Ever wonder how seat belts work on your car? I took apart some seat belts from my Corolla to find out just how and made a short video:
Three point seat belts are meant to restrain the occupant from smacking the dashboard during a collision. Most seat belts are free to extend and retract so while driving your free to move around. However according to various conditions, the seat belt can retract itself and lockup, restricting passenger movement. Here’s what the seat belt mechanism looks like from the back seat: Opening the over, there’s a central cam that locks out the gravity ball (right), so you get some slack in the beginning to fasten the belt. There’s also a white tooth (top) that locks the seat belt from reeling out more webbing when it reaches the end. Inside there’s a gear that drives a slider. The slider locks out the centrifugal “sudden stop” mechanism we’ll see next.

When you tug on the seat belts rapidly, these two cams move outward due to centrifugal force. The teeth lockup and engage with the housing, preventing the belt from extending further.

This is the torsional spring, it has a lot of stored energy.

This is the front seat belt mechanism. It has an explosive canister that will retract the seat belt during a collision, preventing occupants from contacting the dashboard. Before opening the pretensioner, I had to disarm it by blowing the explosive. Quite loud and violent, see the video for full time action. Inside the pretensioner is a piston that pulls a wire. The wire is reeled around a pulley that reels in the retractor. Pretty simple, but it all happens within a few milliseconds of a collision. And that’s all the components that go into making the seat belts work on your car.

Enjoy!