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Hydraulic oil is a fluid that has several functions. It serves as an energy transfer or power transmission medium, lubricant, and sealant. Also, it is a fluid that cools the equipment and carries contaminants away.
Based on the division of hydraulics into hydrodynamics and hydrostatics, we have different hydraulic fluids. Firstly, hydraulic fluids for hydrodynamic applications are called power-transmission oils. Secondly, hydraulic fluids for hydrostatic application are called hydraulic oils. Moreover, in the latter application, applied pressures are high and the flow rates are low. That is why hydraulic oils have to be non-compressible fluids that transfer power within a system or piece of equipment.
When we think of lubricating fluids, we can say that hydraulic oils are the most important lubricants following engine oils. As the three major areas of hydraulics are stationary, mobile, and aviation hydraulics, we can say that each of these applications requires a particular hydraulic medium that is in accordance with the operating demands. If applied optimally, hydraulic oils save energy, reduce machine and component wear, hence, extending maintenance intervals and increasing machine life.
As with the majority of lubricants, we have the primary categorization into mineral and synthetic hydraulic oils. The first originate from petroleum, while the second are synthesized or created artificially. In addition, there are also fire-resistant fluid-based hydraulic oils that are particularly used in sensitive environments, biodegradable, food-grade hydraulic oils, STOU and UTTO universal mobile hydraulic oils, and aircraft hydraulic fluids.
As we mentioned at the beginning, hydraulic systems are omnipresent and can be found in any industry. There is a wide range of hydraulic machinery, equipment, and components that employ hydraulics and require hydraulic oil to maintain operations.
The selection of hydraulic oil and hydraulic oil specifications depends on several factors. These include the design and type of hydraulic system and hydraulic pump, working temperature and pressure ranges, and environmental considerations.
Mineral-based hydraulic oils are derived from crude oil fractions. Furthermore, they are refined to a level at which they achieve adequate lubrication properties. Then they are enhanced with a system of additives. These additives serve different functions and can have anti-wear properties, rust and oxidation inhibition properties, viscosity index improvement properties, and others.
Since they are petroleum-based, they represent a lower-cost option compared to their synthetic counterparts. Depending on the quality of the base oil and additive package, these mineral lubricating hydraulic oils can offer high performance. The types of additives found in mineral-based hydraulic oils protect the hydraulic systems against corrosion, wear, water contamination. Also, they improve the demulsifying properties of the oil and the viscosity index which improves their resistance to temperature changes. Additionally, in this category, we can find hydraulic oils with detergent/dispersant qualities.
Synthetic-based hydraulic oil was designed to compensate for the shortcomings of mineral hydraulic oil. They are made of chemically-produced base oils which accounts for their superior performance compared to mineral hydraulic oils. What is more, they have proven performance at high temperatures, excellent oxidation stability, and biodegradability. However, as synthetic fluids, they come at a much higher cost, can be highly toxic, and are potentially incompatible with some seal materials.
Both mineral and synthetic base oils are used in the formulation of fire-resistant hydraulic oils that biodegrade at very quick rates. As the name implies, these hydraulic oil types are intended for applications with a high probability of fire including aviation, mining, steel mill, and die-casting applications. Thanks to this important feature, they are used in a range of applications and are less expensive than synthetic hydraulic oils. However, when it comes to their anti-wear protective properties, they are behind the synthesized fluids. On the other hand, the system of additives used in fire-resistant hydraulic oils improves corrosion protection, reduces friction, and foaming.
Fire-resistant hydraulic oil can be mineral oil-based oil-in-water emulsions, synthetic water-based solutions, mineral oil-based water-in-oil emulsions, water-based, and water-free.
This category of hydraulic oils was developed with rising standards with regard to environmental protection. In addition, they provide ecological compatibility and can be used for stationary and mobile applications in industrial, forestry, mining, tunnel building, and earthmoving equipment.
In recent years, biodegradable hydraulic oils premium synthetic hydraulic fluid have been replacing mineral oil-based hydraulic lubricants. Additionally, they can be divided into several groups including:
Due to their features, biodegradable hydraulic oils are also used for marine applications and hydraulic systems of boats and ships.
This category is based on numerous international guidelines which regulate which base oils and additives can be used in the formulation of these hydraulic fluids. Food-grade hydraulic oils have subgroups that indicate whether the lubricant is suitable for use in the food industry where no direct contact with the food can happen and the subgroup where such contact might occur. The base oils used for these hydraulic lubricants are special white oils, special polyalphaolefin grades, and special polyglycols.
The universal tractor transmission oil (UTTO) and super tractor universal oil (STOU) are hydraulic fear oils and fluids for gearboxes and hydraulic systems of agricultural machinery and tractors with and without “wet” brakes.
These oils are used in the hydraulic systems in aviation and can handle high pressures and extreme climatic conditions with drastic temperature changes. Therefore, these fluids have to be exceptionally thermally stable and have a high viscosity index. Also, they have to be fire-resistant and must be free from contaminants.
The lubricating property of hydraulic oils is achieved through the addition of additives. Additionally, these systems of additives or additive packages, additive mixtures, or add packs as they are also known, have different properties with different functions. The creation of an additive package is a very delicate process as additives need to be compatible and complement each other.
Some of the properties of base hydraulic oils that can be improved are stability, anti-corrosion, anti-wear, viscosity-temperature stability, foaming, detergency, water separation, and friction coefficient, to name the most important ones.
Based on their function, there are two elementary categories of additives:
An additional classification that is based on the system of additives used in the formula of the hydraulic oil is as follows:
As we can see from the aforementioned categorization, hydraulic oils have to possess certain properties to be able to meet the requirements of the hydraulic systems and operating conditions. Whether it is heavy-duty or an application that does not have such extreme working conditions, hydraulic oil needs to be:
Additionally, the choice of hydraulic fluids has to be cost-effective for the owner of the hydraulic machinery or equipment.
A single hydraulic oil cannot usually meet all the aforementioned points. That is why hydraulic oils are specially formulated based on the conditions in which they will be applied.
The viscosity of hydraulic oils is a highly important feature as it represents its resistance to fluctuations in temperatures. The viscosity index of mineral-based hydraulic oils can be improved by the addition of additives. For example, hydraulic oils with a high viscosity index improve the performance of hydraulic systems and pumps at low temperatures. At the same time, they provide protection against wear at high temperatures.
The density of hydraulic oil has a very important role in the lubrication and performance of hydraulic machines. Typically, systems can only pump fluids of specific density. Therefore, if the density of the hydraulic oil is affected, so is the hydraulic pump's operating efficiency. For instance, if the density of the fluid increases, the fluid's erosive potential increases as well. In systems with high turbulence, the hydraulic fluid can wear valves, piping, and other parts and surfaces that come in contact with the fluid.
Hydraulic oil with high density improves the system's contamination control as they help the suspension, transport, and removal of contaminants. In this case, contaminants remain in the suspension for a longer time and filters remove them more efficiently resulting in a system that is much easier to clean.
Each hydraulic oil is different and based on its intended application and additives package, it can offer different benefits.
In general, mineral-oil or synthetic-oil based hydraulic oils can provide the following benefits:
Hydraulic systems that harness the power of fluids are found in an impressive and wide range of hydraulic machinery and components. These systems are used in almost all existing industries and are engineered to operate in various working and environmental conditions. The fluids that are used in hydraulic systems are hydraulic oils that can be categorized into several groups based on their formulation and/or application. Moreover, they serve as mediums for transferring energy, lubricants and coolants of hydraulic systems, sealants, and eliminators of different types of contaminants.
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