Bolted Connections

That's what ya gotta love about specifications; they seldom tell the reasons. (Admittedly, if they explained everything, the book would be a lot larger.)

It sounds like a rationale of commonsense to me.
You obviously want to engage the full thread of the nut and thus need at least one thread showing, but someone might put two washers on or think no threads is ok...so lets say 3 threads minimum, that even caters for some thread damage.
Now you don't want too much sticking out else I may whack my elbow on it. It will also inhibit effective quick assembly.
I'd guess the spec was written by Goldilocks on her day off from the 3 bears house.
Del

Have you read through this thread:

http://cr4.globalspec.com/thread/48673/Bolt-Length ?

My company also uses 3 threads exposed on both side, it is not exposing too much threads out and it indicated that the nut is completely in. Although I think that anything above 1 thread is actually enough, don't really need 3.

So, effectively, both here and in the thread (no pun intended) to which JohnDG kindly directed my attention, there is no standard that can be applied. There are many opinions though and most, if not all are sound and logical. My next problem is to convince my Client's inspector that it is not going to accellerate the end of the world if more than one thread is exposed (but not an infinite number) because a standard bolt length was used.

I've found this reference to BS 5950-2: 2001, which specifies one full thread pitch after tightening.

Are you sure you signature qute isn't from Ambrose Philips?
Del

The article says "at least" so two or more would seem to be deemed acceptable.

True - it was a minimum specification. Some applications could actually need more than one, as, for instance, when plastics caps are to be fitted later to cover sharp edges (possibly to protect against abrading cable insulation, or to prevent personal injury).

Having been there, I suggested to the inspector that, although the extra threads did nothing to the connection, the use of standard bolts was both a time and cost consideration, and if he still had a problem to please fail the connections (several hundred in this case) and we'd make them to his specifications.

None of the connections were undone or, to my knowledge, failed.

Bill

Did any one check this?

I don't have access to DIN 78 , so can not comment, but it seems as per the above link,

protrusion for standard parts = nut height + 2P (P is thread pitch, so "2P" effectively is 2 threads)

protrusion for lock nuts = nut height + 3P

And as per SAE (sorry can not confirm again) it is 1.5 threads.

Here in the USA, for high strength structural bolts, A325 & A490, it is only required that the end of the bolt be flush with the surface of the nut. The detailers try for a small extension.

In my view, the number of threads beyond the top of the nut is not performing any structural function, so other than eye appeal and uniformity, it is immaterial the number of threads beyond the nut.

At one time in the Marine Industry the thread was flush with the nut to allow removal without the problem of corrosion in the threads preventing removal. That was some time ago in the 80's.

This sounds like a Department of Energy or Naval Sea Systems requirement. The "inspector" works to a set of specifications that have parameters for acceptance, called tollerances.

Your arguments have been heard many times by the inspector; however, you will soon find that arguing with an inspector is like wrestling with a pig in mud. The pig enjoys it.

You say you used a "standard bolt." I hope that you have the complete metallurgicals to support your claim, because I, having been an inspector, would, at the Government's cost, have a representative sample of the bolts removed, tested, and inspected for compliance to the parameter specification, as is my right. If the bolts do not meet the specification and the certifications are fraudulent, you would have more trouble than you can possibly imagine (try reading United Stated Code [USC 18] title 18), not to exclude your added cost of having to redo all fo the hardware. You might even be lucky enough to have the contract terminated "for cause." I belive the penalty for product substitution and false testing is around $250,000 per count. in a flange with 24 bolts, that could be $6,000,000. Ah laddie, "What a wicked web we weave, when first we practice to decieve."

I might go so far as to have the piping and welding reinspected, because, if you cut corners on the bolts, what else did you cut corners on? This reinspection and retesting would be at "your" cost.

After bolts are installed in any flanged pipe connection in Nuclear power station and Petroleum installations, all bolts or studs should be sized or(measured) so that an equal number of threads are exposed beyond the nuts, the reason, as explained to me, was the repeated heating and cooling of the flanged pipe connections, will cause the studs to stretch, Scheduled retorking of the all bolts or studs to keep the flanged pipe connection from Leaking is many applications is required. Measuring the length of each stud or bolt after retorting, from nut to end of threads, or original measurement, will determine the amount of total stretch of each bolt or stud, and if each stud or bolt stills meets the required Engineered Specifications for this application. Seeing Bolts and Studs being removed from large Steam and Chemicals Systems where the diameter of the center or the 2" bolt or stud is only 1 3/8" diameter from stretch or corrosion scares the hell out of me, Remember Safety is #1 Thank God For Engineers

Possibly in the case of gaskets settling. But I would not like to think of continuously stretching and retorking bolts to be a well designed system. I would think the reference is expansion by heat causing the nuts to work loose, this would require a scheduled re tightening to be a more correct assumption. This is what you meant I think.

Hello Larry,

Being focused on bolted joint reliability in many heavy industrial sectors, I've wrestled copious numbers of porcine Luddites. Sometimes I've even been able to open their eyes a bit. Unfortunately, old habits don't die easily.

Extended threads in a closed joint are for the most part, superfluous. It's only the first three threads that see the majority of the load. Aesthetics and personal (corporate) preference often dictate the degree of thread extension. However,the following are practical considerations:

• Threads that do not meet the nut's surface may allow pooling of liquids and accumulation of debris within the nut
  

• Where flanges are to be drawn together (without applying undue stress onto the system's components), extra stud length is required. The length is based on the original separation distance. In many cases such as this, the original "longer" studs are replaced by studs of the "proper" length once the joint is closed

• Any bolts that are tensioned (rather than torqued) require at least the equivalent of one thread diameter extending beyond the nut. On certain joints, it will be noticed that every-other stud is longer than its neighbour on one side while the arrangement is indexed by a factor of "1" on the other. This enables rapid assembly completion because it allows simultaneous tensioning of all of the bolts in the joint.

Inspectors and their ilk have often donned their gloves and readied the mud pits upon encountering such unfamiliar arrangements.

"... one thread diameter..." ???

I was obviously still suffering from the effects of my last mud wrestling escapade. I had meant to say "...equivalent of one nominal bolt diameter..." when bolt tensioners are used. Ooops

I am surprised with a nick name like Boltintegrity that you do have heard of the 3-6 thread rule that most pipe fitters, QC people and construction companies in general go by when bolting up flanges and valves and such. I went back and checked a few piping specifications and although it is not required by a code or standard I know of many times the client does include " a minimum of 3 but not over 6 threads exposed past the nut" in the piping specs for most piping services. This would be Industrial type construction job piping specs. used in paper mills, power plants and oil refineries Etc. and probably just the clients going by the same general unwritten rule that I am talking about but including it in the spec. so that it is followed in the field.

Indeed, I've heard of 3,6. Quite often, in fact. However it's precisely because of my moniker that I choose not to subscribe to certain groundless rules of thumb and other wives tales. Slavishly following these in the field just because everybody does can lead to rework, schedule delays and cost overruns. For example, how would one tension a flange where the studs only have 3 threads extending beyond the nut? Can't be done. One would have to pull the joint, order new studs and gasket and, possibly assemble the thing a few days later.

I don't think this is a steadfast rule you can find in a code book somewhere but more of a general good practice rule that people use. I always follow this 3-6 thread rule and I also turn all stud designation end stampings to the same side for each flange and of course make sure the nuts are not screwed on backwards. There are other unwritten rules such as the 1.5 times a tube OD as a minimum length for Dutchmen which I have never found in a code or any other book although I follow it because it is a good practice to use.There are many more in the construction world and some of the old ones like the 7:11 rule for stairs tread run and set spacing, electrical switch and outlet spacing and the bottom and top hinge spacing on doors. The door hinge 7:11 rule is not followed by all door manufactures and many will space the hinges at 6-3/4" on top and 10-3/4" on bottom or some other dimension close to the 7:11 rule I assume to prevent anyone from installing their door in another company's frame.