Shear Pin Question

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Joe Rocket 97

Well-Known Member
Joined
Jun 29, 2018
Messages
190
Reaction score
51
Looking to purchase nylon screws, new to high power rocketry. What length does every one use? 2-56 diameter if it makes a difference .
 
Length has little bearing on the shear strength, just how far it sticks through the parts. Mine are 1/2" iirc.
 
They need to be long enough to pass through both the outer body tube and the inner coupler or nosecone shoulder wall. Depending on what you’re building that can vary. If you’re using screws, don’t tap the inner layer. Leave it so you can just push the sheared stub through.
 
Several rocketry vendors sell them in packs small and large. I buy them by the 100 from McMaster-Carr.
 
Screws that stick into the tube too far can create snags getting laundry out. For any rocket small enough to use #2-56 screws, I use 5/16".
 
I have some that are about 1" long. I usually just put them about 1/8" into the rocket and clip the end off to use on more flights.

Only cost about 8 cents each at Amazon when you buy 100 and free shipping. So you're not saving much by reusing the screw
 
Does anyone use M2 (.0787") or M2.5 (.0984") nylon screws vs #2 (.086")?

Most of us use the #2 and #4 nylon screws because of some testing that was done to show that they reliably shear at a certain point, I believe some testing was also done with polystyrene rod as well. Not sure if it was Mr. Damerau's testing or what.

Edit: Here is a link to the Wayback Machine link to rocketmaterials.org
https://web.archive.org/web/2015021...ls.org:80/datastore/cord/Shear_Pins/index.php
 
I plan on using 0-80 nylon screws to hold a 38mm nosecone. Anyone know the shear strength ?
 
Last edited:
I use solids ABS rod in a tight fit hole. I have a number of diameters that approximately match nylon screw shear strengths. I precut them into small lengths and bag them. Works great and costs next to nothing.
 
Does the ABS rod actually shear or does it rock/pull out sideways in the hole? What is the wall thickness of the tubes and materials? I have seen people do this before but am looking for actual experience regarding what happens. The threads on the pin in a tapped hole with light torque lock it in and cause a cleaner shear.
 
I picked up a bunch of nylon screws, stand offs, etc from a local Grainger. I found 2-56 nylon flat head screws and drilled a counter sink hole in the air frame for a smooth surface.

~John
 

Attachments

  • Screenshot_20181018-215233.png
    Screenshot_20181018-215233.png
    175.3 KB · Views: 48
The ABS shears nicely, with fiberglass airframes. It's softer and easier to shear than nylon so it should perform better than nylon screws in most materials. The holes are snug enough that the rod needs to be pushed in, you can either use a full length of rod, push it in, then it can be broken it or cut off or you can pre cut the rod a bit longer than the thickness of the airframe plus coupler and push it in flush by hand or with a plastic tool. Having the holes snug mean that the pin cannot pivot or rock and will shear cleanly. I had lots of issues with screws in their clearance side bending into the gap between the tubes and then smearing and damaging the tubes, the ABS fixed my problems. Also since the holes are smooth through holes I can easily use a small hex key, miniature screw driver or punch to push the remains of the pins out.

I get 1/8" ABS rod, which if very close in shear strength to a 4-40 6/6 nylon screw, from mcmaster for $0.50 per foot which works out to $0.007 per pin. I get 3/32" and 1/16", equivalent to 2-56 & a bit over 0-80 nylon screw respectively, from Stewart MacDonald for $1.37 per foot or $0.02 per pin. Stewart MacDonald sells guitar making supplies and the ABS is sold for side dots in guitar necks. I use a shear cutting wire cutter to pre cut pins about 3/16" long and bag them. A note if you order the 1/8" from McMaster, it comes in 5 ft lengths which they will ship in a mailer tube which costs more. If you request they coil it into a box with other items you may be purchasing your will save shipping costs.

If there is interest I could take pictures of my installation.
 
I sometimes take an xacto and cut the protruding head off the shear pin. Guess that might add a couple of feet of altitude. I prolly would not countersink because of the thickness of the thin wall tubing I use. Most of the time I just leave the screw heads on which gives me something to grab onto when removing them.
 
I plan on using 0-80 nylon screws to hold a 38mm nosecone. Anyone know the shear strength ?

Shear Strength of 6/6 nylon is 10,000 PSI. The minor diameter of a 0-80 screw is 0.0465 inches.
Shear strength*cross sectional area = 10,000 PSI * (0.0465/2)^2*3.14 in^2 = 17 lbs
For a 2-56 screw: 10,000*(0.0667/2)^2*3.14 = 35 lbs
For a 4-40 screw: 10,000*(0.0849/2)^2*3.14 = 57 lbs
 
One thing I have wondered is, are all nylon screws created equally? Are there different formulations of the nylon or manufacturing techniques that can affect the shear strength? That is, to keep this relevant, in the 2-56 and 4-40 sizing which is arguably the two most common sizes for our application.
 
Nylon 6/6 varies, if I'm not mistaken it can range from 8ksi to 12ksi so plus or minus 20%. Also there are a large number of different types of nylon including various reenforced types but most screws are 6/6.
 
One thing I have wondered is, are all nylon screws created equally? Are there different formulations of the nylon or manufacturing techniques that can affect the shear strength? .

For what it's worth, the ones I get in bulk [2500] were labeled "aircraft grade' or something like that & made from virgin raw nylon.
Been using them for years and have been consistent throughout.
Where they would use 2-256 screws on an aircraft is only a guess for me???
 
Back
Top