As is known, a current trend of automobile manufacturers is to replace the traditional starting motor and generator with a single reversible electric machine, which is connected to the engine shaft by means of a drive belt. During the starting step, said reversible electric machine functions as a motor and drives the engine shaft of the internal-combustion engine; when the vehicle is in motion, instead, the electric machine is driven by the internal-combustion engine and generates electric current for recharging the battery.
The belt that connects the electric machine to the engine shaft of the internal-combustion engine may be used also for driving one or more further auxiliary members, such as, for example, the compressor of the air-conditioning system.
As is known, normally associated to belt drives is a belt tensioner, i.e., a device provided with an idle pulley mounted on a mobile arm loaded by a spring in the direction of the belt in order to compensate for the variations of tension of the belt itself. The belt tensioner acts on the slack branch of the belt, i.e., on the less-tensioned branch, located downstream of the drive pulley with reference to the direction of motion.
In the case where a reversible electric machine is used, which has the function of starting motor and current generator, the branches of the belt have a different tension according to the mode of operation: the branch that is tensioned during the starting phase, in which the electric machine is the driving member and the internal-combustion engine is the driven member, becomes slack in the stage of normal running, in which the internal-combustion engine is the driving member and the electric machine is the driven member.
To overcome this problem bi-directional belt tensioners or two-arm belt tensioners, i.e., which comprise two arms provided with pulleys, each acting on a respective arm of the belt, have been proposed. The two arms can be mounted on one and the same pin, in order to turn about a common axis and be loaded in the direction of one another by a spring so that the respective pulleys will co-operate with respective branches of the belt, so ensuring their tensioning.
WO-A-00/77422 discloses a belt tensioner including a fixed portion, a first and a second arm hinged to the fixed portion about a common axis and carrying respective idle pulleys, and a spring biasing the arms towards one another to maintain the pulleys in contact with respective branches of a transmission belt.
Stop means are provided to limit the travel of each arm both in the direction of the spring force, so as to define an assembly position of the arms, and in the opposite direction to prevent overtravel of the arms under dynamic pull variations of the belt.