Research On The Behaviour Of Grease Migration And Lubrication Mechanism Under Starved Condition

Grease has been widely used in bearings working in a large variety of mechanical devices.In many applications,grease lubricants provide better performance than lubricating oils.For example,it can adapt to high pressure,ultra high temperature,low speed and radiation.Basically,the re-lubrication period for grease lubricated bearings are very long which make grease lubrication saves energy and costs.Most grease-lubricated bearings work in starvation phase.The mechanism of film formation under such condition is still not controversial.The lack of knowledge on the properties of lubricating film and the behaviours of lubricant migration makes it even harder to calculate film thickness and to estimate bearing life.In this work,a series of grease samples were designed,and the characteristics of lubricant migration and film thickness formation was studied.The replenishment mechanism and its determinants were studied in detail.A real bearing torque device was then developed to test the theories derived from this work.First of all,the behaviour of film thickness versus disc revolutions under starved condition was studied based on the optical interference nanometer lubricating film measuring technology.Central film thickness showed three phases namely the fluctuation phase,the decay phase and the recovery phase.The finger-shaped grease remnant along the rolling track was the only lubricant supply under starved condition and it would bleed oil to form a “second-phase grease reservoir” to replenish the contact during operation.However,the recovery phase always appeared later than the “second-phase grease reservoir”.By using atomic force microscope(AFM),the presence of nano thickener deposited film in the rolling track was detected.Results of dynamic contact angle showed that such deposited film could decrease the wettability of rolling track,which prevent base oil from reflowing back to the contact.This was the primary cause of the hysteresis of film thickness recovery.However,the deposited film could be worn during operation,which enhanced base oil reflowing and led to film thickness recovery.The adhesive ability of thickener greatly influenced its deposited film and replenishment ability.In addition,the in fluence of thickener type,base oil viscosity and polarity,load and the amount of grease supply on the mechanism of film thickness formation and replenishment was studied.Results showed that with the increasing of base oil viscosity and polarity,the oil bleeding ratio decreased.Oil bleeding ratio,load and grease supply significantly influenced grease replenishment ability.At high temperature at 120 oC,base oil and thickener would age successively.Base oil oxidation in the early stage would cause higher friction and more severe surface damage.However,the microstructure of thickener would be destroyed which led to an enhancement of refplenishment ability caused by thickener oxidation.Finally,traction torque of different grease samples was measured on the home-made thrust bearing torque test device,and the in situ observation of grease distribution inside the worked bearing was made.Results basically supported the theory derived from the above tests.The observation of grease fingers along the rolling track provided a new evidence to evaluate bearing failures.