02/04/2026 by Joel Thompson
Hydraulic systems require sufficient lubrication to work effectively and stay protected against damage.
As a result, leading brands like Shell, Mobil and Millers (among many others) produce high-quality hydraulic oil specifically designed to meet requirements and deliver top performance.
Over time, hydraulic oil can become contaminated during active service. However, contaminated oil is the enemy of hydraulic systems, and is attributed as the root cause of around 75 to 90 per cent of all failures.
In this in-depth article, we’ll investigate the most common outcomes of contamination, what causes the issue and how to mitigate it.
Understanding the consequences of contamination
Hydraulic oil contamination causes mechanical wear as it disrupts the oil film designed to separate moving components. This introduces abrasive particles that can damage metal surfaces and trigger chemical degradation, resulting in corrosion.
Reduced performance levels and efficiency are also common outcomes of contaminated lubricants, as insufficient lubricity leads to increased friction, meaning that parts can’t move freely at optimal speeds. Larger solid particles can clog valves, causing them to stick and block filters.
Overheating is also a serious problem when contaminants accumulate in lubricants. They stop oil flow and increase internal friction, making systems run far hotter than their design permits. Left unchecked, parts ultimately fail and require expensive repair work and replacements.
Not all contamination is solid, like particles of silt. Water and air are also contaminants that cause havoc for lubricants and reduce their usefulness.
Water in oil leads to corrosion, causing metal components to become pitted and lose their structural integrity. This often leads to breakdowns during operation. Water contamination also impacts the chemical composition of hydraulic oils, leading to fluid degradation that lessens their valuable properties, which can lead to harmful bacteria growth in systems.
Air contamination can cause erratic performance, foaming and excessive heat, leading to oxidation.
In worst cases, when contaminated fluid is kept in use, it can lead to catastrophic failures at considerable expense to operations. Hydraulic equipment and systems represent a significant investment that can be decimated by unresolved issues like overheating, friction and wear and corrosion.
Common indicators that oil is contaminated
Operators in charge of maintaining hydraulic systems and managing the lubricants that are their lifeblood should watch out for key signs that call out to contamination issues.
A visual inspection of the oil itself can offer valuable information on its condition. For instance, when hydraulic oil is contaminated with water, it takes on a milky or cloudy appearance. However, monitoring the system can also provide many early warning signs.
Excessive noise is a strong indicator of air contamination, while grinding sounds and frequently clogged filters are a sign of solid contaminants like silt and larger particulates in lubricants.
Finally, performance issues like slow or inconsistent operation are also often connected to contamination problems.
What are the most common causes of hydraulic oil contamination?
One of the most frequently experienced causes of hydraulic oil becoming contaminated is poor maintenance practices.
Hydraulic systems require proactive maintenance routines to ensure they stay in optimum working order. Oils must be stored and used correctly to avoid contamination. Seal degradation can occur over time, leading to the contents of unopened containers becoming compromised. As a result, attention to detail with inventory is essential, and oils should have their use by date clearly marked, and older stock always used before new.
During changes, oils should have dedicated filling equipment, so no cross contamination occurs with other lubricants. Drums holding hydraulic oil should only be opened immediately before it is needed to avoid excess air contamination. Oils should always be stored indoors in a dry and clean environment to protect against solid and water contamination.
Hydraulic system seals and filters should also be monitored regularly. If systems seals have deteriorated, they should be replaced immediately, and clogged filters cleaned and replaced.
High-quality hydraulic breathers are advisable. These advanced contamination control devices, remove both moisture and solid particles and stop air from entering hydraulic reservoirs. Breathers can therefore help prevent component wear, corrosion and oil degradation.
To avoid contamination issues in hydraulic oil, regularly monitor and test lubricants in use and always observe best practices for maintaining hydraulic systems and the solutions that serve them.
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