4. The thread is suddenly broken during sewing, and the upper thread has a curved shape.

       Analysis: The position of the shuttle bed was not loaded with the upper thread and rolled into the shuttle bed. The bobbin case is locked or suddenly displaced during sewing. The quality of the shuttle is poor, and the heart spring is too long or the heart is oval.

       Treatment: Re-adjust the position of the shuttle, reinstall the shuttle, and change the swing.

       5. Suddenly break the bottom line during sewing. The bobbin case is unqualified and the inner diameter is elliptical.

       Analysis: The bobbin case is unqualified, and the inner diameter is elliptical, causing the bobbin to rotate. The bobbin spring screw is tightened too tightly so that the bobbin pressure is too small. The bobbin is too full or too loose. The bottom line is crunchy and has a knot, so that the bottom of the heart cannot pass through the bobbin.

       Treatment method: replace the bobbin case, loosen the bobbin heart spring screw, rewind the bobbin thread, and replace the bottom thread.