Dissecting a DC axis motor

[Andy Fritz]

Hi Guys!

During the last 2 years my 4020 has eaten three Glentek AMP-0021's and it looks like poor insulation in the axis motors is causing this. I will order three new Baldor equivalents to the Glentek GM6000B's I am replacing. In the mean time I would really like to learn how these things happen and see if it is possible to clean out the motors so I can use them as back ups. There are a few things I cant figure out;

1. Despite eating amps there has not been any signs of degradation during cutting. I had the Y motor make some small "jumps" a while ago, but that went away by just blowing out carbon and replacing brushes. Is it "normal" that a motor can degrade to the point it consumes amps but still work fine for cutting?

2. One tech said that if I get less than 5 megohms megging at 2000VDC between motor lead and chassi the motor will eat amps. My megger only goes to 1000VDC and I am getting 0,05 megohm on X, 0,8 meg ohm on Y and 1,3 megohm on Z. It is very tempting to think that the megger is shot, but unfortunately I am getting 75 megohm on the A motor which is rarely used. But that is a Glentek 4020 motor, so not the same.
Does anyone know what is considered normal insulation resistance on a Glentek GM-6000B motor?

3. I pulled the X motor off the machine and I really want to pull it apart. If nothing else, just to see if I can learn something. I keep reading about magnets loosing their magnetism if a motor is pulled apart, but as there are people re-building these I am sure there must be a way to do it. Any comments?

4. What is the proper procedure of opening the motor? I have removed the main cover, removed the resolver and removed the 4 long bolts from the front. There is now a small gap between the main casing and the section where the motor holders are, but I cant pull them apart. Should I also remove the tach?

On this particular motor I am getting 78 megohm between the black motor lead and chassi with brushes removed, but only 0,05 megohm between the red lead and chassi. I was hoping I could find something sticking out of the winding or maybe clean the winding, bake it and spray some type of varnish on the winding and see it that would help.

As I am in Thailand, sending the motors to the US for re-build is not viable so either I try to fix them or they will end up corroding away on a shelf.

Any help would be greatly appreciated!

Cheers,

Andy

[Al_The_Man]

I have rebuilt motors without inserting a metal 'keeper' in place of the armature with no ill effects.
If you want to you can slide an iron bar in as you draw the armature out.
Warning, as soon as the armature leaves the bearing, the tendency is for it to be immediately attracted to the permanent magnets, care has to be taken not to chip or fracture the magnets.
I am not sure how your tech came up with high currents being possible with 5meg at 2000v, at 200v it will be the same or alot less, most likely.
Blowing the carbon out is a quick fix, but if the build up is high or the brushes worn right down, the motor should be disassembled and the brush holders completely cleaned or tracking can occur.
If the motors are controlled by analogue input drives, they can be replaced by easily available Advanced Motion or Copley Controls.
Al.

[Andy Fritz]

Hi Al,

Thanks for replying.
I removed a nut which locks the tach commutator to the shaft, but I still cant remove the main armature. The tach stator / brush holder does not seem to have any screws holding it to the motor. Should I just pull it out with some polygrips?

Andy

[Al_The_Man]

You should not have to remove the tach armature, it should come out with the main armature in one piece.
After removing the tach brushes, the end plate should come off after removing the screws which often go the length of the motor, remove the tach connections first.
Unless the endplate comes off with the stator, but that is unusual?
Al.

[Andy Fritz]

I removed the four long bolts and can only separate the main cylinder 1 - 2 mm from the section where the brush holders are creating a small gap as show in the picture.

It feels like something else besides the tiny tach brushes are preventing the two parts from separating.

I can remove the tach brushes, but it looks like it will be difficult to put them back.

Andy

[Al_The_Man]

You have to remove the tach brushes, regardless.
I am not sure what should be preventing the end cover from coming off?
It may take some very close scrutiny, maybe with a strong light on the subject?
Al.

[Andy Fritz]

I removed the tach brushes, but that did not make much difference.
When you write "end cover" do you mean the part holding the motor brushes or do you mean the shaft end plate?

Is the motor shaft normally pushed through the front bearing? If that is the case maybe it is just a matter of the shaft corroding to the bearing.

Andy

[carbidecraters]

You can eat amps when your 2 bearings go bad and the windings start to rub on the magnets on the inner part of the case. You will usually get a squeeking noise from the motor if you rotate the shaft.

[Andy Fritz]

Ok I made some progress,

I managed to tear off the brush holder casing from the rest of the frame. As I got the same 0,05 megohm between any of the three positive brush holders and case I figured either a broken motor wire or carbon tracking. Eventually I found that the rubber compound the brush holders are insulated with must have absorbed carbon dust over the years. As soon as all the rubber insulation was scraped off I got over 100 megohms megging at 1000VDC.

Now the question is what to use to insulate the bottom of the brushes and wires. I think varnish would crack due to expansion when the brush holder gets hot. The rubber insulation obviously does not work well.

I thought maybe the best is to leave it bare and just blow out dust more frequently. What is used on a Fanuc DC motor? Any advice?

At least the top bearing, a Fafnir 205PP, does not have any radial play and is silent but if I spin it by hand it only spins 1/2 turn after release. Should they spin like a bicycle wheel?

Are bearings used in axis motors typically high preceision or of the standard tolerance USD 15 type?

Cheers,

Andy

[Al_The_Man]

There are a few things you can use to clean and treat them with, but the trick with carbon tracking is to stop the build up in the first place.
Even on a varnished surface, if the build up is great enough and tracking starts it burns off the varnish and creates a nice conductive path.
I have seen quite a few older machines that motors were lost through lack of regular maintenance. Older machines with DC motors are often overlooked, and the last time I checked, Mitshubishi wanted $6,000 for a large DC servo motor with 4 weeks delivery.
As to the bearings, the sealed type do not spin when new, of course they could have lost the grease and are rusting!
Usually if you have gone to the trouble of dismantling, bearings are relatively cheap and worth replacing at this stage.
Al.

[Andy Fritz]

Hi Al,

The bearings are at least two years old as that is how long I have owned the machine. If corroded would they not feel rough? They actually feel very smooth, just not spinning more than half a turn.

If I can find similar bearings locally I will replace them for sure.

What are the options for varnish or other compounds?
Anything readily available that may work? Nail varnish maybe:-)

What about leaving the brush holders bare?

Andy

[Al_The_Man]

The bearings sound like they are still OK.
A non-residue contact cleaner can be used for cleaning, and then either Glyptol or Conformal coating could be used to coat them.
Leaving them as-is is an option with regular checking of brush wear.
Al.