Vehicle Restraint System: Seatbelts

Jan. 1, 2020
All seatbelts are woven from manmade materials, and in most systems, the web is made to stretch a specified amount with a specific force.
The first recorded use of seatbelts was 1913 when a French aviator used a simple lap belt to hold himself in place so he could fly inverted. In 1955, Ford began offering lap belts as optional equipment, and in 1959, Volvo began installing three-point seatbelts in all their vehicles. In 1968, lap belts were required in all passenger cars sold in the United States, and three-point seatbelts were required in 1972.

Unlike the French pilot's equipment, passenger car seatbelts don't help the driver control the vehicle. They are strictly safety devices designed to prevent the second collision, and the newest seatbelt systems are designed to minimize the third collision.

In every crash, there are three collisions. The first is the vehicle itself. The second is the occupants colliding with the interior of the vehicle, or being ejected from the vehicle and colliding with whatever is in their path. Seatbelt systems (as opposed to lap belts) are intended to hold the occupants in their seated position during the first collision to eliminate the second collision. The third collision is inside the occupants' bodies, as organs collide with each other and with bone.

The inertia reel seatbelt retractors used in every seatbelt system today were introduced in the 1970s. Although there are different designs, they all function alike. Normally the spring-loaded reel can move freely in either direction, but sudden deceleration of the vehicle causes the reel to latch so the belt cannot unreel further. The earliest latch was a simple ratcheting mechanism that used a pendulum to move the pawl, and some variation of that design is still used on most seatbelt systems.

Compared with the inertia reel latch, the belt itself is very high-tech. It must be light and flexible but strong enough to withstand peak loads of 30 tons when new. The material must be able to accept color dyes and withstand years of heat, cold and sunlight.

There are different manufacturers, but all seatbelts are woven from manmade materials, and in most systems, the web is made to stretch a specified amount with a specific force. Because the belt winds around a spool like a roll of film, a fair amount of belt still plays out as the belt wraps tighter around the reel after the reel locks. To improve control over belt stretch, a new latch was designed. Called a web grabber, it clamps the web outside the reel instead of latching the reel itself, so only the web that's already played out can stretch.

Belt stretch affects "impact ride-down" forces. Impact ride-down is a time-speed-distance calculation that measures the force of impact. At any speed, spreading the impact over a greater distance will reduce the peak force of impact, and it doesn't take much distance to make a big difference. This is why cars are made to deform in a collision. In a 30-mph collision with an immovable object, an unbelted occupant striking the dashboard will stop moving in about 2 /10 of a foot. An occupant weighing 160 pounds will experience a deceleration force of about 150 g's; 40 g's is usually lethal.

If the occupant is belted and the belt does not stretch, the ride-down distance is whatever the whole vehicle experiences. If that distance is 1 foot, the occupant experiences 30 g's, which is survivable. If the belt stretches only half a foot, the occupant's ride-down distance is 1.5 feet and impact force is reduced to 20 g's. Not bad, but at higher impact speeds, more ride-down distance is needed to survive the third collision.

Load-limiting or force-limited seatbelts are designed to work with airbags. They allow the seatbelt to unreel in a controlled manner during the collision, allowing the occupant to move forward into the airbag and transfer some of the restraining force from the belt to the airbag.

There are two benefits. Sharing the restraint force with the airbag reduces bruising or more serious injury where the seatbelt touches the body. More importantly, it also allows a greater ride-down distance.

The most common method of force limiting is to put the inertia latch next to the reel and connect it to the axle with a torsion bar. Like "tuned" web stretch, the torsion bar can allow a specific amount of belt to unreel for a given force. But unlike the web, which tightens as it stretches, the torsion bar can be tuned to loosen the further it twists, gradually transferring the restraining force from the belt to the airbag at just the right time.

The latest development in seatbelt technology is the pretensioner. A cable is wrapped around the inertia reel or connected to the buckle, and the other end is connected to a piston that rides in a tube. The piston is driven with either a strong spring or by a pyrotechnic squib similar to those used in airbags. When activated (usually by the airbag control unit), the piston is driven down the tube, pulling the cable to tighten the seatbelt. Whether just removing the slack or actually tightening the seatbelt, this device alone can add several inches to the ride-down distance.

As sensor technology improves, electronically controlled seatbelts are sure to come. But that will be the subject of another Teardown.

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