Bobbin Tension and Balanced Stitches

We really needed to shed some light on the lower tension or bobbin tension. Regardless of how we refer to it the ability to control the tension allows us to stop the loops and bird nests and make a perfectly balanced stitch. I try not to be brand-specific because I intend to help others learn the science of stitch formation to apply the concepts to the type of machine they are dealing with. That way no matter if it is a Singer, Kenmore, Elna, Necchi, Brother, Pfaff, and the list goes on and on the science is the same for forming a stitch. Tension is nothing more than a means of controlling the top and bottom threads during the stitch formation. It doesn’t matter how tension is applied,(type or style) as long as it controls the thread accurately and consistently.

Bobbin Tension

Although the exact duties of the lower thread are vastly different than the top thread. The tension is just as important in controlling the stitch quality. If the lower tension is too loose, the top thread will pull the loops through the fabric, causing them to appear on the top side. It will cause excess thread to hang haphazardly under the deck in the bobbin area just waiting to snarl around the hook. It will tie the mechanism into a creative knot that will stop even a freight train of a machine and can do seriously ugly things to the timing of the machines.

If the lower tension is too tight then the thread will stretch tight. It will not allow the top thread to pull back up with the needle so the stitch has a ridge on the underside of the fabric at every intersection of the two threads. The resulting stitch will then lose its ability to flex and move with the fabric. This can cause premature wear and thread breakage at the seams.

Balancing Stitches

The solution is to balance the stitch. That means we match the tension of the top thread with the bottom thread. Pushing our top thread through the fabric with the needle so it intersects with the lower thread. As the needle rises it pulls the lower thread halfway through the fabric as the feed dogs advance for the next stitch. The takeup spring lifts the top thread to follow the travel of the fabric. It also slows down the bounce created by the take-up lever travel. The lower tension keeps the bobbin from spinning off too much thread as the needle pulls the lower thread up. This would then result in a perfectly balanced stitch.

Thread Size

Sounds easy enough,until we start adding all the variables which can change the delicate balance. Starting with thread we need to keep our top thread and bottom thread the same diameter, size, and weight. Mixing types of thread can create different tensions on the top or bottom, requiring you to re-adjust the balance.

Needle Size

Matching our needle size to our thread size will also help. Choose the proper needle for the desired fabric being sewn. These are things which must be accounted for before we try to balance tension issues.

As you can see tension is not a cut-and-dry type of situation. Instead of easy black-and-white solutions, we find ourselves dealing with a lot of grey solutions. So much grey is involved that I can’t possibly tell you exactly what to do in every situation. Instead, I am going to share with you my routine. Hopefully, you will be able to adapt my technique to accommodate your continually changing needs and specific applications.

Setting A “New” Sewing Machine Tension

Whenever I set the tension on a new to us machine I always clean the machine thoroughly and oil or lubricate all of the appropriate places. If the upper tension is numbered or has graduated increments, I adjust it until zero tension touches the thread, allowing it to pull through without any drag.

Setting Bobbin Case Tension

I then use a full bobbin, properly installed and threaded in the bobbin case. I hold the thread and do a drop-bounce test. Then adjust the tension screw until the drop-bounce test allows the bobbin and case to fall 1-5 inches before it stops releasing thread and holds in place. The heavier the bobbin case the farther I let it slide. Most bobbin cases are well-matched in weight so I expect them to slide about 3 inches for the drop-bounce test.

I install the bobbin case into the machine and move to the upper tension. Once the machine is threaded properly I set the tension knob on about 2-3. If it doesn’t have numbers or increments then I turn the tension knob 1 full revolution for the rough adjustment.

In the Quilting Room with Mel, we generally sew a lot of cotton fabric. I use the fabric we sew to set the standard for the tension. Same thing with the choice of needles, for me, it is generally a #12, and for Mel, it would be #14. This gives me a pretty decent starting place and from there, it is all about balance.

Fine Tuning The Stitch Balance

I adjust the top thread first then the bottom. If for some reason my starting parameters do not allow me to adjust to a balanced seam I start over by setting my top tension back at a 2 and adjust the bobbin tension until a tight balanced seam appears. Keep in mind the grey variables and don’t be discouraged if a balanced seam doesn’t appear the first time. Persistence will pay off greater dividends.

Drop-In Bobbin Tension

When the machine uses a drop-in bobbin the process slightly differs because the bobbin tension cannot be preset with a drop-bounce test. So I adjust the lower tension for a slight drag approximately double the tension or drag as the top tension has when it is at its starting preset and from there the routine is exactly the same. As with any other learned skill the more you do the easier and better it becomes.

I hope this helps some of you and of course if you get a good laugh from my futile attempts to balance a seam it is still all about enjoying the love of sewing and the machine. Until next time may you all be blessed with perfect seams and unused seam rippers.