Snug Tight 1

[haggis]

Hi all,

I work in an industry where we hang overhead conveyors from building trusses and joists using bolted clamp connections. I have always believed in torquing a bolt to specs and ensuring that the clamping components are strong enough that they dont bend under these conditions. However, the new thing is that contractors and even some Structural Engineers are saying it is acceptable to specify the bolt tightening as "snug tight with a standard spud wrench" or "snug tight with the turn of the nut method" where the nut is turned at least another 1/3rd revolution after snug.

The thing that makes me uneasy with this, is that if the Secretary tried one, the results in torque would differ greatly from the Ironworker's.

Any help in this area would be appreciated.

Thanks in advance.

 

[unclesyd]

This a tightening procedure that is used on all connections.
If it needs to stay in the snug tight condition it will be called out on the print. All connection should be a least snug tight using "systemic tightening", tightening from the ridged side to the free edge.
The essence of a snug tight joint is that there is no gaps in the members being connected around the bolt holes. The contact area is usually one nut dia. This is one important point that most inspectors will check.

Snug tight should always be achieved prior to tightening for a pretensioned or slip critical connection.

 

[arto]

The ASME B&PV Code Sec VIII Div 1 App. S-1 gives the "probable bolt stress developed manually, using standard wrenches" as:

S = 45000/sqrt(d)

S = psi
d = inches
[Oh, so that's why those 1/4" bolts keep breaking... ;-) ]

Shigley's Machine design Book relates bolt force & Torque by:
F = 5T/d

In turbine land, we usually call for "tighten bolts to 30ksi" or 45ksi bolt stress

 

[UcfSE]

Snug tight is defined, paraphrasing, as the full effort of a man using an ordinary spud wrench or a few impacts of an impact wrench. I agree that it seems like it is bit too ambiguous or "voodoo engineered" but it does at least eliminate the little old lady from trying to make a bolt snug tight, lol.

 

[evelrod]

As a general rule in the construction trades, Ironworkers tend to be pretty tough cookies. No problems in 'snugging up' a 1/2" , 5/8" or even 3/4" bolt but, in practice, the ungraded bolts of large dia are seldom used. It is difficult to get a good 'snug tight' on a one inch or better bolt, even by a husky Ironworker. In the good old days of Bethelehem Steel and American Bridge you would aften find 'erection bolts' of 7/8" to 1" used but they would eventually be replaced by the "bolt up crew" with A-325 or greater rated bolts.
The specs are ambiguous no doubt. In actual practice it is seldom a problem.

Rod

PS: I did have one man, Louis from South Africa, who could twist off an A-302 3/4" bolt with what appeared to be little effort!!!

 

[krus1972]

Haggis,

Be very careful who tells you what is exceptable and what is not. Contractors ARE NOT the engineers of the job and ARE NOT responsible for the design. They are responsble for building what you as the engineer have designed and what you specify. The contractor's method of design is "We always do this way" or "We did it on this OTHER job this way". This is not acceptable design practice by any means.

The actual analysis & design of the clamp you are talking about is more complicated then most people realize. It's a tension hanger/clamp that undergoes tension, prying action, and bending. What most people forget is that you are hanging conveyors which subject the clamp to fatigue (repitional /cycilic)loading. Even if all components of the clamp are way over designed a small cyclic loading on the bolts CANNOT be overlooked. Remember this occurs for as long as you run the conveyors on the connection(s).

Without getting deep into the connection design your question addressed concerns as to the actual bolt tightening. The "Snug-Tight" is not good enough. Please see PG. 5-107 of the 1989 ASD (Steel Construction Manual". As directed proceed to table J3.7 at the bottom pg 5-77. Here you will find the required pre-tension load that MUST be achieved when tighting the bolts. I am sure you can find the same table in the LRFD manual. Just look up "fatigue" in the index.

You can convert the required pretension load to an actuall bolt torque to specify to the contractors. It's only one formula and I do not have it at my finger tips while I am writing this. If you need it let me know and I will look it up.

You can specify the "Turn of the Nut Method" and then specify that after this is accomplished the specified bolt-torque must be achieved. Since you are specifying this in your design the contractors shall do this. The inspectors will inspect the bolts based on what YOU specify and advised the contractors in the event they find bolt(s) that do not meet your spec. On top of this you can add lock-washers or use two (2) nuts to further insure the bolts never come loose. If contractors complain about the required torque specifications then tell them to buy the "Twist-Off" bolts. These bolts automatically let the person know when the torque is achived.

Remember YOU are designing the job not the contractor, inspector, or other people in other trades. Don't let no one convince you in these arm-waving "we did it on other jobs this way" methods of design.

Please ask me any questions anytime.

Good-Luck

Jeffrey A. Krus P.E.

 

[haggis]

krus1972,

Thanks for your most cadid reply.

You echo my feelings on this exactly. Yes, I've heard them all about how it was done elswhere. I've even heard the one about "don't worry, when I'm done tightening, they will be over torqued" and " I've popped the head off a 5/8" grade 8" , all while flexing 22" bicepts.

Well, that's not what I ask for either. Bolt sizes and grades have pre-tension and torque numbers for good reason.

I don't give in, and have become known among the construction crews as an obstinate hard nose. Another argument I constantly get is when I specify that holes may be punched in components up to 3/8" thick but thicker than that, I want them drilled or punched undersize and reamed out.

It sounds as if you are familiar with the clamping method I described in hanging conveyors and it's exposure to repetitious/cyclic loading. I work in a major car assembly plant where the conveyor loads are high due to fully assembled vehicles in process at the rate of 540 jobs per shift/3 shifts per day. As you can imagine, this leaves very little time for sceduled inspections or preventative maintenance. There is no doubt, it has to be done right, no ambiguity.

You mentioned a formula for converting pre-tension loads to actual bolt torque. I would be interested in seeing it and if you can post it, it would be greatly appreciated.

Again, thanks for the response.

 

[jetmaker]

haggis,

The issue at hand here is whether you feel the bolts need to be torque to specific values, or just snugged up.

Where I work, everytime we call out a specific torque value, some QA guy has to come over and watch the mechanic apply the torque to the bolt, plus it has to be done with a calibrated torque wrench. Furthermore, that number is often based on a calculation that makes several assumptions in is only marginally accurate unless actuall conditions meet those of the assumptions. So, we only specify torques if it is absolutely necessary, otherwise we rely upon our assy standards to specify the torque.

There are many equations available for converting preload into torque. Look up MIL-S-8879 and I believe that has a standard calculation. But you may need to do some minor testing just to get reliable numbers for your situation.

Once you have calculated which ones really need to be torqued, then call those out specifically. For those that only need to be snug tight, convinvce the company to invest in some pneumatic torque guns that have limiters. Then even little old ladies can get the same torque as Louis from South Africa.

My 2 cents worth.

Regrads,
jetmaker

 

[haggis]

jetmaker

My issue is that I want these bolts torqued to specific values. As you mentioned, everytime you specify torque, a QA guy has to watch, well, as i said, I work in the auto industry and critical fasters on the vehicles are electronically "torque monitered" (because of shear volume)for quality and safety standards assurance. Just as it is calculated in product engineering which fasteners are critical and need to be torqued for obvious reasons, I feel that the fasteners that hold the conveyor system up are just as critical.

True, a lot of conditions have to be met for accurate torque results, and we do the best we can, but better torquing than having a variety of conditions from not being tight enough to the guy who uses a pipe on the end of his spud and overtighens creating a situation just as dangerous.

Not being derogatory, but in our respective industries, I think the guys who build aircraft have a different attitude towards the seriousness of their work and have a different mindset than that of the ironworker in a car plant.

I have to be very vigilant, believe me.

 

[evelrod]

Haggis---
In general terms I stick by what I said in my previous post.
Specifically, I supervised desigining, erecting, maintaining and, monitoring the ship loader and conveyor systems of the L.A. plant for US Borax for nearly four years. I installed several conveyor systems at the old South Gate and Van Nuys GM plants over a two year period in the early 80's. In addition to that, I supervised and installed MANY conveyor systems at rock plants in the SoCal area. In that time and on every conveyor or conveyor related system save hand rail or floor grating, I never specified nor installed ANY ungraded fasteners. The minimum was A 325 or automotive grade 5 ! ( TS bolts are very common in structural steel erection but not at all common in conveyor systems, at least up to the time I retired in the mid 90's.) Snug tight was just for the ungraded fasteners or, as I mentioned, to "snug up" the TS bolts prior to "torquing" with the electric "guns". Torque applied to graded fasteners was usually done with calibrated pneumatic tools or hydraulic tools and all assemblies checked on a random basis (generally) with a torque wrench (with a current calibration sticker, mandatory in the Los Angeles area) overseen by the deputy building inspector.
As to your vigilance---most IW's I have worked with over a 38 year period were no slackers and would do what you wanted but, since your 'stamp' is on the prints---I don't blame you a bit. Most engineers I have met try not to 'micro manage' the jobs but it's tough to do if your reputation or job is on the line. Best of luck.

Rod

 

[haggis]

evelrod,

Seems we have a little in common. I am familiar with South Gate and Van Nuys, although I am in another plant, have worked about the same number of years and no doubt you know exactly what kind of connection I'm refering to.

I may have misled somewhat about the bolt grades, we use nothing but A325 for conveyor structural support.

I was afraid the comment about ironworkers may be misconstrued and I assure you and the rest of the people participating in the forum, that no offence was intended.

Haggis

 

[krus1972]

Haggis,

Just like yourself I have worked in automotive assembly plants. This is the type of work I have been doing for over 5 years consistantly. I have designed supports for conveyors, robotics, and many other machines. My work was contained in many of Ford's North American assembly plants.


If you are using A-325 grade structural bolts and not machine bolts then the method I've outlined above using the "Steel Construction" manual IS acceptable and works. In your case the "Turn-of-the-Nut" is NOT acceptable all by itself even if each bolt is loaded only partially. Fatigue(cyclic) & vibration loading is a not so obivous load condition you must consider before specifying bolt tightening. If you look at the pages in the "Steel Construction" I have mentioned in my preivous post you will see this issue addressed. The "Turn-of-the-Nut" method requires judgment from the contractor, the inspector and anyone else who as to put their 2 cents into these bolts. The judgement of each party is not always the same and causes disagreements among them. They eventually end up in the design engineer's lap (YOURS). At this point YOU will be required to inturrpet the "Turn-of-Nut" method. Everyone is going to provide thier opinions and views ALSO.

If all other connection components are designed correctly and you specify a torque number there will be no question in judgement from any of the parties that have their 2 cents into tightening the bolts.

If you specify double nuts or lock washers in addtion to the torque spec. Then you will be fine. Remember you (and your associates) are designing the project no one else.


Jeffrey A. Krus P.E.