Tension is the term we give to the process of balancing the top and bottom threads so the machine will sew a good stitch.
Problem: The top thread frays.
Probable Cause: The needle is too small, the top tension is too tight, or there is a burr or rough in the thread path.
Problem: The bobbin thread shows through on the top.
Probable Cause: The bobbin tension is too loose or the top tension is too tight.
Problem: The top thread loops on the bottom.
Probable Cause: The bobbin tension is too tight or the top tension is too loose.
Problem: The top thread snaps.
Probable Cause: The top tension is too tight.
Problem: The thread gathers under the needle plate.
Probable Cause: There are two reasons why thread gathers under the needle plate. Either the top tension is too loose or the machine is threaded incorrectly, bypassing the take-up lever. It is not a thread problem. If the thread were inferior, it would simply break and not be strong enough to gather under the needle plate and jam the machine. What is occurring is that when your take-up lever raises to pull the thread back up through the fabric and form the stitching knot, it is instead simply pulling thread off the spool because there is not sufficient top tension. This situation results when the tension is set too loose, the tension disks are held apart by lint or thread debris or the thread is not pulled all the way down between the disks. The other condition occurs when the take-up lever is not pulling the thread back through at all because it is not threaded. Clean your machine regularly.