20pcs Bobbin Case Spring Sheet Fit most Industrial Single needle Straight Stitch Industrial sewing machine sewing machine. Suitable for bobbin case with a diameter of 2.1cm No-Back-Lash brake spring which puts constant , light pressure on bobbins to overcomemomentum. Bobbins stop spinning faster, preventing unraveling or tangling of bobbin thread when sewing operations stop and start. Fit most Industrial Single needle Straight Stitch Industrial sewing machine sewing machine. No-Back-Lash brake spring which puts constant , light pressure on bobbins to overcomemomentum. Bobbins stop spinning faster, preventing unraveling or tangling of bobbin thread when sewing operations stop and start.