Porous
polyimide (PI) materials are one of important bearing retainer
materials in space applications due to the storage and continuous
supply of a lubricant through the porous structure. Understanding
the lubricant recycling process in porous polyimide retainers is of
vital importance to improve lubricant supply performance of bearing.
In this work, through molecular dynamic simulations, coarse-grained
models are built to study lubricant recycling processes on porous
and solid surfaces. A spontaneous imbibition behavior is observed
when the lubricant is present on the porous surface. The dynamic change
in the contact angle in this process and the deviation of the effective
radius from the volumetric radius because of the molecular structure
of polyimide causes the classical Lucas–Washburn (L–W)
equation fail to describe the process. By fitting dynamic contact
angle and effective radius, a modified L–W equation is developed,
which well predicts the process of imbibition. Furthermore, it is
found that the lubricants between the porous polyimide surface and
the solid surface are recycled by extrusion, and spontaneous imbibition
does not occur. In this case, the accumulation of lubricant pressure
and weak interfacial interaction between the lubricant and the solid
surface are also the main factors that promote lubricant recycling.
The retainer of a space rolling bearing widely made of porous polyimide (PI) materials is oil-impregnated and can continuously release lubricants for lubrication. Understanding the lubricant supply mechanism in porous polyimide retainers is important to improve the lubrication performance of space bearings and therefore extends the bearing life. In this work, molecular dynamics simulations are adopted to model the lubricant outflow process from the pore of the PI material. Coarse-grained models are constructed to investigate the lubricant migration behaviors with different pore sizes and radii of rotation. At rest, a lubricant within the pore fails to outflow due to the capillary effect, which decreases with the increase of the pore size. However, for the rotating pores, if the inertial forces generated by the rotational motion exceed the capillary forces, the lubricants will begin to accumulate and some of the lubricants will flow out. Furthermore, the lubricant in the larger pore is easier to outflow due to the smaller capillary forces. This study quantifies the inertial effect and reveals that the centrifugal force is the main mechanism of lubricant outflow from the pores.
Oil-impregnated porous polyimide (PI) materials can provide continuous lubricant supply, which is widely used to manufacture space rolling bearing retainers. The lubrication performance of porous polyimide materials mainly depends on their ability to release and recycle lubricants, which is closely related to pore size. In this paper, to investigate the effect of pore size, porous polyimide materials with different pore sizes were prepared by preheating the retainer tube billet during the limit pressing process. The lubricant content rates at each stage were measured by the lubricant immersion and centrifugal release experiment to show the variation of the lubricant content rate in the porous PI sample during a working cycle. At first, the lubricant can be absorbed into the pore. It is found that the absorption rate is faster for lubricants with a smaller viscosity. Moreover, lubricant thinning caused by temperature rise also improves the absorption rate. After lubricant absorption to saturation, the lubricant is released under the centrifugal effect to provide the lubricant. Increasing pore size and using low-viscosity lubricants are the main ways to improve lubrication. Finally, the lubricant on the surface can be recycled into the pore by capillary effect. The smaller the pore size, the faster the lubricant recycles to saturation. These insights gained in this study can provide guidance for the choice of an oil-impregnated porous retainer in different working conditions.
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