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Found during condition inspection. Gear leg bolt n Nut

Danny King

Well Known Member


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Bob Dean found one of his gear bolts hanging down when he pulled the cover plate for gear bolt torque check.

Here are the fact given to me by Bob.

1) Time on aircraft: 617 hours.

2) The torque on these bolts are checked once a year at my annual inspection.

3) I have never notice a problem, perhaps one of the bolts needed a little tightening over the years.

4) I had marked both bolts on both sides with Torque-seal to help detect movement.

5) The first picture with the bolt protruding is just the way I found it when I removed the inspection plate.

6) Inside the gear tower, the nut was totally off of the bolt. I cannot remember if the washer was still on the bolt or not. The bolt was sticking up through the gear tower; it seems like at least 3/16 of an inch, perhaps 1/4 inch.

7) I was using the the proper low profile metal lock nut. The threads were stripped, and you could see little metal filings.

8) The washer was the one that came with the kit. I am assuming that it is an AN960-616. As you mentioned they have replaced it with a 062 25783054.

9) The washer was bent. Hopefully, you can see it from the second picture.

10) The other bolt on this gear leg was loose. It took about a full turn of the bolt to get it properly torqued.

11) I have not properly check the other gear leg yet, but I do not see any problems from the visual inspection.

I am going to replace all four bolts with new bolts, new hardened washers and a new nuts.
 
wow

I recieved my fuselage kit in June 2001. I am going to check to see which washers I have. This is a concern for sure. I wonder what caused this. Maybe they need to be inspected more often.

bird
 
Good catch

Good job doing a complete high quality annual. Seems some people skim through it with cursory looks etc. Taking the time and really going through each item start to finish and putting a wrench on things is the key.

Glad you found this and are fixing it properly. Hope your post gets others to dig deep and check.

Blue skies.
 
If this is a bolt that holds the landing gear to the airframe, I know of one RV-8 that had a gear collapse during early take-off roll due to an APPARENT bolt(s) failure.

Airframe rebuild. Engine rebuild. New CS prop.
 
I was using the the proper low profile metal lock nut. The threads were stripped, and you could see little metal filings.

Doesn't this imply incorrect initial assembly? A cross thread or over-torque?

Stripping threads is an unusual failure mode for a correct bolt joint isn't it?

If the nut is actually overloaded in an axial direction, then perhaps it should be a normal high profile nut and not a low profile one....
 
A couple of years ago, Steve Smith and I ran a series of tensile tests on the various types of nuts applicable to the RV-8 landing gear. The results are discussed in this thread:

http://www.vansairforce.com/community/showthread.php?p=320444

Edit Add: Here's the money shot:

nut_test2b.jpg


Thanks, Bob K.
 
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one more point

There is one essential item that hasn't been discussed in this scenario. Bob and I discussed the failure and one point mentioned was that the washer was bent. In this case, the washer was ground down on the side so it would fit inside the radius of the longeron angle, as is often the case. Somewhere along the line, probably at initial installation, the washer got turned and was sitting at a slight angle when the bolt got torqued. I'm guessing that over time, the washer began to bend more to make itself fit into the radius of the angle, thus slightly loosening the nut and allowing it to work and wear. It doesn't take much to make this happen. At any rate, regardless of retorquing the nut every year, if the washer allowed even the tiniest gap, in my opinion, that would cause the failure of the nut over time
As a point of interest, Lycoming calls out that no paint is allowed beneath the cylinder nuts when torquing down cylinders since the failure of the paint over time could destroy the proper torque of the nut and actually cause the cylinder to come off the engine. (It has happened!)
At any rate, this discussion is a good lesson in why we need to do thorough condition inspections at regular intervals. You just never know what will happen next.
 
Tom Brink @ GAHco has (or had) the required one size longer bolts if you want to use the easily obtained MS21045-6 nuts.

....and if I may....good call Smith and Kuykendall!
 
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Bolts, nuts

Just drilled and install gear legs. Looking at some problems others have had with gear hardware I paid close attention when aligning the gear and proper drilling angle. I think drilling the 3/8 holes in the outboard leg clamp might be the source of nut problems. If the hole is not "square " with the gear tower weldment the nut will not seat evenly around the circumference of the washer.
Mine came out pretty good ,but I plan on installing the nuts on the bottom as the clamp is factory drilled and looks like it would support the bolt in shear with more support.
Is .030 the minimum gap ? what is the maximum.? I don't want to have to remove the clamp after flying a few hours to mill off a few thousandths .
 
Inspection

Another good example of a good inspection revealing something needing attention, regardless of the cause, (which should be identified.)
 
And Another

Similar to Danny, I found this yesterday.
I tighten mine ~300hrs annually. They usually require a 1/4 to half turn to tighten on occasion.
This terrible picture is in the tower is how I found it. Nut nearly off! Threads not stripped. Nut looks fine. You can see how close it was to coming off compared to the other one in the picture. The bolt can not come out due to the access plate. In any event, this is troubling.
Its a complete head scratcher how this happens.
Left gear, fwd outboard bolt. ~1500hrs on this plane.

I can usually tell when its time to tighten. I hear creaking sounds when taxiing. This time the creaking was on the right side, not the left where this nut was found loose. Im replacing all the hardware and adding the 1804-6 nuts.
gearlegbolt.jpg
 
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Thanks for sharing this info. I'm approaching 1400 hrs and have only had to retorque mine once at ~100 hrs. And that was 25 or soo late, but that was back in the early days of learning the model specific maintenance.

I'm coming due for annual next month I believe. Will certainly check closely based on seeing that pic of the loose fastener.

Now a followup question for those finding loose fasteners (totally disengaged). Are you replacing the components? When I found mine loose (gear was deflecting fore and aft), I replaced them. I see these as critical and having a loose fastener being loaded in an unintended fashion made me unconfortable leaving it in service. Was probably conservative but based on my engineering experience, having investigated failed fasteners, and thinking about the consequence of failure, I replaced with new. Just something to consider.
 
Are you replacing the components? When I found mine loose (gear was deflecting fore and aft), I replaced them. I see these as critical and having a loose fastener being loaded in an unintended fashion made me unconfortable leaving it in service. Was probably conservative but based on my engineering experience, having investigated failed fasteners, and thinking about the consequence of failure, I replaced with new. Just something to consider.

Yes new hardware and the 1804-6 nuts are on order.
 
OK you got my attention

I hear creaking on our RV-6A and as unpleasant as it seems I think a near term removal of the seat pans and the cowl and torque check are in order. I have though it was a brake sound but maybe it is loose landing gear mounting bolts.

Bob Axsom
 
Nut nearly off! Threads not stripped. Nut looks fine.
Its a complete head scratcher how this happens.

You can say that again. Seems impossible. You have the nut and bolt out and in your hand now?
 
...Threads not stripped. Nut looks fine...

As I recall from our hardware tensile tests, the NAS679 nuts were rather chewed up, but had considerable threads left inside them after they'd failed. The failure mode seems to have been that under tension the entire nut expanded enough to allow the bolt to slip thread by thread, and returned to something close to its original size after it slipped.

So I bet that if you look closely you'll see that the tops of the threads are rounded over. It probably looks worst when you look in from the side of the nut that bears against the fastened parts.

Thanks, Bob K.
 
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You can say that again. Seems impossible. You have the nut and bolt out and in your hand now?

No. Waiting for hardware to arrive early next week before I take anything out.
Until then, she is out of service.
 
It doesn't actually look like a nut strength issue - but is there any damage to the threads on the bolts?

It looks more like a locking issue. Now why these are unscrewing, I haven't any idea unless someone has run a tap through the nut or something, eh?

Has anyone tried a drilled bolt and cotter pin?

Dave
 
It doesn't actually look like a nut strength issue - but is there any damage to the threads on the bolts?

It looks more like a locking issue. Now why these are unscrewing, I haven't any idea unless someone has run a tap through the nut or something, eh?

Has anyone tried a drilled bolt and cotter pin?

My money is on this actually being a nut strength issue. Analysis has shown that the nuts and bolts are under some pretty large loads. Testing has shown that the stock nuts don't leave much margin. Service history has shown that the margin is occasionally exceeded.

On the other hand, there are no rotational factors at play; certainly nothing of the magnitude required to overcome the self-locking feature built into the nuts used here.

Thanks, Bob K.
 
With a new set of bolts and nuts in hand, I removed the offending gear bolt and found the nut threads stripped and the gear bolt threads a mess. Im betting as others have mentioned a nut strength problem. Reinstalling with NAS 1804-6 nuts. I can only assume that as a result of braking, there is enough tension to strip the nut. Yikes! This seems to me to be a problem. This plane and Danny's, have not been in and accident to cause this. Again this was the fwd outboard nut. Left gear.
gearbolt.jpg
 
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Maybe I'm speaking out of turn,

but in the Supercub world, when the tail wheel mount bolt needs to be re-torqued, it's because the bolt stretched. I tell my operators to tighten it only until in can be replaced. A Nas type bolt is stronger (180 ksi vs 125 ksi) but the tend to snap instead of stretch. So that's what I do with any bolt that could have been stretched. Replace. Not being familiar with the -8 gear, I know I may be speaking out of turn. If so , please delete.
 
You are doing fine, a couple tweaks;

but in the Supercub world, when the tail wheel mount bolt needs to be re-torqued, it's because the bolt stretched. I tell my operators to tighten it only until in can be replaced. A Nas type bolt is stronger (180 ksi vs 125 ksi) but the tend to snap instead of stretch. So that's what I do with any bolt that could have been stretched. Replace. Not being familiar with the -8 gear, I know I may be speaking out of turn. If so , please delete.

You are doing fine, a couple tweaks;

The most common NAS bolts NAS62xx and NAS66xx go from 3/16" dia to 1&1/2" dia. The tensile strength ranges from 160 Ksi to 180Ksi.

http://www.gen-aircraft-hardware.com/images/pdf/nashhb.pdf

This should help. PS if I had an 8 I would use the NAS1804 nuts (properly torqued) maybe the MS21250 bolts too.
 
Have you measured the bolt to see if it's stretched?

Dave

Neither of my fwd outboard bolts had stretched measured ~.001 of new.
I left the rear outboard bolts in. There is only so much gear tower abuse I can take in one week.
With these new nuts being only 1/2" 12pt socket, and rework of the -8 and -3 nuts and bolts in the tower in the area of the fwd outboard bolts, torquing these in the future will be easier.

I tell you this, my next 8 will have a complete redesign of nuts and bolts in this area. Its just ridiculously brutal in there to work and get anything done.

Doesn't make it any easier when your helper on the outside, laying down on a cushioned creeper, is laughing at you whilst your on the inside busting knuckles, cursing, dropping everything etc. Im over it!
 
I just don't understand how that particular thread damage could be caused by an overload. Seems that if it were, the other sides of the bolt threads would show more damage than the side closest to the end of the bolt.

It kind of looks like galling of the threads.

On the other hand, there are what look like either marks from rotating the bolt during installation or removal or they might be from a shear load - kind of hard to tell, although they don't appear to be accompanied by any noticeable permanent shear strain.

Dave
 
RV8A G3X

How do you like G3X, ? Aerotronics is building my panel as we speak, ready next month, I'm anxious to speak with someone who has used one.
 
This should help. PS if I had an 8 I would use the NAS1804 nuts (properly torqued) maybe the MS21250 bolts too.

Agree, you'll never get full use of the NAS1804 nuts without MS21250 bolts. This will also allow a much higher pre-load which should help a lot of this. You should be able to get to a point where you're torquing to 600 in-lb (50 ft-lb!) with that bolt and nut.

In my experience, MS21250 bolts and NAS1804 nuts usually fail by stripping the thread off the BOLT at about the pitch diameter; leaving what looks like a perfect helicoil in the nut, and a almost smooth bolt. Granted these were on 3/4", not 3/8" but the result should be the same. (I can't remember if it was the 180ksi NAS1804 (a.k.a. H93-6) nuts or the identical looking 220ksi BACN10HR6 version (a.k.a. H20-6). I'd get the 220 ksi version if you can, if GAC doesn't have them, PM me for a source).
 
I will put a second post (after Oshkosh)

Agree, you'll never get full use of the NAS1804 nuts without MS21250 bolts. This will also allow a much higher pre-load which should help a lot of this. You should be able to get to a point where you're torquing to 600 in-lb (50 ft-lb!) with that bolt and nut.

In my experience, MS21250 bolts and NAS1804 nuts usually fail by stripping the thread off the BOLT at about the pitch diameter; leaving what looks like a perfect helicoil in the nut, and a almost smooth bolt. Granted these were on 3/4", not 3/8" but the result should be the same. (I can't remember if it was the 180ksi NAS1804 (a.k.a. H93-6) nuts or the identical looking 220ksi BACN10HR6 version (a.k.a. H20-6). I'd get the 220 ksi version if you can, if GAC doesn't have them, PM me for a source).

I will be prospecting for RV8 builders/ owners who want the real deal beefier hardware! MS21250-06027 or 06028, and appropriate compatible nuts & Washers.

I will put a second post (after Oshkosh) just for that, so I can get a head count so I can know what the whole batch of hardware will cost. The more we get the less the cost to Gahco and the RV8 owners.

It will take a little bit of time;)
 
landing gear bolts

I have the Grove gear. Why not put the bolts in from the top? I know this does not address the nut issue, but wouldn't this make maintenance easier. The nut side of the connector is also more stable it would seem as it is the portion through the large portion of the gear clamp. Down side is the extra length to cover with a fairing, but with the difficulty people seem to have with annuals, it may be worth it.
 
Agree, you'll never get full use of the NAS1804 nuts without MS21250 bolts. This will also allow a much higher pre-load which should help a lot of this. You should be able to get to a point where you're torquing to 600 in-lb (50 ft-lb!) with that bolt and nut.

You can't increase the torque on this assembly without damaging the outboard mounting brackets ("U" brackets).

MS1804 nuts are definitely beneficial here, but coupling them with MS21250 bolts is going to be overkill in this assembly.

Skylor
 
What RV8 Builders Really Need

....is a nut retainer that stops rotation of the heads (bolt or nut) inside the outer steel gear leg structure so the torquing can be done from the underside of the leg. I'd personally prefer to insert the bolts from the inside, wit the nuts on the outside and can live with the bolt protruding thru the cover.

I thought I was going to be the one RV8 builder to not complain, I adjusted the position of those holes just slightly more inboard ~1/32 than the 3/16 print dimension, based on those that have done this before. But even with cut down wrenches, ground sockets and many of the previously installed AN3 nuts/screws removed from the areas, it was painful enough cuss.

My worry is that this installation is so hard that mis-assembly is easy and might explain some of the loose/striped condition inspection results.
 
You can't increase the torque on this assembly without damaging the outboard mounting brackets ("U" brackets).

MS1804 nuts are definitely beneficial here, but coupling them with MS21250 bolts is going to be overkill in this assembly.

Skylor

I assumed it was just a clamped up joint. I have a 7, never messed with the gear on an 8.....
 
Barrell nuits

These are standard NAS 577 engine mount locking barrel nuts in the pylon of most jets and Helo's. Perhaps someone need to machine a mount for them to slip inside the tower and presto you're done.

$(KGrHqMOKi8E5!MUO-1yBOZZ5yPfUg~~60_12.JPG
 
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These are standard NAS 577 engine mount locking barrel nuts in the pylon of most jets and Helo's. Perhaps someone need to machine a mount for them to slip inside the tower and presto you're done.

$(KGrHqMOKi8E5!MUO-1yBOZZ5yPfUg~~60_12.JPG

The also hold the outer wings on C-130's. 15 -14's (7/8") and 13 -12's (3/4")!
 
Outside bracket clamp gaps

I just switched over to the 12 point NAS1804-5 nuts, and also the NAS6606-27 bolts, which are slightly longer, thus giving a few threads through the new taller nuts. These nuts are certainly easier to get on with a socket to torque, but I have another issue. I am not the builder, but am slowly going over what was a beautiful job on my newly purchased RV8. Upon gear inspection prior to the nut and bolt replacement I found that neither gear clamp had the .035" recommended gap in the bolt areas on the front bolt end, but did on the rear bolt end. While installing the new bolts and nuts, we were careful in the torquing process, alternating back and forth as we reached the 240in/lb required and ended up with about .010 gap, both front and rear on the left, but on the right .003 front and .014 rear, and all gaps varied between the gap at the inside, closest to the gear leg, and on the outside of the bolts, away from the gear legs, an obvious sign that the clamps are bending. I've read about removing them and taking material away from the face of the bracket to ensure the .035" gap, but is shimming a possible solution? I don't know about other RV-8's, but getting the bracket completely out requires the complete removal of my vent and fuel fittings in the landing gear bracket area. I am thinking that hardened shim stock in the recessed area of the bracket, between the leg and bracket, would increase the gaps, but I am not sure the same quality of results.
 
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