Testing Carbon Brushes

[redwoodguy]

And maybe it is, (especially to you bench techs out there) But here goes , is there a way to test brushes or do you only check them by their general appearance ? If so , what does a defective or worn out brush look like ?








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[Mike]

There are a few things to look for. First, make sure the brush spring still has good tension. If the spring gets too hot it can lose its strength and the brush won't make good contact against the armature. You'll also want to check the carbon brush itself for any cracks or pitted surfaces. It's also possible that the brush is just too scrubbed away and needs replacement. The brushes start out pretty long in your Makita saw. They should look a lot like the ones in this picture:

http://www.ereplacementparts.com/car...154-p-376.html

[redwoodguy]

Thanks for the reply Mike. My set of brushes do look noticeably shorter, and there is some scoring on the contact ends. I also see some mild corrosion for lack of a better term on the "comutator?" , or whatever that part of the armature is called that the brushes contact. I think that I read on another post somewhere that a suggestion was made to clean up that surface with very fine grit sandpaper. And the local electrical motor shop here said the same thing. Is this recommended ? It sounds kinda touchy to me.
Anyway, thanks again , I really do appreciate it.




stayin cool in the Redwoods
Keep on Truckin

[Mike]

A commutator is always going to have a dark band around it from contacting the brushes. If it's just a discoloration I wouldn't worry about it. Install the brushes and see how it runs. If there's a lot of arcing then you may need to clean up your armature.

Instead of sandpaper, the ideal method is to clean it up with a commstone to seat the brushes. This is really only necessary on a very worn armature though.

[redwoodguy]

Hi mike , Thank You (again) for your reply about the commutator. You are awesome, (I hope that didn't come off as sounding a little gay, ha ha) I really appreciate your input.

[Mike]

Hehe, no problem!

[BigDyl]

Hey redwood guy, the commutator should be almost a chocolate color where the brushes ride. And you can also clean a commutator with an eraser. That won't get any scoring out of it though.

[redwoodguy]

I've been busy, i know that it's been a week since my last reply.
OK, I put a new set of brushes in my saw and.....still nothing Mother _ _ _ _ !!!!!!
I'm not sure if I'm testing the field coils correctly, do I touch one of the test leads to the hot wire or the neutral wire ? (and touch the other test lead to the brush contact ( where the field wire is connected to the brush's seating) 'IF' I did the test right like I described above for each side of the field coils, the resistance wasn't the same, one side put the ohmmeter pegged and the other field coil showed about half as much. AND that's like really, really bad news for my saw isn't it ? The cost of an armature and a new field ass. isn't very practical
$152.94 before shipping. when you get time to reply to this , as before Mucho Grassyas.

Scott

[cmoorejack]

Hey Scott,
just thought I would throw my two cents in here..[or maybe a nickel..!]..you might want to start over with your trouble shooting..sometimes just an ohm reading isn't enough..a cord or switch can check good with an ohm check but they will break down under live voltage..so check your cord with live voltage..also the switch..[be careful...!] if they check ok..move on to the field...[if you have any kind of ohm reading..it probably is ok..i assume that your problem is that the just won't run at all..? as long as you have any kind of reading [ohm] on the commutator/armature..it is ok..[if your saw was sparking really bad you would isolate it to a bad arm,field,brush...]
hope this helps..?
Craig

[redwoodguy]

hi Craig , thank you for the quick reply. Yes, you are correct in assuming the saw won't run at all. Prior to this the saw was never sparking or running erratically at all. When you say to check various components under live voltage , which setting should I set the meter at ? Mine has 3 settings , Rx1 , Rx100 and Rx10,000. And it won't damage my meter to test resistance with the tool plugged in ? On the tests , should there be continuity between all 3 wires on the switch when the switch is in the on position ? And on the field test , are you testing for continuity between the field leads and the hot wire ? And I'm not really sure how to test the armature/ commutator itself , where do I touch the meter's probes to ?
And again , Thank you so much for you time. I REALLY APPRECIATE IT !!!
Scott

[cmoorejack]

Hey Scott,
Yes..it will damage [or at least blow the fuse]on your meter if it is not switched over to ac volts on any setting that is 115 or higher...it would be a good idea to read up on the instructions on your meter or look up tips or techniques online...
once you have your meter on the correct setting..with the saw plugged in[blade removed for safety!] take your two probes from the meter and touch both wires going into the switch from the cord[one probe on each connection terminal] you should get a reading of 115 or 120 volts..if not the cord is bad..if it checks with the correct voltage move on up to the other end of the switch..[switch locked on] and check to see if the voltage reads on that side..if so..that means your switch and cord is in working order.if not then your switch is bad..
To check the armature..[with the tool unplugged]..switch back to your ohm setting..set one of the probes on any part of the comm and then take the other probe and move it slowly around the rest of the com to the other side meeting up with the first probe..if you keep a reading of any kind it means your armature doe's NOT have an open circut and is ok..[your armature could still be shorted to itself but can only be tested with a growler test.]. .a sign of a shorted armature is a slow speed,funny noise.smells bad[kinda like my mother-in law].another check is to see if there is small separation between one or two of the bars indicating an open... anyhoo..to check your field with the ohm test you have to check continuity between one side of the winding and the other [on each side] ..since your not checking the field for a short or grounded out to itself the meter scale wont mean much..you just want to make sure you have continuity...there is a lot more testing info I could go into but I might end up confusing myself..
hope this didn't seem like too much rambling and is helpful to you..!
most likely you have a bad switch..but im sure you will figure it out..
Craig

[redwoodguy]

Craig , once again Thank you , Thank You. I tested the field and it is ok , I'll check the voltage at the switch on each side like you described and then I'll disassemble the saw to test the armature at the commutator. That part will be easy because I had the saw completely apart earlier , but I didn't know how to test the armature. I reassembled the saw though, no problem. I can guess that some brands of tools are probably a real bitch to work on , thankfully this makita isn't one of them!! BTW , have you ever checked out this web site:
Notes on the Troubleshooting and Repair of Small Household Appliances and Power Tools ?
http.//repairfaq.ece.drexel.edu/sam It's full of info for repairs at various skill levels, alot of it is over my head , but it is the best site I've found that covers just about every type of electronic device you can think of , from small appliances of every kind imaginable , to every type of power tool to universal garage door opener remotes and receivers !! It even explains motor armature tests , the growler !! But , like I said it doesn't explain alot of procedures in layman's terms. that's how I came to find this site and the great people like you and Mike.
I'll get back with my results. Scott