Figure 1 | 3 Red-hot slab: "final product" on the continuous casting plant
Figure 2 Custom hydraulic cylinder in the continuous casting plant: oscillation at three hertz prevents the steel strand sticking to the inner wall of the ingot mould.
They oscillate during normal production with 80 vibrations per minute and a stroke of ±3-4 mm. The cam profile - like the amplitude and frequency – can be freely modified during operation to keep the relative movement of each single oscillation between the ingot mould and strand exactly constant. Since the strand itself has an inherent movement on account of the casting speed, the downwards movement of the ingot mould must be much faster than the strand's own movement. Hänchen thus designed the cylinder as highly dynamic and practically friction-less. The movement is recorded by a magnetostrictive position transducer system and controlled with an industrial PC, which in turn ensures the connection to the process control computer.
The patented floating annular gap seal in a Servofloat® design is used in the controlled servo hydraulic cylinders, that exert a force of 400 kN. A steel liner hereby follows the radial deflection of the piston rod with a seal gap of only a few 1/100 mm that is kept practically constant over the entire pressure range. However, this method only works with a production accuracy of a few μm since the leakage would otherwise lead to high hydraulic losses.
Cylinders in a Servofloat® quality offer maximum positioning and repeating accuracy thanks to their negligible friction, are stick-slip-free and suitable for very slow and fast movements. Thanks to the practically friction-free design, this type of cylinder does not wear, even after extremely long periods of use. This corresponds to a requirements profile where maximum availability takes top priority. High demands are placed in particular on the purity of the fluid: the filter is replaced as necessary, the two pumps that generate the operating pressure of 260 bar are replaced every 35,000 hours. Particles are counted and the servo valve replaced once a year. 3D measurements on the elevating platform with an accuracy of 10 μm that record and show any change to the oscillating movement are also used for quality control. „We originally planned to service or replace the cylinders each year“, reports the technical coordinator for maintenance at ThyssenKrupp Steel AG, Dipl.-Ing. Marc Ramacher. „But the replacement cycle for a general overhaul was initially increased to two, then four and now six years. Even after this period of time we still do not have to completely replace the Hänchen cylinders. They only have to be sent to Hänchen for a check and general overhaul.