How seat belts are made

Weave on a needle loom

Polyester multifilament warp runs into a narrow-fabric needle loom, where a reciprocating needle carries weft across. Needles scrape warp far less than old shuttles did. A typical strap has about 300 warp threads in its 46 mm width.

Dye and heat-set

The woven strap goes through dyeing and heat setting. Some makers add a surface finish of resin or oil, which helps it stand up to abrasion.

Sew in the pin

One end of the strap is folded into a loop around a strong steel pin and sewn shut. That pin stops the end from pulling through the spool under load.

Thread the spool

The webbing is threaded through a slot in the spool's core, often a one-piece die casting, until the pin seats in the slot and leaves a smooth surface to wind on. One patent notes that two-slot designs spread the load but are harder to automate.

Fit the rewind spring

A fully wound spiral spring can arrive sealed in a casing, held by a U-shaped clip until it is installed. Its inner end drives the shaft, and it pulls the strap back onto the reel.

Add the locking mechanism

An inertial lock goes on the reel. One type answers a fast pull on the strap through a centrifugal clutch, and the spool stops paying out.

Fit the buckle and latch

A stamped steel latch plate goes on the webbing, and the buckle it clicks into is built as its own unit from steel and plastic.

Test to the standard

The US federal safety standard requires at least 46 mm width on webbing that touches the wearer, and a lap belt may stretch no more than 20 percent at 11,120 newtons. After abrasion, webbing must keep 75 percent of its required strength, and hardware must resist corrosion.

26,689 N minimum breaking strength for a lap belt