The magnet has important advantages against other means of lifting. The large area of contact prevents damage to the coil, the stores can be more densely loaded, the magnet is not subjected to wear and is more economical. It has now been in 3-shift working since 1987 without faults or repairs.
 
Service conditions in the coil store. A store for sheet coils weighing up to 50 t contains horizontally laying and vertically standing coils; the former being destined for continuous and the latter for hood type annealing. The safety of the staff is of the highest priority. Consequently, a magnet was built that can transport coils in both positions without conversion work, figure 1. The coil diameters vary between 1,280 and 2,780 mm and the widths between 600 and 1,700 mm. The coil is only circumferentially strapped. However, the inner windings of the vertically standing coils must also be safely held during transport. The load carrying capacity must also be ensured with an edge offset of up to 20 mm, extending over a number of windings. With horizontally laying coils allowance has to be made of the fact that the strip surface can be slippery due to rolling emulsion. However, the coil should not slip laterally during transport. The strip temperature is below the Curie point and is thus ferromagnetic and suitable for transport by magnets, although the magnet itself is additionally heated by thermal radiation from the coil. Allowance has to be made for this in the design of the electrical part. Asymmetrical contact between the lifting device and the load must be accepted within specific limits. Accelerations for lifting, lowering, approach and braking are prescribed, these factors resulting in the load-carrying forces to be allowed for.The load-carrying capacity was established as the coil weight times a factor of 2, this having had to be confirmed. The usual way of confirming the load-carrying capacity of lifting magnets is a breakaway test, the load being fixed to the ground. The holding force is measured whilst the crane is pulling the magnet upwards. With large loads, e.g. 50 t coils, such a test is expensive and dangerous. On the one hand a suitable force measuring device is required, the force having to be applied from the crane lifting mechanism and, on the other hand, the high rate of change in the force occurring at the moment of breakaway can result in unacceptable accelerations on the crane. Consequently, a new method was used to confirm the agreed load-carrying capacity. 
  
Figure 1.  Lifting magnet for the transport of upright (left) and laying down (right) sheet coils weighing up to 50 t