The Traffic Accident Reconstruction Origin -ARnews-

Seatbelt Burst Load Limiters

Ed Phillips (edphill@aol.com)
Sat, 11 Jan 1997 11:24:36 -0500 (EST)

Burst Load Limiters

This post is motivated by the thread "Seat Belt, Reclining seat, and rear collision" by Dick Smith. That discussion had moved toward the function of Load Limiters. This new post will serve to focus exclusively on that topic.

The "crash pulse" is a time-velocity curve of a collision event. Smaller cars tend to have more steep or severe crash pulses than larger vehicles. Load limiters, sometimes called fused belts, or tug restraints, have been around since the 1970's. I believe some restraint designers used this system in an effort to lower the chest G's experienced by occupants (FMVSS 208). When loaded and at some design peak, the stitches fail under tension and absorb some energy. They also increase the distance and the time of the acceleration change the occupant is undergoing. If space is limited this may not be wholly advantageous, but unreasonably high G's are likely at such a level and the trade off is reasonable.

Exhibit #1

This photograph shows the open passenger side door of a 1996 model year import sub-compact. At the lower left quadrant of the photograph we see a portion of the type 2 restraint. The passenger was afforded an airbag, three point restraint, and a load limiter.

Exhibit #2

This photograph is oriented looking down with the B' pillar to the right. We see the sleeve which covers the lower portion of the webbing. This area typically covers the load limiter. The load limiter is shown with burst stitches.

Exhibit #3

This photograph shows the same passenger side restraint. We again see the plastic sleeve, and the unraveled stitches which failed under tension, as intended.

Exhibit #4

This photograph illustrates a type of warning label stitched onto the webbing visible as the load limiter unravels indicating the seat belt should be replaced as it will no longer operate as designed.

Exhibit #5

The photograph of the return loop of the passenger side further illustrates the magnitude of the force applied to the restraint system. The webbing has also burst and become unraveled horizontally about 2/3 rds the width of the belt.
All Photos Copyright Ed Phillips. Use allowed with permission.

This ARnews post is a good example of how illustrations can be used in this space. Have a photo that would clarify your post? Contact TARO to make the arrangements.

Ed Phillips
edphill@aol.com

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