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  #1  
Old 04-23-2009, 05:47 PM
scsmith scsmith is offline
 
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Location: Ashland, OR
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Default Breaking News: tests on landing gear nuts and bolts

I have been a little skeptical about the strength of the NAS679A6 nuts that Van's supplies for the main landing gear bolts on the RV-8. So, I decided to do some tests of various different nuts to see what the different strengths are.

We wanted to simulate the tension load that gets put on the bolt and nut when the landing gear gets a fore-and-aft load, which puts a twisting moment on the gear which tries to pry apart the mounting saddle ( U803) and puts the bolt and nut in tension.

I will describe the results here, then describe the test procedure in a second post.

So, we tested four different nut types, as follows:

MS21042-6 low profile nuts with 7/16 wrench flats
NAS679A6 low profile nuts ( stock style) with 9/16 wrench flats
MS21045-6 full-height metal lock nuts with 9/16 wrench flats
NAS1804-6 mid-height nuts with 12-point 7/16 wrench flats

Below is a picture that shows each of the four nuts, and the cut-open failed nuts. The legend shows the breaking strength that we found. All the nuts failed by some or all of the threads stripping. The NAS679 tended to have a couple of threads un-stripped, as if they had radially stretched enough to leap-frog over the mating thread on the bolt. The MS21045 showed significant swelling of the base of the hex, so that a wrench would no longer fit on the flats. This was also true to a lesser extent with the MS21042.

In summary, the strengths were as follows:
MS21042-6 11,600 lb
NAS679A6 12,500 lb
MS21045-6 15,800 lb
NAS1804-6 16,200 lb

The expected strength of the 3/8-24 NAS close-tol bolts is about 14,000 lb. so only the last two types of nuts are capable of developing the full tensile strength of the bolts. We used Unbrako socket-head bolts (better than grade 8) to be sure that the nuts broke and not the bolts.

here's the link too: http://www.hpaircraft.net/rv8/nut_test2b.jpg

A couple of other builders in other threads have mentioned using the MS21042 nuts in place of the ones Van's supplies. An advantage is that they take a smaller size socket which helps get a socket on them in the gear tower. But as you can see, they are not as strong. The same benefit of smaller socket can be gained using the high strength NAS1804 nuts. You can order them from GAHco, they are pretty pricey but worth it for the peace of mind in my view.
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Last edited by scsmith : 01-01-2018 at 06:41 PM. Reason: added description of failure mode
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  #2  
Old 04-23-2009, 05:55 PM
scsmith scsmith is offline
 
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Default More BREAKING NEWS on landing gear nuts and bolts

OK, I promised a description of how we did the tests.
Above I mentioned that I wanted to simulate the prying action on the bolts when the gear arm is pulled fore and aft.
A second issue is that our home-made tensile testing machine is only capable of about 6000 lbs of tension, so we needed a way to amplify the load with some leverage. Here's how we did it:



You can see that this arrangement gains a 4:1 leverage on the bolts. The bars are 3/4 x 3" hot-rolled steel bar stock. The bolts and nuts were assembled with a washer under the nut, and torqued to 16 ft-lb. (correct for AN bolts, a bit low perhaps for the NAS close tol bolts, but the preload will not affect the single-load tensile strength of a test like this significantly).

As I mentioned above, we used Unbrako socket-head bolts. Each nut style was tested twice, new bolts and washers (and obviously nuts too) were used for each test.

The home-made tensile tester is shown here,with the bolted assembly at the bottom. The large "S" shaped device hanging between large eye bolts is the electronic load cell.



If you google "break-o-tron" I think you will find some U-tube videos of Bob Kuykendall ( hpaircraft.com) doing some tests on climbing gear and such.
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Last edited by scsmith : 01-01-2018 at 06:42 PM.
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  #3  
Old 04-23-2009, 06:12 PM
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Bubblehead Bubblehead is offline
 
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Default

Are the bolts in tension or shear?
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  #4  
Old 04-23-2009, 06:24 PM
scsmith scsmith is offline
 
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Default Both!

Of course the bolts also see very large shear forces. And for that, the nuts have no effect on the overall strength. But as I described, the tension in the bolts from the twisting of the gear leg is also very significant and in that case, its all about the nuts.
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  #5  
Old 04-23-2009, 06:34 PM
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jsharkey jsharkey is offline
 
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Default

Quote:
Originally Posted by Bubblehead View Post
Are the bolts in tension or shear?
Tension and some bending from the looks of it.
Jim Sharkey
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  #6  
Old 04-23-2009, 06:44 PM
scsmith scsmith is offline
 
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Default Actually rather minimal bending

The bars we used were pretty stiff. The tension is obviously reacted by the contact pressure right at the edge of the bars, but they did not bend much, nor gap very much. The gap at the far edge of the bars was less than 1/4 inch. None of the bolts were bent so much after the test that they could not be pulled through the holes in the bars.

Bolt bending is actually another serious question about the RV-8 gear attachment. The U803 saddles are pretty thin, about 0.25" thick, where they transition into the end blocks. And remember, the end blocks are supposed to have a 0.030" gap to the wear plate when the bolts are torqued. So already the saddle is bent slightly. Under the prying load I am concerned about, that saddle will try to bend even more, causing a combination of double-shear and bending in the bolt shank.

I believe my load fixture put substantially less bending into the bolt than the actual RV-8 assembly. But none of that is relevant for the question of the nut strength.
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  #7  
Old 04-23-2009, 07:01 PM
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chrispratt chrispratt is offline
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Default

Steve:

So what conclusion am I to draw from this? Are you saying that the nuts Van's supplies are not adequate or does the test show they will do just fine? What are the loads on the nuts in real-life flying? I need a reference point to draw a conclusion.

Chris
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  #8  
Old 04-23-2009, 07:44 PM
scsmith scsmith is offline
 
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Default Conclusions? well that depends....

Well, I could analyze a typical, although hypothetical load case on the gear assembly to show what the loads are that the attachment hardware is expected to carry. I have not done that at this point.

I can not say that the current assembly is inadequate - from the ground-loop damage I have seen, the gear tower seems to tear out of the fuselage with the gear leg still bolted on. I have not examined any of those to see if there have been any failures of the nuts.

I can only say that from my engineering experience I was rather skeptical about the supplied nuts, which is why I did the tests. I can say that it is typical engineering practice to use hardware that allows you to develop the full tensile strength of the bolts, when possible. ( of course you would not bother to do that in a pure shear loaded situation). But in a situation where gear loads can go from normal levels to catastrophic levels easily in a ground loop or wheel falling in a squirrel hole on landing, I think I want all the strength I can get.

My OPINION is that I think it is better to use a higher strength nut, and I will. It may be that the existing nuts are fully adequate, I don't have any evidence that says otherwise. It is likely that there are other weak links in the assembly that would usually fail first. It is also possible that a particular scenario would lead to failure of the nut before any other part failed. I might as well go for all the strength I can just in case....
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RV-8 N825RV
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WW 200RV
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Hobbs 515 in 9 years (would have flown more this year if not for fire smoke)
also LS-6-15/18 sailplane
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  #9  
Old 04-23-2009, 10:54 PM
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az_gila az_gila is offline
 
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Smile The design strength...

Quote:
Originally Posted by scsmith View Post
......
My OPINION is that I think it is better to use a higher strength nut, and I will. It may be that the existing nuts are fully adequate, I don't have any evidence that says otherwise. It is likely that there are other weak links in the assembly that would usually fail first. It is also possible that a particular scenario would lead to failure of the nut before any other part failed. I might as well go for all the strength I can just in case....
...used by the designer should be from MIL-Handbook-5.

The applicable page is here, but I don't know which bolts are specified, so I can't give the design ultimate tensile strength.

How do these numbers fit in with your tests?



Note "d" certainly fits in with your conclusions.
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  #10  
Old 04-24-2009, 12:55 AM
RatMan RatMan is offline
 
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Default Very interesting!

Very interesting indeed.

Seems you have come up with a solution that has evaded a problem!

Unless I've missed something, I haven't heard of one incident where these nuts have failed. Is this something no one is speaking of?
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