meshes with a bevel gear 98 which is fixed by a key 99 fig1 on a shaft 100 rotatably mounted in the drive unit housing 93 a further gearwheel 101 is fixed on the end of the shaft 100 remote from the bevel gear 97 by the same key 99 and meshes with a gearwheel 102 which together with a further gearwheel 103 is on a shaft 104 spaced from and parallel to the shaft 100 the gearwheel 103 meshes with a gearwheel 105 which is freely rotatably mounted on the shaft 100 on the side of the gearwheel 101 facing away from the bevel gear 98 the gearwheels 101 102 103 and 105 are preferably spur gears the shaft 100 and the gearwheel 105 are mounted rotatably in the drive unit housing 93 by bearing units 106 to 109 for mutual support and stable journalling according to fig1 to 12 the shaft 104 is rotatably mounted in an oscillating frame 112 by means of bearing units 110 111 the frame for its part being rotatably mounted by means of bearing units 114 and 115 on the shaft 100 or axially extending collars of the gearwheels 98 101 and 105 and being capable of swinging to and fro about an axis 113 fig1 12 of the shaft 100 the oscillating frame 112 is provided with teeth 116 on an outer wall surrounding the shaft 101 in ring manner the teeth 116 being in engagement with teeth 117 on a rack 118 which can be moved to and fro perpendicular to the axis 113 in a guide 110 fixed in the drive unit housing 93 and in the direction of an arrow z fig1.