31 replies to this topic

[stix]

This is where I'm at. Crappy quality, but given time, you can see what I mean.

 

 

70rpm is the fastest the motor will go.

Edited by stix, 19 September 2021 - 11:06 AM.

[stix]

It's early days, but the concept is there.

 

I've got to attach some sort of "tilt switch" so when it breaks, it doesn't (unnecessarily) wreck the whole thing. I've got other things to do as well with making parts for the final product.

 

This is my new "cross slide vise" attached to my basic drill press.

 

 

Well, not actually attached in that image, but now so. Cheers.

Edited by stix, 21 September 2021 - 09:53 AM.

[stix]

I've made some modifications. I've extended the radius of the arm and included a swivel type elbow which hopefully gives more randomness. Also very importantly, the device is now securely attached.

 

I thought I should clarify some things. The reason for the "stress" test is to determine where the weakness is in the rocket motor mount. My view is that there should be a weakness on purpose for safety reasons and also to protect the load cell in case of a RUD (Rapid Unscheduled Disassembly) 

 

The attached image (red circles) shows where I think/thought/predicted that there may be a failure of the mount:

 

 

That's my reasons for doing this "stress" test.

 

The Aluminium is 1.6mm (1/16th") thick V-shaped angle. Scored and folded under to create the mount feet.

 

I'm concerned that over time their would be fatigue on those points and they would break - fail.

 

So, here is the last video of the updated test device before I let the test run for hours. Personally from what I've seen, I think the dc motor will fail first.

 

 

Cheers.

[stix]

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Edited by stix, 29 September 2021 - 10:15 PM.

[Richtee]

A heat treat on the bent edges would help the cause. Or pop rivit/bolt on steel flanges I guess. But... you have more time on yer hands than I do

[stix]

A heat treat on the bent edges would help the cause. Or pop rivit/bolt on steel flanges I guess. But... you have more time on yer hands than I do

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Thanks Richtee. I'm a bit confused about what you mean.

 

Heat treating the Aluminium makes it softer - which is not good for what I want. Yes, I can use pop rivets and steel to make it stronger, but that's not what I want. Steel pieces makes it way way less safe. Who wants steel pieces flying through the air after an accidental RUD? I don't.

 

Maybe we are having a mis-understanding. The V-Mount which holds the rocket motor should be expendable, but also have good strength to securely hold the motor during many firings.

 

My "stress test" is to determine when, or if, the V-Mount fails over time. That is, it fails due to metal fatigue. I chose thin Aluminium on purpose. If I wanted to ensure it never failed then I would "over-engineer" it using thick Aluminium or Steel. It's a safety issue as well.

 

Maybe I'm looking at this whole scenario completely the wrong way??

 

Nevertheless, I'm going to start the test in the next 10 minutes, run it for a few hours and see what happens. I'm hoping the V-Mount will fail at some point. I'll try to keep the test going for as many hours as it takes. In post #23 you can clearly see the wobble and stress forces applied the the base "feet". It should break eventually.???

 

After that, regardless if it breaks or not, I will have to do an actual rocket motor test. A rocket motor test that is designed for an RSD. Rapid "Scheduled" Disassembly. The test will be done properly "in the field" with no danger to others. I'm hoping that that test breaks/bends the V-Mount without destroying the load cell.

 

The load cell I'm using is capable of measuring 50kg (110lb) of thrust - which is substantial for mid-powered rocket motors. These load cells cost around AU$50 a piece, so maybe, in case of RUD, best to destroy the aluminium mount than the load cell. Also, as I have said previously, "SAFETY" is paramount and part of my current design process.

 

If I haven't explained things well, please ask questions. Time to start the proper test.

 

Cheers.

 

[EDIT]

I realise, I always, use too many commas

Edited by stix, 01 October 2021 - 05:35 AM.

[stix]

After 1 hour, there is no failure of the mount itself. But, it seems like the DC motor is becoming worn out. I can hear it.

 

Like i said in a previous post, the dc motor will likely f*ck up before the V-mount does.

 

Nevertheless, at this point, I'm happy with what I'm seeing within the constraints described.

[stix]

On the 2nd hour, it still seems like the DC motor is screwing up. But it is still working and putting force onto the point of question. I'm still getting the bending of the aluminium mount, which is good.

 

At this point, something has to give sooner or later.

[stix]

On the 3rd hour, it's time for an evaluation.

 

The dc motor seems fine. Using the Elbow arm that swings around and about (post #23), I thought was random as much as I made it. But it still has a bizarre consistency to the randomness. I didn't expect that. I tried to make random, but is seems like within random, there is consistency. Weird indeed.

 

Occasionally, there is a cyclical thing that does happen where it sits there for a while - thinking?... Nah, there is a point of too much weight at a particular point.

 

Anyway, I've got it set for 8hrs. It's almost 1am here. I must sleep on the couch in lounge-room just in case something happens.

[stix]

It's going to be part of my dreams.

 

https://soundcloud.c...9cdb935387fe511

Edited by stix, 01 October 2021 - 09:07 AM.

[Richtee]

I dunno the difference between steel and AL flying around. They all hurt. So build it so that does not happen. Is that not the point?

[stix]

I dunno the difference between steel and AL flying around. They all hurt. So build it so that does not happen. Is that not the point?

.
Yes Richtee, I understand what you are getting at. But my point was about if there was a RUD, then the Aluminium piece is likely to deform and bend, than break off flying through the air, therefore safer. I certainly wouldn't use steel. If I wanted to go stronger, then I would use thicker Aluminium, but I don't need to, which my test has proven.

 

In fact, I'm considering drilling small holes in the back of the "V" like a perforation to ensure it bursts apart in that way, in case of an unfortunate RUD.

 

Anyway, the TEST IS COMPLETE!

 

The testing method I refer to, is the video at the end of post #23.

 

I had it running for 12 hours straight. Nothing broke!!! I was very surprised indeed.!!

 

I was initially disappointed, I wanted breakage. But I'm happy that the mount stood up to the forces and stresses applied to it. What that means to me, is the "V-Mount" design is way strong enough for the purpose it was built for. I was concerned that it wouldn't be. I no longer have that concern.

 

The motor did have some wear on the brass bushing that surrounds the shaft. This made the shaft loose and caused the whole "arm" to droop slightly, and no doubt the reduction gearing inside the motor would have been wearing out as well. I suspect over a longer period of time, the motor would have failed before the mount did.

 

In conclusion:

Firstly, I'd like to thank everyone that has contributed to this thread. I know it started out as an electronics question, but the purpose was always about doing the "stress test". I've learned a bit more about electronics and brushed up on my coding skills. Also making the pieces required, like the arm and bearing elbow was enjoyable.

 

I'm not working atm, so it's good to use and exercise my brain - for better, or worse. Also, it's interesting to understand the many paths that pyrotechnics can lead you down. I mean, I don't have to be making BP etc. I can still enjoy this hobby (for me) creating test equipment etc.

 

If anyone is interested in the Code (Arduino, C#) that I used to get the motor spinning up and down, then I will post it here.

 

Cheers.

 

[EDIT] In the end, it wasn't a pointless exercise at all

Edited by stix, 02 October 2021 - 08:33 AM.