Scuffing on spiral bevel pinion. Photo courtesy GEARTECH © 2013.
Definition: Scuffing is severe adhesion that causes transfer of metal from one tooth surface to another due to welding and tearing. The damage typically occurs in the addendum, dedendum, or both, away from the operating pitchline, in narrow or broad bands that are oriented in the direction of sliding. Scuffing may occur in localized patches if it is due to load concentrations. The scuffed area appear to have a rough or matte texture. Under magnification, the scuffed surface appears rough, torn, and plastically deformed.
Identification: Scuffing leaves rough, torn streaks in the direction of sliding. It does not occur at the pitchline, a fact that can be used to identify scuffing damage.
Discussion: The term “scoring”, which was incorrectly used in earlier gear nomenclature for scuffing, is in reality scratching and is now classified as a form of abrasive wear. Scuffing is not a fatigue phenomenon and it may occur instantaneously. Based on the severity of the damage, scuffing can be categorized as mild, moderate, or severe. Scuffing is classified as mild if it occurs only on small areas of the teeth and is confined to the peaks of the surface asperities. It is generally nonprogressive. Moderate scuffing occurs in patches that cover significant portions of the teeth. If the operating conditions do not change, moderate scuffing may be progressive. Severe scuffing occurs on significant portions of the gear tooth (for example, the entire addendum, the entire dedendum, or both). In some cases the surface material may be plastically deformed and displaced over the tip of the tooth or into the root of the tooth. Unless corrective measures are taken, severe scuffing is usually progressive.
Prevention: Scuffing damage can be prevented through design changes or through operational/break-in changes. Design related changes include optimizing gear geometry/accuracy and the use of nitrided steel. Operational and break-in changes include the use of high viscosity lubricants with anti-scuff additives, reduced lubricant temperatures, and the running in of new gearboxes at reduced loads.