Abstract: A sewing machine designed to stitch together two stacked fabric layers advancing at different speeds comprises an upper and a lower conveyor belt coupled with a common drive shaft by a differential gearing. The relative speed of the two conveyor belts can be manually adjusted with the aid of a friction coupling comprising two mutually parallel larger disks with relatively offset axes and overlapping surface areas contacted by a smaller disk perpendicular thereto, the latter disk being displaceable in a common axial plane of the two larger disks to vary their transmission ratio.