Experimental Study On Impact-sliding Composite Wear Under Grease Lubrication Conditions

In industrial chain drives,the sleeve slides on the pin and impact loading occurs due to the polygon effect,while the collision between the ball and cage usually produces an impact-sliding motion in the rolling element bearings.Aiming at investigating the occurrence of surface damage caused by the impactsliding motion,a ball–disk test rig using optical interference technology was designed to realize load variation.The shape and thickness of the grease film under grease lubrication conditions were observed by light interference experiments.It should be noted that due to the existence of processing errors,there will be a point contact area between the sleeve and the pin shaft,and this paper mainly studies the point contact.The main research contents are as follows:Impact tests are carried out to study the grease film evolution on a point contact optical interferometric test rig using a self-made lithium grease.The initial distance between the steel ball and the glass disk is zero and the load repeatedly varies in the form of a triangle wave.The results indicate that during the first impact period,there exists big lump of thickener fibers in the contact,which results in an entrapment of the grease film.With the increase of the impact periods,the big lump thickener disappears,the film thickness is gradually reduced and lubrication state turns to be thin film lubrication.At last,surface damages occur in the surfaces of both the steel ball and the glass disk.In each impact period,either central or minimum film thickness mainly takes a constant value.The central film thickness decreases with the increase of the impact periods.The minimum film thickness varies only slightly in the first 100 periods and gradually decreases to zero.On the basis of impact motion,the sliding working conditions are taken into account to further explore the influence of kinematic parameters on impactsliding composite wear.Two kinds of commercial grease,Klüber Centoplex 3 and Centoplex 2EP,were used in the experiments when the glass disk slides at a constant speed while the steel ball impacts on them.The sliding and impact motions were controlled by PLC programming.After the experiments,the midsection grease film distributions were measured using DIIM software.The results show that surface damage can rapidly occur even in the first working cycle,and that the phenomenon is affected by the sliding speed,maximum load,and grease consistency.When the sliding speed is low,multiple contacts of asperity peaks occur in the interior contact region and develop into adhesive wear.When the sliding speed increases,surface wear starts to occur at the side-lobe position of the elastohydrodynamic lubrication(EHL)horseshoe shape and extends with time accompanied by obvious surface scratches.The wear mechanism investigated provides valuable visible information for the further exploration of impact-sliding composite wear.It is suggested that great attention should be paid to impact-sliding wear occurring with grease lubrication since such working conditions are very common in industrial applications.Since some components and parameters of commercial greases are not known,in order to further study the factors that affect the impact-sliding motion under grease lubrication conditions,the author used four self-made lithium-based greases to conduct impact-sliding experiments,combined with field emission scanning electron microscope and dual-light interference technology,visually explored the influence of base oil viscosity,thickener content and the morphology of thickener fiber clusters on the lubricating effect.It was found that higher base oil viscosity is beneficial for elastohydrodynamic lubrication.Among the components of grease,the factor that has the greatest influence on the thickness of the grease film is the viscosity of the base oil.Under the same conditions of other parameters,the higher the viscosity of the base oil,the thicker the central film thickness of the grease.The content of thickener in grease needs to be controlled within a reasonable range.In the experiment,the lubricating performance of 8% soap content grease is better than that of 16%.The optimal range of content needs further study.The volume and density of the thickener’s fibrous structure affects the flow and load carrying capacity of the grease.The larger the volume of the thickener fiber,the worse the fluidity and recoverability;the larger the distribution density of the fiber structure,the stronger the bearing capacity of the grease.In order to achieve the optimal anti-friction performance,one cannot only pursue fluidity or load-bearing capacity,and the volume and density of thickener fibers must be controlled within an appropriate range.