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I'm totally unfamiliar with DC brushed motors but bought one for my watchmakers lathe. I was surprised how hard it is to rotate the spindle by hand and how noisy it is when running. With it running around 2000 rpm and my ear about 2" away from it, it sounds like a faint grinding noise.

So I took off the section with the brushes and the bearings seem perfect. But all four brushes have a raised rib contact surface and the commutator has rub marks that match the ribbed brush pattern.

enter image description here

I did some research on DC motors. One common remark was that brush contact surfaces must be smooth flat and shiny. So I did a search for ribbed contact surface brushes and nothing came up.

My questions are:

(1) Is it typically hard to turn the spindle by hand

(2) Could the brushes have been installed backwards, because the opposite end is smooth, flat and shiny. And there is no perceptible depth to rub marks on the commutator so I'm hoping the motor will be usable.

enter image description here

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    \$\begingroup\$ I've never seen them like that but they probably will wear smooth fairly quickly. \$\endgroup\$
    – Transistor
    Commented May 24, 2021 at 16:37
  • \$\begingroup\$ Thanks for the reply. Even the tech at the company that sells them said he had never seen ribbed brushes. \$\endgroup\$
    – henrylr
    Commented May 24, 2021 at 17:29
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    \$\begingroup\$ Is either end curved? The end that is curved, if it exists is definitely the one that should go on the motor. But if not, the ribbing might be a cost-cutting alternative to making it curved by making it wear in faster. I'd just run it in honestly (though I wouldn't expect it to take too long if that was the intention), as long as the ribs are perpendicular to the direction of rotation. Also, note that I mentioned curved. You don't want your brushes to actually be flat because the commutator ring is not flat. Smooth and shiny yes, flat, no. \$\endgroup\$
    – DKNguyen
    Commented May 24, 2021 at 18:02
  • \$\begingroup\$ Thanks for the reply. The back end is flat. Only the grooved contact surface is curved and the surface at the bottom of the grooves is also curved. They are also properly aligned with the direction of rotation. And are DC brushed motor spindles typically hard to turn by hand? I have to squeeze the spindle hard, with my fingers, to rotate it. And now about the noise...is that also typical and just the sound of the brushes rubbing the commutator? \$\endgroup\$
    – henrylr
    Commented May 24, 2021 at 18:43
  • \$\begingroup\$ @henrylr I can't tell how big your motor is but you have no leverage if you're just grabbing the shaft. I wear earplugs at work so I couldn't tell you if I can hear brush noise or not. \$\endgroup\$
    – DKNguyen
    Commented May 24, 2021 at 19:07

1 Answer 1

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I'm guessing the ribs have to do with tthe motor break-in period. Those ribs will likely quickly wear off, and after time make full contact with the commutator.

Yes, brushed motors have quite a bit of drag, and are usually squeaky when spun. Your commutator looks normal, and in good condition.

Does the user manual suggest a break-in period of letting the lathe run no-load for a number of hours? If not, you could possibly just let it run for a few hours or even a day with no load, and I bet things will be much smoother, and the motor brushes will arc less.

If you decide to take the motor back apart after that, just make sure you put the brushes back in the same orientation as they were removed.

Search "RC motor brush break in" for deeper info.

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  • \$\begingroup\$ You guess is correct. The ribs allow for 'fast' bedding in (which could take many hours if the motor is lightly loaded). \$\endgroup\$
    – Bruce Abbott
    Commented Jun 7, 2021 at 7:10

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