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Author Topic: Gears for measuring flow  (Read 538 times)
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Mattk
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« on: November 08, 2018, 05:05:09 PM »

I would like to build a gear flow meter similar to the one in this image attachmen t.
These gears are only about 1/2" diameter with 23 teeth, im not exactly sure how to draw this in gearotic, what would be the best tooth design to mesh well enough to keep liquid from flowing through the teeth?
I found this article talking about the principal behind how these meters work here: http://www.flowmeters.co.uk/flow-technology-spotlight-comparison-oval-gear-flow-meters-standard-gear-flow-meters/

Any tips for me would be appreciat ed!
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ArtF
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« Reply #1 on: November 08, 2018, 05:36:05 PM »

 Looks like just 2 standard involutes meshed. Involutes should give you the best mesh ,
I suspect these leak somewhat and are only approxima tions of volume?


  Ahh, on reading I see what you mean. I suspect you dont want very tight mesh, since these
are driven by differenc es in pressure, whatever you build the gears should be on good bearings
and not a tight mesh at all, just a loose  coupling that makes them move with only sligth pressure.


Art
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Mattk
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« Reply #2 on: November 09, 2018, 04:42:18 PM »

Thanks Art,
I think I should go for a tighter mesh to prevent leakage through the gears, the operating pressure of the flowmeter is over 300 psi so I do not think I will have a problem getting the gears to spin.
Here is my first try at Gearotic:
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ArtF
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« Reply #3 on: November 09, 2018, 05:00:40 PM »

Looks good.

   I was more worried about just how much different ial pressure develops from the flow.
As the gears rotate backwards to the flow I was thinking it wasnt much as a %. I dont think
operating pressure would mean much, the pressure different al will be between input and output
flows, so I wouldnt think it would be much more than a small % of the actual flow rate.

  I kinda pictured them running very easy but with one on an encoder or something for
a frequency readout. Ill be intereste d in how much force they seem to have when you
get them running, Ive never experimen ted with flow. Once you get it running let me
know how you make out.

Art
 
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Mattk
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« Reply #4 on: November 09, 2018, 05:09:43 PM »

The flow rates are really low so there should not be a tremendou s pressure diff.

The way I was going to read the flow was to press a magnet into the top of one of gears and pick up the magnet with a hall effect sensor.
This way I could seal the chamber completel y and have the hall sensor in a separate cavity to prevent leaks.
Body of the meter will be stainless which will allow magnetic flux through.
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ArtF
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« Reply #5 on: November 09, 2018, 11:15:10 PM »

If its high pressure with low flow, Id suspect you need them so finely meshed that they spin just by
blowing on them, but then I really have no idea. Just sounds right to me that thge pressure wont matter
so low flow is low energy for spinning. Magnets do sound good for transmiss ion of a frequency .

Art
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Mattk
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« Reply #6 on: November 19, 2018, 11:11:26 PM »

I have decided to use an oval gear for the flow meter as I have read that they work better with the lower viscosity fluids that I will me measuring .
I am trying to draw a tiny oval gear similar to the one in this picture:
 

At its widest point it is 7mm and at its narrowest is is 5mm and the shaft is 3mm  22 teeth.

Is there a setting in Gearotic for me to enter these measureme nts? Or do I just have to adjust the DP untill I reach these measureme nts?
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ArtF
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« Reply #7 on: November 20, 2018, 08:23:49 AM »

Hi Mat:

    I checked on this to see if I could just give you the numbers and I can see
the screen numbers are misleadin g, Ive added a developme nt note to revise them.
 The major axis radisu is incorrect on the screen, its actually the radius of the originati ng
DP pitch radius creating the gears.

   Untill I fix this and rerelease, here is how to get your gears made.

 First, you want 7mm major and 5mm minor axis sizes. Im assuming these are
the pitch radius, if they are outside radius, you really want to know the shaft distance
instead.

  Lets assume a 12mm shaft distance for the sake of this example with 7mm/5mm
pitch radius gears.

  So set Mod to 1 and eccentric ity to 1 - ( 5/7). So subtract minor over major desired
from 1. This gives an eccentric ity of .285 . Regen the gear and youll see the distance
between shafts is 43.5mm's. The desired in this case is 12mm.

  So the Mod is linear to shaft distance. If we take a new mod number from the
setmod multiplie d by the ratio of desired/actual, we get mod = 1 * 12 / 43.5370 = .275

 Entering .275 into the desired MOD and pressing regen you'll now have a center distance
of 11.999mm. with the two meshed radii of 7mm and 5mm.

  Useing the shaft distance as your primary measurmen t, not the outside radius of the gear
will make this a very accurate. The eccentric ity is the reciproca l of the ratio of major/minor end radii. The gears I get doing this are 5mm/7mm with center distance of 12. Heres a photo
of the end design, but Id encourage you to play with the numbers to see the effects
of the ratio and mod. I have made a note to replace the reading of major radius with the proper
one instead of the pitch radius of the DP circle at an order of 1.

Thx
Art






* eccentric.jpg (342.86 KB, 1434x1481 - viewed 73 times.)
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ArtF
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« Reply #8 on: November 20, 2018, 09:13:19 AM »

Matt:

 Just a note, the elliptica l displays have been repaired for next release to show both actual minor and major radii
of the elliptica l pitch radius of each gear. Shouldnt be too long before a release, but if your finding it a problem, yell
and Ill send you a fixes version.

Thx
Art
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Mattk
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« Reply #9 on: June 28, 2019, 06:26:54 PM »

Hi Art,
Sorry for not getting back to you, I have used your advice and created some gears that I think will work.
1 issue that I see with these small gears that I am making is when I export them to dxf I see some jogs in the lines like the attached pic. Is there a way to smooth this out? The full dxf file is here: https://drive.google.com/file/d/1IS3cE6DMsU1zoQR32YtjiRv1tmlg911a/view?usp=sharing


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ArtF
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« Reply #10 on: June 28, 2019, 07:30:04 PM »

Hi Matt:

   I suppose many cad programs will filter the lines, they result from the way
these gears are created, though the envelope of iterative positions all subtracte d
from the gear blank, a type of digital hobbing. This type of generatio n ensures
higher accuracy than most analytica l creation of elliptica l gears, but it has the
side effect of the small variation s you see. They typically though are so small
a variation as to not affect the output.
  I dont have anything in gearotic to smooth them, I always figured the smoothing
can actually make a worse mesh. If you measure the variation you'll usually find
its a small enough number to discount as a percentag e of the overall gear tooth
and doesnt typically affect the mesh.

Art
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