Seat Belt Tensioner: Description and Operation

PRETENSIONER

CONSTRUCTION
The driver's and front passenger's seat belts are equipped with seat belt pretensioners. The pretensioners use the front sub sensors and the airbag control module inside sensors to control their operation. If the sensors detect an impact exceeding the predetermined level during a frontal or frontangled collision, the front seat belts are quickly rewound by the retractors to take up slacks for maximum restraining of the seat occupants. If the load placed on a seat belt exceeds the predetermined level, the torsion bar twists to allow the belt to be payed out, thus lessening the load imposed on the belt wearer's chest. Once the seat belt pretensioner has been activated, the seat belt retractor remains locked.

FUNCTION:

Operation of pretensioner:

1. Initial state
- The piston is held in place by the shear pin in the case gear, and the pinion is held in place by the shear rib in the case gear (when operated, the shear pin and shear rib are sheared off).
- The sleeve rotates while transmitting torque of the spring to the spool, and the roller and sleeve rotate freely because a gap is maintained.

2. Pretensioner starts operating and the belt is retracted
- When the gas generator is triggered in response to the ignition signal from the airbag control unit, the piston moves upward by gas pressure. As a result, the rack gear of the piston becomes engaged with the pinion and the linear motion is converted into a rotating motion.
- When the pinion rotates, the slant of the pinion causes the roller to move in an axial direction and be caught in the sleeve engaging the clutch. The pinion, sleeve, and spool starts to rotate together as a unit and retract the belt.

3. Load limiter
When the passenger restraining load increases and the torsion bar begins to be twisted, the rotation of spool, sleeve and piston is reversed thus returning the piston. The piston moves downwards while releasing pressure from the gas vent hole, and when the piston reaches the bottom position it is disengaged with the pinion enabling it to rotate freely.







a. Initial state
The draw out of the belt is detected and the lock mechanism operates. When the belt is drawn out further, the spool and lock mechanism try to rotate relatively.







b. Belt extraction (when wire and torsion bar operating)
When the belt is drawn out further, the torsion bar is twisted and the wire begins to be rubbed. Plastic deformation occurs in the wire and torsion bar and the belt extracting load is kept constant. When this occurs, the wire gradually comes out from the spool and gets wound around the lock mechanism.

c. Belt extraction (when only torsion bar operating)
After the wire comes out from the spool completely, the load is kept constant by the twist of the torsion bar only.