Page 38 - Fluid Power Engineering

CHAPTER 2

Hydraulic Oils

and Theoretical

Background

2.1 Introduction
Hydraulic fluids are used in hydrostatic power systems to transmit
power. The power transmission is carried out by increasing, mainly,
the pressure energy of the fluid. In addition to the power transmission,
the hydraulic fluids serve to lubricate the contact surfaces, cool differ-
ent elements, and clean the system. Water was the first fluid used for
the transmission of fluid power. The main advantages of water as a
hydraulic fluid are its availability, low cost, and fire resistance. On the
other hand, water is of poor lubricity, has a narrow range of working
temperature, and has a high rust-promoting tendency. These disad-
vantages limited its use to very special systems.
Although mineral oils were readily available at the beginning of
the twentieth century, they were not practically used in hydraulic sys-
tems until the 1920s. In the 1940s, additives were first used to improve
the physical and chemical properties of hydraulic mineral oils. The
first additives were developed to counter rust and oxidation. How-
ever, mineral oils are highly flammable, and fire risk increases when
operating at high temperatures. This has led to the development of
fire-resistant fluids that are mainly water-based, with limitations on
the operating conditions. The need for extremes of operating tempera-
tures and pressures led to the development of synthetic fluids.
This chapter is dedicated to studying the properties of hydraulic
fluids and their effect on a system’s performance. It also explores the
theoretical background needed for studying the topics of this text.

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