The A-Z Guide to Sewing Machines: Tension

Contrary to popular belief there is no great mystery about setting tensions and in practice it's not difficult. If your tension is poor then simply correct it as explained below.

The machine tension, correctly set so that the bobbin thread and top thread lock in the middle of the fabric, needs very little adjustment for the life of the machine. There are a few exceptions, however, e.g. when making pin tucks or sewing machine embroidery. For general sewing it's not necessary to frequently alter your tension. Modern machines are capable of sewing most fabrics with the same tension setting, and it's the needle size, pressure and the length of stitch which needs to be adjusted to suit the fabric, not the tension. As a general rule the thinner the fabric the shorter the stitch length.

Ignore books which say the bobbin tension never needs adjustment — if the tension is poor you must correct it.

Step-by-step method of setting tensions

1. Thread the bobbin with one colour thread and the top with a contrasting colour thread of the same thickness and manufacture.

2. Set the top tension indicator to the middle of its range, e.g. if the machine has a knob marked 0-9 set it between 4 and 5.

3. Use a small piece of medium weight fabric, e.g. cotton twill or calico, to test the tension by stitching a line of straight stitch and a line of zig-zag. (It is often easier to see if the tension is correct on zig-zag stitching).

4. Check to see if the threads are locking in the middle of the fabric (fig. 200). You should not be able to see any ‘dots’ of the bobbin colour on the top, or any ‘dots’ of the top thread on the underside of the stitching. The same applies to the zig-zag sample. If the two threads don't lock in the centre of the fabric correct the bobbin tension as below.

Fig. 201 A ‘front loading’ bobbin case indicating the tension adjustment screw

Fig. 200 Good tension: the top and bottom threads lock in the middle.

Fig. 202 A ‘drop-in; bobbin showing the tension adjustment screw

5. To tighten the bobbin thread. If the bobbin thread colour appears on the top of the fabric the bobbin thread is too loose and needs tightening by turning the screw on the bobbin case, or if your machine has a drop-in bobbin, turn the screw located to the side of the bobbin. (If figs. 201-202 are not applicable to your machine, consult your manual). To tighten the bobbin tension, turn the screw clockwise with a small screwdriver. Try turning it a quarter turn at a time and then sew a sample. It is a case of trial and error and you may need to tighten the screw in several stages before you get the threads to lock in the centre of the fabric.

6. To loosen the bobbin thread. If the top thread shows on the underside of the fabric the bobbin tension needs loosening. This can be achieved by turning the screw on the bobbin case, or for the drop-in bobbin, by turning the screw alongside the bobbin, anti- clockwise. Use a small screwdriver and try turning the screw a quarter turn at a time anti-clockwise. Then sew a sample of straight stitch and a zig-zag sample. You may need to adjust the screw several times until you are happy with the stitching. Occasionally, especially with an old, well used machine, the threads on the screw become worn and the bobbin case needs replacing.

Achieving a good tension with the top tension knob set in the middle of the dial allows room for adjustment. To increase the top tension turn the knob to a higher number, to decrease the top tension turn to a lower number. Adjustment is necessary when two different weight threads are being used, i.e. the top thread is a different thickness from the bobbin thread. For instance, if you were using a metallic thread on the top and a synthetic thread on the bobbin the top tension would need adjustment to balance the threads. The same would apply if a thin silky embroidery thread was being used on the top and a synthetic thread was wound on the bobbin.

Often when a block satin stitch is being sewn the top tension needs to be loosened. You will soon become familiar with altering the tension to achieve a good stitch quality but always remember to turn back to the middle of your dial for normal sewing.

NOTE: If, whilst sewing, your stitching suddenly be comes erratic, don't reach for the tension knob! Look at the faults page. It is never the tension which needs adjusting.