“…Krauter [13] analyzed torsional vibrations in water lubricated bearings and authors in [2,3,16,21,24,25] analyzed torsional vibrations in drill-string systems. In those papers, it is concluded that torsional vibrations are caused by negative damping in the friction force in the bearings [13], the friction force at the contact between the bit and the borehole rotor [2,3,16,21] and by the friction force at the rotor [24,25].…”