with the reference symbol 1 , a hook base of a hook assembly 3 is designated . in fig1 , a bobbin case 5 with a bobbin 7 inserted therein lies above the hook base 1 . the bobbin case 5 with the bobbin 7 is thus not yet inserted into the hook base 1 . this is a state after the replacement / filling of the bottom thread bobbin 7 . in the hook base 1 , a connection element 11 is placed on a spindle 9 constructed centrally . this element is connected rigidly to the spindle 9 but can rotate axially and can be made from plastic or metal . the periphery of the connection element 11 can have a cylindrical construction ( no illustration ) or , as shown in the figures , it can have a shoulder - shaped neck section in the upper region . the bobbin case 5 comprises , in the center , a borehole 13 that has a cylindrical profile in the upper part a and that has a cylindrical construction in the lower part b over a partial region of the periphery of the borehole 13 , e . g ., over 180 °, and the opposite region is formed by a cylindrical section whose axis lies at an angle to the axis of the other region ( cf . fig1 - 4 , in each , left side of the borehole 13 ). in the end region of the bobbin case 5 , the borehole 13 can have a closed construction or its cross section can have a smaller diameter than the cylindrical section a of the borehole 13 lying underneath . on a circular shoulder 15 or a flat end face on the upper end of the connection element 11 , a coaxially arranged coil spring 17 is supported . the spring 17 is attached with its upper end on the bobbin case 5 , e . g ., clamped or bonded . when the bobbin case 5 is placed on top , the upper end ring 19 of the coil spring 17 therefore contacts the bobbin case 5 . advantageously , the end ring 19 is larger in diameter than the rest of the windings . the coil spring 17 is dimensioned such that , in the case of a bobbin case 5 locked completely in the hook base 1 , the coil spring 17 is under compressive force and , when the bobbin case 5 is released , the coil spring 17 lifts or ejects the bobbin case 5 at least partially from the hook base 1 , in order to simplify the grasping of the bobbin case 5 . on the lower end of the connection element 11 , also called the case carrier , a circular groove 21 is countersunk . the axial height of the groove 21 is slightly greater than the axial height of a crescent - shaped flange section 23 directed inward and running over a partial region of the borehole 13 on the bobbin case 5 . the distance of the inner edge 25 of the flange section 23 relative to the opposite , non - parallel wall section b is slightly larger than the diameter of the connection element 11 ( cf . fig6 ). below , the functioning of the hook assembly 3 will be explained in greater detail . the bobbin case 5 equipped with a new bobbin 7 , i . e ., a bobbin filled with thread , is pushed axially from the top over the connection element 11 . here , the coil spring 17 comes into spring contact with the upper end of the connection element 11 . a marking 29 is formed eccentrically on the end face 27 of the bobbin case 5 . the marking 29 can be coloring or a region provided with grooves by means of , for example , a laser beam , or can consist of a recess . for engaging or locking the bobbin case 5 , the person sewing presses with one finger approximately in the center on the bobbin case 5 ( arrow f 1 ); for further pressing , the bobbin case 5 becomes slightly tilted by the flange 23 provided only over a partial region ( ca . 180 °) on the lower end of the borehole 13 , as is visible in fig2 . through this inclined position , it becomes possible that the crescent - shaped flange 23 on only part of the periphery can slide past the lateral surface of the connection element 11 and can then engage in the groove 21 , when the bobbin case 5 tilts back to the horizontal . for releasing the bobbin case 5 from the hook base 1 , the procedure is reversed , in that , a pressure according to arrow f 2 is exerted with a finger on the eccentrically formed marking 29 and thus the bobbin case 5 tilts and the edge 25 of the flange 23 moves out from the groove 21 and then the bobbin case 5 is lifted by the force of the coil spring 17 until it can be raised with two fingers from the hook base 1 . in fig5 , a bobbin 7 is shown in the axial section that is not supported in a bobbin case 5 . so that it can rotate within the hook base 1 during sewing , a sleeve 31 is inserted so that it can rotate in the center of the bobbin 7 . the sleeve 31 comprises , both at the top and also at the bottom , means , e . g ., a raised edge or flange 33 , wherein these means hold the sleeve 31 in the bobbin 7 axially with play . the borehole 13 in the sleeve 31 is constructed analogous to the borehole 13 in the bobbin case 5 in the examples 1 to 4 . in other words , the upper region a is cylindrical and approximately half of the lower region b has a profile of a cylinder - barrel surface inclined toward the axis of the bobbin case 5 ( cf . also fig2 ). the upper end of the sleeve 31 is closed and is used as a contact for the coil spring 17 not shown in fig5 . in the ring - shaped flange of the sleeve 31 lying at the top , a marking 29 is formed eccentrically , as in the first embodiment , in order to indicate to the operator at which point he must apply a pressing force f 2 for unlocking . in another construction of the invention shown in fig8 , the connection element 11 is part of the spindle 9 , i . e ., the periphery of the spindle 9 has the shape of the periphery of the connection element 11 in the first embodiment . the function of the insertion or ejection of the bobbin case 5 with the bobbin 7 takes place in the same way .