3d printer filament for shear pins?

[GrouchoDuke]

Nylon screws probably followed by styrene rod seem to be the shear pins of choice. I'm curious. Has anyone tried using 3d printer filament? I have piles of PLA, ABS & PETG in 1.75mm diameter that all seem like they'd work. They make 3.0mm filament too.

Anyone tested it yet?

 

[patelldp]

Nylon screws probably followed by styrene rod seem to be the shear pins of choice. I'm curious. Has anyone tried using 3d printer filament? I have piles of PLA, ABS & PETG in 1.75mm diameter that all seem like they'd work. They make 3.0mm filament too.

Anyone tested it yet?

Really anything can be used with proper testing.

 

[GrouchoDuke]

Yup. I was just curious if anyone had tried it yet. I may have to do some pull tests.

 

[Woody's Workshop]

Why not use hot melt glue? That's pretty darn cheap!

 

[GrouchoDuke]

Yeah, that would be cheap. Filament seems like it'd be easier to deal with in the field though.

I guess I wasn't really concerned about price (what's that motor cost anyway?). I was more interested in the differences in shear strength between the different options. My current projects are 29mm min-diameter rockets with tiny bays. I don't need much strength, I just want it to be easy to use and predictable. Something that shears in the 10lb range would be perfect.

 

[dhbarr]

Slick fitment + predictable shear, makes sense. 2-56 nylon's ~20# and fork tines are too variable.

Love to see those pull tests when you get around to it. Article?

 

[jderimig]

I have found PLA filament to vary widely in its brittleness with age. Moisture absorbance? May not be the best for consistency of the shear pin break forces.

 

[Nytrunner]

My initial thought is that things like filament or hotglue are a tad more ductile than I'd like to see for a breaking component.

I've just got this image in my head of a hotglue sliver strrrretching and jamming the cone.

 

[Oberon]

The minor diameter of a 2-56 screw is pretty close to 1.75mm, so I don't think you'd see a major reduction in shear strength (which it sounds like is your goal). But it would probably work.

What if you 3D printed shear pins, rather than using raw filament? You could probably reduce the diameter, and 3D printed parts tend to shear pretty cleanly between the layers. You could even print at lower-than-ideal temperature to create intentionally poor adhesion between the layers to further reduce the shear strength.

Then again it could all be moot - I've been using a single 2-56 nylon screw as a shear pin for a 54mm diameter Punisher, and it's worked great, with little (like 0.5g) BP required to reliably separate. I've been containing the charges in small centrifuge vials which would easily fit in a 29mm airframe.

Still, it's an interesting project and I'm curious what you'll find!

 

[ttabbal]

Printer filament is going to vary a fair bit. Material (not always exactly what they say it is, fillers, colorants, etc..), age, moisture exposure, and probably other things. The nylon screws are so cheap and well understood, I don't see any reason not to use them, even though I have piles of filament. If the nylon screws are too strong, I think I would try small diameter styrene rod. It's likely to be more reproducible.

If you do use filament, make sure to ground test the exact filament you plan to use to fly with, preferably not too long before the flight. Perhaps even keep a bunch of it cut and ready, stored in a ziplock or similar.

 

[GrouchoDuke]

Thanks everybody. Good point about filament potentially changing properties as it absorbs water. A stringy mess with either filament or hot glue would certainly not make me happy when I'm hoping things separate.

Different types of filament absorb different amounts of water. I live in the desert & haven't had any of the printing issues with older filament that people in wetter climates report. That, of course, is not data that's worth anything for deployment gear. So, hmm.

2-56 nylon screws are too strong for what I want for my 29mm rockets. I have some 0.060" styrene rod that should be here tomorrow. I'll do some testing with it and maybe print some 0.060" pins in PLA, ABS & PETG to try those too. A printed pin will have a different internal structure than a piece of raw filament, so that might just make things completely unrepeatable. We'll see.

 

[tim cubbedge]

Groucho - Where did you get your 0.060" styrene rods?

 

[GrouchoDuke]

Someone on Amazon is selling the Plastruct 0.060 styrene rods: https://www.amazon.com/gp/product/B0000WS9N8/?tag=skimlinks_replacement-20

 

[ttabbal]

A printed pin would, I expect, be limited in shear strength based on the bonding strength between layers, assuming you oriented it that way. If you print them at the same temperatures every time, I would think it would be somewhat repeatable. I suspect it's cheaper and easier to just buy some plastic that works well for your needs and save the printing for more complex parts though.

 

[GrouchoDuke]

Yeah, that's my guess too. 3d printing pins is way into the "research for research's sake" kind of thing. I'm hoping this styrene rod gives me a good ~10 pound shear.

 

[Ravenex]

I use 3/32" and 1/8" ABS and 1/8" nylon for shear pins in different situations. The 3/32" ABS is about the same shear strength as a 2-56 nylon screw, the 1/8 ABS is about the same as a 4-40 nylon screw, the 1/8" nylon is about equivalent to a 6-32 nylon screw. I pre-cut them about 3/16" long and put them in plastic bags in my charge box.