What is the brownish spot found on spool valves commonly referred to as?

The brownish spot found on spool valves is commonly referred to as varnish. Varnish is a term used in the context of hydraulic systems and machinery to describe a buildup of residues and deposits that can form when oil ages or when it is exposed to heat and contaminants. Over time, these residues can accumulate and appear as brownish spots on metal components, particularly in areas where oil circulates, such as in spool valves.

Varnish can lead to operational issues, such as sticking valves or reduced performance, and it is important for millwrights and maintenance personnel to recognize and manage this condition to ensure the reliability of the machinery. Understanding varnish and how it forms helps in selecting appropriate lubrication and maintenance practices to prevent its buildup and ensure smooth operation of hydraulic components.