Introduction to hydraulics
Objectives
Upon completing this chapter, one should be able to:
• Understand the background and history of the subject of hydraulics
• Explain the primary hydraulic fluid functions and also learn about the basic hydraulic fluid properties
• Understand how important fluid properties like velocity, acceleration, force and energy are related to each other, and also learn about their importance m relation to hydraulic fluids
• Understand the concepts of viscosity and the viscosity index
• Explain the lubrication properties of a hydraulic fluid.
Introduction and background
In the modern world of today, hydraulics plays a very important role in the day-to-day lives of people. Its importance can be gaged from the fact that it is considered to be one part of the muscle that moves the industry, the other being Pneumatics. The purpose of this book is to familiarize one with the underlying principles of hydraulics as well as make an effort at understanding the practical concepts governing the design and construction of various hydraulic systems and their applications. Additionally the functional aspects concerning the main hydraulic system components as well as the accessory components have been dealt with, in detail. The final part of the book is devoted to the general maintenance practices and troubleshooting techniques used in hydraulic systems with specific emphasis on ways and means adopted to prevent component/system failures.
The Greek word ‘Hydra’ refers to water while ‘Aulos’ means pipes. The word hydraulics originated from Greek by combining these words, which in simple English means, water in pipes. Man has been aware of the importance of hydraulics since prehistoric times. In fact even as early as the time period between 100 and 200 BC, man had realized the energy potential in the flowing water of a river. The principles of hydraulics were put to use even in those early times, in converting the energy of flowing water into useful mechanical energy by means of a water wheel.
Ancient historical accounts show that water was used for centuries to generate power by means of water wheels. However, this early use of fluid power required the movement of huge quantities of fluid because of the relatively low pressures provided by nature.
With the passage of time, the science of hydraulics kept on developing as more and more efficient ways of converting hydraulic energy into useful work were discovered. The subject of hydraulics which dealt with the physical behavior of water at rest or in motion remained a part of civil engineering for a long time. However, after the invention of James Watt’s ‘steam engine’, there arose the need for efficient transmission of power, from the point of generation to the point of use. Gradually many types of mechanical devices such as the line shaft, gearing systems, pulleys and chains were discovered. It was then that the concept of transmitting power through fluids under pressure was thought of. This indeed was a new field of hydraulics, encompassing varying subjects such as power transmission and control of mechanical motion, while also dealing with the characteristics of fluids under pressure.
To distinguish this branch of hydraulics from water hydraulics, a new name called ‘Industrial hydraulics’ or more commonly, ‘oil hydraulics’ was coined. The significance behind choosing this name lies in the fact that this field of hydraulics employs oil as a medium of power transmission. Water which is considered to be practically incompressible is still used in present-day hydrotechnology. The term water hydraulics has since been coined for this area of engineering. But by virtue of their superior qualities such as resistance to corrosion as well as their sliding and lubricating capacity, oils which are generally mineral-based are the preferred medium for transmission of hydraulic power.
The study of ‘Oil Hydraulics’ actually started in the late seventeenth century when Pascal discovered a law that formed the fundamental basis for the whole science of hydraulics. The concept of undiminished transmission of pressure in a confined body of fluid was made known through this principle. Later Joseph Bramah, developed an apparatus based on Pascal’s law, known as Bramah ‘spress while Bernoulli developed his law of conservation of energy for a fluid flowing in a pipeline. This along with Pascal’s law operates at the very heart of all fluid power applications and is used for the purpose of analysis, although they could actually be applied to industry only after the industrial revolution of 1850 in Britain.
Later developments resulted in the use of a network of high-pressure water pipes, between generating stations having steam-driven pumps and mills requiring power. In doing this, some auxiliary devices such as control valves, accumulators and seals were also invented. However, this project had to be shelved because of primarily two reasons, one the non-availability of different hydraulic components and two, the rapid development of electricity, which was found to be more convenient and suitable for use.
A few developments towards the late nineteenth century led to the emergence of electricity as a dominant technology resulting in a shift in focus, away from fluid power. Electrical power was soon found to be superior to hydraulics for transmitting power over long distances.
The early twentieth century witnessed the emergence of the modern era of fluid power with the hydraulic system replacing electrical systems that were meant for elevating and controlling guns on the battleship USS Virginia. This application used oil instead of water. This indeed was a significant milestone in the rebirth of fluid power hydraulics. After World War II, the field of hydraulic power development has witnessed enormous development. In modern times, a great majority of machines working on the principle of ‘oil hydraulics’ have been employed for power transmission. These have successfully been able to replace mechanical and electrical drives. Hydraulics has thus come to mean, ‘the science of the physical behavior of fluids’.
Classification
Any device operated by a hydraulic fluid may be called a hydraulic device, but a distinction has to be made between the devices which utilize the impact or momentum of a moving fluid and those operated by a thrust on a confined fluid i.e. by pressure. This leads us to the subsequent categorization of the field of hydraulics into:
• Hydrodynamics and
• Hydrostatics.
Hydrodynamics deals with the characteristics of a liquid in motion, especially when the liquid impacts on an object and releases a part of its energy to do some useful work.
Hydrostatics deals with the potential energy available when a liquid is confined and pressurized. This potential energy also known as hydrostatic energy is applied in most of the hydraulic systems. This field of hydraulics is governed by Pascal’s law.
It can thus be concluded that pressure energy is converted into mechanical motion in a hydrostatic device whereas kinetic energy is converted into mechanical energy in a hydrodynamic device.