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Hydraulic fluid: It's mission and types of hydraulic fluid

Hydraulic fluid is a key component in the hydraulic system. It’s the fluid that transfers power from the pump to various parts of the hydraulic system. In addition to this it also lubricates the system, acts as a heat-transfer medium, and prevents wear and corrosion. The fluid comes into contact with every component in the system, so it must be compatible with the materials in it, ensuring that the surfaces, seals and other components are not at risk of being altered. It must also have a good ability to deal with water and air, which can otherwise cause severe problems in hydraulic systems – whether in mobile devices or the processing industry.

A hydraulic fluid is comprised of a base oil and additives. The base oil could be various kinds of mineral oil, vegetable oil or synthetic oil. Synthetic oils include polyalphaolefins (PAOs), polyglycols and synthetic esters. The additives take various forms and help to enhance or improve particular properties in the fluid. This could be anything from viscosity, friction, temperature or pressure, to wear, corrosion, filterability or the fluid’s ability to separate water and air. The additives hone the fluid’s performance for specific applications, and it is the additives that determine which product you should use.

Types of hydraulic fluid

Synthetic esters are produced by a chemical reaction between alcohol and fatty acid. The environmental properties vary significantly depending on which alcohols and fatty acids are used. Modern hydraulic fluids that are biodegradable and have an environmental classification tend to have better technical properties than conventional mineral oils, and they can also reduce energy consumption and thereby contribute to lower carbon dioxide emissions. This makes them an efficient choice for many hydraulic systems.

Mineral oil is the predominant base oil and is generally divided into three groups: I, II and III. Group I is conventional solvent-refined mineral oil. Groups II and III are hydrocracked mineral oils, purer in terms of sulphur impurities and with a higher viscosity than Group I. Over the next few years we will see a transition from Group I to Group II, as more refineries that manufacture Group I oils are converted to produce Group II or close down.

A vegetable hydraulic oil is based on raw materials from the plant kingdom, such as oil from rapeseed, turnip rape, sunflower seeds or soy beans. Vegetable oils are always natural and have very good environmental properties, but they are not currently widely used due to their limited technical properties.

There are also hydraulic fluids designed for hydraulic systems in particularly demanding areas. One example is the food industry where, for obvious reasons, the fluids have to meet the very strictest demands on purity (they must be approved for unintentional contact with food), safety and technical performance. Other examples are mining, steel and aviation, which all have extremely high demands on fire safety.

A lot of the prejudices from the first generation of inefficient environmental oils, which were based on rapeseed, still remain today. Forget them! Things are very different today. Modern biodegradable, environmentally classified hydraulic oils, which are usually based on synthetic esters, generally offer better technical performance than conventional mineral oils (which are not environmentally classified), and now represent the most efficient choice for most hydraulic systems.

In hydraulic fluids that meet the environmental requirements of Swedish Standard SS 15 54 34 and are published on what’s known as the SP List both the base oil and the additives have been selected to produce the lowest possible negative impact on nature in the event of a leak or spill. Moreover, they should not cause skin irritation or other allergic reactions in the people who use or maintain machinery.

The EU has the EU Flower Ecolabel. The environmental criteria and technical requirements are very similar in both systems, but there are differences in the fine details.

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