Gears have been used in transmission for a long time. As early as 300 BC, the ancient Greek philosopher Aristotle, in his *Mechanical Problems*, discussed the use of bronze or cast iron gears to transmit rotational motion. The ancient Chinese south-pointing chariot already utilized a complete gear system. However, ancient gears were made of wood or cast metal, and could only transmit rotational motion between shafts, failing to guarantee smooth transmission. Their load-bearing capacity was also very small.
Plastic gears are mainly used for transmission. Made of plastic, they are meshing toothed mechanical parts, and their application in mechanical transmission and the entire mechanical field is extremely widespread.
Modern gear technology has achieved: gear modules of 0.004~100 mm; gear diameters of 1 mm~150 mm; power transmission up to 100,000 kW; speeds up to 10,000 rpm; and maximum circumferential speeds of 300 m/s.
