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What Causes a Seatbelt To Lockup?

January 25, 2013 - We have all felt the seatbelt lock up on us when we have to slam on the brakes and come to a sudden stop, or more likely when we reach down too quickly to grab a dropped french-fry. Whenever I ask someone, “what makes the seatbelt do that?” They always answer with, “There is something that locks up your seatbelt when you move forward too quickly.” Well yes, I understand that, but what is that “something” that locks it up. I did some digging and discovered there are a few systems that could cause the seatbelt to lockup and save your life during a crash, or unnecessarily choke you.

The first system relies on a pendulum with a weight on the bottom. This pendulum is below the spool that your seatbelt wraps around. This spool has a metal plate on the outside of it that has teeth. Don’t worry, these teeth are not going to eat that french-fry you dropped. When your car stops abruptly the force causes the weight on the bottom to move forward quickly. This causes the top of the pendulum to move backwards and lock into the teeth of the spool. Once the top of the pendulum is locked into the teeth, the spool cannot spin and it is unable to let out any more slack. You my friend, are locked in place.

A centrifugal clutch is the second method that locks up your belt. This weighted pivoting lever is mounted to the same rotating spool as we talked about before. When the spool gently spins the lever does not move at all. However, when the spool spins very quickly centrifugal forces drives the weight end of the lever outward. Once the lever gets forced too far outside, it hits another plate with teeth and locks up the spool. Imagine someone with their arms firmly against their sides spinning really quickly in circles next to a wall. If they spin fast enough their arms will be forced out from their sides and eventually hit the wall causing them to stop and perhaps curse. Although, you probably won’t hear your seatbelt shout out a four letter word, it will stop.

Those are the two methods of stopping your belt, but there is a new device that instead of just locking, it pulls in. This in the industry is called a pretensioner. A pretensioner accompanies one of the two methods we previously discussed. It does not replace them. It is supposed to pull in a driver or passenger to the optimal crash position. This device is activated by the same process that sets off your airbags in a crash. Pyrotechnics blow gas up a cylinder. Inside the cylinder is piston that has, you guessed it, more teeth on one side. When the cylinder gets blown upwards the teeth grab onto the teeth of the spool causing it to wind up and tighten the belt.

So, how many of you are going to slam on the brakes this weekend while your friends are in the car so you can sound smart explaining why their seatbelts just locked up? 

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