VEHICLE MECHANISM · ORIGINAL 3D STUDY
Why does a seatbelt move easily, then suddenly lock?
Easy movement. A different response to sudden payout.
Start with the narrated walkthrough.
See the whole mechanism, then inspect it yourself below.
Take it apart and inspect every system.
Click a part to highlight it and read why it matters.
A familiar object. A hidden system.
Easy movement. A different response to sudden payout.
The rewind spring and the lock do different jobs.
Inspect one webbing-sensitive design: a spool stores the belt, a spring draws it back, and a separate sensing chain can engage the lock. Open the same assembly to see how its parts fit together.
Rewinding and locking are different jobs. A small sensing movement can engage a separate stop.
Read the explanation
One continuous belt, wound around a spool.The webbing winds around the drum and passes through a fixed guide. This teaching view stays at rest: it does not pretend that a rigid belt can simulate flexible payout.
A spring links the shaft to the case.In this model, the spiral strip has one end attached to the turning shaft and the other to the fixed case. Payout stores elastic energy; the spring can then draw the belt back. The cover is removed for inspection, and the strip is shown at rest.
Relative motion starts the locking chain.In this selected design, sufficiently rapid belt acceleration makes the inertia disc lag the spool. Its cam moves the small programming pawl toward the lock cup’s internal teeth. The inertia disc and front support are ghosted here so you can inspect the follower, pawl and teeth. This is a webbing-sensitive trigger; vehicle-sensitive sensing is a distinct mechanism. The model stays in its released pose.
The small sensor pawl is not the main stop.Engaging the lock cup can move a separate frame-mounted pawl into the spool ratchet, stopping further payout. Here the pawl remains released; other parts are ghosted to expose the relationship. Neither the locking event nor restraint forces are simulated. Pretensioning and load limiting are separate functions, not demonstrated by this model.
Sources and safety
- Autoliv: A retractor stores webbing on a rotating spool and allows it to pay out during normal use.
- Autoliv: Webbing-sensitive and vehicle-sensitive locking respond to different inputs.
- US patent publication US4917325A: In the selected architecture, inertia-disc lag moves a programming pawl into a lock cup, which operates the main locking pawl.
Seatbelts are safety-critical equipment. This lesson is not a test, repair, calibration or disabling procedure. Do not open, modify or experiment with a vehicle restraint or pyrotechnic pretensioner. Follow the vehicle manual and use qualified service for faults.
Scope and limitations
Original simplified webbing-sensitive retractor with spool, rewind strip, inertia disc, programming pawl, lock cup and separate main pawl. Not a product replica, certified restraint, crash simulation or service model.
- Original webbing-sensitive educational geometry; no copied OEM CAD, dimensions, product certification, calibration, crash dynamics, repair or bypass guidance.
- Vehicle-sensitive locking is a distinct trigger type, not the inertia-disc mechanism depicted. Pretensioners and load limiters are additional functions not modelled internally.
- Small wire-to-pin spring end seats are simplified local attachments; they are not product-exact fastening details. Receiving pockets for both rewind-strip tangs are actual open geometry.
- This teaching model supports rest-pose inspection and a continuous parts explosion. Flexible belt payout, spring deformation and locking motion are outside its scope.
- Guided views ghost surrounding parts for inspection; those parts have not been removed from an operating mechanism. No force, acceleration threshold, certification or protection rating is calculated.
A retractor stores webbing on a rotating spool and allows it to pay out during normal use.Applies to: Original simplified webbing-sensitive retractor with spool, rewind strip, inertia disc, programming pawl, lock cup and separate main pawl. Not a product replica, certified restraint, crash simulation or service model. Uncertainty: The original displayed model is one illustrative layout, not a current Autoliv product.
Webbing-sensitive and vehicle-sensitive locking respond to different inputs.Applies to: Original simplified webbing-sensitive retractor with spool, rewind strip, inertia disc, programming pawl, lock cup and separate main pawl. Not a product replica, certified restraint, crash simulation or service model. Uncertainty: Only the webbing-sensitive architecture is represented here; not every retractor has the same mechanism.
In the selected architecture, inertia-disc lag moves a programming pawl into a lock cup, which operates the main locking pawl.Applies to: Original simplified webbing-sensitive retractor with spool, rewind strip, inertia disc, programming pawl, lock cup and separate main pawl. Not a product replica, certified restraint, crash simulation or service model. Uncertainty: The source supports this causal arrangement, not the authored dimensions, tooth shapes, timing or physical performance.
