Abstract: A tensioner is provided for a power transmission belt comprises a tensioning roller with which the belt cooperates, a return spring for controlling the displacement of said roller when the tension of the belt decreases and means for damping the vibrations likely to occur in the belt and comprising two variable volume chambers filled with a liquid and communicating together at one of their ends through a valve device through which the passage of the liquid entering and/or leaving said chambers introduces the damping effect, wherein the sidewall at least of one of the chambers is a membrane made from an elastomer material, such as rubber, having a general bell-shape and without spring, and the return spring associated with the other chamber is formed by a coil spring working under compression, prestressed in its rest state.

Abstract: A tensioner is provided for a power transmission belt comprising a return spring associated with the axis of a tensioner roller co-operating with the belt to control the displacement of said roller under the action of said spring with hydraulic type means for the dampening of vibrations generated in the belt. The spring is covered with an elastomer forming a sleeve closed at one of its ends by an obturator for defining said hydraulic dampening means which comprise at least one variable volume chamber filled with a liquid and closed at one of its extremities by a dissymmetrical valve device.