Seatbelt System

Seatbelts function to keep an occupant safely in their seat during both regular automobile operation and a collision. The Seatbelt System was chosen for its importance in automobile safety, despite its simplicity. The Seatbelt has become one of the oldest automobile safety devices that are still used today. It plays a crucial role in the safety of its occupant during the operation of a vehicle. The Seatbelt has become far more than just a belt used to restrain an occupant during an automobile collision. The seatbelt system has developed through the years to become a part of the Safety-Restraint System (SRS), which works in effort with the Airbag system to minimize the risk of fatal injury during an automobile accident. The seat belt is the single most important restraint system in the vehicle. When worn correctly, it reduces the possibility or severity of the occupant striking the interior of the vehicle or the likelihood that the occupant will be ejected from the vehicle in an accident. A properly worn seat belt also helps to hold the occupant in the proper position in relation to the air bag so that the occupant can benefit from its deployment, if required.

Seatbelt History – The First Belt

Seatbelt like devices have been around since the early 19th century, although they were not used in automobiles. In the early 1950's lap belt type seatbelt's began to be offered in automobiles. The modern three-point seatbelt utilized today was invented by Nils Bohlin in 1949. In 1959 Volvo became the first automobile manufacturer to implement it in production vehicles. The three-point seatbelt offered tremendous improvements over the previous lap belt design.

Seatbelts – How do they work?

The seatbelt is specifically designed to keep the occupants body safely in the seat as well as fit comfortably over the occupant’s shoulder and pelvis with minimal restriction. Most modern seatbelts use locking retractors to stop a belt from extending during a quick deceleration. These retractors use a pendulum mechanism that locks a belt in place when the pendulum swings too far away from it’s rest position. A feature present on newer seatbelts is pre-tensioners. These remove the slack from the seat belt at the moment of impact. The mechanism then releases the seat belt slowly, absorbing the momentum of the moving body to reduce the risk of seatbelt injury.

Features:

Emergency tensioning devices and seat belt force limiters.

Locking retractors

Driver and front passenger air bags.

Special features of dual-stage front air bags.

Special features of an Occupant Classification System.

Side impact air bags.

Head protection systems.

Window curtain air bags.

Head-thorax side air bags.

Seatbelt Mechanism: 

In a typical seatbelt system, the belt webbing is connected to a retractor mechanism. The central element in the retractor is a spool, which is attached to one end of the webbing. Inside the retractor, a spring applies a rotation force, or torque, to the spool. This works to rotate the spool so it winds up any loose webbing.

When you pull the webbing out, the spool rotates counter-clockwise, which turns the attached spring in the same direction. Effectively, the rotating spool works to untwist the spring. The spring wants to return to its original shape, so it resists this twisting motion. If you release the webbing, the spring will tighten up, rotating the spool clockwise until there is no more slack in the belt.
The retractor has a locking mechanism that stops the spool from rotating when the car is involved in a collision. There are two sorts of locking systems in common use today: systems triggered by the car's movement

The first sort of system locks the spool when the car rapidly decelerates (when it hits something, for example). The diagram below shows the simplest version of this design.

The central operating element in this mechanism is a weighted pendulum. When the car comes to a sudden stop, the inertia causes the pendulum to swing forward. The pawl on the other end of the pendulum catches hold of a toothed ratchet gear attached to the spool. With the pawl gripping one of its teeth, the gear can't rotate counter-clockwise, and neither can the connected spool. When the webbing loosens again after the crash, the gear rotates clockwise and the pawl disengages.

The second kind of system locks the spool when something jerks the belt webbing. The activating force in most designs is the speed of the spool rotation. The diagram shows a common configuration.

The central operating element in this design is a centrifugal clutch -- a weighted pivoting lever mounted to the rotating spool. When the spool spins slowly, the lever doesn't pivot at all. A spring keeps it in position. But when something yanks the webbing, spinning the spool more quickly, centrifugal force drives the weighted end of the lever outward.
The extended lever pushes a cam piece mounted to the retractor housing. The cam is connected to a pivoting pawl by a sliding pin. As the cam shifts to the left, the pin moves along a groove in the pawl. This pulls the pawl into the spinning ratchet gear attached to the spool. The pawl locks into the gear's teeth, preventing counter-clockwise rotation.

In some newer seatbelt systems, a pre-tensioner also works to tighten the belt webbing. In the next section, we'll see how these devices work.