This invention relates to a belt tensioning device for a belt drive, which comprises a continuous belt and at least two belt pulleys, of which one serves as a drive and one as an output of the belt drive. Such belt drives are especially used in combustion engines of a motor vehicle for driving auxiliary units, wherein a first belt pulley rests on the crank shaft of the combustion engine and drives the belt. Further belt pulleys are assigned to the auxiliary units, as for example a water pump, an electric generator or an air conditioning compressor and are rotatably driven by the belt drive. In common belt drives, the auxiliary units are designed as consumers, which means, they are driven by the belt pulley of the crank shaft via the belt. In this case, the undriven side of the belt is arranged between the crank shaft and the unit following in circumferential direction of the belt, generally the generator. To ensure a sufficient contact of the belt around the belt pulley, the belt is pre-tensioned by means of a tensioning pulley of the belt tensioning device.
Also belt drives are known, in which a starter-generator unit is integrated into the belt drive as a further auxiliary unit, i.e. an electro motor, which can be operated depending on the operational condition as a starter (car starter) or a dynamo (generator). During the normal- or engine operation, the belt pulley on the crank shaft is the driving pulley, while the starter-generator as well as the other residual units are driven. During the starting or cranking operation, the starter-generator drives via the corresponding belt pulley the crank shaft, to start the combustion engine. In such belt drives with starter-generator as auxiliary unit a change between the driving side and the undriven side to both sides of the belt pulley of the starter-generator unit takes place between the engine operation and the starting operation. Thus it is necessary, to provide spring-loaded tensioning pulleys for both named sides, of which respectively one pulley is acting onto the undriven side by means of a spring force, while the other pulley is pushed back by the tensioned driving side.
From EP 2 128 489 A2 a belt tensioning device for a belt drive with a starter-generator unit is known. The belt tensioning device has a housing, in which two tensioning arms are pivotably supported around a common pivot axis. The tensioning arms are supported on each other by means of spring means. The housing can be mounted while the drive belt pulley has already been mounted on the starter-generator
From U.S. Pat. No. 4,758,208 B a system for a belt drive with a starter-generator unit is known. The housing of the starter-generator unit is mounted on the engine block and such, that the housing is rotatable in a limited manner around the axis of rotation of the drive pulley. The belt tensioning device comprises two tensioning arms, which are pivotable scissor-like relative to each other around the drive shaft. Between the tensioning arms a tensioning spring is arranged, which acts on both the tensioning arms towards each other.
From DE 10 2011 053 869 A1 of the applicant, a belt tensioning device with exactly one tensioning arm is known, which is supported via a helical spring relative to the base member in circumferential direction.
The available installation space around the generator axis as well as before and behind the belt plane is especially limited for two tensioning devices or a double arm tensioning device. Furthermore, the mounting of the belt is difficult, not at least because of the installation space conditions. Depending on the design of the belt tensioner high torques acting on the belt tensioner can result, which may lead to an increased wear.