PSA: Falcon Failure Mode

Ooh, I’m glad we bought those ThriftyBot output shafts, and had the kiddos Loctite all the screws when they installed them!

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19-708850 is the CTRE part number.

You should be looking at the bottom line of text which would start with a 19 or 20 and not have any dashes.

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That’s what threads like this are for. We’re only using one Falcon this year but, I happened across this thread and now I know that we’ll need to tear it down tomorrow to avoid trouble.

I’m sure there are FRC teams using Falcons that do not browse Chief Delphi. It’s great that this thread exists to warn teams before an official message can be sent out by VEX but they are the only party who would have the ability to notify everyone who has purchased a Falcon of this issue, which they should.

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So, where would we find this date code?

I went through a pile of 9 motors today and most have two or more loose fasteners indicating that it is ESSENTIAL to do the fix VEX recommend. I found maybe one out of five that had Loctite and decent torque.

I used the 290 loctite. I did not have any medium strength at hand but the 290 is about the same strength as 243.

I strongly encourage anyone working with this to get a ground drill-rod 2mm hex driver like the ones used by the RC car guys. The regular rolled steed hex drivers are not up to the job. Wera makes good ones too. Do not try to use ball end drivers. Get a Wera or similar Torx T10 driver too. It is quite easy to put the motor together with a well fitting driver.

One thing that I noted on the three motors that had been run was that the bearing outer rings rub against the screw head generating enough heat to anneal the outer race blueing the ring. Over time this will be an issue. We really need to find a 8 by 11mm, 0.3-0.4mm thick shim washer to put behind the inner ring to prevent this. The best would be if VEX could make the counter bore deeper for the screws, add a shoulder and put a outer ring spring washer in the endbell. A fix like that could be retrofitted and would greatly extend the life of the motors.

I also saw a lot of fretting on the shafts. The shaft is already way to loose at about 7.95mm shaft diameter. It will continue to open up. The architecture with a replaceable shaft makes this manageable but I would recommend that you look over this during the season to prevent failures of the other bearings.

For now I bonded the outer ring in the front endbell with a tiny layer of 290 Loctite to hopefully eliminate the risk. I also ordered a pile of 8x14x4mm bearings si we can replace them between the events.

Here is a picture of one of the two blued bearings. You can also see how the shaft is wearing.

Ulf Jonsson, 177 Bobcats.

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We opened all of ours tonight to loctite our shaft bolts and we also found our bearing doing this and also the grease from inside the bearing also had leaked from it. Our motors never got hotter than I’d say 90° F because we could hold onto them so I’m not sure how that is hot enough for this to happen.

Man, was having a great day until I came across this post. Our practice is pushed back by 1 day. I guess it’s better to have found out now than mid-match. Thanks!

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We went through 5 of our Falcons last night. 4 of them had threadlocker on the shaft screws and 1 had no sign of it anywhere.

I went through 5 last night. A couple of them had 1-2 bolts that were difficult to get out, but all of the others came out rather easily, with no sign of loctite.

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Yikesyikesyikes

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+1 on the Wera tools. I use them for work, and they are hands down the best tools I’ve used. They’re worth every penny.

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we pulled all 3 of our motors apart last night as well ( 1 spare and 2 on the drive with about 5 hours of drive time on them) and none had any signs of loctight on them and the 2 drive motors each had a few screws starting to loosen up. Glad we found it now, thanks for the info.

We made the modification on all 10 of our Falcons last night. None of them had any sign of loctite. It was not easy to remove the pinion gears to take the motors apart; we need to Leonardo da Vinci* them to take them off. When we put them back on, we found that one of the motor shafts was installed so that it was no longer concentric with the motor rotation. So to any teams doing this, make sure that your shafts are all concentric when you reinstall them.

* Leonardo da Vinci (v): the act of using a mallet and screwdriver like a hammer and chisel to separate two press-fit objects
 

I would assume this is what the bearing on our motor looked like before it exploded. Good thing you caught it in time :slight_smile:

 
We’re thankful to Vex for agreeing to replace our broken motor, but this seems like a pretty big design mistake on their part. That a plurality of motors would want to self-destruct within a few hours of life says to me that not enough quality control testing was done. Combining this with the set screw problems, shaft concentricity issues, and pinion press fit difficulty, it seems pretty obvious to me that design and testing during the development of these motors was rushed. I don’t necessarily blame Vex for this, there are certainly factors here outside of their control. But this experience has seriously soured me towards using new products in their first season. In the past I’ve been hesitant on things like this, but we decided to go for it this year because we wanted to use brushless motors and we’d rather buy into Vex’s ecosystem than REV’s. Because of this experience though, I doubt we’ll be using first-year products again any time soon.

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You should send this to Vex customer service.

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Do you have documented evidence of this? Not doubting your findings, but this isn’t one of the issues I’ve heard of.

I was just ranting about this to some other robotics mentors at the shop I work at. Some of the design decisions that were made on this product seem borderline at best and its disappointing to see them being made by such an experienced robotics company.

The output shaft was the first thing I had serious questions and concerns about when the falcons first got to our shop. The spline shaft doesn’t really make sense to me from a team standpoint because its such an arbitrary geometry and is pretty much impossible for 99% of teams to make a female interface with. I feel like a small hex or even keyed round would have been far more appropriate but Vex wouldn’t have sold as many ancillary parts after the fact if they didn’t essentially force teams to buy them so I guess good business decision by them?

Then the fact that the output shaft is aluminum and face bolted on to the motor??!! Its not the most alarming design decision I’ve seen made, but its up there. Right off the bat, that’s not conducive to maintaining concentricity whatsoever. Secondly is that it results in something like what we’re seeing here and that should have been patently obvious after any appreciable amount of testing by Vex, but, well here we are. On top of that, these shaft mounting bolts are obviously going to be subject to large amounts of vibration which is known to cause loss of clamping load on bolts even when proper thread locking agents are applied, and even more so when absolutely NOTHING is done to mitigate the loss of clamp load. Combine that with there they are located in the motor assembly and you get what has been detailed in this thread. The shafts being aluminum is also somewhat concerning to me and it would seem that such a small part could very easily have been steel to avoid any chance of fretting or shearing without causing much gain in weight.

On to the mounting screws for the whole motor. I love the decision to have a bolt pattern in the face of the motor for mounting, it has been so easy…except the bolt holes go through directly into the armature of the motor and using a bolt even 1/4" too long completely seizes it. Not a huge deal if everyone knows what bolts to use to install it but I think we all know that it is seldom the case that everyone knows something, and idiot proof should always be the way to go for idiots like me. Why not slightly expand the motor housing so that these bolt holes don’t interfere with moving parts? Or make the overall casing shape a hexagon and use the points of the hexagon for the mounting screw holes in the same manner as the versa planetaries are squares on the outside? This seems like another fairly major oversight that should have been pretty obvious with proper beta testing by the company selling them rather than the customers who bought them.

Last but not least, the grub screw covering the cooling port. I can see how that would be an attractive choice for sealing that port…but again it leads straight through into the motor armature and not everyone will know not to over tighten it. How about a button head screw?? It really doesn’t seem like the space saved using a grub screw is worth the risk to me.

Add on to all of this the overall lack of availability (I know, I know Chinese new year and Corona Virus, but they should have left themselves enough production time to avoid unknown complications; think an FRC team asking for more time before stop build day because they had snow days) and I’ve got one heck of a sour taste in my mouth. Under normal operating conditions, these have been great motors and motor controllers. Our programmers are happy, our electronics people are happy, even I’m happy with their performance. They just seem to have been seriously rushed, to the detriment of teams who were happy to throw down their money and gamble on a new product. I understand that any new release is going to have its problems but I think we should all expect that every effort should be made by the company producing it to minimize the problems and at the very least be transparent about any that may still be present.

[/rant]

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Are you saying that this was not an issue brought up by beta teams?

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When we were first assembling our Falcons a few weeks ago we found that one of the shafts was visibly not concentric right out of the box. We took the shaft off and put it back on again and the problem went away. When we were reassembling the motors yesterday we had the same problem and same solution. I suspect another non-concentric shaft may be causing problems on our practice robot’s drivetrain, but we need to look into that further before I will say that for sure.

We didn’t measure the amount by which the concentricity was off, but the shaft was visibly non-concentric. As the motor was spinning, we could see the shaft vibrating back-and-forth. We probably should have documented our findings better, but we have been rushing to get the robot done so if we could fix the problem ourselves we preferred to just move on. We haven’t yet reported this problem to Vex either for the same reason.

These are for sure the right size bearings? I can’t find the size or anything in the manual.