Thread tension is a bit of a challenge: without it nothing works. Getting it right seems to take a bit of fussing. There are many good descriptions available of how to adjust the thread tension. The point of this post is less about setting the perfect tension than it is about understanding what is going on with thread tension. My goal was to measure the thread tension directly. Having access to a force sensor and some other incidental equipment that I usually use in the physics lab where I teach, it occurred to me that I could use the force sensor to measure the thread tension on the sewing machine.
As one would expect, the higher the tension setting number the higher the thread tension. The standard unit for measuring tension is Newtons (or mN): here I have converted the measurements into grams, another common way to measure tension.
The data here are the numbers I measured. I have no idea of what they should be. I have not been able to find any clear reference as to what should be expected for the top tension.
I did find a reference for this: the TOWA bobbin tension gauge.
It suggests the bobbin tension should be 25-30g
Having calibrated my top tension and the bobbin tension, I made various sample stitches to determine which setting gave the best results. The sample consisted of three layers of cotton muslin, Gutermann Mara 100 polyester thread, and a 65/9 needle. The best results were achieved with a tension setting of 5 which corresponds to a top tension of approximately 68g and a bobbin tension of 35g (I used a microscope to photograph the following pictures).
The microscope tends to reduce depth perception so I attempted to cut away the fabric to look at a cross section. You can just see the loop formed inside the fabric.
My best guess as to why there is a difference between the top tension (68g) and the bobbin tension (35g) is the resistance of the fabric. That is, some additional top tension is necessary to pull the lock-stitch into the fabric. However, I am no expert, these are only my observations.
Based on the above observations, I’m assuming that the correct thread tension depends on the following parameters:
- Thread type and size
- Fabric and weave
- The desired properties of the stitch to be sewn
My next challenge is to explore these other combinations.
Thanks for reading.