PEM Nut Kits and Shear Pins Kits?

[rfjustin]

I've got 3-4 builds on my horizon:
- Composite Warehouse Concept 125
- Composite Warehouse Concept 98
- Composite Warehouse Lotus 5 (changing the fins slightly and stealing the conical cone for the Concept 125)

I need to re-up supply of PEM nuts and shear pins, so a McMaster-Carr order is needed. Since the question about PEM nuts and shear pins comes up somewhat regularly, would you have interest in a PEM nut kit? Same question for shear pins, if a kit were available, any interest?

Not looking to make money, more of a public service if there is interest.

Assuming there is interest, what size PEM nuts? What thread and length shear pins?

Thanks for the feedback!

 

[mrwalsh85]

I might be interested... Varying sizes. Is the tool one-size-fits-all, with different inserts?

 

[jd2cylman]

Deal me in on PEM nuts kits. 6 or 8-32.
I think I’m set on shear pins currently.

 

[crossfire]

JF
I would be in for a kit. PEM nuts and pins. Also have a C125 to build. I purchased a red one but I will guess you will paint yours.

 

[rfjustin]

JF
I would be in for a kit. PEM nuts and pins. Also have a C125 to build. I purchased a red one but I will guess you will paint yours.

I plan to paint the fin can area only. Paint the fin can area black, and the nose cone black. Orange tubes look too nice to paint over.

 

[crossfire]

I plan to paint the fin can area only. Paint the fin can area black, and the nose cone black. Orange tubes look too nice to paint over.
View attachment 460042

Will look nice. Most of the time I clear coat it. Dirt gets into colored tube and it just will not clean up. Its really nice a decal is included.

 

[rfjustin]

I might be interested... Varying sizes. Is the tool one-size-fits-all, with different inserts?

I have always used socket head cap screws, (2) flat washers, and a hex wrench to walk the PEM nut in place. More often than not, I also epoxy the threads before installation.

Once I place the mcmaster carr order, I'll post a "how-to" here with some pics.

 

[tsmith1315]

Justin, I'd be interested in a PEM nut kit.

6-32 and smaller would be more useful for me.

 

[rfjustin]

Justin, I'd be interested in a PEM nut kit.

6-32 and smaller would be more useful for me.

2-56, 4-40, and 6-32? Or is 2-56 too small in your opinion?

 

[mtnmanak]

I would be interested in PEMs and pins. For PEMS, I use everything from 2-56 all the way up to 1/4-20.

 

[Tobor]

Hmmm, I've never used PEM nuts before but, it would be nice to have the option. So okay I am interested. I'll take some of every size PEM you order. Oh, and the same for shear pins.

 

[watheyak]

I'd be interested in a #6 or #8 kit. Maybe even a #4

 

[tsmith1315]

2-56, 4-40, and 6-32? Or is 2-56 too small in your opinion?

Not too small for me, that mix sounds great.

 

[jd2cylman]

Justin,
Add a couple of 4-40 PEM sets to my list. Say, 4 of each set.
Thanks

 

[crossfire]

JF
Just give me a good mixture of sizes not really sure what sizes I need.

 

[T34zac]

I’m a bit confused as to what you mean by a kit?

I’ve been using 8-32 PEM nuts in my 54mm up to 4” rockets for a while now. The screws countersink nicely into the airframes

 

[rocketcharlie]

I understand and use sheer screws, but would someone explain how PEM nuts are used? They are new to me.

 

[T34zac]

I understand and use sheer screws, but would someone explain how PEM nuts are used? They are new to me.

In place of plastic rivets or other methods of joining airframe sections that aren’t supposed to separate during flight. Like an Av-bay and payload tube

 

[rfjustin]

I understand and use sheer screws, but would someone explain how PEM nuts are used? They are new to me.

Pics incoming.

 

[rfjustin]

I’m a bit confused as to what you mean by a kit?
I’ve been using 8-32 PEM nuts in my 54mm up to 4” rockets for a while now. The screws countersink nicely into the airframes

I think you mean the PEM nut itself countersinks into the airframe?

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.

I prefer to use socket head cap screw, (2) washers, and "walk" the PEM nut in. Once the PEM nut is fully seated, back out the socket head cap screw, rinse and repeat.

 

[T34zac]

I think you mean the PEM nut itself countersinks into the airframe?

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.

I prefer to use socket head cap screw, (2) washers, and "walk" the PEM nut in. Once the PEM nut is fully seated, back out the socket head cap screw, rinse and repeat.

No I mean the screw itself.
Attached is one that I just did the other day into a 4” fiberglass airframe

 

[rfjustin]

No I mean the screw itself.
Attached is one that I just did the other day into a 4” fiberglass airframe

So there is a PEM nut on the backside of said countersunk screw?

 

[T34zac]

So there is a PEM nut on the backside of said countersunk screw?

Yes. It’s inside the coupler

 

[mtnmanak]

I understand and use sheer screws, but would someone explain how PEM nuts are used? They are new to me.

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:




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:





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.

 

[rocketcharlie]

The PEM nuts sound and look good. I have never seen one as far as I know, I have always used plastic rivets. Are PEM nuts difficult to work with? Do they require being counter sunk? How far do they protrude through a fiberglass tube?

 

[rfjustin]

The PEM nuts sound and look good. I have never seen one as far as I know, I have always used plastic rivets. Are PEM nuts difficult to work with? Do they require being counter sunk? How far do they protrude through a fiberglass tube?

Hang loose, pics coming in the next few days, likely before my next order is placed.

 

[mtnmanak]

Another use for PEM nuts - attaching your camera. I have seen Justin post his shrouds and use a screw that goes all the way through to a PEM in the body tube.

I like to use them with keychain cameras (808's, for example). I drill out a slightly larger hole on the back where the keyring goes and put a 6-32 bolt through there to a PEM in the body tube. I use nuts on the bolt between the camera and the PEM to adjust the angle. Velcro on the bottom of the camera keeps it from shaking a lot. I have flown that config supersonic numerous times. Quick, easy and interchangeable.

 

[Idunno]

Can someone explain to me why pem nuts are superior to rivets? They're more expensive to buy, heavier, harder to install and yet they serve the same purpose. What's the upside? Durability? Aesthetics?

 

[rfjustin]

Can someone explain to me why pem nuts are superior to rivets? They're more expensive to buy, heavier, harder to install and yet they serve the same purpose. What's the upside? Durability? Aesthetics?

More thread sizes/options, depth (panel thickness) options, robust mounting, and consistency. Nothing wrong with rivets, I just prefer PEM nuts, especially in fiberglass tubing.

 

[mtnmanak]

Can someone explain to me why pem nuts are superior to rivets? They're more expensive to buy, heavier, harder to install and yet they serve the same purpose. What's the upside? Durability? Aesthetics?

Justin's points are all valid and I would add that I personally use rivets when doing cardboard construction.

I would counter some assumptions, though -

After a quick search of about 10 rocket supply sites, the best price I found for plastic rivets was about $3 for a pack of 10, or 30 cents a piece.

A box of 50 PEM nuts at Mcmaster is $10.64 for a box of press-fit steel PEMs and a box of corresponding steel 1/4" screws is $4 for 100. So total cost per install is about 25 cents.

However, even though the per PEM/screw cost is a 17% savings over the plastic rivet, I am not sure the PEM or rivet is breaking the bank on a rocket that costs hundreds of dollars. More important is that I can get a box of PEMs and screws that are not only cheaper, but they store smaller in my shop and are good for 15-20 rockets with one order.

For weight, I weighed a few plastic rivets and they averaged about 0.19 grams. I weighed some 4-40 PEMs (for me, these are the most common ones I use for 4"-5" rockets and below) and they came out to .16 grams each. Even adding in the screw weight for some 1/4" long 4-40 screws only brought the total weight up to 0.3grams.

So, the PEM itself is lighter than the rivet, but a set of 3 PEMs and screws would add about 1-2 grams over the weight of rivets. Once again, the additional weight on a multi-pound rocket would be negligible.

I am not trying to criticize you, just wanted to make sure that there was no confusion that PEMs/screws are more expensive or that much heavier. In the end, it is a wash in those factors for a vast increase in strength, options and efficiency.

Valid point on "harder to install". I would say that the smaller you go, the more difficult PEMs are to work with (2-56 PEMs are made by the Devil himself...), but when you get to larger sizes, it is really easy. 4-40's aren't bad and by the time you get to 6-32, I don't find them that much more difficult to install versus rivets.

Finally, there are some applications that PEMs can accomplish that rivets can't. I would not attach tube fins to a high power rocket with rivets, for example. Ditto for cameras/camera housings.

At the end of the day, I don't see it as an "either/or" choice - I have many sizes of PEMs/screws available and I have plastic rivets. They are all useful.