Karbonite gears

Karbonite gears are a reinforced plastic composite material used in Hitec RC servos that show almost five times the strength of nylon gears and better wear resistance. Cycle times of over 300,000 have been observed with these gear trains showing virtually no wear. Karbonite-geared servos are more expensive than those using nylon ones but are highly durable. Hobbyists are advised avoid use of thread locking compounds on Karbonite as this will cause the plastic to fail.

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Wikipedia