practical hydraulic & pneumatic systems: …1.5 the energy transfer in a hydraulics field 2 2...

Practical Hydraulic & Pneumatic Systems: Operations and Troubleshooting

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Practical Hydraulic & Pneumatic Systems: Operations and Troubleshooting

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Contents

1 Introduction to Fluid Power 1 1.1 Introduction 1 1.2 History of hydraulics 1 1.3 Advantages over electrical devices 2 1.4 Advantages over pneumatics 2 1.5 The energy transfer in a hydraulics field 2

2 Physics Fundamentals for Hydraulic and Pneumatic Systems 5 2.1 Physical properties 5 2.2 Mass 5 2.3 Weight 5 2.4 Volume 6 2.5 Density 7 2.6 Relative density or Specific gravity 7 2.7 Force 8 2.8 Work 9 2.9 Energy 9 2.10 Potential Energy 10 2.11 Kinetic Energy 10 2.12 Internal Energy 11 2.13 Power 12 2.14 Horse Power 12 2.15 Torque 12 2.16 Heat 13

3 Pressure, Pressure Laws and Flow 15 3.1 Pressure 15 3.2 Vapor Pressure 18 3.3 Hydrostatic principles 20 3.4 Hydrostatic laws 20 3.5 Static fluid pressure 21 3.6 Fluid pressure measurement 21 3.7 Pascal’s Law 22 3.8 Flow and flow rate 22 3.9 Fluid velocity in hydraulic circuits 23 3.10 Types of flow in hydraulic circuits 24 3.11 Bernoulli’s Equation 25 3.12 Friction 28 3.13 Viscosity 29 3.14 Absolute or dynamic viscosity 30 3.15 Kinematic viscosity 30 3.16 Viscosity index 31 3.17 Lubrication and lubricity 32

4 Hydraulic Pumps 33 4.1 Introduction 33 4.2 Pumping Principles 33 4.3 Pump classifications 34 4.4 Standard parameters of pump selection 34 4.5 External gear pumps 36 4.6 Functional description of external gear pumps 36 4.7 Direction change facility in external gear pumps 37 4.8 Sizing of external gear pumps 37 4.9 Characteristics curves – external gear pumps 39 4.10 Applications, advantages and disadvantages 39 4.11 Functional description of internal gear pumps 40 4.12 Sizing of internal gear pumps 40 4.13 Characteristics curves – internal gear pumps 42 4.14 Applications, advantages, and disadvantages 42 4.15 Vane pumps 43 4.16 Functional description of single-chamber vane pumps 44 4.17 Functional description of double-chamber vane pumps 44 4.18 Single-cartridge vane pumps 45 4.19 Double-cartridge vane pumps 45 4.20 Sizing of vane pumps 46 4.21 Characteristic curves – vane pumps 47 4.22 Applications, advantages and disadvantages 48 4.23 Vane pumps – variable displacement 49 4.24 Characteristic curves – variable vane pumps 51 4.25 Gerotor pumps 51 4.26 Functional principles of gerotor pumps 52 4.27 Sizing of gerotor pumps 52 4.28 Applications of gerotor pumps 52 4.29 Characteristic curves – gerotor pumps 53 4.30 Lobe pumps 53 4.31 Functional principles of lobe pumps 54 4.32 Sizing of lobe pumps 55 4.33 Characteristic curves – lobe pumps 56 4.34 Applications, advantages, disadvantages 56 4.35 Radial piston pump –fixed displacement 57 4.36 Functional description of radial piston pumps 58 4.37 Sizing of radial piston pumps 59 4.38 Applications, advantages, and disadvantages 60 4.39 Characteristic curves – radial piston pumps 60 4.40 Axial piston pumps 61 4.41 Axial piston pumps – fixed displacement 61 4.42 Characteristic curves – fixed displacement piston pumps 62 4.43 Functional description – axial piston swash plate variable pumps 63 4.44 Sizing of axial piston swash plate pumps 64 4.45 Characteristic curves - axial piston swash plate pumps 65 4.46 Axial piston bent - axis pumps 65 4.47 Functional description of bent - axis pumps 66 4.48 Sizing of axial piston bent-axis pumps 67 4.49 Characteristic curves - bent-axis axial piston pumps 68 4.50 Application of bent-axis axial piston pumps 69

5 Hydraulic Motors 71 5.1 Introduction 71 5.2 Performance characteristics of hydraulic motors 71 5.3 Classification of hydraulic motors 73 5.4 External gear motors 73 5.5 Functional description of external gear motors 74 5.6 Sealing arrangements in external gear motors 74 5.7 Additional features in external gear motors 75 5.8 Sizing of external gear motors 76 5.9 Characteristic curves of external gear motors 77 5.10 Applications, advantages and disadvantages 78 5.11 Internal gear motors 79 5.12 Functional description of direct drive gerotor motors 79 5.13 Functional description of spool valve type gerotor motors 80 5.14 Functional description of spool valve type geroler motors 80 5.15 Functional description of disc valve type geroler motors 81 5.16 Functional description of valve-in-star geroler motors 83 5.17 Sizing of internal gear motors 84 5.18 Characteristic curves for gerotor and geroler motors 85 5.19 Applications, advantages and disadvantages 85 5.20 Vane motors 86 5.21 Functional description of vane motors 86 5.22 Sizing of vane motors 88 5.23 Characteristic curves for vane motors 89 5.24 Applications 89 5.25 Radial piston motors 89 5.26 Telescopic type radial piston motor 90 5.27 Functional description of telescopic radial piston motors 90 5.28 Characteristic curves for telescopic radial piston motors 90 5.29 Applications 91 5.30 Multi-cam lobe radial piston motors 91 5.31 Functional description of multi-cam lobe radial piston motors 91 5.32 Sizing of radial piston motors 93 5.33 Applications, advantages and disadvantages 94 5.34 Axial piston motors 94 5.35 Axial piston In-line fixed displacement swash plate motors 94 5.36 Axial piston bent-axis motors 95 5.37 Axial piston fixed displacement bent-axis motors 95 5.38 Axial piston variable displacement bent-axis motors 96 5.39 Axial piston dual displacement swash plate motors 96 5.40 Sizing of axial piston motors 97

6 Hydraulic Cylinders 99

6.1 Introduction 99 6.2 General classification of cylinders 99 6.3 Functional types 99 6.4 Cylinder construction types 104 6.5 Cylinder parts and material specifications 105 6.6 Cylinder mounting styles 109 6.7 Cylinder mounting considerations 111 6.8 Cylinder mounting accessories 113 6.9 Cylinders for special applications 115 6.10 Cylinder speed control 115

6.11 Cylinder piston rod strength in buckling 118 6.12 Sealing systems used in cylinders 122 6.13 Cylinders in automation 123 6.14 Common problems in cylinders 124 6.15 Sizing of hydraulic cylinders 127

7 Hydraulic Directional Control Valves 129 7.1 Functions of directional control valves 129 7.2 Classification of directional control valves 129 7.3 Number of service ports 130 7.4 Spool positions configuration 130 7.5 Valve element construction 130 7.6 Type of actuators for directional control valves 132 7.7 Spool valve element characteristics 133 7.8 Spool position changeover in valve operation 135 7.9 Direct-operated directional control valves 136 7.10 Pilot-operated directional control valves 141 7.11 Subplates and manifolds porting patterns 144 7.12 Non-return valves 147 7.13 Pilot-operated check valves 148 7.14 Pre-fill valves 150

8 Hydraulic Pressure Control Valves 153

8.1 Introduction 153 8.2 Pressure relief valves 155 8.3 Characteristics of pressure relief valves 159 8.4 Pressure-reducing valves 161 8.5 Pressure sequence valves 164 8.6 Counterbalance valves 165 8.7 Unloading valves 167 8.8 Brake valves 169

9 Hydraulic Flow Control Valves 171

9.1 Function of a flow control valve 171 9.2 Types of throttling points 172 9.3 Types of flow control valves 173 9.4 Application of two-way flow control valves 179

10 Hydraulic Filters 185

10.1 Function of filters in hydraulic systems 185 10.2 The contaminants 186 10.3 Mechanisms of filtration 187 10.4 Filter locations in hydraulic systems 190 10.5 Selection of filter media and applications 191 10.6 Constructional aspects of filters 195 10.7 Filter circuits 202 10.8 Accessories for filters 202

11 Electro-Hydraulic Systems 203 11.1 Introduction 203 11.2 Operation of proportional valves 205 11.3 Use of DC coils in proportional valves 205 11.4 Amplifier cards 206 11.5 Classification of proportional valves 207 11.6 Proportional directional control valves 207 11.7 Proportional pressure relief valves 211 11.8 Proportional pressure-reducing valves 213 11.9 Proportional flow control valves 215 11.10 Servo valve technology 218 11.11 Comparison of proportional and servo technology 218 11.12 First stage of servo valves 219 11.13 Application of a flapper jet design 221 11.14 Servo valve calculations 225 11.15 The difference between proportional and servo valves 228

12 Hydraulic Accessories 229

12.1 Hydraulic oil reservoirs 229 12.2 Hydraulic tubes and fittings 236 12.3 Hydraulic hoses and fittings 241 12.4 Hydraulic accumulators 249 12.5 Heat exchangers 258

13 Hydraulic Fluids 265

13.1 Introduction 265 13.2 Functions of hydraulic oil 265 13.3 Performance characteristics of hydraulic oil 266 13.4 Hydraulic oil groups 273 13.5 Oil additives 276 13.6 Common problems with hydraulic oil 279 13.7 Oil sampling points 285 13.8 Oil cleanliness 286 13.9 Oil condition monitoring and contamination removal 288 13.10 Fluid change intervals 289 13.11 Storage and maintenance of oil stock 290

14 Seal Design in Hydraulic Components 295

14.1 Introduction 295 14.2 Seal Types 296 14.3 Piston Seals 296 14.4 Gland (headend) seals 299 14.5 Guide rings in piston and gland assembly 301 14.6 Cylinder barrel seals 302 14.7 Seals used in telescopic cylinders 303 14.8 General seal materials for piston, rod, wiper and O-ring seals 306 14.9 Storage and shelf life of seals 309 14.10 O-rings and their applications 310 14.11 O-ring materials 313 14.12 Practical aspects of seal failures - descriptions 316 14.13 Practical aspects of seal failures - illustrated 317

14.14 O-ring failures 319 14.15 O-ring failure analysis 320 14.16 Practical aspects of O-ring failures 321 14.17 Typical seal configuration from manufacturers 326

15 Basic Hydraulic Circuits 337

15.1 Types of hydraulic circuits 337 15.2 Basic understandings of open circuits 339 15.3 Hydraulic circuit illustration in documents 340 15.4 Integrated hydraulic circuits 342 15.5 Sub-circuits 343

16 Maintenance and Troubleshooting 375

16.1 Introduction 375 16.2 Construction of hydraulic systems 375 16.3 Commissioning preparation 379 16.4 Cleanliness in hydraulic systems 381 16.5 Hydraulic system maintenance classifications 382 16.6 Troubleshooting hydraulic components 387 16.7 Hydraulic maintenance documentation 401 16.8 Maintenance equipment 402

17 Air Preparation, Generation and Distribution in Pneumatic Systems

407 17.1 Characteristics of air 407 17.2 Characteristics of pneumatic systems 408 17.3 Air generation, preparation and distribution 409

18 Pneumatic Symbols and Standards 415

18.1 Standards 415 18.2 Symbols 416

19 Pneumatic Elements 427

19.1 Basic Structure of Pneumatic Control System 427 19.2 Components of pneumatic systems 428 19.3 Compressors 429 19.4 Directional control valves 432 19.5 Flow control and pressure valves used in pneumatic circuits 438 19.6 Other valve types 441 19.7 Actuators 444

20 Basic Pneumatic Circuit Design 451

20.1 Operation of single and double acting cylinders 451 20.2 Timing system for cylinder extend and retract cycle 453 20.3 Speed and safety control systems 453

21 Troubleshooting and Fault Finding in Pneumatic Systems 457

21.1 Maintenance requirements for pneumatic systems 457 21.2 Guidelines for maintenance of system components 458 21.3 Troubleshooting pneumatic system problems 460

Appendices 467 Appendix A: Facts Worth Knowing About Hydraulics 467 Appendix B: Hydraulic Circuit Symbols 469 Appendix C: Practical Hydraulic and Pneumatic Problems 501 Appendix D: Solutions to Practical Hydraulic and Pneumatic Problems 507 Appendix E: Practical Exercises using FluidSIM Software 521 Appendix F: Solutions to Practical Exercises using FluidSIM Software 525 Appendix G: Pneumatic Exercises 535 Appendix H: Solutions to Pneumatic Exercises 541

1

Introduction to Fluid Power

In this chapter, we summarize how the term “hydraulics” was derived, the history of hydraulics along with the researchers who have contributed to its advancement. A brief comparison of advantages of hydraulics over electrical devices and pneumatics is made. The inherent characteristics and the energy transfer in the field of hydraulics is discussed. A brief summary of ensuing chapters sketches the basic principles involved, various hydraulic component functions and applications, mathematical calculations for sizing components, explanatory figures, hydraulic circuits and symbols, practical examples and many more.

1.1 Introduction

The term fluid power generally refers to the power generated by fluid substances like liquids and gases. The power generated by the pump is controlled at various stages with the help of valves. Finally, the power generated is applied to the end user to obtain force or motion in the form of an operating mechanism. In our explanation in ensuing chapters, we emphasize that power from liquids (mainly hydraulic oil) will invariably become the operating medium in power transmission. Power from gases, means the transfer of power by air, that has been compressed to a pressure higher than atmospheric air that is put to work in operating mechanisms. To summarize, the use of hydraulic oil, mainly because of its incompressibility characteristic, in energy transfer leads to the term “HYDRAULICS”. The use of atmospheric air’s compressibility characteristic in energy transfer is called “PNEUMATICS.” Both these categories are used in “FLUID POWER SYSTEMS.” Up to Chapter 16 we will be explaining the functionality of hydraulic systems and controls. Chapters 17 to 21, with the common Chapter 2, will deal with pneumatics.

1.2 History of hydraulics The science behind modern hydraulics dates back 2000 years, when water was the only liquid medium available for experimentation. There were many scientists and, mathematicians whose inventions led to the stage-by-stage development of modern hydraulics.

2 Practical Hydraulic & Pneumatic Systems: Operations and Troubleshooting

Aristotle 384–322 BC Theory of motion of liquid Archimedes 287–212 BC Theory of floating body and displacement Leonardo da Vinci 1452–1519 Jet, waves, eddies, continuity and velocity of flow Simon Stevin 1548–1620 Hydrostatic paradox Galelio 1564–1642 Gravitational acceleration Castelli 1577–1644 Principles of continuity Torricelli 1608–1647 Vacuum theory Edme Mariotte 1620–1684 Wind and water pressure and elasticity of air Robert Boyle 1627–1691 Gas laws Blaise Pascal 1623–1662 Principles of hydrostatics Isaac Newton 1642–1727 Inertia, principles of momentum Johann Benoulli 1667–1748 Kinetic theory of liquid and gases Hendri de pitot 1695–1771 Pitot tube and rotating arm Osborne Reynolds 1842–1912 Theory of laminar and turbulent flow Joseph Bramah 1748–1814 Hydraulic press

1.3 Advantages over electrical devices

Hydraulic actuators provide high force transmission at low speeds, whereas electrical motors or devices transmit low force or torque at low speeds. Even though high torque electrical motors are available, they require high current, but the speed is drastically reduced.

Hydraulic actuators can be located in harsh environments. Hydraulics can operate in explosive atmospheres; whereas electrical devices can

generate sparks can cause serious accidents. Constant holding force or torque in hydraulics can be easily achieved, even when the

power system is not running, whereas electrical motors draw large current to maintain the torque even when stopped.

Most electric motors overheat and burnout easily when overloaded. Hydraulic power transmission is practically noiseless, whereas electrical transmissions

are noisy. Many hydraulic components are self-lubricating. Hydraulic maintenance and troubleshooting activities do not need licensed electrical

personnel.

1.4 Advantages over pneumatics Hydraulics are high pressure systems (may be up to 70MPa); whereas pneumatics have

low maximum working pressures (0.7 to 1. MPa). A hydraulic system is practically noiseless, its actuators operate smoothly, whereas in

pneumatics, the system is noisy when compressed air exhausted. Incompressibility characteristics of hydraulic fluid allow the transmission of high force

at low speeds, whereas with air’s compressibility the force out of actuators is limited. When precision control is required, in most cases only hydraulics can satisfy the

requirement.

1.5 The energy transfer in a hydraulics field

Phase-1 Electrical energy is obtained from an electric motor or diesel engines (rotary motion).

Phase-2 Mechanical energy is transferred by a coupling, v-pulleys or gear drive (rotary motion).

Phase-3 Hydraulic energy is generated by hydraulic pumps. Then it is directed by valves with spools having a rotary or reciprocating motion.

Phase-4 Mechanical energy is available from actuators (cylinders or hydraulic motors). They provide reciprocating or rotary motion in the form of pushing, pulling or twisting.

Introduction to Fluid Power

3

Although, various fields like industrial, mobile, marine and aerospace utilize hydraulic systems and controls; consequently, emphasis in this book is placed primarily on the theory, functions, characteristics, applications, and maintenance aspects of industrial hydraulics systems. Many applications presented in this manual are representative in nature to explain the function and operating characteristics of different hydraulic systems and components that commonly exist in this field. It does not promote a particular model or maker. A summary of this book’s contents follows next.

4 Practical Hydraulic & Pneumatic Systems: Operations and Troubleshooting

2

Physics Fundamentals for Hydraulic and Pneumatic Systems

In this chapter, we will learn the basic physical principles that are relevant to hydraulic and pneumatic systems. These are very important to understand. Their applications are widespread in these systems.

2.1 Physical properties

The common physical properties used in other engineering disciplines are also used in hydraulics and pneumatics and are explained in the text below.

2.2 Mass

Mass is a property of a physical object or a body, irrespective of its surroundings, position, or shape which measures the amount of matter contained in it. The mass of a body is constant and measured in kilograms or pounds. The mass of 1 l of water at 4C is taken as 1kg.

A mass will remain at rest. It has resistance to change its position, unless acted upon by an external force. The related energy concepts are potential energy and kinetic energy. Mass due to gravity is the object’s property to interact with the gravitational force on the earth. An object with a smaller mass at rest can be moved with a smaller force, whereas, a large mass requires a large force to move it. The measure of gravitational forces varies between planets. The mass remains same on the earth and the moon, whereas the strength of gravitational forces that attracts the mass is greater on earth than on the moon.

2.3 Weight

Weight is a measurement term used to refer to the mass of an object or the force due to earth’s gravity acting on the object of a given mass. The gravitational force acting on any body or object is directly proportional to the mass and is given as: W = mg, Where “g” is the acceleration due to gravity.

Practical Hydraulic & Pneumatic Systems: Operations and Troubleshooting 6

The value of “g” is taken as 9.81m/s2 in SI units and 32.2ft./s2 in imperial units. Since “g” is on earth, 1kg of mass or object weighs 9.81N.

In everyday use, when we weigh a mass using a scale, we obtain the apparent weight of an object. The weights of the object’s mass or the downward force due to gravity on different planets are shown in Table 2.1 below.

Table 2.1

Acceleration due to Gravity on Different Planets

Earth 1.000 Moon 0.166 Mars 0.377

Mercury 0.378 Uranus 0.889 Venus 0.907 Saturn 1.064

Neptune 1.125 Jupiter 2.364

Weight of a mass on earth is taken as one. Weight of a mass on other planets, multiplied by other factors is given for respective

planets. The unit of weight in SI units is the Newton or simply N. The unit of mass in SI units is the kilogram or simply kg (The imperial unit for mass is

the pound (lb).

2.4 Volume

The volume of a solid object is measured as the space occupied by the three dimensional value of the object and usually expressed as cubic metres (m3) in metric units and cubic feet (ft.3) in imperial units. The volume of a liquid substance is measured as how much space is occupied or displaced in a container at static condition or the capacity of a container to hold the liquid and is usually measured in litres (l) or as cubic metres (m3) in metric units and cubic feet (ft.3) in imperial units (see Figure 2.1).

Figure 2.1 Volume measurement

In describing a pump’s characteristics, the term “Capacity” illustrates the volume of a fluid handled in a specific time. The measurement for capacity is usually expressed as litres per minute (lpm), as cubic metres per minute (m3/min) in metric units or cubic feet per minute (ft.3/min) in imperial units.

Fundamentals of Hydraulics and Pneumatics

7

2.5 Density

The density of an object or a substance is a measure of mass per unit volume and is denoted by the symbol “ρ” (rho). m ρ = ----------- V In SI units ρ = The density of an object measured in kg/m3 m = The total mass of an object measured in kg V = The total volume of an object measured in m3 In Imperial units ρ = The density of an object measured in lb/ft.3 m = The total mass of an object measured in lb V = The total volume of an object measured in ft.3 It is to be noted that when an object’s density is greater, it’s mass per volume is also greater. Comparing substances like iron with water, it is seen that iron is denser than water and occupies less volume.

Density changes with the temperature. The maximum density of pure water at atmospheric pressure and at a temperature of 4C is 1kg/l. When water is cooled to 4C, it contracts, i.e. it’s volume decreases and results in increase in density. The density of solid loose materials, such as powders, sand, etc. is termed as “bulk density” and it varies according to packed conditions. Table 2.2 below shows the density of air at different temperatures.

Table 2.2

Air Density at Different Temperatures

T (C) (kg/m3)

10 1.341 5 1.316 0 1.293

+5 1.269 +10 1.247 +15 1.225 +20 1.204 +25 1.184 +30 1.164

2.6 Relative density or Specific gravity

The term “relative density” is a measure to calculate the density of any material and is dimensionless. It is the ratio of its density to the density of some standard material. The relative density of solids and liquids (s) Density of substance = ------------------------------------- Density of water at 4C The relative density of gases (s) Density of substance = --------------------------------------- Density of air (at particular temperatures)


The terms “relative density or specific gravity” are often ambiguous and have the same meaning. Mostly, when calculating the volume of an irregular object, geologists use relative density, whereas in the field of engineering and medicine, the term “specific gravity” is used.

Specific weight

The specific weight or weight density of a fluid is defined as the ratio of the weight of the fluid to its volume. The letter ‘w’ denotes it. Thus, the weight per unit volume of a fluid is called the Weight density. Weight of fluid Weight density (w) = Volume of fluid

Mass of fluid (m) g =

V Since m / V is density (), the equation for weight can be written as: w = g So, weight density (w) = mass density () acceleration due to gravity (g). Specific weight of water is given by = 1000 9.81 = 9810 N/m3 (in SI units)

2.7 Force

Force can be defined as a pull or a push acting on an object as a result of another object’s interaction trying to create a push or pull or trying to change the present state of rest or motion. Force is a vector quantity and has both magnitude and direction and can also be defined as the rate of change of momentum of an object. In everyday life, we come across different types of forces, either direct interaction or indirect interaction between two objects.

Direct Interaction

Frictional forces exist between fixed and moving parts, air resistant forces like wind and a moving vehicle or an airplane, applied forces like push open a door, pulling a luggage trolley, tensioning forces existing on a string, like a rope or wire supporting a vertical load, hydraulic cylinder piston rod pulling a load, the spring force applied on an object either in compression or tension mode, etc. are some of the examples of force with direct interaction. The magnitude of the force is different in each case and is dependent on the size and mass content of the object.

Indirect Interaction

The gravitational pull between an object and the earth, between planets like the earth and the moon, electrical forces between protons and electrons, magnetic forces like two permanent magnets attract each other when opposite poles come together, all constitute indirect interaction forces. If a force (F) acts on an object of mass (m), the object accelerates in the direction of the force. The acceleration is directly proportional to the force and inversely proportional to the mass of the object. F = m a This relationship is also referred to as “Newton’s second law of motion.”


9

The force of gravity is directly proportional to the mass of the object. The force due to gravity on a mass of 1kg is called a 1-kg f, 9.8N.

2.8 Work

Work is defined as the force through a distance. Work is said to be done on an object, when an object is displaced from an initial position to a final position at a distance apart. Force is applied to move the object. When a force is applied on an object, energy is being transferred and work is being done on the object (see Figure 2.2).

Figure 2.2

Work

The amount of work done is calculated by multiplying the amount of force and displacement Work done = F d

The unit of work is Newton-metres in SI Units and 1 Newton-metre is equal to 1 joule in terms of energy transfer (see Figure 2.3).

Figure 2.3 Work and work done

2.9 Energy

A body is said to possess energy, when it is capable of doing work. Therefore, energy can be broadly defined as the ability to do work. In other words, energy is the capacity of a body for producing an effect. In hydraulics and pneumatics, the method by which energy is transferred is known as fluid power. The energy transfer takes place, from the prime mover to the actuators in three different phases. In the process of energy transfer, heat energy and electromagnetic energy are also associated with it. Since energy can neither be created nor destroyed, it can be transferred from one form of energy to another form, like electrical to mechanical, mechanical to air or oil, and then to mechanical.


Energy can be classified into many different forms of energy, like physical energy, mechanical energy, chemical energy, thermal energy, biological energy, meteorological energy, geological energy, radiation energy, nuclear energy, etc. but all of them are finally reduced down to two important categories (1) Potential energy (2) Kinetic energy. The unit of energy is the Joule (J), and equals a force of 1 Newton moving through a distance of `1 metre.

2.10 Potential energy

Potential energy is the energy stored by an object by virtue of its position or height at which it is stored. An object possesses gravitational potential energy, if it is lifted vertically against the gravity (see Figure 2.4). Examples of potential energy:

A stretched coil spring A waterfall A load hanging on a crane above the ground

Figure 2.4

Potential energy

As seen earlier, work done (WD) equals force times the distance. When lifting an object above the ground, the force of lifting at constant velocity equals the weight of the object. The weight of the object is its mass times the acceleration due to gravity. The displacement (d) of the object becomes the height (h). The work done, the Potential Energy (PE), the work becomes the stored energy at that height. Therefore, the formula is stated as follows: WD = F d, WD= Fgravity d, Fgravity = m g and h=d Potential energy P.E = m g h

2.11 Kinetic energy

Kinetic energy is the energy possessed by an object by virtue of its motion. This energy remains stored in an object moving with constant velocity as long as it is in motion. When the velocity of the moving object becomes zero, the kinetic energy also becomes zero.


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Therefore, the formula derived is as follows: The average acceleration of an object during a particular interval of time:

a = (v v0)/Dt, The distance travelled by an object during the particular interval of time:

x = (v + v0)/2 Dt Work = force distance and according to Newton’s second law, Force = mass acceleration The work = mass acceleration distance “x” W = mass (v v0)/Dt (v + v0)/2 Dt. (v v0) (v + v0) = m ---------- -------------------- Dt Dt 2 Simplifying the above equation: W = w = 1/2 m v2 1/2 m v0

2. Since we consider the kinetic energy at maximum, the formula becomes, K.E = ½ m v 2 The standard measurement of kinetic energy in metric units m2

1J = 1kg ------------------ s2

Kinetic energy is a scalar quantity and does not have any direction. Therefore the kinetic energy of an object is described by magnitude alone. The example of kinetic energy is a car with a definite mass moving at a velocity, the flow of liquid from a higher elevation to a lower elevation, etc.

2.12 Internal energy

Internal energy is defined as the energy associated with the motion of molecules of liquids and gases. For example, a glass of water on a table is said to possess, neither potential energy nor kinetic energy. However, when examining by microscope, at room temperature, water contains molecules, which are moving with high velocity. These molecules have a large amount of kinetic energy stored in them. Any change in the room temperature results in a change in the molecular kinetic energy, since the molecular velocity is a function of temperature. In addition, the molecules in the solid state are attracted toward each other by forces, which are quite large. These forces tend to vanish once the molecules attain a perfect gas state. In processes such as melting of a solid or vaporization of a liquid, it is necessary to overcome these forces. The energy required to bring about this change is stored in the molecules as potential energy. The contents of any sealed vessel also possess internal energy.


The sum of these energies is called internal energy and is stored within the body. This energy is sometimes called as thermal energy.

2.13 Power

Power is defined as the time rate of doing work. The same also can be defined as the amount of energy utilized to accomplish work. Sometimes, the forces may not be acting in the direction of velocity. Therefore to calculate the instantaneous power, the following formula is being used Power “Paverage” = F cos Vaverage The SI unit of power is “J/s.” If the amount of work done is 1J in 1s, then the power is 1W. Therefore, 1W = 1J/s, 1kW = 1000W, 1MW = 106W

The definition of power is being used in many applications, such as mechanical, electrical, and thermal.

2.14 Horse power

The first practical unit of power previously used in engineering was the horse power (HP). Why was it so called? The engineer James Watt invented this term to impress upon his potential customers the abilities of his steam engines. The HP equals 33,000ft.-lbf/min The Watt (W) is 1N x 1m/sec or the Joule/s One HP converts to 746Watts The unit of power is expressed in larger unit as: 1kW = 1000W 1MW=1 000 000=106 W

2.15 Torque

Torque also known as twisting force is measured in kg-metres or foot-pounds (see figure 2.5).

Figure 2.5

Principle of torque


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In the illustration shown, a 10 kgf-metre torque is produced when a force of 10 kg’s is applied to a 1-metre long wrench. This is the theory that finds application in all motors. For a given supply pressure, hydraulic and air motors are rated at specific torque values. The specifications of a hydraulic motor in terms of its rpm at a given torque define its energy usage or power requirement.

2.16 Heat

Due to the conservation of energy concept, energy can neither be created nor destroyed, but transferred from one state to another. In physical terms, heat is a form of energy transfer, associated with the motion of atoms and molecules within an object, irrespective of whether the object is hot or cold. Heat can be transferred from one object to another by the processes of conduction, convention, and radiation.

The units for the measurement of temperature are C, or F, or K

practical hydraulic & pneumatic systems: …1.5 the energy transfer in a hydraulics field 2 2...

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