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Thread: Testing arms questions

  1. #1
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    Testing arms questions

    Hey Guys,
    You know me with the questions Well hey,, you cant learn if you dont ask right?? Im trying to figure out, or should I say learn, how to test arms for shorts or other problems. As of right now, all I can do is look at them very closely and see if there are any visible issues. It really sucks to get a motor all together and find out you have a dead or crapy arm in it only to have to tear it all down again and install another arm with your fingers crossed I do understand that motor building is alot of trial and error, but is there a better way to check arms? Will an ohm meter work? or is there a better way to test these things?

    Vic
    Keep your finger on the trigger ,and your eye on the slot

  2. #2
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    OLNEY MD.
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    468
    you said it best cross your fingers and hope for the best just keep loading arms and pull the trigger, if its slow put another arm in it thats what the fast racers do lotta time and effort but when you find the right combo, it will all be worth it. good luck
    When will you guys ever get your car to go around YFS

  3. #3
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    St Charles, Illinois, USA
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    There are Ohm meters and there are really GOOD ones. That is the problem - buy a typical cheap one that only reads to one decimal point and you just cannot get a reliable reading. You CAN tell if it is a dead short, but you may not have enough precision to see EQUAL readings among the three poles.

    1) Short test: Set meter to continuity and see if you have any from the commutator of each pole to the unpainted stacks or shaft. There should NEVER be continuity.

    2) Electrical balence: Set to lowest Ohm rating and measure between two commutator segments. Check each combination. They should read the same to as low as possible or 2-5% .
    l.d. kelley, M.A. Ramcatlarry@aol.com

    60 year pin 1959-2019
    Racing slot cars in America
    USRA 2019 member
    IRRA, ISRA/USA, Hardbodies 1/24 &
    1/32 - Great Lakes Slot Car Club
    retired raceway owner 1992-2007
    Omni/Cidex service center

  4. #4
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    trickyvic3

    I remember Monty touching on that in a discussion in the motor forum maybe 1-3 weeks ago. It wasn't an answer directly but he was outlining to one member the steps he goes thru and he covered that. If I remember correctly he said it wasn't a guaranteed result, ie may/may not tell you if there is a short, not sure. If you grab his profile and look up his recent posts you might find it.


    ps, your avatar throws me every time I see it.
    Guy Middleton
    Everything I was taught in Engineering school I had already learned from slot cars....

  5. #5
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    I see Ramcat posted while I was composing. I'd say he covered it.
    Guy Middleton
    Everything I was taught in Engineering school I had already learned from slot cars....

  6. #6
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    OH - if one pole is wound backwards. it would test fine and not run at all. No metering inspection is perfect and no gear ratio screw-up will prove a bad arm either. I always err in gear ratio on the too low side - let them scream than watch them burn.

    I have had more arms ailing due to a burr on the comm after turning than shorts in the coils.
    l.d. kelley, M.A. Ramcatlarry@aol.com

    60 year pin 1959-2019
    Racing slot cars in America
    USRA 2019 member
    IRRA, ISRA/USA, Hardbodies 1/24 &
    1/32 - Great Lakes Slot Car Club
    retired raceway owner 1992-2007
    Omni/Cidex service center

  7. #7
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    Thank you guys
    I know theres no magic bullit, but mostly Im just looking to test for dead shorts.
    Ramcat,
    What do you usualy use to clean out the slots after cutting a com?
    I never really gave that a thought, but your right ! I should also be checking that.

    Vic
    Keep your finger on the trigger ,and your eye on the slot

  8. #8
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    Usually a sharp body pin or xacto blade and then burnish the groove with an old ball point pen to make the brushes smoother and last longer.
    l.d. kelley, M.A. Ramcatlarry@aol.com

    60 year pin 1959-2019
    Racing slot cars in America
    USRA 2019 member
    IRRA, ISRA/USA, Hardbodies 1/24 &
    1/32 - Great Lakes Slot Car Club
    retired raceway owner 1992-2007
    Omni/Cidex service center

  9. #9
    Join Date
    Jun 2002
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    882
    I would highly recommend NOT using an exacto blade on your comms. It can lift the edges of the comms at each slot and is NOT good. Use a peice of paper or .005 lexan and gently clean the slots.

  10. #10
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    Now that you mention it, I do remember Monty talking this in a past thred.
    See.. I do pay attention in school !! Hahaha !!

    Thanks guys

    Vic
    Keep your finger on the trigger ,and your eye on the slot

  11. #11
    Join Date
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    Wesley Chapel, FL.
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    1,385

    Thumbs up


    I think what has been said many times is that the windings can move around and sometimes in this moving. They can short. I have had some show no problem untill a good power hit and then they will show a short. Most rebalancers do not want to do a NEW arm as they will move with RPMs and sudden stops. Most recomend 3-5 races before rebalancing. Checking all arms will not guarentee a runner but will show a dud....

    OLPHRT
    PHIL I.

  12. #12
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    Quote Originally Posted by Phil I.

    I think what has been said many times is that the windings can move around and sometimes in this moving. They can short. I have had some show no problem untill a good power hit and then they will show a short. Most rebalancers do not want to do a NEW arm as they will move with RPMs and sudden stops. Most recomend 3-5 races before rebalancing. Checking all arms will not guarentee a runner but will show a dud....

    OLPHRT
    PHIL I.
    Well Thats all Im looking to do Phil, is try to pick out the duds. Last week I built up a 16-D using all new parts except the arm. I spent hrs blueprinting everything and carfully piecing everything together, only to find out when all was said and done, It was a bad arm It did run, but I couldnt get it to wind up/ no rpm. So I pulled the arm and went with another one. After alot of swearing/two cigarettes/ and an ice cold beer, the second arm ran like poop through a duck If I had a way to check the bad arm first, I could have saved myself alot of grief

    Vic
    Keep your finger on the trigger ,and your eye on the slot

  13. #13
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    St Charles, Illinois, USA
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    Some folks are inept enough to use an xacto like a prybar. I have not had any luck cutting copper with plastic.

    I used to true commutators on automotive starters and generators in the old days also....
    Last edited by Ramcatlarry; 02-03-2010 at 11:26 PM.
    l.d. kelley, M.A. Ramcatlarry@aol.com

    60 year pin 1959-2019
    Racing slot cars in America
    USRA 2019 member
    IRRA, ISRA/USA, Hardbodies 1/24 &
    1/32 - Great Lakes Slot Car Club
    retired raceway owner 1992-2007
    Omni/Cidex service center

  14. #14
    Join Date
    Jan 2010
    Location
    michigan
    Posts
    2
    I have a arm and gauss meter in the classifide listing check it out,then email me

  15. #15
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    Location
    Victorville, CA
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    urban legends etc.

    I have ALWAYS used the back edge of an exacto (with the sharp point honed down so as not to gouge the phenolic deep in the slot), thousands of armatures, and NEVER damaged one. It ain't hard - unless you get one where the maker couldn't even keep epoxy out of the slot. In that case, you GOTTA dig the slot out, or the arm will slow rapidly.

    Never try to bevel the edge of the slot (the pen thing), you're just rolling a burr back into the slot. Looks good from a foot away, but a pen doesn't cut, it just squashes the edge.

    Vic, that arm that was struggling to rev sounds like one with a metal sliver stuck in the slot, or a carbon bridge over epoxy stuck in the slot. Could also be comm contacting hoods or underplate, but I hope your blueprinting skills have progressed to where you avoid that sort of thing. Still, a new comm can be fairly large at minimum cleanup.

    Phil, what rebalancing services have you been talking to? I have "preconditioned" lots of arms, and most racers feel ONE race is enough to heat cycle an arm. Many get reconditioning done after a test run designed to do nothing but heat the arm up.
    A clean slotcar is a happy slotcar!!

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