thread against the adjacent portion of the external surface of the cylindrical wall is adjustable by a screw the tensioning member is tiltably installed in the pan shaped portion of the spring so as to adjust its inclination relative to the adjacent portion of the external surface of the cylindrical wall in dependency on the position of the thread application_number US 26816488 A description fig1 shows a bobbin case 1 which comprises a tubular preferably cylindrical wall 3 one end of which is closed by a bottom wall 5 the wall 3 surrounds a customary thread bobbin which is not shown in the drawing the central portion of the bottom wall 5 carries a hollow pin shaped mandrel 7 which is coaxial with the wall 3 and carries the core of the properly inserted bobbin for a supply of lower thread 33 the open end portion of the wall 3 is formed with a v shaped notch 9 forming the inlet of a suitably configurated slot 10 through which the thread extends from the bobbin during threading the closed end of the slot 9 constitutes a substantially circular opening 11 in the wall 3 a relatively large window 13 is adjacent but angularly offset with reference to the opening 11 the improved bobbin case further comprises a resilient thread tensioning element 19 hereinafter called spring whose curvature approximates but need not exactly match the curvature of the periphery 17 of the wall 3 this spring 19 is adjustably secured to the wall 3 by a fastening means 15 here shown as a threaded fastener screw which can be turned by a suitable tool to thereby change the amount of energy which is stored by the spring 19 and hence the.