The Endless Chain Drive
The world's oldest illustrated depiction of an endless power-transmitting chain drive is from Su Song's horological treatise. It was used in the clockworks for coupling the main drive shaft to the armillary sphere gearbox (rotating three small pinions), as seen in Needham's Fig. 410 and Fig. 652. This belonged to the uppermost end of the main vertical transmission shaft, incorporating right angle gears and oblique gears connected by a short idling shaft. The toothed ring gear called the diurnal motion gear ring was fit around the shell of the armillary sphere along the declination parallel near the southern pole. Although the ancient Greek Philo of Byzantium (3rd century BC) featured a sort of endless belt for his magazine arcuballista, which did not transmit continuous power, the influential source for Su Song's chain drive is most likely the continuously-driven chain pump known in China since the Han Dynasty (202 BC–220 AD). From his horological treatise, Su Song states:
The chain drive (lit. celestial ladder) is 19.5 ft long (5.9 m). The system is as follows: an iron chain with its links joined together to form an endless circuit hangs down from the upper chain-wheel which is concealed by the tortoise-and-cloud (column supporting the armillary sphere centrally), and passes also round the lower chain-wheel which is mounted on the main driving-shaft. Whenever one link moves, it moves forward one tooth of the diurnal motion gear-ring and rotates the Component of the Three Arrangers of Time, thus following the motion of the heavens.In addition, the motion gear rings and the upper drive wheel both had 600 teeth, which by Su's mathematical precision carefully calculated measured units of the day in a division of 1/600. These gears having 600 teeth would thus ensure the division the day into basic unit measurements of 2 minutes and 24 seconds.
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