Seat Belt Tensioner: Description and Operation

Design of Emergency Tensioning Retractor (ETR):

K18/2 Left front seat belt lock relay module
K18/3 Right front seat belt lock relay module
R12/1 Left front seat ETR squib
R12/2 Right front seat ETR squib
4 Eye with Bowden Cable for keeping belt free
6 Threaded mount for attaching ETR Unit
9 Inertia Reel with ETR Unit
10 Plunger
11 Cable
12 Tube
21 Inspection Window

Operating Principle





The complete unit is installed in the left and right backrest and connected electrically by a two-pin connector.

Before the ETR unit is installed in the backrest it is automatically short-circuited electrically with a short-circuit bridge. The short-circuit bridge is opened automatically when the connector is plugged in.

If the control module [N2/2] transmits an electrical pulse to the squibs [R12/1, R12/2]. A high pressure propellant gas is produced. This propellant gas passes along a Tee piece to the plunger which is then accelerated down. The cable connected between the tube produces a torque at the cable pulley. The shear pins break off and the cable pulley is connected positively through the coupling to the shaft of the inertia reel.

The shaft of the inertia reel is turned back and in this way eliminates the belt slack and the film reel effect. The belt is then held tight against the person to be protected and provides an enhanced protective effect.

The emergency tensioning retractor is designed in such a way that it is not capable of applying forces which cause the seat belt to pull the occupant back into the seat -in contrast to traditional belt inertia reels in which the belt feed is locked by mechanical ball sensors. The belt inertia reels [9] of model 129 feature two electro-mechanical relays [K18/2, K18/3]. The relays are activated by the roll-over bar control module and lock when de-energized. To fasten the belt the inertia reel has a device controlled by a bowden cable for releasing the belt lock [4].





An inspection window [21] is provided in the housing of the emergency tensioning retractor through which a cable is visible. It no cable is visible the emergency tensioning retractor has been activated and should be replaced.