PEM Nut Kits and Shear Pins Kits?

[debonairbear]

Been crazy busy as of late, but finishing a modified Composite Warehouse Lotus 5, 10-24 PEM nuts in booster for rail guides, and 4-40's in nose cone.
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For the implementation of the shear pins, do you use a washer, drill a smaller sized hole for the outer tube, or just let them sit recessed?

 

[rfjustin]

For the implementation of the shear pins, do you use a washer, drill a smaller sized hole for the outer tube, or just let them sit recessed?

Drill smaller size hole in the outer airframe, just large enough to allow the nylon screw/shear pin to pass.

 

[COrocket]

If you are bulk buying PEM nuts to then divide into kits, I've had good luck with Mouser Electronics. At least for the stainless ones, they are anywhere from 1/2 to 2/3 the cost of McMaster, and the price/unit gets better if you order 100,250,500,1000 etc

 

[Bill Owens]

I use PEMs for a variety of things on a rocket.

As Justin noted, they are primarily used to fasten two parts that you do not want to separate in flight, but do want to separate on the ground. Most commonly, this means fastening the ebay coupler to the payload bay instead of plastic rivets:

4-40 PEM on a 3" rocket:
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I also use them for a similar purpose to attach the coupler to a nosecone when I want to put electronics in the NC Bay:

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Another thing I use them for is to attach tube fins. I have a 4" Neon upscale with 6 Tube Fins and 6 straight fins. The tube fins are all held on by 2 x 6-32 screws with a PEM in the airframe and they are also epoxied to three points of contact (each adjacent fin and the body tube). One set of the tube fin PEMs is 10-32 to accommodate 1515 rail buttons inside the tube. In addition to the PEMs I used for the ebay and nosecone, this rocket required the use of 17 PEMs of various sizes. I am currently working on 6" and 8" versions of this rocket, each needing a similar number of PEMs up to 1/4-20.

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I like how you've used the PEM and would like to replicate it. I see how you've used a PEM nut inserted from the inside of the ebay/coupler. But what is the part you have on the outside of the airframe? It looks like a countersunk washer but appears to sit down in the airframe nicely.

 

[mtnmanak]

I like how you've used the PEM and would like to replicate it. I see how you've used a PEM nut inserted from the inside of the ebay/coupler. But what is the part you have on the outside of the airframe? It looks like a countersunk washer but appears to sit down in the airframe nicely.

That was a product from a company called LumaDyne. A while back, as far as I know, Apogee became the sole reseller for these:

https://www.apogeerockets.com/Building-Supplies/Misc-Hardware/Aluminum-4-40-Tube-Fasteners-3-pk
They have been sold out of all the sizes for a while now. I talked to Apogee about them a couple time this year and they told me LumaDyne still exists and does plan to send them more, but they don't know when.

I have searched for a replacement - I would love to find one, but, as of yet, have not been able to. One of these days, I may try to mill some myself. I think I may be able to manage the 6-32 version and possibly 4-40, but I think the 2-56's would be too small for me to mill. I don't have a 3D printer (yet), but I think it may be worth looking at using a regular PEM inside the coupler and 3D printing a version of the outer "washer".

 

[FlyBy01]

Finally getting free time to get back to building and ordering these sets, let me know if interested.

Full PEM Kit (2-56, 4-40, 6-32, 8-32, 10-24): $80.69
2-56 PEM Kit (25 PEM nuts): $15.29
4-40 PEM Kit (25 PEM nuts): $15.24
6-32 PEM Kit (25 PEM nuts): $17.12
8-32 PEM Kit (25 PEM nuts): $19.99
10-24 PEM Kit (10 PEM nuts): $13.05

Please confirm:
mrwalsh85 - Full PEM kit (2-56, 4-40, 6-32, 8-32, 10-24)
jd2cylman - 4-40, 6-32 & 8-32 PEM kits
tsmith1315 - 2-56, 4-40, 6-32 PEM kits
mtnmanak - Full PEM kit (2-56, 4-40, 6-32, 8-32, 10-24) + #2 and #4 shear pins
Tobor - ***Please confirm PEM kits sizes if still interested***
watheyak - 4-40, 6-32, 8-32 PEM kits
Pete.D - 2-56, 4-40, 6-32, 8-32 PEM kits
teepot - 8-32 PEM kit

Kit would consist of a pack (25 or 50) PEM nuts of a given thread, 2-3 hex/allen keys, some socket head cap screws, and some flat washers. If shear pins are needed, I can order those as well. If you are not interested, no worries, Mcmaster Carr part numbers are below for reference.
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Is it too late? I would like one of these kits.

 

[rfjustin]

Is it too late? I would like one of these kits.

Sorry, never got around to putting these together. Work and life and what not. All McMaster Carr part numbers are in the thread though.

 

[mtnmanak]

Lumadynes are back in stock at Apogee:

https://www.apogeerockets.com/Building-Supplies/Misc-Hardware/Aluminum-4-40-Tube-Fasteners-3-pk

 

[Tobor]

Lumadynes are back in stock at Apogee:

https://www.apogeerockets.com/Building-Supplies/Misc-Hardware/Aluminum-4-40-Tube-Fasteners-3-pk

Thank you for the heads up!!!

Edit: FYI, they come in 3 sizes 2-56, 4-40 & 6-32.

 

[rfjustin]

Lumadynes are back in stock at Apogee:

https://www.apogeerockets.com/Building-Supplies/Misc-Hardware/Aluminum-4-40-Tube-Fasteners-3-pk

Based on your experience, do you like Lumadynes better than PEM nuts? Curious as I've never used the Lumadynes.

 

[mtnmanak]

Based on your experience, do you like Lumadynes better than PEM nuts? Curious as I've never used the Lumadynes.

I am going to go with "yes and no" - kind of depends on what you want to do. If your goal is to fix a coupler in place in a composite tube, I think the Lumadynes work very nicely in anything 6" diameter or less. Very clean solution. Above 6" gets a bit too big for the lumadyne, in my opinion. They will work, but the thinness of the lumadynes leaves a large gap in the 8" and larger frames (because the FG is thick) and I am not all that confident a 6-32 screw is big enough for an 80+ pound rocket. I am not so worried the screws will shear off, but I am concerned the smaller diameter will zipper in the frame of a heavy rocket. In my big rockets, I go back to 1/4"-20 PEMs.

For most other applications of a PEM - like anchoring a rail button or a camera - lumadynes won't work. They also aren't that great in cardboard tubes. I still think plastic rivets work well in cardboard for about 4" and smaller tubes, but PEMs work great once you get bigger than that. Over time, cardboard has a tendency to warp a bit, especially in places you drill holes. Trying to get a screw into a single PEM in cardboard that has warped a bit is not that difficult, but the lumadynes need to be lined up precisely to work - one side is threaded and tolerance for deviation is very low.

And, obviously, the lumadynes are stupid expensive. Paying $16-$18 just for some hardware to attach a coupler does sound ludicrous. Considering Apogee is the sole supplier of these things now, I imagine the markup is about 1000%. However, if you have a project that you really want to be streamlined - like a slick MD rocket, these are a good option since they are almost flat to the frame.

I guess I would assess Lumadynes as "nice to have" in your build box for the occasion when you want to use them, but not necessarily better than PEMs when you consider cost. If they were cheap(er), I might consider using them all the time - they do give you a very nice end product.