Engaging and disengaging gear for a bicycle pedal crank axle

An engaging and disengaging gear for a pedal crank axle of a bicycle, said axle being rotatably mounted in a five-way tube on a frame of the bicycle with two ends thereof projected from the five-way tube to connect to two pedal cranks, one end of said axle further having a chain wheel mounted thereto between the five-way tube and the pedal crank, said chain wheel being able to bring a small chain wheel at a rear wheel of the bicycle to rotate via a chain wound around said chain wheel and said small chain wheel, said engaging and disengaging gear comprising:

• a stepped thimble being fixedly connected to an end of the five-way tube facing said chain wheel and including a diameter-narrowed inner part and a diameter-expanded outer part;
• a transmission gear having teeth radially spaced on a portion of its outer surface and an internal thread provided on a wall of a central hole thereof, said transmission gear being mounted in said expanded outer part of said stepped thimble with said teeth facing toward an inner side of said chain wheel;
• chisel grooves being formed on the inner side of said chain wheel corresponding to said teeth on said transmission gear for detachably engaging with said teeth, and a bearing being mounted in a central hole of said chain wheel;
• an external thread being provided at least on a section of said axle between said stepped thimble and said chain wheel for engaging with said internal thread of said transmission gear when said axle is sequentially extended through said bearing on said chain wheel, said internally threaded central hole of said transmission gear, and said narrowed inner part of said stepped thimble into said five-way tube;
• and
• a ring spring being put around another portion of the outer surface of the transmission gear, such that said ring spring slightly contacts its outer periphery with an inner circumferential surface of said expanded outer part of said stepped thimble to provide a suitable resistance to a maximum static friction force between said transmission gear and said axle that are screwed together;
• whereby when said axle is caused to turn forward, said transmission gear is guided by said external thread to move along said axle toward said chain wheel until said teeth on said transmission gear engaging with said chisel grooves on the inner side of said chain wheel for said transmission gear to bring said chain wheel to rotate forward;
• and when said chain wheel or said axle is caused to rotate reversely from a forward rotating state, said transmission gear is guided by said external thread to move along said axle toward the five-way tube to disengage said teeth thereon from said chisel grooves on the inner side of said chain wheel, so that said axle and accordingly said pedal cranks connected thereto are not backward rotated by said reversing chain wheel.