Su Song - Su Song's Escapement Mechanism

Su Song's Escapement Mechanism

In Su Song's waterwheel linkwork device the action of the escapement's arrest and release are achieved by gravity exerted periodically as the continuous flow of liquid would fill containers of limited size. In a single line of evolution, Su Song's clock therefore united the concept of the clepsydra and the mechanical clock into one device run by mechanics and hydraulics. In his memorial, Su Song wrote about this concept:

According to your servant's opinion there have been many systems and designs for astronomical instruments during past dynasties all differing from one another in minor respects. But the principle of the use of water-power for the driving mechanism has always been the same. The heavens move without ceasing but so also does water flow (and fall). Thus if the water is made to pour with perfect evenness, then the comparison of the rotary movements (of the heavens and the machine) will show no discrepancy or contradiction; for the unresting follows the unceasing.

In his writing, Su Song credited, as the predecessor of his working clock, the hydraulic-powered armillary sphere of Zhang Heng (78–139 AD), an earlier Chinese scientist. Su Song was also strongly influenced by the earlier armillary sphere created by Zhang Sixun (976 AD), who also employed the escapement mechanism and used liquid mercury instead of water in the waterwheel of his astronomical clock tower (since liquid mercury would not freeze during winter and would not corrode and rust metal components over time). However, Su Song stated in his writing that after Zhang's death, no one was able to replicate his device, much like his own.

The mechanical clockworks for Su Song's astronomical tower featured a great driving-wheel that was 11 feet in diameter, carrying 36 scoops on its circumference, into each of which water would pour at uniform rate from the 'constant-level tank' (Needham, Fig. 653). The main driving shaft of iron, with its cylindrical necks supported on iron crescent-shaped bearings, ended in a pinion which engages with a gear-wheel at the lower end of the main vertical transmission-shaft.

Joseph Needham gives a general description of the clock-tower itself:

(Su Song's) clockwork, driven by a water-wheel, and fully enclosed within the tower, rotated an observational armillary sphere on the top platform and a celestial globe in the upper story. Its time-announcing function was further fulfilled visually and audibly by the performances of numerous jacks mounted on the eight superimposed wheels of a time-keeping shaft and appearing at windows in the pagoda-like structure at the front of the tower. Within the building, some 40 ft. high, the driving-wheel was provided with a special form of escapement, and the water was pumped back into the tanks periodically by manual means. The time-annunciator must have included conversion gearing, since it gave 'unequal' as well as equal time signals, and the sphere probably had this. Su Sung's treatise on the clock, the Hsin I Hsiang Fa Yao, constitutes a classic of horological engineering.

That was figure Fig. 650, while Fig. 656 displays the upper and lower norias with their tanks and the manual wheel for operating them.

Fig. 657 displays a rather miniature and scaled-down pic for the basics of the escapement mechanism in an illustration (from Su's book), with Needham's caption here in this quote: "The 'celestial balance' or escapement mechanism of Su Sung's clockwork (Xinyi Xiangfayao, ch. 3, p. 18b)". The latter figure carefully labels:

• a right upper lock
• upper link
• left upper lock
• axle or pivot
• long chain
• upper counterweight
• sump
• checking fork of the lower balancing lever
• coupling tongue
• main (i.e. lower) counterweight.

Figure 658. displays a more intricate and most-telling half-page scale drawing of Su Song's large escapement mechanism, labeling these individual parts as they interact with one another:

• arrested spoke
• left upper lock
• scoop being filled by
• water jet from constant-level tank
• small counterweight
• checking fork tripped by a projection pin on the scoop, and forming the near end of
• the lower balancing lever with
• its lower counterweight
• coupling tongue, connected by
• the long chain with
• the upper balancing lever, which has at its far end
• the upper counterweight, and at its near end
• a short length chain connecting it with the upper lock beneath it;
• right upper lock