Hydraulic Fluids

Hydraulic Fluids

Hydraulics is a technology for transferring of potential or kinetic energy (pressure and movements) using a fluid as the energy carrier. The fluids which are used for this purpose are known as hydraulic fluids. The hydraulic fluid frequently being referred as ‘hydraulic oil’, creates volume flow between pump and hydrostatic motor, and is in contact with all the components in a hydraulic system. Hydraulic fluid is a complex liquid which has to serve many different purposes and posses many different characteristics. Fully formulated hydraulic fluids consist of a blend of a base fluid and an additive package. Hydraulic fluid plays a very important role in the operation of machines.

The operating practices of yesterday in industry have changed a lot. But steady and dependable, hydraulic fluid technology did not change much for decades. But today, the pressure is on hydraulic systems. Hydraulic systems are expected to deliver optimum performance while operating at higher pressures, temperatures, and tougher operating conditions.

Originally hydraulic fluid used was water. Beginning of the 1920s, mineral oil began to be used more than water as a base stock due to its lubrication properties and its ability to be used at temperatures above the boiling point of water. Today most hydraulic fluids are based on mineral oil base stocks. Natural oils such as rapeseed are used as base stocks for fluids where bio-degradability and renewable source is considered important. For specialty applications such as fire resistance the hydraulic oils used are of four types. They are dilute emulsions (oil in water), invert emulsions (water in oil), water glycols and synthetic preparations such as chlorinated hydrocarbons, organophosphate easters, polyalphaolefin, propylene glycol, and silicone oils.

Hydraulic fluid is a medium to transfer power in the system or the machinery. It transmits power, lubricates moving parts, prevents corrosion and wear, provides sealing between clearances, and moreover acts as a heat transfer medium. At the same time, the hydraulic fluid transports solid contaminants to the filters of the hydraulic system. In a hydraulic system, the fluid is just as important as any of the hardware components.

The properties needed from hydraulic fluids are basically classified into two categories namely (i) physical properties needed for normal operation, and (ii) chemical properties needed for stability during long term operations. The properties normally required from the hydraulic fluids include (i) suitable viscosity and a preferably high viscosity index, (ii) pouring characteristics, (iii) lubrication characteristics, (iv) chemical stability of additives, (v) resistance to carbonizing due to heating, (vi) oxidation stability, (vii) anti-rust and anti-corrosive properties, (viii) high shear strength, (ix) high degree of demulsibility, (x) anti-foam properties, (xi) very low compressibility, and (xii) compatibility with seals. Common types of hydraulic fluids are shown in Fig 1.

Fig 1 Common types of hydraulic fluids

The important characteristics needed from a hydraulic fluid are given in Tab 1. For most of the identifying characteristics listed in the Tab 1, there already exist standards or at least preferred testing procedures which allow a numerical classification of these identifying features.

Tab 1 Characteristics of hydraulic fluid
Necessary characteristics of hydraulic fluid
Volume stabilityAdequate capacity to separate air
Wear protection capacityFor a hydrodynamic or hydrostatic fluid layer between sliding surfaces
Adequate viscosity at operating temperature for all others wear reducing additives
Corrosion protection capacityNon–aggressive toward customary materials and rust protection additives
Desirable characteristics of hydraulic fluid
Only slight change in usage Adequate oxidation resistance
Adequate de-emulsification capacity for some cases of application
Adequate shear stability, if polymer viscosity index improvers are used
Viscosity–temperature behaviour So that oil changes due to summer and winter operation become redundant
Adequate viscosity–temperature behaviour
Interaction with seals / gasketsStandard sealing materials can be used
Minimal characteristics changes of standard elastomers

The fluids are classified on the basis of their viscosity, which is useful for the industries to select the fluid for the particular function. The classification ranges from ISO (International Organization for Standardization) classification to the classifications as per various national standards.

According to ISO there are three different types of fluids according to their source of availability and purpose of use. These are (i) mineral oil based hydraulic fluids, (ii) fire resistant fluids, and (iii) Environmental acceptable hydraulic fluids.

Mineral oil based hydraulic fluids – These fluids have a mineral oil base. These fluids have high performance at lower cost. These mineral oils are further classified as HH, HL and HM fluids.

Type HH fluids are refined mineral oil fluids which do not have any additives. These fluids are able to transfer power but have lesser properties of lubrication and unable to withstand high temperature. These types of fluid have a limited usage in industries. Some of the uses are manually used jacks and pumps, and low pressure hydraulic system etc.

Type HL fluids are refined mineral oils which contain oxidants and rust inhibitors which help the system to be protected from chemical attack and water contamination. These fluids are mainly used in piston pump applications. HM is a version of HL type fluids which have improved anti-wear additives. These fluids use phosphorus, zinc, and sulphur components to get their anti-wear properties. These are the fluids mainly used in the high pressure hydraulic system.

Fire resistant hydraulic fluids – These fluids generate less heat when burnt than those of mineral oil based fluids. These fluids are mainly used in situations where there are chances of fire hazards. These fluids are made of lower heat value compared to those of mineral oil based fluids, such as water-glycol, phosphate ester, and polyol esters. ISO have classified these fluids as HFAE (soluble oils), HFAS (high water-based fluids), HFB (invert emulsions), HFC (water glycols), HFDR (phosphate ester), and HRDU (polyol esters).

Environmental acceptable hydraulic fluids – These fluids are basically used in the application where there is a risk of leakage or spills into the environment, which can cause some damage to the environment. These fluids are not harmful to the aquatic creatures and they are bio-degradable. These fluids are used in forestry, lawn equipment, off-shore drilling, dams and maritime industries. The ISO have classified these fluids as HETG (based on natural vegetable oils), HEES (based on synthetic esters), HEPG (poly-glycol fluids) and HEPR (polyalphaolefin types).

Some of the main types of the hydraulic fluids are described below.

HH fluids – These are basic circulation fluids, normally without additives. These fluids have a relatively short lifetime as they are not oxidation-stable and are consequently broken down. These oils are presently no longer in widespread use in many countries.

HL fluids – These are the hydraulic fluids with additives against oxidation and corrosion. On the basis of improved oxidation stability, the fluids have a relatively longer lifetime. These fluids are used in hydraulic systems which have no specific requirements as regards to the anti-wear properties of the oil and for systems which operate under low pressure.

HM or HLP fluids – These are hydraulic fluids with additives against oxidation and corrosion, as well as additives which reduce wear and / or improve the high-pressure properties (EP properties). This is the most widely used type of hydraulic fluid and is a universal hydraulic fluid for a large group of applications which need a long lifetime and good protection against corrosion and wear. These fluids have a viscosity index of around 100.

HLPD fluids – These are hydraulic fluids which, in addition to additives, as in the HLP fluids, include a cleaning additive (detergent).

HV or HVLP fluids – These are hydraulic fluids which, in addition to the additives against oxidation, corrosion, and wear, also contain additives which improve viscosity. These fluids have a viscosity index of greater than 140 and hence have good viscosity / temperature properties. In addition, the HVLP fluids contain a pour point improver. The high viscosity index is achieved through the addition of additives and / or by using base oil with a naturally high viscosity index.

A naturally high viscosity index in the base oil is preferable, since this avoids shear-losses. If a viscosity-improving additive is used, it is important that it has a high mechanical stability so that there is no shear-loss in the molecules, which leads to a viscosity reduction. Shear stability is a measure of the ability of the fluid to withstand viscosity reductions due to the breaking down of the viscosity index improver. These fluids are used within a wide temperature range.

HG fluids – These fluids have additives to improve their stick-slip and anti-stick-slip properties. These additives prevent jerky movements, which can arise in the event of very low sliding speeds and high loads. HG fluids are used for example in hydraulic elevators and cranes.

Important properties of hydraulic fluids

Various properties of the hydraulic fluid affect the ability of the fluid to perform different functions. While selecting a hydraulic fluid one has to be aware of hydraulic fluid properties and its effect on hydraulic system. Normally the hydraulic fluids have many properties and some of the important properties are density, viscosity, viscosity index, stability, bulk modulus, toxicity, and bio-degradability. Beside all the properties cost is the important factor both for the producer of the hydraulic fluid and the producer of the equipment having hydraulic power transmission.

Viscosity – Viscosity, which describes a resistance of the fluid to flow, is the most important. It accounts for hydrodynamic / boundary lubrication, volumetric efficiency, mechanical efficiency, cavitation, quantity of lubricants\ reaching lubricated parts, heat generation, and many other properties like air release, heat dissipation, and filterability.

Viscosity is a measure of the internal friction of the hydraulic fluid or the flow resistance. It is frequently being said that the viscosity indicates the thickness of the fluid. The higher the value, the thicker or more viscous the fluid is. Viscosity is specified as kinematic or dynamic. Kinematic viscosity corresponds to the dynamic viscosity of the fluid divided by the density. For hydraulic fluids, it is most common to specify the kinematic viscosity in centistokes or sq mm/s, at 40 deg C. The hydraulic fluids are classified with regard to viscosity in accordance with the ISO 3448 standard. This standard contains 18 main groups with the designation ISO VG (viscosity degree) (Tab 2). The viscosity is influenced to a large extent by the temperature, as the viscosity increases as the temperature drops and reduces as the temperature rises.

Tab 2 ISO viscosity grades (ISO VG) based on  kinematic viscosity
UnitCentistokes at 40 deg C
ISO VG Minimum Maximum

Viscosity index – The change in the viscosity of the fluid in relation to the temperature is indicated with the viscosity index of the fluid. The viscosity index is an index measure which relates the kinematic viscosity at 100 deg C to the viscosity at 40 deg C. The higher the viscosity index a hydraulic fluid has, the lesser is the viscosity change as the temperature changes. The temperature dependence of the viscosity is determined by the chemical structure of the fluid and any content of viscosity index improving additives.

Mineral oil-based hydraulic fluids normally have a viscosity index of around 100. Some synthetic and vegetable hydraulic fluids have a viscosity index of over 200. Mineral oil-based hydraulic fluid, which is primarily being used in mobile installations or within a large temperature range, has a viscosity index and improving additive. The viscosity index for these fluids then is normally in the range 150 to180. Hydraulic fluids with such a high viscosity index cover a large viscosity range. In practice, this provides the potential to replace several fluid viscosities with a viscosity index of around 100 and with one hydraulic fluid with a viscosity index of around 160. By adding additional viscosity index improving additives, fluids can be produced with a viscosity index range of 350 to 400.

It is important that the polymer which is used is shear stable so that it does not break down after a short period of use, since this lowers the viscosity. The viscosity is also affected by the pressure, although to a much lesser extent than by the temperature. As an example, the pressure in the fluid rises from atmospheric pressure to around 350 bar before the viscosity of a normal hydraulic oil doubles.

Cold properties – When the temperature drops, a fluid becomes more viscous. It is hence important to determine the cold properties of the fluid. It is normal to state the lowest pour temperature for the oil, i.e. the temperature at which the fluid, under certain specified test conditions, cannot flow any more. By selecting a hydraulic fluid which has a pour point of at least 10 deg C below the lowest anticipated starting temperature, it is probable that the system functions well. When a mineral oil is cooled down to minus 30 deg C, for example, it takes around 24 hours for the viscosity to stabilize, while for a vegetable fluid this can take upto 5 days to 6 days. In the Swedish standard SS 155434, a requirement for maximum viscosity after 3 days of storage at minus 20 deg C and minus 30 deg C respectively is specified.

Compressibility (Hydraulic fluid under pressure) – Compressibility (where the volume of the fluid is reduced) is dependent on pressure and temperature. At pressures upto 400 bar and temperatures upto 70 deg C, compressibility is of little importance to the system. At pressures from 1,000 bar and upwards, changes in the compressibility can be registered. The compressibility is normally of little importance, but when dimensioning filters for example, it can be very important. If the hydraulic fluid is used under very high pressure, the fact that the fluid acquires a higher viscosity is to be taken into consideration. As an example, the viscosity of the fluid doubles when the pressure is increased from 1 bar to 400 bar.

Wear-inhibiting properties – In order to improve the ability of the fluid to counteract grinding wear between heavily loaded contact surfaces, the hydraulic fluid is provided with a wear-reducing additive. Even if the hydraulic equipment producers do their utmost to achieve the best possible operating conditions in the hydraulic system, there are frequently a series of unfavourable contacts which make hydrodynamic lubrication difficult. The most common wear-reducing additive used in hydraulic fluids is zinc dialkyl dithiophosphate (ZDDP).

Zinc-free wear-reducing additives have been available and have been used for many years. One important factor, which is increasingly the focus of attention, is that the zinc-free wear-reducing additives have less of an impact on the environment than those containing zinc. Zinc is very toxic to aquatic organisms. The regulatory authorities in various countries are tightening up the national requirements regarding the use of chemicals which have an impact on the environment, and it is expected that the use of hydraulic fluids which contain zinc will be regulated in the future. Many industrial organizations, as well as off-shore installations, do not now wish to use hydraulic fluids containing zinc. Zinc-free hydraulic fluids produce no ash during combustion, and are hence referred to as ash less. Zinc-free hydraulic fluids can also be used in the organizational ‘green accounts’.

The lubricating capacity of a hydraulic fluid, anti-wear properties and high-pressure properties can be tested in several ways. The most common test methods are Vickers’ pump tests V 104C, Vickers 35VQ25, FZG gear rig, 4-Ball test, AFNOR E48-603, DIN 51350 and DIN 51354.

Oxidation stability – Oxidation stability is an expression of the ageing resistance of the fluid. When the fluid comes into contact with the oxygen in the air, a chemical process begins. The hydrocarbon molecules in the mineral oil react with oxygen and form compounds such as organic acids, hydro peroxides and alcohols. The oxidation products can be viscous, sticky deposits or varnish-like, which for example can result in valves becoming stuck. The oxidation process is primarily affected by the temperature, the access to oxygen (air) and catalytic metals such as copper and iron.

The ideal working temperature for a hydraulic installation is between 60 deg C to 70 deg C, and at this temperature hydraulic fluid which contains anti-oxidation additives can have a lifetime of several thousand hours. However, the oxidation speed displays an accelerating course in relation to the temperature, and can be said to double every 8 deg C to 10 deg C. Based on this, it can also be said that the lifetime of the fluid is halved for every 10 degrees that the oil exceeds around 65 deg C. There are several laboratory tests which can analyze oxidation stability. The TOST test (DIN 51587) is one of the most common. The oxidation stability is characterized by the increase in the neutralization value when the fluid is exposed to oxygen, water, steel and copper for 1,000 hours at a temperature of 95 deg C. The maximum limit for the neutralization value is 2 mg KOH/g after 1,000 hours.

Air separation – Mineral oil-based hydraulic fluids normally contain 7 % to 9 % by volume of dissolved air at atmospheric pressure. As long as the air is dissolved in the hydraulic fluid, it is of secondary importance. However, pressure changes in the system can result in the formation of free air bubbles, and the consequence is noticeable in the form of noise, unreliable operation and possibly damage to pumps and other components. Non-dissolved air is a common cause of cavitation on the suction side of hydraulic pumps.

The reason for the absorption of non-dissolved air can be leaks in the suction line, connections, or contamination with another fluid grade containing additives which reduce the ability of the hydraulic fluid to separate air. Foam forms on the surface, and this is a result of the air separation in the fluid. However, a foam-inhibiting additive is added to the fluid, which is to prevent the build-up of surface foam. The addition of extra foam-inhibiting additive to a hydraulic system experiencing foam problems can reduce the foam on the surface in the tank, but cannot prevent free air being bound in the fluid. In the event that the foam-inhibiting additive is overdosed, the air separation ability of the fluid is reduced and operating problems arises, despite the fact that there is a low level of foam in the tank. If is hence important to identify the cause of the foam problems and take the necessary action, instead of the short-term solution of adding more foam-inhibiting additive.

Air release is a measure for the time needed to release air bubbles (free air) contained in the fluid to the surfaces. Air typically enters the circuit through the suction line if the seals are not tight. The rate of air release varies based on different viscosities and temperatures. At a given temperature, air is released faster with lower viscosity fluids, as shown in Fig 2. As the temperature increases, air is also released faster for each fluid.

Fig 2 Air release and foaming characteristics of hydraulic fluids

Water separation – An important property of the hydraulic fluid is the ability of the fluid to separate water. Water contamination can for example result from leakage from the cooler, condensation, or through leaking gaskets. Water in the hydraulic system can cause corrosion, cavitation in pumps, filter problems, an increase in friction and wear, and can also have a negative effect on the durability of the gaskets. It is important that free water can be drained from the system, and it is a requirement that emulsified water in the hydraulic fluid can rapidly be separated. The demulsification capacity of the fluid differs between new and used fluid. Used fluid has a poorer demulsification capacity than new fluid. Contamination by another type of fluid, such as engine oil, can greatly reduce the ability of the hydraulic fluid to separate water. Contamination with other fluid can also lead to sludge formation and cause forming of deposits  in valves and filters.

Rust and corrosion-prevention properties – Corrosion can occur in the hydraulic system when water is present. Even small amounts of corrosion products can have catastrophic consequences for a hydraulic system. Hydraulic fluids hence need anti-corrosion additives, which are intended to counteract corrosion. The risk of corrosion attacks is also reduced if the water penetration is prevented and the system is drained regularly.

Hydrolytic stability – A few types of base oil, such as natural or synthetic esters, as well as certain types of additive, demonstrate a tendency to react with water. During a chemical reaction between a fatty acid and an alcohol, ester and water are produced. This reaction is reversible, in other words if water comes into contact with the ester, it can revert to the original state, i.e. fatty acid and water. The fluid has a lower flash point and acidic constituents are formed (the acid value increases). Filterability becomes poorer, and there is an increased risk of filter penetration. The acidic products which are formed during hydrolysis can attack sensitive metals in the hydraulic system. Esters, which are used in high-quality environmentally adapted hydraulic fluids, are selected on the basis of requirements for the best possible hydrolytic stability, but it is still important to ensure that a fluid is selected with documented, good hydrolytic stability.

Elastomer compatibility (sealing materials) – One important requirement for hydraulic fluids is that they are to be compatible with sealing materials and hydraulic hoses. The optimum in order for the hydraulic system to remain sealed is a moderate swelling (2 %). Hose and gasket producers are always be contacted if there is any doubt as to whether the hydraulic fluid is suitable for the hydraulic system. One of the most common materials in hoses (internal) and gaskets is nitrile rubber (NBR). Many other materials are used in hydraulic systems, which include fluoro-rubber (FPM with the trade name Viton), polyester urethane (AU) and polyether urethane (EU). Materials which are not suitable include natural rubber, polychloroprene (neoprene) and isobutylene isoprene.

In order to choose the right hydraulic fluid, it is advisable to get as detailed information about the system as possible. Factors which are to be evaluated are (i) within what temperature range the system is to work, (ii) the upper and lower viscosity limits, (iii) The recommendations of the manufacturer with regard to the most critical components (in particular the hydraulic pumps), (iv) any specific requirements regarding lubricating properties in conjunction with any of the components, (v) some limitations for any of the construction materials of the system, for example seals, and (vi) whether there is a requirement for the fluid to be biodegradable.

A hydraulic fluid is to be able to accomplish all of its particular work duties, ideally with a good safety margin. Although economy is important, it is frequently not profitable to select the fluid which only satisfies the minimum requirements of the pump producer. The choice on each occasion is dependent on what it costs to purchase a slightly better fluid and what benefits are achieved in the form of less wear and greater operational reliability.

Optimum viscosity grade – The optimum viscosity for a hydraulic system is a compromise between the demands for lubrication capacity and mechanical and volumetric efficiency. This balance can best be determined through practical trials, and frequently it is the nearest the lowest viscosity which is necessary to avoid wear in the pumps. Different pump types place different demands on the viscosity of the hydraulic fluid, and the climatic conditions can represent a significant challenge in connection with hydraulics for outdoor use. As a general rule, the viscosity of the fluid which is used is to be within the viscosity range 10 sq mm/s to 1,500 sq mm/s at start-up of the system, in order for cavitation problems and wear are to be avoided. For optimum efficiency, the viscosity at operating temperature is kept in the range of 20 sq mm/s and 50 sq mm/s.

The performance of pumps and motors used in a hydraulic system is important for the overall efficiency of the system (Fig 3). There are two types of hydraulic efficiencies. One is mechanical efficiency and other is volumetric efficiency of the system. Mechanical efficiency relates to frictional losses in system and volumetric efficiency relates to flow losses in the system. Both of these depend on the viscosity of the hydraulic fluid. The viscosity of fluid is directly proportional to volumetric efficiency and inversely proportional to mechanical efficiency of the system. Hence, the hydraulic fluid is to be selected satisfying both these efficiencies for the maximum overall efficiency of the system.

Fig 3 Optimum operating range of a hydraulic system

The most common method for selecting hydraulic fluid is temperature operating window (TOW) method. Under this method, during the selection of the hydraulic fluid the lowest ambient temperature and the highest operating temperature of the system are determined. Then the hydraulic fluid which falls in this range of temperature is selected.

Normally hydraulic fluid lasts in service for many years, if it does not get contaminated and degraded. Contamination by other fluids, external dirt, wear metals, rust particles and water reduces the life of the hydraulic oil. The characteristics of the hydraulic fluid most likely to change in service are visual appearance, smell, water content, solids content, foaming, acidity and viscosity. Regular quality checks of the hydraulic oil are necessary to identify the potential problems for corrective actions in order to avoid malfunctioning and failure of the hydraulic system.

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