A tensioner for traction means such as a belt, includes a housing which accommodates a cylinder for guiding a hydraulically dampened, spring-loaded plunger in an axial direction for tensioning the traction means. The interior space of the housing is closed by a cover which has a guide opening for passage of the plunger. A seal is placed outside the cover to seal an annular opening bounded by the housing and the plunger. In order to prevent a gas cushion under pressure and trapped between the cover and the seal, from pushing the seal from its seat, the cover is formed with a vent passageway which communicates with the interior space for release of the gas cushion via the vent passageway into the interior space.
In an auto tensioner, a high pressure oil chamber sectioned by a leading end portion of a cylinder and a plunger is provided within the cylinder. A gas chamber and an oil reservoir chamber are sectioned in a sealed manner in an outer periphery of the cylinder and the plunger. A first oil passage communicating the gas chamber with the high pressure oil chamber is provided within the plunger. A second oil passage communicating the high pressure oil chamber with the oil reservoir chamber is provided in the cylinder. The first oil passage is provided with a pressure side damping force generating means, (an orifice hole) and a first check valve closing during expansion. A second check valve closing during compression is provided in the second oil passage, and a locking means (an electromagnetic valve) closing the first check valve is provided.
