Strength of a sewn seam v thread size

[chrisash]

I have often wondered about the requirement of heavy thread on  items like holsters or any other heavy use items, without considering cosmetics

My question is is the strength of say #138 at 22 pound breaking strain, (bonded nylon) on say a 10cm length of stitched leather, at a nominal stitch interval, likely to break down at 22 pound applied pressure, or is the pressure divided by the number of adjoining stitches, and therefore require a far heavier pressure to break, i.e. each single stitch would break at 22 pounds but is the load shared by other stitches

The second question is, if you have glued the leather as well as sewn, what do the experts expect to give way first

Presumably someone must have done some real life tests in the past, or is it in reality just a case of always been overengineered and people just followed tradition

[johnv474]

I am not an expert but here is part of the answer. If you use synthetic thread, your leather is more likely to tear than using a natural thread.  That makes for a holster that will be stronger, but when it breaks it will be in worse condition (torn' plus more stretched leather because more stress on seams didn't break them)... which makes them harder to repair.  It's easier to replace a few stitches than to fix torn leather generally.

 

Every choice has pros/cons.

 

When pulling apart a seam, only the first few stitches (plus any leather, adhesive, etc) take the great majority of the stress. The stitches further from whatever force is applied sense virtually nothing, until the first stitches break that is.

 

 

[chrisash]

yes that's obvious,

Not sure about natural thread though as far weaker breaking strain, and do you have proof it resists cutting better than Bonded

What amount of the pressure do the adjoining threads absorb from the total before one in this case #138 breaks at 22 pounds. also does the glue break first

I just feel there must be some scientific research done somewhere and we may well be over engineering the strength of thread required on some items

Cosmetic decoration is a different matter

[Arturomex]

This is written for fabric but it might be a starting point to consider:

Stiches Per Inch.pdf

Regards,

Arturo

 

[chrisash]

Thanks thats handy knowledge, but i wonder if it applies to leather as well, with little give or stretch

Rather than making you download the article the main line of thought using cotton and fabric is

Formula for Estimating Seam Strength on Woven Fabrics 301

Lockstitch - Estimated Seam Strength = SPI X Thread Strength (lbs.) X 1.5*

* 1.5 is a factor based on the average loop strength ratio of most sewing threads.

= 10 SPI X 4.0 lbs. X 1.5 = 60 lb. strength

Certainly gives food for thought

I wonder if any of the machine suppliers on the forum have any input they can give

Edited May 15, 2020 by chrisash

[nylonRigging]

On 5/13/2020 at 1:01 AM, chrisash said:

I have often wondered about the requirement of heavy thread on  items like holsters or any other heavy use items, without considering cosmetics

My question is is the strength of say #138 at 22 pound breaking strain, (bonded nylon) on say a 10cm length of stitched leather, at a nominal stitch interval, likely to break down at 22 pound applied pressure, or is the pressure divided by the number of adjoining stitches, and therefore require a far heavier pressure to break, i.e. each single stitch would break at 22 pounds but is the load shared by other stitches

The second question is, if you have glued the leather as well as sewn, what do the experts expect to give way first

Presumably someone must have done some real life tests in the past, or is it in reality just a case of always been overengineered and people just followed tradition

Don't know ??  if you going to find that detailed depth info with Leather and Thread .
In the Para. Rigging industry there is books full of this mind numbing info  . The Rigging topic has intense in-depth study of all, structural load strengths, shear strengths, shock load strengths, abrasion strengths and failures of fabrics, webbing's and all the Threads, and with all hardware's the fabrics and threads come into contact with being sewn . All done over the years because of aircraft air speed/exit speeds and structure and failure with parachute, harness container systems, cutaway systems, ejection systems ..etc. . with all the history with Gov. contracts and FAA TSO and other Certs. with all fabrics and threads involved used with constructed of life saving equipment .
.

[chuck123wapati]

That's a great wonderment , I've toted a lot of holsters and haven't had a one tear out due to stress. I've had sheaths and holsters thread give out due to rot and or wear. Don't know why it would be of much concern in holster fabrication. Now if your worried about the belt I can see that because its under stress and any belt holster combo should use the same thread just to match. 

[johnv474]

On 5/14/2020 at 3:52 AM, chrisash said:

yes that's obvious,

Not sure about natural thread though as far weaker breaking strain, and do you have proof it resists cutting better than Bonded

What amount of the pressure do the adjoining threads absorb from the total before one in this case #138 breaks at 22 pounds. also does the glue break first

I just feel there must be some scientific research done somewhere and we may well be over engineering the strength of thread required on some items

Cosmetic decoration is a different matter

I can't give exact answers because they would require calculus, which I do not know.  I am not sure which part is obvious.

 

Natural threads break under tension easier than common synthetic sewing threads.  I would expect they would be cut easier as well.

 

Of the things that can give way/fail, we have the leather, the glue, the thread, and, possibly, some other reinforcement or hardware like a rivet.  A glued/cemented holster would fail first, then a stitched but unglued probably (depending on details like whether backstitched, whether ends face the opening or away,), then the stitched and glued, then stitched and glued and reinforced.  Within those, leather type, tannage, thickness, and construction (eg rolled edge near the seam) can cause a wide range of strengths, and so can adhesive strength and bond quality, as well as thread type, sewing procedures, stitch spacing, thread tension, leather conditioning, quality and installation of reinforcement, the type and direction of the force applied to it at failure, etc.  The range and overlap of those areas makes giving a numerical or specific answer difficult except in broad terms.

 

I expect many larger manufacturers have done some stress testing to failure but would expect it to not be very available to their competitors.

 

If you want suggestions on very strong construction, many here could make recommendations on how to overbuild.  Being able to know how much each part plays in the failure may not be an easy or universal answer to find.