Adding to the list of complex devices that require the use of a simple mechanical spring, today we're going to talk about the "check spring." What is the check spring? If you haven't used a sewing machine, you probably have no idea just how crucial this very simple device is to the machine that is responsible for keeping your clothes together.
Most people (even experienced sewers) do not know how a sewing machines works; it's just one of those inventions that people take for granted. You see a needle drive into a piece of fabric as the fabric is pulled through the machine and a stitched piece of fabric comes out on the other side. Though this seems like it is a tremendously complex machine, the way it works is quite simple.
As you can see from the animation, the real action of a sewing machine occurs out of sight; just below the stitch plate. The needle carries the thread through the fabric to meet up with another device called the hook (as represented in red in the animation). While the needle is carrying one supply of thread, the hook is guiding another supply of thread from a smaller spool known as the bobbin. As the upper thread is brought down by needle, the hook helps guide the lower bobbin thread to form a stitch with the upper thread. In space between where the needle and the hook interact is often measured in fractions of a millimeter to ensure proper stitch formation. Even though this process seems highly technical, the action itself is not much different than someone tying a form of knot or stitch by hand, but rather the entire system is automated to be able to form as many as 3,200 stitches in a minute on an industrial machine and around 1,500 stitches a minute maximum on a sewing machine for the home.
Though this seems complex enough as it is, none of this could take place properly without this simple device:
Though it looks relatively straightforward, the check spring (as it is referred) is crucial for proper stitch formation. Located on the upper "head" of the sewing machine typically near where the upper thread tension is adjusted, the check spring keeps tension on the upper thread as the take-up lever (an arm-like device also on the upper "head" of the sewing) is in the process of pulling the upper thread up to tighten the stitch. Without the check spring, the additional slack in the thread would cause the take-up lever to throw the thread around and get tangled on other components of the sewing machine and negatively impact the desired tension of the stitch.
|A check spring just below the take-up lever.|