feeler roller 10 of the double lever 9 rests on the elevated lobe of the interval cam 11 so that the cam sleeve 4 is in its furthermost right end position the feeler roller 21 then rests on the cylindrical cam 4a which corresponds to a pivoted position of adjusting lever 20 and thus the adjusting crank 15 which with the rotation of the arm shaft crank 13 leads to a constant maximum crank radius and thus to a maximum needle shaft stroke in sewing the needle at the lowermost stroke position penetrates through the material and the loop catcher lying therebelow catches the upper thread carried along by the needle which then leads to the normal formation of a stitch when the feeler roller 10 rolls down from the elevated lobe as the interval cam 11 is further rotated which as previously mentioned happens when the adjusting lever 20 and thus the feeler roller 21 rest on the upwardly positioned section of the cam 4a which corresponds to the top dead center position of the needle shaft 4 the control sleeve 4 is pushed far enough to the left by a corresponding pivoting of the double lever 9 clockwise in fig1 so that the feeler roller 21 is located on the cylindrical section of cam 4b as the arm shaft crank 13 continues to rotate the feeler roller 21 rolls up onto the cam lobe of the cam 4b causing a corresponding pivoting of adjusting lever 20 and thus adjusting crank 15 i e in the sense of a reduction of the crank radius which.