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    The ultimate hydraulic oil guide

    Hydraulic oil

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    Hydraulic oil is one of the most common fluids found in a hydraulic system and it’s imperative that it’s regularly checked and maintained.

    Hydraulic oil plays a fundamental role in transporting power through a hydraulic system and lubricating system components.

    Features and functions:

    • viscosity: how well the fluid flows in certain temperatures;
    • low temperature fluidity: ease of flow under low temperatures;
    • thermal and oxidative stability: preventing contamination through a sludge build-up;
    • hydrolytic stability / water tolerance: preventing contamination through water;
    • cleanliness and filterability: regular maintenance ensures reduced contamination;
    • demulsibility: ability of the component to filter out water from the oil;
    • anti-wear features that improve the life of components;
    • corrosion control: preventing corrosion of the components;
    • biodegradability and managing environmental impact.

    What does hydraulic oil contain?

    Hydraulic oil includes mineral oil and esters. The latter is a carbon-based compound, which is created by substituting the hydrogen of an acid with an alkyl. Other
    main elements in hydraulic oil include: glycol, silicone and ethers. The latter is a highly flammable, volatile liquid containing an oxygen atom which links two alkyl.

    Other chemical additives are added to most hydraulic oils in order to maintain or improve the performance of both the oil and the equipment within the hydraulic system. They can prevent corrosion, rusting and also water contamination. It is vitally important to always choose the appropriate oil for your system. In fact, the wrong oil can cause performance issues and in more severe cases, permanent damage to your system and components.

    When oil should be avoided

    Most hydraulic components and systems use oil-based hydraulic fluids. There are, however, circumstances when oil should be avoided. For example in applications with potential ignition sources; sparks, open flames, hot metal. In such hazardous environments, a leak from a high-pressure hydraulic system could create an explosion or a serious fire. The term “ATEX” comes from the French “atmosphere explosibles” and is the name commonly given to the framework for controlling explosive atmospheres.

    Hydraulic oil pouringWhich type of oil should be used?

    There are many different types of oil for a wide range of applications and it is important to choose the right oil for your system.

    There are three main groups of equipment and areas that hydraulic oil can be found in: mobile, industrial and aviation. Each one of these requires different types of hydraulic oil depending on the conditions they are operating in and the demands placed on the equipment.

    Hydraulic oil for low temperatures

    There are many different oils for applications that are utilised in harsh, cold environments. Moreover, anti-freeze additives are often added to the oil to ensure it doesn’t freeze.

    Heat-resistant hydraulic oil

    If the temperature of oil is increased it has a lower viscosity, meaning the oil will flow easier. This increases the risk of leaking of the loss of the oil’s properties. In the event that the system is exposed to high temperatures additives are added to the oil in order to maintain a viscosity suitable for the system.

    Oil for high pressures

    If oil is placed under a large amount of stress in high pressure conditions, it still needs to be able to function and enable a hydraulic system to run smoothly. This is where heavy-duty oil is necessary, usually containing additives that prevent wear. These types of hydraulic oil can be found in a wide range of applications including those used in the construction industry.

    Environmentally-friendly oil

    Some applications may pose a potential risk of oil spills or leaks. This could lead to environmental contamination. And therefore, most environmentally-friendly oils are biodegradable and comprise of rapeseed and other vegetables oils.

    Hydraulic oil life

    It’s important to maintain healthy hydraulic systems and components and there are proven methods to help you look after your hydraulic oil.

    Modern hydraulic systems are typically a lot smaller and more compact than they used to be. This means that less oil is used during operation. Pumps can also produce a lot more output, subsequently producing higher pressures. Less oil within the system also means higher fluid temperatures – which in turn, increases oxidation and thermal stress on the additives on the oil. However, engineers still want the most cost-effective oils that last as long as possible and still operates faultlessly.
    Hydraulic Oil Life
    The hydraulic oil longevity depends on a few influences: the quality of the oil, working conditions and possibly even contamination. However, within the right environmental factors, a high quality oil should be able to last at least six months. And with the right maintenance and up-keep, the hydraulic oil life should be able to last even longer.

    Oil Temperature

    Hydraulic oil life can be greatly impacted by the temperature of the oil during operation. The most common temperature in most industrial applications is around 60°C. However it may fluctuate and even rise to 85°C, which is normal. If the oil temperature remains at 85°C for a long period of time, there is a risk of restricting the life of your oil, as the oxidising process is enhanced. For every 10°C that the temperature increases, the more the oxidation increases and thus, significantly reducing the hydraulic oil life.


    Oxidation is the process of the conversion of hydrocarbon molecules in to carboxylic acids. As we’ve said before, higher temperatures can reduce the life of the hydraulic oil, as it quickens the oxidation process. The by-products made from oxidation eventually form a varnish, which subsequently can block filters and suction strainers. If oxidation becomes a big issue, changing the oil immediately is the best course of action.

    Moreover, contaminants (such as water, air, dirt, fuel, and other oils or lubricants) are a common issues that can have a detrimental effect on hydraulic oil life.

    Ways to make your oil more cost-effective

    1).  Ensure that any oil you have in your system is of high quality and from a supplier who can provide technical support when needed;

    2).  Keep an eye on operation conditions. Make sure the fluid is always kept clean;

    3).  Always ensure oil is changed if oxidation or contamination become too much.

    Whilst there are ways the user can help preserve oil life, there are also various methods of ensuring long hydraulic oil life within the oil itself. For instance the usage of additives. The additives used in the oil are dependent on what the oil is used for in the system. This allows the oil to perform in diverse conditions.

    • Anti-wear: lengthens the service life of hydraulic components and machinery;
    • Cold flow: these additives enable a system to operate in extremely cold environments;
    • Anti-foaming: an anti-foaming agent for hydraulic oil reduces foaming within the fluid which is sometimes caused by detergents. This foaming can reduce the lubricating quality of the product thus causing damage;
    • Anti-oxidant: allows for longer periods of use without an oil change and also reduces sludge deposits;
    • Anti-rust: forms a protective coating which reduces the risk of rust damage from oxygen contact.

    Hydraulic filters and oil life

    A hydraulic filter is an essential component of any hydraulic system. Moreover, selecting the right hydraulic filter is absolutely vital to protect oil life and ensure that particle contaminants are removed from the hydraulic fluid before components are jammed or damaged through abrasive wear.

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