Webbing
Thread & machine
Pattern
| Segment | Qty | Each | Inches |
|---|---|---|---|
| Total sewn | |||
| Required | |||
| Margin |
| Stitches per inch | |
| Thread strength | |
| Lockstitch loop factor | 1.5 |
| Seam strength per inch | |
| Seam holds at total sewn |
Conservative: one thickness of webbing carries everything.
Only if the loop is genuinely loaded as a closed loop, both legs sharing.
| Benchmark | kN | lbf |
|---|---|---|
| Aerial straps, recommended MBS | 22.0 | 4,946 |
| 3 × measured peak (E1.43) | 15.9 | 3,575 |
| Measured peak, straps | 5.3 | 1,192 |
Read this before you trust a number This works out stitching, not certification. It assumes the thread strength you entered is real, the machine is making full lockstitches, and the webbing is undamaged and unrated for nothing else. Proof-test samples from every roll and every pattern change on a load cell before anyone hangs from them.
How the numbers are worked out, and where they come from
Seam strength
stitches per inch × thread strength × 1.5 = seam strength per inch.
The 1.5 is the average loop strength of thread on a lockstitch machine. Stitches
per inch comes from your machine's stitch length: 25.4 ÷ mm, carried
unrounded — rounding 6.35 up to 6.4 would overstate the seam and ask for less
stitching than the load needs.
Sailrite's break testing found the stitch pattern doesn't decide strength —
the total inches of stitching does. So this tool counts inches and compares them
to what the target load needs.
(Sailrite, How to Sew Webbing Loops)
Geometry
The box contributes two long sides plus your crosswise runs — the two box ends and any extra bar tacks. Diagonals are computed from the box, not measured off one.
An X runs corner to corner, so each leg is
√(length² + width²) and there are two.
A V runs from each corner to the midpoint of the far end —
same length, half the rise — so each leg is √(length² + (width/2)²)
and there are four. That makes a V leg fractionally shorter than an X leg
on the same box, not equal to it.
Why the target is conservative
The stitch target defaults to the webbing's full rating, and that is roughly twice what the seam strictly has to carry. In a folded-back loop the bight splits the load: the standing part sees the whole load, but each leg of the loop — including the stitched-down tail — sees about half, and half is all the box has to transfer. Targeting the full rating leaves the seam around 2× the webbing rather than matched to it.
That margin is kept on purpose. The even split assumes the bight is loaded symmetrically over a decent radius; a loop pulled across a narrow edge or loaded off-axis doesn't share evenly, and there is no separate allowance anywhere else in this tool for that.
Why 22 kN
Cossin, Ross and Gosselin instrumented single-point apparatuses at the National Circus School of Montréal. Aerial straps produced the highest force of anything they tested: 5.3 kN, or 7.9 × the performer's bodyweight. They recommend a minimum breaking strength of 22 kN for straps and for every component in the hanging chain — 17 kN for rope, 12 kN for silks, hoop and dance trapeze. (Making single-point aerial circus disciplines safer, 2017)
The same number arrives independently from EN 566:2017, the European standard for sewn mountaineering slings — structurally the same object you're making. It requires 22 kN minimum. Two unrelated bodies of work, one figure.
ANSI/ESTA E1.43 Performer Flying Systems is the governing US standard. For flexible lifting medium — rope, chain, band, webbing — it is reported to require 10 × working load limit, 6 × characteristic load and 3 × peak load. Against a 5.3 kN measured peak that's 15.9 kN, which 22 kN clears. Treat the E1.43 figures as secondhand until you read the standard yourself.
What this tool won't catch
- Commodity webbing sold with a strength claim and no test certificate, batch traceability or derating data. A supplier's number is a starting point for your own testing, not a substitute for it.
- Thread and webbing lose strength to UV, abrasion and age. Nothing here derates for service life.
- Bends and edges. Webbing loaded over a small radius or a sharp edge fails well below its rated strength.
- Skipped stitches, wrong bobbin tension, needle damage to the weave. Only inspection finds those.
| Fold line | |
| Box far line | |
| Hand loop line | |
| Rig loop line | |
| Sleeve fabric |
| Cut length | 5 in |
| Straight stitch | 3/8 in from end |
| Then | zigzag to seal |
| Fold line | |
| Box far line | |
| Rig loop line |
- Fold and cut the suede into 5 in strips.
- Face two strips together and sew the faces, making one long strip — the seam ends up inside the tube.
- Fold the strip in half, faces together, and sew the outside. You now have an inside-out tube.
- Sew a rope or cord into one end so you have something to pull on when you invert it.
- Close that end to push against, and start inverting. The sleeve needs to run a few inches longer than the strap.
- Sew the sleeve 1.75 in wide — there's a mark on the machine for it, and it's wide enough to pass the webbing.
- Flip it right side out with the stick and rope.
- Cut the closed end off.
- Run the fish through the tube, attach the strap, and pull it through.
- With the webbing inside, fold the ends in and stitch shut as close to the webbing as you can.
- Run polyester line down the centre to lock the webbing to the sleeve — 5 mm stitch length.
- Sew the box X's with heavy thread to form the loops.
- Safeties: cut 5 in of webbing, sew closed 1/8 in from the cut end, then flip inside out.
Sleeve fit — what's been tried
| Material | Webbing | Cut | Sewn | Result |
|---|---|---|---|---|
| Headliner | 2 in | 6 in | 2.25 in | Too snug — webbing wouldn't pass |
Idea on the shelf: microsuede wrap, neoprene on the hand loop only, poly core.
Open question
The body loop's stitch box marks in the sheet were typed in by hand as 3 in, 5 in and 7 in rather than calculated. This view derives the fold, box and rig lines from your inputs instead. If that three-line scheme means something specific, it needs its own rule.