A lot of people are intimidated by the necessity to adjust thread tension on their sewing machine. Yet, as every sewing machine manual tells you, both bobbin and upper tension need to be checked and adjusted for every project, sometimes several times during the project, because these settings depend on many factors, such as the type of thread, the type of needle, the foot, the foot pressure and of course the material you’re sewing, and how many layers of it you’re sewing.
And no, it is not just for vintage machines. Some modern machines have sensors and clever electronics to adjust tension automatically. This works well enough only on very expensive, top of the range machines. On others – based on the experiences of people crying for help – you’re better off with a manual control.
But be it as it may, people are still intimidated by tension adjustments. If you want a detailed guide to tension adjustment, see this page by Alex Askaroff. It used to be free, but now there’s a charge, but perhaps you can still find it somewhere out on the Internet free, as before.
Personally, I think there is nothing scary or mythical in tension adjustment, and also it is not so complicated as to require a whole book to describe it. After all, your manual only dedicates one or two pages to it – and there is nothing else to tell!
It’s all about balance
On a lockstitch machine, there are two threads that form the stitch, and their tensions need to be the same to have a balanced stitch. This is what it is all about: balance.
My method of balancing tensions is to start with the bobbin, and then to adjust upper tension to match.
Adjusting bobbin tension
The method is the same for all bobbin cases that have a spring that presses down on the thread, and a screw that adjusts the pressure. Also machines with drop-in bobbin have such a spring and a screw – check your manual if unsure.
You loosen the screw to lighten the tension, and tighten to tighten. The bobbin case should deliver a full range of tensions – from free moving to completely stuck. If it doesn’t, it’s probably dirty – when was the last time you cleaned under the spring? It is fiddly but needs to be done: remove the screw (or screws) completely, take off the spring and behold layers of solid lint both on the underside of the spring and on the bobbin case. Ugh! Scrape it off and polish the spring and the bobbin case. Reassemble and don’t wait another 50 years before cleaning it again. 😉
If your bobbin case still doesn’t deliver a full range of tensions, the spring became deformed due to all that lint under it. You need to bend it back into a better shape, by trial and error. I talked more about it in a previous post.
Ok, now that your bobbin case is working properly, start by setting it to a “medium” tension. Put the bobbin in, thread it and let the bobbin case hang by the thread. Adjust the tension screw so that the bobbin case with the bobbin just only manages to hang without unravelling. It’s not very precise, I know, but it’s a start!
Adjust the upper tension
Put the bobbin into the machine and try sewing. Now adjust the upper tension until you get a passable stitch.
Hint: if you get big loops on the underside of the fabric, your upper tension is way too loose. Tighten it significantly. Those big loops may cause the mechanism to jam too – it should all be sorted once you increase tension.
If the upper thread breaks, the tension is probably way too tight. To be sure, lower the foot, grab the thread with your fingers just before it comes into the needle and pull on it – this is the tension with which you are trying to sew. If you can’t get any thread there, neither will the machine, and the thread will break. (If the thread pulls out very easily, lower the foot properly and try again.)
The upper tension assembly must be able to deliver a full range of tensions – from free moving to completely stuck. If it doesn’t, there is something wrong with it, and may be that’s why you’ve been having so much tension trouble!
But first of all, re-thread the machine. Get your manual out and make sure that the thread passes through all the guides correctly, and that it passes between the disks of the tension unit and not next to them. Old machines had easy to thread tension units where you could see exactly what was happening, but new machines have a “clean” design with the tension unit hidden from view – I can never be sure I got the thread between the disks correctly! Hope and pray, hope and pray… 😣
Simple old tension assemblies are difficult to mess up, so they usually always work (although they could do with cleaning and polishing to make sure that thread doesn’t catch on anything). Newer “clever” tension assemblies however have a lot more parts which can get lost or damaged over time, so these “improved” tension units often get broken and need replacement. Some improvement! 😣 (I often see on eBay from people selling their sewing machines: “Fully working but tension needs adjustment“. First, it is not “fully working” if you can’t get the tension right, and second, it most likely means that the upper tension unit has been messed with or is broken and needs replacement. It is never that the owner hadn’t tried adjusting the tension – there is always a bigger problem. Such has been my experience.)
Sorry, there is no quick fix guide for the upper tension assembly. They vary quite a lot, so need to be considered individually.
Assuming that your tension assembly is fine, try to get the threads to interlock inside the material rather than on either side of it, don’t worry at this point whether the actual stitch is good or not. Make it balanced – it’s all about balance!
Now check your stitching. Is it too tight? You need to rebalance at a lower tension. Is it too loose? Rebalance at a higher tension.
Start with the bobbin tension again. Loosen or tighten it a bit depending on which way you need to rebalance. Then adjust the upper tension to match, such that the threads interlock inside the material again.
A simple way to check whether the tension of the threads is balanced, is to pull your stitching sample along the seam. If one of the threads becomes more taught than the other, your stitching is not balanced – and you now know how to fix it! 😀
Tuning tension as you sew
No, I don’t mean literally during sewing. 🙂 But as you start paying attention to your stitching, you’ll notice that – as I said straight away – correct tension depends on so many factors. Tension setting often needs to be different for straight stitch, zig-zag and fancy stitches, and sometimes you are looking for special effects like satin stitch or purl buttonholes. These are done with skewed tension, and often with special thread. This is one of the reasons I use several machines during a project: so I wouldn’t need to change settings, but only to switch machines.
But even on the same machine, tension may need small adjustment as you go on. If you set it for the usual two layer seam, but suddenly find yourself sewing over eight layers, don’t be surprised if the stitch comes out too tight. A thicker material needs looser tension! A quick fix here is to slightly loosen upper tension before sewing that extra thickness. The seam won’t be perfectly balanced – you’d need to adjust bobbin tension for that as well – but at least it won’t be too tight, and it will be good enough in most cases. And it is only a small adjustment that is easy to reverse back to normal.
Another common thing is gathering – either slight to “set” the fabric, or full for proper gathers. The easiest and most flexible way of doing it is to run a gathering stitch along the edge, then pull one of the threads to gather, and then to sew your gathered panel in place with a separate stitch.
The gathering stitch is a straight stitch with the maximum stitch length and increased upper tension. The higher the tension, the more gathering you get “automatically”, and then you can adjust the gathers by pulling the ends of upper thread (make sure you leave long ends!).
Again, this is the kind of “on the fly” tension adjustment that is easy to revert when you are returning to regular sewing.
There are quite a few urban myths surrounding tension adjustment. They all will have you believe that it is something mysterious, possibly coming directly from the Allmighty! No, it’s not. 🙂
“Tension is set correctly at the factory and must not be touched.” No, just no. See all of the above.
“Bobbin tension may only be adjusted by a certified engineer.” Huh? No. ☺
“Tension setting does not depend on the type of thread.” 😝
“Tension must be set exclusively by your repair shop.” Someone is seeing dollars here! 🤑
I’m sure you’ve heard a few more yourself. Don’t be fooled! Tension adjustment is not that difficult and is for you to do, not some certified engineer! You adjust stitch length and zig-zag width all by yourself, don’t you? Thread tension is no different.