Bolt Loosening

BOLT/NUT LOOSENING

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I recently received a question concerning bolt loosening:

 

Crane rails are suspended from structural steel by fully tensioning 1" A325 bolts in a hanger type connection. The installation has been in place approximately 5 years and on at least 2 occasions the Owner has inspected the bolts and found the nuts to be loose. He is in the process of replacing the bolts. It has always been my belief that when a bolt is fully tensioned it will not "back off" and that if the original installation had been properly pretensioned he would not be seeing a loosing of the nuts. Our recommendation was to replace the bolts and use either a "twist off" type bolt or to use a DTI so that the installation could be properly inspected and verified to be properly pretensioned. The Owner understandably wants to make sure that this doesn't re-occur. He has suggested that the nut be tack welded to the bolt. We have advised against this. He is not fully convinced that there is anything wrong with this procedure. As a followup suggestion, he would like to predrill and tap the nut to install a set screw or predrill the nut and field drill the bolt to install a cotter pin - both after pretensioning the bolt.

 

  1. Will fully tensioned nuts "back off"?

  2. Is it a good idea to tack weld the nut to the bolt after pretensioning?

  3. Is predrilling the nut and installing a set screw a good idea?

  4. Is there any other method the owner can use to stop the "backing off" such as double nutting?

I answered as follows:

 

This is a subject I have been looking into quite a bit. The AISC and RCSC Specifications and the Manual do not really address this issue directly. I will therefore try to provide some guidance, much of which is opinion based on sources I have consulted. Ultimately you must make your own decisions.

 

It is intended that properly pretensioning the bolt will prevent (or at least delay) bolt loosening. This is supported (though in an oblique manner) by statements in the Specification, which states, A325 bolts are allowed to be installed snug-tight... "where loosening or fatigue due to vibration or load fluctuations are not design considerations." (J3.1). This is later repeated in the Commentary to M2.5. This implies that proper pretension will prevent subsequent loosening.

 

Section 4.1 of the Bolt Guide also states: "Proper tightening also prevents loosening of the nut."

 

AISC Engineering FAQ 6.5.1 addresses the question this way:

 

“In general, when properly installed, the high-strength bolt-nut assembly will not loosen. When snug-tight bolts are used, the loading will be such that loosening of a nut will not occur. When fully tensioned bolts are required, as for slip-critical connections subjected to vibratory or fatigue loading, the installed tension and the attendant friction on the threads will prevent the nut from loosening.

 

In some other cases, such as nuts on anchor rods (for which full-tensioning is generally inappropriate), further consideration may be required. In such cases, an additional jamb-nut or second nut may be provided. Alternatively, the threads can be spiked or marred or the nut can be tack-welded to the base metal to prevent it from turning. Note that the latter two solutions are permanent actions. There also exist proprietary nut devices with locking features to prevent the nut from backing off. “

 

From the above, it might be assumed that pretensioned bolts will not loose, ever.

 

However, bolts come loose. This is attested to by a statement in AISC’s Design Guide 7, which states in Section 18.8:“The bolted butt joint is the most commonly used rail joint. Butt joint alignment is created with bolted splice plates. These plates must be properly maintained (bolts kept tight). If splice bars become loose and misaligned joints occur, a number of potentially serious problems can result, including chipping of the rail, bolt fatigue, damage to crane wheels, and as a result of impact loading, increased stresses in the girders. Girder web failures have been observed as a consequence of this problem.”

 

If pretensioned bolts come loose in practice, which they do, then my telling you that they don’t won’t accomplish much for either of us. So, we should think about the possible causes.

 

First, we could assume that a properly pretensioned bolt will not come loose. This would lead us to believe that the bolts, which have come loose, have not been properly pretensioned. If this is that case, then we should make sure that the bolts are properly pretensioned. You are trying to do this by using either TC (twist-off) bolts or DTI’s. Both these methods have been shown to provide consistent and economical results and are widely used, and I do not want to say anything that would lead you to believe these are not acceptable methods of pretensioning. However, from the data I have seen Turn-of-Nut Pretensioning as described in Section 8.2.1 of the RCSC Bolt Spec. provides the “gold standard” in terms of pretensioing. When properly applied the turn-of-nut method will provide higher mean pretension and less scatter than the other methods. It would probably be a good idea to use match-marking as well. Match-marking is usually applied only to the turned element, but “match-marking” at both the head and the nut, with as permanent a mark as possible would also presumably allow long-term monitoring of the relative movement of the bolt and nut. It would also logically provide a means to readjust the bolt later, if it did start to loosen. Of course the bolts must also be kept clean prior to installation to ensure proper pretension. You should read through Section 8 of the Bolt Spec. You may also find some useful information under the Education heading of the RCSC website (boltcouncil.org). Proper installation is key to achieving proper pretension.

 

Another thing to consider is if pretension helps to prevent loosening, then increased pretension should be better. I realize the more is better line of reasoning does not always work, but here I do not see anything to negate it. Providing pretensioned A490 bolts might provide better resistance to loosening than A325 bolts. There may be trade-off in ductility and fatigue resistance though.

 

Now, if properly pretensioned bolts will not loosen, as is commonly stated, then by ensuring proper pretension, we will have solved the problem. But just in case – let’s consider some other options.

 

Some common methods used to prevent bolt lossening are: lock-nuts with some sort of insert or special threading, lock washers with either a split in them or some treatment to increase friction, and double nuts. There are various data available about each of these methods, some supporting their use and some refuting it. In general the reports I have seen have not to my knowledge been peer reviewed and are sometimes poorly documented. A further problem with these methods is that they all will probably have a detrimental effect on the pretension in the bolt. Pretension helps to prevent loosening, but it also increases the ductility of connections subjected to fatigue, by effectively reducing the stress range the bolt is subjected to. We would not want to solve a loosening problem only to find we have exacerbated a fatigue problem. I have had people argue that double-nutting does not reduce pretension, but from personal experience and everything I have read double-nutting involves jamming one nut against the other by first installing the original nut, then installing the second nut. The action really takes place though when the first nut is backed into, or jammed against, the second. This has to have some detrimental effect on pretension.

 

Another option that has been suggested to you is the use of a pin to stop the bolt from loosening. An Introduction to the Design and Behavior of Bolted Joints by J. H. Bickford states: "Lock wires, keyes, and cotter pins are often used. These can effectively prevent total loss of the nut - which may be extremely important - but they are not very effective in preventing substantial loss of preload within the fastener." To be fair Bickford’s assumed application probably does not involve a hole through both the nut and bolt in which to insert the pin, as has been suggested. I do not know of any data on this application and do not have any feel for what the process of drilling the hole in the bolt might do to the pretension in the bolt.

 

Tack welding is another commonly proposed solution, but I personally do not recommend it. AISC Design Guide 21 states: “As a general principle, welding should not be done on bolts or nuts. However, if essential, the composition of the bolt (and nuts, if involved) must be carefully considered… ASTM A490 bolts should never be welded upon, given their very high strength (150 ksi minimum). Welding on nuts and washers is also problematic.” Again, I think be welding the nut to the bolt, you are trading one problem for a different and potentially more serious problem.

 

Another factor, in addition to pretension, that seems to affect the likelihood of bolt loosening is slip. Now we are more into opinion. I cannot point you to an AISC or RCSC reference, and it is not mentioned in the texts that I know of, but it does seem to play a role. Since the threads are essentially an inclined plane and the pretension set-up a situation where the bolt and the nut have a tendency to rotate in one direction (loosening) and not the other (tightening), preventing slip, wherever it might occur would seem to be a good thing. The pretension should eliminate much of the slip between the threads, but some sources suggest that the relative movement of the plies may also precipitate loosening. If this is the case, then roughening the faying surface might be beneficial.

 

Some sources also suggest that increasing the grip is beneficial, but I tend to be more skeptical of this, since the same level of pretension should be attained regardless of grip using the tensioning methods in the Bolt Spec. Some suggest piling up washers to increase the grip, but to me this would seem to encourage slip.

 

Just in case the bolts do loosen despite all your efforts, it is probably a good idea to inspect the joints periodically. It would seem to me that match–marking, as suggested earlier would aid this process. If the bolts have loosen “a little” they could probably be retightened a few times before they had to be replaced, but eventually the retightening could lead to fatigue problems. This behavior is discussed in section 4.5 of the Bolt Guide (a free download from boltcouncil.org). The fatigue would occur with both A325 and A490 bolts, but would be more of a problem for the A490’s. If significant loosening occurred the bolts should probably be replaced and not retightened.

 

To summarize some points:

  • Pretension is important to resist loosening and to prevent fatigue.

  • Pretension should not be sacrificed in an attempt to prevent the bolt from loosening.

  • The general consensus seems to be the pretension is sufficient.

  • It seems logical (to me at least) that preventing slip between parts might help prevent loosening.

  • Since the consequences of loosening could be great, periodic inspection is probably warranted.

 

Sorry for such a long response, but I wanted to put all the information in front of you so you could make your decision and hopefully be better prepared to refute any options you were uncomfortable with. I hope you find some of this useful.

Larry S. Muir, P.E.