Root cause of Z-Axis humming

Hi Guys!

Repost here, as Im hoping this gets a little bit more traction here

So:

The Z-Axis on my MV Junior has started (several months ago) to make a humming noise, when it is near zero return or at zero return.
The noise starts suddenly and sometimes disappears from itself.

I made this video to record it:

YouTube


It clearly comes from the Z servo.

As soon as I move the axis a little bit away from the zero return position, the noise disappears, as shown in the video. In the video I then return to home, and the noise does not re-apear. Only to come back some minutes later. I really dont know where to check for a problem. Its like the servo is oscillating at this position, but why should it?


The Z-Axis on this machine does also have two other strange issues, but which do not trouble me much, but maybe are connected:

1. Often when I turn the machine on and want to home all axis, I get a z-axis soft limit error, when the axis is homing. Only way arround it is to restart the machine in IPL mode, disable soft limits, home, then restart und home again, and it usually does home without a problem then.

I confirmed, that is is due to the Z-Axis moving a little bit down, when the machine is powered off. This is movement which is not seen, and thus the control errors out. Not such a big deal.

2. I cannot run the Z-axis with 100% rapid speed toward the Z+ direction (towards zero return position). When I do this, the axis abruptly stops (after approx 1 second) and the machine throws a alarm (Z AXIS EXCESS ERROR (MOVE)). It only alarms with 100% rapid. If I go with 50% rapid, everything works fine.


When I move the axis by hand (by turning the powered off servo, it moves freely to my opinion. I cannot feel more resitance or a problem near the home position.

Has anyone and idea, what this could be? Or what to check?

As these issues are all related to this axis, there must be something off. But is it mechanical? Or a wrong parameter (i got the machine with a defective masterboard and got it back to life, with all new parameters set from the internet).

Or shall I start messing arround with the speed and positon controller setting potentiometers?

Thanks for any advice.

Normally I'd say that it's gain drift on the amplifier, but that wouldn't cause it to alarm at rapid.

You have brushed servos, so I'd start by removing the brushes and blowing out any carbon dust that has accumulated. Clean and DRY air is mandatory for doing this...

Thanks alot for your advice!

I will check the brushes!

I also rechecked the manual again about the z axis excess error (move) at 100% rapid. the maintenance manual also mentions brush check as a remedy fir this error.
Honestly I did not realise that I have brushed servos..

Really sounds like it could be the problem!

At my work they had a mori that would buzz really loud on startup and fault out. They finally got it fixed but, it took 2 mori factory repairman (from Japan) and multiple other repairman and lots of new parts that didn't fix it.