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Post 1 of 9

Helix around a helix?

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06-26-2009 11:15 AM
Hello!

Using Inventor 2010 in Vista 64...

I'm trying to model a helical tube-and-fin heat exchanger, where a single fin extends radially off of a tube, is wound in a helix, and then that finned tube is wound into a helical shape. So, what I have is one big helix around a vertical axis and a second helix that uses the first helical curve as its axis. Using these two curves, I can sweep all of the necessary features. Please see the attached JPEG to see what I'm trying to do.

I can't find a way to generate a helix using another helix as its axis. I was able to do a single turn of the fin helix at one end of the tube and then create a single-axis aligned rectangular pattern along the big helix (thanks to JD Mather's instructions in "Becoming an Autodesk Inventor Professor in 90 Minutes"), which gets me really close, but if there's a way to get that curve and then generate one sweep I'd rather do that.

Alternatively, if there's a way to make the heat exchanger as a straight tube with a fin helically wound around it and then somehow "bend" the entire part into a helix after the fin is swept, that would work too.

That is, unless using a pattern instead of a complex sweep offers significant performance increase... I'm thinking it might, and it's already slow on my 4 GHz machine with 8 GB of DDR3 RAM and a RAID 0 hard drive array. The rectangular pattern I'm using repeats the fin 1400 times.

Thanks for any help!

-Rodney Sparks
Creative Spark Engineering Edited by: rodneysparks on Jun 26, 2009 11:15 AM
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Post 2 of 9

Re: Helix around a helix?

06-26-2009 11:52 AM in reply to: rodneysparks
Someone from Autodesk posted an Excel to create points for this.
You set Inventor to automatically create a 3D Spline through the points on import for your sweep path. I meant to keep this one as this question comes up often, but now I can't find it.
If you find the url to the thread please post it here.

I only tried it with a circular profile, not a rectangular profile like your fin. I would be concerned that the fin orientation might twist. If it does then sweep with guide surface might be the ticket. The other concern I would have would be the number of loops.

Hope someone here turns up that Excel.

JD

Here is one, but this isn't the one by the 'Desker. At the time I didn't realize that Inventor could automatically create the spline. http://discussion.autodesk.com/forums/thread.jspa?messageID=6053371?

Edited by: JDMather on Jun 26, 2009 3:00 PM

Here is a solution. I didn't experiment with the dimensions to try to get to look like yours. Good luck. Edited by: JDMather on Jun 26, 2009 4:17 PM
Please mark this response as "Accept as Solution" if it answers your question.
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Post 3 of 9

Re: Helix around a helix?

06-26-2009 01:48 PM in reply to: rodneysparks
I am not sure it is known. There are quite a few curve sample Excel spreadsheet in \Samples\Models\Parts\Curve Samples\.

johnson.shiue@autodesk.com


Johnson Shiue (johnson.shiue@autodesk.com)
Senior Principal SQA Engineer, Inventor
Mechanical Design
Autodesk, Inc.

*Brian Ekins \(Autodesk\)
Post 4 of 9

Re: Helix around a helix?

06-26-2009 02:09 PM in reply to: rodneysparks
This isn't an Excel sheet but a VBA program that generates a curve that
represents a helix wrapping around another helix. It's something I put
together quickly so there isn't a user interface. Instead you'll need to
change some of the variable values to controls the various sizes. Hopefully
it's useful.
--
Brian Ekins
Autodesk Inventor API Product Designer
http://blogs.autodesk.com/modthemachine


"JDMather" wrote in message news:6209445@discussion.autodesk.com...
Someone from Autodesk posted an Excel to create points for this.
You set Inventor to automatically create a 3D Spline through the points on
import for your sweep path. I meant to keep this one as this question comes
up often, but now I can't find it.
If you find the url to the thread please post it here.

I only tried it with a circular profile, not a rectangular profile like your
fin. I would be concerned that the fin orientation might twist. If it does
then sweep with guide surface might be the ticket. The other concern I
would have would be the number of loops.

Hope someone here turns up that Excel.

JD

Here is one, but this isn't the one by the 'Desker. At the time I didn't
realize that Inventor could automatically create the spline.
http://discussion.autodesk.com/forums/thread.jspa?messageID=6053371?

Edited by: JDMather on Jun 26, 2009 3:00 PM

Here is a solution. I didn't experiment with the dimensions to try to get
to look like yours. Good luck.

Edited by: JDMather on Jun 26, 2009 4:17 PM
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Post 5 of 9

Re: Helix around a helix?

06-26-2009 03:18 PM in reply to: rodneysparks
Wow, thanks for the great suggestions! I'll give them a shot on Monday.

I should have thought about the spreadsheet approach... I actually used that approach to generate a sinusoidally-oscillating planar spiral curve (think the edge of a rolled up piece of corrugated sheet).

I was, however, very disappointed with Inventor's performance with regard to the surface extruded from that spline. I think there were about 150 points in the spline, and it took a VERY LONG TIME for Inventor to process it, I think when generating the spline (via an add-in that someone passed on to me back then). I also tried generating the spline in AutoCAD and then bringing into Inventor, which helped on processing time. When I extruded that spline into a surface, regardless of which method I used to generate it, it behaved very poorly, i.e. faces were being rendered that bridged the "windings" of the spiral, etc. In contrast, when I used SolidWorks via its built in XY point import-to-2D-curve function, it both imported the same exact points and extruded the surface VERY efficiently and had no difficulty in rebuilding the model or flipping it around. Considering all that Inventor does so well, I was surprised at how badly it did this.

So, if that experience was any indication of what I'll see with a spline generated from a minimum of 2800 points, I think I might stick with the arrayed single winding in order to keep any semblance of efficiency. ;-) I'll give the other method a shot though, if only to add it to my arsenal for smaller applications than this monster.

Thanks!

-Rodney
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Post 6 of 9

Re: Helix around a helix?

06-26-2009 05:35 PM in reply to: rodneysparks
>I think I might stick with the arrayed single winding in order to keep any semblance of efficiency.

I would use a cosmetic texture rather than try to model this.
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Post 7 of 9

Re: Helix around a helix?

06-28-2009 01:01 PM in reply to: rodneysparks
You might also improve the performance if you make the single winding a part and do the array in an assembly. Someone brought that up in another radiator discussion a month or so ago.

I did something similar a while ago but mine were just discs, I'm not sure if the real part was spiral or not (and it was just some straight finned pipes with a cross pipe on the end so it was easier all around). I didn't believe they did that at first but a web search found that the disc method was the exception, but the ones I found tended to either be cut on the outer edge or crumpled at the tube. The crumples would be a real challenge - a sine wave on a spiral on a spiral, but I guess if you get the right formula you can get the points for a 3D spline and loft with the spiral spiral rail on the outside and the sine spiral spiral rail on the inside - I'd ask for a much faster computer before trying it though.
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Post 8 of 9

Re: Helix around a helix?

07-28-2009 02:00 PM in reply to: rodneysparks
JD -- Please explain how to automatically create a 3D (or 2D) spline upon point insertion. This is a new one...
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Post 9 of 9

Re: Helix around a helix?

07-28-2009 02:22 PM in reply to: rodneysparks
>This is a new one....

Been around for at least two releases.

Click Options when importing from Excel.
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