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Conventional hydraulic fluids are
satisfactory for low-pressure and low-speed applications.
However, hydraulic fluids for high-pressure (over 6900 kPa
or 1000.5 lb/sq in) and high-speed (over 1200 rpm)
applications that use vane or gear pumps must contain
antiwear additives. These applications do not permit the
formation of full fluid film lubrication to protect
contacting surfaces--a condition known as boundary
lubrication. Boundary lubrication occurs when the fluid
viscosity is insufficient to prevent surface contact.
Antiwear additives provide a protective film at the contact
surfaces to minimize wear. At best, use of a hydraulic fluid
without the proper antiwear additives will cause premature
wear of the pumps and cause inadequate system pressure.
Eventually the pumps will be destroyed.
Quality assurance of antiwear properties is
determined through standard laboratory testing. Laboratory
tests to evaluate antiwear properties of a hydraulic fluid
are performed in accordance with ASTM D 2882. This test
procedure is generally conducted with a variety of
high-speed, high-pressure pump models manufactured by
Vickers or Denison. Throughout the tests, the pumps are
operated for a specified period. At the end of each period
the pumps are disassembled and specified components are
weighed. The weight of each component is compared to its
initial weight; the difference reflects the amount of wear
experienced by the pumps for the operating period. The
components are also inspected for visual signs of wear and
stress.
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