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Author Topic: Tensile strength of 4-40 aluminum blind nut  (Read 2802 times)

Offline Matt Piatkowski

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Tensile strength of 4-40 aluminum blind nut
« on: September 24, 2018, 08:40:51 AM »
Hello,
Has anybody ever tested the tensile strength of the 4-40 aluminum blind nut and the matching steel screw?
Assuming the blind nut support will not yield, the strength of this connection is defined by the shear stress in the aluminum thread.
Thank you,
M

Online Gerald Arana

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #1 on: September 24, 2018, 09:39:20 AM »
What are you using it for? If you want to "save" weight and use it for the engine, I'd say "No way!" use steel.
But, if you are using it on something like a hatch cover, I'd say "Sure"!

Good luck, Jerry

Online Brett Buck

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #2 on: September 24, 2018, 10:03:59 AM »
Hello,
Has anybody ever tested the tensile strength of the 4-40 aluminum blind nut and the matching steel screw?

I was unaware that something like this existed, what with engineers not being particularly given to bizarre flights of fancy.  Do you have a link to such an item?

   I know this is tilting at the same windmill (but Howard may be amused...) - just use regular blind nuts or a nut plate. You are having problems with very basic of fundamental issues of construction, trim, and power that everybody else has. You do not need to be screwing around with strange fringe topic -  get the basics correct first, then worry about some trivial non-issues if you must.  Stunt is hard enough without making it harder on yourself. Holding engines on airplanes is a fully-solved issue, do it like everyone else does it, and deal with your other problems first.

       Brett

Offline Tim Wescott

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #3 on: September 24, 2018, 10:44:58 AM »
I was unaware that something like this existed,

Matt, are you mistaking a galvanized T-nut with aluminum?  Get a magnet close to it, see what happens.

what with engineers not being particularly given to bizarre flights of fancy.

Uh, Brett -- haven't you worked with guys with PhD's and titles like "director of advanced development" or "systems engineer"?
« Last Edit: September 24, 2018, 12:20:01 PM by Tim Wescott »
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Offline Scott Richlen

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #4 on: September 24, 2018, 12:12:07 PM »
At Thiokol the engineers would work in a "bullpen" with the PhDs in offices on the perimeter.  Usually with doors closed.  They were not high in inter-personal skills and did best when you you didn't disturb their flights of fancy.  Best method was to write down your request on a sheet of paper and then slide it under their door.... ;D

Online Brett Buck

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #5 on: September 24, 2018, 12:49:00 PM »
Matt, are you mistaking a galvanized T-nut with aluminum?  Get a magnet close to it, see what happens.

Uh, Brett -- haven't you worked with guys with PhD's and titles like "director of advanced development" or "systems engineer"?

     You mean the Advanced Development types who gave use the U-2, A12/SR71, F117, and who had designed a fully functional spy satellite that could take pictures from 100 miles at 18000 miles an hour - a year before anyone had launched a satellite of any sort and when most computers ran on rack after rack of 6SN7 tubes?    Yes, all of those and more. They are rarely the problem. In a strictly engineering sense, we could have put a man on Mars in 1980 using 50's/60's technology. The engineering is not the problem. There are plenty of "absent-minded professor types around", we know where they need to go and how to manage them.

     As you appear to be associated with the fringes of engineering somehow, here's some advice from someone nearing the end - to be able to think "out of the box" successfully, you first need to know why people built the box that way in the first place. Only then can you conceive of where the limitations of The Box might be exceeded or overcome. The box was built that way for a very good reason by people as smart or smarter than you and I over many decades. Unless you really, really understand it, you aren't going to operate outside it successfully, unless you are just lucky.

    Matt (and many other people here) tend to fall prey to these side issues without first having mastered the basics, like putting "aluminum blind nuts" (which probably isn't what he really means, although I could be wrong) to save 1/2 a gram while possibly creating durability issues - on a rudimentary profile trainer that is grossly out of trim. It's like wanting to enter a Formula 1 race and saying "I can save 1/2 ounce of unsprung weight by drilling out the center of the suspension bolts!" and failing to note that the car is a stock 67 Impala. "Can't see the Forest for the Trees", "polishing the cannonball", whatever aphorism resonates.

   This is certainly not intended to be insulting, I was in the same boat in stunt for maybe 15 years. But you must overcome it, accept that other people have figured out a very good system, and that mastering the simplest things has to be done before you can move on to the more complex or obscure. That means your efforts have to be first directed towards studying and experimenting within The Box to ensure you really master it.

       Brett

Offline Tim Wescott

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #6 on: September 24, 2018, 01:00:38 PM »
As you appear to be associated with the fringes of engineering somehow...

Yes, Brett, I'm "associated with the 'fringes' of engineering somehow", since I hold a Master's degree in EE, I've been earning money doing circuit, software, and systems design for nearly 30 years now, and I have one professional book, a book chapter, and a couple of articles published.  In much the same way, you're somehow associated with the "fringes" of control line stunt.
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Offline pat king

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #7 on: September 24, 2018, 01:08:53 PM »
One should be very careful using any steel screw in an Aluminum nut. Without proper antisieze the Aluminum will gall and the screw will become a permanent fixture in the nut. Steel screws in tapped holes in Aluminum parts must be lubricated with the proper material to ensure being able to disassemble the two. Some heat treated Aluminum is stronger than mild steel. Heat treating any metal does not make it stiffer. Heat treated Aluminum is still as flexible as non heat treated Aluminum. As everyone has said, use a steel nut. If you want to save weight, go with a steel 2-56 nut and screw.

Pat
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Offline Tim Wescott

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #8 on: September 24, 2018, 01:20:00 PM »
One should be very careful using any steel screw in an Aluminum nut. Without proper antisieze the Aluminum will gall and the screw will become a permanent fixture in the nut.

This has not been my experience with using tapped holes in 2024 and 6061 aluminum for various bits & bobs over the years, including engine mounting pads for profiles.  For that matter, there's a whole lot of aluminum model airplane crankcases out there that have backplates & heads bolted to them that have not seized.

If you want to save weight, go with a steel 2-56 nut and screw.

Especially if you're holding an engine on.  When your 40FP flies off of the nose, the airframe will be a LOT lighter.
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Offline Dave Hull

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #9 on: September 24, 2018, 03:21:59 PM »
Matt,
I have not seen any aluminum T-nuts—but I sure did not go looking for them, either!

Therefore I have not tested any, and do not intend to. Here is why.

Four DuBro 4-40 T-nuts (P/N 135) weigh 1.8 grams. If these were made of aluminum, they would weigh 0.62 grams, and you would have saved 1.18 grams.  A better place to save that amount of weight would be the landing gear you are using.

The strength of aluminum varies greatly with alloy and temper. You don’t say what the proposed parts are made from or their temper. Without that information, all anyone will give you is a bounding number. So here is mine. One of the companies that specializes in aluminum fasteners uses ASTM 211 materials for nuts. (They do not offer aluminum T-nuts in their catalog, but they might have them as “specials.” They may not go down to size you are interested in, though.) The simplest approximation would be to assume exactly the same geometry and derate the aluminum part based on the ratio of shear strengths. This means the aluminum part would be half the strength of the steel part using likely materials values, such as 1018 for the comparable steel part.

More rigorously, use the shear area of the major diameter and properties for 6061-T6 with a shear strength of around 25ksi. If you don’t do stress analysis, then you are stuck breaking test parts if you want a real answer. If you find yourself stripping the steel .112-40UNC T-nuts that hold your engine down, you are not going to be happy with an aluminum replacement. I use T-nuts when I bury them in structure, so stripping a nut takes work to repair and refinish and is grief I just don’t want. All to save 1.18 grams. Bad choice. If you are using them on the outside of a profile, then there is no repair work. Just keep a few extras in your flight box. Since four of them weigh less than a gram, you won’t notice the extra weight in the toolbox.

Be aware that how threaded fasteners work is that the nut is always of “softer” material than the screw. The nut micro-yields as it is fully torqued, which allows load sharing. Otherwise, all of the tension preload would be in the first thread or two. Since the screw stretches under this same load, you can see that something has to give to result in a useable bearing area. The nut.

If you are going to mix steel and aluminum you need to be aware of galling. A stainless screw in a soft aluminum nut with no lubrication is going to be problematic. I suspect an alloy steel screw with a black oxide finish would be slightly better. This is not a combination used in aerospace—for a dozen good reasons—so you are going to discover each of these things yourself. By the way, LocTite is considered a lubricant when you calculate torques and preloads for fastener sizing. It would probably reduce galling issues with aluminum and save you some grief.

A typical hobbiest never uses a torque screwdriver. Since wrench handles scale with drive size (whether hex, or Phillip, or torx or…..) and the fastener drive size is scaled to the fastener strength, using an aluminum component is going to mislead you…especially when the nut is aluminum. Making stripped threads even more likely. Cross-threading is also going to be a greater issue.

I use custom hardware on racing planes and put 4-40 and 2-56 threads into 6061-T6 aluminum (and some 2024 an 7075 when I have it) all the time. In general, I don’t count on threading an aluminum engine mounting plate for attaching the engine. I use it in a through and through mount—with steel blind nuts on the far side, usually buried under a balsa tripler covered with glass and epoxy. In short, I don’t want stripped threads!

I get the idea of trying new stuff. I also sense in others and sometimes personally feel the frustration of thinking someone wants help, but actually is just floating ideas. The tricky thing is that people want their hobby to be one of discovery. “Hey, I found a great new way to……”  But they find out most of the hobby has advanced 40 years beyond the point they want to start from. It is disappointing to be that person who wants to discover new stuff and come to that realization. So my thought is this:  go do the tests and the experimenting and feel the discovery.  But realize that getting on the web and then asking guys who have already done that very thing—maybe 20 years ago—is tantamount to looking in the back of the textbook for the answer and cheating yourself of your own personal discovery.

I kind of came to this epiphany when I read and began to participate in the discussion on lubricating oils for alcohol fuels. For his own reasons that initially made no sense to people, a forum member wants to find a better/cheaper/more available oil than one of the proven standards that is just as cheap and available as the alternatives. In short, he wants to discover something new and valuable. But instead of doing the experimenting, he polled the forum—in effect, cheating himself of the discovery. And, as an unintended side effect, kind of frustrated the folks who attempted to save him time, money, and his own frustrating journey by giving him the answer. Because he didn’t really want the answers he got—he wanted to experiment and discover something.

It’s your hobby, go where it leads you. But don’t be upset if no one wants to go along that particular path you are taking…because they may have already walked it, or they may have taken prior knowledge and applied it already.  Square earth society, you know….

Dave

PS—For reference, I have had no issues with quality using the DuBro T-nuts. I have had to replace a lot of T-nuts in ARFs that I built. Just not impressed with the stamped parts.

Offline Curare

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #10 on: September 24, 2018, 03:35:59 PM »
I believe I have come across these little beauties before. While they may not be aluminium, they are from something equally soft - maybe chewing gum.

Hobbyking sell them (or sold them, once was enough). They'll strip out at the drop of a hat.
Greg Kowalski
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Offline Tim Wescott

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #11 on: September 24, 2018, 04:01:44 PM »
I believe I have come across these little beauties before. While they may not be aluminium, they are from something equally soft - maybe chewing gum.

Hobbyking sell them (or sold them, once was enough). They'll strip out at the drop of a hat.

I can't speak to the Hobbyking ones, but just from experience with modifying them, all the T-nuts I've used are of a harder alloy than dead-soft steel.  A t-nut made from really cheap steel, with some equally cheap threading process, would be pretty bad, and would still attract a magnet.
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Offline Curare

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #12 on: September 24, 2018, 05:29:01 PM »
I just checked the ones I have (and refuse to use for anything other than hatch hold downs) - they're magnetic, but weakly so.
Greg Kowalski
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Offline Matt Piatkowski

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #13 on: September 25, 2018, 03:12:02 PM »
Hello,
These are steel 4-40 blind nuts from Du-Bro - they stick to the magnet.

I am a little ashamed by such fundamental mistake...old age perhaps?

Thank you for your comments.
Regards,
M










Offline Tim Wescott

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #14 on: September 25, 2018, 03:23:41 PM »
Hello,
These are steel 4-40 blind nuts from Du-Bro - they stick to the magnet.

To answer your implied question -- I haven't, but the quality does vary.  Buy from reputable sources.  I like RTL for bulk buys, and DuBro for ordinary.  If you want to go outside of normal model channels, McMaster is absurdly expensive compared to other machine shop supply houses, but their service and quality is top notch.

The only failures that I've experienced are the nuts spinning on the wood -- I haven't even had one fall prey to cross-threading, that I know of.

I routinely hold down 46LAs and Tower 40s with 4-40 hardware, and have yet to have one part ways with an airframe.
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Offline Mike Haverly

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #15 on: September 25, 2018, 04:45:18 PM »
Hello,
These are steel 4-40 blind nuts from Du-Bro - they stick to the magnet.

I am a little ashamed by such fundamental mistake...old age perhaps?

Thank you for your comments.
Regards,
M

Over thinking causes a lot of trouble.  I think it might be a curse of the over-educated.  My high school education didn't take me that far, I just follow along. 

Only kidding here.
Mike

Online Dave_Trible

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #16 on: September 25, 2018, 04:56:01 PM »
The way this stuff usually works out is you use the aluminum nuts and save a gram or two- then you have to add 1 1/2 ounces of lead to the nose to balance.....how many Fox .35s had the backplate filled with lead?  :-))).

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Offline Tim Wescott

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #17 on: September 25, 2018, 04:56:41 PM »
Over thinking causes a lot of trouble.  I think it might be a curse of the over-educated.  My high school education didn't take me that far, I just follow along. 

The highly educated apply heuristics to empirical data.  But you can give it a whirl, and use what you learn to make a guess.
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Online Ken Culbertson

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #18 on: September 25, 2018, 05:56:44 PM »
I just use 6-32's  Even the crappy ones hold better.

For what it is worth, I ran the machines that make this stuff when I was in college.  It was very difficult to tap aluminum,  It dulls the taps quickly and will snap them in a heartbeat if you don't have your rpm's right.  Steel is much easier to tap.  Dies were different.  Don't know why but it was not hard to make an aluminum screw.

Ken
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Offline FLOYD CARTER

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #19 on: September 25, 2018, 06:58:30 PM »
I like Dave's response.  It's longer than Brett's.  I guess it took longer to compose.

Long ago, I was accused of writing technical reports that were too long.  I then learned to be concise.
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Offline Howard Rush

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #20 on: September 25, 2018, 10:36:03 PM »
Has anybody ever tested the tensile strength of the 4-40 aluminum blind nut and the matching steel screw?
Assuming the blind nut support will not yield, the strength of this connection is defined by the shear stress in the aluminum thread.

1. If you are not designing it, but testing an existing part, who cares about that assumption or the assertion about the thread?

2. The nonferrous blind nuts I used broke where the barrel attached to the flange. 
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Offline Lauri Malila

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #21 on: September 26, 2018, 10:12:34 AM »
I have allways hated the usual blind nuts, I prefer my own aluminium (#2024-t3) ones.
If thread lenght is more than 6x pitch, there is usually no problems with strenght or longevity. But I allways make them full lenght through the material where they are glued. That way they won't crush the wood and come loose.
In aluminium 4-40 is slightly better than metric equivalent M3, but made with good tools to good material, both are fine. So fine that in my torque tests the screw allways fails first.

Lauri

Offline Howard Rush

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #22 on: September 26, 2018, 10:38:50 AM »
Don't know why but it was not hard to make an aluminum screw.

It was hard for me to make anything screw.
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Offline Paul Smith

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #23 on: September 26, 2018, 12:02:26 PM »
I have often been disappointed with the high failure rate of 4/40 blind nuts.

Two failure modes:

Thread fouled with glue.  This is best dealt with by screwing in a lubricated screw whilst gluing is in process.  If the has failed, a heated bolt will often clear the threads.

On the 4/40, the diameter (and also the radius) of the prong circle is too small compared to the torque needed to secure the bolt.  The prongs often tear loose from the wood.  In contrast, 6/32 and M3 blind nuts have a bigger and more reliable prong circle.

Paul Smith

Offline Dane Martin

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #24 on: September 26, 2018, 12:04:24 PM »
It was hard for me to make anything screw.

High school was difficult indeed.

Online Ken Culbertson

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #25 on: September 26, 2018, 12:14:25 PM »
High school was difficult indeed.
Yes but you still can be on the Supreme Court.  Well maybe not.  Remember that girl that you never had time for?  She is out there ready to take you down.  Flying Airplanes and playing Tennis sure kept me out of a lot of trouble in HS!  The lesson to be learned from this is to use 6-32.

Ken
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Offline phil c

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Re: Tensile strength of 4-40 aluminum blind nut
« Reply #26 on: September 29, 2018, 06:16:30 PM »
I have often been disappointed with the high failure rate of 4/40 blind nuts.

Two failure modes:

Thread fouled with glue.  This is best dealt with by screwing in a lubricated screw whilst gluing is in process.  If the has failed, a heated bolt will often clear the threads.

On the 4/40, the diameter (and also the radius) of the prong circle is too small compared to the torque needed to secure the bolt.  The prongs often tear loose from the wood.  In contrast, 6/32 and M3 blind nuts have a bigger and more reliable prong circle.



I think microfasteners.com has 4/40 blind nuts that are larger, ~1/4 inch.  I've gotten something similar from SIG also.
phil Cartier


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