Gripper for a double lockstitch sewing machine with stable tension of the bobbin thread

Abstract

A hook for a double lockstitch sewing machine, comprising a tension spring (80, 90, 100, 110, 120) with an inwardly curved main bending radius relative to its receiving means on which it is attached, in this case specifically the bobbin case (8) or the bobbin holder (6) of the hook (11, 12, 13, 14) and a bobbin (4) on which the bobbin thread is wound, characterized in that the tension spring has an outward curvature, relative to the receiving means on which it is mounted and relative to the main bending radius of the tension spring, at the end of the tension spring which is responsible for pressing on the bobbin thread, and is designed such that the pressure point (P) on a bobbin thread is the apex between the radius (R) of the outward curvature at the end of the tension spring and the receiving means in order to generate a stable tension on the bobbin thread;wherein the tension spring (110, 120) is an extension of the flat extension of the tension spring sheet near the end of the tension spring, with the effect of pressing on the spool thread, and the excess material (111, 121) resulting from the extension is bent outwards with the radius (R) of the curvature, relative to the receiving means on which the tension spring is mounted and inwards relative to the main bending radius of the tension spring.

Description

[0001] The invention relates to a hook for a lockstitch sewing machine (with one needle thread and one bobbin thread), both as a rotary hook and a central bobbin hook, commonly known as a CB hook, and both suitable for both household and industrial sewing machines. It features an improved tension spring compared to the prior art. A more uniform thread tension is achieved during sewing, and the tension generated is less sensitive to the type and diameter of the bobbin thread.

[0002] The invention further relates to a lockstitch sewing machine with such a hook having such an improved tension spring compared to the prior art in order to ensure a more uniform thread tension during sewing and to make the tension generated less sensitive to the type and diameter of the bobbin thread.

[0003] The improved tension spring can be used instead of the known tension spring, which, depending on the design, is mounted on the bobbin case in the case of the CB hook, and on the bobbin case in the case of the rotary hook, either on the bobbin case or directly on the bobbin carrier.

[0004] The rotary gripper can be a type with either a horizontal axis of rotation or a vertical axis of rotation.

[0005] The lockstitch sewing machines and the respective CB or rotary hooks are generally well known and will therefore not be described in detail here, but rather their schematic composition will be presented.

[0006] The CB hook comprises a hook body onto which the bobbin case, containing the bobbin with the bobbin thread wound on it, is locked. The bobbin thread is threaded through the thread guide holes on the bobbin case to below the tension spring, so that the bobbin thread lies between the tension spring and the bobbin case shell, creating tension on the bobbin thread as it unwinds during the sewing process. This tension can be adjusted by adjusting the thread tension adjustment screw.

[0007] The bobbin-bobbin system works very similarly in rotary hooks, with the tension spring mounted on the bobbin case always creating the tension on the bobbin thread, similar to what is described for the CB hook. The difference, however, is that in rotary hooks with a bobbin case, the bobbin case is attached to the bobbin case carrier, which is mounted inside the hook body, rather than directly on the hook body.

[0008] In rotary hooks without a bobbin case, however, the bobbin is inserted directly into the bobbin carrier, and the tension spring, which creates tension on the bobbin thread, is mounted on the bobbin carrier. The bobbin thread is threaded through the guide holes on the bobbin carrier up to below the tension spring. This creates tension on the bobbin thread during unwinding during the sewing process. This tension can be adjusted by adjusting the thread tension adjustment screw.

[0009] Regardless of where the tension spring is mounted (bobbin case or bobbin holder), its operating principle remains the same: the tension spring consists of a thin metal sheet, typically between 0.2 mm and 0.5 mm thick, elongated in shape with a major axis several times longer than the minor axis, and curved in a corresponding arc along its major axis. This forces the bobbin thread to press against the surface of the receiving means on which the tension spring itself is mounted (bobbin case or bobbin holder, respectively), creating friction on the bobbin thread, which is then tensioned as the bobbin thread is unwound during the sewing process. The pressure with which the tension spring presses against the bobbin thread can be adjusted using an adjusting screw.

[0010] By acting on this adjusting screw and adjusting the pressure, the tension of the bobbin thread is changed during sewing. The tension spring is firmly connected to its take-up device, to which it is mounted by means of this adjusting screw and a fixing screw or, alternatively to the fixing screw, by means of a locking system on the take-up device (or the bobbin case or the bobbin carrier). The stability of the thread tension depends on the point of contact between the tension spring and the bobbin thread. In the standard design according to the state of the art, this point of contact is located at the edge of the tension spring. In the absence of bobbin thread, it is obvious that the arcuate bending of the tension spring causes the tension spring itself to rest on the take-up device on which it is mounted, along the ends of the tension spring, i.e., on the edge of the metal sheet from which it is made.In a second design (actually very rare), the contact point is shifted rearward from this edge and is determined by the apex of the tension spring's bending arc and the profile of the support member to which the tension spring is mounted. In this case, however, it is much more difficult to identify the contact point with certainty, and the reproducibility of such an arrangement is much more difficult to maintain in series production of the tension spring, since it is always a thin, bent sheet of metal and, as such, has discrete machining tolerances.

[0011] The first case, where the pressure point corresponds to the edge of the tension spring plate, is the one used universally and has the advantage of being simple to implement. A first disadvantage, however, is the bobbin thread tension, which is unstable during sewing (i.e., it exhibits fluctuations) because it is very sensitive to thread irregularities: when pulling the bobbin thread by hand, one has the impression that the tension spring is "scraping" against the bobbin thread. This effect is more noticeable with certain types of thread, such as those that are rough and of lower quality, and is a significant disadvantage when sewing requires low thread tensions, since irregularities negatively influence knotting during stitch formation. A second disadvantage of this design becomes apparent when changing the type of bobbin thread, since this also changes the thread tension generated by the tension spring.Therefore, every time the bobbin thread type is changed, the pressure acting on it must be readjusted by adjusting the adjusting screw. This effect is obviously even more detrimental where bobbin thread changes occur more frequently, such as in household sewing machines and small commercial businesses.

[0012] The second case, in which the pressure point corresponds to the apex of the tension spring's bending and the profile of the support member on which it is mounted, partially overcomes the first disadvantage mentioned, as it reduces tension fluctuations as well as sensitivity to irregularities and roughness of the bobbin thread. However, with a change in the diameter of the bobbin thread, the pressure point can also shift, thus also changing the tension. The main disadvantage of this second case, however, is the difficulty of mass-producing such a tension spring and the critical, insufficient reproducibility from one tension spring to another. These factors generate significantly higher production costs, while a residual intrinsic uncertainty regarding the exact pressure point remains.

[0013] US 6,152,057 A and US 6,901,871 B1 describe a system in which the tension of the bobbin thread is at least partially wrapped around the tension-generating element. The tension of the bobbin thread is thus maintained by friction of the bobbin thread on the tension-generating element.

[0014] US 6 895 879 B1 describes a tension spring with a different cross-section between the section where the adjusting screw acts and the section where the tension spring presses on the bobbin thread. The purpose is to provide greater flexibility in the section where the tension spring presses on the bobbin thread.

[0015] CH 658 273 A5 refers to a shuttle for textile machines and embroidery machines with a cover on which a leaf spring arranged parallel to the cover is attached to give tension to the bobbin thread.

[0016] DE 10 85 754 A (D1 in the German examination procedure) discloses (reference numerals are taken from there) a hook for a lockstitch sewing machine, comprising a spring (here "leaf spring" 7), with a main bending radius (in the area of ​​the screws 6 there). It is attached to its receiving means (here designated hook 3, moved by the driver 1). In addition, the screws 6 there can also be regarded as receiving means. Furthermore, it can be seen from this document that the spring 7 has an outward curvature, relative to the receiving means to which it is mounted, and relative to the main bending radius of the spring, which has the task of pressing on the hook, so that the pressure point on the hook is the vertex between the radius of the outward curvature of the spring and the receiving means, wherein at least in principle (e.g.With appropriate thread guidance, a stable tension of the bobbin thread could be generated, but is not. It should be noted that the spring 7 there has nothing to do with the tension of the bobbin thread. This spring 7 of the prior art may have a shape corresponding to the shape of the invention, but the spring 7 there has nothing to do with the tension of the bobbin thread. The spring 7 must not press on the bobbin thread, since the hook 3 and the hook driver 1 (on which the driver spring 7 is mounted) have an alternating movement. The spring end 9 presses on the outgoing hook and releases from the returning hook. Conversely, the spring end 11 presses on the returning hook and releases from the outgoing hook. The tension spring that causes the tension of the bobbin thread can only be mounted on the bobbin case or the bobbin case support, which (both) do not rotate and do not have an alternating movement.Instead, it is the loop of the needle thread that passes through slot 5 once with each stitch and should therefore run freely (i.e. without tension).

[0017] In DE 199 30 026 A1 (document D2 in the German examination procedure), a tension spring (reference number 30 there), fastened with a screw (31 there) on the flange 7 there, with a radius near the end 32 of the tension spring 30, presses on the bobbin thread (it is designated by 4 there), which Fig. 5 shows. The shape of the tension spring 30 there is completely different from that of the invention.

[0018] The object of the invention is to create a hook having a tension spring that can be manufactured just as easily and economically as the standard tension spring corresponding to the prior art, but which produces a much more stable tension of the bobbin thread during sewing (i.e. with little fluctuation) because it is not influenced by the irregularities of the thread itself. When pulling the bobbin thread by hand, there must not be a feeling that the tension spring is "scraping" against the bobbin thread, but rather the bobbin thread must run "smoothly" under the tension spring. A second advantage of the invention is that when the type of bobbin thread is changed, the tension difference remains unchanged or only slightly different. This eliminates the need to readjust the thread tension by adjusting the adjusting screw every time the type of bobbin thread is changed.

[0019] The gripper of independent claim 1 solves the problem. Claim 4 also solves this problem. Claims 7 and 8 also do so.

[0020] Further advantageous features are the subject of the respective dependent claims.

[0021] The gripper according to the invention has a tension spring which is firmly connected to the bobbin carrier or the bobbin case and generates a friction on the bobbin thread which is as far as possible independent of the irregularities and properties of the thread, for example its thickness, material, manufacturing method and surface roughness.

[0022] In the following we refer to these better performances as more stable voltage.

[0023] This more stable tension is achieved in a preferred embodiment by curving the tension spring outwards (relative to the receiving means on which the tension spring is mounted and relative to the main bending radius of the tension spring) at the end of the tension spring, so that the pressure point is no longer the edge of the tension spring, but rather the apex of the radius of said curvature at the end of the tension spring. In this preferred embodiment, the radius of curvature at the end of the tension spring is opposite to the main bending radius of the tension spring (which is directed inwards towards the receiving means on which the tension spring is mounted) and is of very limited dimensions (typically an order of magnitude smaller than the main bending radius of the tension spring).

[0024] In a preferred embodiment, the flat extension of the tension spring plate is extended at the end of the tension spring, which is responsible for pressing on the bobbin thread, so that the excess material can be bent outward with a radius of curvature (relative to the receiving means on which the tension spring is mounted and relative to the main bending radius of the tension spring), while the point of contact of the tension spring with the bobbin thread remains unchanged. The advantage lies in the fact that, whereas in the prior art design, the edge of the tension spring plate falls at this point, in the invention, the apex of the radius of curvature is located at the end of the plate.

[0025] In a further preferred embodiment, said radius of curvature outwards at the tension spring end and opposite to the main bending radius of the tension spring (which is directed inwards of the receiving means on which it is mounted) is of very limited dimensions and less than 2 mm.

[0026] An advantage of the hook is the fact that it can be used with all existing sewing machines without having to change their sewing elements and without any modification of a sewing machine available on the market, since a hook manufactured according to the invention is completely interchangeable with a conventional, known hook and has all the design features to implement the invention.

[0027] Even the bobbin case or the bobbin carrier, including the described tension spring, are the subject of the present invention and one of their advantages is the fact that they can be mounted on all existing hooks and without having to change the hooks available on the market, since the bobbin case or the bobbin carrier manufactured according to the invention are completely interchangeable with a conventional, known bobbin case or a bobbin carrier and have all the design features to implement the invention.

[0028] A tension spring is also claimed. Its advantage lies in the fact that it can be mounted on all existing hooks, bobbin cases, and bobbin carriers without having to modify the hooks, bobbin cases, and bobbin carriers available on the market. This is because the tension spring manufactured according to the invention is completely interchangeable with a conventional, known tension spring and possesses all the design features necessary to implement the invention.

[0029] The invention will now be described by way of example but not by way of limitation in the figures. Fig. Figure 1 is a schematic exploded view of a commonly known CB hook, complete with bobbin case. Fig. Figure 2 shows a schematic exploded view of a rotary gripper with horizontal axis, complete with bobbin case, according to the current state of the art. Fig. Figure 3 shows a schematic exploded view of a rotary gripper with vertical axis, complete with bobbin case, according to the current state of the art. Fig. Figure 4 shows a schematic exploded view of a rotary gripper with vertical axis, in this version without a bobbin case, according to the current state of the art. Fig. 5 to 7 show some embodiments of the tension spring according to the current state of the art.

[0030] Fig. 8 to 10 show an embodiment of the tension springs of the Fig. 5 to 7 according to the invention, in which an outward curvature of the tension spring (relative to the receiving means on which it is mounted and relative to the main bending radius of the tension spring) is provided at the tension spring end, so that the pressure point on the bobbin thread is no longer the edge of the tension spring plate, but the apex of the radius of curvature at said end.

[0031] Fig. 11 and Fig. 12 show another embodiment of the tension spring of the Fig. 8 according to the invention, which provides that the flat extension of the tension spring plate at the tension spring end, which has the task of pressing on the bobbin thread, is extended so that the excess material can be bent with a radius of curvature outwards (opposite the receiving means on which the tension spring is mounted, and opposite the main bending radius of the tension spring), whereby the contact point of the tension spring with the bobbin thread remains unchanged. This other embodiment of the tension spring of the Fig. 8 according to the invention, in which it is provided that the flat extension of the tension spring plate is extended near the tension spring end, which has the task of pressing on the bobbin thread, can also be realized in a simple manner in the other embodiments of the tension spring described, but the graphic representation is omitted, since it does not noticeably differ from the Fig. 9 and Fig. 10 differs.

[0032] Fig. 13 shows in cross-section and in detail only the point from the Fig. 5, Fig. 8, Fig. 11 along the section line AA, where the tension spring presses on the bobbin thread, with comparison a) the implementation according to the state of the art in which the pressure point is located at the edge of the tension spring plate; b) the first embodiment according to the present invention, in which the tension spring end is bent outwards, thus creating a radius and moving the pressure point backwards; c) the second embodiment according to the present invention, wherein the flat extension of the tension spring has been extended so that when the excess material at the end of the tension spring end has been bent outwards, the pressure point still remains the original pressure point of the tension spring according to the prior art.

[0033] In the accompanying figures, the corresponding elements are identified by the same reference numerals.

[0034] Fig. Figure 1 shows a schematic exploded view of a CB gripper 11, with horizontal rotation axis α, in a known design, in which the numerical references only identify the elements relevant to the present description: - a hook body 2, containing a trough 18 in which the bobbin case 8 complete with bobbin is stored; - a means for receiving the tension spring, in this case a bobbin case 8 housed in the trough 18 of the hook body 2 and rotating freely within the hook body 2, complete with a hinged carriage 15 for axially locking the bobbin case 8 on the hook body 2 to prevent accidental release during sewing, a hinged flap 16 which, by means of a lever system on the hinged carriage 15, allows the operator to release the hinged carriage 15 from the axial lock on the hook body 2 and thus remove the bobbin case, and a tension spring 9 for imparting tension to the bobbin thread (the graphic representation of which is omitted) wound on the bobbin 4 housed in the bobbin case 8; - a bobbin 4 on which the bobbin thread (not shown) is wound is accommodated in the bobbin case 8 and is limited therein by the installation of the bobbin case 8 in the hook body 2.

[0035] Fig. Figure 2 shows a schematic exploded view of a rotary gripper 12 with a horizontal axis of rotation α, in a known design, in which only the elements relevant to the present description are identified by the numerical references: - a gripper body 2; - a bobbin case carrier 6 which can rotate freely inside the hook body 2, and comprising: a trough 18 of the bobbin case carrier 6 in which the bobbin case 8 is stored complete with bobbin 4; - a means for receiving the tension spring, in this case a bobbin case 8 housed in the trough 18 of the bobbin case support 6, complete with a hinge slide 15 for axially locking the bobbin case 8 on the bobbin case support 6 to prevent accidental release thereof during sewing, a hinge flap 16 which, by means of a lever system on the hinge slide 15, allows the operator to release the hinge slide 15 from the axial lock on the bobbin case support 6 and thus remove the bobbin case, and a tension spring 9 for imparting tension to the bobbin thread (the graphic representation of which is omitted) wound on the bobbin 4 housed in the bobbin case 8; - a bobbin 4, on which the bobbin thread (not shown) is wound, is accommodated in the bobbin case 8 and is limited therein by the installation of the bobbin case 8 in the bobbin case carrier 6.

[0036] Fig. Figure 3 shows schematically an exploded view of a rotary gripper 13 with vertical axis of rotation α, in a known design, in which only the elements relevant to the present description are identified by the numerical references: - a gripper body 2; - a bobbin case carrier 6 which can rotate freely inside the hook body 2, and comprising: a trough 18 of the bobbin case carrier 6 in which the bobbin case 8 is stored complete with bobbin 4, a hinged flap 16 which allows the operator to release the bobbin case 8 from its axial locking with the bobbin case carrier 6 and to remove the bobbin case 8; - a tension spring receiving means, in this case a bobbin case 8 housed in the trough 18 of the bobbin case support 6, complete with tension spring 9, for imparting tension to the bobbin thread wound on the bobbin 4 housed in the bobbin case 8 (the graphic representation of which is omitted); - a bobbin 4, on which the bobbin thread (not shown) is wound, is accommodated in the bobbin case 8 and is limited therein by the installation of the bobbin case 8 in the bobbin case carrier 6.

[0037] Fig. Figure 4 shows a schematic exploded view of a rotary hook 14 with vertical axis of rotation α, in a known design without a bobbin case, in which only the elements relevant to the present description are identified by the numerical references: - a gripper body 2; - a tension spring receiving means, in this case a spool holder 6, which can rotate freely inside the gripper body 2, and comprising: a trough 18 of the spool holder 6 in which the spool 4 is housed, a hinged flap 16 allowing the operator to release the spool 4 from its axial locking on the spool holder 6 and thus to remove the spool 4, and a tension spring 9 for imparting tension to the spool thread wound on the spool (the graphic representation of which is omitted); - a spool 4 on which the spool thread (not shown) is wound and which is mounted in the spool holder 6.

[0038] The Fig. 5 to 7 show various tension springs in the state of the art design in a perspective view a) and in a side view; b) with the detail; c) enlarged and in section along the AA axis to highlight the pressure point P of the tension spring on the bobbin thread.

[0039] The gripper 11, 12, 13, 14, subject of the present invention comprises a tension spring which is firmly connected to the bobbin carrier or firmly to the bobbin case, which creates a stable tension on the bobbin thread, as far as possible independent of the irregularities and properties of the bobbin thread itself.

[0040] In a preferred embodiment of a gripper 11, 12, 13, 14 according to the invention, described in the Fig. 8 to 10, the tension spring, which is responsible for generating this stable tension, comprises an outward curvature (relative to the support means to which it is mounted and relative to the main bending radius of the tension spring itself) of the tension spring end, so that the pressure point is no longer the edge of the tension spring plate, but the apex of the radius of curvature at the tension spring end. In this preferred embodiment, the radius of said curvature at the tension spring end is opposite to the main bending radius of the tension spring (which is directed inward of the support means to which it is mounted) and is of very limited dimensions (typically an order of magnitude smaller than the main bending radius of the tension spring).

[0041] The Fig. 8 to 10 show a first preferred embodiment of the present invention. In the prior art embodiment, the tension spring 50, 60, 70 has an edge at its end that creates the pressure point P on the bobbin thread. In the embodiment according to said first preferred embodiment of the present invention, the tension spring 80, 90, 100 includes an outward curve at its end, whose radius of curvature R is responsible for exerting pressure on the bobbin thread.

[0042] The Fig. 11 and Fig. 12 show a second preferred embodiment of the present invention. As mentioned in the prior art embodiment, the tension spring 50 comprises an edge at its end that creates the pressure point P on the bobbin thread. In the corresponding embodiment according to said second preferred embodiment of the present invention, the tension spring 110, 120 comprises an extension of the flat extension of the tension spring plate near the tension spring end, which has the task of pressing on the bobbin thread, so that the excess material 111, 121 can be bent outwards with a radius of curvature R (opposite the receiving means on which the tension spring is mounted and opposite the main bending radius of the tension spring), wherein the contact point of the tension spring with the bobbin thread remains unchanged.In a very obvious manner, corresponding extensions can be realized for other embodiments of the tension spring according to the prior art 60, 70, the illustration of which is omitted here.

[0043] Fig. 13 shows only the detail in the cross section of the section line AA of the tension springs 50, 80, 110 of the previous Fig. 5, Fig. 8, Fig. 11, the point P, at which the tension spring presses on the bobbin thread, with comparison a) the design of the tension spring 50 according to the prior art, in which the pressure point is located at the edge of the tension spring plate; b) the first embodiment of the tension spring 80 according to the present invention, in which the tension spring end is bent outwards, thus creating a radius R and the pressure point P is displaced rearwardly compared to the original position of the tension spring 50 according to the prior art; c) the second embodiment of the tension spring 110 according to the present invention, wherein the flat extension of the tension spring has been extended so that, when the excess material at the tension spring end has been bent outward, the pressure point P remains in the same position as the original pressure point of the tension spring 50 according to the prior art. Similar considerations apply to the other embodiments of the tension spring 60, 70.

[0044] In a further preferred embodiment, said radius R of curvature outwards at the tension spring end, and opposite to the main bending radius of the tension spring (which is directed inwards of the receiving means on which it is mounted), is of very limited dimensions and less than 2 mm.

[0045] The grippers according to the examples of the invention differ from those according to the prior art 11, 12, 13, 14 only because of the use of the tension spring 80, 90, 100, 110, 120, which is manufactured according to one of the preferred embodiments.

Claims

[1] Hook for a lockstitch sewing machine, comprising a tension spring (80, 90, 100, 110, 120) with an inwardly bent main bending radius towards its receiving means on which it is fastened, in this case specifically the bobbin case (8) or the bobbin carrier (6) of the hook (11, 12, 13, 14) and a bobbin (4) on which the bobbin thread is wound, characterized bythat the tension spring has an outward curvature, relative to the receiving means on which it is mounted, and relative to the main bending radius of the tension spring, at the tension spring end which has the task of pressing on the bobbin thread, is designed such that the pressure point (P) on a bobbin thread is the apex between the radius (R) of the outward curvature at the tension spring end and the receiving means in order to produce a stable tension of the bobbin thread; wherein the tension spring (110, 120) is an extension of the flat extension of the tension spring plate near the tension spring end, with the effect of pressing on the bobbin thread, and the excess material (111, 121) created by the extension is bent outwards with the radius (R) of the curvature, relative to the receiving means on which the tension spring is mounted and inwards relative to the main bending radius of the tension spring. [2] Gripper according to claim 1, wherein the radius (R) of the outward curvature at the tension spring end is an order of magnitude smaller than the main inward bending radius of the tension spring (110, 120). [3] Gripper according to one of the preceding claims, wherein the radius (R) of the outward curvature at the tension spring end is less than 2 mm. [4] Tension spring, suitable for a hook (11, 12, 13, 14) of a lockstitch sewing machine, wherein a tension spring (80, 90, 100, 110, 120) has an outward curvature, opposite a receiving means on which it is mounted, and opposite a main bending radius inward of the tension spring, of the tension spring end, with an action to press on a bobbin thread, so that the pressure point (P) on the bobbin thread is the apex between the radius (R) of the outward curvature at the tension spring end and the receiving means of the tension spring, in order to produce a stable tension of the bobbin thread;wherein the tension spring (110, 120) is an extension of a flat extension of the tension spring plate near the tension spring end, with the effect of a pressure on the bobbin thread to expel the excess material (111, 121) created by the extension outwards, with the radius (R) of curvature opposite the receiving means on which the tension spring is mounted and opposite the main bending radius inwards of the tension spring; [5] Tension spring according to claim 4, wherein the radius (R) of curvature at the tension spring end outwards is of a smaller order of magnitude than the main bending radius inwards of the tension spring. [6] Tension spring according to one of claims 4 or 5, wherein the radius (R) of the outward curvature at the tension spring end is less than 2 mm. [7] A lockstitch sewing machine comprising a tension spring (80, 90, 100, 110, 120) according to any one of claims 4 to 6 for generating a stable tension of the bobbin thread. [8] Use of a tension spring (80, 90, 100, 110, 120) according to one of claims 4 to 6, for generating a stable tension of the bobbin thread of a hook (11, 12, 13, 14) of a lockstitch sewing machine.

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