Bolting Truss Together

[highschooltech]

So today i was working on a load in for school and the TD flipped on me for asking for and impact driver and wrench to bolt truss together. He said that it was bad for the truss. This is conflict with several professional shows where we've bolted hundreds of feet of truss together using an impact driver under the supervision of an ETCP certified rigger.

So my question is, is it acceptable to use an impact driver to bolt truss together saving time or use ratchets which is much slower?

[derekleffew]

What's the correct torque for truss bolts? How does one know whether he/she is under- or over-tightening? How does use of an impact driver affect this?

[Footer]

Impacts should only be used to break truss apart. You never really know the exact torque you are getting on an impact. Also, you run the risk of crossthreading the bolt. Impacts to that rather easily. If you have ever dealt with a stuck truss bolt, you know you should avoid that at all costs.

I prefer this tool for all my truss bolting needs....


Klein 8238

[Lotos]

Personally, for all my truss bolting needs, I will use a ratchet. To check torque, I will use:
[media]http://evergreen-rentals.com/images/Torque%20Wrench.jpg[/media]

Impact Drivers are great for removal... But I simply don't trust them not to cross thread my bolts.

[len]

While on the subject, some have told me it's important that all the bolts face the same direction. True or false?

[church]

The bolts only need to face the same direction for esthetics. What is important is the correct size and grade of nut bolt are used. Also you should use bolts with the same size head as the nut.

As an aside if you use an impact wrench with stainless steel fastners you run the risk of galling (cold welding) the nuts onto the bolts before you reach the required torque (Assuming you are using torque bars with the impact gun to avoid overtorquing). It is then possible that you do not realise that you have not achieved the required clamping force on the joint and the load is now spread to three fastners instead of four. As has already been said use a ratchet to instal the bolts or if you must use an impact you need the type professional auto mechanics use where you can adjust the torque by a selector switch and also has a variable speed trigger and select the lowest value and run the nuts on at the slowest speed. Alternatively use an air powered ratchet they have low torque output.

[JD]

Two points:
One, if the truss connects via flat plates (rare these days), then proper torque is a must. If the truss connects with cross bolts, then all of the force is shear force and there is more latitude.

Second, every pound of stress put on the bolt from tightening gets subtracted from the capacity of the bolt. The following example uses low numbers to make a point:
Lets say the tinsel strength of a bolt is 1000 pounds. When tightening the bolt, 200 pounds of force accumulate on the bolt shaft. The bolt will now only have 800 pounds of holding force before its tinsel strength is reached. Clarification- We are not talking about pounds of torque here, but the actual stretching of the shaft of the bolt, which tends to be a multiple of the torque level applies to the bolt. (depending on the thread ratio.)

The point here is that as many (if not more) problems can occur by over tightening a bolt then under tightening. Don't do much rigging at my advanced age (and strangely modified center of gravity ), but always used hand wrenches. Always want to feel what the bolt is telling me.

[jonliles]

Don't do much rigging ...modified center of gravity

+1

On a side note, there are certain structural torque requirements for bolt when that bolt is carrying a structural load. Though JD's numbers are low, they do represent the basic physics. In the Structural Iron World, we start with Grade 8 bolts - and they ain't cheap. If the loads were static, it is relatively easy to calculate what your strutural fasteners need to be, but now that we add movers to our trusses, the loads become dynamic increasing the forces applied to the bolt, thusly the need to use a higher rated bolt. Physics. Cool stuff.

Some one mentioned galling earlier. Do you not use some form of anti-sieze on the threads since these are meant to be used over and over again? It would certainlyminimize calling whether you use an impact, hand wrench, or ratchet wrench.

[church]

I mentioned galling in the context of stainless steel fastners being assembled with the aid of an impact gun. Stainless steel is very prone to galling when fastners are installed at high rpm - other steels do not have the same degree of issue. The problem with antiseize is also modifies your torque value. the dry torque value and the torque value with antiseize can be quite different.

[jonliles]

True. There is a difference between dry and wet torque values.

[mstaylor]

I agree that you hand tighten with sockets on the in, you can break with an impact on the out to speed things along. I have worked with certain outfits that impact on the in and generally they aren't very good companies. As far as the bolt direction, I always insist that bolts all go the same way but it is simply to make it faster to tighten and loosen. You don't have to keep switching your socket and box wrench around. I also insist that you put the socket on the nut, box wrench on the bolt. If you are using two sockets, my preference, you still need to tighten the nut.

[len]

Lets say the tinsel strength

To clarify, it's tensile. Of or pertaining to tension.

[JD]

To clarify, it's tensile. Of or pertaining to tension.

Sorry! Must have been thinking of hanging my Christmas lights again

[derekleffew]

What's the correct torque for truss bolts? ...

Since no one answered...
A post on BlueRoom (that I have not confirmed), states:

If you follow that rule and do some research you find that a selection of bolt manufacturers' websites state that grade 8.8 iron M16 24 mm bolts with clean dry threads should be tightened to approx. 140-146 ft.lbs. which converts to just under 200 Nm (Conversion factor of 1.36).

FWIW, I've been working with plated-connection truss for about twenty years, and have NEVER seen anyone apply a torque wrench to truss bolts.

[KyleLeonardMT]

One thing I haven't seen mentioned is the reusability of nuts. Just don't do it.
Nuts are manufactured for the threads to "mold" to the bolt. Every time a nut is retorqued it deforms more. This creates a higher probability of failure.

I used to sell fasteners for a manufacture of these bolts. We guaranteed all of our fasteners for ever as long as they were properly torqued and nuts were not reused.

All distributors of fasteners have torque sheets. Ask your supplier for one, it should be free.

As for using impact wrenches, the light duty electric ones on the market today don't have enough torque to over torque anything better than a grade 5 SAE or 8.8 metric. Air wrenches vary widely by model and manufacturer, as well as how much air is being used. As stated earlier, they are great for removing but not installing.

If you're buying fasteners at the local hardware store, stop. Buy from a manufacturer or supplier that provides warranties. Companies like Kimball-Midwest, Barnes Distribution, Lawson Products and Fastenal. Most will also provide storage and organization at no extra charge.

Kyle

[highschooltech]

So if a bolt needs to be tightened to approximately 140ft.lbs., then a Milwaukee 18v Impact Driver (the tool i use normally) which delivers 116.7 ft.lbs would actually not be able to tighten the bolt all the way and defiantly would not deform the bolt or damage the truss?

[coldnorth57]

I am sorry but that is then UNDER torqued and with a impac you do not know at what torque you are at, unless you have proper torque sticks that slip at the torque that they are rated for (used by auto mechicanics to put on wheel nuts)

[mstaylor]

One thing I haven't seen mentioned is the reusability of nuts. Just don't do it.
Nuts are manufactured for the threads to "mold" to the bolt. Every time a nut is retorqued it deforms more. This creates a higher probability of failure.

I used to sell fasteners for a manufacture of these bolts. We guaranteed all of our fasteners for ever as long as they were properly torqued and nuts were not reused.

All distributors of fasteners have torque sheets. Ask your supplier for one, it should be free.

As for using impact wrenches, the light duty electric ones on the market today don't have enough torque to over torque anything better than a grade 5 SAE or 8.8 metric. Air wrenches vary widely by model and manufacturer, as well as how much air is being used. As stated earlier, they are great for removing but not installing.

If you're buying fasteners at the local hardware store, stop. Buy from a manufacturer or supplier that provides warranties. Companies like Kimball-Midwest, Barnes Distribution, Lawson Products and Fastenal. Most will also provide storage and organization at no extra charge.

Kyle

Kyle, not to be a smartaleck but there is no way a touring company could do that. They reuse bolts every day of their lives. A touring company or rental company will buy bolts for their truss and reuse them hundreds of times before they are discarded. If you are saying use them once and toss them you are talking a serious amount of money.

[tecgod13]

Kyle, not to be a smartaleck but there is no way a touring company could do that. They reuse bolts every day of their lives. A touring company or rental company will buy bolts for their truss and reuse them hundreds of times before they are discarded. If you are saying use them once and toss them you are talking a serious amount of money.

I'll agree. There are some bolts that are manufactured to be torque-to-yield, and stretch when they are properly torqued. They are often used in head bolts in engines. I've also heard that nyloc nuts (ones with nylon inserts that won't loosen over time) are only one time use, but thats do to the nylon forming to the bolt, not a strength issue.
If the threads are deforming in a nut, or bolt, when they are used, you are either overtorqueing or they are poorly threaded from the manufacturer. Do you replace the nuts (or bolts) every time you change/rotate the tires on your car?

Research torque values and you'll find out a lot about how fasteners are supposed to be used and tightened. There is a factor of safety involved when designing the trusses and spec'ing the proper size bolt. That easily covers any minor wear on nuts and bolts when reused. Its perfectly safe to reuse whatever nuts and bolts.
To add to this perspective, I've never worked with trusses, but I do have a lot of experience working on vehicles, where 95% of the time you reuse a fastener thats in good shape (often times you can't do to rust, but thats another thing altogether). I also have a degree in mechanical engineering, and never once heard anything about not reusing nuts and bolts...?

[church]

this is one of the best discussions we have had in while.

Although manufacturers provide torque values for their fastners ( a selection of bolt manufacturers' websites state that grade 8.8 iron M16 24 mm bolts with clean dry threads should be tightened to approx. 140-146 ft.lbs.) it is inappropriate to tightene the fastners to these values without consideration of the application. These values are realted to the maximum torque for a fastner. If you just torque fastners to these values then you are going to cause a lot of damage to hardware. when the engineer designs a bolted joint they have to take into account the materials used to manufacture the part, the required clamping force required to survive the vibration, the shear loads, the tensile loads, the quantity of fastners, the fastner grade, if the surface is painted, the hole size, the use of helicoils, the number of threads that will be engaged and the thread type and possibly the thread pitch. The significance of each of these items changes with the application.

Remember truss is usually made from aluminium and the fastners are steel aluminium yields before steel but depending on the fastner size and whether you are using bolts with gussets or bolts with inserts that fit inside the truss ends the torque required is different.

The best approach is to contact the truss manufacturer - after all truss is engineered to carry UDL and PDL loads over spans so the engineer must have addressed this issue.

[Lightguy48]

I have to concur, I would NEVER use an impact tool to bolt together truss. I have seen over-torqued bolts have to be snapped after a show to get the truss apart, always caused by someone over tightening.

[wolf825]

Impacts should only be used to break truss apart. You never really know the exact torque you are getting on an impact. Also, you run the risk of crossthreading the bolt. Impacts to that rather easily. If you have ever dealt with a stuck truss bolt, you know you should avoid that at all costs.

I prefer this tool for all my truss bolting needs....


Klein 8238

Just for clarification--I think that is a Klein 3238 construction model....or is there a new 8200 series I have not seen in my catalogs yet?



-w

[JD]

Just for clarification--I think that is a Klein 3238 construction model....or is there a new 8200 series I have not seen in my catalogs yet?
-w

Looks more like what we used to use setting steel. This is the ONE tool everyone feared being dropped! (of course, judging by the nicks, they used to rotate and hit head down, as compared to the Edger Allen Poe position.) It always amazed me just how much weight you could move around getting the bolt in.

[porkchop]

Just to add some wood to the fire, while bolting together some fly track a few weeks ago one of our local workers claimed to do some work in the realm of truss engineering and went off on how since we are a touring show and we do reuse nuts and bolts all the time that grade 5 bolts would be better. I didn't really inquire as to why because I was certain that we weren't going to go buy all grade 5 stuff, but he said something about the softer metal conforming better over time and having more consistent long term rigidity. Sorry I don't have a source, just something a guy I worked with claims to have studied and had XSF and Tomcat agree with. Thoughts anyone?

[Lotos]

While we're speaking of Nut and Bolt grades... I try and keep a copy or two of this posted in handy locations.

[mstaylor]

I have nothing formal or official but I would never bolt anything that would fly, especially fly track, with anything less than grade eight. I saw a horizontal lifeline installation that was done with grade five eyes. With nothing more than tension on the bolts, they straightened out the eyes. I went through and replaced them pronto.
I just went through the Tomcat site and they say grade 8 only.

[derekleffew]

What's the correct torque for truss bolts? ...

Since no one answered...
A post on BlueRoom (that I have not confirmed), states:

If you follow that rule and do some research you find that a selection of bolt manufacturers' websites state that grade 8.8 iron M16 24 mm bolts with clean dry threads should be tightened to approx. 140-146 ft.lbs. which converts to just under 200 Nm (Conversion factor of 1.36).

Wildly varying responses from TRUSS and Bolt Torque Specs - Cinematography.com :

good point made about the actual bolt size.. referencing the mechanical trades pocket manual; 3rd edition
3/4"grade 8 bolts require 296 foot pounds torque
5/8"grade 8 bolts require 180 foot pounds torque

Note: these differ from the values listed in the PDF Link [ http://www.teleamerica.net/reference...olt_torque.pdf ] mentioned above.

Spoke with a represenative of Tomcat Global USA today. They called me back after checking with (I assume) the shop staff and told me 80 ft/lbs of torque.

[65535]

Unless you carry around a 2' breaker bar I don't see how you could properly torque a 5/8" or 3/4" grade 8 bolt, you would have to weigh over 200lbs and stand on the end of a 12" ratchet to achieve 200 ft-lbs.

Even with a 2' breaker bar you would have to apply 150lbs. to the end to achieve proper torque.

No battery powered impact driver would get near either figure, an impact wrench might get pretty close if it's a pneumatic 3/8" or better square.

[JD]

Sounds like bolt torque specs, and torque specs for the use of a bolt in the application of joining truss sections together are being confused.
I would go by specs issued by the truss manufacturer, who's application the bolt is being used in, not the bolt manufacturer's general spec.

[avkid]

I do carry around a 18" breaker bar, and truss bolts are usually 15/16".

[derekleffew]

... and truss bolts are usually 15/16".

While most have a 15/16" hex head, bolts are specified by diameter, TPI, and length, not the head size/type.
-----

... and stand on the end of a 12" socket to achieve 200 ft-lbs.

Ratchet. The base is part of the lamp. Wait, wrong pedanticiasm.

[gafftapegreenia]

Ok heres another question: the washers one uses. I think I see at least three different diameter washers intermingled almost daily, along with D washers on truss that doesn't require them.

[gafftapegreenia]

I do carry around a 18" breaker bar

Every time I think that I really might be "that guy", you remind me of what I can aspire to be.

[Pie4Weebl]

I just saw this thread now and OP made me want to find something to throw at my monitor...

CSB time:

I once had an ox of a stagehand tighten a bolt so tight he stripped the threads off leaving it stuck on. I found out during load out, and had to take 30 min to find a saw to cut it in two. Not a good time when your load out involved five guys leading crews in different parts of load out that all have to fall together near the end.

[65535]

stand on the end of a 12" ratchet to achieve 200 ft-lbs.
.

Ratchet. The base is part of the lamp. Wait, wrong pedanticiasm.

I was going to make a quip about standing on a 12" socket but the biggest I know of or could find using Google was a 2 1/2" Dr. 8" 6-point impact socket. Then I thought to myself; self, that would be a hell of a conversation starter, alas it runs over $2,000 and weighs better than 80#.