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General. The terms scuffing and scoring are
frequently interchanged. The following definitions are
provided to assist in correctly ascertaining the type of
damage observed. The ASM Handbook Vol 18 defines scuffing as
localized damage caused by the occurrence of solid-phase
welding between sliding surfaces. It defines scoring as the
formation of severe scratches in the direction of sliding.
The handbook also stipulates that scoring may be caused by
local solid-phase welding or abrasion, but suggests that
minor scoring be considered as scratching. Gear scuffing is
characterized by material transfer between sliding tooth
surfaces. Generally this condition occurs when inadequate
lubrication film thickness permits metal to- metal contact
between gear teeth. Without lubrication, direct metal
contact removes the protective oxide layer on the gear
metal, and the excessive heat generated by friction welds
the surfaces at the contact points. As the gears separate,
metal is torn and transferred between the teeth. Scuffing is
most likely to occur in new gear sets during the running-in
period because the gear teeth have not sufficient operating
time to develop smooth surfaces.
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