25 each have a square hole 23 through which the two screws 26 are respectively threaded into the threaded holes 142 on the square nuts 13 at the two opposite ends of the shaft 14 and therefore the upper and lower swinging plates 24 25 rotate with the shaft 14 the cylindrical casing 10 has a bolt hole 101 on the peripheral wall near the top end thereof corresponding to a bolt hole 181 on the upper bush 18 into which a tightening up screw 27 is threaded to stop against the adjusting ring 19 the tension of the compression spring 15 is adjusted by adjusting the adjusting ring 19 the adjusting ring 19 has an adjusting hole 192 on its peripheral outside wall by loosening the tightening up screw 27 and then inserting a pointed object into the adjusting hole 192 and rotating it the tension of the compression spring 15 is adjusted referring to fig5 and 7 rotating the upper and lower swinging plates 24 25 causes the shaft 14 and the stop plate 16 to rotate a hydraulic buffer mechanism is formed by matching the rotary motion of the stop plate 16 with the hydraulic buffer 12 the hydraulic buffer 12 is comprised of a cylindrical container 30 a spring 31 a piston 32 a piston rod 33 and a cap 34 the piston rod 33 and the piston 32 are made in a unitary piece and inserted with the spring 31 into the holding space inside the container 30 the cap 34 is covered on an opening not shown.