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• December 6, 2016 Create Date
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National Advisory Committee for Aeronautics, Technical Notes - Friction at High Sliding Velocities of Surfaces Lubricated with Sulfur as an Additive

Friction occurring at high sliding velocities of‘ sulfur-type
extreme-pressure lubricants was investigted. Experimental evidence
is presented that indicates the existence of a limiting and critical
condition of sliding velocity.

Tue emeriments were performed with an apparatus incorporating
means for measuring sliding friction that consists basically of an
elastically restrained spherical rider sliding in a spiral path on
a rotating disk, Ellie disk specimens were lubricated with cetane
(normal hexadecane) and with cetane containing free sulfur or benzyl
disulfide. Elie effects of concentration of free sulfur on friction
and surface damge were determined. The experiments were conducted
over a range of sliding velocities between 50 and 8000 feet per min-
ute with loads from 269 to 1545 grams (126,000 to 225,000 lb/sq in.,
initial Hertz surface stress). Supplemental studies of friction
specimens were mde using standard physical, chemical, and metal-
lurgical equipment and techniques. ’

Ellie theory is advanced that rate of chemical reaction between
surfaces and additives is a limiting factor in lubrication by extreme-
pressure additives of surfaces operating at high sliding velocities.
In support of this theory, the emeriments indicated lubrication
failures at high sliding velocities for solutions‘of sulfur in cetane.
Above the critical velocity, friction increased and mes surface
welding occurred. Variations in load and in sulfur concentration
had no appreciable effect on the sliding velocity at which the initial
lubricant failure occurred.

Experiments with pure cetane on clean steel and. on a solid
ferrous-sulfide film indicate that cetane may fail as a boundary
lubricant by oxidation and that it influences the chemical and physical
processes in sliding, which determine the degree and severity of
surface failure.