US3578770A

US3578770A - Sewing machine

Info

Publication number: US3578770A
Application number: US784770A
Authority: US
Inventors: Johannes Franciscus Hazendonk
Original assignee: Jong & De Vlieger Machinehande
Current assignee: De Jong & De Vlieger Machinehandel Nv; Jong & De Vlieger Machinehande
Priority date: 1967-12-29
Filing date: 1968-12-18
Publication date: 1971-05-18
Anticipated expiration: 1988-05-18
Also published as: DE1816353A1; BE726006A; DE1816353B2; CH477591A; FR1601538A; NL6717780A; DK120061B; GB1194993A; SE348240B; IE32599B1; IE32599L

Abstract

A sewing machine is disclosed having a treadle-operated main clutch, an electromagnetic clutch and brake unit, a thread cutoff and an electric control circuit comprising a rotating needle positioning device. On pressing the treadle forwardly the main clutch is actuated whereby the driving motor of the machine drives the latter at the full operating speed and on subsequent disengagement of the treadle the electromagnetic clutch is actuated whereby the drive motor drives the machine through a reduction gear at a reduced needle positioning speed until the needle has reached its lowest position and is stopped by the actuation of the electromagnetic brake. When the treadle is moved from its forward position through its rest position to a rear position, the electromagnetic clutch remains energized until the machine has moved the needle to its highest position and is then stopped. The thread cutoff is electrically actuated during the movement of the needle from its lowest to its highest position.

Description

United States Patent [72] Inventor Johannes Franciscus Van Hazendonk Zwanenburg, Netherlands [21] Appl. No. 784,770 [22] Filed Dec. 18, 1968 [45] Patented May 18, 1971 [73] Assignee De Jong & De Vlieger Machinehandel N.V. Zwanenburg/Haliweg, Netherlands [32] Priority Dec. 29, 1967 [3 3 Netherlands [3 1 1 67 17780 [54] SEWING MACHINE 5 Claims, 1 Drawing Fig.

[52] US. Cl 112/219,

1 12/252 [51] Int. Cl D05b 69/26, D05b 65/02 [50] Field of Search 112/219, 219 (A), 219 (F0), 220, 67, 252,130,129

[56] References Cited UNITED STATES PATENTS 2,961,591 11/1960 Frankel eta] 112/219X 3,156,204 11/1964 Hannish et al..... 112/219X 3,174,450 3/1965 Becker et al. 112/219 3,386,402 6/1968 Ross 112/252 Primary Examiner-H. Hampton Hunter Attorney-Waters, Roditi, Schwartz and Nissen a rotating needle positioning device. On pressing the treadle forwardly the main clutch is actuated whereby the driving motor of the machine drives the latter at the full operating speed and on subsequent disengagement of the treadle the electromagnetic clutch is actuated whereby the drive motor drives the machine through a reduction gear at a reduced needle positioning speed until the needle has reached its lowest position and is stopped by the actuation of the electromagnetic brake. When the treadle is moved from its forward position through its rest position to a rear position, the electromagnetic clutch remains energized until the machine has moved the needle to its highest position and is then stopped. The thread cutoff is electrically actuated during the movement of the needle from its lowest to its highest position.

SEWING MACHINE BACKGROUND OF THE INVENTION The invention generally relates to sewing machines and I more in particularto the electrical control of a sewing until the needle of the sewing machine has been moved to its lowest position at said reduced speed whereupon the needle positioning device causes the machine to stop through the actuation of the electromagnetic brake. If thereupon the treadle is pushed rearwardly, the electromagnetic clutch is again actuated whereby the needle is moved to its highest position at said reduced speed and is then again stopped by the actuation of the electromagnetic brake. During the upward movement of the needle from its lower to its upper position the thread cutting device is actuated.

In the known sewing machines of this type, if the thread has to be cut, the treadle must first be moved from its forward position to its rest position so as to cause the machine to stop with the needle in its lowest position and thereupon the treatdle must be pushed rearwardly to have the needle travel upwardly to its highest position and to actuate the thread cutting device as soon as the needle has moved outside the needle plate of the machine. In the known sewing machines the thread cutting device is generally controlled mechanically or electropneumatically through a separate treadle-operated cam or microswitch. In these machines the operator has thus to make two separate foot movements when he wishes to cut the thread since he should first return the treadle to its rest position and wait until the machine has stopped and should then push the treadle rearwardly. This procedure reduces the working speed of the operator. The needle positioning speed of these known machines is, furthermore, relatively low, i.e. approximately ll 80 revolutions per minute, since the treadle controlled actuating means for the thread cutting device generally does not allow higher speeds.

SUMMARY OF THE INVENTION The principal object of the present invention is to provide a sewing machine of the foregoing type, and more in particular an electric control system therefor, which allows.a greater needle positioning speed and a quicker thread cutoff, whereby the output of the machine can be increased.

Another object of the invention is to provide a sewing machine of the kind indicated allowing the operator, when he wishes to cut the thread, to move the control treadle from its forward position to its rear position in a single continuous motion.

According to the invention, the electric control circuit of the sewing machine comprises a first control relay which is energized through a treadle-operated contact in the forward position of the treadle and closes the circuit of the electromagnetic clutch, and a second control relay having its relay coil connected in series with a second treadle-operated contact closed in the rear treadle position and with a normally closed contact of the first control relay and a contact of the needle positioning device controlling the upper needle position which second control relay on its excitation energizes the electromagnetic clutch whereby the needle on reaching its lowest position and the subsequent deexcitation of the first control relay continues its travel to its highest position to be stopped there, the thread cutting device being electrically controlled by an additional contact of the needle positioning device to cause the actuation of this cutting device during the needle movement from the lowest to the highest needle position.

Preferably, the thread cutting device is pneumatically actuated through electromagnetically controlled valves.

If the thread has to be cut, the operator may thus move the treadle in a single motion from its forward position to its rear position since at the moment the needle reaches its lowest position and the first control relay is deenergized, the second control relay immediately takes over and holds the electromagnetic clutch energized until in the highest needle position also this second control relay is disconnected and the electromagnetic brake is actuated. This possibility to move the treadle in a single motion from the forward to the rear position leads to an important increase in working speed. The electrical control, through the needle positioning device, of the pneumatic actuation of the thread cutting device leads to a quicker operation of this latter device whereby it is possible to increase the needle positioning speed of the machine to for instance 220-300 revolutions per minute.

BRIEF DESCRIPTION OF THE DRAWING design and comprises an electric drive motor 1 having a motor shaft 2, a main machine shaft 3 for driving the moving elements of the machine including the needle in its up and down movement (not shown). The motor shaft 2 can be coupled to the machine shaft 3 by means of a clutch mechanism generally indicated by 4 and operable by a threadle 5 which may be turned by the operator from its shown mid position to a forward position and to a rear position, respectively. The clutch mechanism may be of known design and, as shown, comprises a friction disc 6 fixedly mounted on the motor shaft 2, a fric tion plate 7 secured to one face of a worm wheel 8 mounted for free rotation on an intermediate shaft 9, and a spring mounted coupling disc 10 which is nonrotatably but axially slidably mounted on the shaft 9. By known means not shown on an anticlockwise turning movement of the treadle from its mid position to its forward position the coupling disc 10 is moved tothe left into engagement with the friction disc 6 so as to connect the motor shaft 2 to the intermediate shaft 9 whereas in the mid position of the treadle S and also when the treadle is turned clockwise to its rear position the coupling disc 10 engages the friction plate 7, thus coupling the worm wheel 8 to the intermediate shaft 9. The shaft 9 is connected to the machine shaft 3 through suitable coupling means, such as a belt drive 11.

An electromagnetic clutch and brake unit generally indicated by 12 is mounted on a shaft 13 carrying a worm Mengaging the worm wheel 8. The unit 12 comprises a magnetic coupling member 15 mounted for free rotation on the shaft 13 and carrying a coupling winding 16 for energizing the member 15. The unit 12 further comprises a magnetic brake member 17 fixedly mounted on a stationary part of the machine and carrying a brake winding 18 for energizing the magnetic member 17. A coupling disc 19 is nonrotatably but axially slidably mounted on the shaft 13 and is made of magnetic material whereby on the excitation of the coupling winding 16 the disc is moved to engage a friction plate on the member 15, thereby coupling this member to the shaft 13, and on the excitation of the brake winding 18 is moved to engage a friction plate on the stationary member 17, thereby holdingthe shaft 13 against rotation. The magnetic coupling member 15 further carries a pulley 20 which is connected by means of a suitable belt drive such as 21 to a pulley 22 secured on the motor shaft 2. The magnetic coupling member 15 is thus directly and continuously driven by the motor 1.

When the treadle is pressed to its forward position and the coupling disc is thereby moved to engage the friction disc 6, the machine shaft 3 is driven by the motor 1 through the coupling mechanism 4 at its full operational speed. When, on the other hand the treadle 5 is in its mid position or in its rear position in which the coupling disc 10 engages the worm wheel 8, the machine shaft 3 is connected to the shaft 13 through the worm gear 8, 14 whereby on the excitation of the coupling winding 16 the machine shaft 3 is driven at a reduced speed, the needle positioning speed, and on the excitation of the brake winding 18 is immediately stopped and held against further rotation. The above-described drive system with clutch mechanism 4 and electrically controlled electromagnetic clutch and brake unit 12 cooperating therewith is as such well known in the art.

A needle positioning device generally indicated by 23 comprises a switch drum 24 connected to the machine shaft 3 for rotation therewith and thus making one revolution for each up and down movement of the needle of the sewing machine. The switch drum 24 has a body of insulating material and four circumferential contact strips 26429 arranged thereon cooperating with four contact brushes 3033, respectively. The contact strips are electrically interconnected, the strip 27 extending completely around the drum and, together with the contact brush 31, serving for feeding current to the other three contact strips. The

latter strips

26, 28 and 29 extend only over part of the circumference of the drum, the circumferential gap between the strip ends having a different angular position and a different angular width for each contact strip.

The contact strips and the contact brushes are so arranged with respect to each other that the brush 33 leaves the strip 29 in the angular position of the drum 24 corresponding to the lowest position of the sewing machine needle and that the brush 32 leaves the strip 28 in the angular position ofthe drum 24 corresponding to the upper needle position. The brush 30 contacts the short strip 26 during the latter part of the needle movement from the lower to the upper position thereof.

The control circuit is fed from a suitable current source 25, such as the normal network, through a reducing transformer 34 having a secondary voltage of 30 v. and a rectifier arrangement 35. The positive terminal of the rectifier is directly connected to the contact brush 31 of the positioning device 23.

A first control relay 36 has its relay coil 37 connected to the terminals of the rectifier 35 in series with a treadle contact 38 which is closed in the forward position of the treadle 5 and is open in the mid and rear position of the treadle. The relay has a normally open switch contact 39, a normally closed switch contact 40 and a reversal switch contact 41. When the treadle contact 38 is closed and the relay coil 37 energized, the contact 39 closes a holding circuit for the relay coil 36 from the positive terminal of the rectifier 35 through the contact brush 31, the switch drum 24, the contact brush 33, the relay contact 39, the relay coil 37 to the negative terminal of the rectifi- A second control relay 42 has a relay coil 43 which has one end connected to the negative terminal of the rectifier 35. The other end of the relay coil 43 is connected through the normally closed switch contact 40 of the relay 36, a second treadle switch 44 which is closed in the rear position of the treadle 5, the contact brush 32, the switch drum 24 and the contact brush 31 to the positive terminal of the rectifier 35.

When both relay coils 43 and 37 are deenergized, a circuit is closed from the positive terminal of the rectifier 35, through a reversal switch contact 45 of relay 42, the reversal switch contact 41 of relay 36 and the brake winding 18 of the clutch and brake unit 12 to the negative terminal of the rectifier. When the relay 36 is excited, a similar circuit is closed but now through a reversal switch contact 41 to the coupling winding 16 of the unit 12. When the relay 42 is excited, a circuit is closed from the positive rectifier terminal through the reversal switch contact 45 directly to the coupling winding 16.

The control relay 42 has a further normally open switch contact 46 which is connected across the rectifier 35 in series with a relay coil 48'of a third control relay 47 and with the contact brush 30 cooperating with the short contact strip 26 of the drum 24. The control relay 47 has a reversal switch contact 49 which in its rest position, as shown, connects the field winding. 51 of an electromagnetically controlled valve 50 across the rectifier 35 and which on the excitation of the relay 47 connects the field winding 53 of a similar electromagnetically controlled valve 52 across the rectifier. The

valves

50 and 52 may be of known design, for instance of the type known by the trade name Electrojet, and are connected on their inlet side to a common air inlet tube 54 in which is connected to a source of compressed air (not shown). On their outlet sides the

valves

50 and 52 are connected through

tubes

55 and 56, respectively, to a cylinder 57 with piston 58 and piston rod 59 which is connected to a thread cutoff unit. Thread cutofi' units are well known in the sewing machine art and the unit here employed has, therefore, not been shown as forming as such no part of the present invention. It suffices to say that when as shown, the winding 51 is energized the valve 50 is open whereby pressure on the right-hand side of the piston 58 keeps the thread cutoff unit in its rest position. When the contact 49 is switched to deenergize the winding 51 and to energize the winding 53, the valve 50 is closed and the valve' 52 opened whereby the piston 58 carries out its operating stroke and actuates the thread cutoff. Upon reversal of the switch 49 the piston 58 is again returned to its rest position.

The above-described described needle positioning and thread cutoff control circuit functions as follows.

Assuming the motor 1 to be running, on pressing the treadle 5 forwardly the coupling disc 10 is moved against the friction disc 6 whereby the mean machine shaft 3 is driven at its normal operating speed of for instance 5,0006,000 revolutions per minute. By this actuation of the treadle 5 the treadle contact 38 is closed whereby the control relay 36 is energized and the reversal switch contact 41 interrupts the circuit of the brake winding 18 of the clutch and brake unit 12 and closes the circuit of the coupling winding 16 of this unit. The excitation of the coupling winding 16 causes the coupling disc 19 to engage the rotating member 15 which thus drives the shaft 13 and, through the worm 14, the worm wheel 8. This has however, no effect at this moment since this worm wheel may rotate freely on the intermediate shaft 9.

When the operator removes his foot from the treadle 5 and the latter returns to its rest position, the relay 36 remains energized through its closed holding contact 39 and the contact brushes 31 and 33 of the switch drum 24 as long as the needlt of the sewing machine has not reached its lowest position. Tht return of the treadle 5 to its rest position also causes the coupling disc 10 to disengage the friction disc 6 and to engagt the friction plate 7 of the worm wheel 8 thereby coupling this worm wheel to the intermediate shaft 9. This shaft and thereby the machine shaft 3 is then driven through the worm gear 14, lat a reduced needle positioning speed of for instance 30( revolutions per minute. As the needle reaches its lowest posi tion the contact brush 33 leaves the contact strip 29 whereby the holding circuit of the relay 36 is interrupted and this relay is deenergized. The reversal switch contact 41 thereby interrupts the circuit of the coupling winding 16 and closes the cir cuit of the brake winding 18 whereby the machine is immediately stopped with the needle in said lowest position.

If thereupon the operator presses the treadle 5 rearwardly the treadle contact 44 is closed. Thereby the circuit of thr winding 43 of the control relay 42 is closed through the con tact brushes 31 and 32 of the switch drum 24 and the no closed contact 40 of the deenergized relay 36 whereby the relay 42 is excited switching its contact 45 to interrupt the circuit of the brake winding 18 and to close the circuit of the coupling winding 16. The coupling disc 19 is thus again disen gaged from the brake member 17 to engage the coupling member 15 whereby the machine shaft 3 is again driven at the reduced needle positioning speed moving the needle from its lowest to its highest position. As soon as the needle reaches this highest position, the contact brush 32 leaves the contact strip 28 of the drum 24 interrupting the circuit of the relay winding 43 so that the relay contact 45 is again switched over to open the circuit of the brake winding 18. The machine is thereby immediately stopped with the needle in its highest position.

The excitation of the control relay 42 also causes the closure of the contact 46 preparing the circuit for the'winding 48 of the relay 47. During the movement of the needle from its lowest to its highest position the contact brush 30 engages the contact strip 26 of the switch drum 24 causing the relay 47 to I be energized for a short interval and the reversal switch contact 49 to be switched. Thereby the previously closed circuit of the control winding 51 of the electromagnetic valve 50 is interrupted and the circuit of the control winding 53 of the valve 52 is closed causing the actuation of the thread cutoff mechanism in a manner as above-described. When the control relay 47 is again deenergized, the switch contact 49 causes the winding 51 to be again energized so as to return the thread cu toff mechanism to its rest position.

The above-described described control circuit makes it possible for the operator to move the treadle 5 in a single continuous motion from its forward position through it mid position to its rear or tread-cutting position. In that case first the control relay 36 is energized and as soon as the needle has reached its lowest position and this relay is deenergized, the control relay 42 is energized. This causes the coupling winding 16 to remain energizes without interruption whereby the needle is allowed to travel in a continuous movement through its ofthe following claims.

lclaim:

' 1. An electrically controlled sewing machine of the type including a main machine shaft, an electric driving motor, a treadle having a neutral mid position, a forward operating position and a rear position, first and second normally open treadle-operated contacts each arranged to be closed in the forward and rear treadle position, respectively, a thread cutting device having electrically controlled actuation means, main clutch means actuated by said treadle in the forward position thereof to couple said motor to said machine shaft for driving the latter at a first operating speed, reduction gear means, an electromagnetic clutch and brake unit having a coupling winding and a brake winding, respectively, and arranged to connect said motor through said reduction gear means to said machine shaft for selectively driving the 'latter at a second reduced speed when, in said neutral or rear treadle position, said coupling winding is energized, and for braking said machine shaft when, in said latter treadle positions, said brake winding is energized, respectively, and an electrical control circuit for said clutch and brake unit windings and said thread cutting device comprising a needle positioning device including a rotary switching device operated in synchronism with said machine shaft and having a first normally closed contact means adapted to be opened each time the sewing machine needle reaches its lowest position, a second normally closed contact means arranged to be opened each time the needle reaches its highest position and a third contact means arranged to be temporarily actuated in the course of the needle travelfrom the lowest to the highest needle position; first relay switch means arranged to be actuated on the closure of said first treadle contact and cooperating with said windings of said electromagnetic clutch and brake unit and with said rotary switching device to hold said coupling winding energized when said treadle, after having been moved to its forward position, is returned to its midposition whereby said machine shaft continues to advance the needle at said reduced speed, and to deenergize said coupling winding and energize said brake winding as soon as the needle has reached its lowest position; and

second relay switch means arranged to be actuated on the closure of said second treadle contact and the opening of said first contact means of said switching device and cooperating with said windings of said electromagnetic clutch and brake unit to energize said coupling winding when said treadle is moved to its rear position whereby said machine shaft further advances the needle at said reduced speed, and to deenergize said coupling winding and energize said brake winding when the needle has reached its upper position; and

said second relay switch mean in its actuated position controlling said thread cutting device actuating means through said third contact means of said switching device whereby said thread cutting device is actuated during said movement of the needle form its lowest to its highest position.

2. The sewing machine of claim 1 further comprising pneumatic means for driving said thread cutting device, said thread cutting device actuating means including electrically con trolled valve means for controlling said pneumatic means.

3. The sewing machine according of claim 1 in which said first relay switch means comprises a first reversal switch contact for alternately energizing the brake winding and coupling coupling winding.

4. The sewing machine of claim 1 further comprising a third relay switch means having a relay coil and a switch contact, said second relay switch means having a normally open con tact in series with said third relay switch means relay coil and said third contact means of said rotary switching device said thread cutting device actuating means being controlled by said switch contact of said third relay switch means.

5. An electrical control circuit for a sewing machine of the type including a main machine shaft, an electric driving metor, a treadle having a neutral mid position, a forward operating position and a rear position, second normally open treadleoperatedcontacts each arranged to be closed in the forward and rear treadle position, respectively, a thread cutting device having electrically controlled actuating means, main clutch means actuated by said treadle in the forward position thereof to couple said motor to said machine shaft for driving the latter at a first operating speed, reduction gear means, and an electrically controlled electromagnetic clutch and brake unit having a coupling winding and a brake winding, respectively, and arranged to connect said motor through said reduction gear means to said machine shaft for selectively driving the latter at a second reduced speed when, in said neutral or rear treadle position, said coupling winding is energized, and for braking said machine shaft when, in said latter treadle positions, said brake winding is energized, respectively, said control circuit comprising a source of current,

a needle positioning device including a switch drum coupled to said machine shaft and cooperating with a first contact brush arranged to interrupt its contact each time the sewing machine needle reaches its lowest position, a second contact brush arranged to interrupt its contact each time the needle reaches its highest position and'a third contact brush arranged to make contact during a short interval in the course of the needle travel from the lowest to the highest needle position,

a first control relay having a relay coil and a first reversal switch contact normally connecting said brake winding to said current source and, on the excitation of said relaycoil, connecting said coupling winding to said current source, a normally open second relay switch contact arranged in series with said first switch drum contact brush in a holding circuit for said first control relay, and a normally closed third relay switch contact, said relay coil being connected in series with said first treadle-operated contact across said current source, and

a second control relay having a relay coil connected in series with said second treadle contact, said normally closed first control relay third switch contact and said second

Claims

1. An electrically controlled sewing machine of the type including a main machine shaft, an electric driving motor, a treadle having a neutral mid position, a forward operating position and a rear position, first and second normally open treadle-operated contacts each arranged to be closed in the forward and rear treadle position, respectively, a thread cutting device having electrically controlled actuation means, main clutch means actuated by said treadle in the forward position thereof to couple said motor to said machine shaft for driving the latter at a first operating speed, reduction gear means, an electromagnetic clutch and brake unit having a coupling winding and a brake winding, respectively, and arranged to connect said motor through said reduction gear means to said machine shaft for selectively driving the latter at a second reduced speed when, in said neutral or rear treadle position, said coupling winding is energized, and for braking said machine shaft when, in said latter treadle positions, said brake winding is energized, respectively, and an electrical control circuit for said clutch and brake unit windings and said thread cutting device comprising a needle positioning device including a rotary switching device operated in synchronism with said machine shaft and having a first normally closed contact means adapted to be opened each time the sewing machine needle reaches its lowest position, a second normally closed contact means arranged to be opened each time the needle reaches its highest position and a third contact means arranged to be temporarily actuated in the course of the needle travel from the lowest to the highest needle position; first relay switch means arranged to be actuated on the closure of said first treadle contact and cooperating with said windings of said electromagnetic clutch and brake unit and with said rotary switching device to hold said coupling winding energized when said treadle, after having been moved to its forward position, is returned to its midposition whereby said machine shaft continues to advance the needle at said reduced speed, and to deenergize said coupling winding and energize said brake winding as soon as the needle has reached its lowest position; and second relay switch means arranged to be actuated on the closure of said second treadle contact and the opening of said first contact means of said switching device and cooperating with said windings of said electromagnetic clutch and brake unit to energize said coupling winding when said treadle is moved to its rear position whereby said machine shaft further advances the needle at said reduced speed, and to deenergize said coupling winding and energize said brake winding when the needle has reached its upper position; and said second relay switch mean in its actuated position controlling said thread cutting device actuating means through said third contact means of said switching device whereby said thread cutting device is actuated during said movement of the needLe form its lowest to its highest position.

2. The sewing machine of claim 1 further comprising pneumatic means for driving said thread cutting device, said thread cutting device actuating means including electrically controlled valve means for controlling said pneumatic means.

3. The sewing machine according of claim 1 in which said first relay switch means comprises a first reversal switch contact for alternately energizing the brake winding and coupling winding, respectively, of the clutch and brake unit, said reversal switch contact being connected in series with a second reversal switch contact of said second relay switch means, said second reversal switch contact on the actuation of said latter relay switch means closing an energizing circuit for said coupling winding.

4. The sewing machine of claim 1 further comprising a third relay switch means having a relay coil and a switch contact, said second relay switch means having a normally open contact in series with said third relay switch means relay coil and said third contact means of said rotary switching device said thread cutting device actuating means being controlled by said switch contact of said third relay switch means.

5. An electrical control circuit for a sewing machine of the type including a main machine shaft, an electric driving motor, a treadle having a neutral mid position, a forward operating position and a rear position, second normally open treadle-operated contacts each arranged to be closed in the forward and rear treadle position, respectively, a thread cutting device having electrically controlled actuating means, main clutch means actuated by said treadle in the forward position thereof to couple said motor to said machine shaft for driving the latter at a first operating speed, reduction gear means, and an electrically controlled electromagnetic clutch and brake unit having a coupling winding and a brake winding, respectively, and arranged to connect said motor through said reduction gear means to said machine shaft for selectively driving the latter at a second reduced speed when, in said neutral or rear treadle position, said coupling winding is energized, and for braking said machine shaft when, in said latter treadle positions, said brake winding is energized, respectively, said control circuit comprising a source of current, a needle positioning device including a switch drum coupled to said machine shaft and cooperating with a first contact brush arranged to interrupt its contact each time the sewing machine needle reaches its lowest position, a second contact brush arranged to interrupt its contact each time the needle reaches its highest position and a third contact brush arranged to make contact during a short interval in the course of the needle travel from the lowest to the highest needle position, a first control relay having a relay coil and a first reversal switch contact normally connecting said brake winding to said current source and, on the excitation of said relay coil, connecting said coupling winding to said current source, a normally open second relay switch contact arranged in series with said first switch drum contact brush in a holding circuit for said first control relay, and a normally closed third relay switch contact, said relay coil being connected in series with said first treadle-operated contact across said current source, and a second control relay having a relay coil connected in series with said second treadle contact, said normally closed first control relay third switch contact and said second switch drum contact brush, said second control relay having a reversal switch contact normally connected in series with said first control relay reversal switch contact and so arranged that on the excitation of said second control relay it opens the circuit of said brake winding and connects said coupling winding directly across said current source, said second control relay further having a second normally open contact in series With said third switch drum contact brush, said thread cutting device electrical control means comprising said latter normally open contact.