Page 72 - Fluid Power Engineering

Hydraulic Oils and Theor etical Backgr ound 49

2.4 Additives
The largest class of hydraulic fluids consists of refined hydrocarbon
base oils (petroleum oils) and suitable additives to improve the base
properties. The main types of additives used in hydraulic fluids are
the following.
• Oxidation inhibitors improve the ability of liquid to withstand
chemical reaction with oxygen/air and avoid subsequent
degradation. They are of prime importance when operating
at elevated temperatures.
• Corrosion inhibitors form molecular layers that are bound to
the surface by electrostatic forces and form an effective barrier
against penetration by oxygen and water, which is necessary
for rust-forming electrochemical reactions.
• Antifoaming agents, which form small heterogeneous areas
within the bubble walls of surface foam. Due to their low
surface tension, these small areas are weak spots and result in
bubble bursting.
• Anti-wear additives of high thermal stability.
• Viscosity index improvers
• Pour point depressants
• Friction modifiers are necessary under certain conditions to
ensure smooth operation, free from juddering (stick/slip).
• Detergents are substances providing a cleaning action with
respect to the surface deposits.

2.5 Requirements Imposed on the Hydraulic Liquid
The following are the main requirements imposed on hydraulic
liquids:

• Satisfactory flow properties throughout the entire range of
operating temperatures.
• A high viscosity index that ensures moderate viscosity
variation in relation to the temperature fluctuations.
• Good lubricating properties are a prerequisite to reduce the
wear and increase the service life of the system.
• Low vapor pressure to avoid cavitation.
• Compatibility with system materials since the fluid should
not react chemically with any of the used materials or
deteriorate their physical properties.
• Chemical stability is necessary to increase the service life of
liquid and avoid performance deterioration.