The needle Bar Movement

In some early models the needle bar just went up and down but this meant that the shuttle did not always pass inside the thread. However, if the needle bar rises slightly after reaching its lowest point a small loop is formed in the thread for the shuttle point to enter.

In the diagram, the plate with the bird-wing slot is fixed to the needle bar, which can only move up and down. The rotating disc is on the same axis as the flywheel. It has a bushed cylinder on its rim which fits into the bird-wing

As the disc is rotated the needle goes up and down with a small bump at the bottom caused by the dip in the shape of the slot.

This mechanism is not needed on a chain stitch machine so the needle bar is often moved by a simple lever.