Size reduction is of cardinal importance in the recycling of scrap and waste. The choice of size reduction machines is critically dependent on the properties and bonding conditions of the components present in the waste or scrap. Shredders have become widely used for the size reduction of various kinds of scrap. Size reduction of metals in shredders requires sufficiently deformation of the pieces of material (bending to compaction), which leads to cracking. Tensile loads con combination with bending and torsion can enlarge the cracks and ultimately lead to break-up of the pieces of metal. Reduction of the circumferential velocity of the shredding hammers will increase the energy required for the compaction preceding size reduction. Hence, shredders should be operated with maximum possible circumferential velocities.