Hydraulic fluid is the lifeblood of any hydraulic system, from massive heavy-duty excavators to compact skid-steer loaders. But how much do you really know about hydraulic fluids?

Below are six essentials that every equipment operator, technician, or fleet manager should understand about hydraulic fluid.

1. What Is Hydraulic Fluid?

Hydraulic fluid (generally oil) is a flexible medium that transfers power through a circuit, enabling the system to perform work. It also:

  • Lubricates moving parts, reducing friction and abrasion
  • Cool system components by transferring heat

Common examples of hydraulics are the brake and power steering fluids in automotive vehicles.

2. What Are the Different Types of Hydraulic Fluid?

Hydraulic fluids are engineered to meet the demands of the equipment and application you may need:

  • Water-based or petroleum-based fluids
  • Natural/bio-based or synthetic options
  • Biodegradable fluids for environmentally sensitive areas
  • Flame-retardant fluids for high-risk environments

Tips:

  • Stick with one manufacturer’s fluid, don’t mix brands, even of the same grade.
  • Sample the fluid routinely to check for contamination or water.
  • Follow OEM recommendations for hydraulic fluid properties.

3. How Does Hydraulic Contamination Happen?

Approximately 70% of hydraulic system failures can be attributed to contaminated fluids. It can result from:

  • Foamy fluids may indicate air contamination impacting cycle times, heat dissipation, lubricity, and performance.
  • Milky fluids suggest water contamination impacting lubricity, oil life, and performance
  • Particulates like dust, sand, silt, or soot generate wear. If oil can get out, the chances hard particulates are getting into your oil.
  • Chemical contamination occurs when aging hydraulic fluid begins to degrade (oxidize) and break down. It can also happen if different hydraulic fluid types are mixed: incompatible additives may have unwanted chemical reactions.

Best practices:

  • Fix leaks immediately.
  • After replacing hoses or hard parts, replace the filters and flush the fluid if needed.
  • Use only high-quality filtration and deliquescent breathers.

4. Do Hydraulic Filters Help?

Hydraulic filters are a key contributor when tackling contamination. Investing in quality filters helps:

  • Protect sensitive system components
  • Improve equipment longevity
  • Minimise costly repairs and downtime

Donaldson filtration solutions include:

  • Tank top, suction, return line, low, medium, and high-pressure filters
  • Steel mesh, high-efficiency Synthetic, cost-effective cellulose, and water-absorbing media.
  • Regenerative deliquescent T.R.A.P. breathers for airborne moisture and particulates

5. When Should You Flush or Change Hydraulic Fluid?

Flushing is not just draining and refilling fluid. It's a more thorough process that:

  • Should be done after major component failures or system rebuilds
  • Involves using a low-viscosity fluid at high pressure to clear debris
  • Prepares the system for clean, fresh fluid

Tip: 

Change fluid regularly based on the manufacturer’s guidance, and flush only when necessary.

6. What Is Hydraulic Fluid Injection?

Hydraulic fluid under pressure can penetrate the skin.  This is a serious, often underestimated, and potentially fatal injury. It may only feel like a bee sting or pinprick, but it can cause severe complications.

If checking for leaks:

  • Never use your bare hands; rather, use the appropriate PPE and assess the area with a dry piece of cardboard or other material.
  • If you are unfortunate enough to be stung/stabbed and suspect an injection injury, play it safe: seek medical attention! The injury might seem insignificant, but the potential consequences are not!

Final Thoughts

Hydraulic fluid does much more than power your system. It plays a critical role in performance, efficiency, and long-term durability. To protect your investment:

  • Use the right fluid and filters
  • Sample and replace fluid as recommended
  • Perform flushing when appropriate
  • Watch for contamination and act quickly to correct it

Your hydraulic system will reward you with faster cycle times, fewer breakdowns, and longer service life.