a transmission mechanism includes a first gear unit having an axially - movable ring gear and a second gear unit having an axially - movable fixing ring . the fixing ring includes first and second protrusions on two sides , respectively . when the ring gear moves , the first inner teeth and the outer teeth of the second planet gear disk are engaged with each other or disengaged from each other . when the fixing ring moves , the first protrusions and the first positioning ridges are engaged with each other or disengaged from each other , or the second protrusions and the second positioning ridges are engaged with each other or disengaged from each other . by the different combinations of the statuses due to movement of the ring gear and the fixing ring , the input speed from the motor gear is transferred into four different speeds which is output from an output shaft .