Why Snap Rings

[edwardw]

Who do most casings use snap rings. From an engineering point of view it seems that you are taking away material at at place where the circumference could expand and that isn't good.

Why not instead take 4-40 machine bolts and screw into a retaing block with hole through it for the nozzle?

I know snap rings are prevalent and are reliable, but how did this come about? And machining snap ring grooves can be a bear, but a block with screws is easier.


Edward

 

[Johnnie]

In my opinion, it is for safety concerns...should the motor over pressureize, the end closures would let loose, before the sides would burst. I witnessed it happen to 2 of my own motors...kinda sounds like a shot gun blast

Sometimes even snap rings will hold long enough to fragment the casing on a motor that builds pressure fast.

I prefer snaps to threads, as there is very little groove clean-up like the threaded casings...I do not have any experience with the bolted style of motors...yet!

 

[edwardw]

When I have seen motors CATO usually they are 100% gone. Even with snap rings I have seen a 54mm motor cato and the splits started at the ring groove and traveled down. I can see a 'shotgun' blast type thing - does this render the casing unusable?

 

[Johnnie]

The casing generally swells like pregnancy before the closures let loose, rendering the the remainder of the casing useless...

 

[Ryan S.]

well, I think snap rings are easier to use, and they are also easier to machine into the case. If you use screws you also need to machine nozzle making more work there.

Just a few thoughts....

 

[bobkrech]

All commercial consumer motor casing are required to fail in a controlled manner should a massive overpressurization occur due to a mechanical grain failure. (See NFPA 1125) Since the airspace above and below the rocket is clear before launch, the forward and aft closures are designed to fail at pressures that are 2 to 3 time higher than the expected maximum chamber pressure, but 2 to 3 times lower than the case rupture pressure. This spits out the nozzle and/or the forward closure and the fuel grains, preventing the generation of shrapnel in a CATO. All commercial consumer casing must be made from 6061T6, a strong aluminum alloy that yields well below it tensile strength so it will not fragment into shrapnel.

Almost anyone can make a decent C-ring groove on a lathe, whereas it take a reasonably good machinist to cut quality threads. If you setup in a lathe, you could easily cut 2 internal C-ring grooves in a minute. It will take several minutes to cut in internal thread unless you have a CNC machine. It's simply cheaper for low volume operations to go with C-rings.

Also lathe work is much faster and cheaper than drilling and tapping. Threading fine threads in graphite is a job for the pros and certainly 4-40 threads would pull out unless you had an ungodly number of them. And in the end, you don't need to buy the taps or the screws with C-rings.

Machining nozzles from graphite is really simple when it's all lathe work. Once you have the OD finished and the graphite cut to length, it should take only 10 to 15 minutes to make a 54 mm nozzle if you are set up on a lathe to do it: Drill the throat in 2 or 3 steps, put a slight dish on the chamber side, a 15 degree half angle cone on the exit, and pop in 2 o-ring grooves on the od.

The only disadvantage with C-rings is the need to have a good set of snap ring pliers and the need to being careful when inserting or removing them. A slip of the compressed ring could result in personal injure, especially to the eye, if safety glasses and precautions are not taken.

In practice, motors with snap rings go together a whole lot quicker than with threaded fitting and with fewer parts. The Kosden/Animal Works approach is simpler for the Ex guy making his own casings. A cheap source of C-ringed hardware is the purple stuff from https://www.aeroconsystems.com/motors/diy.htm

Bob Krech

 

[edwardw]

I am asking because even though I have access to a decently equipped machine shop, they don't have a lathe long enough or with a big enough through hole to mount some aluminum and cut the grooves. I was then thinking of using an aluminum washer with three 4-40 screws or maybe even three 2-56 screws in it to retain the nozzle like a snap ring. First I am going to machine a small 6" casing and hydrotest it so that I can get small enough screws so that they will shear around 1500 psi. From the tool stand point it is easier to make a nozzle out of graphite and a retaining washer out of aluminum and then just have three screws into the aluminum.

Edward

 

[bobkrech]

Edward

The standard snap ring design is commercially available from a number of sources, Loki Research, Animal Motor Works, Kosden, etc.

From your website I think you're looking for a 54 mm case that is about 24" long. AMW has a complete motor for $175 and Loki has a a similar one for $180.

https://www.animalmotorworks.com/hardware01.htm

https://secure.consumersinterest.com/loki/store.asp?groupid=521200312291168

That includes the casing, forward closure, nozzle, snap rings, o-rings, everything. I don't think potential cost saving to build one and hydrotest it yourself, if any, is worth it.

If you do make your design, the base ring should be steel or stainless steel. Aluminum looses all strength by 300 C, (570F). Your motors tend to have a high chamber pressure. I would think that the design failure point should be 3000 psi, and the minimum design case failure at 10,000 psi.

For material strength of alloys go to https://www.matweb.com/

BTW It's not as easy, but you can make the grooves on a milling machine with a boring head and a rotary table, but it's a lot of work.

Bob Krech

 

[AZ_Ron]

For EX hardware, you can also check out

www.lokiresearch.com and
www.proxhardware.com

Jeff Taylor is Loki Research, and "Woody" Wood is Pro-X.
Both great guys to deal with, with quality as good as Kosdon or AMW.

Ron