The present invention discloses a pivotal disc is pivotally coupled to a first brace and a second brace. The pivotal disc has a motion-limit member and a positioning member limiting the angle by which the second brace rotates with respect to the first brace with the pivotal disc being the axis and fixing the angle between the first and second braces. The present invention is characterized in that the motion-limit member uses a range-adjust member to push a slide block and determine the range that a block portion of the second brace is blocked by the slide block, and that the positioning member uses a switch knob actuates on end of a positioning rod to engage with or depart from a positioning notch on the pivotal disc, and that the pivotal disc has two limit elements to define the maximum movement range of the first brace and the second brace.