Nose Gear Bolt Failure. Argh!

[NTex]

Originally Posted by AlexPeterson

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).

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

[DavidBurton]

Originally Posted by RKellogg

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

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

[n468ac]

Originally Posted by Walt

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

I would too be interested

[pa38112]

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!

[YellowJacket RV9]

Originally Posted by pa38112

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!

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

Chris

[tomhanaway]

Sounds like cheap insurance against an expensive mishap.
Sent pm. Count me for one.
Thanks,
Tom h.

[Mike S]

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

[rhill]

Originally Posted by Walt

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

I'd like one for all three locations.Do they come in interference fit sizes?
RHill

[Bill Boyd]

If the 6A takes the same size bolt, I will definitely take one.

[BillL]

Originally Posted by pa38112

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!

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.