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Nose Gear Bolt Failure. Argh!

Kahuna

Kahuna

Moderatoring
This past weekend one of our teammates, our only nose dragger left on the team, had a nose gear bolt failure resulting in a prop strike. I had seen this before on VAF, but had never seen one myself.
Fortunately he was not moving, or barely moving at the time, pulling out from a run-up, holding a brake to turn to square up on the taxiway. As soon as the plane started to rotate, the bolt failed, the gear leg rotated, and she nosed down and caught a few inches of the prop. We are very thankful for composite props, a little luck, and some heads up wingman that were around him at the time. A couple of weeks earlier, he had the tail down, nose gear up, greasing the nose wheel bearing, and had no play in the gear. Its also checked at condition. This hidden bolt, probably doesn't give much warning before failure. Had it failed in another phase of flight other than barely moving, it could have been much much worse for him and the plane. I think he has ~1500 hours or more on the plane. Probably worth considering a periodic change of this important bolt. Or stick it in your tool bag of reasons to own a tail dragger:) Thought Id share a couple of pictures.

leggsnose.jpg


boltfail2.jpg
 
Interesting

Thanks for sharing the essential information.

One thought is to check the torque on that bolt at inspection. Literally click the wrench on it at near the specified torque. This should ( no data here) reveal a weak shaft without adding any additional loads. It seems this is similar to a couple of 6's we have seen here with similar failures. If there is loss of torque, then it should be removed and replaced, as the crack could allow enough stretching to lose torque but still be intact enough to accept full torque. I have seen that on occasion, rare, but real.

I don't know if it allows a practical torque check, but every 100 hrs or annual should do it as a fatigue failure would take a while to progress. It would not hurt for all nose bolts to be checked now (next inspection trigger) for this condition. We could use some more good reports.

Glad it failed in nearly the most benign way. Pulling from the hangar would have been the absolute "best" way to fail.
 
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Mike, thanks for the report. Tell "Leggs" sorry to hear that happened to him. So glad that he is alright and the plane had minimal damage. As you said it could have been much worse. Thank the good lord for minimal damage . . . expensive but fixable.
 
Ouch! I wonder if the taper pin would be stronger, and not allow any movement?
 
Since I'm about to put the legs on my rv8a, this thread is major for me.
Separate from checking torque, is this a metal fatigue issue?

Does a bolt made of higher strength make sense?
Are they available?
I've read the threads about a tapered pin, would this address the issue?

Thanks,
Tom H.
 
magical torque wrench?

This sounds like a good check of integrity, but can you show me the torque wrench for this application? I can hardly get an open end wrench on mine!
In the armchair engineering dept.....would some kind of additional key or pin keep the shaft in alignment even after the bolt shears?
In theory, the leg could still 'fall out', but in most cases, staying in alignment, and the static load would keep the leg in the socket.
 
This is not the first nose gear bolt to break. We had one a couple of years ago at the Capital City Air Show on one of the show formation aircraft (RV-6A). The nose gear bolt broke without the engine running while moving the aircraft if I remember correctly. It was fixed in less than an hour. The aircraft did fly in the air show after the fix. Since the engine was not running, the prop did come in contact with the ground.

IMHO, the hole size is critical. It must be just the right size else you have some play in the connection and the bolt will eventually shear.

I had the single AN3 (3/16") bolt shear years ago in my Aviation Products tail wheel mount to the tail wheel spring. I eventually replaced it with a Tapper Pin and have not had any issue since. The Tapper Pin was a change that the RV Guys flying out of Livermore Kalifornia came up and I borrowed the idea.
 
I am glad the damage is not any more than it is and all are safe.

I believe it was last summer that some one?s bolt had sheered and as a cautionary measure, I pulled mine to inspect and replaced it with another close tolerance bolt. The original bolt did not, or at least I could not, detect any sign of stress (close to head) that would break.
 
Does anyone have this bolt size handy? I am going to order some and just change it out every condition inspection. This issue got my attention last time it came up. Now I have seen enough that I feel I need to take preventative action.
 
Bolts

AN bolts 125,000 psi strength, NAS bolts 160 or in some cases 180,000 psi.
To get a valid check for play in the landing gear bolt, the nut MUST be loosened.
The hole in the socket and gear absolutely MUST be reamed to a very close tolerance.
 
jrs14855 said:
AN bolts 125,000 psi strength, NAS bolts 160 or in some cases 180,000 psi.
To get a valid check for play in the landing gear bolt, the nut MUST be loosened.
The hole in the socket and gear absolutely MUST be reamed to a very close tolerance.
Click to expand...

I hope we're not suggesting that AN bolts, at only 125ksi shear, are inadequate. That's over 9,400 lbs single shear capability for a .310" nominal diameter bolt. Since the nose wheel bolt is in double shear, let's call that almost 19,000 lbs. There must be some other, or additional, failure mode at work here. Simple fastener shear can't be the only mechanism.

It really stinks that this happened. Are there perhaps any photos of the bolt itself and gear socket area? It may help in identifying a cause and cure.
 
nose wheel torque...Loctite 641 Retaining Compound

Kurt,

IIRC, the 125,000 psi number is ultimate tensile, not shear. Shear stress is typically half of UTS.

Data on Skybolt.com lists AN-5 single shear at 5750 lbf, full diameter.
(http://www.skybolt.com/Catalog-AN-MS-Hardware.pdf)

Did the nose gear rotate before it slid? A lateral force at the nose wheel contact patch will generate a torque about the nose gear socket axis. Guess 14 inch moment arm. The radius of the shear interface is ~.625 inches?? (1.25 inch diameter) So we can calculate that a lateral load of 513 lbf would shear the bolt in two places. Still hard to imagine a 500 lbf lateral load at the nose wheel of a 1100 lb plane... maybe there is some other failure mode here.

I have been contemplating use of Loctite 641 Retaining Compound on the LG sockets and bolts to forestall this type of failure. Got any perspective?

- Roger
 
combined tension and shear

...don't remember how to calculate it, but a bolt under tension will be easier to shear than on that isn't under tension... The tighter the bolt is stretched, the more likely it is to fail in shear. Will check old textbook.
- Roger
 
It would be interesting to see where the sheer had taken place relative to the grip portion of the bolt.

Also, was the nose wheel turned fully to one side during the run up? I usually take care to come to stop when the nose when is more or less in a straight position. I would imagine that more force gets applied to the assembly if the nose wheel is turned fully to one side or the other then trying to taxi with more and power.
 
One AN bolt between normal ops and a new prop and engine overhaul? It would be at the top of my critical annual inspection list. Or upgrade fasteners. (Comment based upon 20/20 hindsight here in the cheap seats, and having seen the same failure another time. Not trying to be smart.)

Condolences, btw.
 
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I want to mention that I just had an annual and this bolt they mentioned was either loose or was showing wear. Def a super critical inspection point !
 
RKellogg said:
Kurt,

IIRC, the 125,000 psi number is ultimate tensile, not shear. Shear stress is typically half of UTS.

Data on Skybolt.com lists AN-5 single shear at 5750 lbf, full diameter.
(http://www.skybolt.com/Catalog-AN-MS-Hardware.pdf)

Did the nose gear rotate before it slid? A lateral force at the nose wheel contact patch will generate a torque about the nose gear socket axis. Guess 14 inch moment arm. The radius of the shear interface is ~.625 inches?? (1.25 inch diameter) So we can calculate that a lateral load of 513 lbf would shear the bolt in two places. Still hard to imagine a 500 lbf lateral load at the nose wheel of a 1100 lb plane... maybe there is some other failure mode here.

I have been contemplating use of Loctite 641 Retaining Compound on the LG sockets and bolts to forestall this type of failure. Got any perspective?

- Roger
Click to expand...

You're right, Roger. I didn't look up the shear value and foolishly used someone else's stated number. According to the Skybolt document you referenced it does look like margins can be increased by using a NAS1100 (superceded by NAS6200, or for close tolerance NAS6600) series bolt here. Your calculation of 513 lbf at the tire to shear the AN bolt by a rotating load would be increased to about 640 lbf with a NAS6200/NAS6600 bolt at a 95ksi minimum single shear. Almost 25% stronger. I like those odds.
 
did the bolt fail in the thread

It would be beneficial to understand where the bolt failed. A picture of the removed bolt would help to understand if it failed in the thread, ie a short bolt.
 
Steve Melton said:
It would be beneficial to understand where the bolt failed. A picture of the removed bolt would help to understand if it failed in the thread, ie a short bolt.
Click to expand...

Great idea. The team is S. Florida for the NASCAR race at Homestead right now. We'll get Legg's airplane back together soon after and will do some forensics on the bolt. Stay tuned.
 
I too am glad minimal damage was done.

While I need to research this further, I believe a chamfer in the gear leg hole would help reduce the problems. For those interested, I dug up this paper.

The situation being tested is reasonably similar to our application, and their data showed an improvement in shock load cycles to failure with the addition of a chamfer on one or both of the shear plates. The basic theory is that more energy can be absorbed by the bolt during shear loads.

This may or may not apply to our gear bolts (the same design applies to most gear legs in the RV series). I will continue to look into whether or not the chamfer helps or hurts.

As I believe I've written in these forums before, a very minimal preload is a good idea on these bolts. The nut's only real function is to prevent the bolt from falling out. Pre-load on these bolts is an additive stress to those encountered during shear loads. IF the bolt's preload was adequate to cause friction between the gear leg and mount, it would be a different story. My experience is that these bolts, torqued to spec., do not apply enough squeeze to the mount to create any friction.

It could perhaps be argued that some sort of a split housing might be better, as the bolt's primary function would change from shear to clamping (which would create friction to keep relative movement at zero).
 
Bolts

I am amazed at the number of times I read posts on this forum where builders seem to think they can ream, or much worse, drill to size a hole in a critical area using a fractional size drill or reamer. Nominal AN5 bolt would be considered 5/16" diameter. I just measured a few AN5 bolts, size varied from .3097 to .3119.
Obviously a bolt that is almost .003 undersize if not satisfactory for the nose gear. Furthermore it is not unusual to find substantial variation in the unthreaded portion of an AN4 bolt. This is especially true of the area immediately adjacent to the thread which may be substantially smaller than the rest of the shank. I compensate for this in critical areas by using a bolt that is one dash number longer than required.
A fractional reamer is very likely to cut slightly oversize, which further complicates the issue.
I have only a limited knowledge of the RV nose gears/mounts as to whether some or all are drilled/reamed to size by the factory.
When drilling a Wittman main gear, I drill in the heat treated condition. I use a clamp in place drill jig with removeable hardened drill bushings. I start with a 3/16 pilot drill, then a 1/64 undersize drill, then .309, 310, and 311 reamers. I select bolts that are a medium drive fit on an AN5 bolt.
If using a NAS bolt, these are usually .001/.002 over the fractional size, so would require a different set of reamers.
I use bolts that are one dash number extra length and use all metal lock nuts.
 
Just two observations ...

Common with formation flyers, wouldn't flank turns with a brake locked up put a lot of twisting stress on the nose wheel gear since the front tire can't really roll into position ? We have never really discussed if you should let the locked wheel move just a little bit ? (which would seem scary up that close)


And, that particular bolt doesn't really set up correctly because it's a flat head on a round surface which means it only has two bearing/pressure points under the bolt head and on the nut as well. It might be that one of those special washers with a flat surface on one side and the other matched to the nose gear barrel might help ? I think if you look at the picture, it sort of looks like a stress failure and not shear but it's hard to tell ?? It had to shear somewhere otherwise it wouldn't spin in the barrel but I wonder which happened first ?

Just thinking someone might offer an expert opinion :confused:
 
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Looking at those mattresses, I hope you had gloves on!

Sorry to see another thing I have to worry about with the nose wheel. It looks like you will have to dress the ends of the prop. When you take the prop off and mount it on your wall, I vote for a much better story than a broken bolt. After the expense and trouble, you deserve a good story to tell!!

Best of luck with your repair.
 
I feel your pain. This doesn't only happen to the RV clan. I just had almost the same bolt failure on my KIS4 nose gear assembly back in August.

IMG_0507_zps1a1ff04a.jpg


IMG_0513_zpse1fe940e.jpg


:(
 
krw5927 said:
You're right, Roger. I didn't look up the shear value and foolishly used someone else's stated number. According to the Skybolt document you referenced it does look like margins can be increased by using a NAS1100 (superceded by NAS6200, or for close tolerance NAS6600) series bolt here. Your calculation of 513 lbf at the tire to shear the AN bolt by a rotating load would be increased to about 640 lbf with a NAS6200/NAS6600 bolt at a 95ksi minimum single shear. Almost 25% stronger. I like those odds.
Click to expand...


Given the relatively low factor of safety, it is a good place to add a bit of margin. I was surprised that the FS is that lean. My static empty nose gear weight was just under 300 lb. Will measure and order tomorrow.

Do the mains experience the same failures?

Roger
 
Upgrade

I think we will take this step up on the 7A to the NAS6605-27 standard bolt for now. Just makes me feel better. :)
 
Ditto

I agree with Jim .... Nov. 1964 Flying Magazine says it all ....

jrs14855 said:
I am amazed at the number of times I read posts on this forum where builders seem to think they can ream, or much worse, drill to size a hole in a critical area using a fractional size drill or reamer. Nominal AN5 bolt would be considered 5/16" diameter. I just measured a few AN5 bolts, size varied from .3097 to .3119.
Obviously a bolt that is almost .003 undersize if not satisfactory for the nose gear. Furthermore it is not unusual to find substantial variation in the unthreaded portion of an AN4 bolt. This is especially true of the area immediately adjacent to the thread which may be substantially smaller than the rest of the shank. I compensate for this in critical areas by using a bolt that is one dash number longer than required.
A fractional reamer is very likely to cut slightly oversize, which further complicates the issue.
I have only a limited knowledge of the RV nose gears/mounts as to whether some or all are drilled/reamed to size by the factory.
When drilling a Wittman main gear, I drill in the heat treated condition. I use a clamp in place drill jig with removeable hardened drill bushings. I start with a 3/16 pilot drill, then a 1/64 undersize drill, then .309, 310, and 311 reamers. I select bolts that are a medium drive fit on an AN5 bolt.
If using a NAS bolt, these are usually .001/.002 over the fractional size, so would require a different set of reamers.
I use bolts that are one dash number extra length and use all metal lock nuts.
Click to expand...
 
220KSI bolt

There is a bolt made that is far tougher than AN/NAS bolts. They are the "big plane" industry standard for ultimate strength..I have a few on my plane. They are 220KSI (220,000psi) tensile, and made from Inconel. They just don't break..period. The MFG designation is a Boeing trade designation, but you can google around and find them. Not knowing exact size for the A model application, I am only guessing you would be looking at a BACB30US5K27. The "BACB30" is the style, the "US" is the Inconel alloy, the "5" is the diameter, the "K" is for the coating of aluminum (long story I wont go into) and the "27" is the grip length. I don't intend to even try and re-engineer the current product, but that's what my day job is...and Len, stop out at 8A7 next time your around.
 
fixnflyguy said:
There is a bolt made that is far tougher than AN/NAS bolts. They are the "big plane" industry standard for ultimate strength..I have a few on my plane. They are 220KSI (220,000psi) tensile, and made from Inconel. They just don't break..period. The MFG designation is a Boeing trade designation, but you can google around and find them. Not knowing exact size for the A model application, I am only guessing you would be looking at a BACB30US5K27. The "BACB30" is the style, the "US" is the Inconel alloy, the "5" is the diameter, the "K" is for the coating of aluminum (long story I wont go into) and the "27" is the grip length. I don't intend to even try and re-engineer the current product, but that's what my day job is...and Len, stop out at 8A7 next time your around.
Click to expand...

Good idea, maybe GAHCO could track some of these down for us?
 
careful...

AlexPeterson said:
I too am glad minimal damage was done.

While I need to research this further, I believe a chamfer in the gear leg hole would help reduce the problems. For those interested, I dug up this paper.

The situation being tested is reasonably similar to our application, and their data showed an improvement in shock load cycles to failure with the addition of a chamfer on one or both of the shear plates. The basic theory is that more energy can be absorbed by the bolt during shear loads.

This may or may not apply to our gear bolts (the same design applies to most gear legs in the RV series). I will continue to look into whether or not the chamfer helps or hurts.

As I believe I've written in these forums before, a very minimal preload is a good idea on these bolts. The nut's only real function is to prevent the bolt from falling out. Pre-load on these bolts is an additive stress to those encountered during shear loads. IF the bolt's preload was adequate to cause friction between the gear leg and mount, it would be a different story. My experience is that these bolts, torqued to spec., do not apply enough squeeze to the mount to create any friction.

It could perhaps be argued that some sort of a split housing might be better, as the bolt's primary function would change from shear to clamping (which would create friction to keep relative movement at zero).
Click to expand...

Alex,

Don't take this personally but I want to caution people looking for a fix from exploring this idea further.

I would strongly discourage you (and anyone else) from doing any of the 2 things you mentioned: adding a chamfer to the hole and reducing the preload.

First, the article you referenced is for the design of submarines and naval vessels to withstand nuclear or depth charge attacks. This is a different phenomenon involving hydrodynamics and significantly larger materials (2 inch steel plates bolted together) and significantly larger dynamic shock loads. Again, this is substantially different The material here is pretty thin already, don't reduce it by adding a chamfer. Note the loads in the paper you referenced are on the order of 20,000 lbs. Different animal.

Second, I think this bolt is definitely in tensile and shear, and a high preload is needed to resist the dynamic (cyclical) loads. The preload on the bolted connection resists the torsional moment - the nut is not there to keep the bolt from falling out. Keep in mind the weight on the nose gear is limited by vans (275 lbs, I think?). So the loads you are worried about are of course landing loads and maybe inflight loads. Those are dynamic in nature and higher magnitude than the static load. Maybe you are right in that the bolts are not torqued enough, but I don't agree that reducing the preload further is a step in the right direction.

I think an acceptable alternative has been proposed with the use of a tapered bolt (essential a dowel with threaded ends) that does not allow eccentricity. For the aircraft in question, I'd be curious how the gear holes look (see if they are out of round) and if the bolt preload was checked at some interval.

Third, I'd double check with Vans before modifying the structure.

My 2 cents. Don't take my word for it, this is the internet after all, but carefully explore your options before modifying anything.

Thanks
Chris
 
RKellogg said:
Kurt,

IIRC, the 125,000 psi number is ultimate tensile, not shear. Shear stress is typically half of UTS.

Data on Skybolt.com lists AN-5 single shear at 5750 lbf, full diameter.
(http://www.skybolt.com/Catalog-AN-MS-Hardware.pdf)

Did the nose gear rotate before it slid? A lateral force at the nose wheel contact patch will generate a torque about the nose gear socket axis. Guess 14 inch moment arm. The radius of the shear interface is ~.625 inches?? (1.25 inch diameter) So we can calculate that a lateral load of 513 lbf would shear the bolt in two places. Still hard to imagine a 500 lbf lateral load at the nose wheel of a 1100 lb plane... maybe there is some other failure mode here.

I have been contemplating use of Loctite 641 Retaining Compound on the LG sockets and bolts to forestall this type of failure. Got any perspective?

- Roger
Click to expand...

It will be interesting to see what the forensic investigation of the broken bolt reveals. I would suspect that fatigue played a part in this failure. The huge numbers quoted for tensile strength make it seem difficult to believe that a bolt like this could be loaded enough to break it. What often happens is that the bolt receives a shock load large enough to cause a microcrack to form. This crack propagates through the bolt over time until a relatively small load can fracture it completely. Just a nick or sharp radius under the head of a bolt can start a fatigue crack growing.

I think we all have seen video of RV nose wheels oscillating wildly at certain takeoff and landing speeds. That kind of loading is ideal to cause a fatigue crack to propagate.

The investigation should tell the story.

David
RV6
 
I just got a quote from one of my suppliers for the Inconel bolts for $75 each. There is a minimum order of 20. If 19 or more people PM me that they would like one, I'll place an order. - If someone else found a cheaper source, let me know and I'll take one!
 
pa38112 said:
I just got a quote from one of my suppliers for the Inconel bolts for $75 each. There is a minimum order of 20. If 19 or more people PM me that they would like one, I'll place an order. - If someone else found a cheaper source, let me know and I'll take one!
Click to expand...

Getting close to mounting the gear, I'll get in line for one.

Chris
 
In for Iconel bolt

Sounds like cheap insurance against an expensive mishap.
Sent pm. Count me for one.
Thanks,
Tom h.
 
So, has anyone asked the factory what they think of using a vastly stronger bolt in this location??

Anyone considered that this may be a designed in failure point to protect other parts of the airframe??

No idea one way or the other, just rambling.............but might be good to know;)
 
pa38112 said:
I just got a quote from one of my suppliers for the Inconel bolts for $75 each. There is a minimum order of 20. If 19 or more people PM me that they would like one, I'll place an order. - If someone else found a cheaper source, let me know and I'll take one!
Click to expand...

I must have missed something. How did Inconel bolts become the solution? I am surprised that a high temperature alloy is the answer to a reamed assembly and fitted bolt. The high strength, precision diameter NAS bolts listed earlier at $2.86 each.

edit: Yes, I missed this:

There is a bolt made that is far tougher than AN/NAS bolts. They are the "big plane" industry standard for ultimate strength..I have a few on my plane. They are 220KSI (220,000psi) tensile, and made from Inconel. They just don't break..period. The MFG designation is a Boeing trade designation, but you can google around and find them. Not knowing exact size for the A model application, I am only guessing you would be looking at a BACB30US5K27. The "BACB30" is the style, the "US" is the Inconel alloy, the "5" is the diameter, the "K" is for the coating of aluminum (long story I wont go into) and the "27" is the grip length. I don't intend to even try and re-engineer the current product, but that's what my day job is...and Len, stop out at 8A7 next time your around.

Bill E.
 
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