Spreading magnets

Trouble-Effects

Apart from the restrictions described there are some more points of trouble which are of major importance and affect the spreading function much more considerably.

• The Sticky Effect
  
  Spreading magnets are applied to lift sheets lying above each other resp. to isolate the upper one in a pile from the one lying directly below. As the sheets lying below are slightly oiled before the piling to prevent early corrosion, the continuously increasing number of sheets also increases the pressure due to the own weight. These piles bound together like bundles are frequently put on pallets. The pressure on single sheets arising from that can easily amount to several tons, because the lower sheet of an average bundle of an area of 1 m² and a piling height of 300 mm is pressed by a weight of 2.3. tons of its own.
  
  
• The Formation of Burr
  
  Due to natural wear cutting tools become blunt resulting in a larger amount of burr. Therefore the piling cannot be performed evenly; as a consequence the pile forms an inclined plane. Additional sheets piled lead to an increase of this effect. A more inconvenient characteristic of burr can be observed, when the blanks are not cut evenly, but look ‘fringed’. This is the result of tools being so blunt that the sheets are not cut, but are more or less torn. The fringes resulting from that tend to be hooked on the sheet at the bottom. In such a case it is nearly impossible to spread the sheets, since there is a partially mechanically firm connection. Only the strong deformation of single sheets can loosen the hooking.
  
  
• Attractive Forces
  
  Spreading magnets do not only spread ( fan ), but also have an attractive effect. They attract the sheets. On the one hand this effect is wanted for the alignment of the sheets, on the other hand it is not, when the blank leaves the area of the spreading magnets. In this case the spreading magnet tries to hold the sheet, though it is to be removed. The effect of alignment, of course, only occurs, if all spreading magnets are found on one side of the pile. The arising attractive force generates a high friction at the spreaders, which in turn weakens the spreading effect. Attaching the spreaders to opposite sides of the sheet bundle is often impossible due to construction. Thus, however, the clinging forces would be compensated considerably and therefore would add the spreading forces. Therefore electromagnetic spreaders, which can be switched off, as required, are frequently applied. Their performance, however, depends on the operating time and is not so effective in many applications.

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