Molecular Dynamics Simulations of Lubricant Outflow in Porous Polyimide Retainers of Bearings

Abstract

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.

Design and Pore Properties Tuning of Two‐Scale Porous Polyimide Fabricated by Isostatic Pressing for Improving Oil Storage and Tribological Properties

“…[ 16 ] The higher the speed, the more lubricant would be thrown out at the same time, which is in good accordance with the simulation results. [ 21 ]…”

“…[ 8,19 ] Theoretical simulations indicate that the accumulation of lubricant pressure and weak interfacial interaction between the lubricant oil and the solid surface are also the main factors that promote lubricant recycling, and centrifugal force is the main mechanism of lubricant outflow from the pores. [ 20,21 ] Therefore, PPI with a single‐scale pore structure can only meet the continuous oil supply, but oil storage requires a certain speed within a narrow speed range. In other words, a larger pore size is suitable for a lower speed, while a smaller pore size is suitable for a relatively higher speed.…”

“…the pores. [20,21] Therefore, PPI with a single-scale pore structure can only meet the continuous oil supply, but oil storage requires a certain speed within a narrow speed range. In other words, a larger pore size is suitable for a lower speed, while a smaller pore size is suitable for a relatively higher speed.…”

Study on Lubricant Release-Recycle Performance of Porous Polyimide Retainer Materials

“…On this basis, Marchetti et al , quantified the centrifugal effect and proposed that the lubricant will be transported from the pore only when the rotation speed exceeds a certain threshold. Our recent works compared the centrifugal effect and capillary effect to further confirm that only when the centrifugal effect overcomes the capillary effect, the lubricant will be released . It was also found that the lubricant will migrate to the surfaces with the increase of ambient temperature in working conditions .…”

“…Our recent works compared the centrifugal effect and capillary effect to further confirm that only when the centrifugal effect overcomes the capillary effect, the lubricant will be released. 23 It was also found that the lubricant will migrate to the surfaces with the increase of ambient temperature in working conditions. 13 Some researchers believe that the thermal effect rather than the centrifugal effect is the primary factor affecting lubricant release.…”