Topic Author's Original Post - Jul 13, 2014 - 09:02pm PT
I have found very little information about sewing webbing yourself with a home sewing machine. Everyone seems to blindly agree that you just can't do it yourself and if you do it's never going to be strong. I decided to give it a try using and old Sears/Kenmore sewing machine and #69 polyester thread that is used for upholstery. My reasoning for sewing my own webbing is based on the need for webbing products that I'll be using for slacklining that aren't available for purchase.
I spent a few days getting used to the machine I have and how to control stitch patterns, speed, thread tension, and foot pressure. Once I figured out all the details I made several slings using a bar tack like stitch that you see on all climbing slings. I used the calculation of 1.8 x thread strength(11 lbs for #69) to figure out the strength of my stitches which made it easy to figure out how many stitches I needed to get full strength on 1" tubular(4500lbs).
I made a hand full of slings and have been testing them at work using a forklift. I have had to refine my stitch patterns slightly but now all my slings break at the webbing and not the stitches.
This has been a fun experiment and I have been able to create several new items that are slackline related that I now use daily. I don't plan on using these slings for climbing since store bought/certified slings are so cheap but they are obviously just as strong and in most cases stronger than any other gear used in climbing.[photo[photo[photo[photo[photo
Sewing a bight means your bar tacks are seeing only half the applied load. So a break in the webbing can be understood. You have, however, shown your sewing does not reduce strength by half.
When you sew an overlap you might also try inserting one end inside the other end of the tubular sling. Back before Lindbergh's flight I did that and got noticeably higher strength. Watch and see how a regular overlap fails and you will see why the insertion may make for higher strength. Sewing caused weakening of the sling material is something you need to keep an eye on. Make some test parts with way too many threads. See where you begin to run into trouble.
Don't bet your life on anything I say. Take lots and lots of data.
Get a copy of a book called On Rope. It's a guide for technical caving that includes a large amount of sewing your own gear information. You can get spools of nylon #69 tread that will be stronger than polyester and work with a decent sewing machining. There are stiches that are both easier to do and stronger than bar tacks. See that book!
My father was an auto and boat upholster. He made some of my first harnesses when I was still a young lad. Cavers were the first to make their own harnesses, by the way. I made 3 harnesses my self through the years. I sewed my own runners and slings and field tested them all. And one thing that my father taught me, was that polyester thread was so much better for standing up to both UV radiation, and abrasion. It is even more resistant to chemicals than nylon. The strength difference is only marginal between nylon and polyester from the git-go. Polyester thread, with a properly adjusted machine, is able to pull the thread much tighter into the nylon webbing and lock much tighter than nylon. Nylon stretches if pulled and worked, thereby allowing the "lock stitch" to slowly work at the "lock". In time nylon not only looses its strength quicker than polyester, due to UV radiation, but can exhibit somewhat of a sawing action at the "stitch lock" and weaken there. There has actually been tests performed on all of this if you do the research.
If the climbing industry uses predominantly nylon thread, I might suggest that the reason would be for ease of sewing, as it is far more forgiving in the application. The industry, as a whole may not care if your slings rot quicker than they need to. I'm sure they would be happy to sell you a new one.
The strongest stitching is simple back and forth longitudinal stitching. Bar tacks are longitudinal stitching, but this bound section is more flexible than just straight back and forth stitching. A size 16 or 18 round point needle needs to be used, full tension on the foot. The "stitch lock" needs to be pulled as close as possible to the center of the webbing, this is key. Try this simple test once you get the tension just right. If you weigh about 170lb stitch just 7-8 stitches in a row in the dead center of a sling joint, and see if it won't hold your body weigh when you slowly weight it. I use to do this test and it was kind of mind boggling that this was even possible!
Great info two toes! I have both #69 nylon and #69 polyester and have been primarily been using the polyester due to the UV and rot issues you mentioned. I tested the strength of both threads by seeing how many steel carabiners I could pick up with a single strand and they both seem to be about the same strength. I'm also curious about using dyneema thread which can be bought at the sporting goods store in the form of braided fishing line. My only concern with this is that the webbing would stretch and the stitches wouldn't since dyneema is so static. Anyone have any insight on this?
The thing you said about the lock stitch being as close to the center of the webbing makes a ton of sense, I have had several of my early test slings start failing at the side where I ended my stitched and work there way across blowing stitch by stitch. I fixed this by starting and finishing the stitch closer to the center of the webbing.
I think you're on the right path slacker30. Checking out the prior vertical caver articles that wivanoff had posted, it's a little had to read, but I notice that the strongest splices are done with the straight longitudinal joins. They are not as pretty as the bar tacks though and are not as marketable, you see. The best way to start a splice like this is to first stitch right down the middle of the webbing, to first center the pieces. Then cross over a stitch and then right up the other side, all vertical stitches, until you work the one side. And then back to the middle to work the other side. It will bind your splice to such a degree that the whole splice will be very stiff. I never cared about that because I knew that there was such a great degree of strength in this method of a join. The #69 thread works well and is most easily obtained.
May I reiterate, as a simple test, by making only 8-10 stitches longitudinally in a row, if the stitches are properly locked in the middle, will hold body weigh if you slowly step into the sling! The more stitches you put into the system the strength just goes up exponentially.
I still re-sling all of my old cams by sewing them myself and I've taken some real wingers on them through the years. I use an ancient Singer 31-20 that had been set up for upholstery, and drive #69 black colored thread, as it should stand up to the most sunlight since it is so opaque.