sewing nylon tow straps

[ketch]

Hello everyone, been a member for a while now but this is my first post. I repaired a loop on a nylon tow strap using a zig zag pattern sewing from one edge angling down a little and to the other edge working to the end of the loop then back up using the same method ending where I started.  This gave me a total of 351 stitches. Is it safe to say 351 stitches using a thread with a breaking strength of 45 lbs will give the repair a breaking strength of 15795 lbs (351 x 45 = 15795) ? I am using 277 thread with a #25 needle and a Cowboy 4500 machine. The needle is a diamond point but I ground it to a point so as not to cut the fibers on of the strap. I can't find a #25 spear point needle. How are other people sewing tow straps? I would appreciate to know your set up.   Thanks Martin

  

[MadHatter]

not how you would calculate the strebgth of the loop.  consider the tensile, tangential and stress distribution of the forces in play as it would ne in use. also the stitch length will effect the strenth of any seam.

 

but l would say your strap would hold as you describe

[Wizcrafts]

I sew webbing tow straps on occasion. I sew a series of vertical lines about 1/4 to 1/3 inch apart, at 4 to the inch. I back tack at both ends, into the same holes for at least 1 inch, covering about 5 inches of folded length.

In the past I would sew a rectangle with a X pattern inside it. But, I found that the row going across the front end was the weakest point because it went across from side to side. I looked a heavy tow strap that was used on bigger trucks and it was sewn with multiple rows inline. This is similar to the technique used in heavy horse tack, where the buckle and squares ends are never sewn across the strap, but inline along the sides.

Bob Kovar sells round point system 7x3 needles in most sizes up to #27. I have some packs I bought from him in #25, 26 and 27.

[Tejas]

Practical sailor, April 2015, did test of various hand sewing techniques for webbing and concluded that seven parallel stitch patterns were the strongest, and that the joint, either the stitching or the webbing, would start failing from the ends of the joint. The objective was to discover stitching that had approximately the same strength as the the webbing.

The article stated that rock climber's slings are load tested and that parallel "Machine-stitched bar-tacking is the norm for professional rock-climbing gear." The following link depicts an example.

https://www.rei.com/product/830935/mammut-contact-sling-dyneema-cordelette-80

 

[Wizcrafts]

I neglected to use the term parallel when describing the lines of stitching I sew into tow straps.

[TinkerTailor]

I would imagine that the rule about never stitching across a harness strap mostly applies to leather because the holes are actually punched through and weaken the strap considerably, as well as letting in gunk that will rot it. With webbing the whole goal is to sew it without any fiber damage. If the fibres are not damaged, it should not matter which direction the stitching goes provided the pull direction on the strap is as designed. In reality once you get that much stitching the textile to textile friction due to surface area is what holds it. Of course higher spi has more hold in this respect but also more damage to material due to more holes. Not that one shouldn't sew parallel to the strap as wiz suggested, as this probably makes a difference in the straps ability to handle the shear loads on it due to wind and such going down the road. I also imagine that re-sewing a dirty old strap will cause more damage to the fibers than when sewing fresh clean stuff. Basically i am saying that the leather to webbing comparison isn't the best one due to major differences in materials and their stitchability.

Edited October 31, 2016 by TinkerTailor

[Tejas]

41 minutes ago, TinkerTailor said:

In reality once you get that much stitching the textile to textile friction due to surface area is what holds it.

As for increasing surface friction, inserting a reinforcing piece of webbing between the two webbing layers apparently does that. Here is a link about  sailboat jack-lines (safety lines attached to the deck to which a harness tether can be attached while on-deck, especially in heavy weather).

http://westpacmarine.com/blog/category/straps-and-webbing/

I think I recall a similar article about strengthening parachute webbing joints but can't locate it.

[Colt W Knight]

The heavy duty nylon slings we used in the mines were all sewn parallel with one row stitching at the loop going perpendicular. Plus, there is a certification tag sewn onto the strap as well, and its sewn all the way around like a box. These were all tested and certified. Technically, they couldn't be used if any of the fibers were torn, and definitely couldn't be repaired and used unless they were re certified. Not saying, you can't or shouldn't repair them, but in the mines we fell under OSHA and MSHA regulations, and lifting/towing/pulling straps were all highly scrutinized.

Edited October 31, 2016 by Colt W Knight

[TinkerTailor]

Nerd stuff ahead:

Friction is a funny beast. I was watching a video where an engineer placed a square bar across a gap between two blocks, and placed a given load in the middle, measuring the deflection. He then took another pair of  bars, both too short to span the gap, and placed them overlapped held together with a couple of rubber bands. This assembly was such that neither bar was supported by the bricks,  He then placed the same load on the top and the deflection was half as much........The only thing holding the  bars together was a couple rubber bands........The friction between the two bars, provided by the rubber band tension, was sufficient to prevent sliding and allow the bars to reinforce each other.

What i take from this is if a strap is designed and used properly so the load is lengthwise, the seam should not fail until the pieces slide which would require overloading or abrasion. I look at the thread in a seam as operating similarly to the rubber bands here. If the seam is not tight enough ie. thread tension too low, the friction co-efficient will drop and so will the seam strength.  Basically the max tension within the limits of the thread will give the highest co-efficient of friction and thus the maximum strength. As well, the minimum stitch length within the materials limits will give the most strength.

If you apply force in the wrong direction, and pull the seam apart in use, all bets are off

 

[Colt W Knight]

I have used a lot of nylon slings in the mining industry to lift and drag things that weigh Tons and tens of thousands of pounds, and I have never seen a sling fail at the stitching. Normally it rips in the loop portion.

[TinkerTailor]

Only time in the oilfield i ever saw one fail at the stitching, they were winching a truck out of the ditch off of a couple of trees. They connected the loop to two trees at the same time with two chains in a Y configuration, because they thought one tree was not strong enough. The stitch failed right away, because the chains were pulling it apart. Thing is, all they needed was a shackle or a steel ring  to hook the chains to and take the side load off the strap loops.

[Colt W Knight]

2 hours ago, TinkerTailor said:

Only time in the oilfield i ever saw one fail at the stitching, they were winching a truck out of the ditch off of a couple of trees. They connected the loop to two trees at the same time with two chains in a Y configuration, because they thought one tree was not strong enough. The stitch failed right away, because the chains were pulling it apart. Thing is, all they needed was a shackle or a steel ring  to hook the chains to and take the side load off the strap loops.

Yeah, I can see that. Technically, we wouldn't be allowed to use a sling like that because it is not proper rigging, but we all know stuff gets done the wrong way all the time.

 

[Tejas]

If with stretchy material such as webbing, the failure of stitches under load tended to be towards the the direction of the load, then sewing parallel to the edge of the webbing versus orthogonal to the edge of the webbing seems to be counter intuitive. Wouldn't breaking the end of a parallel to the edge stitch-line tend to compromise the entire stitch-line?

[TinkerTailor]

Not if the stitch has as much stretch or more as the webbing, both due to tension and similar materials. Nylon thread in nylon webbing should stretch close to the same.

[ketch]

Thanks for the responses, I learned a lot.