62 of each linking member is articulated with the bulges 53 of the respective ring wheel by means of a pintle the first and second wedges 71 72 of each wedge tooth is expanded by a resilient member 10 and the distance between their rotation center and the circular arc shaped top surfaces is slightly larger than the relative spacing of the inner walls of the annular recesses of the drive wheel such that the circular arc shaped top surfaces and the inner walls of the annular recesses create a reverse frictional contact when the drive wheel rotates in the present embodiment the first and second wedges 71 72 respectively have a hinge gaine and wedge teats 73 74 axially arranged at outer circumferential surfaces of the gaine the hinge gaine 75 of the first wedge is located at the middle of the wedge teat and the second wedge has two hinge gaines 76 77 located at both sides of the wedge teat the hinge gaine 75 at the middle could be matchably inserted into the hinge gaines 76 77 at both sides such that they are aligned with a common axis while their wedge teats 73 74 are opposite to each other up and down the lateral surfaces 78 78 u2032 of the wedge teat are planar.