Question about Reedy 729 brushes in a high-wattage motor

[fyrstormer]

I've been running a custom Holmes Hobbies 11-turn 550 motor in a short-course truck for...about a year or so? Maybe longer at this point. I had some pretty severe growing-pains as I learned about brush compounds and spring tension, resulting in a totaled armature with a severely burned commutator. That's all in the past now -- the motor has been working well for months. However, I took it out for a run last night and it ran as though it had a dead coil in the armature, which happened before -- but then it started slowing down quickly and the motor completely stopped responding a few seconds later. There was no smoke, the motor was cool, and a test motor plugged into the ESC ran fine, so it wasn't a burned-out ESC. I took apart the motor to examine it, and I found the brushes and commutator to both be in a very strange condition. They had the usual wear-grooves on them, but in addition, the "smooth" areas between the wear-grooves were no longer smooth -- they were rough as if some kind of abrasive had suddenly gotten sucked into the motor and sandblasted both the commutator and the brushes. (there was no evidence of any actual abrasive powder inside the motor, for the record -- just the normal accumulation of brush dust.) In addition, the brushes are not excessively short, nor were they jammed inside the hoods, and the brushes did not spark excessively during a test-run after lathing the comm.

I was under the impression that Reedy 729 brushes are a copper/graphite composite, so I'm not sure what could've caused this kind of wear. After lathing the comm everything is back to normal, but if this happened once it could happen again, and I'd rather it didn't. Has anyone ever seen a motor where the brushes and comm both had a rough surface after a run, and more importantly, does anyone know what would've caused it? If nobody has any brilliant ideas, I might just switch back to Reedy Plutonium brushes in this motor, because I don't recall them ever having these kinds of problems, though I didn't use them very long in this motor before switching to the 729s. Maybe I'm just expecting too much from the 729s?

 

[fyrstormer]

Hmm...

I took the truck for another run today, and the + brush shortened by almost 1mm during the course of that run. Looks like 729s are just too soft for the wattage I'm pushing through this motor. Strange that the wear accelerated so much in the past few runs. I wonder if the brushes were further softened by heat-cycling during previous runs? I have no idea if that's even possible, but it's my best guess since the comm is freshly-turned, so it's nice and smooth and non-abrasive.

It also appears that the cutting-out problem is being caused by buildup on the brush faces. I'm not sure why this is happening now, but maybe it's a side-effect of the increased dust from the accelerated wear on the + brush.

 

[fyrstormer]

I switched back to the Plutonium brushes. I lost about 1mph of top speed on pavement, but the top speed on grass is the same, and the motor seems happier now. I probably should've just stuck with Plutonium brushes for this highly-loaded motor in the first place, but...live and learn. Hopefully they wear slower than the 729s did, even though I'm running XXX springs instead of XX springs now. (the XXX springs increased the top speed of the truck by 4mph and almost completely eliminated sparking.) They're more aggressive against the comm than the 729s are, but if they don't suddenly turn super-abrasive near the end of their lives then that will still be an improvement.

 

[Hydrocarbon92]

I had to dbl-check the date on this post. Are you racing in some historical class or just a glutton for punishment? The abrasive-like wear, if near the end of life, could be the braid basically poking thru. It's not bonded to the edge, it's cast into the material. Industrial brushes usually are done after 1/3 is gone. They commonly have a spring cast inside so when they were down, it springs out and pushed the brush from the comm. Dummy-proofing them in a way.

 

[fyrstormer]

I like brushed motors in some of my vehicles. Brushed motors run really smooth, and once you know what you're doing they're pretty reliable. I'm pushing the envelope with a brushed 550 motor in a short-course truck, but that's part of the fun for me. I could run totally-reliable sensored brushless motors in all my vehicles, but I'd probably fall asleep from boredom, because I wouldn't have to actually pay attention to the vehicle to make sure it's working right.

I think RC brushes are supposed to be usable for 1/2 their length instead of only 1/3, but who knows. I wondered if the braid was scraping the comm, but it looked like the entire face of the brush had become rough, not just one spot near the middle.

That's a good idea with the spring inside the brush. Too bad RC brushes don't have that. But I guess the racers would hate it.

 

[Hydrocarbon92]

Tekin is saying 1/2 is replacement time, but that concerns the brush only. Beyond that brush stability goes down because so little is supported by the hood/slot. If there's any wiggle to the brush, 1/2 might be overly optimistic.

Think of it another way. You've worn 2 sintered graphite/metal blocks down about the thickness of a pencil by grinding it on a cylinder of copper. How smooth do you expect it? It's very common for the leading edge of the comm pads to get heavily oxidized from arcing. That oxide is twice the hardness of copper, so when the "good" comm surface wears down you have a raised ridge of jagged oxide. Diamond saw blades use the same principal to cut concrete.

Maybe you should get into boats if you love maintenance. Any 50mph+ boat requires the shaft pulled & lubed every few packs, at least once at the end of the day. Little variations in surface conditions require trim tap and strut adjustment. Then you need to always look for hull cracks, particularly if it flips at speed. If I had to add brushed motor maint. to that mix, I'd fill it with concrete and sink it lol.

 

[fyrstormer]

LOL nope, not going anywhere near boats. I don't want to have to swim out to get it if it stops working.

I've had brushes with significant wear that were still quite smooth, because the graphite component lubricates the brush/comm interface. The + brush in this motor looked more like sandpaper, even though the - brush had recovered and was smooth and shiny after a couple runs with the lathed comm. Your point about the rough leading edges of worn comm pads is well-taken, though. I suppose if the brush was wobbling due to being too short, even though I was using the stiffest springs I had available, then there could still be enough extra arcing to pit the face of the brush and basically cause a feedback loop. I've had Reedy Plutonium brushes wear-down shorter than this without incident, so I guess it was exacerbated by the soft no-silver compound of the 729 brushes.

The 729s still work fine in my other motors, but I started with Plutonium brushes in this motor, and I'm going to stick with Plutonium brushes in this motor from now on. Clearly, in this motor, the potential for increased comm wear caused by silver brushes is less problematic than the potential for comm damage caused by copper brushes malfunctioning when they wear-down enough.