Page 70 - Fluid Power Engineering

Hydraulic Oils and Theor etical Backgr ound 47

• Water/polymer—water glycol (35% water minimum, 80%
maximum)
• Pure chemical fluids; water-free
• Phosphate esters
• Chlorinated hydrocarbons
• Mixture of phosphate esters and chlorinated hydrocarbons

2.3.2 Mineral Oils
Mineral-based oils are the most widely used hydraulic fluids. They
are relatively inexpensive, widely available, and can be offered in
suitable viscosity grades. They are of good lubricity, noncorrosive,
and are compatible with most sealing materials with the exception of
butyl rubber.
Mineral oils are chemically stable for reasonable operating tem-
peratures. At higher temperatures, however, they suffer chemical
breakdown. Premium grade mineral oils contain a package of addi-
tives to combat the effects of wear, oxidation, and foam formation,
and to improve viscosity index and lubricity. There are, however,
certain disadvantages of mineral oils that cannot be remedied by in-
corporating additives. The two most important are the flammability
and the increase in viscosity at high pressures. Fire risk excludes the
use of mineral oils in hazardous areas such as injection and plastic
molding machines, coal mines, and near furnaces. The viscosity-
pressure characteristics limit their use to pressures below 1000 bar
(see Fig. 2.3).

2.3.3 Fire-Resistant Fluids

Oil-in-Water Emulsion
This hydraulic fluid consists of tiny droplets of oil dispersed in a con-
tinuous water phase. The dilution is normally between 2% and 5% oil
in water, and the characteristics of the fluid are more similar to water
than oil. It is extremely fire-resistant, is highly incompressible, and has
good cooling properties. Its main disadvantages are poor lubricity
and low viscosity.

Water-in-Oil Emulsion
The water-in-oil emulsions are the most popular fire-resistant flu-
ids. They have a continuous oil phase in which tiny droplets of
water are dispersed. Their lubrication properties are very much
reduced. This is partially overcome by running pumps at reduced
speeds. Therefore, larger displacement pumps are necessary to
obtain the required flow rate. The usual dilution is 60% oil + 40%
water. For optimum life, the operating temperatures should not
exceed 25°C, but intermittent operation up to 50°C is permissible.