The cylindrical and conical rubbing motions of a symmetric rigid rotor in high speed rotating machinery are investigated.When both journal bearings of the rotor are simultaneously under starvaton lubrication condition

both ends of the rotor contact locally with the bearings so that the cylindrical rubbing motions occur.The stability margin of the cylindrical rubbing motions is determined

and the relationships of instability with the radial clearance

the bearing/rotor friction coefficients

and the angle of incidence during successive rubs are obtained.When one of the journal bearings is under starvation lubrication condition

the conical rubbing motion of the rotor occurs.The analysis shows that in this case the stability of rotor depends on its physical parameters

the transcritical bifurcation is analyzed.Then the first order approximation formula of whirling angular speed is given

which provides theoretical analysis for diagnosis of rotor systems.