Door closer

A door closer, comprising a door shaft, a housing, a piston shaft, a sliding cylinder and a door-closing spring, the sliding cylinder slidably disposed in the housing of the door closer in a manner that the sliding cylinder meshed with the door shaft, the door-closing spring located between the housing and the sliding cylinder, wherein:

• the sliding cylinder, an outer periphery of which is formed with teeth for meshing with a plurality of teeth formed on the door shaft, the piston shaft includes a shaft and a piston, the piston is moveably mounted on the shaft in such a manner that an outer surface of the shaft closely abuts against an inner surface of the piston, and the piston is slidably received in the sliding cylinder in such a manner that the outer surface of the piston shaft closely abuts against an inner wall of the sliding cylinder both ends of the piston shaft are inserted in an leak-proof assembly at both ends of the sliding cylinder, inside of the piston shaft is formed an outlet in which being received an one-way valve, the piston is interiorly formed with a guiding passage in which is formed an operating hydraulic space containing a steel ball for the one-way valve, hydraulic oil in the sliding cylinder flows between the inner wall of the sliding cylinder and the piston shaft via the one-way valve,
• the piston of the piston shaft is further formed with an axial passage which is connected between the operating hydraulic spaces at both ends of the piston, in the axial passage is provided with flow-adjusting screw.