How Is Varnish the 'Cholesterol' of Machinery? The Hidden Threat in Oil Systems

Common Customer ConcernsThe machine is still operating, the oil in the sight glass still looks clear, and basic parameters such as viscosity or TAN may remain within limits. Yet servo valve performance becomes inconsistent, filters plug quickly, or bearing temperature gradually rises. These symptoms may share one underlying cause: varnish forming in the system.

Technical Explanation

How does varnish form? - Lubricating oil reduces friction, removes heat, protects against wear, and carries contaminants away from critical areas. In service, the oil is exposed to heat, oxygen, water, contaminants, and operating stress. These factors gradually degrade the base oil and additives, producing oil degradation products.

From dissolved material to deposits on machine surfaces - At first, some degradation products can remain dissolved in the oil, so no obvious abnormality is visible. As their concentration increases, or as the oil flows into cooler areas, solubility decreases. The products then precipitate from the oil and begin to adhere to surfaces. These tenacious deposits, which are difficult to remove, are called varnish.

Why compare it with cholesterol? - Varnish resembles cholesterol buildup in blood vessels. It can accumulate slowly while the system appears to operate normally. Once enough deposit has formed, it begins to restrict flow or interfere with the movement of critical components such as servo valves and oil passages.

Impact on Machinery

• Servo valve stiction or hysteresis
• Reduced oil flow as oil passages become restricted
• Higher filter differential pressure and shorter filter life
• Reduced heat-transfer efficiency
• Increasing bearing temperature and wear
• Greater risk of alarms or unplanned shutdowns

Points to Consider

• Assess oil condition, machine condition, and operating conditions together
• Correlate varnish-related oil analysis with bearing temperature, filter differential pressure, and valve response
• Review oil-change, top-up, shutdown, and filter-replacement history to identify trends
• Avoid making decisions based only on oil color or a single routine test result

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