Machinery Lubrication July August 2008

There is another solution: New additive technology for
hydraulic oil can reduce the need to frequently clean or
change the servo valves. Such technology works to prevent
varnish from depositing on critical work surfaces. The result
is longer component life and a hydraulic system that stays
clean. That translates into improved equipment efficiency
and reduced cost.
A major use for industrial lubricants is in hydraulic equipment,
and varnish formation is a major issue
for many of these applications.
is most obvious at low pressures when there is little
centrifugal force and low fluid pressure.
In the case of piston pumps, varnish can increase piston
land friction against the wear plate, leading to leakage and
possible seizure. It also is well known that sticking valves,
such as the regulation valves on piston pumps, can cause
unscheduled stoppages during equipment use. Finally, the
varnish that forms can block filters, leading to high use of
Varnish Defined
As oil ages, it degrades through oxidation
and thermal decomposition. The additives,
which are performance-enhancing chemicals,
are consumed during the life of the
fluid. The decomposition by-products of
aging increase over the life of the oil, ultimately
forming varnish.
Because varnish is polar, it is attracted to
metal surfaces, including servo valves. It
starts as a sticky, soft residue and attracts
wear debris, forming a sandpaper-like
surface. In time, it ends up as a tenacious,
hard lacquer.
Why It’s Bad
Oil that has oxidized generally does not
lubricate well. It can reduce oil flow, plug
filters, cause valves (especially proportional
and servo types) to stick, increase friction,
inhibit heat transfer and elevate operating
temperature. Because varnish acts as an
insulator, cooling capacity can be diminished.
In addition, oxidation shortens
component life, affecting valves, filters,
pumps, bearings and seals. The result is
diminished hydraulic system performance.
For example, when varnish adheres to
vanes in high-performance vane pumps, the
vanes can stick in the rotor slot. This can
result in increased noise, decreased volumetric
and mechanical efficiency with an
equivalent increase in energy consumption,
side plate scuffing, rotary seal damage and
possibly bearing damage. The phenomenon
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