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Old Jan 12, 2013, 10:19 AM
Registered User
dk31's Avatar
Athens
Joined Oct 2009
727 Posts
Help!
Mounting a big electric motor

I have the Turnigy G46 motor (300gram weight)
http://www.hobbyking.com/hobbyking/s...arehouse_.html
in a pusher trainer, and I had it mounted on the GP Large motor mount (140gr weight)
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXLCL5

Now I am considering to change this heavy motor mount, and instead use the following layout:
-Four 4mmX60mm screws that will hold the "X" type stock motor mount at around 55mm distance from the fuselage. I will use four 55mm long pieces of Aluminium tube 5,0/4,15 mm (OD/ID) as spacers (screws will be inside the rube)
-Reverse the motor shaft, so that the motor will be mounted in front of the "X" mount (In the old layout, motor was behind the GP mount)

I am not sure if the 4mm screws of alu tube will withstand the forces of this motor. Would 5mm be more wise for this motor size? (4mm is what I have in hand, otherwise I have to look for screws and tubing )
Also I think I have read that mounting in front of (outside) the mount is not good for big motors.
Any advice?

PS: Expected maximum thrust is below 3kg, and airplane weght will be ~4kg
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Old Jan 12, 2013, 12:36 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,288 Posts
The screws will more than withstand the thrust of the motor. But the torque and vibration is another issue. Those skinny scews and tubes do not have a lot of resistant to side loads. And going up to 5 mm won't be all that much better.

A better option would be to build up the spacers from a good hard wood that is about 10mm square with holes for the screws down through the middle. The larger footprint of spacers like this will produce a LOT more side to side rigidity.
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Old Jan 12, 2013, 08:01 PM
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dk31's Avatar
Athens
Joined Oct 2009
727 Posts
Thanks Matthews, helpful as always.
Well you are right..side loads, didn't think of that.
I doubt I will manage to drill a 55mm hole that would be reasonably straight.

However, I already have these big spacers (stand off mounts):
http://www.hobbyking.com/hobbyking/s..._4pc_30mm.html
The inner diameter is 5.1mm and the outer should be around 10mm (I haven't mesured yet). I could stack a 30mm long piece and a 20mm smaller one to get me to 50mm. Do you think it would be ok, even though it's not wood?
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Old Jan 13, 2013, 04:29 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,288 Posts
Thanks to the larger "flanged" ends those spacers would be ideal. To keep them centered slip a little heat shrink or tape around the screws where the spacers join to act as a light push fit sort of "bushing".
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Old Jan 14, 2013, 12:59 AM
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dk31's Avatar
Athens
Joined Oct 2009
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Heat shrink is a nice idea. Indeed, the 5mm screw needs a little something to fit tightly in the spacer.
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Old Jan 16, 2013, 10:14 PM
Thinking about flying
Rhea's Avatar
United States, MN, Eden Prairie
Joined Oct 2008
900 Posts
I have used this mount for that motor. http://www.innov8tivedesigns.com/pro...roducts_id=411
It's not as heavy as the GP mount.
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Old Jan 16, 2013, 10:49 PM
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dk31's Avatar
Athens
Joined Oct 2009
727 Posts
Thanks for the link. I will give a shot to the stand-off mounts as that's what I have in hand.
Trouble so far is that the 5mm bolts are too heavy and the whole assembly gets 80gr (I was hoping to save more weight).
Next plan is to insert 5-OD/4-ID alu tubing in the standoffs and then try 4mm bolts. Or instead of alu tubing use 2-3 layers of heat shrink on the 4mm bolts.
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Old Jan 19, 2013, 10:55 AM
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Harry H's Avatar
Los Angeles
Joined Oct 2010
1,519 Posts
I like to make my own standoffs so I can have the length right. I use different size aluminum stock and dill it out, thats if you have a drill press. Strong and is relatively light for what it is. Could not find a photo from a smaller motor, this is kind of extreme.

Harry
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Old Jan 20, 2013, 12:02 PM
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dk31's Avatar
Athens
Joined Oct 2009
727 Posts
I don't have a drill press (wish I had). A nice thing about HK standoffs though, is that they are "flanged", so they have a larger diameter at the hot spots while they reamain slim & light in the middle. You 'd need a lathe too, to replicate one.

Anyway, the heatshrink solution was a success. I used 4mm screws, two layers of 4mm heatshrink, and the whole assembly reached 65gr. I didn't post any pic, as I still need to reverse the motor shaft, and I had an accident with the set screw (I m waiting for a new hex wrench )
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