how to adjust serger tension

Tension disks

The term “thread path” means all of the parts of the machine that the thread passes through on the way to the fabric. For a serger to function properly the threads must be under the correct amount of tension. To provide the proper amount of tension a serger has adjustable disks in the thread path. The tension disks pinch the thread to create tension. You can demonstrate this by squeezing thread between your two fingers. As you squeeze your fingers you can control the tension depending on how hard you squeeze.

The tension disks of a serger work the same way except that the tension is controlled by a knob instead of by changing finger pressure.  The knob is on top, next is the tension spring and on the bottom are the two tension disks that the thread is going though. As the knob is turned it moves down and compresses the spring which puts more pressure on the disks. Most new machines have the tension disks hidden inside a slot on the front of the machine.

There are separate tension adjustments for the needle threads and the looper threads, each thread has its own adjustment knob and disks. The knobs or dials on most machines are numbered from 0 to 9 with 0 being almost no tension and 9 being the greatest amount of tension possible. A tension dial is shown on the right side (you can see one of the tension disks to the left of the dial). Some newer home machines have dials, most older home machines and all industrial machines have knobs.

Correct tension

Correct thread tension is the LEAST AMOUNT OF TENSION needed to form the stitch reliably and with the desired seam strength. There are several reasons for this:

  • Your machine will run better and last longer with lower tension settings.
  • During adjustment It is much easier to start with the thread tension too loose and adjust it tighter than to start with the tension too tight. This is because it is easy to see loose stitches and then tighten them until they are correct, but it is much more difficult to see if the tension is too tight.
  • Excessive tension can cause a variety of problems such as weakening of the thread or fabric and poor seam strength.
  • Loose tension also causes poor seam strength, but it is much easier to recognize loose tension.
  • As a general rule when you are adjusting tension, start with a lower setting than you think will be needed and work your way up to the correct tension.

Default tension

All sergers have recommended (default) tension settings that are given in the owners manual for the machine. Some sergers have a mark or special color on the tension dials that indicate a default or middle position. The default tension is used as a starting tension and will produce a usable stitch in a medium weight fabric when using regular serger thread and a medium stitch width and medium stitch length. The owners manual will also have tension recommendations for other stitch types and stitch widths.

Most home machines have numbered tension knobs or dials that go from 0 to 9. The default tension will usually be 3 or 4 on the dial (unless your owners manual says otherwise, in that case follow your owners manual). Set the tension for all threads to the same setting unless your owners manual says otherwise. For machines that do not have numbered dials the default tension is usually about 1/4 to 1/3 of the maximum.

Setup, test, inspect

Setup – Make sure that the machine is properly threaded. Set the stitch length to a medium-long setting. Set the thread tension knobs to the default setting for your machine (see the section above).

Test sew – Sew a test seam about four inches long at a slow or medium speed and then remove the fabric from the machine and cut the chain. Mark the fabric with a pen to keep track of which side is the top side.

Inspect – Look at the stitches under a bright light. Use a magnifying glass or reading glasses if needed so that you can clearly see the individual threads.

If the stitch is forming correctly but the stitch looks too loose or too tight then the tension will need to be adjusted, so proceed to the next step. If the stitch is not forming correctly check that the machine is threaded properly. Your machine should form an acceptable stitch if it is threaded properly and the tensions are set as recommended in the owners manual.

Tension adjustment procedure

First look closely at the stitch and determine what part of the stitch is too loose or too tight and then go to the appropriate section below.

Needle thread loose

The needle thread is loose when it has loops or is pulled away from the stitch line by the looper thread. For most types of stitches the needle thread should remain in the stitch line as shown in the example on the right. If the needle thread is loose then tighten it slightly and test again. For machines with a numbered dial try increasing the tension about 1/2 number at a time. If it is still too loose tighten again and test again until it is no longer too loose.

Looper threads loose

. If the looper tension is correct it will look like the example on the right side. If the looper threads are loose, tighten both looper threads by an equal amount and test again. For machines with a numbered dial try increasing the looper tensions about 1/2 number at a time. If they are still too loose tighten again and test again until they are no longer too loose. After the looper threads look good recheck that the needle thread is not now too loose.

Stitch unbalanced

For most stitches the point that the two looper threads interlock or cross each other is supposed to be at the edge of the fabric. If the crossing point is not at the edge then the tensions must be adjusted to move the crossing point to the edge of the fabric.

  • If the crossing point is being pulled to the top side of the fabric the upper looper thread is too tight. In this case loosen the upper looper tension a little and re-test. Re-adjust and re-test if the crossing point is still not right. If the crossing point is now balanced (on the edge of the fabric) but the stitch is too loose then slightly tighten both the upper and lower looper threads the same amount and then re-test.
  • If the crossing point is being pulled to the bottom side of the fabric then the lower looper is too tight. In this case loosen the lower looper tension a little and re-test. Re-adjust and re-test if the crossing point is still not right. If the crossing point is now balanced (on the edge of the fabric) but the stitch is too loose then slightly tighten both the upper and lower looper threads the same amount and then re-test.

Looper threads too tight

  • If the looper threads are too tight the fabric will roll or be deformed. If you are intending to make a rolled edge then this is good, but other wise this indicates that the looper tension is too tight.
  • Even if you are intending to make a rolled edge you don’t want the tension too tight because the machine will not run well and you will be more likely to have miss-stitches and thread breakage. The tension should just be tight enough to roll the edge and get the result you want but no tighter.
  • If the looper threads are too tight it tends to make thread ends from the weave of the fabric stick out from the side of the seam and this is ugly and makes the seam look frayed.
  • As a general rule use only enough tension to get the job done! If you are not sure then back off the tension until the stitch becomes too loose then re-tighten just enough so that the stitch forms reliably.

Needle thread too tight

It is difficult to see when the needle thread tension is too tight, but over tension is a serious issue.

  • Over tension can cause the fabric to bunch up, cause the seam to weaken or could cause the needle thread to break. Over tension can also cause skipped stitches and and inconsistent seams.
  • With a three or four thread overlock stitch the easiest way to be sure that the needle thread is not too tight is to loosen it a little at a time until it starts to pull away from the stitch line or form loops. Then tighten it enough to bring it back to the stitch line or eliminate the loops. The goal is for the needle thread to be tight enough to reliability make the stitch but not much tighter. It is best to use the same tension setting for both needle threads if you are using a four thread stitch, or you can have a slightly higher tension on the left hand needle.

Tension Tips

Fabric and thread for testing – To test a machine for correct thread tension and stitch formation it is best to use different colors of thread and fabric so that the stitches are clearly visible. Some people prefer to use different thread colors for the needle thread and looper threads so that it is easier to see the point where the threads cross each other. For testing, it is recommended to use two layers of a medium weight cotton or polyester material (like a scrap from a bed sheet or pillow case).

Readjustment may be needed if you change stitch length – If you make a large change in the stitch width or stitch length you may need to readjust the looper tensions. In general changing to a longer stitch length may cause your looper tensions to increase, in this case you should decrease the looper tensions slightly. If you go to a shorter stitch length the looper threads may become loose, in this case increase the looper tensions slightly.

Adjust one at a time – Change the thread tension for one thread at a time and make small adjustments, then test sew to see if the adjustment gave you the desired results. If your machine has numbered tension knobs (lets say from one to nine) then a small adjustment increment to use would be 1/4Th or 1/2. After you are close to having the perfect stitch you may want to adjust in increments of 1/4Th.

Right direction – Be sure you are turning the tension knobs or dials the right direction – It is easy to get confused and loosen a thread when you really should be tightening it! For knobs clockwise tightens and counterclockwise loosens. For vertical dials move the dial down is to tighten and up to loosen.

Seam strength – For construction seams it is a good idea to always test the seam strength. Do this by pulling apart a seam and seeing if you can cause it to fail. If you find that more tension is needed so that the stitch will not pull apart then increase the tension enough to meet the requirement, but no more than needed. To gain stitch strength you can also try to decrease the stitch length, in some cases that will work.

Stitch balance – The looper thread crossing point does not always have to be perfectly on the edge of the fabric. Don’t be overly obsessive and continually check and re-adjust the thread tension for every seam. Sergers are not prefect and the crossing point will move towards the top or bottom of the fabric to some degree as you sew different thicknesses of fabric, as you adjust the stitch length or stitch width.

Tension problems – If the tension is always changing and causing stitch problems for no apparent reason then the machine will need troubleshooting and may need repair. After a while you will learn the difference between acceptable variations in the stitch and when there is a problem that requires troubleshooting and repair.

Observing tension as you sew – You should be aware of the stitches that the machine is making as you are sewing and occasionally check the tension and stitch balance (just by giving a quick look at the seam).