Investigation On The Lurication Behaviour Of High-speed Grease Lubricated Bearings

Bearing lubrication is one of the crucial factors that determines the reliability and efficiency of the mechanical transmission system in aerospace fields.The accurate prediction of lubrication behavior of bearings is therefore important for controlled and reliable operation of machine parts.This thesis is concerned with efficient numerical solution of elastohydrodynamic lubrication(EHDL)problem.It approaches the lubrication of nonconforming contacts from numerical aspect.Numerous numerical schemes for elastohydrodynamic lubrication problems have been presented including inverse,forward iterative,partly coupled,fully coupled.Similar,coupled computational technique is analyzed here with the use of the Reynolds equation and the Hertzian contact theory.Governing equations of EHDL are derived for general elliptical contact case,to be used later for more specific line and point contact cases.FEM based EHDL codes are implemented on available commercial software COMSOL Multiphysics for analyzing EHDL behavior of greaselubricated bearings.Numerical results of the EHDL of bearings are presented both for line and point contact problems to allow the performance comparison.Two typical cases corresponding lightly(ph =0.5 GPa,um=0.25 m/s)and heavily(ph =3.2 GPa,um=1 m/s)loaded contacts are considered respectively.The findings for line as well as circular contacts form the basis of the discussion,on how the different pressures influence the film thickness.The results showed that the magnitude of lubricant film thickness at the heavily loaded condition is smaller than that at the lightly loaded condition,even though the high velocity at the heavily loaded condition,thus bearing higher pressures in contact zone.Well qualitative agreement is found between theory and simulation analysis.In addition,the effect of three parameters upon EHDL behavior are evaluated: entrainment speed,contact load and lubricant viscosity.The results showed that lubricant film thickness decreases with increasing load,while increase in entrainment speed and viscosity both leads to an increase in film thickness.Finally,the research discussed in this thesis contributes significantly to a deeper understanding of the effect of grease rheology on efficiency and lubrication conditions of rolling element bearings.