Pre-Tensioner Systems: Design & Function

Subject: Supplemental Restraint Systems

Topic: Seatbelt Pre-tensioners

Objective: Understand the design and function of seatbelt pre-tensioner systems.

Task: While visually inspecting a current model-year vehicle selected from the accompanying list, identify the location of all potential seatbelt pre-tensioner systems on the vehicle and explain how common vehicle rescue tasks would be performed by your department to avoid contact with the pre-tensioner units.

A seatbelt pre-tensioner is a device installed in late-model vehicles to reduce slack that may exist in a seatbelt system at the moment of a frontal collision. Having the seatbelt system snug allows the supplemental restraint airbag system to work most effectively. A loose seatbelt system does not keep the occupant in the proper position during the forces of a crash. In order for frontal airbags to reduce or eliminate injury, the occupant must be seated, belted and in the correct position in the vehicle.

Pre-tensioners may activate mechanically by the force of deceleration or may be activated by a small electrical current similar to that which deploys frontal airbags. In order to pull the slack out of the seatbelt, the pre-tensioners must be directly connected to the seatbelt system.

There are two pre-tensioner types. One design attaches at the seatbelt buckle end; the other type attaches to the retractor spool mechanism. Either type of pre-tensioner system will tighten the belt during crashes that involve frontal airbag deployment only.

Pre-tensioners working at the buckle typically use a small explosive charge to generate a pressurized gas. The pressure of the gas forces a piston down deep into a tube that is about the size of a cigar. As the piston slides down the tube, it pulls a cable which pulls the seatbelt buckle downward. As the buckle moves down, it snugs the seatbelt being worn by the occupant.

The second more common type of pre-tensioner is the retractor-type. There are rotary-piston designs and piston-tube designs. With a piston tube system, a small chemical charge again generates gas to propel a piston down a small tube. The piston is attached to the retractor spool of the seatbelt with a cable. The cable "backwinds" the retractor spool, snugging the seatbelt. With the rotary piston design, the piston actually spins the large seatbelt spool backwards to snug the belt against the occupant.

In a crash test performed at the Vehicle Research Center of the Insurance Institute for Highway Safety, engineers found that a Volvo pre-tensioner retracted two inches of seatbelt slack at the moment of impact.

You will know that the seatbelt pre-tensioners have deployed when you arrive at a crash and see deployed frontal airbags. With the buckle-type pre-tensioner, you can confirm this by noticing that the top of the seatbelt buckle is at or below the top of the seat cushion.

If your patient is still seated and belted when you conduct your primary survey, a good sign confirming that a pre-tensioner has activated along with the frontal airbags is the presence of some slight tension in the lap/shoulder belt system.

Pre-tensioners are available for front driver and passenger seatbelts. Selected model vehicles also provide pre-tensioners for rear seat passengers as well. The new Volvo S80, for example, has five lap/shoulder seatbelts, two for the front seat occupants and three for the rear. All five positions have seatbelt pre-tensioners. The rear pre-tensioners are located under the rear window shelf.

If in a crash situation the front airbags have not deployed, it should be anticipated that none of the pre-tensioners have activated either. Use the same precautions you do for undeployed airbags. Disconnecting power from the battery drains the airbag’s electrical system capacitor and also disables the pre-tensioner's electrical system if it is an electrically fired design.

Extrication crews should avoid cutting into a seatbelt pre-tensioner unit. To do this, we must first locate the units. Typically they are on or inside the vehicle’s B-pillar or are secured to the rocker channel at the seatbelt spool mechanism.

The best way to identify them is to strip away the interior plastic trim of the B-pillar before cutting into it. Any cutting or spreading along the rocker channel that is near the seatbelt spool should also be done with caution.

General Motors publishes an 18-page booklet explaining its airbag systems and their pre-tensioners. The publication, titled Air Bags and Pre-Tensioners in GM Vehicles, is available free of charge by calling the toll-free telephone number 800-393-4831 and following voice prompt 2. The booklet is item number 22008.22-Q2 and is free of charge for single copies to an agency.

Ron Moore, a Firehouse contributing editor, is a battalion chief and the training officer for the McKinney, TX, Fire Department. He also authors a monthly online article in the Firehouse.com “MembersZone” and serves as the Forum Moderator for the extrication section of the Firehouse.com website. Moore can be contacted directly at [email protected].