Belt drive for accessories of an internal combustion engine with friction wheel

A friction wheel is held out of engagement with a belt (2') or drive wheel (7), dependent on rotation speed and/or load. This is performed by a control device (9) with a pivot arm (10) carrying the friction wheel, and mounted on a cam (12). The control device is provided with an external servomotor (13) which is rotatably connected to the cam (12). An independent claim is included for a control device.