As Craig3DR stated in this post:
“On March 15th our motor supplier changed the motor base and now when the set screws for the legs are installed, the motor is pushed away from the base of the arm preventing the screws from fully engaging the motor. Customers have been informed and longer screws have been sent. You can also correct the issue by loosening the set screw on the legs and tightening the motor screws.”
As a result we have seen numerous people in this forum as well as over at DIYDrones, where the motor disintegrated from the arm, resulting in crashes of their IRIS as well as substantial damage.
Reading through Craig’s text would make one assume that IRIS manufactured before the given date shouldn’t have an issue.
Even though my IRIS was manufactured in February, I went ahead and swapped the screws. And I recommend to everyone to do the same. Here’s is why:
I went ahead measured the various dimensions of the arm and motor mount and here is what I found:
- The motor mount screw has to go through approximately 6.5 mm of plastic arm before it touches the metal of the A2830 motor. With the standard M3x8 screw that was used with IRIS this means that the screw is only inside the thread by about 1.5 mm.
- As the thread leads into the magnetic coils of the engines, the inserted screw needs to stay short of these coils. Thus the screws need to be shorter than 12 mm. (An M3x12 screw would touch the coils and therefore can’t be used).
- You can see that with a M3x10 screw, about 3.5 mm of the screw are inserted into the motor thread. As there are no M3x11 screws, that’s the best you can do.
After taking measurement of the motor mount on my IRIS, I have to come to the conclusion that using an M3x8 screw for IRIS is a design mishap - even for quads build before March 15th. M3x10 screws should have been used from the beginning.
While this mishap is something that can easily be fixed with a longer screw, the next mishap is something that cannot be fixed as easily:
As this document clearly shows, the IRIS motor is only mounted to the arms by 2 screws. Yet, as the technical specs for the used A2830 motor show, there are 4 threads that can be used for mounting the motor to an arm or frame.
While the technical spec. doesn’t state how many screws should be used, keep in mind that the entire weight of the IRIS Quad including any additional payload rests on these motor mount screws. Quick ascents or longterm vibrations will place additional stress on these screws and will lead to quicker failure.
You are basically doubling the weight that each screw needs to support by only using 2 instead of the 4 screws.
As some of the after-crash pictures show, that when a single screw fails, the other screw is not able to support the weight of the Quad against the motor and will also fail. At least with the short M3x8 screws.
With that it’s in my opinion very short-sighted by 3DR to only use 2 screws for the IRIS motor mounts.
I invite everyone: Please go ahead and measure the dimension on your IRIS to confirm or refute what I measured and documented above. If you find out that my measurements make sense, please replace your motor screws (don’t forget the loctite).
As this is the official 3DR support forum, I would like to ask 3DR why your IRIS arm design only uses 2 screws per motor for mounting. Are you looking into re-designing the IRIS arm for a future version with the ability to use 4 screws per motor for mounting?