"Barrer" in the context of gears and screw threads refers to the wearing down of the teeth (in the case of gears) or the "stripping" of the thread (in the case of nuts/bolts), such that the mating pieces start to slip against each other.

Recognizing gear failures

Failure modes typically have distinct features. Here's what to look for.
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Wear is a continuous, abrasive process of material removal from matinggear teeth that happens with or without abrasive particles in the oil. For example, hard asperities on gear flanks can remove material from mating flanks. Removal of the hardened layer from surface-hardened gears accelerates wear. Extremely worn spur gears have pointed teeth and a reduced profile contact ratio. Continual wear of tooth roots weakens the gear until it breaks.

Wear typically happens under boundary or mixedlubricating conditions lacking a thick supporting oil film that would otherwise separate tooth surfaces. Mild antiwear additives that help protect surfaces with adsorbed or reacted layers under critical lubricating conditions lessen wear.
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