The billet handling crane is a customised design crane that meets the technical specifications of a steel mill. The large capacities of billet handling cranes are used for handling long sized billets that help operators to increase their production. These cranes consist of a "C" hook assembly to support billets, rods, pipes, and other elongated products. The hook assembly includes a spreader beam suspended from the rope drum through a pair of cables. These cranes are available in various capacities and come with various attachments, such as rectifiers, controllers, a cabin with video monitoring system and VVVF (variable-voltage, variable-frequency) drive. But, the most important design factor of a billet crane is that it is designed with a rotating spreader beam to reduce handling, waste and energy consumption. This is feasible and by incorporating these design features, yield is increased and cost is minimised. Most of the higher capacities of billet handling cranes are supplied with magnets.
Anupam has supplied a number of billet handling cranes and slab handling cranes to various steel plants of the Steel Authority of India Ltd. (SAIL), Jindal Steel & Power Ltd. (JSPL), and Raigarh, Monnet Ispat & Energy Ltd. (MIEL). Also side the ladle crane, the billet handling crane is one of the most critical pieces of equipment in a steel mill. Its design and operation can directly impact the efficiency and output of the mill.
The billet handling crane with a rotating trolley is considered to be one of the best designs and used by almost all the major steel plants in the world. In this design, the hoist unit is mounted on the trolley frame through slew bearing cum gearing and rotated by geared brake motor through an open gearing. The slew bearing cum gearing is installed on the trolley structure after machining the upper surface of the trolley with the help of bolts. Therefore, any breakdown directly contributes to the loss of production of steel plants. The production capacity of steel in India is not as high as China or Brazil, adding pressure to the private and public sector steel plants in India to avoid down time.
The critical components of this billet handling cranes are its structure, drive system, rotating trolley mechanism and the electromagnets.
Girder sections
The structure of billet handling cranes will be of double girder construction like other cranes and the major emphasis is given for design of rotating trolley mechanism and for the selection of the magnet.
Bridge and trolley structures are generally fabricated together by welding structural shape. Manufacturers find problem with weld joints between rolled structural members because of the subsurface lamellar tearing that occurs in the low joints during fabrication. Lamellar tearing should not be allowed at all. The weld joints must be passed through radiography and ultrasonic inspection.
Trolley mechanism
The next design dimension of billet handling crane is its trolley mechanism. The trolley mechanism is rotated for the purpose of shifting the slab or billets from one place to another. The rotational movement is a must. This rotational movement is carried out by the following three methods:
– Rail bending – A rail section is bent to make the curve path for the movement of the trolley. This design is not very effective for fine tuning the control of rotation, which is very much important when handling and shifting the billets.
– Spreader beam with slew mechanism – A spreader beam of adequate size is used and through the help of slew mechanism the crane rotational movement is brought in the trolley mechanism.
– Slew gear box – Motors are used in combination with slew gear box to maintain the ratio and speed so that the rotational movement is possible for the purpose of lifting and shifting the billets.
Rotating trolley
A rotating trolley is widely considered the best solution and almost all the major steel plants in the world follow this design. The hoist unit is mounted on the trolley frame through slew bearing cum gearing and the unit is rotated by a geared brake motor through open gearing. The slew bearing cum gearing is installed on the trolley structure, with the help of bolts, after machining upper surface of the trolley. There are a number of slew bearing manufacturers but two of the biggest serving the hoist industry are Germany’s Rother Erde and China’s LYC. This design has some salient features:
– Robust design and with no radial play during operation.
– A very smooth slew operation as the hoist unit is mounted on slew antifriction bearing.
– Maintenance free operation.
– Due to the height of slew bearing, low head room is required in the building.
Wire rope and reeving system
While operating the crane, side loads are generated on the reeving system if the hoisting is carried out at angles departing from a normal vertical lift. This may result damage in from excessive wear on sheaves and wire rope. Furthermore, the wire rope may be cut by jumping its groove barrier on the drum. If side loads or track loading cannot be avoided, then the reeving system should be kept with a guard, which will keep wire rope properly located in the grooves on the drum.
The reeving system is very important on a billet handling crane. It needs special consideration when designing the system. The load carrying rope is subject to accelerated wear if it rubs excessively on the sides of the grooves in the drum and sheaves. If the alignment is not proper or large, fleet angles between the grooves become very large. Load carrying rope further suffers excessive loading if it is partly held by friction on the groove wall. Generally, rope is protected by conservative fleet angles. Ropes also suffer damage due to excessive strain developed if the load construction and pitch diameter of the sheaves are not properly selected. Consideration of fatigue stress in rope is to be given in such a way that it is minimised and the pitch diameter of the sheaves is selected large enough to produce only nominal stress.
Magnet
The magnet is arguably the most important component for a billet handling crane. Most of the steel plants prefer imported magnets such as those from SGM of Italy. There are few good manufacturers in India today, but when it comes to the operational efficiency of the crane, many steel plants in India prefer imported magnets. The magnet plays very important role in overall performance of billet handling cranes.
There are two types of magnets. One is a ‘circular type’ and another ‘rectangular type’ magnet. The circular type magnet is used to lift the scrap whereas the rectangular type magnet is used for lifting the billets, slabs, blooms and plates, etc. The size of the circular magnet is around 1,200mm to 1,600mm diameter and the size of the rectangular magnet is around 700mm x 1,700mm. The magnet should be an electromagnet of a two-pole design with chains and surge suppressor. Overall dimensions of the magnets is as per the mutual decision of the buyer and manufacturer, to suit the dimension of handling components.
An electronic controller type is used with electromagnet to feel, control and protect the magnets. Flux is regulated with potentiometer. A suitable device is used to deactivate the 100 per cent of magnet power when the latter has automatically been activated and operated with partial flux regulation. Along with the magnet, a drop prevention device in the form of demagnetised switch and an overrating device is used. An air condition system is used because the ambient temperature of a billet handling crane is of no more than 45°C.
A master controller is installed in the crane cabin with all necessary push buttons and indicating lamps.
There are very few good manufacturers of billet handling cranes particularly when it comes to the rotating trolley type. Anupam Industries is one of the leading manufacturers for these types of cranes in India. In the last couple of years, the company has strengthened the design by developing sophisticated design software for billet handling cranes and is also taking expert advice from its technical associate Mitsubishi Heavy Industries of Japan, wherever needed to augment the sophistication in design of such cranes. There is a huge gap between the manufacturer quality and the buyers’ expectations, but manufacturers are taking every measure to fill that gap.