Hydraulic oil additives are special products that are added to hydraulic oil. They can help to protect your hydraulic system by reducing wear and tear, stopping the oil from becoming rancid, and preventing bubbles from forming. You may be wondering: What exactly are additives? What types are there? Do different systems require different formulations? This article will clearly explain how they work, what they are used for, how to measure how well they work, and how they are used differently in high-pressure systems.
Definition and Role of Hydraulic Fluid Additives
Hydraulic oil additives are important parts of the hydraulic oil mixture, and make up between 1% and 30% of the total mixture. Their job is to stop things from wearing out, stop rust, and stop foaming.
Base Oil and Additive Package
When you use the product, you use a mix of “base oil and additive package”. This means the base oil and a complete set of additives are designed in specific proportions.
Additives are not just one chemical, but a mixture of chemicals that have been combined to work well together. Different systems need different amounts of additives. High-pressure systems need more anti-wear agents, while high-temperature systems need more antioxidants.
Purpose of Hydraulic Oil Additives
From an engineering point of view, the job of hydraulic oil additives is to make sure that the system stays working properly.
Without additives, oil breaks down more quickly, metal surfaces wear more easily, and foam stops pressure from being transmitted. Over time, the pump and valve will wear out more quickly.
So, additives are not optional extras but important parts that make the system reliable.
Not a Simple Aftermarket Chemical
Hydraulic oil additives are not just any old chemicals that can be added at any time.
The complete formulation is designed during the production stage. Adding one additive at a time might change the balance and affect how the oil works.
When doing maintenance, focus on the overall condition of the oil instead of adding individual components.
How Hydraulic Fluid Additives Work
Hydraulic oil additives reduce wear and keep the oil stable. They do this by creating protective layers on metal surfaces, slowing down reactions where the oil and metal react with each other, and reducing the formation of bubbles. They first change the oil, then affect how well the parts are working, and finally decide whether the system will work smoothly.
Anti-Wear Additives and Boundary Lubrication
If the system is under a lot of pressure or running slowly, it might start to use more lubrication. Here, the oil film becomes less thick, which means metal surfaces are more likely to touch each other.
Anti-wear agents form a protective layer on metal surfaces. This film is strong and can handle pressure, which helps to protect hydraulic systems from wear and tear.
If it doesn’t have enough anti-wear capability, the pump will wear out more quickly, there will be more clearances, and it will be less efficient.
Oxidation Inhibitors and Oil Oxidation Control
Oil slowly becomes rancid when it is exposed to high temperatures and air. Oxidation makes the oil thicker and leads to the build-up of deposits.
Antioxidants are good for this because they slow down the process. If you slow down the oxidation process, the oil will last longer, and the system will be more stable.
Anti-Foam Agents and Air Release
If air gets into the hydraulic system when it is switched on, this can cause the pressure to go up and down a lot and the system to not work properly.
Defoamers make bubbles collapse faster and make air easier to release. This keeps the oil stable, reduces unusual noise, and minimises response delays.
Main Types of Hydraulic Fluid Additives
Hydraulic oil additives are mainly made up of anti-wear agents, antioxidants, anti-foaming agents, rust inhibitors, and viscosity modifiers. Each additive category deals with different risks to the system. So, when you’re looking at different types of hydraulic oil additives, you need to think about the actual conditions the oil will be used under.
Anti-Wear Additives
Additives that reduce wear and tear stop metal surfaces from rubbing against each other directly.
In high-pressure pumps or systems that are under a lot of pressure, the surface pressures on metal parts are very high. Anti-wear additives form a protective layer on these surfaces, slowing down how quickly they wear. If they don’t have enough anti-wear capability, they won’t last as long.
Common anti-wear systems include zinc-based additives. Some systems use ashless additives to meet environmental or material compatibility requirements.
Oxidation Inhibitors
Oxidation inhibitors stop oil from going rancid.
When oil is left in hot places for a long time, it slowly changes and forms lumps. Antioxidants reduce the rate at which this happens, meaning you don’t have to change your oil as often.
Anti-Foam Agents
Anti-foam agents control foam and air issues.
If there is too much air in the oil, it can cause the pressure to go up and down a lot, or the system can stop working properly. Anti-foam agents make bubbles disappear faster, which helps to keep the pressure steady.
Corrosion Inhibitors
Corrosion inhibitors stop metal from rusting.
If there is moisture or humidity in the system, metal surfaces can be damaged by corrosion. Rust inhibitors form a protective layer on surfaces, reducing the risk of damage.
Viscosity Modifiers
Viscosity modifiers keep oil from becoming too thick or too thin.
Big temperature changes can make oil go thick or thin. Viscosity modifiers reduce these differences, making sure the system stays stable when it is cold or when it is hot.
Zinc-Based vs Ashless Additives
In anti-wear systems, zinc-based additives contain a certain amount of phosphorus. The amount of phosphorus in hydraulic oil affects how well it protects parts from wear and how well it works with other materials.
Ashless additives do not contain any metals, making them better for systems that need to be compatible with the environment or materials. When choosing, it’s better to look at the equipment’s specifications instead of just comparing its advantages and disadvantages.
The table below will help you understand the main functions and impacts of different additives.
| Additive Type | Main Function | Engineering Impact | Typical Application |
| Anti-wear additives | Reduce friction on metal surfaces | Extend pump and valve lifespan | High-pressure, high-load systems |
| Oxidation inhibitors | Slow down oil oxidation | Extend fluid service life | High-temperature or continuous-operation equipment |
| Anti-foam agents | Control foam and air | Ensure stable pressure transmission | High-speed circulation systems |
| Corrosion inhibitors | Prevent metal rust | Reduce internal corrosion risk | Humid or moisture-prone environments |
| Viscosity modifiers | Stabilise oil viscosity | Maintain flowability across varying temperatures | Operating conditions with significant temperature differentials |
Performance Impact and Evaluation Criteria
To see how well additives work, we look at things like how well they stop oxidation, how much foam they make, how well they release air, and how well they wear things. We don’t just look at the product specifications.
Oxidation Stability
When it is hot and in the air, oil slowly changes. The oxidation stability index is a measure of an oil’s resistance to oxidation.
If the oil becomes less able to resist oxidation, it will change colour more quickly and may form deposits. These deposits can stop valves from working properly or block filters.
Foam Resistance and Air Release
If there is air in the oil, it can affect how well the system works. The ability of oil to form foam is an important property.
At the same time, air release performance shows how quickly oil expels trapped air. If air can’t be released quickly, the system might have pressure problems or be slower.
Additive Concentration and Depletion
How well they work also depends on how much of the additive is used. As the system is used more, the additives are used up bit by bit.
As the amount of additive in the oil goes down, it can become more likely to go rancid and form foam. So, when checking the condition of your oil, it’s important to keep an eye on how it’s performing and how much additive is in it.
Additive Compatibility and System Interaction
There might be compatibility issues with different additives, as well as with additives and seals, filtration systems, and base oils. Using the wrong mix of oils can make the oil perform badly and cause problems with how the system works.
Seal Compatibility
Additives can affect seal materials, so it’s important to check that they are compatible with the seals.
Some additives can change how quickly rubber seals expand. If the materials don’t match, the seals can harden, swell, or lose their elasticity, which can cause leakage.
When choosing fluids, make sure they are compatible with the sealing materials used in your equipment.
Filterability Performance
Filtration systems need stable fluid conditions, so the filters must work well.
If the additive is not stable enough, it can form deposits in the oil when it is heated or contaminated. These substances can block filter elements and restrict oil flow.
Oil Mixing Risks
Different oils may not mix well together.
If two oils don’t mix well together, they can cause problems like more foaming or limescale. So, don’t mix oils randomly when you top up or change the oil.
Additive Conflicts Between Oil Systems
Different oil systems use different types of additives. Some oils use zinc-based anti-wear systems, while others use ashless formulations.
Mixing these systems can cause reactions that affect the stability of the oil. When choosing oils, it’s best to stick to a system of additives that matches the equipment.
Hydraulic Fluid Additives for High-Pressure Systems
In high-pressure hydraulic systems, such as axial piston pumps operating at pressures over 3000 psi, the fluid needs to perform even better. So, additives need to be able to stop things from wearing out and be more stable at high temperatures.
Boundary Film Protection Under High Load
If there is a lot of pressure on the engine, the oil film might become thin. At this point, the lubrication state is more like boundary lubrication.
Anti-wear additives form a protective layer on metal surfaces, reducing direct friction. This helps to keep the fluid performing well under pressure and reduces the risk of wear and tear on pumps and valves.
Thermal Stability of Hydraulic Additives
High-pressure systems usually have higher operating temperatures. Higher temperatures speed up the process of oil going rancid.
If hydraulic additives are not stable enough, the oil will age more quickly and may form deposits. These deposits can cause problems with valves or filtration systems.
Why Stronger Anti-Wear Performance Is Required
In high-pressure systems, the parts that make up the system are under more pressure.
If they don’t wear well, they will wear more quickly, and the spaces between them will get bigger. The system might not work as well over time. Therefore, high-pressure equipment usually needs a stronger anti-wear additive system.
Additive Depletion and System Risks
When the things that are added to hydraulic fluid run out, the system may experience increased wear, more foaming, and the fluid will age more quickly. As additives decrease, the fluid’s ability to protect the system declines.
Common Signs of Additive Depletion
As additives gradually decrease, the fluid typically changes quite a lot.
For example, the fluid may become darker, foam more, or leave more deposits. These things usually show that the product is losing its ability to resist oxidation or to stop foaming.
If these changes continue, the system may wear out more quickly.
Potential System Failure Risks
When there are no more additives, the fluid can’t protect the system as well.
If anti-wear ability is reduced, pumps and valves will wear out more quickly. The fluid ages faster because it has less oxidation resistance. If you don’t, you risk more serious system failure.
Why Additives Should Not Be Added Randomly
If the performance of the fluid gets worse, adding one additive isn’t usually a good idea.
The additives in hydraulic fluid are designed as a complete formula. Adding one more thing might change the balance.
It’s safer to check the fluid’s condition by running a fluid test and then deciding whether to replace it.
FAQ
Can hydraulic fluid additives be added separately?
It is not usually a good idea to add hydraulic fluid additives one by one. Additives in hydraulic fluids are already mixed in during production to form a complete additive package. Adding an additive randomly might change the original balance of the mixture, affecting its anti-wear or anti-oxidation properties. It is usually best to test the oil before changing the hydraulic fluid.
What is the typical percentage of additives in hydraulic oil?
Additives in hydraulic fluids usually make up about 1% to 30% of the overall mix. The right mix depends on the type of hydraulic oil. For example, anti-wear hydraulic fluids usually have higher levels of additives. The base oil is the main component, but additives determine how well the fluid works.
Are zinc-free additives better?
Additives that don’t contain zinc are not always better than those that do. The main difference is in how they are made and used, like what the environment needs or if the materials can be used together. Some equipment works better with ashless additives, but high-load systems may still use zinc-based anti-wear systems.
Do additives affect hydraulic seals?
Hydraulic fluid additives can affect sealing materials, so it’s important to check that they are compatible with the seal. Some additives can change how much rubber seals expand, which can affect how well they seal. When choosing hydraulic fluid, make sure it is compatible with the equipment’s sealing materials.
How long do hydraulic additives last?
How long hydraulic fluid additives last depends on the temperature, pressure and how long they are used for. As equipment is used, the additives gradually break down. The longer you drive, the more quickly the anti-oxidation and anti-wear properties decrease. This means you need to test the oil regularly.
Are additives different in synthetic hydraulic fluids?
Yes, synthetic hydraulic fluids usually contain different types of additives. The base oils used in synthetic hydraulic fluids are different, so the additives are designed to make sure they don’t go rancid or decompose when heated.
Can too many additives damage a system?
Yes, adding too much to the hydraulic fluid can affect how stable the oil is. If there is too much additive, it can cause deposits or foaming problems. The right amount of additive is important in hydraulic fluids. You can’t just add more and more.
Final Thoughts
Things like additives in hydraulic oil can make things wear out faster, and can also affect how stable the oil is and how long the equipment will last. Knowing the different types of additives, how they work and how to check how well they are working can help you look after your hydraulic systems better.
Target Hydraulics has over 10 years of experience in hydraulic components and systems. We can help you choose the right hydraulic system and provide technical support. If you have any other questions, please contact us.
