Design criteria of ladle crane

August 11, 2014 12:01 pm

 Ladle cranes have utmost importance with respect to production efficiency of steel melting shop.  Hence, the selection of ladle cranes is vital.  However, enough wisdom is not meted out in its selection with respect to its specification, choosing of materials and components.
Any steel plant will be requiring ladle cranes whether it is of 3MT or 5MT.  Capacity of the ladle crane may also vary depending on the ultimate size of the SMS (steel melting shop).
When the procurement of these cranes is initiated by smaller size steel plant, the price becomes the essence which to my mind is not correct.  The price hurdles are to be ignored to a lesser extent if not to a greater extent not to sacrifice the operational efficiency of the steel plant i.e. SMS as well longevity of the crane.  By saving money during the time of initial procurement stage will not be a wise decision because it’ll pilferage more and more money in the long run because of troublesome operation of the crane.
Design standard to be followedThere are various concepts of selection of this crane are prevailing. In Indian market, IS-4137 is employed and in international market, people recommend FEM and CMAA standard.  There are distinct advantages with respect to FEM standard because; it reduces the size of wheel, optimising the size of hook.  But, the  feels that from all the standards available for this particular crane, FEM is the best one for some of the following reasons:
• Wheel load calculation is not based on mean load and ultimately the manufacturer is able to derive the optimisation of wheel. • This design standard provides accurate compatibility of wheel and rail size.• Classification of mechanism is distinctly spelt out and so the design has a wide choice to select.• Provides very detailed guidelines to check fatigue.• Rope and rope drum selection is very accurate.
Following are the parts which are very critical while selecting a ladle crane for 3 MT to 5 MT SMS.
VVVF drive: Nowadays, most of the modernised steel plants use this drive against the conventional slip ring motors concept.  The usefulness of VVVF drive is discussed as below.
Wire rope:  Wire rope of adequate strength and construction is to be used to withstand the rigorous operation. 
Hook block assembly:  Stresses are generated in wire rope and hook block assembly pin which causes the premature failure of the crane. So, adequate design criteria must be kept in consideration. The better composition of materials of EN series is employed as pin for the hook block assembly to counter the stress.  
Wheel: This component is also very important as far as longevity and smooth operation of this ladle crane is concerned. The higher hardness of around 400 BHN or 58/RC is nowadays desired by many of customers to counter wear and tear arises out of operation. Another aspect is to use EN-28 material for better life and strength of the wheel to withstand the impact load for continuous operation and also offers the smooth performance but only difficulty is that EN-28 material is scarcely available. 
Mostly, C55Mn75 with hardness of 250 to 280 BHN or SAE5160 material is used for good health of the ladle crane. However, when it comes to bigger size of the ladle crane and criticality of the operation is severe, then use of EN-28 material is recommended.
Design aspect of ladle crane:Kinematics of the four motors V/s two motorsAcross the world, there is an ongoing debate between the two kinematics: four motors and two motors. Some people still prefer two motors kinematics whereas some prefer four motors kinematics.  Though there is nothing wrong to go for two motors kinematics, there is a significant advantage of using four motors kinematics which are discussed in Table 2 owing to the fact that main hoisting mechanism performs well in this kinematics.
After incorporating all the above features in manufacturing, selection of components, selection of painting supported by the full fledged quality system for incoming and outgoing materials, we have got break through in overseas market but crux of the situation is that the buyers / customers procurement departments’ approach relevant for Indian steel plants should be more pragmatic.  They must give more insight in the greater reality of procurement of this critical crane exercising their wisdom.