Page 71 - Fluid Power Engineering

48 Cha pte r T w o

At the higher temperature, water content is affected owing to evap-
oration, which decreases the emulsion’s fire-resistance properties.
When the system has been idle for long periods, there is a tendency
for the oil and water to separate. However, during running, the
pump will re-emulsify the fluid.

Water-Glycol Fluids
These fluids were developed primarily for use in aircraft because of
their very low flammability characteristics. However, their applica-
tion is limited since they cannot be used at high temperatures because
of their water content. Their lubricating ability is inferior to that of
mineral oils; they attack most paints; they are very stable with respect
to shear because of the low molecular weight of their constituents;
and their good anti-freeze properties make them particularly suitable
for low-temperature applications.

Synthetic Oils
Synthetic oils, such as phosphate esters, have remarkably good fire-
resistance properties. They are used in industries such as plastic
molding and die-casting, where unusually great fire risks occur. Their
lubricating ability is similar to that of mineral oil.
Elastomers used in conjunction with phosphate esters must be
chosen carefully. Some silicone polymers and butyl rubber are suit-
able. Certain metals, particularly aluminum, and most paints are
susceptible to attack.
Synthetic oils are superior when compared with mineral-oil–
based fluids in one or more of the following respects:

• Thermal stability
• Oxidation stability
• Viscosity-temperature properties (VI)
• Low temperature fluidity
• Operational temperature limits
• Fire resistance

However, mineral oil–based fluids may be advantageous regarding:
• Hydrolytic stability
• Corrosion protection
• Toxicity
• Compatibility with elastomers and construction materials
• The solubility of additives
• Frictional characteristics
• Cost and availability