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Old Sep 24, 2010, 09:37 PM
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Truglodite's Avatar
Carmichael, CA
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Quote:
Originally Posted by Stall'n_N_Haulin View Post
On a side note, is there an easy way to calculate motor efficiency from ET data?

Also the outside loops on every other tooth (on pic) not being able to be flush at the base, does it cause any concerns/issue's? I've seen many motor pic's with such, but asking anyway. As you can see, I could fit 9T on one tooth before going to next layer, but only 8 on the next tooth.
There are many commercially available programs that can calculate efficiency based on RPM/V/A. If all you're interested in is finding efficiency from a single prop test, drivecalc simple prop calculator is probably your best bet. It's free, simple to use, and reasonably accurate. The best part about Dcalc is lots of people use it. So your numbers will be comparable to the majority of thrust and efficiency numbers you'll find on these forums.

No real concern with 8T (1st layer) on one tooth and 9T on the next, as long as the total turn count is correct. There is a very small difference in resistance, since the 9th turn has to be longer on the poles with 8T on the first layer. Still, the difference is probably something 99.9% of us would never notice.

Quote:
Originally Posted by Stall'n_N_Haulin View Post
Here's some ET from a 4cell run up. Same prop, 6x5, 2200mah 25c. 10P 10T 20awg YY. Approx. 2030kv from unloaded averaged 5sec run data. 15624rpm/7.58v - 21952rpm/10.81v - 26615rpm/13.10v.

It should unload in the air a bit too since its only 3" off the floor. Just to compare, I plugged up a storage charged (3.85v) 3cell for some #'s. 10T 3cell was 16042rpm - 8.80v - 21.77amps - 191watts.

4cell 10T about the same rpm of the 3cell 7T at 15 less amps. Runs strong and cool.
Based on the RPM's, I'm guessing the first set of numbers you gave were all no load tests, not with a 6x5. The motor gods would be much happier if you included currents (Io) for those tests. You ran a 6x5 prop an 3cell, but the voltage is very low. My esc's would be on the LVC at that point with a 3s pack. For that reason I generally scrap data if it falls near 3V/cell, besides, I wouldn't want to be flying my 3s packs that far down. Not sure what 6x5 you used, but plugging in an EMP 6x5, Dcalc SPC says 68%. Not great, but heck, a 90gm motor isn't going to overheat with 70W of heat, and the performance isn't too shabby for a torque motor used in a high rpm application. Curious to see prop data on 4s.

Cheers,
Kev
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Old Sep 25, 2010, 10:27 AM
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I'll look up dcalc, thanks. I looked at the formula, but didn't know power in and wondered if ET's no load amp draw was accurate also at .49amps I think it was.
Motor Efficiency = Pout/Pin - [Pout = (Vin - Iin * Rm) * (Iin - Io)]

Yes, those numbers were unloaded. Ah yes, I didn't put the amps. I'll get those and add to post. (2cell 3.94amps - 3cell 5.26amps - 4cell 6.02amps)

The 3cell wasn't even charged, I didn't want to wait before posting 4cell data, so I ran it up on a stored 3cell in case it helped.

The 4cell data is the graphs. I had picked them off of two different FT runs over 10sec and zoomed in to get averages as mentioned earlier. I can post the whole file(s) if anyone wants them.

68%... not so good, but for a basically free motor and the motivation to bother learning some winding, I'm happy for now. Anyone remember the old Mad TV dating service - Lowered Expectations...
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Last edited by Stall'n_N_Haulin; Sep 25, 2010 at 10:50 AM. Reason: add amp data
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Old Sep 25, 2010, 11:00 AM
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Carmichael, CA
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Pin = V*A, or simply Watts in from ET. ET does get a bit "grainy" at low amp draws, but it's accurate enough to tell motors apart. Yep, those Io's are a bit high... what we'd expect from a torque motor spinning faster than it wants to. On the flipside, I bet Rm is very low.

Kev
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Old Sep 25, 2010, 11:14 AM
JustFlying
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I only saw this thread now and did not read through it all, but I see there are some crazy winds on this motor. I also have three of these motors which I got from a guy that replaced it with other motors in his planes. One is still standard and two I played with different windings, up to 413 watt. I did not use any in a plane yet.

What I want to know, did anybody report bearing failures here? With this motor all I see is well but for the bearings, I doubt it will last long with high power input.
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Old Sep 25, 2010, 12:46 PM
Jack
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BP Hobbies had the bearings on a closeout at 50 cents a pair, I bought 10 pairs and will change them as often as needed.

I've started lubricating my bearings before I install them and they are doing fine. I have a number of hours on the set in my SpaFFFnutz and they are still dead quiet and very smooth.

I use Mobil 1 10W-50 and a suction pump to fill the bearings, I don't think they put enough lube in them when they make them.

Jack
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Old Sep 25, 2010, 01:31 PM
JustFlying
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Quote:
I use Mobil 1 10W-50 and a suction pump to fill the bearings,
How about melted grease?

Did any of these DT750 motors bearings failed before?
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Old Sep 25, 2010, 08:02 PM
Jack
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There has been some muttering here about noisy bearings on some of them. I think the bearings are a little undersized but I like them keeping the bearing tube size down a little. So it is a trade off of sorts.

I'll bet if you made up a bearing tube and doubled the bearings at the prop end or went to a wider bearing there you might get better bearing reliability.

I don't care for the mess of doing the melted grease thing but it might work good too.

I have a Mityvac hand vacuum pump I use for bleeding brakes and stuff. To lube the bearings I put them in a small zip lock bag with a little bit of Mobil One and then put the bag in the suction catch bottle. Then I pull a little vacuum (10-12 inches or so) and you'll see a good sized bubble come out of each bearing as it is filled with oil. So there is not much oil in them to begin with is my guess.

I take them out of the ziploc bags and lay them on a rag to drain the excess and then mount them. I have not found any inordinate messes or anything else from doing it that way. And the bearings are working good.

Jack
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Old Sep 25, 2010, 10:07 PM
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Most motors, including DT7xx's, ship with a thick greased packing. Having greased bearings on our little motors is like running uphill in the sand... oiled bearings are more like ice skating.

Here's the tool I use:
First I use "Magnum brand" electric motor cleaner spray to blow the bearings out (readily available in most rc car shops, Magnum is what my LHS carries, any similar product will work though). To use it effectively, I made a few "bearing adapters" from various Aluminum and CF tubes with ODs that match my motor shafts (DT7xx's are 4mm). The adapters are each attached and sealed with heatshrink to short pieces of red plastic tubing that came with the can of spray (the red tubing comes with the can and fits in the nozzle, just like cans of WD40). The opposite ends of the tubes are sealed with a dab of epoxy. I used my dremel to cut small slits on the sides of each tube, about 1" from the epoxy sealed end.

To use:
Push the adapter on the nozzle of the cleaner spray, insert the adapter through the motor so the slit is located between the bearings, and spray while rotating the stator back and forth. The cleaner comes out of the slit, creates high pressure inside the bearing tube, and forcefully blows out any grease, oil, or dirt that might be in the bearings... you'll see the spray squirt out at high velocity between the inner race and sheild... wear eye protection... don't ask me why.

Once dry (motor spray dries in less than a minute), I add a small drop of mobil 1 5w-20 full synthetic to both bearings between the inner race and sheild. Really any lightweight oil will work. The lighter the better, and full synthetic sticks around longer than others. You just want a film on the balls. Flood the bearings is not a good idea, because it attracts more dust. Too much oil also may cause the bearing if they "hydroplane", which creates huge hydraulic pressure that creates friction and causes pits in the races.

It sounds pretty complicated I guess, but if I could show you a 1min video you'd see how simple it is. It takes less than 30sec to sterilize and oil a stator this way, and a 20oz can of spray will clean about 100 motors. I've found this procedure makes a noticeable difference in efficiency, particularly with high RPM stuff.

IIRC, there is a post earlier in this thread (with photos) from a guy who upsized his front bearing. For sure the loads resulting from 400W are going to be hell for the microscopic stock bearings. Regardless if it's a tractor or pusher, most of the radial load acts on the front bearing. If it's a pusher, the front bearing takes the axial load; if it's tractor, the rear bearing carries the axial load. So the front bearing mod is perfect for pushers, but on a tractor setup the tiny rear bearing will still fail early. That's why I think the small bearing tube is a major design flaw on this motor. On the other hand, even if it did have a bigger rear bearing, I still would have a hard time trusting a 4mm e-clip to hold on to 4lb of thrust. Of course, if you have a lathe, you could cut out a bigger bearing tube, and replace the e-clip with a stepped wheel collar. *sigh* I digress, I wish this were already done at the factory... I'd pay double if they had reasonably sized bearings. (hint-hint)

Cheers,
Kev

Cheers,
Kev
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Old Sep 26, 2010, 07:22 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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A good and easily done bearing mod for the stock bearing tube would be to ream the bearing counterbore deeper and seat a pair of the 4 x 7 x 2.5mm ball bearings.

Or even better, go from a ball bearing to a 4 x 7 x 5mm or so roller bearing.

The roller bearing would give you a huge decrease on the loaded area in the bearing. But as always, there is no free lunch. It would bring a small amount of additional bearing drag with it but I'll be it would be almost impossible to measure.

I like the suction trick because it is easy on the seals and is quick. When I drain them a get a small amount of oil back out and I think I'm only leaving the interior well coated.

Mobil One, in any weight, is really a good lubricant. And it used to be better. A few years ago Mobil One was a pure all synthetic oil, then the cheap buggers started diluting it with petroleum oils to "improve it's performance." But that was a bald faced lie and they tried to make the change in a very low key manner hoping that consumers would not notice it.

Adding the dinosaur oil just made it cheaper to manufacture. If you want good pure synthetic oil pick up a bottle of AMSOIL, you can find it in motorcycle shops if nowhere else.

There is little or no difference in the viscosity of 5W, 10W or even 50W oil as far as room temperature flow or drag. The Weight spec goes more to the operating temperature performance of the lubricant than the thickness of the oil.

Jack
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Old Sep 26, 2010, 12:11 PM
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Mobil 1 makes "synthetic" (a natural/synthetic blend), and "full synthetic" (100% synthetic). My local Walmart and most of the auto supply stores around here sell full synthetic. It's more expensive than the partial synthetic stuff, but well worth it.

Cheers,
Kev
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Old Sep 30, 2010, 11:48 AM
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Replace bearings

Can anyone help me?
I need to replace the bearings on my dt 750, how can I do?

Thanks
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Old Sep 30, 2010, 12:50 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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Remove the circlip from the back of the motor and pull the magnet housing forward and the shaft will slide out of the bearings and stay with the magnet housing.

Take a small (3mm or so) fibreglas or metal rod that is about 3"/75mm long and push it through one bearing and to the side so that it will contact the outer race on the bearing on the opposite end of the bearing tube.

Hold the rod and bearing tube together and tap the end of the rod on the bench top. Move the rod around the bearing a little and repeat the tap. Keep doing that until the bearing slides out of the tube.

Then repeat that for the opposite end. Or you can use a 1/4"/6mm wooden down on the second bearing once the first has been removed.

The bearings you want are the 4mm x 7mm X 2.5mm (I.D. x O.D. x width) bearings that are called MR74ZZ bearings in their most common grade. Here is once source for them and more info about them:

http://www.rc-bearings.com/catalog/p...roducts_id=431

I recommend they be lubricated as I described above before they are installed. They will be much quieter and last longer.

Jack
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Old Oct 01, 2010, 01:37 AM
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Thanks Jack!
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Old Oct 05, 2010, 01:56 AM
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rewind

Hi, I replaced the bearings and the crankshaft as well, now I rewind my dt 750 but first I need some advice.
I use 4 for the propulsion of a multicopter.
I would use the 10x4.7 or 11x4.7 APC, powered by 3cell lipo.
The maximum efficiency of the engine I would like about 350-400g of thrust (the model weighs 1400-1600gr) with a maximum thrust of about 900-1100 grams.

What kind rewind do you recommend?

thanks
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Old Oct 05, 2010, 03:37 AM
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Mexico, BC, Mexicali
Joined Aug 2004
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for 400g is best, i think for AX2210, AX2213.
one HXT750 to 20turns is good for around 900g.

one Stok HXT with 2s is for around 400g with APC 11x4.7.

Manuel V.
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