One of the most critical pieces of equipment in a steel mill is the ladle crane, but too many steel mill operators don’t pay enough attention to the design and procurement of the crane. Correct selection of crane materials, components and specifications is crucial to ensuring high levels of productivity and an efficient steelmaking shop.
However, during the procurement process, heavy equipment such as turbines, boilers and to some extent pumps and valves are given more emphasis, while ladle cranes are neglected. Ladle crane purchasing decisions are made hastily, with little scientific thinking applied, so the technical specifications only roughly meet the application requirements.
Additionally, budget constraints often make matters worse. Since ladle cranes are typically procured after a factory's caster, steel shop, and furnace, there is little money left for procurement, limiting appropriate crane specifications.
To ensure efficient and correct commissioning, procurement, design, manufacture and installation of ladle cranes, collaboration between manufacturer and customer is crucial.
For smaller mills, price is always an important factor in the procurement process and total funding for expansion needs to be carefully balanced.
However, cost cutting at this stage is likely to have a knock-on effect on the plant's productivity in future years, whether through higher and more frequent repairs and maintenance than necessary, or a shortened life cycle due to being unsuitable for the task at hand.
In the long run, whether it is a 3m ton rolling mill or a 5m ton rolling mill, a crane is required. If a factory wants to ensure the highest level of efficiency, price cannot be the driving factor in the purchasing process.
Design standards for ladle cranes
There are several concepts to consider when selecting a ladle crane. India adopts IS-4137, while international markets recommend FEM and CMAA standards. The FEM standard has clear advantages as it reduces the size of the wheels while optimizing the size of the hook. Of all the available standards for ladle crane design, FEM is the most suitable standard for several reasons.
First, since wheel load calculations for cranes do not use average loads, wheel optimization is actually possible. This design standard allows for accurate comparisons of wheel and rail dimensions.
In addition to detailed guidance on how to check fatigue and accurately select ropes and rope drums, the classification of each available mechanism is clearly explained, allowing design teams to well understand the various options before them.
Looking at ladle cranes for 3 million ton or 5 million ton steel plants, there are five important areas that must be considered in the crane design.
The first, and arguably most important, is the drive system. Most modern steel mills use variable voltage variable frequency (VVVF) drives instead of the traditional slip ring motor concept.
The motors will be standard AC squirrel cage motors requiring no maintenance, while the absence of rotor contactors and rotor cables means less complexity.
To further reduce the number of complexities, overload protection, undervoltage protection and on/off operation are built-in, so external contactors and protective relays are not required.
Continuously variable speed from minimum to maximum speed also means any speed setting is possible, even micro-speeds between 5% and 100% without requiring any additional hardware and allowing for smooth movement.
Another aspect of ladle crane design that can reduce jerky operation is reducing starting current. Start-up current is lower than traditional direct-on-line (DOL) start, improving operation and saving power consumption.
Features such as electrical braking reduce wear on brake shoes and linings compared to simple mechanical braking, meaning less mechanical fatigue on crane components, extending the life cycle of its mechanical components.
Other features such as tandem control functionality with good RPM control, as well as feedback options to facilitate tandem control, parameter display and diagnostic display on the LCD, can also be used while the crane is online, thus reducing downtime. Moving to the second point of note in ladle crane design, fail-safe brakes should always be imported from reputed suppliers. While domestic brakes perform well, when safety considerations are questioned, brakes made by Sime or Sibre are often preferred in India. However, the indigenous brakes manufactured by Galvi are ideal for ladle cranes.
Wire ropes of sufficient strength and construction must be used to withstand the rigorous operation of a ladle crane. Anupam uses 2160n/mm2 strength steel wire rope to achieve good operating efficiency. In India, Usha Martin wire ropes are recommended.
During operation, stresses can develop in the wire rope and hook set assembly pins, which can lead to premature crane failure. Anupam uses a better EN series material composition and is used as the pin for the hook block assembly to resist stress.
Wheels are an important component when it comes to the longevity and smooth operation of a ladle crane. Many customers typically require a higher hardness of around 400 BHN or 58/RC to handle the wear and tear that occurs during operation. Another important aspect is the use of EN-28 material, which increases the longevity and strength of the wheels, helping them withstand impact loads for continued smooth operation. However, EN-28 materials are scarce.
More commonly, C55Mn75 or SAE5160 material with a hardness of 250 to 280 BHN is used for good health on ladle cranes and there is nothing wrong with this. But for larger size ladle cranes of 400t and above, the criticality of the operation is very strict, so it is strongly recommended to use EN-28 materials.
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