A general-purpose bridge crane primarily consists of four main components: the bridge structure, the trolley, the crane travel mechanism, and the electrical equipment. The bridge structure typically features a double-girder design composed of box-section main girders and end carriages; rails are installed on the top flange of the main girders to facilitate trolley movement. When narrow-section main girders are used, the electrical control equipment is housed on a walkway alongside the girder; when wide-section main girders are used, the equipment is generally housed inside the girder itself. The trolley is the core component of the crane. One or more trolleys are mounted on the bridge structure; they can operate independently, or-in multi-trolley configurations-be designed for either independent or synchronized operation. Each trolley is equipped with a hoisting mechanism (including the load-handling device) and a travel mechanism. Trolley travel mechanisms may utilize either centralized drive or independent drive systems. The crane travel mechanism typically employs independent drive systems, utilizing either a two-corner drive or a four-corner drive configuration. Figures 2-1 through 2-7 illustrate examples of various general-purpose bridge crane models, showing their typical structural configurations; users can refer to the specific drawings provided with the equipment to understand the exact construction of their crane. Examples of the general structure of hook-type bridge cranes are shown in Figures 2-1 and 2-2. Figure 2-1 shows the outline of a low-headroom crane with small-to-medium lifting capacity; the trolley is equipped with both main and auxiliary hoisting mechanisms and utilizes a centralized drive system for travel. The main girders are of the offset-rail, narrow-section type, and the end carriages are connected to the main girders via hinged joints. Independent drive systems are used for crane travel, with "three-in-one" drive units powering both the crane and trolley travel mechanisms. A service cage located outside the left-hand walkway is provided for the installation and maintenance of the current collectors.
Figure 2-2 shows the outline of a high-capacity, low-headroom crane. The trolley is equipped with both main and auxiliary hoisting mechanisms, with trolley travel driven by independent "three-in-one" drive units. The main girder is a wide, offset-rail type, connected to the end carriages via articulated joints. The crane's longitudinal travel is powered by independent drive units featuring parallel-shaft gear reducers, which are installed inside the main girder. An electric hoist mounted on an electric trolley is suspended beneath the left main girder, while the electrical control system is housed within the right main girder.
Examples of the construction of electromagnetic bridge cranes are shown in Figures 2-3 and 2-4. The overall construction of vacuum-suction bridge cranes is similar to that of electromagnetic bridge cranes.
As shown in Figure 2-3, a lifting electromagnet is suspended from the hook of the crane's hoisting mechanism, and a cable reel on the trolley supplies power to the magnet. A backup battery is typically provided for use in the event of a power failure; its capacity is sufficient to ensure the load can be safely lowered to the ground (with the exception of permanent magnet lifting magnets). The trolley travel mechanism utilizes a centralized drive system powered by a vertical parallel-shaft gear reducer. The main girder is of the wide, offset-rail type, and the end carriages are rigidly connected to the main girder. The crane travel mechanism employs independent drives-using horizontal parallel-shaft gear reducers-mounted inside the main girder.
In Figures 2-4, the suspension beam is of a non-detachable type; the hoisting mechanism's wire ropes suspend the beam via pulleys mounted on it, with a lifting electromagnet positioned beneath the beam, which is oriented parallel to the main girder. The layout of the hoisting mechanism accommodates the requirements of the long suspension beam. The main girder is narrow, and the end carriages are rigidly connected to it.
The general structure of a grab-bucket bridge crane is shown in Figure 2-5.
In the Type I configuration, the opening and closing direction of the grab bucket aligns with the crane's direction of travel. A dual-drum hoisting mechanism controls the lifting, lowering, opening, and closing of the four-rope grab bucket, while the trolley travel mechanism utilizes centralized drive via a vertical parallel-shaft gear reducer. The main girder features a narrow-flange design with a centrally mounted rail, and the end carriages are welded to the main girder.
An example of the construction of a dual-purpose bridge crane is shown in Figure 2-6.
This example features a crane capable of operating with either a grab bucket or a hook; the grab bucket is of the hydraulic type. By removing the grab bucket, the hook can be used to lift and transport loads.
Figure 2-7 shows an example of the construction of a multi-purpose bridge crane.
This example is a crane designed for three types of operation: hydraulic grab, hook, and lifting electromagnet. The grab must be removed before using the lifting electromagnet.
