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Tube Compression / Shrinkage Due to Pressing

compression shrinkage tube rockets pressing

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#1 Tinderbox

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Posted 02 May 2017 - 12:09 AM

After about 20 minutes of searching I didn't find anything relating to the problem of tubes loosing length after pressing. Apologies if I missed it. i'm not sure what the correct term for this phenomenon is.

 

I am working on a 3/4" 1lb motor design after purchasing some tools from Caleb at Woody's. I live in the UK but just can not find decent tooling over here so spent a fortune on some quality stuff.

 

Anyways, my tubes are 7.5" long but lose close to 1/4" in length after pressing the whole thing using a 3 ton arbor. The nozzle presses in one increment, fuel grain in about 14 increments, delay takes about 5 and the bulkhead takes 2. I'm using a clam-shell support and when the finished motor is extracted it looks fine. No tube wall crimping or rippling and the things take off like lightening.

 

However, there is this shrinkage thing going on. What is causing this?

 

Tubes are convolute from Stell. Had 500 made up. They tell me they are made from the same materials their pyro clients use.

 

I am fairly new to pressing rockets. I have a PtoF gauge but I find just pulling by feel works well so far. It's a 3 tonnes so maybe I'm exerting way too much force without realising? 

 

Anybody else experience this sort of thing before?



#2 Rip Rap

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Posted 02 May 2017 - 08:15 AM

I have experienced this in the past too. I think the short answer is that you are using convolute tubes. Even with thick walls, once you start exerting very high pressures, the internal wall starts to wind itself up, effectively pulling the tube in on itself and shortening. That is my take on it anyway.

You could try slightly less pressure or parallel wound tubes.


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#3 maxman

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Posted 02 May 2017 - 09:00 AM

The problem arises as each increment grips the wall of the tube when you press it, pulling the tube down and making it shorter. This also makes the outer tube unsightly and wrinkled also. After the tube has "relaxed" some time later...maybe months you will likely get an explosion of a motor that would otherwise have worked just after pressing. This is because cracks then manifest in the pressed comp grain.

 

To fix all these issues you need to treat your tubes with paraffin wax!

 

Tubes will now not shrink, they will press more smoothly, they will not wrinkle, they will store better and you will also be able to use  a faster fuel without the rocket going bang at ignition.

 

Rod



#4 Tinderbox

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Posted 02 May 2017 - 10:24 AM

I have experienced this in the past too. I think the short answer is that you are using convolute tubes. Even with thick walls, once you start exerting very high pressures, the internal wall starts to wind itself up, effectively pulling the tube in on itself and shortening. That is my take on it anyway.

You could try slightly less pressure or parallel wound tubes.

Interesting theory. Hoping the wax will sort it out.

 

The problem arises as each increment grips the wall of the tube when you press it, pulling the tube down and making it shorter. This also makes the outer tube unsightly and wrinkled also. After the tube has "relaxed" some time later...maybe months you will likely get an explosion of a motor that would otherwise have worked just after pressing. This is because cracks then manifest in the pressed comp grain.

 

To fix all these issues you need to treat your tubes with paraffin wax!

 

Tubes will now not shrink, they will press more smoothly, they will not wrinkle, they will store better and you will also be able to use  a faster fuel without the rocket going bang at ignition.

 

Rod

Cheers Rod. Defy going to treat with wax. I was going to get two cheap pans and two small metal funnels. One funnel closed up with a bung of some kind and then inverted inside one pan for the tubes to sit on whilst the other funnel is used to introduce the wax from the top. lift the tube when full so all wax pours out into bottom pan. Other pan used on low heat for primary wax melting. 

 

I saw a video of Ned Gorski doing this and it looked like a great method. No wax on outside of tubes. Just inside the entire length. Thanks!


Edited by Tinderbox, 02 May 2017 - 10:30 AM.


#5 starseeker

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Posted 02 May 2017 - 07:51 PM

Also if it helps' press my 3/4 motors to about 1 3/4 tons on the gauge .



#6 Tinderbox

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Posted 02 May 2017 - 10:51 PM

Is that a prefered pressure rating? Does your press go higher than that? I have a home-made ptof but no idea what actual pressure I am applying. Not sure how to calibrate it tbh.

#7 starseeker

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Posted 05 May 2017 - 06:23 PM

Yes,that is the pressure that i have found gives me consistent results, yes ,my press will go to ten tons but i think that would be a rather large motor :)



#8 cooperman435

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Posted 05 May 2017 - 10:26 PM

Has anyone ever tried a slightly convex base to the rammer?

My thought being it would encourage the loose powder to consolidate while not pushing outwards at the start, once solid it would then push outwards and consolidate against the wall of the tube too but not be pushed down after this point.

If the convex was too strong I imagine it would end up leaving an uncompacted layer around the outside (against the tube wall) which would undoubtably lead to failures though.

#9 Tinderbox

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Posted 07 May 2017 - 07:10 AM

OK. I went and got all the bits for waxing. Waxed 10 tubes. Internal walls only.

My tooling is a very snug fitting with my tubes without the wax and now it seems impossible to get the rammers into the things. Getting them out is an ordeal I never wish to repeat and I only did it once!

This waxing thing just isn't going to work for me. Not with these tools and tubes anyway. Does anybody have any suggestions? What am I doing wrong?

One idea I have is to use a graphite wash. Just ordered 50g of fine graphite powder on ebay. Going to try dusting the tubes with it to see if that improves things any. Hopefuly it will stick and then be forced into the fibre grain of the paper during the pressing process providing lubrication?

Worth a bash.

#10 Tinderbox

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Posted 07 May 2017 - 07:40 AM

Has anyone ever tried a slightly convex base to the rammer?

My thought being it would encourage the loose powder to consolidate while not pushing outwards at the start, once solid it would then push outwards and consolidate against the wall of the tube too but not be pushed down after this point.

If the convex was too strong I imagine it would end up leaving an uncompacted layer around the outside (against the tube wall) which would undoubtably lead to failures though.



#11 maxman

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Posted 07 May 2017 - 08:28 AM

Was the wax hot enough? Needs to be just about smoking 250f so it coats the tube very thin.
Rod

#12 Tinderbox

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Posted 07 May 2017 - 08:24 PM

I'll give it another go then. Thanks.

#13 icarus

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Posted 09 August 2017 - 11:41 PM

dissolve your wax in a solvent mould release wax or even beeswax furniture polish diluted with acetone rinsed through the tube to leave a very fine coating  inside the tube .I tried hardening and flame proofing my tubes with sodium silicate solution .I would not recommend it the tubes are very hard but brittle and an exploding motor case could be hazardous .It also becomes nessecary to coat  the bottom inside of the tube with adhesive using a cotton bud then sprinkle in dry coarse sand it sticks to the nozzle area so that the  pressed nozzle is not blown out so easily .This problem however of poorly bonded bentonite nozzles may also  be due to my addition of porcelain powder from a local refractory to the bentonite .   Porcelain powder helps prevent erosion of the nozzle .caused by long burning end burners


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#14 PeteyPyro

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Posted 03 September 2017 - 12:50 PM

Perhaps the addition of some 'grog' into your clay would help your nozzles to bite into the sidewalls, and not be blown out as easily. Less than 5% of crushed floor tiles or clay flower pots, added to your clay, may be worth trying. I too, wax my tubes to reduce internal sliding friction of the compressed increments of powder. This reduces the linear crushing stress/strain on them, as well as providing a modest degree of resistance to absorbing moisture from the surroundings.

Edited by PeteyPyro, 16 December 2017 - 04:14 PM.


#15 Tinderbox

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Posted 20 October 2017 - 03:17 PM

The beeswax in acetone sounds interesting. Might give that a whirl.

I use cheap Tesco cat litter for nozzles and bulks/bungs. It's absolutely brilliant. Powders up nice in a grinder and never had a problem with blow-outs. Even on long burning high powered drivers.




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