Spiral spring is also called the power spring , which is made by the steel ribbon, it can be divided into the natural type power spring and the prestress force type power spring due to its relaxed shape. The natural type power spring is made directly by the wrapped steel,  while the prestressed force type power spring will finalize the shape first, then reverse the direction into a spiral shape, its shape after spread out just looks like the letter S , hence it is also known as the S shaped spring.
When we use the spiral spring, we need to use a case to confine its outer diameter, the middle of the spring is connected to the line shaft. Once the line shaft and the spring do relative rotation by the external force, the spring will be rolled onto the line shaft and store power. After the external force disappears, the spring generates torque force to drive the line shaft, and output the storage power via the line shaft at the same time.
Comparing under the same specification spring, the prestress force type can store more torque force of  25%~40% , it can provide more rotation turns under the same space.