveredpoweractual
leakagenogassupoweroutputpumpm
min=η
Using English units and horsepower for power yields
63000/
1714/
NT
pQ
A
Tm =η 3.1.6
In metric units, using watts for power,
NT
pQ
A
Tm =η 3.1.6M
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 18
The parameters of equations (5.8) and (5.8M) are defined as follows in conjunction with figures
3.31.
=p Pump discharge pressure (psi, Pa)
=TQ Pump theoretical flow-rate (gpm, m3/min)
=AT Actual torque delivered to the pump (in. lb, N.m)
=N Pump speed (rpm, rad/s)
A
Tm T
T
pumptodeliveredtorqueactual
pumpoperatetorequiredtorquelTheoretica==η 3.1.7
Note that the theoretical torque required to operate a pump (TT) is the torque that would be required
if there were no leakage. Equations for calculating the theoretical torque and the actual torque are
as follows:
Theoretical torque
( ) ( ) ( )π2
3 psipinVlbinT D
T
∗=∗ 3.1.8
Or
( ) ( ) ( )π2
3 PapmVmNT D
T
∗=∗ 3.1.8M
Actual torque
( )rpmN
pumptodeliveredhorsepowerActualTA
000,63∗=
3.1.9
( )sradN
pumptodeliveredActualTA /
= 3.1.9M
Where
( ) ( )rpmNsradN60
2/
π=
3. Overall efficiency (η0). The overall efficiency considers all energy losses and hence is
defined as follows:
pumptodeliveredpoweractual
pumpbydeliveredpowerlTheoreticaefficiencyoverall =)( 0η 3.1.10
The overall efficiency can also be represented mathematically as follows:
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 19
mv ηηη ∗=0 3.1.11
Substituting equations (3.7) and (3.8) into equation (3.13), we have (for English units):
000,63/
1714/0 NT
pQ
Q
Q
A
T
T
Amv ∗=∗= ηηη
Canceling like terms yields the desired result showing the equivalency of equation (3.1.10) and
equation (3.1.11).
pumptodeliveredhorsepoweractual
pumpbydeliveredhorsepoweractual
NT
pQ
A
A ==000,63/
1714/0η 3.1.12
Repeating this substitution for metric units using equations (3.7), (3.8M) and (3.13) yields:
pumptodeliveredpoweractual
pumpbydeliveredpoweractual
NT
pQ
A
A ==0η 3.1.12M
Note that the actual power delivered to a pump from a prime mover via a rotating shaft is called
brake power and actual power delivered by a pump to the fluid is called hydraulic power.
Pump selection
Pumps are selected using two basic criteria: (1) those relating to the actuator; and (2) those relating
to the operation and efficiency of the pump itself. The basic parameter relating to the actuator is
volume flow rate. Those relating to the operation and efficiency of the pump include system
pressure, drive speed, cost, reliability, maintenance, and noise. A system pressure should be selected
that will provide existing and future actuators with the force necessary to move the load resistance.
Higher system pressures allow for smaller system actuators and higher efficiencies, but are less
tolerant of contaminants in the system and other lower quality components. Working pressure
should also be compatible with the rest of the system should exchange of components be necessary.
Volume flow rates are selected to provide the necessary fluid and power to move specified load
resistances required distances within available time limits.
1714
QpPhp
∗= 3.1.13
That is, must be equal to or greater than the anticipated output of the system.
The next parameter to be defined is drive speed. Standard speeds of 1,200 and 1,800 rpm are
common. Higher speeds are associated with higher pump efficiencies, but noise, vibration, and
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 20
cavitation are usually associated with high suction lifts and air entering the system. Accepted
practice places the pump as close to the reservoir as possible and provides for cooling of the fluid
(adequate reservoir size and design) as well as filtration. If HWCF fluids are used, the reservoir is
placed above the pump inlet.
Another consideration is whether the pump will be of the non positive or positive displacement
type, and, if positive, whether the displacement will be fixed or variable. System pressure is used to
determine whether a non positive displacement pump should be used, whereas cost and circuitry are
used to determine if the pump should have a fixed or variable displacement. Fixed displacement
pumps require pressure relief and flow control valves to set the pressure and flow limits and prevent
component rupture. Variable displacement pumps, on the other hand, may be pressure-, flow-, or
load-compensated to limit system pressure and volume flow rate simultaneously.
Cost factors, system reliability, and required maintenance level are used to determine if the pump
should be of the gear, vane, or piston type, although gear and vane types are usually associated with
lower pressures. Some vane pumps have replaceable cartridges that allow rebuilding within minutes
without removing the pump from the circuit. Gear pumps are usually less expensive and more
tolerant of adverse conditions such as contaminants in the fluid, but have lower volumetric and
overall efficiencies than vane pump counterparts. Piston pumps offer maximum volumetric and
overall efficiency, as well as reliability, but cost is more than proportionally higher.
Selection of an appropriate pumping unit typically follows the sequence:
1. Determine the flow rat e.
2. Select an appropriate drive speed, direction and unit.
3. Circuit conditions determine whether the pump will be of the positive or non positive type, and if
positive, if the displacement will be fixed or variable. This will depend to a large extent upon the
circuit selected.
4. Determine system pressure.
5. Select an appropriate reservoir and plumbing, including the use of a heat exchanger, if necessary.
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 21
6. Compute cos t and other factors such as noise levels, vibration, wear characteristic s, fluid
filtration and periodic maintenance schedule necessary to assure the longest possible life of the
pump.
Pump Specification
The following list shows the basic specifications that should be available to specify the pump
precisely:
• Size (displacement)
• Speed (maximum and minimum speeds)
• Maximum operating pressure (continuous/intermittent)
• For open/closed circuit
• Direction of rotation (viewed to shaft end; clockwise [R], counterclockwise [L])
• Controller (for variable displacement pumps)
• Seals (oil)
• Drive shafts
• Port connections
• Mounting type
• External dimensions
• Installation position
• Operating temperature range
• Further details in clear text
Pump noise
Noise from pumps, measured on the decibel db(A) scale or sones scale, may be used over time to
evaluate performance and service conditions. The relationship between sound level s on the sones
and db (A) scale is seen in Table 3.1.1. Increases in noise usually indicate increased wear and
imminent failure, unless a pipe or tube can be found vibrating against a machine or mounting.
Sometimes the coupling is misaligned where the pump connects to the drive motor. While noise has
been traditionally associated with pump wear and failure, recent redefinition of the term as
unwanted or excessive sound recognizes the phenomenon as a form of environmental degradation.
3.1 Hydraulic pumps 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 22
Department of Labor standards for noise levels in work areas also must be considered when
specifying pumps, with or without enclosures, to meet occupational safety and health standards.
Pump vibration is influenced by both mechanical out of balance forces associated with the pump de
sign and by the surges of fluid which exit the outlet port in the case of positive displacement pumps.
The frequency of pump vibration for piston pumps is defined from the cycle rat e (N ) of the pump
and the number of pistons (n) such that
( ) ( ) ( )npistonofNoNrateCycleffrequencyPump .∗=
And
nNf ∗= 3.1.14
Pressure impulses or ripples in the system resulting from fluid surges from the pump have been
shown to be equal to the pump frequency for pumps with an even number of pistons and to twice
the pump frequency for pumps with an odd number of pistons.
Table 3.1.1 comparison of sones vs. DB (A) sound scales (Courtesy of Sperry Vickers).