Main Mechanisms of a Quay Container Crane

1. Lifting Mechanism

While bridge-type grab unloaders typically operate at full load, the load on the lifting mechanism of a quay container crane varies considerably. Since the current container load capacity is generally only 50%–60% of the rated capacity, for example, if a crane's rated lifting capacity is designed for 30.5t, lifting a 20ft container, even at maximum gross weight, only reaches 67% of the crane's rated lifting capacity. To improve loading and unloading efficiency, the lifting speed must vary with the load. If the lifting speed for the rated load is constant, then when the load is less than the rated load, the lifting speed should increase inversely proportionally. Generally, the unloaded speed is 2 to 2.5 times the loaded speed.

2. Trolley Travel and Anti-sway Mechanism

The trolley of a container crane travels a long distance and at a high speed. When the trolley starts and brakes, the cargo sways in the trolley's direction of travel. The higher the trolley speed, the more severe the swaying, thus affecting loading and unloading efficiency and operational safety. Therefore, anti-sway devices must be installed.

3. Trolley Traveling Mechanism

During container ship loading and unloading, the trolley needs to be frequently moved to align with the containers on the ship, preventing collisions with adjacent containers or the ship's superstructure. Therefore, the trolley traveling mechanism requires good speed regulation, micro-motion, and braking performance, and is typically driven by a DC motor.

4. Drive and Power Supply Methods

To meet the operational requirements of quay container cranes and achieve good speed regulation performance, a DC continuously variable speed control system is generally used, with each mechanism driven by a DC motor. There are three types of DC power supply methods: The first is the AC motor-DC generator method. This method is widely used, as it is relatively reliable, and the supply voltage is largely unaffected by grid voltage fluctuations, making it relatively stable. The disadvantages are the heavy weight of the unit, high price, and relatively high noise. The second is the thyristor rectification method. It has high electrical efficiency, good speed regulation performance, light unit weight, small footprint, and convenient maintenance, but requires a large grid capacity and minimal voltage fluctuations. Furthermore, it requires a high level of technical expertise for maintenance. The third type is a diesel engine powered by a DC generator. It is not affected by external power sources, which improves the mobility of container cranes. It is especially suitable for docks where power supply is inconvenient and can save on investment in power supply facilities. However, this type of unit is very noisy, the diesel engine is more complicated to maintain, and it is not suitable for the current stringent environmental protection policies.