## 1 Introduction  
During the recent rapid advances in science and technology, bearing technology has been extensively used in mechanical structures. However, as a hydrodynamic bearing-rotor system rotates at a critical frequency [1; 2], the interactions of the surrounding fluid in the bearing-rotor system will produce subsynchronous self-excited vibrations of large amplitude that track running speed and make the system be unstable. The fluid-induced instability behaviors are oil whirl and oil whip. These interactions will make the rotor impact the wall of the journal bearing, and cause dry friction that damages the mechanical components. In view of this, how to cope with fluid-induced instability for the hydrodynamic bearing-rotor system has attracted much attention in the recent years.  
In the past, there have been many solutions proposed to eliminate the instabilities of a fluid bearing. Some articles have discussed how to enhance the dynamic bearing performance using a groove design. [PERSON] et al. [3] discussed the influence of a circumferential groove for a hydrodynamic short bearing. [PERSON] et al. [4] showed the performance of a journal bearing with a single and twin axial groove configuration. [PERSON] and his research team [5; 6; 7; 8] showed that dynamic stiffness and viscosity are unstable factors in a rotor system which will affect the rotating of the whirl and whip, and they also showed that the oil whirl phenomenon will occur at about 0.47 times the frequency of the rotation speed, indicating the effect of bearing rigidity and pressure. Furthermore, some researchers have used an extra actuator to change the stiffness of the rotor-bearing system. [PERSON] et al. [9; 10; 11] used electromagnetic exciters to increase the stiffness of the rotor-bearing system to raise its threshold to a higher stability and eliminate the whip instability.  
Ferrofluids are colloidal liquids made of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid. The properties of ferrofluid are similar in nature to the carrier liquid in the absence of an external magnetic field. When an external magnetic field acts, the ferrofluid exhibits the characteristics of a magnetic material and has controllability and rheology. Ferrofluids exhibit many special effects, and have high academic value and application prospects [12; 13]. In recent years, with the development of nanotechnology, many applications have been proposed, and they have mainly been used in three major areas: space technology, damping mechanisms, and in shaft sealing technology and hydrodynamic bearings. A dynamic equation has been proposed for a hydrodynamic bearing-rotor system lubricated by ferrofluid, which has shown that ferrofluid applied on the oil-film bearing can improve the viscosity and the load-carrying capacity in a magnetic field [14; 15; 16; 17; 18; 19; 20; 21; 22].  
Several researchers [23; 24; 25] have discussed the change of the bearing capacity of a sliding bearing under the influence of an external magnetic field. [PERSON] et al. [26; 27] have studied the influence of external magnetic field on the friction performance of journal bearings. The experimental results show that the addition of magnetic fluid can effectively reduce the wear.  
This study builds a single-oil wedge hydrodynamic bearing-rotor system as the test platform, and uses ferrofluid as the lubrication oil which can be controlled by the magnetic field to improve the performance of the hydrodynamic bearing. Experimental results showed that the bearing stability threshold conditions will be increased by the external magnetic fields and the stiffness of the magnetic fluid is also increased.