Brush and commutator polishing.

[Inspector86]

Background:

I currently have five HH Crawl Master Pro 550 12T motors powering five of my rigs with the HW1080 ESC. Recently one of them stopped running while out trailing and after some investigation I discovered the commutator had worn to the point it lost one segment of contact. This concerned me as this motor had less than 30 hours of run time and had never been in sand, water or mud. I sent the motor in to HH for a rebuild and $80 later my motor was returned with a new armature and new brushes. It had obviously been run in when it came back with the start of a really ugly wear pattern on the com. So I disassembled the motor and noticed the contact surface of the new brushes is ribbed intentionally. This ribbing on the brushes strikes me as a really bad idea for long term wear.



modification:
Using a 4mm diameter round ceramic sharpening rod, I removed all traces of the ribs from the contact surface of the brushes leaving a nice polished surface in place of the ribs. Next, using a 1/2x1/4" 1000 grit flat honing stone I chucked the armature in my mini-lathe and polished the commutator to a smooth polished finish. After reassembling the motor and driving the truck for a little over an hour the wear pattern on the com is MUCH smoother with no trace of rib or groove wear and will likely last a long time without destroying the com. I have since done this polishing on my remaining four CM Pro motors as well. One of these four has significant wear on the com with around 20 hours of run time and will likely need to have the armature replaced on it as well but at least the polished brushes should slow the wear rate.



conclusion:
From a performance stand point can I understand about getting as much contact surface between the brushes and com as possible. But from a wear stand point this ribbed brush design feature is really sub optimal. From my perspective, I expect my motors to last a good long time without undue wear and I certianly don't expect a high end motor like this to die in 30 hours of runtime. After this, I plan to monitor my motors more closely and hopefully avoid having to replace another armature anytime soon. I actually tried calling HH to discuss this issue with them but no one answers the phone and a recorded message says to send an email. I would love to discuss this with JRH at some point and get his insight.

 

[drew_2872]

I am unsure of ribbed brushes, but back in the 80's and 90's, yes I dated myself, they sold brushes that were cut in different shapes. The cut shape determined how we the brush worked and distributed the power (voltage and/or current) to the comutator. Some where really narrow, some were normal width. Something about a narrower cut created a faster motor. I am trying to remember the difference but the last time I dealt with brushes was in the early 90's on my RC10T 1.0.

Sent from my SM-A326U using Tapatalk

 

[EddieO]

They are call serrations and they actually assist in breaking the brush faster with less wear on the comm. We discovered way back the motors ran better with many compounds with the serrations. Companies even made units to put new serrations back on brushes after they wore away but the brush was still good.

So, you are not correct they are bad idea for long term wear, nor is it sub optimal. John wouldn't use or sell them if they didn't work.

And don't polish a comm, that is sub optimal if anything. You CUT a comm, buy the correct bit for your lathe and CUT it. That's the optimal surface for the brushes, not a polished comm. We have 50+ years of data on a what does and does not work for a comm surface. Polishing is not, nor has it ever been the correct thing to do, it in fact make the motor run worse typically because the brushes don't have the needed friction to commutate as well.

If you want your motors to last longer.........clean them more often, cut the comm and replace the brushes as needed, especially if they get burnt or too short. Many people run the brushes too short and when you do that, spring tension changes. When you lose tension, you get brush bounce, which accelerates wear. Gearing is critical as well, a majority of arm failures we see are from improper gearing. Lastly, have reasonable expectations for your motors. They only last if you take care of them and certain things can speed up wear regardless of what you expect, you need reasonable expectations on these things. All of these motors mostly use tech from the days when brushed ruled the racing scene and in those days we literally cut comms and replaced brushes before every run......yes, every 5 minute run we rebuilt the motor. Some people did it after each race day, some did it pre race day and then before the main, but it was constant motor rebuilds.....and those these can be slower, we are also running them on more voltage, which accelerates wear as well. Expecting them to last 20 hours with no maintenance is a bit on the unreasonable side.

Later EddieO

 

[usert_l]

Something you may also want to think about is the tension of the motor's springs and how much friction they create on the comm. You can get springs with different tension ratings from sources like Integy. Going to a lighter tension will reduce comm friction and wear.

I find that closed endbell motors usually have a very long brush/comm life, and I think one of the reasons for that is their integrated brush holders have much lighter tension and don't push as hard on the comm. In addition, their brushes wear slower because they're harder than the brushes used in rebuildable motors.