A damper includes a cavity and a rotation shaft. The rotation shaft is inserted into the cavity of the body in sealing way. An oil cavity is disposed between the cavity and the rotation shaft. The external wall of the rotation shaft is disposed with a pair of protruding ribs axially. Each protruding rib is a groove. One side of the groove is a first retaining wall and the other side is a second retaining wall. A transitional arc is disposed between the side wall of the groove and the top surface of the first retaining wall. A first shape is formed between the other side wall of the groove and the top surface of the second retaining wall. A blade is disposed between the groove of one protruding rib and an internal wall of the cavity, and the blade is movable between the two retaining walls of the groove.