What you have is a tube that slides within a tube mounted the the control surface. The ball link is mounted to the servo output wheel, so that neutral has the ball link, hinge line and slide tube all inline. When the servo rotates, the ball link follows and the sliding tube pivots along with the control surface. Since the distance between the ball link and the control surface varies with the angle of the control throw, the tube in a tube has to be able to slide.

I'm building one with a 4-40/2-56 ball link (Dubro 369) threaded onto a 4-40 threaded rod. The T-rod is epoxied into a 5/32" K&S brass tube, and that slides into a K&S 3/16" brass tube that is mounted into the control surface.

CAD away!