The operating mechanism of a single-girder bridge crane generally only uses four active and driven wheels. If the lifting capacity is large, the method of increasing the wheels is often used to reduce the wheel pressure. When there are more than four wheels, an articulated balanced frame device must be used to distribute the load of the crane evenly on each wheel. The metal structure of the bridge is composed of main beams and end beams, which are divided into two types: single-main-girder bridges and double-girder bridges. A single main girder bridge consists of a single main girder and end beams located on both sides of the span, and a double girder bridge consists of two main girder and end beams.
The main beam and the end beam are rigidly connected, and the ends of the end beam are equipped with wheels to support the bridge to run on the elevated. Rails are welded on the main beam for the crane trolley to run. There are more typical structural types of bridge main beams, such as box structure, four truss structure and hollow truss structure. The box-shaped structure can be divided into two types, such as regular box-shaped double beams, off-track box-shaped double beams, and off-track box-shaped single main beams. The positive rail box-shaped double beam is a widely used basic form. The main beam is composed of upper and lower flange plates and vertical web plates on both sides. The trolley rail is arranged on the center line of the upper flange plate. Its structure is simple, It is convenient to manufacture and suitable for mass production, but has a large self-weight.
The section of the off-track box-shaped double girder and the off-track box-shaped single main girder is composed of upper and lower flange plates and main and auxiliary web plates of unequal thickness. The stiffening plate can be omitted. Among them, the off-track box-shaped single main beam is replaced by a wide-flange box-shaped main beam instead of two main beams. The self-weight is small, but the manufacturing is more complicated. The four-truss structure is composed of four plane trusses to form a closed space structure. The surface of the upper horizontal truss is generally covered with a walking platform, which is light in weight and high in rigidity. However, compared with other structures, it has large dimensions, complicated manufacturing and fatigue. The strength is lower and has been less produced.