Selecting the Right Hydraulic Fluid: More Complicated Than It Sounds

Finding the proper hydraulic oil for your fluid power system might seem like a simple task, but there is a dizzying number of hydraulic oils available and you will need to find the specific one that works with your fluid power system.

The wrong oil will damage your fluid power system over time and create the need for part replacement earlier than expected. Other poor results include increased heat within the system as well as sludge and corrosion.

What seems like a small matter requires care and research to avoid costly consequences.

With the health of your fluid power system at stake, it’s important to have a firm grasp of the basics. 

Here are a few things to note as you decide which oil to use for your system.

Viscosity — How Thick is the Oil?

Viscosity — generally, how thick the oil is — is the key factor here. If the hydraulic fluid is too thick, the machine will have to work harder to make its parts move. If the oil is too thin, the parts might wear down from added friction.

Viscosity grade is dictated by the operating temperature of a hydraulic system. The correct viscosity will help the system manage heat, release air, and helps with efficiency by allowing parts to move smoothly.

For the most part, as oil gets hotter, it gets thinner. So, knowing the right viscosity oil for a system that operates at a specific temperature is a major factor in selecting a hydraulic fluid.

Additives — Keeping Friction Low & Part Life Long

There are a number of additives contained in hydraulic oils. The two most common types add anti-wear properties to the oil, creating a thin layer on metal surfaces which prevent them from coming into contact.

ZDDP or Zn based additives are the most common. However, there are Zn-free additives that are more environmentally friendly, and help avoid corrosion issues that Zn-based products sometimes cause or worsen.

Rust inhibitors, the second most common additive, coat the metal and seal it against water’s corrosive effects.

Stability—Prevent Oils from Oxidizing

Oils can mix with oxygen and create acidic by-products. These substances contribute to corrosion. Stable oils resist oxygenation at high temperatures which is a key factor when choosing the right oil for your system.

Water Release — Fighting Corrosion & Rust

Humidity in the air surrounding a hydraulic system can cause water to mix with the system’s oil. The oil will also need to release this water, because, if unattended, water in oil will cause rust and corrosion. Some oils have water-expelling properties that help. Rust inhibitor additives are also helpful with this issue.

Filterability — An Uphill Battle

While hydraulic oils can be filtered, in many systems it’s likely that some contaminants will get in. This is especially the case when you’re working in the field with a garbage truck or an excavator, where dirt is just part of the job. Filtering will help keep your system operating smoothly, and will help avoid costly repairs or replacements.

Protect Soft Parts — Make Sure Your Oil is Easy on Seals

It’s also important to select hydraulic oil that will not deteriorate the seals or other plastic or rubber materials. Be careful to choose one rated for your system’s thermal output, and for the parts that your system uses.

Hydraulic Fluid Choice Worth Investigating

The factors above make it seem a little complicated to select hydraulic oil, but it’s not terribly difficult and easy when you use common sense. Your machinery should come with a suggestion for lubricants, and you can also talk to a qualified hydraulic specialist to find out just what to expect and which would be the best option for your particular system.

Remember, there are a surprising number of factors to consider when choosing a hydraulic fluid for your fluid power system, but ultimately, it’s worth considering a wide range of factors before committing to a solution.