Abstract:
A sewing machine having differential material feed, with a main feed dog and differential feed dog being mechanically connected to one another in such a manner that if the feed stroke length of the first feed dog is changed, the feed stroke length of the second feed dog is changed inversely in a predetermined ratio.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to sewing machines with differential feed. 
     Sewing machines having differential feed are generally known for example from German Patent Specification 10 71 461 corresponding to U.S. Pat. No. 2,965,056. In sewing machines having differently adjusted feed stroke lengths of main and differential material feed dogs, there is a particular ratio between these two feed stroke lengths. If this ratio alters as a result of an increase in the feed stroke length of the differential feed dog, in the case of known sewing machines with differential feed, the length of the stitch in the material to be sewn will also increase. Such a result is disadvantageous. In order to maintain the same stitch length, it is necessary, given a change in the differential ratio, to make a time-consuming readjustment of the main feed. 
     SUMMARY OF THE PRESENT INVENTION 
     A principal feature of the present invention is the provision of an improved sewing machine with differential feed. 
     The sewing machine of the present invention comprises, a main feed dog, and a differential feed dog. 
     A feature of the present invention is the provision of means for adjusting the feed stroke length of the differential feed dog while maintaining a constant stitch length of the sewing machine. 
     Thus, another feature of the invention is that the sewing machine is adjusted in such a way that the stitch length remains constant when the differential ratio is changed. 
     A further feature of the invention is that when the differential feed dog is adjusted, the feed stroke length of the main feed dog is automatically adjusted inversely in a predetermined ratio. 
     Thus, a feature of the present invention is that it is no longer necessary to perform a time-consuming readjustment of the main feed when the differential feed is adjusted. 
     Further features will become more fully apparent in the following description of the embodiments of this invention and from the appended claims. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a plan view of a differential feed device of the present invention for a sewing machine. 
     FIG. 2 is a sectional view taken substantially as indicated along the line II--II of FIG. 3; 
     FIG. 3 is a sectional view taken substantially as indicated along the line III--III of FIG. 1; 
     FIG. 4 is a sectional view taken substantially as indicated along the line IV--IV of FIG. 1; 
     FIG. 5 is a front view of the device of FIG. 1; and 
     FIG. 6 is a sectional view taken substantially as indicated along the line VI--VI of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The differential feed device is disposed in a sewing machine housing 1, and is driven by a shaft 2. Two eccentric arrangements are disposed on the shaft 2 and each comprises a lifting eccentric disc 3, a stroke eccentric disc 4 and a counterbalance disc 5. The lifting eccentric disc 3 is enclosed by a sliding block 6 which is mounted in a fork 7 of a feed dog carrier 8. One end of the feed dog carrier 8 is pivoted on a respective frame 9 and the other end carries a feed dog 11. The frame 9 is mounted on an axle 12 which is fixed to the housing 1, and is articulated by way of a pivot point 10 with a fork 13. A sliding block 14 lies in the fork 13 and embraces the stroke eccentric disc 4. On the fork 13 there is an arm 15 which carries a bolt 16 on which a sliding block 17 is mounted. The sliding block 17 is guided in an arcuate guideway 18 which is provided on a respective adjusting shaft 19. The adjusting shaft 19 is mounted in the sewing machine housing 1 by means of a bushing 21. 
     Parts 3a to 21a are the same as parts 3 to 21, apart from 8a and 11a, and some are disposed as mirror images of one another. Parts 3 to 21 for driving the feed dog 11 form the differential feed device, and parts 3a to 21a for driving the feed dog 11a form the main feed device. 
     As shown in FIG. 5, a double lever 23 is clamped to the adjusting shaft 19a by a screw 20, and a tension spring 24 acts between the lever 23 and the sewing machine housing 1. A lever 25 is pivotally mounted on the adjusting shaft 19a. The levers 23, 25 have cam tracks 26 and 27, which are coupled together by way of a bolt 28. The bolt 28 is located on a pivotable adjusting lever 29. On the opposite side of the cam track 27 to the bolt 28 there is a stop face 31 which abuts against a stop 32. The stop 32 is formed by the free end of a bolt 33 which moves in an adjusting screw 34. 
     As shown in FIG. 6, a double lever 35 is clamped to the adjusting shaft 19 and a spring 36 acts between the lever 35 and the sewing machine housing 1. The double lever 35 is connected by way of a connecting rod 30 to a lever 37 on an auxiliary shaft 38. The end of the auxiliary shaft 38 projecting out of the sewing machine housing 1 carries a lever 39 (FIG. 5) to which the adjusting lever 29 is pivotally connected. The adjusting lever 29 is provided with a longitudinal slot 41 into which projects a locking screw 42 which is also guided in a slot 43 in a housing cover 44. The slot 43 contains adjustable stops 45, 46 to limit the pivoting angle of the adjusting lever 29. 
     Starting from the initial position shown in FIG. 5, the stop 32 is drawn back in the direction of the arrow X by turning the adjusting screw 34. As a result, the lever 25 turns in an anti-clockwise manner about the central axis of the adjusting shaft 19a, as the double lever 23 presses, as a result of the force of the spring 24, by way of the bolt 28 onto the lever 25. The double lever 23 also turns the adjusting shaft 19a in an anti-clockwise manner. As a result, the arcuate guideway 18a is moved out of its concentric position about the pivot point 10a (not shown) of the fork 13a and the arm 15a thereon (cf. FIG. 4). With the guideway 18a in this position, the up and down motion of the sliding block 17a conveys a reciprocating motion to the fork 13a which sets the frame 9a into a rocking motion. The rocking motion of the frame 9a causes a longitudinal motion of the feed dog carrier 8a, on whose free end the main feed dog 11a is secured. 
     By adjusting the adjusting screw 34 as described, the adjusting lever 29 is moved by way of the bolt 28 in the direction of the arrow Y and hence the auxiliary shaft 38 is turned in the clockwise direction. The auxiliary shaft 38 turns the adjusting shaft 19 by way of the connecting rod 30 in the clockwise direction, as a result of which the same adjustment of the guideway 18 takes place as in the above-described adjustment for the main feed dog 11a. Hence, both feed devices have the same feed stroke length. 
     In order to increase the feed stroke length of the differential feed dog 11 over that of the main feed dog 11a, the adjusting lever 29 is moved in the direction of the arrow Z. As a result, the bolt 28 is moved between the levers 23 and 25 in the direction of their free ends. As the bolt 28 is pressed against the lever 25 by way of the double lever 23 as a result of the force of the spring 24, said bolt 28 moves along the cam track 27 of the lever 25, as a result of which the adjusting lever 29 is moved further in the direction of the arrow Y. This displacement of the adjusting lever 29 causes the auxiliary shaft 38 to be further rotated in the clockwise direction and hence the adjusting shaft 19 to be further rotated in the anti-clockwise direction, as a result of which the eccentricity of the arcuate guideway 18 with respect to the pivot point 10 is further increased and the differential feed device is given a longer travel. At the same time, displacement of the bolt 28 in the direction of the free ends of the levers 23 and 25 causes the double lever 23 and the adjusting shaft 19a to rotate in the clockwise direction, because the gap between the cam tracks 26 and 27 becomes similar in the direction of the free ends of the levers 23 and 25. As a result, the previously set feed stroke length of the main feed dog 11a is reduced by a predetermined amount and the stitch length is kept constant for different feed lengths of the differential feed dog 11. 
     The foregoing detailed description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.