Principles of Hydraulics E-Book

Horst Walter Grollius

• Herausgeber: Books on Demand
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 71

Veröffentlichungsjahr: 2017

Beschreibung

To maintain the efficiency and competitiveness of industrial products, it is important to rationalize manufacturing process with the aim to increase automation. Oftentimes this is achieved by the application of fluid systems, subdivided in hydraulik and pneumatic systems. With this book the author especially intends to introduce the reader in the principles of hydraulics. Reference is made on the book "Grundlagen der Hydraulik" published by the CARL HANSER-Verlag. This book is in the 7th-edition. The book presented here, offers the possibility to familiarize with the topic of hydraulic in a condensed manner by keeping the time effort limited. This particularly applies for students at universities and technical schools, but it is also a beneficial help for technicans in professional practice who want to refresh their skills in the field of hydraulics. The last chapter the reader will finds ten exercises with a detailed presentation of the solution approach by use of the "step by step"-method. Each step is commented to provide highest clarity of the solution approach.

Leseprobe

“If you can`t explain it simply, you don`t understand it well enough.”

Albert Einstein

Univ.-Prof. Dr.-Ing. Horst Walter Grollius

Cologne

Preface

To increase the efficiency of production, knowledge and its application in various engineering disciplines is required. This also includes the fluid technology which is subdivided in hydraulics and pneumatics.

With this book the author especially intends to introduce the reader in the principles of hydraulics.

Recourse is made on the book “Grundlagen der Hydraulik” (chapter 2) published by the author in the German language. This book appears in the CARL HANSER-Verlag and is now in the 7th edition.

The book presented here, offers the possibility familiarizing themselves without spending too much time with the principles of hydraulics. This particularly applies for students at universities and technical schools. In addition the book will also be of help for those readers which are as technicians in professional practice and want to refresh their basic skills in the field of hydraulics.

In the last chapter the reader will find 10 examples with the detailed presentation of the solution path by the “step by step” method (each step is commented); clarity of the path to find the solution is thus given.

May the study of this book not only make effort, but rather have also motivated the reader to delve with additional literature in this fascinating and economically important field of technology.

Furthermore, many thanks to the company TENADO GmbH (Bochum, Germany); the TENADO CAD software of this company has been used for the creation of all figures shown in the book.

Cologne, November 2017

Horst Walter Grollius

Contents

Introduction

Physical Principles

2.1 Pressure Definition, Absolute Pressure, Overpressure. Pressure Units

2.2 Law of

Pascal

2.3 Hydrostatic Pressure

2.4 Hydraulic Press

2.5 Pressure Transmission

2.6 Hydraulic Work, Hydraulik Power, Efficiencies

2.7 Equation of Continuity

2.8

Bernoulli-

Equation

2.9 Laminar and Turbulent Flows

2.10 Viscosity

2.11 Pressure Losses in Pipes, Fittings and Valves

2.12 Flows through Throttling Devices - Flow Measurement

2.13 Gap Flows

2.14 Hydraulic Resistance

2.15 Compressibility and Compression Module

2.16 Cavitation

Basic Structure of a Hydraulic System

Circuit Diagrams

Examples

Example 1: Container with two pistons

Example 2: Water conducting channel with drain pipe

Example 3: Pump delivers water from a dam into a container

Example 4: Oil flows from a tank into a container

Example 5: Water flows from a reservoir into a channel

Example 6: Hydraulic press with pressure transmission

Example 7: Two cylinders which are connected by a pipe

Example 8: Cylinder to whose piston rod a rope is fastened

Example 9: An oil filled pipe in different states

Example 10: Forces acting on piston and piston rod

Sources of Literature

Symbols

Symbols used in the book and not found in the following list will be explained by the book text.

A

Area

m

2

B

Width

m

b

Correction factor, gap width

-, m

C

Flow coefficient

-

d

Inner diameter (hydraulic cylinder)

m

d

A

Area (infinitesimal small)

m

2

d

F

Force (infinitesimal small)

N

d

e

Hydaulic diameter

m

d

PR

Piston rod diameter

m

E

Modulus of elasticity

N/m

2

F

Force

N

F

P

Piston Force

N

G

Weight

N

g

Acceleration of gravity

m/s

2

h

Height coordinate, gap height

m, m

I

Electrical current

A

K

True compression module

bar

K

S

Average compression module

bar

k

Absolute wall roughness, correction value

m, -

k

/

d

Relative pipe roughness

-

l

Pipe length, gap length

m, m

m

Mass

kg

Mass flow

kg/s

P

Hydraulic power

kW

p

Pressure

N/m

2

p

abs

Absolute pressure

N/m

2

p

amb

Atmospheric pressure

N/m

2

p

e

Overpressure (or gauge pressure)

N/m

2

p

I

Inlet pressure (hydraulic pump, hydraulic motor)

N/m

2

p

O

Outlet pressure (hydraulic pump, hydraulic motor)

N/m

2

Q

Volume flow or flow rate

m

3

/s

R

Spring rate, hydrostatic resistanse

N/m, kg/(m

4

-

s

)

R

tot

Total hydrostatic resistance

N/m, kg/(m

4

-

s

)

Re

Reynolds-

number

-

Re

crit

Citical

Reynolds

-number

-

s

Way

m

T

Torque

Nm

t

Time, temperature

s,

o

C

U

Perimeter, electrical Voltage

m, V

V

Volume

m 3

υ

Velocity

m/s

υ

m

Average velocity

m/s

υ

max

Maximum velocity

m/s

υ

Plate

Plate velocity

m/s

υ

crit

Critical velocity

m/s

W

Hydraulic work

Nm

G

Ratio of diameters

-

ß

P

Isothermal copressibility coefficient

1/bar

Δ

p

Pressure difference

N/m

2

ζ

Flow resistance coefficient

-

η

Dynamic viscosity

N-s/m

2

NOTE: For the physical variables used in this book the International System of Units (SI) is used. For conversion into units used in Anglo-Saxon countries, conversion tables have to be used, which are available in the web.

1 Introduction

Fluid power is the generic term for the areas of hydraulics and pneumatics. In the area of hydraulics the fluids are liquids; in the area of pneumatics gas is used, namely air. In the beginnings of the hydraulics water was used as the fluid for energy transfer. Since the beginning of the 20th century oils are used. These have lubrication- and corrosion protection in addition. For some years water is also reused as the fluid for energy transfer in individual cases for reasons of environmental protection and costs, also called “water hydraulics”. The present book deals mainly with the physical principals relevant for oil-operated hydraulic systems (usually mineral oils are used).

The oil-hydraulic is divided into the areas of hydrodynamic and hydrostatic energy transfer.

The hydrodynamic energy transfer uses an impeller in order to transfer mechanical energy to the oil. The flow energy of the oil is used to drive a turbine wheel. These systems are called hydrodynamic drive systems (for example Föttinger converters and Fluid couplings).

In the case of the hydrostatic energy transfer, a mechanically driven pump (hydraulic pump) produces a mainly pressure-loaded volume flow which is supplied to a hydraulic cylinder or a hydraulic motor. Therein, the pressure energy is reconverted into mechanical energy. These are called hydrostatic drive systems.

The kinetic energy is negligible in systems with hydrostatic transfer energy compared to the pressure energy. Conversely, the pressure energy contained in the flow can be neglected in hydrodynamic energy systems. In mechanical engineering, the hydrostatic drive systems have a much greater importance than the hydrodynamic drive systems.

2 Physical Principles

2.1 Pressure Definition, Absolute Pressure, Overpressure, Pressure Units

For the explanation of the pressure definition a volume section from a fluid shall be considered as shown in Figure 2.1.

Figure 2.1: For the explanation of the pressure definition

The characteristic fluid point O is equal to a point located on the surface of the part fluid (Figure 2.1). At point O the surface element dA is situated, where the force dF is acting vertically. The pressurep is the quotient of dF and dA:

The pressure value is independent of the cutting sectional plane direction touching point O . That means the pressure is a scalar physical quantity; its numerical value depends only on the place in the fluid.

Below, the terms absolute pressure and overpressure (= pressure measured relative to atmospheric pressure) will be explained based on Figure 2.2.

Figure 2.2: Absolute pressure scale and overpressure scale

The absolute pressure scale (upper scale in Figure 2.2) starts at pabs 0 (pressure at vacuum). The difference between the absolute pressure pabs and the local (absolute) atmospheric pressure pamb is the atmospheric pressure difference:

This pressure difference is called overpressure (or gauge pressure).

If the absolute pressure pabs is higher than the local (absolute) atmospheric pressure pamb the overpressure became positive value

If the absolut pressure pabs is lower than the actual (absolute) atmospheric pressure pamb the overpressure became negative value

The minimal (theoretical) overpressure value pe,min is determined by the actual (absolute) atmospheric pressure pamb. For example, if there is a pressure with pamb =1,05bar as shown in Figure 2.2 the minimal overpressure value is

The example shows: The numerical value of the minimal overpressure value is depending on the actual (absolute) atmospheric pressure value pamb.