How do seatbelts save lives?

It’s estimated that seatbelts have collectively saved over one million lives, making them one of the most cost-effective public health inventions ever made. These incredible devices can reduce fatalities in car accidents by about half. Yet their development and implementation as standard equipment was surprisingly rocky. Let’s see how do seatbelts save lives?

How do seatbelts save lives

How do seatbelts save lives?

​​Seat Belt History Timeline

For something that seems so straightforward, the humble seatbelt is more sophisticated than you may think. Even though wearing a restraint in a vehicle moving 100 kilometers per hour may seem like common sense to us today. It took many years to convince the driving public to buckle up, a problem that persists in many countries around the world. This ingenius device saves lives, and this is how it does it.

In 1903 race car driver Barney Oldfield became the first person to drive over 100 km/h. He was all too aware of the dangers he was subjecting himself to and commissioned Leslie Leroy Irvin, another dare devil of the 1920s, to develop a harness for his racing cars. Irvin had developed his own static line parachute to jump out of airplanes, and 1919 became the first American to jump out of a plane and manually deploy a parachute from a backpack in midair.

Challenges in wearing a seat belt

A design he helped the US military to develop. The harness they came up with did not catch on however. Despite its life saving potential, it had a number of problems. First, they were simply uncomfortable, which discouraged drivers and passengers from wearing them.

Then there was the very real problem of “seat belt syndrome,” injuries directly related to wearing a seatbelt during a collision. Early seatbelts only went across the lap. In the event of a crash, this rigid single point restraint, laying in line with vital organs and the lumbar spine, could cause severe injuries.

The prevailing thought at the time was that it would be better to be ejected from the vehicle, which might catch on fire or go under water, than to remain inside of it during a crash by being stopped forcefully by a thin piece of fabric. Despite rising fatalities and injuries during the rise of the automobile from the 1920s to 1950s, the general public showed little interest in wearing seatbelts.

The evolution of the seat belt

Even though Nash Motors installed seat belts in 40,000 cars, only about 1000 of them were actually used by drivers. When Ford led a marketing campaign in 1955 to promote seat belts and other safety options, they too faced public backlash. However, in the 1950s resistance began to ebb away with a new innovation by Swedish engineer Nils Bohlin.

Bohlin first began developing safety features for Saab, including an ejection seat, but was then poached by the president of Volvo. He experimented with several designs, including a seatbelt that fastened between the legs. He ultimately settled on the design we’re familiar with today. The three point harness.

This set up more evenly distributes the forces of a crash across the shoulders and hips. Bohlin was able to prove the effectiveness of this design and received the U.S. patent for it in 1962. But rather than profit from it, the design was released for free, paving the way for a new and incredibly important safety feature.

While certain elements of the seat belt have been modified and improved over time, the main design remains the same. So Do you know how do seatbelts save lives?

How Do Seatbelts Work?

1 . False alarm in the Seatbelt

Let’s see the first mechanism of How do seatbelts save lives. We need a way for the strap itself to wind and unwind under normal circumstances. Allowing passengers to easily move around, and take it on and off without adjusting its size for different passengers and drivers. We have all experienced that annoying phenomenon where you try to put on a seatbelt too quickly causing it to jam.

This happens as a result of a false alarm in the seatbelt mechanism. It has a number of components, the simplest being this spiral torsion spring. When the seatbelt is unwound it spins a cylinder at the center of this torsion spring, pulling the spring into the center into a compressed state, storing energy that can retract the seat belt tightly against your body once the the belt buckle is locked into the buckle socket.

How do seatbelts save lives

This mechanism is usually attached to one side of the webbing spool, while the locking mechanism that will prevent the spool from being unwound at all during a crash is located on the other side. For this mechanism to work we need a way to detect a crash extremely quickly. One kind of locking mechanism reacts to the momentum change of the vehicle.

Just as our bodies want to continue moving forward in a crash, a weighted mechanism inside the seatbelt will too. First we need a ratchet gear, like this, attached to the same cylinder at the center of our torsion spring. This gear will be engaged to prevent rotation. Now we need a mechanism that can engage this gear when the car decelerates.

How do seatbelts save lives

This can be done with a weight pendulum. When the car decelerates this weight will swing forward, causing its opposite end to jam into the ratchet, preventing it from rotating any further.

Like any good safety mechanism there is a redundancy in its design, being backed up by another locking mechanism. One you can test without crashing your car, by tugging on your seatbelt hard enough this mechanism will activate.

How do seatbelts save lives

As it depends on centrifugal force to activate. Two weighted levers are attached to the central spool. When the central spool unwinds at a normal speed, these levers do not create enough force to overcome the force of a spring that pushes them towards the center. But, if the webbing comes out in a sudden jerk, the centrifugal force exceeds the force of the spring and the levers push outwards engaging the locking teeth and prevent further rotation.

2 . Pretensioner Mechanism

There is another final safety mechanism added to make seat belts even safer. When driving we often slump forward slightly, leaving slack between our body and the seat. In the event of a crash it’s best if we are tightly held against the seat, so there is a mechanism within seat belts that actually forcefully pulls us against the seat during a crash. This mechanism is fired electronically by the same sensor that initiates the explosive that expands airbags, and this mechanism is also fired by an explosive charge.

As shown in above image It’s called the pretensioner. A chamber at the bottom of the device has two electrodes, an igniter material and a piston. When the car’s central processor detects a collision, it applies an electric current that ignites the explosive and the sudden burst of pressure forces a mechanism to actuate.

To quickly retract the seat belt and pull the passenger tightly against the seat. Finally, the seatbelt has several mechanisms built in to reduce the risk of seat belt syndrome–injuries. Some webbing has a fold sewn into it, with stitches that will break when a set amount of force is applied, allowing the person to move forward a bit more.

3 . The Torsion Bar

There is also the torsion bar, a length of metal attached to the rotating spool. This torsion bar is designed to twist and absorb some of the force from a crash, instead of allowing that energy to be transferred to the passengers body.

How do seatbelts save lives : The Three-point belt system Saves Life

Despite the clear benefits of this three-point belt system, it took decades before the use of seatbelts really became widespread. Although Wisconsin was the first state to require all cars to install seat belts in the front in 1961, it wasn’t until 1984 that another state, New York, required passengers to actually wear them.

I remember growing in Ireland in the 90s and seeing the change in opinions about seatbelts, and I especially remember moving to Malaysia, a country with a terrible road safety record and where a friend of mine died because he wasn’t wearing a seatbelt, and arguing with local co-workers who refused to wear them.

The National Highway Traffic Safety Administration estimates 15,000 lives are saved by seat belts every year in the U.S. alone, so please put your seatbelts on. They do save lives. Of course there are plenty of other technologies helping to make cars safer.

We have front and side airbags, crumple zones, driver assist, and automatic emergency braking. And Volvo is now proposing a blanket-style seat belt for its autonomous cars that would keep passengers safe and comfortable even when they’re napping. But that’s still something of a pipe-dream, since we’re still a ways away from self-driving cars.

For now we are stuck with the three-point harness, a simple and cheap device to keep passengers safe. Sometimes simple solutions really are the most effective. Other times more complicated solutions are needed. One concept is so complicated that I struggle to wrap my head around it. So, this is How do seatbelts save lives.

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