Take Up Spring
The take up spring has three functions. As one function it acts as an electrical contact for the thread break detector for the upperthread (above). It also holds the thread as the needle comes down into the fabric until the eye of the needle penetrates the fabric. This prevents the thread from flopping in front of the needle. In the third function the take up spring acts like a little shock absorber while the stitch is being formed. When the hook is in the six o'clock position (below) the take up spring lets out the thread being pulled by the hook and relieves the strain on the thread. The spring continues to act like a shock absorber as the loop of the upper thread is being pulled into the fabric by the take up lever (bottom). This prevents the take up lever from pulling too hard on the thread.
The six o'clock position of the hook, the point at which the maximum thread is being pulled off by the hook.
Upper thread tensioning and take up lever. The take up lever lets out thread according to the demand of the hook.
Both the length of the movement of the take up spring and the strength of the spring effect how the stitch is formed. Too long of a movement and the take up lever does the pulling and the spring is collapsed too much. Too strong of a spring and the machine will pull up a tight stitch but it may break the thread at the six o'clock position.
With Polyester thread that is looping often adjusting the spring to a longer and stiffer movement will help control the looping. Looping is caused by the poly thread stretching and not returning to its original shape. By increasing both the length of the movement and the strength of the movement, the take up spring moves instead of the thread stretching, thus avoiding the creation of a loop.