By Jim Fitch
Hydraulics & Pneumatics Magazine
Most oil-related problems in machinery lead to wear or corrosion. However, there is one very serious problem – the presence of sludge and varnish – that may cause neither. Instead, these substances plug flow passages and cause valve sticking. Sludge and varnish can occur even In the most well maintained machines. Surprisingly, they may also appear when oils are not particularly old or contaminated. And they can occur in even though most thermally robust synthetic hydraulic fluids. While there a:re many well-known reasons why oils will throw sludge, there are an equal number that are unknown or misunderstood.
First, what is the distinction between varnish and sludge? Both can form directly or through n sequence of intermediate steps. For simplify lion, we will define varnish as tough, adherent oxide or carbonaceous material that coats internal machine surfaces. Heat and/or time will cure varnish to a hard, brittle consistency. On the other hand, sludge, which is sometimes a precursor to varnish, is soft and sticky in form. Sludge can move around a system until it finally comes to rest at sump bottoms, troughs, strainers, filters, and narrow fluid passages. Other common names for varnish and sludge include deposits, lacquer, tars, pigments, gums, and resins.