Contamination of hydraulic fluids is one of the most common reasons for hydraulic equipment failure. Let’s take a look at the types of hydraulic contamination that can affect your hydraulic systems.

The Purpose of Hydraulic Fluid

First, let’s talk about the purpose of hydraulic fluid. Hydraulic fluid is used to create a barrier between precision parts that move and function, this thin lubricating barrier keeps these precision parts from contacting each other and wearing, leading to equipment failure.

Types of Contaminants

There are four major types of contaminants. Particle, air, water, and heat. Each of these contaminants can affect a system differently and can cause different warning signs and adverse effects.


Water is the most common chemical contaminant for hydraulic systems. Water can cause a plethora of problems from reduced lubrication characteristics, additive depletion and deposition, oxidation, sludges, rust in tanks, and accelerated wear due to the thinning of the lubricants.

Another major issue with water contamination, in particular to hydraulic systems that are outdoors is freezing, water in lubricants will freeze, turn into ice crystals and cause damage to the system.


Air contamination can be in two different states when it contaminates hydraulic fluid, dissolved and undissolved. Dissolved air may not be an issue in many situations as long as it stays within the solution. Undissolved air can cause serious issues. Undissolved air can cause issues due to compression, causing rapid heating of the trapped air, with enough air this compressibility can cause system failure and loss of system control.


The issues that heat can cause to a hydraulic system is very straight forward. Excessive heat will result in additive depletion as well as causing chemical changes to the hydraulic fluid. This will cause damage in the long run due to wear caused by parts losing the protection of fluid in the same manner as water contamination, or low fluid levels.

Sources of Contamination

There are 4 major ways that contaminants can get into your hydraulic system, let’s take a look at what these are.

Native Contamination

These contaminants are typically left in the system after manufacturing processes or repairs and can include detritus such as weld slag, machining swarf or shavings, Teflon tape, or excessive sealant use.

Contaminated New Fluid

Even though it’s a new fluid, it can still easily be contaminated due to the manufacturing process, or subsequent handling or storage of the fluid which may introduce air or water.

Ingressed Contamination

This typically occurs when the hydraulic system is in use and can introduce contaminants in a number of ways. Moisture can ingress through breather caps and the reservoir, as well as operation in humid conditions, as there is no way to really prevent it from coming into the reservoir.

Particles can enter over time from a cylinder rod, although there are seals to protect against this no seal is 100% effective at stopping all particles, and damaged seals will most certainly allow particles to enter the system.

Anytime a hose is disconnected, or fluids are topped off there is a chance that contaminants can enter the system, in particular air.

Internally Generated Contamination

This is generally the result of introduced contaminants wearing the internals of a hydraulic system. Once internals become contaminants it continues a cycle of wear, removing more of the internal surfaces and ultimately leading to system failure.


Hydraulic contamination is something users must be aware of and checked for on a regular basis. This can be achieved by regular inspection of the equipment, as well as regular and recommended fluid changes. Proper maintenance of your hydraulic system will ensure decades of reliable use.

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