Cooking G320's all of a sudden

[tomindallas]

Has anyone ever run into a situation where one axis just started cooking G320's? I though the Gecko went bad, swapped in the spare and it cooked itself until it burned my fingers (literally) in about 5 minutes.

Nothing changed and last time the machine was run it went months & 100's of hours without problems. I then swapped the original drive back in after it cooled and it ran perfectly for about 20 minutes until it also cooked itself. I noticed when the axis became unstable during rapids and finally just started oscillating until I could get the DC supply turned off.

Help!

[John3]

Originally Posted by tomindallas ...... I noticed when the axis became unstable during rapids and finally just started oscillating until I could get the DC supply turned off. Help!

An oscillating feedback loop is sure a way to heat things up.... I'd check for causes of that.

How tightly coupled is the encoder to the motor? Is there a loose belt, coupling with loose set screw? Motor brushes in poor shape could cause intermittent loss of servo control and oscillations.

Or did the load dynamics change? More or less inertia? Maybe even different motor supply voltage.

If not these, could the motor have a partially shorted armature?

Is there any difference in static / steady state loads, more friction, more weight on Z axis, etc.....

John

[tomindallas]

John, thanks for the reply so quick. It's good to know that oscillation will cause it to heat up, not the other way around.

I used forced-air cooling on the original G320 to finish the job and it ran fine for another 10 minutes to complete with no oscillations.

More information, it's a Series II with the air balanced knee and the Z drive is having the problems. I'll verify the air pressure is still balancing the knee correctly later today and make sure the lube is getting to the knee adequately.

Since the motor runs fine with a cool G320 I'm guessing it may be an encoder problem. I've had problems on this axis before with the encoder (loose on the shaft) and I'll just order up a new one tonight and trade it out. This motor has always been problematic to tune because of all the inertia from the knee and table. It tunes in OK, but the range of tolerance on the pots is very narrow. I think I'll also make a new bracket and install a monster servo next week too...