Watch Out! Udrones ARFs need threadlock.


I'd like to preface this post with this statement: I love my (lab's) Udrones Hexa C, but I feel that this simple fix would save a lot of grief for Udrones flyers.

So!  I have noticed that the Udrones Hexa C that I use has a nasty habit of vibrating its motor mounting screws loose.  Twice before, I have landed my hexa only to find that one of the motors was dangling by one screw, because the other had vibrated loose.  Well this time it finally happened!  During the landing sequence from 80m, the rear left motor came unscrewed and hit the arm, flying off.

Here's the sequence of events:

18:25:45: Hexa is returning to launch.  I notice the it sounds rattle-y.  I've heard that sound before, it means that a motor has lost a screw.  Since it was already about to land, I let it continue.  Landing sequence commences at 80m above home.

18:26:06: Motor comes off at 55 meters.  At this point, the copter stays airborn but cannot stay in one spot.  Missing one motor, it yaws at a good rate, making manual flight very very difficult.  I can't compensate for the yaw, because it makes the copter tip over.

18:26:18: At this point the height reading in the tlog says that the copter very nearly touched the ground.  I can assure you it did not, I was trying to get control of it in the air, no lower than 20m at the very least.  At this point I was deathly afraid of the copter landing on a roof, or much worse: the pool.  I attempted to stear it towards the safest place I knews, the forest.

18:26:37: Not quite reaching the forest, the copter is flying over the tennis courts at a frightening clip.  The telemetry says it was underground at this point, but I assure you it was about 20-30m up.  

18:26:46: At this point I am still trying to hit the forest but barely miss it.  At this point I am running through the tennis courts.  I see that the copter is going towards a medium sized road.  Under no circumstance do I want the copter to crash anywhere near moving cars, it would be dangerous to everyone involved.  So I lowered the throttle until the copter entered a descent.

18:27:01: From my point of view, the copter vanishes behind a stand of trees, still descending.  I give it two seconds, and then cut the throttle completely.

18:27:04: Impact.  From that point, I dash over and am immensely relieved to see that the copter landed upright on a grassy field with nothing broken beyond the missing motor and a chipped prop.  I went back to the launch site and recovered the top half of the motor, complete with propeller still attached.  The bottom half of the motor was still dangling from the copter, its mount cracked.

The moral of the story is: apply threadlock to your motor mounts, and check them before every flight!  Otherwise the screws will vibrate loose and possibly detach your motor.  I urge Udrones to add threadlock to the motor mounting screws of their ARF copters.







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  • Found this thread searching on "threadlock".  I am first time builder of a 3DR Y6.  After the "compassmot" test, was switching props back to normal and found one motor wobbly.  Took the assembly apart, got too much threadlock on, which I didn't see until I tried turning the prop and it resisted.  I took it apart and cleaned it off.

    Will be heading to the store this morning to get the "stick" type threadlock, which should let me apply it with more control.  Hopefully, I didn't cause an issue with the motor.

    Thanks for the tip on the stick type threadlock.


  • Admin

    Most of the motor shaft are not notched for the prop grub screws to bite , grinding/cutting a small notch without debalancing the motor is all the is required for fixing the prop,  never thought that some might thread lock the prop to the shaft :))))  Red thread lock for those you don't want them to get loose ever or dangerous  if they become loose and fly off..

    Blue thread lock for those you don't want to get loose but may need to change later.

  • This is good information all around.  I had the same failure on my tricopter today.  I'm hovering about 6 inches up, trying to see which way it wants to go first and then Bam, left motor comes off and twists up into a knot and breaks the prop.  I was lucky I found some extra motor screws.  I had no threadlock anywhere and was just going over everything before each flight.  I guess that's ok until you forget to check that one. 

  • T3

    Oliver, I know this very well!  Some members of my lab took three of our Mikrokopter Hexas (retrofitted with APM2.5's of course) to costa rica for some photo collections.  Unfortunately for them, one of them decided to threadlock all of the propellers onto the mounts!  The threadlock ate away at the core of the props and caused several of them to break in flight; ruined all of them.  Luckily they brought enough backup props!  The point is: never ever threadlock plastic!

  • It was mentioned (once) above, correctly, that all metal-to-metal threads should be threadlocked. But not mentioned is that non-metallic joints should NOT be threadlocked. The threadlock compond may attack and degrade a non-metallic material, resulting in failure of the joint/connection. Some manufacturers warn about this (for example HiTec, whose various servos come with metal and non-metal output shafts). 

  • Oh, and I should point out, try not to get the shaft to hot.  If it turns blue, you're doing it wrong and could cause problems.  Take it easy with the dremel.  Let the cutter do the work and don't push it.  Maybe take a break halfway through.

    It would be hard to cut those shafts using a metallic cutting tool, because they are hardened.  The dremel disk is ceramic and uses heat to remove the metal.

  • Yup, I have a lathe and other tools, but I used a Dremel anyway.  I recommend taking some paper, push the shaft through the paper making a small hole, wrap the paper around the motor.  Cut the shaft, then careful take the motor out of the paper.  This prevents any metal shavings getting into the motor or bearings which would be bad.

  • That's how I did it as well.
  • Should have read "Dremel" - darned autocorrect.
  • I use a dreamed rotary tool with a cutoff wheel followed up with a fine tooth file.
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