Hydraulic Systems Analysis

Frontmatter

1. Fluid Flow Calculations

Abstract

The reader who is familiar with the traditional subject of ‘hydraulics’ will know most of the things dealt with in this first chapter; the aim here is to summarise those aspects of the subject which are particularly relevant when dealing with power hydraulic systems.

John Stringer

2. Dynamic Analysis

Abstract

The response of hydraulic systems and servomechanisms can be much quicker than the response of their electrical or other counterparts. This is one of their advantages. For example, a hydraulic motor can accelerate or change its speed more quickly than an electric motor of the same torque rating (mainly because it has a far lower inertia). The response of a high-performance hydraulic system is usually measurable in terms of milliseconds with the permissible degree of oscillation closely restricted.

John Stringer

3. Hydraulic Frequency

Abstract

This chapter is concerned with the primary source of oscillations in hydraulic systems, namely the interaction between the inertia of the moving parts and the compressibility of the fluid.

John Stringer

4. Variable Pump Systems

Abstract

This chapter makes use of the concepts already introduced to show how the dynamic characteristics of one type of complete hydraulic servo system can be predicted. The technique involves analysing the component parts separately and then combining the analyses to predict how the system will behave when the components are coupled together. In this chapter the only two components considered are a pump and a motor. As with all linearised analyses, the aim is to obtain a mathematical model which is readily dealt with but at the same time gives reasonably realistic predictions of actual system behaviour.

John Stringer

5. Linear Control Theory

Abstract

This chapter outlines ways of extending the methods of dynamic analysis used in chapter 2 in order to cope with the characteristics of complete servo systems. It begins with an indication of the behaviour to be expected from systems governed by third-order equations (such as that analysed in chapter 4). Then it deals with some generalisations and techniques involving block diagrams and vectorial representations of harmonic response. These lead up to a summary of the criteria which can be used to assess the stability of servo systems in general and of hydraulic servomechanisms in particular.

John Stringer

6. Pumps

Abstract

This chapter represents a respite from the mainly analytical nature of the rest of the book. It is intended simply to outline the types of pump or motor which may be used in hydraulic systems and to stress the similarities between the different types rather than their differences. Some analytical work is included dealing with flow pulsations mainly to indicate a feature which could be a source of vibration or oscillation in a completed system.

John Stringer

7. Flow Through Valves

Abstract

Most hydraulic servomechanisms or other high-performance systems rely for their operation on the metering of fluid through a valve. This chapter deals with a linearised method of analysis for ‘four-way’ valves. They are called this because they have four connections, one for the supply pressure, another for the exhaust, plus two control ports through both of which fluid may be metered, either from the supply to the system or from the system to the exhaust. (A valve may have five ports but these include either two pressure ports or two exhaust ports which can be connected together.) Such valves may be of the spool type as sketched in figure 7.1 or of the nozzle-flapper types. Other sorts of valves are used and the theory in this chapter is extended in appendix B to cover ‘three-way’ valves whilst a little information about poppet valves is given in appendix C.

John Stringer

8. Valve-controlled Systems

Abstract

This chapter deals with the linear method of analysing hydraulic devices which are controlled by using metering valves. Special attention is given to pistons controlled by metering valves of the four-way type but the method is equally applicable to systems employing motors or other types of valve (for three-way valves, see appendix B). The aim of these linear or small perturbation analyses is to derive mathematical models which give reasonably accurate representations of practical system behaviour without being unduly complicated.

John Stringer

9. Electrohydraulic Servo Valves

Abstract

Electrohydraulic systems use low-power electrical signals (of less than say 1 W) for precisely controlling the movements of large power hydraulic pistons and motors (which may be rated at say 10 hp—7460 W—or more). The ‘interface’ between the electrical (control) equipment and the hydraulic (power) equipment is the so-called ‘electrohydraulic servo valve’. These valves are used on systems which must respond both quickly and accurately: aircraft controls are one example and numerically controlled machine tools another, although increasingly stringent specifications for other types of plant are extending their use into most fields. Many mechanisms which use other methods of control particularly if they already employ hydraulics could benefit from incorporating electrohydraulic techniques.

John Stringer

10. Electrohydraulic Servomechanisms

Abstract

Electrohydraulic servo systems combine together the versatility and preci­sion available from electrical techniques of measurement and signal proces­sing with the superior performance which high-pressure hydraulic mechanisms can provide when moving heavy loads and applying large forces.

John Stringer

11. Conclusion

Abstract

The practical development work needed to produce a satisfactory high-performance hydraulic system will by no means be eliminated when the system has been designed by using the methods of linear analysis described in this book. The same would be true even if more complicated methods of calculation were adopted. It is doubtful whether any such system has ever been developed from slide rule and drawing board alone. It is equally doubtful whether a system could be produced by trial and error methods alone. A satisfactory system will probably always need both practical and analytical treatment.

John Stringer

Backmatter