Patent ID: 7094168

Claim:
A belt tensioner positionable between a side of a stationary member and a side of a pulley, arranged and constructed to provide tension to a belt, comprising: a plunger slidably inserted into a cylinder; the cylinder being mountable to the side of the stationary member positioned in an upper side, and the plunger being mountable to the side of the pulley positioned in a lower side; a high pressure oil chamber sectioned by the cylinder and a pressurizer of the plunger being formed within the cylinder; an outer tube sectioning an oil reservoir forming a gas chamber in an upper portion being disposed in an outer periphery of the plunger and the cylinder; a first oil passage communicating the gas chamber with the high pressure oil chamber being disposed in an upper portion of the cylinder; a second oil passage communicating the high pressure oil chamber with the oil reservoir being disposed in the plunger; an orifice hole being provided in the first oil passage and a first check valve closing during expansion being disposed in the first oil passage; and a second check valve closing during compression being disposed in the second oil passage, wherein the outer tube comprises a dust boot attachable respectively to the side of the plunger and the side of the cylinder in upper and lower base end portions, and being expandable and contractable in an axial direction.