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Varnish is an undesirable by-product of using hydrocarbons as lubricants and hydraulic fluids. Oxidation of hydrocarbons is a natural process accelerated by heat, pressure, light and mechanical stressing. Varnish coats metal surfaces with a thin orange carbonaceous film that reduces clearances and, in extreme cases, causes stuck hydraulic valves in large frame gas turbine engines and automotive automatic transmissions, as well as failure of journal bearings and thrust surfaces from reduced clearances and high temperatures. These effects can cost down time for equipment causing lost revenue and expense to address varnish related failures. One method of removing varnish is the use of chemical cleaning compounds that are flushed through the system to soften, dislodge and remove varnish using filtration. However, comparisons of varnish removal effectiveness between cleaning compounds have not been possible because there is no consistent method used for evaluating varnish removal and, more generally, a poor fundamental understanding of removal mechanisms. This research included design and implementation of a varnish removal test system combined with a new method of characterizing cleaner fluid effectiveness. In the newly developed method, the fluids remove artificial varnish applied to a coupon within the test system and removal is characterized by mass loss, filter inspection and in situ imaging of the removal process. The results are analyzed in terms of varnish removal mechanisms based on multiple types of diffusion. This research allows cleaner fluids to be directly compared and provides a greater fundamental understanding of varnish and varnish removal mechanisms.

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