# Checking Ohms of Arms



## Ligier Runner (Nov 29, 2004)

Can anyone enlighten me HOW to check the ohms on armatures? I understand that you check all three arms and you're looking for equal ohms rather than just the lowest but where do you place the leads of the meter?


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## *MAYHEM* (May 4, 2007)

Ligier Runner said:


> Can anyone enlighten me HOW to check the ohms on armatures? I understand that you check all three arms and you're looking for equal ohms rather than just the lowest but where do you place the leads of the meter?


Place the leads on two of the commutator contacts at a time, one lead on each contact. Check all three combinations that way.


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## SwamperGene (Dec 1, 2003)

Pictorial here.


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## roadrner (Jul 21, 1999)

SwamperGene said:


> Pictorial here.


 
Great link SG! :thumbsup: :thumbsup: rr


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## SwamperGene (Dec 1, 2003)

Thanks RR  ! It's always amazed me that for a question that's asked so often there is so little easy to understand (and with pics) answers available. In my experience, even face to face, most people that ask want to see how it's done.


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## Ligier Runner (Nov 29, 2004)

That's an awesome post! I was more or less on the right track but I'm the type that hates to be only half right and get it all wrong. Much thanks!!


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## Grandcheapskate (Jan 5, 2006)

SG,
Assuming a factory wound armature, when one of the readings is vastly different than the other two, what does that indicate? A broken wire?

Joe


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## Bill Hall (Jan 6, 2007)

Grandcheapskate said:


> SG,
> Assuming a factory wound armature, when one of the readings is vastly different than the other two, what does that indicate? A broken wire?
> 
> Joe


Most times Joe, However before I gong an arm, I fire up the pencil tip iron and carefully re-do the solder joints and recheck the reading. It doesnt always work but some times you can rescue an arm or two and only takes a minute. 

Sometimes you can repair a broken wire if it's the wire on the outside end end of the pole. You can take a turn or so off (no more than you need) and pick the wire through then re attach it. Obviously if the inside wire of the post is broken your looking at a rewind because it's held captive by the windings.


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## martybauer31 (Jan 27, 2004)

Grandcheapskate said:


> SG,
> Assuming a factory wound armature, when one of the readings is vastly different than the other two, what does that indicate? A broken wire?
> 
> Joe


It could indicate a broken wire, but if it spins up, then it's still a good arm and the poles are just off.

I went to the fray last year and according to everything I read, people recommended that the poles should not be off by more than .1 ohm between each pole and be as close to 16 ohms for each pole as possible. After the fray was over some folks listed the specs of the winning arms:

1st place: 
motor- red wire ohms at 16.4 15.8 16.5 balanced not trued

2nd: 
motor- christmas tree ohms at 18.2 18.0 18.2

5th:
Armature - red wire, 16.7-16.8-16.9, trued

6th:
motor- red wire, ohms at 16.2, 16.3, 16.5 balanced and trued.

So you can see that some motors adhere to this policy, but that 1st and 2nd were definitely out of the range of the supposed good arms.

It was a good lesson for me, after I figured this out, I went back through a bunch of arms that I had set aside that either ohmed out too high or were the dreaded Christmas tree arms you were supposed to stay away from. At this point my fastest 2 cars are a Christmas tree arm and one that ohms in the mid 18's. The only reason I ohm my arms at all now is to make sure they meet the minimum 16 ohm average for the fray spec.

It's more important how they spin up than how they ohm out I believe at this point.

Marty


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## Ligier Runner (Nov 29, 2004)

Okay. Now Marty has me posing another question. How do you balance and/or true an armature? My background as a racer was just to throw together the best car you could and womp on everyone with it. We never bothered with this high tech crap! LOL


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## SCJ (Jul 15, 1999)

Ligier Runner said:


> Okay. Now Marty has me posing another question. How do you balance and/or true an armature? My background as a racer was just to throw together the best car you could and womp on everyone with it. We never bothered with this high tech crap! LOL


JMHO, but a balanced arm is *MUCH* more important in our opinion the a low ohmed arm as ohmage can be changed by gauge or wire and or number of winds per pole which is NOT always consistent from pole to pole.

*Balancing an ARM:* One of the cheapest/easiest ways to set up an arm balancer is probably as follows:

Set up a little tech block (just the width of the shaft running through the arm) and place a old fashioned razor blade on each side (careful they are sharp!) then place the set up on a flat and level surface....confirming with one of those little torpedo levels. Now place the arm (free from the arm plate of course) one shaft on each of the two blades.....whichever pole drops to the bottom is the heaviest, drill or sand the metal painted edge on that side until each pole will stick straight up or down..........now your arm is balanced. A little work bench vice (if you can make it level) works as well.



But, don't forget to measure your magnet strength………this is also very important! How do you measure this you ask? Well you can go out a buy a $300+ piece of machinery, or make one!



*Poor Mans Gauss Meter:* Take an old fashioned plastic film canister (remember those) and drill a small hole through the lid placing a small bolt and nut through said hole with the threads of the bolt sticking upward. Now get several pennies placing several into the canister and closing the lid. Now, separate your N & S magnets. Using the curved part of the magnet (the part that pulls on the arm) and see if it will pick up the film canister with say 5 pennies in the canister. Those that won't set aside. Add another penny and repeat the process, repeat again and again until you have only one magnet that picks up the canister with __ number of pennies in it. Repeat for the other group of magnets until you have one N and one S magnet….you know have your strongest set of magnets!



Hope this helps,



----------------------------

www.SlotCarJohnnies.com


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## martybauer31 (Jan 27, 2004)

To "true" an arm I chuck it into my dremel and then spin it up and grind very carefully the edges on a piece of sandpaper, this just evens out any inconsistencies in the outside diameter of the arm.

For balancing, I am not an expert by any means, but I use a watchmakers poising tool I won on ebay. You set the arm onto the tool and if there is a heavy side to the arm it will drop to the bottom. I will then take the dremel with a mini cutting disk and remove a bit of material from the outside of the pole, checking to see when it no longer favors dropping to any one particular pole.

I highly recommend practicing both of these on a known crappy arm or two before doing it on one you really want to use, as if you aren't careful you can ruin the arm.

Hope that helps, there is a good article on this on HO WOrld:
http://howorld.fsmra.com/archives/howto/rewind/balance/poising.html

Marty


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## afxgns (Jul 6, 2006)

To Add to the ballancing thing:
After you find the heavy pole, do you filing (I prefer to use a dremel with a cuttoff wheel, notching up and down) Place the arm in your chassis sans the gear, and spool it up. If it is still out, take off some more 'till it's prefect. This is a dyamic ballance and it works much better.
Good luck

Tim Leppert


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## SwamperGene (Dec 1, 2003)

Joe...those guys pretty much answered your question.  


SCJ, you just touched on my latest project, lol... how 'bout a DELUXE Poor Man's Gauss Meter...A quickie Clamp, a magnetic flat washer, some 1/16" brass rod, and a piece of 1/2" copper tube with a plug siliconed into the bottom. Fill tiny "bucket" with some weight of choice til bucket fall. Then, remove the last added piece, rehang and fill with drops of water til bucket falls, then weigh it.:










The bucket hangs from a hole drilled in the washer, and to stick it to the mag I use a piece of brass rod as shown in the first pic, all the weight is always in the same spot, you want as much consistency as possible (hence the blue "front" dot on the bucket).










I test in grams to .1, and take three readings per mag to get the average. You have to keep the eyedropper at a fixed height, the force of the drops falling can make a difference. For stronger mags, I stack stainless spacers evenly on each side of the bucket's handle. Works pretty slick, actually!


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## jamiestang (Dec 15, 2006)

Where can i get 3 ohm tyco armatures hot wheels armatures ?


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## Ligier Runner (Nov 29, 2004)

Thanks guys for all the great info. I *always* forget about HOWorld.net and their archives for info.


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## Grandcheapskate (Jan 5, 2006)

Another poor man's magnet strength tester I saw once used a compass. I think this was how it was set up.
Lay the compass flat and put a wooden ruler next to the compass at either the east or west end of the compass on a line with the East-West axis. Now take a magnet and put it against the compass on the ruler - the arrow should point either at or away from you. Find out how far away from the compass the magnet can get while still holding the pointer at the east or west pole. Once the magnet is moved too far away, note the distance. Match up those magnets that have the same distance reading.

Joe


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## Bill Hall (Jan 6, 2007)

This was won of my favorite pieces of Arkansas engineering!


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## jamiestang (Dec 15, 2006)

Anyone know where to get 3 ohm slot car armatures ? i need them for my tyco's thanks


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