Failure of Nose Gear (now with bolt pics)

[DavidBurton]

+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=fati...l=np&source=hp

Good luck!
David

[Bavafa]

I am curious, aren?t AN bolt typically bent instead of sheer in situation like this?

[Rootski]

Originally Posted by RV6_OKLA

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

I love the plug! 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!)

[Rootski]

Originally Posted by ty1295

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.

[Bob'sRV6A]

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.

[Rootski]

Originally Posted by Bob'sRV6A

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.

[Mike S]

Nose gear, not main gear.

[az_gila]

Originally Posted by Mike S

Nose gear, not main gear.

Correct, but my -6A plans show the nose gear attach as an AN5-20A bolt, same as mentioned, just slightly shorter.

[N941WR]

Originally Posted by flytoday

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.)

[rivetshaver]

Curious what bolt was actually installed, what kind of marking does the head have?