May 11, 2021

The Roles and Characteristics of Hydraulic Fluid

The Roles and Characteristics of Hydraulic Fluid

No matter how familiar you might be with hydraulic machinery, it’s easy to overlook the importance of the hydraulic fluid itself. There are a surprising number of hydraulic fluid characteristics to consider in order to ensure that your system is able to work at full efficiency. 

In this article, we’ll look at the various roles that hydraulic fluid plays in your hydraulic system, along with the different factors to consider when choosing the right fluid for your needs.

The Roles of Hydraulic Fluid

high-pressure hydraulic system

First and foremost, hydraulic fluid is an energy transfer medium, which allows pistons, vanes, and other pumps to power your devices. Unlike air, hydraulic fluid is non-compressible, making it ideal for applications that require fast action and more power than could traditionally be achieved with pneumatic systems.

While energy transmission is the primary role of hydraulic fluid, it also performs four other secondary roles. Each of these roles can be affected when you select different hydraulic fluid characteristics, either by selecting a different base or through the use of additives.

Heat Transfer

When a hydraulic pump is functioning normally, it creates high pressure within the pump. This pressure forces hydraulic fluid into the spaces between parts within the system. While this is an important part of the way that a hydraulic system works, the friction of the molecules in the fluid rubbing against the machinery can build up a tremendous amount of heat.

In order to protect your machinery, hydraulic fluid carries away this heat as it returns to the large reservoir that services the entire system. Typically, the hydraulic fluid passes through a cooler that extracts much of the heat, although, in some low-use situations, the heat simply dissipates in the reservoir.

This heat transfer can also work in reverse. If your equipment needs to work in cold temperatures, your reservoir can be outfitted with a heater, which then warms the machinery in your hydraulic pump and protects your equipment from damage.

Removing Contamination

While the perfect hydraulic system would be entirely sealed, the simple truth is that no seal is perfect. That means very small amounts of water may occasionally be introduced into your hydraulic fluid. You may also find dirt or metal particles in your fluid, which is usually the result of wear over time.

These impurities are removed through filters, which catch and trap water and particles to prevent them from damaging your machinery. This also keeps your system working at peak performance since any water can promote foaming and add to the compressibility of the fluid.


While O-rings and other seals do the lion’s share of the work in sealing your hydraulic system, the hydraulic fluid helps to deliver the final seal, filling the minute spaces that exist between these seals and other metal parts. This prevents foreign materials from getting into your system while ensuring that pressure remains constant.


One of the most important roles that hydraulic fluid plays is ensuring that your machinery is properly lubricated. Without this lubrication, the metal parts of your equipment would rub against each other, quickly wearing out while producing less effective power. 

This is why hydraulic fluid often uses mineral or synthetic oil as a base. Depending on the temperature of the work being performed, the size of the machinery, the internal pressure, and other factors, you can choose the right level of lubrication to keep your hydraulic system working efficiently without putting additional strain on your system.

Hydraulic Fluid Characteristics

hydraulic hose leak

With these different roles in mind, it’s time to look at the hydraulic fluid characteristics that you should consider when choosing the right fluid for your system. Each of these characteristics has a range of possibilities. But to select the correct fluid means that you must be familiar with the requirements specified for your equipment.

There is no perfect hydraulic fluid, but there is a wide range of options available to you, and you should be able to find one that works for your needs. Here are several of the most important hydraulic fluid characteristics you need to be aware of.


Viscosity describes how resistant a fluid is to flow — the higher the viscosity is, the thicker or more flow-resistant the fluid will be. Viscosity changes with temperature, with fluids becoming less viscous as they get warmer.

Viscosity is an important factor in choosing a hydraulic fluid. While you need a fluid that is viscous enough to properly create seals and form a lubricating film over your internal mechanical parts, if the viscosity is too high, the fluid won’t flow at all. In this case, your equipment would not only fail to work correctly, but it would likely cease to function at all.


Related to viscosity, a hydraulic fluid’s lubricity indicates how well it lubricates the machinery in your hydraulic equipment. With the right viscosity, your fluid should form a thin film on everything within your hydraulic pump, including the small spaces between tightly fitting pieces. The surface tension of your oil should be able to stand up to the constant friction between the moving pieces, providing a small cushion of lubrication.


While lubrication is responsible for reducing the friction within your equipment, anti-wear is another closely related hydraulic fluid characteristic you should consider. Anti-wear additives, such as zinc dithiophosphate (ZDP), create a film over metal parts that minimizes metal loss from close contact with other moving parts. 

It helps to think of these additives as a kind of sacrificial layer of material. When temperatures rise higher than normal or pressure increases for short bursts of time, the additive film takes the brunt of the excess wear, preserving the machinery from damage.

Foaming Resistance

Foam can be a serious problem in a hydraulic system. Foam occurs when air becomes mixed into your hydraulic fluid. Because air is compressible, it can throw off the normal operation of your hydraulic pump. 

The most important step in eliminating foam is to ensure that your system is completely sealed. However, two types of foam are still often found in hydraulic fluid. The first, surface foam, usually appears on the surface of your fluid reservoir. With defoaming additives, you can limit the amount of foam that makes its way into your system.

Entrained air, on the other hand, is air that has been absorbed into your system, which can lead to significant damage to your equipment, including cavitation. The best option for avoiding this phenomenon is to make sure that you pay attention to the viscosity and base oil that your equipment is designed for. You should also avoid overusing defoaming additives.

Corrosion Resistance

Most hydraulic fluids contain additives that limit the amount of corrosion that will occur in your system. Called R&O (rust and oxidation) inhibitors, these additives protect ferrous metals from rusting when they come into contact with the water that eventually infiltrates almost every hydraulic system.

More troublesome is oxidation, which tends to occur more frequently when your fluid runs at extremely high temperatures or cycles from cold to hot to cold again. Oxidized hydraulic fluid can eat away at softer metals, including brass and bronze, and creates sludge that keeps your system from working properly. Antioxidizing additives help to prevent this from happening.

Thermal Stability

cavitation in hydraulic pump strainers

The ideal operating temperature for hydraulic equipment is usually somewhere between 100˚ and 140˚ F, with 150˚ F usually being considered to be the maximum. That doesn’t mean that hydraulics won’t function at higher temperatures. But it does mean that your hydraulic fluid will be less efficient, and you’ll find that you need to change your fluid much more frequently.

Thermal stability is one of the most undervalued hydraulic fluid characteristics, despite its importance. While low-grade fluids will transfer heat, a hydraulic fluid with high-grade thermal stability tends to resist breaking down in high temperatures, making it ideal for high-pressure work.

Fluid Life

The longer your hydraulic fluid maintains its working characteristics, the less money you will have to spend on replacing fluid. Because higher-quality hydraulic fluid tends to resist breakdown much longer, you’ll spend less on repairs, as well.


Now more than ever, responsibly disposing of spent hydraulic fluid is incredibly important. When you’re choosing a fluid for your hydraulic system, you should consider how you can safely dispose of it while staying in compliance with state and local laws.


While it’s not one of the inherent hydraulic fluid characteristics we’ve already mentioned, it’s important to remember that while higher quality hydraulic fluids may cost more, they usually come with more additives. You’ll need to decide which additives are necessary for your equipment, then make sure to balance cost with the operation of your equipment.

Choosing the Right Hydraulic Fluid Characteristics for Your Needs

When you’re choosing an appropriate hydraulic fluid to keep your equipment functioning at its best, it’s important to consider all of the aspects of your system, from operating temperature and workload to the ambient temperature and pump type. This will help you to choose the right hydraulic fluid characteristics for your system.

At MAC Hydraulics, we have the knowledge and experience to make sure you get the right hydraulic fluid for your equipment. Contact our experts today to discuss the variables in your hydraulic system, and we’ll take care of your hydraulic fluid needs.

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