Design for a Thruster pod, V1.0


#1

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2013-04-05 Update: This post is now out of date. To see the most recent version of the thruster, see this post.

-Ben

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I've been working on designing a magnetically-coupled, pressure compensated, and ducted thruster pod. here's a picture of the current design:


I'll discuss the details more later in the post, but here's the important part: The design is ready for a production build and some in-water testing, but I'm looking for some help manufacturing parts. If you are interested in helping to produce a set of the machined parts, please let me know. The particularly tricky part, if you have not guessed already, is the nozzle, which needs to have a precise fit against the prop maximize efficiency. Anyhow, if you have some good machine tools, a good 3D printer, or are an ace machinist and want to give it a shot, get in touch!

And now, on to the details!

  1. The overall length is approximately 124 mm, and the maximum diameter is approximately 85 mm.
  2. I've used an H105 hydrofoil as the surface of revolution in the shroud. In theory, it offers better performance (less cavation) than the usual Kort nozzle, but in reality, I've guessed on so many things - like the angle of attack, that it's hard to predict if it'll be better or worse. My solution was to plan for adjustments after the fact, which brings us to:

  3. There are two versions of the nozzle. There's a single-piece version and a split version. The two piece nozzle is held together with three 2 mm steel pins, which can be removed, and allow other shrouds to be attached. If possible, I'd like to get a minimum of one split version and three single-piece versions to play with - this will let me make side-by-side comparisons during normal use and swap in other nozzle designs if for comparison testing.

  4. For any aspiring hydrodynamicists out there, professional advice on the bulb and nozzle design would be appreciated!

  5. I'm still experimenting with the magnet mounts in the coupling. I haven't figured out the best way to make them yet (preoccupied with other problems), but all that really matters is that they hold the right number of magnets. (and that they fit, but who's asking . . .)

  6. There's a thrust bearing in the back to ensure that everything works smoothly.

  7. Magnets: I use ten .250" x .250" x .100" magnets for the motor mag mount, and ten .125" x .125" x .125" magnets for the prop mag mount. Initial tests indicate that the coupling is plenty strong, though I have yet to do any in-water testing. I procured the magnets from Magcraft via Amazon.

  8. I use the E-flite spinner nut that's designed to work with the prop. The major benefit of this is that there's no trouble with aligning the prop on the shaft, and that the nut has a nice tapered shape, improving the hydrodynamics.

  9. Right now, there is no geometry locking the nozzle to the rear end cap. Until I've had some time to play with it, I plan on using rubber cement (or something else that can be removed if I feel like checking on the bearings). In the long run, (v2) I'm going to either put in a thread or some snap-locking geometry - depending on how I end up using it.

  10. The final assembly will be filled with light mineral oil (food grade), and put in a vacuum chamber to degas it before it is sealed. This should render the motors corrosion proof and pretty much immune to pressure. A membrane (red in the picture) allows for some compression and expansion due to temperature differences or slight compresability.

If you've read this far, you deserve some reward for your work, so here are some pictures of parts that I've built to start testing out this design:


A motor with half of a magnetic coupling


The other half of the coupling. you can see the thrust bearing that prevents reverse thrust from pushing the prop out of position.


The two assembled sub-assemblies. It looks great, now all I need is the hard parts . . .

As I said earlier, if you are interested in making some of the more complicated pieces, send me a note - I'll send you the CAD and we'll see what happens.

-Ben


#2

I've got a pretty nice mill(Tormach PCNC1100) that I somewhat know how to use and a lot of random stock material, can you send some stuff to scubasonar at gmail and I will check it out? If there are lathe parts a friend of mine has those.


#3

Beautiful work! :D Out of curiosity, how much torque does your magnetic coupling design enable? Also in regards to the shroud design, some CFD might be able to help pin down shroud parameters. I'm no CFD expert but I can try running a few simulations! Could you pop the CAD files onto someplace that I could publicly access it? (like in the thread or something)


#4

I will be working on this, on these following two weeks.

R&D process is going on right now. I have CNC and all kinds of machines. I just need a cad maybe
I am with 3ds max for now.


#5

I like solidworks and sprutcam for CAD/CAM but just some recommendations. It will be cool if we can build up a nice quick prototyping network for openROV!


#6

thats a good idea! I'll work on setting up a wiki page listing people with prototyping capabilities


#7

All,

Thanks for all the interest!

For those of you who want files, here's what I would like you to do: Send an email request to mccand at gmail. I'll reply back with the CAD. I apologize for the inconvenience, but if we do it this way, I know who to tell when the models change, and I can make sure that everyone has the most up-to-date files. I'll also send some notes that may be useful.

I should have prepared better for distributing the files (version numbers in everything), and I'll do that next time. Until then, I hope you'll bear with me.

-Ben


#8

Colin,

Great question. My rough assessment - by feel - was that the torque was that it was probably "good enough", but I actually haven't got around to measuring the torque yet. I'll try to get a measurement for you by tomorrow.

For the files, send me an email (see the post below), and I'll reply back with files.

-Ben


#9

One more note: the native files are Solidworks 2012. If you need a different file type, let me know in your email.

-Ben


#10

We will get pro after this first round! I am having a friend come over tomorrow night so we can try and learn how to use the 4th axis on my machine for tricky parts.


#11

There might be a MakerSpace that has the tools to build this.


#12

this will be my first try, I am totally amateur on this


#13

I think Thruster Motor Mag Mount v1.0 is the one I am going to try or Thruster Prop Mag Mount v1.0 They look like good two operation 4 axis mill parts. Thruster Rear Cap looks the next easiest on a friends lathe. The remaining parts besides Thruster Bulb look pretty tricky.

Here's my hope for success: http://www.youtube.com/watch?v=CYTMk3ZuQG0&playnext=1&list=...


#14

Update: A rough test with a load sensor indicates that the coupling slips when roughly 4.4 N is applied at the end of the prop blade, which is approximately 32 mm from the axis.


#15

I ended up making the Thruster Motor Mag Mount in a couple of pieces. What I'd recommend doing is fixturing the part with its axis in the vertical direction and cutting the slots by passing across the top (rotate, then repeat). Then you can glue the magnets in place and place a ring over the top.

I've been using ABS for these parts, and I've found that it's much more difficult to bond the magnets in place than I expected. Even after roughing up the surface, neither superglue nore the two part epoxy I've been using forms a particularly strong bond. This is then complicated by the fact that the magnets desperately want to move.


#16

For the CFD, if anyone has the Flow Simulation module available (since the file are available in Solidworks2012), a quick assessment of the shroud parameters can be made easily.

From what I remember of the software, mesh generation is kind of automatic which might be useful in our case. However I don't know its performance in terms of computation speed and accuracy, but we are more interested in the tendency more than really optimizing the shroud.


#17

I have some PEEK plastic clamped into the 4th axis and a friend coming over tonight with more machining experience than me so we can try it out. For the more complex pieces it might be easier to have them 3d printed and then cast them in polyurethanes if you are worried about strength. I have the mold making and casting materials but not the $$ to get the part printed. My friends are very good at mold making (one of them teaches mold making and the other is his assistant) that can help us.



#18

Fortunately, none of the parts needs to be particularly strong, since the thing will be pressure compensated. Any plastic will probably work fine, with the possible exception of the mag mounts. Those need to be made of something that bonds nicely with whatever adhesive is used to hold the magnets in place.

For casting, the only thing to worry about is shrinkage of the shroud - it's a pretty close fit between the prop and the plastic. Let me poke at a couple of people and see if we can have some nice high-res SLA models made. We could then cast those. What resin do you have? If I know the shrinkage, I can plan ahead for it.


#19

Hi,

looks great! Good work so far!

Excuse me for the question but why didn't you think about a rotary shaft seal?

Would'nt this be much easier than magnetic coupling?

http://de.wikipedia.org/wiki/Wellendichtring

- Tobi


#20

Tobi,

There are a couple of attraction for magnetic couplings.

  • First, a worn part does not result in a leak. Avoiding leaks is really ideal, as once the motor unit is sealed up, there's no provisions to check for leaks or to replace the part (in this design).
  • They allow us to divide the thruster into two independent and replacable sub-assemblies There's the "Motor Unit" and the "Drive Unit". If one of these units fail, it can be replaced without affecting the other. If we have an oil seal, this becomes much more complicated. With the current design, every part exposed to seawater is either immune to corrosion or is easily and (relatively) cheaply replaceable.
  • Finally, magnetic couplings are far more forgiving than shaft seals. Dirt, grit, and muck that would destroy a shaft seal and compromise the motor scarcely bother a magnetic coupling, and in the worst case will merely jam the drive unit. This shouldn't result in motor damage, since a properly sized coupling will slip before the motor is stalled, and repairing the problem is a snap - simply remove the drive unit from the thruster and clean out the obstruction.

If I could do have built this any way I wished (money and equipment was no object) then I could simplify this design even further! There are actually two magnetic couplings in this thruster: the one we've been talking about, and the motor itself. My preference would be to have the motor built with sealed windings (epoxy impregnated or somethign similar), and then the outrunner would be mounted directly to the propeller. Care would have to be taken to ensure that the shaft bearings were protected from muck, corrosion, and comtamination, but then you'd essentially have a two piece thruster. For another variant on this idea, you should check out Eric Stackpole's "Ring thruster" comment on this post: http://openrov.com/forum/topics/brushless-motors?commentId=6365107%3AComment%3A7404