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spar-rib torque

propsync

Well Known Member
Hi group,

On to torque'ing the ribs to the spar, 12-07 step 2.

I'll be torque'ing AN3 bolts to MS21042-3 nuts. Using the chart on 5-12 (vans mistakenly says 5-19 and then later 5-20), standard nuts get a torque of 20-25 in/lb and the self locking gets 28 in/lb.

But their explanation of what to add for friction drag doesn't make sense. I'll quote right out of the book.

"AN3 bolts and MS20142-3 all metal lock nuts testing showed the friction drag torque at 14 in-lbs. This results in a final torque setting on our wrench of 14 plus 20 or 34 in-lbs."

They are using the number from standard nuts, not self locking.

Shouldn't the number be 14 + 28 = 42 in lbs.??
 
The key here is that you want to pre-load the bolt to a certain load; the torques listed are how you get there.
Typical number needed is 20 to 25 in lbs on a standard, non locking nut.
The typical nylon insert locknut has a drag of about 5 in lbs, 5+23=28.
The metal locknuts have a lot more drag, around 14, so use 14+(20 to 25) = (34 to 39) in lbs.

If you wanted to be precise, you could measure the drag of each nut individually, then add that to (20 to 25) in lbs.
 
corrected torque value for drag?

I don't know for you guys but I have decided to change almost ALL the lock nuts on the main spar bolts. Frankly, I fail to understand why Van's installed these on the spar in the first place since you've got to remove them in order to install the ribs.

Out of 44 bolts and nuts removed (22 of ea per spar), at least 5 were totally ruined. I could screw them down by hand with no effort. Another 12 or so required only very little effort by hand equipped with small wrench. My point is to be careful with these torque correction values, they will apply to a brand new bolts (and nuts) that still have their designed friction/retention capabilities.

IMHO, I would prefer they spend a little more time on parts deburring than installing bolts that needs to be removed anyway.
 
The key here is that you want to pre-load the bolt to a certain load; the torques listed are how you get there.
Typical number needed is 20 to 25 in lbs on a standard, non locking nut.
The typical nylon insert locknut has a drag of about 5 in lbs, 5+23=28.
The metal locknuts have a lot more drag, around 14, so use 14+(20 to 25) = (34 to 39) in lbs.

If you wanted to be precise, you could measure the drag of each nut individually, then add that to (20 to 25) in lbs.

I'm afraid I still don't understand it. The chart publishes a value of 28 for MS21042-3 nuts. Doesn't anything get added to it?

I guess I don't understand why you are adding 14 to 20(25) instead of 14 to 28?
 
I don't know for you guys but I have decided to change almost ALL the lock nuts on the main spar bolts. Frankly, I fail to understand why Van's installed these on the spar in the first place since you've got to remove them in order to install the ribs.

Out of 44 bolts and nuts removed (22 of ea per spar), at least 5 were totally ruined. I could screw them down by hand with no effort. Another 12 or so required only very little effort by hand equipped with small wrench. My point is to be careful with these torque correction values, they will apply to a brand new bolts (and nuts) that still have their designed friction/retention capabilities.

IMHO, I would prefer they spend a little more time on parts deburring than installing bolts that needs to be removed anyway.

The bolts are installed as part of the spar assembly process (to hold parts while riveting), not so they can be prelocated for the builder...
 
I totally agree with you and I am confused as ****. why would the baseline torque for a self locking nut be 28, while the regular nuts are 20-25? One would think that a nut is a nut and that regular and self locking would be 20-25, but for the self locking there would be a value which you would need to add for the friction. Why would there be a range for regular nuts (20-25) and a specific number for the self locking nuts?

This is weird.
 
I totally agree with you and I am confused as ****. why would the baseline torque for a self locking nut be 28, while the regular nuts are 20-25? One would think that a nut is a nut and that regular and self locking would be 20-25, but for the self locking there would be a value which you would need to add for the friction. Why would there be a range for regular nuts (20-25) and a specific number for the self locking nuts?

This is weird.

I do not have the chart in question, but here is what I think is going on:
You are correct, you should measure the torque it takes to turn (not to break loose, but to keep turning) a self locking nut, and add that to 20 to 25 in lbs. I think Vans' is estimating the drag for a nylon-type (e.g., AN365) lock nut to be 3 to 8 in lbs, so using a setting of 28 in lbs would work for all cases. For metal locknuts the drag is much higher; I suggest measuring it because I found a lot of variation. However, Vans is suggesting using 14 in lbs for the drag and then using the lower torque number (20 in lbs) to guard against over stressing the bolt, should the actual drag from the nut be less. That's my take, I can easily be wrong. As I said, I don't have the chart in front of me.
 
Here is the chart

fp357c.png
 
It says in section 5.19 that you should lube up the shank and the nut. What kind of lube should one use for this purpose?

Also, would it be easier to just buy a bunch of nuts that are already lubed? They are only 0.35 each from aircraft spruce.
 
Who wrote this chart? I would complain to Vans.

An AN365 IS a self locking nut!
You can't say 28 in lbs in the chart, and 34 in lbs elsewhere, for the same metal locknut!!

These are the proper torques for UNlubricated threads!!!
 
I do have a torque wrench scaled in "in-lb" and I tried to "measure" the drag induced by the lock nut and in no way I could detect a 14in-lb drag value. Cracking your knuckles will be a zillion time more audible than that. I don't know how your torque wrench looks alike but mine has its smallest graduation indicating 30 in-lb. Perhaps one exist at Toys'R"Us?? Right now, it is absolutely unnoticeable with the one I have.

Also, I've check in the ADVISORY CIRCULAR AC 43.13-1B (and in many other reference books for that matter) and there's nothing valuable I could find there about how to deal with friction drag for bolts or nuts. Not really satisfied with my poor findings, I've consulted with a follow builder who owns a very notable local machine shop and he did confirm the need to take in account the friction drag for proper torque application. As a note, don't forget that your torque wrench is only telling you how much force is required to turn the bolt or nut. The force required to "elongate" the bolt stem that will effectively fasten the parts together is a relationship with the torque.

That is, I do believe that we have to consider the friction drag but how significant that is with respect to what we're doing here?. To make it simple, I've ordered the whole set of bolts and nuts and will replace them all (about 30$ with plenty of spares). Secondly, I'm going to use the value proposed of 35 to 40 in-lbs as a torque value for all these new bolts. This is well within the "elastic" portion of the stretch value tolerable for a bolt of that size and alloy type.
 
the other issue is that most of the in-lb torque wrenches measure down to 20, so how can you measure a nut drag lower than 20?

The torque wrenches that measure down to 10 are not cheap.
 
I don't have enough experience here to say what to do so I am going to torque them to 42 in-lb. According to vans plans, the mil spec is rated to 65.

Unless you think that is out of line......:confused:
 
Do not over torque the bolts! Doing so reduces their load carrying capability.
Just play with the nuts a bit, threaded onto a bolt. Put a 4" wrench on the nut, and use a fish scale to pull on the wrench. If it takes two pounds to smoothly move the nut, that's 8 in lbs. Or hold the wrench horizontally, and put a weight on the end. If a one pound weight keeps it moving, that's 4 in lbs. Figure out the drag and add it to 22 or 23 in lbs. Use the lower (20) limit if the bolt is only in shear.
 
I am going with Marc's idea. Buy all new nuts and bolts for both wings (probably don't need the bolts, but who knows). $24 for order including shipping.

All of the current nuts have friction drag from zero to who knows. So it is a guessing game with the current nuts and bolts. Hopefully all the new nuts will have close the the same drag if new.
 
Group, I've just gone ahead and sent an email to vans support. I'll let you know what they say. PS - I've given them the link to this thread so they can see what all the commotion is about.
 
Hi Group, it turns out it is an error in the plans. We are to add a friction variable to 28. It may or may not be 14 of course. Here is Sterlings actual reply.

" I am not exactly sure what the question is after reading this thread but I believe you are asking about the typo in the RV-14 manual section 5.20 where we added 14+20=34, the text suggest it should have been 14+28=42. The example should have read 14+28=42, this is only an example and the number 14 represents the friction drag torque that needs to be added when using lock nuts. Keep in mind that this 14 pounds is not set number that you will add to your torque value every time. The 14 is only an example and you will need to find out for yourself what the drag is on the nuts you have.

Sterling "
 
OK--here is what I did for determining nut drag. I used the following torque wrench to get nut drag:

http://i1335.photobucket.com/albums/w663/Kenneth_Stockman/photo-9_zpsd02f8cdf.jpg

http://i1335.photobucket.com/albums/w663/Kenneth_Stockman/photo-11_zpsd0965ba0.jpg

Then I used the following wrench to set the nuts after determining the drag:

http://i1335.photobucket.com/albums/w663/Kenneth_Stockman/photo-10_zpsa56eda13.jpg

Both from Amazon for about $40 each.

I determined that the nut drag on the ranged from almost nothing (3-5 in lbs) to 25 in lbs. So the final torque on the nuts ranged from 31-33 to 53. That is a huge range. I did buy new nuts and bolts, but I think with this technique, the ones that came with the kit are fine.

I assume that I could have used the first wrench to set the final torque, but I bought the second one first.

cheers
Ken
 
Ken,

Do these variations were observed on new nuts and bolts or on those removed from the spars?

I've ordered a brand new torque wrench as well and I'll do my own testing this week end. Will post the results.

Bottom line is, these bolts should've been send in a bag in the first place and since these seems to be the only bolts of that type and of that size, a single torque value should've been indicated in the plans.

I'm a certified car mechanic and I've done countless engine and frame modification on motorcycles and else and I never thought that I'd have to go into such details for aircraft building.
icon8.gif
I guess this is part of the job.
icon10.gif
 
these were the nuts and bolts that came on the spar. I did buy a new set, but I don't know that I will change them out at this point. I will check a few of the new ones to see how much variation there is between bolts.

I might change out one of hte bolts on the first 3 inboard ribs since I did not tighten them yet. It basically has zero nut drag.

ken
 
these were the nuts and bolts that came on the spar. I did buy a new set, but I don't know that I will change them out at this point. I will check a few of the new ones to see how much variation there is between bolts.

I might change out one of hte bolts on the first 3 inboard ribs since I did not tighten them yet. It basically has zero nut drag.

ken

Every one of mine had drag, enough so that they could not be hand tightened beyond the first turn.
 
So kind of along this thread I was just checking the drag on my AN3 nuts on the spar and it was below 7 lbs after I had torqued them previously. That's as low as my wrench goes.

The other thing I noted is once the wing leading edge is installed there will be no way to check torque settings on most of the bolts again without removing the leading edge.

So Does it make sense to use Loctite on these Nuts/bolts?

Pete
 
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