Failure of Nose Gear (now with bolt pics)

[LarryT]

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

[David Paule]

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

[flytoday]

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

[Low Pass]

Originally Posted by LarryT

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.

[LarryT]

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

[jrs14855]

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.

[OLDSAM]

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.

[Mike S]

Originally Posted by OLDSAM

Was the bolt head still on the long end?

Re-read post 29.

[OLDSAM]

Originally Posted by OLDSAM

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.

[krw5927]

Originally Posted by LarryT

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?