Numerical and experimental study on the static performance of externally pressurized thrust bearings lubricated with refrigerant gases

Purpose Oil-free heat pumps that use the system refrigerant gases as lubricants are preferred for thermal management in future space applications. This study aims to numerically and experimentally investigate the static performance of externally pressurized thrust bearings lubricated with refrigerant gases. Design/methodology/approach The refrigerant gases R22, R410A and CO2 were chosen as the research objects, while N2 was used for comparison. Computational fluid dynamics was used to solve the full 3 D Navier–Stokes equations to determine the load capacity, static stiffness and static pressure distribution in the bearing film. The numerical results were experimentally verified. Findings The results showed that the refrigerant-gas-lubricated thrust bearings had a lower load capacity than the N2-lubricated bearings, but they presented a higher static stiffness when the bearing clearance was less than 9 µm. Compared with the N2-lubricated bearings, the optimal static stiffness of the R22- and CO2-lubricated bearings increased by more than 46% and more than 21%, respectively. The numerical and experimental results indicate that a small bearing clearance would be preferable when designing externally pressurized gas thrust bearings lubricated with the working medium of heat pump systems for space applications. Originality/value The findings of this study can serve as a basis for the further investigation of refrigerant gases as lubricants in heat pump systems, as well as for the future design of such gas bearings in heat pump systems for space applications.

Planetary Gearbox Dynamic Modeling Considering Bearing Clearance and Sun Gear Tooth Crack

Planetary gearbox systems are critical mechanical components in heavy machinery such as wind turbines. They may suffer from various failure modes, due to the harsh working environment. Dynamic modeling is a useful method to support early fault detection for enhancing reliability and reducing maintenance costs. However, reported studies have not considered the sun gear tooth crack and bearing clearance simultaneously to analyze their combined effect on vibration characteristics of planetary gearboxes. In this paper, a dynamic model is developed for planetary gearboxes considering the clearance of planet gear, sun gear, and carrier bearings, as well as sun gear tooth crack levels. Bearing forces are calculated considering bearing clearance, and the dynamic model equations are updated accordingly. The results reveal that the combination of bearing clearances can affect the vibration response with sun gear tooth crack by increasing the kurtosis. It is found that the effect of planet gear bearing clearance is very small, while the sun gear and carrier bearing clearance has clear impact on the vibration responses. These findings suggest that the incorporation of bearing clearance is important for planetary gearbox dynamic modeling.

Analysis and Experimental Validation of the Tribological and Dynamic Characteristics of Journal Bearings

Sliding bearings are the main kinematic pair of a Stirling engine, and one of the main sources of friction power loss and excitation force. There is a coupling effect between the lubrication state and the excitation force. In this paper, the dynamic model of a Sapphire bearing fatigue test bed, which includes thermo-elasto-hydrodynamic (EHD) property, is established and the accuracy of the model is verified by the experimental data. Based on the model, the effects of the load, the oil supply temperature, the oil supply pressure, the bearing clearance on the tribological and dynamic characteristics are studied. With an increase in the load, the orbital path of the journal moves downward, the lubrication state changes from the full hydrodynamic lubrication to the boundary lubrication, the friction power loss increases, and the bearing excitation frequency increases. Due to the decrease of the oil viscosity, the hydraulic friction power loss decreases, but the friction power loss derived from the asperity contact increase, so does the total friction power loss. With the increase of the radial clearance, the shape of the orbital path and the minimum oil film thickness remain unchanged. The increase of the bearing clearance results in the collisions between the journal and bearing bush, thus the amplitude of the frequency multiplication component in the bearing load increases, and the asperity and the total friction power loss increase.

Mechanism and Characteristics of Global Varying Compliance Parametric Resonances in a Ball Bearing

Varying compliance (VC) is an unavoidable form of parametric excitation in rolling bearings and can affect the stability and safety of the bearing and its supporting rotor system. To date, we have investigated VC primary resonance in ball bearings, and in this paper other parametric VC resonance types are addressed. For a classical ball bearing model with Hertzian contact and clearance nonlinearities between the rolling elements and raceway, the harmonic balance and alternating frequency/time domain (HB–AFT) method and Floquet theory are adopted to analyze the VC parametric resonances and their stabilities. It is found that the 1/2-order subharmonic resonances, 2-order superharmonic resonances, and various VC combination resonances, such as the 1-order and 2-order summed types, can be excited, thus resulting in period-1, period-2, period-4, period-8, period-35, quasi-period, and even chaotic VC motions in the system. Furthermore, the bifurcation and hysteresis characteristics of complex VC resonant responses are discussed, in which cyclic fold, period doubling, and the second Hopf bifurcation can occur. Finally, the global involution of VC resonances around bearing clearance-free operations (i.e., adjusting the bearing clearance to zero or one with low interference) are provided. The overall results extend the investigation of VC parametric resonance cases in rolling bearings.

Optimum Design of Journal Bearings Dimensions for Rotating Machines

The values of Many parameters which involve in the design of fluid film journal bearings mainly depend on the bearing applied load when using the conventional design method to design the journal bearings, in this study, as well as applied bearing load, the dynamic response and critical speed have been used to calculate the dimensions of journal bearings. In the field of rotating machine, especially a heavy-duty rotating machines, the critical speed and response are the main parameters that specify bearing dimensions. The bearing aspect ratio (bearing length to bore diameter) and bearing clearance have been determined based on rotor maximum critical speed and minimum response displacement. The analytical solution of rotor Eq. of motion was verified by numerical solution via using ANSYS Mechanical APDL 18.0 and by comparing the numerical solution with the preceding study. The final study results clearly showed that the bearing aspect ratio has little effect on the critical speed, but it has a high effect on the dynamic response also the bearing clearance has little effect on the critical speed and considerable effect on the dynamic response. The study showed that the more accurate values of bearing aspect ratio to make the response of rotor as low as possible are about 0.65 - 1 and bearing percent clearance is about 0.15 - 0.2 for different rotor dimensions.

The Thermo-Hydrodynamic Analysis of the Plain Journal Bearing With Centered Circumferential Groove: Numerical Simulation vs. Experiment

In order to increase the reliability of the oil lubricated journal bearing, the thermal characteristics of the plain journal bearing are investigated and optimized. In this paper, the steady state thermo-hydrodynamic analysis of the plain journal bearing with centered circumferential groove is studied by numerical simulation and experiments. The diameter of the journal bearing is 190mm, and the load of the bearing is 6150N, which is operated with the rotating speed of 5560 r/min. The flow characteristics including the temperature distribution in the clearance of the bearing are simulated by ANSYS CFX, where the Walther viscosity temperature relation and the cavitation effect are considered. The test rig for measuring the pressure and temperature distributing in established, which is driven by electrical motor and certain bearing load can be exerted on it. In the experiments, the temperature distribution of the bearing is measured by 10 temperature sensors, which is arranged on three cross sections along the axial direction. The temperature of the lubricating oil is obtained by the sensors on the shell of the bearing, which is installed in the Babbitt metal. In order to obtain the circumferential distribution of the temperature, the sensors locate at different circumferential angles. The influence of geometrical parameters on the temperature distribution is studied, including the bearing clearance, the bearing length and the groove width by numerical simulation and experiment. The bearing clearance ratios of 1.8‰ and 2.5‰ are compared, and as the increment of the bearing clearance ratio, the temperature of the bearing decreased, which is benefit from the flow mass into the bearing increases. For the bearing length increasing from 90mm to 94mm, the maximum temperature decreases about 3K. For the groove width varying from 20mm to 19mm and 18mm, the temperature of the bearing decreases. The comparison of the numerical results and experiments are presented, and they show similar tendency for the temperature distribution, but the difference of temperature values exists. Based on the results from variation of bearing clearance ratio, bearing length and groove width, the optimized parameters of the bearing are proposed by determining the groove width with 18mm, bearing length with 94mm and the bearing clearance ratio with 2.1‰. The thermo-hydrodynamic analysis from the CFD and the experiments for the optimized bearing are carried out. The results indicate that the thermal performance of the bearing is improved. Compared with original design of the bearing, the maximum temperature is reduced by approximately 5K for optimized design of the bearing.

Suppression of Complex Hysteretic Resonances in Varying Compliance Vibration of a Ball Bearing

It is traditionally considered that, due to the Hertzian contact force-deformation relationship, the stiffness of rolling bearings has stiffening characteristics, and gradually researchers find that the supporting characteristics of the system may stiffen, soften, and even coexist from them. The resonant hysteresis affects the stability and safety of the system, and its jumping effect can make an impact on the system. However, the ball bearing contains many nonlinearities such as the Hertzian contact between the rolling elements and raceways, bearing clearance, and time-varying compliances (VC), leading great difficulties to clarify the dynamical mechanism of resonant hysteresis of the system. With the aid of the harmonic balance and alternating frequency/time domain (HB-AFT) method and Floquet theory, this paper will investigate the hysteretic characteristics of the Hertzian contact resonances of a ball bearing system under VC excitations. Moreover, the linearized dynamic bearing stiffness of the system will be presented for assessing the locations of VC resonances, and the nonlinear characteristics of bearing stiffness will also be discussed in depth. Our analysis indicates that the system possesses many types of VC resonances such as the primary, internal, superharmonic, and even combination resonances, and the evolutions of these resonances are presented. Finally, the suppression of resonances and hysteresis of the system will be proposed by adjusting the bearing clearance.

Effect of CSL on the characteristics of six-pocket hybrid irregular journal bearing

This article examines the effect of couple stress lubricant on the characteristics of six-pocket hybrid irregular journal bearing system. Various shapes of irregular journals viz. barrel, bell-mouth, and undulated journal are considered in the analysis. To model the behavior of the flow of couple stress lubricant in bearing clearance space, the modified form of Reynolds equation is derived by using Stokes theory. The unknown pressure field in Reynolds equation is determined by using Galerkin's method. To illustrate the effect of couple stress lubricant on bearing system, the results for characteristics parameters of journal bearing system are presented. The results noticeably reveal that the presence of different geometrical irregularities in journal may ominously influence the performance of bearing system. Further, the use of couple stress lubricant instead of Newtonian lubricant offers a significant improvement in the value of bearing characteristic parameters of geometrically irregular hybrid journal bearing system viz. [Formula: see text], [Formula: see text] and [Formula: see text].