The Death of a 775Pro...

We didn’t smoke it, but we sure did stop it from turning!

We’ve had some interesting hardware failures from vibration this year, two shorted compressors and two “exploded” 775pros along with a host of tread delamination and bearing ruptures. Great learning year for us! I just wanted to show off the innards of the 775pro as we took it apart.

Holly!! Could you share a little bit more on how did it happen?

We cracked the magnet on the inside of a CIM ¯_(ツ)_/¯

You might consider sending VexPro support an email with the pictures and a description of the failure. Usually they like to hear how their products fail so they can find flaws in the designs and improve future products. If they are interested in your failure they may even ask you to ship them the broken motor so they can take a look at it themselves.

We had a 775 all of a sudden lose a whole lot of efficiency. Took us a while to figure out that the brushes had fallen off. We’re guessing it happened when our rope failed after a climb.

Here’s what that looked like. You can see that the copper brush holder broke at the bend. Click to embiggen.

Looks almost the same as the failure you guys had.](

The 775 pro on our climber burned out after climbing in our second quarterfinal match at Pitt. Then in the process of replacing it we lost track of a sun gear and we couldn’t climb in the tiebreaker (we would have lost anyway even if we climbed though). We now have a climber repair kit for this situation.

We’ve had brushes fall off two different 775Pros recently. Not a failure mode I remember seeing before on similar motors.

Yes, the failure was a break in the brush holder at the bend.

In simple terms, we shook it to death.
It was mounted inside our shooter drum, which had balance prob…er, issues.

What I found interesting also is the pattern on the brush and the wear on the commutator. Looks to me like it was bouncing quite a bit before the holder broke. Makes sense if you ever the drum spinning.

Another thing I noticed is that we, 5511, need a better hand model. :slight_smile:

For sake of clarity, let’s call the carbon block the “brush” and the metal strip the “brush spring”. Were your failures with the brush coming off the spring, or with the spring breaking?

Our symptoms were the following: During a competition, we were shooting balls with power, and working on tuning our auto path to the hopper after hanging a side gear. Suddenly, we started shooting with much less power, not able to reach the boiler. We kept tweaking the path closer and closer, just trying to get some balls in, and also messing with the angle and compression of the hood. We couldn’t figure out what had changed so drastically, since previously we could blast the balls 15 feet over the boiler. At the shop, we tweaked and tweaked the hood, getting worse and worse results until the motor just stopped working. At that point we pulled the motor and noticed that the brush was missing and the spring had worn to the point that it no longer reached the motor armature.

My current theory is that at some point, most likely during our only robot drop, the brush became detached from the spring. The spring itself can lay on the armature, so the motor still worked, but with less efficiency. As the armature rubbed on the spring, it slowly wore through the spring until the spring was no longer tangential, but the tip was rubbing. Then the tip continued to wear back until the the spring no longer reached the armature, and the motor stopped working.

Does that square at all with what you observed?

That’s called a “fatigue failure”.

Search that phrase on the internet for some interesting reading.

Upon closer look, it appears to me as the spring breaking (at the bend right before the brush.) I attached photo.

I did poor failure analysis on this, and the first failed motor got thrown away. I do recall the spring looking the same from outside (I thought, “that’s strange, brush fell off, must have been a fluke, trash”.) In both failures I could see a loose brush in the case, and I’d take a picture of that, however it seemed to have made it’s way out the vent holes in the meantime cause it’s sure not there now.

Both failures were on the practice bot, in different mechanisms. It appeared to suddenly just stop working, however there could have been a drop in efficiency beforehand that wasn’t noticed. No robot drops, or extra hard hits that I recall prior to failure, but I could have missed something.

“Fatigue Failure” … similar to what happens to many teams near the end of the season…

ToddF - the spring broke. The brush remained attached to the spring. The armature side of the brush had significant amounts of wear (for the relatively short life span of this motor), along with a distinct wear pattern.

Ether - yes, exactly.
My guess is the motor ran about 20-30 minutes at “all” RPMs with the majority in the 7-9000 range. I don’t know where that puts us on an s-n curve with all of the speed variability and the force (w^2) from the imbalance, but my rough guess puts the cycle count between 100-250k. Assuming the spring itself wasn’t resonating.

I guess the moral of the story is to balance first, then spin, and don’t go too fast. We have a second motor that also failed in this mechanism, but I haven’t taken it apart…yet.

That’s nothing, we managed to shatter the steel mounting face plate by overtightening when not slotted into the sheetmetal properly. At least the inside is super cool!

In every event that we have done this season, we have had 775 Pro fatigue in our drive and replaced them at the end of practice match days at our 2 subsequent events this season.
We realized the problem at our practice matches during the 2nd event on one side of our drive, swapped it out and noticed observable improvements when our driver went full speed on the field.
From that point on, we decided to just swap both sides for every subsequent event.
In the past, we’ve used the 775 Banebot motor in our drive and never had to swap them out for an entire competition season.
I’m not sure if we still have the ones we replaced, but after reading this thread, perhaps we should investigate also.

Unfortunately for us, it happened again in Hawaii, us or our alliance partners broke and missed matches in 2013, 2016 and this past weekend.
368 had two burnt 775 Pros on one side of their swerve drive in QF-3. The smell was so strong, everyone in the stands could smell it, including those standing right outside the stadium way above.:ahh: The Hawaii regional venue is pretty big by regional standards.
As a result, they missed our first SF-1 and were never the same in SF-2 since they were unable to score gears effectively based on an auto release and drive back feature they programmed.
Their swerve drive was very impressive when it worked though.

We had the same failure on 2 different 775s in different time periods. Because they were used in tandem with our climber, and the failure was so hard to detect instantaneously - we ended up roasting 2 other 775s as they were carrying the brunt of the load.

We’ve been using these motors everywhere for the past 2 years, but this is a new failure mode we’ve seen.


We have a process for monitoring the 775 that is powering our climber, and recently we have been seeing the current drop as the motor sees more use. “Unloaded” through a 81:1 reduction we usually see around 2.5A monitored through code, and we would see it drop by ~0.5A or more over time. We suspected either that the gearbox was getting worn in (after a gearbox change) or that we may have been experiencing issues with the brushes.

Haven’t investigated yet, but I’ll try to update when we crack them open.

I thought WCP was the originator of the 775Pro after BaneBots discontinued the 775 18V? RC, correct me if I’m wrong.

Still a valuable input however Ari.

There is two things I would like to take a look at, one is the end plate with the bearing for the brush end of the shaft for motor one. The second would be to get a good look at the brush spring that failed. It may have gotten so hot that it became plastic and deformed. That failure would have allowed the brush to get caught by the commutator and then the spring. If the motor was running very hot and at high current, the second photo shows the break at where I would think that failure would occur. The other brush does not seem to be damaged in that photo.
I would also like to see the armature windings just because…

I love 775 Pros. We have never had one break and used 12 this year.