adding main beam的用法和样例：
- The crane is connected with the main beam through its under-frame and the truck chassis.
- Luminaire designed to be suspended from a cable with its main beam axis at right angles to the axis of the road.
- The main beam of the roof of this palace built in the Ming dynasty was made of precious wood.
- The extended part is internal frame, which is pushed by hand to orbit for sending or receiving cargos inside main beam.