Standard Shear Pin Size?

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Kruegon

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I'm building a rocket that'll use shear pins. Mine aren't here yet. What diameter are they? I want to have everything ready once they arrive.
 
What and what size did you order? Some folks use 2-56 nylon machine screw, other opt for 4-40 which is what I use on a 6" diameter fiberglass rocket. Some folks use various diameters of polystyrene rod.
 
Right. Depends on your tube material, diameter, wall thickness, etc. They have to do two things: keep the pieces together on the way up (which really doesn't take much effort, especially if you vent the tube), and break when the charge goes off.

In the biggest rocket I have, I use two #2-56 X 1/4" nylon screws in the nose, and two in the booster. Drill with a #50 bit and tap with a 2-56 tap. Or, as many do, don't tap. Drill slightly bigger and push/shove them in.

Bigger/heavier rockets with thicker walls and more base drag may need to bump up to #4-40, and more of them.

If using them on any kind of cardboard tube, you'll want to add a square of metal to mount them through to ensure breakage and save your tubes.
 
On cardboard rockets you can just harden the hole with a 2-3 thin CA soaks. I generally use 3 2-56 nylon on my 4" cardboard rockets, and after 15+ deployments (flight + ground tests) the holes show no elongation at all. I do drill and tap, and I always make sure that if the NC has raised rings that I drill through those. I don't know how well CA/cardboard would hold up to 4-40 screws, but CA/blue tube has no problem with 4-40 at all.

David
 
Ok. I just ordered shear pins lol. Didn't know they came in varying sizes. So I'm using them on an Estes mean machine and an Estes argent.

The argent is being DD via an av bay in the transition. The BT is being CA reinforced, inside, outside and through the hole.

The mean machine is being done through the center coupler. The coupler and BT are being CA reinforced the same way. The screws for the av bay are #4-40 x 1/4".

So it looks like neither bird can fly tomorrow unless someone has the 2-56 screws at the launch tomorrow. Seems I'll need a tap as well. Dang this hobby was expensive before. Now it's getting ridiculous.

Neither is DD just yet, but will be soon.
 
So why are you bothering with shear pins?

I know shear pins are the new "in" thing, but I never used shear pins til I flew my L3 rocket, and only because my TAP said I needed them (not that I argued). A properly fitted friction fit will work on any rocket. I've flow L1 and L2 DD rockets for years without shear pins and still do. My opinion; don't use shear pins unless it is a heavy fiberglass rocket, friction fit will work for the lighter paper tube rockets just fine.
 
Well for now I need something to keep it from separating at the coupler that will be the av bay in a couple of months.

Another is that I have a couple of rockets that are tall. Too tall for transport. Like my mean machine I finished tonight. I need to use something very light weight to pin the coupler for launch.
 
Well for now I need something to keep it from separating at the coupler that will be the av bay in a couple of months.

Another is that I have a couple of rockets that are tall. Too tall for transport. Like my mean machine I finished tonight. I need to use something very light weight to pin the coupler for launch.

That's what masking tape on the coupler is for. I use X patterns. The thickest part is where the two pieces of tape cross. It is easy to control the amount of friction and has worked very well for me since 2003. If you want to prevent a joint from opening at all, a wrap of tape on the outside works great. It also doesn't leave permanent holes in your rocket like shear pins would.
 
+1 to what Handeman said. On models as small and light as the Argent and Mean Machine shear pins really are not necessary a little tape goes a long way, I generally use the good ole blue masking tape since it removes easily later.
 
+2

If you can hold the rocket vertically by the nose cone and the booster doesn't fall off, you're golden. Shear pins are good to prevent drag separation, which happens at higher altitudes.
 
What are you pinning? How much does it weigh? What G-forces are you estimating?

I'm pretty sure Bat-mite meant "at higher thrust" or "at higher G's."

For what it's worth, I don't think your Mean Machine needs #4-40's. That, to me, sounds excessive by a significant factor, as two #2-56 pins hold back 3 lb of shifting material at 20G's. I don't think your Mean Machine weighs that much pad-ready, does it?

Neither rocket you described --at least, not without significant modifications-- sounds like it would need shear pins at the nose for dual-deployment or at the mid-point break for drag separation. Friction fit will do just fine.


Later!

--Coop
 
Drag separation usually occurs at, or shortly after motor burn-out, between fin can & payload section. Higher drag is on the fin can than the upper parts & pull them apart if conditions are right. [usually high thrust motors coming to an abrupt stop...V-max...Warp-9...Blue Thunder etc.]
Shear pins can be used to alleviate this, so can tape, till a charge fires.

Shear pins are also used to hold NC's on, until a charge fires to shear them.

The " higher" altitude part that is confusing, is due to pressure change as you gain altitude. Think ears popping as you travel up or down in mountains.
You must have vent hole in airframe of fin can & payload to bleed off the internal pressure, which will be higher than external. If pressure has no way to escape it will "push" the sections apart at the coupler/payload or nosecone/payload joint.

Depending on how fast you are reaching "higher" altitude determines 1. how large vent must be 2. how fast you must bleed pressure off.

I have seen problems occur with fast moving rockets at 2500-3000 ft where vents were forgotten. [not to be confused with altimeter vents in the av-bay.]
Slower speeds will allow the pressure to bleed of through all the places rocket leak due to not tight fit.

Drag separation can occur at very low altitudes [my definition anyhow] A N-10,000 burns out at under 800 ft. [my flight did] should the upper section be heavier than the lower, the inertia imparted to it, can keep it moving faster than the fin can, should the fins be draggy enough to slow dow quickly.

So under those situations pins keep things together, whether it's high altitude pressure change or any altitude drag separation.
So don't forget your airframe vents when building high power rockets.

Most likely I forgot something, but that's the very basic explanation.

PS. Plastic rivets or metal screws are generally used to hold coupler/av-bays in place when desired to be removable, not shear-pins.
I use a 5/64 bit for 2-256 pins & a 7/64 bit for 4-40's not the perfect wire # bit...but the very close standard bit that comes in most sets. Just push into place.
 
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So why are you bothering with shear pins?

I know shear pins are the new "in" thing, but I never used shear pins til I flew my L3 rocket, and only because my TAP said I needed them (not that I argued). A properly fitted friction fit will work on any rocket. I've flow L1 and L2 DD rockets for years without shear pins and still do. My opinion; don't use shear pins unless it is a heavy fiberglass rocket, friction fit will work for the lighter paper tube rockets just fine.

I disagree with you on this. Kicked a couple main at or near apogee with shear pins early on. The nose cones were friction fit suck that you could pick the rocket up by the nose.
 
Well part one of all this is that the nosecone and transition fit pretty loosely. It would take more tape than I prefer to friction fit them.

Second. The nosecone will eventually be used for DD but for now, it's just in the tube. Shear pins will keep it there for the non-DD flights till then.

Third, the booster is WAY heavier than the upper section. Depending on atmospheric changes at 3000' plus, the transition might separated if friction fitted.

Lastly, friction fitting, in thin wall cardboard, slowly expands the tubing. Eventually, often sooner rather than later, it will expand past viable friction fit. The paint will only help so much.

With all this considered, that's why I'm opting for shear pins. This rocket will one day fly on an I200. I figure it's easier to make the adjustments now rather than wait till its painted and all.

I'm using 3 #6-32 SS screws to hold the upper side of the av bay/transition. They are working very well. The bulkheads should be ready soon and then I can start building my av bay into the transition. I'm flying her first flight on a G40. Low and slow. Second flight will be an H128. Cert flight.

Friction fit may work well on the mean machine clone. I was worried about using shear pins on a balsa cone anyway.
 
For any Estes rocket, you want to use nothing larger than 2-56 nylon screws.

A 50 +/- 3 pound shear load is required to fail (2) 2-56 shear screws. To insure that you develop enough force to shear the screws, you should have an ejection that will develop a 75 pound shear load.

A 81 +/- 3 pound shear load is required to fail (2) 4-40 shear screws. To insure that you develop enough force to shear the screws, you should have an ejection that will develop a 120 pound shear load.

A 1.6" Mean Machine has an internal area of ~2 square inches. Using (2) 2-56 screws, it takes ~50/2 = 25 psi internal pressure to shear the screws so you would want 75/2 = 37 psi internal to make sure. That is a lot of pressure for 1.6" Estes airframe and I think it may fail the airframe. Using (2) 4-40 screws, it takes ~80/2 = 40 psi to shear the screws so you would want 120/2 = 60 psi to make sure. I'm almost positive this will fail 1.6" Estes airframe.

A 2.5" Argent has an internal areal of ~5 square inches. Using (2) 2-56 screws, it takes ~50/5 = 10 psi internal pressure to shear the screws so you would want 75/5 = 15 psi internal pressure to make sure. That should be ok for 2.5" Estes airframe. Using (2) 4-40 screws, it takes ~80/5 = 16 psi to shear the screws so you would want 120/5 = 240 psi to make sure. I think this is marginal for Estes 2.5" airframe.

Bob
 
Awesome Bob. Thanks. I was wondering about pressure. I may not even try shear pins on the mean machine. It was more theory for DD than a determined course. The argent is almost absolutely going to need them. The transition is so loose it's scary. I totally overbuilt the booster. I'm taking no chances on failure for my L1 cert. I'm not too worried about failing the BT on the booster, it's stouter than I would have expected. But failing the payload seems much more possible.

Have you ever augmented the deploy charge on your motors? I'm wondering about the feasibility of adding another few mg of bp to the stock charge to ensure pressure.
 
Have you ever augmented the deploy charge on your motors? I'm wondering about the feasibility of adding another few mg of bp to the stock charge to ensure pressure.

To me that sounds like you are compensating for one design issue by adding to the BP. That smacks of the "Blow it out or blow it up" school of thought. I would recommend you rethink the way the rocket come apart before you add more BP to the ejection. If more BP is really needed, just realize it will probably also require heavier shock cords and recovery parts which makes the whole rocket heavier. There is a point of diminishing returns.

There are cases where adding to the BP is needed, usually large volumes and small ejection charges, like HobbyLine motors in a G-Force modified with the nose cone being the separation point.
 
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Awesome Bob. Thanks. I was wondering about pressure. I may not even try shear pins on the mean machine. It was more theory for DD than a determined course. The argent is almost absolutely going to need them. The transition is so loose it's scary. I totally overbuilt the booster. I'm taking no chances on failure for my L1 cert. I'm not too worried about failing the BT on the booster, it's stouter than I would have expected. But failing the payload seems much more possible.

Have you ever augmented the deploy charge on your motors? I'm wondering about the feasibility of adding another few mg of bp to the stock charge to ensure pressure.
IMO you are taking the wrong approach, and making a problem that doesn't exist. simply add tape to the transition to tighten the fit, and drill a small (1/16") hole to equilibrate the internal pressure in the booster. Since you "over-built" AKA "made a lead-sled" out of the booster you should not drag separate after burnout and before motor ejection, and there shouldn't be any need to augment the motor if you haven't altered the physical volume inside the rocket.

Add tape, shove in a motor and launch!

Bob
 
I was just looking at the motor ejection vs volume. The same ideas used in calculating DD variable charges for deployments. Some rockets use .4g for deploy. Some use 1.2g. Was simply asking since it could affect shearing the pins. It's just questions to learn. Theory for now. Not practice.
 
I was just looking at the motor ejection vs volume. The same ideas used in calculating DD variable charges for deployments. Some rockets use .4g for deploy. Some use 1.2g. Was simply asking since it could affect shearing the pins. It's just questions to learn. Theory for now. Not practice.

I think what Bob was trying to say is that you don't need shear pins and extra BP for an Estes Argent. You can certainly use them, but I think you are causing yourself more headaches then it's worth. That's just my opinion.
 
Well there will be many more rockets than just the argent. This info is needed, if not for the argent, then for future rockets. Just info gathering, learning and data storage for reference.
 
You are basically building a DD rocket but want it to hold together during single deploy flights? You can use small plastic reusable rivets at the sections you want to hold together or just go ahead and tap it and use the 2-56 nylon screws that you intend to use for shear pins in DD. With no charge separating the pinned sections, they shouldn't shear. Like was mentioned earlier though reinforce the holes somehow. (I am finishing my Estes Partizon with future DD in mind and that is how it is set up. But then again I've neveri flown DD myself sooo....)
Mike
 
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You are basically building a DD rocket but want it to hold together during single deploy flights? You can use small plastic reusable rivets at the sections you want to hold together or just go ahead and tap it and use the 2-56 nylon screws that you intend to use for shear pins in DD. With no charge separating the pinned sections, they shouldn't shear. Like was mentioned earlier though reinforce the holes somehow. (I am finishing my Estes Partizon with future DD in mind and that is how it is set up. But then again I've neveri flown DD myself sooo....)
Mike

Same here. It's a plan in motion. And I'm prepping for both options.
 
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