How does a seat belt know when to lock in a crash?

I mean sometimes when I lean forward in the seat it locks and I can’t move forward but this rarely happens. I am able most of the time to move freely and the seat belt moves forward with me. So how does it know when to lock and hold you back? Is it so sudden quick pull on the belt or something? Thanks.

Typically, seatbelts consist of a lap belt and a shoulder belt, which are securely attached to the frame of the car in order to keep passengers in their seats.

The seatbelt applies most of the stopping force to the rib cage and the pelvis, widening the area of stopping force so the damage is less.

The seatbelt itself, made out of webbing, has more give than a windshield or dashboard making the stop not quite so abrupt.

Seatbelts also extend and retract, except when the car comes to an abrupt halt, and then they tighten up and hold the passenger in place.

The belt webbing connected to a retractor mechanism creates the seatbelts extending and retracting movement.

This retractor mechanism contains a spool attached to one end of the webbing and a spring, located inside the retractor.

The spring applies a rotation force to the spool rotating the spool and causing the webbing to wind up.

When pulling the seatbelt out, the spool rotates counter-clockwise, turning the attached spring in the same direction. When releasing the seatbelt, the spring tightens up and rotates the spool clockwise until all the slack is taken out of the belt.

The retractor also has a locking mechanism that halts the rotating of the spool when the car is in an accident. There are two locking mechanisms used in cars, the car’s movement triggers one while the belt’s movement triggers the other.

Locking mechanisms triggered by the car’s movement locks the spool when the car stops abruptly. In this system, the main operating element is a weighted pendulum that swings forward when the car is stopped suddenly.

A pawl located on the other end of the pendulum catches a toothed ratchet gear attached to the spool. When the pawl grips the teeth, the gear is unable to rotate counter-clockwise and consequently the spool cannot rotate either.

When the seatbelt is loosened after the incident, the gear rotates clockwise and the pawl disengages, releasing the ratchet gear teeth.

The second system that locks the spool when the jerking the seatbelt contains a centrifugal clutch, which is a weighted pivoting lever mounted to the rotating spool.

As the spool spins slowly, the lever does not pivot and a spring keeps it in position. When jerking the seatbelt the spool is spun quickly, the centrifugal force drives the weighted end of the lever outward, and the lever pushes a cam piece mounted to the retractor housing.

When this cam, connected to a pivoting pawl with a sliding pin, shifts left, the pin moves along a groove in the pawl. This movement pulls the pawl into the spinning ratchet gear attached to the spool and locks into the gear’s teeth, preventing counter-clockwise rotation…………….

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