In hammer mills, the grinding process consists of hitting the material by hammers, articulated on a rotor. It is important, however, that the percussions applied to the hammer are not transmitted to the articulation. The position of the hammer articulation should be determined so that the percussion given by the collision with the particles of material is applied in the centre of percussion. Because the conditions of the percussion center are not always accomplished, unbalances depending on the connection percussions appear and produce supplementary perturbations. These effects are estimated by the transmission ratio of the percussion to the articulation. This is checked by measuring the appearance of some amplified vibration levels in conditions of increased transmission ratio of percussion.