Tensioner for an accessory drive of a motor vehicle and accessory drive including such a tensioner

A tensioner for an accessory drive of an internal combustion engine, the drive including at least a first pulley connected to a drive shaft of the engine, at least a second pulley connected to an electric machine and a belt wound at least on the first and second pulleys, the tensioner comprising:

• a base configured to be fixed to a casing of the electric machine;
• a first ring rotating with respect to the base about a first axis (A1);
• a second ring rotating with respect to the first ring about a second axis (A2) distinct from the first axis (A1);
• a first tensioning pulley carried by the first ring and rotating with respect to the first ring about its own axis (PA1);
• a second tensioning pulley carried by the second ring and rotating with respect to the second ring about its own axis (PA2);
• and
• an elastic member acting on the first and second rings to push the first and second tensioning pulleys into contact with respective spans of the belt,
• wherein under nominal conditions a plane passing through the first and second axes (A1;
• A2) is inclined with respect to a bisector of a belt winding angle (0) on the second pulley by an angle (a) that is a function of the belt winding angle (0) on the second pulley according to the relation:
• α=−0.21660+97.267+c,
• where α and θ are expressed in degrees (°) and c is a variable comprised in the range between +10° and −10°.