Film-Tech Forum ARCHIVE: Potts A3 Brushes wearing out

This is topic Potts A3 Brushes wearing out in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Tim Sherman (Member # 569) on 05-10-2009, 11:37 PM:

I have a Potts A3 microswitch platter. Problem I seem to be having is that one brush on the motor is wearing away completely, while they other brush looks brand new. When mounted on the tree it is the upper brush (opposite the diode) that is wearing away to nothing. It isn't something that happens very fast, takes maybe a couple of years. But I was just wondering why one brush would be gone entirely and the other looks brand new on the same motor. This is happening on more then one motor and is also happening on the other identical platter as well. Any ideas? for now i'll just keep replacing brushes. Other then that they run flawlessly and have since 2000 when we bought them.

Posted by Louis Bornwasser (Member # 3063) on 05-11-2009, 08:24 AM:

I have noticed this as well. I put it down to some brushes are better than others since wear is not the same. Perhaps the spring is pushing harder on the one that wears out. Louis

Posted by Monte L Fullmer (Member # 2797) on 05-11-2009, 03:19 PM:

Same here on a AP3 unit - I replaced the motor (winding opened up) with a Grainger 2M262 motor back in 2004, and this wknd, the top brush went completely down to nothing while the lower brush was only slightly used.

Luckilly, I've got a good stock of those 1R236 brushes from Grainger to replace with and I replaced the pair.

See what happens..

-Monte

Posted by Mark Gulbrandsen (Member # 72) on 05-11-2009, 09:49 PM:

Actually boys it happens because the commutator gets a tad bit rough... Clean it up and give it a reasonable polish(not mirror finish or the brushes will never seat!).
The armature assy if shot is available seperately if needed at well stocked theater supply dealers.

Mark

Posted by Tim Sherman (Member # 569) on 05-12-2009, 12:34 AM:

Mark, why would it only wear one brush then and not both? Both brushes contact the same commutator.

Posted by Monte L Fullmer (Member # 2797) on 05-12-2009, 02:33 AM:

quote: Tim Sherman
why would it only wear one brush then and not both

(a guess here) Maybe, it's how the current is going on one brush since the diode is in series with the one leg.

Posted by Louis Bornwasser (Member # 3063) on 05-12-2009, 10:47 AM:

Monte: I think you have a valid point! Louis

Posted by Mark Gulbrandsen (Member # 72) on 05-12-2009, 10:48 AM:

Exactly!

Posted by Gordon McLeod (Member # 33) on 05-12-2009, 11:08 AM:

The diode is 1/2 wave rectifier and it would effect both the current going in and out of the commutator as it is onlya 2 wire device

Posted by Stephen Furley (Member # 1277) on 05-12-2009, 11:13 AM:

Are the contact surfaces on the brushes shiny and polished, or pitted and burned? Maybe there's arcing on one of the brushes, brush sticking in holder or weak spring.

Posted by Monte L Fullmer (Member # 2797) on 05-12-2009, 01:03 PM:

Sometimes, the carbon content can be not up to snuff and can cause the quicker weardown..