Though I use home-built tethers tied from 1-inch tubular climbing webbing, I've been questioned about sewn eyes. My reasons for staying with tied webbing? I am still playing with design and length, I know the knots are strong enough (force is limited by a Yates Screamer because I value my ribs and spine), and because knots are less vulnerable to UV and abrasion. I will sew a pair when I am satisfied with the final design.
The standard knot for webbing is an over hand loop. While a relatively weak knot in rope (40%), it does better in webbing (65%) because webbing is thin and the short radius turns less important. As for concerns that webbing can slide through an over hand loop with repeated cycling, this occurrence is exceedingly rare among climbers (that is all we had for many years, before sewn sling became common, setting the knots hard and using sufficient tail works, and I place ~ 6 stitches in the tails just to be certain. Knots can be very safe. The reason for the move to tied slings was twofold: they are easier to handle and narrower webbing can give the same strength because of the higher joint efficiency. Weight really matters to climbers.
tacks; some things seem logical, but are not. If anything, testing shows that rows of stitching straight across give the best load distribution, and failure is always at the last row so how could what is happening in the X-region matter? No logic in that.
(2-inch minimum). In principle, squeezing a few extra stitches in the throat row is probably good, to control spreading--there is little risk of weakening the webbing there, since each side is carrying 50% of the load (technically a bit more than that on the load side). There is little tape-to-tape or over the carabiner friction with webbing, so we stitch moderately tightly so that all stitches bear an equal load. We sew in straight lines and maintain uniform stitch size, because the webbing will not deform and spread the load as a rope will. We do place fewer stitches in the last row to form a sort of taper, but we do stitch the full width. There is an urban legend among some non-climbing gear manufacturers and web sites that sewing in an X pattern helps spread the load an increase strength. I have never seen testing to support that, my testing does not support that, and climbing gear companies use straight bar
As before, all stitching is with doubled 50-pound waxed whipping twine and a sail makers needle. The webbing used was 1-inch tubular climbing webbing, 17.8 KN (4000 pounds) rated . A stitching awl is not recommended because is it slower, cannot use doubled twine, and makes a bigger hole which weakens the webbing. Awls are for leather work.
Stitched Webbing Eyes. All failures were sudden, with a sharp bang, suggesting all stitches failed nearly simultaneously.
Trial 1: 12 passes (24 strands) failed at 1200 pounds, or 50 pounds/strand.
Trial 2: 16 passes (32 strands) failed at 1650 pounds, or 52 pounds per strand.
Trial 3: 24 passes (48 strands) failed at 2550 pounds, or 53 pounds per strand.
Trial 4: 36 passes (72 strands) failed at 3750 pounds. The webbing failed at the last row of stitching. 86% efficient splice.
I'm guessing that as the number of strands increases, slight imbalances in stitching become less important. The bottom line is that holding power = (strands x line test). I will use 50 stitches spread over 9 rows (8x6 + 1x4) be very confident that he connection is sound.
Vulnerability of the stitching to UV? A very good question. I've been working off the same large spool for nearly 20 years, and I have never had the stitching fail on any project, but I'm going to hang some loops out in the sun a test break them over the next few years, just to see. I'll report back. However, tethers are not in the sun very much. They are stored below decks. Often they are used at night and in gray weather. I suspect the wax offers some UV protection. I'm not too worried about it.
How critical is the size of the eye? Much more so than for rope eyes, as it is impossible to seize the throat to resist spreading forces. Fortunately, webbing does not require a thimble to provide a safe radius--a carabiner is enough--yet even so, a 2-inch minimum eye length is required. The maximum spreading force a machine bar tack can sustain is known to be about 750 pounds, while the maximum spreading force (pealing force) hand stitching can sustain (tested thrice with 16 strands of 50-pound twine) is about 350-450 pounds.
Examine the table to the right and you can see that the pealing force reaches the failure point at a 20 degree throat angle. But when we consider that those stitches are also under longitudinal strain, the design point becomes ~ 6-8 degrees, which is the range we tested and the range created by carabiners if a 2-inch eye is used. Keep your eyes long enough to avoid creating significant pealing force, since hand stitching is most efficient in shear.
All pictures are Trial 2:
- at 85% strain
- close-up of failure