Principles of Hydraulics

Horst Walter Grollius

• Publisher: Books on Demand
• Year: 2017
• ISBN:
  9783746003115
• Edition: 2
• Topic: Tecnología e ingeniería
• Subtopic: Ingeniería mecánica

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.

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 20^th 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 pressure p 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 p_abs = 0 (pressure at vacuum). The difference between the absolute pressure pabs and the local (absolute) atmospheric pressure p_amb is the atmospheric pressure difference:

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

If the absolute pressure p_abs is higher than the local (absolute) atmospheric pressure p_amb the overpressure became positive value

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

The minimal (theoretical) overpressure value p_e,min is determined by the actual (absolute) atmospheric pressure p_amb. For example, if there is a pressure with p_amb =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 p_amb.

NOTE: Often the indices “abs” and “e” are omitted for clear identification of absolute pressure and overpressure. From the context it is to find out whether absolute pressure or overpressure is of importance.

A commonly used unit of pressure based on the International SI-System is Pascal (unit symbol: Pa)

(Pa = Pascal, N = Newton, kg = kilogram, m = meter, s = second)

An also often used unit is Bar (unit symbol: bar ):

Small pressure values are given in millibar (unit symbol: mbar) or hectopascal (unit symbol: hPa)

The unit used in Anglo-Saxon countries is Psi (unit symbol: psi):

2.2 Law of Pascal

The law of Pascal is the fundamental law of hydrostatics. It is valid f...

Table of contents

• Epigraph
• Notes
• Preface
• Table of Contents
• Symbols
• 1. Introduction
• 2. Physical Principles
• 3. Basic Structure of a Hydraulic System
• 4. Circuit Diagrams
• 5. Examples
• Sources of Literature
• Copyright