a sleeve, having an axial through hole, and comprising a body having an inner peripheral wall; and

a connecting piece, comprising at least two connecting sheets, wherein the connecting sheets can circumferentially rotatably disposed in the inner peripheral wall of the body relative to the body of the sleeve, the connecting sheets enclose a connecting hole, the connecting sheets are provided with threads on sides close to an axial centerline of the connecting hole, and a driving structure capable of driving each of the connecting sheets to radially move toward the axial centerline of the connecting hole when the connecting sheet circumferentially rotates relative to the sleeve is located between the connecting sheet and the inner peripheral wall of the body;

wherein the driving structure comprises a bulge disposed on the inner peripheral wall of the body of the sleeve in an axial direction and an inclined guide surface disposed on the connecting sheet in the circumferential direction, the inclined guide surface has a decreasing distance from the inner peripheral wall of the body of the sleeve from a starting point to an ending point, and the bulge can relatively move in the circumferential direction on the inclined guide surface to drive the connecting sheet to radially move toward the axial centerline of the connecting hole.