RV-8 gear nuts and bolts--a thread-sheared nut.

I was most interested in Steve Smith's tests on landing gear nuts and bolts! 220 hours since completion 2004 on my RV-8 QB. I'm not a hamfisted pilot and I annually inspected my RV-8 gear tower and torqued the bolts faithfully. (I'm very careful about not over-torquing.) Sure, I've had some bounces, but nothing spectacular. At my last annual inspection, before torquing the gear tower bolts, I jacked up the left gear to rotate the tire. To my surprise, the gear leg was not rigid, but would rotate backward 5 degrees easily. I have the Grove gear (non-aerodynamic). I looked in the left gear tower and saw the front MS21042-6 nut totally off the bolt and sitting by itself in a corner. The nut thread was totally stripped out. I called Grove and they sent a new set of bolts and the NAS679A6 nuts. I replaced both sides (nuts and bolts) and inspect the gear tower regularly--every few flights. I will definitely replace the nuts soon with the NAS1804-6, as suggested by Steve. Cheap insurance! I'm posting this to possibly prevent someone from wrecking their airplane. I almost did not post this account due to the unkind comments which may follow--take this post for what it's worth to you and keep the cheap shots to yourself....

Hmm

Did you ship the failed nut and bolt off to Grove or to Vans for them to have a look at? If there were not any extenuating circumstances, it would be good to know what the possible cause of the nut failure was since the gear had not been over-stressed.

I for one, wouldnt want any of us to experience the same failure in the future.

Chris

Don, I for one appreciate the post.

Steve and I talked about a possible contribution from oversize washers. Can you contibute anything there? In particular, did the hardened washer under the nut fit the bolt closely, or was the ID a bit oversize?

BTW, forget pilot error. A bounce would not strip this nut; wrong location. In order to strip the forward clamp bolt, I think the most likely application of force would be something like dropping the tire in a hole, forcing the tire rearward and twisting the gear stub.

stitches

chepburn said:

Hmm

Did you ship the failed nut and bolt off to Grove or to Vans for them to have a look at? If there were not any extenuating circumstances, it would be good to know what the possible cause of the nut failure was since the gear had not been over-stressed.

I for one, wouldnt want any of us to experience the same failure in the future.

Chris

Click to expand...

Chris,
I did send the failed nut and bolt to Grove. These were the original nuts and bolts supplied with the original grove rectangularly shaped gear. After a couple of months I called and talked to the head honcho. They said they didn't know what was a possible cause of the failure. I questioned whether the metallurgy or hardness of the nut was up to standard, but they could not give me a definitive answer. To their credit, they replaced all the bolts and nuts at no charge and were, as always, pleasant and willing to please. It is interesting that the new nuts were the NAS679A6 nuts (slightly more resistant to tension loads) and not the originally supplied MS21042-6.

RV-8 gear nuts and bolts--a thread-sheared nut

DanH said:

Don, I for one appreciate the post.

Steve and I talked about a possible contribution from oversize washers. Can you contibute anything there? In particular, did the hardened washer under the nut fit the bolt closely, or was the ID a bit oversize?

BTW, forget pilot error. A bounce would not strip this nut; wrong location. In order to strip the forward clamp bolt, I think the most likely application of force would be something like dropping the tire in a hole, forcing the tire rearward and twisting the gear stub.

Click to expand...

Dan,
The ID of the washer was not oversized. Good question, but I do not recall dropping the tire in a significant hole.

Very important and GREAT report Don - if anyone gives you a cheap shot on this one, I will make sure it doesn't stick around!

As a "high time" RV-8 driver, I'll be interested in anything else you learn about this.

Paul

Dan as a good point...

...in stating that a hole (or runway lip on a cross runway) could exert enough rearward force.

As an aside, our Air Tractors have life-limited gear bolts and gear legs that are to be replaced at given hourly intervals and I've often wondered about the -8's since the -6, -7, -9 bolts don't carry any tensile loads to speak of.

Would it be prudent to just replace them at say 200-300 hours?

Interesting thread,

Strictly to avoid confusion, a quick repeat of data.....

Note that Don's failed nut was an MS21042-6, a popular substitute for the Van's specified nut because of its 7/16" wrench size. Per Steve's tests, the MS21042-6 is weaker than the stock NAS679A6 (11600 lbs vs 12500 lbs). Neither will match the full strength of the bolt, reported at around 14500 lbs.

<<our Air Tractors have life-limited gear bolts ..>>

Most of us are EAA members and may get the new Experimenter e-mail publication. There is an excellent article about bolt prestress and fatigue in the most recent edition, written by the late Vance Jaqua. You can read it here:

http://www.eaa.org/experimenter/articles/2009-05_bolts.asp

That said, I don't recall ever reading anything regarding the fatigue failure of threaded nuts. Fact is, until these recent threads I don't recall ever reading about nut stripping; seems like the bolt always broke first.

Mmmmm..

I have been getting small qty orders for NAS1804-6 on my webstore, is this a coincidence? or zer may be intelligent life formz on zee intervebs

Best nut you can buy for the money and still keep your legs (pun intended)

Rush, check your PM's

Hi Rush, I sent you a PM with recommendations on nuts and washers,

Steve.

DanH said:

Strictly to avoid confusion, a quick repeat of data.....

Note that Don's failed nut was an MS21042-6, a popular substitute for the Van's specified nut because of its 7/16" wrench size. Per Steve's tests, the MS21042-6 is weaker than the stock NAS679A6 (11600 lbs vs 12500 lbs). Neither will match the full strength of the bolt, reported at around 14500 lbs.

Click to expand...

According to the Genuine Aircraft Hardware tech info, the MS21042-6 nut has a tension rating of 160ksi as opposed to 125ksi for the NAS679A6 nut. Robby Grove recommended that I change to the MS21042 nut from Van's supplied nut to gain the extra strength. The NAS1804 nut sounds even better.

Paul

Clarification

The numbers 125, 160, 180 ksi, are the material strengths.

The more threads a nut has the more tension it will withstand.

The more meat it has the more abuse it will take before deformation.

And yes, the NAS1804, has both those qualities, plus a high tensile value of the material.

Grove gear nuts

I used the NAS679A6 nuts.