One of the most critical pieces of equipment in a steel mill is its ladle crane, however too many mill operators do not devote sufficient attention to the design and procurement of the crane. The correct selection of materials, components and specifications for the crane is vital to ensure high levels of productivity and an efficient steel melting shop.
However, heavy equipment such as turbines, boilers and to some extent pumps and valves are afforded greater importance during the procurement process, while the ladle crane is neglected. Purchasing decisions for ladle cranes are taken hurriedly, and with scientific thinking rarely applied so that the technical specifications only loosely meet the application requirements.
In addition, budgetary constraints frequently conspire to make matters worse. With ladle cranes often being procured after the mill’s caster, steel melting shop and furnace, this leaves precious little funding for its purchase, hamstringing proper crane specification.
To ensure efficient and proper commissioning, procurement, design, manufacture and erection of ladle cranes, cooperation between the manufacturer and the customer is paramount.
For smaller steel mill operations price is always an important factor in the procurement process that entails careful balancing of total funding for expansion.
However, cost cutting at this stage can have knock-on effects on the productivity of the plant for years to come, whether through higher and more frequent than necessary repair and maintenance costs or a decreased lifecycle due to its unsuitability to the task at hand.
In the long run, whether it is for a 3m tonne mill or a 5m tonne mill, a crane is needed. Price cannot be the driving factor during procurement if a plant means to ensure the highest levels of efficiency.
Design for life
There are various concepts to consider during the selection of a ladle crane. In India, IS-4137 is employed, while FEM and CMAA standards are recommended across the international market. The FEM standard has distinct advantages as it reduces the size of the wheels while optimising the size of the hook. Of all the available standards for ladle crane design, FEM is the best suited of all for a number of reasons.
Firstly, as a mean load is not used for the crane’s wheel load calculation optimsation of the wheel is actually achievable. The design standard allows accurate comparison to be made between the wheel and rail size.
Along with detailed guidelines on how to check for fatigue and make accurate rope and rope drum selection, the classification of each mechanism that is available is distinctly explained, giving the design team good understanding of the wide array of choices before them.
Looking at ladle cranes for 3m tonne or 5m tonne steel mills, there are five significant areas of crane design that must be considered.
The first, and arguably most vital, is the drive system. Most modern steel plants use variable voltage variable frequency (VVVF) drives instead of the conventional slip ring motor concept.
These will feature a standard AC squirrel cage motor, which requires no maintenance, while the absence of a rotor contactor and rotor cable means fewer complexes.
To further reduce the number of complexes the overload protection, under voltage protection and on/off operation are in built, hence no external contactor and protection relay needed.
Stepless speed variation from minimum to maximum speed also means that any speed setting is possible, even micro speeds of between 5% and 100% do not require any additional hardware, and result in smooth motion.
Another aspect of ladle crane design that can reduce jerky operation is the reduced starting current. A starting current of an amount lower than conventional direct on-line (DOL) starting, improves operation and saves power consumption.
Features such as electrical braking, which reduces wear on the brake shoe and liner when compared to simple mechanical braking, mean less mechanical fatigue on crane components, and therefore extension of the lifecycle of its mechanical parts.
Other features, such as a tandem control function, enabled by good RPM control, and feedback options that aid tandem control, parameter display and diagnostics display by LCD can also be used while the crane is on line, reducing downtime. Moving on to the second point of note for ladle crane design, the fail safe brake should always be imported from a reputed vendor. While indigenous brakes are good, brakes manufactured by Sime or Sibre are often preferred in India when the safety factor comes into question. However, Galvi makes indigenous brakes that are very good for ladle cranes.
Wire rope of adequate strength and construction must be used to withstand the rigorous operation expected of ladle cranes. Anupam uses 2160n/mm2 strength wire rope to achieve good operational efficiency. In India Usha Martin’s wire rope is recommended.
During operation, stress is generated in the wire rope and hook block assembly pin, which can lead to premature failure of the crane. Anupam uses a better composition of materials of the EN series, which is employed as a pin for the hook block assembly to counter the stress.
The wheel is an important component as far as longevity and smooth operation of a ladle crane is concerned. A higher hardness of around 400 BHN or 58/RC is often desired by many customers to counter wear and tear arising from operation. Another important aspect is the use of EN-28 material for better life and strength of the wheel, helping it to withstand the impact load for continuous smooth operation. However EN-28 material is scarce.
More often C55Mn75, with a hardness of 250 to 280 BHN, or SAE5160 material is used for good health of the ladle crane, and nothing is wrong with that. But when it comes to bigger sizes of ladle crane of 400t and above, where the criticality of the operation is severe, then use of EN-28 material is highly recommended.
Kinematics matters
There is an ongoing debate within the realm of kinematics today, with some people falling on the side of two motor kinematics whereas others prefer four motor kinematics. There is nothing wrong with either approach, but there are significant advantages to the use of four motor kinematics for the main hoisting mechanism. With the four motor system, four motors drive through two gearboxes with in-built planetary gearbox systems, as opposed to two motors driving through one gearbox. The rope drums are mechanically connected through bull gears, which synchronise drum motion. This means that the selection of motors falls into a standard size category, which in turn reduces maintenance costs and the initial cost of the motors compared to a conventional two motor arrangement.
A conventional two motor arrangement will result in a non-standard size of motor whenever a horsepower increase is required, as well as a singular gearbox requiring large size standard input planetary gears, and this significantly increases the costs associated with its size.
In the case of a hoist motor failure the four motor system exhibits other advantages that a two motor system does not have, such as the ability to continue crane operation at half speed with the other two motors, even at 100% of the load in situations where a two speed system would have to cease operation. This obviates problematic lowering of the load.
Also, in the case of a failure involving the input pinion of the main hoist’s gear box, a four motor system allows the crane to continue operation in a similar manner, where the lack of redundancy on a two motor system would simply see the emergency brake cut in, requiring break down maintenance before operations can resume at all.
Indeed, even the differences in the emergency brake arrangement of each system make a significant contribution to the hoist sizing and its safety performance.
With a two motor system one set of emergency disc brakes is typically mounted on each rope drum, along with a suitable over speed protection device. However, a mechanical failure on any part of driving system between motor and rope drum will initially cause the load to free fall.When such a mechanical failure occurs, the speed of the load will exceed the rated maximum speed and the emergency brake would need to be applied immediately to hold the load. During such a release of the load from any height, the falling load will generate a very high speed and torque, and under such conditions the emergency brake will be applied with an additional impact.
This abnormal impact often causes damage to the crane’s main structure, specifically the end carriage necks between rail and wheel, which may shear off.In addition, to accommodate the emergency disc brake on the rope drum flange additional space is required, which necessitates more room for the hook approach and more headroom.
By contrast, mechanical failure leading to the same over-speeding situation on a four motor system is sensed, either by encoder mounted on motor shaft or by the limit switch mounted on the rope drum shaft. In this case the power is shut off immediately through one or both devices, and the service brakes on input shaft come into action immediately to hold the load.
Also, a number of redundant service brakes is provided on input pinion shaft, so even in case of failure of one brake, another will hold the total load and stop free fall. Therefore, in this system, under any situation, there is no free fall.
Rope drums on a four motor system are mechanically connected via gear rings, and on the input side of the gearbox an independent shaft is provided that connects to the output gear directly through an in built gear train. A service brake is mounted on the input shaft and acts as an emergency brake in this system.
This emergency-cum-service brake is connected parallel to the MH motor through the encoder, which is mounted on hoisting motors. Therefore no emergency disc brake is required on the rope drum, and the hook approach and headroom no longer needs to be reduced.
All in all, some of the most distinct advantages of four motor kinematics over two motor kinematics pertain to safety.
Two motor systems rely on very thorough and frequent maintenance of the emergency disc brake to provide adequate safety, as it only ever comes into action in the case of mechanical failure so it must be sure to work first-time even after long periods of inactivity. With no redundant braking feature, it is not 100% failsafe.
But with four motor systems the service brakes and rope drum gear train contribute to the standard electromechanical braking action in case of any mechanical failure in driving system between motor and rope drum.
Incorporated redundancy makes the system 100% fail-safe, with no free fall under any circumstances even despite the absence of an emergency disc brake on the rope drum.
Fatigue calculation, FEM analysis and resonance elimination are applied in the structural and mechanical design for such extra large size heavy-duty ladle cranes, which have fewer spare parts, meaning less complicated maintenance.
