The earliest hydraulic systems were based on using water as the fluid medium.    Water has the prime advantage of fire resistance and is sometimes still used today with additives to reduce corrosion effects.     Hydraulic fluids based on castor oil and alcohol were also used as these are compatable with the natural rubber seals available at the time.     The introduction of more exotic seal materials based on synthetic rubber enabled the introduction of mineral oils which are used today for the majority of hydraulic transmission systems.    The exceptions being applications where fire resistance is of paramount importance.    The notes below relate to hydraulic oil, the design of the oil tanks (reservoirs ) used to contain the oil in hydraulic power packs, and the filters used to remove small particles and contaminants from the circulating oil.

In hydraulic systems the fluid is of primary importance for transferring the energy of motion from the pump to the actuators.    The hydraulic fluid characteristics are important in maintaining the equipment performance and operating life and it is important to use a clean high-quality fluid to ensure an efficient hydraulic system operation . A hydraulic fluid has four principal functions:

Transmission of power: This is enabled by having a fluid with low compressibility and low resistance to flow.

Lubrication of moving parts: The hydraulic fluid fills all the spaces between adjacent moving surfaces reducing friction and wear

Sealing of clearances between mating parts:  The fluid between the vanes / pistons enclosing surfaces acts as sealant

Dissipation of heat: Heat generated in the pumps, motors, and actuators due mechanical losses is removed by the flowing fluid and dissipated to the environment by heat transfer from the pipe /reservoir/ cooler surfaces.

Fluid properties which are relevant to successful operations of hydraulic systems are listed below

The principal advantage of using mineral based hydraulic oils is the enhanced lubrication properties of the fluid which reduces the friction between sliding surfaces.     There are two recognised lubrication regimes :

Hydrodynamic lubrication which occurs when a thick layer of lubricant is present between the surfaces and the fricton is directly related to the fluid viscosity:

Boundary lubrication when a very thin layer of fluid lubrication is present.    This occurs when the conditions are extreme and can result in contact at surface irregularities.

Even under ideal operating condition with hydrodynamic lubrication there can be occasions e.g. at stopping and starting and transient overloading when boundary lubrication can occur.

Detailed information about Viscosity is found at my webpage. Viscosity .   
A viscosity converstion table is provided at viscosity conversion Table

Thicker oils have higher viscosities and result in increased friction in the piping and hydraulic devices.    Thinner oils of low viscosity cause increase leakage and increased wear of sliding parts.

The modern method of grading the viscosity of hydraul oil is to use the ISO 3348 viscosity classification as shown below.

The reference temperature of 40 �C relates to the hydraulic system operating temperature.    Each Viscosity grade (VG) within the classification has approximately a 50% higher viscosity than the preceding one. The the minimum and maximum values of each grade ranges �10% from the identified mid point.    For example, ISO VG 32 refers to a viscosity grade of 32 cSt � 10% at 40�C.    The viscosity at another temperatures can be calculated using the viscosity at 40�C and the viscosity index (VI), which represents the temperature dependency of the lubricant.    The higher the VI the less the viscosity changes with termperature.    Special oils with VI of 125 to 150 have been developed with a very low dependency of the viscosity on the temperature.

The viscosity of an oil also varies with pressure : it increases with increasing pressure see figure below as an example of a typical industrial oil

Chemical stability is another property which is important in the selection of a hydraulic fluid.    It is defined as the liquid�s ability to resist oxidation and deterioration over time.    Modern industrial oils have been formulated to ensure that under arduous operating conditions the oil does not break down resulting in contamination of the fluid and reduction in the lubrication properties.

All hydraulic fluids are, to a small extent, compressible and this is generally expressed in terms of the bulk modulus which is the reciprocal of the compressibility and has the units of pressure.    The bulk modulus K is basically [the change in pressure dp] / [the proportional change in volume (dv/V)].    The equation for the bulk modulus K and the compressibility β is shown below.

The Bulk modulus of is different when a sudden change in pressure occurs (isentropic) to when the change in pressure is slow (isothermal ).    A typical isentropic value of bulk modulus, of an hydraulic oil, is K = 24.1 kBar, for an isothermal value of the same fluid under the same conditions a K = 18,8 kBar results.

The only fire resistant hydraulic fluid is water which is now rarely used.   However a range of mineral based hydraulic oils are available which have some degree of fire resistance in that they are difficult to ignite and resist the tendency to propogate flames.    In most hydraulic systems there is virtually no fire risk and so this is not a problem.    However there are high pressure systems in which a small leak can result in fine spray /mist escaping which can be easily ignited in an engineering environment.    It is therefore vital that if a hydraulic system is used in a region with the risk of hot surfaces and / or sparks /flames then a fire resistant fluid is used.

An hydraulic fluid should clearly have good thermal capacity and conductivity.     The operation of the pump and motors and other hydraulic actuators are not 100% efficient and the losses result in heat being generated.    This heat results in the temperature of the oil increasing . This hot oil is then cooled as it flows back to the reservoir and it is cooled within the reservoir.  The hotter the oil the greater the heat transfer through the piping walls and the walls of the reservoir. Generally there is sufficient heat transfer capacity to remove the heat generated with the oil remaining at a suitable working temperature (Say 60-70 deg. C.). If there is not sufficient heat transfer capacity there is a need for a cooler in the fluid return circuit.

The density of a typical mineral oil is about 0,9 kg/litre. (sg = about 0.9).

Foam is an emulsion of gas bubbles in the fluid. Foam in a hydraulic system results from compressed gases in the hydraulic fluid. A fluid under high pressure can contain a large volume of air bubbles.    When this fluid is depressurized, as when it reaches the reservoir, the gas bubbles in the fluid expand and produce foam.    Any amount of foaming may cause pump cavitation and produce poor system response and spongy control.     Any tendency of an hydraulic fluid to foam, as is moves round the hydraulic circuit, or is agitated as it returns to the reservoir, or is drawn from the reservoir is a great problem.    This could result in poor performance and failure of the pumps and a significant reduction in the performance of the hydraulic actuators

All hydraulic fluid has a limited life and has to be disposed.    Maintenance/ installation/ decommissioning personnel all come in intimate contact with hydraulic fluid when undertaking their work.     It is therefore necessary that there is no toxicity risk involved in the handling and disposal of hydraulic fluid.

In selecting a suitable hydraulic fluid, cost and availability, can be and important factor especially when it need to be regularly replaced or replenished, as on machinery/ vehicles used in agriculture, construction and heavy goods vehicles.    For more critical applications requiring fire resistance and in precision electro-hydraulic servo systems the fluid cost/ availabiliy may not be of prime concern but is is one of the important factors which need to be taken into account

Low volatility is desireable in hydraulic fluids . Volatility is the rate at which fluids vaporize.    High volatility can lead to fluid loss and potential equipment damage.

Related to volitility is the fluid flashpoint.  This is the temperature at which a liquid gives off vapor in sufficient quantity to ignite momentarily or flash when a flame is applied.     A high flashpoint is desirable for hydraulic liquids because it provides good resistance to combustion and a low degree of evaporation at normal temperatures.    Desired flashpoint minimums vary from 150�C for the lightest oils to 265�C for the heaviest oils.

There are numerous types of fluid used in hydraulic systems.    Currently, liquids used include mineral (petroleum based) oil, water, phosphate ester, water-based ethylene glycol compounds, and silicone fluids.     The three most common types of hydraulic liquids are petroleum-based, synthetic fire-resistant, and water-based fire-resistant.

The most common hydraulic fluids used in general/mobile systems are the petroleum-based oils.    These fluids contain additives to protect the fluid from oxidation (antioxidant), to protect system metals from corrosion (anticorrosion), to reduce tendency of the fluid to foam (foam suppressant), and to improve viscosity.    Petroleum-based fluids are used in mobile systems, electrohydraulic servo systems general machinery systems, aircraft systems, shock absorbers, brakes, control mechanisms, and other hydraulic systems using seal materials compatible with petroleum-based fluids.

Petroleum-based oils contain most of the desired properties of a hydraulic liquid.    However, they are flammable under normal conditions and can become explosive when subjected to high pressures and a source of flame or high temperatures.

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Phosphate ester fire-resistant fluid are often used when fire resistance is important.     These fluids will burn if sufficient heat and flame are applied, but they do not support combustion.     The disadvantage of phosphate ester fluids are that they will attack and loosen commonly used paints and adhesives, deteriorate many types of insulations , attack many gasket and seal materials.

Phosphate ester fluid is used in aircraft elevators.

This type of fluid contains a controlled amount of neurotoxic material.     Because of the neurotoxic effects that can result from ingestion, skin absorption, or inhalation of these fluids should only be used if safe working practices are adhered to to ensure operator safety:

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The most widely used water-based hydraulic fluids may be classified as water-glycol mixtures and water-synthetic base mixtures.     The water-glycol mixture contains additives to protect it from oxidation, corrosion, and biological growth and to enhance its load-carrying capacity.    Fire resistance of the water mixture fluids depends on the vaporization and smothering effect of steam generated from the water.    The water in water-based fluids is constantly being driven off while the system is operating.    Therefore, frequent checks to maintain the correct ratio of water are important. Water glycol based fluids contain 35-60% of water in form of solution (not emulsion) .

A disavantage of this type of fluid is that the temperature range is relatively low (0�C - 49�C).

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Silicone synthetic fire-resistant fluids are frequently used for hydraulic systems which require fire resistance, but which have only marginal requirements for other chemical or physical properties common to hydraulic fluids.    Silicone fluids do not have the detrimental characteristics of phosphate ester fluids.     These fluid do not provide the corrosion protection and lubrication of phosphate ester fluids, but they are excellent for fire protection.

Below are tables showing properties of some typical ISO classified hydraulic oils

The design of the reservoir should be of sufficient capacity to contain all of the fluid in the hydraulic system with at least a 10% excess margin.     For static hydraulic transmission systems the reservoir capacity of about 6 x the pump flow/minute should be available.     On mobile units it is often necessary to have a smaller reservoir.

The return line to the reservoir should be at the furthest end from the pump inlet feed to allow solid particale the drop out and entrained air to be released to the open surface.    The return pipe can include a pepper pot arrangement well below the fluid surface level to encourage dispersion of flow    The reservoir should be designed with a safe working level such that the the pump inlet and the system return pipe are continuosly immersed at all times during the operating cycle.     The oil flow through the reservoir should be at a low rate and preferable through perforated baffle plates to encourage air and precipitation of contaminants.     The reservoir normally provides a cooling function, often eliminating the need for an oil cooler.   I have produced a webpage identifying crudely the heat loss from a gearbox . These notes could be used, with judgement, to ROUGHLY estimate the rough heat dissipated from a reservoir. Gear Thermal .    I have also provide a simple calculation for heat loss on my webpageHydrostatic Calculations

The construction of the reservoir is generally based on a simple rectangular box with a floor sloping down to a drain plug.     The tank should have internal corners suitably designed to ensure convenient cleaning and the surfaces should be desecaled and painted with a paint which is corrosion resistant and suitable for the oil contained.    The tank should include a sealed lid which includes a Filler/breather cap with air filter included and a sight level gauge on the side.

Filters are an important part of hydraulic systems.     Metal particles are continually produced by mechanical components and need to be removed along with other contaminants.

Filters may be positioned in many locations.    The filter may be located between the reservoir and the pump intake.    Blockage of the filter will cause cavitation and possibly failure of the pump.    Sometimes the filter is located between the pump and the control valves.    This arrangement is more expensive, since the filter housing is pressurized, but eliminates cavitation problems and protects the control valves from solids originating in the pump.    The third common filter location is just before the return line enters the reservoir.    This location is relatively insensitive to blockage and does not require a pressurized housing, but contaminants that enter the reservoir from external sources are not filtered until passing through the system at least once.

The current convention is to provide the following filters in a hydraulic system.

1)The reservoir will be sealed and provided with a filler cap with an air filter included to minimise contamination of the fluid from the environment.
2) The pump suction will be provided with a course filter /strainer to protect the pump from large solids with have arrived into the reservoir in error.
3) If the system includes servo-valves or items with extremely close gaps between the moving parts the pump delivery line should include a fine filter
4) In general the return line to the reservoir should include a high quality filter to remove particles in the piping and generated by the motion of the pump /motor/actuator
5) Precision components, with small gaps between moving parts often include fitted fine filters

It is also good practice to circulate the oil continuosly for a period of time, with the components with very small gaps being out of circuit.     When the oil has been suitably cleaned the filters elements are replaced and the sensitive components are introduced into the circuit.

Typically, manufacturers of hydraulic equipment specify the required cleanliness level for oil in systems using the ISO4406:1999 Standard. This scheme estimates contamination by counting the number of particles larger than 4, 6 and 14 �m in a 100ml sample of fluid.

There are many types of filter in use including felt, woven wire, impregnated paper. and sintered metal.    Those which are capable of being easily cleaned are usually above 25 μm are used as suction filters.    the most popular throw away filters are impregenated paper filters which are supplied in ranges < 2 to 40 μm.. these are used in suction returen and also high pressure filters.     The allowable flow through a filter element depends on the permeability of the oil and the area of the exposed media to the oil flow.    A crude measure of the permeability is shown on the graph below.

It is important to note that all hydraulic filters have to be regularly serviced .    This generally involves replacement of the filter element.    With some filter systems this may involve cleaning the element and replacing it.    Of all maintenance tasks this is probably the most important.    Some filter systems have warning instruments to identify that the filters are blocking.    The frequency of replacement is clearly dependent on the duty.