The Tension Spring is used to regulate the pressure placed on thread while sewing with an MG-3U emblem edging machine. Tension discs squeeze the thread as it passes between them, while the Tension Spring controls the amount of pressure on the discs. When edging with the MG-3U, the needle thread is typically paired with a heavy tension spring, the outer cover thread with a light tension spring, and the inside locking thread with a very heavy tension spring.