Seat Belts and Pretensioners: Safety Systems in Modern Vehicles
Seat Belts and Pretensioners
Conventional Seat Belts
The primary function of a seat belt is to restrain passengers during a frontal collision or any event involving strong deceleration.
This restraint minimizes physical harm, particularly to the cervical region, head, and chest.
Additionally, the seat belt supports the body against the seat, preventing ejection from the vehicle.
A standard seat belt typically features two or three anchor points. The belt itself is made of durable fabric wound around a spool. A buckle slides along the belt and secures into a quick-release closure. An automatic retractor system, utilizing a coil spring, maintains tension on the belt, keeping it snug against the occupant’s body and retracting it when not in use.
The locking mechanism allows for slow, controlled forward movement. However, in the event of an impact or sudden deceleration, it activates a system of checks and balances within the reel, causing it to lock. This prevents the occupant from colliding with the steering wheel, dashboard, or other interior components.
The locking mechanism also incorporates a steel ball or inertia sensor designed to detect abrupt movements or rapid deceleration. This triggers the ball to lift from its seat, engaging a pin that locks a sprocket.
Key components: Buckle, Belt, Reel, Mechanisms, and Spring.
Pretensioners
Pretensioners eliminate slack in the seat belt, ensuring it is taut during a collision.
This action keeps the occupant firmly secured in their seat, enhancing the effectiveness of the seat belt. Pretensioners are integrated into either the belt reel assembly or the closure and utilize a mechanical or electrical detonator.
Mechanical Activation Tensioner
These tensioners operate mechanically, employing a pyrotechnic or explosive charge with a hammer for activation. Upon collision, the charge detonates, driving a cable or rotating a spool to tighten the belt. This ensures the passenger remains firmly against the seat.
These systems require no maintenance.
Various Types of Pretensioners
Cable Tensioner
This type features a piston moving within a cylinder. A steel cable is attached to the end of the piston and secured to the spool recovery device.
Beneath the piston is a housing containing the pyrotechnic charge and a detonator responsible for its activation. When activated, the piston moves, pulling the cable and tightening the belt by approximately 10 to 15 cm. This occurs due to the force of deceleration, which releases the firing pin from the retention tooth, allowing the tension spring to push the plunger towards the detonator. These can be oriented vertically or horizontally.
Movement by Ball
This system consists of a tube filled with balls in a row, a sprocket connected to the spool recovery, and a container to collect the balls.
When the pyrotechnic charge is activated, the balls circulate and are deposited into the container. This movement causes the sprocket to spin, winding the belt and providing the necessary tension.
Wankel Tensioner
This tensioner operates by sequentially detonating three pyrotechnic charges. The first charge is triggered by the deceleration detected by a mechanical driver. Upon detonation, it produces a gas that enters a chamber, spinning a rotary piston attached to the reel, resulting in the initial belt tightening. As the piston rotates, it uncovers a conduit through which the gas enters the second striker, activating the second charge and causing further belt tension. The same process occurs with the third charge, providing a third stage of tightening.
Electrical Activation Tensioner
This type utilizes an electric detonator that receives a signal from a control unit, often the same one used for the airbag system.
Anti-Pressure System
The anti-pressure system allows a degree of forward displacement for the occupant, relieving excessive pressure from the tensioner. This prevents the occupant from being held too rigidly against the seat. The system incorporates toothed wheels and a torsion axle within the reel. The torsion axle is connected to the sprockets, allowing the pretensioner to act upon impact, initially tightening the belt and then allowing the occupant to slide slightly over the tape, pressing on the torsional reel shaft. This reduces pressure by up to 10 cm.
Security System for Transportation
This system deactivates the pyrotechnic system during transportation or when a child seat is installed, preventing the pyrotechnic charge from potentially harming young children.