Design of hydraulic systems for lift truck

Design of hydraulic systems for lift truck

Ivan Gramatikov

Design of Hydraulic Systems

for Lift Trucks

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

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: gramatik.publishing@abv.bg

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)

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

Hydraulic Lock 27

Obliteration 28

Stiction 29

Cavitation 29

The Bernoulli Equation 30

The Torricelli Equation 31

Chapter 3: Hydraulic system components 33

1 Flow Restrictors 34

2 Pressure Relief Valves 36

3 Check Valves 37

4 Reduction Valves 39

5 Pressure Compensated Flow Controls 40

6 Directional Control Valves 42

7 Hydraulic Pumps 48

8 Hydraulic Motors 59

9 Hydraulic Cylinders 60

10 Pressure Sensors 64

11 Hydraulic Accumulators 66

12 Hydraulic Filters 70

13 Hydraulic Reservoirs 77

14 Hydraulic Lines, Fittings and Couplings 83

15 Manifold blocks 88

16 Hydraulic Fluid 90

17 Fluid Cleanliness 95

18 Electric Motors 98

Chapter 4: Management and quality of hydraulic system design process 101

Brief history of quality 101

Introduction 103

Factors 104

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

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 211

I Servicing the hydraulic systems 211

Troubleshooting principles 212

System Life 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

II Components layout- general considerations 222

III Common problems 223

IV Contamination of the hydraulic fluid 225

V The future of the hydraulics 229

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

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

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:

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 regulated through provincial organizations The governing body is comprised of engineers chosen, through a voting process, by members of the engineering organization

self-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

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

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)

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:

C

g

lbm

slug =

There are two gallons: British and US gallon

1 British gallon = 4.546 litters

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

τ Shear stress [N/m2] and [Pa]

ω Angular velocity [rad/s]

θ Angle [rad], [º]

Electrical control, proportional

Pump, constant volume, one direction of flow

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

Pressure relief valve

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

Where: 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/ft 3 where the

mass is in slugs

] [ 174

.

W

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

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

Where: 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

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

SG

2

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 (du dy) and the shear

stress (τ ) by the equation:

du

µ

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:

cm

s m s

mm

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

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 (http://www.webtec.co.uk/)

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

of Elasticity shows the resistance of the fluid to compression and is defined as:

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 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

dρ is differential change in density of the fluid;

ρ is initial density of the fluid

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 E v 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 and the temperature for two types of fluid v

Fig 2.2

Reynolds Number and Types of Flow

Fluid flow can be laminar, turbulent or a mixture of both The factor that

determines which type of flow is present is the ratio of inertia forces (v s ρ)

to viscous forces (µ/L) within the fluid This ratio is expressed by the

non-dimensional Reynolds Number:

When the flow is in a pipe with a circular cross-section, the lineal

characteristic L is equal to the pipe diameter D Then the equation can be

4000, the flow is transitional (between laminar and turbulent) and it has elements of both flow types For flows within circular pipes the critical Reynolds 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 This allows us to obtain relatively simple solutions to most engineering problems

Pressure distribution (for incompressible fluids) is called hydrostatic

with pressure p 2 This distance is called pressure head and it is interpreted as the height of a column of fluid of specific weight γ required

to give a pressure difference (p 1 - p 2) If we have a surface exposed to the atmospheric pressure it is convenient to use a point on this surface as

reference point 2 Thus, we let: p 2 =p 0

In SI, unit pressure is expressed as Pa (Pascal), where: 1Pa=1N/m² In some cases we use the unit bar (1bar = 0.1 MPa)

In BG, units are lb/ft² or lb/in² (psi) The relationship between the metric and the English systems is: 1 bar = 14.5 psi

In mobile truck hydraulic systems, positive displacement rotary pumps are used to create pressure A disadvantage of using these type pumps is that 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 atmospheric

pressure is called gage pressure Absolute pressure, on the other hand,

is measured relative to the perfect vacuum (absolute zero) Absolute pressure is always positive while the gage pressure can be either positive

or negative A negative gage pressure is also referred as a vacuum

Hydraulic systems used in the industrial trucks are classified according to the 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 Since the observation of all molecules is almost impossible, we are describing the flow as motion of part of the fluid, called small volume (or unit volume) Small volume contains numerous molecules Flow is created when 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 in the conductor and displace them Displaced molecules move by pushing their neighbours and so on So, the ejected fluid volume from the conductor at the opposite end will be the same as the one entered The movement of fluid molecules causes a pressure wave traveling at the speed of sound (about 1400 m/s) The speed of sound in fluids is:

ρ is the density (kg/m3)

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

c = (1.7x 109 / 870)1/2 = 1398 (m/s)

The density values are given in Appendix B

When calculating the parameters of the hydraulic hydrostatic systems we

assume that the velocity, v, at a given point in space does not vary with time dv/dt = 0 Such flow is called: steady flow In a system with a steady

flow, rapid closure or opening of a hydraulic component can cause unsteady effects, which have to be considered when a hydraulic system

is designed For example the “water hammer” affect, which results in loud banging 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 through

an observation cross section

V time Unit

volume

Unit

Q

3_

_

In SI units flow rate can be expressed either in cubic meters per minute

in gallons per minute [gpm]

In systems working with incompressible fluids we use volumetric flow rate

in the calculations In our further calculations, we are going to use exclusively this type flow rate

• Mass flow rate, q

Mass flow rate is the unit mass per unit time

m time

It can also be defined as: q = ρ Q , [ kg / s ]

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

F time Unit

force Unit

gQ

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:

Work and Power

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

If we further replace V = Ax [m³], we receive the formula most commonly

used for solving fluid power problems

]

[Nm

pV

Where, V is the fluid volume

In SI units work is expressed in Newton meters [Nm] or in Joules (1 J 1 = Nm)

Power is work per unit time

pV

Where:

][

3

s

m t

V

Q= is the flow rate;

p [Pa] is the pressure

The most convenient form of this formula for calculating the input power

on the pump shaft is:

] [

P

η is pump’s overall efficiency

Power, in Hydrostatics, is transmitted on the bases of applying pressure

on 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 as work We know that energy cannot be created or lost In other words, we cannot get something without giving up something else We can only transfer energy from one form to another and from one point to other In mobile hydraulic systems, the fluid transfers energy from one location (the hydraulic pump) to another location (linear or rotary actuator) We put energy into the system and get energy out of the system, but there are always losses of energy due to friction, heat loss, etc So, we can never get out more energy than we put in Energy that we lose to friction

is 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 forces

along the fluid layers As we know, any fluid moving inside hydraulic lines (tubes or hoses) experience drag Total drag is a function of the magnitude of the shear stress, τ, and the orientation of the surface on which it acts

Pressure loss is the energy that hydraulic fluid loses to overcome the

friction 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 velocity through the hydraulic components and connectors, fluid viscosity, hydraulic line inside wall roughness, etc

Lineal pressure loss

Lineal pressure loss is the pressure loss of laminar flow (with Re<2320)

moving along the straight sections of the pipes For laminated flow the

pressure loss (pressure drop) due to friction is calculated with the

D'Arcy-Weisbach equation:

( )2 L

2

ρ d

l

λ

∆p = v 2.26

Where:

l is the length of the pipe;

d is the diameter of the pipe;

v = Q/A is average flow velocity in the pipe;

λ [lambda] is the coefficient of lineal flow resistance

Re

64

n

=

λ , for round cross sections n=1 2.27

In many cases, pressure drop (∆p L) for different lengths can be

determined faster graphically by using nomograms There are two type

nomograms for determining: 1) in straight pipes and 2) in flexible hose

Local pressure loss

Local pressure loss is a result of turbulence in the fluid when the flow

changes its direction and velocity This turbulence occurs inside hydraulic

fittings

Local resistance occurs in the hydraulic fittings and it is a result of a

change in the flow speed and direction The pressure drop is calculated

with the formula:

Where:

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

ζ [Zeta] is the coefficient of local flow resistance

Zeta depends on the geometrical shape, cross section and surface

roughness of the local restrictor Approximate values of Zeta are given in Table 2.1, Appendix D

Hydraulic Shock

A Hydraulic Shock is also called: “water hammer” It is caused by quick closure of the hydraulic component causing pressure increases in the pressure side of the closing element When the free flow is closed the kinetic energy of the moving fluid is transformed to potential energy, which in turn creates a pressure wave (shock wave) In order to absorb shock waves due to valve closure we use flexible hydraulic hoses as hydraulic lines In the full power brake systems where hydraulic lines are metal tubing and a brake valve is used to redirect fluid to the wheel cylinders, the shock waves can be absorbed by an accumulator

Hydraulic Lock

One of the most common causes for failures in plunger type valves is excessive frictional force between the plunger and the housing Frictional force (Fr) is due to uneven pressure distribution in valve clearances (fig 2.4a) Different pressures on both sides of the plunger create a force perpendicular to the plunger axis This force pushes the plunger off its center position against the housing increasing friction between internal surfaces Friction force higher than the control force causes seizing of the

plunger This failure is called hydraulic lock Valve designers add

balancing grooves to equalize the pressure distribution around the plunger circumference (fig 2.4b)

called obliteration It is caused by the adhesion forces between metal

surface and the fluid which results in the buildup of layers of molecules on the surface Adhesion force is an interaction at an atomic level and depends on the chemical composition of the fluid Experiments show that obliteration exists in openings smaller than 0.01 mm and causes both surfaces to stick together plugging the opening When the opening is plugged, the plunger is seized This condition appears in plunger type hydraulic components with small internal clearances To eliminate the stickiness and seizure of the valve, the plunger is subjected to vibrations with frequency higher than 30 Hz The high frequency input to the valve is

called dither signal

Stiction

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

Cavitation

Cavitation in fluids is a process of formation and collapse of air or vapour bubbles This leads to micro jets of oil pounding and eroding adjacent surfaces Cavitation occurs when the absolute pressure of the fluid becomes 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 tension

of the fluid

When cavitation is formed at the suction of the pump, several things happen 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 to many 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 and component design issues or related to service

1 Tank design issues Whirlpools in the tank churn the air into the oil or simply don't allow air to be released from the oil This can be caused by turbulence in the returned fluid, low fluid level, reservoir that is not deep enough, lack of proper baffling, etc

2 Suction-line leaks Leaks between the tank and the pump can introduce air into the system Often this is associated with the shaft seal

at the pump that allows air to leak in

3 Suction-line restriction Sometimes suction lines are too long, too narrow 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 line conditions, the pump must be located at a sufficiently low elevation to enable 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 air release properties become impaired This means that once air bubbles are formed they stay locked into the oil and do not detrain out of the oil in the reservoir Moisture contamination and oxidation are the main originators of this problem ASTM D3427 is a test for air release properties

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

The Bernoulli Equation

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

2

const p

1 ρϑ is the dynamic pressure The dynamic pressure is

the kinetic energy of the particle

Third term γ z = ρ gz is the weight of the fluid

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

2

2 2 2

1

z p

The Torricelli Equation

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

Fig 2.6

From, 2

2 2 2

1

2 1 1

2

1 2

1

z p

z

p + ρϑ + γ = + ρϑ + γ

1

ϑ at the surface is very small therefore, ϑ12becomes negligibly small

and it can be ignored Pressures p 1 and p 2 are equal to zero because they are equal to the atmospheric pressure

Then, the equation can be simplified to

2 2 2

1 )