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    Hydraulic Oil Guide

    Hydraulic oil is one of the most common fluids found in a hydraulic system, so it’s imperative that it is looked after with regular maintenance.

    As well as playing a fundamental role in transporting power through a hydraulic system and lubricating system components, hydraulic oil has many other features and functions including:

    • 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.

    The main elements usually found in hydraulic oil are: mineral oil, esters (a carbon-based compound created by substituting the hydrogen of an acid by an alkyl), glycol, silicone and ethers (a highly flammable, volatile liquid containing an oxygen atom which links two alkyl).

    Most hydraulic components and systems use oil-based hydraulic fluids, hydraulic oil. There are, however, circumstances when hydraulic oil should be avoided, e.g. 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 and / or serious fire. The term “ATEX” comes from the French “atmosphere explosibles” and is the name commonly given to the framework for controlling explosive atmospheres that may exist and that standards of equipment (and hydraulic fluid) used in them.

    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 – the wrong oil can cause performance issues and in more severe cases, permanent damages of your system and components.

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    Hydraulics allow energy to be transferred through oil in a hydraulic system. There are many different types of hydraulic oil for a wide range of applications and it is important to choose the right oil for your system. This may also include a bespoke mixture made for specific requirements.

    Most hydraulic oils are made up of mineral oils, synthetic and fire-resistant fluids, which are used in all types of applications.

    There are three main groups of equipment and areas that hydraulic oil can be found in: mobile, stationary / 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 hydraulic oils for applications that are utilised in harsh, cold environments; anti-freeze additives are often added to the oil to ensure it doesn’t freeze.

    Heat-resistant hydraulic oil: If the temperature of hydraulic 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. Additives are added to the oil in order to maintain a viscosity suitable for the system, when it is exposed to high temperatures.

    Hydraulic oil for high pressures: If hydraulic 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 hydraulic oil: Some applications may pose a potential risk of oil spills or leaks. This could lead to contamination within the environment, so most environmentally-friendly oils are biodegradable and comprise of rapeseed and some other vegetables oils.

    Other features of hydraulic oil to consider when looking into the different types of hydraulic oil include:

    • thermal steadiness;
    • durability;
    • hydrolytic balance;
    • continuous viscosity;
    • decreased chemical corrosives;
    • high anti-wear characteristics;
    • total water rejection;
    • decreased likelihood of cavitating;
    • cost-effectiveness.

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    Hydraulic oil viscosity must be considered when choosing the right hydraulic oil for your system.

    In hydraulics, there’s a direct relationship between hydraulic oil viscosity and its temperature – if the temperature of the oil increases then the viscosity decreases. For example, when you put cooking oil into a pan and heat it up, the oil moves faster the longer it is heated. If the temperature drops, the oil will move less easily and it becomes more viscous.
    As applications become more demanding for hydraulic fluids, hydraulic oil viscosity needs to be able to work effectively to enable smooth operation no matter the temperature.
    Hydraulic oil viscosity explained
    How viscose hydraulic oil is depends on how smoothly it flows. If viscosity increases, then it will take longer for the oil to pass through the hydraulic system. High viscosity means the oil is thicker and is more difficult to transport through a system, whereas lower viscosity means the oil is easier to pass through the system. The measurement of viscosity in hydraulic oil is taken in Centistokes (cSt) and most commonly at temperatures of 40°C and 100°C.
    Hydraulic oil viscosity index
    The hydraulic oil viscosity index helps us to measure the change in temperature in hydraulic oil. A lower viscosity index means that the oil may be more susceptible to change than if it were a higher viscosity index. If the hydraulic oil viscosity is high, the oil will be better suited to an application which is used in harsher environments, exposed to a variety of operating temperatures.
    The viscosity must be accurate for your application and the temperature, even if the oil has properties including anti-wear, anti-oxidisation or anti-corrosion – an incorrect viscosity could result in damage to hydraulic equipment, problems during operation and reduced service life.
    The table below shows the classifications of hydraulic oil viscosity against different temperature ranges:

    Viscosity Index Classification
    0-35°CLow
    35-80°CMedium
    80-110°CHigh
    110°C and greater

    Very High

    Pumps and Viscosity Requirements

    Hydraulic Oil ViscosityDifferent pumps require different viscosities. There are three main types of pump that can be found within a hydraulic system: vane pumps, piston pumps and (internal and external) gear pumps. Each pump is utilised in different ways for different applications.

    Vane: there are rotors inside vane pumps that include slots attached to a shaft that is rotating erratically to a cam ring. When in operation, the vanes can wear out, due to the constant contact between two surfaces. Because of this, vane pumps are usually less cost-effective when it comes to maintenance, however they are useful for keeping a stable flow. Vane pumps work best with viscosities ranging between 14 to 160 centistokes (cSt) at normal operating temperatures.

    Piston pumps are more suited to harsh conditions, manufactured for durability in design and during operation. These pumps are able to operate at higher pressures, usually up to 6,000 psi. Piston pumps work best with viscosities ranging between 10 to 160 centistokes (cSt) at normal operating temperatures.

    Gear: Although gear pumps are classed as the most inefficient out of all of the pumps, they are still able to function through a large amount of contamination. They work by expelling fluid once it has been pressurised between the meshing teeth in the pump. Gear pumps come in two variations: internal and external.

    Internal gear pumps are usually available in a variety of viscosities, ranging up to 2,200 cSt. With high efficiency and reduced noise, these pumps typically operate under pressure from 3,000 – 3,500 psi.

    Although external gear pumps are less proficient than there counterpart, they still offer easy maintenance, stable flow and are cost-effective when it comes to purchasing and repairs. Similarly to the internal gear pumps, external gear pumps produce pressure between 3,000 – 3,500 psi – however, their viscosity range only reaches 300 cSt.

    Leakage in a hydraulic system can result in unreliable performance, increased energy consumption, increased maintenance costs and overall reduced cost-effectiveness. Unfortunately, leaks aren’t always easy to spot – often not until after operation has been negatively affected.

    Hydraulic Oil LeakHowever, some hydraulic systems have leaks that have been planned; allowing an equipment manufacturer to record the maximum amount of leakage during normal operation. Deliberate leakage allows a fluid to travel from a highly pressurised zone to a lower pressurised zone to clean and cool specific areas of a component. The fluid is unable to exit, leaving no obvious sign of its existence.
    Most leaks are a direct result of wear along the surfaces of hydraulic equipment. However, they can also occur through poor design, inappropriate equipment choice and poor quality control during the manufacturing process. The first obvious signs of leakage could be decreased performance, unreliability and increased working temperatures.
    If the viscosity of hydraulic oil is low or the temperature is too high, this may increase the chance of leaking. Untimely wear of surfaces and fluid properties in a component can be a consequence of this. Many original equipment manufacturers recommend a maximum viscosity in order for their equipment to operate fully. Choosing the correct oil and sustaining a sensible temperature to retain this viscosity is usually achieved by the end user.

    A hydraulic oil leak can be found in one of two ways:

    Infrared thermometers: these are useful for taking unobtrusive measurements of equipment temperature. Abnormal temperatures in relief valves are examples of anomalies that could go undetected, due to the hydraulic system’s cooling or distribution of heat throughout the equipment.
    Ultrasonic detection localises the internal leakage, however it does not confirm how much the component is leaking. To gain a better understanding of the hydraulic oil leak, the best measurement would be taken from a flow meter or similar equipment.
    In many cases replacing fluid can be a lot less cost-effective than buying brand new oil; safety and environmental issues also need to be considered, however.
    Hydraulic Oil LeakTo make it easier to detect an external hydraulic oil leak, dyes that are sensitive to black light are good ways of determining the location of external leaks. This dye is designed to be compatible with both the hydraulic fluid being used and the component surfaces themselves without contaminating the oil. When mixed with the oil, the dye creates a bright green/yellow glow when exposed to the black light. This enables you to see where the leak in coming from in the hydraulic system.

    Hydraulic oil life is important to maintain healthy systems and components. There are methods to help look after your hydraulic oil.

    Modern hydraulic systems are typically a lot smaller and more compact than they used to be, meaning that less hydraulic 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 longevity of hydraulic oil depends on a few influences: the quality of the oil, working conditions and possibly even contamination. However, a high quality hydraulic oil should be able to last at least six months if the environments it is in aren’t harsh. With maintenance and up-keep, the hydraulic oil life should be able to last much longer.
    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 oi 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 has the consequence of sticking servo valves and sludge, which would build up and block filters and suction strainers. If oxidation becomes a big issue, changing the oil immediately is the best course of action.
    Contaminants (such as water, air, dirt, fuel, and other oils or lubricants) are a common issue of oils in hydraulic systems, but it can cause many issues.
    Water contamination is the most common form of contamination in hydraulic oil (see ‘WATER IN HYDRAULIC OIL’). Water is harmful to components because it doesn’t lubricate the surfaces and can cause wear. Corrosion may also occur if the water reacts to the additives in the oil. Rusting can also occur to component interiors, again accelerating the oxidation process.
    If contamination arises in the air, oxidation can occur, adding to the viscosity of the oil. Over a long period of time, this can also varnish the surfaces. The contamination in the air may cause pumps to cavitate as the air in the oil dissolves due to the heat, which can damage the pump.
    Other hydraulic oil contaminants may include dirt, metal particles and soot. They can cause corrosion on component surfaces and low viscosity, with lower viscosity reducing lubrication effectiveness.
    All of the conditions mentioned above can have a detrimental effect on hydraulic oil life. Although there is no guaranteed way to make it last a lifetime, there are ways to make your hydraulic 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 hydraulic oil life, there are also various methods of ensuring long hydraulic oil life within the oil itself, one of which is to use additives. The additives used in the hydraulic oil are dependent on what the oil is used for in the system. This allows the hydraulic 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 hydraulic oil life:

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

    Hydraulic oil contamination is a common problem amongst hydraulic components and systems, but there are ways of preventing it.

    Hydraulic Oil ContaminationIt is vitally important that your hydraulic oil is contamination-free, as it can degrade the fluid and, eventually, cause system failure. It poses the risk of internal leakage as well as decreasing the control of flow and pressure in valves, which could ultimately waste horsepower and produce too much heat. Contamination can form from hydraulic oil, environmental exposure, system wear, the manufacturing process and servicing.

    Contaminants: Water can cause corrosion within the hydraulic system and in turn, its components. Water may be able to get into a system through design flaws, poor maintenance or fluid-servicing procedures.
    With other hydraulic fluids, contamination may occur with both miscible and immiscible liquids. This can occur when a hydraulic fluid is put into the hydraulic system without first being cleaned. Some oils are compatible however, and can be mixed without causing harm to the system, as they do not form insoluble solid materials. This could still be classed as contamination though, as their properties are not retained within the mixture.

    Effects of contamination: Hydraulic oil contamination starts in the oil itself and is then carried through the system; having a hugely negative effect in a closed or sealed system. Contaminants produce solids in components and over a long period of time and usage, can reduce productivity and damage equipment.

    Particle sizes determine how damaging the contaminant can be. Smaller particles form silt and gradually erode the interior surfaces of the equipment, eventually preventing operation. Particles that are similarly sized to the clearance between two surfaces can cause the equipment to jam and wear. Large particles block ports, resulting in malfunction and preventing a mechanism from operating.

    Hydraulic system malfunctions are categorised in three ways:

    • Degradation (wear of the equipment);
    • Transient (sporadic failures);
    • Catastrophic (complete failure of a system or component).

    Reducing contamination: Contamination can become a recurring issue for some hydraulic systems. However there are preventative actions you can take in order to prevent catastrophic failure:

    • Choosing the right hydraulic filter: A hydraulic filter is an essential component of any hydraulic system and selecting the right hydraulic filter is absolutely vital to ensure that particle contaminants are removed from the hydraulic fluid before components are jammed or damaged through abrasive wear.
    • Choosing the right oil: It is vital to ensure that the hydraulic oil you use is the right one for your system – see ‘TYPES OF HYDRAULIC OIL’, above. It is also important to refrain from mixing oils, even if the system stipulates that other fluids can be used. If other oils have to be used, the system must be thoroughly cleaned before another fluid is added. In some instances, mixing oils can be detrimental to the system. When choosing a fluid, environmental and hazardous material regulations must be kept in mind.
    • Proper fluid handling and storage: Before use, oils should be kept in their sealed containers.  The lids of these containers should be checked on a regular basis and secured. If the temperature is constantly fluctuating, it can cause the containers to expand and contract the air and liquid inside. Temperatures changes can also result in moisture in the container, producing water in the oil. Some liquids are able to absorb moisture, but require extra procedures to prevent contamination.
    • Maintaining a hydraulic system: Hydraulic systems sometimes need to be repaired due to frequent use. During maintenance, hydraulic oil contamination should always be checked for and preventative measures should be placed to keep contamination from damaging the system.

    Water in hydraulic oil is one of the most common causes of contamination in a hydraulic system.

    Water can find its way into the fluid in a number of ways including worn seals, breathers or condensation. It may also enter the hydraulic oil through leaking heat exchangers or coolers from the water used during operation of the equipment. To prevent this your equipment must have high quality design and regular maintenance, however it can be costly and time-consuming to prevent all possible sources of water from getting into the oil.
    Water can have hugely damaging effects on hydraulic components. Corrosion of the surfaces in the system is the most prominent effect and is caused by free water. Bearings can be directly affected by this corrosion through metal surface wear and may still be affected even if the water is the fluid has dissolved.
    Not only does water wear the components of the system, but it can also have an impact on the oil itself, changing its physical and chemical properties. Physical properties include:

    • viscosity – see ‘HYDRAULIC OIL VISCOSITY’, above;
    • lubrication and load-carrying;
    • compression in hydraulic systems.

    The absolute content and the relative content are two different ways to measure the extent of water present in the hydraulic oil and system. Absolute content measures how much water is in the oil in parts per million (ppm), whilst relative content allows you to see what the fluid water content is in relation to its saturation at certain temperatures. This method warns you of the imminent creation of free water.

    • Draining – the separation of contaminants usually happens in the hydraulic reservoirs; the air rises to the surface, whilst the water falls to the bottom. Draining should be a regular occurrence; automatic drain valves reduces time-consuming maintenance.
    • Free and emulsified water can be removed by using absorbent filters. The filters contain absorbent polymers which can be found in the filter matrix. This method isn’t recommended if large amounts of water exist in the oil; they also won’t remove dissolved or strongly emulsified water.
    • Vacuum dehydration purifiers are able to dry water in hydraulic oil by exposing it to a partial vacuum. Flash distillation vacuum dehydration and mass transfer vacuum dehydration are the two common ways of achieving this. Flash distillation applies heat to boil as much water as it can and produces a stronger vacuum; this particular method is more effective because it removes more water from the oils than the mass transfer vacuum. The only disadvantage to this technique is that the combination of the high temperature and strong vacuum can cause thermo-oxidative fluid degradation as a result of the loss of volatile additives.

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    Overheating is one of the most common issues in a hydraulic system; caused by inefficiencies in the system affecting the hydraulic oil temperature. It can lead to a loss of input power, as it is converted to heat.

    If hydraulic oil temperature increases beyond 82°C, hydraulic component seal compounds can be damaged accelerating oil degradation. You can usually tell if the temperature is too high if the viscosity is too low for the hydraulic components. Viscosity can also be negatively affected in temperatures below 82°, depending on the oil’s viscosity index. To ensure a stable oil temperature, the hydraulic system must be able to dissipate heat much faster than it is built up.
    The two most common ways of preventing overheating in your system are to either reduce the hydraulic oil temperature or increase heat dissipation.
    Heat dissipation occurs in the hydraulic reservoir. By checking the reservoir, you can ensure that the fluid level is correct and if it is too low, fill it to the correct level. You must also make sure there is nothing obstructing the air flow into the reservoir (this may be a build-up of dirt or debris).

    Hydraulic Oil Temperature
    In checking heat exchangers, make sure that the core is not blocked; the heat-exchanger relies on the flow-rate and temperature of the hydraulic oil and the coolant in order to disperse heat suitably. If you experience issues with the cooling circuit then they need to be replaced.
    You can use an infrared thermometer to ensure that the performance of the heat exchanger is optimal, but you must be aware of the flow-rate of the hydraulic oil through the exchanger.
    If the pressure in a system drops, heat is being generated. If any of the components in the system are leaking, they will increase the heat generation.  You should be able to identify any components that are leaking internally; they could be anything from a cylinder leaking fluid to a valve that hasn’t been adjusted properly.
    If a relief valve is below, or too close, to the pressure setting of a pressure-compensator in a closed-centre circuit, it can lead to increased heat generation; the system pressure cannot reach the pressure compensator setting. The component will continue to move oil thorough the system, passing over the relief valve, which produces heat.


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