The Need for Fluid Power

In applications for which large forces, torques, or both are required, often with a fast response time, it is inevitable that oil-hydraulic control systems will be called on. They may be used in environmentally difficult applications because the drive part can be designed with no electrical components, and often they are the only feasible means of obtaining the forces required, particularly for linear actuation. A particularly important feature is that they almost always have a more competitive power–weight ratio when compared with electrically actuated systems, and they are the inherent choice for mobile machines and plants. Fluid power systems also have the capability of being able to control several parameters, such as pressure, speed, and position, to a high degree of accuracy and at high power levels. The latest developments are now achieving position control to an accuracy expressed in micrometers and with high-water-content fluids. In practice, there are many exciting challenges facing the fluid power engineer, who now must preferably have skills in several of the following topics:

• Materials selection, water-based fluids, higher working pressures

• Fluid mechanics and thermodynamics studies

• Wear and lubrication

• The use of alternative fluids, given the environmental aspects of mineral oil, together with the extremely important issue of future supplies of mineral oil

• Energy efficiency

• Vibration and noise analysis

• Condition monitoring and fault diagnosis

• Component design, steady-state and dynamic

• Circuit design, steady-state and dynamic

• Machine design and its integrated hydraulics

• Sensor technologies

• Electrical–electromagnetic design

• Computer control techniques

• Signal processing and associated algorithms

• Modern control theory and artificial intelligence

Introduction to Basic Concepts, the Hydromechanical Actuator

The interconnection of components to form a closed-loop control system introduces new features, primarily the consideration of steady-state error and the speed of control. Steady-state error is a function of component design, such as the existence of a servovalve spool underlap, and the speed of control is a function of the way control is dynamically achieved combined with the dynamic characteristics of the system. It has been shown in Chapter 5 that fluid compressibility and load mass–inertia play the dominant part when characterizing system dynamics, and these aspects cannot be neglected when the dynamic stability of a closed-loop system is considered. Increasing system gain – for example, by increasing a servovalve servoamplifier current gain in a closed-loop servovalve–actuator position controller – will eventually lead to closed-loop instability. This will result in severe oscillations that could rapidly lead to component damage.

The hydromechanical actuator is one of the simplest forms of closed-loop control with applications reaching back to the very earliest days of industrial manufacturing using cast-iron components and low-pressure water as the working fluid. It incorporates a spool valve and an actuator – for example as shown in Fig. 6.1.

It will be seen that as the handle is moved to the right, the spool valve opens, allowing pressurized fluid to move the actuator in the same direction as the handle movement. In its basic form, it therefore acts in a manner similar to that of a servovalve–actuator system.

Introduction

These studies represent a variety of mathematical and simulation solutions for a range of components and systems and also include much experimental testing with some novel measurement techniques and practical limitations. They are intended to bring together the various aspects of fluid power theory introduced in earlier chapters, but in a more comprehensive manner usually required for more complex systems studies involving the integration of components and control concepts.

Performance of an Axial Piston Pump Tilted Slipper with Grooves

Introduction

This study was undertaken by Bergada, Haynes, and Watton with experimental work in the author's Fluid Power Laboratory at Cardiff University as part of a comprehensive study on losses within an axial piston pump. It was concerned with a new analytical method based on the Reynolds equation of lubrication, with experimental validation, to evaluate the leakage and pressure distribution for an axial piston pump slipper, taking into account the effect of grooves.

The analytical work was developed by JM Bergada (UPC, Terrassa, Spain) with experimental work undertaken by JM Bergada and JM Haynes. Additional CFD analysis and test-rig design was undertaken by JM Haynes and J Watton. Further CFD results by R Worthing and J Watton are also presented in this overview.

The equations consider slipper spin and tilt and are extended to be used for a slipper with any number of grooves.

This book is aimed at undergraduate students as a second-year and beyond entry stage to fluid power. There is much material that will also appeal to technicians regarding the background to fluid power and the operation of components and systems. Fluid power is often considered a specialist subject but should not be so given that the same would not be said for electrical power. In fact, there are many applications for which fluid power control is the only possibility because of force/torque/power/environmental demands. In the past 20 years, a number of groups around the world have made significant steps forward in both the understanding and the application of theory and control, complementing the R&D activity undertaken within the manufacturing industry. Details of just one organization involving many participating fluid power centers around the world are available at www.fluid.power.net. I embarked on this book ostensibly as a replacement for my first book, Fluid Power Systems – Modelling, Simulation, Analog and Microcomputer Control, published by Prentice-Hall in 1989 and now out of print. However, the result is a much different book and perhaps not surprising, given the developments in fluid power in the past 20 years. Following many constructive comments by undergraduate students, friends in industry, and academic friends who still use my first book for teaching, it was clear that a new book was needed.

Fluid Types

The preferred working fluid for most applications is mineral oil, although in some applications there is a requirement for a water-based or synthetic fluid, mainly for reasons of fire hazards and increasingly for environmental considerations. The drive toward nonmineral oil fluids has seen a renewed attitude to pure water hydraulics together with the emergence of biodegradable and vegetable-based fluids. Fire-resistant fluids in use fall under the following classifications:

1. HFA 5/95 oil-in-water emulsion, typically 5% oil and 95% water

2. HFB 60/40 water-in-oil emulsion, typically 60% oil and 40% water

3. HFC 60/40 water-in-glycol emulsion, typically 60% glycol and 40% water

4. HFD synthetic fluid containing no water

5. HFE synthetic biodegradable fluid

The use of water-based fluids has implications for component material selection – for example, the use of stainless steel, plastics, and ceramics. In addition, serious consideration of fluid properties must also be given, particularly viscosity, which can be very high at low temperatures in some cases. Fluids are being continually developed, and the following information is intended to reflect the general trend and is not considered as definitive because this would require an overview of many suppliers from many countries around the world – for example, see www.shell.com.

Type HFA 5/95 oil-in-water emulsions are fire-resistant emulsions that exhibit enhanced stability, lubrication, and antiwear characteristics and have the following important aspects:

• They have much improved stability toward variations in temperature, pressure, shear, and bacterial attack.

• The performance limitations become obvious for systems operating well above 70 bar, reliability and efficiency often being sacrificed where fire resistance is of paramount importance.

• […]

Introduction

The preceding chapters considered the steady-state behavior of common fluid power elements and systems. In reality, fluid power systems handle significant moving masses, and the combination of this with fluid compressibility results in system dynamics that usually cannot be neglected. In addition, individual components such as PRVs require a finite time to accommodate flow-rate changes. This also applies, for example, to a servovalve that again requires a finite time to change its spool position in response to a change in applied current. The combination of these issues means that the design of both open-loop and closed-loop control systems must take into account these dynamic issues. In particular, a closed-loop control system will almost certainly become unstable as system gains are increased because of such dynamic effects. Instability can lead to disastrous consequences if severe pressure oscillations occur. Instability in axial piston motor speed control systems, for example, can result in severe repetitive lifting and impact of the pistons on the swash plate.

Consider the design of a servoactuator that forms one of four to be used to provide the “road” input to the wheels of a vehicle sitting on a rig commonly called a “four-poster.” Figure 5.1 shows one of the servoactuators and a block diagram of the position control system.

Determining the dynamic performance of the position control system only is relatively straightforward once the important dynamic features have been identified.

The Ramjet and the Supersonic Combustion Ramjet (Scramjet) Engine Cycle

An invention attributed to René Lorin of France in 1913 (Hallion, 1995), the ramjet is a remarkable air-breathing engine in its conceptual simplicity. Lacking moving parts and achieving air compression only through internal geometry change, it is capable of extending the operation beyond flight speed when the gas-turbine engine becomes inefficient. The ramjet does not, however, operate from takeoff, and its performance is low at subsonic speeds because the air dynamic pressure is not sufficient to raise the cycle pressure to the efficient operational values.

Above a flight speed of around Mach 3, cycles using rotating machinery, i.e., compressors, are no longer needed to increase the pressure, which can now be achieved by changes in area within the inlet and the diffuser leading to the combustion chamber. Engines without core rotating machinery can operate with a higher maximum cycle temperature as the limit imposed by the turbine presence on the cycle maximum temperature can now be increased. The ramjet cycle with subsonic air speed at the combustion chamber entrance becomes more efficient. As the speed further increases, the terminal shock associated with subsonic combustion leads to both significant pressure losses and elevated temperatures that preclude, in great part, recombination-reaction completion, thereby resulting in considerable energy loss. It becomes more efficient to maintain the flow at supersonic speed throughout the engine and to add heat through combustion at supersonic speed.

Inlets

Introduction

Air intakes for any air-breathing engine-equipped vehicles must

These general requirements for all air-breathing engine inlets would place particular emphasis on some of the stated functions or others, depending on the specific characteristics of the propulsion system used and the vehicle mission. Some of these requirements are of general applicability; minimum pressure losses and least possible drag induction fall into this category. Other inlet characteristics have more or less significant influence, depending on the particular engine used. For example, dynamic distortions induced by an inlet can create serious difficulties for a gas-turbine-engine compressor because they reduce the stall margin, thus limiting the operational range. The extent to which the dynamic distortions affect a scramjet engine operation, on the other hand, is not entirely clear because increased flow unsteadiness could accelerate mixing but may also have a negative effect on momentum losses. This is not the case for the steady-state flow nonuniformities that have been shown to cause significant effects on the scramjet flow field, as they do on other engines.

Design considerations derived from mission requirements lead to specific inlet characteristics.