a limit signal application_number US 68848291 A description a tensioning device 2 for the needle thread coming from a thread reserve not shown is arranged on the stand 1 of the double thread chainstitch sewing machine shown in fig1 a transducer 3 which is fastened to the sewing machine via a damping element 4 fig2 made of e g rubber is provided behind the tensioning device 2 in the direction of thread pull the transducer 3 has a bending bar 5 whose width is reduced toward the free end beginning from the clamping point at its free end the bending bar 5 is designed on the underside with a needle thread receiving eye 6 on it wire strain gauge elements hereinafter called wsg elements 7 are provided to receive the tension of the needle thread a first wsg element 7 is fastened on the top side and a second wsg element 8 on the underside of the bending bar 5 close to the point of clamping of the bending bar the wsg elements 7 and 8 are connected to a power source fig3 and are connected to form a half bridge 9 which is connected to an amplifier 10 the output of the amplifier 10 is connected to a voltmeter 11 with a display unit 12 and to a comparator 13 with an adjusting device 14 serving to set its switching threshold the output of the comparator 13 is connected to one input of and elements 15 and 16 each whose second input is connected to a position transmitter 18 that counts the revolutions of the main shaft 17 this position.