Since three- point harnesses are the most common seat belt that people use today, here is a quick list of the main parts used in a typical shoulder seat belt system:
How Seat Belts Work
How a seat belt works isn't too difficult to understand, the genius is in it's design. The typical shoulder belt is anchored to the seats frame. This is a fixed attachment to minimize flexibility in the belt. Along the belt you'll find a seat belt clip; the clip slides along the belt so it can strap down people of all sizes. The clip secures into the buckle, which acts as the second anchor point to the harness. The buckle is secured to the frame on the other side of the seat. The belt then crosses back diagonally over the seat up to the B-, C- or D-Pillar. Although the seat belt can loosen or tighten the belt from the retractor mechanism, this location acts as the third anchor point in the harness. The retractor mechanism allows the seat belt to extend or retract in order to make the occupant comfortable, but can lock the seat belt in place when the vehicle stops abruptly or if the web belt moves too quickly.
The retractor mechanism retracts the seat belt by using a wound spring. As the seat belt is pulled out, the mechanism rotates counterclockwise, which makes the spring expand. This makes the spring want to coil back into its original shape, thus it pulls on the belt if there is extra slack. The retractor mechanism locks the seat belt in place by using a pendulum counter weight and a locking system. If a vehicle stops abruptly enough, the counterweight will slide forward. The weight leverages a blade into a ratcheting gear, forcing it to halt the seat belts movement.
In most modern vehicles, pretensioners are also used to maximize the safety of the occupants. Where a retractor is used to stop the seat belt from loosing up in a crash, a pretensioner is used to actually tighten the belt up. This tightening can occur either before or after the ratcheting gear has been locked by the pendulum mechanism. This minimizes the person's body movement, helping to avoid injury and death. Most pretensioners work by using an electric motor or solenoid to tighten the belt via a ratcheting mechanism. Pretensioners are activated by a sensor in the vehicles body, which means they are only activated during a collision.
Common Seat Belt and Safety Restraint Components
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