9imod Brushless Servo motor separated

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ih8nixon

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Arrma RC's
  1. Kraton EXB
I have a 45kg 9imod brushless servo in my 1/8th Kraton EXB V1. After one too many hard flat landings the servo completely froze. Opening the servo up and playing with the gears I managed to get the servo to move again, but the next time running it the servo started whirling like I had stripped the gears. Opening the servo up again I could see the gears were still in good shape. Pulling the bottom part of the case I could see that the stator of the motor was soldered to the board and it was able to separate from the bell housing and bind.

To fix this I took a peice of foam tape about 1.5 mm thick stuck it to the lower part of the case where the part of the board where the motor FETs are to push up on the motor so it can't seperate. I've run two packs and so far it's working great. A piece of adhesive thermal pad would probably be a better solution as the foam insulates the FETs.

I'm considering it a tempory fix so I can bash until I get a new servo, and I'm posting this as I couldn't find any solutions online. Not sure if this problem is specific to 9imod brushless servos or if it happens to other brands brushless servos as well.
 
So I spoke too soon. It completely stopped working today after a good chassis slap. After pulling it apart I couldn't get the gears to mesh no matter what I tried. So I figured with nothing to lose I would find out why it failed and give the board a good tug to get it off.

Taking a look at the first picture you can see the base of the inrunner is soldered and glued to the board. The can of the motor is secured to the plastic base by 4 small indentations around the outside of the base. The second picture shows the permanent magnet rotor that has a brass bushing around it. The motor itself is firmly glued into the case. The board hangs freely supported by the motor.

Pretty sure what happened is overtime the forces from jumping causes the weight of the rotor to wear on the indents allowing the rotor some vertical movement. The rotor at this point becomes a slide hammer. First punching the base of the motor free and then knocking the brass bushing loose. In my first post the base was knocked free and the bushing was partly dislodged. If I had used 3 to 4 more peices of foam tape at that point I might have been able to keep it going a while longer.

So if you have this style of servo and do a good amount of chassis slaps I would consider opening it up and put 6-8mm of foam tape or something similar below the brushless motor to support the rotor before it punches its base off. Given the size of the gap between the FETS and the base it's clear they're not dissapting heat through the bottom of the case.

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Edit: So an addendum I actually managed to fix this thing. If you unsolder the three solder joints at the end of the board. This is easy as the holes are huge compared to the pins. After doing so the motor PCB will easily separate from the motor base (The motor base stays attached to the control PCB, mine literally fell apart). If your motors interior wires are still attached you will still need to do this to press the base back in. I pressed the bushing back in by using a small socket to push on the rotor. I then resoldered the three interior wires to the motors pcb. Next I unbent the indents in the motorcase so the motor base would go in far enough when reattaching the control board. Resoldered the three motor pins and its good to go. As there are no longer the indents to hold the motor together I used a 6mm thick peice of TPU in addition to the foam tape to press on the controll board and hopefully hold the motor together.
 
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