Chinese engineers are tackling a significant challenge in manufacturing the main bearing for the world’s largest tunnel boring machine, utilizing the ancient technique used to create the sword of King Goujian of Yue.
An ancient technique used 2,500 years ago to forge the sword for King Goujian of Yue is now being applied to a tunnel boring machine nicknamed the “king of construction machinery.” While striving to produce the largest machine in the world for tunnel construction, scientists and engineers on the project faced a major hurdle: no country had been able to manufacture steel hard enough for this massive machine.
The 2,500-year-old sword of King Goujian of Yue employs a special forging technique. (Photo: WordPress)
The Tunnel Boring Machine (TBM) features a main bearing with a diameter of 8.61 meters, equivalent to the height of a three-story building. This bearing drives the gigantic machine, which weighs over 10,000 tons—similar to the weight of the Eiffel Tower. All that weight is supported by a bearing surface less than one centimeter thick. The primary issue in producing such a large bearing is that temperatures in different areas change at varying rates during the process. This inconsistency can weaken the steel. To address this problem, the project team relied on techniques from the past.
An ancient technique used to create the 2,500-year-old sword of Goujian provided a solution for the engineers, according to Science and Technology Daily. Renowned for its extraordinary sharpness and exquisite design, the bronze sword was excavated from an ancient tomb in Hubei Province in 1965. The sword is believed to have belonged to Goujian, the king of Yue from 496 to 465 BCE during the late Spring and Autumn period of the Warring States.
To forge the bronze sword, ancient blacksmiths hardened it using a process known as “quenching,” where the metal is heated to high temperatures and then rapidly cooled in water, air, or other liquids to achieve certain properties, such as increased hardness. While quenching is quite common in sword production, Chinese blacksmiths applied a significant difference when crafting the sword of Goujian. They covered the sword with clay during the quenching process to ensure a uniform temperature change across all parts of it. Using clay as an intermediary medium for the TBM bearing effectively addresses the issue of uneven quenching temperatures, according to the project team.
Tunnel boring machines are widely used in underground construction. They can cut through all types of materials and terrains, from soft clay to the hardest granite, but this depends on the main bearing. Previously, the main bearing of the largest TBM had a diameter of under 8 meters, setting a world record when manufactured in 2021. This machine was produced by Liebherr, a German-Swiss equipment manufacturer based in Bulle.
Currently, that record has been overshadowed by a bearing produced by the China Railway Heavy Industry Group (CRCHI) last October. The project leader, Xie Jindong, recently shared the process they undertook to overcome significant technical challenges in creating the “king of construction machinery.” With over 30,000 components, the TBM marks a milestone in China’s capability to produce underground construction machinery.
In July 2019, CRCHI decided to establish a research institute and design bearing systems for TBMs. The following year, the Chinese Academy of Sciences also initiated a collaborative project to produce premium bearings. With the emergence of this record-sized bearing, CRCHI stated that China could maintain its leading position in the manufacturing of ultra-large diameter main bearings. The project overcame manufacturing difficulties through hundreds of experiments with various processes and materials, resulting in a bearing with a long lifespan and high precision.
