Shear Pins

[Bruce R. Schaefer]

Okay, new project… this rocket will be 2″ over ten feet, two 48″ sections and a 26″ nose cone, 5 inch diameter, all PR G-10. It will fly on L to N motors. Last time, I used 6-32 shear pins for the nose on a slightly longer (12′) but similar bird, as wisely advised by Conway. This nose cone, for stability reasons, will have to be even heavier. Last one was just over 2 pounds, this one will be a pound heavier. Close to the same nose cone, but a shorter rocket needs more weight to remain stable. We all know that. I’m wondering if I can “get away with” (4) 4-40’s or perhaps (2) 6-32’s and (2) 4-40’s? Has anyone tried to mix shear pin sizes? I never have, and I’m just wondering if it is possible. Granted, ground testing will show me, and I can always drill bigger for the extra (2) 6-32’s, if need be. The (2) 6-32’s would give that added protection from the nose popping off from acceleration and a “relatively” sudden stop (will be using my typical VERY long harness though). I appreciate your thoughts and experiences. 😉

[SCOTT EVANS]

Im new to this and I just found out what shear pins or for today.
At what point do you decide to start using them?
A wieghted nose cone?
A 60 foot harness??!!!

Scotte

[Conway StevensParticipant]

First off Bruce, Keep them all the same size for each joint. don’t mix sizes. It will create more issues then its worth. Also I always use a minimum of 3 that are evenly spaced. Also, the first thing you need to do before determining shear pin size and quantity is to determine the exact size of the airspace you will have in each section to pressurize. Without that knowledge first its impossible to say how big or how many you will need. So once we have that then I can recommend what I think you should use..

[Conway StevensParticipant]

Im new to this and I just found out what shear pins or for today.
At what point do you decide to start using them?
A wieghted nose cone?
A 60 foot harness??!!!

Scotte

Scotte,

Ive used Shear pins in many different sized rockets. Always HPR though. Most anything that I have thats either going very fast, Very high, Both, or fairly large or even that I just want to make sure it sticks together I use shear pins. It has nothing to do with the weight of the cone or the harness length. I shear pin about everything I do in flights like I mentioned above.

Conway

[Bruce R. Schaefer]

I agree with what you’re saying, Conway. Just looking to see what people have to say. I just wanted to find out if anyone had ever tried to use two, offsetting sizes of screws. One = failure, two = possible failure, 3-4 = good to go!

[slipstick]

I’m very much interested in this topic as I am building my 3″ diam G10 level 2 bird (dual deploy) and am struggling with the number of screws vs. the volume vs the amount of pwoder used for deployment.

From an engineering standpoint, the cross section of a 4-40 at the minimum diameter is .081″.which derives an area of 5.153×10-3 as compared to a 6-32’s minimum diameter of .099″ which has an area of 7.698×10-3, however the double shear strength of a nylon 6-32 is 97 lbf almost twice that of a 4-40 which is 50 lbf.

In essence you need two 4-40’s to equal the shear strength of a 6-32

Ref: http://www.anixtercomponents.com/images/uploads/Nylon_screws_strength_data.pdf

Its safe to say, I’ll probably default to whatever the lexperts in the club tell me to use. Which is……?

[Anonymous]

Screws are great, and I have used them often. Having said that, consider this stuff http://www.evergreenscalemodels.com/Shapes.htm which I have used many, many times. The 1/16″ round rod (their item #222) is terrific, and available either online or at most hobby stores.

My general rule of thumb – use polystyrene rod on projects 54mm and under, 4-40’s on 3″-4″ diameter projects, and 6-32’s on the biggest, most massive stuff. I think you would find #6 screws to be a bit much on your 3″ project.

I have used the polystyrene rod to over 20,000′ and I’ve seen other guys take it over 30K. It works…

Merry Christmas all-

[Warren B. MusselmanModerator]

On smaller birds I use the round stock that John Wilke mentions in his post. On 3″ and 4″ birds I use 2-56 nylon screws although if the nose is heavier I’ve been known to go up to 4-40. I always use three equally spaced. On the UprOar I believe we used 6-32 nylon screws which I felt were overkill.

Warren

[Conway StevensParticipant]

Like JW I to have used the Styrene rod for shear pins. Mostly in my 38mm and under rockets. But what you need to use will depend on the size of the rocket, drag and weight. Also the forces applied at say motor burn out or when the apogee deployment happens on a dual deploy. You want to have it all stay together when it needs to but come apart when it needs to as well. I also base what I use on what the actual rocket is made from.

I used 4 #4-40 nylon screws to hold my booster section on of my L3 rocket Full Throttle. I used 10 grams of powder to get the unit apart. Thats with the pins It took 9 grams to get it apart with no pins as the section is just very large 6″ diameter and roughly 6 to 7 feet long. For a safety measure I added a gram and made my actual recovery with 11 grams. Better to get that apogee out for sure. But having a large rocket like that coming out of Mach as well as a large charge popping I didn’t want the nose cone to come popping off either. So for that I used 4 #6-32 nylon screws. That end is fully packed and still has a little room (about 2 feet @ 6″ diameter) so i ended up with a 5 gram charge to safely deploy my mains. Again the rocket is CWFG tube and much stouter then most other tubes and can handle those sizes of charges.

There is a really awesome site that I recommend called High Power Strength of Materials. That have a great chart on this of tested strength. The owner of the site is an engeneer and has some great info on alot of stuff with more new stuff comming.

Here is his chart on test done with Nylon screws and some styrene products.

http://www.rocketmaterials.org./data/cord/Shear_Pins/index.php

Now remember a lot of what should determine the size you use will be the forces applied to the rocket. What happens when you take a rocket that weighs say 50lbs and make it go Mach then slow it quite rapidly? Anyone know what the negative g forces could be coming out of Mach? It would of course vary depending on the mass and aerodynamic drag from rocket to rocket. As well as how fast and how many g forces it was attaining forward and then to how much slowing factor from all of that put negative forces again back on the model.

You could end up with a rocket thats 50lbs and when it slows due to the forces implied on it weighs closer to 200lbs (or more or less) as an example.

Anyhow, those should all be the things that one takes into mind when figuring what size of shear pins to use. Or at least thats what I do..

Have a Merry Christmas!!!

Conway

[Bruce R. Schaefer]

I’m very much interested in this topic as I am building my 3″ diam G10 level 2 bird (dual deploy) and am struggling with the number of screws vs. the volume vs the amount of pwoder used for deployment.

I have the same rocket, and I’ll either use (3) 1/16″ polystyrene rods, or (3) 2-56’s. First, I’ll ground test it with 1/16″ PS, then if I don’t feel that will hold the nose, I’ll just enlarge the holes for 2-56’s. Although you should never use just one or two shear pins, I have used two, and it worked fine. But then again, I tend to go heavy on the charges. Which is why I, like you, went with G10 tubing. I’d like to keep my birds around for a while. 8) For the bigger rocket mentioned earlier, I’m going with (4) 4-40’s for the booster, and (3) 6-32’s for the nose cone. I probably could get by with (4) 4-40’s on the nose, but at that power level (and speed), I just won’t take any chances. It took 6 grams to blow the nose off a similar rocket at ground test, but in flight it took 7 grams. If the rocket is G10, the G10 can take a little more charge. Even though I have taken the old NAR axiom, blow it apart or blow it up perhaps too literally, it’s always better to have more than needed than not enough. 😉

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