[0001] The present invention relates to domestic sewing machines, in particular to the zig-zag
mechanism of such a machine..
[0002] In conventional zig-zag sewing machines the needle bar performs a vertical movement
and a horizontal movement of which the lattet is controlled by a guiding means which
keeps the bar in an adjusted position during the downward movement of the needle which
is effected by a crank and a connecting rod. When the material to be penetrated by
the needle is hard or thick a torque on the needle bar created by the resistance in
the needle movement can be so great that the needle is laterally moved from the adjusted
position, whereby the needle is broken. Several solutions for eliminating this drawback
have been suggested, for instance, an enforcement of the guiding means so that it
can resist the torque, and further, a shortening of the lever arm of said torque and
the use of universal joints etc. By a step motor arrangement for the zig-zag movement,
which is more and more used in modern sewing machines, the guiding means has a rather
limited force of keeping of the adjusted position and therefore it is important to
make an arrangement having a very small torque on the needle bar when moving the needle.
[0003] The upper arm of the invention has a crank shaft perpendicular to the zig-zag movement
of the needle. The problem forming the basis of the invention is how to achieve a
simple propulsion on the needle bar when the crank shaft is positioned In this way
including links and connecting means so that the guiding means of the zig-zag movement
will not be disturbed by the driving force of the needle bar. The solution of the
problem leads to a positioning of the connection between the connecting rod and the
needle bar at a point which approximately coincides with the fulcrum of the zig-zag
movement in the moment when the needle penetrates the cloth. The properties thus introduced
into a sewing machine according to the invention are stated in the characterizing
clause of Claim 1.
[0004] An embodiment of the invention is described in the following with reference to the
accompanying drawings which show in Fig. 1 a sketch of a needle bar mechanism with
a top journalled needle bar, Fig. 2 a sketch of a mechanism with a center journalled
needle bar, Fig. 3 a sketch of a mechanism with an oval-shaped movement path of the
connecting rod, Fig. 4 the same mechanism as in Fig. 3 in another position and Fig.
5 a perspective view of such a mechanism.
[0005] The sewing machine according to the invention is provided with a hollow body having
an upper arm, in the end of which there is a horizontal pin 10 and a needle bar frame
11 journalled thereon to be swingable in a direction perpendicular to the feeding
direction of the machine. The adjustment of the frame, the zig-zag position, is effected
by a guiding means 12 shown in a block in Figs. 1-4. A connecting rod 13 is connected
at a point 14 to a needle bar 15 journalled in the frame. A crank shaft 16 is journalled
in the body perpendicular to the zig-zag direction, its crank pin 17 is connected
to the connecting rod, and a force is transferred from the crank shaft to the needle
bar via the connecting rod when the needle hits the material. As this rod forms an
angle v to the frame, which is journalled at its top (Fig. 1), there will be a lever
arm LIe This one and the force of the connecting rod form a torque at the pin 10 with
a magnitude depending on the resistance to penetration of the material. This torque
is stressing to the guiding means 12 which must be dimensioned to resist the torque
which may be great.
[0006] In Fig. 2 the mechanism is drawn as in Fig. 1 with the difference that the bearing
pin 10 of the frame 11 in Fig. 1 is moved from the top to the center, i.e. to the
same level as the point 14. In Fig. 2 this bearing pin is denoted 18. This means that
the lever arm L
1 in Fig. 1 now is zero meaning that the needle bar frame does not obtain any torque
from the force of penetration of the needle at this moment of the stitch forming procedure.
When the needle has penetrated the material the lever arm increases somewhat in relation
to another lever arm L 2 which is the distance between the pin 18 and the means 12,
however, L
1 is still small so the horizontal component of the force of the connecting rod stressing
the guiding means will be essentially less here than in the device shown in Fig. 1.
[0007] The mechanism in Figs. 3 and 4 involves still another improvement of the device.
The propulsion of the connecting rod is here effected by means of a link system creating
an oval movement 19. The system consists of a pin 20 disposed in the body and a link
21 journalled thereon and a crank arm 22 carrying a crank pin 17. An arm 23 connects
the crank pin to two bearings 24, 25 of the link and the connecting rod, respectively.
When the crank shaft 16 rotates the pin 17 describes a circular path 26, the bearing
24 makes an arc of a circle, and the bearing 25 makes the said oval path 19. As this
path is narrowed in the horizontal direction the angle u between the connecting rod
and the needle bar frame is less than the angle v in Fig. 1. In Fig. 4 a position
of the stitch forming procedure is shown where the ratio L,, L
2 is most disadvantageous. L
1 is here very small which brings a slight influence of the force in the connecting
rod on the guiding means 12. This means can therefore be dimensioned for a much less
keeping force of the position than what is the case in the device according to Fig.
1. The pin 20 may be provided with an adjustable attachment so it may be raised or
lowered in the body, thereby allowing an adjustment of the needle level in a simple
way.
[0008] In Fig. 5 the mechanism is shown as a module in a plate frame 28 which either can
constitute a part of the upper arm of the machine or be an independent part which
is mounted in the arm. Most of the members in Fig. 4 appear in Fig. 5 which shows
the principle of the design of the module. An arm 29 is provided on the- crank pin
17 and has a pin 30 which carries a bearing of a sliding block 31 on a rod 32 which
is fastened to a thread take-up lever 33 journalled on a shaft 34 in the module frame.
The end of the arm has a thread guide 35 which in the usual way takes up the upper
thread during a phase of the stitch forming procedure. The arm is driven by the crank
shaft in a vertical, oscillating movement; when the crank pin moves in its circular
path the sliding block 31 slides up and down on the rod 32 swinging the same to and
fro and the arm is following in the oscillating movement.
[0009] The adjusting force for the zig-zag movement, as mentioned before, can be reduced
in a device according to Fig. 4. In the module, Fig. 5, the zig-zag movement is effected
by a step motor 36 and a gear and link system connected thereto. A pinion 37 is in
mesh with a rack 38 which in one end has an angular recess 39. The rack carries a
pin 40 which is entered in an extension 41 of the needle bar frame which thereby follows
the movement of the rack. The pin is normally kept in the bottom of the angular recess
thanks to the two sloping edges 44, 45 thereof and to a couple of compression springs
42, 43 which on the opposite side of the rack are fastened to the frame. However,
it may happen that the pin slides up on one of the edges when the needle is subjected
to a lateral force. The pin will return to the bottom of the recess when the force
ceases. A similar guiding means is priorly described in the Swedish patent publication
No. 423,912.
[0010] The mechanism now described is an example how to exercise the invention. The design
of links, knee levers and fulcrums etc. can, of course, be changed without departing
from the inventive idea as generally defined in the following claims.
1. A zig-zag mechanism in a sewing machine provided with a laterally swingable needle
bar frame (11), a guiding means (12) for providing zig-zag movements, a connecting
rod (13) connected to the needle bar (15) at a connecting point (14) performing a
vertical reciprocating movement and a crank shaft (16) extended perpendicularly to
the zig-zag movement, characterized in that a center position in said reciprocating
movement of the connecting point approximately coincides with the fulcrum (18) of
the needle bar frame.
2. A zig-zag mechanism according to Claim 1, characterized in that the said center
position of the connecting point corresponds to a position of the needle tip when
hitting the sewing material.
3. A zig-zag mechanism according to Claim 1, characterized in that the upper end of
the connecting rod is journalled on a crank pin (17) projecting on the crank shaft.
4. A zig-zag mechanism according to Claim 1, characterized in that the upper end of
the connecting rod is journalled to one end of an arm (23), the other end of which
is journalled to a link (21), and is therebetween in driving connection with the crank
shaft by means of a crank arm (22) and a crank pin (17).
5. A zig-zag mechanism according to Claim 4, characterized in that the upper end of
the connecting rod describes an oval movement path (19) the longitudinal axis of which
is extended in the longitudinal direction of the needle bar.
6. A zig-zag mechanism according to Claim 4, characterized in that the said link (21)
has a bearing (20) in the body of the machine which bearing is adjustable in the vertical
direction making the level of the needle adjustable in relation to the hook of the
machine.
