How do seat belts function?

Question

How do belts function (work)?

A diagram could be useful and a description of how seat belts function.

How do you pull the belt?

How do you buckle the belt properly?

Answer 1

I am assuming that you don't mean to ask how the belt holds you in place. :) I'm assuming you are asking how the mechanism works. I'll break this into two parts. The simple mechanism and the pretensioner.

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Simple mechanism

Here's the diagram:
(Image source)

And here's the explanation:

The actual belt is attached to the a drum that is attached to the blue wheel you see in the image. Most of the time, it is free to rotate. It is also spring loaded so that it will automatically retract.

In the case of an impact (or bumpy roads!), the metal ball (grey) attached to the red claw slides forward. Suddenly, the red claw catches on the blue wheel. Not shown in the picture is a block for the red lever so that it can't pivot totally. That block also forces the metal ball forward such that the only way to disengage the lock on the belt is to release the pressure on the lever.

If you have removable seats in any of your vehicles, you will notice that if you tip the seat in certain directions, the belt will lock. In my case, that is useful for picking up the bench. :)

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Pretensioner

Here's the diagram:

(image source)

And here's the explanation:

The car has sensors on it that send a signal to the pretensioner in the case of an impact, or, some vehicles, rapid acceleration and deceleration. When the pretensioner receives these signals, a rapidly expanding gas basically explodes (red and orange). This forces the metal balls to advance, forcing the grey wheel also to advance.

What this accomplishes is to remove any slack in the seat belt, holding the passenger (or driver) against the seat. This grey wheel would be attached directly to the blue wheel in the other diagram. In this particular set up for pretensioners, the pretensioner can only be used once.

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Special thanks to @DucatiKiller for pointing out the existence of pretensioners.

Answer 2

To add to the other great answers, I must point out that seat belts 'work' to reduce the impact on the passenger by stretching. In a large impact the fabric stretches quite significantly, don't quote me, but perhaps a centimetre or two. This 'extra controlled travel' in the seat belt gives the human body a few more milliseconds to decelerate (ie a little more slowly), so that the impact of the actual belts against the body does much less damage.

Further, that is why you should discard the seat belts every time they have been used in a large impact. That is - if they have already stretched, they will not stretch again and are more dangerous.