They say you learn something new every day. Well, this is one of the things I learned today. I was read the section in here about sewing webbing, about 2/3 of the way down. Keep in mind if you read it that they are trying to achieve a breaking strength of 4500 lbs, I'm happy with 1/3 of that.
Let me try to summarize what I learned. First of all, there are different forces at work if you are sewing a end loop, like in tree straps, vs joining two lengths of webbing together. It turns out that the handy writeup and chart that many of us have been referencing from Nylon Highway is for end to end joints and not end loops. With end loops, the bulk of the force is applied to the stitching nearest the end of the webbing. The overall strength of joint is a function of the tensile strength of you thread x 125% x the number of stitches, that is the overly simplified version anyway. There also seems to be a factor or the density of the stitching. Too close together and you weaken the material. Too far apart and there aren't enough stitches to share the load.
So, the conclusion is that multiple bar tacks is the strongest solution. If you don't have a ZZ machine, through as many lines of stitching into the last inch of the webbing that you can. The stronger the thread the better, but there are limitations on what our home machines can handle.
The factors that I'm really not sure of at this point are the actual clamp friction factor on our webbing as there seems to be quite a bit of variability between combinations of thread and material. I used the 125% as it seems to be a safe number. Also I have not been able to find the tensile strength of the Gutermann thread types that many of us commonly use for projects.
Mara 70 is a V69 and if it has the same tensile strength of the V69 used in testing (10.6 lbs) you would need 113 stitches to equal the rating of 1500 lb webbing.
I have been know to misread things in the past. So please read the information for yourself and let me know if I've made any errors in my summery.
Bookmarks