Seat Belt Protection

A driver or passenger travelling in a car is moving at the same speed as the car. If the car suddenly stops, the body of the rider inside will keep moving forward at the same speed. This demonstrates inertia - the tendency of a moving object to keep moving, or of a stationary object to remain at rest. An inertia-reel seat belt works on the same principle. Its mechanism includes a pendulum, which hangs vertically under ordinary driving conditions. If the car should suddenly stop, however, it swings forward, and a locking lever resting on the pendulum is released. The lever engages a toothed ratchet that locks the shaft around which the belt is wound. The locked seat belt then prevents the body from being flung forward. When the seat belt is fastened, it winds out from the reel against slight tension from a spring. This keeps it taut during normal traveling, but allows enough free movement for a driver to reach forward as necessary. If the driver tugs on the belt abruptly while winding it out, the locking mechanism will engage and stop the action of the spring. Slackening the belt releases the spring and the locking lever.