The Important Components Of The Lock Sewing Device Are The Shuttle Hook And The Bobbin Assembly

A spool is simply a roll of thread placed underneath the fabric. The latter rotates driven by a motor, synchronized with the movement of the needle.
As with the chain stitch, the needle pulls one loop through the fabric, rises again as the feed dog moves the fabric forward, and slips another loop through. However, instead of joining different coils together, this stitching mechanism joins them with another length of thread that has been unwound from the spool.
As the needle slips the thread into the loop, the rotating shuttle catches the loop with the crochet hook. As the shuttle rotates, it pulls coils around the thread coming from the spool. This makes the stitching strong.
This kind of rotary shuttle also evolved from the straight shuttle. The loop stitching principle of sewing machines has evolved from straight shuttle to rotary shuttle, and has entered a mature stage.