View Full Version : Motor Brushes

12-18-2005, 03:52 PM
I need to find DC Servo Motor Brushes. Where can I find these animals. My X axis servo keeps kicking out on my mill, it is a non-manufactured motor as of now. What specs do I need to know, are these basically standard between vendors, and what vendors are reasonably friendly. MSC does not have them.

The motor keeps kicking out in rapid traverse, I tried all the things the service tech from the control company asked me to check, then he referenced me to an area tech. We isolated the propbelm to either a bad motor or bad brushes. The service man has been great, he has told me exactly what he would do if he came to see me, and is doing gratis work on the phone to help me as best he can before he makes the expensive trip up. The new brushes are the missing piece of tyhe puzzle, as the machine manual says change brushes at 1/3 wear, and they are at about 40% wear. Thus my problem, hopeful solution, and my plea for help here.

12-18-2005, 03:59 PM
Batteries.........these contain the parts you want .
now do some thinking ....
all the best.mark

12-18-2005, 06:30 PM
Hi Spope,

I know you can find some at the local vacuum cleaner repair place. These motors use brushes of about 1/4" square.

I know that some drill motors use them. Another source maybe.

Good luck.

12-18-2005, 07:27 PM
There's literally thousands of variations on brushes. To match them up, you'll need width, height and length. After that, you'll need to know if they are plain, have a pigtail or pigtail with spring.

They're probably carbon brushes but If yours have a pigtail, it will be hard to add one to a "D" battery anode. Besides that, motor brushes probably have graphite or some lubricity compound too. A dry cell battery anode is pure carbon and brittle. (But a good thought Mark.)

12-19-2005, 10:33 AM

You will need dimensions of the brushes as mentioned earlier. I would then try one of the servomotor manufacturers to see if they heve what you want. A possibly better option would be a motor repair shop. As a servomotor mfg, try Dynetic Systems, 612-441-4300.

Jack C.

12-19-2005, 12:17 PM
Mcmaster Carr sell the by size.


12-19-2005, 08:27 PM
Brushes are certainly made in different compounds. Vacuum cleaner brushes are generally a little harder, and lower voltage dc motors generally use a softer compound. It's usually pretty safe to choose based on the size of the wire coming off the brush, since it's usually fatter wire for lower voltage motors. A brush made for alternator field current is generally quite workable to use for other applications, and should be available. Something I've done many times is buy a set of brushes that a counter man can actually show me (I know it's in stock then, and I can assess it for suitability) then carefully sand it to size to suit the motor I would be putting it in. This is messy, but is fairly quick, and it beats trying to find the proper ones if you've already struck out a few times. Visit someone who repairs small appliances or power tools.

12-23-2005, 05:26 PM
"motor kicking out in rapid traverse" -Exactly what is happening?. As you know, the brushes are conducting current to the armature, if they are too short, they are either not making contact, cos' the spring pressure is too low or they are jamming diagonally across the brush holder. The other causes of a dodgy motor are:- short circuit armature windings or a commutator with either no connection to its coils or a short between adjacent sections. Buy a $10 multi meter, switch it to ohms, turn of all power to the machine and put your test prods, one to each brush holder. Rotate the motor by hand. What you should see is a low resistance, .1 - 2 ohms between the brush holders with a dead spot as you switch over the commutator insulations. Any high resistance spots should be investigated by tweaking the springs with a little screwdriver, if this cures it then brush replacement is the answer. Buy some for a similar motor, i.e. 12 - 24 V. Sand them down. The length is fairly critical. How do you know that you have used up 40% - you have only got the worn ones to compare to? The end terminations can be cut off, a hole drilled through them and the new braid passed all the way through and soft solderd on the OUTSIDE. Be very careful with the end terminations and the springs, they will have to be pre-tensioned and the little b******s will fly apart and loose them selves in your work shop. The only other job that is within the scope of the home workshop, is to recondition the commutator, turn the armature between centres, using a "finishing" tool(large radius end) take a very light cut over the commutator, clean out all the segments with a Stanley knife or such like. Could it just be that the motor is taking too much current cos' the mechanical loading is too high?

12-23-2005, 08:19 PM
Scott,send me your brush measurements and I'll drop you a set in the mail for the postage cost.

12-24-2005, 03:00 PM
"axis drive kicked out" -blew the fuse?, motor stopped ? -??. How can you tell if your brushes are 30% worn, unless you compare them to a new one? As far as I can tell there are the following faults with DC motor armatures :-
Poor brush contact (worn brushes/overheated springs) - intermittent contact & sparking.
Brushes jammed - motor stops but can be re-started after whacking it with a mallet!
Segments(between the commutator contacts) fouled up. - high current and overheating. - clean out with scalpel.
Burnt commutator - motor won't start every time, indicates something worse normally. Clean with glasspaper.
Short circuit armature coils - too much current & overheating causing burnt commutator. - replace/rewind motor.
Something mechanically wrong, too much load on the motor - causing high current & overheating.
If the brushes are suspected to too short, increas the spring tension temporarily by inserting a 1/16" thick piece of copper/ silver paper under the end of the spring AWAY from the armature end. i.e. this will increase the compression of the spring, if the motor then runs correctly Fault Found! When modifying the brushes from a different source, be VERY careful with the spring & end cap, tie a piece of cotton say 12" long on each and a piece of paper ( to be removed when the soldering is done) because these little varmits will shoot away on re-assembly , never to be seen again.
best of luck Frank