Lubricants

Squeeze-Film Dampers (SFDs), with their fixed structural parameters, can easily induce detrimental nonlinear dynamic behaviors, resulting in rotor instability and fatigue damage. This paper proposes a novel partitioned oil film control method to adjust oil film stiffness and damping, thereby reducing the nonlinear vibration level of the rotor. The SFD is designed with an outer ring comprising three independently adjustable bearing pads (referred to as SSFD). The radial position of each pad is controlled by a piezoelectric actuator (PZT), enabling localized regulation of oil film clearance in different circumferential regions. This study involved developing a dynamic model of the SSFD-rotor system, which was experimentally validated. Through analysis of oil film pressure and force, we revealed the controllable mechanical properties of the SSFD. The nonlinear dynamics were thoroughly investigated. The proposed partitioned control method proved superior to the SFD, demonstrating a significant reduction in system nonlinear vibrations.

Reliable identification of drill-string–wellbore friction is important for evaluating mechanical load transfer and contact behavior during drilling. However, friction estimation from surface hook-load measurements can be affected by non-frictional contributions from the hoisting system and drilling-fluid-induced forces. This study develops a multi-physics hook-load correction framework incorporating sheave efficiency, traveling-block and hook weight, mud-hose catenary force, drilling-line weight, annular and internal viscous forces, swab/surge effects, and pump-on hydraulic lift. Field data from a 3500 m directional well were analyzed using a yield-power-law (YPL)/Herschel–Bulkley fluid model. For representative slide- and rotary-drilling intervals, measured average hook loads of 442.21 and 481.01 kN were corrected to 294.92 and 332.89 kN, corresponding to correction ratios of 33.31% and 30.79%. Corrected off-bottom hook loads were then used to identify state-dependent effective drill-string–wellbore friction factors of 0.180, 0.154, and 0.104 at three calibration states. These factors were transferred to subsequent drilling intervals for surface-torque reconstruction. After one-point baseline alignment, the remaining torque samples yielded mean relative reconstruction errors of 2.28%, 4.22%, and 3.21%, respectively. Removing these non-frictional contributions allows the effective drill-string–wellbore friction factor to be identified from a corrected surface load and subsequently compared with the measured torque response.

Micro flapping-wing air vehicles are increasingly used in low-altitude and aerospace applications, creating a growing need for accurate motion and stable dynamic performance. This trend motivates a detailed investigation of the dynamic behavior of their actuation mechanisms. Manufacturing and assembly tolerances inevitably introduce small clearances into the kinematic joints, which reduce the motion accuracy of the mechanism, induce high-frequency vibration, and accelerate interfacial wear. Most existing studies on the dynamic behavior of mechanisms with clearances focus primarily on conventional planar mechanisms or single-clearance scenarios. However, the nonlinear dynamics, wear behavior, and chaotic responses of high-frequency flapping-wing mechanisms with multiple clearance joints remain insufficiently understood. Consequently, this paper investigates the dynamic responses and kinematic joint wear characteristics of a high-frequency planar flapping-wing actuating mechanism featuring multiple revolute-joint clearances. First, a clearance model for multiple planar revolute joints is established. Second, the Lankarani–Nikravesh normal contact force model and a modified Coulomb friction model are integrated to characterize nonlinear impact and frictional behaviors at the clearances. Subsequently, we construct a dynamic wear model for the revolute-joint clearances based on Archard’s law. Finally, a multibody dynamic model of the high-frequency flapping-wing mechanism considering multiple revolute-joint clearances is developed. The nonlinear dynamic responses of the mechanism are analyzed, and the effects of clearance size and driving speed on the flapping trajectory, velocity fluctuations, and joint wear are quantified. The results provide a theoretical basis for tolerance design, dynamic optimization, and service-life prediction of high-frequency mechanisms such as those used in micro flapping-wing air vehicles.