In a typical seatbelt system, the belt webbing is connected to a retractor mechanism, also called a seatbelt spring. The central element in this mechanism is the spool, which is attached to one end of the webbing. Inside the retractor, a spring applies a rotation force (torque) to the spool which then works to rotate it so that it winds up any loose webbing.
When the webbing (seatbelt) is pulled out, the spool rotates counter-clockwise turning the spring in the same direction. Effectively, rotating the spool untwists the seatbelt spring and because the spring wants to return to the original shape it was in, it resist the twisting motion. This results in the spring tightening up, when loosened, until there is no more slack in the belt.
These retractors are vital in a car because they have a locking mechanism that stops the spool from rotating when a car is involved in a collision.
Generally seat belt springs are made from textured, rolled, hardened & tampered high carbon steel strips in coil form, ranging from 0.18 mm to 0.35 mm thick & 5.00 mm to 12.00 mm wide.
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