35 replies to this topic

[havlicek]

This won't be "the arm", but it helped steer me in the right direction.  Based on having done this one, I *think* I'll be able to make "the arm".  Ultimately, I'm dealing with a "ES" (*eurosport ) setup, and I did a little "R&D" as an "investment".  In the past, I had only wound arm blanks for these provided to me already spaced and ready-to-wind, so I have some catching-up to do.  One thing is clear though, there is precious little (*make that basically zero) room to screw-around with.  The stack here is around .280"-ish long and would have to be O.D. ground way down if it were "the arm".

[Dave Crevie]

OK, John. I want to hear this from you. Back when I was running eurosport, I ground a few .513 arms down to .480. I

felt that I lost too much of the "tops" of the poles, so I didn't do it anymore. What is your take on this?

[havlicek]

OK, John. I want to hear this from you. Back when I was running eurosport, I ground a few .513 arms down to .480. I

felt that I lost too much of the "tops" of the poles, so I didn't do it anymore. What is your take on this?

 

Hi Dave,

     First off, I am FAR from an authority on all this, but as the crown of the arm gets narrower (*in this case from OD grinding), an arm will spin faster, and spin-up faster, all other things being equal.  There are some laminations that start off at a smaller OD and would require less grinding, but there would be less room inside the web for winding, so that can be a limit.  Here, I'm only shooting for a final OD of .500" which would leave much more of the original lamination out there.  This particular lam wouldn't be suitable for grinding down as far as you mention, but there's another issue that can come up.  Even with plenty of crown left, the tips of the  crown can get thin and start to deflect outwards because of the force exerted on the outside by the arm spinning so fast.  Because of that, when these arms get reconditioned, they can need (*I'm told) a light OD grind just to bring them back to round.  In a worst case scenario, and with a tight airgap, the arm can contact the magnets...no bueno...and there goes your several hundred dollar motor!

***Here's the thing, the general format here (small diameter arms with hot winds in tight spaces provided by really small motors using very strong magnets) is proven by the eurosport guys.  If grinding the arm down so small were a disadvantage, no one would be doing it!    From what I understand, arm ODs of .480" are pretty typical.

[Geary Carrier]

Hi John,

 

 

R&D is fun as even failures tell you something, in certain situations more than success.

 

The cap is anodized Al?

[havlicek]

Hi John,

 

 

R&D is fun as even failures tell you something, in certain situations more than success.

 

The cap is anodized Al?

 

Yep.  By doing all this. I hope to avoid a failure Geary...but that remains to be seen.  Yes, the cap is anodized aluminum, and the cap is one of the big issues that cuts way down on the room you have to build these things.  The hardware on the setup (*Cahoza, very nice indeed!) has vertical hoods.  Between the height of the brushes oriented this way and the space lost on the com by the cap...then also the hard limit under the end bell to the top of the arm tabs, there's very little room to move the arm.

Then there's the whole thing about installing multi-segment magnets...oy veh.  I'm sure that if I did a bunch of these, I'd figure out ways to make things easier, but doing just this one makes me NOT want to do a bunch of these things.  There are all kinds of other obstacles to get beyond besides *just* what I mentioned above too.

[Kim Lander]

That arm looks fun as it is....keep in mind when you grind the Lams you open the gap and that will inavertantly change the timing some because the magnetic field changes slightly on the leading and trailing edge. Should be fun finding out your results.

[havlicek]

Hi Kim,

     There's really nothing to keep in mind.  This has all been done before, and with these very lams.  On the grinding thing, the effect will be the same as using a narrower crown lam.

[Dave Crevie]

Good answer, John. The arms I ground had a slight radius at the leading and trailing edges of the crowns of the lams. That

went away when I ground the O.D. While knocking off the burrs left from the grinding with a polishing stone, I broke those

edges slightly. Three of the four arms I did ground well, but one had a few lams spin on the shaft, and ruined that attempt. It

also screwed up the dress of the grinding wheel. I used an 8" dia. 80 grit aluminum oxide wheel in a Harig surface grinder,

and a St. Mary's Spin-roll set up with a tailstock to support the comm end. Any finer grit wheel burned the edges of the crown. 

 

The whole idea was to reduce rotating mass out away from the shaft. My theory was that I would have a more responsive motor. 

I can't say definitively that it helped. The arm was used in a Koford quad set-up, and probably any arm would have been good in

that. I wound them 16/24. 

[olescratch]

WOW!  16/24, definitely drag arm zone, or was it for a wing car.  This had to be a hot one!

[Dave Crevie]

For a 1/24th eurosport. It was hot. Ran it in a Horky designed JK chassis, on the MTT track. It was a handfull, but I

really needed an edge. After I outgrew the high-horsepower kick, I put it in an open wing car. But I was never into

wing cars, so I just played with it some on Jerry's black king. 

[havlicek]

Oooooo...Kaaaaaay.....   So, I honed the mags in this thing, and blew off a couple of man-sized chunks.   Round two, remove the magnets, clean everything up (*no small thing), and assemble/install another set.  This time, because I'm working with what I have, I make up two sets of four segments per side...top and bottom segments with the arc ends, and two inner segments.  No, this won't be the super-current 24 (or more!?) segments per side type deal, but it will work.  Mags are honed, and only some very minor chipping happened at the ends.  Getting the cobalt magnet dust out of there is a real job in and of itself, especially when some of the segments were pre-charged.  Now I have to build and wind "the" arm and that's going to be something of a task because:

*The vertical brush hardware PLUS the com cap makes for almost zero room for spacing error.  I was thinking that, as a "plan B" I could space the com cap a little off the end of the arm and still have enough overlap with the com segments to help protect the com from exploding...maybe.

*These setups don't have much room at the tail spacer as it is, so shortening the tail spacer doesn't get you much extra room at all.

***Because the arm OD is going to be way larger than the magnet hole before OD grinding (*which happens after winding), I can't even test the spacing before building the arm, because If I rough grind the arm before winding, I don't think I'll be able to clamp the darned thing.  I may have to space the arm visually and HOPE it's correct.  Given the very limited room to move the arm fore and aft, that's one heck of a crap shoot!

[havlicek]

Heck with it...I built an arm just for spacing, out of cheap Chinese lams from a Parma D motor, grinding the OD down small enough the get the size of the com and tail spacers figured out.  Then I can remove those from the "jig arm" and put them in place on the actual arm.  That way...the arm should fit when it comes back from grind/balance...hopefully.  The jig-arm lams are made from a different thicker lamination steel, so I will have to get as close (*or slightly under) the exact same stack length when I press a new arm out of the better laminations.  This is an awful lot like work.  Matter of fact, it's just like work.    I'm sure there will be more failures ahead, but that's how we learn.

[havlicek]

Soooo...I *think* this is "THE" arm.  Of course, I can't say for sure until it gets back from a heavy grind and balance, because as it stands, I can't install it in the setup to double-check the spacing.  For the com cap, I also cut the com (*which didn't take much of a cut to get it true), and then only cut the end where the cap goes, forming a shoulder.  I did that because getting the cap on the com would have required enough more of a cut to shorten the use-able life of the arm.  I don't know how this is generally done, but this is how I did it.

Arm is conventionally-wound at 20/25, and comes in at precisely thirty thousandths of an ohm (*depending on ambient temp ).  Stack length is a little over .280", and timing is set at +15 degrees advance CCW.  I could have got to this same wind with three layer coils by "reverse-winding", but the four layer coils clear the back end of the can, so I went with it.  Out it goes for grind/balance.

[wbugenis]

  I don't know how this is generally done, but this is how I did it.

 

 

Yes,  that is how it is "generally done".     A press   fit with a tiny dab of Loctite 609. 

[Geary Carrier]

Sweet arm John,

 

Not much room to wind, not much room for anything, that had to be fun...

[Geary Carrier]

 

Yes,  that is how it is "generally done".     A press   fit with a tiny dab of Loctite 609. 

 

That's interesting, 609 is not that heat resistant...

[wbugenis]

 

That's interesting, 609 is not that heat resistant...

 

You may be right, Greary  It's what I  use to secure the commutators before winding so it's what i have around.

 

If you get the press fit right you may not need anything.

[havlicek]

Hi Guys and thanks for looking-in.  Really, there's nothing "different" here, other than the stupid-tight spaces and tolerances, but boy do those make a difference.  The real PITA comes from spacing the arm when you can't just put the arm in that you're winding (*in my case).  Also, apart from the arm, the magnets are a real challenge, but "trying and failing" leads to better methods. 

I'm going to do the same wind as a three-layer coil since I have the "jig-arm".  It will be an opportunity (*once again) to see what the comparable resistance measurements work out to be.  So the motor will be supplied with two arms...one reverse,  and one conventional. 

In the end, these motors, more than any other I've done, really require setting-up and running-off multiples of parts, because there's so much test-fitting involved that these things would be stupid-expensive to produce.  Actually, they are stupid-expensive to buy, and at least part of that is understandable, considering what goes into them.

[havlicek]

As per above, I did the same arm, but this time as a reverse-wind, so now it's a three-layer coil.  This makes for a bit more space at either end of the arm...it's only a little, but when things are this tight, even a little is noticeable.  Resistance for this one is slightly down, at twenty eight thousandths of an ohm, or two thousandths under the first one.  Like I said, when these things wind up lower, it's not by enough to probably matter.  On the other hand, the coils are tighter and more compact, which might matter a teensy bit.  So the difference here is a little more room and tighter coils, resistance can go either way.

[gotboostedvr6]

They look great John!

[havlicek]

Thanks Dave.  I do try! 

[gotboostedvr6]

We need to talk.
2019 Nats coming quickly

[wbugenis]

Better order up some 0.040"  cobalt segments, Dave.   

 

The 0.070"  stuff that John is using is just too much to hone.

[havlicek]

Better order up some 0.040"  cobalt segments, Dave.   

 

The 0.070"  stuff that John is using is just too much to hone.

 

Boy is that ever an understatement Bill. 

[Phil Smith]

I'm sure I've seen open cars that had capped arms under the hood, but never just a capped arm. What are caps made out of?