Shear pin shear force higher than expected

Warning: long post about something that isn't a huge deal so be forewarned

Short version:
I'm seeing meaningfully higher forces required to shear shear pins than predicted by rocketmaterials.org. Have other folks on the forum measured this themselves and what kind of results did they get? My experimental setup isn't perfect by any means, so I'm just as curious in other folks' results as I am in discussing all the reasons my setup is flawed

Long version:
Like many people, I've relied on rocketmaterials.org (https://web.archive.org/web/2011081...rials.org/datastore/cord/Shear_Pins/index.php) for shear pin shear forces.

Motivated by Matt's unfortunate loss of his L3 rocket, I decided to manually test the force required to shear my L3 build's nose cone shear pins, where I use 4x 2-56 nylon shear pins. rocketmaterials suggests this should take 85lb-90lb to shear.

I bought this 110lb luggage/fishing scale and hung the nose cone and payload section vertically. I attached some shock cord to a forward bulkhead in the nose cone (and ran it out through the nose tip) as well as to the rivet holes on the bottom of the payload section, such that I could add weight to the bottom and all forces would be applied longitudinally with no tilt and such that the forces are balanced.

I get the same results regardless of whether I use 2-56 shear pins from apogee or regular 2-56 nylon screws from mcaster-carr.

It was hard to do this very precisely with my setup, so I'd say the following results are plus/minus at least a couple of pounds. The results are different enough though from rocketmaterials that there is definitely something going on.

1x shear pin: ~33lbs
2x shear pin: 57lbs, 65lbs (mounted on opposite sides of rocket)
3x shear pin: 105lbs, 97lbs (mounted in 3 of 4 holes spaced 90 degrees apart)
4x shear pin: at least 125-130lbs (this exceeded my scale's capacity, so I did this by hanging a large cooler and filling it with water until the pins broke. The water splashed out when they broke so this is a rough estimate done by filling it back to about where it was)

As you can see, these values are substantially higher than what rocketmaterials claims.

My first thought was that my scale was inaccurate, but I measured out 5 gallons of water by volume and the scale was within 0.1lbs of the predicted weight (around 43 pounds including the container weight)

The next theory was that my pins aren't shearing cleanly, but that seems unlikely: the body tube and coupler are FW fiberglass that fit very snugly, and the shear pin holes are very tight and mate perfectly*

Another theory is that I have imbalanced forces, causing some sort of binding. It's hard to prove this wasn't the case, but I did do my best to make sure all forces were balanced and applied exactly along the lengthwise axis of the rocket. Everything hung straight and the cords attaching the loads to the rocket were hung symmetrically around the perimeter.

Another theory is that because I couldn't apply a steady increasing load, perhaps the pins bound or jammed in some way? This too seems unlikely though given the tight tolerance of the shear pins.

So, what have other folks found when measuring shear pin forces? My results do partially explain why I had to use more powder than predicted to shear these pins when ground testing a month ago....



* The shear pin holes were created by 1) drilling a single hole through tube and coupler with a #51 bit 2) using a 2-56 tap to thread the outer hole 3) widening the inner hole so that a shear pin can just barely be pushed in if pushed hard 4) Inserting a screw to lock the assembly and then repeating (1)-(3) for the other 3 holes.

Any skew (perhaps caused by small airframe deflections away from each other) in the pins as they attempt to shear will increase the surface area, and thus the force needed to shear them. Some people use metal shear plates glued onto the airframes at the sliding interface to improve the shearing action.

It is possible that tolerance on the shear strength of the material is giving you the low-end number for shear force, which might be applicable at the high-end temperature spec. Depends on how the data sheet was written sometimes. Strength figures (like shear), if not quoted at a specific temperature, are the minimum over the range provided by the data sheet. In the case of a lot of plastics it will be at the high temperature limit. Also, most data sheets will quote the minimum specs, but the actual material may be stronger. That's what most people care about.

Sometimes if M3 or M4 shear pins are too retentive for my liking I drill out the center (after fitting to the rocket) with something like a 1mm or 1.5mm drill. That makes them a little easier to shear.

Thanks for the thoughtful reply. Comments inline

OverTheTop said:

Any skew (perhaps caused by small airframe deflections away from each other) in the pins as they attempt to shear will increase the surface area, and thus the force needed to shear them. Some people use metal shear plates glued onto the airframes at the sliding interface to improve the shearing action.

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Yup, or even worse can cause tearing as opposed to a clean shear. In my case (and rocketmaterials) I'm using very tight fitting fiberglass tubes and couplers, whereas I'd only heard of folks using metal for cardboard (or other soft material) tubes. I'm having a hard time imagining how the forces could be off-axis enough to cause such a substantial delta in the shear area. The shear area for a pin tilted by angle θ is approximately (1/cos θ)* the shear area for a perpindicular pin, so a 40% increase in shear force would require a 44 degree tilt if my math is correct. This seems unlikely to happen given the tight fit of all components, right?

OverTheTop said:

It is possible that tolerance on the shear strength of the material is giving you the low-end number for shear force, which might be applicable at the high-end temperature spec. Depends on how the data sheet was written sometimes. Strength figures (like shear), if not quoted at a specific temperature, are the minimum over the range provided by the data sheet. In the case of a lot of plastics it will be at the high temperature limit. Also, most data sheets will quote the minimum specs, but the actual material may be stronger. That's what most people care about.

Click to expand...

Huh interesting. I haven't actually calculated the expected values based on spec sheets, but was just comparing my empirical results to those found empirically by rocketmaterials. Mcaster-carr doesn't have a rating for the strength of the nylon screws, though I could try and dig deeper. Maybe the nylon used these days is just tougher than when rocketmaterials did their tests?

I have read that a 2-56 shears at around 30 pounds, and your results are right in line with that. I think this is more an issue with rocketmaterials being off than anything else.

Flyfalcons said:

I have read that a 2-56 shears at around 30 pounds, and your results are right in line with that. I think this is more an issue with rocketmaterials being off than anything else.

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30lbs matches what I'd read, and so my results don't seem crazy. That being said, rocketmaterials' analysis was done in a very rigorous fashion with much better process, equipment and documentation, so I'm loathe to assume it's as simple as I did it right and they did it wrong

One thing that occurs to me: a loose or not-perfectly-centered fit for some pins would cause a sub-linear relationship between # of pins and total shear force needed since the loading on the pins would be unbalanced. Is it possible that the shear pin holes in the rocketmaterials work were looser, causing a sub-linear relationship between #pins and force? It's hard to know for sure since they didn't publish the shear strength for a single pin (or at least I can't find it), so I can't tell if the difference is due to different pin materials, mistakes in my setup, or their setup having more sub-linear behavior than mine.

This is why I'm curious if anyone else has tried to measure this and what results they got.

Your numbers seem right on the money to me.

See this link on nylon 6/6 fastener strength. 2-56 not tested but 4-40 was and shows 50# break strength in double shear.

A 2-56 screw has about 63% of the area of a 4-40, so 0.63 x 50 = 31.5#. Close enough.

Huh interesting. I haven't actually calculated the expected values based on spec sheets,

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Specs for even something like Nylon 66 vary quite widely, depending on manufacturer. Remember specs quoted in spec sheets but not at a specific point (eg temperature) have to apply over the full specified range usually (unless they are being deceptive in their datasheet).

When sizing black powder charges, I like to be conservative... like most. I use 10,500 psi as the shear strength for 66 nylon screws, and then use the pitch diameter to calculate shear area.
Using this, the maximum shear strength is ~46lbs for 2-56 nylon screws and ~76 lbs for 4-40 nylon screws.
Another online reference for shear pin strength is https://www.feretich.com/rocketry/Resources/shearPins.html.

rocketsam2016: In your build thread, when I mentioned the charges I was planning to start with for my Frenzy XL, it was based on those numbers, the ideal gas law and a safety factor.

kevinkal said:

When sizing black powder charges, I like to be conservative... like most. I use 10,500 psi as the shear strength for 66 nylon screws, and then use the pitch diameter to calculate shear area.
Using this, the maximum shear strength is ~46lbs for 2-56 nylon screws and ~76 lbs for 4-40 nylon screws.
Another online reference for shear pin strength is https://www.feretich.com/rocketry/Resources/shearPins.html.

rocketsam2016: In your build thread, when I mentioned the charges I was planning to start with for my Frenzy XL, it was based on those numbers, the ideal gas law and a safety factor.

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Conservative is good, though we need to be conservative in the right directions. In terms of min shear pins needed to keep the rocket together against drag separation, pressure deltas, and apogee events, we need to place a conservative lower bound on shear strength, whereas for how much BP to use we need an upper bound.

What motivated me to test my shear pins manually (and start this thread) was I wanted to feel good about the lower bound on shear strength, and my tests showed a consistently higher shear force than what I previously considered the canonical resource (rocketmaterals.org). It does sound like the consensus is that assuming say ~25lbs per pin as a lower bound on the pins I'm using is probably ok. I agree with your math for how to size the BP charge.

Great discussion everyone!