The thermal elastohydrodynamic (EHD) analysis of lubrication was applied to investigate the friction factor of spiral bevel gears in helicopter's main transmission system.The calculation was based on the simultaneous solution of the Reynolds

energy and elasticity equations

including variations of lubricant viscosity and density in relation to pressure and temperature.The real contact load and velocities of sliding and entrainment between meshing tooth surfaces were treated based on loaded tooth contact analysis (LTCA) of spiral bevel gears.A thermal finite element analysis based on one-tooth sector model developed by real tooth profile was actualized

with calculated heat flow rate generated by sliding friction as thermal load and heat convection coefficients of all exposed surfaces as thermal boundary conditions.The tooth temperature field in steady state and flash temperature of contact location in transient state over several revolutions were investigated.The obtained results were reported and discussed.The comparison with analytical and experimental results from references and gear bench test was given in the end.