Page 62 - Fluid Power Engineering

Hydraulic Oils and Theor etical Backgr ound 39

2.2.6 Lubrication and Anti-Wear Characteristics
The fluid must be capable of covering the contact surfaces of all mov-
ing parts with a thin and continuous lubricating film. The lubricating
film may be destroyed, as a result of high loading forces, insufficient
oil delivery, and low viscosity. This would result in wear due to fret-
ting. The lubricating power and film strength of a liquid are directly
related to its chemical nature and can be improved by the addition of
certain chemical agents. In addition to the wear due to fretting, there
is also wear due to fatigue, abrasion, and corrosion.

2.2.7 Compatibility
The fluid must be fully compatible with other materials used in the
hydraulic system, such as those used for bearings, seals, paints, and
so on. It should not react chemically with any of these materials, nor
change their physical properties. Moreover, the fluid leaks out from
the hydraulic system and encounters other system parts, such as elec-
trical lines, mechanical components, and others, so the fluid must
also be compatible with the materials of these parts.

2.2.8 Chemical Stability
Chemical stability is an important property of the hydraulic liquid. It is
defined as the ability of the liquid to resist oxidation and deterioration
for long periods. All liquids tend to undergo unfavorable changes under
severe operating conditions. Some metals, such as zinc, lead, brass, and
copper, have undesirable chemical reactions with certain liquids. These
reactions result in the formation of sludge, gums, carbon, or other de-
posits, which clog the openings and cause valves and pistons to stick. As
these deposits are formed, certain changes in the physical and chemical
properties of the liquid take place. The liquid usually becomes darker,
the viscosity increases, and damaging acids are formed. The stability of
liquids can be improved by the addition of oxidation inhibitors, which
must be compatible with the other required properties of the liquid.

2.2.9 Oxidation Stability
The oxidation stability is the ability of the fluid to resist chemical deg-
radation by reaction with atmospheric oxygen. It is an extremely
important criterion concerning the quality of hydraulic fluids, particu-
larly in high-temperature applications. It determines the resistance to
aging. The degradation of hydraulic fluids by oxidation can result in
significant viscosity increases, development of corrosive organic acids,
and lacquering of critical surfaces by resinous oxidation products.

2.2.10 Foaming
All fluids contain dissolved air. The amount of dissolved air depends
upon the temperature and pressure. Typically, mineral-based oil can
contain up to 10% by volume of dissolved air. As the temperature of