The water level behind the dam is up to 113 m higher than the river downstream. To overcome this great height, a ship lift with a basin sized 120 m (length) x 18 m (width) x 3.5 m (depth) is required. The lock gate, the mechanic systems and the water itself weigh more than 15,500 t in total. The ship lift was installed as an addition to the five-storied lock, which is also the largest of its kind in the world and has been in operation since 2003. The freight volume on the dam has since quadrupled from close to 35 million t to over 140 million t/a. This rise is due to the booming water transportation sector in the mountain regions around the Three Gorges and the adjacent south-western Chongqing region. Before, it took between three and four hours for a ship to cross the dam through the lock. The shiplift reduces this time to between 40 min and 1 h. The drive of the lock gate is via four toothed wheels, each running on a toothed rack with a length of 113 m. For the heat treatment of this large metal construction, EMA Indutec GmbH, the Aichelin subsidiary from Meckesheim, supplied a hardening machine that was specially designed for the hardening of the tooth gaps of the weighty toothed rack segments.
The trough to lift the vessels hangs between four towers made from reinforced concrete, with a height of 169 m. A pinion system was chosen for the trough drive; four toothed wheels run on the toothed racks along the solid construction of the towers. For the hardening of these toothed racks, EMA Indutec has developed, built and manufactured the largest ring hardening machine for track hardening and single tooth hardening to date. The individual segments of the toothed racks are placed on separate workpiece fixtures; the individual teeth are hardened in a single-tooth hardening process. While size and weight of one single segment (approx. 5 m long x 0.8 m wide; weight approx. 12 t) were relatively easy to adapt by the construction department, the gear module of 62.7 t presented a big challenge for the process development department at EMA Indutec. A gear tooth depth of 140 mm was still unheard of. As the machine needed to be available for other hardening applications after the manufacturing of the toothed racks, a portal with two processing stations was chosen. Conventional applications for this type of machine are, for instance, track hardening of large-diameter bearings, or internal and external gearing of large toothed wheels. For this purpose, two toothed racks each were "mounted" onto a solid workpiece fixture with a weight of approx. 16 t. The entire loading unit has a weight of approx. 34 t and consists of the workpiece fixture and the two back-to-back toothed racks. It was placed between the two processing stations, onto a solid mount, and each rack is handled by the station opposite it. For the unusual module size of 62.7, a total tooth depth of 140 mm, and the required hardness penetration depth of 6 to 8 mm, a new inductor concept had to be developed, which was based on the design of the known single-tooth inductors. The desired process was the so-called "tooth-by-tooth hardening", where the contour is spin-hardened, but the tip remains unhardened. Since tension cracks occurred on the contour during the development phase (one of the biggest risks with such large tooth systems), the spray concept had to be revised as well.
_{(Source: EMA Indutec GmbH)}