Varnish formation in hydraulic systems: causes and mitigation

 The formation of varnish in hydraulic systems happens in the process of thermally degrading and oxidizing the molecules of hydraulic fluid, polymerizing to form sticky insoluble deposits that cling to the surfaces of internal components. In contrast to sludge, varnish forms an extremely thin, hard, lacquer-like layer that is formed in fluids that are clean and clear, which makes it difficult to discern by routine inspection of fluids. Varnish is deposited on spools of valves and orifices of servo valves along with bearings, 

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