Sewing Machines - Tension Guidelines

To get the best results and form a line of stitches that appear the same on each side of the fabric, an equal amount of thread needs to run from the spool and bobbin at the same time. This is achieved by running the threads through a number of tension devices such as: tension discs, tension regulator on the machine head, thread guides and the bobbin-case spring for the bobbin thread. On some sewing machines, there is also a tiny hole in the bobbin-case finger through which you can feed the bobbin thread in order to increase the tension for a better stitch appearance when embroidering, satin-stitching or top-stitching. The tension assembly is made up the tension discs and the tension regulator. The way the tension discs work is by how they squeeze the thread as it flows in between them, and the tension regulator changes the level of pressure on the discs.

When turning the tension regulator clockwise to a higher number, the tension discs are moved nearer together which increases the pressure on the thread, whilst turning the tension regulator anti-clockwise will have the reverse effect. An important point to note here is that if you change your thread to a thicker thread than you were previously using, you will be increasing the tension on your thread if you don't reset the tension regulator correspondingly, resulting in the upper thread flow to decrease unless your machine automatically adjusts this for you. Bobbin tension is not however automatically adjusted and may be required to be manually changed to match.

Guide threads also help increase pressure on the thread, and it is important to make sure your all of your thread guides are threaded before sewing.

Pressure is applied to the thread that comes out of the bobbin case by the flat bobbin case spring. The amount of pressure can be adjusted by a tiny screw at the back of the spring. To increase the pressure on the thread, the screw should be turned clockwise and anti-clockwise to decrease the pressure. Only turn the screw in very small increments, typically no more than a quarter turn, then test your switch before making any more adjustments. Keep in mind that changing your thread size will also increase the amount of pressure.

To determine when your tensions are balanced, your stitch line will look good on each side of the material, with the seam at its strongest. If your bobbin thread is showing on the right side, either your top thread tension is too tight or your bobbin thread is too loose, as is the case if your needle thread is showing on the wrong side. Even when your tensions are balanced, they may still be too loose or too tight. If your tensions are too loose, gaps will show in the seam and expose the threads between the pieces of fabric. If your tensions are too tight, the seam may bundle and pucker up or break really easily when stretched. To undertake maintenance tension adjustments, find two different colour threads in the particular brand and size that you use. Use one colour for the bobbin thread and the other for the top thread, using a new needle in the most common size that you use. Set your stitch length for the most common length that you intend to sew. Set your upper-tension regulator around the middle of its range. Sew a test seam on two layers of lightweight material and then examine the stitches carefully. If the tension is not perfect and the bobbin thread shows on the right side then tighten the bobbin thread or loosen the bobbin thread if the upper thread shows on the bottom layer.

A handy idea once you have your stitching balanced is to start your own journal in which you can log the different settings on your upper regulator and bobbin screw for different thread brands, sizes and types. If you intend on switching your standard thread to another one, thread your machine and test the same setup to see if you might only need to do a upper tension dial change. Sew a test seam, and try and find a balance by only changing the upper tension regulator. If that fails, use a second bobbin case and start making adjustments, then testing, and repeating until your happy with the stitch. Remember that so many different aspects can influence the tension on your thread so before you feel that you have to start making lots of adjustments, check the following things first:

1. Make sure your machine is threaded correctly.

2. Make sure your bobbin is filled correctly according to the machine instructions. Remove any thread from the outside of the bobbin.

3. Make sure your machine is clean.

4. Make sure all machine parts aren't damaged, such as bent needles and bobbins.

Kathy Simpson

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