Patent Publication Number: US-7905189-B2

Title: Bobbin thread winder and sewing machine equipped therewith

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims the benefit of priority from the prior Japanese Patent Application No. 2007-130120, filed on May 16, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     The present disclosure relates to a bobbin thread winder which is provided in a sewing machine arm to be drivingly coupled to a timing pulley driven by a sewing machine motor, thereby winding a thread supplied from a thread supply source on a bobbin, and a sewing machine equipped with the bobbin thread winder. 
     2. Related Art 
     Various types of sewing machines such as lock stitch sewing machines include an arm in which a rotary hook is provided. A bobbin on which a bobbin thread is wound is detachably attached to the rotary hook so that a bobbin thread is supplied thereto. When the bobbin thread has been consumed in a sewing operation, the bobbin is detached and a thread of thread spool is wound on the bobbin by a bobbin thread winder provided in an arm of the sewing machine. 
     For example, Japanese Patent Application Publication, JP-A-H09-313763, discloses a conventional bobbin thread winder which is drivingly coupled to a timing pulley driven by a sewing machine motor, thereby winding a thread supplied from a thread supply source on a bobbin. The above-noted publication corresponds to U.S. Pat. No. 5,816,512. More specifically, the disclosed bobbin thread winder includes a thread winding shaft protruding upward from an upper surface of the arm, a rubber ring provided on a lower end of the thread winding shaft, and a bobbin presser provided near the thread winding shaft. 
     In winding the bobbin thread, the bobbin which has been emptied is put onto the thread winding shaft, and the distal end of the thread drawn from a thread spool is wound onto the bobbin by a small amount. Thereafter, the thread winding shaft is moved to the bobbin presser side. A spring force of a torsion coil spring presses the rubber ring on the lower end of the thread winding shaft against a side of a timing pulley. When the operator then operates a start/stop switch, torque developed by a sewing machine motor rotates the rubber ring, the thread winding shaft and the bobbin together via a pulley, whereby the thread is wound on the bobbin. 
     When an amount of thread wound on the bobbin is increased with progress of the bobbin thread winding, an outer periphery of the wound thread is brought into contact with the bobbin presser. The rotation of the bobbin is continued even after contact of the wound thread with the bobbin presser. Accordingly, when an amount of wound thread is further increased, a relative pressing force is exerted on the thread winding shaft by the bobbin presser. As a result, the thread winding shaft is moved away from the bobbin presser against the spring force of the torsion coil spring. Consequently, a contact force between the rubber ring and the pulley is gradually reduced. When a predetermined amount of thread is finally wound on the bobbin, the rubber ring is moved to a location where the rubber ring departs from the side of the pulley. At last, a rotational force of the pulley is not transmitted to the rubber ring such that rotation of the thread winding shaft is stopped. 
     However, rotation is transmitted by a frictional force between the side of the pulley and the outer periphery of the rubber ring in the above-described conventional bobbin thread winder. Accordingly, since slippage between the side of the pulley and the outer periphery of the rubber ring results in attrition, partial wear of the rubber ring is increased with use for a long period. As a result, rotation of the rubber ring produces rumble and the winding shaft results in irregular rotation. Consequently, there is a possibility that the bobbin thread winding may be disrupted. In this case, a countermeasure such as replacement of the rubber ring needs to be taken. 
     SUMMARY 
     Therefore, an object of the present disclosure is to provide a bobbin thread winder which can ensure stability and durability of a bobbin thread winding operation for a long period of time without use of a consumable part such as the rubber ring and a sewing machine equipped with the bobbin thread winder. 
     The present disclosure provides a bobbin thread winder which is provided in an arm of a sewing machine so as to be supplied with a driving force from a timing pulley to wind a thread supplied from a thread supply onto a bobbin, the bobbin thread winder comprising a thread winding shaft to which the bobbin is detachably set and which is rotated by the timing pulley, the thread winding shaft having an end, a thread winding shaft gear attached to the thread winding shaft and having a first gear portion rotatable relative to the thread winding shaft, a second gear portion provided integrally on the timing pulley so as to be mesh-engageable with the first gear portion of the thread winding shaft, and a torque limiter mechanism transmitting rotation of the thread winding shaft gear to the thread winding shaft when a load exerted on the thread winding shaft is below a predetermined value, the torque limiter mechanism cutting off transmission of rotation of the thread winding shaft gear so that the rotation of the thread winding shaft gear is prevented from being transmitted to the thread winding shaft when the load exerted on the thread winding shaft is not less than the predetermined value, wherein the torque limiter mechanism includes a closely wound portion on a periphery of the thread winding shaft and a coil spring having a spring end engaged with the thread winding shaft gear, the rotation of the thread winding shaft gear is transmitted to the thread winding shaft by a frictional force of the closely wound portion when the load exerted on the thread winding shaft is below the predetermined value, and when the load exerted on the thread winding shaft is not less than the predetermined value, the closely wound portion is loosened such that the rotation transmission is cut off. 
     In the above-described construction, the thread winding shaft gear in mesh-engagement with the second gear portion of the timing pulley is rotated when the timing pulley is rotated during the winding of a bobbin thread. In this case, when a load exerted on the thread winding shaft is below a predetermined value, rotation of the thread winding shaft is transmitted to the thread winding shaft so that the thread from the thread supply is wound on the bobbin attached to the thread winding shaft. On the other hand, when the load exerted on the thread winding shaft is not less than the predetermined value with progress in the winding of the thread onto the bobbin, the torque limiter mechanism prevents transmission of rotation of the thread winding shaft gear to the thread winding shaft. Consequently, rotation of the thread winding shaft and accordingly rotation of the bobbin are stopped, whereby the bobbin thread winding is stopped. 
     Rotation is thus transmitted to the thread winding shaft by mesh-engagement of the second gear portion of the timing pulley and the thread winding shaft gear. Accordingly, differing from the case where rotation is transmitted by a frictional force of a rubber ring, the above-described bobbin thread winder can ensure stability and durability of a bobbin thread winding operation for a long period of time without use of a consumable part such as the rubber ring. Moreover, the bobbin thread winding can automatically be completed by the torque limiter mechanism and accordingly, the bobbin thread winding can be prevented from being excessively carried out. Furthermore, the torque limiting mechanism can be realized by a simple construction. 
     In an embodiment, the thread winding shaft is disposed so as to extend in a direction intersecting a direction of a shaft center of the timing pulley, and each of the first and second gear portions comprises a bevel gear. Consequently, a smooth rotation transmission can be realized. 
     In further another embodiment, the bobbin thread winder further comprises a bobbin presser provided near the thread winding shaft so as to come into contact with a periphery of the wound thread thereby to increase a rotating torque of the thread winding shaft when a diameter of a thread winding on the bobbin is increased. When an amount of thread wound on the bobbin is at or above a predetermined value, a frictional force between the bobbin and the bobbin presser rapidly increases the torque of thread winding shaft or load, whereupon the torque limiter mechanism can be operated. 
     Incidentally, the torque limiter mechanism is desirable to operate when an amount of thread wound on the bobbin reaches a range from 70% to 90%, for example. In this case, the rotational load ranges from 130 to 160 gf·cm, for example. 
     The disclosure also provides a bobbin thread winder which is provided in an arm of a sewing machine so as to be supplied with a driving force from a timing pulley to wind a thread supplied from a thread supply onto a bobbin, the bobbin thread winder comprising a thread winding shaft to which the bobbin is detachably set and which is rotated by the timing pulley, the thread winding shaft having an end, a thread winding shaft gear fixedly provided on the end side of the thread winding shaft and having a first gear portion, a timing pulley gear provided on a side surface of the timing pulley so as to be rotatable relative to the timing pulley, the timing pulley gear having a second gear portion mesh-engageable with the thread winding shaft gear, and a torque limiter mechanism transmitting rotation of the timing pulley to the timing pulley gear when a load exerted on the thread winding shaft is below a predetermined value, the torque limiter mechanism cutting off transmission of rotation of the timing pulley so that the rotation of the timing pulley is prevented from being transmitted to the timing pulley gear when the load exerted on the thread winding shaft is not less than the predetermined value, wherein the torque limiter mechanism includes a cylindrical coil holder formed integrally on the timing pulley and a coil spring having a closely wound portion closely wound on a periphery of the coil holder and an end engaged with the timing pulley gear, the torque limiter mechanism transmits rotation of the timing pulley to the timing pulley gear by a frictional force of the closely wound portion when a load exerted on the thread winding shaft is below a predetermined value, and when the load exerted on the thread winding shaft is not less than the predetermined value, the torque limiter mechanism cuts off transmission of rotation of the timing pulley so that the rotation of the timing pulley is prevented from being transmitted to the timing pulley gear. 
     In this construction, the thread winding shaft gear in mesh-engagement with the second gear portion of the timing pulley is rotated when the timing pulley is rotated during the winding of a bobbin thread. In this case, when a load exerted on the thread winding shaft is below a predetermined value, rotation of the thread winding shaft is transmitted to the thread winding shaft so that the thread from the thread supply is wound on the bobbin attached to the thread winding shaft. On the other hand, when the load exerted on the thread winding shaft is not less than the predetermined value with progress in the winding of the thread onto the bobbin, the torque limiter mechanism prevents transmission of torque of the timing pulley to the timing pulley gear. Consequently, rotation of the thread winding shaft and accordingly rotation of the bobbin are stopped, whereby the bobbin thread winding is stopped. 
     Rotation is thus transmitted to the thread winding shaft by mesh-engagement of the first and second gear portions of the thread winding shaft gear and the timing pulley. Accordingly, differing from the case where rotation is transmitted by a frictional force of a rubber ring, the above-described bobbin thread winder can ensure stability and durability of a bobbin thread winding operation for a long period of time without use of a consumable part such as the rubber ring. Moreover, the bobbin thread winding can automatically be completed by the torque limiter mechanism and accordingly, the bobbin thread winding can be prevented from being excessively carried out. 
     In one embodiment, the thread winding shaft is disposed so as to extend in a direction intersecting a direction of a shaft center of the timing pulley, and each of the first and second gear portions of the thread winding shaft gear and the timing pulley gear respectively comprises a bevel gear. Consequently, a smooth rotation transmission can be realized. 
     In further another embodiment, the bobbin thread winder further comprises a bobbin presser provided near the thread winding shaft so as to come into contact with a periphery of the wound thread thereby to increase a rotating torque of the thread winding shaft when a diameter of a thread winding on the bobbin is increased. When an amount of thread wound on the bobbin is at or above a predetermined value, a frictional force between the bobbin and the bobbin presser rapidly increases the torque of thread winding shaft or load, whereupon the torque limiter mechanism can be operated. 
     The disclosure further provides a sewing machine comprising an arm, a sewing machine motor, a timing pulley transmitting torque of the sewing machine motor to a main shaft, a bobbin thread winder which is provided in the arm so as to be supplied with a driving force from the timing pulley to wind a thread supplied from a thread supply onto a bobbin, the bobbin thread winder comprising a thread winding shaft to which the bobbin is detachably set and which is rotated by the timing pulley, the thread winding shaft having an end, a thread winding shaft gear fixedly provided on the end side of the thread winding shaft and having a first gear portion, a timing pulley gear provided on a side surface of the timing pulley so as to be rotatable relative to the timing pulley, the timing pulley gear having a second gear portion mesh-engageable with the thread winding shaft gear, and a torque limiter mechanism transmitting rotation of the timing pulley to the timing pulley gear when a load exerted on the thread winding shaft is below a predetermined value, the torque limiter mechanism cutting off transmission of rotation of the timing pulley so that the rotation of the timing pulley is prevented from being transmitted to the timing pulley gear when the load exerted on the thread winding shaft is not less than the predetermined value, wherein the torque limiter mechanism includes a closely wound portion on a periphery of the thread winding shaft and a coil spring having a spring end engaged with the thread winding shaft gear, the rotation of the thread winding shaft gear is transmitted to the thread winding shaft by a frictional force of the closely wound portion when the load exerted on the thread winding shaft is below the predetermined value, and when the load exerted on the thread winding shaft is not less than the predetermined value, the closely wound portion is loosened such that the rotation transmission is cut off. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become clear upon reviewing the following description of one embodiment with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a sewing machine of a first illustrative example; 
         FIG. 2  is a perspective view of the sewing machine as viewed at a different angle from  FIG. 1  with a face plate being detached; 
         FIG. 3  is a longitudinal front section of the bobbin thread winder; 
         FIG. 4  is a top view of the bobbin thread winder during winding of the bobbin thread; 
         FIG. 5  is also a top view of the bobbin thread winder during sewing; and 
         FIG. 6  is a perspective view of a sewing machine of a second illustrative example. 
     
    
    
     DETAILED DESCRIPTION 
     A first embodiment in which the invention is applied to a household lock-stitch sewing machine will be described with reference to  FIGS. 1 to 5 . Referring to  FIGS. 1 and 2 , an overall construction of the lock-stitch sewing machine M is shown. The sewing machine M includes a body comprising a bed  1  extending in the right and left direction (the X direction), a pillar  2  standing upward from a right end of the bed  1  and an arm  3  extending leftward from an upper end of the pillar  2  all of which are formed integrally. In the following description, the X direction will refer to the right and left direction of the sewing machine M and the Y direction will refer to a front and back direction of the sewing machine M as shown in  FIGS. 1 and 2 . 
     A main shaft  12  is mounted so as to extend in the right and left direction in the arm  3  as shown in  FIGS. 2 and 3 . The arm  3  has a distal end or left end on which a head  3   a  is provided. A needle bar  6  having a needle  5  is mounted on the head  3   a . Furthermore, in the arm  3  are provided a needle bar drive mechanism which converts rotation of the main shaft  12  to vertical movement of the needle bar  6 , a needle thread take-up drive mechanism which vertically moves a needle thread take-up  7  (see  FIG. 1 ) in synchronization with the needle bar  6 , and the like. Switches are mounted on a front of the arm  3  as shown in  FIG. 1 . The switches include a start/stop switch  4  for instructing start and stop of a sewing work. 
     A needle plate  1   a  is mounted on an upper surface of the bed  1  so as to correspond to the needle  5 . In the bed  1  are provided a rotary hook which is located below the needle plate  1   a  and forms stitches in cooperation with the needle  5 , a feed dog drive mechanism and the like although not shown. A bobbin  8  on which a bobbin thread is wound is detachably attached to the rotary hook.  FIG. 3  shows the bobbin  8  while the bobbin thread is wound. A sewing machine motor  19  is mounted on a bottom of the pillar  2  as shown in  FIG. 2 . A timing pulley  11  is provided in an upper interior of the pillar  2  for rotating the main shaft  12 . A timing belt  10  extends between the sewing machine motor  19  and the timing pulley  11 . When the start/stop switch  4  is operated for start of the sewing machine M, the sewing machine motor  19  is driven so that the timing pulley  11  is rotated via the timing belt  10 . When the start/stop switch  4  is operated for stop of the sewing machine M, the sewing machine motor  19  is stopped. 
     A driving force of the timing pulley  11  is adapted to be transmitted via a clutch mechanism  15  (see  FIG. 3 ) to the main shaft  12 . In this case, the timing pulley  11  is coupled to the main shaft  12  by the clutch mechanism  15  in a normal sewing mode (the state where a thread winding shaft  21  which will be described later assumes a left sewing position) so that the driving force is transmitted to the main shaft  12 . As a result, the main shaft  12  is rotated in the direction shown by arrow A in  FIG. 3 . Furthermore, when the bobbin thread is wound with the thread winding shaft  21  assuming a right bobbin thread winding position, the timing pulley  11  is decoupled from the main shaft  12  so that the thread winding shaft  21  of a bobbin thread winder  20  is rotated. 
     The clutch mechanism  15  will now be described. The clutch mechanism  15  comprises a clutch member  16  as shown by two-dot chain line in  FIG. 3 , a transfer pin  17  mounted on the main shaft  12  and a compression coil spring  18  as shown by two-dot chain line in  FIG. 3 . The aforesaid timing pulley  11  has a cylindrical support  11   a  formed integrally on a central part thereof and fitted on the right end of the main shaft  12  so as to be rotatable relative to the main shaft  12 . The timing pulley  11  is positioned by the transfer pin  17  and a retaining ring  13  with respect to the right and left direction as will be described later and supported so as to be immovable in the right and left direction relative to the main shaft  12 . The cylindrical support  11   a  has an outer circumference formed with a plurality of axial (horizontal) gear (not shown). 
     The clutch member  16  is formed into a cylindrical shape and extends in the right and left direction. The clutch member  16  has a left end with a flange  16   a  which is formed integrally with the end and has a large diameter. The clutch member  16  further has an inner circumference formed with a plurality of axial (horizontal) recessed grooves (not shown) corresponding to the gear of the cylindrical support  11   a . As a result, the recessed grooves of the clutch member  16  are engaged with the gear of the cylindrical support  11   a  such that the clutch member  16  is splined to the cylindrical support  11   a , whereupon the clutch member  16  is slidable in the direction of shaft center of the main shaft  12 . 
     The transfer pin  17  is secured to the main shaft  12  so as to extend therethrough in order to transfer rotation of the timing pulley  11  via the clutch member  16  to the main shaft  12 . On the other hand, the flange  16   a  of the clutch member  16  is formed with a coupling recess  16   b  into which the transfer pin  17  is fitted. The aforesaid compressed coil spring  18  is interposed between the timing pulley  11  and the clutch member  16 . The clutch member  16  is normally biased by a spring force of the compressed coil spring  18  leftward or in the direction of engaging the transfer pin  17 . In this case, the thread winding shaft  21  is spaced away leftward from the flange  16   a  when located at a left sewing position as shown in  FIG. 5 , so that the coupling recess  16   b  of the clutch member  16  engages the transfer pin  17 . Consequently, the driving force of the timing pulley  11  is transferred to the main shaft  12  so that the main shaft  12  is rotated together with the timing pulley  11 . 
     On the other hand, when the thread winding shaft  21  is manually moved to a right bobbin thread winding position by the operator or user, a lower end of the thread winding shaft  21  presses the flange  16   a  rightward thereby to move the clutch member  16  rightward, as shown in  FIGS. 3 and 4 . As a result, the clutch member  16  is disengaged from the transfer pin  17 . Thus, the driving force of the timing pulley  11  is not transferred to the main shaft  12  but is transferred to the thread winding shaft  21  via a torque limiter mechanism  23  which will be described later. 
     The bobbin thread winder  20  is provided on an upper right end of the arm  3  for winding a thread T on the bobbin  8 . A thread spool set portion  9  is provided on the upper surface of the arm  3  as shown in  FIGS. 1 and 2 . A thread spool (not shown) serving as a thread supply source is set on the thread spool set portion  9 . 
     The bobbin thread winder  20  will now be described with reference to  FIGS. 3 to 5 . The bobbin thread winder  20  comprises the timing pulley  11 , the thread winding shaft  21 , a thread winding shaft gear  22 , the torque limiter mechanism  23 , an attachment plate  26 , a swing arm  27  and a bobbin presser  33 . A ring-shaped gear portion  11   b  comprising a bevel gear is coaxially formed on a left side of the timing pulley  11  so as to be located outside the cylindrical support  11   a  as shown in  FIG. 3 . 
     Referring to  FIGS. 4 and 5 , the attachment plate  26  is formed into an approximately inverted C-shaped plate as viewed on a plane. The attachment plate  26  is fixed to a frame of the arm  3  in a horizontal state as shown in  FIG. 3 . The swing arm  27  has a central part nearer to the rear thereof, which part is supported on a rear side of the attachment plate  26  so as to be pivotable about a shaft  27   a  in the direction of arrow C or D, as shown in  FIGS. 4 and 5 . A two-position switching spring  29  extends between a front end side of the attachment plate  26  and a front end of the swing arm  27 . The switching spring  29  comprises a toggle spring and is adapted to be switched between a right swing position as shown in  FIG. 4  and a left swing position as shown in  FIG. 5 . 
     The vertically extending thread winding shaft  21  is rotatably mounted on the front end of the swing arm  27 . The thread winding shaft  21  extends in a vertical direction intersecting an axial direction (horizontal direction) of the timing pulley  11  and provided with the thread winding shaft gear  22  mounted on the lower end side of the timing pulley  11  for relative rotation. A pair of retaining rings  35  are fixed to an outer circumference of the thread winding shaft  21  so as to vertically sandwich the swing arm  27  and the thread winding shaft gear  22  therebetween, whereupon the retaining rings  35  are held so as to be axially immovable relative to the swing arm  27 . Furthermore, a positioning cam  30  is fixed to an upper part of the upper retaining ring  35  of the thread winding shaft  21 . 
     A cam abutment plate  28  is fixed to an upper surface of the attachment plate  26  by a fixing screw  28   a  so as to correspond to the positioning cam  30  as shown in  FIGS. 4 and 5 . When the swing arm  27  is located at the left swing position as shown in  FIG. 5 , the positioning cam  30  abuts against the cam abutment plate  28  such that rotation of the thread winding shaft  21  is stopped. When the swing arm  27  is located at the right swing position as shown in  FIG. 4 , the thread winding shaft  21  is situated at a bobbin thread winding position. When the swing arm  27  is swung to the left swing position as shown in  FIG. 5 , the thread winding shaft  21  is situated at a sewing position. 
     A circular bobbin receiving plate  24  is mounted on a portion of the thread winding shaft  21  near the upper end thereof as shown in  FIGS. 1 to 3 . The bobbin receiving plate  24  receives the underside of the bobbin  8 .  FIGS. 4 and 5  show the thread winding shaft  21  with the bobbin receiving plate  24  being detached therefrom. A bobbin holding spring  25  is mounted on the upper end of the thread winding shaft  21  as shown in  FIGS. 4 and 5 . Accordingly, when the bobbin  8  is attached onto the bobbin receiving plate  24  of the thread winding shaft  21 , the bobbin holding spring  25  engages an engagement recess (not shown) provided in the bobbin  8  so that the bobbin  8  is locked so as not to fall off from the thread winding shaft  21  thereby to be rotated together with the shaft. 
     The thread winding shaft gear  22  has a cylindrical portion  22   c  which is fitted in a cylindrical portion formed in a front-end side underside of the swing arm  27  as shown in  FIG. 3 . The thread winding shaft gear  22  is attached to the lower end of the thread winding shaft  21  so as to be capable of relative rotation. The thread winding shaft  22  is held by the lower end of the swing arm  27  and the lower retaining ring  35  so as to be axially immovable. The thread winding shaft  22  has a lower end on which a gear portion  22   a  is formed integrally. The gear portion  22   a  comprises an annular bevel gear. As a result, when the thread winding shaft  21  is moved to the right bobbin thread winding position, the gear portion  22   a  of the thread winding shaft gear  22  is adapted to be brought into mesh engagement with the gear portion  11   b  of the timing pulley  11 . When the gear portion  22   a  is thus in mesh engagement with the gear portion  11   b , rotation about the horizontal shaft of the timing pulley  11  in the direction of arrow A is converted to rotation about the vertical shaft of the thread winding shaft gear  22  in the direction of arrow B. 
     The torque limiter mechanism  23  is provided between the thread winding shaft gear  22  and the thread winding shaft  21 . The torque limiter mechanism  23  transfers rotation of the thread winding shaft gear  22  to the thread winding shaft  21  when the load of the thread winding shaft  21  is less than a predetermined value. When the load of the thread winding shaft  21  is no less than the predetermined value, the torque limiter mechanism  23  cuts off rotation of the thread winding shaft gear  22  so that the rotation is not transferred to the thread winding shaft  21 . 
     In the embodiment, the torque limiter mechanism  23  includes a coil spring  32  wound closely on the outer circumference of the thread winding shaft  21  in order that rotary torque may be transmitted from the thread winding shaft gear  22  to the thread winding shaft  21 . More specifically, the coil spring  32  is wound on the outer circumference of the thread winding shaft  21  counterclockwise as viewed from the top and is disposed so as to be housed in the cylindrical portion  22   c  of the thread winding shaft gear  22 . The coil spring  32  has a lower spring end in engagement with an engagement groove  22   b  formed in the cylindrical portion  22   c . Furthermore, the coil spring  32  in a free state has an inner diameter of the coil set to be slightly smaller than an outer diameter of the thread winding shaft  21 . Accordingly, since the coil spring  32  is attached to the thread winding shaft  21  so as to tighten up the shaft  21 , the coil spring  32  rotates the thread winding shaft  21  while receiving a spring loosening force directed so as to loosen the coil spring (the direction in which the inner diameter of the coil spring is enlarged). 
     When the timing pulley  11  is rotated in the direction of arrow A in  FIG. 3  during winding of the bobbin thread, the thread winding shaft gear  22  in mesh engagement with the timing pulley is rotated in the direction of arrow B in  FIG. 4  (clockwise as viewed on a plane). When load of the thread winding shaft  21  is smaller than the predetermined value with rotation of the gear  22 , rotation of the thread winding shaft gear  22  is transferred via the coil spring  32  to the thread winding shaft  21  so that the thread winding shaft  21  is rotated in the direction of arrow B. As a result, the thread T is drawn from a thread spool set in the thread spool set portion  9  to be wound on the bobbin  8  attached on the thread winding shaft  21 . A predetermined amount of thread T is wound on the bobbin  8  in a bobbin thread winding completion state. The predetermined amount of thread T ranges from 70% to 90% of an amount of thread wound on the bobbin  8 , for example. When the bobbin  8  comes to the bobbin thread winding completion state, the periphery of the thread T on the bobbin  8  is brought into contact with the bobbin presser  33 . Then, a braking force is applied to the thread winding shaft  21  as the result of frictional resistance between the bobbin presser  33  and the thread T. Consequently, a rotational load of the thread winding shaft  21  is gradually increased. When the rotational load of the thread winding shaft  21  is equal to or larger than a predetermined value (ranging from 130 to 160 gf·cm, for example), the inner diameter of the coil spring  32  is enlarged. As a result, a tightening force of the coil spring  32  against the thread winding shaft  21  is reduced such that the coil spring  32  starts to slip on the thread winding shaft  21  and finally runs idle. Consequently, rotation of the thread winding shaft gear  22  is no longer transmitted to the thread winding shaft  21 . 
     The bobbin thread winder  20  constructed as described above will operates as follows. When the bobbin thread of the bobbin  8  has been used up, the operator detaches the bobbin  8  from a rotary hook (not shown) and attaches the bobbin  8  to the thread winding shaft  21  while the sewing machine M (sewing machine motor  19 ) is in a stopped state. With this, the operator sets a thread spool in the thread spool set portion  9 , drawing out the distal end of the thread T from the thread spool to be wound on the bobbin  8  several turns. The thread winding shaft  21  is manually moved to the right bobbin thread winding position (see  FIG. 4 ). In this case, the swing arm  27  is moved from the left swing position (see  FIG. 5 ) to the right swing position as shown in  FIG. 4 , and the thread winding shaft  21  is displaced to the right bobbin thread winding position. Accordingly, the thread winding shaft gear  22  is brought into mesh engagement with the gear portion  11   b  of the timing pulley  11 , whereas the clutch member  16  is moved rightward thereby to be disengaged from the transfer pin  17 . 
     When the operator turns on the start/stop switch under the above-described condition, the sewing machine motor  19  is driven so that the timing pulley  11  is rotated via the timing pulley  11  in the direction of arrow A in  FIG. 3  and the thread winding shaft gear  22  in mesh engagement with the timing pulley  11  is rotated in the direction of arrow B in  FIGS. 3 and 4 . In this case, when winding of the bobbin thread starts, an amount of thread T wound on the bobbin  8  is small and the rotational load of the thread winding shaft  21  is accordingly small. As a result, the rotation of the thread winding shaft  21  is transmitted via the coil spring  32  to the thread winding shaft  21  such that the thread winding shaft  21  is rotated, whereupon the thread T is kept wound onto the bobbin  8 . 
     Subsequently, when the bobbin  8  comes near to the bobbin thread winding completion state, the braking force is applied to the thread winding shaft  21  as the result of frictional resistance between the bobbin presser  33  and the thread T, whereupon the rotational load of the thread winding shaft  21  is gradually increased. When the rotational load of the thread winding shaft  21  becomes equal to or larger than the predetermined value (ranging from 130 to 160 gf·cm, for example), the inner diameter of the coil spring  32  is enlarged. As a result, a tightening force of the coil spring  32  against the thread winding shaft  21  is reduced such that the coil spring  32  starts to slip on the thread winding shaft  21  and finally runs idle. Consequently, when the predetermined amount of thread T has been wound on the bobbin  8 , transmission of rotation of the thread winding shaft gear  22  is cut off by the coil spring  32  (the torque limiter mechanism  23 ) so that the rotation of the thread winding shaft gear  22  is prevented from being transmitted to the thread winding shaft  21 . The operator turns off the start/stop switch  4  when confirming that rotation of the thread winding shaft  21  (the bobbin  8 ) has been stopped such that winding of the bobbin thread has been completed. Subsequently, the bobbin  8  on which the thread T has been wound is detached from the thread winding shaft  21 , and the thread winding shaft  21  is moved from the bobbin thread winding position to the leftward sewing position. As a result, the swing arm  27  is switched to the left swing position as shown in  FIG. 5  such that the thread winding shaft gear  22  is released from the mesh engagement with the gear portion  11   b . Furthermore, the clutch member  16  is displaced to the left so that the timing pulley  11  and the main shaft  12  are coupled together, whereby the sewing operation is allowed. 
     According to the bobbin thread winder  20  of the embodiment, rotation is transmitted to the thread winding shaft  21  by the mesh engagement of the gear portion  11   b  of the timing pulley  11  and the gear portion  22   a  of the thread winding shaft gear  22 . Accordingly, differing from the conventional construction in which rotation is transmitted by the frictional force of a rubber ring, the above-described bobbin thread winder  20  excludes a wear-out part such as the rubber ring. Consequently, the stability and durability of the bobbin thread winding operation can be ensured for a long period of time. In this case, since each of gear portion  11   b  of the timing pulley  11  and the gear portion  22   a  of the thread winding shaft gear  22  comprises the bevel gear, smooth rotation transmission can be realized even when the axes of the timing pulley  11  and the thread winding shaft gear  22  are at right angles to each other. Moreover, the bobbin thread winding operation is automatically completed by the torque limiter mechanism  23 , the bobbin thread winding operation can be prevented from being executed excessively. Since the torque limiter mechanism  23  comprises the coil spring  32  particularly in the foregoing embodiment, the torque limiter mechanism  23  can be formed into a simple construction at lower costs. Furthermore, the bobbin presser  33  is provided so as to be brought into contact with the outer periphery of the wound thread T, thereby increasing the rotational torque. Consequently, when the bobbin thread winding completion state comes near, the rotational torque or the load of the thread winding shaft  21  is quickly increased such that the torque limiter mechanism  23  can quickly be operated advantageously. 
     The bobbin thread winder  20 A of a second embodiment will be described with reference to  FIG. 6 . In the second embodiment, identical or similar parts are labeled by the same reference symbols as those in the first embodiment, and only the difference of the second embodiment from the first embodiment will be described. In the bobbin thread winder  20 A of the second embodiment, the rotational force of the timing pulley  11 A is transmitted via the coil spring  34  serving as the torque limiter mechanism  23 A to the timing pulley gear  14 . The thread winding shaft gear  22 A and the thread winding shaft  21  are driven by rotation of the timing pulley gear  14 . 
     The clutch mechanism  15 A is provided on the timing pulley  11 A. The rotational force of the timing pulley  11 A rotated by the sewing machine motor  19  is transmitted to the main shaft  12  during sewing. On the other hand, the timing pulley  11 A is decoupled from the main shaft  12  during the bobbin thread winding and the thread winding shaft  21  is rotated. The clutch mechanism  15 A comprises a coupling pulley  11 B coupled to the left side of the timing pulley  11 A, the clutch member  16 A is splined to the coupling pulley  11 B, and the transfer pin  17  provided on the main shaft  12 . The coupling pulley  11 B is formed into a cylindrical shape and is rotatably fitted with the main shaft  12 . The coupling pulley  11 B has an arc protrusion  11   d  formed integrally therewith so as to protrude rightward from the right end surface. On the other hand, an arc recess  11   e  is formed in a right side of the timing pulley  11 A. The protrusion  11   d  of the coupling pulley  11 B is engaged with the arc recess  11   e  so that the coupling pulley  11 B is rotatable together with the timing pulley  11 A. The timing pulley  11 A and the coupling pulley  11 B coupled to the timing pulley  11 A are positioned in the right and left direction by the transfer pin  17  and supported so as to be immovable in the right and left direction relative to the main shaft  12 . 
     The coupling pulley  11 B has an outer circumference formed with a plurality of axially extending gear (not shown). The clutch member  16 A has an inner circumference formed with a plurality of axially extending recessed grooves (not shown) and Corresponding to the gear of the coupling pulley  11 B. The gear of the coupling pulley  11 B are splined to the recessed grooves of the clutch member  16 A so that the clutch member  16 A is slidable in the direction of the shaft center of the main shaft  12 . Furthermore, the transfer pin  17  is secured to the main shaft  12  so as to extend through the main shaft  12 . The flange  16   a  of the left end of the clutch member  16 A is formed with a coupling recess  16   b  into which the transfer pin  17  is fitted. The clutch member  16 A is normally biased by the spring force of the compression coil spring  18  leftward or in such a direction that the clutch member  16 A engages the transfer pin  17 . The compression coil spring  18  is interposed between the coupling pulley  11 B and the clutch member  16 A. 
     The transfer pin  17  engages the coupling recess  16   b  of the clutch member  16 A since the swing arm  27  is located at the swing position (see  FIG. 5 ) and the thread winding shaft  21  is located at the left sewing position during sewing. Accordingly, the rotational force of the sewing machine motor  19  is transmitted via the timing belt  10  to the timing pulley  11 A and the coupling pulley  11 B. The rotational force of the sewing machine motor  19  is further transmitted via engagement of the clutch member  16   a  and the transfer pin  17  to the main shaft  12 . 
     On the other hand, when the thread T is wound on the bobbin  8 , the swing arm  27  is swung to the right swing position (see  FIG. 4 ) so that the thread winding shaft  21  is moved to the bobbin thread winding position. The lower end of the thread winding shaft  21  presses the flange  16   a  rightward so that the clutch member  16 A is moved rightward. As a result, the clutch member  16 A and the transfer pin  17  are released from engagement, whereby the rotational force of the timing pulley  11 A is not transmitted to the main shaft  12 . Consequently, the main shaft  12  is stopped. Under this condition, the rotational force of the timing pulley  11 A is transmitted via the torque limiter mechanism  23 A to the timing pulley gear  14  and the thread winding shaft gear  22 , so that the thread winding shaft  21  is rotated. 
     The bobbin thread winder  20 A of the second embodiment comprises the timing pulley  11 A, the timing pulley gear  14 , the thread winding shaft  21 , the thread winding shaft gear  22 A secured to the lower end of the thread winding shaft  21 , the torque limiter mechanism  23 A, the attachment plate  26  and the swing arm  27 . On the attachment plate  27  are provided the swing arm  27 , the cam abutment plate  28 , the two-position switching spring  29 , the thread winding shaft  21 , and the thread winding shaft gear  22 A. The thread winding shaft gear  22 A has a cylindrical portion  22   c  which is fitted in a cylindrical portion formed in a front-end side underside of the swing arm  27 . The thread winding shaft  22 A has a lower end on which the gear portion  22   a  is formed integrally. The gear portion  22   a  comprises an annular bevel gear. The thread winding shaft gear  22 A is secured to the lower end of the thread winding shaft  21 . 
     On the other hand, a cylindrical coil receiving portion  11   c  formed integrally on the left side of the timing pulley  11 A. The coil receiving portion  11   c  is located at the outer circumferential side relative to the coupling pulley  11 B so as to be coaxial with the coupling pulley  11 B. A timing pulley gear  14  is provided on the left side of the timing pulley  11 A. The timing pulley gear  14  is generally formed into a cylindrical shape as a whole and has a cylindrical portion  14   a  rotatably fitted in the inner circumference of the coil receiving portion  11   c , an annular engagement portion  14   b  extending from a right end of the cylindrical portion  14   a  toward the inner circumference of the coil receiving portion  11   c , a bent portion  14   c  bent to the side of an outer circumference, and a gear portion  14   d  formed on the left outer circumference of the bent portion  14   c  and comprising an annular bevel gear. These portions  14   a  to  14   d  are formed integrally. 
     The annular engagement portion  14   b  is held between the timing pulley  11 A and the coupling portion, whereupon the timing pulley gear  14  is supported on the timing pulley  11 A so as to be axially immovable and rotatable relative to the timing pulley  11 A. The gear portion  22   a  of the thread winding shaft gear  22 A is adapted to be brought into mesh engagement with the annular gear formed on the left end side of the timing pulley gear  14  when the thread winding shaft  21  is moved to the right bobbin thread winding position. 
     The torque limiter mechanism  23 A is provided between the timing pulley  11 A and the timing pulley gear  14 . The torque limiter mechanism  23 A comprises a coil spring  34  closely wound on the outer circumference of the coil receiving portion  11   c . The coil spring  34  is wound on the outer circumference of the coil receiving portion  11   c  counterclockwise as viewed from the right (the same direction as a rotational direction of the timing pulley  11 A). A left spring end of the coil spring  34  is in engagement with an engagement groove  14   c  of timing pulley gear  14 . Furthermore, the coil spring  34  in a free state has an inner diameter of the coil set to be slightly smaller than an outer diameter of the coil receiving portion  11   c . In this case, too, since the coil spring  34  is attached to the thread winding shaft  21  so as to tighten up the coil receiving portion  11   c , the coil spring  34  rotates the timing pulley gear  14  while receiving a spring loosening force directed so as to loosen the coil spring (the direction in which the inner diameter of the coil spring is enlarged). When load of the thread winding shaft  21  is smaller than the predetermined value, rotation of the timing pulley  11 A is transmitted via the timing pulley gear  14  and the thread winding shaft gear  22 A to the thread winding shaft  21 , so that the thread winding shaft  21  is rotated in the direction of arrow B in  FIG. 6 . Thus, the bobbin thread winding operation is carried out. 
     On the other hand, the frictional resistance between the bobbin presser  33  and the thread T in the bobbin thread winding complete state in which a predetermined amount of thread T is wound on the bobbin  8 . The rotational load of the thread winding shaft  21  is gradually increased by the frictional resistance between the bobbin presser  33  and the thread T. When the rotational load of the thread winding shaft  21  is equal to or larger than a predetermined value (ranging from 130 to 160 gf·cm, for example), the inner diameter of the coil spring  34  is enlarged. As a result, the coil spring  34  starts to slip on the coil receiving portion  11   c , finally running idle. Consequently, rotation of the timing pulley  11   a  is no longer transmitted to the thread winding shaft  21 . 
     According to the bobbin thread winder  20 A thus constructed, the gear  22   a  of the thread winding shaft gear  22 A and the gear  14   b  of the timing pulley gear  14  are brought into mesh engagement with each other when the operator attaches the bobbin  8  to the thread winding shaft  21 , winds the distal end of the thread T from the thread spool on the bobbin  8  and moves the thread winding shaft  21  to the bobbin thread winding position, in the same manner as in the first embodiment. Consequently, the clutch member  16 A and the transfer pin  17  are released from the engagement. 
     When the operator turns on the start/stop switch under the above-described condition, the sewing machine motor  19  is driven so that the timing pulley  11 A is rotated via the timing pulley  11  in the direction of arrow A in  FIG. 6 . Since the rotational load of the thread winding shaft  21  is small in the initial stage of bobbin thread winding, the rotation of the timing pulley  11 A is transmitted via the coil spring  34  to the timing pulley gear  14 . As a result, the thread winding shaft  21  is rotated via the thread winding shaft gear  22 A so that the thread T is wound on the bobbin  8 . Thereafter, when the rotational load of the thread winding shaft  21  is equal to or larger than a predetermined value (ranging from 130 to 160 gf·cm, for example), the inner diameter of the coil spring  34  biased in the direction of loosening the spring is enlarged. As a result, the coil spring  34  starts to slip on the thread winding shaft  21  and finally runs idle. Consequently, when the predetermined amount of thread T has been wound on the bobbin  8 , the rotational force of the timing pulley  11 A is cut off by the coil spring  34  (the torque limiter mechanism  23 A) so that the rotational force is prevented from being transmitted to the thread winding shaft  21 . The operator turns off the start/stop switch  4  when confirming that rotation of the thread winding shaft  21  (the bobbin  8 ) has been stopped such that winding of the bobbin thread has been completed. The thread winding shaft  21  is then moved from the bobbin thread winding position to the left sewing position. The thread winding shaft gear  22 A and the timing pulley gear  14  are then released form the mesh engagement, and the clutch member  16 A is moved leftward so that the timing pulley  11 A and the main shaft  12  are coupled together, whereby the sewing operation is allowed. 
     According to the bobbin thread winder  20 A of the second embodiment, too, rotation is transmitted to the thread winding shaft  21  by the mesh engagement of the gear portion  14   d  of the timing pulley gear  14  and the gear portion  22   a  of the thread winding shaft gear  22 A as in the first embodiment. Accordingly, differing from the conventional construction in which rotation is transmitted by the frictional force of a rubber ring, the above-described bobbin thread winder  20 A excludes a wear-out part such as the rubber ring. Consequently, the stability and durability of the bobbin thread winding operation can be ensured for a long period of time. In this case, since each of gear portions  14   d  and  22   a  of the timing pulley gear  14  and the thread winding shaft gear  22 A comprises the bevel gear, smooth rotation transmission can be realized even when the axes of the timing pulley  11 A and the thread winding shaft gear  22 A are at right angles to each other. Moreover, the bobbin thread winding operation is automatically completed by the torque limiter mechanism  23 A, the bobbin thread winding operation can be prevented from being executed excessively. Since the torque limiter mechanism  23 A comprises the coil spring  34  particularly in the foregoing embodiment, the torque limiter mechanism  23 A can be formed into a simple construction at lower costs. Furthermore, the bobbin presser  33  is provided so as to be brought into contact with the outer periphery of the wound thread T, thereby increasing the rotational torque. Consequently, when the bobbin thread winding completion state comes near, the rotational torque or the load of the thread winding shaft  21  is quickly increased such that the torque limiter mechanism  23 A can quickly be operated advantageously. 
     Several modified forms of the foregoing embodiments will be described. Although the bobbin presser  33  is provided in each foregoing embodiment, the bobbin presser  33  may be eliminated. In this case, tension of the thread T supplied from the thread spool increases the rotational moment as a winding diameter of the thread T wound on the bobbin  8  is increased. This means that the rotational load of the thread winding shaft  21  is increased according to the winding diameter of the thread T wound on the bobbin B. Accordingly, the torque limiter mechanism can be configured so as to operate when an amount of thread wound on the bobbin reaches a range from 70% to 90%, for example. 
     Regarding the coil springs  32  and  34  of the torque limiter mechanisms  23  and  23 A, a spring wire diameter, an inner diameter of coil and a spring constant are set in view of various thread winding conditions including rotational speeds of thread winding shaft gears  22  and  22 A during thread winding, the magnitude of rotational load of the thread winding shaft  21  at the time of completion of bobbin thread winding and the like. Various types of springs can be employed. 
     The foregoing second embodiment describes shapes and the coupling relationship of the timing pulley  11 A and the timing pulley gear  14  and the location of the coil spring  34 . However, these are mere examples and can be changed according to use conditions. 
     Although the invention is applied to the household lock-stitch sewing machine M in each foregoing embodiment, the invention may be applied to various types of sewing machines. 
     The foregoing description and drawings are merely illustrative of the principles of the present invention and are not to be construed in a limiting sense. Various changes and modifications will become apparent to those of ordinary skill in the art. All such changes and modifications are seen to fall within the scope of the invention as defined by the appended claims.