embodiment shown in fig9 includes a number of elements common to the arrangement shown in fig2 and these common elements are denoted by reference numerals used in fig2 plus 34 100 34 to avoid a repetitive description the embodiment shown in fig9 is characterized in that the coil springs 18a and 18b are replaced by plate springs 30a and 30b the plate springs 30a and 30b are fixed at one of their respective ends to spring force adjusting levers 123a and 123b respectively the other ends of the plate springs 30a and 30b abut against the first tension disks 113a and 113b therefore the embodiment shown in fig9 is characterized in that the spring support disks 16a and 16b and the spring holders 17a and 17b shown in fig2 are not used either since the spring support disks 16a and 16b are not used as described above the first tension disks 113a and 113b are designed to have the function of the spring support disks 16a and 16b that is the first tension disks 113a and 113b are formed with cam follower surfaces 126a and 126b as shown in dotted lines while the shaft 111 is provided with pins 125a and 125b the embodiment shown in fig9 is not provided with the dials 6a and 6b shown in fig2 instead the plate cams 121a and 121b have knobs 31a and 31b directly attached thereto respectively these knobs 31a and 31b are exposed from the sewing machine frame 1 as shown in fig1 in the embodiment shown in fig9 by rotatively operating the operating lever 108 to rotate the shaft 111 around its axis the first tension disks 113a and.