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Re-use of Nuts and Bolts

dshevick

Member
I had a prop strike, and I had to remove my wings (close tolerance spar bolts, gear bolt, alieron linkage bolts, forward and aft spar bolts, etc.). Can I re-use the bolts? Seems like I should not re-use the nylon locking nuts, correct?

My nose gear leg was bent and curled up. I am getting a factory new-vans/lycoming o-320. Can I just visually inspect the engine mount cage for cracks?

Thank you,
David
415.794.2084
 
I'd use a 10 power magnifier on all the engine mount joints, at least. And more likely, I'd send it out for a magnetic particle inspection.

Dave
 
Most (well, some) of us would reuse nyloc nuts as long as we can't turn them by hand with the nyloc portion engaged on the threads. Kinda your call on the spar bolts. If they're inserted or removed without chilling them, it tends to remove the cad plating.
 
I know the bolts are expensive, but I would go ahead and replace them with new bolts and nuts. It's cheap insurance that you don't weaken the old bolts by re-stretching them when torqued a second time.
 
I know the bolts are expensive, but I would go ahead and replace them with new bolts and nuts. It's cheap insurance that you don't weaken the old bolts by re-stretching them when torqued a second time.

Is that really an issue, when a bolt is loaded in shear?
 
Bolts in shear

Is that really an issue, when a bolt is loaded in shear?

In a bolted joint (generally speaking), the bolt itself isn't (or at least shouldn't be) loaded in shear. It's the tension force in the bolts that clamp the pieces together, then friction between the pieces is really what does the work in stopping the pieces moving. Hence if there isn't enough tension, parts can move and then you will end up with a bolt loaded in shear that's now trying to hold the joint together. I know this is less of an issue with close-tolerance bolts, but for super critical applications (like spar attachment bolts) it's probably best to replace with new.

Yes there are bolts used in shear, and if properly designed they will function quite OK.
 
In a bolted joint (generally speaking), the bolt itself isn't (or at least shouldn't be) loaded in shear. It's the tension force in the bolts that clamp the pieces together, then friction between the pieces is really what does the work in stopping the pieces moving. Hence if there isn't enough tension, parts can move and then you will end up with a bolt loaded in shear that's now trying to hold the joint together. I know this is less of an issue with close-tolerance bolts, but for super critical applications (like spar attachment bolts) it's probably best to replace with new.

Yes there are bolts used in shear, and if properly designed they will function quite OK.

Well, I don't have the credentials to respond to that. But I sure hope an actual engineer will address it. All I know to say is to assign value to interweb info based, at minimum, on what you pay for it....
 
The assertion that friction, rather than the shear strength of a threaded fastener, is what carries the load is simply wrong - at least in aerospace. Since the surface condition of the mating parts is rarely tightly specified, the frictional qualities of a particular joint are both unknown and uncontrolled. Added to that, the preload established from torque is particularly variable.

There are plenty of methods to specify and control both the surface finish of a part and the preload that a threaded fastener can develop. For general aviation aircraft, these techniques are very rarely used, because of both cost and the time they involve, as well as the quality assurance effort needed to verify the joint. Plus, often enough, components are disassembled and reassembled and to accommodate that, standard techniques are generally applied. Certainly that's the case for the joints I've seen on RV aircraft.

Now it might be that friction is the primary shear load-carrying path in other forms of equipment. I don't know about that. Before I retired, I was an aerospace structures analyst, and am really only familiar with that type of hardware.

Dave
 
In a bolted joint (generally speaking), the bolt itself isn't (or at least shouldn't be) loaded in shear. It's the tension force in the bolts that clamp the pieces together, then friction between the pieces is really what does the work in stopping the pieces moving. Hence if there isn't enough tension, parts can move and then you will end up with a bolt loaded in shear that's now trying to hold the joint together. I know this is less of an issue with close-tolerance bolts, but for super critical applications (like spar attachment bolts) it's probably best to replace with new.

Yes there are bolts used in shear, and if properly designed they will function quite OK.

Sorry, but a seriously wrong assertion. Read any aircraft structures text and the friction of a shear joint is ignored in the calculation. In a glider, the wing spar pins are in shear and don?t even use nuts.
 
On the mount... mag particle is required (before I would use it again). Cracks that might form during a prop-strike event can close up when no longer under load so you can?t see them, but the weakness is still there. I would visually inspect at 10x first, and if you don?t find any cracks, send it out for a mag particle inspection.
 
I'm glad enough people responded to the joint friction nonsense.

I would replace the wing main spar bolts. OK to reuse the nuts. Ordinary AN hardware (e.g. aileron pushrod connections) can be re-used.

I agree to get the engine mount magnifluxed. Those bolts should probably be replaced too.
 
Re-use them

I wont elaborate on my day job, but 40 years as a structural specialist and liaison engineer on Heavy Jets, I can say in design, nearly all load bearing joints and assemblies are SHEAR. Then one can go into single,shear or double shear depending on load path, but tension joints are few and far between, limited to section joining and places where shear type connections are not practical. Bolts are often re-used unless coatings are degraded or the removal has damaged them. The torque levels (if followed)for a shear application are a good margin away from bolt stretching or thread distortion. I don't believe there is one single Tension rated fastener in an RV kit, as the head design and nut design are different, and no reason for it. I can recommend in the RV builders arena, when installing permanent structure, not meant to come apart again,such as spar bolts, its good practice to install them "wet" with primer or assembly lube to prevent corrosion. I built mine to last 100 years at the bottom of the ocean because that's what my day job has done to me!
 
In a bolted joint (generally speaking), the bolt itself isn't (or at least shouldn't be) loaded in shear. It's the tension force in the bolts that clamp the pieces together, then friction between the pieces is really what does the work in stopping the pieces moving. Hence if there isn't enough tension, parts can move and then you will end up with a bolt loaded in shear that's now trying to hold the joint together. I know this is less of an issue with close-tolerance bolts, but for super critical applications (like spar attachment bolts) it's probably best to replace with new.

Yes there are bolts used in shear, and if properly designed they will function quite OK.

Hi Geoff, we need to clarify a few things here.

Firstly you are correct that in some cases shear forces are purely resisted by friction between the surfaces. In structural steelwork HSFG bolts (high strength friction grip) are commonly used to perform exactly this function whereby the bolts are tightened to a shank tension so that the transverse load across the joint is resisted by the friction between the plates rather than the bolt shank's shear strength. At the design engineer's discretion other joints in shear can be resisted by both friction AND the bolt's shear strength (ie. the bolt shank is bearing on the hole).

In the case of an RV however HSFG bolts are not used and analysis calculations would be based solely on the shear strength of the bolt shank and an appropriate safety factor would be incorporated.

However, the fact remains that any bolted fayed joint in shear will generate some friction between the plates if the bolt is properly torqued, including on our RVs. What this means is that proper torquing of all bolts on an RV will increase the safety factor of the joint by some indeterminate amount. Look on it as a safety bonus reward for the correct application of bolt torque.

But having said that, I believe that the biggest danger for RV builders is not in under-torquing bolts but in over-torquing them....particularly the smaller sizes.
 
I built mine to last 100 years at the bottom of the ocean because that's what my day job has done to me!

Good on you Bill. I really respect guys who build for the expected life of the aircraft rather than for their own limited flying life. I used Duralac passive jointing compound on all my bolts...but let's hope that neither of our aircraft end up at the bottom of the ocean. :)
 
The assertion that friction, rather than the shear strength of a threaded fastener, is what carries the load is simply wrong - at least in aerospace. Since the surface condition of the mating parts is rarely tightly specified, the frictional qualities of a particular joint are both unknown and uncontrolled. Added to that, the preload established from torque is particularly variable.

Dave, while the spirit of your comment is correct, the wording could have been better. There most certainly exist joints widely used in aeronautics that transmit essentially all the power between parts through friction (pedantically: "stiction"). One common example is the mating of a wood prop to steel crank flange. The dead giveaway is that the wood prop bolts are typically specced to be torqued to high fraction of the bolt's ultimate tensile, with a crush plate distributing the load onto the much softer wood. The tiny amount of wood applying shear to the thin bolts would not absorb the pulses from the engine without being crushed. From the moment that stiction to flange fails (from neglecting to maintain proper nut torque, usually), one might have minutes before the prop departs the airplane.

This is not true for spar bolts and most other aluminum-to-aluminum joints, but here the specced tensile stretch is a fairly small fraction of the ultimate strength of the bolt, limited by the strength of aluminum not steel.

Yes, replace the spar bolts with new ones every time they are removed; don't overtorque or your spars might crack from excess fatigue much sooner than designed.
 
I don?t fully understand why you would replace spar bolts every time they are removed. We don?t replace prop bolts, even at overhaul unless they don?t pass. I know many folks who have removed wings for transport and re-used the bolts.
Leads to another question, would there be a difference in that decision with close tolerance bolts/system like the newer models or the less critical older laminated spar bolts?
I am all for playing it safe, but I removed a couple spar bolts to check for wear as was discovered by another VAF?r and found nothing wrong with the bolts visually, as he did, and I put them back in. What would cause a perfectly good bolt torqued to spec to suddenly be no good after it is removed?
 
Dave, while the spirit of your comment is correct, the wording could have been better. There most certainly exist joints widely used in aeronautics that transmit essentially all the power between parts through friction (pedantically: "stiction"). One common example is the mating of a wood prop to steel crank flange. The dead giveaway is that the wood prop bolts are typically specced to be torqued to high fraction of the bolt's ultimate tensile, with a crush plate distributing the load onto the much softer wood. The tiny amount of wood applying shear to the thin bolts would not absorb the pulses from the engine without being crushed. From the moment that stiction to flange fails (from neglecting to maintain proper nut torque, usually), one might have minutes before the prop departs the airplane.

This is not true for spar bolts and most other aluminum-to-aluminum joints, but here the specced tensile stretch is a fairly small fraction of the ultimate strength of the bolt, limited by the strength of aluminum not steel.

Yes, replace the spar bolts with new ones every time they are removed; don't overtorque or your spars might crack from excess fatigue much sooner than designed.

I know; even Sensenich says that in their literature. But I also know (empirical data) that if you fly without drive lugs, the prop *will* move on the face of the prop flange, unless you're flying something with incredibly low power pulses, like, maybe a turbine.

Again, I hope someone with actual credentials will respond to this. Tell you what. Remove all the drive lugs (there's a clue in the name) from your prop flange. The center ridge on the flange will keep your prop centered. Replace the prop, torquing the 6 bolts to spec. Fly for 10 hours, and let us know how it goes.

I don't know what wood prop you're using, but the ones I've used and/or checked (including Sensenich) spec lower than typical torque for whatever bolt diameter is used with the prop. My layman's assumption is that there's a reason for lower than typical torque when we're clamping wood instead of metal.

Sensenich wood torque specs:
https://www.sensenich.com/wp-content/uploads/2018/01/Installation_Instructions_Wood_Aircraft_Installation_1452800693.pdf

AN bolt normal torque specs:
http://www.supercub.com/pdf/AN%20Bolt%20Torque.pdf

Charlie
 
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Certainly there are a few instances where friction plays a role in carrying shear, and if you look hard, you can identify them.

For most cases, all except a very, very few, friction is not the load path.

If it were, the engineer would need to define surface treatments and establish factors of safety for friction under load, and perhaps even under dynamic load. There would need to be a means of inspection to verify these things. If it can't be quantified, it can't be relied on.

Dave
 
I don’t fully understand why you would replace spar bolts every time they are removed.

In a word: galling. Cadmium lubricates. The price of new bolts is not worth the sweat of maybe having to re-ream the spars...

Edit: also, cadmium narrows the effective torque transfer; old nuts/bolts won't clamp as hard for the same wrench torque.
 
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I don't know what wood prop you're using, but the ones I've used and/or checked (including Sensenich) spec lower than typical torque for whatever bolt diameter is used with the prop. My layman's assumption is that there's a reason for lower than typical torque when we're clamping wood instead of metal.

Sensenich wood torque specs:
https://www.sensenich.com/wp-content/uploads/2018/01/Installation_Instructions_Wood_Aircraft_Installation_1452800693.pdf

AN bolt normal torque specs:
http://www.supercub.com/pdf/AN%20Bolt%20Torque.pdf

Charlie

You meant higher? From the documents you linked (same ones I use):

Prop AN4 torque 120-140 in*lbs
Regular AN4-on-aluminum torque 50-70 in*lbs

So, same bolt on prop is set closer to 80% of tensile limit of steel, and only to 30-40% on aluminum. I cringe every time I go to retorque my Sensenich--the Belleville washer actually mars the aluminum crush plate!
 
You meant higher? From the documents you linked (same ones I use):

Prop AN4 torque 120-140 in*lbs
Regular AN4-on-aluminum torque 50-70 in*lbs

So, same bolt on prop is set closer to 80% of tensile limit of steel, and only to 30-40% on aluminum. I cringe every time I go to retorque my Sensenich--the Belleville washer actually mars the aluminum crush plate!

Never seen a 1/4” prop bolt on an RV. Need to compare the qouted torque spec to the standard for a 7/16 or 1/2” bolt

My prop calls for 35 ft lbs, which I believe is well below yield strength of a 7/16 bolt. I recall the 7/16 bolts on my chevy’s main journals being close to 100 ft lbs and those still weren’t torque to yield bolts. Most prop bolts are grade 8 and I don’t believe that an spec bolts are.
 
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For the OP.
Give Wayne at Phlogiston a quick call. Nobody knows Vans spars better than Wayne. He can advise you on bolt replacement or not. There is a big difference between the laminated spar and extruded spar designs. I would suspect you need to be pickier with the newer spars, and less with the older.

I hate to see you throw away perfectly good expensive bolts that may be just fine. To do it just for the reasons some have listed here without a good understanding why doesn?t teach you are any of us anything. If there is good reason, he will know and you can share with VAF?rs.

For what it?s worth, I would trust Wayne over most folks at Vans.
 
I agree about saving/reusing spar bolts; back in post #3, I was trying to say that the only reason to replace is if they're damaged, and it's user's call on whether they consider cad plating damage actual damage to the bolt. (Yes, I'm aware of the purpose of cad plating on bolts used in aluminum.)

You won't know it unless you remove one, but unless you used the 'freeze in dry ice' method to allow them to just drop in place, you can bet that the cad plating was getting partially removed when you drove them in. The later, 'solid' spar wings (-7, -8, etc) have known issues where the bolt holes in the web sometimes don't perfectly align with the 'meat' of the spar caps. If the builder assumes they are perfectly aligned and drives the bolts home, the cad plating gets scraped pretty bad upon insertion.
 
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