How big is the main bearing of a tunnel boring machine? The main bearings in tunnel boring machines (TBMs) are quite large to accommodate the forces involved in excavating through rock and soil. The size of the main bearing of a tunnel boring machine varies depending on factors such as the type of boring machine, application scenario, and design requirements. Here are some size information of common main bearings for tunnel boring machines:

　　1. Main bearing for 6-meter-level hard rock boring machine:

　　The outer diameter is up to 6010 millimeters, the inner diameter is up to 4920 millimeters, and the weight is nearly 34 tons. The precision reaches P5 level and can be equipped for hard rock boring machines with a diameter of 12 to 14 meters.

　　2. Main bearing with a diameter of 8 meters for shield machine:

　　China has developed a main bearing with a diameter of 8 meters for super-large shield machines, weighing 41 tons.

　　3. 7.6-meter main bearing:

　　The 7.6-meter main bearing independently developed by China Railway Construction Heavy Industry provides the "heart" for shield machines with a diameter of 16 meters.

　　Generally speaking, the size of the main bearing of a super-large boring machine is usually between φ3m and φ7m. But, some exceptional projects require extra-large TBMs. One notable example is the TBM used for the Doha Metro project in Qatar, which had a diameter of nearly 15 meters. The main bearing for this TBM was reported to be approximately 7.8 meters in diameter!