How Do Seatbelts Work?

Seatbelts have been proven to save lives when used correctly, but how do they work?

Seatbelts have been proven to save lives when they are used, even though they do cause injuries occasionally. The question here is how can a piece of fabric become the difference between life and death?

Everyone knows that a seatbelt keeps a person from flying through the windshield or being thrown into the dashboard when a vehicle comes to a sudden stop. Inertia is the cause of the person flying through the car after the car has come to an abrupt halt. Inertia keeps a car moving in the same direction at a constant speed while the road friction and air resistance work together to slow the car down making the car's engine compensate to keep the car traveling at the determined speed. All objects located in the car, including the people have their own inertia that is separate from the moving car. Inertia causes the objects in the car to be moving the same speed as the car, making them feel like they are one single moving unit. The difference in the two is obvious when the car come to an abrupt halt and the objects inside it continue with the speed that the car was traveling before the halt, and would not stop until it were exposed to an object such as a windshield, dashboard or such. A seatbelt applies the stopping force to more durable parts of the body over a longer period, helping protect the body from more serious injuries.

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.

Newer vehicles have a pretensioner that tightens the seatbelt. The pretensioner tightens any slack in the seatbelt when the car is stopped quickly. The pretensioner actually pulls in on the belt, moving the passenger into the optimum crash position. Pretensioners are used with conventional locking mechanisms to create a safer seatbelt system. There are several different pretensioners available from ones that pull the entire retractor mechanism backward to ones that rotate the spool itself. Usually pretensioners are wired to the central control processor that activates air bags. This processor monitors sensors that respond to the abrupt halt and activates the pretensioner first, then the air bag.

Seatbelts have saved many lives and will continue to save lives as long as people will use them.

© High Speed Ventures 2011