Stitch Strength in Custom Fabrication of Strap Assemblies

The strength of a custom strap is a major concern for most individuals who order them. Several factors control the strength of a new strap; webbing, hardware, design and stitching strength. Once a strap has been put in to service its strength will decrease due to abrasion, ultraviolet light exposure, chemicals and a host of other factors beyond our control.

How does one calculate the strength of a particular stitching pattern? There are two major factors; the number of stitches and the pattern itself. Figuring out the theoretical strength based on the number of stitches is pretty easy. You need to know the strength of the tread being used, they type of stitch and the number of stitches. We purchase our thread from A&E (a domestic manufacturer) and our primary thread is their Anefil Poly®. The two main sizes we use are Tex 90 and Tex 70. You can look at their technical specification page and look up this thread and you will discover that the Tex 90 size has a strength of 13.3 pounds and Tex 70 is rated at 10 pounds. A&E also offers a page on how to calculate seam strength which provides the formulas we need.

All of our sewing machines produce a number 301 lockstitch so the formula for estimated strength is:
Estimated Strength = Stitches X TS x 1.5
TS stands for Thread Strength
1.5 is a factor based on the average loop strength ratio of most sewing threads

If you were using Tex 70 thread and had 20 stitches the estimated strength would be:
20 x 10 x 1.5 = 300 pounds

The same 20 stitches using Tex 90 thread would increase the estimated strength to:
20 x 13.3 x 1.5 = 399 pounds

As you can see it would not take very many stitches of either thread to exceed the strength offered by most of the plastic hardware we sell. But this is only part of what needs to be considered. Will the stress on the seam be in a shear or peel direction? Is the load static or dynamic?

The drawing above provides examples of shear or peel. In shear the red and black pieces of webbing are being pulled in a flat plane. Properly sewn every stitch is taking its share of the load so the calculations should be a good indication of our actual strength. Peel mode is a different beast. Here the bulk of the load is at the 90 degree joint and stitches further to the right of that will provide very little strength to the assembly. Think of putting a piece of masking tape on the kitchen ledge. The tape will break if you pull it in line with the ledge (shear mode) but you can easily remove it from the ledge if you pull in a peel direction. Please don't try this if your ledge is painted or varnished.

When designing a strap the sewing pattern needs to take the direction of load in to consideration, the favorite Box-X pattern might not be the right choice in many applications.

Finally, at least for this page, one needs to consider if you are dealing with a static (constant pull) or dynamic (jerk) load. Dynamic loads are much more difficult to design for and are often underestimated. While we do not offer products like rescue harnesses or lifting slings we still have to deal with both types of loads. When walking the dog is he/she always straining on the leash (static) or does she/he see a cat and want to chase after it even though the leash had been slack (dynamic). While this might be a silly example it shows how complex strap design can be.

When you get ready to contact us regarding a custom strap we will need to know about your strength requirements. This is not something we can figure out for you. In the dog leash example we don't know how big/heavy the dog is or if she/he loves to chase cats. There is no way we can come up with a strap that is stronger than its weakest link (pretty obvious but we have had customers that want the stitching to be ten times the strength of the buckle) so please give this some consideration. Earlier we discussed "estimated strength" and those calculations are just estimates. If your project requires a specific strength you will need to order a prototype and have it tested. This is something you will have to do, we are unable to write your test protocol for you.

So get all your requirements figured out and give us a call at (253) 627-6000. We would love to work with you.

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revised 10/2019