Experimental Study On The Performance Of Lubricating Films Using A Thrust Bearing Test Device

A ball-on-disc single contact model test device is widely used to evaluate the lubrication properties of rolling element bearings fundamentally.However,the model test device is so simple that most bearing features are ignored,e.g.,the cage,contact geometry,etc.This weakens the correlations between the model test results and the fullbearing performance.Alternately,a thrust-bearing structured test device contains all the full-bearing features,and a transparent disc could directly replace the upper race,providing an effective method that links the model test and bench test or full-bearing performances.The former optical thrust bearing device has limitations in the speed range,loading range,and coefficient of friction(COF)measurement.Therefore,this paper aims to develop a new thrust-bearing device to realize the lubricating film and COF measurement.Using the new device,studies on the lubrication properties of thrust bearing are experimentally conducted.The film thickness and COF are two main parameters to characterize the lubrication properties of the contact pairs.The new device can measure the film thickness and COF with two test modes respectively under different conditions.The main studies include:(1)An optical thrust-bearing device to measure film thickness and COF is established,which overcomes the limitations of the test condition and COF measurement of the former device.At low speeds,film thickness and COF can be measured simultaneously.At high speeds,to measure film thickness,a relative motion mode is used by fixing the cage and letting the disc free.To measure the COF,the disc has to be fixed and makes the cage motion.(2)Reliability test of the new device.Comparisons of test results between multicontact and traditional single-contact are conducted with an analysis of the test errors.The results from the two methods agree well.The trends of COF curves are similar to the Stribeck curves.The test results confirmed the measurement reliability of the new device.(3)Influence of replenishment intervals on the lubrications under a fixed amount of lubricant supply.Different replenishment intervals are obtained by regulating the arrangement of rolling elements in a cage.The influence of replenishment interval,lubricant amount,and viscosity on lubrications are observed.The results show that under a given lubricant amount,the number of rolling elements and arrangements will change the replenishment intervals and hence contribute to the lubrication states.The increase in the replenishment interval will improve the inlet lubricant supply and lubrication states.Under the same replenishment interval,an increase in lubricant amount will contribute to the film formation,but an excessive amount will result in the COF rise.That means there exists an optimal lubricant supply amount.A higher viscosity will help to get a higher film thickness,but it is also apt to cause starvation at higher speeds.The results are consistent with the real bearing performance.(4)Influence of contact geometry on lubrications.The contact geometry is induced by replacing the real upper race with a glass disc.The raceway is manufactured on a glass disc with the same dimensions as the real bearings.The results show that under fully flooded conditions the film thickness of the grooved disc is a little higher than that of the flat disc.But under a fixed amount of lubricant supply,the film thickness of the grooved disc is obviously thicker than that of the flat disc,and the differences are enlarged at higher speeds.The friction torque decreases with speed rise followed by an increase.The fiction torque of the grooved disc is much higher than that of the flat disc at high speeds.(5)Influence of lubricant structure on the lubricants.Two types of lubricant mineral oil and synthetic oil with the same ambient viscosity are employed.The results show that the lubrication behaviors are quite different depending on the lubricant types.Under the same test conditions,the synthetic oil contributes to the film formation,mainly attributing to the higher viscosity-pressure index.The difference in the polarity of the lubricant will cause the different adhesion abilities of the lubricant to the solid surface,causing different slip degree and friction torques.