Scheffer magnet lifting systems
Magnets for steel bar transportation
The demands on magnet lifting technology vary greatly when transporting steel bars. The elongated goods are sometimes stored in stacking cradles or high racks, but also often just on the ground. Each of these individual storage types places a specific requirement on the magnets, crossbeams and controls. To enable easy access to the stacking cradles, the crossbeams are equipped with either folding or swivelling cradle holding fixtures. Furthermore, retractable pole shoes on the magnets can be used for picking up individual bars.
How material becomes magnetised:
when ferromagnetic steel (e.g. 42CrMO4) is transported magnetically, the material still has a high level of residual magnetism after the magnetic field has been switched off. This is due to the orientation of magnetic domains known as ‘Weiss domains’. If these domains have the same orientation, the material is measurably magnetic.
This magnetic field has a major impact on the further processing of the material:
- Metal chips and grinding dust stick to the workpiece;
- Sintered tools wear out faster;
- Downtimes for robots/automatic feeding systems due to parts sticking together;
- Magnetic field sensors are activated by mistake;
- Measurement errors generated by highly sensitive measuring instruments;
- Defective weld seams;
- Defective electron beam welding;
- Uneven thickness of hard chromium or titanium nitride coatings;
- Edges break off during wire EDM;
Demagnetisation during switch-off.
Scheffer magnet systems feature a demagnetisation function that can be selected to prevent these negative effects of ‘residual magnetism’. When this function is selected, the material is exposed to a strong alternating magnetic field (polarity reversal of + and -) when the magnets are switched off. The strength of this field is consistently reduced to zero. By measuring the current quickly, the duration can be reduced to a minimum. The homogeneous orientation of the ‘Weiss domains’ is destroyed and the magnetic effect reduced externally.
Once the demagnetisation process is complete, the crane’s hoisting unit is enabled once again and the material can be processed without any disruptive magnetic effects.
The current and thus the field of the magnets on the Scheffer magnet system can be infinitely adjusted by using Siemens DCM converters.
One converter is used per magnet group.
The magnetic force in partial-load operation can be controlled in the following ways:
- As a fixed value
- As step switching with binary signals or as a BCD code
- As infinitely variable pre-selection using a potentiometer or decimal value
In full-load operation, it is not possible to manually adjust the magnetic force.
However, in two-handed operation, it is possible to return to partial-load operation. Then, the magnetic force can be adjusted as described above.