| In modern industry,grease is widely used to lubricate mechanical systems and is one of the most common lubricating materials.The study of the film-forming mechanism of grease lubrication and its failure is of great significance to improve the reliability of mechanical systems,and is also an important way to develop high quality,long-life greases to achieve tasks energy of saving and emission reduction.Bearings,gears and chain drives and other important mechanical components are often work under the non-steady state.The lubrication behaviour of grease under nonsteady conditions and its failure law may no longer conform to traditional lubrication theory.When the stroke length of reciprocating motion decreases,the reciprocating motion becomes oscillation.In fact,the lubrication mechanism of grease has not yet reached a consensus,and the difference and link between the lubrication effect of grease under reciprocating motion and oscillation have not yet been studied.In addition,the film formation of grease lubrication in a single cycle can no longer meet the guiding role of actual long-term work.These problems seriously hinder the research and development of grease.Aiming at the bottlenecks in the application of the above technology,this thesis focuses on the film formation characteristics of greases under reciprocation-oscillation.Combining both lubrication and wear aspects in tribology,the study of lubricant film state transition is used as a prediction and the surface wear results as a final verification,aiming to enhance the link between the two fields.The specific research method is to start from the response of grease film at low speeds under pure rolling reciprocating motion,then based on oil film variation and surface damage in the process of reciprocation-oscillation transformation,the characteristics of grease lubrication in this transition was also studied,and finally considered the slide-roll ratio and free water influence,the results and important conclusions obtained are as follows:The long-term film formation law of grease under steady-state and transient-state pure rolling motion is studied.It is found that as the work-cycle increased,the reciprocating motion attenuated the accumulation of grease thickener and the maximum film thickness is formed near the stroke end.After 1000 cycles,severe starvation occurred,resulting in the central or minimum film thickness value remaining almost constant over the long time.When the stroke length is shortened,the thickener is spread,rather than disappearing from the contact zone at the 1000 th working cycle.When the maximum velocity of the reciprocating motion increases to a certain value,severe grease starvation occurs rapidly,causing surface damage.The experimental results demonstrate that the life of grease lubrication under transient conditions is much lower than under steady-state conditions.Effects of slide-to-roll ratio and varying velocity lubrication performance of grease at low speed under steady-state and reciprocating motion were considered.Under steadystate conditions,the thickener fiber agglomeration can exist for a while and the time becomes shorter with the increase of the slide-to-roll ratio above the critical speed.Below the critical speed,the thickener fiber can exist in the contact in the form of a quite thick film for a very long time under pure rolling conditions but that time is decreased with the increase of the slide-to-roll ratio.Compared with the failure of the grease,the fatigue life of the thickener should be considered when designing and developing the grease,and the suggestion that the viscosity of the grease should be paid attention to is put forward.The film formation mechanism of oil lubrication in the transition from reciprocating motion to fretting under the pure rolling and simple sliding motion is investigated,and the transition process of elastohydrodynamic lubrication film and boundary lubrication is revealed with emphasis.The tribological performance during the transition from reciprocating motion to oscillation under sliding conditions is also evaluated using an SRV wear tester.The experimental results show that the boundary lubrication area in the contact zone gradually increases as the stroke length decreases,the area of the entrapped oil film produced by the squeezing action gradually decreases,and the wear test results indicate that the surface damage increases as the stroke length decreases.The performance of two different thickener microstructures of grease during the transition from reciprocating motion to oscillation was investigated,and corresponding wear tests were carried out to explore the potential tribological performance of the surface texture + DLC coating + grease lubrication system.The experiments reveal that the shorter the stroke length,the lower the average film thickness in the contact zone during the transition,and the phenomenon of grease thickener replacing the entrapped oil film is found.It is concluded that the grease with the shorter thickener fiber structure has better lubricating effect under long-period reciprocating motion and oscillation.Under grease lubrication,the DLC coating makes the anti-wear effect better,but the anti-wear effect is poor.At 80 °C,the mixed lubrication system has the best anti-wear and anti-wear effect.Aiming at the current situation that the influence of free water in the contact zone on the formation of grease film has not been studied,the experiment has observed the influence of free water on the thickness of the grease lubricating film and the grease lubricating track.It is found that the real reason for the temporary increase of film thickness caused by free water is that the oil-water mixture connects the oil band on both sides of the oil pool.The experiment determined that the state of the mixture before entering the contact is water-in-oil.At the same time,it is found that free water enhances the fluidity of the grease under reciprocating motion,which leads to more obvious film formation characteristics of reciprocating-oscillation grease lubrication. |
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