Water was the first fluid used for the transmission of fluid power.

The main advantages of water as a hydraulic fluid its availability, low cost, and fire resistance.

Disadvantages of water:

corrodes,

evaporates,

support growth of bacteria,

contaminates,

poor lubricity,

low viscosity

Mineral oils were readily available at the beginning of the 20th century, but were not practically used in hydraulic systems.

1940s: First additives were used to improve the physical and chemical properties of hydraulic mineral oils. The first additives were developed to counter rust and oxidation.

But, 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 temperatures and pressures led to the development of synthetic fluids.

Power transmission

Lubrication

Sealing – thin hydraulic film act as sealing that reduce leakage

Cooling – capable to absorb heat generated by moving part

Cushioning of oscillations caused by pressure jerks

Corrosion protection

Viscosity Oil density Oil compressibility Thermal expansion Vapor Pressure Lubrication and Anti-Wear Characteristics Compatibility Chemical Stability Oxidation Stability Environmentally Acceptable Hydraulic Oils

Viscosity describes the resistance to the laminar movement of two neighboring fluid layers against each other.

Simply, viscosity is the resistance to flow. It results from the cohesion and interaction between molecules

E.g. The lower plate is fixed, while the upper plate is moving at a steady speed v

Important to keep the oil viscosity within a certain range during the system’s operation; otherwise, the operating conditions will change with temperature.

The viscosity index (VI) of oil is a number used in industry to indicate the effect of temperature variation on the viscosity of the oil.

A low VI signifies a relatively large change of viscosity with temperature variation.

A high VI means relatively little change in viscosity over a wide temperature range.

The best oil is the one that maintains constant viscosity throughout temperature changes.

Effect of Viscosity on Hydraulic System Operation

The oil viscosity influences the function of hydraulic power systems as it introduces resistance to fluid flow and to the motion of bodies moving in the fluid. Herein, the following effects are studied: ◦ Hydraulic losses in transmission lines

◦ Resistance to fluid flow in narrow conduits

◦ Viscous friction forces and damping effect

Density - the mass per unit volume: ρ=m/V.

The hydraulic oils are of low compressibility and volumetric thermal expansion. Therefore, under ordinary operating conditions, the oil density is practically constant.

The density of mineral hydraulic oils ranges from 850 to 900 kg/m3

The oil density affects both the transient and steady state operations of the hydraulic systems.

Local Losses ◦ Result from a rapid variation in the magnitude or

direction of the velocity vector. E.g. throttling elements, elbows, and T connections area

◦ Directly proportional to the fluid density

Hydraulic Inertia ◦ Occurs during the early running stage

◦ Affects the transient response of the hydraulic transmission lines, but it has no significant effect on its steady state behavior

Defined as the ability of liquid to change its volume when its pressure varies.

Liquids are of very low compressibility, compare with gas. Therefore, liquids are usually assumed incompressible.

But this is applied when the liquid compressibility has no significant effect on the performance of the studied system.

The hydraulic oil compressibility has a direct impact on the transient behavior of the hydraulic system.

Generally, the reduction of oil volume by 1% requires an increase of its pressure by 10 to 20 MPa.

The hydraulic liquids are subjected to volumetric thermal expansion. Generally, the volume of liquids changes with temperature as follows:

The fluid must be capable of covering the contact surfaces of all moving 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.

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.

The fluid must be fully compatible with other materials used in the hydraulic system, such as those used for bearings, seals and paints.

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 electrical lines, mechanical components, and others, so the fluid must also be compatible with the materials of these parts.

The hydraulics fluid must be stable in term of chemical and oxidation.

Some metals, such as zinc, lead, brass, and copper, have undesirable chemical reactions with certain liquids.

Can be improved by the addition of oxidation inhibitors

An oil leak may result in considerable pollution of the surroundings and ground water.

Mineral oils are composed of relatively stable hydrocarbon compounds, and are only very slowly broken down by microorganisms in the environment.

Thus, pollution by conventional mineral hydraulic oils can disturb the ecological balance for long periods.

Led to a growing interest in biodegradable products including hydraulics fluid.

Mineral Oils

The most widely used hydraulic fluids.

Relatively inexpensive, widely available, and can be offered in suitable viscosity grades.

Good lubricity, noncorrosive, and are compatible with most sealing materials with the exception of butyl rubber.

Chemically stable for reasonable operating temperatures. At higher temperatures, however, they suffer chemical breakdown.

Premium grade mineral oils contain a package of additives to combat the effects of wear, oxidation, and to improve viscosity index and lubricity.

Disadvantages of mineral oils: ◦ Cannot be remedied by incorporating additives.

◦ High flammability

◦ 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

This hydraulic fluid consists of tiny droplets of oil dispersed in a continuous 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.

Nain disadvantages - poor lubricity and low viscosity.

The water-in-oil emulsions are the most popular fire-resistant fluids.

They have a continuous oil phase in which tiny droplets of water are dispersed. Their lubrication properties are very much reduced.

For optimum life, the operating temperatures should not exceed 25°C, but intermittent operation up to 50°C is permissible.

At the higher temperature, water content is affected owing to evaporation, 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.

These fluids were developed primarily for use in aircraft because of their very low flammability characteristics.

However, their application 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;

Good anti-freeze properties make them particularly suitable for low-temperature applications.

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. Synthetic oils are superior when compared with mineral-

oil–based fluids in term of Thermal stability, Oxidation stability, Viscosity-temperature properties (VI), Low temperature fluidity, Operational temperature limits and Fire resistance

Disadvantages: ◦ Elastomers used in conjunction with phosphate esters must be

chosen carefully. ◦ Certain metals, particularly aluminum, and most paints are

susceptible to attack.

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.

Corrosion protection by adding effective corrosion inhibitors.

Rapid de-aeration and air separation. Good thermal conductivity is required to rapidly

dissipate the heat generated due to friction between elements and due to hydraulic losses.

Fire resistance is essential in some applications. Electrically insulating properties can be

significant in a number of modern designs. Environmental acceptability.