Experimental Study On The Fluid-wall Boundary Slippage Velocity Characteristics Of Highly Stressed Friction Pair

Classical mechanics fluid lubrication theory are based on the no-slip boundary assumptions.With the rapid development of micro and nano testing technology and related fields of science and technology,boundary slippage can not be ignored at micro and nano scales.Gears,worm gears,cam,bearings,pumps,motors and traction drive system,high friction in vehicle transmission systems,usually work in high pressure and high speed.Lubrication film thickness in these higher pairs is very thin and pressure is on the order of10⁹ Pa.What's more,the shear strain rate is up to 10⁶ s^-1.So the oil film easily reaches its limit shear stress,which is prone to slippage of the border.After that,the velocity boundary condition of film is changed and the velocity of film adjacent the wall doesn't equal to the velocity of the wall.The film thickness,bearing capacity and fractional force would change as well.Thus,boundary slip has great effect on the dynamic characteristics of lubricating oil film.Unfortunately,these proposed boundary slippages can not be used at present and the lack of testing methods obstructs the research progress in the field of boundary slippage of EHL films.Aiming at boundary slippage characteristics of highly stressed friction pair,the existence of boundary slippage is verified using experimental method.Then quantitative analysis is achieved by using slip length.Lastly,factors,like shear strain rate,is analysised,which affect the slip length.This paper presents a experimental method to measure the fluid velocity near the wall based on optical interferometry.Then designed and built the bench of EHL film thickness.On the base of Lubricating oil film thickness distribution using image analysis software,used H-D empirical formula thrust reversers to obtain the actual lubricant entrainment speed,then get the fluid velocity near the wall.The experiment at the pure rolling condition verified the experimental principle feasibility and accuracy.Considering the difference between actual entrainment velocity and theoretical entrainment velocity,which is obtained on the assumption of no-slip boundary phenomenon,the difference verified the existence of boundary slippage.What's more,the experiment results of the ball slide and plate slip further verified the existence of boundary slippage.At the same time,conclusion can be obtained that the slippage at the ball interface can be ignored compared to the degree of slip happened at glass disk interface under the experimental conditions,that is the boundary slippage only existed between the Cr film and the lubricating oil.A general slip length formula was defined to quantify slippage.To clarify the debate on the dependence of slip length on the shear rate,the measured slip lengths were compared to the corresponding shear rates.Their relation displays highly non-linear.Under low nominal shear rates,the slip length increases with shear rate,but approaches to a constant with further increase in shear rate.This non-linear curve is quite similar to the observation from MD simulations.It is demonstrated that high pressure,relatively poor wettability and high viscosity can lead to a large slip length.Lastly,related interpretations were made to exploit influence of different factors on the slip length.It provide inspiration and support to the theory of the influence of a specific factor.