From the Manufacturer

The HS-322HD Standard Heavy Duty Servo utilizes Hitec’s revolutionary Karbonite™ gear train which is four times stronger than our standard white nylon gears, and even after hundreds of thousands of cycles they don’t show any signs of wear. Karbonite™ gears and our custom IC make the HS-322HD a great choice for an economical sport servo. Great for Heli's and Aircraft up to 72” or up to 12lbs when using just one servo per control surface. Be aware that high vibration gas (not glow) engines combined with large control surfaces and large throws (3D models) can put undue strain on the gears. Metal geared servos are recommended for these applications, even if under 12lbs. Warning: Do not use thread lockers on Karbonite geared servo as it will cause the output shaft to fail.