21 replies to this topic

[Gator Bob]

This has probably been discuscused... but I'll ask:

What is the science behind the positive motor brush wearing faster then the negative?

[Ron Hershman]

It probably has more to do with acceleration and braking than anything else. In most motors, the positive side "arcs" more under both go and stop than the negative side brush,

More arching eats away the brush material.

In all my years of building motors, I have never seen what I would call a problem with what you are describing. I can't ever recall taking apart a used/raced motor and saying/thinking to myself... wow, that positive side brush is a lot shorter than the negative side brush.

I have never seen a "noticeable" difference between brush lengths in a motor after it was raced for any length of time... if both were similar size when new.

So how much length difference are you seeing in your motor brush length, Bob?

Is this a problem on one track you may be racing on or on all tracks you are racing on?

[Ron Hershman]

Bob...

Check these links out... maybe of some help and may not... some of it applies to our motors and some does not.

Non-uniform brush wear

Motor facts: maintaining DC motors

Probably the closest and best explaination here...

Timing test

Brushed electric motor break-in guide

[Gator Bob]

Ron,

All tracks, oval, Hillclimb, King, etc. All motors, too, PDs to Deathstars.

I know I run my motors a long time, probably a much longer time on the brushes than you are used to running. If they keep up with the pack I just flush them (with CRC brakecleaner..hahaha) and keep running them. I'm running a Deathstar with the original stock brushes and springs for over 30 races in an FCR with 17 grams of extra weight no less and and run fast time in the race and within within .005 to .010 all the time. Did I say my driving sucks...

My PD has run a four-hour enduro and two regular races... hahaha. Still strong and the car had one win and a second in those two with a hired driver.

I never actually took a measurement but it is easy to see the positive is in the hood and at that point the negative is still sticking out about .020".

I was thinking it had to do with the electron flow or some sacrificial property of the brush material due to the charge.

Will check out the links that you provided. Thank you!

Maybe this question was not asked after all.

[MSwiss]

When we ran 40 minute one-motor G7 races in the Midwest USRA, the front brush always was the one that wore out first.

Stu explained it as the direction of the current flow and more arcing on that brush.

Conversly the braid that wore more (again due to arcing) is the one connected to the opposite buss bar (looking at the car from the bottom pointing forward, the right one) or in 1/1 American car terms, the "drivers" side.

[Gator Bob]

Yep, Mike... I see the same thing you describe with my flag brushes: the negative wears faster. I don't change them either... LOL.

Personally... Don't like new guide brushes even when flattened down the front of the car feels spongy for a lack of a better term. Longer gives more contact area but may be offset by the additional friction/drag.

I prefer they are worn down to 2/3 length and fluff up the ends slightly. I switch them side to side and keep them fairly even as they wear. I guess I could trim and comb a new set, but I just clean them with lighter fluid about every two lanes run to get everyones nasty braid juice off.

Boy, am I cheap.

Whatever dynamics that cause the arc is what is causing the wear.

BTW: Did the little ceramic capacitors that guys ran help with the phenomenon?

[MSwiss]

Again quoting my old boss, Stu, he claimed the miniscule size/value of those capacitors across the guide clips couldn't possibly help a G7 car.

[Rick]

Mr Swiss, you have made an error, DC elecric flows from neg towards positive. Your example is good but for opposite reasons...

[MSwiss]

No, I didn't make an error.
I never said which way the current flowed.

I said, "Stu explained it as the direction of the current flow and more arcing on that brush".

[Rick]

[MSwiss]

Apology accepted.

[HarV Wallbanger III]

Yep, Mike... I see the same thing you describe with my flag brushes the negative wears faster. I don't change them either...LOL.

Bob! Change your guide brushes whenever you see burning! When they arc chances are the brushes are arcing as well and this makes more heat.

Also on the P/S mini motors make sure you do not allow the brushes to wear down too short so the short leg hits the brush holder (and stops pushing on the brush) ... because when that happens that brush will arc more until you will not have any spring pressure and stop running. It again will put more heat in the motor until it stops...

Good luck.

[Gator Bob]

Current flow is from the + terminal to the - side terminal. Electrons flow the opposite direction from the - to the +.

So it seems the current flow toward the negative and thus "giving up" material from the positive motor brush and for the same reason the the negative braid (flag brush) gives up more material. Like a galvanic action.

The carbon in the motor brush passes more current (less resistance) as it is heated.

The further the timing is advanced, more current is drawn and the speed increases. The combination of the higher current draw and increased speed accelerates the the degradation of the motor brushes but even more so on the positive brush (and negative braid). A DC motor with zero timing does not care which way the current (or electrons) flow or even the voltage applied (to a certain limit).

Personal experience. While restoring vintage and antique farm tractors they were all set up as 6 volt positive ground. The starters labored and got pretty hot with longer cranking to start the beasts. I converted most all of the ones the did not need to be 100% Korrect to 12 volt negative ground. The cranked faster and cooler as when the voltage is doubled the current draw is cut in half. There weren't any "electronics" on these old machines so all that needed changing was the 6v bulbs to 12 volt bulbs.

[Michael Rigsby]

Bob,

I have noticed this also, more so on my .560" diameter S16D arms in my GTP cars, than in the 16D Florida arms in the My Series NASCAR motors. I use the same Golddust Pro brushes and Koford springs in both, and am very careful to match the Koford springs I use in the S16D motors.

It all has to do with current flow as these motors also burn guide braid quicker (yep, the negative braid). I can get a full race out of a set of the new soft JK braid on my NASCAR My Series cars with the 16D motors. The GTP cars I have to keep a new pair of braid handy to change as soon as they start to show wear in the middle of the negative braid. Those braid then go to my Hawk 7 powered cars (I don't waste braid since the price went out of sight).

I don't think what you are seeing is that unusual on the higher amp draw motors. I don't even see this on the Hawk 7s, Falcons, or Puppy Dogs. They don't draw that many amps. (I'm lucky to get a Hawk 7 to last a race anyway before the brushes wear away... those motors have gotten terrible).

MR

[marc]

MR: Just a couple questions on your Hawk 7. What voltage do you guys run them at? Do you find they surge at high speeds?

And last... do you find they either break-in fast right away or are dogs from the start?

Just asking to compare notes. Wonder if I'm seeing the same.

[Michael Rigsby]

Marc,

We are running them on our Hillclimb which I know has at least one 90 amp Bulldog power supply with a 1farad cap. Voltage hangs pretty close to 13.8 with no ripple. A lot of the guys do run in to the Hawk 7's surging as you call it (we call it "two speeding"). To stop this, I've been using a break in box I got from some RC guys called the MuchMore Motor master. What it does is cycle the break in voltage from 2V to 8V with a constant 12V input source. I set it up for three 5 minute cycles of running the motor from 2V to 8V with a three minute cool down in between run ins. You can hear the motor become smoother as the brushes were in to the comm, the amp draw changes, and the motor picks up RPM. I then put it in the car and run it for two five minute cycles with a cool down in between runs. I find at that time it's going to be as good as it's going to get. Since I've started doing my Hawk7's this way, I have not had a single one two speeding on me.

One of the RC racers who also is one of our top slot racers just happened to have this at the shop one day and was using it, we heard it doing a pulsating break in and all got curious and it kinda caught on at our place and a few guys have them. I also use it to break in Falcon 7's, but I zap the Falcon 7's in my BOW zapper after the break in is finished before they go into the car.

With either motor, after the machine controlled break in, and the running on the track, the motor is going to be as good as it ever will be. The Hawk 7's will either be consistent in the 4.3's on our track, or it will be a dog and hang around the 4.5 range, but it will be a consistent, on way or the other. Now be aware, even after a break in like this, it may take about 10 laps for that motor to build up heat inside, and it will picked up a half tenth once it does that. That isn't considered abnormal on these motors. Thats the reason I don't cool them down between heats.

Even being very careful with break in on the Hawk 7's, I have to keep an eye on brush wear. It seemed the original motors the brushes lasted a long time, but here lately, it got so bad I started running Hawk 6's with everything completely stock except the springs and brushes and they would outrun the Hawk 7's. I pushed for the Hawk 6 to be allowed in the My Series GT-1 class, but because of a couple of guys sending the Hawk 6's out to be super balanced and having a lot of work done to the chinese arm, two track owners shot down the idea because they didn't feel it could be caught in tech keeping the motors legal.

I just used this method to put a Hawk7 in the car I am going to run at the My Series race in Jax next month, and it was consistently lapping at 4.3's with some laps in the 4.2 range after about 10 laps of running. Hoping to find another strong one to go into my Group F car to keep it in the 3.7's consistently.

MR

Bob---sorry for the thread drift

[Guy Spaulding]

The Boulean Algebraic answer:

High performance + low brush wear = Positive
Low performance + high brush wear = Negative

[Phil Hackett]

One of the curious side effects of switching from ceramics to cobalt magnets (this is the late 70s) was the motor brushes lasted ***hours*** instead of **maybe** a 5 minute heat and sparked less. So, by that effect we can say magnets and their strength also affect the brush life and performance.

[SST]

The reason for more wear on the front is the front spring is pushing the brush into the leading edge of the commutator slot. If you shorten or put a bend in the spring to contact the brush in it's center or slightly toward the trailing side, most of the wear will go away. I have used this in OMO and seen a drastic reduction in brush wear on the front to the tune of the same as the rear.

[Gator Bob]

Hi David, welcome to slotblog and thanks for replying.

While you make a good point, I see this wear on Deathstar motors with stock springs and brushes and the springs only contact the brushes at a point in the center of the brush due to the way that Parma has the springs bent.

[SST]

Bob the springs on Deathstars usually contact near the bottom. You can adjust to center but with the looseness of the hoods as the rpm increases and brush drag increases this tends to shift the brush and causes the leading edge to dig into the comm. The next time you run a deathstar try bending the spring to contact a point approximatley 65-75% toward the traioling edge to see the results for yourself. there are other factors that aggravate this condition due to manufacture and build tolerances.

[SST]

Bob your understanding of current flow is incorrect. Yes the part about electron flow is correct ( from negative to positive) but the other theory of flow from positive to negative is taught as "hole flow". that is the movement of a hole (lack of an electron) from positive to negative. All the mass that moves is in the electrons thus all flow is only negative to positive. This means the flow of electrons is from the left braid to the negative motor brush through the armature to the positve brush and out the right lead wire and braid.