Are higher grade bolts more brittle?

Sure I'm right! ;-) No, I got the point now. We had different pictures in our minds. The grade-5 fraction had this in mind: Overlapping sheet that is held together with rows of bolts. You have that picture of several rows of rivets? OK, here it really pays to have soft bolts. Because what happens when you do have overload is, that the bolt most stressed and being at his second failure (plastic deformation) still takes some load but also is partially giving in and thus handing the overload to his neighboring bolts/rivets. If that bolt would be too strong, the sheet would start to tear and this is certainly more catastrophic than a bent bolt with an oval hole in the sheet.

As soon as you are going away from sheet metal and do have more solid constructions with longer bolts things are getting different and the grade

5 fraction is getting wrong.

Is that acceptable? :-)

Nick

Reply to
Nick Mueller
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How do you drill through a tube the wrong way?

Reply to
Dan

Cripes, there goes the next three years of spare time.

Thank God for FEA, eh? I used to try to optimize spaceframe structures by hand. I spent most of a year, when I was 18, trying to optimize a little Lotus-type frame that I designed, using a slide rule and paper. Now I can change an element or two and run the whole stress/strain analysis in less than 30 seconds. And if I spent serious money on the software I could even let the program do the optimizing for me.

However, you still have to know what it is you're analyzing. You have to be able to look at a design and smell where there could be trouble.

-- Ed Huntress

Reply to
Ed Huntress

Yes! And you've identified an issue here that most people miss: Those rivet-bonded wing skins and so on are *not* designed to share the load between the glue and the rivets. The rivets are just there to prevent peel and cleavage, failure modes in which high-strength epoxy is very weak. But the glue is much stronger at resisting shear than an all-riveted joint would be, even one that's optimized for resolving the shear loads in the rivets.

Yes, but that's only one of the issues with ductility in fastening. As I mentioned, I'm too rusty to get into the whole schtick, but there are other reasons you need ductile fasteners, as well. Sometimes.

-- Ed Huntress

Reply to
Ed Huntress

============= And herein lies the rub.

Proper lap joints *NEVER* rely of fastener shear strength. The proper function of the fastener is to clamp the surfaces together such that friction between the two is what resists the load. For location in such a situation you would use dowel pins, but still rely *ONLY* on the friction the fasteners generate by clamping the surfaces together for strength.

In correctly designed and assembled lap joints, any axial failure should be in the base/joint material. If this is not the case (i.e. fastener shear failure), using more but smaller fasteners to more evenly distribute the clamping (and friction) across the joint, or "washers" to distribute the clamping force is indicated, not harder bolts.

While stronger bolts may allow increased assembly torque and therefore higher clamping force/pressure on the joint members at assembly and thus "solve" the problem, it is due to the higher clamping force and not the "harder" bolt.

Fine v course threads, assembly techniques [thread lube/antiseize] and applied torque are also factors. Harder bolts are also less prone to cold flow/creep under stress, and thus may maintain joint clamping pressure better than lower grade fasteners. Joint member material can also be a problem in this regard if enough cold flow occurs to reduce the clamping pressure and thus the friction, and periodic re-torquing may be required to maintain joint strength.

Reply to
F. George McDuffee

You're thinking of thick materials, George. There are lots of structural designs in which the shear strength of the fastener is the issue, such as riveted aircraft skins.

Reply to
Ed Huntress

Had me busting up..the ol lady was looking at me like I had gone nuts.

Great job!

Gunner

"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.

Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner

Reply to
Gunner Asch

He doesn't like to see holes drilled vertically in a spar tube.

Actually, he's right in that removing material from the top and bottom do reduce the amount of bending load the tube can ultimately take before bending permanently.

The reason is that the top and bottom are the highest stressed areas. The top skin is normally in compression and the bottom in tension.

Drilling here reduces the amount of material to carry the load.

But, like the sleeved bolt holes he mentioned, it is unnecessary - if the stresses at that point are below what the structure will stand.

Now, the two seaters that Clare mentioned (there is no such thing as a two seat ultralight in the US) are considerably heavier airplanes.

At that point they probably do need the sleeves (bushings?) in the tube to take the compression load that the wing imposes on the root connection.

At the other end of the spectrum, the mast on my sailboat (and on the bigger boats too) have only a simile bolt pinned through an unsleeved aluminum tube to secure the base of the mast. AND - the compression loads on it are right near the same as the wing root compression loads on the airplane's wing root! Interesting.

To bring it back to the thread topic...

I used grade 8's on my airplane - for the landing gear axles.

5/8" diameter by 5 to 6 inches - to fit the wheels you want ti use. The heads are cross-drilled for a 3/16 (AN-3!) bolt that attaches to the shock absorber tubes (telescoping tubes with bungees).

After drilling a 3/16 hole through the heads, there is not a lot of metal left - but we've never had a failure there.

So we can conclude that the stresses imposed here (gear loads are the highest on the whole airplane) are lower than what the bolt head can safely stand.

And - that - is all that matters.

Richard

Reply to
cavelamb himself

Excellent, Nick!

Well, better anyway, We can't judge this part right or wrong without putting numbers on it.

Like the example one fellow mentioned - hard bolts on an oil pan that started breaking when they replaced the old hard gaskets with silicone.

But this outta be a good place to start for general sizing to loads...

formatting link

Richard

Reply to
cavelamb himself

This is interesting. I know that in bolted steel-framed buildings, the shear force is intended to be carried entirely by friction. But these structures use relatively thick steel (probably 1/4" minimum) and have high factors of safety. I'm also under the impression that it's more acceptable for rivets to carry a shear force than bolts, because the shank of the rivet expands to completely fill the hole when the rivet is set.

In airframe construction, do the rivets carry a shear force in normal operation, or only when the structure is overloaded?

Best wishes,

Chris

Reply to
Christopher Tidy

I'm definitely interested in doing some reading. I'm building a small library of engineering books at the moment.

That looks like a really useful set of books. I'd like to get a copy of those. Anyone got a set they'd like to sell?

Indeed. That's where I'm building my knowledge at the moment.

Thanks for an interesting discussion!

Best wishes,

Chris

Reply to
Christopher Tidy

It's just not true that the joints in structural steel must always be designed such that the joint will never slip. It may have been true, due to conservatism, early in the transition (in the 1950s & 1960s) from rivets to bolts in steel erection. But even when I took my structural courses in the early 70s there was a design procedure for joints where the bolts bear on the periphery of their holes.

It appears the use of bearing connections has become much more common, and accepted in more situations, in the last 35 years. This is the AISC spec for bolted joints. Section 4 includes comments on the history and suitability of bearing connections.

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(I don't mean to single you out, Chris. This has come up several times before and is another candidate for the RCM dogma file. )

Reply to
Ned Simmons

That's an interesting document, Ned. Am I still right in thinking that the bearing type of joint is less common that the friction grip type of joint in structural engineering today? I'm a mechanical engineer rather than a structural engineer, so I don't deal with architectural structures on a day-to-day basis.

Best wishes,

Chris

Reply to
Christopher Tidy

I'm an ME as well and haven't had any contact with actual structural practice since I graduated in 1974. Certainly at that time friction type connections were more common, but after looking around the web a bit, I'm not sure that's true anymore. If I remember, I'll ask my daughter to ask one of the structural engineers she deals with.

Reply to
Ned Simmons

Thank you, Ned. Fascinating...

I wonder if the whole argument is not explained on page 9 of that doc?

Hydrogen embattlement of hot dipped galvanized bolds???

The "brittle" hard bolt legend (not myth?)...

Richard

Reply to
cavelamb himself

I can't answer that with authority, but the literature on joint strength in aircraft design and testing is loaded with references to rivet shear strength. When they talk about bearing in this context it's almost always about the bearing of the rivet on the hole. And the references to bearing of the faying surfaces of skin are mostly about fretting and corrosion. Also, thermal cycling is an issue with jet airliners, and maintaining sufficient rivet clamping tension, just to prevent the rivets becoming loaded in tension in service, is a big problem. Certainly a rivet loaded in tension by displacement of the skins is not contributing to friction between the skins by its clamping force.

So it looks to me like they are designed to be loaded in shear. Maybe you can find an aircraft engineer who can confirm it. If I had to research it for an article I'd be calling the engineers at the rivet manufacturers.

-- Ed Huntress

Reply to
Ed Huntress

That's pretty much my understanding of it too, Ed.

Also note that the rivets work harden from the first stroke of the gun.

But the skins don't really until the rivet is way overdriven.

Don't have a clue how to over analyze that...

Richard

Reply to
cavelamb himself

Can't tell for airframe, but rivet constructions are designed for shear. Or maybe we should say the *were* constructed for shear. Don't forget, that the holes have to be drilled (or even reamed) in place, so the two bores align perfect.

Nick

Reply to
Nick Mueller

There do exist bolts for both. Those for shearing do have a tighter tolerance on their shaft and the two mating bores have to be drilled in place while erecting the building (-> architectural).

Here's a link (in German), but you also get the picture:

Interesting enough, shearing (called SL) is not allowed with dynamically loaded constructions (cranes, bridges etc). They work with friction (called HV) and have to be precisely torqued (including rules how to regularly check the torque and the number of samples measured).

Nick

Reply to
Nick Mueller

On Sat, 19 Jan 2008 22:26:32 -0500, with neither quill nor qualm, Ned Simmons quickly quoth:

Your post reminded me of Take 6, but here are all the fun eng. jokes: Enjoy!

--snip--

Comprehending Engineers-Take One

---------------------------------------------------- A pastor, a doctor and an engineer are waiting one morning behind a particularly slow group of golfers. They see the course marshal and ask why he isn't doing something to expedite play. "They're blind firefighters," says the marshal, "They lost their sight saving our clubhouse from a fire last year, so we let them have free access to the course anytime they want." After a moment's reflection, the group responds: Pastor: "That's so sad. I think I will say a special prayer for them tonight." Doctor: "I'm going to contact an ophthalmologist friend, and see if there's anything he can do for them." Engineer: "Why can't these guys play at night?"

------------------------------------------------------------------ Comprehending Engineers-Take Two

----------------------------------------------------- In a high school gym class, all the girls are lined up against one wall, and all the boys against the opposite wall. Every ten seconds, they walk toward each other exactly half the remaining distance between them. A mathematician, a physicist, and an engineer are asked, "When will the girls and boys meet?" Mathematician: "Never." Physicist: "In an infinite amount of time." Engineer: "Well... in about two minutes, they'll be close enough for all practical purposes."

------------------------------------------------------------------ Comprehending Engineers-Take Three

------------------------------------------------------ There was an engineer who had an exceptional gift for fixing all things mechanical. After serving his company loyally for over 30 years, he happily retired. Several years later his company contacted him regarding a seemingly impossible problem they were having. One of their multi-million dollar machines wasn't working and no one knew how to fix it. The engineer reluctantly took the challenge. He spent a day studying the huge machine. At the end of the day he marked a small "x" in chalk on a particular component of the machine and proudly stated, "Replace this part and the machine will work." The part was replaced and the machine worked perfectly once again. The company received a bill for $50,000 from the engineer. They demanded an itemized accounting of his charges. The engineer responded: One chalk mark ........ ..... ..... $1 Knowing where to put it ... $49,999

------------------------------------------------------------------ Comprehending Engineers-Take Four

----------------------------------------------------- Three engineers and three mathematicians are traveling by train to a conference. At the station, the three mathematicians each buy tickets and watch as the three engineers buy only a single ticket. "How are three people going to travel on only one ticket?" asks a mathematician. "Watch and see," replies an engineer. They all board the train. The mathematicians take their respective seats, but all three engineers cram into a restroom and close the door. Shortly after the train departs, the conductor comes around collecting tickets. He knocks on the restroom door and says, "Ticket, please." The door opens just a crack and a single arm emerges with a ticket in hand. The conductor takes it and moves on. The mathematicians see this and agree it is quite a clever idea. After the conference, the mathematicians decide to copy the engineers on the return trip and save some money. They buy a single ticket for the return trip, but are astonished to see that the engineers don't buy any ticket at all. "How are you going to travel without a ticket?" asks one perplexed mathematician. "Watch and see" is the answer. They board the train, the three mathematicians cram into one restroom and the three engineers cram into another one nearby. Shortly after the train departs, one of the engineers leaves his restroom and walks over to the restroom where the mathematicians are hiding. He knocks on the door and says, "Ticket, please."

------------------------------------------------------------------ Comprehending Engineers-Take Five

----------------------------------------------------- The Top 10 Things Engineering School didn't teach

  1. There are at least 10 types of capacitors.
  2. Theory tells you how a circuit works, not why it does not work.
  3. Not everything works according to the specs in the databook.
  4. Anything practical you learn will be obsolete before you use it, except the complex math, which you will never use.
  5. Always try to fix the hardware with software.
  6. Engineering is like having an 8 a.m. class and a late afternoon lab every day for the rest of your life.
  7. Overtime pay? What overtime pay?
  8. Managers, not engineers, rule the world.
  9. If you like junk food, caffeine and all-nighters, go into software.
  10. Dilbert is a documentary.

------------------------------------------------------------------ Comprehending Engineers-Take Six

--------------------------------------------------- Q: What is the difference between Mechanical Engineers and Civil Engineers? A: Mechanical Engineers build weapons, Civil Engineers build targets.

------------------------------------------------------------------ Comprehending Engineers-Take Seven

--------------------------------------------------- Two engineering students were walking across campus when one said,"Where did you get such a great bike?"

The second engineer replied, "Well, I was walking along yesterday minding my own business when a beautiful woman rode up on this bike. She threw the bike to the ground, took off all her clothes and said, 'Take what you want.'"

"The second engineer nodded approvingly, "Good choice; the clothes probably wouldn't have fit."

------------------------------------------------------------------ Comprehending Engineers-Take Eight

--------------------------------------------------- To the optimist, the glass is half full. To the pessimist, the glass is half empty. To the engineer, the glass is twice as big as it needs to be.

------------------------------------------------------------------ Comprehending Engineers-Take Nine

--------------------------------------------------- Normal people ... believe that if it ain't broke, don't fix it. Engineers believe that if it ain't broke, it doesn't have enough features yet."

------------------------------------------------------------------ Comprehending Engineers-Take Ten

--------------------------------------------------- An architect, an artist and an engineer were discussing whether it was better to spend time with the wife or a mistress.

The architect said he enjoyed time with his wife, building a solid foundation for an enduring relationship.

The artist said he enjoyed time with his mistress, because of the passion and mystery he found there.

The engineer said, "I like both."

"Both?"

Engineer: "Yeah. If you have a wife and a mistress, they will each assume you are spending time with the other woman, and you can go to the lab and get some work done."

------------------------------------------------------------------ Comprehending Engineers-Take Eleven

--------------------------------------------------- An engineer was crossing a road one day when a frog called out to him and said, "If you kiss me, I'll turn into a beautiful princess".

He bent over, picked up the frog and put it in his pocket.

The frog spoke up again and said, "If you kiss me and turn me back into a beautiful princess, I will stay with you for one week."

The engineer took the frog out of his pocket, smiled at it and returned it to the pocket.

The frog then cried out, "If you kiss me and turn me back into a princess, I'll stay with you and do ANYTHING you want."

Again the engineer took the frog out, smiled at it and put it back into his pocket.

Finally, the frog asked, "What is the matter? I've told you I'm a beautiful princess, that I'll stay with you for a week and do anything you want. Why won't you kiss me?"

The engineer said, "Look I'm an engineer. I don't have time for a girlfriend, but a TALKING frog, now that's cool."

------------------------------------------------------------------ Comprehending Engineers-Take Twelve

--------------------------------------------------- Several engineers are standing around one day trying to decide what type of engineer must have designed the human body. (All right, for the purpose of the joke there is an assumption of some sort of higher being that actually designed the human body.....work with me people.)

The chemical engineer says "the human body was designed by a chemical engineer. Look how the body takes in nutrients and then turns them into energy and body parts just by re-organizing a few chemical bonds."

The electrical enginner says "the human body was clearly designed by an electrical engineer. Just observe how tiny electrical impulses cause the muscles to move, cause the person to feel, see and listen to all that is happening around them. And finally look how a few very tiny tiny electrical impulses can store a memory for a lifetime, and yet bring that information back at a moments notice. Clearly the work of a brillaint electrical engineer."

The mechanical engineer says "bahh! The human body was designed by a mechanical engineer. Notice how the muscles and the bones work to make the body move. Notice how the organs work to move the food and other nutrients around to the places where they are needed."

Finally the Civil engineer pipes up and says "you're all wrong. The human body was designed by a civil engineer. Who else would put a waste treatment plant right next to a recreational facility?"

--snip--

-- You cannot depend on your eyes when your imagination is out of focus. -- Mark Twain

Reply to
Larry Jaques

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