Symptoms of Incorrect Tension
Tension that is too tight or too loose causes stitches that look uneven. These stitches often have loose loops on one side of the fabric or are so tight that the fabric begins to pucker. Improper tension also causes the machine to periodically skip stitches. Thread breakage, coming from either the upper thread or the lower bobbin thread, is another common sign of tension that is far too tight.
It is important to test your tension before starting your sewing project so you don't waste any valuable fabric. Cut a small scrap of your fabric and use the machine to sew the stitch pattern you plan to use for your project. Remove the swatch from the sewing machine and carefully inspect the stitches on both sides of the fabric. If your tension is correct, the stitches should be identical in size and will appear to be interlocking with each other. If the stitches on the top side of the fabric appear straight and are not interlocking with the bottom stitches, your upper tension on your spool thread is too tight. If the stitches on the bottom side of the fabric are straight and not interlocking with the top stitches, the upper tension on your spool thread is too loose.
You can adjust your sewing machine's tension by manipulating either the upper tension on the spool thread or the lower tension on your bobbin thread. Most machines do not feature tension dials for lower tension, and you will only be able to adjust the lower tension by tightening a screw. For this reason, unless that screw is unusually loose, you should try to correct tension by manipulating the upper tension dial, which is usually located on the control panel of your machine. If your upper tension is too tight, turn the dial to a lower number. If your upper tension is too loose, turn the dial to a higher number. Sew another test swatch to see if your tension is now correct.
Other Sources of Tension Problems
Incorrect tension dial settings are not the only cause of unbalanced tension. If changing the tension setting does not solve your tension problem, look closely at your machine to make sure you don't have a different problem. Incorrect threading from either the spool or the bobbin will cause tension problems. You will also experience tension difficulties if your machine is not clean or if lint is present on the tension discs.