An X-ray tube assembly having bearings rotatably connecting a rotor shaft and a stator. A pair of circumferential protrusions each radially extend from the shaft and are spaced apart a first radial distance from the stator. In a first example, the protrusions are longitudinally outward of, and to one longitudinal side of, the pair of bearings, and one of the protrusions is longitudinally and radially proximate one of the bearings. In a second example, a line may be drawn which is parallel to the axis and which intersects each of the protrusions and each of the bearings. A substance, which includes metal (such as gallium) and which is liquid at the assembly's operating temperature, is located longitudinally between the circumferential protrusions. The substance radially extends a second radial distance between, and in conductive thermal contact with, the shaft and the stator.