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#1 Robert BG

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Posted 05 June 2016 - 12:32 AM

I have always wondered if anyone has experimented with trimming the ends to reduce the contact patch on the comms. Did it help at all?

 

I would think you could run higher spring pressure to reduce bounce while reducing the friction on the comm. Or possible get a small increase in RPM, etc.


Robert Fothergill




#2 havlicek

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Posted 05 June 2016 - 05:40 AM

This is done on some motors, but I would guess more for reducing brush overlap between comm segments and maybe slightly changing timing than increasing RPM, although all "could" be the result of trimming the brushes.  With the physics involved though, just trimming the brushes increases the pressure on the brush face without increasing spring tension. Think of it this way, the pressure a spring exerts on the brush face where it contacts the comm would be expressed in units of force per square unit of measure. With the same spring force applied to a smaller area, the force per square unit of measure has increased. A practical example of all this is... say a woman steps on your foot with her heel while wearing a pair of sneakers... no big thing. Say the same woman steps on your foot with her heel while wearing very skinny high heels... ouch!

Anyway, reducing the brush face and increasing spring tension may not be such a good idea because the pressure on the brush would effectively be increasing a whole lot. There's a lot in play in all this. Generally, the size of the brush face is important compared to the size (diameter) of the commutator. Small commutator and big brushes = no bueno. Big commutator and small brushes = OK, but maybe not optimum. Small commutator and small brushes = bueno, big commutator and big brushes = bueno.

All this gets into the general subject of "commutation," which, to my mind, is probably the most important aspect of how and why a motor works and performs the way it does. The brushes (and the material they're made from), the size of both the brushes and the commutator, timing, how (and how well) the coil wires are connected to the comm tabs, spring tension and material... as well as the number of coils, the angle of the legs, even the metal the comms are made from (*most often copper, but tin-plated and even silver and I guess others), even the body of the comm (usually phenolic, but I remember that even anodized comms have been (or maybe are?) used... all sorts of minutiae have a really large effect on the motor's performance. I have a set of the old LaGanke "silver" brushes I never tossed just because they reminded me of all the old stuff we tried for that extra edge. The label says 93% silver and 7% graphite, and while both copper and silver are soft, malleable, ductile metals, I figure silver brushes are probably pretty hard and would take longer to break-in and might cause premature comm wear. Even among brushes of the same basic composition, there can be changes in performance characteristics that are huge because the sintering process used to make them can have some variations if not done well or something.

Anyway, this is one huge subject, when you look at everything involved with brushes and commutators. Anyone can wind an armature, but producing race-winning armatures is literally rocket science. 
 
-john
 
PS: Since all the current a motor "needs" must pass through only the brush contact area, it's possible that a motor with reduced contact area brushes would not be able to run optimally being "starved for current."


John Havlicek

#3 havlicek

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Posted 05 June 2016 - 03:50 PM

O..........K.........you're welcome!

 

-john


John Havlicek

#4 Robert BG

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Posted 06 June 2016 - 12:16 AM

Sorry,I went to reply and in the middle of my post a transformer went poof in front of the house.

 

The short of it is that I'll probably experiment a little with this.It should be fairly easy to model and with some research I can probably figure out the calculations. Although I'll probably just wing it at first ;) With lighter springs and thinner brush faces. 

 

I do need to figure out how big a brush is needed to pass the needed current and how to reduce tension.But all of our motors use the same brush size yet have different amp draws chances are it can be thinned down some.

 

The other part of my question was that a thinner brush face shouldn't affect the timing like a smaller brush.If I'm not mistaken.

 

There was more but I'll try and piece the rest together tomorrow

 

Thanks

Robert


Robert Fothergill

#5 havlicek

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Posted 06 June 2016 - 05:30 AM

No biggie.  If you find a particular size that works, making (or having someone make) a jig/fixture to trim the brushes the same will give you repeatable, predictable results.  Until or unless you're using high-amp-draw motors, I wouldn't worry much about the brush being able to pass enough current.

***What I can say about this is that, with two otherwise identical (*of course there's no such thing as truly "identical") motors in the <2amp (*6VDC, no load) range...say a #28/#27 wind, the one with the smaller brushes AND smaller diameter com will run noticeably faster.  So, this is *only* RPMs, and without track testing, actual positive or negative results are impossible to know.  Some of this may be because the smaller brushes and com setup has less friction, but I don't know how much (if any) of a factor friction is here.  Put the small brushes in a motor with the large com and the motor may not run as well as the large brushes with the large com...pretty much wiping out the whole "friction" concept.

I have done many motors where some will get the larger "36D" size brushes (mounted horizontally) which are the "standard" and a typical modern com , and some will get the old 16D size brushes and the thinner (ie: Tradeship-type) com.  The ones with the smaller com and brushes will always run faster. There's not a question, and it's entirely predictable.  Of course, a person who knows what they're doing setting up a car could (and would!) use a slightly different gear ratio for the two motors, even in the same car at a given track.  That could somewhat, if not totally mitigate any advantages/disadvantages one setup might have over the other.  ***It is possible though that one would have an absolute advantage over the other, and just because the larger brushes/com combination is more the standard, doesn't mean it's better.

 

I always come back to the realization with these things that most anything you can dream of has been tried over the decades, and certainly obvious things like modifying brushes hasn't been overlooked.  That's never a reason not to try things for yourself, because some "givens" in all this are based on fairy tales and/or just other people saying "that won't work", even though they never tried it!

 

-john


John Havlicek

#6 Dallas Racer

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Posted 06 June 2016 - 10:01 AM

I have always wondered if anyone has experimented with trimming the ends to reduce the contact patch on the comms. Did it help at all?

 

I would think you could run higher spring pressure to reduce bounce while reducing the friction on the comm. Or possible get a small increase in RPM, etc.

 

I remember seeing cut brushes in RC car magazine ads back in the day. It seemed like there were many variations, but all I remember is brushes like in the pic and brushes with a hole drilled down the center. It seems like a good idea, but you would think it would in common use today if it worked.

 

I would guess that cut down brushes would require less spring tension, not more. Less brush weight equals less spring tension.

 

I found the brushes below at PCH: http://e-slotcar.com...brushes-ke-994/

 

KE_994__37344.1429127884.1280.1280.jpg?c


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#7 MSwiss

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Posted 09 June 2016 - 01:22 PM

Been meaning to respond to this.

I've tried taller, shorter, narrower, and even talked Stu in making a few, high quality, small diameter comms, for me to experiment with, years ago.

Other than shortening the height of horizontal brushes, a bit, on high cog, small dia. comm motors, like the JK Hawk 6 and P/S 4002FK, and getting better start up and a little more efficiency, everything I tried worked the same, or worse.

If anything different from the norm, worked, believe me, someone would be marketing it.
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Mike Swiss
 
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#8 Mr. M

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Posted 21 June 2016 - 07:25 PM

Mike,

Why is a small diameter com in some cases a higher performer?

Chris
Chris McCarty

#9 Robert BG

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Posted 22 June 2016 - 09:31 AM

Mike,

Why is a small diameter com in some cases a higher performer?

Chris

I was wondering the same thing myself.

A wild guess would be due to timing but that is just a wild guess.

In the past I've had setups get faster as the coms got smaller,they even put down better times with black phenolic showing through the coms than they did when new.People used to think I was crazy for running them like that but I was pitting next to Keeler once and mentioned how my best arm was showing plastic through it.He said dont worry,if its that fast you can still use it to qualify a few times or run a heat.At first I thought he was just pulling my chain but I tried it and he was right,it was still fast and lasted long enough to qualify a few times.


Robert Fothergill

#10 MSwiss

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Posted 22 June 2016 - 01:42 PM

Mike,
Why is a small diameter com in some cases a higher performer?
Chris

I don't really know.

Lots of theories that I really can't back up.

Less mass?

Less friction?

An arm that can be cut small, is the result of being drilled real on center?

After I had a solid 27 arm that really got exceptional after it got below .183"(?) and switched to vertical brushes, I had Stu make me a few arms with comms that were already small.

They were just too average to persue it any further.

Mike Swiss
 
IRRA® Components Committee Chairman
Five-time USRA National Champion (two G7, one G27, two G7 Senior)
Two-time G7 World Champion (1988, 1990)
Eight-time G7 King track single lap world record holder
17B West Ogden AveWestmont, IL 60559, ( 708) 203-8003
mikeswiss86@hotmail.com (also my PayPal address) 
Note: Send all USPS packages and mail to: 5858 Chase Ave., Downers Grove, IL 60516
Make checks out to Chicagoland Woodworking, Inc.


#11 Jay Guard

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Posted 22 June 2016 - 02:39 PM

One explanation for cutting the brushes as shown in post #6 was that a much larger portion of the comm was exposed (not covered by the brushes) and thus the comm runs cooler. This does make sense although I don't have any technical data to prove it. I recently cut the brushes like this on a Pro Slot FK motor and it did improve performance noticeably.

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