Abstract: In an aspect, the invention relates to a decoupler that is positionable between a shaft (eg. for an alternator) and an endless power transmitting element (eg. a belt) on an engine. The decoupler includes a hub that mounts to the shaft, and a pulley that engages the endless power transmitting element, an isolation spring between the hub and the shaft. The decoupler provides at least a selected damping torque between the hub and the pulley.

Abstract: A tensioner (24) for tensioning an endless drive member (14) on an accessory drive system or a cam shaft drive system is provided with a vent structure (99) that opens into a cavity (68) of the tensioner (24) to permit reduction of any pressure in the cavity (68) and the ambient environment of the tensioner (24), while inhibiting ingress of contaminants into the cavity (68). In some embodiments the vent structure (99) includes one or more of a mechanical one-way valve (152), a semi-permeable membrane (311) or a passageway with a circuitous path.