The backiron compensates the magnetic pull on a driven clip, in the area where motors are mounted on the bottom level only. Otherwise the forces on the roller bearings are too high, which leads to a short lifetime. The backirons for 0 idler and 2 ilder lines consist of metal sheet in a rectangular shape, for 1 idler lines in shape of the motor iron core.Furthermore the 1idler backirons are covered with a stainless steel housing, in order to avoid dirt inside the grooves and make the whole construction more rigid.