No single set of adjustments on an embroidery machine contribute more to overall embroidery quality than ensuring the thread is run at the optimum tension. Productivity will also be enhanced as poor tension is one of the leading causes of thread breakage on the machine.
It can be a complicated affair as there are two aspects to consider:
Bobbin thread tension: only one thread needs to be tensioned and it is best to set the bobbin thread tension first as this will control the tensions of all the top threads
Top thread tension: as many as fifteen different top threads need separate adjustment to ensure perfect tension on each needle - being not too loose and not too tight.
The program illustrated above get each different needle to embroider the letter 'H' to demonstrate the top thread tension of that needle.
It is important that you ensure the bobbin tension is adjusted correctly (see table below) so that the top thread tension rings can be adjusted individually without needing to touch the bobbin check spring again.
All tension settings are best assessed by turning the embroidery over and judging the appearance from the back.
The ideal settings show that the bobbin thread should capture 36% of the available width on the back of the lettering.
Assuming the bobbin thread tension to be correct each top thread can be individually adjusted until correct tension is achieved.
Analysis of the above:
You can see that the quality of embroidery varies considerably between each row of needles; assuming the bobbin tension is set at 35g (for Coats alcazar) then only the red row of 'H's' have the correct top thread tension; the top row of blue 'H's have too tight a tension and the yellow row have too loose a tension and require adjustment so that the bobbin thread occupies 36% of the letter width as illustrate in the center row.
The device illustrated above is the most accurate way of assessing thread tension.
The correct thread tension is as follows:
| Coats Thread Type | Top thread tension (g) | Bobbin thread tension (g) |
| Coats Alcazar - viscose rayon | 135-140g | 35g |
| Coats Sylko - trilobal polyester | 160g | 40g |
| Coats Sylko Metallic - metalised polyester | 150g | 40g |