Shear Pins

[khabuda]

Do all #2-56 nylon screws have similar shear strength, or is there something special about the screws that rocket suppliers sell as "shear pins"? I ask because you can get them in large quantities on Amazon for much cheaper. They're anonymously branded and have no specs, but it's still a really thin plastic screw. I'm going to ground test them, but it's easy enough to make a post here too.

 

[LGM-25C]

same thing

 

[mtnmanak]

You can get them anywhere, but if you want to ensure you are getting what you paid for, just buy them from McMaster-Rocket, I mean McMaster-Carr:

https://www.mcmaster.com/products/phillips-screws/material~nylon-plastic/rounded-head-style~pan/
As the URL notes, you are looking for Nylon Pan Head Screws. Two recommendations:
- Get Philips head screws - a lot easier to put in and take out than trying to fumble around with a tiny straight slot screw on a windy day at the field
- Get them longer than the rocket vendors sell them - I like a 1/2" for 2-56 and 4-40 screws. They cost the same as the 1/4" screws and it is a lot easier to get the sheared off part in the ebay out when you can reach in and have something to grab onto.

 

[waltr]

I also buy them from McMaster.

 

[tim cubbedge]

This table was provided by @blackjack2564 a few years ago.

 

[cls]

Above table shows exactly why I have had zero good results using 2-56s, vs nearly 100% perfect using 4-40s.

 

[mtnmanak]

This table was provided by @blackjack2564 a few years ago.
View attachment 591043

Also check out the tables on the Rocket Calculator BP Estimation Calcualtor page:

https://rocketrycalculator.com/rocketry-calculator/bp-estimator/
Their numbers are slightly higher at the low end for the pins, but they also provide a window for 6-32 pins, which I use for 8 inch and 9 inch diameter rockets. I have found their estimations on the shear strength of the pins to be good enough when I wanted to calculate estimated BP charges.

 

[Titan II]

I have had 100% success in everything from 38mm to 6" using only 2-56.

 

[mtnmanak]

For a lot of flights, 2-56 is fine. My experience is that it has less to do with the size and weight of the rocket as much as the type of motor I am using.

At launch, when using a large, fast burning motor, the high initial G load can take even a tiny amount of slop in the nosecone/coupler fit and slam the nosecone back enough to shear smaller pins. I was having a hell of a time figuring out why I was getting early deployments of my main on an 8" diameter rocket with 3 x 4-40 shear pins in the nosecone when I used an N or O motor. I pointed a Runcam 4K camera forward and watched a couple flights. By slowing the video way down, you could see the that shear pins were already busted before the drogue ejection charge went off at apogee. The nosecone wouldn't come out at apogee, but it would slowly wiggle its way out as the payload bay flopped around in the wind during drogue descent.

After I replaced the nosecone shear pins with 6-32 pins, I have not had an issue.

Point of the story is that there are a number of factors in determining shear pins, not just size and ejection charges. If you tend not to fly very high-G flights on heavier rockets, then smaller pins should work great.