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Old Mar 16, 2012, 01:01 PM
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Question
Remote drive shaft

I dont know what to call it really, but the model I'm building is supposed to be a pusher and it is tail heavy even before the motor is mounted. The biggest battery I have is a 2200 mah 3s and it wont balance it and a bigger battery wont fit.

The idea I have is to mount the motor near or in front of the CG and then use a heavy rubber band like a vacuum cleaner belt to connect to a cdrom motor at the back of the wing. The cdrom motor wouldnt have any magnets or windings, just there to hold the prop and for the bearings.

I'm not too sure how to do that(no machine tools), so if there is a better solution like a ready made system for that... Maybe a rubber model prop thrust bearing setup backwards??
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Old Mar 16, 2012, 01:46 PM
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I should make it clear that the heavy rubber band would get twisted by the front motor and then the prop would spin when there is nothing anchoring it. It wouldnt be meant to drive the prop from winding like a rubber model, but now I see that it could spin the prop backwards when throttle is removed. That could be kinda funny like an unintended airbrake.

Anyway, I've got a cdrom motor and looking at it I can see how to mount it and there's enough shaft sticking out the back that I could braze a hook onto the end.

Still interested in any other ideas on this, so thanks.
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Old Mar 16, 2012, 03:52 PM
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Serendipitydooda! I have some super thin carbon arrow shafts and some GWS prop inserts and I managed to fit a prop onto the arrow shaft and the inner diameter of the arrow shaft fits the motor shaft.

So now what I need are bearings which will fit the outer diameter of the arrow shaft and I can make my shaft driven prop and balance my model.

How do I figure out the inner diameter of those GWS prop inserts? They came with the prop on a plastic tree but theyre not labeled or anything.

Thanks
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Old Mar 16, 2012, 05:00 PM
If it flies - I want one!
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Werribee, Victoria, Australia
Joined Jul 2008
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Drill bits?

HELmodels,
Sounds as if you are having fun!
Re your last question, the easiest way to get a rough sizing for the holes is to poke the back end of a few drill bits in them. If you have a set of metric and imperial drill bits, one should be a snug fit.
If you don't have drill bits - you need some!
Cheers,
PeteM
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Old Mar 17, 2012, 05:59 AM
Reduce the drama...
rick.benjamin's Avatar
USA, OR, Damascus
Joined Apr 2004
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Vernier Caliper
http://www.physics.smu.edu/~scalise/apparatus/caliper/
http://www.northerntool.com/shop/too...3656_200303656
Digital Vernier Caliper
http://www.harborfreight.com/6-inch-...per-93293.html

Conversion chart mm decmal fraction
http://mdmetric.com/tech/cvtcht.htm
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Old Mar 17, 2012, 06:55 AM
Reduce the drama...
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USA, OR, Damascus
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Bearings
http://www.rcgroups.com/forums/showthread.php?t=1602228
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Old Mar 17, 2012, 01:23 PM
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Thanks, you guys, great suggestions on proper tools. I'm going to try the local hobby shop today. I might get lucky and they'll have a size that works. They cater to cars and helis mostly and I know they have bearings. We'll see.
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Old Mar 17, 2012, 04:09 PM
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For $2.00 I got two 5mm inner diameter bearings. I found a caliper and the shaft is a solid 3/16, but just shy of 5mm like maybe 4.8mm. Rick's conversion chart shows 3/16 equals 4.725 mm.

It's off by the thickness of exactly one layer of scotch tape.
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Old Apr 17, 2012, 02:10 AM
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Just to give this some closure, I built the drive 3 times. First time it worked, but the shaft needed the bearing moved more to the rear to stop flexing and vibration. Second time, I moved the bearing to the rear and messed up the alignment which made vibration really bad and the motor almost unscrewed itself. Third time I started over with a new shaft, got the bearings lined up right and gave it better support.

There was one other iteration in there and that was I tried the original idea of rubber between the motor and prop. That was a disaster.

The final version works much better than the first and the vibration is minor at low RPM and no problem at high. The motor and mount screws are still tight. If this one goes bad then I'll go with an EDF.
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Old Apr 17, 2012, 10:49 PM
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United States, MD, Elkton
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I'm working on a similar problem,but on a grander scale.I need to run a .91 four cycle through a 17" x 3/8 dia steel shaft...I plan on using a crankcase with just the crank in it to drive the prop,at the nose,with the engine in the cockpit,17" away.
I had considered at least one support bearing about half-way back.Thanks for sharing....now there's TWO crazies...lol
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Old Apr 18, 2012, 12:15 AM
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Quote:
Originally Posted by epoxyearl View Post
I'm working on a similar problem,but on a grander scale.I need to run a .91 four cycle through a 17" x 3/8 dia steel shaft...I plan on using a crankcase with just the crank in it to drive the prop,at the nose,with the engine in the cockpit,17" away.
I had considered at least one support bearing about half-way back.Thanks for sharing....now there's TWO crazies...lol
Good luck with that. I got it to work finally, alignment was critical obviously. I relied on the precision of somebodyelse's manufacturing and the magic of self-leveling.. If you have a machine shop, something like this is probably routine. I have a dining room table, some good and bad luck, and a tube of epoxy.
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Old Apr 18, 2012, 12:27 AM
Art Schmitz
United States, TN, Crossville
Joined Jan 2012
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I would be looking at model boat inboard drives with a prop adaptor if it were my project.
The older J-57 jet engines had a mid shaft bearing on the shaft connecting the N1 compressor to the last two turbine rotors...good for 10,000+ rpm. FWIW. art
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Old Apr 18, 2012, 12:45 AM
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Quote:
Originally Posted by lindart View Post
I would be looking at model boat inboard drives with a prop adaptor if it were my project.
The older J-57 jet engines had a mid shaft bearing on the shaft connecting the N1 compressor to the last two turbine rotors...good for 10,000+ rpm. FWIW. art
FWIW? Quite a bit, thanks. I went with what I had available first but, I remember eyeing the graupner catalog years ago and thinking a boat drive could be useful one day.
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Old Apr 18, 2012, 07:05 AM
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At first,I thought of using a universal on each end,so alignment wouldn't be a problem,as used in cars. But I'm concerned about "whipping",so I'm installing the forward part of the shaft directly into the fuel transfer tunnel in the hogged out crankcase,centered and pinned.
I don't have a machine shop,so this is a file and drill press operation.
If it fails,I've lost some time.....if it works !.....
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Old Apr 20, 2012, 02:51 PM
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United States, CA, Clovis
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I commend your intuitiveness, and creativity.. and don't want to burst a bubble.. as some folks like to take the challenges that we face with RC modeling.. but.. it would seem to me much easier and possibly a lighter approach just adding nose-weight as needed to balance the plane.. as weight is being added with extra motor cases.. shafts.. bearings..etc.. anyways..

So I wonder after all that, if the outcome would have been the same weight or even less if the correct weights were added to the nose to start with. I wish you luck, and again there's some very cool engineering going on here.. this was just a thought I had.
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