The root is hinged by the horizontal pin and the slewing platform, and can be freely rotated in a vertical plane (ie, a variable plane). The variable amplitude hydraulic rainbow is installed in the middle and lower part of the basic arm, and the two are also hinged by the horizontal pin. Therefore, the boom can be regarded as a simply supported outrigger in the plane of the variable amplitude. The external loads acting on the boom mainly include the weight of the boom with uniform hook distribution, the weight of the hoisting weight considering the dynamic load coefficient, and the pulling force of the hoisting rope. They can be converted into concentrated loads acting on the ends of the boom.