Bolt failure

Stan

Grub

Stan

Dusty

Mechanical is the branch of engeering that deals specifically with bolts and their strength. There is an optimal length of engagement of thread with either a nut or threaded hole that develops the maximum strength of the bolt (or stud). If the depth of engagement of the threads is less than optimum, the threads will fail in shear rather than the bolt failing in tension. In other words, the threads will strip. If the engagement is greater than optimum, the bolt will fail in tension, assuming a direct tensile load on the bolt (not a shear failure due to torque on the bolt). This optimum length of thread engagement varies but is generally 1/2 to 1 times the bolt thread root diameter for ductile materials like steel. This is assuming the material of the bolt is the same as the material of the nut or part that has the threaded hole. In other words they need to have the same physical strength properties. There is more than one formula, depending on how precise one wants to be and how ductile the material is. The basic "quick & dirty" would take the shear strength to be 1/2 the tensile strength, the thread shearing force as equal to 1/2 x tensile x pi x root diameter x engagement and set that equal to the bolt tensile force of tensile x pi x root diameter squared x 1/4. That gives engagement = 1/2 root diameter of bolt.

BTW, here's a good list of bolt standards, markings and strengths.

Rod

Mechanical engineers are schooled in machine design and often deal with this problem, It requires knowlede of Strength of Materials and Mechanics. I'd almost bet that soemone on these forums could give you a detailed, easy to understand answer to your question. But it ain't this guy. My training is in aeronautical engineering and fluid flow was my specialty when I worked for the big kitemaker in Seattle.

Regards,

A former propulsion engineer

I had a very picky, discerning outside consultant/inspector analyze structural connections on some fairly large metal windows severl years ago, and the subject of thread engagement came up along with means to prevent the nut from backing off the fastener, field conditions can really toss a wrench in the works when an inspector looks at the particular connection as shown by the engineer in the stamped calculations for the job and sees things that vary, even slightly, due to miscoordinated details, interferences and or dimensions that change, don't work or what have you. Any time something like that arised it was reported to our engineer, with a suggested corrective detail for them to calculate and usually became an addendum to the stamped calculation/connection details provided for our work, for record.

What you are talking about gets into the nitty/gritty of things, whereas I believe a structural engineer and or mechanical engineer would be best to analyze, check, and or calculate a specific problem, or just explain it in theory, as it's beyond my knowledge, with a scant few occurrances on connections we did while I worked as a sr. project manager for metal and glass specialty contractor.

I'm not sure as far as reference standards like ASTM and what information may apply to that, do know that they are one reference standard for fasteners, high strength bolts used for bolted flange connections of structural steel are designated as A-325 right on the head of the bolt for an inspector to see and believe they are equal to grade 8, there are other designations, A-307 I believe is grade 5, also recall an A-304 for stainless, not sure what grade it was equivalent to, just remembering these from things I've worked on in the past.

What you might find handy is a fastener supplier like Hilti or others that include engineering data in their catalogs, they show ultimate strengths in shear, tension etc. of all their fasteners, although Hilti is mostly Construction related fasteners, the same information should be available for more industrial applications, I've never seen, any information regarding thread engagement, and it's assumed that the fastener used, with a nut holding the assembly, that it is fully engaged onto the threads, I think that is why the inspector had some issues as partial engagement may lessen the ultimate strength in tension. Fully engaged, tension strengths are usually quite strong, same is true in shear. They do perform tests, some to actual failure of these kinds of things. In construction applications we do them on the job with the fastener supplier, but the fastener is usually installed into a substrate like concrete and is checked to make sure it performs as called for, I have also done them to failure on occasion and have accompanied an engineer with the test equipment onto a swingstage scaffold hanging many many floors above grade to test various applications of fasteners.

I rememeber solving the problem above by designing an alternate detail with our engineer to get the thread engagement so the fastener was flush with the outside face and the concern about the nut backing off was dealt with by using a wicking grade loctite, lockwasher negated the thread engagement and the dimensions were too tight, another wonderful miscoordinated structural detail provided on the contract documents, that ended up in my lap, but was eventually accepted. These kinds of problems are commmon place on high rise jobs, designers sometimes do a poor job on the contrac documents and wait until contractors find them or arise during construction, at which time a smart contractor issues an R.F.I. request for information and or clarification of said detail, upon which usually becomes a cost add to the contract because of a problem it creates, requiring extra work, engineering, and can very well be an uncoordinated or omitted detail on the documents, in short they can be a real pain in the arse to deal with, create significant risk and be very costly to a contractor, owner or designer. I'm sure it applies to many other industries, figred I'd add my take on it from a construction standpoint, little beyond what you were asking and the usual long winded response LOL !