With the Quad 3DR frame availability increasing, I wanted to take a moment to offer a "Build Quick Tip". Skip to the bottom of this post to get to the "quick" part of this tip.


The Backstory


The ArduCopter2 frame uses narrow aluminum arms, and so the design calls for motor mounts. The 3DR frame uses 20mm, 3/4" square tube arms, much larger than those on the other frame, and so it offers a different mounting method. Included in the 3DR frame kit, for each arm, are metal screws and a bag of washers/spacers. The 3DR uses a direct mount method. Each arm has two holes spaced about 1.5 cm apart along the center line of the tube. Screws are placed up through the arm holes and connect directly to the bottom of the motors. Only two of the four motor mounting holes are used, but it is plenty solid, and the X-shaped motor mounting brackets included with most brushless motors are not used at all.


The 3DR frame assembly instructions discusses motor mounting, and I encourage you to read all of the instructions carefully, twice before picking up a single tool. Once you are done reading those instructions, I recommend you take the small bag of spacers and ... put them aside. Do not even open them. 


"But if I do not use the spacers, don't I run the chance of damaging my motors?" you might ask. The truth is, no. You will not run the chance of damaging your motors, you are virtually guaranteed to damage your motors. The screws are simply too long, "engineering variances" aside, for virtually any motor you are likely to be using. The point of my recommendation, however, is that even with the included spacers, you are still running the risk of damaging your motors. So perhaps I should explain myself?


The 3DR frame is my first "kit" built frame. It is my fourth quad, but the first frame I purchased. When yours arrives, it may well have three or four times the spacers/washers that were included in mine. I purchased one of the earliest 3DR frames available from the DIYDrones store, and it included about 20 spacer/washers. I used all the washers and supplemented some small nuts besides, and I still wasn't sure I was getting the spacing right. Ever eager, I "got by" with fewer of the spacers on one of the arms. And you can see the motor from that arm in the picture above. It is a disaster waiting to happen. It functions around 45% throttle, just enough to get the quad off the ground. Then it stops. I discovered this during routine testing, fortunately, and not during a flight test. I am confident that jDrones shipped me a good motor, so what went wrong? The screw was just long enough that, under load, there was some shorting (think, spark gap transmitter!) inside the motor that damaged part of the stator wiring. The motor will spin up and then will short and fail. Backing the screw out will not help, the motor damage is done. Fortunately for me, my last order from jDrones was a batch of sixteen. What can I say, I am an addict. 


The "Quick" Tip


The washers/spacers will be useful, so keep them. For use elsewhere. The problem is that there are just not enough of them, and even if you do have enough of them, it is a pain to count them out (you need at least five per screw, 40 in all, to be safe, and I suspect they are also heavier than this solution.)


Included in your propeller bag (and, if you are using the standard or heavy ArduCopter motors, and the 3DR frame, I really do recommend you use the 10x45 or 12x45 props; unless you know the math better, of course...) are some plastic inserts used, along with the prop adaptor, to fit the propeller to the motor shaft. You can see them in this photo.


At the top of the photo, you will see the full set of  

unadulterated spacers. You get one of these sets with each propeller.


The second set of spacers, in the middle, shows one item missing. That is the plastic insert that is pressed into the center of the propeller.


Notice on the bottom, two additional spacers are missing along the right hand side. Counted collectively, you will have four spacers of just the right size for use in your props themselves. Then, for each prop, you will have two spacer of very slightly different sizes which will work perfectly for our purposes. 


After you have fitted the correct spacer into the propeller, to avoid any mixups, remove these additional spacers, and use them, rather than the included 3DR frame washers, to back the motor screw off so that it cannot damage your motor wiring. Inspect your motors well  (we can now speak of "engineering tolerances.") It is possible that you may still need to use an extra spacer or two (rather than a minimum of five per motor.) I did not need to use any of the 3DR-included spacers when I used the propeller inserts, one insert per motor screw. 


Total (additional) cost: $0.00. Please let me know which country you are from in the comments and I will happily provide conversions for the additional cost into any currency of your choice. ;) This tip is so simple that you might just be annoyed with me for writing such a lengthy blog about it. But, I suspect, not nearly as annoyed as you would be with four secretly unreliable, damaged motors. 


I leave you with a photograph of the final product. I hope that it brings you the same peace of mind and happy motors that I now enjoy.


Views: 1631

3D Robotics
Comment by Alan Sanchez on September 26, 2011 at 10:34am

Hi Mike, glad that worked out for you. You are the second person to have this issue, not sure if there has been some tolerance issues with our 25mm screws or what happened. The 3 3DR frames we fly here all have the stock screws with the washers without hitting anything inside the motor. But since this is the second instance of this happening and we want this to be an easy out of the box solution, all future shipments will include 8 22mm screws to use for the motors. I like your idea of using those plastic donuts there, it could also help with vibrations. Maybe even some softer rubber washers under there would work as well. Post videos when you get your quad flying :)

Comment by UnmannedTechShop.co.uk on September 26, 2011 at 11:15am

Thats a great and simple idea to solve the issues of they screws not fitting the motors for your Arducopter.  Thanks for sharing.  Looking forward to see your flight videos :)

Comment by Matthew Tang on September 26, 2011 at 12:35pm

Thanks for posting this. This happened to me I think. One of my motors is simply not putting out enough power compared to the rest. Compared to my previous quad it was feeling under powered. 

Comment by Scott Plunkett on September 26, 2011 at 1:52pm

Did I read that right?  Did he say read the instructions...twice?? - MAN CARD REVOKED.  Just Kidding, my wife always tells me I am the anti-man because I read the manual and am willing to ask for directions.  This is sage advice.  I recently did a conversion of a plane from an OS two stroke engine to an electric and ran into a similar issue.

Comment by Brad Carter on September 26, 2011 at 2:27pm

The screws worked for me but I was worried they were a little long so I put in a lock washer along with the regular one the kit comes with.


Alan, one thought for you guys is to include one spare of each of the screws, at least the nylon ones.  So far out of the two kits I've built, (one Jdrones and the other the DIY store) I've had a bad nylon spacer.

Comment by steve on September 26, 2011 at 4:08pm

I use the larger kit motors from my jdrones first kit and could easily see the screws were too long so I went to the grinder and shortened them and also added lock washers under them and in between the motors and the frame. Looked closely when screwing in and they have enough space so should be ok.

Comment by Michael Pursifull on September 26, 2011 at 8:40pm

I haven't really posted many videos because they are just not all that interesting. My crashes either happen when I am not filming (often not even flying! Only time I have damaged a prop so far, it was on the ground, not even armed. That is another story...) or are too far from the camera to make good entertainment, and I cannot pilot well enough yet for anything entertaining during a good flight.


I don't have most of the 3DR videos uploaded to my computer, I'm juggling HDD space right now. And I started using a different camera, it keeps storing in broken AVIs that I have to transcode - youtube doesn't like them, iTunes and iMovie and Quicktime complain about errors, they play, but do not export. 


Anyway, enough with excuses, just some context for why I don't have more videos to share at the moment. I record the flights, but there is not a good enough reason to share to bother with these boring issues. I did just locate a clip from a week or so ago I was using to sort out the conversion issues. This was my initial 2.0.42 testing for this frame. It is tethered, but you can see it runs pretty well. Stabalize, no TX input, and it just sort of hangs there in the air. I have a little trim on to push it back into the garage, but it just flys, no fuss. 


This is "AAR QuadFour", it has APM/IMU, Xbee, and RX only. 850Kv/20Amp ESCs from jDrones, I think it is a 4400mAh 15C 3S FlightMax Zippy. The sonar, uBlox GPS, magnetometer, AttoPilot current sensor, RemzibiOSD, and FPV camera are all swapped over to QuadThree during this filming. I have more parts on order so I don't have to swap these all the time. 


It has flown before this, tethered and untethered, I have maybe a hour on it by the time of this video, but this was the most stable I had it up to that time. I've put another 20 minutes or so on it, with 2.0.43 and 2.0.44. I have since swapped the magnetometer, GPS (now with batteries!), and sonar back to this quad, and have 2.0.45 loaded, and am in the process of adding a 500mW 900Mhz video transmitter and WDS600 FPV camera before rebalancing it - then I might have more interesting videos. But until then, here is a quick clip from about a week ago.



Comment by Martint BuildYourOwnDrone.co.uk on September 26, 2011 at 11:25pm

Hello Mike,

Thank you for posting this information, I will be adding a note to my store section for the 3DR Quad, were I stock the motors & ESC's to make the complete kit, I think this tip will be of great use to them!


I have not had any of my customers comment on this issue to date, however pre-warned is pre-armed as they say!







Comment by Greg on September 28, 2011 at 5:29am

Mike:  Did you pull the bad motor apart (remove rear circlip) and look under the coils near the screw mounts? The reason I ask is I have had 4 bad 880Kv motors from jdrones. Even in my third shipment one vibrates too much compared with the others without a prop fitted. My latest shipment are supposed to have new bearings.  I have found all sorts of metal burrs left over inside the motor casing.  One in particular was at the end on the screw mount hole as a round cap which matched the diameter of the screw hole.  You get these sometimes when drilling through alloy as the drill exists the hole.  Perhaps the longer screw before you fixed it with custom washers, dislodged a burr or drill/ tap shaving into the wiring?  hence reason for checking your motor which has problems.  I like the plastic washer idea using spaces you get with the props.  I measured the jdrones motor base thickness before the screw.exits inside the motor. It is 3.5mm Also  three of the mount holes on the 880Kv motors have a bit more safe clearance but one hole the internal wiring termination point is directly over one hole.


Comment by Michael Pursifull on September 28, 2011 at 5:45pm

@Greg: I havent pulled it apart yet. I tested each of these four motors at full throttle for 30 minutes (a sort of burn in test) before I mounted them. I do not like to waste my time mounting them if they are bad, or unreliable, and have to troubleshoot/take them off/replace them) so I am pretty sure I messed it up during the mounting. But that is a good idea. If I did not know for certain that the screws were too long for my motors, then I would maybe not have posted this. But I measured the clearance from one motor, found the minimum I needed, and added a couple of washers. Then I found I did not have enough, and for two of the motors, I used just the minimum I had measured for.... because I did not want to make a special trip to the hardware store just for my "engineering flub". And that was a mistake. But I was a little annoyed at counting out the washers for each arm, so when I had this motor apparently "go bad", I looked around for what I could use without spending more money (or wasting gas on a trip to a hardware store.) I was happy enough with the solution that I decided to share. 


I will take apart the motor and inspect it in greater detail. But if I build another 3DR frame, I will most likely still use the readily-available prop inserts. 


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