Hollow RC Motor Shaft; extra Astro cooling thoughts

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SoSauty

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Do motor shafts have to be steel? All the motors I've seen have steel shafts. I got Astro 3210 & 3220 in mind, but this could apply to others also. Problem I'm seeing with cooling my 3210 is that heat transfer from inside the motor, to the outside is slow. Even if I get a heat sink to dissapate 5X's via surface area, I'm only looking at being able to apply an extra 5-10% additional amp(or power; uncertain which is more correct). I can put a glorious CPU heatsink on the exterior body.

A fast way to cool my little Astro3210 is to put an icecube on the protruding steel shaft. Then I see SDP-SI sells hollow steel shafts. Cool. Yet when grinding I notice that aluminum transfers heat around much faster than steel. (I'll edit later and back up several of these points). Aluminum should withstand 700f degrees, heat tolerant enough for the inside of an RC motor.

So, if aluminum doesn't mess up electrical fields, or something, and Astro says they can build motors with 12mm shafts, why not use 7075 aluminum rod? Could get it 12mm diameter and put a 4mm hole, maybe larger thru it. Then place some sort of mini fan blade at each end, presto, airflow to carry inner motor heat out and away :idea:

This isn't so hard and doesn't seem anyone else has done this. So what gives? Where I'm off :?:
 
It wouldn't make a significant difference. The heat is generated in the windings with (in the case of the Astro) a good conductive path through the stator laminations to the outside of the case. The only conductive path from the windings to the motor shaft being through the stator laminations, then the case and finally through the bearings..... In any case, very little heat dissipation could be achieved, using air through a 4 or 5mm hole.
 
If you are going for the max, think about oil cooling with ATF (Automatic Transmission Fluid). Bring the oil in on both shaft bearing areas. Allow it to form sheets on the rotor face. Have the rotor fling the oil into the stator winding end turns. Then have gravity drain it to your heat exchanger and pump. This approach was demonstrated by the "Power Dense" electric motor project at Bowling Green State University in the early 2000's.
 
A hollow aluminum shaft sounds like a fun experiment, and I suppose it couldn't hurt. Cheap too.

If motor heat became a big issue for a particular RC project, I think it would be easy to enshroud the baseplate and blow air into the motor. No need to put a lot of work into the escape path, the air will find a way out the holes at the other end. Or, add an end-cap over the outlet that is shaped like the well-designed ventilated front brake discs, which would pull air through the motor...or both?

Lots of options
 
Miles, your comment about the outer windings producing the heat has me hopeful about trying this CPU heatsink. Its amazing, grind a piece back and I can feel the heat transfer. Blow on it and I can feel the warmth being carried off on the far side, recheck the fins and they've cooled. It's about 84mm in diameter after removing the fan, my Astro is closer to 82mm diameter. So I've cut a slit down the fins, it's stiff with little flex but still have hope for a snug fit.
http://www.crazypc.com/products/spinq-50070.html
I'm planning on coating the Astro body with epoxy and fitting this over, then snug it down with twisted copper wires which I'll leave on. The small tubes running perpendicular through the fins are copper but coated, I'm guessing chrome plated. The fins are less than .5mm width with 1/16" spacing. With decent contact, the surface area is almost 10X fold, yet I don't anticipate excellent contact. I just hope the epoxy doesn't hold in more heat than gets transferred out thru this heatsink.

With an outer heatsink, I think the inner motor would still be hot and a hollow aluminum axle would just be cheap, if part of the manufacturing process, and functional way to move heat out from the center. Unneccessary if mounted on an airplane but a bit helpful for e-bikes. Will contact Astro to see if I can ship 'em a hollow axle to build a motor around next time.

Don't think my hobby skills are up to expiermenting with the oil spray, our e-bike motors truly have a long way to advance.
 
Depends on the epoxy you use...you can get stuff with high thermal conductivity for potting transformers etc.

http://www.epoxies.com/therm.htm
 
Good grief jonathanm, now I have to spend hours searching and reading about thermal conducting resins to use LOL, thanks :!:

Ordered 'Silver Artic' epoxy w/ rush shipment. Plan to epoxy the heatsink Thurs before Willow Springs run.

Spinning Magnets, any ideas what's inside an Astro? I started to pull the housing off yet would've needed to clip my connectors off the motor wires so didn't. Think I could open up the ends of an Astro for air to circulate thru?
 
If "silver arctic" is the same as "arctic silver" - They have a good rep as a heatsink compound in the computer field. dunno about their epoxies though.

As for mechanically coupling a soft aluminum shaft to a hardened sprocket....I agree it's not gonna last too long but should see you through a race if that's the aim. I would go for a ground flat and grub screw with loctite....
 
SoSauty said:
any ideas what's inside an Astro? I started to pull the housing off yet would've needed to clip my connectors off the motor wires so didn't. Think I could open up the ends of an Astro for air to circulate thru?
Click to expand...

AF3210-12T-02.jpg
 
Y'kmow, I agreed with you at first, but on second thoughts I don't know if liquid coolant is such a bad idea. In the world of machining, it is quite common to use some kind of coolant spray or mist over the tooling to cool it......it is nothing more elegant than spuffing coolant over the tool/workpiece interface. as long as you have a way to collect, filter and re-spuff it, then it's not such a big deal...........
 
Texaspyro easily made his own thermally conductive epoxy in (I think) one of the lighting threads. Might've been in the CD tab welder thread, though.
 
Given that the magnets on the Astro motors can safely be used up to 550deg F. (288deg C.), with temporary loss of performance being commensurate (I guess the bonding would fail, below that :)). The priority should be retaining the integrity of the winding insulation, I would have thought? Surely, a hollow Alu. shaft would be completely and utterly pointless, with regard to this?

A heat sink with circumferential fins, accurately machined and press fitted to the case with thermal grease would be good. A case with integral fins (as on Matt's version) would be better.

Drilling a few holes around the case, either side of the lamination stack and in the end caps, could also be effective.
 
Yeah Miles! Astro still uses Samarium Co magnets and they can work through a fair bit of heat. Not quite as strong as the Neodyme ones but Neo starts to fail (lose magnetic strength) at about 180C.
otherDoc

edit: The model boat racers use coils of copper around the inrunner can motors with an inlet/pump arrangement and seem to have success. For a bike a small radiator might be effective too. This has been discussed a lot in the forum. Check it out SS!
otherDoc
 
Man boostjuice, those windings are pretty tight, guess 6T as there maybe 3pairs for each of the 8 poles. My Astro is a little different with the wires exiting thru the end cap rather than the can.
So Miles, I just need to get the Astro apart and get to drilling 8) Getting to see Astro innards and know I can open up the can is near priceless to me :!:

AW, I thought about grinding some copper and stir that into epoxy but Artic Silver epoxy removes some of my concerns. Should be in my hands a day or 2 before Willow Springs.

I don't see a great problem with an aluminum shaft fitted with a double set screw aluminum pulley such as I have now. Could grind 2 slightly concave slots near the end of shaft at 90degrees from each other to cup the set screws. Also could use epoxy which isn't a great problem to break loose later on.

Docnjoj, hope to find that thread when a get a bit more time. The heat tubing (copper tubing lined with a wicking material) seems pretty efficient at transferring heat. I think that would work similiar to copper coils around the can. You can special order it sealed with coolant for passive operation. Pricey I bet, but.. Motored Bikes still aren't as costly as some racing formats. The tube would travel up? to a flat aluminum plate similiar to coil in an air conditioner condenser or radiator. Once cooled (heat cast off) revert back to a liquid and wick back down to the motor.

Presently I'm able to run 60-65amps on/off generally WOT for 10-12 miles. I hope 6/18 to run 70amps and still hold the motor temp to 170f or so.
This heat sink was a 1st idea before finding the CPU fine finned heatsink.
EpoxySpok.jpg

Additional notes; the pulley now has 8 holes drilled for cooling, the JB epoxy makes the 9mm wide pulley able to handle the 15mm belt and is holding up so far 100+miles. The pulley & belt soaks and helps shed some heat.
 
I think AussieJester had some suggestions using computer pump and radiator coolers for his insane tower computers that he used to build. Very strange to hear of a water cooled computer but ya do gotta keep them processors cool!
otherDoc
 
OK, wish me luck and skill at Willow Springs tomorrow Sat 6/18 :p
heatsink.jpg
 
Looks great! wonder how it performs - guessing you will find out soon enough, lol. What did you use for the thermal transfer epoxy? Good luck!
 
Hey Man,

I stock heatsinks for Astro motors, you know. :) They slide right on. I would have gladly donated a pair as a sponsorship item. :wink:

Anyway, assuming your heatsinks have good contact with your can, you should have PLENTY of cooling.

As a side note, I am working on something that will make heat questions on Astro motors (that are running my drive) a thing of the past.

Honestly, though, at 94+% efficient, few people have heat concerns. However, for those of us pushing the envelope, any help is welcomed.

Matt
 
Sponsorship items eh? I'm all ears :!: Silver Arctic (Arctic Silver) 14 gr thermal epoxy. Stuff said 5min working time. At 4 min I thought it'd go 10min, then bam, instantly hardened, just a little flex in the material.

Thank you Miles. This heatsink did allow a minimum of a 10amp additional throttle, from 60 up to 70amps. That's conservative, maybe in a few weeks, I'll revise that upward. (edit: ran 70amps, when I could nail it, and the motor indicated a slightly cool 160f after a 7.2mi. main race; think It'll take 75-80amps!)

Really having fun squeaking another ounce of MoJo from this half a soup can motor. But when I look into the gypsy's crystal ball, I see a 3220. Believe with a tight set up, the 3220 could take any motored bike presently out there. Noticed that my little rig was killer in the turns, but (compact Astro 3210) 2.8 horsepower in the unlimited class isn't up to the task. The 3220, capable of real power, is only 3.4" long and can be fitted nice and low, below the BB just inches off the asphalt, gives a fantastic low center gravity. The Schwinn would dart in at every turn, only get ahead a yard or 2, then give it up the on the straights. Spent 3 laps getting around 1 bike then pulled 100' out front of 'em the next 3 laps.

Be even more fun with longer races. Only used 5.5 amp hr. of 12S lipo for the 12 lap race (have 18amp hr, 20amp hr, 24amp hr lipo packs), could've run another 3 races on a single pack :!:
 
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