Design of hydraulic systems for lift truck

Ivan Gramatikov

Design of Hydraulic Systems
for Lift Trucks

Second Edition


Preface to the Second Edition
All information contained in the first edition has been retained. Some
corrections and additions have been made to better serve the purpose of
the book.

Design of Hydraulic Systems for Lift
Trucks
First Edition
Published by Technical University- Sofia, Sofia 1000, Bulgaria
ISBN: 978-954-438-730-3
Printed in Bulgaria

Second Edition
Copyright 2011 by Ivan Gramatikov
All rights reserved. No part of this book may be reproduced, stored in a retrieval system
or transmitted in any form, or by any means, electronic, mechanical, photocopying,
recording or otherwise, without the prior written permission of the author.
For permissions e-mail:

ISBN: 978-1-257-01500-9
Printed in the United States of America
Front cover photos: Courtesy of Balkancar Record ()

Design of Hydraulic Systems for Lift Trucks

i

CONTENTS
Chapter 1:

Introduction

1

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

Definitions for design and system design . . . . . . . . . . . . .

2

Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

Systems of units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

Symbols used in formulae and hydraulic diagrams . . . . . .

5

Chapter 2:

Properties and parameters of the fluids

11

Properties
Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11

Specific weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12

Specific gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13

Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13

Compressibility of fluids . . . . . . . . . . . . . . . . . . . . . .

16

Reynolds number and types of flow . . . . . . . . . . . . .

18

Parameters
Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

19

Flow and flow rate . . . . . . . . . . . . . . . . . . . . . . . . . .

20

Fluid velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23

Work and Power . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23

Drag and pressure loss . . . . . . . . . . . . . . . . . . . . . .

25

Hydraulic shock . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27


ii

Hydraulic Lock . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27

Obliteration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

28

Stiction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

29

Cavitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

29

The Bernoulli Equation . . . . . . . . . . . . . . . . . . . . . . . .

30

The Torricelli Equation . . . . . . . . . . . . . . . . . . . . . . . .

31

Chapter 3:

Hydraulic system components
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.

Flow Restrictors . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pressure Relief Valves . . . . . . . . . . . . . . . . . . . . . . .
Check Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reduction Valves . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pressure Compensated Flow Controls . . . . . . . . . . .
Directional Control Valves . . . . . . . . . . . . . . . . . . . . .
Hydraulic Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hydraulic Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hydraulic Cylinders . . . . . . . . . . . . . . . . . . . . . . . . . .

Pressure Sensors . . . . . . . . . . . . . . . . . . . . . . . . . .
Hydraulic Accumulators . . . . . . . . . . . . . . . . . . . . .
Hydraulic Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hydraulic Reservoirs . . . . . . . . . . . . . . . . . . . . . . . .
Hydraulic Lines, Fittings and Couplings . . . . . . . . . .
Manifold blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hydraulic Fluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fluid Cleanliness . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electric Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

33
34
36
37
39
40
42
48
59
60
64
66
70
77
83
88
90
95
98

Chapter 4:

Management and quality of hydraulic system
design process
101
Brief history of quality . . . . . . . . . . . . . . . . . . . . . . . . . .

101

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

103

Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

104


Design of Hydraulic Systems for Lift Trucks

iii

Structuring the design process . . . . . . . . . . . . . . . . . . . .

106

Definitions of tools used . . . . . . . . . . . . . . . . . . . . . . . . .

108

Description of the design process steps . . . . . . . . . . . . .

110

Design guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

116

Documenting the design activities . . . . . . . . . . . . . . . . . .

117

Project close-out criteria . . . . . . . . . . . . . . . . . . . . . . . . .

118

Failure and failure rate . . . . . . . . . . . . . . . . . . . . . . . . . . .

119

Patents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

120

Designing around an existing patent . . . . . . . . . . . . . . . .

122

Legal aspect of the design process . . . . . . . . . . . . . . . . .

123

Chapter 5:

Hydraulic systems for high lift trucks

125

Elevating system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

126

Hydraulic systems overview . . . . . . . . . . . . . . . . . . . . .

128

Design principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

129

Design requirements . . . . . . . . . . . . . . . . . . . . . . . . . . .

130

Hydraulic system with proportional manual directional valve 133
Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

146

Hydraulic system with electrically controlled proportional

valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

153

Hydraulic system with emergency lowering . . . . . . . . . .

158

Energy recovery systems . . . . . . . . . . . . . . . . . . . . . . . .

160

Hydraulic steering system . . . . . . . . . . . . . . . . . . . . . . .

165

Electro-hydraulic steering system . . . . . . . . . . . . . . . . . .

171

Integrated hydraulic system . . . . . . . . . . . . . . . . . . . . . .

174

Smoothness of the lifting . . . . . . . . . . . . . . . . . . . . . . . . .

176

Chapter 6:

Hydraulic systems for low lift trucks

181


iv

Hydraulic system with independent power steering
and lift circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

183

Integrated hydraulic systems for low lift trucks . . . . . . . .

185

Integrated hydraulic system with accumulator . . . . . . . .

189

Hydraulic system for pallet trucks with long fork attachments 194
Hydraulic power-assisted steering . . . . . . . . . . . . . . . . .

197

Integrated system with power-assisted steering . . . . . . .

199

Chapter 7:

Hydraulic systems for boom-type trucks 201
Hydraulic circuit for boom lift, extend and fork tilt . . . . . . .

202

Hydraulic lift & lower circuit for telescopic boom . . . . . . .

203

Hydraulic circuit with an automatic shut-off valve . . . . . .

207

High-speed extension of telescopic boom . . . . . . . . . . . .

208

Chapter 8:

Selected topics
I.

211

Servicing the hydraulic systems . . . . . . . .

211

Troubleshooting principles . . . . . . . . . . . . . . . . . . . . . . . .

System Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

212
212

Safety Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

213

Servicing the fluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

213

Servicing filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

216

Servicing reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

216

Servicing rotary pumps and motors . . . . . . . . . . . . . . . . . .

217

Servicing hydraulic cylinders . . . . . . . . . . . . . . . . . . . . . . . .

218

Servicing valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

219

Servicing connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

220

Seals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

221


Design of Hydraulic Systems for Lift Trucks

v

II.

Components layout- general considerations

222

III.

Common problems . . . . . . . . . . . . . . . . . . . . .

223

IV.

Contamination of the hydraulic fluid . . . . . .

225

V.

The future of the hydraulics . . . . . . . . . . . . .

229

Appendixes

231

Appendix A

ITA classification

Appendix B

Physical properties of common fluids

Appendix C

Viscosity Classification of Industrial Lubrication
Fluids

Appendix D

Coefficients of local resistance

Appendix E

Decision Matrix and QFD house

Appendix F

Hydraulic system calculation


vi


Design of Hydraulic Systems for Lift Trucks

1

Chapter 1

Introduction
Preface
The purpose of this book is to illustrate design principles and methods for
designing and calculating hydraulic systems for industrial lift trucks.
Determining the main parameters of these systems is based on principles
of hydraulics and mechanics. This book is to be used as a source of
information for mechanical engineers involved in designing, manufacturing
and servicing hydraulic systems for mobile lift trucks. This book can also be
used by engineering students in Industrial Truck Programs. To combine
these two purposes, there is an introductory chapter, “Properties and
Parameters of Hydraulic Fluid”, and a chapter on “Hydraulic Components”

describing the construction and the functions of components used in mobile
hydraulic systems. This book will also be beneficial for engineers working in
areas of design, fabrication and service of any other mobile off-highway
equipment.
In all universities, mechanical engineering students study the theoretical
foundations of fluid mechanics, fluid dynamics, and thermodynamics.
However few universities offer courses in hydraulics and pneumatics (also
called: fluid power), which are the applications of these disciplines. That is
why most design engineers learn the basics of the fluid power on the job.
Fluid power learning time can be reduced significantly if some basic
hydraulic principles are understood up front. This book will describe the
hydraulic principles and operation of the main hydraulic arrangements
which will give you the foundation for designing any system on your own.
It is more difficult to design hydraulic systems for smaller lift trucks. That is
because these systems must have the same performance as the bigger
trucks but they have to be put into a smaller space envelope. The smaller
design envelope is a major challenge to the design engineers. To meet this
and all other challenges through the design process, engineers have to
follow the principles of continuous improvement and design process quality.
Quality of the design process depends on the proper execution of each step


2

Chapter 1: Introduction

of the process. The proper execution requires knowledge in engineering
and management areas. The core necessary disciplines are: Mathematics,
Mechanics of the Fluids, Hydraulic Circuits and Components, Management
of Quality, Project Management, Design for Excellence and Professional

Communication. Some of these courses, in most of the engineering
programs, are not part of the engineering curriculum and therefore,
engineers must take extra courses in order to acquire the right set of
knowledge.
Chapter 4, “Management and Quality of the Design Process”, describes the
managerial aspect and the basic principles of the design process.

Definitions for design and system design
•
•
•

“The best design is the simplest one that works” Albert Einstein
Design is creative problem solving.
System design is finding the balance in system performance that
best satisfies the engineering requirements. This balance has to be
achieved first at the conceptual level and then maintained throughout
the whole design process.

Design of hydraulic systems is built on knowledge of several fundamental
principles. Most fluid power engineers have them as background
knowledge and do not even think about them. For people learning
hydraulics, knowing the fundamental principles is the first step to designing
energy and cost efficient systems. The milestones of the hydraulic
principles are:
• Knowledge of properties and parameters of the fluids
• Velocity-pressure relationship (Bernoulli equation)
• Knowledge of the hydraulic components
Fluid properties, fluid parameters and the Bernoulli equation are described
in Chapter 2. Chapter 3 describes the components used in the system.

Good system designs would also require knowledge of:
•
•
•

The engineering requirements (parameters) for the system
Factors affecting system functionality and system life
Constraints- cost, space, surrounding environment

When designing a system, the engineer must focus on four main aspects:


Design of Hydraulic Systems for Lift Trucks

3

First: maximizing the system efficiency and the system life.
In order to achieve this requirement, the design engineer has to select the
components of the hydraulic system so that they will work together in a way
leading to maximum system efficiency.
Second: design for manufacturability and assembly
Third: design for test and service
Fourth: design a cost effective system
These four aspects are described in chapters 4, 5, 6 and 7.
In addition to designing the hydraulic system, the system engineer has to
also consider how the system interacts with other systems (mechanical,
electrical, control), type of vehicle (ICE or electric) and the ergonomic
consequences of the design (the interaction of the system with the people).
A definition of “system engineering” is given by the International Council of
System Engineers (INCOSE)

Systems engineering is an interdisciplinary approach and means to enable
the realization of successful systems. It focuses on defining customer
needs and required functionality early on in the development cycle,
documenting requirements, and then proceeding with design synthesis and
system validation while considering the complete problem. System
engineering integrates all the disciplines and specialty groups into a team
effort forming a structured development process that proceeds from
concept to production to operation. System engineering considers both the
business and the technical needs of all customers with the goal of providing
a quality product that meets the user needs.

Regulations
In some countries, such as Canada, the engineering profession is selfregulated through provincial organizations. The governing body is
comprised of engineers chosen, through a voting process, by members of
the engineering organization.
In other countries, such as the USA, the state governments regulate the
licensing, the practices of the profession and approve the governing body of
the engineering organizations.


4

Chapter 1: Introduction

Professional organizations develop standards for minimum qualification,
professional ethics and practices. They are also involved in the mediation of
conflicts.

Calculations
Clarity and accuracy of the technical calculations are an important part of a

system design. All data, assumptions, mathematical and physical laws have
to be specified clearly. Calculations are an intellectual asset for a company.
Therefore any other engineer with the same background should be able to
understand and use them. This reduces the development time of future
projects and helps to bring new products to market in a shorter time. A
good practice is to put all calculations on a server in HTML or PDF format.
European countries (except the United Kingdom) use a comma as a
decimal marker. The UK, the USA and English speaking provinces of
Canada use a period as a decimal marker. In this book, since it is written in
English, I am going to use a period.

Systems of Units
International System (SI) of units
This system was adopted in 1960 at the Eleventh General Conference on
Weights and Measures as an international standard. SI is accepted by all
countries in Europe and most countries in the world. In the future, it is
expected to replace all other systems and to be used by all countries.
In this book we will primarily use SI units.
British Systems of Units
• British Gravitational (BG) System
In the past, the BG system was used in the English speaking countries. In
the BG system the unit of length is foot (ft), the unit of force is pound (lb),
the unit of mass is obscure (slug) and the unit of temperature is degree
Fahrenheit (°F).


Design of Hydraulic Systems for Lift Trucks

5

Fahrenheit (°F) = [Celsius (°C) x 9/5] + 32
Celsius (°C) = [Fahrenheit (°F) – 32] x 5/9
• English Engineering (EE) System
The units in the EE system are similar to the units in the BG system. The
unit of length is foot (ft), the unit of mass is pound mass (lbm), the unit of
force is pound force (lbf) and the absolute temperature scale is degree
Rankine (°R).
The equation used to convert slugs to pounds is:

slug =

lbm
gC

There are two gallons: British and US gallon
1 British gallon = 4.546 litters
1 US gallon = 3.785 litters

Symbols used in formulae and hydraulic diagrams
Latin alphabet
A

Area [m2]

D

Diameter [m]

dP

Pump displacement [cm3/rev]

dM

Hydraulic motor displacement [cm3/rev]

EV

Bulk Modulus of Elasticity (Bulk Modulus)

F

Force [N]

G

Gravity force [N]

GQ

Flow rate, weight [N/s]

h

Height, distance [m]

k

Ratio

L

Length or distance [m]

m

Mass (kg)


6

Chapter 1: Introduction

M

Mach number [-]

n

Rotational speed (frequency of rotation) [rev/min]

P

Power [Nm/s] and [W]

p

Pressure [N/ m2] and [Pa]

Q

Flow rate, volumetric [m3/s] and [L/min]

q

Flow rate, mass [kg/s]

RL

Lineal flow resistor

Re

Reynolds Number [-]

SG

Specific gravity [-]

t

Temperature [ºC]

T

Torque [Nm]

v

Velocity [m2/s]

V

Volume [m3] and [litter]

W

Work [Nm], [J]

Greek alphabet
α

Angle [rad], [º]

β

Angle [rad], [º]

γ

Specific weight [N/m3]

δ

Deviation

ε

Angular acceleration [rad/s2]

η

Efficiency

ϕ

Angle [rad], [º]

µ

Dynamic (absolute) viscosity [Pa.s]

ν
ν

ρ

Kinematic viscosity [m2/s], [St]
Specific volume (m3/kg)
Density [kg/m3]

ρ SG

Specific Gravity [-]

τ

Shear stress [N/m2] and [Pa]

ω

Angular velocity [rad/s]

θ

Angle [rad], [º]


Design of Hydraulic Systems for Lift Trucks

Hydraulic symbols
________

Work line (suction, pressure and return)

--------

Pilot line

Flexible line

Crossing lines, junction

Crossing lines, not connected

Plugged line

Venting

Reservoir, open

Reservoir, pressurized

Filter

Accumulator

7


8

Chapter 1: Introduction

Pressure gage

Thermometer

Flow meter

Foot operated

Hand operated

Spring operated

Electrical control

Electrical control, proportional

Pump, constant volume, one direction of flow


Design of Hydraulic Systems for Lift Trucks

9

Pump, variable volume

Pump, pressure compensated

Hydraulic motor, one direction of flow

Hydraulic motor, reversible flow

Pump- motor, reversible flow

Flow restrictor (orifice) fixed

Flow restrictor (orifice) variable

Flow control, pressure compensated, two-way

Flow control, pressure compensated, three-way


10

Chapter 1: Introduction

Pressure relief valve

Relief valve, proportional with indirect (pilot)
control

Pressure reduction valve

Check valve

Pressure switch

Steering valve, type Orbitrol

Torque generator


Design of Hydraulic Systems for Lift Trucks

11

Chapter 2

Properties and Parameters of the Fluids
Fluid in general is any existing liquid or gas. In lift truck hydraulic, brake
and steering systems, only liquids are used as working fluids.
The science of Mechanics of Fluids consists of Hydrostatics and
Hydrodynamics.
Hydrostatics is based on Pascal's law, which states that a confined liquid
that has a pressure placed on it will act with equal force on equal areas at
right angles to the area. In Hydrostatic drives, the power is transmitted on

the bases of applying pressure on the fluid or by the fluid’s potential
energy.
In Hydrodynamic drives, the power is transmitted by the kinetic energy of
the fluid.

Properties
Density
Density of the fluid is defined as its mass per unit volume containing the
mass.

ρ=

m ⎡ kg ⎤
V ⎢ m3 ⎥
⎣ ⎦

Where:

2.1

m

is mass of the fluid in a unit (kg)

V

is unit volume of the fluid (m3)

In SI system density has units of kg/m3). It is designated by the Greek
letter ρ (rho). In BG system density is expressed in slug/ft3 where the

mass is in slugs.


12

m=

Chapter 2: Properties and parameters of the fluids

WO
[ slug ] , WO is the weight in pounds at sea level
32.174

A common reference for fluids is the density of water at 4°C temperature:

⎡ kg ⎤

ρ H 2O = 1000 ⎢ 3 ⎥
⎣m ⎦
A common reference for non-liquids is the density of iron:

⎡ kg ⎤

⎡ t ⎤

ρ IRON = 7850 ⎢ 3 ⎥ or ρ IRON = 7.85 ⎢ 3 ⎥
⎣m ⎦
⎣m ⎦
Density can also be expressed as:

ρ=

1 ⎡ kg ⎤
v ⎢ m3 ⎥
⎣ ⎦

Where:

2.2

υ is specific volume (m3/kg)

Unlike gases, the density of the fluids depends little on pressure and
temperature. Densities of different fluids are given in Appendix B.

Specific Weight
Specific weight is a characteristic for bodies under the influence of the
gravitational field. The gravitational field is not a force (because it is
massless) but it produces a force when it interacts with mater. As a result,
mater receives a gravitational acceleration which does not depend on the
physical state of the mass.
Specific weight of fluid is equal to the product of fluid density (ρ) and
gravitational acceleration g = 9.806 m/s² (g = 32.174 ft/s²). It is defined as
fluid weight per unit volume containing it.


Design of Hydraulic Systems for Lift Trucks

13

⎡N⎤

γ = ρg ⎢ 3 ⎥
⎣m ⎦

2.3

Specific weight is designated by the Greek letter γ (gamma). In the SI
system it has units of N/m3 or kN/m3. In the BG system the units for
specific weight are lb/ft³.
The intensity of the gravitational field is stronger at sea level and
diminishes farther away from earth which means that the gravitational
acceleration changes. For engineering application the variation of the
gravitation (g) is neglected therefore, only the variation in the fluid density
causes variation in its specific weight. Specific weights of different fluids
are given in Appendix B.

Specific Gravity
Specific Gravity is the ratio of the density of the fluid to the density of the
water at the same temperature.

ρ SG =

ρ
2.4

ρ H 2O

Specific Gravity is a dimensionless parameter and it has the same values
in both SI and BG systems.

Viscosity
Viscosity of the fluid is a measure of resistance against friction between
fluid layers. It is related to the velocity gradient (
stress ( τ ) by the equation:

du

dy )

and the shear


14

Chapter 2: Properties and parameters of the fluids

τ =µ

du
dy

[Pa.s]

2.5

Where, the constant of proportionality, µ (mu), is called dynamic (or
absolute) viscosity of the fluid. Fluids, for which the velocity gradient is
linearly related to shearing stress, are called Newtonian fluids (all
common fluids). Graphically, the slope of shearing stress vs. velocity

gradient is equal to the viscosity. The value of the viscosity depends on
the fluid chemical content and temperature. In most fluid problems,
viscosity is combined with the density in the equation:

µ ⎡ m2 ⎤
ν=
ρ ⎢ s ⎥
⎣ ⎦

2.6

Where, the Greek letter ν (nu) is called kinematic viscosity. The
dimension of kinematic viscosity in SI units is m²/s.
The units Stocks (St) and Centistokes (cSt) are also used.

1St = 1cm 2 / s = 10 −4 m 2 / s

1cSt = 1mm 2 / s = 10 −6 m 2 / s

The values of ν for different fluids are given in Appendix B.
In the ISO classification system viscosity is related to ISO grade. There
are 18 viscosity grades covering a range from 2 to 1650 centistokes.
Viscosity of the ISO grades is measured at 40° C temperature. ISO
system for viscosity measurement was adopted by The American
Petroleum Institute and American Society for Testing and Materials
(ASTM). Today all petroleum companies and manufacturers use this
system as a standard for viscosity measurement. Prior to ISO adoption,
viscosity of the ASTM grades was measured at 100° F (37.8° C) in SUS
(Saybolt Universal Seconds) units.
SUS unit range

To convert to cSt units

from 32 to 99

cSt = 0.2253 x SUS - (194.4 / SUS)

from 100 to 240

cSt = 0.2193 x SUS - (134.6 / SUS)

more than 240

cSt = SUS / 4.635


Design of Hydraulic Systems for Lift Trucks

15

Because of the small temperature difference, ISO grades are a little more
viscous than the corresponding ASTM grades in SUS units. Viscosity
grade classification is given in Appendix C.
Another characteristic given by fluid manufacturers is the Viscosity Index
(V.I.). This index is a number that indicates changes of viscosity over
change of temperature. High V.I. means that there is little change in
viscosity with temperature change and vice versa. Fluid viscosity is a
main factor that determines the amount of friction between the fluid
layers, the boundary layers thickness along the inside walls and the
friction between metal surfaces of the hydraulic components. Viscosity

changes with the change of temperature, pressure and contamination.
When the pressure on the fluid increases, the shear stress increases
leading to viscosity increase. Also, when the fluid temperature increases
its viscosity decreases. The effect of temperature on kinematic viscosity
of some fluids is shown in Figure 2.1.

Fig. 2.1 Source: Webtec Products Ltd. ( />

16

Chapter 2: Properties and parameters of the fluids

Compressibility of fluids
Compressibility of a fluid is a measure of how easy a fluid volume can be
changed under pressure. Compressibility is characterized with the Bulk
Modulus of Elasticity (Bulk Modulus or Modulus of Elasticity) EV. Modulus
of Elasticity shows the resistance of the fluid to compression and is
defined as:

Ev = −

dp ⎡ N ⎤
dV / V ⎢ m 2 ⎥
⎣ ⎦

2.7

Where:
dp is differential change in pressure needed to create a differential
change in volume dV;

V is the initial volume of the fluid;
∆V/V is specific volume.
Because the specific volume is dimensionless, Modulus of Elasticity has
the same units as pressure. The negative sign shows that an increase in
pressure will cause a decrease in volume. In SI units Ev is given as N/m²
(Pa). In BG (English) units it is given as lb/in² (psi). Some values of Ev are
given in Appendix B.
In the case of using hydraulic oil, the value of ∆V/V is very small (large
Ev). For this reason, for the engineering applications we accept that fluids
are incompressible and disregard the compressibility factor. Large values
for the bulk modulus indicate that the fluid needs a great amount of
pressure to make a small change in the volume. In other words, the
bigger the number is the bigger resistance to compression the fluid has.
Modulus of Elasticity can alternatively be expressed as

Ev = −

dp
ρ / dρ

Where:
dρ is differential change in density of the fluid;
ρ is initial density of the fluid.

2.8


Design of Hydraulic Systems for Lift Trucks

17

For most engineering applications we consider the fluids as
incompressible. In doing so, we always have to keep in mind
compressibility factor when designing or redesigning a system. In any
hydraulic system, we have to look at not only rigidity of the fluid but also
rigidity of the whole system. Bulk Modulus of the fluid is one of the main
factors that determine the rigidity of the system. There are a number of
cases when compressibility must be considered.
•
•

•

Compressing and decompressing large fluid volumes in hydraulic
actuators such as piston cylinders.
Presence of air in the fluid. Presence of air decreases fluid Bulk
Modulus, which in turn increases compressibility of the whole
system. Contents of 1% insoluble air can reduce Ev with 40%.
Presence of air in the fluid usually is caused by improperly
designed reservoir, incorrect selection of hydraulic components or
damaged suction line.
Use of an accumulator in the system.

For lift truck hydraulic systems compressibility is considered a negative
characteristic because it reduces the rigidity of the system. Volume
reduction as a result of compressibility of hydraulic oil is approximately
1% for every 15 MPa (2000 psi) pressure. Fig. 2.2 shows the relationship
between Bulk Modulus E v and the temperature for two types of fluid.

Fig. 2.2