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Gear train systems used in the lunar environment should be able to endure limiting conditions such as ultra-high vacuum with minimal energy dissipation. From this point of view, we have obtained the optimum thickness of a silver coating that can influence friction mechanisms of the gear and pinion. To clarify the sliding friction between the gear and pinion, a pin-on-disk tribometer that could be implemented in space tribological applications was constructed according to the specifications of ASTM G99-95a using a solid lubricant at a vacuum pressure of approximately 10-7 Torr. The friction characteristics were compared for atmospheric and vacuum environments. Through these results, an improved understanding of the friction mechanisms between the gears coated with silver is obtained. Moreover, an analysis of energy dissipation due to differences in the gear backlash was conducted.

The exact understanding of the dynamic characteristic of the seal is a crucial parameter for designing the system. This article presents an experiment that estimates the dynamic characteristics of a brush seal under a super-heated steam environment for a steam turbine. The super-heated steam facility makes super-heated steam (523.15 K) to replicate the real steam turbine environment. Two brush seal units are utilized with a housing connected by springs to measure the modal parameters of the system. To extract the brush seal characteristics, the modal parameters of the pure housing were subtracted from the overall dynamic system. Moreover, to predict modal parameters more accurately, the least-squares method and the instrumental variable method were used to reduce the noise caused by the steam. Two major effects were experimentally investigated by varying the operating speed (0–16,900 r/min) of the rotor and the injection flow of super-heated steam. The results showed that the direct stiffness and damping of the brush seal increased significantly when the steam was injected. Under steam exposure, obtaining the modal parameters using instrumental variable method was confirmed to reduce more noise than obtaining the parameters via least-squares method.

Type 316 L stainless specimens were creep rupture tested at 620°C in a vacuum. The FeO type wüstite phase with the lattice parameter of about 4.056 Å was identified on the creep cavity wall. The {200} planes of the oxide phase have shown the epitaxial growth on the {111} planes of the matrix phase. As the oxide film forms on the creep cavity wall, the surface energy term of the cavitation could decrease, making the critical nucleation radius of the creep cavity smaller and the growth rate faster. [ABSTRACT FROM AUTHOR]

This study aims to verify the effectiveness of the 250 HP class compressor system to which the integrated hybrid air-foil magnetic thrust bearing (i-HFMTB) proposed in (Ha et al., 2023, "Integrated Hybrid Air Foil-Magnetic Thrust Bearing (i-HFMTB) Part I: Preliminary Experimental Test for Rotordynamic Parameter Identification and Behavior With PD Control," ASME Paper No. GT2023-102860) [1] is applied, and the vibration and instability problems of air-foil journal bearing (AFJB) occurring in the rigid mode are controlled by i-HFMTB. The compressor rotor is supported by two AFJB (journal diameter = 60 mm) and an i-HFMTB, and the length and mass of the rotor are 550 mm and 15.24 kg, respectively. i-HFMTB has a structure in which air-foil thrust bearing (AFTB) pads are inserted into eight slotted active magnetic thrust bearings (AMTB), and PD control (proportional gain: 4000; differential gain: 10) is applied. The operating area was identified through AFJB's dynamic coefficients and rotordynamic analysis. As a result, in the experiment with the 250 HP compressor system supported only by two AFJB, sub- and super-synchronous vibrations were generated owing to the AFJB's insufficient load capacity and damping in the rigid mode (7000 rpm) region, and this instability did not disappear even upon increasing the speed to 15,000 rpm. However, when i-HFMTB was turned on in rigid mode, it was confirmed that the sub- and super-synchronous vibrations were significantly reduced. The thrust collar tilt angle was calculated through orbit trajectory analysis of the impeller and cooling side, and it was confirmed that the tilt angle of the thrust collar was reduced during i-HFMTB operation. [ABSTRACT FROM AUTHOR]

This study presents the dynamic motion of a ball bearing cage submerged in a cryogenic fluid under high-speed conditions. The dynamic motion of the cage has been studied as a function of the race land-cage and ball-cage pocket clearances for different inner race rotation speeds under light load conditions. In addition, this study conducted computational fluid dynamics (CFD) analysis using commercial software to analyze the fluid dynamic forces on the cage. The hydraulic force obtained from the CFD analysis was coded in commercial ball bearing analysis software as a function of the eccentricity ratio and rotation speed of the cage. Finally, the dynamic motion of the ball bearing cage considering the effects of fluid dynamic forces has been studied. The results include the cage whirling amplitude, fluctuation of cage whirling speed, and cage wear for various cage clearances and rotation speeds. The cage whirling amplitude decreases as the outer guidance clearance decreases, and it decreases as the rotation speed increases up to 11,000 rpm because of the increasing hydrodynamic force of the liquid nitrogen (LN2). However, the probability density function curves indicate that an increase in the rotor speed increases the standard deviation in the cage whirling frequency. The wear loss of the cage was greatest for the largest race land-cage and the smallest ball-cage pocket clearances. Consequently, the analysis results for various operating conditions (inner race rotation speeds, cage clearances, traction coefficients, etc.) are in good agreement with the reference results. [ABSTRACT FROM AUTHOR]