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Anyone know the shear strength of 1/4" plastic rivets?

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Tonimus

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In short, I do not. But they are going to have different shear resistance if the are oriented like a "+" or a "x" with the direction of travel.
 

Bat-mite

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In short, I do not. But they are going to have different shear resistance if the are oriented like a "+" or a "x" with the direction of travel.
I don't follow you. I use them to hold my payload tube to my coupler. I'm using five #8-32 nylon screws for shear pins, and I want to use enough rivets to make sure it shears at the screws and not the rivets. I was planning to go with five unless I can get some real numbers.

Maybe this is a better question: what is "tensile strength" vs. "double shear" on a 6/6 nylon data sheet?
 

Graduator

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Here is an explanation of single vs double sheer http://www.engineeringarchives.com/les_mom_singledoubleshear.html
The style of plastic rivets that Madcow has are commonly known as Click-Lock Shanks.
See this for another example http://www.mcmaster.com/#standard-nylon-rivets/=13oh1p2
I searched around a bit, but could not find any specs. for their shear strength.
How big is your rocket and what material is it? I've never seen anyone use five #8-32 nylon shear pins.
A typical arrangement would be three #2-56 nylon shear pins, then three #4-40 pins once your NC gets over maybe 10lbs.
All of my 2-4" fiberglass rockets are set up with four #4-40 Steel screws at the coupler junction and three #2-56 nylon shear pins at the NC.
 

mccordmw

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I was planning on doing the same by securing the top airframe to the coupler with steel screws and #2-56 shear pins in the nosecone. Are we allowed to use metal screws like that? I'm always fuzzy on how much metal we can use.
 

dixontj93060

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I was planning on doing the same by securing the top airframe to the coupler with steel screws and #2-56 shear pins in the nosecone. Are we allowed to use metal screws like that? I'm always fuzzy on how much metal we can use.
Metal screws are fine.
 

Onebadhawk

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John,,
As rockets get bigger forces get greater...
Those plastic rivets that worked fine on your 3" build aren't strong enough for your 5" build...
Take a steel 8/32" screw,, coat the threads in grease..
Put a 8/32nd tee nut on the inside of the coupler tube upside down, so the flat surface is against the wall of the coupler tube...
Screw in the 8/32" screw from the outside of the coupler, snug the tee nut,, a bit more grease on the protruding male threads of the screw...
Bury the tee nut in epoxy ( thicken with chopped glass if necessary to keep it from running )...
After the epoxy hardens, the screw should unscrew out easily as the epoxy won't stick to the greased threads..
You now have a perfectly aligned steel bolt threading into steel threads....

( Of course,, all holes must be drilled while the coupler tube is installed in the airframe tube so you have perfect alignment there as well )....

Master this technique and you'll never use plastic rivets again......

Teddy
 

Bat-mite

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Here is an explanation of single vs double sheer http://www.engineeringarchives.com/les_mom_singledoubleshear.html
The style of plastic rivets that Madcow has are commonly known as Click-Lock Shanks.
See this for another example http://www.mcmaster.com/#standard-nylon-rivets/=13oh1p2
I searched around a bit, but could not find any specs. for their shear strength.
How big is your rocket and what material is it? I've never seen anyone use five #8-32 nylon shear pins.
A typical arrangement would be three #2-56 nylon shear pins, then three #4-40 pins once your NC gets over maybe 10lbs.
All of my 2-4" fiberglass rockets are set up with four #4-40 Steel screws at the coupler junction and three #2-56 nylon shear pins at the NC.
It's a Formula 200 -- 127 inches tall, 8 inches diameter, all G12 and G10. Finished nose cone weighs 10.8 pounds.

Where did you come up with those numbers?

The minimum shear strength of a #8-32 6/6 nylon screw is 103 PSI. If I have to retain 10.8 pounds of NC at the apogee charge, then five pins at 103 PSI = 515 PSI. Divide by the 10.8, and that means my acceleration at deployment is limited to about 47G. My home computer with OR on it is down, so I don't know the expected acceleration due to the apogee charge. But I'm guessing it is even more than 47G.

Three #4-40 pins only gets you 150 PSI of retention at apogee. Divide that by 10.8, and you get a max allowable acceleration of 13.8G.
 

MCriscione

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John, It sounds like you're concerned about the shock load transmitted to the nosecone shear pins during apogee ejection.

If you haven't already, consider minimizing that load instead or in addition to looking at different materials for retention. Using a nylon harness instead of Kevlar (or part nylon at least), lengthening the harness or bundling small segments with tape that will break away during ejection. All of these will reduce the shock load. If my understanding is correct, switching Kevlar to Nylon could reduce it by 3X or more (depends on TK/TN size / tensile strength to load ratio).

I know, I'm new to all this, but it came to mind, and if you hadn't already thought of it, it's worth considering. Sorry for telling you what you probably already know!
 

Bat-mite

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John, It sounds like you're concerned about the shock load transmitted to the nosecone shear pins during apogee ejection.

If you haven't already, consider minimizing that load instead or in addition to looking at different materials for retention. Using a nylon harness instead of Kevlar (or part nylon at least), lengthening the harness or bundling small segments with tape that will break away during ejection. All of these will reduce the shock load. If my understanding is correct, switching Kevlar to Nylon could reduce it by 3X or more (depends on TK/TN size / tensile strength to load ratio).

I know, I'm new to all this, but it came to mind, and if you hadn't already thought of it, it's worth considering. Sorry for telling you what you probably already know!
No, actually I'm trying to make sure that the main ejection charge does not cause the payload tube to separate from the coupler. IOW, I want it to shear at the pins, not at the rivets.

Yes, I can use metal; but there has to be a "right number of rivets" to get this to work. They are so much easier.
 

MCriscione

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Sorry, I suppose it works both directions though. If you could use #4 shear screws instead of #8's you wouldn't need such a large charge, and 1/4" rivets would work for you. And you could consider using smaller shear screws if your drogue deployment shock was smaller, right?
 

Bat-mite

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Sorry, I suppose it works both directions though. If you could use #4 shear screws instead of #8's you wouldn't need such a large charge, and 1/4" rivets would work for you. And you could consider using smaller shear screws if your drogue deployment shock was smaller, right?
I'm with you on the last sentence. No need to yank on the cone if I have the right size charge. But it is really hard to get a super-accurate charge size on the ground as compared to in the air.

Well, if I miscalculate, I'll have a really looooong walk, but it will come down safely.
 

manixFan

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...Those plastic rivets that worked fine on your 3" build aren't strong enough for your 5" build....
I've used plastic rivets exclusively on all my builds including a couple of 50# (loaded) rockets, a 6" and a 7.5"flown on N motors. I've never had a failure with any of the rivets in any of my launches.

Your method seems very simple and I'm sure produces great results. As do plastic rivets for me.

What works, works.


Tony
 

bobkrech

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Attached is the results of a spreadsheet calculation of the forces and velocities of a Formula 200 rocket with a 10.8 NC and a 10 psid ejection charge pressure.

View attachment Formula200-10psid.pdf

A 10 psid ejection charge pressure will generate ~500 pounds of force (thrust). It will provide a ~45.5 G initial separation acceleration. Drag and shock chord length will lower this value. If your value for the shear strength of the 8-32 shear screws is correct, I strongly suggest you use only 3 of them which would hold off 27 G deceleration forces created by the shock chord.

I personally think that's too much force because the ejection charge pressure is too high. Since the diameter is large, you should be fine with 5 psid. This lowers the peak separation thrust to 250 pounds and the peak acceleration to ~22G. This would allow you to use (4) 4-40 shear screws that would require 150 pounds to shear and provide shock retention to 14 G or 150 pounds force.

The shear resistance of the 1/4 rocket rivets is going to be higher than the 8-32 shear screws probably by approximately the diameter ratio squared. If you have 1 rocket rivet per shear screw you should be fine.

View attachment Formula200-5psid.pdf

YMMY
 

Tonimus

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In short, I do not. But they are going to have different shear resistance if the are oriented like a "+" or a "x" with the direction of travel.
I don't follow you. I use them to hold my payload tube to my coupler. I'm using five #8-32 nylon screws for shear pins, and I want to use enough rivets to make sure it shears at the screws and not the rivets. I was planning to go with five unless I can get some real numbers.

Maybe this is a better question: what is "tensile strength" vs. "double shear" on a 6/6 nylon data sheet?
The plastic rivets I use have four fingers on the larger piece. The ones you have may have two, three or four. If the gap between the fingers is aligned with the direction of shear, the shear strength will be less than if the gaps are not aligned with the direction of shear. The gap in effect gives shearing a head start.

If you're using 5 #8-32 screws, I'd use 10 rivets. It'd be symmetrical and match the pattern of the shear pins.
 

Bat-mite

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Attached is the results of a spreadsheet calculation of the forces and velocities of a Formula 200 rocket with a 10.8 NC and a 10 psid ejection charge pressure.

View attachment 298815

A 10 psid ejection charge pressure will generate ~500 pounds of force (thrust). It will provide a ~45.5 G initial separation acceleration. Drag and shock chord length will lower this value. If your value for the shear strength of the 8-32 shear screws is correct, I strongly suggest you use only 3 of them which would hold off 27 G deceleration forces created by the shock chord.

I personally think that's too much force because the ejection charge pressure is too high. Since the diameter is large, you should be fine with 5 psid. This lowers the peak separation thrust to 250 pounds and the peak acceleration to ~22G. This would allow you to use (4) 4-40 shear screws that would require 150 pounds to shear and provide shock retention to 14 G or 150 pounds force.

The shear resistance of the 1/4 rocket rivets is going to be higher than the 8-32 shear screws probably by approximately the diameter ratio squared. If you have 1 rocket rivet per shear screw you should be fine.

View attachment 298816

YMMY
Thanks, Bob. Super, super helpful as always.
 

Bat-mite

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The plastic rivets I use have four fingers on the larger piece. The ones you have may have two, three or four. If the gap between the fingers is aligned with the direction of shear, the shear strength will be less than if the gaps are not aligned with the direction of shear. The gap in effect gives shearing a head start.

If you're using 5 #8-32 screws, I'd use 10 rivets. It'd be symmetrical and match the pattern of the shear pins.
I think I have the same ones (4). Makes sense about the gaps. If I follow Bob's research above, I may get it down to three 8-32, and I guess 3 rivets aligned with them. Or I could double them in line and use six.
 

Onebadhawk

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I've used plastic rivets exclusively on all my builds including a couple of 50# (loaded) rockets, a 6" and a 7.5"flown on N motors. I've never had a failure with any of the rivets in any of my launches.

Your method seems very simple and I'm sure produces great results. As do plastic rivets for me.

What works, works.


Tony
Tony,
Don't they degrade over time and launches for you ??
The little side flaps that expand and hold would break off after a few launches..
Do you reuse then at all ?? Or new on each flight / expendable ??
I find the installation I described more time consuming and labor intensive of course,,
then just drilling for the rivets...
But once the tee nut is epoxied in place you can't get quicker, more convenient or solid then a screw in a nut...

Teddy
 

Bat-mite

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Tony,
Don't they degrade over time and launches for you ??
The little side flaps that expand and hold would break off after a few launches..
Do you reuse then at all ?? Or new on each flight / expendable ??
I find the installation I described more time consuming and labor intensive of course,,
then just drilling for the rivets...
But once the tee nut is epoxied in place you can't get quicker, more convenient or solid then a screw in a nut...

Teddy
I never reuse mine.

I'm using your method for my Unistrut rail buttons. I guess I don't like the look of machine screws sticking out of my payload bay. A shiny black rivet just fits the blue/black theme of the rocket better.

True, your method is more work up front and cheaper down the road, and guaranteed not to shear. Decisions, decisions. Maybe I won't be able to decide, and I'll just scrap the whole thing.

Anyone want a half-built Formula 200? :facepalm:
:point: Kidding, of course.
 

manixFan

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Tony,
Don't they degrade over time and launches for you ??
The little side flaps that expand and hold would break off after a few launches..
Do you reuse then at all ?? Or new on each flight / expendable ??
I just ordered 400 rivets from Amazon so I'd have plenty for BALLS. So the answer is I don't reuse them. My total cost shipped for 400 rivets was $20.68. Pretty cheap for an ample supply.

Not to belabor the point but in a race between screwing in several screws and pushing in an equal number of rivets, I think I'd stand a good chance of winning. And not worrying about losing the screws required to hold my rocket together is just one less thing to deal with. I have small packages of rivets in several locations in my range gear.

The only downside is getting the rivets out can be a challenge without damaging the surrounding paint. I've gotten pretty good at using a sharp pocket knife to slide under the rivet and pull it up. Since I don't reuse them I'm not concerned about damage.

As I mentioned I think your method sounds very good and clearly screws are an excellent solution. But for me the plastic rivets work well and since that's what I learned, that's what I've stuck with. I have found consistency to be the key to success. The fewer variables from rocket to rocket the less likely I am to goof something up.


Tony
 
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