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Old Nov 22, 2004, 08:41 PM
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billystiltner's Avatar
United States, VA, Clintwood
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Graham
Your gearbox is much lighter than mine even with that huge 1/32" bearer.
Mine weighed in at 0.62g without the prop and 0.8g with the obeche prop.


I reckon I could loose the aluminum tube if I could find a glue that would stick teflon to ply or make the teflon a very tight fit in the ply or switch to brass bushings. Would be nice to find some light 0.020" ID ball bearings.

Billy
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Old Nov 22, 2004, 08:48 PM
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Brandon Mississippi/Independence Kentucky
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Billy:

I don't remember if Gordon made that comment publicly or not but I do remember him mentioning it in an email he sent me. My feeling was that the difference he was seeing was as a result of less drag. It's an opinion based on my experience with the Ikara Firefly conversion but Gordon's results seem to support it. With the 4 MM motor we are dealing with very low power so mechanical issues can cost us a lot of performance. Weight is not the only issue we need to pay attention to.

Ralph
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Old Nov 22, 2004, 09:35 PM
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state college, pa
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Billy,

My digital camera's closest focus point is about 2 feet. I do have some close-up lenses for a Nikon SLR, maybe I can hold them up to the lense on my digital and take pics that way. If that doesn't work, I'll either take pictures with a friend's digital cam or dig up a role of film for the Nikon.

Getting the gear mesh right isn't all that hard with the technique David Dewitt posted. You put both disk/bearing assemblies over the shaft in the appropriate spots, and glue in the one in front so that the gear teeth are pressed together. Then you adjust the final mesh with the rear disk. You could probably use a scrap of condenser paper to gauge the mesh, but it would probably be safer to just do it by eye. The most important thing I found to getting the mesh right is getting the shaft to fit snuggly in the bearings. I used little bits of the thin tubing you find to dispense tiny amounts of CA, then stripped down 0.010" carbon rod until I was happy with the fit. If you go slowly and thin the rod a few strands at once you can get a fit with no noticable play very easily. I also found that after a few minutes of motor run the rough cf shaft seems to smooth the burs out of the nylon (or teflon) bearings and it runs much smoother.

Ben
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Old Nov 22, 2004, 10:02 PM
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Ben

I agree it is best to get a tight fitting propshaft. If it's loose at all the thing goes sideways. I accomplished that by using 1/64" ID teflon tube and drilling out with .029" drill. This ends up a perfect fit for .025" shaft. I guess the teflon stretches when you drill it.

Billy
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Old Nov 22, 2004, 10:30 PM
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Ben
I get .0758 for the module of your gears. Isnt it OD / (number of teeth + 2).

Billy
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Old Nov 23, 2004, 08:12 AM
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Ralph,

Thanks for pointing that out. I focus on weight so much that I overlook other things. This is a similar analogy to actuator hinges, a place where maximum efficiency is most important.

Dave
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Old Nov 23, 2004, 11:04 AM
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One other thing to consider on the friction in the bearings: as I understand going to a smaller shaft reduces the friction considerably, Jochen posted something on this I think. Has anyone considered using watch jewels (the type with the hole not dimple) for bearings?
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Old Nov 23, 2004, 12:01 PM
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Has anyone found a "retail" source for these size gears? Like some sort of cheap timer that can be had at Walmart or something like that?

Dave
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Old Nov 23, 2004, 04:46 PM
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I finally got a smooth running gearbox. I added about .015 - .02mm for good meshing. I used the method posted by Robert Guillot to mark my ply. ie mark out 10 times the distance on a line then step out your divider 10 paces adjust caliper, repeat till you get it right. I put the gearbox on a 13 ohm motor and it ran a little 2" prop for a long time without heating up. I broke the uprights while assembling. I misplaced my 4mm drill so had to use a no 11 knife for the final drilling of the motor hole. Holes are oblong so I'm not so sure my spacing is right.
Have another set of uprights in the works. If I hold the motor in front of the monitor while its running I can see a definate wobble that is not apparent in normal light. I'm not sure if the shaft is bent or it's the oblongness of the wire insulation the gear sits on, or an unbalanced prop(how do you balance a 2" prop anyways? - hahahaha). What glue is best to hold pvc insulation to metal? My spur is really loose and have glued it several times with duco.
It is a treat to watch and listen to these little gearboxes run. Even if it doesnt go in a plane it is worth building one to experience it running. I'm not sure what the gearbox will go in or current draws and stuff. Maybe the thing will fly a really light pistachio with a larger prop and the 40 maH cell. Anyone have any thrust figures for the 13 ohm long 4mm motor at 6:1 gearing?

Attached are some more photos.

Ben
Nice photos and awesome little motor gearbox.
Which coil are you using? 24 ohm or 10 ohm?

Graham
I didn't see any jewels small enough for the shaft I'm using but did think about making some quartz or ruby bushes when I seen the dimpled jewels.

Billy
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Old Nov 23, 2004, 04:48 PM
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Quote:
Originally Posted by Graham Stabler
One other thing to consider on the friction in the bearings: as I understand going to a smaller shaft reduces the friction considerably, Jochen posted something on this I think.
Yes, as you will easily understand. The radial force is approximately equal to torque at the shaft divided by radius of the gear. The friction force of the bearing is radial force times friction coefficient (this in turn dependent on the materials and the lubrication). This force is independent of the diameter of the shaft but takes effect on the outer diameter of the shaft, so the friction torque is growing proportionally with the shaft diameter.

By the way, technically spoken a shaft is an axle that carries torque. An axle does not carry torque and is just a pivot point, but may carry radial loads (at least this way it is taught in German engineering schools ).

Regards, Jochen
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Old Nov 23, 2004, 06:09 PM
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A few more photos.
In one you can see what I have been talking about the eylets on the ends of the litz wire.
All it takes to make them:
fold the litz over a pin(about 2 or 3 mm length of wire)
hold 2 strands with finger and thumb
rotate pin same direction litz is spun
remove pin
apply flux to eylet with toothpick
get a dab of solder on your iron tip(needs to be smaller dia than eylet)
push iron tip into eylet
wait till the smoke dissapears
twist the eyelet around a few times
pull eylet from iron tip
touch up the solder and fill up the eylet if it did not fill up on removal
your done - it takes less time to do than to explain and you can keep your iron in one hand when you get good at it
Once you have the eylet filled with solder all you have to do is touch it to whatever you want to solder it to the touch the iron to it , remove and let cool then you have a really nice strong solder joint.

Billy
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Old Nov 23, 2004, 07:40 PM
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state college, pa
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Billy,
That's a nice looking gearbox. I'll agree that it's worth building one of these just to hear it run, the metal gears make a wonderful sound. It's also a great excercise in very precide building, even if you do "go crazy".

I used a 24ohm coil on that motor, my last one in fact. I'll get some more when I scratch up a $25 order from Bob, unless someone wants to send me one in exchange for some 0.0034" flexible dbl sided pcb...

Ben
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Old Nov 23, 2004, 08:12 PM
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Here's the next one
I left the uprights a little thicker this time for handling. The thinner ones were plenty strong during operation but easy to brek when inserting the motor and bushing tube. With the 1/64" uprights the thing weighs in at 0.94g. With the thinner uprights it weighed about 0.9g. Might be some glue buildup on the shaft contributing to the weight as well. I done some current draw testing with this one. It runs smoother than the last and the spur stayed glued during the test but came loose afterards as I was spinning the prop.

Here are the results:
Didel 13 ohm 4mm long motor
0.125 module gears 60t spur 10t pinion 6:1 ratio
2" x 1.5" obeche prop
3.76V 90 to 100 ma

Not setup for thrust measurements yet.
The thing was pushing some air though.
I still have to find a glue to hold the wire insulation to the shaft.
Would like to know a glue that would hold teflon to wood as well then I could get rid of the bushing holder tube to make a much lighter gearbox. Or figure out how to make thin round carbon tubes.

Ben
thanks

Billy
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Old Nov 24, 2004, 03:08 AM
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Paris - France
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Quote:
Originally Posted by billystiltner
...I used the method posted by Robert Guillot to mark my ply. ie mark out 10 times the distance on a line then step out your divider 10 paces adjust caliper, repeat till you get it right....
Billy,

Have you any link to Robert Guillot's post you are referring to ? I did a search on his name with no success.

Thanks.

jean-Michel.
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Old Nov 24, 2004, 03:51 AM
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Oxford/England
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somewhere in this thread:

http://www.rcgroups.com/forums/showt...0&pagenumber=1
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