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Failure of Nose Gear (now with bolt pics)

The color and coating of the fractured head end looks aged. Can't find the bolt head because it's been gone a while?

Judging from the number of threads exposed between the nut and the break, what looks like rounding/peening of the exposed thread near the break, and the polishing of the shank near the break, I'd say the bolt was too short...the root thread was in the shear plane.

Internet analysis, worth what you paid.
 
How is the condition of the hole in the gear AND the mating part on the engine mount? Is the hole has enlarged by much to have much play between the two parts?
 
jrs14855 said:
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.
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I have to admit I don't follow all these comments and recommendations. Could you make a 5 minute youtube video holding parts, touching and pointing, and explain more completely why too short, correct threads below a nut, ideal bolt and washer stack-up, etc?

Thx for everyone's posts!

Carl
 
Bavafa said:
How is the condition of the hole in the gear AND the mating part on the engine mount? Is the hole has enlarged by much to have much play between the two parts?
Click to expand...

Might be awhile, and be relatively difficult, to get data on the "collar" holes. Would a small amount, like 5% enlargment matter?

Carl
 
bolts

I explained the bolt diameter issue in thread #36. Measuring from the head along the shank as you approach the thread the bolt diameter will become slightly smaller. You don not want this smaller diameter engaging the socket tube in this particular application. So the unthreaded portion should extend thru the socket tube at least 1/8".
A medium drive fit means the hole and the bolt diameter are virtually the same. The bolt could even be .0002/.0003 thousands larger than the hole. A 5% oversize hole is completely unacceptable.
The comments I have posted apply ONLY to the gear bolts and do not necessarily apply to any other application.
 
jrs14855 said:
I explained the bolt diameter issue in thread #36. Measuring from the head along the shank as you approach the thread the bolt diameter will become slightly smaller. You don not want this smaller diameter engaging the socket tube in this particular application. So the unthreaded portion should extend thru the socket tube at least 1/8".
A medium drive fit means the hole and the bolt diameter are virtually the same. The bolt could even be .0002/.0003 thousands larger than the hole. A 5% oversize hole is completely unacceptable.
The comments I have posted apply ONLY to the gear bolts and do not necessarily apply to any other application.
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How does this square with the taper pin modification? Would a carefully chosen +.001-3 NAS Bolt and well prepared undersize hole along with molded washers and a all metal locking nut be a better option?
 
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One additional data point... The Whittman Tailwind, when configured as a tricycle, uses RV6A gear legs in all three locations. A local Tailwind builder purchased his gear legs from Vans. He reported experiencing one shopping-cart shimmy on a touch-and-go landing. Next landing the nose gear attachment failed, allowing the nose gear to rotate 90 degrees, thus the side of the wheel pant became a skid while the aircraft ground its way down the runway after having turned its gorgeous Ed Sterba prop into toothpicks.

The nose gear attach bolt failed in the same manner as shown by the OP. All 3 parts were found, head, body and "nut end". Shear pattern on body section of the bolt looked very similar to pics provided by the OP. Body section of the bolt had to be driven from the gear leg.

That same builder installed a taper pin; its installation resulted in the builder emitting an extreme quantity of profanity, often at high volume levels. The taper pin is still in service today; no further gear problems were had on that aircraft.
 
Canadian_JOY said:
One additional data point... The Whittman Tailwind, when configured as a tricycle, uses RV6A gear legs in all three locations. A local Tailwind builder purchased his gear legs from Vans. He reported experiencing one shopping-cart shimmy on a touch-and-go landing. Next landing the nose gear attachment failed, allowing the nose gear to rotate 90 degrees, thus the side of the wheel pant became a skid while the aircraft ground its way down the runway after having turned its gorgeous Ed Sterba prop into toothpicks.

The nose gear attach bolt failed in the same manner as shown by the OP. All 3 parts were found, head, body and "nut end". Shear pattern on body section of the bolt looked very similar to pics provided by the OP. Body section of the bolt had to be driven from the gear leg.

That same builder installed a taper pin; its installation resulted in the builder emitting an extreme quantity of profanity, often at high volume levels. The taper pin is still in service today; no further gear problems were had on that aircraft.
Click to expand...

Well you can't argue with success,and the taper pin achieved the interference fit that had not been there before the incident.I guess my question is more about the hardness of the taper pin vs a +.001-3 NAS bolt captured by properly squared head+nut. When the SB on the gear leg came out I can't recall anyone who cut there own thread saying they would do it again.
 
Bolt Failure

From a quick look it seems that the position of the nut and washer would indicate that a significant portion of the bolts threaded area would have been in sheer with the gear mount. I am not a mechanical engineer nor did I sllep at Holiday Inn last night, but I was always told that it is a NO NO to have the threaded portion of the bolt captured inside the captive area in a sheer condition.
 
Thank you Mike, I did not know about these washers, but you can bet the next time I put on my exhaust I will have them under my bolts.
 
Be sure you still have enough stud sticking out past the nut----these washers are thicker than standard. Also, I think it would be wise to check into the heat tolerance of them.

Tom at GAHCO might have the correct ones, my prior link to Grainger was just a reference as to type, and the ones I linked are rather big for your need. http://www.gen-aircraft-hardware.com/default.asp
 
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grade 8?

Does anyone else think the threads on the bolt look "coarse" instead of the fine machine threads found on AN bolts? Have to wonder if this was a grade 8 bolt and more brittle than AN hardware. Should be easy for the OP to find out what type threads. I agree it appears the bolt was too short and had threads under shear.
 
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Canadian_JOY said:
One additional data point... The Whittman Tailwind, when configured as a tricycle, uses RV6A gear legs in all three locations. A local Tailwind builder purchased his gear legs from Vans. He reported experiencing one shopping-cart shimmy on a touch-and-go landing. Next landing the nose gear attachment failed, allowing the nose gear to rotate 90 degrees, thus the side of the wheel pant became a skid while the aircraft ground its way down the runway after having turned its gorgeous Ed Sterba prop into toothpicks.

The nose gear attach bolt failed in the same manner as shown by the OP. All 3 parts were found, head, body and "nut end". Shear pattern on body section of the bolt looked very similar to pics provided by the OP. Body section of the bolt had to be driven from the gear leg.

That same builder installed a taper pin; its installation resulted in the builder emitting an extreme quantity of profanity, often at high volume levels. The taper pin is still in service today; no further gear problems were had on that aircraft.
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I bet that was an expensive fix all around, accounting for a new prop and all the other work that comes with a prop strike.

A next-best-alternative recommendation to the high degree-of-difficulty of the taper pin solution: if the bolt holes are indeed enlarged (and I'll bet my bottom dollar they are) ream them out to accept the next-highest AN bolt gauge, likely an AN6 or AN7 depending on the condition of the holes, and take care to make sure your bolt is long enough that the threads aren't captive. A straightforward solution providing some extra material for sheer and fatigue strength.
 
Since you are not the builder, it may not be a bad idea to check other bolts to see if they are of the right type and setup. The picture, one to the last, it shows a bolt with relatively too many threads showing past the nut. It is not clear what it is being used for but that might have bottomed out based on the number of threads showing.
 
Bolts

NAS bolts are 180,000 psi strength compared to 125,000 for AN bolts.
The only NAS bolts I had available to measure were 3/16 diameter and measured .189 or .0015 over the nominal size.
Except for the NAS, the AN3 bolts were the closest to the nominal size.
As the diameter of the AN bolt increases, apparently the manufacturing errors increase. The AN 5's ironically were the worst, some with slight taper, some with substantial taper immediately adjacent to the thread. They were consistently up to .0025 under the nominal size.
 
Failure of Nose gear

Interesting to read through all the comments, curious how many are from RV builders vs other types of homebuilts? The main point is all the folks talking about interference fit, reamed holes, etc. That all makes perfect sense to me as a Starduster builder where holes are typically drilled undersized and them reamed to get that close tolerance fit.

However going back and looking at my RV7A plans it clearly states that grease should be applied to the gear leg before inserting it into the socket because the parts are not corrosion resistant. Obviously they are also are also not a press fit. I didn't find anything in the instructions about reaming the hole to get a close tolerance fit on the bolt either.

From what I've heard this was most likely a corrosion issue and the bolt failed. Sounds like people should look at this area a little more closely since I doubt many of us ever remove it once installed. Secondly it would be nice to get a read from Van on installing a second bolt? This is the first failure of this type I've heard of but that may be a factor of the age of the fleet as much as anything. Obviously a good design, but a little redundancy can't hurt either as long as there is enough material to support it.

D2
 
bolt

I paint the gear except for the two places where it makes contact with the socket. Some type of anti corrosion treatment other than grease would be appropriate for the inside of the socket and the two unpainted areas on the gear leg. In my opinion this was not a corrosion issue but rather bolt too short, possibly over or under torqued, possible loose fit. Not to be disrespectful, but the suggestion of a second bolt shows a serious lack of understanding of this issue. WHERE WOULD YOU PUT THE SECOND BOLT?????
 
The Cure?

On an A model there just isn't any room. And again your right back to a modification that may or may not have a failure mode of its own.This is the first failure of this type I'm aware of,not to say it's the only one,its just not common the way the bent strut/back flip is.This is productive thread and as I see it,may be worth an SB of its own.Outside of shipping there is no reason not to mate the engine mount&strut on the bench after the mount holes are marked and drilled on the firewall. I'm debating should I do an NSA +.001-3 interference fit now or the taper pin?If the hole becomes hogged out you may not have that option.Soap base grease is hygroscopic(wheel bearing grease),so another question is what is the best coating for this application.Short version bolt or taper pin? Best area prep method?
RHill
 
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Rootski said:
A next-best-alternative recommendation to the high degree-of-difficulty of the taper pin solution: if the bolt holes are indeed enlarged (and I'll bet my bottom dollar they are) ream them out to accept the next-highest AN bolt gauge, likely an AN6 or AN7 depending on the condition of the holes, and take care to make sure your bolt is long enough that the threads aren't captive. A straightforward solution providing some extra material for sheer and fatigue strength.
Click to expand...

A number of us, including myself, have installed the tapered pin. I'm not sure it's fair to characterize it as any more difficult than reaming the hole and installing a new close-tolerance bolt. It seems to me that the tapered pin offers many advantages, if you think about it and evaluate it carefully yourself.

Here's a thread in the forums discussing the topic in more detail, for anyone who may not have seen it:

http://www.vansairforce.com/community/showthread.php?t=105870
 
jrs14855 said:
NAS bolts are 180,000 psi strength compared to 125,000 for AN bolts.
The only NAS bolts I had available to measure were 3/16 diameter and measured .189 or .0015 over the nominal size.
Except for the NAS, the AN3 bolts were the closest to the nominal size.
As the diameter of the AN bolt increases, apparently the manufacturing errors increase. The AN 5's ironically were the worst, some with slight taper, some with substantial taper immediately adjacent to the thread. They were consistently up to .0025 under the nominal size.
Click to expand...

Your numbers seem to meet the specifications

The AN5 diameter is 0.309 to 0.312 inches

The NAS bolts are much better - 0.3110 to 0.3120 if cad. plated.
 
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