Understanding Thread Tension
by Claire Shaeffer
excerpted from Threads #78, pp. 39-41
Many sewers avoid the tension dials on their machines like the plague, certain they'll only make matters worse if they make adjustments. In fact, there's nothing very mysterious about setting and adjusting thread tensions on your sewing machine, whatever its make and model. What's potentially more confusing is that many apparently tension-related problems are caused by factors other than misadjusted tension dials. Let's look closely at how to identify and correct "tension" problems, both with and without touching the tension settings.
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|Tension devices and proper threading
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You can't get proper tension without correct threading. All machines have basically the four tension devices shown here- thread guides, tension discs, tension regulator for upper thread, and bobbin-case spring for bobbin thread- which ensure that the same amount of thread flows simultaneously from the needle and the bobbin, producing a symmetrical stitch.
Meet your tension tools
In order to form a row of stitches that looks the same on both sides of the fabric, the same amount of thread needs to flow from the spool and the bobbin simultaneously. This is accomplished by running the threads through various tension devices, including the thread guides, tension discs, and tension regulator on the machine head for the upper thread(s), and the bobbin-case spring for the bobbin thread (see "Tension devices and proper threading"-right). Some machines also include a small hole in the bobbin-case finger, through which to feed the bobbin thread to increase the tension for improved stitch definition when topstitching, satin-stitching, and embroidering, without touching your tension settings.
The tension discs and tension regulator together are called the tension assembly. The tension discs squeeze the thread as it passes between them, while the tension regulator controls the amount of pressure on the discs. On older ma- chines there are only two tension discs, controlled by a screw or knob. On newer models there are three discs controlled by a dial or key pad on the front of the machine, which can regulate two threads at once.
In either case, the tension regulator is elementary: When adjusted to a higher number (turned clockwise), the discs move closer together, increasing the amount of pressure. Turned to a lower number (counterclockwise), the discs move apart, decreasing the pressure. Using a thicker thread without resetting the dial will increase the pressure and cause the upper thread flow to decrease, unless you've got a newer machine that makes automatic upper-tension adjustments. Since the bobbin tension is not self-adjusting, the lower tension may need to be adjusted manually to match.
In addition to guiding the thread along its path, each thread guide exerts a small amount of resistance on the thread, adding to the tension from the discs to achieve balanced tension. Bottom line: Always make sure all guides are threaded before stitching.
The flat bobbin-case spring exerts pressure on the thread as it comes out of the bobbin case. The amount of pressure is regulated by a small screw at the rear of the spring. Both the spring and screw are easy to locate when the machine has a separate bobbin case. When the machine has a drop-in bobbin with a built-in bobbin case, locating the tension screw can be more challenging. Both types are shown in the drawings below. In either case, to increase the resistance, use a small screwdriver to turn the screw clockwise (to a higher number) or counterclockwise (to a lower number). Turn the screw in small increments and never more than a quarter-turn between tests. This helps you keep track of how much you're changing your settings and reduces the risk of losing this very small screw.
|The bobbin-spring screw regulates bobbin-thread tension, whether your bobbin is a separate, drop-in unit (left) or is built into the machine (right).|
As with the tension dials, the amount of pressure will be increased when thicker threads are run under the bobbin spring. To eliminate the need to fiddle with the bobbin-case screw, many sewers (myself included) have two bobbin cases: one set for general sewing and the other for adjusting to less frequently used threads.