Siemens DC Servo trouble

AlfaGTA · Apr 18, 2018

I have a machine that uses older perm. magnet DC servos.
The one on the machine 's "Z" axis is giving me issues.
Servo is a Siemens HU 3078 0AC01

The problem is one of noise and seeming vibration that i can feel in the machine when the axis is moved.
Makes a rumbling sound , bets louder with speed.
Makes the noise although somewhat different in tone going up or down.

Have a similar machine with similar servo that i have swapped and the noise clears up with the exchanged servo...so i am focused on the motor....

I have had the servo apart and inspected the stator and commutator...all looks fine, no burned spots of discoloration to my eye.

I have replaced all the bearings in the servo, both on the stator and on the final drive support.

I am at a loss at what to do from here.....

Servo has a 24v brake and a DC tachometer as part of the package...as well as a right angle drive that is used for the hand wheel....

On re-assembly with the new bearings i was able to freely rotate the servo by hand by energizing the brake...no unusual noises

I really don't have a way to run the servo out of the machine (183 VDC)

Looking for any wisdom here as this is making me nuts......

Cheers Ross

adamm · Apr 18, 2018

I've had a problem that sounds like it was a similar noise on my Siemens DC servo equipped lathe. My problem ended up being the encoder cable failing. The machine is from 1981, and I think it had the original wiring. The insulation was failing (brittle and breaking off). I think it was causing subtle feedback error. Too subtle to trigger an alarm on the control, or cause dimensional problems, but it would make the motor vibrate.

BugRobotics · Apr 18, 2018

Hey AlfaGTA,
I have a similar issue when cold with my Siemens Z-axis motor on my Hermle UWF 851. Scratchy movement until it gets warmed up (roughly 20 seconds). I haven't investigated in depth but did the same preliminary checks you mentioned (except bearing replacement) and all was well. My next step would be to hook up a scope to the tachometer output and check the quality of the signal.

Mud · Apr 18, 2018

The problem is one of noise and seeming vibration that i can feel in the machine when the axis is moved.
Makes a rumbling sound , bets louder with speed.
Makes the noise although somewhat different in tone going up or down.

Can you swap tachs between the servos? My FP7NC made similar noise/vibration in the spindle drive, cleaning the tach and reseating the brushes made it quiet and smooth.

Vancbiker · Apr 18, 2018

Mud said:
Can you swap tachs between the servos? My FP7NC made similar noise/vibration in the spindle drive, cleaning the tach and reseating the brushes made it quiet and smooth.

+1 on the suggestion above. Cleaning and new brushes for the tachogenerator is the first move on machines equipped with them when having servo problems. There's a good reason they disappeared from use on modern machines....

AlfaGTA · Apr 18, 2018

Thanks guys for the responses....will try swapping the tach assembly...easy enough to try.

Cheers Ross

thermite · Apr 18, 2018

Vancbiker said:
+1 on the suggestion above. Cleaning and new brushes for the tachogenerator is the first move on machines equipped with them when having servo problems. There's a good reason they disappeared from use on modern machines....

Disappeared from use?

I cannot disagree that digitals can be better, have no idea how many "design wins" if ANY they still gain, new systems, but...

The classical analog tachogenerator yet lives, new production, made in USA.. even if ultimately run by a banker:

Desco | Team

ServoTek Products | DC Tachometers and Encoders

And.. ServoTek has pricing that isn't at all onerous.

Is there a ServoTek drop-in for whatever Siemens used? Above my current pay-grade.

But even ServoTek's catalog & data sheets carry a great deal of information that might be helpful.

Pathogen · Apr 18, 2018

The Permanent Magnet DC Motor | electric equipment

I learned the hard way when I pulled the armature out of a permanent magnet dc motor. The magnets lost some of their magnetism and the motor was no longer useable. I posted the link above so you can investigate for yourself. The speed - torque curves of these motors are easily influenced by the magnetic field of the magnets.

If in your attempted repair you removed the armature, you probably can kiss that motor goodbye.

sable · Apr 18, 2018

I have a machine with the same problem in one direction on the X axis ,it has been like it for years, machine does not get used much and one day I might fix it ,especially if I know what needs doing.

sable · Apr 18, 2018

Pathogen said:
The Permanent Magnet DC Motor | electric equipment

I learned the hard way when I pulled the armature out of a permanent magnet dc motor. The magnets lost some of their magnetism and the motor was no longer useable. I posted the link above so you can investigate for yourself. The speed - torque curves of these motors are easily influenced by the magnetic field of the magnets.

If in your attempted repair you removed the armature, you probably can kiss that motor goodbye.

I am no expert but I think you just need to follow the armature up with a steel tube to act as a keeper.

Vancbiker · Apr 18, 2018

thermite said:
Disappeared from use?

I cannot disagree that digitals can be better, have no idea how many "design wins" if ANY they still gain, new systems, but...

The classical analog tachogenerator yet lives, new production, made in USA.. even if ultimately run by a banker:

So.... how many tachogenerators do you see on current CNC machines???

Using pulse frequency to determine axis velocity has lower potential for trouble than an analog feedback. You already have the encoder for position control. Why add another device for velocity control when the encoder signal can be used for that too?

Tachogenerator was good technology in its day. Sure, you can still get them, just like I can still get parts got my 1963 tractor. Some designers like to use "time-proven" technology. I live in the past sometimes too.

Mud · Apr 18, 2018

If in your attempted repair you removed the armature, you probably can kiss that motor goodbye.

Disclaimer - I'm not a servo expert. But I don't think it's as dire as all that. In discussion with a few companies who repair/rebuild servos I asked about this because I had a fairly large one disassembled that I wanted repaired. All answered that it probably was not an issue if it had not been apart for a long time - months to years. Two did reply that they liked to put something steel in the case in place of the armature to ensure the magnets were not weakened. One said they sometimes used a length of chain ("anything steel, even a pile of chain") if nothing else was available over a length of time. Another said "We've never seen it happen".

BTW, I use an abrasive eraser to clean the tach commutator.

thermite · Apr 18, 2018

Vancbiker said:
So.... how many tachogenerators do you see on current CNC machines???

Did

"design wins" if ANY

Miss your notice when I first posted it?

When you then quoted it?

Or just BOTH?

I'd not expect to see ANY since resolvers first became affordable. Far easier to make them really, really, REALLY repeatable, also "absolute", if need be, they are inherently linear, there is little degradation, if any, from wear or age, just for for openers.

Analog? "none of the above" goodness is inherent. Nor is that "goodness" even easy to approach, if one can approach it at all.

All that said? Somewhere around $250-$450 still gets you a new "vanilla" ServoTek. Somewhere around $20 to $100 still gets you a used-but-good one - higher-end even with a form of warranty.

I think Ross has a "decent" repair budget, but not so huge as justifies scrapping this machine for brand-new, all-digital, CNC, so.. we can expect to see him fix what he has,

'coz "obsolete" does not mean useless.

Some among us do have to remind our wives of that now and then, of course, but it can be a pleasant project!

AlfaGTA · Apr 18, 2018

Pathogen said:
The Permanent Magnet DC Motor | electric equipment

If in your attempted repair you removed the armature, you probably can kiss that motor goodbye.

Armature was never removed....Not my first rodeo.....inspected the field and armature by pushing it half way out one end of the field enclosure , then reversed to expose the opposite end ....
Never out of the magnets.....

Bearing change can be done with the armature fully nested in the field magnets...which is what i did.

I do posses a factory service and parts pamphlet for that servo .
There are precautions within describing the process to remove the brake. (carries the bearing at one end) how you must energize (24VDC) the brake to disassemble so as not to damage the brake.
There is however nary a mention of needing to use a pole piece in the magnets when removing the armature....
Cheers Ross

adama · Apr 18, 2018

Its a DC motor, although rated at 183V they will normally run - spin on a bit over 12V, think car battery! You don't want to get any were much over a 1/4 rated voltage on a totally unloaded dc motor or they can self destruct through over speeding.

thermite · Apr 18, 2018

AlfaGTA said:
nary a mention of needing to use a pole piece in the magnets when removing the armature....
Cheers Ross

I'd suspect that few have been that fragile since shortly after abandoning AlNiCo.

DC Drive manuals cover damage or NOT to PM motors. Those can be waaaay more rude in reversing & c. as to possible damage to magnetic state than passive dis-assembly, yet seldom become a Real World // real application problem.

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