How does retractable tape measure work?

The tape measure is a tool that best reflects the principle of energy storage of a clockwork spring. The end of a 5-meter-long steel rod is connected to the front end of the clockwork spring, and the end of the clockwork spring is fixed in the middle of the tape measure. The difference between the longest and the shortest received by the mainspring will be greater than 5 meters, which means that every time the ruler is reset to zero, the mainspring has room to tighten the ruler; when the ruler is stretched to 5 meters, it will not The spring exceeds the deformation limit and cannot be retracted.