Shear pins

[BM1993]

Hi Forum,

I just got a delivery of fun parts for my rocket project, including a starter set of shear pins and tap. At first glance the shear pins seem to be too thick, and I am concerned that they might tear/rip a 'zip' through my phenolic tubing of the rocket body and coupler (ebay section).

I do not have experience with shear pins, since this is my first build. Would anyone suggest a solution if these shear pins might in fact damage the tubes?

Should I buy smaller ones? Are rocket shear pins the same thing as ordinary nylon screws which I can get online (i live in Poland) instead of ordering them from specialist rocketry shops?

I will check the packaging on the pins and update you on the description.

Thanks,


B.

 

[user 30299]

I just so happened to have a picture that shows the typical nylon shear pins we use for rockets; 2-56 and 4-40. They are ordinary nylon screws.



With cardboard and phenolic tubes I usually strengthen the area at and around the shear pin hole with a few
applications of CA (Super Glue). I will drill the hole and apply more CA. I find this works fine up to a 4" diameter
body tube, where the rocket doesn't weight more than 15 lbs.

Some people will add little tabs of metal on the inside of the body to help shear the pins.

There's a lot of variations in the rocket's design that can affect the cutting of the shear pin.

The weight I mention is just my own personal rule of thumb for when I start to think about how
I want to shear the pins, and where they may be located. The rocket's weight, how it's distributed,
the size of the deployment charges all have an effect. (People's build experiences vary . . . . )

Always do a pop test to see how the deployment charges and shear pins perform before the 1st launch.

Good luck on the build,

 

[waltr]

BM1993,

What size nylon screws did you get? I'm sure they are metric.

What size (weight) is your rocket, the Nose cone and the AVbay +chute section + nose cone.

We use Imperial 2-56 (for smaller rockets) or 4-40 (for larger).
I use one 2-56 nylon screw for a 2.2" (54mm) diameter LOC kit with a nose cone weighting 100 gram. This needed 1.2 gram BP for reliable shearing and ejection. The LOC BT is thick wall cardboard that is hardened with thin CA.

2-56 diameter is 0.08 inch or 2mm
4-40 diameter is 0.11 inch or 2.8mm

 

[BM1993]

I checked the packaging and they're M3 size (3mm). I was thinking of gluing some really thin brass pieces (from 0,5mm sheet) onto the outside of the airframe and on the inside of the altimeter bay, that way the screws can shear properly. I have checked online and I only found m3 size, which I already have :/

Airframe is 2.5" in diameter, with cessaroni nose cone. I haven't assembled the rocket yet so unfortunately I cant tell you the exact dimensions and weight.

 

[waltr]

OK, for a 2.5" airframe the Nose could be in the 100-150gram range if plastic. A bit more if fiberglass.

A recommend 2mm nylon screws as shear pins and you may need only one (like I use on the LOC 2.2").

 

[Titan II]

I have checked online and I only found m3 size, which I already have :/

This is from Amazon Poland..........

2mm x 5mm nylon.

 

[teepot]

I put copper blades on the inside of the BT. I use a Dremel with a sanding wheel to make a depression in the tube wall deep enough for thin copper pieces. Then drill thru the BT, the copper and the nose cone and tap. On tubes to thin to make a depression, I put the blade on the outside before painting. I tried brass but couldn't find any thin enough. The copper is thin and flexible.

 

[BM1993]

@Titan II could you send me the link to the auction site itself?

 

[BM1993]

I found the following on Allegro (polish amazon/ebay)

M2 5mm

M2,5 6mm

M2,5 8mm

Which would be the most suitable?

Is 5mm enough to cover two layers of phenolic? Body tube + coupler tube?

 

[Titan II]

@Titan II could you send me the link to the auction site itself?

https://www.amazon.pl/

 

[tsmith1315]

An alternative would be a piece of styrene rod.

Here are some discussions on the subject that could be helpful to you or others reading:

https://www.rocketryforum.com/threads/shear-pins-vs-nylon-screws.108620/post-1245956

https://www.rocketryforum.com/threa...r-pins-rivets-or-nothing.166076/#post-2207288

 

[waltr]

I found the following on Allegro (polish amazon/ebay)

M2 5mm

M2,5 6mm

M2,5 8mm

Which would be the most suitable?

Is 5mm enough to cover two layers of phenolic? Body tube + coupler tube?

5mm length should be ok.
What are the Wall thicknesses of the tubes? Guessing maybe 1.5-2mm