The reason is as follows; A portion of lubricating oil for spiral oil wedge hydrodynamic bearing drains from oil return hole due to the oblique thrust of spiral oil groove, and the pressure flow resistance and shear resistance are larger, so the temperature is higher around the oil return hole; In axial direction, the temperature is lower around the oil feed hole, but the temperature is larger around the oil return hole, it is the same as above-mentioned reason. The higher temperature at high speed and around the oil return hole is easy to lead to the deformation of bush and the bad lubrication condition, even it can lead to shaft-braking.