Falcon 500 Failure Mode Completely Sheared Shaft Screws


So, upon investigating a wobble on one of the shafts of our SDS MK4i drive motors, I discovered the image above. They have only been through one competition. All 5 bolts had been completely sheared, leaving the shaft to rotate freely and the ends had been seemingly melted into the motor. The bolts that were in the shaft were the older hex socket ones, which Vex has since replaced with “harder materials”. We took apart each motor and loctited them per guidance from Vex while building these modules. Has anybody seen this before? It is extremely concerning to say the least and I would prefer if we didn’t destroy the rest of our Falcons… If we over tightened or did something else wrong, I would appreciate somebody telling me.
Thanks

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Impressive

Hard to tell from the photo, but the screws seem to be sheared from a single high impact event.

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Paging @Jon_Jack . Have you seen this before?

The motor has no where near the required torque to shear those. I would wager good money that they worked loose and the forward /back cycles of the drive train sheared them off. Then it the motor continued to spin and flattened them off nicely.

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Yes. This is a result of the screws coming loose.

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Has VEX considered revising the shaft attachment itself to remove the bolts entirely, in light of the amount of trouble they seem to cause?

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Have you seen this failure mode for falcons that have undergone the suggested instructions for loctite and reassembly? These falcons were modified per manufacture instructions.

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I’d be curious to hear a better option. For all the trouble it causes I’m not sure I want a replaceable output shaft anymore but I have seen teams use it.

The only real reason I’ve seen teams around here use it is because they needed the “industry standard” 8mm keyed shaft, rather than the “proprietary” splined shaft (yes, I know the spline is standard and can be manufactured by other companies. But that’s far and few between at the moment IMO).

Being able to replace a previously cut shaft though is definitely nice. Especially considering if you need a new/non-cut shaft on a NEO, you replace the $45 motor. Whereas with a falcon (if the shafts weren’t replaceable), you’re now replacing a $180 motor

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Maybe another direction to pursue is not having the controller become a pumpkin if/when the motor fails.

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Often when you want to reinforce something like this you can put in two or three dowels so the Fasteners can’t shear and are only loaded in tension (from the manufacturer).

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I find it odd that that this wasn’t done from the beginning. It’s a pretty standard thing to do and wouldn’t be hard to implement.

Output shaft runout should be controlled by the pilot fit, to maintain parallel and angular alignment to the rotor. Dowels as you describe would need to be clearance fit to avoid disturbing that alignment. Dowel clearance will allow shock loads if the screws loosen, just as the current system does.

All screws loosen eventually. (See: safety wire.) When these particular screws loosen, shock loads may break them. Dowels would be harder to break but they won’t stop the screws loosening. VEX has already provided guidance on how to make it take longer for that to happen.

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If the problem is the FOC encoder calibration, they probably don’t want to deal with teams taking it apart, messing up cal, and destroying it before they realize they have to recalibrate. Easier to just tell them to never take it apart and RMA it when they do.

Thanks Richard! Always good to have actual engineering knowledge to understand what is going on.

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The problem is, we actually followed this guidance, and still experienced this failure mode. When we opened up the motor all 5 screws were still in place and did not fall off until we rotated the shaft, it was almost like the only thing holding it in place was loctite or something…

I guess you have already contacted Vex support? @Jon_Jack already confirmed the failure mode is a result of screws coming loose.

Was the motor that failed in your drive train, or on another mechanism?

It was a drive motor from the swerve module (MK4Is).

I haven’t studied that module in enough detail yet. Which motor is the drive? The one on the left, or the one on the right?
image

the drive motor is on the left in that image