Device for adjusting a lower thread in a sewing machine

A lower thread tension adjusting device for a sewing machine comprising first and second tension arrangements mounted on a bobbin carrier of the sewing machine for tensioning a lower thread with a predetermined tension and a predetermined additional tension, respectively. A positioning device is mounted on the bobbin carrier and movable between first and second set positions in which the positioning devices moves the second tension arrangement to operative and inoperative positions wherein the second arrangement provides and does not provide the predetermined additional tension of the lower thread. A guide assembly guides the lower thread to positions adjacent to one end and a center of a laterally elongated needle hole and simultaneously guides the positioning device to the first and second positions thereof. A control unit moves the guide assembly between first and second guide positions thereof in accordance with selected patterns of straight and zigzag stitches, respectively.

FIELD OF THE INVENTION 
The invention relates to a sewing machine and more particularly relates to 
a device for adjusting tension of a lower thread and, also, for changing a 
lower thread path from a thread source to a fabric to be sewn in 
dependence upon selection of a pattern of straight stitches or of a 
pattern of zigzag stitches, to thereby selectively provide a most 
appropriate stitching condition for the pattern of straight stitches and 
for the pattern of zigzag stitches. 
BACKGROUND OF THE INVENTION 
Generally a sewing machine is designed to produce stitches with an upper 
thread (needle thread) and a lower thread which are locked with each other 
to form generally so called lock stitches. Especially in case of the 
straight stitches, if the upper and lower threads are normally locked as 
shown in FIG. 10, the normal lock stitches are formed as desired. On the 
other hand, if the upper and lower threads happen to be irregularly locked 
as shown in FIG. 11, the irregular lock stitches, which are generally 
called hitch stitches, are formed. If the hitch stitches are mixed in a 
series of the normal lock stitches, the stitches will lack uniformity and 
will have a lower quality of stitches as shown in FIG. 14. 
Further the normal lock stitches are the products of the upper and lower 
threads locked with each other in accordance with the twist of the 
threads. On the other hand, the hitch stitches are the products of the 
upper and lower threads locked with each other in a manner as to untwist 
the thread, especially the upper thread. 
The formation of normal stitches or hitch stitches depends on a position of 
a lower thread with respect to a needle which drops to one point at all 
times. As shown in FIG. 12, if the lower thread 40 is positioned on the 
left side of the needle from a view of a machine operator, the loop taker 
is rotated counter-clockwise as shown by an arrow, and catches a needle 
thread loop formed on the right side of the lower thread and locks the 
needle (upper) thread to the lower thread through a stitch formation 
phases, FIG. 12(a)-(g). As the result, the normal stitch is formed. On the 
other hand, if the lower thread is positioned on the right side of the 
needle as shown in FIG. 13, the loop taker is rotated to catch a needle 
thread loop formed on the left side of the lower thread and locks the 
needle thread to the lower thread through a stitch formation phases, FIG. 
13(a)-(g). As the result, the hitch stitch is formed. 
It is therefore required to provide the condition in FIG. 12 to secure the 
formation of the normal stitches at all times. This requirement may be 
satisfied by supplying the lower thread in a manner that the lower thread 
is drawn out from the bobbin carrier at a cut-out as shown in FIGS. 1 and 
3, which is located leftward of the left end of a laterally elongated 
needle dropping hole of a needle plate. 
As particularly shown in FIG. 15(a), the lower thread is supplied to the 
left end L of the laterally elongated needle dropping hole from a position 
opposite to the needle hole and leftward of the left L of the needle hole, 
because the straight stitches are generally formed with a needle position 
set adjacent to the left end L of the laterally elongated needle hole 18a 
within which the needle is laterally swingable from minimum to maximum for 
zigzag stitches. The needle position adjacent to the left and L of the 
needle hole is so set as to reduce the up and down movements of a fabric 
which may otherwise be caused as the needle penetrates into and out of the 
fabric to be sewn. 
However with references to FIG. 15(a), in case of zigzag stitching, the 
needle is swingable between the opposite needle positions L and R over a 
distance D. It is therefore observed that the required amount of lower 
thread is different depending upon the needle positions L and R. Much more 
amount of lower thread is required when the needle come to the right end 
needle position R. The thread tension due to drawing out an additional 
amount of lower thread will pull the needle thread down onto the underside 
of the fabric to be sewn in contrast to the right-side lock of threads as 
shown in FIG. 16. Namely the tread locking positions are unblanced. 
In order to form the zigzag stitches of balanced thread locking positions 
as shown in FIG. 17, it is required to provide a condition for supplying 
the same amount of lower thread to both needle positions L and R from the 
lower source, that is, a condition as shown in FIG. 15(b), providing the 
thread paths of a same distance from a thread supply to the needle 
positions L and R, respectively. 
Japanese utility model laid-open application 56-23074 discloses a sewing 
machine having a lower thread normally drawn out from a thread source 
through an opening located at a position corresponding to a center of a 
laterally elongated needle dropping hole of a needle plate, wherein a 
thread guide member is operatively connected to a pattern selecting device 
including a plurality of pattern selecting keys, and is operated in 
response to selection of a straight stitch pattern to guide the lower 
thread toward a left side with respect to the left end of the laterally 
elongated needle dropping hole from a view of a machine operator. 
The prior art is actually successful in preventing the formation of hitch 
stitches in connection with the straight stitches. However as far as the 
zigzag stitches are concerned, which require the needle thread to extend 
laterally of the fabric feeding direction, the fabric is easily shrunk in 
the lateral direction due to the zigzag stitches if the lower thread 
tension is kept the same as in the case of the straight stitches. It is 
therefore required to separately adjust the upper thread tension to 
prevent such shrinkage of fabric. It is, however, insufficient to 
completely prevent such shrinking phenomena of fabric only by separately 
adjusting the upper thread tension. 
SUMMARY OF THE INVENTION 
The object of the invention is to eliminate the defects and disadvantages 
of the prior art. It is, therefore, a primary object of the invention to 
provide a device to be operated in response to selection of a pattern of 
straight stitches to adjust a tension of a lower thread to a first set 
value and, simultaneously, set a first lower thread path from a thread 
source to a fabric to be sewn, the device being operated in response to 
selection of a pattern of zigzag stitches to adjust the tension of the 
lower thread to a second set value and simultaneously set a second lower 
thread path from the thread source to the fabric. 
It is another object of the invention to provide a device which is compact 
in structure and easy in operation to give most appropriate stitching 
conditions each specific to the straight stitches and to the zigzag 
stitches. 
In short, the present invention relates to a sewing machine having a needle 
plate having a laterally elongated needle hole formed therein, stitch 
forming means including a needle vertically reciprocable through the 
needle hole and laterally swingable within the laterally elongated range 
of the needle hole, and a loop taker having a bobbin carrier positioned 
therein, the bobbin carrier carrying therein a bobbin loaded with a lower 
thread, the loop taker being rotated to cooperate with the reciprocating 
needle to form a stitch with the upper and lower thread, and pattern 
selecting means including pattern selecting keys selectively operated to 
select different patterns of zigzag stitches including a pattern of 
straight stitches stored in a memory in the form of pattern data, the 
lower thread tension adjusting device comprising first tension means 
mounted on the bobbin carrier for tensioning with a predetermined degree 
of tension the lower thread; second tension means mounted on the bobbin 
carrier and movable between an operative position in which the second 
tension means tensions with a predetermined tension the lower thread in 
addition to the tension provided by the first tension means, and an 
inoperative position in which the second tension means does not tension 
the lower thread; positioning means mounted on the bobbin carrier and 
movable between a first set position in which the positioning means moves 
the second tension means to the operative position, and a second set 
position in which the positioning means moves the second tension means to 
the inoperative position; guide means movable between a first guide 
position in which the guide means guides the lower thread to a position 
adjacent to one end of the laterally elongated needle hole and 
simultaneously guides the positioning means to the first set position, and 
a second guide position in which the guide means guides the lower thread 
to a position adjacent to a center of the laterally elongated needle hole 
and simultaneously guides the positioning means to the second set 
position; and control means responsive to a selective operation of the 
pattern selecting keys selecting the pattern of straight stitches to move 
the guide means to the first guide position, and responsive to selection 
of some of the patterns of zigzag stitches to move the guide means to the 
second guide position.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
Embodiments of the invention will be explained in detail with reference to 
the attached drawings. 
With reference to FIG. 5, a sewing machine has a machine frame 1 having a 
key board 5 as a pattern selecting means for selecting a plurality of 
different patterns stored in a memeory. 
With reference to FIG. 6, a needle bar 3 is mounted on the machine frame 1 
and is vertically movably in connection with an upper shaft 2 which is 
rotatably mounted (journalled) on the machine frame. Namely as generally 
known, the needle bar 3 is slidable by a needle bar support 11 and is 
fixedly connected to a needle holder 10 which is connected to a needle bar 
crank 8 by a crank rod 9. The crank 8 is operatively connected to a 
balance weight 7 secured to one end of the upper shaft 2 for rotation 
therewith. The needle bar supporter 11 is swingable mounted on a vertical 
shaft 12 secured to the machine frame 1, and is prevented from vertical 
movement. The needle bar support 11 is connected to one end of a rod 13 
which has the other end connected to an arm 15 secured on an output shaft 
14a of a stepping motor 14 for controlling the needle position. 
A feed dog 19 is mounted on a horizontal feed arm 21 to be driven by the 
drive shaft 1. The movement of the horizontal feed arm 21 is adjusted by 
adjusting the angular position of a member 22 fixed to one end of an 
adjusting shaft 23 which has the other end secured to an arm 24 connected 
to a crank 26 mounted on the output shaft of a feed control stepping motor 
25 by way of the crank 26 and a link 27. 
A loop taker 28 serving as the needle thread catching means, is rotatably 
supported on the machine frame 1 under the needle plate 18, and a gear 30 
is in mesh with a gear 31, the former being secured on a lower shaft 2 to 
be rotated in synchronism with the needle bar 3 by the upper shaft 29, and 
the latter being secured to a rotation axle of the loop taker 28. 
The construction and operation of the lower thread adjusting device will be 
explained below. 
FIGS. 1 and 2 show that a bobbin carrier 32 is rotatably support within a 
loop taker 28. The bobbin carrier 32 contacts rotation checkers 33 and 34 
secured to the machine frame 1, and is restrained from rotation. A base 
plate 35 having a lower thread D draw-out opening 35a, is fixed on an 
outer circumference of a bobbin housing 32a provided in the bobbin carrier 
32. A lower thread tension spring 36 comprises a thin sheet and is secured 
to the frame 35 at its one end by a screw 37. An end 36a of the tension 
spring 36 is positioned on a slit of the opening 35a, and the spring 36 is 
formed with a hole at its center for inserting a headed screw 38. 
The headed screw 38 passes through the hole of the spring 36 and is screwed 
into a threaded hole of the base plate 35, and the headed screw 38 presses 
the spring 36 with its head so as to adjust a pressure exerted between the 
end 35a of the tension spring 36 and the lower thread drawing opening 35a. 
A bobbin 39 with the lower thread 40 is rotatably fitted in the bobbin 
housing 32a of the bobbin carrier 32. The lower thread 40 is drawn from 
the opening 35a through a thread guiding slit formed in the base plate 35, 
and is effected with tension by the pressure of the lower thread spring 
36, so that it is drawn toward the fabric from laterally elongated needle 
hole 18a of the needle plate 18 (see FIG. 5). 
A level 41 is rotatable at its center around by a pin 42 fixed to the 
bobbin carrier 32, and is formed with a presser 41a at its one end to be 
pressed against a free end of the spring 36 (FIG. 1) and has an opposite 
end 41b bent down into an elongated recess 32b formed in the bobbin 
carrier 32 and extending in parallel with the needle hole. 
A small spring 60 has one end held in a hole 35b formed in the base plate 
35, and has an opposite end pressed against the lever 41 so that the lever 
41 is normally turned in a direction shown by arrow A. 
A slider 43 has a width for smoothly moving in length of the oblong groove 
32b of the bobbin carrier 32. The slider 43 has a standing part 43a at its 
end, and a guide member 44 has a groove 44a for moving the standing part 
43a in the length of the groove 32b, is fixed to the bobbin carrier 32. A 
pressing spring 45 is positioned between the end 41b of the lever 41 and 
the slider 48. 
A stopper 46 slidably implanted within the bobbin carrier 32, and is 
normally biased in an upper direction by a spring 47. The stopper 46 has a 
semicircular head for selectively engaging holes 43b and 43c formed on the 
lower side of the silder 43. An actuating lever 48 is supported by the 
machine frame 1 above the elongated hole 32b and is pressed against the 
frame 1 by a presser pieces 49 and is slidable in a direction of an arrow 
B and in the opposite direction thereof above the length of the elongated 
hole 32. The lever 48 has a cut-out 48a formed on a lower side thereof for 
engaging the standing part 43a of the silder 43. The actuating lever 48 is 
fixed at its one end with an end of an actuating body 50 supported by the 
machine frame 1 sildably along the length of the elongated hole 32b. The 
lever 48 has a pin 51 at its other end, which passes movably through a 
hole 52a formed in one end of an actuating pawl 52. 
A cam 53 is secured on an output shaft 54a of a stepping motor 54 fixed to 
the machine frame 1 for adjusting the lower thread, and is defined with an 
operative part 53a and an inoperative part 53b on its outer circumference. 
The inoperative part 53b is formed in a large diameter part of the cam 53 
with respect to the output shaft 54a. The cam 54 has a projected part 53c 
formed just on a side of a rotating direction C of the stepping motor 54 
from the largest diameter part 53b as shown in FIG. 2. The operative part 
53a is formed in a small diameter part with respect to the center of the 
output shaft 54a. 
The actuating pawl 52 has a pawl portion 52b at its one end, and is 
connected with the actuating body 50. A spring 55 is connected at its one 
end with the actuating body 50 and is connected at its other end with the 
machine frame 1, so that the actuating pawl 52 is rotated in an direction 
of an arrow D via the pin 51, and the pawl portion 52b contacts the cam 
53. 
FIG. 7 is a control block diagram where a reference numeral 4 is a pattern 
selecting means, 56 is a memory storing pattern forming information, 57 is 
means for discriminating a type of selected pattern, and 58 is a memory 
for storing a selected pattern. These means are connected to a central 
processing unit 59. 
An explanation will be made of actuation of the lower thread tension 
adjusting device constructed as stated above. 
A key board 5 of the pattern selecting means 4 is operated to select a 
desired pattern. Pattern forming information and pattern code 
corresponding to the selected pattern are read out from the pattern 
forming information memory 56, and stored in the selected pattern memory 
58. THe discriminating means 57 discriminates if the selected pattern is a 
straight stitching or a pattern of zigzag stitches, and the straight 
stitching is assigned with "1", and other patterns of zigzag stitches are 
assigned with "0". 
A reference will be made to a case that a selected pattern is a straight 
stitching. 
Since the pattern code is "1", the discriminating means 57 outputs to the 
central processing unit 59 a signal of "the pattern is a straight 
stitching, and the lower thread adjusting means should be set to the 
straight stitching condition." 
Then, the CPU 59 is operated to rotate the stepping motor 54 in the 
direction of the arrow C until the projection 53c of the cam 53 is rotated 
past the pawl portion 52b of the actuating pawl 52 as shown in FIG. 2. The 
inoperative part 53b is at the maximum biasing position with respect to 
the output shaft 54a of the stepping motor 54, and since the projection 
53c is at a more remote position then the maximum biasing position, the 
pawl 52 is once rotated in the direction opposite to the direction of the 
arrow D, and the actuating body 50 and the actuating lever 48 are moved in 
the arrow B aginst the action of the spring 55 via the pin 51 engaging in 
the hole 52a. 
By the movement of the actuating level 48, the right side 48b of the 
cut-out 48a therefore engages the standing part 43a of the slider 43 and 
moves the slider 43 in the same direction until the silder 43 presses down 
the stopper 46 against the pressure of the spring 47, and the hole 43c is 
engaged by the stopper 46 at the position shown in FIG. 2. The stepping 
motor 54 is further rotated in the arrow C and stops at a position where 
the pawl portion 52b of the actuating pawl 52 comes down the projection 
53c and contacts the inoperative part 53b of the actuating cam 53. Since 
the inoperative part 53b is set lower than the projection 53c, the 
actuating pawl 52 rotates a bit in the direction of the arrow D and stops 
at the position shown with the solid line of the same. 
Therefore, the actuating lever 48 moves a bit in the direction opposite to 
the direction of the arrow B to the position shown with the solid line of 
FIG. 2, and as the result, a space "a" is formed between the side 48b and 
the standing part 43a for passing there through the upper thread to be 
lifted up after interlocked with the lower thread. 
When the slider 43 comes to the position shown with the solid line of the 
same, it presses the spring 45 against the end 41b of the lever 41. Since 
the pressure of the spring 45 is larger than that of the small spring 60, 
the lever 41 rotates in the direction apposite to the direction of the 
arrow A, and a front presser 41 a is pressed against the lower thread 
tension spring 36 so as to increase the pressure against the lower thread 
40 held between the tension spring 36 and the base plate 35. 
Further with the actuating lever 48 at the set position as shown in FIGS. 
2, the left side 48c of the cutout 48a is designed to allow the lower 
thread 40 to extend from the lower thread draw-out position 35a set at the 
left side, more than the left end L of the needle amplitude range, to the 
needle position adjacent to the left end L of the laterally elongated 
needle hole 18a as shown in FIG. 1, and the formation of hitch stitches is 
prevented. 
Next explanation will be made to a case when the key board 4 of the pattern 
selecting means 4 is operated to select stitching patterns other than the 
straight stitching, that is a pattern of zigzag stitches. 
The pattern-forming information and the pattern code are read out from the 
pattern-forming information memory 56, and stored in the selected pattern 
memory 58. 
Since the pattern code is "0", the pattern discriminating means 57 outputs 
to the central processing unit 59 a signal of "a pattern of zigzag 
stitches and the lower thread adjusting means is set to the pattern 
stitching condition." 
Then the CPU 59 is operated to rotate the stepping motor 54 for adjusting 
the lower thread, and it is rotated in the direction of the arrow C until 
a concave 53d of the cam 53 engages the pawl portion 52b of the actuating 
pawl 52 as shown with a dotted line in FIG. 4. The operative part 53a is 
at the minimum biasing position with respect to the output shaft 54a of 
the stepping motor 54, and since the concave 53d is at a lower position 
than the maximum biasing position, the pawl 52 is rotated in the direction 
of the arrow D, and the actuating body 50 and the actuating lever 48 are 
moved in the direction opposite to the direction of the arrow B by the 
action of the spring 55 via the pin 51 engaging in the hole 52a. 
By the movement of the actuating lever 48, the left side 48c of the cutout 
48a thereof engages the standing part 43a of the slider 43 and moves the 
slider 43 in the same direction until the slider 43 presses down the 
stopper 46 against the pressure of the spring 47 and the hole 43b is 
engaged by the stopper 46 at the position shown in FIG. 4. The stepping 
motor 54 is further rotated in teh direction of the arrow C until the 
concave 53d disengages from the pawl portion 52b of the actuating pawl 52 
and the operative part 53a of the cam engages the pawl position 52b. Since 
the inopertive part 53a is set higher than the concave 53d, the actuating 
pawl 52 rotates a bit in the direction opposite to the direction of the 
arrow D and stops at the position shown with the solid line of the same. 
Thereby, the actuating level 48 moves slightly in the direction of the 
arrow B to the position shown with the solid line and therefore a space 
"b" is formed between the side 48c and the standing part 43a for passing 
the upper thread to be lifted up after the thread is interconnected with 
the lower thread. 
When the slider 43 comes to the position shown with the solid line of the 
same, it decreases the pressure of the spring 45 against the end 41b of 
the lever 41 so that the pressure of the spring 45 is smaller than that of 
the small spring 60, the lever 41 is therefore rotated in the direction of 
the arrow A, and the presser 43a releases the pressure to the lower thread 
tension spring 36 so as to decrease the pressure against the lower thread 
held between the tension spring 36 and the base plate 35. 
Further with the actuating level 48 at the set position as shown in FIG. 4, 
the left side 48c of the cutout 48a comes to a position for guiding the 
lower thread 40 to the center of the laterally elongated needle hole 18a 
to provide the condition as shown in FIG. 15(b) which prevents the 
formation of the unbalanced thread locking phenomena of zigzag stitches as 
shown in FIG. 16. 
The other embodiment will be explained, where the present device is applied 
to a mechanic sewing machine as shown in FIGS. 8 and 9. 
In FIG. 8, a pattern selecting dial 104 is provided on a dial shaft 159 
rotatably pivoted on a machine frame 101. A plurality of pattern cams 114 
for controlling the needle positions are fixedly mounted on a cam shaft 
(no shown) rotatably supported to the machine frame 101 together with feed 
adjusting cams (not shown). 
A gear 115 is mounted on the cam shaft 159 and is in mesh with a worm 116 
secured to an upper shaft 102 pivoted rotatably to the machine frame. 
The pattern cams 114 are selectively engaged by a pawl 117 which is 
connected to one end of a rod 113 via a transmission means 118, and the 
rod 113 is connected at its other end with a needle bar supporter 111. 
The dial shaft 159 has a lower thread tension adjusting cam 153 and a guide 
cam 156 mounted thereon for rotation therewith. The cam 156 guides the 
pawl 117 to select one of the pattern cams 114. 
The lower thread adjusting cam 153 is, as seen in FIG. 9, formed with an 
inoperative part 153b which has a large diameter, and two projections 153c 
on the larger diameter part 153b. The other part of the outer 
circumference has a small diameter and is formed with an actuating part 
153a. Concaves 153d, 153d are formed on both sides of the projection 153c, 
which are lower than the small diameter part 153a. The lower thread 
adjusting cam 153 is engaged with a pawl part 152b of the actuating 152. 
The remaining structure of the lower thread adjusting means is the same as 
in the first embodiment, and the explanation will be omitted. 
When the straight stitching is selected by the dial 104, the pawl part 152b 
of the actuating pawl 152 contacts one of the projections 153c of the 
lower thread adjusting cam 153 and the inoperative past one after another. 
When a pattern of zigzag stitches other than the straight stitching is 
selected, the pawl part 152b, of the actuating pawl 152 engages one of the 
concaves 153d and the actuating part 153a of the cam 153 one after 
another. Therefore, when the pattern selecting dial 104 is rotated 
clockwise or counterclockwise to rotate the cam 153 to select the straight 
stitching, the pawl 152 is largely rotated in the direction opposite to 
the direction of the arrow D by one of the projections 153c of the cam 153 
to the position shown with the dotted line in FIG. 9. With further 
rotation of the cam 153, the pawl part 152b of the pawl 152 contacts the 
inoperative part 153b and rotates a bit in the direction of the arrow D to 
the position shown with the solid line of the same. 
This position is the same as when the straight stitching is selected in the 
first embodiment as shown in FIG. 2. 
The pattern selecting dial 104 is rotated clockwise or counterclockwise to 
select a pattern of zigzag stitches other than the straight stitching, the 
pawl part 152b of the pawl 152 contacts one of the concaves 153d of the 
cam 153 and is rotated in the direction of the arrow D to the position 
such as shown with the dotted line of FIG. 4. With further rotation of the 
cam 153, the pawl part 152b contacts the actuating part 153a and is 
rotated in the direction opposite to the arrow D and is set at the 
position such as shown with the solid line in FIG. 4. 
This position is the same as when a pattern of zigzag stitches other than 
the straight stitching is selected in the first embodiment as shown in 
FIG. 4. 
Namely in this embodiement, the tension adjusting cam 153 is used in a 
mechanical sewing machine having a plurality of pattern cams selected by 
manual rotation of the dial 104, and therefore, the cam 153 is designed to 
have the configuration as shown in FIG. 9, which is different from the 
tension adjusting can 53 in FIGS. 2 and 4 used in an electronic sewing 
machine having a memory storing pattern data for a plurality of different 
patterns including the straight stitches.