How a small sensor stops a long belt
Inside a seatbelt retractor, the belt is wound around a spool that can turn as the webbing is pulled out or drawn back in. A small weighted pendulum sits beside a locking mechanism. When the car slows sharply, the pendulum’s inertia makes it swing relative to the car instead of moving in perfect step with it. That motion nudges a lever into position.
The lever engages a toothed wheel connected to the spool. The teeth can turn in the ordinary direction while the belt is being used, but the engaged lever blocks the spool from paying out more webbing. The belt then restrains the occupant rather than unwinding freely.
Why it stays out of the way in ordinary driving
The mechanism is designed to leave the spool free during routine starts, stops, and turns. The pendulum moves enough to trigger the lock only when the vehicle’s motion changes sharply. This makes the sensor a mechanical switch: it converts a sudden change in motion into a physical barrier at the spool.
That direct connection is the key to the design. A small movement inside the retractor can stop a much larger length of belt from extending, helping the belt hold its position when restraint matters most.
