EP0420856B1 - Thread-cutting device for sewing machines - Google Patents

Abstract

In a prior art sewing machine, the tensioner is released in both directions of the axially mobile gear element (21), and hence the thread tensioner is also opened when the needle thread is seized. In the new design, the motion of the gear element (21) is transmitted via a one-way coupling (53), and therefore the thread tensioner is open only while the needle thread is being tensioned.

Description

The invention relates to a thread cutting device according to the preamble of claim 1.

Through a sewing machine design, which was taken into account in the formation of the preamble of claim 1 (see for example document DE-A-3 244 996), a thread cutting device has become known in which an axially displaceable gear element in its one direction of movement the needle and Gripper thread is gripped and pulled out in its other direction of movement and fed to the cutting knife. To control the trigger mechanism for the thread tension, a control cam is attached to the gear element, which actuates the trigger mechanism. Although this measure results in an extremely simple drive of the tension release, it requires an opening of the spring tension during the movement of the gear element in both directions of movement. With critical thread conditions, this results in difficulties in loop detection and also uneven needle thread ends after the cutting process.

The invention has for its object to form the trigger mechanism for the thread tension in a thread cutting device according to the preamble of claim 1 so that the thread tension is only opened during the tightening of the needle thread.

According to the invention, this object is achieved by what is stated in the characterizing part of claim 1 Feature solved.

With the control according to the invention, the thread tension now remains closed during the penetration of the thread catcher into the needle thread loop, so that the stability of the loop is retained when it is caught by the thread catcher. The thread tension is only opened at the beginning of the pulling of the needle thread and only closes when no further thread menr is pulled.

Claim 2 contains a structurally simple design of the one-way clutch.

The measure according to claim 3 ensures a jerk-free opening of the thread tension and its immediate closing after sufficient tightening of the needle thread.

• Fig. 1 einen Ausschnitt einer Nähmaschine mit der Steuerung der Schneideinrichtung und der Fadenspannung, teilweise geschnitten, wobei die Fadenspannung selbst schaubildlich in Fig. 1a dargestellt ist;
• Fig. 2 eine Draufsicht auf einen Teil des in Fig. 1 dargestellten Antriebs der Fadenschneid- und der Spannungsauslöseeinrichtung;
• Fig. 3 bis 5 verschiedene Stellungen des Steuernockens und der Einwegkupplung zur Steuerung der Fadenspannung.

The invention is explained with reference to an embodiment shown in the drawing. It shows:

• 1 shows a section of a sewing machine with the control of the cutting device and the thread tension, partially cut, the thread tension itself being shown graphically in FIG. 1a;
• FIG. 2 shows a plan view of part of the drive of the thread cutting and tension release device shown in FIG. 1;
• Fig. 3 to 5 different positions of the control cam and the one-way clutch for controlling the thread tension.

In Fig. 1, the base plate (1) is shown of a sewing machine, under which the horizontal hook drive shaft (2) is mounted. is. This is driven by a lower main shaft (4) via a transmission gear (3). The gripper (5) is attached to the front end of the gripper drive shaft (2). It works together with the needle (6), which can be moved up and down and which is fastened in a needle bar (6a), which is only indicated.

A thread cutting device (7) is arranged below the base plate (1), the structure and mode of operation of which is described in DE-PS 11 25 742. The thread cutting device (7) has a thread catcher (8) which is arranged coaxially to the gripper (5) and which interacts with a cutting knife (9) attached to the underside of the base plate (1). The thread catcher (8) is fastened to a support arm (10) which is connected to a ring (11) which loosely surrounds the hook drive shaft (2). This is secured in the axial direction and rotatably mounted in an annular path (12) which is attached below the base plate (1). A link (13) is articulated on the support arm (10) and is connected to an arm of an angle lever (14) which is supported by a bearing block (15) fixed to the housing. The other arm of the angle lever (14) is connected via a rod (16) to a drive device (17) for the thread catcher (8).

To drive the thread catcher (8), a control cam (18) is attached to the main shaft (4), which has a control groove (19) and a control segment (20). Parallel to the main shaft (4) (see also FIG. 2) is a gear element designed as an axially displaceable shaft (21) mounted in a carrier (22) fastened below the base plate (1). A control lever (24) provided with a roller pin (23) and a release lever (25) are fastened to the gear element (21). A pawl (28) equipped with a locking lug (26) and a stop (27) is loosely mounted on a bushing (30) fastened in a side wall (29) of the carrier (22) and is detached from the side wall (29) of the carrier ( 22) and an end flange (30a) of the bushing (30) against axial displacement. A spring (31) wrapped around the gear element (21), which rests on one end on the control lever (24) and on the other end on the pawl (28), causes the pawl (28) to pivot in the direction of the control segment (20) of the control cam (18 ) and the roller bolt (23) from the control groove (19). At the same time, the spring (31) acts as a compression spring and shifts the gear element (21) into its right end position shown in FIG. 1, in which the roller bolt (23) faces the control groove (19) when the sewing machine is in the needle down position.

An electromagnet (32) is fastened in the carrier (22), the armature (33) of which is connected to an arm (35) of a switch lever (36) via an eyelet (34). This is mounted in the carrier (22) and is turned upwards by a torsion spring (37) until a bent attachment (38) is applied to the locking lug (26) of the pawl (28) or - when the pawl is pivoted off - an end piece (39) pivoted to an adjustable threaded pin (40) fastened in the carrier (22).

Before the threads are cut, they must be drawn out to a length sufficient for the next stitch to be formed. This is done with solved Needle thread tension. The thread tension (42) (Fig. 1a) for the needle thread consists of a sleeve (43) fastened in the machine housing with a longitudinally slotted threaded bolt (44) on which two tension disks (45, 46) are arranged, which are secured by a knurled nut (47). adjustable tension spring (48) are compressed to exert a braking force on the needle thread. A release pin (49) is arranged in the sleeve (43) so as to be longitudinally displaceable, by means of which the tension disk (45) can be raised against the action of the tension spring (48) from the tension disk (46) in order to open the thread tension (42).

A control cam (50) attached to the gear element (21) (FIGS. 1 and 2) is used to actuate the release pin (49) and thus to release the thread tension (42), the surface of which is designed as a cam curve, which is wedge-shaped at one end rising curve section (51) and at the other end a straight and then radially falling curve section (52). A one-way clutch (53) cooperates with the control cam (50) and is arranged on a lever arm (54) mounted on the carrier (22). The one-way clutch (53) consists of a segment-like lever (55), which is rotatably mounted on the lever arm (54) and interacts with the control cam (50), as well as a stop (57), which acts under the action of a torsion spring ( 58) on a pin (59) fastened in the lever arm (54).

The lever arm (54) is actuated by a torsion spring (60) until a lateral contact surface (54a) or the one-way clutch (53) bears against the control cam (50). twisted. The torque emanating from the torsion spring (60) considerably exceeds the torque emanating from the torsion spring (58). The lever arm (54) is connected via a rod (61) to an angle lever (62) mounted on the machine housing. A push rod (63) is articulated to this and carries at its front end a release element (65) provided with an inclined surface (64) (FIG. 1a).

Assuming that the sewing machine is running, the roller bolt (23) being in its outside the control groove (19) of the control cam (18) rotating with the main shaft (4), the thread catcher (8) with its drive element (21) is in its starting position takes up and the thread tension (42) for the needle thread is closed, the device works as follows:
At the end of the seam, the sewing machine is briefly stopped in the needle down position as the starting position for thread cutting by a needle positioning device. The control groove (19) in the control cam (18) assumes such a position that the roller pin (23) on the control lever (24) can be moved into the control groove (19) by a simple pivoting movement. The control cam (50) and the one-way clutch (53) are in the position shown in Fig. 3, in which the thread tension (42) is closed.

By actuating a switch when stepping backwards on the sewing machine, the electromagnet (32) is switched on briefly and immediately thereafter the drive motor of the sewing machine. The switch lever (36) is pulled down by the armature (33) of the electromagnet (32) and presses the roller pin (23) into the control groove (19) via the release lever (25) and the control lever (24).

At the same time, the spring (31) pivots the pawl (28), the locking lug (26) of which lies over the shoulder (38) on the switch lever (36) and thus secures the position of the roller bolt (23) pushed into the control groove (19).

During the following rotation of the main shaft (4) required for thread cutting in the direction of arrow (A), the cam groove (19) running in the axial direction moves the gear element (21) out of its rest position via the control lever (24) in accordance with a first section of the cam groove ( 19) to the left. The gear element (21) moves the thread catcher (8) via the angle lever (14) and the handlebar (13) into the needle thread loop which is gripped and widened by the gripper (5) and thereby detects the leg of the needle thread loop leading to the material to be sewn and the thread Looper thread.

When it is shifted to the left, the cam section (52) abuts the lever (55) and rotates it against the force of the torsion spring (58), so that the lever arm (54) moves through to the left when the gear element (21) is shifted the bearing surface (54a) remains in the rest position due to the curve section (52) and the lever (55) returns to its rest position when the wedge-shaped curve section (51) moves past, in which it rests on the pin (59).

After the loop of the needle thread has fallen off the hook (5), the gear element (21) is moved to the right as a result of the curve of the control groove (19) (FIG. 5). The wedge-shaped curve section (51) of the control cam (50) pushes against the lever (55) of the one-way clutch (53) and pivots thereby the lever arm (54), which via the rod (61) opens the angle lever (62) and the push rod (63) the thread tension (42) for the needle thread, by the trigger pin (49) through the inclined surface (64) of the trigger element ( 65) axially displaced, thereby lifting the tension disk (45) against the action of the tension spring (48) from the tension disk (46).

At the same time, the thread catcher (8) pulls thread from the hook and needle thread supply through the rightward movement of the gear element (21) until shortly before the end of this movement, the cutting knife (9) attached to the underside of the base plate (1) cuts off both threads.

Immediately thereafter, the cam section (52) of the control cam (50) moves past the lever (55) of the one-way clutch (53); the lever (55) can engage behind the cam section (52) under the pressure of the torsion spring (60), so that the lever arm (54) can rest with its lateral contact surface (54a) on the axially parallel surface of the curve section (52) of the control cam (50). This drop of the lever (55) from the curve section (52) causes the thread tension (42) to close immediately via the linkage for tension release.

When the main shaft (4) continues to rotate until the sewing machine is switched off in the needle up position, the control segment (20) pivots the pawl (28) over the stop (27) so that the locking lug (26) releases the switch lever (36) again. This is pivoted upwards by the torsion spring (37) until the shoulder (38) stops on the pin (40).

This allows the spring (31) to push the control lever (24) away from the control cam (18) so that the roller pin (23) is lifted out of the control groove (19). The sewing machine is then stopped in the thread lever high position and is ready for the next sewing process.

Claims (3)

1. Thread-cutting device on sewing machines, having a thread catcher (8), which is activated by an axially displacable gearing element (21) and which executes a first movement stage, which catches the needle- and the catcher thread, and a second movement stage in the opposite direction thereto, which pulls out the threads and delivers them to a locally fixed blade (9), with a thread tension of the needle thread and with an opening element (65), which opens the thread tension during the thread-pulling process and which is operated via a rod (54, 61 to 63) by a control cam (50̸), which is fixedly connected to the gearing element (21), characterised in that betwen the gearing element (21) and the rod (54, 61 to 63) is provided a one-way coupling (53) for the release element (65), which is active during the second movement stage of the gearing element (21).
2. Thread-cutting device according to claim 1, characterised in that the one-way coupling (53) has a singlesidedly movable lever (55) which is pivoted during the first movement stage of the gearing element (21) and remains rigid during the second movement stage.
3. Thread-cutting device according to one of claims 1 or 2, characterised in that the control cam (50̸) is defined at the one end by a wedgeshaped inclined curved section (51) and at its other end by a radially declining curved section (52).

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