Study On The Oil Film Lubrication And Enhancement Of Lubricant Self-Replenishment Of Surfaces Under Limited Lubricant Supply

Lubrication with limited lubricant supply indicates the use of as little lubricant as possible to achieve effective lubrication of components.It not only reduces environmental pollution and waste of resources but also meets the needs of low energy consumption and low temperature rise in the design of precision mechanical components.To achieve limited lubricant supply lubrication requires efficient and accurate oil supply to the lubrication points.Concentrated on lubrication with limited lubricant supply,the present thesis presents new characteristics of the elastohydrodynamic lubrication(EHL)film under the conditions of oil droplet supply.Theoretical and experimental studies have been carried out to reveal load-carrying mechanism and friction in flat-on-flat contact.Based on the unidirectional flow of liquid on the gradient functional surface,surfaces with enhanced lubricant replenishment were designed and fabricated,and their improved lubricating performance in terms of anti-wear and anti-friction has been validated.Meanwhile,this surface with enhanced lubricant replenishment has been tried to be implemented in real rolling bearings.The work completed includes,(1)A measuring system for multi-point-contact EHL film thickness has been developed,which provides a tool for exploring the lubricating film formation in real rolling bearings.And it has been used to study the lubrication films in multi-point contact under limited lubricant supply.(2)The characteristics of EHL films by single oil droplets or discrete multiple oil droplets were studied by a ball-on-disc EHL film test rig.For a single oil droplet supply,a local dimple-shaped film is formed when there is not enough squeeze and spread of the droplet at the inlet.The dimple-shaped film is more easily to be formed when oil droplets are applied of larger size,higher viscosity,heavier loads and higher speed.For discrete multi-droplet lubricant supply if the distance between the oil drops is too large,the film thickness within the contact is discontinuous with significant thickness fluctuations.Moreover,merging of the droplets and therefore film formation are deteriorated when applying small droplet size,high viscosity,large entrainment speed and weak interface affinities.With the same oil quantity,lubricating film formation by ordered oil droplets is better than that by disordered oil droplets,and the oil film demonstrates better uniformity.(3)Lubrication with limited lubricant supply in flat-on-flat contact was studied by using a slider-bearing lubricating film test rig.The results show that the film thickness increased with increase in the oil supply quantity.When the supply quantity was beyond a certain level,the film thickness did not increase anymore and attained to a steady value which was equivalent to that under fully flooded lubrication.However,the frictional force was not much affected by the oil quantity.The interferograms show that the starvation area at the inlet of the contact was in an oil/air two-phase state.By numerical simulations for the lubricating film thickness and friction,load-carrying capacity of the starvation area was confirmed.The enhanced lubricating film generation was modeled using micro-circular dimples on the sliding plane in flat-on-flat contact,and it was demonstrated that the circular dimples could inhibit the development of the starvation area.Numerical simulations are well correlated to the experimental results.(4)Oleophobic thin layers(1H,1H,2H,2H-perfluorodecyltriethoxysilane)and comb-tooth-shaped groove textures,prepared at the two sides of the lubrication track,present two types of surfaces of enhanced lubricant replenishment.Optical measurements showed that the prepared surfaces could prevent lubricant to escape from the lubrication track.The tests in point contact showed that the prepared surfaces with enhanced lubricant replenishment can significantly reduce friction and wear under conditions of mixed lubrication and full film lubrication,while ball-on-disc optical EHL experiments showed that the enhanced lubricant replenishment surfaces can generate larger oil pools around the contact and boosts lubrication film formation.The enhanced oil replenishment by the oleophobic thin layer comes from the surface tension gradient at the two sides of the lubrication track,and that by the comb-tooth groove texture comes from the unbalanced capillary force in the grooves(Lucas-Washburn effect).(5)Practical significance of the thesis was demonstrated with real bearing tests.A virtual rib bearing(VRB)that was featured by the gradient functional surface micro-patterns for enhancing lubricant replenishment was fabricated.Compared to original bearings,the friction torque and temperature of VRBs were significantly reduced in operation.The virtual rib can effectively resist the lubricant escape from the lubrication track induced by the centrifugal force and prevent the loss of oils and greases.Results of film thickness measurement of thrust ball bearings present that the film-formation capability was augmented owing to the relatively large oil pools formed by the virtual rib and the effective replenishment of oils.