For everybody interested in a general overview and basic understanding of thetechnological know-how of Bosch Rexroth, the book series "Compact knowledge"
was developed. In this series, "Hydraulics − Basic principles" offers an overview ofthe basic principles and components of hydraulic systems such as on/off valves,
hydraulic pumps, hydraulic motors and hydraulic cylinders. These topics areillustrated by means of graphics and circuit diagrams.
This book may be used as reference in addition to formal training at Bosch Rexroth
or for independent learning.

www.boschrexroth.com© Bosch Rexroth AG 2016
Material number: R961009449

Hydraulics – Basic principles

Bosch Rexroth is one of the worldwide leading specialists in drive and control
technology with unique technological know-how. This knowledge is passed on by
the Drive & Control Academy to support customized basic and advanced training
as well as qualification of technical personnel.

Compact knowledge

Bosch Rexroth AG
Drive & Control Academy
Bahnhofplatz 297070 Würzburg, Germany
Telephone: +49 9352 18-1920
E-mail: media@boschrexroth.dewww.boschrexroth.com/academy

Compact knowledgeHydraulics – Basic principles

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Table of contentsForeword
1 General information
1.1 Introduction to hydraulics

79
9

1.2 Comparison of drive technologies

12

1.3 Graphical symbols

15

2 Basic principles of physics

16

2.1 Basic principles of hydraulics
2.1.1 Hydrostatics – Pascal's Law2.1.2 Hydrodynamics – Bernoulli's Principle
2.1.3 Pipe flows
2.1.4 Cavitation

16
16
21
24
27

2.2 Basic principles of mechanics
2.2.1 Relation between hydraulic and mechanical quantities

29
29

3 Components

31

3.1 Pressure medium, displacement
3.1.1 Comparison of hydraulic fluids
3.1.2 Displacement principle and its application

31
31
35

3.2 Hydraulic pumps
3.2.1 External gear pumps
3.2.2 Internal gear pumps
3.2.3 Vane pumps
3.2.4 Axial piston pumps – swash plate design
3.2.5 Axial piston pumps – bent axis design
3.2.6 Radial piston pumps with internal support
3.2.7 Pump regulating systems
3.2.8 Comparison of hydraulic pumps

3838
414243475153573.3 Hydraulic valves
3.3.1 Overview of hydraulic valves
3.3.2 Directional control valves
3.3.3 Pressure relief valves
3.3.4 Pressure reducing valves
3.3.5 Pressure sequence valves
3.3.6 Pressure cut-off valves
3.3.7 Non-return valves
3.3.8 Pilot-operated non-return valves
3.3.9 Throttle valves
3.3.10 Flow control valves

595973
75
77
78
79
81
83
8486

6

3.4 Hydraulic cylinders
3.4.1 Basic principles of hydraulic cylinders3.4.2 Plunger cylinders3.4.3 Telescopic cylinders
3.4.4 Differential cylinders
3.4.5 Servo-cylinders
3.4.6 Double-acting cylinders

88889899100102104

3.5 Hydraulic motors
3.5.1 External gear motors
3.5.2 Axial piston motors – swash plate design
3.5.3 Axial piston motors – bent axis design
3.5.4 Radial piston motors with external support
3.5.5 Comparison of hydraulic motors

106
106
108
110
112
114

3.6 Connection technologies	
3.6.1 Comparison of connection technologies

116
116

3.7 Accessories
3.7.1 Hydraulic accumulators
3.7.2 Hydraulic filters
3.7.3 Coolers
3.7.4 Manometers

121
121
125
127
132

4 Basic circuits135

4.1 Circuit diagrams in hydraulics
4.1.1 Graphical illustration of hydraulic circuits

135
135

4.2 Valve control circuits4.2.1 Control with directional control valves
4.2.2 Velocity control with throttle valves
4.2.3 Load-independent velocity control with flow control valves
4.2.4 Rapid traverse circuits

138
138
144
152
159

4.3 Hydrostatic drives
4.3.1 Comparison of hydrostatic operating principles

163
163

4.4 Pressure and force-depending control systems
4.4.1 Pressure limitation in hydraulic systems
4.4.2 Control system with pressure switches
4.4.3 Control of actuators in parallel operation
4.4.4 Circuits with accumulators

167
167
171
172
174

5 Systems
5.1 Energy generators
5.1.1 Hydraulic power units
Alphabetical index

182
182
182
190

General information

91 General information
1.1 Introduction to hydraulics
Definition
Hydraulics is the science of stationary and flowing fluids. In technical practice, hydraulics isgenerally understood as the generation of forces and motion by hydraulic fluids. The term isderived from the ancient Greek words "hydor" (water) and "aulos" (pipe).

Historical development
As far back as the ancient world people were utilizing the energy of flowing water. For example, there is evidence to suggest that the first water wheels were built around 200 B.C. They
are still being used today in watermills and their technical advancement can still be seen in
the water turbines of power stations.
approx. 5000 B.C. People utilize the energy of flowing water.
approx. 200 B.C.

The first water wheels are developed.

as of approx. 1600 Utilization of water pressure as drive power
1653

French physician Pascal (1623-1662) illustrates the hydrostatic principle using thehydraulic press as an example.

1795

British engineer Joseph Bramah (1749-1814) produces a hydraulic press using
water as a hydraulic fluid for generating large forces. He is thus considered to be
the first to use hydraulics in industry.

1851

British industrialist William G. Armstrong (1810-1900) develops an accumulator("weight accumulator") with which large flows can be generated.

1905

Beginning of oil hydraulics: Williams and Janney use mineral oil as a transmission
medium for hydrostatic transmissions for the first time.

1922

Engineer Hans Thoma uses a radial piston pump operated with mineral oil in anindustrial application.

since 1950

Oil hydraulics becomes prevalent in all fields of industrial hydraulics.

Tab. 1: History of hydraulics

10

General information

Fields of application of hydraulics
Hydraulic systems are used in the fields of power transmission and in both open and closedloop control technology. In order to classify the diversity of possible fields of application,a distinction is generally made between industrial hydraulics (or stationary hydraulics) andmobile hydraulics.

Applications in industrial hydraulics
The term industrial hydraulics comprises applications in which hydraulic components areused in fixed installations. Application examples include machine tools, plastics processingmachines and presses in the rolling mill industry.
Plastic injection molding machine

Tower Bridge

Wind turbine

Continuous casting plant with rolling millTab. 2: Applications in industrial hydraulics

General information

Applications in mobile hydraulics
Mobile hydraulics comprises applications in which hydraulic components are installedin mobile machines. For example, here hydraulics is used in hydraulic excavators, wheelloaders, road rollers, snow groomers, tractors, lift trucks, or municipal vehicles.
Mobile excavator

Mobile crane

Wheel loaders

Tractor hitch control

Tab. 3: Applications in mobile hydraulics

1112

General information

1.2 Comparison of drive technologies
Drive technologies
Solutions from the technological fields of hydraulics, pneumatics, electrics and mechanics
come into play when designing drives:▶ In hydraulic drives, motion is generated by means of hydraulic fluid, which displaces
solid bodies. Hydraulic drives are used in many industrial fields as well as in mobilemachines such as excavators.
▶ In pneumatic drives, motion is generated by means of compressed air. Well-established applications include pneumatic assembly tools for screws and nuts in vehicle
garages and the transportation of lightweight goods on production lines.
▶ Electric drive technology works mostly with electric motors, which are being increasingly controlled electronically. Electric drives are used in high-precision machine tools
and printing presses.
▶ Mechanical drives can generate linear or rotatory motion or curves by means of either
crank drives, transmissions with a fixed transmission ratio, stepless transmissions or
cam gears.
Different requirements are placed on drive technology, depending on application and use.
Specific differences emerge between drive technologies with regard to energy supply,
energy distribution, properties of the transmission medium used and characteristics (typeof motion) of the drive itself.

Comparison of drive technologies
The following table contains a comparison of drive technologies according to their most important technical criteria.
Criterion

Hydraulics

Pneumatics

Electrics

Mechanics

Energy carrier

hydraulic fluidairelectrons

motion, position,
deformation

Energy transmission

pipes, hoses, bores

pipes, hoses, bores

electrically conductive material

shafts, rods, belts,
chains, wheels
etc.

Conversion
hydraulic pump,from / into me- hydraulic motor,
chanical energy hydraulic cylinder

compressor,
pneumatic cylinder,
pneumatic motor

generator, battery,solenoid, electricmotor—

Most important Pressure p,
characteristics flow qVPressure p,
flow qV

Voltage U,
electric current I

Force F,torque M,
velocity v,
speed nStorage

compressed air
reservoir

capacitor

weightbladder-type
accumulator, pistontype accumulator,
diaphragm-type
accumulator

General information

13

Criterion

Hydraulics

Pneumatics

Electrics

Mechanics

Power density

very good(high working pressure)good(limited by max.
working pressure)

not very good(power-weight ratioof electric motors
approx. 10 times
higher than that ofhydraulic motors)

good(energy conversion not required;limited where highdegree of controllability required)

Efficiency

dependent onleakage and friction
during energy conversion; losses during
valve control (openand closed loop)

dependent onleakage and friction
during energy conversion; losses during valve control(open and closedloop)

dependent on
dependent on
size of frictional
availability of electricity as primary en- losses
ergy source

Generation oflinear motion

very easy
very easy(by means of cylin(by means of cylinders; start-up and re- ders)versal of motion at fullload possible)

not very easy
(by means of linear
electric motor,threaded spindleetc.)easy(by means ofcrank drives,spindle etc.)

Generation ofrotational motion

easy
easy
(by means of hydraulic (by means of commotor)
pressed air motor)

very easy(by means rotaryelectric motor)

very easy(by means oftransmission)

Generation ofcurves

not very good

not very good

not very goodvery goodfor certain
applications(bendingtechnology)

Path accuracy

very good(fluid is hardycompressible)

very good
not very goodvaries:(by means of form
(air is compressible) not very good withand force fit)asynchronous motors
very good at synchronous and stepping
motorsControllability(open andclosed loop),
signal processing

very goodvery good(by means of valves)(by means of valves
and variable displacement pumps; use ofservo valves in control
technology; furtherimprovements by combining with electricsystems)

Tab. 4: Technical criteria of drive technologies

very good(by means of switches, relays, semiconductors, variable
speed motors, variable resistors etc.)

good(by means oftransmissions,lever systems etc.)

14

General information

Conclusion
The following conclusions can be drawn from the comparison of the various drive
technologies:▶ Hydraulic drives offer the advantage of generating high forces yet with a very compact
design. The disadvantage of this drive technology is the effort required to prevent
hydraulic fluid from leaking into the environment.
▶ Pneumatic drives can generally be used if fast motion and little force are required. Further advantages include comparatively low capital costs and good environmental compatibility. One disadvantage of this technology is the high level of noise development.
▶ Electric drives feature high control dynamics and flexibility. On the other hand, capitalcosts are comparatively high.
▶ Mechanical drives are straightforward to construct and are easily able to generate linear and rotational motion. However, one disadvantage is less effective controllability.
By combining various drive technologies, e.g. the combination of hydraulics and electricalengineering in variable-speed pump drives (Sytronix), the advantages of the respective
technology can be optimally utilized and the disadvantages compensated as required.

38Components

3.2 Hydraulic pumps
3.2.1 External gear pumps
Short description
External gear pumps are displacement machines in which the displacement
chambers are comprised of externally geared toothed wheels.
In the housing bores of external gear pumps, two gears engage, one of which
is driven (e.g. by electric motor). This driven gear drives the second gear in
the opposite direction. The hydraulic fluid is delivered in the toothclearances.

Characteristics
External gear pumps are fixed displacement pumps. They feature a compact design and highpower density. They offer good emergency operation properties so that any temporarily lackof lubrication, e.g. during start-up, does not cause any damage to the pump.
External gear pumps consist of few components, whereas the housing can be produced withlittle production expenses from one extruded aluminum profile.
Disadvantages include leakage and frictional losses, combined with considerable noise development, which can vary depending on the pump design.
External gear pumps can be used across a broad speed range and operated with hydraulicfluids within a large viscosity range. This results in numerous possible applications, particularly in mobile hydraulics (tractors and forklifts) and industrial applications.

Components

39

DesignThe figure illustrates the general design of an external gear pump in longitudinal andcross-section.
1234255S
698P6

7

1Flange (front cover)7

Bearing2

Housing

8

Sealing surface between bearing blocks and gears

3Bearing blocks

9

Shaft seal

4

Housing cover (end cover)

PPressure port

5

Drive shaft with drive gear

SSuction port

6

Driven gear

Fig. 9: Longitudinal and cross-section of an external gear pump

40

Components

Construction types and variants
External gear pump with helical gearing
The most efficient reduction of noise development of external gear pumps is achieved in"Bosch Rexroth SILENCE PLUS" helical gear pumps.
Due to the helical gearing of a round tooth profile, the delivering displacement chambers
overlap to ensure even flow with minimal flow pulsation. The round tooth profile guarantees
that both gear wheels mesh without clearance, thus ensuring that the hydraulic fluid is completely displaced from the tooth clearance. This ensures that, no hydraulic fluid, i.e. compressed oil, remains in the displacement chambers, which would otherwise lead to abrupt
unloading on transition from pressure to the suction area.
At low and medium pressures, the noise of the "SILENCE PLUS" external gear pump isdrowned by the operating noise of the driving electric motor and is barely audible.

Fig. 10: External gear pumps with helical gearing

Components

41

3.2.2 Internal gear pumps
Short description
Internal gear pumps are displacement machines in which the displacement
chambers are comprised of a pinion and an eccentrically mounted internalgear. Pressure and suction area are separated by a crescent shaped element.

Characteristics
Internal gear pumps are fixed displacement pumps. They are characterized by a broad speed
range and can be operated with hydraulic fluids within a large viscosity range. By means ofclearance compensation, internal gear pumps achieve higher efficiencies.
Due to their low noise level, internal gear pumps have many fields of application. In stationary hydraulics, they are used in such applications as machine tools, presses and plastics
processing machines. In mobile hydraulics, internal gear pumps are particularly used forvehicles working in closed rooms (e.g. electric forklifts).

DesignThe figure illustrates the general design of an internal gear pump in longitudinal andcross-section.
12P

3

5
4
6

S

1Housing

5Internal gear

2

Axial washers

6

Crescent shaped element

3Pinion shaft

P

Pressure port

4

Bearing

S

Suction port

Fig. 11: Longitudinal and cross-section of an internal gear pump

42Components

3.2.3 Vane pumps
Short description
Vane pumps are displacement machines in which the displacement chambers are formed between cam ring, rotor and vane.
The rotor of a vane pump comprises sliding metal plates, known as vanes.
The vanes are guided into slots which are installed radially to the drive shaft
in the rotor. On rotation of the rotor, these vanes are pressed onto the innersurface of the cam ring by centrifugal force. Thus, the vanes divide the space
between cam ring and rotor in the displacement chamber. Due to the eccentric position of
the cam ring, the displacement chambers, which in these pumps are also referred to as
cells, change their size during rotation of the rotor.

Characteristics
Vane pumps can be either fixed or variable displacement pumps. They are characterized by
low pulsation and low noise development. Due to their design, the displacement is easilyadjustable.
The properties of vane pumps make them very versatile. In industry, they are often used inmachine tools and packaging applications.

DesignThe figure illustrates the general design of a vane pump in longitudinal and cross-section.
P21L
34S

6

5

1Housing

6Lens plate2

Cam ring

L

Drain port

3

Vane, can be moved radiallyP

Pressure port

4

Rotor with drive shaft

SSuction port

5

Loading plateFig. 12: Longitudinal and cross-section of a vane pumpImprint
Compact knowledge

Hydraulics – Basic principles
Publisher:
Bosch Rexroth AG
Drive & Control Academy
Bahnhofplatz 297070 Würzburg, Germany

Reprinting, reproduction and translation including in extracts is subject to our priorapproval in writing and reference of sources.
We shall not assume any liability for compliance of the contents with applicable legal
regulations.
Subject to change.

2nd, revised edition (May 2016)
Material number: R961009449
ISBN: 978-3-9816219-0-7© All rights reserved by Bosch Rexroth AG, also in case of applications for industrial property protection.
We reserve all rights for disposition including copyrights and distribution rights.

For everybody interested in a general overview and basic understanding of thetechnological know-how of Bosch Rexroth, the book series "Compact knowledge"
was developed. In this series, "Hydraulics − Basic principles" offers an overview ofthe basic principles and components of hydraulic systems such as on/off valves,
hydraulic pumps, hydraulic motors and hydraulic cylinders. These topics areillustrated by means of graphics and circuit diagrams.
This book may be used as reference in addition to formal training at Bosch Rexroth
or for independent learning.

www.boschrexroth.com© Bosch Rexroth AG 2016
Material number: R961009449Hydraulics – Basic principles

Bosch Rexroth is one of the worldwide leading specialists in drive and control
technology with unique technological know-how. This knowledge is passed on by
the Drive & Control Academy to support customized basic and advanced training
as well as qualification of technical personnel.

Compact knowledge

Bosch Rexroth AG
Drive & Control Academy
Bahnhofplatz 297070 Würzburg, Germany
Telephone: +49 9352 18-1920
E-mail: media@boschrexroth.dewww.boschrexroth.com/academy

Compact knowledge

Hydraulics – Basic principles

5A4P

3min.

B0abTmax.

6

2

1