# #4 shear pins

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When you get it back...I use 1.2 g of BP in my wildman jr with 2 4-40 pins from McMaster Carr...feel free to do the math backwards! for that one, perhaps a little metal CA'd to the inside of the tube to make sure you get a good cut & not tear your nice paint job?? :y:

When's the :tree: rescue mission? Trey likes them :marshmallow:

https://www.rocketmaterials.org/datastore/cord/Shear_Pins/index.php

Load required to fail (x)-#4-40 Nylon Screws.

(2) #4-40 = 81 pounds +- 3.3 pounds, +/- 3 sigma = 71 TO 91 pounds

(3) #4-40 = 115 pounds +- 7.6 pounds, +/- 3 sigma = 92 TO 138 pounds

(4) #4-40 = 152 pounds +- 7.1 pounds, +/- 3 sigma = 132 TO 174 pounds

Bob

I am in the process of determining the number and size of shear pins for my first effort at a dual deploy rocket. It's a big one at 5.38 x 8ft, will weight about 18-19 pounds loaded and will fly on K's. I think I have a basic understanding of shear pins and loads but still am missing something here. Bob, can you please explain to me your equations above, in particular the +- pounds and +/- sigma in each? I then still need to determine the required psi for ejection and I know it will be based somewhat on the shear pin calculations but what about "G"s on the rocket at ejection? Anything else I am missing here?

Using 4-40's is overkill!
I only use 2-56 for shear pins.
My L3 used three 2-56, it held a 2+lb x 6.1" nose cone on a 51lb rocket without trouble.

JD

I am in the process of determining the number and size of shear pins for my first effort at a dual deploy rocket. It's a big one at 5.38 x 8ft, will weight about 18-19 pounds loaded and will fly on K's. I think I have a basic understanding of shear pins and loads but still am missing something here. Bob, can you please explain to me your equations above, in particular the +- pounds and +/- sigma in each? I then still need to determine the required psi for ejection and I know it will be based somewhat on the shear pin calculations but what about "G"s on the rocket at ejection? Anything else I am missing here?
Not Bob here, but I understand it and can explain it: The +/- part represents the variation seen in the test results that you should use for design purposes. For the 2 screw example, 81 pounds is the mean (average) of all the tests, 3.3 pounds is one standard deviation, and 81 lbs+/-3*3.3 lbs = 71 lbs to 91 lbs is the range one could expect the results to be within 99.7% of the time. This comes from probability theory in statistics for a normal (bell curve) distribution. So, there is a chance that it could take more than 91 lbs to shear two screws, but it is a very small (0.15%) chance. There is also a 0.15% chance that it could take less than 71 lbs to do the job, which is pretty irrelevant for this application.

I have to agree with JDCluster here, You are only trying to keep the nose cone from popping out during the drogue event basically. Your rocket is fairly light to boot. If your nose cone is most of the weight you may want to be thinking about higher strength fasteners but 18- 19 pounds for the total weight, I would use 2-56 nylon screws and only 2 of them on something that light. Actually one would be sufficient but the rule is use 2 at least so it will shear and come out straight from the air frame. One might jam if it starts to cock to one side as it ejects. I have rockets that weigh in excess of 45 pounds and only use 2/ 2-56 screws to hold the nose in place.

I am betting you will be around 1 to 1.5 grams of bp for a clean ejection depending on your compartment size. Ground testing is a must. If you send the NC all the way across your yard you may be a little on the high side with your charges Three to four feet is pretty good, that way you will not be bouncing the NC back and through your shock cord as it deploys. It's like crocheting your rigging in flight and can make for some interesting recovery's :horse:

I have to agree with JDCluster here, You are only trying to keep the nose cone from popping out during the drogue event basically. Your rocket is fairly light to boot. If your nose cone is most of the weight you may want to be thinking about higher strength fasteners but 18- 19 pounds for the total weight, I would use 2-56 nylon screws and only 2 of them on something that light. Actually one would be sufficient but the rule is use 2 at least so it will shear and come out straight from the air frame. One might jam if it starts to cock to one side as it ejects. I have rockets that weigh in excess of 45 pounds and only use 2/ 2-56 screws to hold the nose in place.

I am betting you will be around 1 to 1.5 grams of bp for a clean ejection depending on your compartment size. Ground testing is a must. If you send the NC all the way across your yard you may be a little on the high side with your charges Three to four feet is pretty good, that way you will not be bouncing the NC back and through your shock cord as it deploys. It's like crocheting your rigging in flight and can make for some interesting recovery's :horse:

2-56 gets my vote Too. Good posts Sandman, Jd! 4-40 are way to big for this.

I am in the process of determining the number and size of shear pins for my first effort at a dual deploy rocket. It's a big one at 5.38 x 8ft, will weight about 18-19 pounds loaded and will fly on K's. I think I have a basic understanding of shear pins and loads but still am missing something here. Bob, can you please explain to me your equations above, in particular the +- pounds and +/- sigma in each? I then still need to determine the required psi for ejection and I know it will be based somewhat on the shear pin calculations but what about "G"s on the rocket at ejection? Anything else I am missing here?

Not Bob here, but I understand it and can explain it: The +/- part represents the variation seen in the test results that you should use for design purposes. For the 2 screw example, 81 pounds is the mean (average) of all the tests, 3.3 pounds is one standard deviation, and 81 lbs+/-3*3.3 lbs = 71 lbs to 91 lbs is the range one could expect the results to be within 99.7% of the time. This comes from probability theory in statistics for a normal (bell curve) distribution. So, there is a chance that it could take more than 91 lbs to shear two screws, but it is a very small (0.15%) chance. There is also a 0.15% chance that it could take less than 71 lbs to do the job, which is pretty irrelevant for this application.

There is always some difference between screws in a box of screws. All I did was a statistical analysis of the data presented on Rocketmaterials.org

I also agree that (3) 2-56 nylon screws are plenty for a 2 pound NC. It takes about 64 pounds to break (3) 2-52 nylon screws. That's a 32G load which is rather high for pyroshock.

Bob

I understand what your data was about Bob, I was not trying to imply you where leading him astray or missing something. Your answer was fine and I appreciated it as well.

I just wanted to reenforce that I thought using 4-40 screws where not a best choice for his application as well as give him an example of my experience with what he is trying to do.

I actually like reading your input and the data that you dig up

First, thanks for the replies. This forum is amazing and helpful. I have learned much here.

I could just ask which shear pins to use or how much 4F to use and forget it but I cannot do that. I am trying to learn the physics of this stuff so that I know what I am doing rather than just doing it because someone else does it that way or said to do it that way.

I pretty much understand all that has been posted. There are just a few details I am missing but I'll get it, eventually.

My biggest reamaining question is, having and understanding of what kind of force is required to separate the nose from the airframe for the main deployment or the fin can/airframe separation for the drogue deployment, and what is required to shear the pins, I need to know how to put those things together to make the proper choice for number and size of pins as well as the amount of charge.

thanks again!

That's good to hear!
Most people new to the hobby come down with a severe case of Kopy Kat Syndrome. I've seen it many times before, and most of them get bored and leave the hobby as quick as they stormed into it. If something goes wrong at least you might have a clue as to why it happened..... where most of the Kopy Kats are lost in space....

JD

First, thanks for the replies. This forum is amazing and helpful. I have learned much here.

I could just ask which shear pins to use or how much 4F to use and forget it but I cannot do that. I am trying to learn the physics of this stuff so that I know what I am doing rather than just doing it because someone else does it that way or said to do it that way.

I pretty much understand all that has been posted. There are just a few details I am missing but I'll get it, eventually.

My biggest reamaining question is, having and understanding of what kind of force is required to separate the nose from the airframe for the main deployment or the fin can/airframe separation for the drogue deployment, and what is required to shear the pins, I need to know how to put those things together to make the proper choice for number and size of pins as well as the amount of charge.

thanks again!

Gary,

The forces required to separate a coupler joint are fairly low, typically 5-10 lbs. The shear pins are placed and sized to guard against transient forces during the flight. In a typical HPR recovery you may see +/-5G during the flight (if nothing major goes wrong, i.e., CATO, etc.). You size the shear pins to keep things in place during that transient. I have a discussion of this force calculation in my recovery section of my L3 build documentation attached starting on the bottom of page 15.

-Tim

My biggest reamaining question is, having and understanding of what kind of force is required to separate the nose from the airframe for the main deployment or the fin can/airframe separation for the drogue deployment, and what is required to shear the pins, I need to know how to put those things together to make the proper choice for number and size of pins as well as the amount of charge.

thanks again!

View attachment Recovery Section.pdf

I stepped up to 6-32 shear pins for my L3. I had flow the rocket 2x before and both times using 2-56 for the first flight and 4-40 for the next ground test and decided that I'd use more than needed. The first times the mach delay wasn't long enough (32 seconds) and the charges fired while it was still traveling 400 fps. That breaks shear pins relatively easy. I also cut the heads off of the pins so that they wouldn't be in the airstream, so any deformation of the tube and they could slip through easily.

Edward

Next time; try using an accelerometer based altimeter for deployment....

JD

I stepped up to 6-32 shear pins for my L3. I had flow the rocket 2x before and both times using 2-56 for the first flight and 4-40 for the next ground test and decided that I'd use more than needed. The first times the mach delay wasn't long enough (32 seconds) and the charges fired while it was still traveling 400 fps. That breaks shear pins relatively easy. I also cut the heads off of the pins so that they wouldn't be in the airstream, so any deformation of the tube and they could slip through easily.

Edward

I just don't like them. All that integration and math. And if you have non-vertical flight then the integration gets all off and it doesn't work. You can keep chanting though...

It's also odd to have a rocket that after 32 seconds is still going fast enough that you'd need a longer mach delay. I switched to a different barometric altimeter that uses a novel apogee detection method that doesn't need mach delay and had no problems.

Edward

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Next time; try using an accelerometer based altimeter for deployment....
JD

Or one of the newer baro altimeters that don't require mach delays.

BTW, 400 fps isn't anywhere close to the Mach transition zone that tricks barometric altimeters. Sounds like you have a bad unit.

I know 400 fps isn't close, there were other issues that had to be resolved. But, just saying that tossing out the drogue at 400 fps isn't a happy thing and 2-56 shear pins didn't stand a chance. Only had a 3" zipper though.

Edward

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