Chapter 2 Hydraulic Pumps and Motors
Objectives:
The purpose of this chapter is to describe:
1. Operation and performance of hydraulic pumps and
motors
2. Gear, vane, and piston pumps.
3. High-speed hydraulic motors and low-speed, high-torque
hydraulic motors.
Upon completing this chapter, you should be able to:
Explain the operation of gear, vane, and piston pumps.
Evaluate the performance of pumps and motors by
determining the volumetric, mechanical, and overall efficiencies.
Distinguish the different types of pumps and motors.
2.1 Introduction
Mechanic energy → Pump → Hydraulic energy
Hydraulic energy → Motor → Mechanic energy
The pump and motor is reversible in operation principle
but slightly different in structure
2.1.1 Basic principles of hydraulic pumps and motors
1 Classification
Hydraulic pumps are broadly classified as positive
displacement pumps or nonpositive displacement pumps.
Nonpositive displacement pumps are used in hydrodynamic
systems to transport fluids from one location to another.
Positive displacement pumps are used in hydrostatic systems
to overcome the pressure resulting from the mechanical loads
on the system as well as the resistance to flow due to friction.
Positive displacement pumps are further classified as gear
pumps, vane pumps and piston pumps according to the
construction
2.1 Introduction
2 Operation principle of the positive displacement pump
6
7
2
1
5
4
3
1.cam 2.piston 3.spring 4.chamber 5.check valve
Cam 1 rotates → piston 2 reciprocates.
Piston 2 is pushed to the right →chamber 4 expands →a partial
vacuum is generated →The atmospheric pressure pushes fluid
from the reservoir into the pump.
Piston 2 is moved to the left → chamber 4 reduces → to push the
fluid to flow into the system.
7
6
5
3
3
2
1
Figure 2.1 Operation of the positive displacement pump
Check valve 5 and check valve 6 are logically reversible, so it is
impossible for them to open at the same time.
They are called valve type oil-coordinating mechanism.
Besides valve type there are disk type and shaft type oilcoordinating mechanism
The volume during the discharge stroke is called the
“displacement volume” of the pump.
The volume during the suction stroke is called the “suction
volume” of the pump.
2.1 Introduction
7
6
5
3
3
2
1
Figure 2.1 Operation of the positive displacement pump
This kind of pump operates on the principle of volume
variation of pumping chamber, hence they are called positive
displacement pumps
7
6
5
3
3
2
1
Figure 2.1 Operation of the positive displacement pump
3 Structure characteristics of Positive displacement pumps:
(1) There are sealed variable volumes;
(2) volumes(pump chambers) can be cyclically increased
and decreased;
(3) High-pressure chamber and low-pressure chamber
must be separated each other
(4) There is a oil-coordinating mechanism.
2.1 Introduction
2.1.2 Performance parameters
1 Operating pressure and rated pressure
a. Operating pressure
The actual operating pressure of a hydraulic pump depends on
p  f (load )
its load
The maximum operating pressure depends on its pressure relief
valve.
b. Performance index
rated pressure and maximum pressure
2 Displacement and flow rate
Displacement (m3/r) is the fluid volume discharged (sucked)
by a hydraulic pump (motor) per revolution.
Flow rate (m3/s or L/min) is the fluid volume discharged
(sucked) by a hydraulic pump (motor) per unit time.
The actuator velocity depend on the oil-supplied flow rate. v=f(q)
2.1 Introduction
Theoretical flow rate qt:
qt  V  n  Vd  
Actual flow rate q:
q  qt  ql  qt v
3 Power, torque and efficiency
theoretical power
Pt  pqt  Tt
theoretical input (output) torque
Tt  p  Vd  p  V /( 2 )
volumetric efficiency
ql
q qt  ql
v  
 1
qt
qt
qt
2.1 Introduction
mechanical efficiency.
T
m 
Tt
overall efficiency
Po
   vm
Pi
to a pump
Pp  pq / 
Tp 
pV
2m
to a motor
Pm  pq
pV
Tp 
m
2
2.1 Introduction
100%
v
(q)
q
100%
m

v
m
q

p
rated point
p
p max
rated point
(a)
qt
v
q
ql
T
m Tt
1
Vd
p
¦ ¤pq
Vd
(b)
pq
¦ ØT
¦Ø
q  qt
v
ql
¦ ¤p
1
¦Ø
Vd
Tt  T
m
Vd
Tf
Tf
(c)
p max
(d)
¦ ØT
(q)
2.2 Gear Pumps
2.2 Gear Pumps
Advantages and disadvantages
Gear pumps are simple in structure, convenient in manufacture,
cheap in cost, compact in size, light in weight, reliable in operation,
and insensitive in contamination.
Gear pumps produce large
pulsation of flow and pressure,
large noise level, and are unable
to vary displacement.
2.2.1 Operation of the external
gear pump
1 the external view of the
external gear pump
outlet
Pump housing
Side wear plate
Driving shaft
2.2 Gear Pumps
2 main components
outlet
B
B
inlet
gears
pump hoursing
2.2 Gear Pumps
driving shaft
driven shaft
side wear plate
2.2 Gear Pumps
3 operation principle
displacement chamber
suction chamber
The gears come out of mesh at
the right suction chamber → the
volume of the sealed suction
chamber expands → a partial
vacuum is created →the oil is
pushed into the chamber by
atmospheric
pressure
→
furthermore transported to the left
chamber.
In the left discharge chamber, the
teeth go into mesh →the volume
decreases → the oil is forced out the
chamber into the hydraulic system.
2.2 Gear Pumps
2.2.2 Flow rate and pulsating rate of gear pumps
The displacement of an external gear pump
V  dhb  (6.66 ~ 7) zm 2b
The output flow rate
q  (6.66 ~ 7) zm2bnv
In practice, because the displacement is a cycle function of
rotating angle, the displacement pulsation and instantaneous
flow rate pulsation exist.
The flow rate pulsation may affect the system smoothness,
cause pressure pulsation, and produce vibration and noise.
Flow pulsation rate
q  qmin
  max
q0
Flow pulsation rate is one important index of positive
displacement pumps.
The less the number of teeth, the bigger the pulsation rate.
2.2 Gear Pumps
2.2.3 Structure character of gear pumps
2.2.3.1 Oil trapped phenomenon and eliminating method
1 Oil trapped cause
In order to make gear pumps operate smoothly, the overlap
coefficient must be more than one, this means more than one
pair of teeth meshing simultaneously. As a result, a quantity of
oil is trapped in the space between the two pairs of simultaneous
meshing teeth. no oil passage connects either inlet or outlet port.
The oil trapped volume changes as the gears move. At fist the
volume decreases gradually then increases gradually.
2.2 Gear Pumps
2
1
2
1
(¢ ñ
)
(¢ ò
)
0
3
3
compress
(¢ ó
)
(¢ ñ
)
expand
(¢ ò
)
(¢ ó
)
2.2 Gear Pumps
2 Oil trapped harm
When the volume decreases, compressing the trapped oil
forces the oil out of the clearance, leading to the instantaneous
high pressure and high oil temperature, and producing
additional shock load on the components such as bearings.
When the volume increases, a partial vacuum generates due to
no auxiliary oil entering, hence, the air dissolved in oil escapes,
and cavitations exist.
3 Eliminating method
The method of eliminating
oil trapped phenomenon is
to
machine
unloading
grooves on the two side
plates.
2.2 Gear Pumps
2.2.3 Leakage passage and automatic compensation of end face
clearance
1 Clearance seal concept:
In hydraulic pumps, the seal between moving components is
achieved by the micro-clearance.
The smaller the clearance is, the less the leakage is, but greater
the friction force is.
The optimum clearance is 0.02~0.05mm.
2 The leakage paths from the discharge chamber to the suction
chamber in gear pumps.
(1) through the contact line of gear mesh;
(2) through radial clearance(between housing and tooth top);
(3) through the end face clearances(between gears and side wear
plates).
Among the three kinds of clearance, the slippage through end
face clearance is maximum( about 75%)
2.2 Gear Pumps
3 Automatically compensating the end face clearance.
The floating bush bearings are used to automatically
compensate the end face clearances.
1
4
6
7
5
2
3
2.3 Vane Pumps
2.3 Vane Pumps
classification
single-acting (hydraulically unbalanced)
double-acting (hydraulically balanced)
advantages and disadvantages
Vane pumps can produce uniform flow, low pulsation, and
low noise level
Vane pumps are sensitive to contamination and slightly
complex in structure.
2.3.1 Single-acting vane pumps
2.3.1.1 Operation of a single-acting vane pump
2.3 Vane Pumps
1
1 Composition
1－rotor
2－stator
2
3－vane
3
4
4－pump hoursing
2.3 Vane Pumps
（1）rotor and vane
B
b
vane
rotor
b
B
2.3 Vane Pumps
（2）stator
2.3 Vane Pumps
（3）port plate
α β
2.3 Vane Pumps
（4）pump housing
2.3 Vane Pumps
（5）assembling relation, inlet port and outlet port
2
Oil entering：
1
3
4
Inlet port→axial
groove →stator
end face →
kidney shaped
port→suction
chamber
Stator center O
Rotor center O1
Eccentric
1－rotor distance
2－statore 3－vane 4－pump hoursing
Oil displacing：
Displacement
chamber→kidne
y-shaped port→
stator end face →
axial groove
→outlet port
2 Operation of a single-acting vane pump
When the pump is rotating , by the action of centrifugal force,
all vanes are in contact with the internal surface of the stator.
（1）Seal volumes： Vleft and Vright
displace
oil
suck
oil
2
2.3 Vane Pumps
（2）Variation of seal volumes：
Vleft decreases and Vright increases
压油
吸油
2
2.3 Vane Pumps
3 Oil trapped phenomenon and
eliminating method
α
β
A
2
B
In order to make
displacement and suction
chamber separated, the
sealing angle between the
intake port and discharge
port in the end plates
must be slightly bigger
than the angle between
two adjacent vanes.
When the rotor is in the
position shown in the
figure, volume A and B
are oil-trapped volumes.
The volumes vary as
the pump rotor rotates.
Consequently,oil trapped
phenomenon occurs.
2.3 Vane Pumps
Eliminating method : to machine triangle relief grooves
α β
2.3 Vane Pumps
4、Characteristics of single-acting vane pumps:
(1) Vanes extend by the action of centrifugal force.
(2) Vanes slope back in the rotational direction .
(3) Varying the eccentric distance may vary the displacement.
(4) Operation pressure is lower because of hydraulically
low pressure region
high pressure region
unbalanced force.
1
θ
2
2
2.3 Vane Pumps
2.3.2 Double-acting vane
pumps
1
2
3
4
R
r
Fig. 2.12 Operation of a double-acting vane pump
1-startor; 2-rotor; 3-vane; 5-pump hoursing
2.3 Vane Pumps
1 Characteristics of double-acting vane pumps
(1) The internal face consists of 4 arcs and 4 transition curves.
(2) Vanes extend by the action of both centrifugal force and
pressurized force.
(3) the vane slots are usually tilted forward a setting angle in
the rational direction. .
(4) Operation pressure is higher because of hydraulically
balanced force.
(5) A double-acting vane pump is not able to vary the
displacement
2.3 Vane Pumps
2 Port plate of a double-acting vane pump
卸荷槽
槽底通油孔
α β
Because the oil-trapped volume does not vary (arc),
theoretically oil trapping phenomenon does not exist. In view of
manufacture error, unloading grooves are machined on the port
plate.
2.3 Vane Pumps
(1) Structure characteristic
Because a motor must be able
3 Double-acting vane motors to reverse, vanes should be
arranged radially.




3
2
4
1
5
8
7


6
(2) Force moment
Because the hydraulic force
acting on vane 3 is greater than
that acting on vane 1, a force
moment to the rotating shaft is
developed.
(3) Starting problem
There are two methods to start a
vane motor: one is to arrange
springs in the bottom of slots, the
other is first to provide

 pressurized oil for the bottom of
slots then to provide pressurized
oil for the operating chamber.
2.4 Piston Pumps
classification
radial type

piston pumps 
swash plate type
axial type 

bent - axis type

advantages and disadvantages
The axial piston pump is high in operation pressure, high in
efficiency, and able to vary displacement.
The axial piston pump is complex in structure and high in
initial cost, additionally produce a little large pulsation of
flow and pressure.
2.4.1 Swash plate axial piston pumps
1 Operation principle of swash plate axial piston pumps
3
2
A
d
1
4
γ
D
5
A
6
7
Figure 2.18 Operation of the swash plate axial piston pump
1-swash plate; 2-piston; 3-cylinder block; 4-valve plate; 5-drive
shaft; 6-inlet port; 7-outlet port
The swash plate and valve plate are stationary.
The cylinder block 3 and pistons 2 rotate. While pistons reciprocate.
3
2
A
4
d
1
D
5
γ
6
7
A
A swash plate pump has a rotating cylinder containing pistons.
A spring pushes the piston against a stationary swash plate, which
sits at an angle to the cylinder.
The pistons suck in fluid during half a revolution and push fluid
out during the other half.
The valve plate contains two semi-circular ports.
http://www.animatedsoftware.com/pumpglos/swashpla.htm
2.4 Piston Pumps
3
2
A
4
d
1
D
5
γ
6
7
A
When a piston rotates from down to up and extends simultaneously,
the sealed volume in the cylinder bore increases, hence a partial
vacuum generates, and oil is drawn in through port 6 .
When a piston rotates from up to down and retracts simultaneously,
the sealed volume in the cylinder bore decreases, and oil is
discharged out through port 7 in the valve plate 4.
3
2
A
4
d
1
D
5
γ
6
7
A
The angleγ of the swash plate determines the stroke of the
pistons and therefore the amount of displacement of the pump.
By varying the angle of the swash plate, variable displacement
can be achieved.
Problems:
(1) The fatigue of springs
(2) The friction and wear between the swash plate and pistons
6
2 Structure characteristics of swash plate
axial piston pumps
15 14
13
12
11
10
9
17
18
19
20
21
8
22
1
2
3 4
5 6
7
2.4 Piston Pumps
（1）cylinder block
There are 7 bores, the central bore is used to connect drive shaft,
图 3-18
缸体
the right end face is in contact
with the
valve plate, and the external
cylindrical face is used to arrange the cylindrical bearing.
2.4 Piston Pumps
（2）piston and shoe
h
I
Ag f
R
1
2
′
relief groove
retracting ring
swash plate
The hollow structure of the piston is used to reduce weight and
inertia, the leakage is used for lubrication and hydrostatic
support, and the relief grooves are used to prevent the piston
from sticking。
2.4 Piston Pumps
（3）Swash plate
tail groove
shaft
shaft
The swash plate may swing around its shaft, a pin inside the tail
groove is used to fix the off angle of the swash plate, the hollow
structure of the swash plate is used to reduce weight and inertia,
and the leakage via central bore is used to lubricate the pin.
2.4 Piston Pumps
（4）Retracting ring
套滑靴
顶钢球
The retracting ring is used to pull out pistons by the acting of
the spring force ( via a steel ball ) .
2.4 Piston Pumps
(5)valve plate
Oil trapped cause：
In order to separate the suction
chamber
with
the
displacement
chamber, the distance a of transient
region must be larger than the port
diameter b of the cylindrical bore.
when the port of the cylindrical bore
is in the transient region, the operation
volume become the oil trapped volume,
furthermore, the oil trapped volume
changes as the pump rotates.
Consequently, oil trapped phenomenon
occurs.
Eliminating method：to machine
triangle relief groove or orifice.
n
m
2.4 Piston Pumps

0
3
1
2

A-A
orifice
A
orifice
a
A
1
2
3
2
2
2
2
0
4
1
2
0
2.4 Piston Pumps
（6）spring
Along axis center line, the spring which does
not fatigue is used to make the shoes be in
contact with the swash plate.
（7）pump housing
There are 4 ports, one is inlet port, another is
outlet port, the others are used for leakages.
It is possible for two leakage ports to cool or
preheat the pump.
2.4 Piston Pumps
3 Variable displacement mechanism
Manual
variable displacement mechanism
Manual servo variable displacement mechanism
Constant horsepower variable displacement mechanism
Overall horsepower variable displacement mechanism
Manual servo variable displacement mechanism
The mechanism consists of a manual servo valve and a
differential piston.
3
b
d
2
e
c
a
1
4
p
4
p
5
4
p
5
5