PEM Nut Kits and Shear Pins Kits?

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deandome

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mtnman, where did you get the metal 'countersink reinforcements', and how are they fastened/implanted in the outer tube/nosecone?
 

mtnmanak

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mtnman, where did you get the metal 'countersink reinforcements', and how are they fastened/implanted in the outer tube/nosecone?
Those are Lumadyne nuts/washers. You can find them at Apogee:


There was a time when you could buy them straight from the manufacturer and I got a whole bunch of them in bulk. Now, it appears you can only get them from Apogee. Too bad, they are pretty slick and Apogee is selling them at a premium.

Once my current supply is gone, I am not sure I will buy them at $5 a pair. I have been looking around for a replacement, but have not found one yet.
 

teepot

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25 or 50, depends on what quantity said thread/size is sold by Mcmaster. I don't plan to split up quantities if that makes sense.
Depending on the cost I would be interested in the sizes mentioned in a previous post.
 

Idunno

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I'm assuming by now you placed the order. But if for some reason you have extras i'll give some a try since i'm almost out of rivets. Current project is 3" so I guess some 4/40? And whatever the next smaller size is for a 54mm.
 

deandome

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Not to poop on the fun & comradeship this kinda thing brings out, but I needed pem nuts now, and got 2 sizes of knockoff 'self cinching nuts' on Amazon @ $8.00 per bag of 50 https://www.amazon.com/dp/B08XYJXJRC?psc=1&ref=ppx_yo2_dt_b_product_details

Delivered free in 2 days, they seem perfect enough for me, 'cept you can select 3 different 'thicknesses' of the toothed, grippy-sections, and I went with the thinnest one. No biggie, but I'd recommend you go with the medium-size
 

AHansom

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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:
View attachment 460172

View attachment 460173

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:

View attachment 460174

View attachment 460175

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.

View attachment 460176
Pem nuts are cool and I have used them to hold payload sections to the AV bay. I don't think they will add any significant strength to you fin attachment. Theses pictures are of large headed rivets on a 6" BlueTube tube fin, it shows they had little if any effect on the tube fins leaving.
 

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mtnmanak

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Pem nuts are cool and I have used them to hold payload sections to the AV bay. I don't think they will add any significant strength to you fin attachment. Theses pictures are of large headed rivets on a 6" BlueTube tube fin, it shows they had little if any effect on the tube fins leaving.
Those pictures show fins fully breaking off on impact. It doesn't matter what you attach fins to a rocket with - if they break off, what's the difference?

I used #6 stainless steel bolts as well as 3 points of epoxy contact for each tube fin. Each tube fin is a 4 inch fiberglass thick wall coupler. If one of those fins breaks, I am seriously doubting my problem will be with the attach points. This is a 25 lbs rocket. On one flight at Red Glare, the deployment bag wrapped around the main causing a total collapse. It burned in on a 15" drogue right onto the fin can. Paint was scratched, but the fins were fine. I flew it again that day on an L1090, no problem. I have also flown it above Mach 1. Flew it again this weekend, perfect flight. I am very confident the PEM nuts and stainless steel bolts helped assure none of my fins have budged on hard flights and hard landings.

Nothing is going to help you if you crash a bird hard enough with blue tube fins.

You can see the scuff marks on the bottoms of the tubes from the hard landings:

PXL_20210517_004452413.jpg


I have also used PEMs and #10 screws to both secure the tube fin and install the 1515 rail buttons inside the tube. Works perfectly every time.

PXL_20210517_005657007.jpg
 

AHansom

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Those pictures show fins fully breaking off on impact. It doesn't matter what you attach fins to a rocket with - if they break off, what's the difference?

I used #6 stainless steel bolts as well as 3 points of epoxy contact for each tube fin. Each tube fin is a 4 inch fiberglass thick wall coupler. If one of those fins breaks, I am seriously doubting my problem will be with the attach points. This is a 25 lbs rocket. On one flight at Red Glare, the deployment bag wrapped around the main causing a total collapse. It burned in on a 15" drogue right onto the fin can. Paint was scratched, but the fins were fine. I flew it again that day on an L1090, no problem. I have also flown it above Mach 1. Flew it again this weekend, perfect flight. I am very confident the PEM nuts and stainless steel bolts helped assure none of my fins have budged on hard flights and hard landings.

Nothing is going to help you if you crash a bird hard enough with blue tube fins.

You can see the scuff marks on the bottoms of the tubes from the hard landings:

View attachment 464518

I have also used PEMs and #10 screws to both secure the tube fin and install the 1515 rail buttons inside the tube. Works perfectly every time.

View attachment 464521
Sorry mtnmanak most definitely not done on impact. About 3 or 4 seconds into an M2020 tore the fins off. Landing was soft and smooth!!! Rebuilt it and still take the boy Scouts out and launch it. Mostly with 54mm IMAX L's or small M's since then;)

 
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mtnmanak

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Sorry mtnmanak most definitely not done on impact. About 3 or 4 seconds into an M2020 tore the fins off. Landing was soft and smooth!!!

Point is, the fins broke. If your fins break, who cares how you adhered them to the frame?
 

AHansom

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;);)
Point is, the fins broke. If your fins break, who cares how you adhered them to the frame?
Correct. A couple bolt's, Rivets or Pem Nuts won't make a difference on fin attachment unless you click your heels 3 times and you really believe.
 
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AHansom

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Who did the music. It sounded like Dick Dale surf music.
I'm needing to go way back teepot. It was a local Santa Barbara / Ventura surf band from the 90's called Phantom Riders. Mostly saw them playing at the local concerts in the park. With all the distortion, they might have used PemNuts to hold the amps together :cool:
 

teepot

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I'm needing to go way back teepot. It was a local Santa Barbara / Ventura surf band from the 90's called Phantom Riders. Mostly saw them playing at the local concerts in the park. With all the distortion, they might have used PemNuts to hold the amps together :cool:
Well, I got the surf part right.
 

rfjustin

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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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