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:

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.