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Failure of Nose Gear (now with bolt pics)

flytoday

Well Known Member
RV6A, 1995, O320, constant speed prop, almost 1200 hrs TTAE

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Failure in shear of the bolt at the top of the strut during taxi. Strut rotated 180 degrees. I'm told by a more experienced RV owner that this may be a rare event. When I'm flying again I'll be checking or replacing the bolt regularly!

Carl
 
Thanks for the post

First time I have heard of this particular failure -- just installed my Antisplat gear leg reinforcement ---- now I need to look at the mounting bolt.
 
Wow that's not fun thanks for sharing Carl. My nose gear takes a beating on regular basis I will check the bolt immediately next time I am in the hangar.
 
That is weird.. I don't know the math but how much pressure would it take to shear that bolt .. thats amazing.
 
I have seen the bolts holding the tailwheel knuckle to the spring shear before - the tailwheel rotates around 90 degrees and the plane settles when that happens. Those are a pair of AN3's. Never heard of Carl's failure before.
 
Failure in shear of the bolt at the top of the strut during taxi.

Did you actually find the bolt sheared, or was the bolt missing and you deduced that it must have sheared???

Have you ever had the "shopping cart" wobble on the wheel??

Hard to imaging how that bolt could get enough loading to shear it.
 
rather than failure in shear, is it possible that the nut came off and the bolt backed out?
 
Could it be possible to overtigthen/overstretch the bolt to weaken enough for shear failure? I can picture myself doing it.
 
I recovered the sheared bolt and a washer from under the plane. Bolt end had the nut still on it. I might be able to informally get a lab to examine the failure plane. No prior shopping cart wobble.
 
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Photos Please

Photos of the gear and socket,please. Has the wheel strut been lose of wobbly of late?
 
On my 6A, the edges of the hole in the gear leg were quite sharp. I took the gear leg off for inspection after a couple years' use, and I could not get the bolt out without significant effort, like a couple hours' worth. It appeared that the sharp edge on the gear leg hole had caused quite a bit of disruption to the bolt. Additionally, the stock AN bolt seems to leave too much clearance, allowing some wobble (even if not perceived). There is discussion elsewhere on these forums about using a close tolerance bolt in this location. I also chamfered the hole in the leg.

It could be causal in this case - very glad nothing worse happened!
 
I'll try to get and post some photos of the sheared bolt, strut, socket, etc. I didn't notice any wobble at all. Generally I'm able to land on the mains and briefly hold the nosewheel off. Less than 10% grass field ops.
 
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I recovered the sheared bolt and a washer from under the plane. Bolt end had the nut still on it. I might be able to informally get a lab to examine the failure plane. No prior shopping cart wobble.

So the "headless" bolt with nut still on was found out of the hole - it fell out?
 
With concern for the wear on both the NLG bolt and the hole thru the fitting I removed mine a couple of months ago during the conditional inspection. With over 325 hours my bolt and fitting still looked like new. This was accomplished in less than an hour and I'll probably inspect this more often since it is easier to replace a worn bolt instead of a damaged mount due to a worn bolt.
 
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Contact Vans. They might be interested in doing metallurgical tests. Maybe a bad bolt?
 
So the "headless" bolt with nut still on was found out of the hole - it fell out?

I found what looked like part of a bolt, with a nut on it, and a washer on the taxiway right under my engine. I'll assume it fell from my plane.

I'm not a builder, pretty ignorant to the internal structures, so since then I'm learning that it isn't just a simple bolt through a hole.

Carl
 
Bolt under engine

Zoomed in on the right edge of the picture, here's the bolt on the taxiway...

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Carl
 
If you get a good closeup picture of the break we can fairly easily see if a crack existed prior.

1 Common way bolts break is they get loose, slop, etc followed by sudden load.

We can calculate shear load force fairly easy with diameter and material type. Not a hard calculation at all.
 
+1 on posting a good closeup photo of the fracture surfaces. Try to resist "fitting the two broken parts together". This can hide the evidence of the mode of failure by damaging the fine structure left by the break. Try to use oblique lighting (from the side) when you are taking the photo.

I'd be suspicious of a fatigue failure. This can be caused by over tightening the bolt, damage or even being loose over an extended period of time. While not impossible, I'd discount the "bad bolt" theory and try to determine the mode of failure and seek a cure for that...

If you can't get a good photo you can compare the broken surface to images you can easily find online:

https://www.google.com/search?q=fat...fficial&client=firefox-a&channel=np&source=hp

Good luck!
David
 
May be a good application for a titanium bolt from Doug's new advertiser!

I love the plug! :D But as a responsible proprietor I should let you know that empirical reports show Grade 5 titanium usage in shear applications is not very well understood, so I recommend 8740 in shear-loading applications. (But Ti is much stronger in tension!)
 
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If you get a good closeup picture of the break we can fairly easily see if a crack existed prior.

1 Common way bolts break is they get loose, slop, etc followed by sudden load.

We can calculate shear load force fairly easy with diameter and material type. Not a hard calculation at all.

If it is indeed an AN3 shear bolt, then it's a #10-32 with a diameter of .19" and it'd be made of 8740 steel with a shear modulus of 80 Gpa. Rule of thumb for 8740 shear strength is .577 x ultimate tensile strength, which is about 128 Ksi * .577 = 73.85 Ksi (for the material, not accounting for diameter).

Another possible failure cause - as the OP isn't the original builder it's possible the incorrect bolt was used. Always use a bolt spec'd as a shear bolt when called for.
 
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If the failed bolt is the one at the top of the weldment (under our legs--inside the cockpit), my -6A plans call for that to be a AN5-21A bolt. It's really difficult for me to believe that any typical landing scenarios could cause enough rotation to shear that bolt without other major damage to the aircraft. Seems like it would take some major sideloads.
 
If the failed bolt is the one at the top of the weldment (under our legs--inside the cockpit), my -6A plans call for that to be a AN5-21A bolt. It's really difficult for me to believe that any typical landing scenarios could cause enough rotation to shear that bolt without other major damage to the aircraft. Seems like it would take some major sideloads.

That's a 5/16"-24 x 2-1/8th, or .3125" diameter. That's a decently chunky bolt, I'm also surprised it failed.
 
I recovered the sheared bolt and a washer from under the plane. Bolt end had the nut still on it. I might be able to informally get a lab to examine the failure plane. No prior shopping cart wobble.

I was there to help Carl move his plane back to his hangar.

We pulled the gear leg off and stuck in the bolt hole on the gear was a section of the bolt.

Both the head and the part with the nut sheared off. We had to use a punch to knock the remaining section out of the gear leg. (It came right out, with nothing more than a tap.)
 
Is the bolt plated?

Plating can result in hydrogen embrittlement. Typically taken care of with a post-plating baking at about 350 degrees F.

In a former career, the automotive brake manufacturer I worked for had this problem on some caliper slide pins. They actually failed during overseas shipping!

The one picture I observed, although not a close-up looked from a distance to be a brittle, as opposed to ductile fracture surface. I second the request for close-up photos, as well as the warning not to mate the fracture surfaces together.

Larry Tompkins
544WB -6A
W52 Battle Ground WA
 
1. You can look up the specification for AN bolts and it'll give the strength for the bolt in double shear, as well as the surface coating.

2. Have there been many accidents like this? If there really is no significant history of this sort of failure, then it wouldn't be very likely to be a design issue, but more related to the manufacturing, installation, maintenance or operation of this particular one.

3. Examination of the fractured surfaces and the adjacent areas, plus the landing gear parts themselves, should help provide clues. And as David suggested, please avoid fitting the broken bolt parts back together.

Dave
 
I'll get back to the hanger and get good photos posted soon. Hope to get an informal exam in a lab. I agree, don't touch the parts together. Appreciate the suggestions and thoughts.

Carl
 
Plating can result in hydrogen embrittlement. Typically taken care of with a post-plating baking at about 350 degrees F.
...
In an atmospheric application (i.e., non-chemical process environment)?? you sure about that?

Yea, Carl, back in my machinery engineering days I was able to look at almost any fractured metallic surface and tell you what caused the failure. The parts do speak after the fact.
 
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Alodizing

Aren't most aircraft fasteners alodized? It has been 30 years, but hydrogen embrittlement was what the metallurgists determination was.

I am certainly open to the inputs of more informed people than me. The bolt diameter seems quite large for a double shear application, but perhaps I am ignorant of the potential loads involved. Perhaps close-up photos will show that the failure was more ductile than my interpretation.

Whenever there are discreet processing steps and one is inadvertently omitted, there can be a significant difference in product performance. Eaton Corp made coil springs for GM cars in the 70s. QC being what it was in those days, the springs in the shot peening tower at the end of a shift frequently got shot-peened either twice or not at all. Some of the cars that used those springs became the original "low-riders."http://www.vansairforce.com/community/images/smilies/biggrin.gif
 
bolt

I am not an RV person but have 50 plus years experience with the Wittman gear. The gear attach bolts absolutely must be an interference fit. In other words, considerable effort with a medium size hammer. The hoes must be reamed to fit the bolt. Usually .0025/.003 under the nominal size. Buy some extra bolts from different vendors and check them with an accurate micrometer. A dial caliper or hardware store micrometer are NOT ADEQUATE. You must me able to accurately measure one ten thousands. The bolt should be one dash number "too long". The shank adjacent to the threads is typically well under the diameter of the bolt adjacent to the head. All metal lock nut properly torqued, with allowance for lock nut torque vs plain nut. Best of all is a close tolerance bolt or NAS bolt. Spruce Catalog lists the additional strength of NAS bolt vs AN, it is roughly 30% stronger.
Once that gear leg moves just the slightest amount it will enlarge the holes in the socket very quickly.
I have four separate reamers plus an expansion reamer just for the gear bolts. I also have a drill jig with hardened interchangeable bushings for drilling and reaming.
I realize that at least some of these on the RV are "factory drilled". The above info is still relavent. A factory simply cannot afford to spend the time to do this right.
 
Was the bolt head still on the long end? If it was, the bolt could not have failed in shear unless the tube holding the leg had an elongated hole on one side, since otherwise the gear leg cannot rotate without shearing the bolt in two places, i.e., +/- 1/4" under the head, and +/- 1/4" inside the nut (i.e., at the perimeter of the gear leg, not at the perimeter of the tube holding the leg. If the head is still on, and the break is up against the nut, (thus at the outer perimeter of the tube) the failure would more likely be a result of some combination of overtightening and/or fatigue; I would guess both.
 
Was the bolt head still on the long end? If it was, the bolt could not have failed in shear unless the tube holding the leg had an elongated hole on one side, since otherwise the gear leg cannot rotate without shearing the bolt in two places, i.e., +/- 1/4" under the head, and +/- 1/4" inside the nut (i.e., at the perimeter of the gear leg, not at the perimeter of the tube holding the leg. If the head is still on, and the break is up against the nut, (thus at the outer perimeter of the tube) the failure would more likely be a result of some combination of overtightening and/or fatigue; I would guess both.

Oops, sorry, did not see the additional pages of posts indicating that the bolt was sheared at both ends.
 
Plating can result in hydrogen embrittlement. Typically taken care of with a post-plating baking at about 350 degrees F.

AN (NASM) bolts that are supplied for this application by Vans are alloy steel that is formed, cadmium plated, baked for hydrogen embrittlement relief, and then given a supplemental chromate (similar to alodine) treatment. The 5/16" nominal diameter bolt is specified to have a minimum 11,500 lbf double shear capacity.

Seriously doubtful that it's even possible for the nosewheel configuration to exert this sort of force on the specified bolt, I'm curious about the specifics of installation, condition of the holes, and definitely the head marking on the bolt itself. Is it possible that that an aluminum AN bolt (they do exist) was used here, or perhaps a hardware store variety? Is there an "X", either inset or raised, on the head?
 
Bolt Photos

I believe my bolt failed into three pieces. I'll call them the HEAD, BODY, and THREAD&NUT. Reference coin is US Dime.

I cannot find HEAD.

Here is a picture of the two pieces I have. THREAD&NUT was found under the plane at the failure place. BODY was in the strut and came out with a relatively light tap with a punch.

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I believe these are the mating surfaces of THREAD&NUT and BODY

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I believe this is the end of BODY that would have been attached to HEAD

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And here are two lousy angle pictures with a temporary bolt to keep my strut in place while sitting in the hanger....

Top of temp bolt

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Bottom of bolt (threads - no nut), odd orientation in upper right corner

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Carl
 
Have Nas Bolts for gear, standard and oversize

Depending on your model.

Nas6605-24 or Nas6605-29 standard diameters

add an (X) at the end for 1/64th oversize

We have the right reamers.

I do not know the hardware for the Ten model.
 
I believe my bolt failed into three pieces. I'll call them the HEAD, BODY, and THREAD&NUT. Reference coin is US Dime.

I cannot find HEAD.

Here is a picture of the two pieces I have. THREAD&NUT was found under the plane at the failure place. BODY was in the strut and came out with a relatively light tap with a punch.



I believe these are the mating surfaces of THREAD&NUT and BODY



I believe this is the end of BODY that would have been attached to HEAD


And here are two lousy angle pictures with a temporary bolt to keep my strut in place while sitting in the hanger....

Top of temp bolt



Bottom of bolt (threads - no nut), odd orientation in upper right corner


Carl

That doesn't look like a ductile shear failure to me, especially that jagged edge on the head side.

If you're interested, PM me your address and I'll send you a titanium replacement, free of charge.
 
Hi Carl,

It's somewhat difficult to see the fracture surfaces in the photos but at first glance it looks like the nut end failed from fatigue and the other end failed by tensile overload. We'd have to be able to examine the surfaces better to be positive.

If this was the failure mode I'm a little surprised to find the nut. What usually happens is that the strength of the fastener reduces as the crack progresses until there is a final tensile overload that causes the ultimate failure. I suppose that both failures could have happened almost simultaneously with the head end being torn off after the nut end failed.

David
 
Here is my theory for the experts to diagnose.......I am guessing there were 2 separate failures. The first one was the head of the bolt failed in tension due to over torqueing and vibration in the engine mount. The bolt was held in place by friction but the sheer strength was significantly weakened. Some time later, due to all the sheer being transferred to the nut end, the nut finally sheared off.
 
As soon as the weather gets above 0 I'll remove the locking bolt in my nose gear for a look see. When landing at Larry Vetterman's fly-in last year my nose gear went into a violent grocery cart wheel shimmy; so bad that the front cap of the wheel wheel pant was thrown off. I don't expect to find anything but it was a pretty violent event.
 
Looks like typical fatigue failure

From the pictures, it looks like the failure is a typical fatigue, about half of the fracture surface is smooth (fatigue zone), while the other half is rough (due to instant fracture). This is typically caused by repeated bolt bending. It is hard to say what really caused this since this type of failure is rare for RV fleet. If I have to guess, it is mostly likely the bolted joint was loosened due to vibration. Especially if the bolt is not re-torqued during annual.
 
bolt

Is the unthreaded portion of the bolt at least 1/8" longer than the od of the gear socket. The failure at the base of the threads is something that is relatively common on AN3 bolts, I have never seen it on a larger diameter bolt. The amount of thread below the nut is indicative of a bolt that was too short. Ideally the number of washers should be such that the nut is about 1-2 washers from bottoming out on the threads. The number of threads protruding beyond the nut is a very basic rule of thumb. The number of threads protruding will vary with the size of the bolt, the larger the bolt diameter the courser the threads. One thread protruding on an AN 3 is not equal to one thread on an AN8.
The technical name for the coating on bolts is dichromate (dip)> It is normally a brownish gold color but other colors could be used. Hydragen embrittlement is most problematic on chrome plating, especially 4130 steel. Not normally a problem with cadmium but I have my cadmium parts baked anyway.
 
Bolt failure

I had trouble with an exhaust bolt that kept breaking just under the head. What I found was the bolt head was not flat on the surface it was clamping. Another words the hole was not perpindicular with the bolt surface.
 
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