Analysis of Flow Models for Aerostatic Thrust Bearings with Porous Material

The numerical determination of static characteristics of bearings allows a cost-efficient and fast pre-design.In this study,two flow models for aerostatic thrust bearings with pressurized porous material are presented and analyzed.The models are based on the coupling of the Reynolds equation for lubricants(REL)and the determination of pressure drop through porous material by Darcy’s law.The simplified model is based on the assumption of a one-dimensional axial flow through porous media.The extended model considers the three-dimensional flow in the porous body.The analysis includes pressurized air from 4 to 9 bar(a)with nominal clearance of 5 to 60μm,Commercial CFD(computational fluid dynamics)software was used to verify the results.The extended model allows a more accurate prediction about the performance in the critical gap range.In total,the results show good agreement with CFD within a short computation time.

MHD Flow of a Non-Newtonian Power Law through a Conical Bearing in a Porous Medium

The problem of analytical study of the MHD effect through a porous medium of the non-isothermal flow of a non-Newtonian power law lubricant through the gap of conical bearing through a porous medium when an external magnetic field is applied. At first, the more general basic equations of motion, continuity and energy in curvilinear form in the width direction are derived. Then, as a special case, a conical bearing gap is considered. By integrating a modified form of Reynolds equation, the bearing characteristics for a non-Newtonian power law lubricant when an external magnetic field is applied through a porous medium are obtained. Numerical results were presented in each of these forms: pressure, temperature and capacity of the conical bearing. The effects of the parameters of the non-Newtonian power law, magnetic field and porous medium are shown and discussed.

Investigation into gas lubrication performance of porous gas bearing considering velocity slip boundary condition

Porous gas bearings(PGBs)have a proactive application in aerospace and turbomachinery.This study investigates the gas lubrication performance of a PGB with the condition of velocity slip boundary(VSB)owing to the high Knudsen number in the gas film.The Darcy-Forchheimer laws and modified Navier-Stokes equations were adopted to describe the gas flow in the porous layer and gas film region,respectively.An improved bearing experimental platform was established to verify the accuracy of the derived theory and the reliability of the numerical analysis.The effects of various parameters on the pressure distribution,flow cycle,load capacity,mass flow rate,and velocity profile are demonstrated and discussed.The results show that the gas can flow in both directions,from the porous layer to the gas film region,or in reverse.The load capacity of the PGB increases with an increase in speed and inlet pressure and decreases with an increase in permeability.The mass flow rate increases as the inlet pressure and permeability increase.Furthermore,the simulation results using VSB are in agreement with the experimental results,with an average error of 3.4%,which indicates that the model using VSB achieves a high accuracy.The simulation results ignoring the VSB overrate the load capacity by 16.42%and undervalue the mass flow rate by 11.29%.This study may aid in understanding the gas lubrication mechanism in PGBs and the development of novel gas lubricants.

Tribological properties of oil-impregnated polyimide in double-contact friction under micro-oil lubrication conditions

Oil-impregnated porous polyimide(iPPI)materials are usually used as retainer for bearings.In these bearings,balls and rings,balls and retainers are two different kinds of contact.In this paper,the friction and wear properties of iPPI were investigated using steel(disc)–steel(ball)–iPPI(pin)double-contact friction test rig for simulating the actual contact in bearings.The results show that compared with that of iPPI–steel single contact,the friction coefficient of iPPI–steel in double contacts is lower and decreases with the amount of additional oil.The surface of iPPI in single contact suffers more wear compared with that in double contacts.Different from single contact,the worn surfaces of iPPI in double contacts are blackened.The Raman spectra of worn surfaces of balls and discs indicate thatα-Fe_(2)O_(3) and Fe_(3)O_(4) were formed during rubbing of the double contacts.Many nanoscale iron oxide particles are found on the worn surfaces of iPPI in double contacts;on the contrary,few particles could be found on the surface in single contact.In double-contact friction,the nanoscale wear debris penetrates inside the iPPI material through the process of extruding and recycling of oil,which is the mechanism of the blackening of the iPPI worn surfaces.The studies show that the double-contact friction method is a new and effective method to study the friction in bearings,especially for those with polymer retainer.

Effects of pore size on the lubrication properties of porous polyimide retainer material

An oil-impregnated porous polyimide(PI)retainer is used in space rolling bearings to improve the lubrication performance,which depends on the release of lubricant from the pores,and therefore is closely related to the pore size.To study the effect of pore size,in this work,PI materials with different pore sizes were prepared by preheating the retainer tube billet during the limit pressing process,and then the friction tests were conducted with the ball-on-ring mode.The results show that the applied load deforms the pores,allowing the lubricant to be squeezed out from the pore;the centrifugal effect induced by rotation also makes the lubricant migrate out of the pore.Therefore,for the same pore sizes,the friction coefficients decrease with the increasing loads and rotation speeds.In addition,it was found that there exists an optimal pore size for the best lubrication properties of porous PI material.Furthermore,the optimal pore size should be larger for lubricants with high viscosity.The microscopic mechanism for lubricant outflow from pores is clarified by molecular dynamic simulations.The insights gained in this study can guide the preparation of oil-impregnated porous retainers under different working conditions.