in 1959, the work by MIT faculty Blackburn, Reethof, and Shearer [2] is perhaps the greatest technical manuscript ever written on fluid power technology, and it has influenced many following books, including this one.
Many other universities followed MIT and established research institutes expressly devoted to fluid power research. Many of these departments are still alive, shaping research on fluid power technology. A list of the most of these research centers can be found in [3].
The authors encourage the reader interested in learning more about the main historical passages of the hydraulic fluid power technology to consult Skinner's book [4].
Figure 1.3 A collage of pictures from historical moments of the fluid power industry.
Sources: Various online sources.
1.2 Fluid Power Symbology and Its Evolution
One of the main barriers in the past for exchanging technical information related to hydraulics has been the lack of a common “language”: a standard way to represent hydraulic circuits and systems. Figures 1.4 and 1.5 show two examples of hydraulic circuits represented in some “old fashion” styles. Even if the circuits are fairly simple, their representation is not of immediate understanding. They have different ways of indicating the components along with their functions in the circuit; one circuit shows component cutaways, while the other one describes the function of every part with arrowheads and text. Also, one circuit tries to give a detailed representation of the system layout and piping, while the other one uses straight lines between the ports.
Figure 1.4 Hydraulic system of a jet blast deflector.
Source: Beasley 1990.
A multitude of different approaches for representing hydraulic circuits were developed in the past by various companies and fluid power institutes. The aim was the same: representing on a single sheet of paper the system functions, the layout, and the design details of the components. However, the result was different. This lack of commonality represented an obstacle to the development of a common science.
The discussion on fluid power symbols and standards was initiated in the United States by the Joint Industry Conference (JIC) in 1944, and the first JIC‐defined hydraulic symbols and standards were released in 1948. Soon after, the national standardization organizations took over: in 1958 the American Standards Association (ASA) released their revised version of the JIC standard. In Europe, the Comité Européen des Transmissions Oleohydrauliques et Pneumatiques (CETOP), founded in 1962, provided a series of recommendations on graphic symbols for fluid power that were received and approved by the International Organization for Standardization (ISO).
Finally, in 1976, the first version of ISO 1219 standard Fluid Power Systems and Components: Graphic Symbols and Circuit Diagrams was released. The ISO 1219 (parts 1 and 2) is frequently revised and updated. It is nowadays considered as the universal reference for representing hydraulic circuits.
ISO 1219 standard is the universal reference for representing hydraulic circuits. In engineering problems involving a hydraulic control system, it is always recommended to represent the system schematic using ISO symbols and following the criteria suggested by the standard.
Figure 1.5 Hydraulic circuit with pilot‐controlled sequence valve.
Source: Harry Franklin Vickers, 1989.
The detailed description of the ISO 1219 standard is beyond the scope of this book; all rules and conventions can be found on these two ISO standards [5, 6]. However, it is still worthwhile to summarize the primary and most important aspects introduced by the standard:
Function representation. The hydraulic circuit schematic should focus only on the functionality of both the components and system, whereas other information such as mechanical details of the parts or physical layout of the system should not be represented.
Symbols. The standard includes precise recommendations about the symbol to be utilized to represent a specific component. The symbol itself contains relevant information necessary to understand the functionality of the component.
Additional information. In the schematic of the system, each symbol needs to comprise information necessary to understand the operation. These include a label that uniquely identifies the component, particular function of the component, and sizing information.
All the systems presented in this book will adhere to the ISO standard of representation. The additional information will be often omitted for the sake of generality. However, it is important to point out how this information should never be neglected when the circuit is designed for an actual application.
These three aspects of the ISO standard can be described with the example of Figure 1.6, which represents a basic system to control a hydraulic cylinder. The components are laid out in a way that is very intuitive: the oil flows from tank to the actuator following a bottom‐to‐top path; vice versa, the return oil flows from top to bottom. This is the most popular way of representing circuits: all the actuators are clearly located at the top, the power source elements at the bottom, and the control elements in the middle. An alternative layout can also follow a left‐to‐right path. Obviously, this is not related to the actual layout of the system.
The circuit comprises a hydraulic reservoir 1 that supplies flow to a fixed displacement pump 3 driven by an electric motor 2. The pump outlet is connected to a four‐way 3‐position (4/3) solenoid‐operated on/off directional control valve 5, which in neutral position connects P to T, while the workports A and B are blocked. The pump outlet is also connected to an adjustable pressure relief valve 4, which outlet is connected to the return line from 5 to tank. The workports A and B of valve 5 are supplying the linear actuator 6: in particular A is connected to the bore and B to the rod side. When solenoid Y1 is energized, the valve shifts in the “parallel arrows” position, and the pump outlet is connected to the cylinder bore, while the cylinder rod is connected to tank. In this situation, the cylinder extends. When solenoid Y2 is energized, valve 5 shifts in the “crossed arrows” position, and the pump outlet flow is connected to the cylinder rod, while the bore discharges to tank; thus, the cylinder is retracted.
The circuit also provides the additional information needed to complete the understanding of the system operation. These include the size of the tank and the
