The occupant protection in a car can be improved by supplementing the three point seat belt with an efficient pretensioner. This is of special importance if there is a lot of slack in the belt caused by e.g. thick clothes.

This paper describes a mechanical pretensioner which has been in production since January 1989. It acts on the seat belt buckle. At the very beginning of a crash the buckle is rapidly pulled down, thereby pretensioning both the lapand chest belt parts.

The pretensionerc omprisesa preloadedt orsionals pring, which acts upon the buckle and a sensor mechanism for triggering. The design, performance and effectiveness of the buckle pretensioner in a three point belt systemw ith and without slack is presented. The main advantagesa re reduction of occupant forward displacement, lower impact speed into the steering wheel, less head injury criteria (HIC) and less risk of submarining.