Why a seat belt extends slowly but can lock when pulled sharply
A retractable seat belt uses a reel that normally lets the webbing move while a spring takes up slack. Its locking mechanism can respond to rapid belt withdrawal or sudden vehicle movement, depending on the design. Once triggered, a catch stops the reel from paying out more belt.
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Free movement with an emergency lock
A seat belt retractor stores webbing on a reel. A spring gently rewinds spare webbing so the belt can follow ordinary body movement without hanging loose.
A locking mechanism can be triggered by rapid movement of the webbing, by vehicle deceleration or tilt, or by a combination of sensing systems. Once activated, it engages a catch that prevents further outward rotation of the reel.
The belt does not depend on the rewind spring to restrain a person. The locking parts and the belt's structural attachments carry that role.
- Slow movement normally allows webbing to extend.
- Certain sudden motions activate the lock.
- The reel then stops releasing additional webbing.
Designs vary, so not every belt responds identically to a hand pull. Many modern systems also include separate crash-response features such as pretensioners and load limiters.