How Does Seat Belt Works?
Everybody knows what happens in an accident if you don’t wear a seat belt what you see in this accident looks really painful in slow motion modern seat belts are not just mechanisms which arrest your motion rather they cleverly allow enough movement to minimize internal injuries and help you land safely on an airbag in this video, we won’t just learn the working of a seat belt we will learn why modern seat belt mechanisms are designed the way they are so buckled your seat belt and let’s start a design journey
let’s start with the simplest form of seat belt design a design using a spiral spring and seat belt webbing the center of the spiral spring is connected to a freely rotating shaft you can see how the spring tenses as you rotate the shaft
the webbing is also connected to the shaft when not in use the passenger lets the webbing go and due to the tension of the spring the webbing will wind automatically
we have just built a basic seat belt retractor using this arrangement we can build a three-point seat belt mechanism in this design the other end of the webbing is fixed as shown a buckle clip is also kept in the webbing as illustrated the passenger can easily buckle the seat belt it will fit nicely the retractor will release sufficient length of webbing depending upon the passenger size the three points in this design are marked here
this design will not cause any passenger discomfort by restricting movement obviously the seat belt design will fail
completely in an accident because it allows the passenger movement even during an accident an ideal seat belt mechanism should block the passenger’s movement in an accident let’s see how to convert the current design into an intelligent seat belt mechanism during an accident the vehicle experiences a huge deceleration or suddenly comes to a halt a clever mechanism can be designed by making use of this
effect using this free-to-roll steel ball and a hinge the ball is placed at the other side of the retractor and a sprocket is connected to the retractor shaft this means that whenever the belt winds or unwinds the sprocket will spin under normal driving conditions the ball is in a resting position.
however, when the vehicle stops suddenly the ball moves forward due to its inertia, this movement will move the hinge and trigger a locking mechanism this locking will prevent any further release of the webbing from the retractor which means the seat belt simply arrests your motion during a crash in the early days of seat belt development it was quite important to win people’s trust so that they would use seat belts regularly the ball movement technology we just discussed is a perfect one but a common man cannot test this mechanism
so engineers came up with an interesting idea develop one more mechanism which is based on the webbing’s speed if the webbing moves fast it should lock if not it shouldn’t lock at all this speed based locking mechanism can be achieved with the help of a few new components first, a central piece is fitted to the webbing shaft and an internal gear is fixed by attaching it to the frame another piece sits over the central piece let’s give it an orange color the interesting thing is that the orange piece is not tightly fitted to the central piece it is slightly loose and free to move as shown
if the central piece rotates slowly the orange piece will be able to move with it however when the central piece rotates fast the orange piece won’t be able to catch up with it resulting in an interesting lock.
obviously, the belt will lock when the passenger moves forward fast most importantly this is an easy mechanism that everybody can test which increases consumer confidence in seat belt usage this second mechanism has another use if the first mechanism
fails it will act as a backup mechanism but the issues related to seat belt design is not fully resolved yet for instance if the seat belt blocks passenger movement completely during an accident
the belt will create tremendous pressure on the passenger’s chest
which can lead to internal organ injury this risk is why modern seat belt retractors come with a torsion bar the torsion bar helps to release the belt a little when it reaches a critical load thus saving the passenger from excessive pressure on the chest
let’s see how it is done practically the end of the seat belt webbing can be connected to this torsion bar.
the concept of this mechanism is simple when the load on the belt reaches a critical limit
the bar will undergo torsion as shown please note that one end of the shaft is fixed due to the locking mechanism note here that the twist of this bar is not uniform throughout its length it is zero at one end and maximum at the other end if we connect the belt directly to this the shaft we won’t get a belt release to overcome this issue the belt webbing is connected to a spindle and the spindle is connected to the free end of the torsion bar
this connection means that during torsion the whole spindle will turn the maximum twist
the angle of the torsion bar this unit replaces the normal shaft of the retractor this way the belt will be released a little when the belt pressure reaches a critical value
which saves the passenger from internal organ injury one more ingenious safety mechanism common in cars nowadays are the pre-tensioner most of us sit in a car in a relaxed way.
so our bodies may not lean on the seat perfectly this posture drastically reduces the amount of time the safety mechanisms have to save your life however a
pre-tensioner makes you sit as close to the seat as possible right before an accident right before impact most of us press the brake pedal hard a smart controller predicts an accident by sensing the fast movement of the brake pedal the retractors with pre-tensioners have an additional piston mechanism
a piston mechanism filled with explosive materials as soon as the controller detects the chance of an accident.
it will ignite the explosive which will push the piston and the seat belt will tighten in short even before you hit the rock you will be ready for the accident with the increased time the safety measures in your car can work better hope you enjoyed this journey through seatbelt safety technologies.
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