###### Abstract  
The rotordynamic stability of journal gas foil-polymer bearings (GFPBs) applied to a dual-rotor bearing system was investigated. The GFPB has a high damping structure, i.e., it has an additional polymer layer, and top/bump/bottom foil structures. Test GFPBs were fabricated with nitrile butadiene rubber polymer layers with a thickness of 2 mm. Static-load-deflection tests of test gas foil bearings (GFBs) and GFPBs were performed to estimate the geometric bearing clearance (200 \(\upmu\)m). The dual-rotor rotordynamic test rig consisted of a motor rotor, test rotor, and beam-type coupling. Two journal test bearings were installed on both the drive-end and non-drive-end sides of the test rotor. Predicting and testing the natural mode characteristics of the dual rotors revealed that the relative error between them was less than 7%, indicating that the first and second natural frequencies were 15 Hz and 160 Hz, respectively, and the third natural frequency was 1835 Hz in the tests. Based on the API 612 standard, the upper limit of the rotating speed for the test rig was limited to be approximately 92 k\(\upmu\)m with a separation margin of 26%. Rotordynamic tests were conducted to examine the stability performance of GFBs and GFPBs, where the adjusted bearing clearance was 150 \(\upmu\)m. The test results indicate that GFPBs have better stability performance in terms of delaying and suppressing unstable vibrations than GFBs. Specifically, GFPBs showed stable synchronous and subsynchronous responses up to a maximum rotating speed of 80 k\(\upmu\)m. As a result, GFPB is a reliable lubricating element that can be used for vibration dampening in machines operating at relatively low temperatures. g 2022 2021 25 January 2022 30 January 2022 30 January 2022 30 January 2022 30 January 2022 30 January 2022 30 January 2022 30 January 2022 30 January 2022  
light weight, and high-temperature operation; they are mainly applied to high-speed and light rotating machines: small turbines, motors, and generators. On the other hand, GFBs become unstable and too sensitive usually in high speeds due to the subsynchronous instability, i.e., the oil whip and oil whirl phenomena. The oil whirl phenomenon corresponding to the 0.5\(\times\) component of rotating speed and the oil whip phenomenon fixed to the natural frequency of the system are the major instabilities experienced by rotors supported by gas foil bearings [6]. One way to inhibit the instability in rotors is to apply mechanical preload to the GFB. [PERSON] et al. [7] conducted an experimental study on the effect of mechanical preload on rotordynamic performance of GFB. Mechanical preload was applied by inserting three metal shims under the bump foil of the GFB at 120\({}^{\circ}\) in circumference. They demonstrated that the mechanical preload improves bearing load-carrying capacity and reduces the subsynchronous instability. Another way to inhibit the instability effectively is the insertion of high-damping materials into the GFB. For example, there is a gas foil-polymer bearing (GFPB) proposed by [PERSON] and [PERSON] [9]. The GFPB is a bearing with an additional polymer layer inserted into the existing GFB structure, which has a thickness of several millimeters. Figure 1 shows the structures of the GFPB and GFB. The polymer layer was inserted between the bottom foil and bearing housing. Technically, it provides additional hysteresis damping of rotor vibrations, which serves to reduce sudden impact or large amplitude vibrations of the rotor. [PERSON] and [PERSON] [9] determined that the GFPB structure has 200% to 400% higher damping properties compared to the GFB structure through structural excitation experiments. Furthermore, rotordynamic experiments with a single rotor system were conducted by applying the GFPB and GFB. They found that the GFPB could delay the occurrence of instability at the same bearing clearance by 35%.