Abstract: A rotary hinge shaft assembly has a movable shaft section, and a stationary shaft section. The movable shaft section is composed of an extending part and a cylinder part with a split groove having a recess ring for receiving a lubricating grease, and an oil groove thereon to communicate with the recess ring for guiding the grease. A collar is between the extending part and the cylinder part. The stationary shaft section has an extending part and a cylindrical shaft with an axial hole to tightly fit with the cylinder part. When the cylinder part is inserted into the axial hole, the split groove is squeezed and slightly distorted to enter the axial hole smoothly. The oil groove offers a complete lubrication such that the friction is reduced and the noise is prevented as the cylinder part rotates with respect to the axial hole.