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May 23, 2013, 06:53 AM
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0.75 gram linear servo using HK board and PZ super lite mechanics


Here's a quick project I did over the past weekend...

I was interested in seeing how light a servo can get using the new micro vapor servo mechanics. I really like the smaller 3.2mm diameter motor, it will be nice when it finds it's way into rotary servos as well. I've read comments on swapping parts like this and not saving that much weight- but I still wanted to try it.

This try was with about an hour's work, trimming plastic, hand drilling holes in the gear and sanding on the board. This was just my first try, I think I can get another 5 mg or so off by machine drilling-milling the gears proper and trimming the plastic and board a bit more.

I considered using a Spectrum servo board, but it weighs (and costs) more than the HK board. The HK servo (part number HK1300) is about $6, the PZ mechanics are $8. Both boards had a lot of excess solder blobs as well, the HK board got down to 0.25g after some more cleanup. The spectrum may have been closer to the HK weight if I had just cleaned it. Another note- the spectrum board happens to have a small hole where the LH side of the rack fastens. It is not big enough for the stock metal screws but would nicely accept thread like I used. On the RH side the hole pattern is the same on both holes, making it easier. On the HK board I used kevlar thread with CA on the tied knot rather than screws to secure the rack. It's easy to remove if needed. I used litz wire dyed with felt tip pens for connection.

The vid test is using a ar6400 aileron signal for power, and my dx7 tx. Seems nice and quick, centers well and has good torque. Not as much as with the 4mm motor, but plenty for me. I'll look forward to doing another or two- mabe getting them down a wee bit lighter. I'd say about 0.70g is as light as it can get.

(0 min 39 sec)
Last edited by shagrug; May 23, 2013 at 07:53 AM.
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May 23, 2013, 11:48 AM
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Vince Herman's Avatar
That is amazing!
May 23, 2013, 01:21 PM
Wanton construction
Pouncer's Avatar
I'm impressed, didn't think it was possible to reduce weight that much!

Looks like it could be possible to make a Micro Racer with servos and a Deltang receiver ..
May 23, 2013, 06:14 PM
Registered User
Hi Dan,

Did you get a chance to check the performance of the new 3.2 motor with a prop yet? The servo looks fantastic.

Joe.
May 24, 2013, 12:17 AM
Registered User
How did you attach the HHservo mechanics to the HK board. It's not clear from the vid. I assume the Mini Vapor servo mechanics' wiper rides on the HK resistance track? (Like how otherwise would it work?)

Otherwise, this is an epic breakthrough!

Ironically, earlier today I lightened an HK "1.7" gram servo (actual weight = 1.92 gm) to 1.52 gm with only simple steps.
May 24, 2013, 03:07 AM
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Thread OP
Jim- I used a strand of kevlar thread to tie it down, with CA glue to hold the knot. Any kind of sewing thread will work, silk is very good and I use it a lot too. The photo shows it just after the CA was applied, the loose ends cut off after it dries.

The PZ and HK brass wipers fingers look different but work the same. You can see the difference in the second photo. All the photos I've posted here will enlarge a few times if you keep clicking on them. You can see the detail better then.

Joe- I finished testing it a few minutes ago, here is a link:
http://www.westknoxrc.com/index.php/....html#msg10698
Last edited by shagrug; May 24, 2013 at 07:00 AM.
May 25, 2013, 08:38 AM
Registered User

0.75 Gram Servo Weight?


I built up servo using mini vapor motor/gear and HK Linear servo. I cut all
the plastic possible on the housing, drilled the spur gear, removed excess
solder from HK servo board, cut down servo board as much as possible,
used ultra thin braided fishing line to attach servo mech. to servo board.
End result was 0.79 grams - Not sure how you got down to 0.75 grams.
See photos attached - Also included photo of dime on gram scale
which weighs 2.24 grams what weight does your gram scale give you
for weighing dime?
May 25, 2013, 08:10 PM
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Thread OP
Hi Scharlach-

I'm a fan of your work- that new pico 3D is very nice! Sometime this summer I want to build something like it.

On the servo I see some more material you can trim or remove, like the metal pin and retainer on the worm shaft. I cut the protruding end down shorter and used a teflon washer instead of the hard black plastic ring.

More of the rack can be removed, there at the motor mount I see yours is longer. Not sure why you removed the front motor strap. I also scraped most of the plastic down thinner all over, using a razor (and #11 blade) at right angle. (like how you would thin a plastic prop) There is a fat divider in the rack middle to thin, along with the outside case. Front and back under the threaded rod there is some area to remove. I also noticed I trimmed down the plastic slider more than yours.

The board can be sanded more, mine is a mm or more smaller on the motor side.(looking at the holes) I should have taken more off mine, there in front of the motor is one spot, in between the holes another.

The excess solder is always a problem, each blob weighs several milligrams. I always use compressed air to blow it all away, then resolder it with just enough. The factory solder is hard stuff and does not flow well. So best to take it all off and just leave the tinning.

The fishing line you used I am not familiar with, but I see too much glue around it. Same on the motor mounting. I use a .010 or .020 cf rod sharpened to a needle point to apply glue. I can apply really tiny amounts that way, just enough to do the job with no extra.

I don't use pennies or dimes for weight reference much, they vary by age nowdays. For example I've got two dimes on my desk now- the 1988 weighs 2.31g and the 2000 weighs 2.25g. My scale may be a centigram or two different than yours too, they are not lab quality for sure. I also have a $300 lab quality scale that agrees with the .75g. A third scale that measures in mg says .761g. It is just a cheap $15 Chinese model that is for stones, but it may be the most accurate. So, my weight may be off, mabe not. I'd say it is close anyway.

Your servo is so close now I think with some more trimming it will be lighter than mine. There is still a lot of excess on the gears that can come off. I wish I had spent more time there, but was in a hurry.
May 26, 2013, 07:11 PM
Registered User

finnaly got to 0.75 gram


Per shagrug's suggestions I took more plastic off housing, enlarged holes in
spur gear, removed excess CA, and took a bit more off the servo board and
finally got to the 0.75 grams (that is without any leads). The servo is still
fairly rugged and I tested force by lifting a 28 gram weight hung from the
servo output slider. It takes a fair amount of careful work to end up with
the .75 gram weight but after doing one anogher one would be a lot easier.
May 26, 2013, 07:23 PM
Registered User

Who will manufacture us a servo baised on these findings


It would be great if one of the servo manufacture's would take the 3.2 mm, motor, a lighter housing, and a lighter servo amp board which would
most likely yield a 0.60 gram servo at an affordable price say $10.
With the DT receivers it is likely that we could have us a 1/4 oz.
7gm 7 inch span 3D/Aerobatic that would fly with authority.

Servo manuf. the Micro Airplane world is waiting!!!
Jun 17, 2013, 09:39 PM
Registered User
Dan,

With all due respect, a 7 gm - 7 in WS plane would be a hoot, however, we shouldn't forget the square law of "resizing". Merely reducing weigh linearly without reducing wing area linearly - will result in the rise of wing loading. At this tiny size, we must be aware of the square law of wing loading - reducing weight by 50% requires reducing wing area by 100%.

The laws of physics are harsh, but if we respect them, we will prevail!!



Quote:
Originally Posted by scharlach
It would be great if one of the servo manufacture's would take the 3.2 mm, motor, a lighter housing, and a lighter servo amp board which would
most likely yield a 0.60 gram servo at an affordable price say $10.
With the DT receivers it is likely that we could have us a 1/4 oz.
7gm 7 inch span 3D/Aerobatic that would fly with authority.

Servo manuf. the Micro Airplane world is waiting!!!
Last edited by 356Jim; Jun 18, 2013 at 09:20 AM.
Jun 18, 2013, 08:33 AM
Registered User
Vince Herman's Avatar
Quote:
reducing weight by 50% requires reducing wing area by 100%.
I'm pretty sure reducing the wing area by 100% will make it difficult to fly.
Jun 18, 2013, 08:48 AM
Gary
gklein's Avatar
Vince, you beat me to it. We know what he meant but it was a good chuckle!
Jun 18, 2013, 09:23 AM
Registered User
Vince and Glenn - glad you caught my mistake. It should have been the other way around.



Quote:
Originally Posted by Vince Herman
I'm pretty sure reducing the wing area by 100% will make it difficult to fly.
Jul 15, 2013, 02:56 AM
Registered User
Thread OP
Here is my second try that I finished up this weekend. More material has been removed from all of the parts, below I'll post lots of pics to show the details.

The test run first:
0.69 gram servo test (0 min 29 sec)


The .weight on two different scales, and comparison pics of a stock servo with both my tests. I don't think I can go much lighter than this second attempt. There is not much more excess material that can be removed easily. Prehaps someone else will have a idea.
Last edited by shagrug; Jul 15, 2013 at 07:06 PM.


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