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Torquing Bolts

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Brett H

Active Member
Patron
I have not previously asked a question on this forum. So, hopefully I am doing this correctly.

My question is about what torque value should be applied to bolts. I suspect that I may be over thinking this. But, I would like to hear what others are doing.

My questions come from reading section 5.20 NUT AND BOLT TORQUES (i.e. on page 05-13 of the manual, revision 1).

I understand that the preference is to torque the nut rather than the head of the bolt. Further, the end being turned (i.e. nut or head of the bolt) is to have a washer installed to reduce the friction and provide a wear surface.

The tables provided are to be used when torquing the nut. Further, that these values are for clean and dry threads. I find it a bit confusing that the AN365 nut (i.e. elastic stop nut) is called a Standard Nut rather than a Self Locking Nut.

Regarding torquing the head of a bolt, it states that the shank of the bolt is to be lubricated and the applied torque needs to be increased from the nut torque shown in the table by the friction torque to turn the bolt. The friction torque is to be determined by measuring the torque needed to pull the bolt head until it nearly contacts the bearing surface. To be able to measure this friction torque one would need a torque wrench capable of measuring very low torque values. These torques are lower torques than the torque wrenches sold by Avery or Cleveland and would require a very expensive torque wrench to measure.

So, how are people determining what torque should be applied to the head of bolts? What torque specification are people using for bolts going into nutplates? Does anyone else find it confusing that in the last paragraph the example in the last paragraph has results in a different value that shown in the table for MS21043-3 nuts?

Just seeking to understand.

Thanks.

Brett H
Columbus, Indiana
Working on the tail kit
Paid in October 2013
 
Brett H said:
I have not previously asked a question on this forum. So, hopefully I am doing this correctly.

My question is about what torque value should be applied to bolts. I suspect that I may be over thinking this. But, I would like to hear what others are doing.

My questions come from reading section 5.20 NUT AND BOLT TORQUES (i.e. on page 05-13 of the manual, revision 1).

I understand that the preference is to torque the nut rather than the head of the bolt. Further, the end being turned (i.e. nut or head of the bolt) is to have a washer installed to reduce the friction and provide a wear surface.

The tables provided are to be used when torquing the nut. Further, that these values are for clean and dry threads. I find it a bit confusing that the AN365 nut (i.e. elastic stop nut) is called a Standard Nut rather than a Self Locking Nut.

Regarding torquing the head of a bolt, it states that the shank of the bolt is to be lubricated and the applied torque needs to be increased from the nut torque shown in the table by the friction torque to turn the bolt. The friction torque is to be determined by measuring the torque needed to pull the bolt head until it nearly contacts the bearing surface. To be able to measure this friction torque one would need a torque wrench capable of measuring very low torque values. These torques are lower torques than the torque wrenches sold by Avery or Cleveland and would require a very expensive torque wrench to measure.

So, how are people determining what torque should be applied to the head of bolts? What torque specification are people using for bolts going into nutplates? Does anyone else find it confusing that in the last paragraph the example in the last paragraph has results in a different value that shown in the table for MS21043-3 nuts?

Just seeking to understand.

Thanks.

Brett H
Columbus, Indiana
Working on the tail kit
Paid in October 2013
Click to expand...

Brett,

You can pick up a 1/4" bar/beam torque wench from a bicycle shop cheap....works great!
 
torque

I suggest getting a couple of offcuts of aluminium and some nuts and bolts and get the feel of different scenarios
ie torque a nut to the correct torque and then put the torque wrench on the bolt and see how much force is needed to loosen it , put a bolt in a vice and see how much torque it takes to install a locknut ( this will give you the amount of torque you need to add to the torque of a normal nut ) you could also experiment with adding oil etc to the thread
 
crashley said:
I suggest getting a couple of offcuts of aluminium and some nuts and bolts and get the feel of different scenarios
ie torque a nut to the correct torque and then put the torque wrench on the bolt and see how much force is needed to loosen it , put a bolt in a vice and see how much torque it takes to install a locknut ( this will give you the amount of torque you need to add to the torque of a normal nut ) you could also experiment with adding oil etc to the thread
Click to expand...

Unless otherwise stated, the FAA torque numbers for airframe bolts are dry, and the Lycoming torque numbers are for oiled bolts.
 
torque

Just purchased a Park TW-1 (beam type) torque wrench. It took 6 lbs. to turn an AN365 nut before it seated. So I torqued to 25 + 6 = 31 lbs. Will use this wrench whenever possible.

At the same time, I bought a Craftsman micro torque (click type) wrench to use where I couldn?t view the gauge on the beam type. It doesn?t actually click at low values, but it does have a slight slip.

I practiced on some scrap. Double checking the Craftsman with the Park, I feel confident using either. Price of the Park TW-1 was $44.99 at a bicycle shop, the Craftsman (in. lbs.) was around $55-60.

David
 
FWIW, Air Force general practices:

- Lubrication on all friction surfaces (including threads)
- Torque the nut to the 50% of the allowable range
- Torque the head (if you can't get to the nut) to 75% allowable range
 
Toobuilder said:
FWIW, Air Force general practices:

- Lubrication on all friction surfaces (including threads)
- Torque the nut to the 50% of the allowable range
- Torque the head (if you can't get to the nut) to 75% allowable range
Click to expand...

Quoting the Air Force is only muddying the waters.

The FAA is very specific on airframe bolts being torqued "oil free" and it is even a caution at the top of the torque table -

Page 9 of 43.13 Section 7 -

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/list/AC%2043.13-1B/$FILE/Chapter%2007.pdf

These are the numbers Vans uses in the instructions, and should apply "unless otherwise stated by the manufacturer".
 
Yep, you're right. That was for LRU maintenance. Structure fasteners are generally dry.

Thanks for keeping me honest.
 
For a decent, low-cost low-range torque wrench, see http://tinyurl.com/adewcw2. It'll take you to Amazon where you'll see the KD Tools 2955 Beam Torque Wrench (0-60-Inch/Pounds, 1/4-Inch drive), for under $50.

It's quite useful for measuring running torque.

Dave
 
rvbuilder2002 said:
It is a typo that has been corrected in the most recent update of section 5.
Click to expand...

Thanks for the response.

So, for AN-365 elastic stop nuts is one to increase the final torque by the friction torque of the elastic stop nut or use the value in the table as is?

Thanks.

Brett H
Columbus, IN
Paid October 2013
 
Brett,

My logic has always been to go to the top of the torque range. That covers the running torque. Just my opinion, but it makes sense to me. Let the debate begin! :)

Rich
 
This is a duimb question I know, but why don't the world's engineers make up a torque chart for bolts with the normal turning torque required built in for each type of nut used? Sure would eliminate all the guesswork and calculations.
Dick Seiders 120093
 
Just a guess: the max torques are probably based on structural limits of the fasteners. The minimum are probably based on enough preload to ensure the nut doesn't back off. The running torque isn't so predictable.
 
First, thanks for the responses to my questions.

Based on the responses and an email to Van?s Builder Support, this is what I now understand:

The Torque Table is the starting point for determining what the Final Torque needs to be. The Friction Torque is added to the value from the Torque Table to determine the Final Torque.

When the nut is turned (i.e. preferred over torquing the head of the bolt) the Friction Torque is the torque that is required to turn the nut when it is fully engaged on the bolt. For a plain nut (i.e. rotates freely on the bolt) the Friction Torque is zero. For any other nuts, one must measure the torque required to turn the nut on the bolt. To do this one needs a torque wrench that can measure very low torque values. A ?? beam torque wrench seems to be a good choice for this (i.e. I have ordered the KD 0 ? 60 in lb beam torque wrench from Amazon, $44.42). When applying the Final Torque there is to be a washer under the nut and it is to be dry.

The ?funny thing? about this is that are little or no plain nuts used in an RV. So, the torque to be applied to essentially all nuts needs to be determined.

Regarding the torquing of bolt heads, the shank of the bolt is to be lubricated and the Friction Torque is to be determined. Just like with nuts, the Friction Torque is to be added to the value from the Torque Table to determine the Final Torque that will be applied to the bolt head. When torquing the head of a bolt there is to be a washer under the head of the bolt.

So, this is the direction I am headed. Well, the torque wrench needs to arrive from Amazon and then I am heading down this path.

Brett H
Columbus, Indiana
Working on the tailcone
Paid in October 2013
 
Brett H said:
When the nut is turned (i.e. preferred over torquing the head of the bolt) the Friction Torque is the torque that is required to turn the nut when it is NOT(?)fully engaged on the bolt. For a plain nut (i.e. rotates freely on the bolt) the Friction Torque is zero.
Click to expand...

I think you meant to write not fully engaged?

Just to clarify... it does need to be engaged enough that the locking feature of the nut is fully engaged on the bolt, but not fully bottomed out to where it is applying a stretch load to the bolt.
 
Good to clarify my wording.

Yes, by fully engaged I meant that the end of the end of the bolt needs to stick thru the other side of the nut (i.e. the nut is fully engaged on the threads of the bolt). But, it is not to be turned to the extent that it touching what it is clamping together or the end of the threads on the shank of the bolt.

In concept, the nut friction torque could be measured with the nut and bolt separate from what it is clamping together. As an example, the bolt head could be held in a vise and the nut turned to measure its friction torque.

Thanks.

Brett H
 
Brett,

I really believe you are over thinking this. I've built 4 airplanes with mostly torquing to "good mechanical judgement" (yes, that is an actual engineering term) and I've never had a bolt shear or an elastic insert nut back off. Can't say the same for the use of lock washers or plain nuts.

Rich
 
If the friction of the locking nut on the bolt is so small that a special small torque wrench is required to measure it, then how is that extra torque going to be added to the final torque when the big torque wrench is incapable of measuring it? The simple way to deal with this is to just torque to the high side of the range.
Technically speaking, nuts and bolts should be tightened until the bolt has reached a certain tension. Torque is an indirect way of measuring bolt tension and is not always correct, but is usually good enough.
Joe Gores
 
How about these torquing specs - -

Type: Bolt Dim: Length: Head: Hexagon Grade:
33 Ft-Lbs
396 In-Lbs
44.74 N-m
Connects: Cylinder Head To Engine Block
Years: 1995-1997 Vote Record: Vote
Note: 3100 - Head Bolts - 2 Steps: 33FtLbs; +90deg

For those of you that do not work with torquing bolts much, you might find this method interesting. GM 3.1 V-6 Head Bolts. 33# + 90 degrees.
 
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