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 buckle 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 as
shown
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
as shown
let's watch the same animation in ultra
slow motion for better understanding
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 are 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
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
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
is 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.
Thank you
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