AUTOTENSIONER

An autotensioner which can keep the tension in a belt at a constant level from the start of an engine. The autotensioner includes a cylinder having a top opening closed by an oil seal to prevent leakage of hydraulic oil in the cylinder. A rod slidably extends through the oil seal and is biased upward by a rod spring. At the bottom end of the rod is a hydraulic damper. A screw rod is slidably fitted on a small-diameter portion at the bottom end of the rod and is urged by a return spring against a shoulder formed on the rod. A nut member is axially immovably mounted in the cylinder while kept in mesh with a male thread formed on the screw rod. When the engine is cut, the belt may be stretched according to the positions of the cam on camshafts. If this happens, the rod is pushed inwardly by the belt. This rod pushing force is borne by pressure flanks of the threads at the thread engagement portions of the screw rod and the nut member to prevent the retraction of the rod. Since the belt is tensioned while the engine is not running, it will never slacken when the engine is restarted.