If you place a suspended (overband) magnet, keep the following in mind:
When positioning the magnet, the construction below the magnet such as the frame, bearing rollers and reversing rollers must be made of non-magnetic material. This is because steel parts will become magnetized, which will adversely affect the separation performance of the overband magnet.
Goudsmit uses the following rule of thumb:
Keep the area extending a distance of 1/3 of the magnet’s width (shown below as 'a') on all sides free of magnetic material. For the area below the magnet, maintain a non-magnetic zone of twice (2b) the block magnet height ('b'). It is important to make all components in this zone out of stainless steel, plastic or other non-magnetic material.