Not really sure what its called but was paying attention to a conversation ( or trying to at least ) The basic discussion was about bolt thread depth and anchoring / holding. That after a certain depth on a bolt ( in relation to the bore i guess ) the rest of the threads were just extra I forget what size the were talk but the guy said that after the bolt had about 3/8 of thread depth ,,the hold was just as good as at a 1/2 .. So is stuff like this true ?? or did I misunderstand ,,,as it seems to me the deeper in you could get the better the hold
I had'nt really thought about it until you mentioned it, then I found this article: http://www.gizmology.net/nutsbolts.htm
i have been told that if you have threads the diameter of the bolt , its good . ie: 1/2" bolt = 1/2" of threads . ................. steve
It's pretty easy actually, just think how thick the nut is that you would use, about the same as diameter right?
In the engineering department where I "apprenticed" (they don't do you much good for practical stuff at the university) the rule was 1.5 times the nominal diameter of the screw - for holes tapped into finished parts. This is more than nuts usually have - but the expectation was that nuts can be thrown away - and the finished machinery was going to get used over and over again. When we used SAE Grade 8 screws - the depth was increased to 2 times nominal - because the base materials were usually not as strong as the capscrews and we didn't want some ham handed mechanic to ruin an expensive piece with an over-torqued screw.
Nice article! I'm no expert, but I noticed a few things that I didn't see in there (or maybe they were and I just missed it - I didn't read it THAT close) are - the practice of USING the SAME GRADE NUT as the BOLT. Seems obvious, but I've seen people ignore that all the time. Also keep in mind this article is STRICTLY about NUTS & BOLTS - not the same animal as threading bolts into TAPPED HOLES. Using the "same thickness as a nut" logic is fine IF the threaded material is the same grade as the BOLT - which of course is pretty unlikely. It is for THAT REASON (I believe) that the machinist's rule has always been 1.5 times the bolt diameter. This willl cover your butt in a vast array of situations. Also not mentioned in the article is tightening in different situations - wet, dry (lube) bolt finish, and of course material type. Meaning that the "standard" bolt torque of 525 in lbs for a 3/8-16 bolt would be ONLY APPLICABLE in a certain situation - not across the board. A bolt torque chart will usually reference - the grade, finish, and wet or dry. They assume you have sufficient material strength to achieve the BOLT prelaod. Meaning IF you are tightening this bolt in cast aluminum you might want to rethink the length of engagement, etc, etc. Bolts are one of those topics we often take for granted. There's alot more to this peeling this onion than I had ever realized. And to the origional poster - when I first read this I thought you were asking about % of thread engagement - as affected by the tap diameter. The article below mentions 75% - which is standard and most charts will or should mention that as well. Happy tightening
Ok,,does this theory change,,if there is a "load" being supported ? Also with types of metals used ? Seen alot of aluminum parts strip ( like with old adapters and bellhousings ) ,,would seem in some cases deep threads would be the way to go
Stress....figure out how to calculate the stress on the material.....that's what mechanical engineering is all about.
Bolts have a rated torque value for dry thread, and this would place the bolt into it's elastic stress "zone". IE the bolt is now a tensed clamp. So, if the load is axial and increases tension, then you are simply trying to stretch the bolt further. Think of a head bolt, holding down against a high comp motor, like Squirrel's BBC it would be undersized, if the "increase" allowed the bolt to go plastic, stretching further and becoming permanently deformed. Certainly it's to our build's advantage to have the nut or threaded region stronger. Deeper thread or helicoiled/inserted AL parts for example.
I went over to the referenced article for a read - and I can tell you that it isn't the sort of engineering information we would use to design "real" machinery here. The data about the distribution of load is correct (the six threads does the work explanation) -but what if the top six threads are worn to imperfect fit -must be the next six huh? And maybe clear through the part (like the well head after its been used a few years) - no one is going to throw one away after the top six threads are gone out of tolerance are they? What about tooling wear if we make a few thousand parts? The torque formula quoted calculates the (very rough approximate) torque in inch-pounds (he forgot to say) - and what percent of "proof load" is a safe allowable for that writer? Might not be what you and I would really trust - especially if the capscrew has to be safely re-used over and over. We count on being able to take our everyday vehicles and machines apart and reassemble them over and over without any big risks - in that situation...... practicality rules - not theory. Textbook minimum calc's aren't what the real world uses for everyday - and be glad of it...
How hard is the bolt, grade 3-5-8, and it fine or coarse thread, fine having more surface area like a rear axle having more splines/surface area, and is the bolt in sheer or tension, tension needing more threads.
I'd go one step further and say if the INCREASE in load allow the joint to "move" or loose clamping force (and this could well be in the elastic region) your joint is improperly designed and fatigue may well become your enemy. Notice I didn't say your bolt was TOO SMALL - bigger is NOT always necessarily better - contrary to popular belief. I forget every detail and am too lazy to look them up, but there are several factors used when designing a bolted joint. One often overlooked factor is the RATIO between the BOLT DIAMETER and the DISTANCE from under the botl head to the first thread of the FASTENED part (be that the nut thread or the threaded hole) this ratio can be critical to bolt life (fatigue resistance). IIRC this ratio is also 1.5 minimum - longer is better. Years ago I went to a seminar on Bolted Joints - initially I was slightly upset my boss sent me as I thought it was going to be a huge waste of time. Boy was I wrong - it was one of the best seminar's I've ever attended. The guy putting it on was BOTH an Engineer AND an in the field as a practical problem solver. Trust me - this guy was the REAL DEAL. As I said before there's a LOT more to this than alot of us realize - myself included. Another thing to keep in mind when designing a bolted joint is the coefficient of expansion and how that affects your bolt's preload. Ya ever wonder why the typical SBC for instance engine's internal fasteners don't come apart and yet there are no lockwashers or even loctite used from the factory? Rod bolts (nuts) don't typically loosen - head bolts don't suddenly back off - yet all these things go through huge temperature variations and tons of cycles. Yeah you can bet your kiester that those engineers weren't winging it - they were book smart & "street" smart. BTW Pitman - I got way OFF TRACK here - I don't mean any of this as a shot at you or anyone else for that matter -I'm just thinking out loud!
A good artical. I tried to find more info but couldn't and this is what i am wondering: how does all this apply to a stud? Ever run head studs into a tfx or kb? they go in a lot further than 6 or 8 threads i can tell you.
That's cause you're workign with aluminum - you adjust the DEPTH OF ENGAGEMENT based on the base material properties. Sometimes more IS better.
Knew a guy, even after being told what type of torque he should put on a bolt, he would tighten a 1/4 inch bolt just the same as say a 1/2 bolt. Break them off all the time and then bitch about it. Got to be kinda funny watching him after awhile. I think his skull must have been made with grade 8 material, but his brain made of oatmeal. OK back to the thread.
I apprenticeed in a machine tool shop and was taught that 1.5 x diameter of the bolt was the correct depth for threads. Less that that weakened the holding strength and more than that not only didn't add strength, but due to any slight mis-match in the thread leads between bolt & nut, it could actually weaken the clamping strength.
Also keep in mind that the first 1-2 threads on a bolt/screw are generally not counted as threads when measuring engagement as they are undersized in most cases (do to the machining/rolling technique and to allow easier starting) and do not provide adequate load bearing area. This goes for the first 1-2 threads of a tapped hole in the parent material as well; unless extreme care is taken and proper tooling is used, the initial threads are slightly "wallowed" due to the tap starting technique or are removed all together by a countersinking operation done before hand that helps bolt starting. Mike
The Jalopy Journal
