Nylon Shear Pins vs. Blue Tube

[Alan Marcum]

Anyone have any experience using nylon shear pins with Blue Tube airframe? Anything needed on the airframe to ensure the pins shear (e.g., some kind of metal cutter), or is Blue Tube stiff enough?

- Alan

[Kendall, Steve]

Hi Alan,

I haven't used Blue Tube specifically, but I've used other treated paper (PML phenolic) tubes with nothing more than a little CA hardening with great success, so my suspicions are that it will work just fine.

But rather than take my word for it, try an experiment (nothing better than firing off deployment charges in your back yard - or in my case the FRONT yard!!) and see. How big of an airframe are we talking about? 2" or 4" or larger? At 4", you could try 3 2-56 screws or 2 4-40's and see how it shears, and how the holes look afterwards.

I like to tape the inside piece (the coupler?) and do a clearance hole for the airframe. Seems to promote good shearing action and also seems to help reduce the wearing on the airframe part, but my evidence is at best anecdotal (I've never made any measurements or taken photos). As always, YMMV. But if you DO do testing, please do take measurements and photos and let us know your experience.

Steve Kendall
Tel +1 408 522 6333
sken...@sjm.com

- Alan

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[Anthony Cooper]

I use a single 4-40 nylon screw in the Pinata Rocket. It is a paper body tube going into a Fiberglass avionics bay. 

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Anthony M. Cooper
31120 Chicoine Ave.
Hayward, CA 94544-7432
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On Fri, Feb 10, 2012 at 4:46 PM, Kendall, Steve <sken...@sjm.com> wrote:

Hi Alan,e

[Gene Engelgau]

If I can add - IMHO sheer pins should be used in sets of at least two and on opposite sides.  While on may work it can lead to the airframe or nose cone jamming to the size and ultimately not releasing.   

Sheer pins in fiber glass is always good.  For paper be sure to reinforce it with CA or some type of resin.

Ground testing is a must!

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- Regards

Gene Engelgau
KI6IBL, NAR 86770 / TRA 12243 - L3
http://fruitychutes.com - Consumer and Aerospace Recovery Solutions

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[Kendall, Steve]

IMO, it's a myth.  If the coupler is sized correctly (adequate length, doesn't rock in the tube) it doesn't happen.  But I've only pinned up to 4" airframes.  Beyond that, I typically see that couplers are shorter, and I guess could potentially rock and bind, but I would think that could happen regardless of pinning.  I have also used a single pin w/o issue on 2" airframe.  But remember, couplers should fit closely in the airframe.  If it's sloppy and rattles around in the tube (like those Firestorm 54 parts used to do) it's just an overall bad thing and symmetrical pins seems (intuitively) better.  But I've no real data to back that up either.

 

Test, prove it to yourself.

 

Steve Kendall
Tel +1 408 522 6333

Mobile +1 408 489 8808
sken...@sjm.com

 

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From: lunar-...@googlegroups.com [mailto:lunar-...@googlegroups.com] On Behalf Of Gene Engelgau
Sent: Saturday, February 11, 2012 8:20 AM
To: lunar-...@googlegroups.com
Subject: Re: [LUNAR general:] Nylon Shear Pins vs. Blue Tube

[Gene Engelgau]

Your better off with two across for each other at a minimum.  You want the break force to be symmetrical.  Anyway I'm sure there are plenty of other opinions about this...

-G

[Anthony Cooper]

I wasn't going to comment on the Myth. But my feelings are the same. The one either breaks or it doesn't. There is 6 inches of coupler to keeps things aligned. One side is not going to move enough to bind things up. 

_______________________________________________________________

Anthony M. Cooper
31120 Chicoine Ave.
Hayward, CA 94544-7432
Home Site UsCoopers.org Picasa Web Album

VP and Membership for www.lunar.org

Webmaster and Membership for mgscv.org
Cell: 510-589-5292
Google Voice: 510-397-8406 Google Plus

[James Marino]

        Nylon screws are cheap, and holes are easy to drill. Non-symmetrical pins does seem somewhat counter-intuitive. Although, it isn't. Coupler fit is responsible for alignment. But in most cases, for better holding, try two at 180°, or three at 120°, and ground test, ground test, ground test.

 

        In any paper type airframe material, drill the hole, soak it with CA, sand it smooth, and inspect it before every flight. Beats drag or pressure separations at speed, or dumping the main at altitude. 8^(

 

James