A high energy x-ray tube includes an evacuated chamber (12) containing a rotor (34) which rotates an anode (10) in the path of a stream of electrons (A) to generate an x-ray beam (B) and heat. Heat is carried away from the anode to a bearing shaft (54) which rotates relative to a stationary rotor (42) on forward and rear lubricated bearings (44P, 44R). The heat is directed away from the forward bearings (44F), by a core (70) of a thermally conductive material, such as copper, disposed in a central cavity (60) within the shaft. Annular insulating regions (74,76) are optionally defined between the core and the bearing shaft adjacent the races to increase the thermal path between the anode and the races. The reduction in temperature of the forward bearings results in a decrease in the evaporation rate of the lubricant (46) and a corresponding increase in the lifetime of the x-ray tube.