design of hydraulic systems for lift trucks

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Ivan Gramatikov

Design of Hydraulic Systemsfor Lift Trucks

Second Edition


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

Design of Hydraulic Systems for LiftTrucks

First EditionPublished 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 systemor 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: [email protected]

ISBN: 978-1-257-01500-9

Printed in the United States of America

Front cover photos: Courtesy of Balkancar Record (http://www.balkancar-record.com)


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Design of Hydraulic Systems for Lift Trucks i

CONTENTS

Chapter 1:

Introduction 1Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 11Properties

Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Specific weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Specific gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Compressibility of fluids . . . . . . . . . . . . . . . . . . . . . . 16

Reynolds number and types of flow . . . . . . . . . . . . . 18

Parameters

Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Flow and flow rate . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Fluid velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Work and Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Drag and pressure loss . . . . . . . . . . . . . . . . . . . . . . 25

Hydraulic shock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27


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Hydraulic Lock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Obliteration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Stiction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Cavitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

The Bernoulli Equation . . . . . . . . . . . . . . . . . . . . . . . . 30The Torricelli Equation . . . . . . . . . . . . . . . . . . . . . . . . 31

Chapter 3:

Hydraulic system components 33 1. Flow Restrictors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342. Pressure Relief Valves . . . . . . . . . . . . . . . . . . . . . . . 363. Check Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374. Reduction Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . 395. Pressure Compensated Flow Controls . . . . . . . . . . . 406. Directional Control Valves . . . . . . . . . . . . . . . . . . . . . 427. Hydraulic Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488. Hydraulic Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 599. Hydraulic Cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . 6010. Pressure Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 6411. Hydraulic Accumulators . . . . . . . . . . . . . . . . . . . . . 66

12. Hydraulic Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7013. Hydraulic Reservoirs . . . . . . . . . . . . . . . . . . . . . . . . 7714. Hydraulic Lines, Fittings and Couplings . . . . . . . . . . 8315. Manifold blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8816. Hydraulic Fluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9017. Fluid Cleanliness . . . . . . . . . . . . . . . . . . . . . . . . . . . 9518. Electric Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

Chapter 4: Management and quality of hydraulic systemdesign process 101

Brief history of quality . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104


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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 194Hydraulic 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 . . . . . . . 202Hydraulic 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 211 I. Servicing the hydraulic systems . . . . . . . . 211

Troubleshooting principles . . . . . . . . . . . . . . . . . . . . . . . . 212System Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

Safety Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

Servicing the fluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

Servicing filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

Servicing reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

Servicing rotary pumps and motors . . . . . . . . . . . . . . . . . . 217Servicing hydraulic cylinders . . . . . . . . . . . . . . . . . . . . . . . . 218

Servicing valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

Servicing connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

Seals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221


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Design of Hydraulic Systems for Lift Trucks v

II. Components layout- general considerations 222

III. Common problems . . . . . . . . . . . . . . . . . . . . . 223

IV. Contamination of the hydraulic fluid . . . . . . 225V. The future of the hydraulics . . . . . . . . . . . . . 229

Appendixes 231Appendix A ITA classification

Appendix B Physical properties of common fluidsAppendix 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


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vi


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Design of Hydraulic Systems for Lift Trucks 1

Chapter 1

Introduction

Preface

The purpose of this book is to illustrate design principles and methods fordesigning and calculating hydraulic systems for industrial lift trucks.Determining the main parameters of these systems is based on principlesof hydraulics and mechanics. This book is to be used as a source ofinformation for mechanical engineers involved in designing, manufacturing

and servicing hydraulic systems for mobile lift trucks. This book can also beused by engineering students in Industrial Truck Programs. To combinethese two purposes, there is an introductory chapter, Properties andParameters of Hydraulic Fluid, and a chapter on Hydraulic Componentsdescribing the construction and the functions of components used in mobilehydraulic systems. This book will also be beneficial for engineers working inareas of design, fabrication and service of any other mobile off-highwayequipment.

In all universities, mechanical engineering students study the theoreticalfoundations of fluid mechanics, fluid dynamics, and thermodynamics.However few universities offer courses in hydraulics and pneumatics (alsocalled: fluid power), which are the applications of these disciplines. That iswhy most design engineers learn the basics of the fluid power on the job.Fluid power learning time can be reduced significantly if some basichydraulic principles are understood up front. This book will describe thehydraulic principles and operation of the main hydraulic arrangementswhich 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 isbecause these systems must have the same performance as the biggertrucks but they have to be put into a smaller space envelope. The smallerdesign envelope is a major challenge to the design engineers. To meet thisand all other challenges through the design process, engineers have tofollow the principles of continuous improvement and design process quality.Quality of the design process depends on the proper execution of each step


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2 Chapter 1: Introduction

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

Chapter 4, Management and Quality of the Design Process, describes themanagerial 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 beachieved first at the conceptual level and then maintained throughoutthe whole design process.

Design of hydraulic systems is built on knowledge of several fundamentalprinciples. Most fluid power engineers have them as backgroundknowledge and do not even think about them. For people learning

hydraulics, knowing the fundamental principles is the first step to designingenergy and cost efficient systems. The milestones of the hydraulicprinciples 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 describedin 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:


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First: maximizing the system efficiency and the system life.

In order to achieve this requirement, the design engineer has to select thecomponents of the hydraulic system so that they will work together in a wayleading 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 toalso 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 ofSystem Engineers (INCOSE)

Systems engineering is an interdisciplinary approach and means to enablethe realization of successful systems. It focuses on defining customerneeds and required functionality early on in the development cycle,documenting requirements, and then proceeding with design synthesis andsystem validation while considering the complete problem. System

engineering integrates all the disciplines and specialty groups into a teameffort forming a structured development process that proceeds fromconcept to production to operation. System engineering considers both thebusiness and the technical needs of all customers with the goal of providinga quality product that meets the user needs.

RegulationsIn some countries, such as Canada, the engineering profession is self-regulated through provincial organizations. The governing body iscomprised of engineers chosen, through a voting process, by members ofthe engineering organization.

In other countries, such as the USA, the state governments regulate thelicensing, the practices of the profession and approve the governing body ofthe engineering organizations.


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

CalculationsClarity and accuracy of the technical calculations are an important part of asystem design. All data, assumptions, mathematical and physical laws haveto be specified clearly. Calculations are an intellectual asset for a company.Therefore any other engineer with the same background should be able tounderstand and use them. This reduces the development time of futureprojects and helps to bring new products to market in a shorter time. Agood practice is to put all calculations on a server in HTML or PDF format.

European countries (except the United Kingdom) use a comma as adecimal marker. The UK, the USA and English speaking provinces ofCanada use a period as a decimal marker. In this book, since it is written inEnglish, 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 onWeights and Measures as an international standard. SI is accepted by allcountries in Europe and most countries in the world. In the future, it isexpected 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) SystemIn the past, the BG system was used in the English speaking countries. Inthe 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 degreeFahrenheit (F).


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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. Theunit of length is foot (ft), the unit of mass is pound mass (lbm), the unit offorce is pound force (lbf) and the absolute temperature scale is degreeRankine (R).

The equation used to convert slugs to pounds is:

C g

lbm slug =

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]

P d Pump displacement [ cm 3 /rev ]

M d Hydraulic motor displacement [ cm 3 /rev ]

V E Bulk Modulus of Elasticity (Bulk Modulus)F Force [N]

G Gravity force [N]G Q Flow rate, weight [N/s]

h Height, distance [m]

k Ratio

L Length or distance [m]

m Mass (kg)


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M Mach number [-]

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

P Power [Nm/s] and [W]

p Pressure [N/ m 2] and [Pa]

Q Flow rate, volumetric [m 3/s] and [L/min]q Flow rate, mass [kg/s]

RL Lineal flow resistor

Re Reynolds Number [-]

SG Specific gravity [-]

t Temperature [C]

T Torque [Nm]

v Velocity [m 2/s]V Volume [m 3] and [litter]

W Work [Nm], [J]

Greek alphabet

Angle [rad], []

Angle [rad], []

Specific weight [N/m 3] Deviation

Angular acceleration [rad/s 2]

Efficiency

Angle [rad], []

Dynamic (absolute) viscosity [Pa.s]

Kinematic viscosity [m 2/s], [St]

Specific volume (m3/kg)

Density [kg/m 3]

SG Specific Gravity [-]

Shear stress [N/m 2] and [Pa]

Angular velocity [rad/s]

Angle [rad], []


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Hydraulic symbols

________ Work line (suction, pressure and return)

- - - - - - - - Pilot line

Crossing lines, not connected

Reservoir, open

Reservoir, pressurized

Filter

Accumulator

Flexible line

Crossing lines, junction

Plugged line

Venting


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Pressure gage

Thermometer

Flow meter

Foot operated

Hand operated

Spring operated

Electrical control

Electrical control, proportional

Pump, constant volume, one direction of flow


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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


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Pressure relief valve

Check valve

Pressure switch

Relief valve, proportional with indirect (pilot)control

Steering valve, type Orbitrol

Torque generator

Pressure reduction valve


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Chapter 2

Properties and Parameters of the Fluids

Fluid in general is any existing liquid or gas. In lift truck hydraulic, brakeand steering systems, only liquids are used as working fluids.

The science of Mechanics of Fluids consists of Hydrostatics andHydrodynamics.

Hydrostatics is based on Pascal's law, which states that a confined liquidthat has a pressure placed on it will act with equal force on equal areas atright angles to the area. In Hydrostatic drives, the power is transmitted onthe bases of applying pressure on the fluid or by the fluids potentialenergy.

In Hydrodynamic drives, the power is transmitted by the kinetic energy ofthe fluid.

Properties

Density

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

=3m

kg V m

2.1

Where: m is mass of the fluid in a unit (kg)

V is unit volume of the fluid (m 3)

In SI system density has units of kg/m 3). It is designated by the Greekletter (rho). In BG system density is expressed in slug/ft 3 where themass is in slugs.


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12 Chapter 2: Properties and parameters of the fluids

][174.32

slug W

m O= , W O is the weight in pounds at sea level

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

=32 1000 m

kg O H

A common reference for non-liquids is the density of iron:

=37850 m

kg IRON or

=385.7 m

t IRON

Density can also be expressed as:

=3

1mkg

v 2.2

Where: is specific volume (m 3/kg)

Unlike gases, the density of the fluids depends little on pressure andtemperature. 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 ismassless) but it produces a force when it interacts with mater. As a result,mater receives a gravitational acceleration which does not depend on thephysical state of the mass.

Specific weight of fluid is equal to the product of fluid density ( ) andgravitational acceleration g = 9.806 m/s ( g = 32.174 ft/s). It is defined asfluid weight per unit volume containing it.


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=3m

N g 2.3

Specific weight is designated by the Greek letter (gamma). In the SIsystem it has units of N/m 3 or kN/m 3. In the BG system the units forspecific weight are lb/ft .

The intensity of the gravitational field is stronger at sea level anddiminishes farther away from earth which means that the gravitationalacceleration changes. For engineering application the variation of thegravitation ( g ) is neglected therefore, only the variation in the fluid densitycauses variation in its specific weight. Specific weights of different fluidsare given in Appendix B.

Specific Gravity

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

O H

SG

2

= 2.4

Specific Gravity is a dimensionless parameter and it has the same valuesin 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 ( dydu ) and the shear

stress ( ) by the equation:


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dydu

= [Pa.s] 2.5

Where, the constant of proportionality, (mu), is called dynamic (orabsolute) viscosity of the fluid . Fluids, for which the velocity gradient islinearly related to shearing stress, are called Newtonian fluids (allcommon fluids). Graphically, the slope of shearing stress vs. velocitygradient is equal to the viscosity. The value of the viscosity depends onthe fluid chemical content and temperature. In most fluid problems,viscosity is combined with the density in the equation:

=

sm2

2.6

Where, the Greek letter (nu) is called kinematic viscosity . Thedimension of kinematic viscosity in SI units is m/s .

The units Stocks (St) and Centistokes (cSt) are also used.

sm scmSt /10/11 242 ==

sm smmcSt /10/11 262 == The values of for different fluids are given in Appendix B.

In the ISO classification system viscosity is related to ISO grade. Thereare 18 viscosity grades covering a range from 2 to 1650 centistokes.Viscosity of the ISO grades is measured at 40 C temperature. ISOsystem for viscosity measurement was adopted by The AmericanPetroleum Institute and American Society for Testing and Materials(ASTM). Today all petroleum companies and manufacturers use thissystem 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


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Because of the small temperature difference, ISO grades are a little moreviscous than the corresponding ASTM grades in SUS units. Viscositygrade 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 overchange of temperature. High V.I. means that there is little change inviscosity with temperature change and vice versa. Fluid viscosity is amain factor that determines the amount of friction between the fluidlayers, the boundary layers thickness along the inside walls and thefriction between metal surfaces of the hydraulic components. Viscositychanges with the change of temperature, pressure and contamination.When the pressure on the fluid increases, the shear stress increasesleading to viscosity increase. Also, when the fluid temperature increases

its viscosity decreases. The effect of temperature on kinematic viscosityof some fluids is shown in Figure 2.1.

Fig. 2.1 Source: Webtec Products Ltd. (http://www.webtec.co.uk/)


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Compressibility of fluids

Compressibility of a fluid is a measure of how easy a fluid volume can bechanged under pressure. Compressibility is characterized with the BulkModulus of Elasticity (Bulk Modulus or Modulus of Elasticity) E V . Modulusof Elasticity shows the resistance of the fluid to compression and isdefined as:

=2/ m

N V dV

dp E v 2.7

Where:

dp is differential change in pressure needed to create a differentialchange 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 hasthe same units as pressure. The negative sign shows that an increase inpressure will cause a decrease in volume. In SI units E v is given as N/m (Pa ). In BG (English) units it is given as lb/in (psi). Some values of E v aregiven in Appendix B.

In the case of using hydraulic oil, the value of V/V is very small (large E v ). For this reason, for the engineering applications we accept that fluidsare incompressible and disregard the compressibility factor. Large valuesfor the bulk modulus indicate that the fluid needs a great amount ofpressure to make a small change in the volume. In other words, thebigger the number is the bigger resistance to compression the fluid has.

Modulus of Elasticity can alternatively be expressed as

d dp

E v /= 2.8

Where:

d is differential change in density of the fluid;

is initial density of the fluid.


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For most engineering applications we consider the fluids asincompressible. In doing so, we always have to keep in mindcompressibility factor when designing or redesigning a system. In anyhydraulic system, we have to look at not only rigidity of the fluid but alsorigidity 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 ofcases 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 wholesystem. Contents of 1% insoluble air can reduce E v with 40%.Presence of air in the fluid usually is caused by improperlydesigned reservoir, incorrect selection of hydraulic components ordamaged suction line.

Use of an accumulator in the system.

For lift truck hydraulic systems compressibility is considered a negativecharacteristic because it reduces the rigidity of the system. Volumereduction as a result of compressibility of hydraulic oil is approximately1% for every 15 MPa (2000 psi) pressure. Fig. 2.2 shows the relationshipbetween Bulk Modulus v E and the temperature for two types of fluid.

Fig. 2.2


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Reynolds Number and Types of Flow

Fluid flow can be laminar, turbulent or a mixture of both. The factor thatdetermines which type of flow is present is the ratio of inertia forces ( v s )to viscous forces ( /L) within the fluid. This ratio is expressed by the non-dimensional Reynolds Number:

VL=Re 2.9

Where:

V is velocity characteristic

L is lineal characteristic

is the dynamic (absolute) fluid viscosity is fluid density

When the flow is in a pipe with a circular cross-section, the linealcharacteristic L is equal to the pipe diameter D. Then the equation can bewritten as:

VD

=Re 2.10

We can also express the equation with the kinematic viscosity =

VD=Re 2.11

This number is named after Osborne Reynolds (1842-1912), whoproposed it in 1883.Laminar flow is characterized with smooth flow and parallel layers. Itoccurs when the viscous forces are dominant (low Re number). Turbulentflow is characterized with turbulent behavior and whirlpools in flow and itoccurs when the inertial forces are dominant (high Re number). ForReynolds Numbers up to 2000, the flow is laminar. Above ReynoldsNumber of 4000, the flow is completely turbulent. Between Re 2000 and


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4000, the flow is transitional (between laminar and turbulent) and it haselements of both flow types. For flows within circular pipes the criticalReynolds number is generally accepted to be 2320.

Parameters

Pressure

Pressure is the normal force per unit area at a given point within the fluid.For most engineering problems we assume that the fluid moves as a rigid

body (dealing with fluid at rest) therefore there is no shearing stress in it.So, the only forces acting on the fluid are pressure and weight. Thisallows us to obtain relatively simple solutions to most engineeringproblems.

Pressure distribution (for incompressible fluids) is called hydrostaticdistribution .

21 ph p += 2.12

Where:

h = z 1 z 2 is the vertical distance from a point with pressure p 1 to a pointwith pressure p 2 . This distance is called pressure head and it isinterpreted as the height of a column of fluid of specific weight requiredto give a pressure difference ( p 1 - p 2 ). If we have a surface exposed tothe atmospheric pressure it is convenient to use a point on this surface asreference point 2. Thus, we let: p 2 = p 0 .

In SI, unit pressure is expressed as Pa (Pascal), where: 1 Pa =1N/m . Insome cases we use the unit bar (1 bar = 0.1 MPa ).

In BG, units are lb/ft or lb/in ( psi ). The relationship between the metricand the English systems is: 1 bar = 14.5 psi


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In mobile truck hydraulic systems, positive displacement rotary pumpsare used to create pressure. A disadvantage of using these type pumps isthat they create pressure and flow pulsations in the discharge port.Pressure variation in a gear pump outlet is explained in Chapter 3,Hydraulic Pumps.

Pressure measurement

Pressure at a certain point measured relative to the local atmosphericpressure is called gage pressure . Absolute pressure, on the other hand,is measured relative to the perfect vacuum (absolute zero). Absolutepressure is always positive while the gage pressure can be either positiveor negative. A negative gage pressure is also referred as a vacuum .

Hydraulic systems used in the industrial trucks are classified according tothe maximum pressure they are designed for: Low pressure system- up to 5 MPa (< 50 bar) Medium pressure system- from 5 to 15 MPa (50 150 bar) Normal high pressure system- from 15 to 25 MPa (150 250 bar) High pressure system- from 25 to 40 MPa (250 400 bar)

Flow and flow rate

Flow is the motion of the fluid molecules from one point to another. Sincethe observation of all molecules is almost impossible, we are describingthe flow as motion of part of the fluid, called small volume (or unitvolume). Small volume contains numerous molecules. Flow is createdwhen a new fluid is pushed into a fluid conductor (pre-filled pipe or hose).The molecules of the new volume push against fluid molecules already inthe conductor and displace them. Displaced molecules move by pushingtheir neighbours and so on. So, the ejected fluid volume from theconductor at the opposite end will be the same as the one entered. Themovement of fluid molecules causes a pressure wave traveling at thespeed of sound (about 1400 m/s). The speed of sound in fluids is:

Ev

c = 2.13

Where:

E v is the modulus of elasticity (Pa)


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is the density (kg/m 3)

For example, the speed of sound in hydraulic fluid (viscosity grade 32) is:

c = (1.7x 10 9 / 870) 1/2 = 1398 (m/s)

The density values are given in Appendix B.

When calculating the parameters of the hydraulic hydrostatic systems weassume that the velocity, v , at a given point in space does not vary withtime dv/dt = 0 . Such flow is called: steady flow . In a system with a steadyflow, rapid closure or opening of a hydraulic component can causeunsteady effects, which have to be considered when a hydraulic systemis designed. For example the water hammer affect, which results in loudbanging of the pipes or tubers.

There are three types of flow rate: Volumetric flow rate, Q

Volumetric flow rate is the unit volume flow per unit time passing throughan observation cross section.

==

s

m

t

V

timeUnit

volumeUnit Q

3

_

_

2.14

In SI units flow rate can be expressed either in cubic meters per minute[m3 /min ] or litters per minute [ l/min ]. In BG units the flow rate is expressedin gallons per minute [ gpm ].

In systems working with incompressible fluids we use volumetric flow ratein the calculations. In our further calculations, we are going to useexclusively this type flow rate.

Mass flow rate, q Mass flow rate is the unit mass per unit time

== skg

t m

timeUnit massUnit

q _ _

2.15


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It can also be defined as: ]/[, skg Qq = In BG units, mass flow rate is expressed as [slug/sec] or [slug/min].

Weight flow rate, G Weight flow rate is the unit gravitational force per unit time

== s N

t F

timeUnit forceUnit

GQ _ _

2.16

It can be defined as:]/[, s N gQGQ = 2.17

In BG, weight mass flow rate is expressed as [ lb/sec ] and [ lb/min ].

An example of flow rate distribution after the pump is shown at Fig. 2.3.

The deviation in the flow rate is defined as:

_[%]100100 minmax ==mm QQQ

QQ

Where, mQ is the flow rate mean value.

Fig. 2.3


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Fluid Velocity

Fluid velocity, in pipes is:

= sm

AQ

v 2.18

Where, Q [m/s] is the volumetric flow rate passing through a crosssection with area A [m 2 ] .

Designers must always consider velocity of flow through the pipes andhoses and maintain the flow velocity within recommended limits.Exceeding maximum flow rate limits may cause turbulence in the flow

and reduce the efficiency of the system. The recommended flowvelocities are shown in Chapter 3.14 (Hydraulic Connectors).

Work and Power

Work, as we know from the course of Mechanics, is defined as force (F)acting through a distance (x).

][ Nm FxW = 2.19

In hydraulics, we also apply force, F, to move a fluid volume at distance x.

The force is equal to the pressure applied on a surface area.

[ ] N pA F = 2.20

If we replace the force in the equation 2.19, work can be expressed as:

][ Nm pAxW = 2.21


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If we further replace V = Ax [m] , we receive the formula most commonlyused for solving fluid power problems.

][ Nm pV W = 2.22Where, V is the fluid volume.

In SI units work is expressed in Newton meters [Nm] or in Joules( Nm J 11 = )

Power is work per unit time

=

s

Nm

t

W P 2.23

In the SI units power is expressed in Watts [W], where:

1W = 1 Nm/s = 1 J/s .

If we replace work with pressure multiplied by volume (equation 2.22), theequation (2.23) can be expressed as:

][W pQt

pV P == 2.24

Where:

][3

sm

t V

Q = is the flow rate;

p [Pa ] is the pressure.

The most convenient form of this formula for calculating the input poweron the pump shaft is:

][60

kW pQ

P P

= 2.25

Where the units are:


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minl

Q is flow rate in the pump outlet

[ ] MPa p is pressure at the pump outlet

9.08.0 = P is pumps overall efficiency

Power, in Hydrostatics, is transmitted on the bases of applying pressureon the fluid or by the fluid. First, the pump transmits energy to the fluid,and then the fluid transmits it to the actuators.

Energy is the capacity to do work and it is expressed in the same units aswork. We know that energy cannot be created or lost. In other words, wecannot get something without giving up something else. We can onlytransfer energy from one form to another and from one point to other. Inmobile hydraulic systems, the fluid transfers energy from one location(the hydraulic pump) to another location (linear or rotary actuator). Weput energy into the system and get energy out of the system, but thereare always losses of energy due to friction, heat loss, etc. So, we cannever get out more energy than we put in. Energy that we lose to frictionis not lost to the universe; it is simply transformed to heat.

Drag and pressure loss

Drag is a force (in a direction opposite to the flow) due to the shear forcesalong the fluid layers. As we know, any fluid moving inside hydraulic lines(tubes or hoses) experience drag. Total drag is a function of themagnitude of the shear stress, , and the orientation of the surface onwhich it acts.

Pressure loss is the energy that hydraulic fluid loses to overcome thefriction between the moving fluid layers inside the hydraulic lines (pipes,tubes or hoses). The pressure loss is quantified as a pressure drop.Pressure drop is influenced by a number of factors such as: fluid velocitythrough the hydraulic components and connectors, fluid viscosity,hydraulic line inside wall roughness, etc.


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Lineal pressure loss

Lineal pressure loss is the pressure loss of laminar flow (with Re


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Where:

v = Q/A is the flow velocity at the outlet of the component;

[Zeta] is the coefficient of local flow resistance.

Zeta depends on the geometrical shape, cross section and surfaceroughness of the local restrictor. Approximate values of Zeta are given inTable 2.1, Appendix D.

Hydraulic Shock

A Hydraulic Shock is also called: water hammer. It is caused by quickclosure of the hydraulic component causing pressure increases in thepressure side of the closing element. When the free flow is closed thekinetic energy of the moving fluid is transformed to potential energy,which in turn creates a pressure wave (shock wave). In order to absorbshock waves due to valve closure we use flexible hydraulic hoses ashydraulic lines. In the full power brake systems where hydraulic lines aremetal tubing and a brake valve is used to redirect fluid to the wheelcylinders, the shock waves can be absorbed by an accumulator.

Hydraulic Lock

One of the most common causes for failures in plunger type valves isexcessive frictional force between the plunger and the housing. Frictionalforce (F r ) is due to uneven pressure distribution in valve clearances (fig.2.4a). Different pressures on both sides of the plunger create a forceperpendicular to the plunger axis. This force pushes the plunger off itscenter position against the housing increasing friction between internalsurfaces. Friction force higher than the control force causes seizing of theplunger. This failure is called hydraulic lock . Valve designers addbalancing grooves to equalize the pressure distribution around theplunger circumference (fig. 2.4b).


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p b p 1

p b

x distancey x y

p 2

[ p ]

p a

p 2

p aF r

p a

[ p ]

p=p 2 -p 1 p m

p 1

p b

Fig. 2.4 a) b)

Obliteration

It has been determined experimentally that flow rates through very smallopenings can gradually diminish and become zero. This phenomenon iscalled obliteration . It is caused by the adhesion forces between metalsurface and the fluid which results in the buildup of layers of molecules onthe surface. Adhesion force is an interaction at an atomic level and

depends on the chemical composition of the fluid. Experiments show thatobliteration exists in openings smaller than 0.01 mm and causes bothsurfaces to stick together plugging the opening. When the opening isplugged, the plunger is seized. This condition appears in plunger typehydraulic components with small internal clearances. To eliminate thestickiness and seizure of the valve, the plunger is subjected to vibrationswith frequency higher than 30 Hz. The high frequency input to the valve iscalled dither signal .


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Stiction

The term stiction is created by combining the words stick and friction.Stiction occurs when the static friction force is higher than the movingforce. It measures the spool resistance to initial motion.

Cavitation

Cavitation in fluids is a process of formation and collapse of air or vapourbubbles. This leads to micro jets of oil pounding and eroding adjacentsurfaces. Cavitation occurs when the absolute pressure of the fluidbecomes close to zero. Cavitation also occurs when the pressure drop is

enough that at a given temperature the air in the fluid starts to evaporate.In this case we say that the pressure becomes equal to the vapor tensionof the fluid.

When cavitation is formed at the suction of the pump, several thingshappen all at once.

The system experiences a loss in capacity The system can no longer build the same head (pressure) The efficiency drops The cavities or bubbles will collapse when they pass into the

higher regions of pressure causing noise, vibration, and damage tomany of the components.

The five basic reasons that form cavitation are: Vaporization Air ingestion Internal recirculation Flow turbulence Vane Passing Syndrome

Cavitation can have several root causes related to system andcomponent design issues or related to service.

1. Tank design issues. Whirlpools in the tank churn the air into the oil orsimply don't allow air to be released from the oil. This can be caused byturbulence in the returned fluid, low fluid level, reservoir that is not deepenough, lack of proper baffling, etc.


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2. Suction-line leaks. Leaks between the tank and the pump canintroduce air into the system. Often this is associated with the shaft sealat the pump that allows air to leak in.

3. Suction-line restriction. Sometimes suction lines are too long, toonarrow or they are plugged (e.g., a plugged suction strainer).

4. Water vapor. When hot oils become contaminated with water,superheated seam will form vapor bubbles in the oil.

5. Insufficient head. Depending on oil viscosity and suction lineconditions, the pump must be located at a sufficiently low elevation toenable oil to flow steadily from the tank to the inlet port of the pump.

6. Air release problems. As oils age and become contaminated, its airrelease properties become impaired. This means that once air bubblesare formed they stay locked into the oil and do not detrain out of the oil in

the reservoir. Moisture contamination and oxidation are the mainoriginators of this problem. ASTM D3427 is a test for air releaseproperties.

7. High viscosity. When fluid temperature in the reservoir is too low, theviscosity may be too high to enable proper oil flow in the suction line andinto the pump. Any other cause of high fluid viscosity can lead to thesame problem.

The Bernoulli Equation

The Bernoulli equation is a statement that the total pressure ( p T ) along astreamline remains constant (fig. 2.5). The assumption is that the fluid isincompressible and steady. Therefore, if the equation is applied for gasesthere will be an error built into it.

.

2

1 2 const p z p T ==++ 2.29

First term p is the static pressure

Second term2

21

is the dynamic pressure. The dynamic pressure is

the kinetic energy of the particle.

Third term gz z = is the weight of the fluid


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The most popular engineering application of the above equation is whenthe equation is applied between two points on a steam line.

22

2212

11 21

21

z p z p ++=++ 2.30

Fig. 2.5

The Bernoulli equation was formulated it in 1738 by the Dutch bornmathematician and physicist Daniel Bernoulli (1700-1782).

The Torricelli Equation

The Torricelli equation can be derived from the Bernoulli equation whenthe equation 2.29 is applied to a stream in a vessel with one free surfaceand one outlet nozzle (fig. 2.6)

Fig. 2.6


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From, 22

2212

11 21

21

z p z p ++=++

1 at the surface is very small therefore,2

1 becomes negligibly smalland it can be ignored. Pressures p 1 and p 2 are equal to zero becausethey are equal to the atmospheric pressure.

Then, the equation can be simplified to

2221 2

1)( = z z

When we replace the specific weight g = , we receive

gh22 = 2.31This equation is called Torricelli's Theorem. It is named after the scientistand mathematician Evangelista Torricelli who in 1843 proved that the flowof liquid through an opening is proportional to the square root of theheight of the liquid.

Torricelli equation 2.26 can be used to find the flow rate AQ =

gh AQ 2= 2.32


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Chapter 3

Hydraulic Components

Hydraulic components can be grouped according to their function:

I. ValvesHydraulic valves are grouped into three general categories: pressurecontrols, flow controls and directional controls. Some valves can havemultiple functions and can fall into more than one category. The mostimportant valve characteristics are flow and pressure drop in the valve.Flow can be calculated based on the port diameter and the flow velocity.Pressure drop is more difficult to calculate accurately. That is why it isusually determined experimentally by the manufacturer.

Based on the construction, the valves can be plunger or cartridge.Cartridge valves are a screw-in type, which offer the designers thepotential of incorporating the valves into manifold blocks or the body ofother hydraulic components, such as cylinders.

II. Actuators

This group consists of pumps, motors and cylinders. Actuators convertfluid energy into mechanical energy or vice versa.

III. AccessoriesIn this group are: Pressure and vacuum switches, accumulators, filters andconnectors.

IV. Hydraulic reservoirsThere are two main types of reservoirs- open and closed. The hydraulicsystems for industrial trucks use open type reservoirs.

V. Hydraulic fluidThe fluid is the single most important component of the hydraulic system.Its main function is to transmit energy.


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34 Chapter 3: Hydraulic Components

In general, all hydraulic components introduce noise, vibration and lossesinto the system. In order to select them properly we have to understandtheir design, function and performance as a separate unit and as part of asystem.

When a component is used, there is always less energy out than energy in.In order to minimize the losses and the component cost, the componentshave to be sized per system requirements. Over-sizing will increase thecomponent cost while under-sizing will increase the energy losses.

1. Flow Restrictors (Orifices)

Flow restrictors (orifices) are local restrictions to the flow. It can beadjustable or non-adjustable. It is also called variable and fixed orifice .

Flow restrictor (orifice) fixed

Flow restrictor (orifice) variable

Although, all hydraulic restrictors create some degree of turbulence, theycan be lineal or turbulent depending on the type of flow passing though thecomponent.

Lineal is when L>d and the flow is predominantlylaminar.

Turbulent is when L


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In hydraulic schematics usually the lineal orifice symbol is used. Thehydraulic restrictor is shown as turbulent in cases when we want to showthat the pressure drop does not depend on the fluid viscosity.

A disadvantage of the lineal restrictor is that its resistance ( R L) changeswith the temperature change of the fluid.

The main function of orifices is to restrict flow and create a pressure drop inthe system. They are used to control the actuators (motors, cylinders)speed. Although orifices main function is to create a pressure drop, theyare also called: flow controls. It is important not to confuse them with thepressure compensated flow regulators which can also be called flowcontrols. Both flow controls have different symbols and the best practice todistinguish them is to look at the components symbol.

Pressure drop, p , in the orifice is proportional to the flow rate Q . Pressuredrop is calculated with the formula 2.26 or 2.28 (Chapter 2) depending onthe type of flow.

When the relationship between the pressure losses and flow rate is nearlylineal, it can be expressed as:

Q R p L= 3.1

Where:

Q is the flow rate through the restrictor

21 p p p = is pressure drop across the restrictor

L R is orifice resistanceIn a system with an orifice, usually there is a varying pressure, p 2, after therestrictor is determined by the variation in resistance of the actuator.

The main flow restrictor characteristic is based on equations 2.26 and 2.28and it is called: the flow-pressure drop characteristic . An experimentalgraph of such a characteristic is shown in Fig. 3.1


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Orifice Flow-Pressure drop Characteristic

0

2

4

6

8

10

12

14

16

18

20

0 10 20 30 40 50

Flow (l/min)

P r e s s u r e

d r o p

( M P a

)

Fig. 3.1 Orifice flow-pressure characteristics

2. Pressure Relief Valves

Symbols:

Pressure relief valves (also called: relief valves) are pressure control typevalves. It is normally closed until it starts to operate. After the pressure isincreased, the valve opens and the plunger (poppet or ball) finds a balanceposition. The balance is created between the pressure on one side andspring force on the other. The valve plunger can have infinite positionsbetween closed and fully opened conditions. The relief valves mainfunction is to protect the system against excessive pressure. It is usually


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installed between the pressure line, after the pump, and the return linebefore the tank. Relief valves can be adjustable or non-adjustable. Valvesare adjusted by changing the spring pre-compression.

There are three types of balancing/closing elements: ball, poppet and spool(plunger). The balancing element used determines the type of the valve.Valves can be divided into two groups 1) Ball and Poppet and 2) SpoolValves.

Ball and Poppet ValvesBall and poppet valves are usually used for the construction of cartridgevalves. Cartridge valves are less expensive and have higher flow rates thanthe same physical size spool valves. Ball and poppet valves are lesssusceptible to fluid contamination because when closed, the valve movingpart is held tightly against a seat in the housing. On the other hand, theyare more sensitive to flow and pressure irregularity. Their positioning is lessbalanced than spool valves, which leads to less accurate metering.

Spool ValvesSpool valves are easier to control and can move at smaller steps because itis easier to proportionally control the stroke of the spool. On the other hand,they are more expensive and more susceptible to contamination. Spoolvalves have higher leakage rates than poppet valves.

3. Check Valves

Symbol:

Check valves are unidirectional control valves. They have two positions:ON or OFF. This valve has free-flow (open) and no-flow (close) direction.When the flow pushes the ball (or the poppet) away, the valve opens andpermits free flow. Flow in the opposite direction pushes the ball against theseat. The built-up pressure keeps the passage sealed and the flow isblocked. These types of valves are designed to have a very small leakagerate when they are closed. Usually, valve seats are hardened steel whichmakes them more resistible to scoring from hard contaminants in the fluid.


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The only difference between the check valve and the ball relief valve is thespring. Check valves have light springs which are used only to return theball (poppet) to its seat when the flow stops. Because of the light spring, thepressure drop in the valve during operation is very small (about 0.05 to 0.1MPa ). There are three general check valve designs: plunger, poppet and

ball design.

The check valve has a relatively small effect on system noise, vibration andlosses. When the check valve is built into another hydraulic component, thepressure loss from it is included in the total pressure loss of the maincomponent. When we use an in-line check valve, it is acceptable todisregard the pressure loss in it. Therefore, it is very important not toundersize the valve. Undersizing it will increase the pressure drop, leadingto inaccuracy in the calculated pressure demand.

Check valves can have an internal or external pilot control. Fig. 3.2 showstwo valves with pilot ports.

inlet inletpilot

outlet outlet

pilot

Fig. 3.2 a) pilot-to-open b) pilot-to-close

Pilot-to-open can be opened by external pressure. When there is nopressure in the pilot port, this valve allows flow only in one direction. Whenpressure is applied in the pilot, the flow can pass in both directions. Theamount of pilot pressure required to open the check valve is:

r p p

p SPRINGOUTLET PILOT += ,

Where:

SPRING p is the pressure on the poppet due to spring forcer is the pilot ratio. It is the ratio of pilot piston area to poppet area.


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Pilot-to-close also allows flow only in one direction in the absence of pilotpressure. When pilot pressure is applied from the pilot port, it overrides thefree flow function and holds the valve closed. This feature is useful tocontrol regenerative flow in a cylinder circuit or in a hydraulic logic circuit.

Minimum pilot pressure required to close the valve is:

p p p SPRING INLET PILOT

= , r is the pilot ratio

Two check valves can be combined together.

Fig. 3.3 Dual pilot check valve

Dual pilot operated check valve is used for load holding applications orcylinder locking.

4. Reduction Valves

Symbol:

A reduction valve is a pressure control type of valve. Its function is toreduce pressure and maintain a pre-set lower pressure value in the outlet.The valve maintains a constant pressure in the outlet regardless ofpressure and flow rate changes in the inlet. This valve is normally open.


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Principle of operationFlow passes through an opening between a balanced plunger (spool) andhousing. Pressure in the outlet is applied under the valve plunger throughinternal connection. A spring force, acting on the other side, balances theplunger. When the pressure in the outlet increases, the plunger is pushed

up and the opening is reduced which, in turn, reduces the flow through thevalve. Spring chamfer is connected to the reservoir therefore external drainto the reservoir is required for this valve.

5. Pressure Compensated Flow Controls

The Pressure Compensating Flow Controls function is to regulate flow rateregardless of the system working pressure. These valves are also shortlycalled: Flow Controls or Flow Regulators.

The flow rate is usually used to control an actuators speed. The valve canbe placed before or after the actuator.

Symbols:

Two-way flow control

Three-way flow control

Construction of two-way flow controls

This valve has two parts: a pressure balanced plunger and an orificeconnected in series. It is called two-way because it has two ports. Thebalanced plunger (between point 1 and 2) controls the opening to maintaina constant pressure drop across the orifice. The flow through the valve iscontrolled by the orifice (between point 2 and 3) and the pressure drop p =

p 2 p 3 (Fig. 3.4 a). The valve will maintain a constant flow rate from point 1to point 2 within a specified pressure range. The valve regulates the flowrate only in one direction from point 1 (valve inlet) to point 3 (valve outlet).


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Pressure in point 1 must be higher than the pressure in point 3. When theflow is reversed, from outlet to the inlet, the valve simply acts as a flowrestrictor.

1

2

3

Fig. 3.4 Two-way flow controls: a) balanced valve before the orifice, b)balanced valve after the orifice.

The flow equation through the orifice is:

p AQ =

2 3.2

Where:

is the flow coefficient

A is the area of the orifice opening

p is the pressure drop in the orifice

The only variable in flow equation is the pressure drop ( p ). The purpose ofthe pressure balanced valve is to maintain a constant p which ensures aconstant flow rate ( Q) through the orifice. The flow is as a function of the

pressure drop in the valve, )( p f Q = which can be obtainedexperimentally. Flow rate vs. pressure drop relationship determines valveperformance and it is called: Flow regulator static characteristic (shown inFig. 3.5). Because of the nonlinear relationship between the flow and thepressure drop across the valve, the flow rate diverges slightly (the curve isnever perfectly horizontal).


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Close center, b), is when the plunger is at a neutral position and theflow is blocked.

Tandem center, c), is when the plunger is at a neutral position andthe flow is unloaded to the tank.

Two other combinations of the first three, float center d) and open-to-three

port e), are also shown in Fig 3.6

Fig. 3.6

Directional-control valves have two primary characteristics: 1) number ofports for the fluid and 2) number of positions for the controlling element.Valve ports are the passageway for fluid in or out of the valve. The numbersof positions refer to the number of distinct flow paths a valve can provide.

There are three types of spool valve laps (fig. 3.7): zero, positive andnegative. Valve lap is the distance the spool travels before valve opening.Valves with large overlaps have less leakage but they have less accurateflow metering.

B AP P

A BP

A B

a) zero lap b) positive lap (overlap) c) negative lapFig 3.7 Spool laps

Port P (pump) is the valve inlet. Ports A and B are valve outlets. Valve isshown in closed centered position.

In terms of plunger (spool) positioning, there are two major groups: 1)discrete/ finite positioning 2) infinite/ proportional positioning


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Discrete positioning

Finite positioning is when the plunger is shifted from one discrete position toanother. For this reason these type valves are called discrete valves.Plunger shift occurs in an instant, causing the fluid to rapidly accelerate ordecelerate. This causes fluid pulsations or in certain conditions it can causefluid hammer.

Switching time for these valves depends on the size of the coil. Actuationtime increases when the coil size and the valve size are increased. Forexample the switching times of directional valves size 6 (20 l/min nominalflow) with DC (direct current) magnet is about 40 milliseconds while theswitching time of size 10 directional valve (80 l/min nominal flow) is about80 milliseconds.

Infinite positioning

In these valves, the plunger is shifted proportionally to the input signal. Thesignal can be mechanical, electrical or hydraulic. The plunger can haveinfinitive intermediate positions, which makes these types of valves ideal forcontrolling speed and acceleration or deceleration of the actuators. Infinitepositioning is illustrated by adding two extra parallel lines indicating that the

plunger can slide inside the valve.

The infinite positioning directional valves can be further classified as:Proportional Valves, Servo Valves and Load Sensing Valves .

Proportional Valves

Proportional valves provide flow and pressure control proportional to thecontrol input device. The control device can be either mechanical orelectrical.

When an electric signal is used to control the flow rate, the flow ratechanges proportionally with the change of the signal to a solenoid. Inside aspool type valve there is a spool (plunger), which is the only moving part.Changing the electrical current, applied to the coil, changes the magnetic


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field, which in turn creates a magnetic force on the armature and makes itmove. The coil is placed in a metal housing which helps to retain themagnetic field. In most valves, a flat spring is used to resist plungermovement. The spring retains the plunger until the magnetic force on thearmature exceeds the spring force. The main reason for the performance

variation from one valve to another is the mechanical and geometricaltolerances that occur in the manufacturing process. Solenoid magnetic fieldcan be adjusted so that it compensates for mechanical tolerances.Therefore, this valve can create a consistent relationship between the flowrate and the electrical current to the valve.

Proportional valves can be an open-loop or a closed-loop construction.

Open-loop valves do not have feedback between the solenoid input andthe valve spool or valve output. They have a lower response time than theclosed loop valves.Closed-loop valves have an outer loop for spool location feedback. Anouter loop can be made by connecting a LVDT sensor to the spool. ALVDT sensor measures small changes (in the range of microns) of spoolmovement and converts them to electrical signals.

Proportional valves can be spool or poppet type . Most of proportionalvalves are spool type designs because they have better control andmetering capabilities. Poppet type proportional valves are less susceptibleto fluid contamination. For this reason they are mainly used in systemssubject to high contamination. To minimize the leakage from a section withhigh pressure to a section with low pressure, the plunger type valves aremanufactured to have as a small gap as possible between the body and theplunger. The servo valves have 0.001 mm to 0.004 mm internal clearanceswhile the discrete directional valves usually have 0.005 mm to 0.012 mminternal clearances.

Servo valvesServo valves have a shorter response time than standard proportionalvalves. They are always closed-loop valves. There is a mechanicalfeedback link between the input command and the valve output. Servovalves usually consist of a two-stage spool. The spool position is controlledby two electromagnetic coils- one from each side. Manufacturing tolerancesof these valves are in the range of micrometers. The tight tolerancerequirements make them expensive to manufacture. Also, the reduced


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clearances between the valve surfaces make this type of valve susceptibleto fluid contamination which can jam the valve. Because of the high costand the high fluid cleanliness requirements, servo valves are rarely used inmobile hydraulic systems. Servo valves are used in applications whereshort response time is critical. For this reason they are manufactured with

zero laps or near-zero overlaps.Servo valves can have a response time as low as 0.0025 seconds (400hertz). Where:

sec11

1 = Hz

In contrast, standard proportional valves have a response in the range of0.1-0.2 seconds (10 - 5 hertz ).

Proportional valve selection

Proportional valves are selected per maximum flow that must go throughthe valve.

Proportional and servo valves execute their control through a high-pressuredrop. The valve's flow rating is usually based on a specific pressure drop.

After we select valve size, it is recommended to measure the pressure dropacross the valve. If (in our application) the pressure drop is significantlydifferent than the rated pressure drop of the valve, we have probablyselected the wrong valve. Usually, engineers end up with an oversizedproportional valve. If an oversized valve is selected, the hydraulic actuatorsare unlikely to get the anticipated proportional performance. In most cases,the valve will open all the way before it is supposed to, providing a differentresolution that we seek.

When selecting the valve, pressure drop should be used to calculate theflow rate.

A

B

OUT R p

pQQ

=

3.3

Where,

QR = valve's rated flow for our applicationQOUT = output flow needed for application

B p = rated pressure drop of proportional valve

A p = actual pressure drop needed for application


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It is recommended that designers use this method to check the flow rate oftheir valve. In most cases, the flow rate they obtain through this method willdiffer from the flow rate in the catalogue.

Load Sensing Priority Valves

Load sensing priority valves are simply called priority valves . They haveinfinitive positioning. There are two types of priority valves: static anddynamic.

Priority valve with static signal Priority valve with dynamic signal

Load sensing priority valves are used to split the flow in open loop systemswhere one branch must have a guaranteed flow supply. This valve senses

the flow requirements and provides metered priority flow to this port. Thevalve has one inlet and two outlets. One of the outlets is for the controlledfluid (CF) and the other one is for the excess fluid (EF).

Dynamic load sensing valves have faster responses than static valves.They have a passage between CF and LS lines. This passage supplies acontinuous pressurized flow to the LS line even when the line is not usedwhich keeps the valve in a ready-to-respond position.

Directional valves can have direct or indirect control. Direct control isapplied directly to the valve control element. Indirect control (pilot operateddesign) is when the input signal controls a small pilot valve which in turncontrols the main valve (fig. 3.8).

Electrically controlled big valves require big and expensive solenoids. Toreduce manufacture cost of these valves, they are controlled indirectly.Small solenoids are used to control the pilot valve which sends pressurized


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fluid to control the main valve. Some proportional valves with indirectcontrol have a course filter (screen) that protects the pilot stage. If a filter isused, the filter should be replaceable or washable.

a) Symbol

b) Detailed symbol

P T

Fig. 3.8 Directional manual valve with indirect manual control

7. Hydraulic Pumps

Symbols:

Constant flow pump Variable flow pump

Pumps are mechanical devices that convert mechanical energy intohydraulic energy. They draw fluid from a reservoir and send it to hydraulicactuators. There are two main types of pumps: positive displacement (vane,piston and gear pump) and non-positive displacement (centrifugal pumps).

By definition, positive-displacement (PD) pumps displace a defined quantityof fluid with each revolution of the pumping elements. This is done bytrapping fluid between the pumping elements and a stationary housing.


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Pumping element designs include gears, lobes, rotary pistons, vanes, andscrews.

Positive displacement (PD) pumps can be either fixed or variabledisplacement. Fixed displacement pumps have a constant relationshipbetween the flow rate and the drive shaft angular velocity. In variabledisplacement pumps, the displacement can be changed so that the flowrate can be independent from the drive shaft velocity. Gear pumps havefixed displacement while vane and the piston pumps can be either fixed orvariable. Lift truck hydraulic systems use only PD type pumps such as:vane pump, piston (axial and radial) pump and gear pump.

Systems with pressure up to 25 MPa usually have a gear or vane typepumps. While high pressure systems 25 to 40 MPa (3600 6000 psi )require using piston pumps.

Gear pumps

Gear pumps can have external or internal gear meshing. External pumpshave one or more sets of two spur gears while the internal have one or

more sets of spur and ring gear. In fork lift application external gear pump ismore popular because of the bigger selection and the lower cost.

External gear pump construction

A gear pump (Fig. 3.9) has a body in which there are two hardened steelgears intermeshing together. One of the gears is a drive gear and the otherone is a driven gear. The drive gear is mounted on a shaft, which extendsoutside and is connected to a motor. Meshing gears create two chambers-the first is the inlet (suction port) the second is the outlet (pressure port).Rotating gears take fluid from the suction port, drive it around the gears andpush it into the pressure outlet. The highest quality gear pumps have zerobacklash gear meshing. Pumps with zero backlash meshing have highefficiency and low noise. Pump main parameters specified by themanufacturers are: flow rating (maximum and minimum shaft speeds),maximum pressure rating, and the type of mounting.


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Fig. 3.9 Gear pump- external gear meshing

Pump body is subject to cyclic loads due to pressurizing and de-pressurizing during pump operation. For this reason fatigue strength is amain requirement for the body design. A gear pump body can be madefrom die-cast aluminum alloy, aluminum alloy bar stock, cast steel or castiron (ductile iron). Ductile iron is usually less expensive and has betternoise and vibration dissipation than aluminum and steel however, it has theworst heat dissipation of the three. Ductile iron and cast steel have identicalyield and tensile strengths. Ductile Irons have small volumetric changesand retain their strength at high temperatures due to the stability of themicrostructure. For high temperature applications, ductile iron alloys withsilicon and molybdenum are used. Silicon content of 4% to 6% provides thebest combination of heat resistance and mechanical properties. Pumpbodies from cast steel and ductile iron are designed to the yield point of thematerial. Aluminum pumps are designed for minimum deflection (highrigidity) because aluminum reaches its endurance limit sooner than steeland it has smaller plastic range and less tolerance to overload anddeflection. Aluminum body pumps are good for low-temperatureapplications because at low temperatures (below 30C), aluminum has alittle change in properties (yield, tensile and impact strength). Cast ironpumps are preferred in wide temperature rage applications because thecast iron and the steel gears have similar expansion properties which,reduces the thermal distortion and the internal leakages.


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Although a gear pump is tolerant to system contamination, themanufacturer must specify the acceptable contamination level. Direction ofrotation of the shaft must be shown on the pump body. The pump can bebi-directional so that it delivers flow from either port. By using such pump,we can eliminate the directional valve in the system. Bi-directional pumps

require a drive motor which is able to rotate in both directions. Flow isproportional to the shaft speed therefore the relationship between the shaftspeed and the outlet flow is linear.

A gear pump can include a built-in relief valve, check valve or both. Therelief valve can be internal, fig. 3.10a, (the fluid is returned to pump inlet) orit can have external relief port, fig. 3.10b, (the fluid is returned to reservoir).

2

1 1

2

3

Fig. 3.10a Fig. 3.10b

Internal reliefs can be used in systems in which the pump works on request.For steering systems in which the flow goes over relief 50% of the time,internal reliefs are not recommended. They heat up the oil and the pumpand can cause leaks through pump inlet seals. When a system have aninternal relief valve, it is important to keep the fluid temperature in itsoperating range. Overheating of the fluid can affect the relief valvesperformance.

Pump delivery (flow rate in litters per minute)

=min1000 Lnd

Q V P

P 3.4


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Where:

d P [cm 3/rev] is the pump displacement. The displacement is ameasure of the pump size and is given by the manufacturer;

n [rev/min] is the shaft input rotational speed

V is pump volumetric efficiency.

In the BG units, flow rate is given in gallons per minute ( gpm ):

[ ] gpmnd Q V P P 231= 3.5

.

3

revin

d P is the pump displacement (in cubic inches) per

revolution.

Gear pump flow rate can be given at 1000 rpm by the manufacturer. Infixed-displacement pumps, the flow rate can be changed only by changingthe drive shaft rotational speed. These pumps are used in open typesystems in which the flow after each work cycle is returned to the reservoir.

Variable displacement pumps are mainly used in closed systems (systemswhere the pump continues to operate at a stand-by in a neutral position)

Torque on the pump shaft

][P

T Nmm =

Where:

P is the hydraulic power in Watts ( Nm/s )

m is pump mechanical efficiency

w is the shaft angular velocity in ( rad/s )

In mechanical formulas, the shaft speed is expressed in radians per secondw (rad/s )

The angular velocity w (rad/s ) can be converted to rotational speed, n (rev/s ):

w = 2 n


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mn

2 P T =

m P

m nn pd

n pQ

T

22

==

)(2

Nm pd

T m P

= 3.6

Slip

Slip is a leakage of fluid from the pressure outlet back to the inlet. Slipincreases with increasing pressure and wear. Increasing slip is referred to

as a loss of efficiency. Slip can be reduced by constructing the pump forpressure and wear compensation.

Pump Efficiency

Overall efficiency is:

M V O Power Input Power Output

== _ _

3.7

It is determined as the ratio between the hydraulic power at the pump outletand mechanical power at the driving shaft at nominal pressure, rotationalspeed, and fluid viscosity (rated power). The overall efficiency has twocomponents: volumetric and mechanical.

Flowrate Rated Flowrate Actual

V _ _ = is the volumetric efficiency.

The actual flow rate is the flow at the pump output when the pump isworking under load. It will vary at different pressures. Rated flow rate is thetheoretical flow at the pump outlet without volumetric losses. Volumetricefficiency range is: v = 0.90 - 0.97. If volumetric efficiency is not known, forinitial calculations we can take the average values for gear pumps: v =0.90 (low speeds 1000 rev/min) and v = 0.97 (high speeds 3000rev/min)


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Power Shaft Input Power Saft Drive

M _ _ _ _ = is the mechanical efficiency.

It is a result of lost power due to friction in the bearings and between the

meshing gears. 93.090.0 = M

The gear pumps overall efficiency is in the range of 82% to 88% dependingon the pressure and rotational speed. An example of pump overallefficiency at different pressures is shown in Fig 3.11.

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300

Pressure [bar]

O v e r a

l l E f f i c i e n c y

[ % ]

Fig. 3.11

A disadvantage of gear pumps is that they create pressure and flow ripples(pulses) in the discharge port. Pumps are one of the biggest sources ofnoise and vibrations in the hydraulic system. Every time the fluid betweentwo teeth is pushed out of the pump, a peak in the pressure appears. Atypical pressure distribution at a discharge port is shown in Fig. 3.12.

Pulsations ( ) can be expressed as the ratio of pulsations amplitude ( p) toan average value ( p):

[%]1001

= p

p P ,

Where: p is peak-to-peak amplitude.


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t [s]

p [MPa ]

p 1

p 3 p 2

p

Fig. 3.12

Where: P 1 is maximum continuous pressure

P 2 is maximum intermittent pressureP 3 is maximum peak pressure

Pump pressure pulsations travel along the hydraulic lines at the speed ofsound (about 1400 m/s in hydraulic fluid) until it is affected by a change indiameter or direction. Therefore, pulsation amplitude depends on thehydraulic lines (length and diameter) and fittings (type and size) in thesystem. Although the direction valve, after the pump, smoothes the flowand the pressure peaks, pulsations created by the gear pumps travelthrough the system to the hydraulic actuators.

Internal gear pumps have smaller pressure pulses than external pumpsbecause the spur and ring gear set have more teeth meshing than twoexternal spur gears. Gear pumps are mainly used in systems with normal-high pressures (from 15 to 25 MPa). For higher efficiency, they should bedriven at speed close to their rated maximum because internal leakage issmaller at higher speeds. At low speeds, gear pumps have reducedlubrication between side plates and gears. Pump manufacturers alwaysspecify the minimum rotating speed.

Intermittent pressure is used for selecting lift pumps that work intermittently.Continuous pressure is used for selecting steering pumps that have to runcontinuously.


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Vane Pumps

There are two types vane pumps: balanced and unbalanced. In thebalanced design the rotor and the sliding ring surface are coaxial. In theunbalanced design they are not.

Construction

Similar to the gear pumps, a driving shaft coming from primary powersource drives the vane pump. Inside the pump, the driving shaft isconnected to a slotted rotor that is placed eccentrically from the center ofthe circular opening of a casting housing. Vanes placed in the rotor slotsslide in and out. Centrifugal force causes them to slide out and the contourof the cavity pushes them back in. Tips of the vanes slide on the insidepump surface and seal the passage between the suction and the pressureports. The vanes push fluid from the inlet to the outlet through the gapbetween the housing and the rotor. Vane pumps have higher efficiency thangear pumps because of less fluid leakage from the pressure outlet back tothe inlet. They have less slip (smaller volumetric losses). Also, theefficiency remains constant over time. As the vane tips wear the slipremains the same because the centrifugal force always keeps the vanes incontact with the housing surface. Pump housing is made from the samematerials as gear pumps. Mechanical efficiency is a result of the friction inthe bearings and the friction between the cam contour and vane tips.

Fig. 3.13 Unbalanced vane pump


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Disadvantages of vane pumps compared to gear pumps are:

1) A high efficiency into a narrow pressure range and 2) pumps are moresusceptive to fluid contamination. When the contamination increases, theirvolumetric efficiency decreases.

Rotary Piston Pumps

Rotary piston pumps have a rotational driving shaft. These pumps havesome advantages over gear and vane pumps. They are used in systemswith higher flow and pressure demands.

Features: High power-to-size ratio. We can get more hydraulic power out of a

piston pump than we can from the same size gear pump. High pressures: some pumps can maintain pressure up to 70 MPa. Low power consumption at stand-by. High overall efficiency: for most pumps it is about 96%.

Construction

There are two main types of rotary piston pumps: radial and axial.In the radial type, pistons are placed in a cylinder block. Pistons moveradially in and out. The cylinder block (rotor) is located inside a fixedhousing (stator) and is rotated by a drive shaft. The rotor centerline is offsetfrom the stator centerline. The amount of offset determines piston strokeand pump displacement.

In axial pumps (fig. 3.14), pistons move axially. They are placed into acylinder block which is rotated by the drive shaft. The piston ends aredepressed against a tilted disk (swash plate). The angle of the disk causescylinders to move axially. If the disk is perpendicular to the axis of rotation(zero angle), pistons will not be compressed and there will be no flowthrough the pump. The disk can have different angles. When the disc istilted to one side of the neutral, flow goes in one direction. When it is tiltedto the other side, flow direction is reversed. When the disc angle is fixedthe pump has fixed displacement. In pumps with variable displacement, thedisk angle is controlled by a yoke. The yoke can have mechanical, electricor hydraulic control. At the released position of the yoke, the disk is


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returned to neutral (zero) position and the pump stops delivering flow.When the yoke is hydraulically controlled, a pressure compensatormaintains constant output pressure at different flow rates. Such pump iscalled: pressure compensated pump.

a Fig. 3.14 Axial rotary piston pump

Pressure compensated pumps are used mainly on internal combustion (IC)engine trucks because the engines speed is controlled by the trucks speedrequi

design of hydraulic systems for lift trucks

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