Special Report: Seatbelt Pretensioners - Excerpt

Introduction

Most modern seatbelt assemblies use three-point anchoring systems in conjunction with a pretensioner, which locks the belt in place during a crash event.
The three types of pretensioners in use today are mechanical, electrical and pyrotechnic. Each will be described here briefly, though the main focus of this report is the pyrotechnic type.

Types

Mechanical

Mechanical pretensioners use an inertial wheel with a pendulum device that moves under the rapid deceleration of the crash to lock the belt into place. Such mechanisms can often be detected by giving a sudden tug on the belt. A mechanical pretensioner will automatically lock the belt into place, with the intent of limiting occupant travel in the event of a crash.

Electrical

Electrical pretensioners replace the mechanical means of sensing deceleration (the pendulum) with an electrical device that may or may not be tied into the airbag ignition circuits.

Pyrotechnic

Pyrotechnic pretensioners use electrically triggered pyrotechnics that tighten the seatbelt a prescribed amount upon sensing a crash event. These devices can operate on either the buckle or ratchet side of the seatbelt mechanism. These are the most highly technical type of pretensioner, and also the most expensive. These will be the focus of this report.

History

Mechanical seatbelt pretensioners came into play at the same time full three-point anchoring systemswere employed. Some means were required to prevent excessive seatbelt slack with systems using non-fixed belt lengths. Excessive seatbelt travel allows too much occupant motion during a crash, thus increasing the chance of contact with components such as the steering wheel, dashboard or windshield. Too much occupant travel also implies high velocities relative to these components, thus allowing the occupant to develop significant energies as contact with the interior is made. All of this translates into increased potential for injury in an accident.

Initially, mechanical pretensioners simply limited the travel in the belt. These designs include:

o	a pendulum that moves under the sudden deceleration of a crash
o	a ratchet mechanism, which rolls and unrolls the belt upon a reel usually associated with the pillar of the automobile.

The pendulum and ratchet interact in such a way as to lock the reel and therefore the belt into place during an accident, preventing any further travel of the seatbelt and occupant. These devices would more accurately be called limiting devices rather than pretensioners, as no true pretension is imposed within the belt itself.

Electrical pretensioners are similar except that the mechanical means of sensing deceleration is replaced by an electronic sensor that may or may not be associated with the deployment of the airbags.

It became apparent that more occupant protection could be achieved if some tension could be developed in the belt. The use of pyrotechnics in automotive airbags helped forge the way for use of pyrotechnics in seatbelt pretensioners. These devices use small pyrotechnic charges to pull the seatbelts into place and actually generate tension in the belt during a crash. This keeps the occupant travel to a minimum and also helps optimize occupant position for effective use of the restraint capabilities of the airbag systems.

Operation

Pyrotechnic seatbelt pretensioners operate on either the buckle or the ratchet side of the seatbelt mechanism. Their action augments or compensates for a variety of seatbelt performance issues, including:

o	Taking up any slack in the belt prior to occupant motion.
o	Compensating for the "film spool effect" of the seatbelt webbing unwinding upon itself on the locked ratchet.
o	Holds the occupant in position for airbag contact and so-called "ride down". Pretensioners are used in conjunction with other seatbelt features, such as:
o	"torque load bars"
o	"web grabbers"
o	breakaway seams

These features help optimize the performance of the airbag systems.

Maintaining proper occupant position reduces the amount of work the airbag must perform on the occupant, and thus increases the level of protection provided by the airbag system. Keeping the occupant in the proper position also helps minimize injury due to the rapid inflation of the airbags.

Pyrotechnic pretensioners are electrically triggered at approximately the same time as the airbags. Mechanical triggering can also be employed.

The combustion of the pyrotechnics within the pretensioner creates pressure that is used to either pull on the buckle side of the seatbelt mechanism or to tighten up on the spool side of the mechanism, thus reeling in some length of the seatbelt webbing.

Some means of mechanical locking is usually employed to maintain belt tightness.

A further description of the operation of pyrotechnic pretensioners can be found in the Technology section of this report, where specific details of the technologies used in these devices are discussed in some detail.

 

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