ROLLER table drives are essential parts of a steel mill’s equipment. They need to be fully reliable, or else entire production lines threaten to stand still, which may result in irreparable damages to the equipment if freshly cast goods stay on the roller tables for too long.
[Image, right: Hot rolled stock after the cooling line.]
The drives must withstand harsh environments, extremely irregular operating modes, heavy loads, and maximum temperatures. Roller drives must deliver high torques. They must be able to instantly reverse the rolling direction and feature variable speed adjustment. This entails high electrical, mechanical, and thermal stress on the drives.
In the Pudong rolling mill, steel slabs are rolled into sheets. The slabs, freshly cast in a continuous caster, are transported into a reheat furnace via roller tables. The furnace spits them out red-hot. They pass through a high-pressure descaler which removes the mill scale. Because of the spray water, the drives in this area provide IP66 protection.
Next, after the slabs have been tapped, the pieces are rolled to the required thickness and size. The sheets then pass through an accelerated cooling line to a cooling bed, and to customisation. The roller tables for slab transport have rollers mounted with one meter distance between them, each powered by one geared motor. The load torque must be high enough for the motor to stand in for a neighbouring machine in case of a breakdown. To ensure that the roller tables do not come to a standstill in any event, five to seven drives per ten rollers must be powerful enough to transport the rolled stock.
More than 800 helical shaft motors alone were installed at the roller tables in the shearing lines which require up to 500 switches per hour and accelerations of up to 3.4 m/s². In addition, Baosteel uses geared motors from NORD in the hot area, for instance in the descaler and as roller drives, but also in the accelerated cooling line.
For maximum loads, NORD supplies industrial gear units which generate torques of up to 200,000 Nm – they are suitable for applications in the cooling bed, on chain conveyors, and on guide rollers.
To support the changing loads and often fast accelerations, roller tables in the steel industry require drives with high starting and breaking torques. If a steel slab gets stuck, the drives must withstand the resulting overload. Because of the demanding tasks with changing duty types and high ambient temperatures, steel factories often prefer ‘time-honoured’ roller table motors, i.e. motors with ring fins which not only provide excellent self-cooling through convection, but are dirt-repellent when mounted horizontally.
NORD’s straight-finned TENV motors have been designed for frequent reversals and provide perfect protection from dust, dirt, and spray water. Featuring a larger amount of material and bigger bearings and shafts, these specific motors withstand greater loads and have a longer service life than conventional industrial motors in the same performance range.
The motors for Baosteel were fitted with a special winding with a temperature sensors and an incremental encoder. For an efficient cooling in rugged environments, they employ natural convection (totally enclosed non-ventilated – or TENV – models) or forced convection (totally enclosed fan cooled – or TEFC – models). The non-ventilated design provides perfect protection from dirt and water.
The new IE2 motors from NORD furthermore allows for innovative drive systems layouts and afford savings opportunities. They generate less heat loss than the preceding generation, and depending on the size, they can, if necessary, permanently be operated at a 150% performance factor without reducing the availability of the drive system. In addition to large power reserves, IE2 energy saving motors from NORD feature increased starting and breaking torques, higher speed, and typically require less power for the rated output.