Coefficient of friction and damage to contact area during the early stages of fretting 1: Glass, copper, or steel against copper Page: 3 of 24
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NACA TN 3011
In recent unpublished research by H. H. Uhlig, it was theorized that fret-
ting is caused by the scraping off of regenerative oxide films and some
underlying base metal. Other investigations (refs. 1 and 2) have indicated
that fretting is caused by adhesion and cold-welding or interlocking,
formation of loose metal particles, and subsequent oxidation of the par-
ticles. Evidently, more detailed information is needed, particularly on
the start of fretting inasmuch as an understanding of the start is
important in considering means of prevention. The initial stages (0 to
500 cycles) of fretting were investigated at the NACA Lewis laboratory
by measuring coefficient of friction p and observing damage. The
results were compared with data obtained during unidirectional sliding
under similar conditions and with data from other investigators.
Fretting was produced by reciprocating a flat specimen in contact
with a convex specimen at a frequency of 5 cycles per minute, an ampli-
tude of 0.006 inch, and a load of 175 grams. A continuous record of the
coefficient of friction was made. The measurements and observations were
limited in most cases to 300 cycles. Copper against glass, copper against
copper, and copper against steel, as well as other combinations using
cuprous and cupric oxide films and powder compacts, were employed. Copper
was also fretted against copper in the presence of hexadecane to determine
the effect of reduced oxygen availability.
The apparatus (fig. 1) was designed to produce fretting at low
frequency so that friction force could be measured and close observation
made of the start of the fretting action. A flat specimen slid back and
forth in contact with a convex specimen under a normal load of approxi-
mately 175 grams, applied by setting weights on the flat specimen. The
flat specimen was held horizontally by a clamp attached to a beryllium-
copper dynamometer ring (1.5 in. diam, 0.5 in. wide, and 1/64 in. thick).
The linear reciprocating motion of the dynamometer ring and flat specimen
holder assembly was achieved by a nut traveling on a screw oscillated by
a synchronous reversible motor. The velocity of 0.156 inch per minute of
the nut was obtained by using a 10-rpm motor having a screw of 64 threads
The amplitude of 0.006 inch was controlled by a pair of adjustable
limit switches, actuating a relay which reverses the motor. This com-
bination of amplitude, sliding speed, and general dynamics of the
apparatus gave a frequency of about 5 cycles per minute. For unidirec-
tional sliding experiments (under conditions identical to those of the
fretting runs), the amplitude was set at a large value by proper adjust-
ment of the limit switches.
The friction force was measured by a strain gage attached to the
dynamometer ring. As the nut pushed and pulled, the ring and the strain
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Godfrey, Douglas & Bailey, John M. Coefficient of friction and damage to contact area during the early stages of fretting 1: Glass, copper, or steel against copper, report, September 1953; (digital.library.unt.edu/ark:/67531/metadc56786/m1/3/: accessed February 19, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.