To quote ServoCity's website:

<span style="font-size: small;"><span style="font-family: Arial;">"<span>There are two things that you need to know about metal gears in servos.First, they offer much greater strength than their nylon or karbonite counter parts.If your application experiences many jolts and heavy vibration, metal gears are usually the way to go. Second, metal gears wear much faster than their nylon or karbonite counter parts.Although stronger, metal gears will develop a certain amount of play in the gears over time.How much depends on how much load is placed on the servo and the cycle time.If your application needs precision over a period of time we highly recommend using nylon or even better, karbonite gears."

</span><span>"Karbonite" gears are four times stronger than our standard nylon gears and offer virtually ZERO wear; even after 250,000 cycles when tested! The result, you get better control with KARBONITE gear technology.

HTH</span></span></span>