Study On Elastohydrodynamic Lubrication Of Point Contact Under Micro Oil Droplet Supply

Oil-air lubrication is an environment-friendly lubrication technology with metered oil supply,where lubricant is transported to the lubricating point in the form of micro oil droplets.This paper mainly focuses on the problem of elastohydrodynamic lubrication?EHL?with oil supply by oil droplets.Theoretical and experimental studies have been carried out on the EHL performance under micro oil droplet supply.This paper started with theoretiacal modeling and computation.An optimized numerical scheme,which provides fast convergence and good numerical stability,has been successfully built not only to simulate the ball-on-plane lubricated contact with very thin oil supply layer?10 nm,pesudo-dry contact state?,but also to deal with the local transient EHL films formed when micro oil droplets pass through the contact zone.An EHL model with oil supply by a single droplet was established,and the whole process of one oil droplet passing through the contact zone with entrainment motion has been numerically simulated.When the oil droplet on the plane is getting into contact with the ball surface lubricant flow across the entrainment direction in the inlet zone develops around the two side gaps,which is termed as“squeeze spread”in this paper.And the spreading distance will directly affect the EHL state of the contact zone.The effects on oil droplet EHL of entrainment speeds,oil viscosity,initial radii of the oil droplet and the surface roughness were discussed.The results showed that excessive entrainment speeds or lubricant viscosity would lead to insufficient squeeze spread of an oil droplet at the EHL inlet,and the oil droplet directly passes through the contact zone without enough deformation,resulting in a dimpe-shaped partial EHL film and thus lubrication failure occurs.In order to obtain efficient EHL,the initial radius of the oil droplet with the same volume must be in a certain range.Calculation also showed that the surface roughness does not change the basic characteristics of oil-droplet EHL,but the texture direction will affect the squeeze spread distance of the oil droplet.As a further step an EHL model with multiple oil droplets was established,using single-row?Type??or double-row?Type??distributions.Continuous EHL films are formed in the contact zone.The numerical results demonstrated that the film thickness shows periodical change.The effects of oil droplet number,spacing distance,entrainment speeds,lubricant viscosity and contact angles were obtained,and the two types of oil droplet distribution were compared.It was found that,in order to achieve the best EHL performance,there is a tradeoff between the number and the size of oil droplet for a constant volume of lubricant,and the optimal number of oil droplet,which can give the longest effective lubrication time,exists for both two distribution types.The spacing distance between discrete oil droplets will affect the continuity of lubricating oil films.Once the spacing distance exceeds its critical value,which is affected by entrainment speeds,lubricant viscosity and contact angles,lubrication failure will occurs.Considering relative sliding between the two surfaces of the ball and the plate,an oil supply equation based on linear sliding deformation of an oil droplet has been proposed.The effects of slide-roll ratios,oil droplet heights,lubrcantion parameter?₀u_e,loads and droplet volumes were investigated.The results showed that the sliding elongates the oil droplet,and then increases the size of the oil pool.Eventually the effective lubrication can be enhanced.However,if the slide-roll ratio is too large lubrication failure may occur.The increase of oil droplet height can further increase the elongation distance of oil droplet,but it also reduce the squeeze spread distance.The effect of the slide-roll ratio is markedly influenced by lubrication parameters?₀u_e,and the increase of?₀u_e leads to a decrease in the allowable range of slide-roll ratio,and the effect of load will be more obvious when the slide-roll ratio is larger.Finally,experiments have also been carried out to investigate EHL by micro oil droplets,using a custom-made optical test rig on the basis of dichromatic interferometry for the measurenment of lubricating film.The effects of slide-roll ratios,and speeds on the EHL performance of micro oil droplet were investigated.The experimental results were in close correlation with the numerical results.